THE MECHANISM OF LIFE 
 
" Of philosophy I will say nothing, except that when I saw that 
 it had been cultivated for many ages by the most distinguished 
 men, and that yet there is not a single matter within its sphere 
 which is not still in dispute, and nothing, therefore, which is above 
 doubt, I did not presume to anticipate that my success would be 
 greater in it than that of others; and, further, when I considered 
 the number of conflicting opinions touching a single matter that 
 may be upheld by learned men, while there can be but one true, 
 I reckoned as well-nigh false all that was only probable." 
 DESCARTES: The Discourse on Method. 
 
 " When I wrote my paper on the thymus gland, I was very con- 
 scientious about the literature on the subject. I found that many 
 memoirs had been written and published, and I looked at them 
 all or, at least, at all of them that I could obtain. There were 
 many German works, not many French and Italian ones, and a 
 number of English papers. I collated and made abstracts of them , 
 and discussed all the results and conclusions, and, generally, 
 rounded off our knowledge with regard to the matter. Altogether 
 I found afterwards that there were fifty-two memoirs on the 
 development of the gland. My paper only made the number fifty - 
 thrv> !" Unpublished Letter from a Young Zoologist. 
 
THE MECHANISM 
 OF LIFE 
 
 IN RELATION TO MODERN PHYSICAL 
 THEORY 
 
 BY 
 
 JAMES JOHNSTONE, D.Sc. 
 
 PROFESSOR OF OCEANOGRAPHY IN THE UNIVERSITY OF LIVERPOOL 
 
 LONGMANS, GREEN & CO 
 LONDON: EDWARD ARNOLD 
 
 IQ2I 
 All rights res i- wed 
 
B1C 
 
 PRINTED IN GREAT BRITAIN. 
 
PREFACE 
 
 IT is possible that the title of this book may be misleading to 
 some readers, and so an explanation may, very appropriately, 
 form the subject of this introduction. Well, then, by " the 
 mechanism of life " is meant nothing more than the results of a 
 scientific analysis of the activities of living animals. First, we 
 must define what is meant by " scientific method," and this is 
 not at all difficult now that Einstein, Eddington, and the other 
 relativists, have persuaded us to think about what we do when we 
 investigate something " scientifically." 
 
 What we do, in that case, is to observe space-time coincidences 
 in a four- dimensional manifold that is really and actually our 
 procedure, though it seems rather dreadful ! It would be very 
 inconvenient, also, to sustain oneself in this plane all the while, 
 and so we proceed to let ourselves down to earth, so to speak. 
 From the space coincidences that we observe (for instance, the 
 coincidences of the top of a column of mercury in a barometer 
 tube with certain marks on the adjoining scale) we infer space 
 measurements, and from the coincidences of the hands of a clock 
 with marks on the dial we infer time measurements. That 
 simplifies the method a good deal. 
 
 Then it is only the relations between series of space-time 
 measurements that form the data of science (its differential 
 equations), but that, again, is very trying, and so we assume 
 that there are things in nature. These things are separated 
 from each other, at the same instant of time, by intervals of 
 space, while they are separated from each other, in the same 
 space, by intervals of time. Thus we have something to lean up 
 against and sustain ourselves in this rather difficult process of 
 apprehending nature. The things that we regard as existing 
 apart from each other in space and time are electrons. But 
 just yet that is rather inconvenient, and so we regard our natural 
 things as atoms and molecules in motion in an arbitrary three 
 dimensional space and an arbitrary one- dimensional time. 
 
 V 
 
 460727- 
 
vi PREFACE 
 
 Thus there are atoms and molecules which exist and move 
 and form configurations that is, constitute physico-chemical 
 systems in space and time. When we speak about a " mechan- 
 ism," we mean the motions and configurations of material 
 particles. Already we have gone a long way, via inferences, 
 from our " real and actual " observations of the passage of 
 nature, which observations are space- time coincidences; but 
 never mind that : let us stick to our notion of mechanism systems 
 of material particles and their motions and configurations. The 
 descriptions of such systems by making use of space measure- 
 ments, and the devising of mathematical relationships between 
 them (the differential equations), are the method of science. 
 What we call " space " may be measured in terms of x and y 
 and z, the old space dimensions, and t the time one ; and so the 
 equations that we make involve the four " variables," x, y, z, 
 and t. 
 
 That is what physiology does; whatever its particular methods 
 may be, they involve the observations of space- time coincidences 
 the readings of the pointers, scales, etc., of instruments. It 
 observes systems of material particles (the atoms and molecules 
 making up tissues) in certain configurations, and then, after 
 intervals of time, in other configurations. Sometimes the differ- 
 ences between the configurations can be thrown into mathe- 
 matical forms, but more often they cannot. 
 
 This, therefore, is what is called mechanism, and it is the 
 method of physiology. It is the study of the successive phases 
 of a material energetic configuration or system. Note that it is 
 not necessarily the study of an organism. Usually what is 
 investigated is a part of an organism, or even the dead material 
 of the latter. And in all cases it is the study of the physico- 
 chemical activities of the organism that is the object of physiology. 
 In the very act of investigation these activities are necessarily 
 dissociated from each other, and the result is a number of partial 
 views of the whole organic activity. Of course, all this is in- 
 dispensable, and so the greater part of this book is really a sum- 
 mary of the main results of physiological science, and is intended 
 to give the reader an attitude (for he must supplement what is 
 said here) in his attempt to understand life. 
 
 It would be inconvenient, and even pedantic, to state these 
 results in terms of the fundamental space-time concepts, and so 
 our analyses of the activities of the living organism must continue 
 
PREFACE vii 
 
 to use the familiar ideas of atoms, molecules, colloids, chemical 
 and physical states of equilibrium, energy-transformations, 
 potentials, radiation, and so on. In the light of modern physical 
 theory, however, most of these concepts are derived ones, and if we 
 use them in speculations upon the nature of life, there may be 
 some crudeness in our statements. Thus, quoting a very good 
 modern statement as to the aims of biology * 
 
 1. " Scientific biology is strictly deterministic. It admits the 
 possibility of only one result from a given set of antecedents." 
 
 2. " Scientific biology endeavours to explain organic pheno- 
 mena on the basis of antecedent physical conditions, though 
 admitting that our knowledge of cause and effect is in the last 
 resort empirical, to the extent that much which happens could 
 not have been predicted in advance." 
 
 3. " Scientific biology declares that vital phenomena are 
 chemico-physical in the sense that they are the inevitable out- 
 come of the particular material aggregations which we call 
 organisms." 
 
 Now whether our knowledge can be regarded as proving the 
 above theses is the subject of the following chapters. We must 
 be very clear as to what is meant by " determinism," " antecedent 
 physical conditions," " cause and effect," and " particular 
 material aggregations." We have really nothing to do with 
 determinism because the concepts that we employ in discussing 
 our results are those of functionality, correlation, and proba- 
 bility. Determinism is a logical category, or perhaps convention, 
 and it is strict only in mathematics, where, since we make 
 the rules, the strictness of result is to be expected. We assume 
 determinism because it is our mental postulate, and also because 
 we find that it works more or less approximately in chemistry 
 and physics, and even in physiology, so that we can construct 
 and use machines and cure some oliseases. But it never works 
 out exactly, and our results always have the form V e, where 
 V is the value we adopt for something or other as the result of 
 experiment, and e is a " probable error." W^e never get a 
 unique biological result from a " given set of antecedents " : thus 
 the bodily characters of an individual animal may surely be 
 regarded as the result of the characters of its ancestry (which 
 are the " antecedents " ; but this individual result is only one of 
 
 * F. B. Sumner, The American Naturalist, vol. lii., 1919, pp. 193-217;. 
 vol. liii., 1919, pp. 338-369 
 
viii PREFACE 
 
 a number of such (the combinations of bodily characters of the 
 brothers and sisters of the individual), and all of these combina- 
 tions differ from each other though they have the same antece- 
 dents. Sometimes we say that the individual results " ought to 
 be " the same if only we could experiment or observe with suffi- 
 cient accuracy ; but in saying so, are we not simply dogmatising ? 
 
 Further, it is said that " vital phenomena are chemico-physical 
 in the sense that they are the inevitable outcome of the par- 
 ticular material aggregations that we call organisms." But they 
 are not " inevitable," and are they the outcome of " material 
 aggregations " ? It is quite certain that it is not material aggre- 
 gations that our method of science observes in nature, but rather 
 space-time coincidences. We do not know about things, but 
 only about relations which are differential equations between 
 dx, dy, dz, and dt, which symbols are, after all, " ghosts " of 
 space and time. No doubt it is very difficult to think in this 
 way, and one naturally leans up against a mentally constructed 
 world of atoms mathematics is so tiresome ! But when we 
 would speculate about the nature of life and so on, we must not 
 lean up against anything, and pur analysis should at least be 
 as penetrating as that of the physicists. On the whole, one 
 prefers the conclusions of some of the latter our knowledge of 
 the world is a knowledge of form and not of content. We know 
 relations only, and the unknown stuff of the world may just as 
 likely be the stuff of our consciousness as something consisting 
 of electrons. 
 
 Anyhow, life is, after all, mainly an affair of organisms acting 
 individually and as entire, undecomposable entities. It is mind, 
 feeling, perception, memory, emotion, pleasure, pain, and so on. 
 To the vast majority of men and women (to say nothing of all 
 the " lower " animals) these states are life, and it would be very 
 stupid not to recognise that in our philosophy. Tropisms and 
 " concatenated reflexes " and colloids and enzymes, and so on, 
 are all very well in their way, and we have to investigate them 
 if we are to get on in the world, and be comfortable, and live long 
 (quite legitimate objects of scientific research); but are these 
 notions anything else than the terms in our description of how 
 the living (or dead) organism cuts up, so to speak ? Is not this 
 common sense ? 
 
 If we do recognise that mind and intuition of living are to 
 count in our speculations, what becomes of determinism (and 
 
PREFACE ix 
 
 prediction, which surely goes with determinism if we cannot 
 predict, why say that events are determined) ? For mind and 
 memory and feeling and perception are certainly not measur- 
 able in terms of space and time (despite the Weber-Fechner 
 " psycho-physical law "). So the problems of free-will and 
 necessity and determinism are meaningless when they are con- 
 sidered with reference to the mind, for the very essence of these 
 notions is measurement, and we cannot measure mind. 
 
 Such, then, are the lines on which the phenomena of life 
 are discussed in this book, and the reader is asked to take the 
 arguments " on their merits," and without conscious clinging 
 to either mechanism or vitalism. 
 
 Vitalism holds that there is something in the living organism 
 which is not present in an inorganic thing. This may be " spirit," 
 or " soul," or perhaps some hitherto unrecognised " biotic energy- 
 form," or some factor which is not energetic in nature, but which 
 confers direction upon the energy-transformations that occur 
 in the living organism. Most people take one or other of these 
 attitudes; thus some may confess to sympathy with those 
 who would " remove organisms from the domain which includes 
 the stars and precious stones," but may not think that mechanism 
 " exhausts the reality of earth and heavens, still less that of the 
 flower in the crannied wall " ; others like to think that " the sun 
 and moon and all the little stars are a glorious mechanism." 
 Either feeling is, of course, permissible, provided it does not 
 influence our judgment. 
 
 Also, no one can think about these questions without becom- 
 ing " metaphysical," even if he does not know it. There is no 
 harm in that either, provided that one does it nicely. So I have 
 taken care that anything " transcendental," or otherwise objec- 
 tionable, has been discreetly relegated to the Appendices. 
 
 J. J. 
 
 LIVERPOOL, 
 1921. 
 

 
 CONTENTS 
 
 , CHAPTER I 
 
 THE NATURE OF ANIMAL LIFE 
 
 PAGE 
 
 The organism as a structure Structural differences Structure and 
 function Integration of activities Co-ordination of activities 
 The organism and the State ... -I 
 
 CHAPTER II 
 
 THE SENSORI-MOTOR SYSTEM 
 
 The meaning of organic movement Mobility patterns The sensori- 
 motor system The motor organs The sensory mechanisms 
 The nervous links An example of sensori-motor activity 
 
 CHAPTER III 
 
 THE PRINCIPLES OF ENERGY 
 
 The nature of a material body Modern theories of matter The 
 capacity for doing work Energy The diminution of available 
 energy Energy in the abstract Potential and kinetic energ}' 
 Releasing transformations The principle of becoming - 
 
 
 CHAPTER IV 
 
 THE SOURCES OF ENERGY 
 
 The inanimate engine The animate engine Digestive process in 
 animals The foodstuffs Absorption and distribution The 
 respiratory interchange The sources of energy The removal 
 of waste Glandular activity - 55 
 
 CHAPTER V 
 
 ON VITAL PRODUCTION 
 
 Animal metabolism The fate of the proteid residues Plant meta- 
 bolism The balance of life Production and consumption - 76 
 
xii CONTENTS 
 
 CHAPTER VI 
 
 BRAIN AND NEEVE 
 
 PAGE 
 
 The general scheme The arrangement of the ganglia The spinal 
 cord The brain The development of the nervous system The 
 human brain Connections within the central nervous system 87 
 
 CHAPTER VII 
 
 THE SPECIAL NERVOUS MECHANISMS 
 
 The sensory mechanisms The motor mechanisms The " simple 
 reflex" mechanism The cranio-spinal reflexes The mechanism 
 of co-ordination Cortical control - 106 
 
 CHAPTER VIII 
 
 THE ANALYSIS OF BEHAVIOUR 
 
 Stimulus and response in general The lower brain activities 
 Nervous inhibition The cortex cerebri The meaning of 
 behaviour The nervous system as a whole - 130 
 
 CHAPTER IX 
 
 THE MECHANISTIC CONCEPTION OF LIFE 
 
 The Cartesian mechanism The influence of chemical and physical 
 
 investigations Modern mechanisms of life - 152 
 
 CHAPTER X 
 
 THE MEANING OF PERCEPTION 
 
 The nature of sensation Perception The problem of free-will In- 
 determination in acting Memory and habit The " categories of 
 the understanding " Space and time - - 164 
 
 CHAPTER XI 
 
 THE NATURE OF LIFE 
 
 The laws of thermodynamics The condition of the universe 
 Dysentropic phases in the universe Digression on orders of 
 magnitude and duration The improbability of life in the uni- 
 verse The physical nature of life Life and energy - - 192 
 
 APPENDICES 
 
 I. A .M MA 1-HYSICAL DISCUSSION . 222 
 
 I'ACE IN THE MODERN THEORY OF RELATIVITY - - 235 
 
 III. THE SUB VI AXILLARY GLAND AN INSTANCE OF ORGANIC 
 
 FUNCTIONING - 237 
 
 INDEX -.--.--- 241 
 
THE MECHANISM OF LIFE 
 
 CHAPTER I 
 ON THE NATURE OF ANIMAL LIFE 
 
 The Organism as a Structure. The student of biology 
 usually begins his studies by dissecting the body of a warm- 
 blooded animal; and, first of all, he notices a division of this 
 into well-marked regions: head and neck, trunk, limbs, and 
 perhaps a tail. Then, taking a knife, he slits the skin over a 
 limb and finds beneath it fleshy masses (the muscles) attached 
 to rigid supports (the bones). He sees that the latter are 
 movable on each other by articulations, or joints, and from his 
 general knowledge of the animal in the living state he recognises 
 that bones and muscles are parts that move in ways that depend 
 on the nature of the articulations. Looking more closely, he 
 sees white, glistening threads beginning in the muscles, joining 
 together and passing into the central parts of the body. These 
 are the nerves. There are also two kinds of bloodvessels: 
 one kind which are stiff and white and apparently empty, and 
 another kind which are rather thicker and softer, and which 
 contain blood. The former are arteries, and the latter veins. 
 All these parts bones, muscles, nerves, arteries and veins are 
 wrapped up in a loose kind of material called connective tissue, 
 and this must always be separated in order to disclose the forms 
 of the muscles and other parts, and the ways in which they are 
 joined together. 
 
 Then, opening the cavities of the body, he discovers the viscera 
 that is, the lungs and heart-, which occupy the cavity of the 
 thoracic region of the trunk and the stomach, liver, alimentary 
 canal, kidneys, digestive glands, and reproductive organs, all 
 of which are contained in the abdominal cavity. Here, too, 
 attentive study shows the existence of bloodvessels and nerves 
 which ramify among the visceral parts. 
 
 Lastly, breaking open the bones of the skull, he finds a white, 
 
2 ' ITE -MECil-AlSM ' OF LIFE 
 
 soft mass in the cavity of the head. This is the brain, and 
 further examination shows that it is connected, by means of nerves, 
 with the great sense organs of the head that is, the eyes, ears, 
 nose and tongue but also with a similar mass of nervous tissue 
 lying inside a tubular cavity in the backbone. This is the spinal 
 cord, or marrow. Looking at it more closely, he finds that the 
 spinal cord gives origin to nerves which can be traced out into 
 the muscles of the trunk and limbs, and also into the skin. 
 
 In short, he finds that the animal body is a structure : a series 
 of parts arranged in a definite way. 
 
 Structural Differences. Now, extending his study to other 
 examples of the animal kingdom, he finds other structures 
 which are analogous, or similar in a general way, to that which 
 he has already examined. There are, however, very notable 
 differences. The fore and hind limbs of the mammal are replaced 
 by the wings and legs of the bird, or by the little side fins of the 
 fish. He will find that all backboned animals possess these 
 two pairs of limbs in some form or other, but that they may 
 differ remarkably in structure. Examining the animals below 
 the vertebrates, he will find still greater differences: thus there 
 are about twenty pairs of limbs (or appendages) in a crab, 
 shrimp, or lobster, but many more in a centipede, and such a 
 creature as a starfish has apparently no limbs at all (though one 
 speaks about its radial " arms "), but beneath the body he will 
 find some thousands of mobile parts which serve for locomotion, 
 and are known as the " tube- feet." The viscera also vary very 
 strikingly. Thus there are distensible lungs in the quadrupeds, 
 but these are relatively compact and inelastic in the birds, and 
 attached to them are a number of long " air-sacs." There are 
 no lungs at all in the fishes, but he will find gills there, organs 
 which are apparently wanting in the warm-blooded animals. 
 The heart, again, differs greatly in the various kinds of animals; 
 thus it has four chambers in the mammals and birds, three 
 in the frog and some other amphibians, but only two in the 
 fishes. The eyes are large and well- developed in all backboned 
 animals, but quite different in structure in insects and shellfish, 
 like the crab and lobster; different, again, in the cuttlefish, 
 rudimentary in the snail and in most worms, and wanting 
 altogether in such creatures as the oyster, mussel and cockle. 
 
 Thus the student of comparative anatomy finds very great 
 <li (Terences of structure in the various groups of animals: differ- 
 
ON THE NATURE OF ANIMAL LIFE 3 
 
 ences so extraordinary that without the key given to us in the 
 hypotheses of evolution it would have been difficult or impossible 
 to compare animal organs one with another. 
 
 Now we might spend a lifetime (as many men have done) in 
 studying the anatomy of animals, and yet one would really 
 never have studied the animal at all ! It is only when we look 
 at the things that organisms do that we are students of life. In 
 spite of the extraordinary differences of structure (as, for in- 
 stance, between a rabbit and a starfish), there is a remarkable 
 similarity in the functions performed by the organs of the body. 
 Limbs, legs and wings, fins, appendages, tube- feet, etc., are the 
 means of locomotion, and they enable the animals possessing 
 them to run or walk, fly, swim, creep, etc. Teeth, claws, horns, 
 spines, stings, poison-glands, etc., are all weapons which are 
 employed to maim, capture, and kill other animals. The 
 alimentary canal and its glands are the organs of nutrition, the 
 means whereby the animal digests and assimilates its food. 
 Lungs and gills are contrivances whereby oxygen is obtained 
 from the atmosphere, or the sea- water, and the heart and blood- 
 vessels are the mechanism by means of which the oxygen, and 
 also the digested foodstuff, are distributed to all parts of the 
 body. Eyes, ears, nose, tongue, feelers, etc., are the apparatus 
 of sensation, and it is by means of these that the animal becomes 
 aware of the changes that occur in its environment. 
 
 Add to all these things the multitudinous contrivances by 
 means of which animals reproduce, bear offspring, and nourish 
 and cherish the latter. 
 
 Structure and Function. Evidently, then, structure varies 
 in a remarkable degree, but the things that animal structures 
 or organs do are very much the same in spite of the structural 
 or anatomical differences. Limbs, wings, fins, appendages, etc., 
 are the means whereby the animal moves, distributes itself over 
 the world, seeks its prey, or avoids its enemies ; and teeth, claws, 
 etc., are its weapons the means of defence and aggression. 
 These organs, with the alimentary canal and its glands, enable 
 the organism to catch and kill and digest its food. All modes 
 of reproduction, whether sexual or asexual or parthenogenetic, 
 have the same purpose and effect that of the perpetuation of 
 the race. Obviously, then, it is not structure that is the 
 essence of animality, for structure is only the means to various 
 ends. 
 
4 THE MECHANISM OF LIFE 
 
 An animal is 
 
 Something that tends to preserve its own individual exist- 
 ence; 
 
 that nourishes itself; and 
 that reproduces itself. 
 
 Self-preservation, nutrition, and reproduction are, therefore, 
 the characteristics or the essential marks of life. We may call 
 them the primary instincts, tendencies, or impulses of animality. 
 
 If we knew only the structure of animals, we might be able to 
 find what were the functions or things done by those structures, 
 but the history of comparative anatomy gives us many instances 
 of our failure to do this. And, conversely, a detailed knowledge 
 of the things that an animal can do does not always enable us 
 to discover the kind of structure or mechanism that is at work. 
 In a sense it is all wrong to speak about an animal as a structure, 
 or even as a thing. It is " something happening." 
 
 And yet we are about to write several chapters concerning the 
 " animal mechanism," " organs," " parts," etc., and so we must 
 point out that it is not in any spirit of paradox that we take the 
 dynamic view of animality suggested above. " If," said Lord 
 Kelvin, " I can make a mechanical model, I comprehend." This 
 is our way of investigating, not only life, but everything of which 
 we can become cognisant. The human mind is essentially con- 
 structive, and it tries to act upon everything that is outside 
 itself; that the animal must act in this way is the reason that 
 there is a mind at all. When we look at a mechanism and try to 
 understand how it works, we either actually take it to pieces or 
 we do so in imagination we make an analysis of an activity of 
 some kind. We cannot describe a marine engine without con- 
 sidering the cylinders, cranks, crank shaft, reversing gear, etc., 
 even though we know quite well that all these parts exist, as 
 parts, only in the case of an engine that does not go. When we 
 think of, or see, the mechanism at work there are no parts, and, 
 of course, there are none after the engineer " assembles " them. 
 And so our conception of the organs and parts of the animal body 
 is only our analysis of the means of life. In a way they are what 
 Lord Kelvin called his mechanical models, and their usefulness 
 is that they enable us to comprehend and investigate. But in 
 the normal, living animal the parts are integrated, and their 
 activity is an indivisible one. 
 
ON THE NATURE OF ANIMAL LIFE 5 
 
 Think about it carefully, and we find that the normal, healthy 
 man or woman has no intuitive knowledge of the separate 
 activities of the parts of the body. Most of us are (or ought to 
 be) quite unconscious that we possess stomach and liver and 
 lungs, and so on, for all these organs are acting together and form 
 a harmony. We assume that these parts are in us because of 
 our knowledge of the structure of the bodies of other animals, 
 but, lacking that knowledge, it is unlikely that we should be 
 able to make an analysis of the activities of our own bodies. It 
 requires some effort of concentration to convince a healthy man 
 that he possesses a liver and stomach. It is true that he would 
 be periodically conscious of a " feeling " of hunger, but that 
 would be something quite different from the empirically acquired 
 knowledge that his stomach was empty. 
 
 Integration of Activities. The healthy living animal is one 
 and indivisible, and the more deeply we study physiology the 
 more clearly do we see that no organ functions by itself and for 
 itself at least, not in the healthy animal. Let us illustrate this 
 by considering the process of respiration. Concentrating our 
 attention on this phase or aspect of our living activity, we find 
 that the chest wall rises and the diaphragm sinks, and so the 
 cavity of the chest is enlarged. But the lungs fill this cavity; 
 they are distensible, and so air rushes into them to fill up the 
 extra space produced by the act of inspiration. Next, look at 
 the composition of the air inhaled and exhaled; it is different in 
 respect of the percentage of oxygen that it contains, and so we 
 find, on further investigation, that oxygen is taken into the 
 blood- stream via the lungs, and is then carried all over the body. 
 At the same time nutritive material is being taken by the blood 
 from the alimentary canal, and is being distributed over the body, 
 and is built up into the substance of the tissues, where it meets 
 with the oxygen that comes from the process of respiration and 
 is oxidised. This oxidation sets free energy, which is then trans- 
 formed into the mechanical work done, and the heat generated 
 by the muscles. But the muscles themselves are being employed 
 to activate the respiratory machinery that supplies them with 
 the indispensable oxygen. 
 
 Now, imagine that the percentage of oxygen in the air is 
 becoming much less than is normal, and that the percentage of 
 carbonic acid gas is increasing; at once everything changes. 
 The respirations become quicker and deeper, and the heart's rate 
 
6 THE MECHANISM OF LIFE 
 
 of beat increases. Other muscles of the chest and abdomen, 
 which before were inactive, now become employed to increase 
 the capacity of the thorax, and so to draw more air into the 
 lungs. The shoulders and arms are used as the air becomes 
 more and more deficient in oxygen and richer in C0 2 . and bend- 
 ing movements of the body occur so as to increase the ventilation 
 of the lungs. Finally, in the distress of asphyxiation there is 
 hardly a muscle in the whole body that is not pressed into the 
 service of supplying this absolutely vital oxygen to the blood. 
 
 That means that most of the organs of the animal body can 
 act vicariously. When one kidney or lung is diseased, the other 
 responds to the strain on the system, becomes bigger, and takes 
 over some of the work that its partner formerly did. The lungs, 
 skin, and kidneys all excrete water, and if one of the three is 
 diseased and cannot properly function, the others increase their 
 activities. When there is local infection by bacteria, the phago- 
 cytes of the blood, which are, normally, fairly evenly distributed 
 throughout the body, now crowd to the injured place and seek 
 to destroy the intrusive organisms. And so on; it is one for all, 
 and all for one. In the healthy animal the activity of any one 
 organ is regulated by those of all the others to produce a harmony 
 of action in which there are really no distinct parts. 
 
 Inco-Ordination of Activities. But there is also disease and 
 pain, and we must enquire into the meaning of these conditions. 
 In the healthy animal every part of the body is in nervous con- 
 nection with the brain and spinal cord, and a multitude of 
 impulses are continually streaming from the tissues to the central 
 nervous system. Generally we do not attempt to make an 
 analysis of this undeveloped sensation, and it blends to produce 
 a vague but satisfactory feeling of normality, and that is (as we 
 shall see later) because all these impulses entering the centres 
 from the periphery are answered or receive appropriate responses. 
 They are verified, co-ordinated, integrated. If they are not, if 
 there are insistent impulses from the tissues that do not end in 
 suitable responses, there is pain. The vague feeling that dinner- 
 time is approaching passes insensibly into hunger a stimulus 
 which ought to find its response in an act of eating. If it does 
 not, if the stimulus remains unanswered, we recognise it as pain. 
 
 Disease is more than this; it is a disharmony, a disturbance of 
 the general, unified functioning of the body the indication of 
 truly partial activity. If there is individualised, uncontrolled 
 
ON THE NATURE OF ANIMAL LIFE 7 
 
 over-production of hydrochloric acid by the gastric glands ol the 
 stomach we have acidity, and if there is under-production of 
 pepsin we have dyspepsia. When there is over-production of 
 secretion by the thyroid gland there is exophthalmic goitre, and 
 when the secretion is deficient there arises the condition of idiocy 
 called cretinism. If the secretion from the pituitary gland is 
 produced in excess there is malformation of the bones of the 
 face, and when there are abnormalities in the functioning of the 
 pineal gland (see p. 99) in young people there are consequent 
 aberrations in sexual development. When there is uncontrolled 
 proliferation of the cells of the mammary glands in women there 
 arises the most sinister of all diseases cancer. 
 
 And so on. The result of operative interference with the 
 animal body and the effects of disease are truly to set up partial 
 activity. Then we deal with a disharmonious complex of organs, 
 whereas the normal healthy animal is a harmony, an " unity in 
 multiplicity," an integration of activities. The diseased man is 
 a little less than animal, for something is wanting, and the dead 
 body is a structure really an assemblage of inactive parts, and 
 not at all an animal. The heart taken from a tortoise will live 
 several days and continue to beat, but it is not an animal, and in 
 studying it we are making an analysis and are not observing a 
 living organism. 
 
 We have insisted on the conception of the animal as an integra- 
 tion of activities, and more and more this is forced upon us, as 
 we shall see in the study of the central nervous system. Now 
 it is interesting to compare the organism with the modern State ; 
 the comparison has often been made, far more often in a 
 spirit of sentiment or as the support for some propagandist object 
 than in the light of an accurate knowledge of biology. We intend 
 this book for the general student of social science, as well as for 
 those who seek culture, and so we make no apology for a refer- 
 ence to the comparison between the State and the organism. 
 
 The Organism and the State. Of course, the modern State is 
 not an organism, not even by analogy. The animal body is a 
 complex of cells, even as a polity is a complex of men and women ; 
 but the cells of the body are structurally connected, while the 
 units of a population are truly individuals, discrete and physio- 
 logically distinct from each other. Now let us neglect such a 
 difference between the two complexes the organic and the 
 social ones and ask the question, Does the activity of the 
 
8 THE MECHANISM OF LIFE 
 
 modern State approximate to the complete integration of 
 activities exhibited by the healthy animal ? 
 
 Obviously it does not. No State so far developed has ever 
 exhibited such complete socialisation of activity as does the 
 normal organism. It is doubtful if in a modern society, even in 
 the most homogeneous one known to us, men and women are 
 all of the same kind ! The results of the study of " genetics " 
 or " eugenics " indicate that they are not. There are " strains " 
 or " races," germinal differences that are persistent. " Stock " 
 is more important than environment or training. " Nature is 
 stronger than nurture." The modern community is not one, 
 but many; and it does not possess organic unity. There were 
 " two nations " in England, said Disraeli, " the rich and the 
 poor." There are many, implies genetics, and obviously the 
 interests of these racially different stocks are not the same. 
 
 That is to say, there is a " class structure." There is a 
 " ruling class," a " bourgeoisie," and a " proletariat." There are 
 idlers and workers, consumers and producers. There is a class 
 consciousness, the social analogue to what we call pain in the 
 animal body, for the harmonious body politic would only be 
 vaguely conscious of a feeling of normality if all its activities 
 blended, and if all stimuli from its periphery met with appro- 
 priate responses. Obviously there is individualistic activity 
 and resentment at attempts towards integration (which is, of 
 course, the tendency of socialistic propaganda). There are 
 opposed interests because of this individualism and the tendency 
 towards " class war." 
 
 In short, if the body politic is an organism it is at the same 
 time a diseased organism, and the analogy with the biological 
 entity points to the way in which the cure is to be obtained. 
 
CHAPTER II 
 THE SENSORI-MOTOR SYSTEM 
 
 must not attempt to set up any absolute distinctions 
 between plant and animal organisms, or even between one group of 
 animals and another one. - Birds, for instance, appear to be quite 
 different in regard to their structure and habits from reptiles, but 
 when one considers the extinct species known to us by their fossils 
 the differences that we see in the living animals are largely 
 obliterated. Typical plants, such as an ash tree or a toad- 
 stool, differ greatly from typical animals such as a fish or a 
 spider, but there is no absolute difference between some of the 
 microscopic alga? (which are plants) and some of the microscopic 
 organisms called infusoria (which are animals). That which we 
 must regard as characteristic of plants on the one hand, and 
 animals on the other, are the tendencies that they display. 
 
 Thus the typical plant tends to be a rooted, sedentary organism. 
 As a very general rule its movements are those of growth, and 
 these movements are orientated. The roots tend to grow 
 downwards in the line and towards the direction of the earth's 
 gravitative force, and the stems tend to grow upwards towards 
 the direction from which light mainly comes. Other kinds of 
 movements occur, such as the turning of tendrils round fixed 
 supports, the opening and closing of some flowers, and the 
 motions by which the pitcher plant traps insects, but these are 
 not very common; they are not typical, and when we analyse 
 them we can place them in the same category as the movements 
 called tropisms (those of the root and green stems). 
 
 Animals, on the other hand, tend to be freely mobile. There 
 are many exceptions, of course; thus the zoophytes, corals, sea 
 anemones, etc., are fixed, sedentary organisms, and so also are 
 many kinds of parasites (fish lice and tapeworms, for instances). 
 But the great majority of animals tend to move about; they are 
 locomotory, and their movements exhibit a certain general 
 character which is fairly well described by the term " animal 
 behaviour " in the popular significance of those words. The 
 motions included in such a term are often deliberate and chosen 
 
 9 
 
10 THE MECHANISM OF LIFE 
 
 that is, the animal behaves as if it knew very well what it 
 wanted to do and how to do it but often there is hesitation, and 
 even what we should call caprice, so that we cannot always 
 predict what the creature is going to do. Often, again, an 
 animal appears to behave precisely like ourselves when we " will " 
 to do something that is, it appears to act spontaneously. 
 
 Now motion in itself is not something that is distinctive of 
 organisms, for the particles of all substances are in a state of 
 continual movement (except at absolute zero of temperature), 
 but this kind of motion is either a fixed, vibratory, or oscillatory 
 one (the regular oscillations or revolutions of the electrons or 
 atoms, for instance); or it is a random movement (as are the 
 molecules of a gas, or the finely divided particles of some sub- 
 stance in suspension in a liquid); or it is such movements as 
 those of the satellites and planets in a solar system, absolutely 
 predictable; or, again, it may be the apparently random move- 
 ments of the " fixed " stars in what has been called their " helter- 
 skelter " flight through space. 
 
 The Meaning of Organic Movement. Organic movements 
 are, however, different in their tendencies from those that we 
 have just mentioned, though it is not easy to put this differ ence 
 succinctly and accurately. Perhaps one may say that the 
 movements of plants and animals are, in the first place, growth 
 movements determined by internal causes, and, secondly, they 
 are adaptive movements that is, useful and purposeful re- 
 sponses on the part of the organism to changes occurring 
 outside itself. Here " useful " and " purposeful " mean that 
 the result of the motion is something to the advantage of the 
 organism. These remarks, it will be noted, apply to both 
 plants and animals. In the case of the former the responses 
 tend to be inevitable responses to simple, natural stimuli, and 
 they can usually be predicted, while in the latter case they are 
 responses to individualised stimuli; they are partly determined 
 by experience, and they cannot usually be predicted. AVe must 
 not linger on these distinctions here, for we shall return to them 
 in a later chapter. 
 
 Animals, then, tend to exhibit a great variety of adaptive 
 movements, and it is by seeing these that we say that we are 
 looking at an animal; for, obviously, the structure of the latter 
 is almost entirely the means by which the movements can be 
 made. We must now consider what is the nature of the latter. 
 
THE SENSORI-MOTOK SYSTEM 11 
 
 Mobility Patterns. Animal movements, then, fall into 
 generalised and individualised patterns: first those of the body 
 as a whole (locomotion), and then those of the parts of the body. 
 We shall take the more complex patterns first, as these will be 
 the more familiar to the reader. There are the walking, running, 
 and leaping of quadrupeds; the walking and running of flightless 
 birds (which are bipedal movements); and the leaping movements 
 of arboreal quadrupeds, such as monkeys. There is the flight 
 of birds and insects (two rather different types), the flight of 
 bats and squirrels, and the incipient flying movements of some 
 fishes. Next, take the movements of locomotion in water: the 
 swimming of typical fishes, seals, and whales; the swimming of a 
 dog or horse ; and the swimming and diving movements of birds. 
 Very different from these are the swimming of crustaceans, such 
 as copepods; the bizarre leaping movements of a lobster or 
 scallop ; or the swimming of a cuttlefish or squid. Burrowing, as 
 in the cases of moles, earthworms, ants, and fossorial wasps, is an 
 entirely different kind of movement, and so is the creeping of a 
 slug or a centipede. The extremely limited locomotion of a 
 mussel or cockle is different again, and still more so the creeping 
 of a starfish. Among microscopic animals we have the peculiar 
 movements carried out by cilia, as in the cases of flagellate 
 organisms, infusoria, rotifers, or even spermatozoa. 
 
 All these are categories (generalised patterns) ; thus the same 
 kinds of locomotory movements are made by all typical fishes, 
 but the swimming of a herring or mackerel shows minor differ- 
 ences from that of a sole or plaice, and so we have subcategories. 
 Again, the patterns may be specific ones ; thus the movements of 
 a panther are rather different from those of a bear. Or they may 
 be subspecific (in the zoological sense); thus the walk of an 
 Airedale differs from that of a fox terrier. And, finally, they 
 may be individual; thus the "carriage" of a man or woman 
 that one knows well is characteristic of that person, and no one 
 else. 
 
 So also with postures and attitudes, and with the movements 
 of the limbs and other parts of the body that are used in defence, 
 aggression, etc. Thus there are biting, slashing, cutting, grind- 
 ing, and gnawing movements of jaws and teeth; clawing move- 
 ments like those of a cat or a bear; goring and tossing and butting 
 actions of horns ; the use of spines, as in a sting ray ; the lateral 
 swinging movements of the tail, as in a crocodile; crushing and 
 
12 THE MECHANISM OF LIFE 
 
 cutting movements of the mandible, and pincers in crabs and 
 lobsters; piercing and sucking by the proboscis of such an insect 
 as a mosquito; stinging by bees and wasps, aided by the injection 
 of poisons; the gripping and sucking action of the cuttlefish, 
 squid, or starfish; snaring movements involving constructive^ 
 devices, as in the web of a spider, and so on. All these are just] 
 as characteristic of groups of animals as are the locomotory 
 patterns. 
 
 Such mobility patterns go roughly side by side with structure 
 patterns; thus, quadrupedal locomotion involves the use of fore; 
 and hind limbs ; flight that of one or two pairs of wings ; swim- 
 ming means that there are fins (or other movable and fixed 
 hydroplanes), or cilia, or flagella ; creeping requires a number ofj 
 serially arranged appendages (like the " legs " of a centipede, or 
 the setae of a worm), and so on. But the parallelism between 
 structure and mobility patterns is not always complete ; thus 
 the dorsal fin of a fish may be a vertical rudder, a poison organ, 
 or a lure; certain bones in the head of a vertebrate animal may 
 be parts of the skeleton of the jaws (in fishes), or the little " audi- ; 
 tory ossicles " of the internal ear (in mammals) ; the same 
 (historical) limb may be a paddle (in a seal), a wing (in birds), 
 or an arm and hand (in man). Knowing what the structure is 
 does not always mean that we can deduce the function, and vice 
 versa. 
 
 And we are mainly interested in either the structure or the 
 function of the structure, according to our point of view. The 
 former is our main object of study from an historical aspect, as 
 when the same structure gradually became modified; thus a ; 
 seal is a modified carnivore which has adopted an aquatic habit, 
 in the course of which a typical quadrupedal limb became 
 modified in structure to serve as a paddle. But here we are ] 
 concerned with an animal mechanism, and so we regard paddles, 
 fins, and limbs as three variants of a generalised locomotory I 
 pattern. 
 
 That means that it is the animal as a whole that we are study- ! 
 ing: its modes of movement that enable it to distribute itself, j 
 to find shelter, to escape from its enemies, to capture and kill 
 the organisms which serve as its food in general, its behaviour i 
 and the ways in which this is expressed. And so we must study 
 the structure of the mobile animal parts as a means of our 
 analysis of living activity. 
 
THE SENSORI-MOTOR SYSTEM 13 
 
 The Sensori-Motor System. 
 
 The organs of mobility include the skeleton, which is a system 
 of rigid and jointed bones to which the muscles are attached; the 
 sense organs, by means of which the animal comes into relation 
 with its environment; and the nervous system, which is the link 
 between the sense organs and the muscles. The animal is placed 
 in the midst of an environment which continually changes, or it 
 moves about from place to place^ and therefore the conditions 
 under which it lives are variable. It must find shelter, and the 
 nature of this will vary with the climate and seasons; it must 
 find food, and the nature and abundance of this also changes; 
 and it must avoid its natural enemies. Therefore it must become 
 aware of the events that proceed outside itself, and that is why 
 it sees, hears, smells, and " feels." The events that occur in 
 the environment affect or stimulate its sense organs; but that 
 is not enough, for the stimuli must result in actions of some kind. 
 Now we can imagine the stimulation of an organ of sense to 
 produce two main kinds of response : First, the response may be 
 mechanical, constant, and predictable, just such a response that 
 would occur in a model that we could easily construct. In an 
 artificial animal or automaton we press a spring, and the machine 
 begins to walk or to lift a limb, or rolls its eyes, but the result of 
 pressing the spring will always be the same. Tropisms that is, 
 the kind of responses to which we referred on p. 9, and which 
 we shall consider in greater detail in Chapter VIII. tend to be 
 responses of this nature, and a large number of the habitual 
 actions performed by animals belong to such a category of 
 relatively fixed, mechanical movements. Secondly, the responses 
 may be adaptive that is, the thing that happens when a sense 
 organ is stimulated depends on the circumstances of the moment 
 and on the experience of the animal. Thus, a man walking 
 along the street may pass a stranger without doing more than 
 look casually at the latter. If, however, the same person sub- 
 sequently becomes known to him he may thereafter nod, or bow, 
 or stop and speak. Yet the physical stimulus of the organ of 
 sense is the same in the two cases, and the difference of the 
 response depends upon the intervention of the cerebral nervous 
 system. 
 
 With these preliminary remarks we may now consider the 
 physical apparatus of movement. 
 
14 THE MECHANISM OF LIFE 
 
 The Skeleton. The skeleton consists of a series of hard, rigi< 
 parts to which the muscles are attached in various ways; thiL 
 the chassis of a motor-car is, in a way, the skeleton of th 
 mechanism. The skeleton is also the mobile carrier of bodily 
 weapons, such as the teeth or claws, and a mechanical analogu 
 to it in this second aspect is the jib and scoop of a steam digging 
 machine, or the carrier of the buckets of a dredger. Not a' 
 animals possess skeletons, but in those which have soft bodie 
 there is always a rather small limit of size, and most of sue! 
 organisms live in water, which itself acts as the support for th 
 soft body parts. 
 
 The animal skeleton varies to an enormous extent in its form 
 and sometimes it is very simple. Thus the skeleton of a musse 
 or oyster is a shell consisting of two valves connected together b] 
 
 a kind of hinge, while tha 
 -Shell ligament of a man consists of a grea 
 
 number of bones jointed o 
 
 articulated together in man] 
 Adductor -,.. TT 
 
 [muscle of the shell clitterent ways. Upon th 
 
 nature of the parts of th 
 skeleton and the shapes of th 
 
 "Shell joints or articulations depenc 
 
 the kinds of movements tha 
 
 Fio. l.-A TRANSYEESE SECTIOK may be carried out. Thus th 
 THROUGH AN OYSTER. skeleton of the oyster is ver 
 
 simple, and so also is th 
 
 nature of the only movement of the body, as a whole, whicl 
 this animal can make. One large muscle is attached to the tw 
 valves as indicated in the figure, and when this contracts i 
 pulls the valves together so that the shell remains closed 
 When it relaxes, the spring or elastic ligament presses th 
 valves apart, and so the shell opens. These opening an( 
 closing movements are practically the only ones that the oyste 
 can perform. Now compare with this simple mechanism th 
 very complex one of the vertebrate body as we are about t< 
 describe it. 
 
 Sometimes the skeleton of an animal has no function in move 
 ment, but is merely the rigid, or semi-rigid, support of the sof 
 parts; thus the common bath sponge is the horny skeleton of ai 
 animal, and supports the fleshy tissues (which have been rottec 
 away in the process of preparing the sponge). This flesh i 
 
arr 
 
 THE SENSORI-MOTOR SYSTEM 15 
 
 anged round a complicated system of canals through which 
 the sea- water circulates, and the larger of these canals would 
 collapse by the weight of the flesh if they were not kept open by 
 the horny fibres embedded in the latter. There is no locomotion 
 at all here, and the only mobile parts are the vibratory hairs 
 which cause the circulation of water through the canal system. 
 Such an animal as the jelly fish (or medusa) has no skeleton, but 
 it floats suspended in the sea-water, which thus supports the 
 soft, fleshy body. The latter has the shape of a bell, by the 
 expansions and contractions of which water is taken into the 
 cavity and then ejected, and so the medusa slowly moves about. 
 There is no skeleton in a common garden slug, and when loco- 
 motion occurs the front part of the broad, fleshy " foot " adheres 
 to the ground, while the latter part becomes detached and is 
 drawn forward and adheres. The front part is then detached, 
 pushed forward, and again adheres, and so on. The earthworm 
 has no skeleton, but the body is provided with muscles which 
 act so that it can be stretched out or shortened. Along the 
 sides there are little projecting bristles, which can be turned 
 forwards or backwards. When the worm burrows, the bristles 
 on the front part of the body are bent backwards and inserted 
 into the soil, and then the hinder bristles are bent backwards also, 
 and that part of the body contracts and is drawn forwards. The 
 worm then bores into the ground in front of it, swallowing the 
 soil, and stretches out the front part of its body, and gets a new 
 grip by its bristles, and so on. 
 
 In all insects, spiders, Crustacea, etc., the skeleton is an 
 external one, consisting of a hard, horny, or limy cuticle. This 
 acts as the support for the muscles. The body and limbs are 
 jointed, or articulated, and the muscles move these parts on each 
 other in ways that are analogous to those which we are about to 
 describe in the case of the vertebrate animal. Such an exo- 
 skeleton would be very heavy if the animal were to attain a large 
 size, and it necessarily prevents growth, which can only occur 
 during the periods when the animal " moults," or casts its shell. 
 That period is one of danger for the crustacean, and so it happens 
 that these animals never have attained to a great size. When 
 the skeleton is an internal one, as it is in all vertebrates, it is 
 mechanically a more perfect structure, and so some of the species 
 in which this kind of skeleton has occurred have evolved to 
 extraordinary sizes, as in the extinct dinosaurs of the Mesozoic 
 
16 THE MECHANISM OF LIFE 
 
 period. But with the increase in mass of the body there occur 
 certain mechanical disabilities, and so we must regard the huge 
 reptiles of past geological times as some of Nature's unsuccessful 
 experiments. 
 
 Now we may turn to the skeleton of the present-day verte- 
 brate animals, and here we find the most perfect mechanism so 
 far evolved. 
 
 The Axial Skeleton. The whole structure is built up round 
 an axis, which is the vertebral column, or backbone. This 
 consists of a series of vertebrae, or bony discs, which are attached 
 to each other by means of muscles and ligaments, and they are 
 usually kept in place by little bony projections that articulate 
 with each other. In man there are pads of gristle between the 
 adjacent vertebrae, and the latter have a certain amount of play 
 on each other, so that the whole column can bend sideways and 
 from back to front. In some fishes, however, the latter is prac- 
 tically one bone; in birds only the vertebrae of the neck are 
 movable on each other ; and in snakes the separate bones can 
 move only in the horizontal plane, because of little interlocking 
 side pieces. In front the vertebral column carries the skull, and 
 behind it is often prolonged backwards to form the tail. 
 
 The skull consists of two series of parts, the bones of the 
 cranium and those of the face and jaws. The cranium is essen- 
 tially a bony box containing the brain and the great sense organs, 
 eyes, auditory and olfactory organs. Added to it are the bones 
 of the upper jaw (which are immovable on the skull in man) and 
 the lower jaw, which articulates with the skull by means of a 
 hinge- joint (but, nevertheless, allows of a certain amount of side- 
 to-side movement). The whole skull is attached to the first 
 (atlas) vertebra by a hinge- joint, so that it can move up and 
 down in a vertical plane, and the atlas is attached to the second 
 (axis) vertebra by a peg- and- socket joint, so that the skull can 
 be rotated through a certain angle in the horizontal plane. The 
 neck vertebras can bend on each other from side to side, and thus 
 the skull has limited freedom of movement in the three directions 
 of space. 
 
 The ribs are imperfect hoops of bone which are articulated to 
 the backbone behind. Some of them (the upper ones) are also 
 articulated in front to the breast- bone, and thus the whole series 
 act as an outer protection for the organs of the thoracic cavity 
 
THE SENSORI-MOTOR SYSTEM 17 
 
 the lungs and heart. They can be raised up by muscles, and as 
 they are inclined downwards the upward movement enlarges the 
 diameter of the thorax. This is the mechanism of respiration. 
 
 The Limb Girdles. The two pairs of limbs, which are charac- 
 teristic (as fins, paddles, legs and wings, fore and hind limbs, 
 arms and legs) of all vertebrates, are not attached directly to the 
 backbone, but to the two limb girdles. Each of the latter is a 
 kind of hoop attached to the column. The shoulder girdle 
 consists of the two collar-bones, which are attached in front to 
 the breast- bone and behind to the shoulder-blades. The latter 
 are not attached to the column, but are moored, so to speak, 
 in the muscles of the back. The skeletons of the fore-limbs 
 are attached to the 
 shoulder-blades. 
 
 The pelvic girdle 
 also consists of 
 several bones, and in 
 man these are fused 
 together to form the 
 two hip-bones, which 
 are immovablv at- FIG. 2. DIAGRAM OF THE VERTEBRAL COLUMN, 
 tached to the column ^ MB G> AND LIMBS IN A BACKBONED 
 , , . , , . . , ANIMAL. 
 behind, and are joined 
 
 together in front at the pubis. The skeletons of the hind-limbs 
 are jointed into the pelvic bones, but the latter, together, also 
 act as a kind of basin-shaped support for the lower bowel, the 
 urinary bladder, and the reproductive organs. 
 
 Note that the articulation of the limbs with the limb girdles 
 means that the former are set well out from the axis of the body, 
 so that the feet rest on the ground on as wide a base as possible. 
 
 The Limb Skeleton. All the higher animals that is, the 
 vertebrates, the Crustacea (such as the lobster, crab, or prawn), 
 the insects and spiders possess true limbs, which are organs of 
 locomotion, and are also bodily weapons. The nature of these 
 limbs is, of course, very different in the various groups of higher 
 animals, but in all those that we have mentioned they are jointed 
 appendages of the body, freely movable on the latter, and as a 
 rule furnished with weapons. The limb may be a walking leg 
 (as in the case of the fore and hind legs of most quadrupeds), a 
 fin (as in the case of a fish), a paddle (in seals, dolphin, and 
 
 
18 THE MECHANISM OF LIFE 
 
 whales), a wing (in birds or insects), a " swimmeret " or paddle- 1 
 like limb (as in many Crustacea), a mobile weapon or tool fur- I 
 nished with claws and cutting organs (as in the case of the fore- j 
 limbs of a cat, the large chela?, or claw limbs of a crab or lobster, I 
 or the arms and hand of a man), and so on. Sometimes its I 
 structure and degrees of freedom of movement are relatively I 
 simple (as in the side fin of the ordinary fish), but in general the I 
 limb is a freely movable appendage of relatively complex struc- 
 ture. 
 
 The reader may easily verify all that we are about to say by I 
 looking at a human skeleton in a museum, and by observing the I 
 modes of motion of the parts of his own body. The fore-limb, or I 
 arm, then, contains a skeleton that consists of a number of bones I 
 connected together in various ways. The bone of the upper arm 1 
 (the humerus) articulates with the shoulder-blade by a ball- and- 1 
 socket joint, so that it is freely movable in every direction, and I 
 the shoulder-blade itself can be moved (as in " shrugging "), so I 
 that the arm has thus additional freedom. The skeleton of the I 
 forearm consists of two bones (the radius and ulna), and the latter I 
 is articulated to the humerus by a hinge-joint, so that the forearm I 
 can be bent on the upper, or extended into the same straight line 1 
 with the latter, giving 1 degree of freedom of movement. 
 
 The radius, however, has a peculiar twisting movement on the 1 
 ulna, and as the wrist and hand are articulated with it they canl 
 be turned round through a half- circle, the elbow- joint remaining 
 immovable. Thus the hand can be turned palm up (supinated) ] 
 or palm down (pronated). The hand itself is capable of] 
 3 degrees of movement that is, it can be*turned in each of the] 
 three directions of space. There are eight wrist bones arranged! 
 in two rows, all articulating with each other, one row being! 
 jointed with the radius and the other with the long bones of the] 
 hand. Thus the hand, as a whole, can move up and down andj 
 from side to side on the wrist, so that it has 3 degrees ofl 
 freedom. Each of its bones, the metacarpals, articulates withj 
 the bone of a finger that is to say, with the first, or proximal* 
 phalanx, which articulates with the middle one, which finally; 
 articulates with the terminal phalanx (the one that carries the] 
 claw, or nail). The articulations between the terminal and 
 middle phalanges and the middle and proximal ones are hinge- j 
 joints, so that the fingers can only bend on each other in one] 
 plane; but the 'articulation between the proximal phalanx and' 
 
THE SENSORI-MOTOR SYSTEM 
 
 19 
 
 its corresponding metacarpal is, to some extent, an universal 
 joint. Thus the thumb can be bent in the same way as the other 
 digits, but it can also be rotated or swung round in the same 
 way that the whole arm can. The fi^st finger has the same kind 
 of motion, but less of it, so to speak, and so also with the others. 
 The hind- limb of a quadrupedal animal, or the leg of a man, is 
 built on the same plan, and there may be very little difference 
 between the two limbs (as in a chimpanzee, for instance) with 
 regard to the degrees of mobility of the parts. In man. however, 
 the freedom of movement of the hind-limb is restricted, first by 
 
 Shoulder bla.de (fixed bone) 
 
 
 FIG. 3. MOVEMENTS OF THE BONES OF THE FORE-LIMBS. 
 
 the rigid attachment of the pelvis to the vertebral column, and 
 second by the rigidity of the bones of the ankle relatively to those 
 of the wrist. 
 
 Such an analysis of the skeleton as we have just indicated (and 
 which can easily be made more precise by the reader himself) 
 shows that the degree of mobility of any part of the body of the 
 higher animal is determined in general by the configuration of the 
 skeleton, and particularly by the shapes of the joints. The latter 
 make it possible for one bone to move on another in one or more 
 ways; thus we have the hinge- joints between the skull and the 
 atlas, the peg- and- socket joints between the atlas and the axis, 
 
20 
 
 THE MECHANISM OF LIFE 
 
 the ball-and-socket joints between humerus and scapula, and 
 femur and pelvis, and so on. There is always a checking action 
 of some kind which restricts the extent to which one segment of 
 the skeleton can move on another ; thus the movement of the head 
 is checked by muscles (and the relaxation of these when a man 
 goes to sleep causes the head to drop, or fall to one side); the 
 olecranon process (or " funny bone ") in the elbow checks the 
 movement when the whole arm comes straight; the patella (or 
 knee-cap) has the same function with regard to the movement of 
 the lower leg on the thigh, and the slipping aside of the patella is 
 the cause of the knee " going out of joint." In other cases the 
 movement is checked by tendons, or ligaments (as in the case of 
 the separate vertebrae). 
 
 The Motor Organs. Now clothe the skeleton with muscles, 
 which are so arranged as to pull on the bones in the ways that the 
 latter are free to move because of the nature of the joints. This 
 arrangement of muscle and bone is the subject matter of anatomy, 
 and it can be treated in enormous detail ; but the reader will easily 
 see that, from a mechanical point of view, it is all very simple. 
 We have, in fact, systems of levers, with the various powers : thus 
 
 w 
 
 w 
 
 t 
 
 W 
 
 v/ 
 
 IF F 
 
 FIG. 4. THE VARIOUS LEVER MOVEMENTS OF THE FOOT. 
 
 Here 1 is a lever of the first order, 2 of the second, and 3 of 
 the third order, and, with various modifications, these are typical 
 of the mechanisms of movement of the parts of the limbs and 
 body. 
 
THE SENSORI-MOTOR SYSTEM 
 
 21 
 
 One example may be given here, that of the bending of the 
 forearm on the upper arm. Four bones are included in this 
 mechanism the scapula, humerus. radius, and ulna. When we 
 consider only the movement of the forearm on the upper by the 
 elbow- joint, the scapula and humerus are the fixed, and the radio- 
 ulna (considered now as one segment) is the movable part. 
 
 -Shoulder blade. 
 Jlead of humerus 
 
 merus 
 
 \J* 7 /ex o r muscle, 
 (biceps) 
 
 Ex. tensor muscle 
 ( Triceps) 
 
 Tendons _ 
 
 of 
 
 insertion 
 
 --Ulna. 
 Radius 
 
 FIG. 5. DIAGRAM OF THE MUSCLES THAT MOVE THE FOREARM* 
 
 Two muscles, we see. are necessary for the movement the 
 biceps, which is the bending (or flexor) muscle, and the triceps, 
 which is the straightening (or extensor) one. In each case the 
 muscle consists of the tendons of origin (those which are attached 
 to the fixed bone), the belly or contractile part, and the tendons of 
 insertion (which are attached to the movable bone). When the 
 radio- ulna is to be bent, or flexed, on the upper arm the biceps 
 contracts, or shortens (that is, it becomes thicker and shorter), and 
 simultaneously the triceps relaxes, or lengthens (that is, it 
 becomes thinner and longer). By the pull of the tendon of 
 insertion of the biceps the radio-ulna is bent on the upper arm, 
 and by the pull of the tendon of insertion of the triceps it is 
 drawn back again into a straight line with the humerus. It is 
 important to'note that the two muscles are antagonistic ones, and 
 that the flexion of one of them is always associated with the 
 
22 THE MECHANISM OF LIFE 
 
 extension of the other. Extension is not merely a passive relaxa- 
 tion, but is the result of a nervous impulse just as much as is 
 contraction. 
 
 Note also that the humerus and scapula are fixed bones with 
 regard to the flexion and extension of the forearm, but the 
 humerus is itself a movable part, and the fixed points to which 
 its muscles are attached are in the scapula. The latter is also 
 movable, and then the fixed bones are the vertebra and ribs. 
 The hand is a movable part, and the fixed bone to which its 
 muscles are attached is the radio- ulna. Thus " fixed " and 
 " movable " bones are relative to the parts that are to be moved. 
 The reader must also note that the movement of any part of a 
 limb is never so simple as we have indicated, for several pairs of i 
 antagonistic muscles are generally in action at the same time. 
 
 - Circular muscles, 
 
 Circular muscles 
 
 >f the 
 Tine 
 
 ^.Longitudinal muscles 
 
 L ongitudina. I 
 muscles contracted. 
 
 FIG. 6. DIAGRAM OF THE MUSCLES OF THE INTESTINAL WALL. 
 
 
 Non-Skeletal Mechanisms. There are many movable parts in 
 the body for which there are no skeletal supports. Thus the 
 heart consists of a very complicated series of muscle bundles that 
 contract and expand automatically. Simpler cases are those of 
 the intestine and bloodvessels. In the former we have two series 
 of muscles, one which is made up of bundles of fibres running 
 circularly in the wall of the bowel, and the other being made up 
 of fibres that run lengthways. When the former contract the 
 diameter of the intestine is decreased, and when they relax and 
 the longitudinal muscles contract the length of a segment of the 
 bowel is decreased and the diameter is correspondingly increased. 
 In this way waves of contraction of calibre of the intestine are set 
 up, and these propel the food contents from place to place. The 
 mechanism of contraction of an artery is very similar, except that 
 the longitudinal muscles are not present. The wall of the artery 
 is elastic, and when the circular muscles contract the calibre is 
 diminished, but when they relax the elasticity of the wall, and 
 
THE SENSORI-MOTOR SYSTEM 23 
 
 the pressure of the blood within, cause the vessel to regain its 
 former calibre. 
 
 There are other special mechanisms of this kind; thus the 
 actions of closing and opening the eyes involve the contractions 
 and relaxations of two antagonistic muscles which are present 
 in the eyelids. One of these, the closing set, consists of fibres 
 that form a kind of ring in the upper and lower lids. When 
 these contract, the opening between the latter is diminished, or 
 obliterated. The closing muscle consists of fibres which originate 
 in^the sheath of the optic nerve, and which are inserted into the 
 cartilage of the upper lid ; when they contract the latter is raised, 
 the ring-like closing muscle then becoming relaxed. 
 
 Ca.frilla.ry 
 FIG. 7. MUSCULAR TISSUE. HIGHLY MAGNIFIED. 
 
 The Minute Structure of Muscle. Just a word about this. 
 Examination of a piece of muscle beneath the microscope shows 
 that it is composed of a very great number of minute fibres. 
 Each of the latter is about If inches in length, and it tapers 
 away to a fine point at each end. Numbers of fibres are bound 
 together by connective tissue to form muscle slips, which are 
 similarly joined up to form bundles which constitute the muscle 
 itself. A nerve fibre terminates in each muscle fibre, as shown 
 in Fig. 7. In addition, arteries carry blood to the muscle, where 
 it is distributed through the capillaries and from which it is 
 carried away by the veins. The muscle fibres act at the same 
 time, all of them thickening or contracting when the muscle 
 pulls on its movable bone, or simultaneously lengthening when 
 the muscle relaxes. 
 
 As the figure shows, the fibres are cross-striped, and this stria- 
 tion is, in some way, part of the mechanism of contraction and 
 relaxation. But the muscle fibres that are present in the intestinal 
 or arterial wall are not cross-striped. This presence or absence 
 of striation goes along with a difference in the control over the 
 muscular'actions ; in^most orthe cases^'where the activity of a 
 muscle is controlled by the will (voluntary muscles) the fibres 
 
2 \ THE MECHANISM OF LIFE 
 
 are striated, while in those over which we have no control (the 
 muscles of the heart, alimentary canal, and bloodvessels chiefly 
 the involuntary muscles) the fibres are " plain," or non-striated, 
 or are otherwise different from the ordinary striated fibres. 
 
 The Sensory Mechanisms. 
 
 So much for the apparatus of mobility. The reader can 
 easily supplement the above very general summary from the 
 textbooks on physiology. Next we have to consider how this 
 apparatus is set in motion, and that leads us to the study of 
 sensation. There are certain mechanisms, mainly those of the 
 heart and respiratory organs, which are automatic that is, 
 they work in the absence of any external cause (though their 
 rate of working is affected by outer events) but in most of 
 the muscular mechanisms of body and limbs a stimulus is 
 necessary in order that they may be set in motion that is, 
 something analogous to the pressing of the spring in our 
 imagined model must occur. 
 
 Sensation, then, is the preliminary to movement, and here we 
 mean by " sensation " only the physical and chemical events 
 that occur in the organs of sense and in the nerves and brain, 
 and not any affection of consciousness; when the latter occurs 
 we have to do with perception, and this we consider later. Things 
 that happen in the environment, then, affect the organs of sense : 
 the movements of material bodies are known to us by vision, 
 for the light reflected from those things enters the eyeballs and 
 forms a picture on the retina, which is the essential organ of vision ; 
 vibrations in the atmosphere are set up by other material move- 
 ments outside ourselves, and these vibrations are received by 
 the auditory organs ; the particles of chemical substances floating 
 in the air are drawn into the nose or mouth, and so affect the 
 olfactory and gustatory organs; while contact of material sub- 
 stances with the skin similarly affects the nerves that terminate 
 there. These sensations we call sight, hearing, smell, taste, and 
 touch. 
 
 Exteroceptive and Proprioceptive Sensation. Changes that 
 occur outside the body thus affect the organs of sense, or, as we 
 shall call the latter, the receptors. The eyes and auditory organs 
 are therefore exteroceptive organs, or distance receptors, for 
 they are affected by events that may happen at considerable 
 
THE SENSORI-MOTOR SYSTEM 25 
 
 distances away from the body. The olfactory, gustatory, and 
 touch organs are near receptors, for they can only be affected 
 by substances that actually come, into contact with the body. 
 Our sensation of temperature may be a near affect (as when a 
 cold or hot substance touches the skin), but it may also be a dis- 
 tance affect (as when we are warmed by the radiation from the 
 sun). Now there are receptor organs which are stimulated, or 
 affected, by changes that occur within the body itself; thus when 
 we lift anything we have a feeling of effort (the muscular sense) ; 
 when we walk we have also sensation arising from pressure set 
 up in the joints; when the body is in different postures there is 
 sense of equilibrium, which is something quite apart from seeing 
 or hearing (for it can arise when a man is blindfolded and has 
 his ears plugged); and there is also general sensation from the 
 viscera, for a man may experience stomach-ache. In these 
 cases receptor organs in the muscles, tendons, joints, ears, and 
 viscera are stimulated, producing what we call proprio-sensation. 
 Emotion (that which moves us), blushing, the pallor of fear, 
 trembling, rigidity, even the " Rabelaisian effect of fear on the 
 bowels " (" affective tone or feeling," as it has been called), are 
 the results of the stimulation of the proprioceptors. 
 
 Here, in spite of the warning given above, we are speaking 
 of sensation as it results in " feeling," or consciousness, because 
 it is very difficult to avoid doing so ; but, nevertheless, the activity 
 of the receptor organs, in so far as we are about to relate it to 
 the initiation of movement, need not be accompanied by any 
 affection of the mind. 
 
 The Nervous Links. 
 
 Certain mechanisms intervene between the receptor organs 
 on the one hand and the motor organs on the other; these are 
 the peripheral and central nervous systems. Proceeding from 
 a receptor organ there is an afferent nerve (" afferent " because 
 it conveys something to the central nervous system), and pro- 
 ceeding from the motor organ there is an efferent nerve (" efferent " 
 because it conveys something from the central nervous system). 
 The reader must understand clearly that a motor organ is (in 
 general) not directly stimulated to act by something that occurs 
 outside the body ; it is stimulated via the central nervous system. 
 Receptors and afferent nerves are therefore " the way into " the 
 brain and spinal cord, while efferent nerves and motor organs 
 
26 
 
 THE MECHANISM OF LIFE 
 
 are " the way out." When the animal does anything in response 
 to some change that occurs in its environment (1) a receptor 
 organ is stimulated by the external event, (2) an impulse is 
 propagated along an afferent nerve into the central nervous 
 system, (3) some change occurs in the latter, (4) an impulse is 
 propagated out along an efferent nerve, and (5) the motor organ 
 is stimulated, and responds by some kind of movement. 
 
 The Minute Structure of the Nervous System. Complicated as 
 this is, its general scheme is easily understood. In a receptor 
 organ there is always an essential part which we call the nerve 
 
 NodA 
 
 FIG. 8. FORMS OF NEURONES. ALL HIGHLY MAGNIFIED. 
 
 1, Transverse section of a nerve fibre; 2, a nerve fibre; 3, nerve cell from 
 the spinal cord; 4, nerve cell from the cerebellum; 5, nerve cell from 
 a sympathetic ganglion. 
 
 termination; this is the retina in the eye, the auditory hairs in 
 the ear, and so on. Between the receptor and the brain or spinal 
 cord there are stretched a series of nerve fibres, and it is along 
 these that the impulse passes. In the central nervous system 
 there are collections of nerve cells and nerve tracts. Between 
 the centre and the motor organ there are other fibres, and in tl 
 former there are. again, nerve terminations. 
 
 All this structural detail is built up of nervous elements called 
 neurones. 
 
 Neurones are structures that vary greatly in appearance and 
 size, but they have all the same general form. Each of them 
 consists of a nerve cell that is, a minute fragment of specialised 
 
THE SENSORI-MOTOR SYSTEM 27 
 
 protoplasm containing a nucleus. From one side of such a cell 
 there arise one or more prolongations of the protoplasm, and these 
 branch repeatedly to form a plant-like growth an arborisation 
 which is known as the dendritic system of the cell. From some 
 other part of the cell there arises a delicate filament which does 
 not usually branch, but is prolonged out into a long thread, the 
 nerve fibre. As Fig. 8 shows, this fibre, which is called the 
 axon of the cell, becomes invested in a sheath, and between the 
 sheath and the axon there is a sort of packing, the medulla. The 
 axon may be very long; thus the cells of the grey matter of the 
 spinal cord send out axons into the sciatic nerve, and some of 
 these may be about 3 feet in length, though the nerve cell itself 
 is only about ^-Q-Q to ^ inch in diameter. The axons, of 
 
 STIMULUS 
 
 Cell 
 Jtferve terminations 
 
 Motor plate J$EURONE 
 
 FIG. 9. DIAGRAM OF THE WAY IN WHICH NEURONES ARE 
 CONNECTED. 
 
 course, constitute the nerves. At the extremity away from 
 the nerve cell the axon always breaks up into a second series of 
 dendrites, or terminations. 
 
 The essential part of a sense organ such as, for instance, the 
 retina of the eye, or the auditory cells and hairs in the organ of 
 Corti in the internal ear, always consists of the proximal part of 
 a neurone, or even of several neurones, each of which has usually 
 a very short axon. It is always the dendrites of a nerve cell that 
 receive the stimulus, and the latter is always conducted along 
 the axon from the nerve cell to the other terminal arborisation. 
 Thus nerves invariably conduct impulses in one direction only. 
 
 Neurones are always connected together by means of synapses. 
 At a synapse the distal dendrites of one neurone come into close 
 proximity to the proximal dendrites of another neurone. As 
 
28 THE MECHANISM OF LIFE 
 
 Fig. 9 shows, the two series of dendrites do not touch each other, 
 but the branches of the arborisations " interdigitate." They 
 approach each other very closely, but there is always a space 
 between them, and this space is, of course, filled by other tissue 
 or by liquids. This mode of joining up of successive neurones 
 to form a chain is universal throughout the nervous system, 
 both in the central and the peripheral regions. 
 
 An Example of Sensori-Motor Activity. We shall take what 
 is, perhaps, the simplest and most convenient common action, 
 that of " winking." When something unexpectedly and rapidly 
 approaches the face, the eyes " instinctively " close in order 
 that they may be protected. Now this action of closing and 
 subsequent reopening involves a receptor organ, an afferent 
 
 To cortex. 
 
 SgQSfL 
 
 otor 
 Centres oculomotor 
 
 Jrorn cortex 
 
 Closing muscle- -3 
 
 FIG. 10. SCHEME OF THE NEURONES CONNECTING THE RETINA WITH 
 THE MUSCLES OF THE EYELIDS. 
 
 nerve, a nerve centre, an efferent nerve, and a motor organ. 
 The receptor is the retina of the eye. Light reflected from the 
 moving object forms a minute picture on the retina, and in some 
 way the proximal dendrites (which in this case, however, are not 
 dendrites in the usual sense, but rather the rods and cones of the 
 retina) are affected by the variations in light and shade (that is, 
 by differences in the intensity and quality of the radiation), and 
 nervous impulses are set up which travel along the axons of the 
 retinal nerve cells in the optic nerve to the brain. There they 
 are received by a nerve centre, and the afferent impulse is con- 
 verted into an efferent one, which travels out from the centre, 
 via the axons of the nerve cells there, through two nerves. One 
 of these is the third cranial, or oculo-motor nerve, and the other 
 is a branch of the seventh cranial, or facial nerve. The former 
 
THE SENSORI-MOTOR SYSTEM 
 
 29 
 
 goes to the lifting muscles of the eyelids, and the latter goes 
 to the closing muscles. The same event in the centre gives rise 
 to both impulses simultaneously, and that travelling along the 
 oculo- motor nerve causes the opening muscles to relax, while 
 that which travels along the branch of the facial causes the 
 closing muscles to contract. The eyelids thus close. Im- 
 mediately afterwards the series of events is reversed because 
 of a second pair of efferent impulses; the opening muscles now 
 contract, and the closing ones relax, and so the eyelids open. 
 
 Representing this series of action in a very schematic way, we 
 get the diagrams given on pp. 28 and 29. 
 
 ^Retina 
 
 Eyelid. 
 enmef 
 
 muscle) 
 
 FIG. 11. 
 
 -SCHEME OF THE CONNECTIONS BETWEEN THE RETINA, BRAIN, 
 AND EYELIDS USED IN THE ACT OF WINKING. 
 
 According to the figure, there is only one proximal set of den- 
 drites in the retina ; in reality, there are three (as is represented in 
 Fig. 30, p. 107). Further, only one centre is shown, but really 
 the optic nerve ends in three centres in the mid- brain (see Fig. 32, 
 p. 115). From these centres other neurones make connection 
 with the oculo-motor centre, and from the latter another set 
 of neurones pass out along the oculo-motor nerves to the opening 
 muscles, while yet another traverses the nucleus (or centre) of 
 the facial nerve, and go out through the latter to the closing 
 muscles of the eyelids. 
 
 In any activity of the sensori-motor system, then, a rather 
 complicated mechanism is involved. Some change occurring in 
 
30 THE MECHANISM OF LIFE 
 
 the environment acts upon a receptor organ by stimulating the 
 proximal dendrite of a nerve cell, and thus setting up a nervous 
 impulse which travels up the axon of this cell into a nerve centre, 
 or nucleus, or ganglion, in the brain or spinal cord. The axon 
 ends in the centre as a series of distal dendrites, which form a 
 synapse with the proximal dendrites of another neurone. The 
 axon of the latter leaves the brain or spinal cord in an efferent; 
 nerve, which goes to the muscles concerned. The impulse 
 travelling out is distributed to the muscle fibres by the distal 
 dendrites of the final neurone, and, entering the muscle, it releases 
 energy and causes the latter to contract or relax, thus causing 
 the movement. 
 
 This, it must be remembered, is only a scheme of the mechan- 
 ism, and we elaborate it in Chapters VL to VIII. Meanwhile, it 
 will suffice to give the reader a preliminary notion of what are 
 the essential activities of the sensori-motor system. 
 
CHAPTER III 
 THE PRINCIPLES OF ENERGY 
 
 So far our description of the animal body has been that of a 
 mechanism which can be actuated, or made to "go." How it is 
 actuated that is, what are the sources of its energy, and how 
 these sources are utilised is the subject of the following chapter. 
 Meanwhile, however, something must be said about the general 
 principles 01 energy in so far as they concern us in our study of 
 life. First, then, we ought to consider what is meant by 
 
 The Nature of a Material Body. 
 
 A material thing or body is something that is heavy ; that has 
 shape, or occupies space; that coheres and is dense in varying 
 degrees; that has heat, also in varying degrees; that has texture, 
 lustre, colour, smell, and taste. In short, a material body is a 
 massive substance which has physical " properties." 
 
 Coherence. Material bodies may be solid, liquid, or gaseous. 
 When they are solid they can be disintegrated by mechanical 
 means that is, they can be crushed, broken, powdered, filed, 
 etc. and therefore they must consist of smaller parts that cohere 
 together, but which can, nevertheless, be separated. There 
 appears to be a limit (in the practical sense) to the degree to 
 which a solid material can be pulverised, though we can reduce 
 it to exceedingly fine particles. By-and-by, however, our 
 mechanical means of disintegration fail to make the particles any 
 finer, but in imagination we can still divide them. 
 
 The chemist can show that the finest particles to which a body 
 can be reduced are still aggregates of molecules, and thus he 
 regards the latter as the very finest particles to which a material 
 body can be reduced without losing its specific properties. 
 Molecules are therefore the ultimate particles of which bodies 
 are made up, and they cohere together more or less strongly, or 
 not at all. When the degree of cohesion is such that the body 
 preserves its shape irrespective of anything that contains it, we 
 say that it is solid; but when the molecules still cohere, but slip on 
 each other, we call the body viscous (as in the cases of pitch or 
 
 31 
 
32 THE MECHANISM OF LIFE 
 
 syrup), or liquid (as in the case of water). When they do not 
 cohere at all, so that the material can take any shape and expand 
 to any extent, we call it a gas. Thus in ice the molecules cohere 
 strongly, but not nearly so much when the ice is melted, and 
 hardly at all when the water is converted into steam. 
 
 Temperature and Heat. Now all bodies of which we have any 
 experience possess some heat. Heat is a " mode of molecular 
 motion " a form of energy and the intensity of heat (but not 
 its quantity) depends on the rapidity of motion of the molecules. 
 At a temperature of 273 C. below the freezing-point of water all 
 movements of the molecules themselves (but not of the parts of 
 the molecules) cease, and this temperature is the absolute zero, 
 and is approximately that of cosmic space. When the tempera- 
 ture rises, the molecules move more rapidly until they become 
 loosened from, but still attract, each other ; then the body melts. 
 Rising in temperature still more, the molecules finally cease to 
 attract each other, and each of them moves freely so that they 
 tend to fly apart; then the body becomes a gas. The tempera- 
 tures at which melting and vaporisation occur are, of course, 
 different ones in different materials, and depend on the nature of 
 the molecules of which the body is composed. 
 
 When bodies are at different temperatures, heat tends to flow 
 of itself from the warmer to the colder body. If, when we touch 
 a body, heat flows from it to our skin, we say that the body is 
 warm, and if the converse happens, we say that the body is cold. 
 
 Density. The more molecules there are in the same bulk the 
 denser, we say, the body is. Thus a square inch of ice contains 
 a certain number of molecules of H 2 0, and the substance has a 
 certain density ; but when the ice is melted and the temperature 
 of the water rises to, say, 65 F., the same number of molecules 
 now occupies a greater volume (for they are further apart), and 
 so the density becomes less. 
 
 When the water is raised to, say, 220 F., it is converted into 
 steam, and it now occupies over 1,700 times the volume it had in 
 the liquid state. Therefore its density is very much less. This 
 means that the density of a body depends on the closeness with 
 which the molecules are packed, or cohere together. But it also 
 depends on the relative weights of the molecules, for some are 
 heavier than others; thus the molecules of quicksilver are, each 
 of them, much heavier than those of water. 
 
THE PRINCIPLES OF ENERGY 33 
 
 Texture and Lustre. This depends on the ways in which the 
 particles are, so to speak, laid alongside each other ; thus polished 
 steel has a smooth texture, but that of a fractured piece of steel is 
 rough and crystalline. Lustre is a kind of texture, the body 
 exhibiting it being very smooth, so that it reflects the light that 
 falls on it. 
 
 Colour. This depends on the chemical nature of the mole- 
 cules, and on the ways in which they are arranged together. Gold, 
 for instance, when highly polished, has bright, metallic lustre, and 
 is yellow in colour, but when it is finely divided it may be purple. 
 Sunlight is a mixture of light of different colours, or wave-lengths, 
 and material bodies can act differently on this mixture. When 
 the proportions of the lights of different wave-lengths reflected 
 from a body are the same as the proportions in sunlight, the body 
 appears white (or grey) to our eyes. When all the light that falls 
 on a body is absorbed by it, none being reflected, we say that the 
 latter is black. When some of the wave-lengths are reflected and 
 others are absorbed, we say that the body is coloured. Thus 
 rouge absorbs all the light falling on it except that which has the 
 wave-length associated with what we call scarlet; it reflects this 
 kind of light, and so it appears coloured. In monochromatic 
 light that is, light of one colour all bodies look as if they had 
 the same hue, only brighter or duller. 
 
 Smell and Taste. Most bodies give off fine particles into the 
 air, and these become inhaled into our nostrils. If such particles 
 can dissolve in the liquid bathing the olfactory mucous mem- 
 brane, they can react upon or stimulate the nerve terminations in 
 the latter, and so they give rise to the sensation of smell. If 
 a substance placed on the tongue can dissolve and affect the 
 termination of the gustatory nerves, we have the sensation of 
 taste. 
 
 Different Kinds o Molecules. The molecules, or ultimate 
 particles of which material bodies are composed, are not all the 
 same; for instance, quicksilver consists of molecules of the 
 chemical substance mercury, and water consists of molecules, 
 each of which consists of three atoms (H 2 0), two of hydrogen and 
 one of oxygen. Molecules are therefore made up of atoms, and 
 there are about 100 different kinds of the latter. All material 
 bodies, of whatever nature they may be, are therefore composed 
 
 3 
 
34 THE MECHANISM OF LIFE 
 
 of molecules, each of which contains a relatively small number of 
 one or more kinds of chemical atoms. 
 
 Now colour, smell, taste, lustre, texture, temperature, and 
 density, are dependent on the chemical nature of the molecules, 
 and on the ways in which the latter are arranged and on their 
 motions. Thus water may be warm or cold to our sense, but it 
 consists in each case of the same molecules moving relatively 
 slowly when the water is cold, and relatively quickly when it is 
 warm. Phosphorus may be yellow or red, and in the former case 
 it is odoriferous, poisonous, and combustible; while in the latter 
 case it has no smell, is not poisonous, and is non-inflammable in 
 the conditions in which yellow phosphorus is inflammable. Yet 
 the substance phosphorus is chemically the same in both cases, 
 only the atoms are in different configurations. The same number 
 of molecules of H 2 may be dense in the form of ice, less dense in 
 the form of water, and less dense still in the form of steam, accord- 
 ing to the distances that its molecules are apart from each other. 
 Something, however, is the same in the case of the red and yellow 
 phosphorus, or the solid, liquid, and gaseous water that is, the 
 mass of the chemical substance itself. Apparently colour, taste, 
 smell, density, temperature, etc., may be variable, while mass 
 remains invariable. 
 
 Weight and Mass. Weight itself is something that may vary,] 
 while mass remains the same. A material body weighs more at^ 
 the earth's poles than it does at the equator, and if it could be* 
 transported to the sun it would be much heavier, or if to the 
 moon much lighter, than it is on the earth. If it could be removed 
 to several millions of miles away from any cosmic body its weight 
 would be almost nothing. Weight depends on the mass of the 
 body, on its distance from some attracting body (such as the sun, 
 earth, or moon), and on the mass of the latter. The mass of our 
 material body would be the same everywhere (we are neglecting 
 some physical results included in the theory of relativity), but its 
 weight would vary. 
 
 Mass. Something, then, seems to be invariable, or nearly so, 
 and this is the mass or quantity of matter in a body. The mass 
 of a cubic inch of iron is so much in all circumstances, and that of 
 a cubic inch of gold is more than that of the same bulk of iron, but 
 is also invariable. By the quantity of matter, or mass of a body, 
 we therefore mean the number of molecules in the body rnulti- 
 
THE PRINCIPLES OF ENERGY 35 
 
 plied by their molecular weight. (The molecular weight of the 
 lightest molecule, that of hydrogen, H 2 , is taken as the standard 
 to which all other molecular weights are applied.) 
 
 Mass and Inertia. Mass is, then, the most fundamental thing 
 in our notion of materiality, but it is not an irreducible concep- 
 tion, and we must seek for some way of defining and measuring 
 it. Note, first of all, that we have a bodily intuition of mass: 
 let there be two exactly similar stoneware, corked bottles, and 
 let one be filled with water and the other with mercury. We 
 cannot say which is which merely by looking at them, but we can 
 distinguish if we lift them, for a greater degree of effort of our 
 muscles is necessary to lift the mercury than to lift the water, and 
 we have the " feeling " of this effort. 
 
 Nevertheless, such an intuition would be mostly useless to us, 
 for it would not apply to great masses which we cannot lift, nor 
 to very small ones, nor to masses which were nearly the same. 
 A\ e must have some way of measuring mass by taking some 
 dimension of space, or by counting (or numbering) something. 
 Now there is a constant and universal property of masses that 
 of gravitation; all material bodies, whatever their nature, or -mass, 
 fall to the surface of the earth when they are free to move. If 
 they are sufficiently far away they will fall 16 feet in the first 
 second, 64 feet in the first two seconds, 144 feet in the first three 
 seconds, and so on, their rate of fall being accelerated by 32 feet 
 per second per second during the time that they are falling. 
 "We do not know in the least what the force of gravitation is, 
 but we know precisely what it does it accelerates the rate of 
 motion of a body free to fall. Consider what this means : there 
 are three factors mass, space, and time; a mass, when it is 
 attracted by the earth, falls with increasing velocity that is, it 
 falls through a greater space in the third second than in the 
 second one, and through a greater space in the second one than 
 in the first. This acceleration of the motion of a mass during a 
 certain time we shall call the work done by the force (whatever 
 the latter may be). 
 
 Place a mass of metal (say a pound weight) in a scale pan ; it will 
 fall, and the other scale pan will rise. Now place another pound 
 weight in the other scale pan, and the two will balance each other 
 so that neither falls. The work done by gravity on the one is 
 equal to the work done on the other: neither is accelerated; the 
 space and time factors are the same, and therefore the masses are 
 
36 THE MECHANISM OF LIFE 
 
 the same. Take a piece of wire of uniform diameter and density, 
 and (say) 10 inches long, and put it in one scale, and put a piece 
 the same wire 1 inch long in the other. The mass of the 10-in< 
 length will be greater than that of the 1-inch piece, for the scale) 
 pan in which it is placed will fall, and the other one will rise.j 
 Put a 1-inch length in one pan and another 1-inch length in the] 
 other, and it will be found that the work done by the earth's] 
 gravity is the same in each case, and the masses are therefore 
 equal. Finally, put the ten 1-inch pieces in one pan and the] 
 10-inch piece in the other, and again it will be found that the work 
 done is the same in each case, so that the mass of each 1-inch 
 length is one- tenth of that of the 10-inch length. 
 
 Thus we can find the mass of a body by measuring the world 
 done upon it, when it is free to fall, by the earth's gravity, and 
 comparing this with some standard amount of work done. In 
 whatever way we measure this work done it will be found that 
 we always measure a space. Even when we measure time it is 
 really a space that we determine. 
 
 A material body that is in a state of rest will continue in aj 
 state of rest, or if it is in a state of uniform motion it will continue , 
 in that state of motion unless work is done upon it. Of it-sol t it 
 is inert. Its inertia varies according to its mass, so if a greater 
 mass that is at rest is to be moved, or if a greater mass that is in I 
 uniform motion is to be stopped, a greater amount of work must 
 be done. Inertia means the tendency of something to remain as 
 it is, unless some external agency acts upon it. 
 
 Modern Theories of Matter. 
 
 We must say a few words about these. Not so long ago it wa* 
 thought that all material bodies, or kinds of matter, were built up 
 of chemical atoms which were the ultimate particles. An atom 
 was regarded as indivisible. But some atoms were known to be 
 heavier than others, and so their masses were not the same ; thua 
 the mass of an atom of platinum is about 194 times greater than 
 that of an atom of hydrogen. Therefore, the heavier atoms ha< 
 more of something in them than the lighter ones, but more o 
 what ? There might be some universal kind of matter containe< 
 in greater quantity in the heavier than in the lighter atoms, but 
 if so the former could hardly be thought about as indivisible 
 which they must be, according to chemical theory. 
 
 The way out from this paradox was found by the discovery o 
 
THE PRINCIPLES OF ENERGY 37 
 
 radio-activity. As the result of the physical research based on 
 this, it became very probable that an atom (which is still the 
 smallest particle of matter as such that can exist) is really com- 
 plex, and consists of a system of electrons. An electron is not 
 material, but is an unit charge of electricity, the smallest charge 
 that can exist. In the centre of the atom there is an electron, 
 and revolving round this there are others, much in the way the 
 planets are revolving round the sun in the solar system. Now 
 one or more of the electrons can be expelled from the system, and 
 then the latter becomes a new kind of chemical atom. Different 
 atoms contain different numbers of revolving electrons. 
 
 What is an electron ? All we can say about it is that it is elec- 
 tricity, and not material. Thus, materiality dissolves into energy, 
 and the latter is the fundamental physical reality. Mass disappears, 
 but inertia remains, only the latter is now electro-magnetic inertia. 
 
 The Capacity for Doing Work. 
 
 Things, then, remain as they are unless something is done to 
 them. A train remains at rest unless work is done upon it, 
 causing it, for instance, to attain a velocity of thirty miles per 
 hour three minutes after it has started to move. Conversely, the 
 train will continue to run after the steam has been shut off unless 
 work is done upon it, causing it to stop, and such work may be 
 done by the application of the brakes, or by the friction of the 
 air, or that of the wheels on their bearings or upon the rails. The 
 mechanical work that is done in both these cases is measured in 
 horse-power. 1 h.p. being the amount of work done by raising 
 a weight of 33.000 pounds 1 foot high in one minute. 
 
 The water in a steam boiler will remain at one temperature 
 unless work is done upon it. Heat must be supplied to the water 
 by the combustion of coal in the furnace. Now we can express 
 the amount of heat in units, which are called Calories, each 
 Calorie generated in a second being 5-62 h.p. Thus to raise the 
 temperature of the water in the boiler that is, to change its 
 state in a certain way so much work must be done. 
 
 An electric tram will stand still upon the rails for ever unless 
 work is done upon it. When the switch is closed electric current 
 enters the motors, and the latter revolve propelling the car and 
 giving it a speed of, say, ten miles per hour in one half- minute. 
 We measure this work done by the current in units, called kilo- 
 watts, each of the latter being 1-341 h.p. 
 
38 THE MECHANISM OF LIFE 
 
 A dynamo will remain at rest and yield no current unless work 
 is done upon it, and this also we measure in h.p. Supply motive 
 power equal to so many h.p. to the dynamo, and the latter, with 
 its conductors, changes its state and develops so many kilowatts 
 of current. 
 
 Finally, a man may climb a mountain, say 5,000 feet high, in 
 six hours, but not unless certain substances in his body become | 
 oxidised, yielding muscular power capable of raising his body 
 against the resistance of the earth's gravity. But, again, he 
 cannot supply this muscular power and do work unless he takes 
 food, and a certain quantity of the latter that is, so much 
 proteid, fat, and carbohydrate estimated in equivalent heatj 
 units, called Calories, must be supplied before his body can do 
 the specified work. 
 
 In all these cases something called the capacity for doing work 
 is added to the thing that changes or does work. Chemical sub- 
 stances (the fuel and oxygen) are burned in the steam boiler, i 
 electric current is fed into the motor, mechanical motion is im- 
 parted to the dynamo, and chemical substances are assimilated 
 into the muscles of the mountaineer. All these things represent 
 the capacity for doing work, and the latter we define more shortly j 
 as available energy. 
 
 Obviously there are different forms of available energy, and' 
 these can be transformed one into the other. There is mechanical 
 energy, that of the motions of material bodies for instance, the 
 motion of the parts of the locomotive engine; heat energy that 
 is, the enormously increased velocity of movement of the mole- 
 cules of something or other ; electric energy, which is the flow of I 
 electrons through a conductor; muscular energy (about which we 
 know very little) ; gravitational energy ; the energy of radiation, I 
 etc. We know little or nothing of what these various forms of] 
 available energy are, although we recognise them as different, for 
 they affect OUT sense organs differently; thus we recognise 
 mechanical energy because we exert muscular power in opposing j 
 it ; we recognise heat through the stimuli of certain sense organs ; I 
 radiation usually by the stimuli of the retina (and also of thej 
 skin); electric energy mainly by its stimulation of the muscles,] 
 and so on. 
 
 But all forms of available energy can be converted one into the 
 other, and all of them are the capacity for doing work, or more] 
 generally the state of something or other. But, again, in what- j 
 
THE PRINCIPLES OF ENERGY 39 
 
 ever form this capacity for doing work may exist, it can always 
 be measured, directly or indirectly, as a certain quantity of 
 mechanical work performed. That means that we do not know 
 what available energy is, we only know what it does. 
 
 Energy Transformations. 
 
 We may profitably expand the result stated in the last para- 
 graph, and give some further examples. There is a heap of coal, 
 and this we shall call available chemical energy. It contains 
 certain substances carbon and hydrocarbons and these can 
 combine chemically with the oxygen of the air to form carbonic 
 acid gas and water (with some other substances). In this act of 
 chemical combination heat is generated. If the burning fuel is 
 confined in a furnace, a large fraction of the heat may be commu- 
 nicated to the water contained in a steam boiler, and its effect is 
 to increase the motions of the molecules of the water. By-and- 
 by these motions become so rapid that the molecules fly apart 
 and the water is converted into steam. Later on the velocities of 
 the molecules of the steam are increased by the further flow of 
 heat from the furnace, and they collide with, and rebound from, 
 the walls of the boiler in which they are contained, thus setting 
 up steam pressure. 
 
 The chemical energy of the fuel is therefore transformed 
 into the kinetic energy of the molecules of steam under 
 pressure. 
 
 The steam is then allowed to expand into the cylinders of the 
 engine, and in so doing it pushes out the pistons and communi- 
 cates a rotatory motion to the crank shafts. 
 
 Thus the kinetic energy of the molecules of steam is trans- 
 formed into the kinetic energy of the large, moving parts 
 of the engine that is, into mechanical energy. 
 
 The motion of the engine is next communicated, by means of 
 shafting or belts, to the armature of a dynamo. When the latter 
 revolves it generates an electric current, and this uses up the 
 mechanical energy; for if the switch of the dynamo is open, 
 relatively little power is required to rotate the latter, and no 
 current -passes. But when the switch is closed, relatively much 
 power is required, and a current is generated. 
 
 Mechanical energy therefore transforms into electric energy. 
 
40 THE MECHANISM OF LIFE 
 
 The current generated can now be used in various ways; for 
 instance, it can be sent through lamps, when the latter glow; or 
 it can be made to decompose clay, producing aluminium; or it 
 can be fed into motors, generating mechanical energy; or passed 
 through electric fires, generating heat. 
 
 Thus electric energy transforms into radiant (light) energy; 
 into available chemical energy; into mechanical energy; 
 and into heat energy. 
 
 And so on ; summarising the example we have given, and many 
 others that might be quoted, we get our first principle of energetics : 
 
 Any form of available energy can be converted into any other 
 form . . (1) 
 
 The Diminution of Available Energy. 
 
 We come now to a very important result. Returning to the 
 series of examples just given, we note that a certain quantity of 
 coal contains chemical energy, and that the latter may be con- 
 verted into heat. The quantity of heat which can be generated 
 by the combustion of the coal can be estimated in the following 
 manner: A small weighed piece is powdered, mixed with some 
 substance which yields free oxygen, and is placed in a kind of 
 bomb, which is put into a vessel containing a known mass of 
 water at a known temperature. The mixture of coal and oxygen- 
 yielding substance is fired electrically, and precautions are 
 taken that all the heat generated goes to raise the temperature 
 of the water. The result is that a certain quantity of water, say 
 1 kilogram (=2-2 pounds), is raised in temperature, say 1 C. 
 That quantity of heat is called a Calorie, and so we estimate the 
 " calorific value " of the coal that is, the quantity of heat that 
 is generated when an unit mass is burned. 
 
 Now let the coal be burned in a steam-boiler furnace, and let 
 the steam produced work an engine. The work developed by the ' 
 latter can easily be estimated, and so we can find the number of 
 h.p. given per ton of coal, or per Calorie. Next, use the engine 
 to drive a dynamo, and use the current generated by the latter 
 to drive an electric motor. Suppose that we utilise the entire 
 power of the engine to drive the dynamo, and the entire current 
 given by the latter to drive the motor. Now estimate the h.p. 
 developed by the motor, and compare it with the h.p. developed 
 by the engine; it is much less. 
 
THE PKINCIPLES OF ENERGY 41 
 
 Or use the entire power of the current to work a series of 
 electric fires, and estimate the quantity of heat generated by the 
 latter; it is very much less than the quantity of heat originally 
 generated by the burning of the coal that raised the steam, that 
 worked the engine, that drove the dynamo, that produced the 
 current, that generated the heat of the electric fire; or, again, 
 estimate the h.p. required to drive the dynamo, use the current 
 given by the latter to drive a motor, and then estimate the h.p. 
 developed by the latter; it is about 5 to 10 per cent, less than 
 that required to drive the dynamo. 
 
 Finally, estimate the number of kilowatts of current required 
 to work a motor, and then employ the latter to drive a dynamo, 
 and measure the current generated by the latter; it is less (5 to 
 10 per cent.) than the current originally used to drive the motor. 
 
 That means that there is waste incurred whenever there is an 
 energy transformation, and it is quite easy to see how this waste 
 occurs. When coal is burned in the furnace of a steam boiler, a 
 quantity of heat is lost by radiation from the boiler and furnace 
 doors, and another fraction is carried away through the flues into 
 the atmosphere ; there is radiation from the steam pipes and hot 
 cylinders, and still another fraction of heat is given up to warm 
 the condenser water. Thus all the heat generated by the com- 
 bustion of the coal does not transform into the mechanical energy 
 of the engine; in fact, only about 10 to 20 per cent, does so. 
 Further, there is friction in the bearings, slides, etc., of the engine, 
 and so mechanical energy is lost. In communicating the engine 
 power to the dynamo more friction is incurred. In the dynamo 
 itself there is friction, and some of the current developed is wasted 
 on heating the parts of the mechanism, while more is lost by 
 imperfect insulation. These latter sources of loss also exist in 
 the means whereby the current is transmitted and then converted 
 into radiant energy, or heat, or mechanical energy. When the 
 current is used to generate light there is great loss, for heat must 
 first be generated until the intensely hot filaments, or arc, or 
 vapour, glow. If light could be generated directly from chemical 
 action as it is by a glow-worm there would be great economy. 
 
 Thus there are a host of ways in which available energy is lost 
 in the course of its transformations. There is friction, imperfect 
 heat conduction and insulation, imperfect electric conduction 
 and insulation, materials which are not perfectly elastic or per- 
 fectly rigid, etc. In all cases this lost energy reappears as low- 
 
42 THE MECHANISM OF LIFE 
 
 temperature heat, but the latter is unavailable energy, and we 
 cannot transform it. 
 
 Thus we get our second principle : 
 
 In all transformations of energy some of the latter becomes 
 unavailable. Thus the capacity for doing work continually 
 diminishes . . . . . . . . . . . . (2) 
 
 Now consider another notion which is really involved in our 
 second principle, but which it is useful to discuss separately; 
 that is, the notion of 
 
 Reversible and Irreversible Transformations. Take a suitable 
 form of dynamo and cause it to revolve, thus generating a current 
 of, say, K kilowatts, by the expenditure of, say, H horse- power. 
 Then take K kilowatts of current, and supply it to the dynamo, 
 when the latter will begin to revolve and will act as a motor. 
 Thus the same machine is reversible; when current is supplied 
 it will generate mechanical energy, and when mechanical energy 
 is supplied it will generate electricity. 
 
 But notice particularly that we supplied K kilowatts of current, 
 and got H horse-power. Now if we supply H horse-power, we 
 only get about 95 per cent. K of current. Therefore our rever- 
 sibility is not quite perfect; if it were we should have got K 
 kilowatts of current, instead of 95 per cent. K. 
 
 Take the case of a steam engine and consider its theory. We 
 have a " working substance," the water in the boiler. Now let 
 this working substance be heated; it expands enormously, does 
 work by moving the pistons, and finally escapes into the con- 
 denser, where it heats up the circulating water. Therefore, the 
 working substance takes heat from the steam boiler, converts 
 some of this heat into mechanical work, and gives up some to the 
 condenser. Now let the water in the boiler cool down, and try to 
 reverse the process by actuating the engine in the opposite 
 direction. If the mechanism were reversible, heat would be taken 
 from the condenser, the mechanical work done on the engine from 
 outside would transform into heat, and these two quantities of 
 heat would be transferred to the water in the boiler, heating up 
 the latter to boiling-point and beyond, and so establishing steam 
 pressure. This cannot be done (in practice), and so the steam 
 engine is an irreversible mechanism. 
 
 Take a very famous experiment and try to reverse it : 
 
 Joule, in 1843, caused a paddje-wbeel to revolve in water con- 
 
THE PRINCIPLES OF ENERGY 43 
 
 tained in a heat-insulated vessel. He took all possible precau- 
 tions, measured the amount of mechanical work done (by causing 
 a falling weight of known mass to actuate the paddle), and 
 measured the mass of water and the rise of temperature produced 
 by the friction of the paddle. He found what is now called the 
 " mechanical equivalent of heat " that is. he found that when 
 a weight of 1 kilogram falls through 427 kilometres, 1 kilogram 
 of water is raised in temperature 1 C. This amount of heat 
 communicated to the water is called 1 (large) Calorie. Now 
 imagine the water contained in the mechanism to be heated up 
 1 C. ; if the latter were reversible the paddle would revolve. But 
 it does not, and the apparatus is therefore irreversible. 
 
 Finally, make the end of a poker red-hot in the fire, and then 
 take it out and let it stand ; it will cool down, and in half an hour 
 or so it will have attained the temperature of the air in the room. 
 Its heat has been lost by radiation and convection, and has gone 
 to raise the temperature of the air, furniture, and walls of the room 
 ever so little. Imagine the experiment to be reversed, so that the 
 heat of the room would flow into the end of the poker and raise 
 its temperature to redness ; such an effect has never been observed 
 to occur (though if it did occur physicists would not be incredu- 
 lous !). Therefore the flow of heat, of itself, is irreversible. 
 
 These examples will enable us to formulate two more statements : 
 
 Some energy transformations are approximately reversible and 
 others are irreversible . . . . . . . . . . (3) 
 
 The flow of heat (of itself) is irreversible, only going from a hot 
 [ to a cold body . . . . . . . . . . . . (4) 
 
 Thus we can, quite easily, cause some energy transformations 
 to occur, but not so easily, or not at all, some others. 
 
 We can easily cause all kinds of energy to transform into heat, 
 and, in fact, they do transform into heat of themselves. 
 Mechanical friction always generates heat, chemical action 
 generally produces heat and sometimes chemical energy com- 
 pletely transforms into heat ; the flow of electricity through a con- 
 ductor, the straining and bending of materials (internal friction), 
 the reception of light by substances in short, all physical reac- 
 tions transform, or tend to transform, into heat. Therefore 
 
 All forms of available energy tend to be transformed into heat, 
 but heat is not at all, or it is only with difficulty and loss, 
 transformable into other forms of available energy . . (5) 
 
44 THE MECHANISM OF LIFE 
 
 And that is why there is always waste, or loss of available 
 energy, and it is why the capacity for doing work tends to 
 diminish indefinitely in the universe as we know it. 
 
 Quantitative Energy Transformations. The following state- 
 ments are the actual results of experience: 
 
 (a) 1 horse-power = 33,000 foot-pounds per minute. 
 
 That is to say, the amount of mechanical work that would be 
 done against the earth's gravity when a mass of 33,000 pounds is 
 raised 1 foot in one minute, or conversely the amount of work done 
 by the earth's gravity when a mass of 33,000 pounds falls 1 foot in 
 one minute, is called a horse-power. Long ago British engineers 
 found, by actual experiments, the strength of an average cart- 
 horse as measured in the above ways, and the quantity so found 
 was, later on, adopted as a mechanical unit. 
 
 (b) 1 h.p. =0-178 Calorie per second. 
 
 This means that when the work done by a mass of 33,000 pounds 
 in falling 1 foot in one minute is completely changed into heat 
 (which it can be experimentally), 10-8 kilogram of water is raised 
 1 C. in temperature. 
 
 (c) I gram of fat =$-3\ 
 
 f . .j . ..Calories, when burnt in 
 
 1 gram of proteid =4-1 V 
 
 J f , , , the body. 
 
 1 gram of caroohydrate=4:'l ) 
 
 These are statements of the convertibility of chemical sub- 
 stances into heat. They mean that when certain quantities of 
 combustible materials are burned in oxygen, certain quantities of 
 heat are generated. Many more instances might be given. The 
 transformations are complete ones that is, all the available 
 chemical energy transforms into heat energy. 
 
 (d) 1 kilowatt=0-23S Calorie per second. 
 
 That is to say, when a current is sent through a conductor 
 against a resistance it transforms into heat. Some of the 
 current (=1 kilowatt) disappears, and a certain quantity of heat 
 (=0-239 Calorie) is developed. The heat developed is energetic- 
 ally equivalent to the current that disappears. 
 
 Now, since 1 kilo watt =0-239 Calorie per second, and 
 1 -h.p. =0-178 Calorie per second, it follows that 
 
 (e) 1 kilowatt= 1-341 h.p. 
 
THE PRINCIPLES OF ENERGY 45 
 
 And by reversing statements (b) and (d) and (e) we get 
 
 (/) 1 Calorie per second=5-62 h.p. 
 (g) 1 Calorie per second=4:-18 kilowatt 
 (h) 1 h.p. =0-746 kilowatt. 
 
 But equations (e), (/), (g), and (h) are only true in theory, and 
 they are not true in practice. They would be if equations (b) and 
 (d) were reversible, which they are not. And that shows that we 
 must make a very clear distinction between available energy 
 (which is the capacity for doing work) and energy in the abstract, 
 which is understood when we disregard experience and consider 
 all transformations as reversible ones. 
 
 Energy in the Abstract. 
 
 When an energy transformation occurs, one kind of available 
 energy is converted into other kinds ; for instance, so many cubic 
 feet of coal gas are supplied to a gas engine, which then does 
 mechanical work. Now the " calorific value " of the gas can 
 easily be determined that is, we can find the quantity of heat 
 into which the chemical energy of the gas is completely trans- 
 formed when it is burned. So, also, the heat equivalent of the 
 work done by the engine could be determined; for instance, the 
 latter could be employed to lift a mass against the earth's gravity, 
 and then this work can be represented as heat by using the 
 mechanical equivalent in heat of this work done [equation (6), 
 p. 44]. There will be a balance, for the heat equivalent of the 
 gas is greater than the heat equivalent of the work done by the 
 engine, and so available energy is lost. But we know very well 
 that this available energy is really lost as heat radiated away 
 from the engine, lost in the products of combustion (which are 
 still hot when they are blown out into the air), and by other 
 " leakages." And so the energy is not really destroyed, but 
 simply dissipated. In many cases it can be traced, and where 
 it cannot be traced we assume that it might be. This leads us to 
 consider energy in the abstract without considering whether or 
 not it is available, or possesses the capacity for doing work. 
 
 First of all, however, we must make the concept of an isolated 
 system. Let there be a large island which has absolutely no 
 commerce or other communications with the rest of the world. 
 It produces all that its inhabitants require, and it utilises itself 
 all that it produces. It is self-sufficient, and is economically an 
 
46 THE MECHANISM OF LIFE 
 
 isolated system. So we might imagine an apparatus, or experi- 
 mental plant, containing its own source of energy and absorbing 
 itself all the energy given off, or wasted, or dissipated. That would 
 be an isolated, energetic system. It is true that such an isolated 
 physical system is a fiction, for there must always be some 
 interchange of energy between any apparatus that we can devise 
 and work and its surroundings; the only really isolated physical 
 system is the entire universe. But let it be possible to take 
 account of the energy lost by the system to without, or received 
 by it from without; then we approximate to our concept of 
 physical isolation. 
 
 Making all allowances, then, for experimental error, friction, 
 loss of heat by radiation, convection, and conduction, imperfect 
 rigidity and elasticity of materials, etc., we can, very approxi- 
 mately at least, trace the quantity of available energy that 
 becomes unavailable, or is dissipated. Thus we arrive at our 
 fundamental principle of energy: 
 
 In all the transformations undergone by an isolated system 
 the total quantity of energy contained is neither increased 
 nor diminished . . . . . . . . . . (6) 
 
 This is the law of conservation of energy, and it means that 
 although the capacity for doing work that is, available energy 
 diminishes, energy in the abstract does not diminish. When 
 available energy diminishes, unavailable energy always appears in 
 its place. Very often we can prove this. If we cannot do so we 
 assume the law, and our assumption is always justified by other 
 experimental results. But we are compelled, anyhow, to make 
 the assumption, because the law of conservation is an a priori 
 mode of our thought. To this matter we return later. 
 
 We may now state the principal result of our work in the 
 form of a quotation: " In all the transformations of a material 
 system considered in this book, there is a certain entity which 
 (1) remains constant in quantity, and (2) is capable, under 
 certain conditions, of assuming the forms of kinetic and potential 
 energy, which are dealt with under the study of Rational Dyna- 
 mics. This Entity is Energy."* 
 
 Several points arising from this definition must now be dis- 
 cussed: first of all the nature of " Rational Dynamics." In this 
 
 * G. H. Bryan, Thermodynamics, Teubner's Lehrbuchen der Mathe- 
 matischen Wissenschaften, xxi., Leipzig, 1907. 
 
THE PRINCIPLES OF ENERGY 47 
 
 department of science we deal with, the behaviour of material 
 bodies that " obey " Newton's laws of motion. There is no 
 friction; bodies are perfectly rigid or perfectly elastic, and 
 everything happens in an ideal world where all events can be 
 described strictly mathematically. We do not consider heat at 
 all, and energy is always mechanical, and is either potential or 
 kinetic. There is no dissipation, and all transformations are 
 reversible ones. In such circumstances " energy " is an entity 
 that can be denned as above. 
 
 Potential and Kinetic Energy. 
 
 Sometimes available energy seems to vanish, although it is not 
 converted into unavailable energy, or dissipated. Thus a grand- 
 father's clock has stopped because its weights have run down, 
 and we proceed to wind up the latter, but do not start the pen- 
 dulum. In doing so we have expended muscular work, which is 
 measured by the masses of the weights and the distance through 
 which they have been raised. But the clock doesn't " go " 
 until the pendulum is given an initial swing, and we have thus 
 to account for the energy which we have expended. Now the 
 system, clock and weights, and the earth, is in a different state 
 from what it was before the weights were wound up, for the 
 latter are now some 5 feet further away from the centre of the 
 earth than they were when the clock had run down, and they are 
 therefore free to fall. So long as the clock is not started, the 
 weights possess potential energy that is, energy of position 
 relative to something else. When the clock is started the weights 
 begin to descend, and the potential energy begins to transform 
 into kinetic energy, which is the energy of a mass in motion. 
 Call M the mass of the weights and V the velocity with which 
 they fall, and we get |MF 2 =the kinetic energy developed. 
 During the time of fall the potential energy decreases, and the 
 kinetic energy passes into the unavailable form, for the friction 
 of the wheels, etc., generates heat, which is radiated away. 
 When the weights have reached the bottom of the case they are 
 no longer free to fall, and the system contains no more potential 
 energy that is available for doing work at least, so far as the 
 clock itself is concerned. 
 
 Think about a mixture of oxygen and hydrogen in the pro- 
 portions of one molecule of the former and two of the latter ; this 
 system contains potential chemical energy. It will remain a 
 
48 THE MECHANISM OF LIFE 
 
 mixture of oxygen and hydrogen for an indefinite period of time, 
 nothing happening in it. But let a spark pass through the gases, 
 and they will combine together with an explosion and generation 
 of heat, and the potential energy which they contained trans- 
 forms into the kinetic form. Prior to the explosion the mixture 
 might be represented thus : 
 
 H 2 , H 2 , 2 , etc., 
 
 the molecules of the gases (each of them consisting of two atoms 
 bound together) being apart from, and not influencing each other ; 
 but after the spark has fired the mixture we have 
 
 H 2 0, H 2 0, etc., 
 
 the molecules now occupying a different position relative to each 
 other. Thus chemical energy is energy of position. 
 
 Now before the explosion the mixture of hydrogen and oxygen 
 possessed kinetic energy (as a mixture and apart altogether from 
 the chemicaT^ature of its constituents). Each molecule was 
 moving with a certain velocity, and from the temperature and 
 pressure of the gas the average velocity of all the molecules can] 
 be determined. The mixture can do mechanical work, for if it 
 were allowed to rush into a vacuum it could drive in a piston 
 just as steam drives in the piston in a cylinder. But as the] 
 explosion occurs, the work that the gases can do is enormously 
 increased (thus the explosion can propel a projectile if it is fired 
 in a suitable vessel), and this energy that appears is the increased 
 kinetic energy of the molecules of steam at a very high tempera- 
 ture that is, the latter are now moving with much greater 
 velocities than were the molecules of hydrogen and oxygen, and 
 so they exert a greater pressure. 
 
 The energy therefore becomes manifest, or visible, in its! 
 mechanical effects; but where was it before the explosion 
 occurred ? We cannot answer this question, and we assume; 
 that it was not in the molecules of hydrogen and oxygen, but 
 was in the medium between them that is, in the ether of space. 
 The hypothesis is one that does not lead us astray, and there- 
 fore we assume its validity or " truth." 
 
 Available chemical energy, the energy of a weight raised above 
 the surface of the earth, and free to fall when it is released, and 
 that of a coiled spring these are examples of potential energy 
 which transforms into kinetic energy, or the motion of bodies, 
 when a releasing transformation occurs. 
 
THE PRINCIPLES OF ENERGY 49 
 
 Releasing Transformations. 
 
 A word about this matter. The grandfather's clock, with its 
 weights wound up, is at rest, but a very small expenditure of 
 muscular effort will cause the pendulum to make a swing, and 
 then the mechanism starts. The mixture of hydrogen and 
 oxygen is also at rest, and nothing happens until a very small 
 spark (involving a very small expenditure of heat) causes several 
 molecules of H 2 and 2 to combine, thus liberating heat, which 
 starts off other molecules, and so on, until the whole mixture 
 fires in a very small period of time. A stone poised on the 
 summit of a hill remains there until a very small push liberates 
 it, and it starts to roll down, acquiring a high velocity, and 
 becoming capable of doing much mechanical work. The coiled 
 spring of a gramophone remains coiled until the touch of a little 
 lever allows it to uncoil and actuate the mechanism. These are 
 examples of releasing transformations. A relatively large 
 quantity of energy is potential, and because of some condition of 
 " false equilibrium," due to friction of some kind, it remains 
 potential; but a relatively small expenditure of kinetic energy 
 suitably applied upsets the equilibrium or overcomes the friction, 
 and releases the energy which was potential. Such releasing 
 transformations are of much importance in the discussion of 
 organic activity. 
 
 The Principle of Becoming. 
 
 Taking a general survey of the results discussed so far, we may 
 ask the question, Why does anything at all happen ? The 
 enquiry is not so foolish a one as it may appear to a " prac- 
 tically-minded " person, for a little consideration will convince 
 such that if anything happens if there is " becoming " there 
 must be an energy transformation. This is true, whether the 
 event that happens is something very great and important from 
 our point of view for instances, the formation of a huge sun- 
 spot with accompanying magnetic storms, a cyclonic disturbance 
 which wrecks a number of vessels, or a stellar collision and it 
 would also be true of quite trivial things, such as the con- 
 densation of a man's breath on the surface of a shaving mirror. 
 Therefore our question becomes this, Why does an energy 
 transformation occur ? And as such it is a legitimate subject 
 for enquiry. 
 
50 THE MECHANISM OF LIFE 
 
 Now in every transformation three things are involved : 
 
 1. The intensity of the energy (intensity factor). 
 
 2. The quantity of the energy (capacity factor). 
 
 3. The direction of the transformation (the sign). 
 
 These factors may be illustrated as follows : 
 
 The System in which the 
 Transformation occurs. 
 
 Intensity Factor. 
 
 Capacity Factor. 
 
 A steam engine 
 A water motor 
 An electric motor or lamp 
 
 A source of heat 
 
 Pressure of steam 
 Head of water 
 Electric potential 
 (volts) 
 Temperature 
 
 Quantity of steam 
 Quantity of water 
 Quantity of current 
 (amperes) 
 Quantity of heat 
 
 In all these cases there is a system in which an energy trans- 
 formation may occur; thus there may be a steam engine and 
 boiler actuating some mechanism, a water reservoir and wheel, 
 etc. Will a transformation occur in the system, and, if so, what 
 will it do ? To be able to predict these occurrences we must be 
 able to specify the conditions under which the system exists. 
 The steam engine will " go " if the pressure of steam in the 
 boiler is considerably greater than that of the atmosphere, and ' 
 it will continue to go if this pressure is maintained that is, if 
 new steam is generated as quickly as steam is drawn from the 
 boiler. So, also, with the water mill : the motor will revolve if 
 there is a sufficient head of water, and to keep it revolving the 
 head must be maintained, or the quantity of water in the reser- 
 voir must be very great, or must be renewed as fast as it is : 
 depleted. If an electric current flows, there must be some way 
 of raising electric potential, for a difference of this is the reason 
 why the current flows. But the quantity of current is also a 
 factor; thus the voltage of an electric stove might be the same 
 as that of a lamp, but a greater quantity of current would be 
 necessary in the former case (for heavier wires are employed). 
 Finally, the temperature of a steam radiator 1 foot square might 
 be the same as that of a radiator 6 feet square, but the quantity 
 of heat distributed by the latter would be much greater. 
 
 So there must be a difference of intensity if there is to be a 
 transformation. If a hot-water radiator were at the .same 
 temperature as that of the room in which it were placed, there 
 
THE PRINCIPLES OF ENERGY 51 
 
 would be no transference of heat, no matter what quantity of 
 hot water were in circulation. Heat of itself flows from a 
 hotter to a colder region that is, there must be a difference of 
 intensity in order that the flow may occur. 
 
 Why does a chemical reaction occur ? Why, for instance, do 
 coal gas and oxygen combine, of themselves, to form carbonic 
 acid gas and water ? The reaction is (assuming that the gas is 
 CH 4 that is, marsh gas; it really contains hydrogen and other 
 inflammable substances): 
 
 CH 4 +20 2 =C0 2 +2H 2 0, 
 
 and the reaction occurs as if the equation were written from 
 left to right. Why from left to right, and not from right to left ? 
 Marsh gas and oxygen combine to form carbonic acid and water, 
 because in doing so a large quantity of heat is evolved; but if 
 CO, and H,0 are to combine, a large quantity of heat (or more 
 generally of energy) must be given to them, or be absorbed by 
 them. Therefore they do not combine of themselves, although 
 they can be made to do so. Chemical reactions occur, as a rule, 
 only if heat is evolved, and so we may predict the occurrence if 
 we know that heat may be generated. We cannot enter into the 
 qualifications of this statement, but it may be made more general 
 by saying that a chemical transformation will occur if in the 
 occurrence work will be done. That is " why " it occurs, and 
 if no work can be done by the chemical substances by reacting 
 with each other they will not react of themselves. 
 
 And if work is done as the result of the occurrence of a trans- 
 formation energy will be dissipated, for all work done tends to 
 Tansform into heat. But, again, all heat generated tends to 
 listribute itself by conduction, radiation, and convection. It 
 annot of itself accumulate in one place, as it tends to become 
 uniformly diffused throughout the system the room, the world, 
 and, finally, the whole universe. And so our experience is that 
 n all transformations energy becomes dissipated that_Jis, 
 ntensity differences become levelled down. Therefore we say 
 hat an energy transformation occurs of if sp-lf _if f - energy can be 
 issipated, or if intensity differences can Ke abolished. 
 
 This is the sign of the transformation. Marsh gas and oxygen 
 eact with the evolution of heat and the production of carbonic 
 cid and water. But C0 2 and OH 2 do not react with the pro- 
 uction of marsh gas, because heat would be absorbed in this 
 
52 THE MECHANISM OF LIFE 
 
 case; or work is done by the system (CH 4 +0 2 ) in the first case, 
 but work would be done on the system (CO., and H 2 0) in the 
 second case. The latter transformation does not occur unless 
 there is a compensatory energy transformation, a matter to 
 which we return later. 
 
 Thus we obtain another principle: 
 
 Energy transformations will only occur of themselves if energy 
 
 r becomes dissipated that is, if intensity differences are 
 
 diminished . . . . . . . . . . . . (7) 
 
 The principles stated in the course of this chapter are, to some 
 extent, repetitions, and they may be summarised and included 
 in the two fundamental laws of science the first and second laws] 
 
 of thermo-dynamics. These are: 
 
 ^^ 
 
 1. The Energy of the Universe is Constant. 
 
 2. The Entropy of the Universe tends to a maximum* 
 
 Now we may, very shortly, examine these statements. 
 
 Matter, we have seen, can only be defined in terms of energy,; 
 so the first law includes the old statements of the conservation 
 matter and " force." 
 
 But energy was defined on p. 46 as the entity that was con- 
 stant in all transforming systems, and so there is something in 
 universe that is constant. This something is energy. Th< 
 fore the first law is a definition of energy in the abstract. 
 
 Thinking about all this critically, the reader cannot fail to 
 that the law of conservation is not true without some qualifica- 
 tion. It says that there is an entity an existence in thej 
 universe that can neither increase nor diminish. He 
 notice that we have experience of nothing but energy trai 
 formations, to which, it appears, all existences must reduce. Bi 
 there are existences which arise from nothing, and are annihilal 
 These are dreams, hallucinations, " spooks," etc. Our dm 
 may be vivid and convincing, and " true while they last," am 
 apparitions, if we may fully believe those who have seen them, 
 can have all the appearances presented to us by material t 
 Obviously disordered sense organs may mislead us, and we ma] 
 hear noises and smell odours which we may be pretty sure d< 
 not come from outside our own bodies. Then there is telepathy, 
 mediumistic control, table-rapping, the planchette, etc., all 
 
 * The concept of entropy is discussed in Chapter XI. 
 
THE PRINCIPLES OF ENERGY 53 
 
 them phenomena about which one hesitates to dogmatise. These 
 things may be, as William James said, " the wild beasts of the 
 philosophical desert," but still the^ are beasts ! We do not get 
 rid of them by saying that they are purely subjective, for we do 
 not seem to be quite clear as to what is subjective and what 
 objective, and whether there is any difference ! 
 
 But this is quite clear : dreams, apparitions, spooks, telepathy, 
 and the like, are phenomena that are a nuisance, for they are 
 existences that are not conserved, and so we cannot investigate 
 them. They are not energy transformations, for if they Were 
 they would continue in some other form after they had dis- 
 appeared, and then we could count and measure, and describe and 
 weigh them. We cannot do so, of course, and so most scientific 
 men simply do not " believe " in spooks. They are existences, 
 but they are not real existences. Real things are the things that 
 are conserved. 
 
 And so the law of conservation applies to some things and not 
 to others, and the things to which it does not apply are unreal. 
 That seems to be the best way out of the difficulty. 
 
 Nothing that science can possibly do will disprove the law of 
 conservation. Energy seems to vanish, but, if so, we only say 
 that it has become potential. It may appear to come from 
 nothing, and then we say that potential energy becomes kinetic, 
 and we invent an ether of space to give us an abiding-place for 
 potentialities. The discovery of radio-activity is a good example, 
 and we may refer to it. A fragment of diamond can be made to 
 burn, giving off heat, and transforming almost immediately into 
 carbonic acid gas, which can no longer give off heat. A fragment 
 of radium of the same size, however, gives off heat spontaneously, 
 and continues to do so ; and a physicist who could observe it for 
 a thousand years would find that it was still giving off heat, but 
 that, like the bush that Moses saw, nee tamen consumebatur. To 
 a chemist of the middle of the nineteenth century such a pheno- 
 menon would have seemed to be almost miraculous, yet it is 
 doubtful if he would have distrusted the law of conservation. 
 He would, like Sir J. J. Thomson and Sir Ernest Rutherford, have 
 invented a new kind of " bound energy." 
 
 He would, like the physicists of our own time, have been 
 justified in his faith. His ether and electrons and bizarre atomic 
 systems are pure inventions, but they are real; they continually 
 lead us back to order and not to chaos, and they are the means 
 
54 THE MECHANISM OF LIFE 
 
 whereby we make discoveries and obtain increased power over 
 material things. They are concepts that can be investigated, 
 inasmuch as they include the notion of conservation. 
 
 All this goes to show, does it not, that the law of conservation 
 is an a priori one it is a form of our thought ? 
 
 Compare it with Kant's " first analogy of experience " : "In all 
 changes of phenomena substance is permanent, and the quantum 
 thereof in nature is neither increased nor diminished." Pheno- 
 mena are, in so far as they are real, energy transformations, and 
 their " substance " that which underlies them is energy in the 
 abstract. If Kant's transcendental logic is valid, the first law is 
 a mode of operation of the mind; it is something that makes 
 experience by arranging sensations. 
 
 And so we are not surprised when a physicist invents a potential 
 energy to account for something that appears to arise out of, or 
 passes into, nothing, and when his invention works out and leads 
 to discoveries and practical applications. He acts upon nature 
 because he has sensory data and " categories of the under- 
 standing " mental operators, we shall say that deal with 
 sensory data. It is because he acts that he has a mind, for the 
 latter is the expression of his action. Since the law of conser- 
 vation is a means of action, it follows, of course, that operations 
 based upon it that is, potential energies, ether, electronic 
 systems, etc. must " work " or have " pragmatic value." 
 
 The second law is quite different : it has nothing at all a priori 
 in it, as it is simply the resume of our experience. It tells us that 
 entropy always increases when anything happens, and that 
 means, for our present purpose at least, that water does not run 
 up hill, a cold poker in a cold fireplace does not become red-hot, 
 chairs and tables do not " levitate," etc. We can imagine all i 
 these things happening, and if we think about it we do not 
 see why they should not happen, except that they never have 
 done so in our experience. With this observation, however, we 
 defer the discussion of the second law till a later chapter. 
 
CHAPTER IV 
 THE SOURCES OF ENERGY 
 
 IT is clear that the animal has a double relationship with its 
 environment. On the one hand it becomes aware, by means of 
 its sense organs, of the changes that occur in the outer world. 
 Events may happen there that menace it, and so there are places 
 and things that are to be avoided; while other things present 
 advantages food and shelter, for instance and these things it 
 must seek and utilise. As the result of sensation and perception 
 it acts upon the external world : its second relation to the latter. 
 
 This acting involves mobility of body and limbs that is, the 
 performance of mechanical work and we have next to enquire 
 into the source of this kinetic energy, or vis viva, of the animal 
 body. Common experience shows that it is obtained from the 
 food that the animal eats, and so we regard this food as a store of 
 potential energy which becomes transformed into the movements 
 that make up the locomotory, defensive, and aggressive actions. 
 The whole series of energy transformations falls into three stages : 
 
 (1) The digestion, assimilation, and distribution of food material; 
 
 (2) the oxidation of the assimilated food substances, with the 
 liberation of the bound energy contained in it ; and (3) the excre- 
 tion of the products of the process of oxidation. All this train of 
 changes undergone by the substances that are taken into the 
 animal body, built up to form its tissues, oxidised and excreted, 
 is called metabolism. 
 
 The Inanimate Engine. Now it will help us greatly in our 
 study of the animal mechanism if we discuss first the way in 
 which energy is transformed in the inorganic engine. In the 
 most common form of the latter, the steam engine, there is a 
 working substance, the water, which is heated in the boiler 
 until it forms steam under pressure. This steam is admitted 
 into a cylinder, where it is allowed to expand, thus forcing out 
 the piston, and so doing mechanical work. It is again expanded 
 in intermediate and low-pressure cylinders, thus doing more 
 work, and by the time it has entered the COD denser, and attained 
 
 55 
 
56 THE MECHANISM OF LIFE 
 
 the temperature of the circulating water there, it has parted 
 with all the energy which is available under the circumstances 
 in which the steam engine works. The condensed steam is 
 then returned to the boiler, reheated, and the cycle of operations 
 recommences. Note that there is a source of energy contained 
 in the coal, and that this energy is converted into heat, which is 
 the kinetic energy of the molecules of steam. With each quantity 
 of steam that leaves the boiler and enters the cylinders a certain 
 quantity of energy is taken from the source (or coal) by the 
 working substance (or steam), and some of this is given up to the 
 condenser (for the water circulating in the latter becomes heated). 
 But much more energy is taken from the source than is imparted 
 to the condenser, and the difference is represented by the me- 
 chanical work done by the engine. Thus, of the total heat 
 generated in the furnace a certain fraction becomes transformed 
 into the kinetic energy (or mobility) of the engine. 
 
 The Animate Engine. Life in general that is, plant and 
 animal life presents a series of events similar to that just 
 described. There is a working substance which is represented 
 by the very simple chemical compounds, water, carbonic acid, 
 and certain mineral nitrogenous salts (which we shall call 
 " nitrate " for short). These things correspond to the water 
 employed in the steam engine. There is a source of energy 
 corresponding to the heat generated in the steam boiler; this 
 is the energy radiated by the sun (solar radiation we shall call 
 it). Just as the burning coal imparts energy to the working 
 substance of the steam engine, so the solar radiation enables 
 green plants to manufacture carbohydrates, fats, and proteids 
 from the water, carbonic acid, and nitrate supplied to them 
 by the atmosphere and soil. Solar radiation acting through 
 the green plants imparts energy to the working substance of life. 
 
 The latter, in the form of carbohydrates, fats, and proteids, 
 all of which substances contain large quantities of chemical 
 energy, is then eaten by the animal organism, and, after the 
 digestive changes, incorporated in the tissues of its body. In the 
 course of the metabolic changes which it undergoes its energy 
 becomes transformed. Steam at a high temperature (and with 
 high intensity of energy) enters the cylinders of the inanimate 
 engine, and leaves the condenser at a relatively low temperature 
 (and thus with low intensity of energy). In much the same way 
 the life working substance enters the animal body while con- 
 
THE SOURCES OF ENERGY 57 
 
 taining much free chemical energy, and leaves it in the form of 
 the excretions while containing almost no free chemical energy. 
 In the steam engine the difference between the quantities of 
 energy possessed by the working substance when entering and 
 leaving the mechanism is represented by the mechanical work 
 done and the heat lost by radiation, etc. Similarly, the difference 
 between the energy of the working substance (as the food) 
 when it enters the animal body and when it leaves it (as the 
 excretions) is represented by the mechanical work done by the 
 animal, and by the heat radiated away from its body and lost 
 in the excretions. 
 
 The working substance, now degraded in respect of the energy 
 it contained, is taken up again by the green plants, reconverted 
 into carbohydrate, fat, and proteid, and the cycle of operations 
 recommences. 
 
 The Digestive Process in Animals. In the meantime, we 
 consider only the animal part of the life energy cycle. The 
 food matters exist, and the animal obtains these by the exercise 
 of its sensori- motor organs. But these food matters must be 
 digested, distributed to the tissues of the body, assimilated, 
 oxidised, and then excreted. It is this train of events that we 
 have now to study. 
 
 Why must they be digested ? The foodstuffs must enter 
 into the blood-stream of the animal that eats them, and in order 
 that this may happen they must be dissolved. Now, as a rule, 
 the food of an animal is a complex and heterogeneous collection 
 of substances, how complex every healthy man and woman knows. 
 In the animal living in the wild it is even more complex, since 
 such creatures usually hunt down, kill, and devour other animals, 
 and often ingest flesh and bone, fur, feathers, scales, hair the 
 edible as well as the inedible parts, which latter civilised man 
 tries to reject in the processes of the preparation of his food. 
 But even when the latter is carefully prepared and cooked it 
 still contains inedible parts the cellulose or woody fibre of 
 vegetables, fruits, cereals, peas, and other plants, and the fibrous 
 substance of meat and certain fats which are not easily dissolved. 
 All these inedible constituents of the food are excreted in the 
 faeces, and, of course, they vary in different animals: thus her- 
 bivores can digest cellulose, while man cannot as a rule. 
 
 Trituration, or mechanical disintegration of the substance of 
 the food, occurs mainly in the mouth. Finely divided particles 
 
58 ' THE MECHANISM OF LIFE 
 
 are thus produced, so that the digestive juices may act all the 
 more easily. These digestive juices play the principal part in the 
 reduction of the food into such a form that it can enter the blood- 
 stream, and we must say something about them. There are 
 certain glands in connection with the alimentary canal (the 
 mouth, oesophagus, stomach; and intestine), and their function 
 is to elaborate liquids which issue from them through ducts, 
 and are conveyed into the cavities of the mouth, stomach, and 
 intestine. The salivary glands in the mouth secrete saliva, the 
 gastric glands of the stomach manufacture an acid liquid called 
 gastric juice, the liver prepares the bile, and the pancreas the 
 pancreatic juice. All along the intestines there are glands 
 which also secrete a digestive juice. 
 
 These various juices are active in virtue of certain mysterious 
 substances called enzymes (or ferments) which they contain. 
 The saliva contains ptyalin, the gastric juice pepsin, the pan- 
 creatic juice trypsin, and the intestinal juices erepsin, etc. The 
 bile secreted by the liver is active mainly in virtue of the alkaline 
 substances that it contains, and the juices of the intestine contain 
 a substance called enterokinase, which " activates " the erepsin. 
 What these enzymes are we do not know in the least, for they 
 have never been prepared in a pure condition, and they are pre- 
 sent in their respective juices in very small quantity. But we 
 know a very great deal about what they do, and there are many 
 inorganic substances which have very much the same properties, 
 so that we cannot say " for certain " that the digestive enzymes 
 are, in any way, substances that are exclusive to the living 
 organism. 
 
 The Foodstuffs. To understand their modes of operation 
 we must consider the chemical composition of the foodstuffs. 
 Anything that we eat consists of a mixture of substances called 
 proteids, carbohydrates, and fats. White of eggs (albumen) 
 is a nearly pure form of proteid containing much water. The 
 lean flesh of all animals is mainly proteid, while the substance 
 of peas, beans, lentils, cheese, and parts of the cereals and of 
 the solids of milk are proteid in nature. Carbohydrates are 
 represented by cane-sugar and glucose, the starchy foods, such as 
 rice, the substance of potatoes, the main constituents of cereals, 
 the sugar of milk, etc. Fats are the fatty parts of flesh meat and 
 the massive fat that occurs in most animals, the cream of milk, 
 butter, margarine (which is now mainly " hardened " or " hydro- 
 

 THE SOURCES OF ENERGY 59 
 
 genated " vegetable oils, plant oils such as olive, cotton- seed, 
 palm, etc.). 
 
 Proteids are innumerable, and the flesh of almost every species 
 of animal contains distinct kinds. They are the most complex 
 of all chemical substances, and it is only during the last quarter 
 of a century that they have been successfully investigated 
 (mainly by Fischer and the German chemists of his school). All 
 of them contain the chemical elements carbon, hydrogen, oxygen, 
 and nitrogen, united together in a very complex way. They are 
 now known to be built up of peculiar substances called amino- 
 acids, and one of the simpler examples of the latter has the 
 following formula and name: 
 
 '>CH CH 2 CH(NH 2 ) COOH. 
 
 Leucine or a-amino-iso-caproic acid. 
 
 Very many of these amino-acids are known, but many more 
 still have yet to be investigated. Now -imagine a number of 
 these united together in this way : 
 
 R' R" R'" 
 
 NH CH CO - NH CH COOH. 
 
 (2) (3) 
 
 Here we have a chain of members, each of which has the form : 
 
 R 
 
 -NH CH CO ; 
 
 the N, H, C, and each represent an atom of nitrogen, 
 hydrogen, carbon, and oxygen respectively; and R represents a 
 complex " radicle " or group of atoms such as (in a simple case) : 
 
 FCH 3 CH 2 CH 2 . 
 In the example we have given the chain consists of only three 
 members, but there may be very many more. It may be bent 
 round to form a ring, and the ring may be joined to other rings, 
 and there may be " side chains " attached in various ways. Now 
 compare all this with the chemical formula of urea (which is the 
 characteristic substance present in the urine): 
 
 NH 
 
60 THE MECHANISM OF LIFE 
 
 and we get an idea of the exceedingly complex chemical structure 
 of the proteids a structure upon which, in some way or other, the 
 phenomena of life depend. Proteids are therefore combinations 
 of amino-acids, and they contain, generally, about 16 per cent, of 
 nitrogen. 
 
 Carbohydrates contain carbon, hydrogen, and oxygen, but no 
 nitrogen. They are also very complex chemical substances; 
 thus " grape- sugar " (dextrose, or glucose) has the formula: 
 
 CH.OH CH(OH) CH(OH) CH(OH) CH(OH) CHO. 
 
 Many other sugars are more complicated, and the starches and 
 gums are more complicated still. 
 
 Fats are still more complicated in chemical structure than the 
 carbohydrates. What we usually call an " oil " or a " fat " is a 
 combination of one or more fatty acids with glycerine. A fatty 
 acid consists of carbon, hydrogen, and oxygen atoms united 
 together to form a chain; thus stearic acid is: 
 
 CH 3 CH 2 15 other CH 2 links COOH, 
 
 and may be written in short by R COOH, where the R stands 
 for the chain CH 3 CH 2 to seventeen terms. Now three 
 molecules of stearic acid combine with glycerine to give us 
 
 R.-CO CH 2 
 
 I 
 R CO CH 
 
 R_CO CH 2 
 
 which is tristearin, the principal constituent of mutton fat. 
 Since there are a great number of different fatty acids there 
 must also be many kinds of fats. 
 
 Now all this seems very technical, but it must be clearly under- 
 stood that no one can hope to get even a slight acquaintance wi- 
 the mechanism of life without a knowledge of at least such detai 
 as we have set out above. And, after all, it is not very difficult ! 
 
 What we eat is therefore a mixture of proteids, carbohydrates, 
 and fats. The proteids, especially when cooked, are insoluble in 
 water, and so are the fats. We can take cane- sugar, and other 
 sugars, into the alimentary canal, but they must be converted 
 into dextrose before they can be used by the tissues. The starches 
 which form the bulk of potatoes, rice, and similar foods, as well 
 as a large part of cereals and the breads that are made from these 
 
 : 
 
 ail 
 
THE SOURCES OF ENERGY 61 
 
 things, are also insoluble, and they must be converted into 
 dextrose. Digestion consists, therefore, of the solution of the 
 proteids and fats, and the conversion of the starches and other 
 carbohydrates into dextrose. 
 
 As soon as food enters the mouth digestion begins. The 
 salivary enzyme, ptyalin, acts on the starchy substances, con- 
 verting them into sugar, and in the stomach the pepsin of the 
 gastric juice acts upon the proteids. The fats are not touched at 
 all until the food enters the duodenum, and then the main diges- 
 tive operations begin. The trypsin of the pancreatic juice and 
 the erepsin of the intestinal juice act energetically on the proteids, 
 carbohydrates, and fats, and the bile secreted by the liver aids 
 in a way that we cannot explain here. The result of the action 
 of all these enzymes is that the proteids are split up into their 
 constituent ammo-acids, the starches are converted into dex- 
 trose, and the fats and oils are split up into fatty acids and 
 glycerine. Now all these substances are soluble in water (or at 
 least in the liquid bathing the internal walls of the alimentary 
 canal), and so they can soak through into the blood. These 
 substances amino- acids, fatty acids, dextrose, and glycerine 
 are what the animal organism wants they are its proximate 
 food principles. 
 
 Absorption and Distribution. They must get into the blood- 
 stream and be carried to the muscles, glands, and other tissues 
 of the body. How ? That leads us to consider the organs of 
 circulation. Everywhere in the body there are minute blood- 
 vessels, called capillaries, forming a close network, and along with 
 these there is a separate system of vessels called the lymphatics. 
 We do not consider the latter here, except to say that ultimately 
 they communicate with the bloodvessels. 
 
 We regard the bloodvessels (from our point of view of the 
 distribution of nutritive matter) as beginning in the walls of the 
 alimentary canal. There the capillaries form a very close 
 network just within the internal lining (the mucous membrane), 
 and the liquids in the alimentary canal are separated from that 
 in the capillaries by the mucous membrane, some very loose 
 connective tissue, and the very delicate walls of the capillaries 
 themselves. So it is easy for the digested, soluble food substance 
 to soak through the mucous membrane, the loose submucous 
 layer, and the walls of the capillaries. Now the " soaking " 
 through is not the same process as the soaking of a few drops of 
 
62 
 
 THE MECHANISM OF LIFE 
 
 soup through a piece of porous blotting-paper; in the latter case 
 the liquid soup goes through unchanged, except for the solid 
 particles, which are kept back by the blotting-paper, while the 
 living cells of the mucous and other membranes act on the 
 digested food matters. We have seen that the digestive enzymes 
 change the composition of the proteids thus: 
 
 Proteids > amino- acids. 
 
 But the same enzymes are contained in the cells of the mucous 
 and other membranes, and as the amino-acids pass through 
 
 External layer 
 , (Serous coat) 
 
 y Artery 
 
 Jtfi/COUS 
 
 membrane 
 Capillary network 
 in submucous /ayer 
 
 Inferior 
 
 Canal 
 
 (ein 
 Muscular layers 
 
 FIG. 12. PART OF A TRANSVERSE SECTION THROUGH THE WALL OF THE 
 INTESTINE. VERY DIAGRAMMATIC. 
 
 they are again acted upon by the enzymes thus : 
 Amino-acids > proteids. 
 
 Thus the activity of the enzymes is reversible. Now it is no 
 use presenting all this to the reader as a finished picture of the 
 processes of digestion and absorption of the food substances, for 
 we have only the vaguest notion of what is the nature of the 
 reversibility; one thing is, however, certain the proteids are 
 split up into amino-acids in the cavity of the alimentary canal, 
 and as these substances are soluble they can pass through the 
 intestinal wall into the capillary blood-stream. In passing into 
 
THE SOURCES OF ENERGY 
 
 63 
 
 From , Fo re- Ifm b s 
 
 head and. neck 
 
 The Caval veins 
 
 iver 
 
 the tissues which utilise them, the same enzymes now reconvert 
 the amino-acids into proteid. But the proteid is now a different 
 one (or ones), for the amino-acids have been rearranged. 
 
 Somewhat similar rearrangements take place in respect of the 
 fats. 
 
 The blood circulating in the intestinal wall thus becomes 
 charged with proteid, fat, and carbohydrate, all three classes of 
 substances being now 
 thrown into forms which 
 are " native " to the 
 body of the animal in 
 which the digestive pro- 
 cess occurs. One eats the 
 proteids characteristic 70 
 of beef, mutton, pork, 
 fowl, fish, etc., but all 
 these become converted 
 somewhere into human 
 proteid, and all the 
 different fats that one 
 eats are similarly con- 
 verted into human fat. 
 That is, the processes 
 of digestion, absorption, 
 and assimilation are a 
 laborious and round- 
 about series of chemical 
 conversions which make 
 up a large portion of our 
 vital activity, and which 
 might be shortened with 
 much advantage. 
 
 Little by little the 
 capillaries in the walls 
 of the stomach and intestine unite together to form veins, and 
 the smaller veins unite further until one large vessel the portal 
 vein drains away all the blood circulating in the alimentary 
 canal from the stomach backward. This great vein carries the 
 intestinal blood, laden with nutritive substance, to the liver, and 
 there it divides up again into small and smaller veins, and these 
 finally break up into capillaries which ramify among the cells of 
 
 FIG. 
 
 . 
 'a.//menTa.ry Canal 
 
 From other 
 Viscera. 
 
 Prom hind limbs and- Trunk 
 
 13. DIAGRAM OF THE GREAT VEENS 
 IN THE BODY OP A MAMMAL. 
 
64 
 
 THE MECHANISM OF LIFE 
 
 the liver. Something is done to the blood there, and then the' 
 capillaries reunite into veins and then into another great vessel] 
 the hepatic vein. 
 
 At the same time the blood which has been circulating in the: 
 capillaries of the muscles of the hind-limbs and trunk, in thd 
 kidneys, reproductive organs, spleen in short, ia all the body ; 
 behind the upper limbs is gathered up into a vein called the^ 
 posterior vena cava, and this is joined by the hepatic vein. All 
 the blood that has been circulating in the fore-limbs, the chest, 
 head, and neck is collected by two veins called the anterior vena- 
 cavce. Finally, all three caval veins unite and pour their blood 
 into the heart. , 
 
 To muscles of- 
 
 From muse let / 
 of trunk 
 
 FIG. 14. DIAGRAM OF THE BLOODVESSELS OF A FISH. 
 i, ii, iii and iv are the gills. 
 
 Thus there is a vascular system, which consists of a series of 
 veins returning all blood that has been circulating anywhere in 
 the body to the heart. Clearly there are at least two kinds of 
 blood in the veins: (1) That which has been circulating in the 
 muscles, and which has, therefore, been depleted of some of its 
 nutritive properties; and (2) that which has been circulating in 
 the intestine, and which has become enriched with substances of 
 nutritive value. Now we have to consider the other half of the 
 blood vascular system the arterial vessels which distribute this 
 nutritive substance received from the alimentary canal (via the' 
 liver) to the body at large. 
 
 First of all we look at the arterial vessels of a fish (for here 
 the conditions are very simple). Much the same kind of venous 
 system is present in the fish as in man (or at least we need not 
 worry about the minor differences), but the arteries are not 
 complicated by the presence of lungs. 
 
 
THE SOURCES OF ENERGY 
 
 65 
 
 The fish respires by taking oxygen into the blood through the 
 gills, and also by giving out carbonic acid in the same way (we 
 return presently to this respiratory process, and only consider 
 just now the path of the blood). In the fish, then, the heart is 
 a fairly simple organ consisting of two main chambers, the 
 auricle and the ventricle. Each of these is a muscular, hollow 
 organ which expands and contracts rhythmically. The blood 
 which has returned from the body via the caval veins is poured 
 into the auricle, which then contracts. There are valves at the 
 entrance to the auricle and others at the opening of the latter 
 
 from the 
 
 Proper right. 
 
 To body 
 
 Lung 
 
 Proper left. 
 
 FIG. 15. THE CONNECTIONS BETWEEN THE HEART AND THE LUNGS IN 
 A WARM- BLOODED ANIMAL. 
 
 into the ventricle, and these structures so act as to prevent the 
 blood from being forced back into the caval veins, but they 
 allow it to flow into the ventricle. There are valves at the 
 opening of the ventricle into the aorta (that is, the great artery 
 that springs from it), and these act so as to allow the blood to 
 pass through the aorta when the ventricle contracts. Thus the 
 contractions and dilatations of the heart send the blood in one 
 direction only from the caval veins into the aorta. 
 
 From the latter vessel it goes to the gills, where the arteries 
 break up into networks of minute capillaries. Then the latter 
 vessels reunite until they form several large arteries, two of 
 
 6 
 
66 THE MECHANISM OF LIFE 
 
 which go to the head, several to the " shoulders " of the 
 fish, and one runs down the body just underneath the back- 
 bone and supplies the viscera and the muscles of the body 
 and tail. 
 
 Thus there is a fairly simple and complete circulation in th< 
 fish. The blood is propelled by the heart into and through th( 
 gills, and then it is distributed through the arteries to all parts 
 of the body. Having traversed every part of the latter, 11 
 returns to the heart again via the great veins. 
 
 The reader will now easily understand the main scheme o: 
 circulation in the warm-blooded animals, including man. Here 
 we have a double heart, one half of which (the right one) is 
 connected with the lungs, and the other (left) half with the resi 
 of the body. 
 
 Tracing out the paths taken by the blood-stream, we will see 
 that the caval veins discharge into the right auricle, which con- 
 tracts and sends the blood into the right ventricle. From there 
 it goes through the pulmonary arteries into the right and lef 
 lungs, and having traversed the capillaries in these organs, it is 
 returned to the left auricle by the pulmonary veins. The lef 
 auricle forces it into the left ventricle, and from there it is 
 propelled all over the body, being distributed by the great aorta 
 and the arteries that branch out from the latter. 
 
 The first complete demonstration of the circulation of the 
 blood must have appealed to physiologists as a perfect proof o 
 the mechanical conception of life. This proof, however, was 
 slow in coming, and many men contributed to it. Servetus, 
 scholar of the early sixteenth century, seems to have discoverec 
 the pulmonary circulation, and, curiously enough, he announce 
 it in 1553 in a theological work called De Restitutio Christianismi 
 for the publication of which he was hunted out from Spain b] 
 the Inquisition, only to encounter the intolerance of Calvin a 
 Geneva, where he was burned. The English physician Harve; 
 discovered the other, or systemic circulation, and gave a com 
 plete and formal demonstration of the whole scheme in 162 
 Fabricius had already discovered the valves in the veins, an< 
 in 1661 Malpighi found the connection between arteries ancl 
 veins the innumerable, minute, hairlike vessels called thej 
 capillaries and that completed the proof. But even before thej 
 latter discoveries the great mind of Descartes had made use oi 
 the incomplete demonstration of Harvey to establish his conJ 
 
THE SOURCES OF ENERGY 67 
 
 ception of an automatic, mechanical, animal body, and no such 
 momentous contribution to a working hypothesis of life had 
 before then, or has since, been made. 
 
 The Respiratory Interchange. Thus food materials are taken 
 into the alimentary canal, digested, and transferred to the blood- 
 stream in such a form that they can be assimilated by the tissues. 
 They are then distributed in the blood-stream to all parts of the 
 body. 
 
 But simultaneously there is an interchange of something 
 between the blood itself and the air that is inspired by the lungs. 
 Here we mention, for the first time, the arterial and venous 
 kinds of blood. That which issues from a cut vein, and in 
 general from a slight wound, is blood that is rather dark red in 
 colour, and which oozes out from the cut vessels, whereas that 
 
 Artery 
 
 X7ab///ary network 
 Air Vesicle 
 
 FIG. 16. AN ALVEOLUS, OR AIR SAC, FROM THE LUNGS, WITH ITS 
 BLOODVESSELS. HIGHLY MAGNIFIED. 
 
 which comes from an artery is brighter red in colour and it flows 
 ut in pulsations. Examining these bloods, physiologists have 
 ound that that flowing in the arteries contains more oxygen and 
 ess carbonic acid gas than that flowing in the veins. Examining 
 tie air that is inspired, they find that this contains the usual 
 1 per cent, of oxygen and about -^ per cent, of carbonic acid, 
 
 while the expired air contains only about 16 per cent, of oxygen 
 nd about 4 per cent, of carbonic acid. 
 
 Both the composition of the blood and that of the air are 
 hanged in the lungs. Now look at the structure of the latter: 
 ach consists essentially of an immense number of air vesicles 
 urrounded by capillary bloodvessels. 
 
 This is a scheme of the essential structure of the lung. The 
 rachea divides to form the two bronchi, and the latter divide 
 nd subdivide again and again until they terminate in multitudes 
 
 of sacs, or vesicles, one of which the greatly simplified figure 
 
68 
 
 THE MECHANISM OF LIFE 
 
 represents. The pulmonary artery (carrying venous blood) enters 
 the lung, and also divides and subdivides again and again until, 
 in the end, each ultimate twig of the artery breaks up into a 
 network of capillaries on the outer wall of an air sac. 
 
 Air is drawn up into the lungs and expelled out from them by 
 the upward and downward movements of the diaphragm (the 
 muscular partition between the chest and abdominal cavities) 
 in men, or by upward and downward movements of the ribs and 
 chest wall in women. Thus the sacs are full of air which is 
 continually being renewed (we do not really draw air right down 
 to the " bottom " of the lungs, but into the upper parts only). 
 The air of the smallest sacs renews itself, however, by diffusion 
 into the fresh air that is continually entering the larger passages. 
 Now the venous blood that enters the air-sac capillaries contains 
 (in solution) less oxygen and more carbonic acid than that which 
 
 ^ 
 
 FIG. 1 7. BLOOD-CORPUSCLES. 
 
 1, White; 2, human; 3, human in " rouleaux "; 4, human, seen from 
 the side; 5, from a frog; 6. from a fish. 
 
 is contained in the air of the sacs, and therefore an interchange 
 occurs such that oxygen goes through the very delicate membrane 
 of the sac and the equally thin wall of the capillaries from the air 
 into the blood, while carbonic acid gas passes from the blood into 
 the air. 
 
 In respiration, then, we take oxygen from the outer air anc 
 transfer it to the blood-stream, and we take carbonic acid froi 
 the blood-stream and transfer it to the air. And so the venoi 
 blood, which goes to the lungs and comes from the body, contains 
 more carbonic acid and less oxygen than the arterial blood, 
 which comes from the lungs and goes to the body. 
 
 Just a word as to how the oxygen and carbonic acid are carried 
 by the blood. The latter, when seen under the microscope, 
 becomes a clear, colourless liquid in which there are enormous 
 numbers of minute, biscuit-shaped bodies called the red blood- 
 corpuscles. 
 
THE SOURCES OF ENERGY 69 
 
 These contain a chemical substance called haemoglobin, 
 which carries the oxygen, while the carbonic acid is carried by 
 the clear liquid part of the blood, or plasma. In all animals 
 there are other corpuscles which are colourless, and some of these 
 are called phagocytes (devouring cells) because they have the 
 power of ingesting foreign substances, such as bacteria, which 
 may enter the blood. The pus which forms when an abscess 
 gathers consists largely of phagocytes. They are a protection 
 against infection. 
 
 We have now seen how the nutritive materials the proteids, 
 fats, and carbohydrates and the oxygen which is to " burn " 
 them, are obtained, and how they are carried to the tissues. 
 
 The Sources of Energy. These proteids, fats, and carbo- 
 hydrates, digested, dissolved, split up, and recombined, are the 
 immediate sources of energy (the ultimate source is, of course, 
 the solar radiation, which enables the green plants to synthesise 
 water, carbonic acid, and nitrate into proteid, fat, and carbo- 
 hydrate). They contain potential, chemical energy which is at 
 a high intensity, and is available for doing work. 
 
 How ? In the inanimate engine there is a working substance, 
 the steam, and we regard this as something different from the 
 engine, which may exist even if there is no steam in it. But in 
 the animate engine the mechanism (muscle and nerve) are the 
 same as the working substance (the proteids, fats, and carbo- 
 hydrates, which are the muscle and nerve). It may be the case 
 that a " living " muscle contains " non-living " substances which 
 are oxidised and yield energy, but since we only define the " life " 
 of the muscle by its irritability (that is, its ability to contract- 
 when it is stimulated), we cannot be sure that there are parts of 
 it w^hich are not alive. It is probable that the substances 
 carried to the muscle by the blood-stream are actually built up 
 into the living tissue they become alive and are then oxidised, 
 or die. 
 
 Further, the inanimate engine steam, gas, or petrol is a 
 thermodynamic machine. It contains a working substance 
 steam at a temperature, say, 120 C., or a mixture of gases 
 resulting from the explosion of gas and air, or petrol vapour and 
 air at a temperature of, say, 700 C. to over 1,000 C. At these 
 high temperatures the molecules of the gases are moving with 
 enormous velocities, and so they exert pressure. Therefore a 
 heat engine is a mechanism which converts the kinetic energy of 
 
70 THE MECHANISM OF LIFE 
 
 a gas at a relatively high temperature into the kinetic energy of 
 a material structure (pistons or turbines) at a relatively low 
 temperature. 
 
 It may be that the muscle of an animal is a heat mechanism of 
 this kind a thermodynamic machine but it is not likely, and 
 it is far more probable that the muscle contracts because the 
 chemical energy of the proteids, fats, and carbohydrates is trans- 
 formed into mechanical work without passing through the stage 
 of heat. There are physical mechanisms that do this. A 
 galvanic battery may produce a current of electricity, and the 
 latter may work an electric motor and no heat at all (or only a 
 trace of it) may be generated. Here chemical energy (that of the 
 zinc and acid in the battery) transforms into electricity, and the 
 latter does mechanical work in the motor. Now all that we can 
 state positively about the events that happen when a muscle 
 contracts are these: there are complex, chemical compounds in 
 the protoplasm of the muscle which break down (or " explode "). 
 Very complex molecules thus dissociate into relatively simple 
 ones, and the energy that held these complex molecules together 
 is liberated and is transformed into the mechanical work done 
 by the muscle when it contracts against a resistance. 
 
 But it is quite certain that heat is also generated that every- 
 one knows who becomes warm when he does hard muscular work. 
 Now, is this generation of heat a necessary step in the doing of 
 bodily work ? In cold-blooded, wholly aquatic animals it is not 
 certain that heat is produced, or, if it is, it is only in very small 
 quantity. A warm-blooded animal preserves a constant tem- 
 perature, which is usually considerably higher than that of its 
 environment, and so it must generate heat. And it is probable 
 that this heat is produced by the oxidation of the relatively ! 
 simple molecules into which the complex protoplasmic substance 
 breaks down. These products of dissociation are oxidised into 
 water and carbonic acid. 
 
 Protein- containing tissues break down into the same products, 
 and also into simple nitrogenous substances that later on become 
 urea in the liver and kidneys that is, into such substances as 
 can easily be removed from the muscle in solution in the circu- 
 lating blood. 
 
 The Removal of Waste Glandular Activity.- In the heat 
 engine the working substance steam in the steam engine, or the 
 products of the explosion in the internal combustion motor 
 
THE SOURCES OF ENERGY 
 
 71 
 
 loses its available energy and must be removed. The steam is 
 passed out from the condenser, and is returned to the boiler to be 
 reheated, and the cylinder gases in the gas engine are blown out 
 into the atmosphere. So also in the animate engine the working 
 substance loses its available energy: protoplasmic substances 
 disintegrate into simpler ones, which are then oxidised to form 
 carbonic acid and water. These latter compounds, with the 
 urea, which is the end product of the disintegration of the nitro- 
 genous part of the protoplasmic substance, must be removed 
 from the animal body. 
 
 So we must say a word or two about excretion. The water 
 which appears in the muscles as the result of the disintegration 
 
 ~Glandu/a.r 
 pa.rtoF kidney 
 
 ,- Cavity 
 
 FIG. 18. THE KIDNEY OF A MAMMAL. 
 
 A, The organ with its bloodvessels and duct; B, the kidney cut 
 through longitudinally. 
 
 of the protoplasm, or of the oxidation of the products of dis- 
 integration of the latter, simply oozes through into the blood- 
 stream, and the carbonic acid similarly produced is also taken 
 up by the blood. The effect of work done by the body is there- 
 fore to add water and carbonic acid to the blood, and so the latter 
 becomes changed, " impure," or venous. As it streams through 
 the capillaries in the lungs the carbonic acid is excreted, passing 
 out into the expired air, while, at the same time, some of its 
 excess of water also passes out in the same way. 
 
 But most of the water, and also the urea, is excreted by the 
 kidneys. Consider first of all the urea. The proteid substance 
 of the muscles is, as we have seen, continually disintegrating into 
 simpler substances, giving up energy as it thus breaks down. 
 These products of metabolism are not yet urea, but they are 
 
72 
 
 THE MECHANISM OF LIFE 
 
 carried to the liver, where they are converted into this latter 
 substance. The blood circulating in the body therefore contains 
 a certain small proportion of urea. This is the waste product 
 of nitrogenous metabolism ; it has no useful function, and it must 
 be removed from the system. 
 
 We consider the kidneys in some detail, because they may be 
 taken as types of glands, structures which we have not, so far, 
 described. 
 
 Each of them, then, is an organ provided with (1) an aiteiy 
 carrying blood into it from the aorta ; (2) a vein carrying blood 
 
 Surface oF fridn* v 
 
 Cavity of 
 the kidney 
 
 FIG. 19. ONE OF THE SECRETORY UNITS (OR 
 URINIFEROUS TUBULES) OF THE KIDNEY 
 REPRESENTED IN A VERY DIAGRAMMATIC 
 WAY. MAGNIFIED. 
 
 away from it into the 
 posterior vena cava ; 
 
 (3) a duct, the ureter, 
 which carries away the 
 water and other sub- 
 stances taken from 
 the blood that flows 
 into the gland ; and 
 
 (4) nerves. When in 
 action, blood contain- 
 ing urea and other 
 waste substances, as 
 well as an excess of 
 water, is continually 
 flowing into the kid- 
 neys through the renal 
 arteries, while the same 
 blood deprived of these 
 waste substances and 
 of a certain quantity 
 of water is continually 
 flowing out through 
 
 the renal veins. Water containing the urea, etc., gathers in 
 the cavities of the kidneys and slowly trickles down the 
 ureters into the urinary bladder, from which it is periodically 
 expelled. The nerves that enter the kidney go to the arteries, 
 and they act by exciting the muscles in the walls of these 
 vessels to expand or contract, thus altering their calibre, 
 and so increasing or diminishing the quantity of circulating 
 blood. If the calibre of the renal veins remains the same, the 
 blood-pressure of the kidneys increases, and so the secretion of 
 
THE SOURCES OF ENERGY 
 
 73 
 
 urine increases, and vice versa. Thus the secretory activity of 
 the kidneys can be regulated. Similar regulatory mechanisms 
 exist everywhere over the animal body, and the nerves mainly 
 responsible belong to what is called the sympathetic nervous 
 system. 
 
 Now look at the minute structure of the kidney. Here we 
 represent, very simply, and on a much bigger scale than the 
 natural one, one of the tubules that excrete the water, urea, etc. 
 First of all the blood in the renal artery becomes distributed 
 through a multitude of branches or twigs of this bloodvessel, and 
 we may regard each twig as going to a peculiar structure called 
 a glomerulus. 
 
 Capillary 
 knot x 
 
 Outer (Jill 
 of glomerulus 
 
 FIG. 20. THE GLOMERULUS OF A URTNIFEROUS TUBULE, WITH ITS 
 BLOODVESSELS. HIGHLY MAGNIFIED. 
 
 This glomerulus is like a bulb at the end of a urinif erous tubule. 
 )ne part of the bulb is pushed in, and a twig of the renal artery 
 enters and breaks up into a little complex knot of capillaries, 
 from which a twig of the renal vein gathers up the blood and 
 takes it away. This little twig of the renal vein again breaks up 
 into capillaries, which surround the rest of the urinif erous tubule, 
 as shown in Fig. 19. Thus we see a rich blood-supply carried 
 by capillaries surrounding the parts of the gland that secrete. 
 AVater is secreted from the blood in the capillaries of the glomeru- 
 lus, and this water oozes through the inner wall of the bulb, 
 and so goes into the uriniferous tubule. Then the cells of the 
 latter take urea, etc., from the blood in the capillaries that sur- 
 
74 THE MECHANISM OF LIFE 
 
 round them, and these waste substances go with the water into 
 the cavity of the kidney, and so down the ureter into the bladder. 
 
 Note that the essential things in this mechanism are the cells 
 of the walls of the glomerulus and tubules. It is these cells that 
 have the power of taking water, urea, hippuric and uric acids, 
 etc., from the blood; but they do not take sugar, albumen, or 
 other things. If they do (as in diseased kidneys), that is not their 
 proper function, and the organ assumes an individuality that is 
 harmful to the organism a disharmony is established. How 
 exactly the cells of the normal kidney function in removing 
 waste substances and nothing else we do not know. 
 
 This is a type of glandular activity, and there are numerous 
 other organs in the body that do analogous things. The gastric 
 glands secrete hydrochloric acid and pepsin into the stomach; 
 the pancreas secretes a mixture of ferments or enzymes called 
 trypsin; the glands of the mouth secrete saliva; those of the 
 skin secrete sweat, water, oil, etc. (all waste substances), and so 
 on. In all these cases there are tubular mechanisms (as a rule 
 much simpler than those of the kidney), and these are provided! 
 with nerves and arteries and veins in much the same way. There! 
 are also ducts, tubes which carry the products secreted by thd 
 glands to the places where they are wanted, or via which they are] 
 eliminated from the body. 
 
 But there are ductless glands also. Such are the thyroid, the 
 thymus, the pituitary, and pineal glands (see p. 99), and 
 the adrenal glands. In such cases we have glandular cells 
 surrounded by capillaries, but there is no duct and there is! 
 apparently no secretion. We know, however, that there is a 
 secretion, and that this goes into the blood itself, and is then 
 carried to the rest of the body. These ductless glands are of! 
 enormous importance in many ways. 
 
 The reader will miss the true understanding of vital activity] 
 if he does not note the character of unification of activity that 
 is implicit in our conception of the animal mechanism. This 
 chapter and the last one deal with mechanisms: the sensori-motol 
 system that is, sensory organ connected with motor organ; the 
 mechanism of alimentary canal and glands that converts the] 
 crude food substances into the specific proteids, fats, and carbo- 
 hydrates that are required by the animal tissues as sources of 
 energy and means of growth; the apparatus of circulation that^ 
 distributes these substances; the respiratory mechanism that 
 
THE SOURCES OF ENERGY 75 
 
 obtains the oxygen that must be distributed along with the food- 
 stuffs; and the excretory organs that remove the degraded food 
 substances and expel them from the body. It has been necessary 
 to consider these things separately, but that is only our method 
 of analysis of a single phenomenon, and it must be repeated that 
 all these activities are co-ordinated and are really one. Nothing 
 in the way of mechanisms that we can make or devise exhibits 
 the unification of activities expressed in the life of a normal 
 animal. The modern State which has, rather foolishly, been 
 called an organism is said to represent the greatest achievement 
 of men, but no one can attempt to analyse its activities dis- 
 passionately, as we have analysed those of the living animal, and 
 see in them anything other than a tissue of disharmonies. The 
 scientifically organised State has still to come into existence. 
 Think of the distribution of foodstuff to the functioning animal 
 body. In severe manual work the bloodvessels of the muscles 
 dilate so as to permit of a more abundant supply of nutritive 
 material and oxygen, and a rapid removal of waste products. In 
 intensive mental work the blood-supply to the brain is similarly 
 speeded up. In digestion the same thing occurs with relation to 
 the intestinal circulation. Such regulations are as yet hardly at 
 all evident in the activities of the modern State. 
 
CHAPTER V 
 ON VITAL PRODUCTION 
 
 WE have now traced the working substance of life through the 
 transformations which it undergoes in the animal body. These 
 are as follows: 
 
 Animal Metabolism. (1) Certain food substances contain the 
 working substance in the form of a mixture of proteids, fats, and 
 carbohydrates, derived from the tissues of animals and plants. 
 This mixture is taken into the alimentary canal, where it is 
 digested and dissolved. As the result of the processes of diges- 
 tion and absorption the proteids, fats, and carbohydrates of the 
 food are made similar, or assimilated, to those substances such 
 as they occur in the body of the animal that eats them. 
 
 (2) They are then distributed by the blood-stream and incor- 
 porated in the protoplasm of the muscles and other tissues. 
 
 (3) These protoplasmic substances are disintegrated into 
 simpler chemical compounds, with the result that energy is 
 liberated. 
 
 (4) The products of the disintegration of the substances of the 
 tissues are finally oxidised and excreted. 
 
 The result of these metabolic processes is that the working 
 substance becomes degraded energetically and chemically. Let 
 us take the energy transformations first. An ordinary day's diet 
 is likely to contain about 125 grams of dry proteid, 125 grams 
 of dry fat, and 400 grams of dry carbohydrate,* and if the 
 heat value of these quantities of dry foodstuff be found it will 
 add up to about 3,500 calories. Now the result of the chemical 
 transformation undergone by this average quantity of food is 
 that a certain quantity of carbonic acid and water is excreted 
 by the lungs and kidneys, and about 42 grams of urea by the 
 kidneys. The water and carbonic acid contain no free (or 
 available) energy, but the urea contains about 105 calories. 
 Therefore about 3,390 calories of the energy contained in the 
 food is utilised by the body. About 12 to 30 per cent, of this 
 
 * In British weights about 4 ounces of dry proteid, 4 ounces of dry 
 fat, and 14 ounces of dry carbohydrate. 
 
 76 
 
ON VITAL PRODUCTION 77 
 
 energy is transformed into mechanical work done by the muscles 
 of the body and limbs, and the rest is transformed into heat. 
 That is the energy transformation. 
 
 Assume an adult animal in perfect health and of stationary 
 weight; then the working substance enters the tissues in the 
 form of proteid, fat, and carbohydrate, and leaves them in the 
 form of water, carbonic acid, and urea. That is the chemical 
 transformation. Looked at both from the point of view of 
 available energy and that of chemical structure there is degrada- 
 tion. First, in that a high intensity of chemical energy becomes 
 a low intensity; and, second, inasmuch as complex chemical 
 compounds become simple ones. 
 
 The working substance of life is now water, carbonic acid, and 
 urea. Let us trace its further history. The water and carbonic 
 acid undergo no further change (just yet, at all events). But the 
 urea (and the other proteid degradation products) do. 
 
 The Fate of the Proteid Residues. The urea (and other 
 nitrogenous substances) contained in the urine find their way, 
 through drains and by other means, into the water of rivers and 
 of the sea, or on to the land, and then it is at once attacked by 
 micro-organisms. Now a few words about the latter. 
 
 Micro-organisms are (in the present connection) bacteria, 
 moulds, yeasts, and infusoria. The bacteria are exceedingly 
 minute, single-celled organisms which cannot be said to be either 
 animals or plants, since their modes of generation are altogether 
 special. Many of them (but still a small minority) are called 
 pathogenic organisms, and are the causes of certain infectious 
 and epidemic diseases, such as cholera, enteric fever, pneumonia, 
 " influenza," diphtheria, catarrh, septicaemia, etc. Being exces- 
 sively minute, capable of living in water, and even of being 
 partially dried, they may be distributed in liquids, in dust, on 
 infected clothing, etc. When they enter a suitable " soil " that 
 is, a liquid containing certain organic substances in solution 
 they multiply at an incredibly high rate. The human body 
 possesses certain defences against these pathogenic bacteria 
 that is, the phagocytes of the blood can ingest and destroy them, 
 or the plasma can form certain substances which can neutralise 
 their poisonous activity. If the infected animal fails to set up 
 such adequate defences, the invading pathogenic bacteria multiply 
 with great rapidity and form toxins which are injurious tojme 
 or more tissues, and so instigate a condition of disease. 
 
78 THE MECHANISM OF LIFE 
 
 Other micro-organisms have, on the whole, beneficial effects.! 
 Yeasts are the causes of the fermentations in brewing and analo- 
 gous processes. Moulds and bacteria cause the ripening of 
 cheese. Fermentative and putrefactive bacteria act upon and 
 break down organic matter, and moulds and infusoria behave in \ 
 similar ways. 
 
 Many substances that we meet with everyday "go bad": 
 milk curdles; broths, soups, etc., "go sour"; and meat of allj 
 kinds becomes tainted and then putrefies. Beers and wines may 
 sour. In the case of meat and fish, the " ripening " (as in thej 
 case of game) and the tainting are the result of the activity of I 
 bacteria and of enzymes that are naturally present in the flesh; 
 itself. When the flesh is alive, the activity of these enzymes are] 
 inhibited in some way, but after general death of the tissues they ; 
 begin to digest the flesh in which they occur. This process is| 
 called autolysis, or self-digestion. At first it produces savoury,! 
 and then noxious substances. 
 
 By-and-by the meat putrefies, with the production of offensive 
 odours, and this is the result of the action of bacteria which) 
 infect the decaying substance. If this activity is allowed to go 
 on unchecked, the offensively smelling meat begins to disappear, I 
 and by-and-by the smell itself passes away. 
 
 Any organic substances exposed to the air or contaminated 
 with dust, " dirt," or impure water, will " go bad," ferment, ori 
 putrefy, and this is because fermentative and putrefactive: 
 bacteria gain admittance, multiply, and break down the organic 
 matter, using the latter as a source of food. If rigorous and 
 successful means be taken to exclude all micro-organisms from 
 the organic matter, the latter will not go bad. Thus if milk be 
 " pasteurised " (that is, heated for several days in succession in 
 closed bottles to a temperature of about 80 to 90 C.), it will 
 remain fresh and sweet for an indefinite period. If meat be 
 sterilised by heating to over the boiling temperature of water in 
 sealed tins for a sufficient period, it will be preserved in good 
 condition for years. If meat be frozen to well below zero centi- 
 grade it will remain good. If salt, boracic acid, formaldehyde, 
 chlorinated water, etc., be added to the organic substance, 
 putrefaction will not occur. Heating to a sufficiently high 
 temperature kills all micro-organisms, as also does antiseptic 
 substances like salt and boracic acid. Freezing arrests the 
 multiplication and vital activities of bacteria, but does not kill 
 
ON VITAL PRODUCTION 79 
 
 them, so that frozen meat will putrefy if it thaws. This is the 
 rationale of all processes of preservation of meat and other food 
 substances: heating destroys the "germs"; salting and drying 
 and freezing arrest their activities; and rigorous exclusion of air, 
 dust, and other media which contain the organisms may delay 
 or prevent the putrefaction. Filtration of water through porous 
 earthenware keeps back the germs, and exposure to strong sun- 
 light, or, better still, to the rays from an electric mercury vapour 
 lamp, is said to destroy them. 
 
 The Mode of Action of Micro-Organisms. In fermentation and 
 putrefaction much the same chemical processes occur as when 
 food substances are digested in the alimentary canal of an animal. 
 In fact, the micro-organisms form enzymes similar in effect to 
 the enzymes that are found in the stomach and intestine. The 
 enzymes split up proteids into amino-acids and ferment carbo- 
 hydrates, but the processes of disintegration of proteids and 
 carbohydrates go much further in bacterial action than in the 
 digestive operations. These processes are very complicated, and 
 are far from being fully understood. We call the breaking-down 
 of fats and carbohydrates by bacteria, yeasts, and moulds 
 fermentation, and that of proteids putrefaction. Many species 
 of bacteria, etc., are concerned in each process, and the rapidity 
 with which the latter occurs depends on the temperature and 
 other conditions. 
 
 The final results are quite clear and well known. In all cases, 
 and if there is time enough, fats, carbohydrates, and cellulose, 
 such as the vegetable substances in grains, grass, leaves, fruits, 
 woody fibre, etc., are decomposed, with the result that their 
 chemical substance transforms into carbonic acid and water. 
 Putrefaction of proteid substance is accompanied or succeeded 
 by what is called nitrification that is, other organisms, called 
 " nitrifying bacteria," also play their part. In the end the 
 proteid putrefies to form ammonia compounds, and then the 
 nitrifying bacteria oxidise the ammonia, converting this into 
 nitrous acid and the nitrous into nitric acid. The lime, soda, and 
 potash in the soil or in streams, rivers, etc., combine with the 
 nitric acid to form nitrate, and this is the way in which Chili 
 saltpetre and other natural stores of nitrate have been formed. 
 
 Thus the action of micro-organisms on all dead organic matter 
 is to convert the latter into carbonic acid, water, and nitrate. 
 This is, in general, the fate of the excretions of animals and of all 
 
80 THE MECHANISM OF LIFE 
 
 dead animal and plant bodies, " offal," or remains. All organic 
 substances whatever are susceptible to bacterial action, and 
 since micro-organisms are universally distributed in nature, ini 
 the air, soil, and in fresh and salt water, organic matter thus* 
 suffers resolution into innocuous mineral substances of very. 
 simple chemical constitution. There are certain conditions im 
 which this bacterial decomposition of organic matter is delayed. 
 At the bottoms of deep oceans the temperature of the sea is very* 1 
 little above freezing-point (or is even below the freezing-point of! 
 fresh- water), and there putrefactive action goes on very slowly., 
 In extreme northern climates the air temperature is also very 
 low, and dead bodies of animals are sometimes frozen in ice on 
 in the soil, and so escape decay. In very dry climates organic: 
 matter also remains in a relatively stable condition, since the 1 
 putrefactive bacteria are unable to function in the absence of! 
 water. Such is the mode of origin of many forms of guano. 
 
 But, in general, all organic matter is ultimately resolved into 
 carbonic acid, water, and nitrate, and these substances tend to be 
 distributed all over the earth. Carbonic acid is contained to the 
 extent of about 0-04 per cent, in the atmosphere, and it is present 
 in rather larger proportions in fresh and salt water. Water itself 
 is distributed everywhere except over desert land areas. Mineral 
 nitrogenous substances, such as nitrites, nitrates, and salts of 
 ammonia, are also contained everywhere in the soil and in fresh 
 and salt water, and oxides of nitrogen, capable of forming nitric 
 acid, are formed in the atmosphere by the combination of 
 nitrogen and oxygen that occurs whenever there are lightning 
 discharges. But mineral nitrogenous compounds which are 
 the indispensable materials of life occur everywhere in exceed- 
 ingly small quantities,* and the abundance of life depends on the 
 quantity of these substances that is available. 
 
 It is therefore convenient roughly to classify all living tilings 
 into animals, bacteria, and plants. The animals consume 
 proteids, fats, and carbohydrates, oxidising these substances 
 into water, carbonic acid, and certain nitrogenous residues such 
 as urea, uric acid, hippuric acid, etc. The bacteria act upon the 
 nitrogenous residues, converting them into nitrate, and they also 
 act upon dead animal and plant tissues, and upon the excreta 
 
 * Except under quite special conditions, as when deposits of " Chili 
 saltpetre " and analogous substances are formed. Here the dry atmo- 
 sphere and soil inhibit plant growth, with the result that nitrate 
 accumulates. 
 
ON VITAL PRODUCTION 81 
 
 of animals, resolving those substances into water, carbonic acid, 
 and nitrate. The plants then utilise the latter substances as 
 crude foodstuffs. 
 
 Plant Metabolism. We have now to consider the further 
 history of the working, substance of life after it has undergone 
 the chemical and energetic degradations that are the results of 
 animal and bacterial metabolism. Returning to our inanimate 
 engine, it may be recalled that the working substance, or steam, 
 expands and does mechanical work on the pistons, and actuates 
 the mechanism. Then it passes through the condenser, having 
 lost its available energy. It is returned to the boiler and is 
 heated, and so takes up fresh available energy, and the cycle of 
 operations recommences. We take the same general view of the 
 animate engine. The working substance, which is a mixture of 
 fats, proteids, and carbohydrates, passes through the animal body, 
 undergoing chemical transformations, doing mechanical work, 
 and (it may be) heating the body. Then it passes out from the 
 body as the excretions, having lost most of its available energy, 
 and it is further acted upon by bacteria, when it loses the re- 
 mainder. It must now be transformed so as to reacquire avail- 
 able energy, just as the cold water entering the steam boiler again 
 takes up energy in the form of heat. 
 
 This absorption of energy by the life working substance is 
 effected in the tissues of green plants, and we must now refer 
 briefly to the metabolism of the latter. 
 
 A green plant is an organism that is, something that trans- 
 forms energy of itself, grows, and reproduces its individual form 
 and mode of behaviour. Its growth is an obvious thing, and so 
 it is clear that it absorbs material from the medium in which it 
 lives. Now, with rare exceptions* the plant organism does not 
 take in visible foodstuff that is, we can see no obvious process 
 of feeding, mastication, digestion, etc. We know that it can 
 only grow under certain conditions a plentiful supply of water 
 to its roots and air to its leaves and it must also have free 
 access to sunlight. In the failure of such conditions the plant 
 does not grow. 
 
 Its foodstuff is necessarily contained in the air surrounding its 
 green leaves and in the water of the soil that contains its roots. 
 Examining the air contained in an enclosed space, we find that 
 
 * Insectivorous plants. 
 
82 THE MECHANISM OF LIFE 
 
 this consists of about 79 per cent, of nitrogen, 21 per cent, of 
 oxygen, and a trace of carbonic acid. If we next examine the 
 air of an isolated space which has been used by plants, we shall \ 
 find that its percentage of carbonic acid has been greatly dimin- 
 ished, while that of oxygen has increased. Evidently the 
 effect of plant life is to rob the atmosphere of its carbonic acid 
 and to enrich it with oxygen just the opposite to that of animal -, 
 life. If, further, we proceed to examine the water w T hich is taken 
 up by the roots, we shall find that this is not pure, but always con-1 
 tains mineral substances like nitrates, chlorides of potash and| 
 magnesium, salts of iron and lime, phosphates, silica, sulphates,] 
 etc., and it can be proved that the effect of plant metabolism is 
 to take such substances from the soil water. 
 
 The materials from which the green plant builds up its tissues 
 are therefore water, carbonic acid, nitrates, and other simple 
 mineral substances. These materials it converts into proteids, 
 oils, and waxes, and carbohydrates such as starch, sugar, and; 
 cellulose. Now all of the latter substances contain much poten- 
 tial energy, for, if we dry them, we can burn them and so obtain 
 heat. But the water, carbonic acid, and mineral salts cannot be 
 oxidised further, and they contain no available energy. In 
 building up its tissues the green plant must therefore obtain 
 energy from somewhere, and it can be proved that it is obtained 
 from light ; if the plant is kept in the dark, there can be no growth.* 
 But it can grow in the light radiated from an electric arc It is, 
 in fact, fairly easy to show that a green leaf exposed to sunlight 
 is continually forming starch, while, if it is kept in the dark, no 
 such thing happens. 
 
 Here, then, we have the source of energy of the green leaf. 
 Outside the latter is water (OH 2 ), carbonic acid (C0 2 ), and solar] 
 radiation; and inside it is the mixture of pigments called 
 chlorophyll. The solar radiation is not heat, although it isj 
 transformed into heat when it impinges upon most material] 
 objects. When it falls upon certain substances, such as luminous 
 paints, it is transformed into light, and in certain conditions it 
 can be transformed directly into mechanical work. When it- 
 impinges on the green leaf and is absorbed by the chlorophyll 
 pigments, it is immediately transformed into chemical energy;] 
 for it can be proved that there is no starch in a leaf which is kept 1 
 
 * No increase in mass, although a seed a potato, for instance may 
 germinate in the dark. 
 
ON VITAL PRODUCTION 83 
 
 in the dark for'some time, while within a few minutes of its 
 exposure to sunlight starch accumulates in the cells. 
 
 It is difficult to convince the non-chemical reader what a very 
 extraordinary thing this process of photo-synthesis of starch by 
 the green plants must be. Let him note that on the one hand 
 there is water and carbonic acid, and on the other there is dex- 
 trose and finally starch. The chemical equation is (probably) : 
 
 6C0 2 + 6H 2 = C 6 H 12 6 -f 60 2 . 
 
 Carbonic acid Water Dextrose Oxygen 
 
 Then the dextrose is converted into starch (C 6 H 10 5 )n, and the 
 latter gathers in the cells of the leaf till it is required by the 
 plant, when it passes into solution as dextrose, and is removed 
 from the leaf by the circulating sap juices. This is the process 
 of photo-synthesis. Look, however, at the first equation and 
 read it from right to left. There is dextrose and oxygen. 
 Dextrose is a highly combustible substance, and when it burns 
 it combines with oxygen to form C0 2 and OH 2 , with the libera- 
 tion of a large quantity of energy in the form of heat. What 
 actually occurs in the plant, however, is represented by reading 
 the equation from left to right, and this shows that when the 
 C0 2 and OH 2 are synthesised in the plant to form dextrose, the 
 same quantity of energy must be absorbed as is liberated when 
 dextrose burns in oxygen to form C0 2 and OH 2 . 
 
 The former kind of reaction an endothermic one can be 
 made to occur. A chemist can decompose C0 2 into C and 0, as, 
 for instance, when a burning piece of magnesium ribbon is placed 
 in a jar containing C0 2 . But the C0 2 is only decomposed into 
 carbon and oxygen if a large quantity of energy is supplied in 
 the form of the heat liberated by the burning magnesium, and 
 so the decomposition only occurs at a relatively high tempera- 
 ture. The following statement is very important in all such 
 discussions as these : reactions like that in which C0 2 is decom- 
 posed into C and 2 only occur when a compensatory energy 
 transformation is brought about that is, when at least as much 
 energy is supplied from outside as would be yielded if the reaction 
 went in the opposite direction. Such reactions an endothermic \ 
 process coupled with a compensatory one do not occur of \ 
 themselves. When they do occur, the coupling is the consequence V 
 of some outside agency. 
 
 In the green plant, however, it is precisely this coupled reaction 
 
84 THE MECHANISM OF LIFE 
 
 that occurs. C0 2 and H 2 are combined together, and the energy 
 necessary to bring about the combination is taken from the 
 sunlight. A chemist could cause the combination to take place 
 at a very high temperature and in quite special conditions, but it 
 occurs in the green plant of itself, and at ordinary temperatures. 
 All the above is very important, and may not be neglected 
 when we are considering the nature of the living process. We 
 return to the matter in a later chapter. 
 
 The Balance of Life. We see now in what way the energy 
 degraded by animal life becomes rebuilt up again. The animal 
 takes proteids, fats, and carbohydrates into its body, disin- 
 tegrates and oxidises these compounds, makes use of their con-' 
 tained energy to obtain heat and do mechanical work, and then 
 excretes the products of disintegration and oxidation in the 
 form of water, carbonic acid, and urea. 
 
 The bacteria convert the urea (and other nitrogenous residues) 
 into nitrate. 
 
 The green plants take energy from solar radiation, and rebuild 
 water, carbonic acid, and nitrate into proteid, fat, and carbo- 
 hydrate, when the cycle of operations recommences. 
 
 Now it is easy to see that all animal life depends upon plant 
 life. Animals are carnivorous (feeding upon flesh), or her- 
 bivorous (feeding upon vegetable substances), or omnivorous 
 (eating both flesh and vegetable substance). There are also 
 some animals which are called saprozoic, with regard to their 
 manner of nutrition, and these can utilise as food liquids or 
 debris containing broken-down organic matter. Here we need 
 not consider the saprozoic organisms, and it is enough for our 
 present purpose to regard animals as either carnivorous or 
 herbivorous, or both. Since the carnivores eat other carnivores 
 or herbivores, or both, and since the herbivores eat vegetable 
 tissues, it is easy to see that all animal life on the earth ultimately 
 depends upon vegetable life. 
 
 On the other hand, vegetable life depends for its continuance 
 on a supply of water, carbonic acid, and nitrate. The quantity of i 
 water on the earth is (for our purpose) unlimited, but not so the ; 
 quantity of carbonic acid and nitrate, and upon the supplies of the 
 latter substances the abundance of vegetable life hangs. What, 
 then, are the sources of carbonic acid and nitrate ? Home 
 quantity of each substance comes from the earth in the exhala- 
 tions from volcanoes, and possibly as the results of the disintegra- 
 tion of certain mineral substances. There is probably far more 
 
ON VITAL PRODUCTION 85 
 
 CO., in the atmosphere and dissolved in fresh and sea water 
 than the plants are able to utilise that is to say, there is a 
 surplus of both water and carbonic acid on the earth. 
 
 But this is not the case with nitrates and other inorganic 
 nitrogen compounds, which are quite indispensable for the 
 nutrition of plant life. There is an enormous quantity of ele- 
 mentary nitrogen in the atmosphere, but this is unavailable, for, 
 in order to utilise it, plants must have it in combination with 
 oxygen (as nitrous and nitric acids), or with hydrogen (as 
 ammonia). Some nitrogen is always being combined with 
 oxygen as the result of electric discharges in the atmosphere, 
 and even under the influence of solar radiation ; but, on the other 
 hand, some nitrate, nitrite, and ammonia are always being 
 decomposed into elementary nitrogen by certain bacteria present 
 in water and soil. Therefore the total amount of nitrogen in the 
 forms of nitrate, nitrite, and ammonia, and thus available for 
 the nutrition of plants, is nearly constant, and certainly does not 
 change appreciably during very long periods of time. That 
 means that the total average quantity of vegetable life on the 
 earth is practically the same from year to year. 
 
 And that being so, the total average quantity of animal life 
 on the earth is also practically the same from year to year over 
 a great lapse of time, for all animal life depends for its continu- 
 ance on vegetable life. There is a certain balance between the 
 two kingdoms of life. In restricted areas of land and sea, and 
 for restricted periods of time, either the plants or animals may 
 predominate; but in the long-run there is a relation between the 
 two masses of animal and plant substance, and this relation is 
 a nearly constant one. If animal life becomes temporarily very 
 abundant, vegetable life must decrease, because it is being used 
 up to a greater extent than is normal, and this condition will in 
 turn lead to a diminution of animal life. 
 
 Production and Consumption. The reader will now see what 
 is meant by " vital production." All animals eat organised 
 substance fats, proteids, and carbohydrates contained in the 
 tissues of other animals and plants. They reduce these sub- 
 stances to the forms of water, carbonic acid, and certain nitro- 
 genous residues, utilising the contained available energy for the 
 production of mechanical work and heat. Then the working sub- 
 stance is thrown out of circulation and is no longer available for 
 the sustenance of the animal organism. The latter is a consumer. 
 
 All the time solar energy is, so to speak, running to waste. 
 
86 THE MECHANISM OF LIFE 
 
 Falling upon the sea, it evaporates water, which rises up into the 
 air as vapour, falls down as rain and snow, and returns to the 
 sea in rivers, having done nothing of itself but wear away the 
 land and generate heat by friction. The heat is radiated away 
 into space, and is for ever lost to the earth. Falling upon rocks, 
 stones, gravel, sand, soil, etc., and upon the atmosphere, the 
 solar energy heats up all these substances to a slight degree ; butj 
 again this heat is radiated away, and is therefore irretrievably 
 lost. Apart from vegetable life, there is therefore a continual 
 dissipation of solar energy. 
 
 But the green plants intervene. They absorb the solar radia- 
 tion, utilising this to recombine the water, carbonic acid, and 
 nitrate which result from animal metabolism. Apart from the 
 activity of the plants, this energy would be dissipated and the 
 products of animal life would remain unavailable for further life. 
 They are, however, recombined by the plants, and the solar 
 energy which would otherwise be wasted is thus fixed. The 
 plants are producers. 
 
 In the past plant life has, on the whole, been more active than 
 animal life, and the result is the accumulations of coal (and 
 perhaps oil) upon which modern industrial civilisation is based. 
 This civilisation we must, however, regard as merely an episode 
 in the history of terrestrial life, for the enormous increase on 
 human population during the last few centuries has only been 
 possible by the utilisation of the materials produced by a former 
 surplus of vegetable life. With the depletion of this surplus the 
 balance will be restored. It would, of course, be very rash to| 
 regard the future reduction of the human population of the earth 
 as inevitable, for there may be other immense accumulations of i 
 energy in that now bound in the atoms of some material sub- 
 stances. Also some supply of energy may be obtained from the 
 tides, winds, and rivers. But, again, it would be very foolish to^ 
 count upon this possibility, for all that we know so far about 
 radio-active transformations suggests that the process is one] 
 which we cannot initiate or control, and that the energy of <lio 
 atoms, all but a very insignificant fraction, is bound. Also, the] 
 practical difficulties in the way of utilising tidal energy are prob-l 
 ably enormous. Now coal and oil are diminishing rather rapidly, 
 and there are no other sources of energy so far available. Therefore 
 it seems safe to assume a return to an agricultural and pastoral 
 civilisation, and with that a great reduction of population. 
 
CHAPTER VI 
 BRAIN AND NERVE 
 
 THE next thing we must consider is the means whereby the 
 various activities of the animal body are linked together and 
 co-ordinated. What has already been said indicated the existence 
 of such co-ordinations, but it will be useful to give an illustration. 
 
 A man. then, engages in sudden vigorous exercise. First of 
 all he flushes and feels warm (though the temperature of his 
 body does not increase appreciably). He breathes more deeply 
 and rapidly than is normally the case. His respiratory move- 
 ments also deepen and quicken. The rate of the heart-beat is 
 increased. The skin becomes moist. 
 
 Apart altogether from the series of nervous impulses that issue 
 from the spinal cord and brain to actuate the muscles that are in 
 movement, and apart also from the series of impulses that stream 
 into the central nervous system from the muscles and skin, there 
 are other mechanisms in operation. These effect regulations of 
 functioning. First of all the mechanical work done by the 
 contracting muscles leads to a production of heat and carbonic 
 acid, both being due to the oxidation of chemical substances 
 contained in the muscle fibres. The carbonic acid is absorbed 
 by the blood-stream, so that the percentage of this substance in 
 solution tends to rise. Now there is a little group of nerve cells 
 (the respiratory centre or " vital knot ") in the medulla, and this 
 is susceptible to changes in the quantity of oxygen and carbonic 
 acid carried in the blood that traverses it. When this per- 
 centage of C0 2 is greater than normal, the automatic activity of 
 the centre is accelerated, and vice versa. Rhythmic impulses 
 issue from it at an average rate, and actuate the muscles of the 
 chest wall and diaphragm. Therefore, when the quantity of C0 2 
 in the blood increases as the result of increased muscular activity, 
 the respiratory movements are automatically accelerated; there 
 is a greater ventilation of the lung cavities, more oxygen is 
 absorbed and more C0 2 is excreted, and the composition of the 
 blood tends to go back to that which is normal. 
 
 The heart-beat is a rhythmic automatic one, and it occurs at 
 
 87 
 
88 THE MECHANISM OF LIFE 
 
 a certain average rate. But the organ is controlled by two sets 
 of nerves, some of which issue from the medulla via the tenth 
 (or vagus) cranial nerve, and are acceleratory ones, while others 
 come from the cervical sympathetic ganglia and are inhibitory 
 in their function (see p. 141). Increase of C0 2 in the blood- j 
 stream affects the nerve cells from which these fibres originate 
 in opposite ways ; it tends to quicken the rate of impulses descend- 
 ing the sympathetic nerves, and so the heart's beat becomes 
 accelerated or quickened. More blood traverses the contracting 
 muscles, carrying nutritive matter and oxygen, and taking away 
 carbonic acid ; and at the same time more blood flows through the 
 capillaries in the lungs, so that more oxygen than usual is taken up 
 from the air in the lungs and more carbonic acid gas is excreted. 
 
 There are other nervous centres which control the calibre of 
 the small arteries carrying blood to the skin. The walls of these 
 vessels contain muscle fibres which diminish the calibres of the 
 arteries, whereupon the latter contract, or they increase it, 
 and then the vessels relax. Two sets of nerve fibres, derived 
 from the sympathetic nervous system, go to the muscles of the 
 cutaneous bloodvessels, one set, which is called vaso-constrictor 
 (constricting the arteries), and another set called vaso-dilator 
 (dilating them). The effect of the sudden generation of heat 
 and carbonic acid is to set up reactions in the central nervous 
 system which lead to the issue of nervous impulses, wa the 
 sympathetic ganglia, to the cutaneous bloodvessels, causing the 
 latter to dilate. More blood then passes through the skin than 
 usual, the latter flushes, and the sweat glands become more 
 active than is normally the case. The skin becomes moist, there 
 is evaporation of water from its surface, and so the heat generated 
 by the muscular activity is eliminated. 
 
 Here we have a series of regulations carried out by the nervous 
 system. One set of organs, the muscles of the trunk and limbs, 
 have been functioning at a greater rate than usual, and the effect 
 of this alone would be to disturb the general balance of bod in- 
 activity. Heart, respiratory organs, and skin are therefore 
 stimulated by the central nervous system also to function more 
 vigorously than usual, so that the increased activity of the 
 muscular organs is compensated. 
 
 The nervous system is therefore the mechanism of regulation, 
 co-ordination, and integration of bodily activities, and it is from 
 this point of view that we study it. But first of all we must 
 
BRAIN AND NERVE 89 
 
 consider it as a structure an assemblage of nerve cells and nerve 
 fibres arranged in a most complex manner. 
 
 The General Scheme. The central nervous system of a verte- 
 hrute animal consists of masses of nerve cells called ganglia, 
 and these are aggregated together as the brain, the spinal cord. 
 and the sympathetic ganglia. They are connected together by 
 coinmissural (or connecting) strands of nerve fibres, and they are 
 also connected with the sense organs, muscles, and other parts 
 of the. body by nerves. The latter are bundles of nerve fibres 
 varying in diameter from about J inch (the great sciatic 
 nerve of the leg), down to fine threads which are just 
 visible to the naked eye. The nerve fibres themselves are just 
 beyond the limits of unaided vision. A nerve issuing from the 
 brain or spinal cord, or from a sympathetic ganglion, breaks up 
 into finer branches, and these branches divide again and again until 
 the whole is decomposed into its constituent fibres. The latter end 
 in muscle fibres or among the cells of a gland or some other organ. 
 In a sense organ, such as the eye or ear, there are great numbers 
 of nerve cells, from each of which a single nerve fibre starts. 
 These individual fibres become united into a nerve, and the latter 
 (as in the case of the optic or auditory nerves) may go directly 
 to the brain, or they may join with the bundles of fibres coming 
 from other sense organs, and so reach the brain or spinal cord. 
 
 Thus we have two main divisions of the nervous system: 
 (1) The central parts, consisting of the ganglia with their con- 
 nections; and (2) the peripheral parts, consisting of the nerves 
 going from the organs of sense into the brain and spinal cord, and 
 of the nerves going out from the brain to the muscles, glands, and 
 viscera. The peripheral nerves, whether outgoing or ingoing, are 
 distributed to every part of the body, forming a network of fibres 
 which is everywhere a very fine one, but which becomes finer and 
 finer the more important is the organ to which they are distributed. 
 
 Now a very diagrammatic and undetailed picture of the 
 nervous system would show the following parts (Fig. 21): 
 
 We see that the whole consists of the brain, which is divided 
 into several principal parts: the cerebrum or great brain, the 
 cerebellum or little brain, and the medulla. The spinal cord is to 
 be regarded as the prolongation backward of the medulla. From 
 the brain there issue out ten pairs of cranial nerves, which consist 
 of fibres either going to or coming from the parts of the head, 
 far <\ and neck (with a few organs in the body cavities). From 
 
90 
 
 THE MECHANISM OF LIFE 
 
 the spinal cord there issue out spinal nerves, one pair at each 
 joint of the vertebral column. These also consist of fibres going 
 to and coming from the body, limbs, and skin. Along cither 
 side of the vertebral column are other two series of outlying nerve 
 ganglia which form the sympathetic nervous system, and these: 
 
 CerVical 
 
 -7 al 
 , b/exus 
 
 (V-Vllcer\f 
 JcLorsal ) 
 
 FIG. 21. A GENERAL DIAGRAM OF THE CENTRAL PARTS OF THE Hi MAN 
 NERVOUS SYSTEM. 
 
 The sympathetic ganglia and their main connections with the spinal 
 nerves are represented in black 
 
 ganglia are all connected with the spinal nerves by communi- 
 cating branches. Most people are familiar with the " solar 
 plexus." which is an important part of the sympathetic nervous 
 system. The cranial nerves go mainly to the muscles of the face.! 
 and eyes, or come from the great sense organs of the head. The 
 
BRAIN AND NERVE 
 
 91 
 
 spinal nerves go mainly to the muscles of the trunk and limbs, 
 or come from sense organs in the muscles, joints, tendons, and 
 skin. The nerves which go out from the sympathetic ganglia 
 are connected mainly with the bloodvessels and viscera. 
 
 Examining these structures with the aid of the microscope, we 
 find two main kinds of tissue. The central part of the spinal 
 cord, the superficial part of the cerebrum and cerebellum, much 
 of the deeper part of the brain and the sympathetic ganglia are 
 all called " grey matter," and here the substance of the central 
 nervous system is made of up nerve cells with their dendrites. 
 The superficial parts of the spinal cord, the deeper parts of the 
 cerebrum and cerebellum, and much of the other parts of the 
 brain are composed of " white matter," and this consists of 
 nerve fibres possessing a medullary sheath. The nerves issuing 
 
 Dorsa.1 
 
 J)o re, a. I 
 
 ~i j ^~^i yi.li "; ^visax- /lerj/e 
 
 Trunk (Ta.ng/jcL 
 
 B 
 
 FIG. 22. THE NERVOUS SYSTEM OF THE EARTHWORM. 
 I A y The head parts; B, a section through the body. 
 
 from the brain and spinal cord are also composed of white matter, 
 but those coming from the sympathetic ganglia are rather 
 different in appearance, since they do not possess medullary 
 sheaths. Two kinds of tissue nerve cells and nerve fibres 
 therefore make up the nervous system, central and peripheral. 
 
 The Arrangement of the Ganglia. Consider first such a very 
 simple nervous system as that of an earthworm. This animal 
 possesses a segmented or jointed body, and every segment is, on 
 the whole, similar to the one in front or behind it. In each 
 segment there is a pair of nerve ganglia connected together 
 across the middle line of the body by a commissure. The ganglia 
 are also connected together, each with the one in front and that 
 behind. In the first or " head " segment of the worm, the 
 ganglia are bigger than those in the body segments, and they are 
 arranged to form a nerve collar round the oesophagus. These 
 
92 THE MECHANISM OF LIFE 
 
 two pairs of head ganglia may be called the " brain " of the 
 animal. Nerves issue from the ganglia and go to the muscles, 
 skin, and other organs of the segment, and (in general) the organs 
 in each segment are supplied with nerves that originate in the 
 two ganglia belonging to that segment. 
 
 This is a very simple and easily understood scheme, and, strange 
 as it may appear, it is essentially the same scheme as that of the 
 nervous system of the higher animal, such as man. There also 
 the body is built up of segments, and though the latter are not 
 very evident in the adult stage, they can, easily enough, be made 
 out in the embryonic condition. Each segment contains a pair 
 of ganglia from which nerves go out to the organs of the body. 
 But the foremost segments have become coalesced to form the 
 head, and the ganglia have therefore fused together as the brain. 
 Further, all the ganglia in the trunk region have been greatly 
 enlarged, and so have coalesced to make the grey matter of the 
 spinal cord. The white matter of the brain and spinal cord 
 corresponds to the commissural nervous tracts (longitudinal and 
 transverse) of the earthworm. There are very many complica- 
 tions, and it would be wrong to press the analogy too closely; 
 still, it is a true one: the central nervous system of the higher 
 animal consists of a series of ganglia grown together to form the 
 brain and spinal cord. But although this grey matter is almost 
 continuous, we can recognise the existence of a very complex 
 system of " commissures " connecting together its centres, or 
 the positions of the original ganglia. 
 
 We may next consider the arrangement of these nervous 
 centres and commissural nervous tracts. 
 
 The Spinal Cord. This is the thick strand of nervous tissue 
 lying within the tunnel formed by the cavities within the joints 
 of the vertebral column. It looks as if it were the drawn- out 
 prolongation of the medullary part of the brain, but it is really 
 to be regarded as that part of the central nervous system which 
 is situated in the trunk, while the brain is the other part situated 
 in the head. Looking at it in section (as if the spinal cord were 
 cleanly cut across), we get the general arrangement suggested 
 in the figure. The central part of the cord consists of grey 
 matter that is, of nerve cells with their dendrites. The peri- 
 pheral part consists of nerve fibres running lengthways in the 
 cord. These fibres connect together the ganglionic matter of the 
 brain with that of the cord, and they also connect together 
 
BRAIN AND NERVE 
 
 93 
 
 the different segments of the grey matter of the cord. Some of 
 them are fibres of the spinal nerves that enter the cord. 
 
 Between every two adjacent vertebrae one spinal nerve goes 
 out from the cord on each side. But each spinal nerve has two 
 roots dorsal (to the back) and ventral (towards the belly). 
 The dorsal root consists of nerve fibres that carry impulses from 
 the skin, muscles, tendons, joints, and viscera into the spinal 
 cord, while the ventral root contains fibres that carry impulses 
 out from the cord to the muscles and viscera. And, therefore, if 
 the dorsal roots of several adjacent spinal nerves are severed, 
 there is loss of sensation in some part of the trunk or limbs, for 
 sensory impulses are thus prevented from entering the cord and 
 passing up into the brain. And if several adjacent ventral roots 
 
 G~a n a lion Po ste r/'or trac fe 
 i f* \ 
 
 Dorsal or . . ... 
 Sensory root^^ ]"# 
 
 matter 
 ( Commissure*/ barT) 
 
 onic 
 nerve-cells. 
 
 FIG. 23. A DIAGRAMMATIC SECTION THROUGH THE SPINAL CORD, 
 
 SHOWING THE ORIGIN OF THE SPINAL NERVES. 
 
 are severed, there is paralysis of the muscles in some part of the 
 body. Several roots must be cut through to produce these 
 effects, for, by a very beautiful arrangement, every part of the 
 body receives nervoub supply from several segments of the cord, 
 and this overlapping of nervous and bodily segments minimises 
 the effect of an accident. Allowing for the complications of the 
 overlapping, we may say that each group of muscles in the trunk 
 and each area of skin is under the control of one or more segments 
 of the cord. The arms and legs, because of their great import- 
 ance in locomotion and general acting, are supplied by a number 
 of spinal nerves which unite together to form the limb " plexuses." 
 To a great extent the spinal cord is a path of conduction of 
 nervous impulses from the brain to the body, and vice versa. 
 But it is also a nervous controlling centre in itself, and when it is 
 
94 THE MECHANISM OF LIFE 
 
 cut off entirely from the brain it can regulate and co-ordinate 
 bodily muscular actions and functions. A man, for instance, 
 may " become a father " even when there is a lesion destroying 
 the nervous connection between the brain and the centres in the 
 cord which control the working of the genital organs that is, 
 those centres are autonomous. Generally speaking, the lower 
 in the evolutionary scale an animal is the greater is the im- 
 portance of the spinal cord relatively to the brain, and vice versa. 
 It is functionally a series of semi-independent ganglia or centres 
 of nervous control. 
 
 The Brain. It is not certain how many segments have 
 coalesced to form the head of a vertebrate animal. There are 
 ten pairs of cranial nerves, and if we suppose that each of these 
 is, in its origin, to be compared with a spinal nerve, there must 
 be ten segments. But the whole question of head segments is 
 very difficult. Certainly a number of the foremost ones form 
 the head. Goethe thought he could recognise several modified 
 vertebrae in the skull, and this was long afterwards the opinion 
 of anatomists, but we now believe the Goethe-Oken homology to 
 be incorrect. However, the concentration of the great organs of 
 sense in the head and the variety and importance of the movements 
 which are carried out by the eyes and jaws must have led to the 
 concentration of central nervous mechanisms in the neighbour- 
 hood of these organs, and so established the lower brain centres. 
 
 For there are three central nervous systems in the brain the 
 " lower brain " (oldest), the cerebellum, and the cortex cerebri 
 (the most recent). We must see how these have been evolved. 
 
 The Development of the Nervous System. The very first 
 thing to be formed in the embryonic development of a vertebrate 
 animal is the central nervous system. A little groove, lying in 
 the future axis of the body, forms in the embryo of a few days 
 old, and then this closes up to form a tube. Soon the foremost 
 (head) end of this tube swells out to form three vesicles, or bulb- 
 like enlargements, and these are the " anlagen " of the brain. 
 The rest of the structure of the animal becomes built up round the 
 primary brain vesicles and spinal cord in a manner which is very 
 difficult to explain, but exceedingly simple and " obvious " when 
 one watches the developmental process in a vertebrate animal. 
 
 Very soon two little lateral vesicles are pushed out from the 
 side walls of the fore-brain, and these become the cerebral 
 hemispheres. The brain thus begins as a hollow organ, and the 
 
BRAIN AND NERVE 
 
 95 
 
 - - ~; Corpora, striata. 
 Vesicle s 
 
 cavities persist all through, life as the ventricles, which are in 
 open communication with the tubular cavity, or central canal, 
 of the spinal cord. 
 
 Next the walls of the brain vesicles begin to thicken very 
 unequally, and bending occurs. Thus, in the human embryo 
 at about the 
 middle of the 
 eighth week 
 we find the 
 brain some- 
 what as given 
 in Fig. 25. 
 
 Fig. 25 re- 
 presents the 
 stage through 
 which all ver- 
 tebrate ani- 
 mals pass in 
 the course of 
 their embryo- 
 geny. The 
 floor parts of 
 the lateral ves- 
 icles thicken to 
 form the cor- 
 pora striata ; 
 the sides of 
 the fore-ves- 
 icle similarly 
 thicken, to be- 
 come later on 
 
 3 rain 
 vesicles 
 
 Lateral 
 Tore - 
 
 Mid. 
 
 Hind 
 
 ^^'^-M id- brain 
 -" (Corbora. 
 
 Cord 
 
 FIG. 24. DEVELOPMENT OF THE BRAIN. 
 
 The upper figure shows the primitive brain tube and 
 its vesicles. The lower figure shows further stages. 
 
 the optic thalami (or the parts in lower vertebrates corresponding 
 to the human thalami); the roof of the mid- vesicle forms the 
 corpora quadrigemina of man, or the lower vertebrate homologues 
 of these ganglia; the roof of the front part of the hind- vesicle 
 becomes the cerebellum ; and the floor and sides of the posterior 
 part become the medulla. The cerebellum is very small in fishes, 
 and becomes more important as we rise in the scale, until it 
 practically becomes a new part added to the primitive brain. 
 The roofs of the lateral vesicles are never more than mere mem- 
 branes, non-nervous structures, in fishes, but they thicken in 
 
96 THE MECHANISM OF LIFE 
 
 animals higher than fishes to become the cerebral hemispheres. 
 The latter progressively increase in mass in the amphibia, 
 reptiles, and birds, until they become the enormous lobes that 
 we see in man, overarching and almost concealing all the other 
 parts of the brain. 
 
 Thus we have the three developmental parts of the vertebrate 
 brain : 
 
 The lower brain : corpora striata, optic thalami, mid-brain, 
 
 incipient cerebellum, medulla; 
 The cerebellum ; 
 The cerebral hemispheres with their cortex. 
 
 S~ I I > 
 
 Corpus obfic 
 striatum Calamus 
 
 FIG. 25. FURTHEB STAGE IN THE DEVELOPMENT OF THE BRAIN. 
 
 The primitive lower brain in a fish, such as the cod, is very 
 simple (Fig. 26). The mid-brain, with its great optic lobes, is the 
 most important part. The fore-brain is the part lying between 
 the two corpora striata. The cerebellum is not greatly developed. 
 In its general characters such a brain as that which we have 
 just described represents what we shall refer to hereafter as the 
 " lower brain " of the higher mammal. Imagine the thin 
 membranous " pallium " which covers the corpora striata to 
 become enormously thickened and to grow backwards over all 
 the rest of the brain, and suppose the cerebellum greatly to 
 increase in mass relatively to the other parts. Then we should 
 have the brain of the mammalia" n animal. 
 
BRAIN AND NERVE 
 
 97 
 
 The Human Brain. The human brain is represented in a very 
 diagrammatic way in Fig. 27. 
 
 It is supposed that the whole organ has been cut through in a 
 mid-vertical plane, and a number of details that are rather 
 difficult to understand have been omitted from the diagram. 
 The parts actually cut through are cross-hatched, and we see the 
 central canal of the spinal cord widening out to form the " fourth 
 ventricle " of the anatomists that is, the primary hind-brain 
 vesicle. Between this and the " third ventricle " that is, the 
 
 I Olfactory 
 
 Sbinal 
 ' cord. 
 
 K.Optic(eyes) 
 
 JJf Oculomotor 
 
 (muscles ofeyesj 
 
 )Y,Trochlea.r 
 
 ( M uscles of eyes) 
 
 -Yf.Abducens 
 , (muscles of eyes) 
 
 Y- Trijgeminal 
 (facet-Jan*) 
 
 yM Auditory 
 (ear) 
 
 facial 
 
 .ossof)ha.ryn$ eal 
 (Mouth ( pharynx., 
 
 X. Vagus 
 (Gills, heart**) 
 ifia/ nerves 
 
 FIG. 26.- THE BRAIN OF A FISH (COD) SEEN FROM THE UPPER OR 
 DORSAL SURFACE. 
 
 primary fore-brain vesicle is a narrow passage which is the 
 " aqueduct of Sylvius," and this represents the primary mid- 
 brain vesicle. The floor, sides, and roof of the hind-brain vesicle 
 are enormously thickened to form the cerebellum and the 
 peduncles which connect it with the other parts of the brain. 
 Similarly the roof, sides, and floor of the mid-brain vesicle have 
 become thickened to form the corpora quadrigemina, and the 
 great crura, or peduncles of the cerebrum, and some other parts. 
 The roof and floor of the " third ventricle " remain thin, but the 
 
 7 
 
98 
 
 THE MECHANISM OF LIFE 
 
 lower parts of the sides are thickened to form the optic thalamij 
 one of which is represented in the lateral wall in the figure. The 
 enormous lobes, called the cerebral hemispheres, are the roofs of 
 the primary lateral brain vesicles thickened to form the cerebral 
 cortex with its systems of projection nerve fibres. In one corner; 
 of the third ventricle a little opening, called the foramen of MonroA 
 is shown, and dotted lines indicate how this leads into a cavity 
 in each cerebral hemisphere, called the " lateral ventricle." The] 
 cross-hatched part, called the corpus callosum, is the junction of 
 
 Junction of R. and L hemispheres 
 
 Corpus , 
 Sthatum " 
 
 n ng 
 
 ^Kl. inTo " 
 
 1st. ventricle 
 
 L atera I ventricle 
 
 _ _ ^Pineal body 
 - Cor bora. 
 
 --Cerebellum 
 
 -Pituitary ', 
 body I 
 
 7eduncle 
 of the cerebrum 
 
 cord. 
 
 FIG. 27. THE HUMAN BRAIN: AN IMAGINARY SECTION ALONG THB I 
 MIDDLE PLANE. 
 
 The cross-hatched areas represent the cut surfaces; the numbers I, lit 
 and IV, show the lateral, third and fourth ventricles respectively. 
 
 the two (right and left) hemispheres. Just round the foramen] 
 of Monro the corpus striatum of the right-hand hemisphere is 
 shown. This ganglion appears to bulge into the third vontricloj 
 but it really belongs to the lower part of its cerebral hemisphereJ 
 Because of the enormous growth backwards of the latter orgaM 
 the relations of the various parts of the brain become difficult 
 to visualise. Thus the corpus callosum appears to form thl 
 roof of the third ventricle, but this is not really the case. The- 
 reof consists of a delicate vascular membrane, which is now 
 easily representable in a diagram, and the corpus collosmn, with; 
 some other parts which are omitted, are the sections of the 
 
BRAIN AND NERVE 99 
 
 of the two cerebral hemispheres which have coalesced by their 
 internal faces. 
 
 The reader must not omit to note the two little bodies that 
 are attached to the roof and floor of the third ventricle. That 
 one attached to the roof is called the pineal body, and it was here 
 that Descartes placed the seat of the soul. It is really a " ves- 
 tigial organ " which has acquired a new function. Early in the 
 history of the primitive vertebrates there were either one or 
 more " cyclopean " eyes, and even in some of the lizards such 
 eyes exist, though they are never functional organs of vision. 
 In modern mammals the pineal body has become a ductless 
 gland that is, an organ that forms some substance which is 
 discharged into the blood-stream. What the substance is we 
 do not know, but it is said that it exercises an inhibitory in- 
 fluence, restraining precocity of growth and a too early maturity 
 of the reproductive organs. 
 
 The organ on the floor of the third ventricle is called the 
 pituitary body, and it also is a vestigial structure consisting of 
 two parts, one of which appears to have formed the primitive 
 vertebrate mouth. The pituitary body is a ductless gland 
 secreting some substance into the blood which inhibits or controls 
 the growth of the skeleton, particularly the bones of the face. 
 Removal of the gland is always a fatal operation, and disease or 
 hypertrophy produce curious exaggerations of growth, some of 
 which, it has been noted, recall in bizarre fashion the charac- 
 teristics of the extinct Neanderthal human race. 
 Connections within the Central Nervous System. 
 
 The white tracts in the spinal cord, in the peduncles of the 
 cerebellum, and in the parts called the cerebral peduncles or 
 crura in Fig. 27 are the great paths along which nervous im- 
 pulses travel within the central nervous system. -It is known 
 that there are, in the white matter of the cord, two main cate- 
 gories of nerve fibres: (1) such as convey impulses to the brain 
 ascending tracts; and (2) those that transmit impulses from the 
 brain to the grey matter of the cord these are the descending 
 tracts. Starting, then, with the white matter of the cord, we 
 may next consider the main paths along which impulses travel 
 in the central nervous system. 
 
 The Sensory Tracts. First we take the ascending paths, 
 those along which impulses originating as the results of stimula- 
 tion of the receptor organs (see p. 106) reach the lower brain. 
 
100 THE MECHANISM OF LIFE 
 
 All sensory stimuli from the skin, the muscles, joints, and viscera 
 pass into the spinal cord via the dorsal (or sensory) roots ofi 
 the spinal nerves, and either go directly up to the brain or they 
 enter the grey matter and undergo " relays "* there that is, 
 they are shunted on to new paths. Some of them pass out from 
 the cord via relays and the motor roots, when reflex actions occur' 
 (see later), but the others travel up to the brain along tracts in 
 the white matter. The great ascending tract in the cord is the! 
 bundle marked " posterior tracts " in Fig. 23, and this is made 
 up mainly of nerve fibres which start from the cells in the grey! 
 matter round which the sensory root fibres terminate. 
 
 The upward path of these bundles is represented very diagram-; 
 matically in Fig. 28 as " great sensory tracts from cord to 
 medulla." The individual fibres run up into the medulla, and end] 
 there as synapses round cells in two centres or nuclei, thus enter- j 
 ing into a second relay. From these centres two new tracts,- 
 called the lemnisd, start. Immediately after leaving the nuclei 1 
 the lemnisci cross or decussate that is, the fibres starting from> 
 the right-hand nuclei cross over to the left-hand side, and! 
 vice versa. Kunning up through the crura of the lower brain, 
 they end in the corpora quadrigemina and optic thalami, ancfl 
 this is their third relay. From these mid-brain nuclei new tracts 
 of fibres start again, and proceed upwards into the cortex. 
 
 Thus the stimuli originating in the sense organs of the skin on 
 the trunk and limbs, and in the muscles, joints, and viscera of] 
 those body regions, pass into the spinal cord, ascend the latteri 
 to end in the medullary ganglia. From there they travel to the! 
 mid-brain ganglia on the opposite sides of the brain, and from] 
 the mid-brain they proceed to the cerebral cortex. It is mostly 
 the impulses giving rise to the sensations of touch, heat and cold, j 
 and pain originating by stimulation of the skin that take this 
 very complicated path. 
 
 The impulses coming from the receptor organs in the muscle 
 and joints mostly take a different path. They enter the greyl 
 matter of the cord via the sensory spinal nerve roots as before, J 
 and end in cells from which new ascending fibres start, or they 
 may travel up in the white matter of the cord without first 
 undergoing a relay in the grey matter. But in either case thefl 
 take an independent course, and proceed to the cerebellum, e 
 
 * A " relay " is an interruption in a nervous tract. One set of neuroneH 
 join on to a new set by means of synapses. 
 
102 V1J.S MECHANISM OF LIFE 
 
 entering the latter via the inferior peduncles. These are the tracts 
 shown in Fig. 28, and called " afferent cerebellar tracts." 
 
 Thus the receptor organs in the trunk and limbs are connected 
 with the ganglia contained in the mid-brain and cerebellum. 
 This is also the case with the great receptors in the head the 
 visual, auditory, and gustatory organs; with the touch, heat, 
 and cold receptors in the skin of head and face; and with the 
 muscular and articular receptors in the same region, but not 
 with the olfactory receptors. With the latter exception, all the 
 nerve fibres starting from the sense organs in the head and face 
 pass into the medulla via the cranial nerves, and end in separate 
 nuclei. The latter are then connected in complex ways with the 
 corpora quadrigemina, the optic thalami, and the cerebellum. 
 The fibres carrying impulses from the olfactory organs are con- 
 nected directly with the cerebrum ; later on we shall consider the 
 connections of the great sense organs in more detail. 
 
 Connections of the Cerebellum with the Other Parts of the 
 Central Nervous System. The cerebellum of a higher mammal 
 is connected with the cord, mid-brain, and cortex by three great 
 pairs of tracts contained in the cerebellar peduncles. The 
 inferior peduncles are connected with the white and grey matter 
 of the cord in the way just suggested, the superior peduncles are 
 connected with the mid-brain (mainly corpora quadrigemina 
 and thalami), and the opposite sides of the cerebellum are joined 
 together by the middle peduncles which form what is called the 
 pons varolii. Thus the cerebellum has a grip, so to speak, on 
 the nuclei or ganglia which receive all the sensory impulses 
 coming from every part of the body whatever. Further, since 
 the grey matter of the cord and the nuclei of the cranial nerves 
 are ganglia from which motor fibres go out to the muscles of body, 
 limbs, and head, the cerebellum has also a grip on the centres 
 controlling movements. 
 
 The Connections of the Cortex Cerebri with the Rest of the 
 Central Nervous System. The cerebral hemispheres are, we 
 have seen, parts of the central nervous system which are super- 
 added to the lower brain, inasmuch as each of them is to be 
 regarded as a great lobe growing out on each side of the original 
 fore-brain vesicle, expanding enormously, and arching over all 
 the other parts of the brain. The base of each hemisphere is 
 formed by the great ganglia called the corpora striata, and these 
 come into close relationship with the ganglia of the fore- brain 
 
BRAIN AND NERVE 
 
 103 
 
 that is, the optic thalami. These two pairs of basal ganglia are 
 present in the brains of all vertebrates, and we are regarding 
 them here as parts of the " lower brain." The roofs and sides 
 of the lateral fore-brain vesicles are either not present at all as 
 nervous grey matter, or are imperfectly developed in the lower 
 vertebrates, and they become very important only in the 
 mammals. There they form a relatively thin sheet of grey 
 matter which constitutes the superficial part of the hemispheres, 
 and this increases so greatly in area that it becomes crumpled 
 and folded in a complex way, so that it may be contained within 
 the cranial cavity. The crumpling leads to the formation of the 
 cerebral convolutions. 
 There is therefore a 
 thin layer of grey 
 matter the cortex 
 cerebri on the surface 
 of each hemisphere, 
 and beneath this there 
 are great bundles of 
 nerve fibres running 
 in various ways. Be- 
 neath these bundles of 
 white matter, again, 
 are the basal ganglia. 
 The white matter of 
 
 FIG. 29. A SECTION THROUGH A CEREBRAL 
 HEMISPHERE NEAR THE MIDDLE PLANE TO 
 SHOW THE MAIN COMMISURAL TRACTS. 
 
 the cerebral hemispheres form two main categories of bundles 
 the commissural tracts and the projection tracts. We have 
 dealt with the latter in Fig. 28. The commissural tracts are 
 represented in Fig. 29. 
 
 One system of commissural tracts, represented chiefly by the 
 corpus callosum, connect together the right and left halves of 
 the cerebrum. Probably corresponding parts of the two hemi- 
 spheres are joined by the fibres of the corpus callosum, but it is 
 also possible that any one part of one hemisphere is connected 
 with most parts of the other. In addition to these transverse 
 commissural tracts there is an elaborate system of internal 
 commissural or *' association " tracts in each hemisphere, and 
 some of these are represented in Fig. 29. They connect together 
 the various convolutions, or groups of convolutions, by short 
 paths, while longer tracts connect distant parts of the cortex 
 with each other. More and more, as cerebral physiology de 
 
104 THE MECHANISM OF LIFE 
 
 velops, do the interactions of one part of the cortex with other 
 parts by means of the commissural and association tracts come 
 to possess significance in the development of the manifestations 
 of intelligence and higher mental faculties.J 
 
 From all parts of the cortex bundles of fibres radiate inwards 
 towards the cerebral peduncles; these form the "projection 
 tracts " those that connect the cortex with the lower brain and 
 spinal cord. Some of the projection tracts are represented in 
 Fig. 28 in a very schematic way. One, which is called the 
 " great sensory tract," connects the parts of the cortex lying 
 behind the Rolandic fissure (see Fig. 38) with the mid-brain 
 ganglia; another tract of fibres, starting from the cortical region 
 well in front of the Rolandic fissure, travels downwards and ends 
 in the grey matter of the pons varolii, where it forms a series of 
 relays with the fibres coming from the two sides of the cerebellum 
 via the middle peduncles of the latter. This is the " tract from 
 the cortex to the pons and cerebellum " of Fig. 28. 
 
 Thus the cortex is connected with the lower brain on the one 
 hand by a direct tract, and with the cerebellum on the other 
 hand by two indirect tracts, one via the mid-brain and the other 
 via the pons and the middle peduncles. 
 
 The most important of all the projection tracts in man and 
 the higher mammals is the pyramidal tract, that represented in 
 Fig. 28, and called " the motor tract from the cortex to the 
 cord." The fibres composing it start in the peculiar pyramidal 
 cells of the cortical region lying in front of, and immediately 
 round and in the depths of, the Rolandic fissure that is, in the 
 " motor area " of the cortex. These fibres are gathered up into 
 two bundles which travel down in the crura of the brain towards 
 the medulla, where they decussate, or cross, to the other side of 
 the body. The crossing is not, however, complete, as the figure 
 shows, and some of the pyramidal fibres remain on the same side 
 of the body as that in which they originate. But most of them 
 cross over. Running down in the white matter of the cord are 
 therefore two descending tracts of fibres, " the direct and crossed 
 pyramidal tracts." All these fibres pass into the grey matter of 
 the cord, and end there as synapses round the cells that give off 
 the fibres of the motor roots of the spinal nerves. 
 
 The pyramidal cortical tracts, with their nuclei in the cortex, 
 are the typically " higher " parts of the mammalian brain. They 
 are relatively small in such animals as the mole or the rabbit, 
 
BRAIN AND NERVE 105 
 
 and more developed in the dog, and still more in the monkeys. 
 In the anthropoid apes and in man they attain the maximal 
 development, and are to be regarded as the paths along which 
 those impulses travel that are the stimuli to what we call 
 " willed " or spontaneous actions and movements. 
 
 This is a very summary account of the main features of the 
 gross anatomy of the central nervous system, and we deal in 
 greater detail later with the special nervous mechanisms. The 
 reader should now be able to visualise the whole as a series of 
 parts developing progressively in the course of the evolution of 
 the vertebrate animals. Primarily there was a double series of 
 ganglia, one pair in each segment of the body, and all of them 
 were connected together by transverse and longitudinal com- 
 missural tracts. Nerves issuing from the ganglia were dis- 
 tributed to the sensory and motor organs of the body, conducting 
 inwards impulses originating in the stimulation of the receptors, 
 and conducting outwards impulses setting the muscles in move- 
 ment or causing glands to function. With increasing complexity 
 of bodily structure and greater freedom of movement the ganglia 
 increased in mass and began to coalesce, thus forming the con- 
 tinuous core of grey matter of the cord, the hind, mid, and fore 
 brain. The great sense organs became concentrated in the head, 
 and so the cranial ganglia increased in functional importance, 
 finally assuming the conditions that have been described. 
 
 There has been a progressive complexity in the movements 
 included in locomotion as we ascend the series of evolutionary 
 phases represented by the fishes, amphibia, reptiles, birds, and 
 mammals. The need for precise adjustment and co-ordination 
 of the impulses issuing from the spinal cord and setting muscular 
 organs in motion therefore led to the development of specialised 
 ganglia carrying out such functions, and so the cerebellum 
 assumed the anatomical importance that it obviously has in the 
 human brain. 
 
 And in the later stages of this evolution that is, in the 
 mammals a kind of activity, connoted by the terms " intel- 
 ligent," " spontaneous," and " volitional," became the charac- 
 teristic one exhibited by these animals. This, we shall see, 
 involved a nervous mechanism other than those of the mid- 
 brain and cerebellum, which are to be regarded as the ganglia 
 controlling movements and activities that are largely " auto- 
 matic." This mechanism, the latest one to be evolved, is 
 contained in the cortex cerebri and its connections. 
 
CHAPTER VII 
 THE SPECIAL NERVOUS MECHANISMS 
 
 THE very general survey that we have just made of the rough 
 anatomy of the central nervous system will enable the reader to 
 study in greater detail the more special mechanisms into which 
 we, rather arbitrarily, decompose the whole. These mechanisms 
 are those of sensation, of motor control, and of co-ordination. 
 
 The Sensory Mechanisms. 
 
 " Sensation " involves the stimulation, by some physical 
 agency, of receptor organs distributed everywhere in the body. 
 We must suppose that all bodily tissues are irritable that is, 
 that they react in some way to stimuli, which may be chemical 
 or physical. From what we know of the irritability of the sur- 
 face tissues of the lower organisms, we may also conclude that 
 this is a general irritability, and that the same tissue is potentially 
 susceptible to light, electric, chemical, and mechanical stimuli. 
 But in the higher animals the general irritability of the tissues 
 of the lower organisms becomes modified by the evolution of the 
 special organs of sense. We must think of this generalised 
 susceptibility of the skin, say, as being restricted, so that any 
 one kind of stimulus must be intense enough to pass over a 
 "threshold"; if it is too feeble, the receptor is not stimulated 
 at all. A special sense organ, therefore, is a part of the skin 
 or other tissue where the height of the threshold is reduced for 
 stimuli of some particular nature, and raised for all others. 
 Thus the retina is a highly specialised part of the embryonic 
 outer surface, which is extremely sensitive to the stimulus of 
 light, but is relatively insusceptible to changes of atmospheric 
 pressure, while its situation is such that it is not usually stimu- 
 lated chemically, electrically, or mechanically. Similarly, the 
 auditory hairs in the cochlear part of the internal ear are highly 
 susceptible to sound vibrations occurring in the atmosphere 
 outside, while they are so sequestered that they are not readily 
 exposed to stimuli of any other kind. The nerve terminations 
 of the olfactory and gustatory nerves in the mucous membranes 
 of the nose and mouth are so placed that they are very readily 
 
 106 
 
THE SPECIAL NERVOUS MECHANISMS 
 
 107 
 
 exposed to stimulation by chemical substances floating in the 
 inspired air or dissolved in the liquids taken into the mouth, 
 and they are highly sensitive to such chemical stimuli. On the 
 other hand, they are not at all affected by light or sound vibra- 
 tions, and mere physical contact with them of some solid in- 
 soluble substance only evokes a sensation of touch. All sensory 
 surfaces are stimulated by electric discharges, as one may find 
 by placing the electrodes from a battery cell on the tongue, but 
 in such cases the sensation has not the quality of that which is 
 evoked by the special agency appropriate to the sense organ. 
 
 A special sense organ consists, therefore, of the terminations 
 (dendrites) of a nerve cell. The axon of this nerve cell forms 
 one of the fibres in the sensory or afferent nerve which conveys 
 into the central nervous system the impulses generated by the 
 stimulation of the dendrites. As a rule the essential or special 
 part of the sense organ is not so simple as we have indicated. 
 Thus we have in the retina, or essential part of the organ of 
 vision, the following structures at least : 
 
 /-' irfi; Inferior of 
 Gan fcell& \ e^- ball 
 
 Thickness of me retina 
 FIG. 30. A DIAGRAM OF THE NERVOUS ELEMENTS OF THE RETINA. 
 
 ["he latter is supposed to be seen in section, the concave surface being 
 that receiving the light from the pupil and the convex surface being 
 that turned to the back of the eye. 
 
 The actual nerve terminations that receive the rays of light 
 are the nerve cells, called the rods and cones (1). From these 
 elements axons go off and make synapses with the dendrites of 
 another nerve cell (2), which gives off an axon that enters into 
 a synapse with a third nerve cell (3). The axon of this is pro- 
 longed out into the optic nerve, and so passes up into the brain. 
 None of these nervous elements, or neurones, is in actual physical 
 contact with another. 
 
108 THE MECHANISM OF LIFE 
 
 This is a very complicated instance, but it is the type of an 
 essential sense receptor. There is always a terminal nerve cell, 
 and the dendrites of this receive the physical stimulus and trans- 
 mit it through synapses to other neurones, and, finally, via the 
 sensory tract, to the cerebral centres. These terminal dendrites, 
 or sensory nerve terminations, are peculiarly modified in each 
 case, and their threshold is lowered, so that they are susceptible 
 to one form of physical stimulus rather than others. 
 
 The non-nervous parts of the sense organ are concerned in 
 transmitting the particular stimulus and in excluding others. 
 Thus the retina is enclosed in the ball of the eye, and the front 
 part of the latter (the cornea) is transparent, so that light can 
 pass through it. Behind the cornea is the lens with the 
 mechanisms for altering its focus, and the changing diaphragm, 
 or iris, which regulates the amount of light that penetrates it. 
 These dioptric parts of the eye are in every way comparable with 
 a camera, lens, and diaphragm, which focus the external light 
 upon a sensitive plate, making a picture there. 
 
 In the same way the ear consists of the outer, middle, and 
 internal parts. Externally there is a stretched membrane (the 
 drum), which is set in vibration by sound waves in the outer 
 atmosphere. The drum communicates its motions to a chain of 
 three small bones (the auditory ossicles), which again transmit 
 the vibrations to the liquid contained in a cavity in the bone of 
 the skull. This vibrating liquid then stimulates the nerve 
 terminations of the cochlear (auditory) nerve in a most complex 
 structure, called the organ of Corti, and the stimuli are trans- 
 mitted along the cochlear nerve to the auditory centres. The 
 organ of hearing thus excludes light, chemical and mechanical 
 stimuli, but allows the periodic variations of atmospheric pressure 
 that we call " sound " to reach the nerve terminations. 
 
 The nerve terminations in the mucous membranes of the 
 mouth and nasal cavities are " bare " that is, they are exposed 
 to all physical stimuli: variations of temperature, some light, 
 mechanical pressure, and chemical activity. But they are 
 highly sensitive to the stimuli of different kinds of chemical 
 substances, so that they can distinguish between the latter. 
 And they are very susceptible in this way, so that, for instance, 
 one easily tastes the difference between the flesh of, say, cod 
 and haddock, or plaice and sole, a distinction which cannot yet 
 be made by chemical analysis. The nerve terminations of the 
 
THE SPECIAL NERVOUS MECHANISMS 109 
 
 olfactory nerve are still more delicate even in man, in whom 
 the sensation of smell is degenerate and chemical substances 
 existing in such small quantity in the air that they cannot be 
 detected by any known methods of analysis are easily distin- 
 guished by smell. Still more incredible is the delicacy of the 
 sense in such an animal as a bloodhound. 
 
 Heat receptors in the skin are nerve terminations that are 
 stimulated by a rise of temperature above a certain limit, but 
 are not affected by a fall below that. Cold receptors are, con- 
 versely, stimulated by a decrease, but not by an increase of 
 temperature. Pressure receptors, or muscular sense receptors, 
 are stimulated mechanically that is, by something pressing on 
 the skin, or by the degree of tension of a muscle, but not (or 
 at least not much) by chemical changes in the skin or muscle, 
 and not by changes of temperature. Equilibrium receptors, 
 which are present in the " vestibular " part of the internal ear, 
 are stimulated by changes in the position of the body with 
 respect to its surroundings. Thus a man who is blindfolded 
 has his ears and nose stopped, and who is lying immobile on a 
 turn-table, can appreciate a noiseless, Motionless change in the 
 position of his body, and can even roughly estimate the magni- 
 tude of the angle through which he is turned. 
 
 Thus the first step in the development of sensation consists 
 in a refinement of the general irritability, or susceptibility to 
 external changes, which we regard as one of the essential pro- 
 perties of living tissues. The " refinement " means that speciali- 
 ties of reaction are evolved: one kind of tissue, or arrangement 
 of nerve terminations, becomes more sensitive to one kind of 
 physical stimulus and less sensitive to all others. These 
 specialised receptors then localise themselves in appropriate 
 parts of the body, and become served by separate nerves. It is 
 improbable that the fibres themselves are different in different 
 sensory nerves that is, could we transplant the auditory nerve 
 fibres to the optic tracts, they would probably conduct optic 
 stimuli just as well as they conducted auditory ones, and vice 
 versa. The nerve fibres are conductors, and nothing more. 
 
 The next step is to place the fibres that transmit impulses 
 from a receptor in connection with fibres that transmit impulses 
 to a muscle or other effector organ, and that is done in the spinal 
 cord, the lower brain, and the cerebellum. The sensory (or 
 afferent) fibres enter into synapses with motor* (or efferent) 
 
110 THE MECHANISM OF LIFE 
 
 fibres, and thus an impulse arising from the stimulus of some 
 receptor owing to a change in the environment becomes con- ] 
 verted into another kind of impulse that stimulates muscles to 
 contract and relax, and so enables the animal to make an appro- 
 priate (or adaptive) response. But why " appropriate " ? We 
 discuss this question (but do not answer it) in a later chapter. 
 
 The lower brain, cerebellum, and spinal cord, are therefore 
 centres (or ganglia) where afferent impulses become converted 
 into efferent ones. They are simply the loti of synapses. 
 
 The last development of sensation is the becoming aware of 
 the external changes that stimulate the receptors. It must not 
 be thought that the development of consciousness of the environ- 
 ment, or of the body itself, is the sole, or even the main, function 
 of the nervous system. What the latter does in the lower 
 animals almost entirely, and what it mainly does, even in man, 
 is to convert a sensory stimulus set up by some change in the 
 environment into an appropriate response. Consciousness and 
 psychical life doubtless accompany this conversion in all animals, 
 though their intensity is the dimmer the lower in the scale of 
 evolution the organism is placed. In ourselves this final develop- 
 ment of sensation is the function of the cortex cerebri. 
 
 General Body Sensation. Remembering, then, that the impulses 
 arising from the stimulation of a receptor organ need not, and 
 usually do not, give rise to changes of consciousness, we may con- 
 sider, first, the paths by which the afferent impulses coming from 
 the skin, muscles, and joints of the limbs and trunk reach the brain. 
 
 All such impulses enter the cord by the dorsal (or posterior) 
 or sensory roots of the spinal nerves, but the paths along which 
 they afterwards travel depend upon their nature. Those that 
 arise as the stimulation of receptors in the deeper muscles and 
 joints enter the grey matter of the cord, and are received by the 
 synapses of nerve cells there. From these nerve cells axons pass 
 out into the white matter, and these form nervous tracts on each 
 side that go up through the medulla, enter the inferior peduncles 
 of the cerebellum, and end in the grey matter of that part of the 
 brain. At the same time other nerve fibres, carrying similar im- 
 pulses, enter the cord by the sensory roots, and travel directly up in 
 the white matter without first forming synapses in the grey matter. 
 
 Thus a large number of nerve fibres coming from the receptor 
 organs in the deeper muscles and joints deliver their impulses 
 into tracts of fibres in the cord, which then run up through the 
 
THE SPECIAL NERVOUS MECHANISMS 111 
 
 medulla and the inferior peduncles of the cerebellum to end in 
 the grey matter of the latter part of the brain. 
 
 But a large number of fibres entering the cord via the sensory 
 roots take a very different course. These do not form synapses 
 in the grey matter, but turn into the white matter at once, and 
 form the two great tracts of fibres represented in Fig. 28 as the 
 " great sensory tracts in the cord." These tracts end in the 
 medulla in four prominent nuclei that is to say, the fibres form 
 synapses with the nerve cells in these ganglia. The axons passing 
 out from the latter cells are collected together to form the two 
 great sensory tracts called the lemnisci, and the latter, after 
 crossing, as indicated in Fig. 28, run up into the corpora quadri- 
 gemina and optic thalami, and end by forming synapses with the 
 cells in those nuclei. There they come into relation with 
 nervous tracts passing out from the brain, and serving as the 
 avenues along which motor impulses go out to the muscles of the 
 body. So far, then, as we have studied it, the great sensory 
 tract from the cord to the brain is one which carries impulses 
 arising in the muscles and joints up into the mid-brain centres, 
 via the ganglia in the medulla and the lemnisci tracts, and which, 
 again, is mainly or solely a means of muscular co-ordination. 
 In order that consciousness may be affected by these impulses, 
 another link must be added to the chain of paths, and this is, 
 we shall see, the tract of fibres passing up from the mid-brain 
 to the cortex cerebri. 
 
 Lastly, impulses arising from the heat, cold, touch, and pain 
 receptors in the skin pass into the cord through fibres in the 
 sensory roots of the spinal nerves. These go into the grey 
 matter directly, and form synapses round the nerve cells there. 
 The axons of such cells go out again into the white matter of 
 the cord, but they do not form a continuous tract of fibres. 
 Instead of that, they run upwards only a short distance, turn 
 back into the grey matter, form other synapses, and then re-enter 
 the white matter to form another short tract. By a series of 
 such linkages the impulses reach the medulla, and then pass up 
 into the lower brain by a devious route. Finally, they become 
 connected with tracts passing up into the cortex, when they 
 undergo full development into psychical affections. 
 
 Thus the paths or tracts " mediating " general sensibility of 
 the limbs and trunk lead up through the sensory roots of the 
 spinal nerves and through the grey and white matter of the cord 
 

 in in 
 
 halami and 
 mid- brain 
 
 3-1 
 
 Cord and medulla 
 
 FIG. 31. DIAGRAM OF THE MAIN PATHS OF NERVOUS IMPULSES 
 FROM THE BODY INTO THE BRAIN. 
 
 Three spinal nerve fibres (I, II, III) are shown entering the cord. Each fibre divides 
 into three branches. 
 
 Left : Impulses resulting from general sensation pass up in the cord as shown, entering 
 into and again emerging from the grey matter ; theconnection isdirectly with the thulamua, 
 
 Centre: The paths taken ly impulses coming from the muscles and joints ; the COM 
 nection is with the thalamus, but indirectly through nuclei in the medulla. 
 
 lit : The paths taken by impulses coming from the muscles and joints, nnd entering 
 in the cerebellum, but also ascending to the cortex via the thalamus and returning to the 
 cord via the red nucleus. 
 
THE SPECIAL NERVOUS MECHANISMS 113 
 
 into the medulla. The greater number of the impulses passing 
 along these paths are of the nature of stimuli, which lead to 
 muscular responses only, and they are rearranged and co- 
 ordinated in the cerebellum and mid-brain, so that the actions 
 resulting from them are purposeful and appropriate. Some of 
 them, but apparently only a small fraction of the total, proceed 
 upwards to the cortex after passing through synapses in the 
 medulla and mid-brain, and give rise to the changes of conscious- 
 ness which we recognise as the sensations of pain, heat, cold, 
 touch, pressure, resistance, etc. 
 
 General Sensibility in the Head. The same kinds of general 
 receptors are found in the skin, the muscles, and the joints in 
 body, limbs, and head. But the impulses coming from the 
 region of the head and face enter the central nervous system 
 through the sensory fibres of the cranial nerves. Some of these 
 are nerves of special sensation, and we consider them separately. 
 Others (the 3rd, 4th, and 6th) are purely motor nerves, and 
 go to the muscles of the eyes. The remainder (that is, the 5th, 
 7th, 9th, and 10th) are mixed that is, they contain both sensory 
 and motor fibres and they may be compared with spinal nerves. 
 They, with the purely motor nerves, convey efferent impulses 
 from the medulla and mid-brain to the muscles of the eyes, face, 
 jaws, and neck, and they also carry afferent impulses, arising 
 as the result of the stimulation of the general receptors of those 
 regions, into nuclei in the medulla and lower parts of the mid- 
 brain. The internal paths in the brain are rather complicated, 
 and we cannot attempt to describe them here ; but there is the 
 same general scheme as in the case of the special nerves : the 
 afferent impulses coming from the face and head go up to the 
 mid-brain, and through this to the cortex, on the one hand, and 
 into the cerebellum on the other. 
 
 The Sensation of Smell. This stands quite apart from all the 
 other sensory mechanisms in that the connection of the olfactory 
 organ is with the cortex direct, and not with the medulla and 
 mid-brain. The organs of smell and vision and hearing have 
 also a different embryogenic origin from that of the other sense 
 organs in that they arise as parts of the brain, which become 
 pushed out, so to speak, and come into relation with other parts 
 arising from the embryonic integument. The auditory organs 
 originate in this way from the hind-brain vesicle, the visual 
 organs from the fore-brain vesicle, but the olfactory organs 
 
 8 
 
114 THE MECHANISM OF LIFE 
 
 arise as outgrowths from the lateral fore-brain vesicles that 
 become, later on, the cerebral hemispheres. Therefore the 
 connection of the nerve terminations in the mucous membranes 
 of the nose are with the cortex cerebri direct, and this may be 
 the reason why the sensation of smell is said to be more 
 " reminiscent " than those of vision or hearing: it sets up more 
 immediate " associations,-' because of its place of entrance into 
 the higher brain. Less, however, is known about the internal 
 tracts along which the olfactory impulses travel in man than 
 in the lower animals, because of the degeneracy of the human 
 organ of smell in comparison with that of most other vertebrate 
 animals. In some fishes, for instance, the great development 
 of the olfactory organs and their nervous tracts suggests that 
 the sensation in question plays a very important part in the 
 general behaviour of these animals. 
 
 The Auditory Organs. Two entirely different sense organs 
 are contained in the " internal ear." " Vibrations of sound " 
 that is, very rapid, periodically repeated alternations of com- 
 pression and rarefaction of the air are made to impinge on the 
 tympanic membranes, or " drums of the ear," and then, by 
 means of the chain of little bones, called the auditory ossicles, 
 these vibratory movements of the drums are transmitted to 
 the fluid contained in peculiarly shaped cavities in bones on each 
 side of the head. Two organs are contained in this cavity, or 
 " bony labyrinth " the organ of equilibrium, and that of true 
 hearing. The former consists of a little membranous sac from 
 which proceed three semicircular canals, which are arranged in 
 the three planes to which we refer all positions in abstract space. 
 That is, one canal is vertical and runs forward and backward, 
 another is vertical and runs from side to side at right angles to 
 the first one, while the third is horizontal and is at right angles 
 to the first and second. The dendrites of one branch of the 
 eighth or auditory nerve project into the fluid contained in the 
 bases of the semicircular canals. Operative interference inj 
 many animals and diseased conditions in man show that destruc- 
 tion of one or other of the canals produces well-marked abnor- 
 malities in locomotion, apparent giddiness, and lack of co- 
 ordination over the muscles of the limbs. The " vestibular " 
 part of the ear is therefore an organ of equilibrium, and its nerve 
 terminates in a nucleus in the medulla, which is connected in 
 ways that are not well known with the mid- brain on the one 
 
THE SPECIAL NERVOUS MECHANISMS 
 
 115 
 
 hand, and the cerebellum on the other. There is, however, no 
 evidence that it has any connections with the cortex. 
 
 The organ of hearing, called the " cochlea," is essentially a 
 long tube bent on itself like a hairpin, and twisted round spirally 
 like the shell of a periwinkle. Within this tube is a fluid which 
 bathes a very peculiar and complicated structure called the 
 organ of Corti. The fibres of the true auditory or cochlear nerve 
 terminate in cells in the organ of Corti, and delicate auditory 
 hairs project out from these cells into the fluid. When the 
 latter is set in vibration, the auditory hairs are stimulated 
 in such a way that 
 afferent impulses are 
 transmitted along the 
 fibres of the nerve into 
 two nuclei in the medulla, 
 where they are received 
 by synapses. The axons 
 of the cells in these nuclei 
 are now gathered up into 
 certain internal tracts, 
 which pass up along the 
 lemnisci (see Fig. 28) into 
 the mid - brain. Other 
 fibres go to the cerebel- 
 lum. From the cells in the 
 mid-brain round which 
 the axons of the lemnisci 
 end other axons go up, 
 along a very obvious tract, 
 called the auditory radia- 
 tions, into the sensory 
 region of the cortex. 
 
 The Organs of Vision. 
 
 We have already said 
 something about the general structure of the eyes. The essential 
 elements are the cells of the retina, which, we have seen, are 
 arranged in layers. Light is received by either the rods and 
 cones, and these are prolonged into axons which form synapses 
 with other " bipolar" cells, the axons of which form other synapses 
 with third, " ganglionic " cells, the axons of which pass out of the 
 retina into the optic nerve. The latter then run up towards the 
 
 Corpora. 
 CjjUadrijgemina. 
 
 Thalamus 
 
 Optic 
 
 " radiations 
 
 FIG. 32. THE CONNECTIONS OF THE 
 RETINAS WITH THE BRAIN. 
 
 Only the tracts on one side are shown. 
 The section of the brain represented dia- 
 gratnmatically passes rather obliquely 
 through the eyes. 
 
116 THE MECHANISM OF LIFE 
 
 mid-brain as the two optic tracts, which then partially cross 
 each other in the middle line of the head. The further course of 
 the optic tracts in the brain is now fairly well known. 
 
 Fig. 32 represents these tracts in a very schematic way. Some 
 of the fibres of the right optic nerve pass over to the left-hand 
 side of the brain, while some keep on the same side (and vice 
 versa). Then all the optic fibres terminate by forming synapses 
 with the cells in three nuclei contained in the optic thai ami and 
 corpora quadrigemina. That is their lower-brain termination, 
 .but a very obvious tract of other fibres, the optic radiations, 
 start off as the axons of cells in the lower visual centres or nuclei, 
 and proceed up to the cortex cerebri. 
 
 Audition and vision are complex sensations. When we hear 
 we distinguish loudness (that is, the amplitude or " intensity " 
 of the sound waves), pitch (which is the frequency of occurrence 
 of the sound waves), and musical quality (which we explain by 
 assuming that the sound waves are complex, and can be de- 
 composed into components). Similarly, in vision we distinguish 
 between intensity of light and quality of light (or colour). By 
 intensity we mean the amplitude of the vibrations of the medium 
 (ether) which transmits that which we recognise as light, and 
 by colour we mean the components of this mixed light as they 
 are separated from each other by the physical media outside the 
 retina itself. How the analyses of sound and visual stimuli are 
 carried out by the auditory and visual organs is, of course, far 
 from being understood, and we cannot discuss the question now. 
 
 So much, then, for a very summary consideration of the ways 
 by which the stimuli of the receptor organs of the body are 
 transmitted into the central nervous system and brought to 
 bear upon the various ganglia there. We must next consider 
 
 The Motor Mechanisms. 
 
 From what has been said in Chapter II. the reader will already 
 know that a motor mechanism includes (1) an afferent nervous 
 path leading into the central nervous system; (2) a nucleus 01 
 ganglion; and (3) an efferent nervous path leading out from the 
 ganglion to the motor organ. Thus we take the case of a pureh 
 special mechanism. 
 
 Some receptor organ is stimulated (say a touch spot in th< 
 skin), and an afferent impulse is set up and propagated along t 
 spinal nerve into the grey matter of the cord. This impulse 
 
THE SPECIAL NERVOUS MECHANISMS 
 
 117 
 
 enters the latter via the dorsal or sensory root. In the grey 
 matter it is received by the dendrites of a nerve cell, and is then 
 transferred (after something has been done with it) to the axon 
 of the cell. This axon issues from the grey matter and passes 
 out from the cord through the ventral or motor root of the same, 
 or a different, spinal nerve, and is transmitted by the latter as 
 an efferent impulse to a muscle which it then sets in motion. 
 
 This is merely a scheme illustrating the simplest conceivable 
 form of nervous motor apparatus. It may be regarded as the unit 
 
 
 FIG. 33. DIAGRAM OP THE SIMPLEST POSSIBLE SPDSTAL REFLEX 
 MECHANISM. 
 
 An afferent fibre is represented as entering the cord, and ending in a synapse 
 round a cell, which then sends out an axon to a muscle fibre. Through 
 an additional cell and synapse another axon goes out to the antago- 
 nistic muscle. The + sign indicates that one muscle contracts, and 
 the sign that the antagonistic one relaxes. 
 
 or element of the sensori-motor mechanism, but the reader must be 
 very clear in his mind that the very simplest actual nerve-muscle 
 mechanism is very much more complicated than Fig. 33 suggests. 
 So we may now proceed to elaborate it till it approximates to 
 the conditions that may be studied experimentally. First, then, 
 one muscle never, by itself, constitutes a motor mechanism. 
 For there are always two antagonistic organs, one of which 
 contracts while the other simultaneously relaxes. Secondly, the 
 nerve that supplies a muscle always contains fibres that carry 
 
118 THE MECHANISM OF LIFE 
 
 efferent or motor impulses from the nucleus (or ganglion) to the 
 muscle, and also fibres that convey afferent or sensory impulses 
 from the muscle to the nucleus. Thirdly, the stimulus that 
 starts the nervous impulse and excites the muscular organs to 
 activity nearly always originates in a different segment of the 
 body from that containing the former, so that there must 
 generally be a path, or tract, in the central nervous system itself 
 along which the impulse travels. Lastly, there are hardly ever 
 only two antagonistic muscles concerned in a movement, but 
 rather a muscle system consisting of several or many such pairs. 
 Thus the simplest actually observable action that can occur in 
 the body of a higher animal includes a rather complicated 
 mechanism, and the complexity of this becomes all the greater 
 when we take account of the connections of the nucleus immedi- 
 ately controlling the action with the higher brain centres. When 
 such connections exist we have the possibility that the action 
 may be modifiable to almost any degree by the volition of the 
 animal, or by its " experience." 
 
 Leaving aside, in the meantime, the factors of volition and 
 experience, we may consider the action as it is performed in an 
 automatic, mechanical manner. Let it be that which may occur 
 when the side of the spinal dog* is tickled and the " scratching 
 reflex " occurs (see pp. 119, 137). 
 
 Fig. 34 represents nervous mechanisms that are involved in 
 this 'action. These mechanisms (to judge from the figure) appear 
 to be somewhat complicated, and yet we have taken account 
 only of those which must be in action, and we have omitted 
 others that are less essential to our present explanation. 
 
 The stimulation, then, of the touch organs in the skin of the 
 body sets up impulses that are transmitted to the spinal cord 
 via the afferent fibres of a spinal nerve. Now this segment of 
 the cord is well in front of that from which are given off the 
 motor nerves supplying the hind-limb, and so there must be a 
 nervous tract connecting centres (a) and (6). The afferent 
 impulse, after being received by the synapses of the grey matter 
 in centre (a), is modified in some way, is retransmitted along 
 a tract of fibres in the white matter of the cord, and is received 
 by motor nerve cells in segment (6). We must think about 
 these motor nerve cells as being arranged in some way or other 
 
 * A " spinal animal " is one in which the brain has either been destroyed 
 altogether (by operation) or has been separated from the spinal cord. 
 Thus the muscles of the body are entirely controlled by the cord. 
 
THE SPECIAL NERVOUS MECHANISMS 
 
 119 
 
 in pairs, some of them being the cells of axons that go to the 
 flexor muscles, while others are the cells of axons that go to the 
 extensors. Now the same impulse breaks upon both these series 
 of axons, but it causes, at the same time, the flexor to contract 
 and the (antagonistic) extensor to relax, thus bending the limb. 
 But the muscle fibres contain receptors which are stimulated 
 by the acts of 
 contraction and 
 relaxation, and 
 thus set up 
 impulses that 
 
 arc propagated 
 
 >/>/' na / 
 
 S final 
 
 B 
 
 along the affer- 
 ent nerve fibres 
 coming from the 
 muscle. These 
 afferent fibres 
 pass into the 
 grey matter of 
 the cord, and 
 are received by 
 nerve cells there. 
 From the latter 
 cells axons pass 
 off and enter in- 
 to synapses with 
 the motor cells 
 which control 
 the movements 
 of the muscles. 
 
 Thus the mus- 
 cular apparatus 
 responds to a 
 stimulus which 
 enters the central nervous system at a place removed some 
 distance from that place from which the motor nerves emerge, 
 so that there are internal paths of communication in the cord. 
 Also the contracting and relaxing muscles " advise " the motor 
 centres (via their own receptors and afferent nerves) what they 
 are doing, so to speak, so that the entire series of actions may 
 be adjusted, or controlled, or accelerated, or inhibited, while 
 they are still in progress. 
 
 -^x Tens or muscle 
 *- -Relaxor muscle 
 
 FIG. 34. THE MAIN MECHANISMS INVOLVED IN THE 
 SCRATCHING REFLEX OP THE DOG. 
 
 An afferent fibre enters the cord, and is connected by 
 various cells, their synapses and axons with two 
 motor cells in a lower segment. The latter send 
 out axons to the two antagonistic muscles, and 
 the muscles send up afferent fibres which come 
 into connection, via synapses, with the motor cells. 
 
120 THE MECHANISM OF LIFE 
 
 Now add to all this (1) a new series of connections between 
 the nerve cells of the grey matter in segment (6) and the cere- 
 bellum, and note that there must be two sets of paths, to and 
 from the cerebellum, included in these connections. Add (2) a 
 series of connections between the segment (a) and the mid-brain, 
 also to and from paths ; these are, of course, the tracts described 
 on pp. 99-104 and in Figs. 28 and 31. Add, finally, (3), the con- 
 nections between the mid-brain and the cortex on the cne hand, 
 and the mid-brain and the cerebellum on the other these also 
 being to and from paths and the reader may get some idea of 
 the complexity of the machinery of nerve cells and nerve tracts 
 that are involved in a relatively simple action when it is the 
 object of control by will or experience. 
 
 The " Simple " Reflex Mechanism. 
 
 The schematic simple reflex mechanism, such as it is figured 
 in p. 117, is to be regarded as a " fiction" in the conventional, 
 legal sense that is to say, it is a " scheme " which is useful in 
 enabling one to understand the maze of nervous tracts and paths 
 along which impulses travel within the nervous system. If we 
 could isolate all the afferent fibres connecting one group of 
 nerve cells in the spinal cord with one group of receptor organs 
 in the skin, these would represent the afferent path of a simple 
 reflex. Then we should have to isolate all the nerve fibres . 
 connecting the same group of nerve cells with some one muscle, 
 and that would be the efferent path. So we should obtain our 
 schematic " reflex arc." 
 
 Nothing like this exists in the higher animal. The afferent 
 impulses originating in any one small group of receptors do not 
 go to one segment only in the spinal cord, but to several such. 
 Each of these segments is connected with the same muscle, for 
 the fibres going out from the cord through one motor root and 
 ending in one muscle are derived from several segments. Thus 
 one efferent and one peripheral afferent path are connected 
 together by various nuclei, and these nuclei are joined by several 
 intraspinal paths. 
 
 Then we have the further complications indicated in Fig. 34. 
 The schematic single efferent paths are really double ones, each 
 of them including a nerve branch going to a muscle and to its 
 antagonist. The same impulse which breaks upon the synapse 
 in the nucleus stimulates the nerve going to the muscle, so that 
 
THE SPECIAL NERVOUS MECHANISMS 121 
 
 the latter contracts, and it also stimulates the nerve going to the 
 antagonist, so that it relaxes. These two antagonistic effects 
 must be regarded as the single functional muscular action 
 that is, their effect is, say, to bend or extend a limb. 
 
 But the movement of the limb is something that varies to 
 a very great degree: the part may be completely or partially 
 bent (or extended), and it may bend or extend against resistances 
 which also vary immensely. The duration of the movement 
 therefore varies, and so also does the quantity of energy trans- 
 formed more or less work is done according to the circum- 
 stances under which the action is performed. To some extent 
 all this regulation is carried out in the nucleus from which the 
 efferent impulses proceed out to the muscle, but it is also the 
 work of the contracting (or relaxing) muscle itself. There are 
 receptors in the latter which are affected or stimulated by the 
 events taking place in it, and by the circumstances of the action 
 the load, for instance, borne by the contracting muscle. These 
 receptors generate impulses which ascend into the nucleus and 
 modify, if need be, the efferent impulses sent out by the latter. 
 A mechanical analogy may make this very important function 
 
 j of the muscle proprioceptors clear. Let us think about a small 
 railway system in which the movements of every train are 
 initiated and regulated by operators working in a central 
 telegraphic control station. But as every train proceeds on 
 its journey, moves from block to block, stops at and starts from 
 
 . a station, it records its position telegraphically (and perhaps auto- 
 matically) on a time and space chart in the control office, so that 
 the operator may see at each movement where it actually is. That 
 would represent the system of efferent nerves going from the 
 spinal cord nucleus to the muscular apparatus, and the receptors 
 of the latter, with their afferent nerves, going back into the nucleus. 
 These, then, are the mechanisms concerned in the simplest 
 spinal reflex arc: several cord segments in receipt of afferent 
 impulses from the skin, and all of them in connection with the 
 same small group of antagonistic muscles and a system of re- 
 ceptors in those muscles in connection with the nuclei from 
 which the efferent impulses start. 
 
 The Cranio-Spinal Reflexes. 
 
 The muscular apparatus concerned in movements are, we 
 have seen, under the immediate control of ganglia or nuclei in the 
 spinal cord, or (what are very similar) the ganglia in the medulla 
 
122 THE MECHANISM OF LIFE 
 
 from which the motor fibres of the cranial nerves issue. Keflexes, 
 or purposeful and useful adaptive movements, may be carried 
 out under such control, and apart altogether from the activities 
 of the brain, although there are probably very few actions into 
 which some amount of brain control does not enter. So far we 
 have supposed that the stimulus to an action is some change in 
 the environment acting on receptor organs situated in the skin, 
 but in the intact, normal animal it is far more likely that the 
 
 Mfd Infra- cerebral 
 bram ^/pafh, cerebellum 
 
 to rnid-u 
 Cerebellum 
 
 -Afferent ftaih. 
 
 Cord To cerebellum 
 
 -Afferent f}a1h, 
 cord To lhalamus 
 
 and m id- bra in 
 
 ---Efferent fait,, 
 mid- brain fd 
 Cord 
 
 FIG. 35. AFFERENT IMPULSES FROM THE SKIN ARE SHOWN GOING UP 
 INTO THE THALAMUS, WHERE VISUAL IMPULSES ARE ALSO RECEIVED. 
 
 Other afferent impulses enter the cerebellum from the muscles. From 
 the cereballum impulses pass to the mid-brain, and from there motor 
 impulses pass down through the cord, and to the muscles concerned 
 in locomotion. 
 
 receptors stimulated will be one or several of the great cranial 
 organs of special sense. Thus an animal usually responds to 
 something which it sees, or hears, or smells, and so we must 
 include among the afferent paths that take part in the reflexes, 
 or other actions, those going from the visual, auditory, and 
 olfactory organs into the mid-brain. Yet the impulses actually 
 setting the muscles in action must issue from nuclei in the medulla 
 and spinal cord, and must go out to the muscles via the motor 
 
THE SPECIAL NERVOUS MECHANISMS 123 
 
 roots of the cranial and spinal nerves. That means, obviously, 
 that there must be paths, confined to the central nervous system 
 itself, along which impulses may travel from the nuclei of special 
 sense in the mid-brain to the motor nuclei in the medulla and 
 cord. The principal intracerebral and intraspinal paths, as 
 well as those which pass from cord to brain, and vice versa, are 
 represented in Fig. 28. Therefore our " simple " reflex, or other 
 action, will (in the intact, normal animal) include all the paths, 
 or analogous ones, represented in Fig. 33, and also another 
 series of paths between the organs of special sense and the mid- 
 brain, and between the latter and the medullary and spinal 
 motor nuclei. Thus the main paths in use in the case of a man 
 walking in a crowded street and " mechanically " avoiding other 
 people must be somewhat as indicated in Fig. 35. 
 
 Here we do not represent the afferent paths between the 
 acting muscles and the nuclei in the cord, and, of course, no 
 details of the very imperfectly known paths between the mid- 
 brain and spinal centres. 
 
 The Mechanism of Co-ordination. 
 
 Nor do we ever suggest the all-important apparatus of co- 
 ordination. In such a complex series of actions as those involved 
 in walking a very great number of muscular systems are at work. 
 Practically all the muscles of the legs are active and immediately 
 concerned in the production of the movements, but the body is 
 also carried erect and balanced, and this is the work of antag- 
 onistic muscle systems belonging to the trunk. The arms swing 
 about. At each step, and with every deviation from a straight 
 line, the centre of gravity of the whole body changes its position. 
 Practically every nucleus, or ganglion, in the whole spinal cord 
 must be concerned in the generation of the impulses going out 
 along the efferent nerves to the muscles, and in receiving the 
 afferent impulses generated in the acting muscles and joints, and 
 giving information from instant to instant as to what is going 
 on there. Now it is unlikely that the relatively simple reflex 
 mechanisms constituted by the sensory and other afferent nerves 
 entering into synapses with the motor nerves is competent for 
 this work of co-ordination, and there is much evidence that an 
 additional nervous apparatus is involved. 
 
 This additional mechanism is the cerebellum and its connec- 
 tions. We have seen that there are tracts of nerve fibres passing 
 
124 
 
 THE MECHANISM OF LIFE 
 
 T halo. us 
 From cortex 
 
 through the superior peduncles and putting the cerebellum in 
 communication with the mid-brain, and so with the organs of 
 
 special sense. The 
 vestibular part 
 of the auditory 
 organ that is, 
 the part which 
 is known experi- 
 mentally to be 
 concerned in the 
 maintenance of 
 equilibrium and 
 posture of the 
 body has also 
 direct nervous 
 connection with 
 the cerebellum. 
 The receptors in 
 the deeper 
 muscles and 
 joints are con- 
 nected with the 
 cerebellum by 
 special tracts (see 
 Fig. 31), and 
 there are also 
 descending tracts 
 connecting this 
 part of the brain 
 with the motor 
 nuclei of the 
 spinal cord. Even 
 the nuclei of the 
 cranial nerves are 
 
 eereb'ellvm 
 ho cord. 
 
 Skin 
 
 FIG. 36. THE CONNECTIONS OF THE CEREBELLUM. 
 
 Impulses pass up from the skin, muscles, joints, etc., 
 into the cerebellum, via I, the inferior, and S, the 
 superior peduncles. From the cerebellum impulses connected with 
 pass to the thalami and red nuclei, and from the 
 latter efferent impulses pass down into the cord. 
 The cerebellum is also connected, via M, the middle 
 
 the cerebellum. 
 It is known 
 
 peduncle, with the cortex, and in other ways, not also, from direct 
 
 shown in the diagram, with the organsof special sense. 
 
 experiments, that 
 
 injuries to, or even removal of, the cerebellum leads to no obvious 
 impairment of sensation, and yet the organ has most conspicuous 
 
THE SPECIAL NERVOUS MECHANISMS 125 
 
 I and direct connections with almost all kinds of receptors. But 
 I such injuries or operative interference do produce serious motor 
 disturbances, so that ordinary, automatic, or customary move- 
 I ments, such as those of gait, locomotion, and posture, are greatly 
 I affected, and the animal when walking, running, swimming, or 
 flying, behaves in an ineffective, incompetent manner. What- 
 ever, then, are the functions of the cerebellum, it is very plain 
 that they must be of considerable importance and are very 
 complex a deduction from the peculiar and most intricate 
 structure of the grey matter of this organ. The general con- 
 clusion attained by the study of all these lines of evidence is that 
 the work of co-ordination of movements performed customarily 
 ! and automatically is carried out in the cerebellum. Impulses 
 arising in the sense receptors are conveyed to their nuclei in 
 the spinal cord and brain, or they may be conveyed directly 
 into the cerebellum, and if the direct path does not exist, there 
 is one connecting the immediate nucleus of the afferent impulses 
 with the latter part of the brain. That is to say, the cerebellum 
 is the recipient of impulses coming from the receptor organs, 
 and particularly from those which have to do with equilibration 
 and those others which come from the acting muscles themselves 
 and from the joints (where the effort involved in the movements 
 of the limbs must be particularly felt). On the other hand, the 
 cerebellum also stands in close connection with the nuclei which 
 give origin to impulses going out to the muscles of the limbs and 
 body. There is, therefore, a mechanism whereby those complex 
 movements which are the means of locomotion may be timed, 
 adjusted, and co-ordinated, and this we suppose to be situated 
 in the grey matter of the cerebellum. Of the nature of the 
 mechanism we have not even a suggestion. 
 
 Cortical Control. 
 
 We shall consider in the next chapter the experimental 
 evidence on which our knowledge of the functions of the cortex 
 cerebri is based, and in the meantime we deal only with the 
 nervous mechanisms themselves, so far as these have become 
 known by anatomical research and experiment. The greater 
 part of each cerebral hemisphere, then, is a core of nerve fibres 
 making such connections as are indicated in Figs. 28 and 29 
 connections, that is, between the various parts of the cortex 
 itself and between it and the other nuclei in the lower brain, 
 cerebellum, and spinal cord. With the exception of the corpora 
 
126 
 
 THE MECHANISM OF LIFE 
 
 striata, the great nuclei of the cerebral hemispheres are situated 
 in the thin sheet of cortical grey matter, and it is the cells of 
 this that are connected with the underlying core of fibres. The 
 latter are either the axons of these cells (in which case they carry 
 impulses out from the cortex) or they end in forming synapses with 
 the cortical cells (in which case they carry impulses to the cortex). 
 Fig. 37 represents, on the same scale, a " pyramidal " cortical 
 cell from the frog's cortex and one from man; the actual cell 
 bodies are very much the same in the two cases, but the dendrites 
 and axons are very much more complex in the human than in 
 
 Principal 
 axon 
 
 
 FIG. 37. Two CORTICAL (PYRAMIDAL) CELLS FROM THE BRAIN OF MAN 
 AND THE FROG. HIGHLY MAGNIFIED. 
 
 the amphibian cortex. Each cell is pyramidal in general form 
 (as it can be seen in section), and from the apex and lower parts 
 there start off a great number of fine fibres which branch in a 
 complicated way and have a structure peculiar to the cortex. 
 From the middle point of the base of the cell there issues a 
 prominent fibre, which is the axon of the cell, and branches pass 
 off laterally from the axon and again branch. These are the 
 " collateral " axons of the cell. Soon after leaving the latter 
 the axons acquire medullary sheaths (see p. 26), and become 
 the commissural or projection cortical fibres represented in 
 
THE SPECIAL NERVOUS MECHANISMS 127 
 
 Fig. 28. Other fibres, commissural or projection ones, enter 
 the part of the cortex in question and break up into terminal 
 dendrites, which then come into relation with the apical and 
 basal dendrites of the cell by means of synapses. 
 
 Thus each cell in the cortex can make a very great number of 
 connections with other cortical cells, or with nerve cells in other 
 parts of the central nervous system. Each of the collateral 
 axons, for instance, can become a commissural or projection 
 fibre, and a number of fibres coming from anywhere else can form 
 synapses with the dendrites of any cortical cell. This is why 
 the latter are so much more crowded in the canine, and still 
 more so in the amphibian, than in the human cortex : there is the 
 increasing tendency, as we raise in the scale of evolution, for the 
 connections between the cells to become more and more numerous 
 and complex, so that the number of paths which an impulse 
 leaving a pyramidal cell may take tends always to increase. 
 
 The higher is tJie type of brain, the more manifold are the ways 
 in which the various centres may communicate with each other. 
 
 Localisation o! Function in the Cortex. We must think of 
 the activity of the cortex cerebri in a twofold way: it is one 
 organ in so far as every part of it is connected with every other 
 part, and it is multiple inasmuch as functions are specialised 
 or localised in it. This specialisation has become known to us, 
 partly by experiment upon the brains of the higher mammals 
 (other than man), partly by the extension of these results to the 
 human brain (which is, of course, very similar in structure to 
 that of the anthropoid apes), and partly by observations made 
 on human subjects suffering from disease and accidents. 
 
 Thus it is possible to distinguish in the human cortex a " motor 
 region," which is concerned with the initiation and elaboration of 
 willed movements; a " sensory region," upon which is dependent 
 the full psychical development of the sensations arising from 
 stimulation of the receptor organs; and a " prefrontal region," 
 about the functions of which we have very little positive knowledge. 
 
 Fig. 38 represents the approximate positions and boundaries 
 of the regions, and the particular areas into which the latter are 
 divided. The two great fissures those of Rolando and Sylvius 
 serve as lines which enable us to divide up the cortex into these 
 regions and areas. One hemisphere (the left one) is seen from 
 the side, but the reader must understand that the cortex dips 
 down into the great median fissure that separates right and left 
 
128 
 
 THE MECHANISM OF LIFE 
 
 hemispheres, as well as into the Rolandic and Sylvian fissures, so ' 
 that such a view as that of Fig. 38 does not show its entire area. 
 Round about, but mainly in front and in the depths of the 
 fissure of Rolando, is the motor region, and this has been divided 
 up into a number of particular areas, each of which controls 
 the movements of some small part of the body. Thus, directly 
 in front of the fissure are the " centres " or " areas " for the 
 movements of the trunk, arm, hand, head and eyes, face, etc. 
 These areas are not very well defined- (for instance, they do not 
 coincide exactly with the areas of the various " gyri " or con- 
 volutions), and they overlap to some extent. When any one of 
 them is stimulated in a suitable manner, usually by application of 
 
 -Muscular 
 rum 
 
 
 impr* 
 
 FIG. 38. DIAGRAM OF THE PRINCIPAL CORTICAL CENTRES IN THE LEFT 
 HUMAN CEREBRAL HEMISPHERE. 
 
 a feeble electric current, certain groups of muscles twitch or con- 
 tract, and this is the evidence that the area in question controls 
 these parts. The movements that are so elicited are not the finished 
 ones that occur in the normal behaviour of the animal, but they 
 indicate none the less the nature of the nervous controls and paths. 
 The cortex is, as we have seen, a thin sheet of grey matter 
 overlying the core of nerve fibres that forms the greater part of 
 the mass of the hemispheres. Directly beneath it we come 
 upon the white matter, and much of this consists of the axons 
 proceeding from the overlying pyramidal cells of the cortex. 
 Tracing these fibres by various means, we find that they run 
 down into the crura or cerebral peduncles towards the medulla. 
 There, as Fig. 28 shows, these " pyramidal tracts " mainly cross 
 each other, that one coming from the left-hand side of the brain 
 
THE SPECIAL NEKVOUS MECHANISMS 129 
 
 going over to the right-hand side of the medulla, and vice versa. 
 The pyramidal tracts are then continued down the spinal cord, 
 and there their fibres enter into the grey matter and form 
 synapses with the nerve cells which give origin to the axons 
 that go out into the ventral or motor roots of the spinal nerves. 
 
 Thus the grey matter of the motor region of the cortex is, on 
 the one hand, in connection with fibres that run uninterruptedly 
 down into the spinal cord and control the nervous centres there 
 which set the muscles of the body and limbs in action. On the 
 other hand, it is in connection with the centres in the other parts 
 of the cortex which are the termini of the sensory paths. 
 
 But it is also in connection with the cerebellum via another 
 specialised cortical region. Figs. 28 and 36 show a prominent 
 tract of fibres descending from the cortex and ending in the 
 region of the pons varolii that is, the middle part of the middle 
 peduncles which unite together the two halves of the cerebellum. 
 Thus our cortical motor cells have communication with the 
 nervous mechanism of co-ordination. 
 
 And just in the same way as the pyramidal tracts connect 
 together the cortical and spinal nuclei, so do other more com- 
 plicated paths connect the motor cortex with the medullary 
 nuclei which give origin to the nerves that supply the muscles 
 of the head and face. Thus every part of the motor system of 
 the whole body is under the control of the cortex cerebri. This 
 is particularly the case in man. In the fishes the great pyramidal 
 tracts are hardly to be recognised, and they become developed 
 to a progressively greater extent as we ascend the scale of evolu- 
 tion represented by the dog, monkeys, anthropoid apes, and 
 man. In the lower vertebrates the nuclei which mediate between 
 the organs of sense and the spinal ganglia are those in the mid- 
 brain, but in the higher mammals this nervous mechanism becomes 
 more and more replaced by the cortex and its connections. 
 
 Almost all the cortex behind the motor area is, as Fig. 38 
 indicates, the seat of sensory functions. Those centres called 
 " visual," " auditory," " music," etc., contain pyramidal and 
 other cells, the dendrites of which form synapses with the tracts 
 of fibres coming up from the mid-brain nuclei in which the 
 nerves coming from the organs of sense terminate. Stimulation 
 of these cortical sensory centres is certainly essential to the 
 development into full consciousness of the impulses arising from 
 the stimulation of the sense organs. This is a matter to which 
 we return in the following chapter. 
 
 9 
 
CHAPTER VIII 
 THE ANALYSIS OF BEHAVIOUR 
 
 OUR analysis of the nervous system has, so far, been that of 
 a very complicated structure built up of superposed reflex arcs. 
 A reflex arc is the series of nervous parts that connect together 
 a receptor and an effector organ; thus the arc that is functional 
 in the simple act of winking connects the retina of the eye with 
 the visual centre in the mid-brain (by the optic nerve), the 
 visual centre with the oculo-motor centre (by an intracerebral 
 tract), and the oculo-motor centre with the muscles of the eye- 
 lids (by the oculo-motor and facial nerves). When an action 
 of any kind occurs, such a reflex arc or arcs become functional. 
 The receptor organ receives a stimulus that is, something 
 happens outside the body: a flash of light, a noise, a current of 
 air carrying odoriferous particles in suspension, etc. and this 
 event causes some change in the receptor organ. The nerve 
 terminations in the retina, the internal ear, or the mucous 
 membrane of the nose are thus stimulated, and they initiate a 
 nervous impulse which is propagated along the sensory nerve 
 to the centres of the brain. 
 
 There the afferent nervous impulse breaks upon a series of 
 synapses, and so enters a number of nerve cells that constitute 
 the centre (or nucleus, or ganglion). Something now happens 
 to the impulse in these cells, and its physical nature is doubtless 
 changed in some way. At any rate it is transferred from the 
 cells to the axons which come from the latter. Those axons 
 constitute a nervous tract leading to another nucleus, where 
 there is also a series of synapses. The impulse, after further 
 modification in the cells of the second nucleus, is now transferred 
 to an efferent nervous tract which is constituted by the axons 
 passing out from those cells. After traversing this third nervous 
 tract (or effector nerve), the impulse is received by the effector 
 organ. Let us suppose that this is a muscle. The latter there- 
 upon contracts or relaxes. 
 
 This is a scheme of what happens whenever a bodily action 
 occurs ; a stimulus is initiated and propagated along an afferent 
 
 130 
 
THE ANALYSIS OF BEHAVIOUR 
 
 131 
 
 nerve into the central nervous system. It is there prolonged through 
 an efferent nerve into the effector organ, where it releases energy. 
 
 Now we may try to make a " mechanical model " of this train 
 of events, premising that whatever it may be that occurs in the 
 actual nervous arc it is sure to be (physically) very different 
 from the things that happen when our model is set in action, 
 but that the energy relations in the actual muscle-nerve structures 
 and the parts of the model will be the same. Suppose, then, that 
 a number of billiard balls, a to g, are suspended by threads 
 so that they almost touch each other; let another ball, h, be 
 poised on the end of a tipless cue. Let the end ball, a. be pulled 
 a little to one side, and then let go so that it hits b very gently. 
 Further, we may assume (as it is generally assumed when logical 
 hypotheses or mechanical models are made) that the balls are 
 perfectly elastic (which they 
 nearly are), and that there is 
 no friction in their suspensions. 
 
 The ball a will then com- 
 municate momentum to b, and OO OO Q O O 
 cause b to hit c, c to hit d, and f 6 d C b 
 so on; c will communicate as 
 much energy to d as it received 
 from b, and a wave of 
 mechanical displacement will 
 travel alqng the row. The ball g will hit h with as much force 
 in the blow as a has hit b, and the blow will cause h to roll off 
 its perch and drop to the floor. 
 
 Suppose still further that a has been lifted up J inch; that 
 the weight of each ball is J pound; and that the height of h 
 above the floor is 6 feet. Now, in falling through J inch a does 
 work equal to its weight X the distance through which it falls 
 that is, oVx|=T2 foot-pound. The quantity of energy so 
 represented is " propagated " along the row of balls and com- 
 municated to h, and is just sufficient to push the latter off the 
 end of the cue jon which it is poised. It then falls 6 feet, and 
 so does work equal to 6xJ=2 foot-pounds. Prior to being 
 displaced it had this quantity of potential energy due to its 
 having been lifted from the floor and put in such a position that 
 it was free to fall 6 feet. A small quantity of energy representing 
 T-V foot-pound of work can therefore be propagated without loss, 
 and can release a much larger quantity that is, 2 foot-pounds. 
 
 FIG. 39. 
 
132 THE MECHANISM OF LIFE 
 
 What we know about the passage of a nervous impulse along 
 a nerve suggests that the former passes without any of the 
 substance of the nerve being used up. Arriving at the first 
 synapses, it passes into the nerve cells there and transforms into 
 the same quantity of energy, which then passes along the intra-j 
 cerebral tract into the second synapses and cells, where another 
 equal quantity of energy is transformed and is propagated along 
 the efferent nerve (again without loss), and thrown into the! 
 muscle. But there it releases a much greater quantity of energy, I 
 which is represented by the force with which the muscle con- 
 tracts or relaxes. The latter contained energy in the potential 
 (chemical) form, and the minute quantity entering it as a nervous 
 impulse sets free or transforms this chemical energy in the same 
 way as a small electric current can release (or fire) the huge 
 quantity of energy contained in an explosive charge. 
 
 Now, from the point of view that we have taken so far, every- i 
 thing that happens in the sensori-motor system of an animal 
 conforms to the structural scheme of reflex arcs and to the 
 dynamical scheme of the billiard-ball model. The peripheral 
 and central nervous system is a means whereby the energy 
 received by the stimulation of the receptor organs is transmitted 
 as a nervous impulse to the nerve centres, and is there trans- 
 formed into another kind of impulse which is transmitted to an 
 effector organ, where, finally, potential chemical energy is 
 released, and muscles contract and relax or glano^s secrete. 
 The impulse which results from the stimulation of a sense organ 
 goes to the central nervous system, but there it may pass along 
 one or more of a great number of different paths. Upon the 
 path it takes depends its effect; thus the stimulation of the 
 retina may lead to a reflex act of winking, or the " mouth may 
 water," or the man may start violently, or run, or sit down, or 
 laugh. Nothing, then, is explained by our structural analysis 
 of the nervous system; all that we have studied is the means 
 whereby one of a number of effects that may be produced by a 
 stimulus is produced. 
 
 Consider now the energetical side of the process of stimulus 
 and response. Just that quantity of energy which is received by 
 the receptor organ is transmitted along the afferent nerve into 
 the centre, and the same quantity again is sent through the 
 central nervous system from centre to centre, and still the same 
 quantity is transmitted along the efferent nerve into the effector 
 
THE ANALYSIS OF BEHAVIOUR 133 
 
 organ, where it is all expended on releasing the potential energy 
 which is to produce the effect in question. Or suppose (for it 
 does not matter to our argument in the least) that some of the 
 energy entering the sensori-motor system is dissipated, or wasted, 
 or " lost " by " friction " (as some would actually be " lost " in 
 the billiard-ball model). This dissipated energy will therefore 
 appear in the form of heat, and the rest will be transmitted as 
 before to the effector organ. Where, then, does the conscious- 
 ness of having seen something and acted upon the stimulus so 
 received come from ? For we must consider the suggestion that 
 an affection of consciousness is the result of an energy trans- 
 formation ; that it comes from the energy of light that stimulated 
 the retina and transformed into a nervous impulse just in the 
 same way as the heating of a metallic filament is the result of the 
 transformation of an electric current which passes through a lamp. 
 We are not going to accept this suggestion, but we notice it 
 in order that the reader may see quite clearly what are the 
 various possibilities (let us say) : 
 
 (1) All the energy that comes from without the body and 
 stimulates a sense organ is transmitted without loss through the 
 peripheral and central nervous system, and is physically trans- 
 formed in the effector organ, releasing potential energy there 
 which does work. 
 
 (2) Some of it is dissipated, and the rest is transmitted as above. 
 
 (3) Some of it is transmitted as in (1), with or without dissipa- 
 tion, and some of it is physically transformed into " consciousness." 
 
 Now what we know suggests that there is extremely little 
 or even no dissipation of energy in the transmission of a nervous 
 impulse through a reflex arc. And the more we think about the 
 third of our three hypotheses, the more unlikely it appears to be. 
 At all events, it does not seem possible even to attempt to verify it. 
 
 We return to this discussion in a later chapter. Meanwhile, 
 let the reader note that all that we have studied so far all, indeed, 
 that it appears that cerebral physiology can study are the ways 
 in which the stimulation of the organs of sense set up nervous 
 impulses, the paths along which those impulses travel in the 
 central nervous system, and the motor and secretory effects that 
 they produce. There is nothing at all about a theory of know- 
 ledge in the results of such an investigation, but very much 
 about a theory of action. Sensation is not something that 
 enables us to contemplate the external world, but is rather that 
 
134 
 
 THE MECHANISM OF LIFE 
 
 which enables us to act upon the latter. It is not so muc 
 
 prolonged or developed into consciousness of our environmen 
 
 as into action upon the environment. 
 
 We must deal further with this aspect of physiology. 
 
 Stimulus and Response in General. 
 
 We consider again a " mechanical model." Let there be 
 little compass, NS, placed in the neighbourhood of a wire int 
 which an electric current may be thrown by depressing a switch. 
 So long as the current does not flow 
 through the conductor, the needle remains 
 in the position NS that is, in the mag- 
 netic meridian; but whenever the switch 
 is put down and the current passes in the 
 (conventional) direction indicated by the 
 arrow, the compass needle deviates and 
 takes up the new position WE. When the 
 current is switched off the needle returns 
 to NS, and no matter how often we do 
 this the effect is always the same. Now 
 call the making contact in the switch a 
 "stimulus" and the deviation of the 
 needle the "response"; the latter, we 
 see, is invariable and inevitable. Given 
 that the same quantity of current flows 
 
 FIG. 40. A COMPASS through the conductor, and that the in- 
 NEEDLE, NS, LAID . ., , ,, ,, , ,. ~ ,, 
 
 ALONGSIDE A WIRE, 06, tensity of the earths magnetic field 
 
 THROUGH WHICH A CUB- remains constant, then the needle always 
 KENT CAN BE PASSED. deviates in the same direction and to 
 
 the same extent. There is strict determinism that is, the 
 " response " (a certain deviation) always follows upon the 
 " stimulus " (a current of a certain value). 
 
 The Muscle-Nerve Preparation. Now let the prominent thigh 
 muscle (gastrocnemius) in the frog's leg be dissected out, leaving 
 its tendon still attached to the femur and an inch of its nerve 
 (the sciatic) attached to the muscle which carries a weight. 
 Place two electrodes carrying a current in contact with the nerve. 
 Throw a momentary electric current into the circuit containing 
 the electrodes, and the nerve will be stimulated at the place a. 
 The stimulus initiates an impulse which traverses the nerve 
 and releases potential energy in the muscle, whereupon the 
 
THE ANALYSIS OF BEHAVIOUR 
 
 135 
 
 latter contracts, lifting the weight against the resistance of 
 gravity. Do this again and again, and the same effect follows. 
 By-and-by the muscle, or the nerve terminations in it, will 
 become fatigued, but until this happens an electric stimulus will 
 always elicit the same muscular 
 contraction or response. Here, 
 also, there is, perhaps, strict 
 determinism. 
 
 Tropisms in Green Plants. 
 
 When a seed germinates, two 
 structures grow out from it. One 
 of these is the original root and 
 the other is the original shoot. 
 The former always grows down 
 into the soil, and the latter grows 
 upwards into the air. When a 
 green plant is placed in front of 
 a window, the leaves tend to 
 
 turn towards the source of the light. The leaves of a tree 
 tend always to place themselves so that their flat surfaces 
 are perpendicular to the principal direction from which the 
 light comes. All these effects are called " tropisms," or directed 
 growth movements of fixed organisms. The tissues of the 
 plant have the general power of growth, but the stimulus of 
 light falling on them is a directive agency, which causes growth 
 to take place in one direction rather than any others. Such 
 directed growth movements in response to the stimulus of light 
 are called heliotropisms. They are invariable and subject to 
 determinism, just as are the deviation of the compass needle 
 or the contraction of the isolated frog's muscle when the nerve 
 supplying it is stimulated. 
 
 Taxis in the Lower Animals. Chip off some barnacles 
 (Balanus) from the stones on the beach between tide- marks 
 during the last week of March or the first ones of April, and place 
 them in a soup-plate containing clean sea-water. In a short time 
 the embryos contained in the reproductive organs will hatch out 
 and swim about in the water. Examine the latter in a feeble, 
 diffused light, and it will be seen that the larvae swim about at 
 random and in no particular direction; but place a lamp close 
 to one side of the vessel, and it will be seen that they swim 
 
136 THE MECHANISM OF LIFE 
 
 towards the point from which most light comes. This response 
 is called a phototaxis, and it may be denned as the directed move- 
 ments of an organism in response to a directed light stimulus. 
 It is invariable and determined, and a logical hypothesis that 
 is, one which is consistent with what we know about light and 
 its general effect on living tissues can be made to account for it. 
 
 The Spinal Reflex Action. " Pith " a frog that is, cut 
 through the spinal cord immediately behind the brain, and then 
 destroy the latter by pushing a blunt wire into it. The body of 
 the animal is now solely under the control of the spinal cord and 
 its ganglia; to convince oneself that this is the case, the head 
 may be removed. If, now (or, rather, after the effects of the 
 " shock " have passed off), a drop of strong vinegar be placed 
 on the back of the animal, one of the hind-legs will be bent 
 forward and the acid will be wiped off. A reflex arc of some 
 complexity is here involved: afferent impulses pass into the 
 spinal cord from the receptors in the skin irritated by the acid, 
 and these impulses are received by the nerve cells in the segment 
 of the cord which innervates the part of the skin stimulated. 
 
 From the segment stimulated tracts of fibres convey the 
 impulses received to the grey matter of the segments from which 
 the legs are supplied with motor nerves. The latter are then 
 stimulated, and the antagonistic muscles contract and relax, 
 carrying out the series of movements described (Fig. 42). 
 
 Now here, again, there is determinism, or, at least, the results 
 of stimulating the skin can be predicted. But there is now a 
 difference between the response in this case and the " mechani- 
 cally " repeated one that occurs in the muscle-nerve preparation: 
 when the drop of acid is placed on the right flank of the headless 
 frog, the right leg is used to wipe it off. Now let this leg be cut 
 away or forcibly held, and the left one is used to make the same 
 kind of response. Something occurs, then, in this case which 
 we have not, so far, observed. Evidently there are two mechan- 
 isms, one for each side of the body, but in what we may call 
 ordinary circumstances one of them is passive. Let, however, 
 the " normal " mechanism be prevented from operating, and 
 then that one which was passive before now responds vicariously. 
 We have to deal here with a " regulation." 
 
 Such a " spinal reflex " that is, a co-ordinated action carried 
 out by the nervous ganglia of the cord can be elicited from 
 higher animals than the frog. When the cord is severed in the 
 
THE ANALYSIS OF BEHAVIOUR 137 
 
 dog and the effects of the shock of the operation have passed 
 away, suitable stimulation of the skin of the side just behind 
 the shoulder is followed by the response called the " scratch 
 reflex": the hind-leg of the same side carries out a series of 
 kicking movements somewhat similar to those performed when 
 the normal animal scratches himself as the result of irritation by 
 a flea. Even in man something of the same kind may occur. 
 Thus, in cases of hemiplegia, when the connection of the cortex 
 
 _ Afferent 
 
 , fibres s^Area of 
 
 f ' \ 
 
 stimulated 
 
 -/<- 
 
 
 1 r 
 
 c/s 
 >inal cord 
 
 ^- 
 
 ^^x 
 
 
 II 
 
 
 
 
 ^ 
 
 
 J ' 
 
 J^ 
 
 -- 
 
 il I 
 
 
 - 
 
 
 nerves 
 
 Seg mefl fs 
 
 _of the cord 
 
 ~ Supplying 
 
 I i mbs 
 
 FIG. 42. THE " SCHEMATIC " SPINAL REFLEX, BEING THE NERVOUS 
 PATHS INCLUDED WHEN THE STIMULUS is THROWN INTO A DIFFERENT 
 SEGMENT FROM THAT INNERVATING THE MUSCLES THAT ACT. 
 
 with the cord is rendered ineffective by a " stroke," reflexes in 
 the paralysed leg may be obtained, and these are doubtless due 
 to the centres in the spinal cord. 
 
 Reflex actions, complex and purposeful in character, may thus 
 be carried out by the nerve centres in the spinal cord. 
 
 In such cases the stimuli (irritation by chemical substance, an 
 electric current, mechanical pinching, pricking, etc.) applied to 
 the skin are simple physical ones, and the response is generally 
 such an action as would be carried out by the muscles concerned, 
 in the normal animal, for some useful purpose. It always has 
 a certain " inevitability " that is, it " comes off," as a rule, 
 when the stimulus is applied. There is determinism, or at least 
 a large measure of determinism. It can usually be predicted. 
 
138 THE MECHANSIM OF LIFE 
 
 Reflexes in the Normal Animal. Now compare with these j 
 spinal reflexes those that may be observed in the normal, intact 
 animals. Such a response is easily observed in the case of a j 
 dog which is lying sleeping on his side with his legs stretched 
 out. When the skin over the ribs is rubbed vigorously, the 
 hind-leg of the same side will often make a series of foolish little 
 kicks, like incipient scratching movements. We may even study 
 such reflexes in ourselves. We usually start violently on hearing 
 a loud, unexpected noise, and the muscular actions involved 
 have meaning, inasmuch as they seem to be such as would place 
 the body in a posture of defence to meet some sudden menace, i 
 When some object makes an unexpected movement towards the 
 eyes the lids are rapidly closed, and here also the action has j 
 purpose the protection of the eyes. As a general rule these i 
 and similar responses are quite involuntary, and it may even ] 
 require some considerable effort of the will to arrest or prevent 
 them. But that they can be arrested or prevented is the charac- j 
 teristic that distinguishes them from the reflexes which are j 
 carried out by the spinal cord alone in the lower vertebrates or 
 in the higher vertebrates deprived of their cerebral hemispheres, j 
 Even the sleeping dog will not always give the incipient scratch 
 reflex when his side is rubbed, and by strong attention and " will- 
 power " a man may keep his eyes open when someone flicks 
 some object towards the face, or when he puts his head under- 
 neath water, and one may easily arrest the start that he naturally 
 makes when he hears a loud, unexpected noise. 
 
 This point is very important. The response that may be 
 elicited by a stimulus applied to the animal possessing its integral 
 nervous system is not a " fatal " or " inevitable " one. It may 
 occur, but it may not. It may occur in one of several ways, and 
 it may occur and immediately be arrested. All that means that 
 the strict physical determinism that one sees in the " response " 
 or deviation of a compass needle when a magnet is brought near 
 to it, or the equally determined tropistic and tactic responses of i 
 the lower organisms, or the spinal animal, do not occur in the 
 higher vertebrate possessing its entire central nervous system. ; 
 In the responses that one studies in such cases there is always 
 what we must call indeterminism : they are usually unpredictable. 
 
 But while this is the case, it is no less clear that we can study 
 a series of responses beginning with the purely inorganic reaction 
 of the compass needle and magnet, and passing through the 
 
THE ANALYSIS OF BEHAVIOUR 139 
 
 muscle-nerve response, tropisms, taxis, reflexes, in animals sub- 
 jected to various kinds of operative interference, and ending 
 with the behaviour of the normal, intact higher animal. At the 
 beginning of such a series there is determinism, and at its end 
 there is indeterminism ; but it would be rather difficult to suggest 
 any place where determined reaction became replaced by some 
 degree of chosen, deliberated behaviour. In such a series of 
 organic mechanisms we should be able to trace the beginning 
 and the gradual elaboration of a central nervous system. Nothing 
 of the kind is, of course, present in the compass-needle-magnet 
 system, and it is very doubtful if it can be recognised in the 
 typical plant organism. It is present in a very simple form (far 
 simpler than that which we have represented as present in the 
 earthworm) in the barnacle larva, and it becomes progressively 
 more complex as we ascend the vertebrate series. Somewhere, 
 then, in the ascending scale of living things indeterminism of 
 response is developed; we emphasise the word "response" so 
 that it may not appear that we exclude indeterminism of func- 
 tioning in the most general sense from the lower organisms. 
 
 The Lower Brain Activities. 
 
 We may next consider how the reactions exhibited by an 
 animal are modified when certain parts of the brain are removed, 
 or isolated from the other parts. Now, in such an animal as the 
 frog, both the cerebellum and the cerebral hemispheres are 
 undeveloped relatively to the medulla and mid-brain. The 
 former organ, may be called rudimentary, and the cerebrum 
 consists of the two corpora striata, each with a rudimentary 
 cortex cerebri. It is an easy matter to remove the two cerebral 
 hemispheres (including the corpora striata), and the animal sur- 
 vives the operation. It is then in possession of a central nervous 
 system, including the parts developed from the three primary 
 brain vesicles, but lacking the fully developed cerebellum which 
 is present in most vertebrate animals higher than the reptiles. 
 
 Now every action that can be performed by the intact frog 
 can equally well be performed by the decerebrate frog provided 
 that a suitable stimulus be applied. It can swim, leap, and crawl, 
 and it assumes, when motionless, the natural posture of a frog. 
 It can avoid obstacles when it is moving, and it is sensitive to 
 light. Its visceral, respiratory, and nutritive organs function 
 normally. It swallows food which is placed in its mouth. If 
 
HO THE MECHANISM OF LIFE 
 
 it is turned over on to its back it can regain its proper 
 attitude. 
 
 But it does none of those things spontaneously, and if it is not 
 stimulated it makes none, or very few, movements of its own 
 accord, and it nearly always remains in exactly the same position 
 if it is left strictly alone. When it is stimulated it responds in 
 an almost invariable way, and the effect of any particular 
 stimulus can be predicted. What is wanting in its reactions 
 and movements is the evidence of spontaneity: there is no 
 " intrinsic stimulus," and nothing that indicates the possession 
 of volition. It is " a machine, and nothing more, while the frog 
 possessing its cerebral hemispheres is a machine governed and 
 checked by a dominant volition." 
 
 Its responses to external stimuli indicate that it possesses the 
 power of completely co-ordinating its movements. That power, 
 we shall see, is dependent in higher vertebrates on the presence 
 of the cerebellum, and on the full connections of this organ with 
 the rest of the nervous system. But in the frog the cerebellum 
 is rudimentary, and we must therefore conclude that the func- 
 tions that it performs in the higher vertebrates are carried out 
 by the mid-brain. Therefore we cannot say that co-ordination 
 is the work of the cerebellum exclusively in all vertebrates. It 
 can be effected by other parts of the brain. 
 
 Now remove the cerebral hemispheres that is, the corpora 
 striata and pallia from a fish, and the animal survives the 
 operation. But there is no absence, " not even the temporary 
 absence." of apparently spontaneous movements. It is there- 
 fore evident that spontaneity of behaviour, which depends, in 
 the frog, upon the presence of the cerebral hemispheres, can be 
 mediated, in the fish, by the mid and hind brain, just as co-ordi- 
 nated movements, which depend, in the bird, upon the presence 
 of the cerebellum, can be mediated by the mid-brain in the frog. 
 
 The cerebral hemipheres are much more highly developed in 
 the birds than in the amphibia, but they can be removed and 
 the animal (the pigeon) can be kept alive for a considerable time 
 after the operation. The same general effects observed in the 
 frog can also be seen in the decerebrate bird. It either remains 
 quiet and impassive, or it moves about restlessly and without 
 any apparent purpose. Of itself it does not attempt to fly, but 
 if it is thrown into the air it will fly and avoid obstacles. It 
 does not spontaneously pick up corn, though it will do so if its 
 
THE ANALYSIS OF BEHAVIOUR 141 
 
 beak is thrust among the grain. Like the frog, it responds to 
 stimuli, and it sees and hears in so far as seeing and hearing are 
 the stimuli to movements; but sight and hearing do not, ap- 
 parently, evoke memories or utilise experience. Its movements 
 are perfectly co-ordinated. There is the same inevitable response 
 to external stimuli, the same automatism of activity, and the 
 same lack of spontaneity and volition (which latter is indicated 
 by spontaneity) that we see in the decerebrate frog. 
 
 Among the mammals the cerebral hemispheres are much more 
 highly developed, again, than in the bird, and their removal is 
 a formidable operation. Yet it has been accomplished in the 
 dog, and when the animal is nursed with the utmost care and 
 solicitude it may survive for over a year. In the few successful 
 experiments that have been made there has been some little 
 difficulty in exactly describing the condition of the patients. 
 As in the frog and bird, there is no paralysis and no lack of 
 co-ordination so long as the cerebellum is intact. All ordinary 
 movements are carried out, and, as in the bird, there is a tendency 
 to restlessness. The sense organs are active in so far as the motor 
 organs can respond to ordinary sensory stimuli, but things that 
 would, in the intact dog, evoke expressions of terror, dislike, 
 and pleasure, do not appear to affect the decerebrate animal in 
 anything like the same degree. Signs of hunger are exhibited, 
 and food brought near to the nose is eaten. Disagreeable food 
 may be rejected (for instance, one of Golz's dogs would not eat 
 the flesh of another dog). Both in decerebrate dogs and cats 
 painful stimulation elicited expressions of anger (growling, 
 barking, and snarling), but no caressing could evoke any indica- 
 tions of pleasure or affection. There was apparently no dream- 
 ing. Finally (a most curious thing), Golz's dog ate much more 
 than the normal animal did. 
 
 Nervous Inhibition. 
 
 We must say more about these results. In the decerebrate 
 mammal there is the same lack of spontaneous activity as in the 
 frog and bird, understanding by this the absence of apparently 
 willed, or deliberated, or intelligent, or chosen activities, and 
 not the aimless, automatic activity to which we referred above 
 as " restlessness." There is the same tendency for a stimulus 
 to evoke an inevitable response the actions of the animal can 
 be predicted. But the inhibitions are still more interesting. 
 
142 THE MECHANISM OF LIFE 
 
 A nervous inhibition is the arrest, entire or partial, of some 
 activity in progress, and such effects are very common and very 
 important. The rhythmic activity of the heart, for instance, is 
 intrinsic that is, it apparently goes on as the result of some 
 periodic stimulus originating in its own substance. But this 
 rhythmic activity is subject to nervous control, the heart-beat 
 being regulated by two sets of nerves, one coming from the 
 sympathetic system, and the other from the tenth (pneumo- 
 gastric or vagus) nerve. The stimulation of the former 
 accelerates, and that of the latter inhibits, the rate of the heart, 
 so that, by suitable stimulation of the vagus, the beat may be 
 slowed down or may even be arrested altogether. 
 
 Now nervous " shock " that is, the great prostration arising 
 from an injury, or the effects of a surgical operation is to be 
 regarded as a series of inhibitions. In some way or other im- 
 pulses passing down from the higher brain centres are blocked 
 (as when the spinal cord is severed), or are not initiated (probably 
 as the result of afferent stimulation in the case of a severe injury), 
 and the stopping of these impulses is the main cause of the 
 prostration and other effects called " shock." In the decerebrate 
 mammal, then, impulses that normally issued from the cortex 
 cerebri cease, and the general activities of the animal are affected. 
 The restlessness referred to above may be traced to the cessation 
 of inhibitory impulses which in normal life arrest or modify 
 aimless, random movements. The abnormally large consump- 
 tion of food may also be traced to the cessation of cortical 
 impulses regulating the metabolism of the tissues and economis- 
 ing energy, and the condition that anger and dislike could be 
 elicited, but not pleasure and affection, may also be traceable to 
 the loss of inhibitions. 
 
 For one may (taking a general survey of organic behaviour) 
 conclude that the feral animal has normally a " bad time." It 
 struggles for its existence both with inorganic nature and with 
 its organic enemies. It must " eat or be eaten." " Softness of 
 heart," dalliance, pity for others, and the like, are feelings that 
 are wanting, or, if present, are likely to be detrimental or fatal, 
 while their opposites make for self-preservation. What we call 
 loosely the altruistic motives must be regarded as the product 
 of the herd instinct, and they are to be interpreted as inhibitions 
 or checks upon the natural, predatory, and highly individualistic 
 modes of beha/iour. They are opposed to most tendencies that 
 
THE ANALYSIS OF BEHAVIOUR 143 
 
 are the products of natural selection, as a study of adaptations 
 among wild animals will show. Insanity in man that is not the 
 result of lesions, it has been argued, is to be interpreted as con- 
 flict between the mental complexes that the struggle for existence 
 has engendered on the one hand, and those newer complexes that 
 arise from the development of the herd instinct on the other. 
 As in Mr. Wells' s Island of Dr. Moreau, there is a " law " (a 
 series of inhibitions) which comes into tragic antagonism with 
 the naturally evolved animal propensities. 
 
 And so, lacking the inhibitory controls acquired by domestica- 
 tion and centred in its cortex, Golz's dog could growl and bark, 
 and manifest anger and displeasure, but not affection which to 
 it was something quite secondary an inhibition of the " currish " 
 nature of the natural dog. 
 
 And so also a study of social evolution and of mass psychology 
 impels one to the conclusion that what we recognise as " good " 
 is mostly the inhibition of what we may call the lower animal 
 instincts that are in us. The result is, of course, clearly demon- 
 strable in much of our conduct, for our codes of private and 
 public morality are, to a great extent, summaries of the things 
 that may not be done inhibitions and which the natural man 
 would often like to do; while our legal systems supply the sanc- 
 tions for those prohibitions and restrictions. What are called 
 " socialist tendencies " are, of course, attempted inhibitions of 
 individualism. The ruthlessness that characterised German 
 methods of warfare far more than those of their opponents was 
 the result of a slackening of inhibitions, a disregard of the 
 amenities of armed conflict (the things that might not be done), 
 and as such it met with general reprobation. The " war 
 psychoses " of European countries during and after the year 1919 
 illustrate the same proposition: one cannot help noticing a 
 tendency to increased dislike of people belonging to other 
 nationalities than our own, and a general indifference to suffering 
 borne by other peoples. Those feelings were potential in us 
 before the war, but they were repressed, and the strains set up 
 by the necessity for natural self-preservation loosened the in- 
 hibition and allowed of their expression. 
 
 The Cortex Cerebri. 
 
 From what has already been said about the progressive 
 development of the cerebral hemisphere in the vertebrate animals 
 the reader will see that we ought not to speak about " the " 
 
144 THE MECHANISM OF LIFE 
 
 cortex. It is not the same thing in the mammals as it is in the 
 birds and reptiles, where it is a more complex organ than it is id 
 the amphibians, and it does not occur at all among the fishes. 
 This disparity of development must always be remembered when 
 we discuss the functions of the cerebral hemispheres so far as 
 these can be made out from observations of the effects of opera-j 
 tive interference. Removal has little or no apparent effect in a] 
 fish, where the " cortex " is non-nervous, but the result is very 
 obvious in the frog, and still more so in mammals such as 
 the rat, rabbit, and dog. In the monkeys and apes, and, of 
 course, in man, the operation is an impossible one, for these 
 animals do not survive it long enough to make clinical study 
 profitable. 
 
 In lower vertebrates, such as the frog, the cortex is relatively 
 unimportant (judged from the anatomical standpoint) when it 
 is compared with the rest of the brain, and the functions that it 
 subserves are also unimportant. When it is removed the change 
 of behaviour is not a profound one, and it is possible that some 
 of the things that were formerly done by the cerebral hemispheres 
 are then done (or partially done) by the mid-brain. To some 
 extent this is also the case with the dog, where vicarious function- 
 ing on the part of the lower brain may be set up when the cortex 
 is removed. That such is the case is suggested by the observa- 
 tion that animals deprived of their cerebral hemispheres tend to 
 act more normally the longer they can be kept alive and in g 
 general health. Now, in the monkeys and anthropoid apes, th 
 development of the means of control over acting by the co: 
 has been carried so far that vicarious functioning by the mid- 
 brain and thalami become impossible, and this is still more the 
 case with man. 
 
 Reviewing the bare summary of the evidence that has been 
 made here we see, however, that with the removal of the cortex 
 cerebri spontaneous movements tend to disappear, while auto- 
 matic and mechanical activities persist. The animal so trailed 
 reacts to stimuli in a blind, inevitable manner,so that its responses 
 can be predicted. There is physical determinism at any rate, 
 much more of such than in the intact, cerebrate animal. Our 
 only criterion of volition, deliberation, and intelligence is this 
 presence of spontaneous behaviour, and therefore we are justified 
 in placing the immediate expressions of will and intelligence in 
 the activities of the cortical mechanisms. 
 
THE ANALYSIS OF BEHAVIOUR 
 
 145 
 
 The Nervous System as a Whole. 
 
 Having made this very bare analysis of the structure of the 
 central nervous system, we are now in a position to consider its 
 working as a whole, but, first of all, we may profitably think 
 about it as a series of superposed mechanisms, thus following the 
 natural path of evolution. Fundamentally, then, the nervous 
 system mediates between the stimulation of the organs of sense 
 on the one hand, and the organs of activity on the other. 
 
 Some physical change in the environment leads to the stimula- 
 tion of a receptor organ, and the initiation of a nervous impulse. 
 The latter is received by the nerve cells in the ganglion, and in its 
 
 A * 
 
 Shin 
 
 Iffector 
 
 Receptor 
 organ 
 
 .Muscles 
 
 FIG. 43. THE SIMPLEST POSSIBLE SENSORI-MOTOR ACTIVITY. 
 
 A spinal reflex mechanism, the line AB, represents the junction of 
 spinal cord and medulla. 
 
 turn initiates a number of impulses which pass out from the 
 ganglion along efferent nerves and set up a co-ordinated and 
 purposeful activity in the muscles to which those nerves go. 
 There need not be (and there usually is not) any perception in 
 such a stimulus and response. The latter is usually determined, 
 and can be predicted. 
 
 Next, the development of the special sense organs in the head, 
 and the concentration of ganglia there, lead to a certain integra- 
 tion of sensory stimuli and their resulting responses. The head 
 ganglia were primarily the centres, or nuclei, of the great organs 
 of sense, but it happens they also become connected with the 
 lower spinal centres, as we have indicated in Fig. 28, and so we 
 get a superposed series of connections. 
 
 10 
 
146 
 
 THE MECHANISM OF LIFE 
 
 The mediating mechanism is now greatly complicated, inas- 
 much as the receptor organs which are in primary connection 
 with the spinal ganglia are also in connection with the ganglia of 
 the organs of special sense in the head, and there are also motor 
 paths between the mid-brain and the cord. Obviously actions 
 immediately carried out by the motor centres in the cord can 
 now become more complex, and may be co-ordinated to a greater 
 extent because of the additional stimuli given by the special 
 
 c 
 
 Receptors 
 
 in 
 head 
 
 Muscles 
 in 
 head 
 
 m 
 x ot motor 
 
 % 
 
 \Mid 
 
 1 brain, 
 J lhalami, 
 
 medulla 
 ^Tracts 
 
 x " between 
 ~ "\" Mid- brain K 
 J j- cord -T\ 
 
 : il 
 
 ~3J 
 
 iclei ** C 
 
 cranial "nerves > 
 
 J?ecefifors 
 
 ,'- n 
 skin t-c. 
 
 Muscles 
 of 
 trunk 
 
 and 
 
 limbs 
 
 v.^ 
 
 ' 
 
 
 Sfiinal 
 cord 
 
 'Afferent 
 spinal nerve 
 
 
 irrerent 
 
 Spinal 
 nerves 
 
 
 FIG. 44. MAIN PATHS BETWEEN SPINAL CORD AND LOWER BRAIN. 
 
 sense organs. Still, the responses may largely be automatic and 
 inevitable ones, although there must be some subtle difference 
 in them when compared with the purely spinal reactions. 
 
 Such a structure and relationships are, in general, those of the 
 fish, where the cortex is absent and the cerebellum is rudimentary. 
 Now consider the additional complexity which is brought about 
 by the development of the cerebral hemispheres and cerebellum, 
 organs which have evolved simultaneously, or nearly so. 
 
 Neglecting the intermediate stages of this evolution (for we 
 
THE ANALYSIS OF BEHAVIOUR 
 
 147 
 
 know relatively little of the nerve tracts in the brains of the 
 amphibia, reptiles, and birds), we may consider the conditions 
 in man. We have already indicated the connections between the 
 cord, cerebellum, mid-brain, and cortex, but a purely schematic 
 figure will be useful. Fig. 44, then, represents the main com- 
 munications between the mid-brain and spinal cord. The parts 
 
 control 
 
 CORTEX 
 
 nerves 
 
 IfferenT 
 Cranial 
 
 muscles of head 
 
 Afferent 
 
 sfiina I 
 
 nerves 
 general bodily sensation 
 
 Efferentl 
 Spina I 1 
 
 nerves 
 muscles of body, 
 
 THALAMl 
 
 ft I'D -BRAIN 
 MEDULLA 
 
 B 
 
 SPINAL 
 CORD 
 
 \--general 
 
 sensibility 
 in muscles 
 
 '-T > ain i heat i cold Touch 
 ~ flu scu la r control 
 
 FIG. 45. MAIN PATHS IN THE NERVOUS SYSTEM AS A WHOLE. 
 
 below the line, AB, we regard as the simple, primitive, 
 central, nervous system, such as it probably was in the ancestors 
 of the vertebrates, while the parts above AB represent the addi- 
 tional connections established when the great sense organs of the 
 head have become evolved. In Fig. 45 we add the mechanisms 
 involved in the cerebellum and cerebral hemispheres. 
 
148 THE MECHANISM OF LIFE 
 
 Now, but for two exceptional paths that of the impulses 
 passing directly into the cerebral hemispheres through the 
 olfactory nerves, and that of the impulses going to the cerebellum 
 from the vestibular part of the auditory organ the parts repre- 
 sented above the line CD in Fig. 45 connect together the various 
 components of the central nervous system they are the integrating 
 mechanisms. 
 
 Thus the cerebellum receives impulses directly from the 
 muscles and joints of the body and others indirectly (via the mid- 
 brain) from the skin ; it receives impulses indirectly (also via the 
 mid-brain) from the organs of special sense in the head, and it is 
 in direct (to and from) communication with the cortex. And so 
 it has a grip on all the afferent impulses, whether those of general 
 or special sensation, arising anywhere in the body. 
 
 The cerebral cortex has conspicuous connections with the 
 cerebellum, as we have just said, but the communications with 
 the nuclei of the mid-brain, in which the nerves of sense end, 
 are just as prominent, and the great pyramidal tract leading down 
 from the motor region of the cortex to the nuclei of the motor 
 spinal nerves is more conspicuous still. And so the grip of the 
 cortex cerebri on both receptor and effector organs is as evident 
 as that of the cerebellum on the receptor organs. 
 
 We may now resume the general working of this whole 
 mechanism. The parts below the line AB constitute relatively 
 simple reflex arcs sufficient in themselves for movements initiated 
 by the stimulation of the sense organs in the limbs and trunk. 
 The parts between AB and CD that is, the mid-brain complex 
 also form reflex arcs, but the distance receptors (visual and 
 auditory organs) are now included, and become incorporated 
 with the spinal arcs by means of the tracts joining mid-brain and 
 cord. The stimuli initiating movements are now much more 
 varied and numerous, and so the latter become more complex 
 than when they depend on the cord alone. 
 
 Finally, when the parts above CD are fully evolved, something 
 quite new is added to the activities of the central nervous system. 
 The cerebellum mediates between the impulses coming from the 
 receptors in general (but particularly those in the muscles and 
 joints), and the impulses that pass out from the cortex to set 
 muscular mechanisms in action. It is not concerned in the 
 psychical life of the animal to any degree that can be recognised, 
 but it " standardises " and co-ordinates a number of very com- 
 
THE ANALYSIS OF BEHAVIOUR 149 
 
 plex movements performed in a customary manner those of 
 locomotion and posture. Something additional to this is effected 
 in the cortex. Looking at the main features of the anatomy of 
 the latter, we see the extraordinary development of the motor 
 region and the tract of fibres connecting this with the spinal 
 nerve nuclei, and studying the results obtained clinically (in the 
 case of man) and experimentally (in the case of other mam- 
 malia), we see that the movements most suggestively connected 
 with cortex and pyramidal tracts are of a special kind : they are 
 skilled movements, and the facility to make them is acquired by 
 training and is ino^vidual that is, they are not transmitted by 
 heredity. Further, they are usually spontaneous movements, 
 or at least they are such when they are being learned. 
 
 Looking again at anatomical, experimental, and clinical 
 results, we see that the cortex is to be associated with sensation 
 that develops into consciousness. The connections between it 
 and the sensory nuclei in the mid-brain are very obvious, and 
 all observational results point to the same conclusions. We 
 return to this matter in a later chapter. 
 
 The Meaning of Behaviour. 
 
 We define " behaviour " (premising that we are now dealing 
 with the higher animals) as the totality of the activities of the 
 entire sensori-motor system a definition which, however, must 
 not be strained. It excludes the activities of the viscera (heart 
 and bloodvessels, respiratory, nutritive, and excretory organs) 
 because these are subsidiary to the proper functioning of the 
 sensori-motor system. It excludes the activities of the repro- 
 ductive organs, because it is the individual animal that we are 
 studying, and not the race. In behaviour in general we see the 
 activity of the entire motor system, as in running, walking, 
 swimming, flying, and other modes of locomotion, as well as the 
 partial activities of bodily weapons, as in biting, clawing, etc. 
 It would include the actions of defence, flight, concealment, 
 aggression, the construction of nests and shelters, etc. It must 
 also include the actions carried out in play, courtship, etc., and 
 generally the " behaving " of the animal in the ordinary, non- 
 technical sense of the word. 
 
 Organic behaviour is to be regarded biologically as constitut- 
 ing a series of adaptations. The things that occur in the outer 
 world cannot, in general, be prevented by the animal living 
 
150 THE MECHANISM OF LIFE 
 
 there. Thus the succession of day and night, the order of the 
 seasons, changes in weather, such as storms, gradual or catas- 
 trophic geological changes, the vicissitudes of climate, tides, 
 ocean currents, winds, etc. all these things are phases in a 
 " cosmic order," and are beyond the control of the feral animal, 
 or even of man. All that the organism can do is to avoid menace 
 to itself, or to take advantage of external changes in so far as it 
 can do so by some variation of bodily functioning. The real 
 meaning of the evolutionary process, from the biological point 
 of view, is the slow acquirement by the organism of the means 
 of varying its functioning, so that it can evade external changes 
 that are inimical to it or take advantage of other changes. 
 
 When winter comes, many arctic animals respond to the 
 external changes by a change in the colour of the fur, say, from 
 brown to white. This renders them less conspicuous in a snow- 
 covered country, and the animal obtains an advantage in that it 
 is less easily seen by its enemies, and can the more easily approach 
 other animals that are its natural prey. Many fishes and other 
 animals hibernate during cold weather that is, they seek shelter 
 of some kind, and lie in a passive condition, economising muscular 
 movements as much as possible. The respiratory and heart 
 movements slow down. Oxidation of the tissues is partially 
 inhibited, and the animal lives on its reserves of fat and proteid. 
 The advantage gained in such cases is that the animal is spared 
 the necessity of seeking food at a time when this is very scarce. 
 When a man passes rather suddenly from a tropical into a 
 temperate climate, he excretes much less water from his skin 
 and much more through his kidneys, and he thus economises 
 heat by an inhibition of evaporation of sweat. These are 
 instances of adaptations of the functioning of organs other than 
 those belonging to the sensori-motor system, and they do not 
 involve " psychological factors " that is, they would be possible 
 in the absence of the higher parts of the brain, or at least they 
 need not include the activities of the latter organs. They do 
 not constitute behaviour. 
 
 Neither do many adaptations that we call instinctive ones. 
 Thus the blowfly lays its eggs in fresh meat, so that when the 
 larvae hatch out they may obtain abundant nutriment. A bird 
 builds its nest in such a place and in such a way that it is difficult 
 to distinguish it from the surrounding objects, and so the con- 
 cealment of the eggs and young is an advantage. A crab casts 
 
THE ANALYSIS OF BEHAVIOUR 151 
 
 its shell, and for a time it is almost unprotected against many 
 natural enemies. In such cases it seeks shelter in a crevice among 
 the stones or weed on the sea floor, and so conceals itself. These 
 are examples of responses involving the activity of the sensori- 
 motor system, but they are fixed, " mechanical " ones that have 
 become part of the organisation of the animals displaying them ; 
 they are transmissible by heredity, and they do not have to be 
 acquired individually. It is true that they must have been 
 acquired some time or other, but in a different way from the 
 cases which we are aljput to mention. 
 
 Young chicks learn to distinguish between small stones and 
 grains of corn. Collie dogs find that they ought to leave sheep 
 alone, and soon do so. Retrievers pick up, and bring back, game 
 without crushing or eating it. Cats and dogs learn to open 
 latches, to beg for food, and to recognise people who take notice 
 of them. A man puts on an overcoat when the weather becomes 
 cold, and the master of a ship alters his course when a sudden fall 
 of the barometer in certain latitudes suggests that a cyclonic 
 storm is approaching. These are all true adaptations, and each 
 of them ends in the animal obtaining advantage to itself of some 
 kind or other. They all involve intelligent acting of the sensori- 
 motor system, and they are based upon experience. Some time 
 in the past similar external changes have occurred, and the 
 responses made have been successful or not that is, they have 
 or have not been advantageous. The stimulus is remembered, 
 and also the nature of the responses made, so that when the 
 same external event, or change, recurs the unsuccessful response 
 is avoided and the successful one is made. It does not matter 
 that the master of a ship which is approaching a cyclonic storm 
 may not himself ever have had this experience; he has acquired 
 the experience of others, and knows what response is the advan- 
 tageous one. 
 
 Behaviour, in the sense that we employ the term, is therefore 
 the functioning of the sensori-motor system which is based upon 
 experience. By " experience " we mean individual acquirements, 
 and not something that is inherited. When experience becomes 
 a factor in the determination of the particular response that is 
 being made to a stimulus, something new that is, something 
 that we have not yet considered is included in the activities of 
 the animal organism, and our conception of mechanistic responses 
 must be re-examined, 
 
CHAPTER IX 
 THE MECHANISTIC CONCEPTION OF LIFE 
 
 THIS seems to be the proper place to say something about the 
 " materialistic " or, as it is now termed, the " mechanistic " 
 view of the living organism, for, although we have assumed such 
 an hypothesis in all that has been said so far, we are about to 
 abandon it, or, at least, we are about to give to " mechanism " 
 a meaning that is rather different from the one that is usually 
 accepted. Our description of the animal body was first of all 
 mechanical, then we had to make use of physical and chemical 
 ideas, and now we must search for some other concept, which 
 must, nevertheless, still be a logical one. 
 
 The body of the higher animal, then, is a system of muscles 
 and other soft parts which are built up round about, or are 
 supported by a system of rigid parts the skeleton. The latter 
 consists of separate bones immovably attached together, as in 
 the case of the parts of the skull and pelvis, or movably attached, 
 as in the case of the vertebral column and the skeletons of the 
 limbs. In the former cases the skeleton acts as the supports of 
 the soft organs thus the skull contains the brain and the great 
 sense organs while in the latter cases the vertebra and limbs 
 are the apparatus of movement. 
 
 Where the bones move on each other they are said to be 
 articulated, and the configurations of the articulations (or joints) 
 determine the ways in which the movements occur. Muscles, 
 ending in tendons, are attached to these movable bones, and by 
 their lengthening or shortening the parts are made to extend or 
 bend on each other. Thus the enormous variety of movements 
 that an animal can carry out depend on the shapes and lengths 
 of the bones, the configurations of the joints, and the modes of 
 attachment of the muscles and tendons. 
 
 Further, the muscles are supplied with nutritive matter by a 
 series of conduits the arteries, capillaries, veins, and lymphatic 
 vessels. At a central point of this vascular system is a propul- 
 sive organ, the heart, which keeps the blood and lymph in 
 motion. The directions in which these fluids move are deter- 
 
 152 
 
THE MECHANISTIC CONCEPTION OF LIFE 153 
 
 mined by valves, some of which are placed in the heart itself, 
 others at the place of origin of the great arteries and veins, and 
 others, again, in the veins and lymphatics. The velocity with 
 which they stream through the vessels depends upon the propul- 
 sive power of the heart, upon the varying calibre of the arteries 
 and veins, and upon the posture of the body, for it is apparent 
 that more power is expended upon circulating the blood when 
 a man stands erect than when he sits down or reclines. 
 
 So far, then, the bodily a^aratus is a purely mechanical one 
 levers of various shapes and " orders," elastic parts that pull 
 these levers, and channels along which fluids are propelled by the 
 action of a force pump and are directed by means of valves. It 
 would be possible to construct a model of the human body which 
 would carry out the same kinds of motions that are made by 
 the living organism. We should have to supply to this model 
 some source of hydraulic power, some automatic contractile 
 vessel worked by a spring that would cause the fluids to circu- 
 late in the vessels, and we should have to devise some 
 means of supplying motive force to the muscles say, compressed 
 air from a reservoir contained somewhere in the body itself. 
 There might be springs here and there on the surface of the 
 model, and these could be made to actuate valves in the reservoir 
 of compressed air, so that by touching them the limbs of the 
 model would perform certain motions. All this is quite possible, 
 and. indeed, automata of such a kind have been constructed. 
 It would, of course, be impossible to imitate the extreme minute- 
 ness of some parts of the animal body, or to duplicate the 
 intricacies of the motions of other parts, but such limitations 
 would be due only to our defective craftsmanship. There would, 
 of course, be an essential and very curious difference between the 
 activities of the living animal and those of the mechanical 
 model a difference which is rather hard to describe in simple 
 language. 
 
 The Cartesian Mechanism. Descartes thought about the 
 animal body as just such a mechanism as that suggested above. 
 The bodies of the lower animals were pure automata, so that if 
 they were touched, or pinched, or otherwise stimulated (as we 
 should say), they would move, or fight, or run away, or cry out, 
 etc., but these responses were as mechanical as would be the 
 movements made by the model when a spring was pressed. The 
 body of a man, he said, was also an automaton, but it contained 
 
154 THE MECHANISM OF LIFE 
 
 a " rational soul," while the bodies of the lower animals did not. 
 The latter could not feel pain or pleasure, and they acted quite 
 unconsciously; but the rational soul in the human body felt the 
 stimulus and response, and could even modify the latter, or could 
 initiate movements of the body by its own volition. Neverthe-j 
 less, the body of a man or woman was an automaton, and it could 
 do all that the body of a lower animal could do even when the 
 rational soul was inactive. 
 
 To understand this conception we must try to forget our 
 modern notions of matter and energy, and we must study I 
 Descartes' cosmogony. We shall return to the latter presently, 1 
 but in the meantime we must consider the physiology that was | 
 current before his time and the ways in which he modified it. 
 Now the science of mechanics was very well developed at the 
 beginning of the seventeenth century, and craftsmanship had j 
 reached a very high level of attainment. Anatomy had long i 
 been studied, so that the general structures of the bodies of man 
 and the lower animals were very well known and had been 
 described in considerable detail. Thus the gross anatomy of the j 
 skeleton, muscles, viscera, bloodvessels, brain, and nerves was 
 known to a degree that is not greatly inferior to our knowledge ] 
 (which excels that of medieval times mainly in its minutia?). j 
 The microscope had not been invented, or at least had not been j 
 improved to such an extent as to render it an aid to anatomy, 
 and therefore such organs as the brain, the muscles, nerves, and 
 viscera had then little of the complexity that we now know them 
 to possess. Our modern chemistry did not exist, so that there is 
 hardly what we should call a single chemical idea in all Descart cs' 
 physiology. Finally, our kinetic energy was then called vi-s viva, \ 
 and there was nothing at all comparable with our essentially 
 modern conception of potential energy. 
 
 The physiology was, then, largely that of Galen, but it wasj 
 modified very remarkably by Harvey's demonstration of the 
 circulation of the blood. The materials of the food taken into 
 the stomach were supposed to be converted by the agitation of 
 their particles into chyle, a turbid fluid which is present in the 
 small intestine. This was absorbed by the bloodvessels of the 
 gut, and was carried to the liver as the " natural spirits/" In 
 the latter organ it became endued with the " vital spirits/' and 
 the blood, containing this fluid, ascended to the heart, and was 
 poured into the right auricle. There was a " fire " or " innate 
 
THE MECHANISTIC CONCEPTION OF LIFE 155 
 
 heat " in the heart similar, Descartes says, to that which may 
 be observed in fermenting, damp hay, and this fire was fed by 
 the vital spirits brought to the heart from the liver. As the 
 blood fell into the right ventricle it became expanded, " just as 
 all liquids do when allowed to fall, drop by drop, into a highly 
 heated vessel " in other words, it was made to boil, but not 
 simply to boil, as we should say, for it yielded nutriment to the 
 fire of the heart at the same *ime. From the right-hand side of 
 the heart the blood went to the lungs, where it lost some of its 
 vapours, and became thick again by contact with the respired air, 
 " without which process it would be unfit for the nourishment 
 of the fire " of the heart. Returning then to the left ventricle 
 it became distilled, or highly rarefied, with the production of a 
 " very subtle wind, or, rather, a pure and vivid flame," which 
 was the " animal spirits." This was the most agitated part of 
 the blood, and as it ascended into the narrowing arteries leading 
 to the brain the grosser parts were left behind, while the " very 
 subtle wind " or " flame " entered the brain to become stored in 
 the cerebral ventricles. 
 
 The more sluggish and thicker parts of the blood, which were 
 nevertheless a hot and nutritive fluid, then became distributed 
 to all the other parts of the body (except the lungs and muscles). 
 Nutrition and the formation of the " humours " (or secretions, 
 as we should say) depended on the existence of sieve-like struc- 
 tures at the extremities of the arteries, so that when the blood 
 was forced through these the larger and more sluggish particles 
 were left behind " in the same way that some sieves are observed 
 to act, which, by being variously perforated, serve to separate 
 different species of grain." The secretions and nutritive parts 
 of the blood were, therefore,* filtered off, in the modern sense. 
 
 The animal spirits are meanwhile stored in the cavities, or 
 ventricles, of the brain. Now the nerves were described by 
 Descartes as minute tubuli which contained axial threads (our 
 axons). They had a twofold function: on the one hand they 
 transmitted something from the extremities to the brain (our 
 afferent impulses), and on the other they transmitted something 
 (our efferent impulse) from the brain to the extremities. When 
 a sense organ was stimulated, the axial nerve thread was affected 
 in the same way as when a wire is pulled or shaken, and this 
 motion acted upon valves in the walls of the cerebral ventricles, 
 allowing the animal spirits to escape. The latter flowed out- 
 
156 THE MECHANISM OF LIFE 
 
 wards through the tubuli of the nerves, and so into the muscles. 
 Expanding in the latter, they produced the contractions and 
 relaxations which moved the parts of the body. 
 
 Now in all this there is nothing but mechanism, in the strict 
 sense. A fluid (the blood) is expanded and rarefied by heat; 
 gross and cold, or slowly moving constituents are separated from 
 finer, more ardent, and more quickly moving constituents merely 
 by the difference of the motions; larger and coarser particles are 
 separated from smaller and finer ones by sieves; liquids flow 
 through tubes, and the thinner parts flow through the narrower 
 tubes more quickly than do the thicker parts; there are valves 
 that are pulled open by stretched organs; there are muscles, 
 which swell or relax according to the way an expansible fluid is 
 thrown into them; these muscles act upon levers, the bones, 
 which thereupon move the limbs and other parts of the body. 
 All this is pure mechanism in our sense. 
 
 What were the animal spirits ? It is very difficult to avoid 
 reading into Descartes' physiology our present-day notions of 
 energy; still, the spirits were a fluid, but a very rare and subtle 
 one, hot, and thin, and ardent that is, they were kinetic in our 
 sense (like a highly heated gas under pressure). But they could 
 be confined in the thin-walled cerebral ventricles and nerve 
 tubuli, so that their energy must have been repressed in some 
 way in our term, the energy was potential until it was released 
 by the afferent nervous impulse. As Huxley says, a relatively 
 slight change in Descartes' terminology would have brought his 
 physiology into line with ours. 
 
 Obviously he did very much what we do in our chemical and 
 physical hypotheses of living activities he pushed his specula- 
 tions to their limits, as the mathematicians would say. His 
 analysis of structure stopped at what could be revealed by dis- 
 sections, since there was no microscope, and therefore he assumed 
 that his mechanism held true, in the parts that were beyond his 
 observation, just as it did in the parts that he could see. It is 
 very doubtful, however, whether he would have modified his 
 speculations greatly even if he had known as much about the 
 minute structure of the animal body as we do. For instance, 
 the histology of muscle and nerve, as we know it, would lend 
 itself quite well to his mechanical explanation, and the structure 
 of the kidney, as we have sketched it on pp. 71-3, can easily 
 be assumed (in the absence of chemical data, it must be noted) 
 
THE MECHANISTIC CONCEPTION OF LIFE 157 
 
 to be such a mechanical filtering apparatus as he postulated. It 
 I is true that he could not see the pores, but neither can we even 
 with the aid of the microscope. 
 
 In short, Descartes assumed that his pure mechanism was 
 operative beyond the limits of actual observation (and, as we 
 shall see, that is also assumed by us in respect of our chemical 
 and physical mechanism). 
 
 The Influence of Chemical and Physical Investigations. 
 
 - Now, apart from the influence of chemical discovery, it is not 
 easy to see how the Cartesian physiology need have become 
 modified. But the great advances made by chemistry towards 
 the end of the eighteenth century changed the point of view 
 entirely. The true theory of combustion, as it developed in the 
 hands of Priestley, Black, and Lavoisier, was very revolutionary 
 in its effect on the conception of the nature of vital activity. 
 It was shown that when a piece of metal was strongly heated in 
 the air a calx (that is, an oxide) was formed, and that when 
 carbonaceous material was also made to glow in air some part 
 of the latter disappeared, and " fixed air " (that is, carbonic acid 
 gas) came into existence. Now the animal body, when dried, 
 was found to consist largely of carbonaceous material, and it was 
 discovered that when air was taken into the lungs some part of 
 it disappeared and was replaced by fixed air, just as in the case 
 of the combustion of carbon outside the body. The inference 
 was soon made that in respiration there was an actual process of 
 combustion, and that this occurred in the lungs, or blood, or 
 tissues, and was the source of animal heat. There was not, 
 therefore, an innate heat of the heart, and the latter became 
 regarded solely as the propulsive organ of the circulation. With 
 these discoveries the Galenic physiology became obsolete. 
 
 The modern idea of energy came later, and developed naturally 
 from the applications of the motive force of steam that were 
 made by Watt and the engineers, and the theory of the perfect 
 steam engine that was worked out by the French physicist, 
 Carnot. This great man showed that an engine did work by 
 taking heat from a source, and giving it up to a condenser, and 
 that there was a transformation of heat into mechanical energy. 
 His investigation became the foundation of our modern science 
 of thermo-dynamics, which, in its turn, became enormously 
 fruitful in the treatment of chemical problems. Later on 
 Rumford pointed out that mechanical friction generated heat 
 
158 THE MECHANISM OF LIFE 
 
 (a thing that many people before must have observed, but did 
 not think about), and by-and-by Joule estimated how much 
 mechanical work was transformable into just how much heat, 
 and he measured this quantity of heat by noting the increased 
 temperature it could impart to a known mass of water. Joule's 
 work had enormous influence on physiology, for it became 
 possible to estimate the " calorific values " of known quantities 
 of various kinds of food substances. Now a certain mass of 
 combustible material thus became associated with a certain 
 quantity of mechanical work theoretically deducible from it, and 
 so it could be assumed that when food substances were eaten 
 and oxidised in the animal body, some of their heat became 
 transformed into mechanical work. Thus chemical energy could 
 pass into the form of heat, and heat could be transformed into 
 mechanical work. Apparently the animal body was a thermo- 
 dynamic machine. 
 
 Let us note, very shortly, the other great ideas borrowed 
 physiology from chemical and physical science. Graham, aboi 
 the middle of the nineteenth century, found that there were t) 
 categories of chemical substances the crystalloids (like commc 
 salt) that could pass through the pores of an animal membrane 
 when they were dissolved in water, and the colloids (like gelatine 
 or albumen solutions) which could not pass through. Later on 
 de Vries showed how to measure osmosis that is, the passage 
 of water through organic membranes. When a vegetable cell is 
 placed in pure water the latter passes through the wall, so that 
 the cell swells up, while, if it is placed in salt solution, which is 
 more concentrated than the sap, it shrinks, because water now 
 passes out from the cell to dilute the stronger salt solution. 
 Graham's research on dialysis and de Vries' work on osmosis have 
 had a profound effect on physiological research so much so that 
 it has been said that the chemistry of life is largely a matter of 
 the chemistry of colloids. Another most fruitful conception 
 was that of catalysis. It had long been known that fermenta- 
 tions in vegetable and animal substances were due to enzymes 
 (or ferments); that an exceedingly small quantity of the latter 
 substances could produce a chemical change which would not 
 otherwise take place; and that the ferment itself need not be 
 used up during the reaction. But during the nineteenth century 
 it was discovered that a great number of purely mineral sub- 
 stances could act in precisely the same way with other mineral 
 
THE MECHANISTIC CONCEPTION OF LIFE 159 
 
 substances. Thus the same kind of reaction was observed to 
 occur both in organic and inorganic materials, and though we 
 still speak of enzyme action in relation to vital chemistry and 
 catalysis in regard to inorganic reactions, we know very well that 
 the two classes of chemical changes are of the same kind. 
 
 Along with all this investigation went on the synthesis of 
 " organic " substances by the chemists. It is almost possible, 
 even now, to feel the thrill that physiologists must have ex- 
 perienced in 1828 when Wohler synthesised urea from mineral * 
 substances. Before then there was an apparently hard line 
 drawn between organic and inorganic chemical compounds, and 
 it was thought that there were many substances, such as urea, 
 starch, sugar, albumen, etc., which were only and could only be 
 found in the cells of a living plant or animal. Wohler's syn- 
 thesis of urea broke through that line, and the far more wonderful 
 syntheses of sugars and polypeptides (the constituents of proteids) 
 by Fischer and his pupils obliterated it altogether. To-day we 
 look forward with complete assurance to the time when starches, 
 proteids, and fats capable of assimilation by the animal body 
 will be prepared, from their elements, by the chemists. 
 
 Finally (and now we come down to a relatively short time 
 ago), Jaccju.es Loeb demonstrated the possibility of artificial 
 parthenogenesis, and thus physico-chemical mechanism seemed 
 to be attacking vitalism in the very citadel itself. Before these 
 famous experiments were made parthenogenesis (that is, virgin 
 generation) was known to occur in a very few of the lower 
 animals, but in most lower and all higher forms it was quite 
 unknown; the ovum formed by the female could only be made 
 to develop when it was impregnated, or fertilised, by the sper- 
 matozoon formed by the male, and apart from the latter there 
 could, apparently, be no reproduction. Now early in the 
 present century Loeb showed that by simply adding certain 
 chemical substances to the water containing the eggs of sea 
 urchins, the latter could be made to undergo normal develop- 
 ment. It is true that the male element in reproduction is not 
 replaced by a chemical substance, for the spermatozoon does 
 much more than simply initiate the process of segmentation and 
 development of the ovum : it adds the paternal characters to the 
 maternal ones. Still, even the initiation of the process of 
 development by a simple mineral salt is a result of the most 
 extraordinary theoretical interest. 
 
160 THE MECHANISM OF LIFE 
 
 So much for the main ideas and methods which have been 
 introduced into biology from the chemical and physical side.1 
 Nothing at all has been said here about the impetus which these] 
 ideas have given to what we*may call utilitarian biology; to! 
 processes of further preparation of foodstuffs; to the breeding 
 and rearing of useful organisms; and above all to methods for 
 the prevention and curative treatment of disease. Even now,] 
 and much more in the future, the relative freedom from disease 
 that may be enjoyed by every sane man and woman is to bel 
 traced back to the investigations made by a few chemists and 
 physicists and biologists and medical men during tile last two! 
 centuries. To many people all this work is very disagreeable,! 
 and they do not like to think about it or even to enquire how itj 
 is carried on. Like Tennyson's Princess, they shudder at 
 
 " Those monstrous males that carve the living hound, 
 And cram him with the fragments of the grave, 
 Or, in the dark dissolving human heart, 
 And holy secrets of this microcosm, 
 Dabbling a shameless hand with shameful jest, 
 Encarnalise their spirits." 
 
 Throughout their lives they live upstairs, so to speak, knowing 
 that most of the things that make living tolerable are the work 
 of those that labour in the kitchen ; and one likes to think that j 
 some time or other science takes its revenge, and that in the j 
 end they have to go down into the basement and found a pathetic j 
 reliance on the rather sordid toil that goes on there. 
 
 It has been said by Michael Foster, and the statement can 
 easily be verified, that the periods when mechanistic conceptions 
 of life have dominated biology have also been those when the 
 greatest advances in our knowledge of the activities of the 
 organism were made; while the periods during which vital istic 
 views were generally current were sterile ones from the same 
 point of view. 
 
 Modern Mechanisms of Lite. Now one may ask what is the 
 difference between the Cartesian mechanism and that which is 
 represented in, say, the writings of Jacques Loeb, or is there 
 really any difference ? To Descartes there was nothing in the 
 activities of the animal body but matter, the configurations of 
 matter, and the motions of matter. Everything was mechanical. 
 There is no doubt at all as to what he meant. " I wish it to be 
 considered," he says in the Discourse on Method, " that the 
 motion which I have now explained " (that of the circulation of 
 
THE MECHANISTIC CONCEPTION OF LIFE 161 
 
 the blood) " follows as necessarily from the very arrangement 
 of the parts which may be observed in the heart by the eye 
 alone, and from the heat which may be felt with the fingers, 
 and from the nature of the blood as learned from experience, 
 as does the motion of a clock from the power, the situation, and 
 shape of the counterweights and wheels." But to the nineteenth- 
 century biologists there seemed to be no activities in the plant 
 or animal body except those of physical and chemical reactions. 
 What happened in the processes of animal and vegetable meta- 
 bolism was the result of the chemical constitution of the sub 
 stances that compose the living organism. 
 
 Here we must go back to Descartes' cosmogony. In the 
 extreme generality of his hypotheses, in prophetic anticipation, 
 and in sheer dynamic mentality no one has excelled the great 
 French philosopher. " His imagination," says Clerk Maxwell, 
 himself a man highly gifted with just the same qualities, " knew 
 no "bounds." He made not only a cosmogony, but a cosmic 
 evolutionary process. There was nothing, he said, but matter 
 in motion, but matter itself was only extension. Figure and 
 solidity were not essential to a material body, for the latter 
 could be melted or dissolved or broken down without ceasing 
 to be material, and its colour and smell and other qualities that 
 make appeal to our senses were clearly not essential to its 
 materiality. Nothing was essential but the condition that it 
 occupied space. (This anticipates the theory of relativity.) 
 There could be no void, or vacuum, he said, because empty space 
 could only be conceived in terms of matter, which is extension. 
 There could not be action at a distance (here he anticipated 
 Newton), and the whole universe consisted of the same kind of 
 matter (anticipating modern spectroscopic research). The 
 universe was full, and was a continuum (thus anticipating our 
 modern concept of the ether). 
 
 Matter originally lay together in closely fitting, angular blocks, 
 but it was set in motion (by God), and so these blocks suffered 
 mutual attrition, grinding each other down and becoming 
 spherical. An exceedingly fine dust or material resulted from 
 this attrition, and this was relatively inert and formed the 
 Cartesian " first element." The rounded particles, which were 
 very small and were in active motion, made the "second element," 
 while there was a " third element " consisting of particula striata 
 that is, particles which had acquired a spiral shape by passing 
 
 11 
 
162 THE MECHANISM OF LIFE 
 
 between the rounded particles of the second element, so that A \ 
 they possessed a rotatory motion as well as a translational one. ] 
 
 The first matter made up the sun and stars that is, the cosmic 
 bodies that are hot and radiate; the second matter made up the ' 
 heavens that is, the atmosphere and cosmic space, which j 
 convey radiation ; and the third matter was that of the earth and 
 other planets, bodies that are cold and are becoming inert.; 
 By vortices in the first and second matters he tried to explain 
 (but with no more success than Kant and Laplace) the evolution 
 of solar systems, while by the motion of the spiral particles he] 
 sought to explain attraction. The particles of the second ele-j 
 ment correspond to our modern atoms (or molecules, as Clerk j 
 Maxwell calls them). To what do the fine particles of the first ; 
 element correspond in our cosmogony ? 
 
 Fifty years ago we should have said that the universe was j 
 made up of atoms, of matter which here and there were aggre- j 
 gated to form stars and planets, comets, nebula, and meteoritic j 
 dust in short, cosmic bodies attracting each other with forces 
 depending on their masses and on the distances between them 
 and that these bodies filled only a most insignificant fraction of 
 empty space. But they attracted each other across this space, j 
 and heat and light and electro-magnetic radiation were trans- 
 mitted through it, so that it could not be empty. There was a 
 medium the ether of space and this conveyed the radiation, j 
 So much had to be postulated merely to explain things and give 
 us a " working hypothesis," but what was the nature of the 
 ether ? It was something that was perfectly elastic, a kind of ; 
 jelly, so to speak, across which energy could pass without dis- 
 sipation. It had to be continuous and dense that is, there \ 
 could be no interspaces in it such as there are even in the densest \ 
 kinds of gross matter, which, Sir Oliver Lodge tells us, has a 
 texture like gossamer compared with that of the ether. Now 
 the latter cannot be absolutely continuous and perfectly elastic 
 according to the modern theory of energy as formulated byJ 
 Planck, for if it were it would possess " infinite degrees of 
 freedom," and thus it would absorb all the energy in a system, 
 leaving none at all to the matter of the system, and that we, 
 cannot believe. So quite lately physics has come back again 
 to the notion of a discrete ether of space something which is 
 particulate, but which must nevertheless be continuous in some \ 
 way or other, how it is difficult indeed to see ! 
 
THE MECHANISTIC CONCEPTION OF LIFE 163 
 
 But that would be Descartes' first matter, the exceedingly 
 fine dust resulting from the attrition of the primitive matter 
 which fills up all tEe interstices between the other molecules. 
 Since the universe was originally quite full, it must have remained] 
 full, even when its motions had evolved the three elements.' 
 Therefore there is continuity and yet a particular structure, and 
 that seems to be what the Planck theory of energy requires. 
 After many vicissitudes, therefore, physics returns to what seems, 
 in essentials, to be the Cartesian cosmogony. 
 
 And, of course, our physico-chemical mechanism reverts to 
 that of Descartes. There is nothing but matter and its con- 
 figurations and motions, and this is true whether we think about 
 Descartes' matter or the Newtonian matter, or the modern 
 matter of electro-magnetic theory. The Cartesian matter was 
 extension, but what is the matter of to-day ? We are gradually 
 accustoming ourselves to think about it as immaterial. Atoms 
 are merely systems of electrons, but electrons are pure energy, 
 and energy, as we have seen, is something about the " nature " 
 of which we do not know anything all that we know is that it 
 is a measure of causality, and that we can measure it in terms 
 of work done, and that, we have seen, comes in the long-run to 
 measuring a distance or space. So our matter is extension, just 
 as was that of Descartes, and we ought now to call it substance 
 in the philosophic sense of the term. 
 
 Anyhow, our mechanism of the organism has come again to a 
 crisis. First of all it was a mechanical explanation of life, and 
 that being insufficient biology resorted to a physico-chemical 
 explanation, which was also insufficient, since physics and 
 chemistry are again becoming mechanical. Looking about for 
 the new conception that biology has now again to borrow from 
 physics, we have little difficulty in finding it, and it would appear 
 as if it were really something new. The concept is given to us 
 in the physical notion of statistical mechanics, and to this we 
 shall return presently. 
 
CHAPTER X 
 THE MEANING OF PERCEPTION 
 
 WHAT we have done so far has been to make a summary of the 
 main results of physiology. Now it is obvious that this summary 
 has been a very bare one, but we have so designed it that the 
 reader should have little difficulty in greatly amplifying his 
 knowledge by the study of a good, recent textbook. Assuming, 
 then, that he has done so, it will be seen that the outcome of 
 both medieval and modern anatomical and physiological research 
 has been to give us an analysis of the means of acting of the 
 animal mechanism an analysis that has become more and more 
 refined as our chemical and physical methods of investigation 
 have become more powerful and penetrating. 
 
 It is quite easy to show this by tracing out the history of i 
 physiology with respect to any one particular mode of function- 
 ing for instance, conduction within the nervous system. Thus 
 Descartes made an explanation of action which was based: 
 entirely upon anatomical studies, and which, as we have just 
 seen, involved only mechanical ideas. The nerve joining a sense \ 
 organ the eye, for example with the brain contained one kind 
 of fibres, which were, in effect, threads stretched between the! 
 receptors and certain valves in the walls of the cerebral ven- j 
 tricles. When these threads were stimulated they were jerked 
 in some kind of way, and this jerk opened the valves and allowed 
 animal spirits to escape from the reservoir in which they w<ro 
 stored. The spirits flowed outwards along nerves which con-j 
 tained another kind of fibres, which were tubes communicating 
 with muscles. When they entered the latter, an expansion in j 
 one direction and a shortening in another one occurred, so that 
 the muscle exerted a pull on the movable bone to which it was 
 attached. Clearly we have here the original conception of 
 afferent and efferent nervous impulses. Much later (in 1821) j 
 Bell verified this hypothesis in part by direct experiment. Each j 
 spinal nerve contains two kinds of fibres, one carrying impulses j 
 to the centre, and others carrying impulses from the centre to I 
 the periphery. Further, each spinal nerve has two roots, and 
 
 164 
 
THE MEANING OF PERCEPTION 
 
 165 
 
 when the dorsal (or posterior) one is severed, there is loss of 
 sensation in some part of the body to which the mixed nerve is 
 distributed. When the ventral (or anterior) root is cut there is, 
 on the other hand, some degree of motor paralysis. Thus there 
 a iv two kinds of nerve fibres, some carrying impulses inwards 
 and others carrying impulses outwards. 
 
 Later on still (in 1852) Waller's researches showed us the 
 anatomical basis for this conception of afferent and efferent 
 impulses. When a nerve fibre is severed from the cell out of 
 which it grows as an axon it dies, and its substance breaks down 
 and degenerates. The cells from which proceed the fibres 
 passing inwards through the dorsal root are in the sense organs, 
 and so inwards from the place of section the nerve fibres de- 
 generate. On the other hand, the fibres passing out in the 
 
 u 
 $fe$ffe 
 
 VoluriTary 
 muscles 
 
 FIG. 46. DIAGRAM OF THE ROOTS OF A SPINAL NERVE. 
 
 A sympathetic ganglion is shown with the two rami communicantes, one 
 branch going from the cord into the ganglion, and the other going out 
 from the ganglion to the involuntary muscles (those of the alimen- 
 tary canal and bloodvessels, in the main). 
 
 ventral root proceed from cells in the grey matter of the spinal 
 cord, so that when they are cut through they degenerate on the 
 side of the place of section away from the spinal cord. The 
 result of this discovery was that it became possible to trace the 
 paths in the mixed nerves, in the spinal cord, and in the brain 
 along which impulses travelled, and so the results suggested in 
 Chapters VI. to VIII. were obtained (mainly by experiments on 
 animals). It was also found that many of the fibres in a nerve 
 distributed to a muscle came from the latter that is, they carried 
 afferent impulses from receptors there. Later, again (almost in 
 our own time), Sherrington's beautiful researches showed that 
 all movements involved antagonistic muscles that is, when one 
 (flexor) muscle contracted, another (extensor) one relaxed, and 
 this in consequence of one efferent impulse proceeding outwards 
 
166 THE MECHANISM OF LIFE 
 
 from the centre and dividing along two distinct nervous paths 
 as it approached the part that was to be set in action. Finally, 
 while all this was being made out, many investigators had been 
 tracing out the microscopic structure of the receptor organs, nerve 
 fibres, and centres, and also the structure of the muscles, while 
 others were working out the nature of the physical and chemical 
 changes that occur when nerve and muscle become active. 
 
 We have still to find out exactly how the physical events that 
 occur outside the body affect the nerve terminations in the sense 
 organs; what is the nature of the nervous impulse itself; what 
 kind of change it produces in the nervous centres; and what are 
 the energy transformations that occur in the muscle when the 
 fibres of the latter shorten or contract. There is no doubt at 
 all that these things will soon become known, and that our 
 analysis of the sensori-motor activity will become much finer 
 than it is. From the point of view of human progress this 
 analysis is all-important, for upon it depends whatever success 
 we shall attain in preventing disease and lengthening life; 
 obviously it is our knowledge of the working of the mechanism 
 that matters from the utilitarian standpoint. 
 
 Now the reader cannot fail to see that the results of physio- 
 logical research are a description of the way in which things that 
 may happen in the animal body do happen, and that the descrip- 
 tion makes use of the same conceptions that physicists and 
 chemists use; no other results than these could possibly come 
 from an investigation that utilises chemical and physical methods, 
 and so, when we say that science finds nothing in life activities 
 but physico-chemical reactions, we mean that those reactions 
 are all that science looks for, or could possibly attempt to dis- 
 cover. We describe how things happen, but we do not explain 
 why they happen. Of course, it may be said that it is not the 
 business of science to ask why anything happens, but that is just 
 what ordinary people do ask, and it is the task of what we call 
 philosophy to try to answer the question. 
 
 Science, then, no matter whether it be physical, chemical, or 
 " natural " science, investigates the ways in which things happen, 
 and it reduces all kinds of events to motions, or, rather, to dis- 
 placements. What does this mean ? the reader will ask. Well, 
 mechanics, in the first place, obviously considers nothing but 
 motions; it deals with material bodies having mass, and with 
 " forces " which " act upon " those bodies. But force is only 
 
THE MEANING OF PERCEPTION 167 
 
 the acceleration of a body possessing mass; what we observe is 
 that something that is at rest begins to move, or that something 
 that is moving already moves more quickly or more slowly, and 
 that is what we mean by saying that some force acts on the body. 
 Mass itself is only measured by observing that something would 
 move towards the earth if it were free to do so, and then by 
 preventing it from moving by balancing it against something 
 else that is, by weighing the body having mass. Flow of heat 
 is only the transference of rapid motions of the molecules of one 
 body to the molecules of another body which are moving less 
 rapidly. Chemical reactions are displacements of the atoms 
 within the molecules of substances; thus, when we add some 
 solution of silver nitrate to solution of common salt a white 
 precipitate is formed thus : 
 
 AgN0 3 +NaCl=AgCl+NaN0 3 , 
 
 the atoms of silver and sodium being mutually displaced with 
 relation to the N0 3 and Cl. Or chemical reactions may be the 
 coming together of atoms to form molecules, as when the gaseous 
 atoms of hydrogen and iodine combine to form hydriodic acid gas : 
 
 H 2 +I 2 =2HI. 
 
 Or they may be the breaking up of molecules into atoms, as 
 when the last reaction is made to reverse itself : 
 
 I 2HI=I 2 +H 2 ; 
 
 or into other molecules, as when the salt, ammonium chloride, 
 
 dissociates : 
 
 NH 4 C1-NH 3 +HC1, 
 
 and so on in a very great number of ways. All chemical changes 
 include displacements or rearrangements of atoms, and such 
 changes are accompanied by various physical effects. Heat is 
 generally evolved, or is absorbed, when the rearrangement occurs, 
 and that means that the rapidity with which the molecules of 
 the reacting substances were moving changes. Or the latter 
 may change their state ; thus, when petrol is set on fire its mole- 
 cules, which were previously in the liquid state, react with the 
 oxygen of the atmosphere to form molecules of water and 
 carbonic acid gas, the atoms being rearranged. At the same 
 time the new molecules become gaseous that is, their freedom 
 and rapidity of motion increase. Or sound may be produced, 
 as when dynamite explodes, and that means that a large quantity 
 
168 
 
 THE MECHANISM OF LIFE 
 
 of gas is suddenly produced (the substance of the dynamite now 
 occupying an enormously greater space than it did before the 
 explosion). This sudden production of gas sets up a sound wave 
 in the atmosphere that is, a series of condensations and 
 rarefactions of the air, which, again, means simply a series of 
 displacements of the molecules of nitrogen and oxygen in the 
 atmosphere. Or, finally, light and other radiant effects may be 
 produced, and these are again displacements of some hypo- 
 thetical " substance," called the ether of space. 
 
 Dense T>en se Dense 
 
 l^a. re. TZare. 
 FIG. 47. DIAGRAM OF A SERIES OF SOUND WAVES IN THE AIR. 
 
 The distance apart of the dots represents the condensation and 
 rarefaction of the air. 
 
 Thus the older mechanics and the classical chemistry deal with 
 the displacements of bodies or of molecules or atoms that is, 
 they deal with movements of material substance while the newer 
 physics and chemistry now deal with radiation which occurs in 
 a medium (the ether) invented to enable us to describe the 
 radiation. Everything now becomes vibratory motions or 
 waves. What is a wave ? "If you ask a mathematician," says 
 Sir Oliver Lodge, " what he means by a wave, he will probably 
 reply that the most general wave is such a function of x and y 
 and t as to satisfy the differential equation: 
 
 while the simplest wave is: 
 
 y=a sin (x vt), 
 
 and he might possibly refuse to give you any other answer." 
 He would, no doubt, be sure that you ought to know what a 
 differential equation means, anyhow ! Well, all science that is 
 quantitative expresses itself in such statements: the ether is 
 really a set of differential equations. When a scientific result 
 reaches such a form it tells us that there are certain ratios of 
 displacements. In the above expression the d's are differentials 
 
THE MEANING OF PERCEPTION 
 
 169 
 
 that is, differences in measurements of y and x and t that may 
 be as small as we like; y is a measurement of space in a certain 
 (vertical) direction; x is also a measurement of space in a 
 horizontal direction perpendicular to that of y ; t is a time 
 
 o 
 
 measurement, and v is a velocity that is, it is a ratio, -. Time, 
 
 in scientific investigations is always a space measurement; 
 if, for instance, we use a clock, time is the position of the hands 
 relative to their zero position. Thus the equation represents 
 nothing but the ratios of measurements of space; the crest of 
 
 longitudinal 
 motion of crest 
 
 (a1~ihe moment t,) 
 
 Fio.^48. DIAGRAM OF A WAVE IN Two POSITIONS. 
 
 the wave, which we may take to be an Atlantic roller travelling 
 uniformly, moves with a certain velocity, v that is, at a certain 
 moment, t lt the crest of the wave, which is travelling in the 
 irection ox, is at a certain position (x=o), and the water level 
 s at a certain height (y^. A moment later (t 2 ) the crest is at 
 a different position, x l9 and the water level, which was originally 
 at the position represented by y v has now sunk to the posi- 
 tion y. 2 . Obviously, then, our differential equation tells us how 
 the crest of the wave is displaced along the direction ox, while 
 the water level at any position in the line ox is being displaced 
 along the up and down direction oy. It takes a moment of 
 
170 THE MECHANISM OF LIFE 
 
 time, t, at which the crest is at a certain position, and then an 
 " interval " of time, dt, after which the crest has attained a new 
 position, dx. But at the moment t the water level was at the 
 position y, and after the interval dt it has dropped to the new 
 level dy. We call the d?s " intervals," but it must be noted that 
 they are " infinitesimal " intervals that is, they may be smaller 
 than any finite lapse of time, no matter how short the latter 
 may be. 
 
 Clearly, then, our differential equation simply gives us a 
 series of connected displacements. As time changes that is, 
 as some standardised mechanism, such as the hands of a clock, 
 are displaced by so much so the crest of the wave is displaced 
 so much in a horizontal direction, and the water level is displaced 
 so much in a vertical direction. Now all this is not at all irre- 
 levant to the work of this chapter, for our differential equation 
 is a perfect example of a scientific " law." 
 
 Science measures motions, or, rather, displacements, and its 
 residts must be expressed in measures of space. 
 
 Next we turn to the study of organic functioning, and we may 
 choose for an example any bodily activity whatever; in the 
 meantime our treatment of the animal is entirely " objective " 
 that is, we are regarding it as a system in the physico-chemical 
 sense, something outside ourselves that we can observe and 
 measure just as we can an ocean wave. We might take the 
 action of the heart, or the interchange of oxygen and carbonic 
 acid between the atmosphere and the bloodvessels of the lungs, 
 or the excretion of urea it does not matter. In all these cases 
 we investigate and express our results as motions, or, more 
 precisely, as measured displacements of something. When we 
 study the circulation we measure the velocity of the blood- 
 stream (that is, the displacement of a blood-corpuscle in an 
 unit interval of time), or the pressure of the blood in an artery 
 (that is, we record the displacement of mercury in a pressure 
 gauge connected with the bloodvessel), and so on. When we 
 investigate respiration we may measure the tension of oxygen 
 in the blood and compare it with the partial pressure of the gas 
 in the air (that is, we observe and measure the volume of a 
 gaseous residue after certain manipulations). When we study 
 the formation and distribution of urea in the blood and kidneys 
 we make quantitative chemical analyses, and in so doing measure 
 the masses of certain substances at certain times and places, and 
 
THE MEANING OF PERCEPTION 171 
 
 compare them with, the masses of the same substance at other 
 times and places. And in identifying these chemical substances 
 we make use of measurements of space (colour, specific gravity, 
 vapour pressure, boiling-point, chemical properties in general, 
 which are all consequences of the positions of atoms within 
 molecules and of the movements of the atoms and the molecules), 
 and so on. To save time we may assume that the reader is 
 thoroughly conscientious, and desires to verify these statements ; 
 he will find that the results of the investigation of animate, 
 no less than inanimate, activities all reduce down to observation 
 of motions and measurement of displacements. 
 
 But we shall not, or we shall very rarely, get differential 
 equations like we do when we investigate the ether and elec- 
 tricity and relativity, and so on. We may illustrate this by 
 considering growth. Think of a crystal of salt growing in ideal 
 conditions in a pure solution of sodium chloride: if we know the 
 length of one of its sides we can find its mass, thus : 
 
 
 Mass=c (length) 3 , 
 
 and the differential equation is - =2cZ 2 (where m=mass, 
 
 dl 
 
 Z=length of a side, and c is a constant depending on specific 
 gravity). Knowing the length of one side of the crystal, we can 
 predict its mass. 
 
 We cannot do this with a growing animal. If it grew like 
 the crystal does, we could use the equations just given to find the 
 weight of the animal from a measurement of its length. We can- 
 not find this, as experiment will show, and all that we can do 
 is to make an empirical equation after we have measured both 
 length and mass. This equation will be : 
 
 Mass=aZ+6Z 2 +cZ 3 -h 
 
 where Z=the length, and some of the constants, a, 6, and c, may 
 be negative numbers. 
 
 Go on now to a study of behaviour, and the same contrast 
 emerges. Nothing in the way of differential equations is ever 
 possible, and we cannot, in general, predict how a normal animal 
 will respond when we attempt to stimulate it to activity of 
 some kind or other. Yet we can think of a series of organic 
 responses which begin by showing a high degree of determinism 
 and end by failing to show any. 
 
172 THE MECHANISM OF LIFE 
 
 Perhaps we had better define physical determinism, or, rather, 
 illustrate it by an example. A compass needle free to move 
 points in London in a certain direction (about 16 degrees to the 
 west of true north), and it does so because the earth's magnetic 
 force is directed along lines which are running in that direction. 
 Now we know this horizontal component of the earth's mag- 
 netism, and we can also find what would be the force exerted by 
 a feeble bar magnet placed at right angles to the magnetic 
 meridian in which the compass needle is situated and (say) a 
 foot away from the pivot of the latter. Then a physicist could 
 calculate the angle through which the needle would deviate 
 when the magnet was placed as we have indicated. This 
 deviation would always be the same provided that the earth's 
 magnetic field and that of the disturbing magnet remained con- 
 stant. Call the effect of the latter the " stimulus." and the 
 deviation of the needle the " response"; then there is physical 
 determinism, and we can always successfully predict to a high 
 degree of approximation that a certain response will follow the 
 application of the stimulus. 
 
 Now take a muscle-nerve preparation (see pp. 1345) and 
 stimulate the nerve by a feeble electric current; a momentary 
 twitch of the muscle, exerting a certain pull upon a weight 
 attached to it, follows. So long as the muscle remains irritable 
 and unfatigued this response occurs, and we can predict it. There 
 is physical determinism, even although this may not be absolute. 
 
 Next take the frog in which the brain has been destroyed, 
 leaving the spinal cord intact. When a drop of acid is placed 
 on its back it will wipe this away with one of its hind-legs, and 
 when other stimuli are applied it will respond in a mechanical, 
 automatic manner. We can, in general, predict what it will do 
 when certain stimuli are applied in certain ways; thus there is 
 determinism. Yet this is not absolute for instance, the spinal 
 frog appears to prefer to use one leg rather than the other, but 
 if it is prevented from employing that leg it will use the other. 
 Generally the behaviour, in response to stimuli, of the spinal 
 animal is of this mechanical, automatic nature it can usually 
 be predicted; and this statement is true of frogs, birds, reptiles. 
 and mammals, whatever animals have been the objects of the 
 experiments. 
 
 It is also true of a certain number of reflex actions in the 
 normal animal that is, it is generally true, for reflexes cannot 
 
THE MEANING OF PERCEPTION 173 
 
 always be elicited. It is true of habitual, learned, and mechani- 
 cally repeated actions which have, so to speak, become customary 
 to the animal that makes them. And it is true (again in general) 
 of the instinctive actions which make up a considerable part of 
 the behaviour of invertebrate animals in particular, and even of 
 the higher vertebrates. Yet habits can be varied, and instinctive 
 actions are not quite invariable. Thus some animals, such as 
 bees, which build structures of certain materials, may alter their 
 methods when different materials are supplied to them. Clearly 
 the responses made even by intact, normal animals are not rigidly 
 determined ones capable in all circumstances of being predicted. 
 
 And such behaviour as we have indicated in Chapter VIII. in 
 dealing with the functioning of the nervous system shows that 
 there may be spontaneity of behaviour on the part of all animals 
 that have been studied with sufficient care. They react to 
 stimuli sometimes in one way, sometimes in others, and again 
 they may not react at all, or they may act in unpredictable 
 ways even when there are no apparent stimuli. One can see 
 this in men and women: a slight irritation may cause a man to 
 cough again and again, but he may easily repress the response, 
 even when the same physical stimulus recurs. Now in all such 
 cases of variable behaviour to the same kind of stimulus it is 
 open to us to say that there may have been other unrecognised 
 stimuli which modified the response; that in cases where the 
 behaviour seemed to be absolutely spontaneous there " must have 
 been " some causes which we could not trace, or that there were 
 " physiological conditions " introducing elements of uncertainty, 
 and so on. All this may be so ; still, the fact is that in a great many 
 cases we cannot say that organic actions are preceded by definite 
 specifiable causes, and when we say that a response " must " 
 always have its appropriate stimulus we simply dogmatise. 
 
 It hardly seems necessary to lay stress on the conclusion that 
 we are about to make; nevertheless, it is well to do so. Inanimate 
 occurrences are always " more or less " determined, and equivocal 
 results invariably suggest defects in the experimental methods 
 employed, or neglect of some condition or other, or a degree of 
 complexity incapable of complete analysis. That a result can 
 be predicted is a kind of test of a successful physical investigation 
 in fact, we disregard all other results; they are not useful to 
 us. To some extent this physical determinism is true of the 
 functioning of the lower animals and of the higher ones that 
 
174 THE MECHANISM OF LIFE 
 
 have been subjected to operations which destroy certain parts 
 of the central nervous system. But in most animals there is 
 some indetermination and spontaneity of behaviour, and the 
 more highly organised is the central nervous system, the greater 
 seems to be the degree of indetermination that is exhibited. 
 
 The Nature of Sensation. 
 
 We are now about to cross over from the " objective " to the 
 " subjective," and the reader is asked to attach no other mean- 
 ings than those that we specify to the technical terms that we 
 shall employ. By " sensation," then, we mean only the train 
 of physical events which occur when a receptor organ is affected 
 or stimulated by some change in the external medium, and when, 
 in consequence of this stimulation, nervous impulses are pro- 
 pagated along the afferent nerve of the receptor organ into a 
 cerebral or spinal centre. Let us examine this train of events 
 in one case ; the reader can easily make similar analyses for others. 
 
 The Sensation of Hearing. A gramophone begins to play- 
 that is, a disc in which there is a spiral groove of uneven depth 
 revolves, raises and lowers a needle, which presses unequally 
 upon a diaphragm and throws the latter into rapid vibrations. 
 These vibrations set up waves of condensation and rarefaction 
 in the air, and the latter impinge on the drum of the ear, setting 
 this into vibrations which resemble those of the diaphragm of 
 the gramophone. The vibratory movements of the tympanic 
 membrane are communicated to the three little bones of the 
 middle ear, which communicate them to a liquid (the peril ymph) 
 filling the bony labyrinth of the internal ear. Then the vibratory 
 movements of perilymph are transmitted to the endolymph 
 contained in the membranous labyrinth, and the latter vibrations 
 are communicated to the hair cells or other terminations of the 
 cochlear nerve. Nervous impulses are now set up in the fibres 
 of the nerve of hearing, and these are propagated to the auditory 
 centres of the lower brain. 
 
 There is therefore a chain of dependent events : the mechanism 
 of the gramophone, the atmospheric vibrations, the movements 
 of tympanic membrane and auditory ossicles, the vibrations of 
 perilymph, endolymph, and nerve terminations, the nervous 
 impulses, and, finally, the changes produced by the latter in the 
 cells of the auditory centre. Throughout all this series of 
 changes, or motions, there is physical determinism. 
 
THE MEANING OF PERCEPTION 175 
 
 Are the changes that occur in the cells of the auditory centre 
 heard ? They may or they may not. If a gramophone starts 
 to play in the house next door just as a man is going to bed he 
 will certainly hear it ! But a man who is intent on some mental 
 work may not hear a knock at the door, while someone who is 
 with him but is not busy may hear it. And, of course, while he 
 walks through a busy street his eyes and ears receive a multitude 
 of stimuli, and, no doubt, transmit these to his brain, but he is 
 not aware of a large fraction of all this stimulation. Sensation, 
 then, may or may not be accompanied by changes of conscious- 
 ness. Now there is a stimulus leading to sensation, but there is 
 no " awareness," and, again, when the same stimulus is repeated, 
 we hear or see. Why in one case and not in the other ? 
 
 There is a " psycho-physical " law which attempts to relate 
 together the intensity of a stimulus that can be measured and 
 the intensity of the feeling of awareness that a sense organ is 
 being stimulated. Let the stimulus be a weight placed on the 
 hand held out straight, and let the subject be asked to say when 
 he feels the increase of weight, as (unknown to him) increments 
 of mass are added to the latter. Say that the weight is P, and 
 let small mcrements'AP, 2AP, SAP, etc., be added to P ; the 
 subject will not recognise an addition to the weight until this 
 addition has become a certain fraction of the original weight. 
 For instance, he will not recognise that P+ AP is greater than P, 
 nor that P-J-2AP is greater. He witt recognise that P-J-3AP is 
 greater than P, though he will not recognise that P+3AP is 
 greater than P+2AP. Suppose, now, that there is a strict 
 determinism, and that every perception is a function of its 
 stimulus; then it is clear that /(P)=/(P-hAP)=/(P+2AP); 
 also /(P)</(P+3AP); and it also follows that P=P-fAP, 
 P+AP=P+2AP, P+2AP=P+3AP, and that P<P+3AP; 
 wherefore a thing which is equal to another thing is also less 
 than it ! This relation of the intensity of a stimulus to the 
 dependent perception is perhaps the only case in psychology 
 where (after taking some liberties with mathematics) we can 
 make a differential equation: 
 
 dP 
 
 and clearly we are led to a logical contradiction when we do so. 
 It is evident, then, (1) that a certain physical stimulus may 
 
176 THE MECHANISM OF LIFE 
 
 give rise to a perception, or it may not do so; and (2) that physical 
 stimuli of different intensities may give rise to the same per- 
 ception. And therefore, when we attempt to relate our own 
 feelings with the physical stimuli which, we believe, " cause " 
 them, we do not find physical determinism. 
 
 Perception. 
 
 Here we must explain what we mean by " perception." The 
 term has a very great number of shades of precise meaning in 
 psychology and philosophy, and we cannot attempt to discuss 
 these, or even to choose from them. Quite arbitrarily, then, we 
 mean by perception the effect in behaviour of the body, or in 
 consciousness, that follows sensation. 
 
 We take the affection of consciousness first in order to clear 
 the way and render our discussion quite simple. The effect of 
 a knock at the door may be nil (when we are absorbed in other 
 work and do not hear it), or it may be that we do hear it. The 
 sensation that is, the train of physical events that we have noted 
 above is the same in both cases, but in the latter -case there is 
 perception : we recognise something outside ourselves. Or in walk- 
 ing along a street we recognise a friend, when there is perception ; 
 or we fail to do so if we are " absent-minded." Yet we may be 
 quite sure that an image of the friend was formed on our retinas. 
 
 The Objective and the Subjective. In perception, as thus 
 illustrated, there is a " sensible object " the man who knocked 
 at the door, or the knock at the door, or the knocker, or the 
 man whom we recognised in the street. A modern, scientific, 
 realistic philosopher would say that there were certainly sensible 
 objects which stimulated our organs of sense, and there were also 
 " representations," or mental images, of those sensible objects in 
 our consciousness, the sensible, external things which originate 
 physical stimuli being " objective " to us, while the images, or 
 mental representations, were " subjective." A strict idealist (for 
 there are many kinds of idealism) would say that it was quite 
 improper to speak of an external, sensible object, and that all 
 that we could be sure about was the change in our consciousness. 
 Really he ought not to explain anything of the kind to us, for all 
 that he ought to be sure about would be his state of conscious- 
 ness, we being only affections of that and having no existence 
 except in his mind. No philosopher goes that length, and yet it 
 is the logical outcome of idealism ! 
 
THE MEANING OF PERCEPTION 177 
 
 Is it not rather absurd to distinguish quite rigidly between the 
 objective and the subjective ? Of course, for idealism everything 
 is subjective, but for realism what is the difference, or, rather, 
 where does the objective region merge into the subjective one ? 
 What is the sensible object, say, in hearing a gramophone play ? 
 Is it the nervous change in the auditory region of the cerebral 
 cortex, or the nervous impulse, or the vibrations of the auditory 
 membrane, or the sound waves in the air, or the vibrations of 
 the diaphragm of the gramophone, or the scraping of the needle 
 along the groove in the disc, or the vibrations of the vocal cords 
 of the person who sang the record, or what in a series of stages 
 still further back indefinitely until one gets tired of enumerating 
 them ? Nevertheless, common sense recognises something that 
 is, in a kind of way, the object of our recognition either the 
 gramophone, or the disc, or the singer. Is it a new " record " ? 
 Then that is the object. Or a new machine with a different kind 
 of door or shutter that makes it more resonant ? That is the 
 object. Or a new song by a well-known vocalist ? Then she is 
 the object. Is it not plain from this analysis (which an ingenious 
 reader can make in other cases) that the objective in our per- 
 ception is that phase of an endless series of phases which interests 
 us, on which we have acted in some kind of way (for we bought 
 the machine, or record), or on which we intend to act (for we 
 may decide to go and hear the vocalist in person) \ 
 
 " Subjective " and " objective " are therefore attitudes on our 
 part to something which is happening, and they are attitudes 
 that are entirely relative to our interests for the time being. 
 And remember that to speak of " something " which is happen- 
 ing is rather a convenient than a strictly accurate way of speak- 
 ing. Is there something which moves or vibrates or causes a 
 phenomenon ? There may be, said Kant, a noumenon, or thing 
 in itself, which moved or vibrated. We do know the movement 
 or vibration, but we cannot possibly know the thing in itself. 
 Why, then, speak about it ? But again common sense does 
 familiarly speak about things in themselves, and so does experi- 
 mental science, and for the same reason: it is convenient to do 
 so, and it assists us in our acting and investigating. Just in the 
 same common-sense way we pick out from a long or indefinitely 
 prolonged train of events some one which is more accessible to 
 our action, or which interests us particularly, and we say that is 
 the sensible object. To a cerebral physiologist the chemical 
 
 12 
 
178 THE MECHANISM OF LIFE 
 
 changes occurring in the cells of the visual, cortical area might be ] 
 the sensible object of investigation, while to an optician the J 
 crystalline lens of the eye and the muscles of accommodation j 
 would be the significant things. 
 
 The Kinds of Perception. The above discussion suggests, I 
 then, that a perception is the prolongation into action of some 
 kind, or into virtual, or nascent, or contemplated action, of a 
 sensory process : it is an actual or potential sensori-motor process. I 
 Now it may be pure (in Bergson's terminology) when it is un- 
 accompanied by the exercise of memory, or it may be mixed ] 
 w r hen memory is a factor in the prolongation of the sensation into ^ 
 action. We may act in many ways reflexly, instinctively, 1 
 automatically, or habitually, as well as deliberately. In reflex j 
 actions the unconsciousness of the outer event, or stimulus, that 
 initiates the acting may be complete, and obviously a man may 1 
 do very difficult (but learned) things without thinking about what 
 he is doing. May we say that automatic or habitual activity is ] 
 unconscious, or that there is some kind of mental or psychical j 
 accompaniment subconsciousness it may be called and that 
 this is pure perception ? There does not seem to be any reason j 
 why we should not say so. A man may do some skilled work in j 
 a shop, for instance, filled with the continual noise of machinery, I 
 and he may think all the time about something quite different, j 
 He may not be conscious (in the usual sense of the word) of the j 
 noise of the machines, but he will certainly note the cessation ] 
 of the latter, or even a change in the rhythm. At any moment, j 
 also, during the performance of the " automatic " work on which 
 he is engaged, some slight deviation from the usual course of j 
 things may compel the attention to it, so that, although he was j 
 not conscious of his own activities in the sense that he deliberated 
 them, some kind of psychical processes certainly was their 
 accompaniment. 
 
 When the work is unfamiliar or difficult, or when we act in 
 circumstances that are unusual, then full and vivid consciousness 
 of what we are doing accompanies sensori-motor activity. Thus 
 a skilled musician may play a scale with complete mental detach- 
 ment from the sensations of musical intervals, but he will attend 
 to the latter with the greatest care if he is playing a composition 
 for the first time. Deliberated and unfamiliar action therefore 
 involves perception that must be distinguished from that which ac- 
 companies reflex, instinctive, or habitual sensori-motor activities. 
 
THE MEANING OF PERCEPTION 179 
 
 The Problem of Free-Will. 
 
 Here we must enquire into the question whether or not volition 
 that is, the freedom to act or not to act, to act now rather than 
 then, or in this way rather than in some other way is truly a 
 factor in our behaviour. That we are really free in this respect 
 is a belief held intuitively by most men and women, and it is one 
 upon which codes of morality and systems of punishment and 
 reward are obviously based. Yet it is no less evident to most 
 people that the majority of their actions are not truly free. 
 Inclinations have been formed by imitation and habit, con- 
 ventions have been established, instinctive tendencies have been 
 transmitted to us by heredity, and material conditions obviously 
 impose restrictions upon our freedom of acting. The consequence 
 is that most of the things that men and women do are the results 
 of '* causation " in just the same sense that physical events are 
 caused ones. We are naturally strongly influenced by the 
 methods of science which show us more or less strict determinism 
 everywhere among inorganic things. All that happens there, 
 we see, happens because something else has previously occurred, 
 and will, in turn, lead to the result that something predictable 
 will happen in the future. 
 
 Further, the strict sequence of cause and effect that charac- 
 terises physico-chemical events is, to some extent, based upon a 
 convention. We agree that the law of conservation applies to 
 all real things, and since there are phenomena to which it obvi- 
 ously does not apply (dreams, hallucinations, and spooks), we call 
 the latter unreal. This is quite justifiable, since the laws of 
 energy are the expressions of our ability to act upon things, and 
 spooks are clearly nuisances to us in that we cannot act upon 
 or control them. But is it not clear fchat the obvious exceptions 
 to determinism are some mental phenomena, and, that being so, 
 is not the logic of applying the law of conservation to mental 
 experience faulty ? 
 
 However, we adopt Bergson's conclusions here, although we 
 do not summarise his argument. We regard any attempt to 
 disprove the freedom of the will as invalidated by the fallacies 
 to which Bergson draws attention. That leaves us free to 
 accept the natural belief in volition held by most people, and to 
 see in what direction it leads us. 
 
180 THE MECHANISM OF LIFE 
 
 Iiide termination in Acting. 
 
 If our earlier summary of the modes of organic action is an 
 accurate one and we think the reader can easily find that it is 
 so by reference to the literature then it appears that we can 
 make a series of actions, studied " objectively," such that at the 
 beginning of this series there is determination, and at its end 
 indetermination. Tropistic, decerebrate, reflex, instinctive, 
 automatic, and deliberated sensori-motor activities constitute 
 the main terms of this series. Apparently tropisms, the reflexes 
 of spinal animals, and some reflexes of normal animals, are 
 characterised by determinism, while the movements of the higher 
 animals acting normally display what we cannot but regard as 
 spontaneity of behaviour. Of course, it can always be said that 
 the spontaneity is only apparent, that it must be determined, 
 and that we could show this if we knew all the factors. Now, 
 to say that, merely means that we dogmatise. 
 
 Reflecting on our own behaviour that is, approaching the 
 problem from the " subjective " side we seem to find a similar 
 series of terms. Reflexes, instinctive actions, and habitual 
 actions really make up the majority of the things that we do. 
 Many of these activities are inherited ones ; such are all forms of 
 organic functioning, customary modes of locomotion, etc. A 
 great number of other actions have been learned during our 
 individual lif etimes, and are repeated mechanically and without 
 deliberation. Others, again, are conventional in the sense that 
 they are the outcome of obedience to custom and law, or are the 
 responses to what is called " mass suggestion " ; thus, most people 
 " instinctively " like to do the things that the crowd does. Now 
 in all such activities true freedom is either wanting altogether or 
 it is only a minimal factor in our behaviour. In organic func- 
 tioning, reflexes and habitual actions, we do not think about what 
 we are doing that is, consciousness of our activities is absent 
 or it is dim. Perception is what we have called " pure." In 
 the limit, as the mathematicians say, our behaviour approximates 
 to that of the muscle-nerve preparation, or even towards that of 
 the reaction of the compass needle to the bar magnet. In the 
 latter case there is full and strict determinism, and everything 
 that is occurring in the magnet and magnetic field is being 
 prolonged in the reactions of the compass needle. 
 
 But in actions that are new to us, in doing things that we are 
 learning, such as playing a scale on a piano, swimming for the 
 
THE MEANING OF PERCEPTION 181 
 
 first time, shooting, drilling, or steering a boat, the conditions 
 are, most obviously, different. There is trial and error, and 
 repeated trials with increasing success. There is hesitation, 
 embarrassment, deliberation, and choice. There is full and 
 vivid consciousness of all the conditions; in short, full percep- 
 tions of the enviroment, of the results of our action upon it, and 
 memory of past actions that are relevant. While the unfamiliar 
 action is being learned there is indetermination of behaviour, and 
 some true freedom, and what we can only call creative activity. 
 This indetermination. expressing itself as hesitation, deliberation, 
 and choice, is our perception; but now it is perception that 
 involves the factor of memory, and which in turn is going to 
 leave behind it both memory and habit. 
 
 And so an evolutionary process is essentially a creative one, a 
 notion that need not alarm anyone who thinks about art as 
 creative, as evolving something new. Of course, it can be 
 argued that, just as the evolution of a planetary system from an 
 original nebula is a physical process which is strictly determined, 
 so also must an organic evolutionary process be a determined 
 one. That means that there is nothing new; that if we knew 
 the differential equations that described the state of the nebula 
 we could predict the orbits and masses of the planets and nebula 
 that are to be evolved. Now does this strict physical deter- 
 minism apply to the process of evolution of species ? 
 
 For any evolutionary hypothesis always starts by assuming the 
 existence of variations, and the latter must be shown to be physi- 
 cally determined. But between the simple, muscle-nerve pre- 
 paration and the behaviour of a higher animal we cannot make 
 any absolute distinction, and variations of form and functioning 
 and habit that is, the materials, so to speak, on which evolu- 
 tion works are activities which are essentially the same as 
 muscular responses, tropisms, reflexes, instinctive and habitual 
 and spontaneous actions. And if the proof that the actions of 
 an animal are not really free, but are determined, breaks down 
 (as we believe that it does under Bergson's analysis), how can 
 we maintain that organic variability is physically determined 
 rather than indeterminate, and thus new ? Generally an action 
 is not wholly free, we saw, and generally also a variation is not 
 wholly new. But just as action may generally show some inde- 
 terminism, so variations generally show something new. These 
 new things (mutations) are the sources of evolutionary change. 
 
182 THE MECHANISM OF LIFE 
 
 Memory and Habit. 
 
 The simplest kinds of organic responses, such as the helio- 
 tropic movements of a plant or infusorian or insect larva, or 
 the responses of a muscle-nerve preparation to artificial stimuli, 
 are therefore physically determined, or are nearly so. Reflexes 
 and instinctive actions are also largely determined, but we find 
 that there are often variations from the usual kind of acting. 
 Spontaneous, intelligent, and deliberate actions are very different 
 from the former categories, and we are quite unable to explain 
 them in the way we describe a reflex. What, then, is the 
 difference ? Plainly that these higher forms of acting involve 
 experience ; the response that an animal makes to a stimulus 
 depends not only upon the physical nature of the stimulus, but 
 also upon the effects of former stimuli, and the responses made 
 thereto under analogous conditions to those in which the present 
 stimulus occurs. Instances illustrating this can quite easily be 
 given, but we leave the reader to find them for himself by reflect- 
 ing upon his own behaviour. A cat, says Mark Twain, which 
 sits down on a hot stove never sits on one again, but neither 
 does she sit down on a cold stove ! 
 
 What, then, is experience in the physical sense, and how is it 
 stored in an animal ? It is stored in two ways : first as motor 
 habits, and next as pure memories. 
 
 The whole contents of Chapters VI., VII., and VIII. of this 
 book deal with the mechanism of motor habits; the receptor 
 organ, afferent nerve, intracerebral centres and nervous tracts, 
 efferent nerve and effector organ such a chain of structures is 
 the unit mechanism that underlies a motor habit. Any skilled 
 technique say the ability quickly and neatly to deal a pack of 
 cards involves a preparation, in the course of which such action 
 was practised somewhat laboriously and clumsily. But with 
 repetition experience accumulates that is, certain afferent 
 nervous impulses become directed easily along certain cerebral 
 tracts, and pass out from the brain into certain definite efferent 
 nerves and muscles, properly timed and co-ordinated, and so 
 produce the required muscular movements. That is to say, 
 definite nervous paths become established or laid down. Ev-ry 
 time the same stimuli, in the same conditions, recur, these paths 
 are used the more easily. This is the formation of a motor habit, 
 and all education or training which leads to a technique of any 
 kind establishes such paths and habits. 
 
THE MEANING OF PERCEPTION 183 
 
 One must think of the brain of a higher animal as the junction 
 of nervous paths which are extraordinarily numerous. All 
 evolution of the central nervous system increases the number of 
 alternative paths which a stimulus entering the brain may take, 
 so that any receptor organ anywhere in the body, may be re- 
 garded as capable of giving rise to a nervous impulse that may 
 enter the brain and become shunted on to the paths leading to any 
 other part of the body. Lay down some " beaten track " in this 
 labyrinth, and we establish a motor habit and so create experience. 
 
 And what is common to all automatic activity, whether it be 
 that of a heliotropic insect larva, an instinctively acting bird or 
 mammal, or a trained worker of any kind, is the existence of 
 experience, which is expressed by these ready-made motor 
 mechanisms. But the responses of the heliotropic and instinctive 
 animals are the results of inherited motor habits, whereas the 
 skilled worker has to make them for himself, and he does not 
 transmit them by heredity. 
 
 Quite plainly, pure memory is something different from this 
 motor habit experience. The latter is the persistence of the 
 mechanism of receptors, nervous tracts, and muscles that were 
 affected in the formation of an action of some kind, but pure 
 memory we must regard as the persistence of the perceptions that 
 accompanM the action when it was being learned. It becomes 
 manifest, apart from the motor habit (which is manifested in 
 action) in pure recollection, in reverie, in dreams, or in the power 
 of visualisation that most people have in some degree. We must 
 think of all the perceptions that we have ever had as continuing 
 to exist somehow, mingled together to form a " multiplicity in 
 unity " that is, as something which is manifold in its nature, 
 but which is not spread out, as it were, in space. The elements 
 (or constituent notes) of a musical arpeggio are heard, one after 
 another, and so are separate from each other in time; but the 
 same notes, played together perfectly, are heard by a musician 
 and recognised as individual, although they blend to form a mani- 
 fold or a multiplicity in unity. Memory, then, is something 
 analogous, the fusion, as it were, in one of all our past perceptions. 
 Where is it, and how is it stored ? Undoubtedly it hangs 
 together in some way with the cerebral substance, and the older 
 hypotheses stated that memories were stored away in the cells 
 of the grey matter of the brain, and could be extirpated by the 
 removal of the latter structures. Now, although few biologists. 
 
184 THE MECHANISM OF LIFE 
 
 would say as much nowadays, this hypothesis exists in almost 
 as crude a form. " Consciousness," says Jacques Loeb, " is only 
 a metaphysical term for phenomena which are determined by 
 associative memory." " All life phenomena are ultimately due 
 to motions or changes occurring in colloidal substances. The 
 question is, which peculiarities of colloidal substance can make 
 the phenomena of associative memory possible ?" 
 
 Now, what do we discover when we investigate the changes 
 in the colloidal substance of the nervous system that follow upon 
 sensation ? Obviously nothing but displacements, as we showed 
 on pp. 16770. A minute quantity of energy enters the body as 
 the result of the excitation of a receptor organ, and this energy 
 is transmitted along an afferent nerve as an impulse. What 
 becomes of it ? 
 
 If the stimulus leads to a reflex, as when we involuntarily 
 contract the pupils of the eyes when we are suddenly exposed to 
 a bright light, then the thing is clear : the energy of the afferent 
 impulse is transmitted (still as a wave of molecular displace- 
 ments) through the visual and oculo-motor centres, along the 
 oculo-motor nerves, and into the muscles of the iris. There it 
 releases muscular energy, leading to contraction of the pupil. 
 Some of the energy (but a very small percentage) is doubtless 
 dissipated as heat during the nervous passage, just as it would 
 be in an ordinary physical process. There is no conscious per- 
 ception here, and we can account for all the energy put into the 
 sensation; but what are the circumstances when the stimulation 
 of a receptor organ gives rise to conscious perception, and also 
 leads to some muscular response ? Again, some of the energy 
 passes on to effect the releasing transformation, and again some 
 is dissipated. But an image, or mental representation (the per- 
 ception), also arises, and can we say that some of the energy trans- 
 forms into this ? That is to say, does physical energy, which we 
 measure as the displacements of material bodies as space magni- 
 tudes really transform into consciousness, or perception, which 
 is certainly not the displacement of material bodies, and cannot 
 be measured in terms of space ? No one ventures to say that. 
 
 The perception has been called an " epiphenomenon," and has 
 
 been compared with the luminous trail left by a meteorite which 
 
 blazes up as it enters the earth's atmosphere; perhaps it is 
 
 unnecessary to point out the difficulties of this explanation ! 
 
 For the perception does not fade away when the action has 
 
THE MEANING OF PERCEPTION 185 
 
 occurred, as the luminous trail of the meteorite fades when its 
 combustible matter has been consumed. It persists in memory, 
 and it crops out again and again in our consciousness, obtruding 
 itself, as Bergson says, even when we would like to forget it. 
 Something, then, comes into existence as the consequence of 
 sensori-motor activity, and we cannot prove that this is a 
 potential energy stored in the brain, for the latter does not 
 appear to be different when memories coexist with it and when 
 they do not; besides, we can account for all the energy of a 
 sensational process in other ways that are more consonant with 
 scientific methods. And this something continues to exist, and 
 it is utilised (as experience) in future actions, and yet it is not 
 wasted (so, again, it is not energy). And it is real, because it 
 does not fool us (as does a spook), but leads us back to results 
 that are useful and can be verified. What, then, is it, since, 
 apparently, it is not energy ? 
 
 One can only say that it is memory, or " spirit," as Bergson 
 calls it the name does not matter so long as we do not associate 
 with it the (necessarily) non-scientific ideas that are connoted by 
 the term " soul." It is something that belongs (as Bertrand 
 Russell says) to the " ultimate furniture of the world " that is, 
 it is indefinable, just as energy itself cannot be defined in terms 
 of anything else. It may not be a concept peculiar to biology, 
 as we shall try to show later on, but in the meantime we may 
 conveniently regard it as such. Physiology is not concerned 
 with it, and investigates only the chemical and physical processes 
 in which mind or spirit becomes materialised; and psychology, 
 disregarding the concept of energy, starts with that of mind and 
 proceeds with its analysis. 
 
 The *' Categories of the Understanding." 
 
 There are, therefore, both body and mind, or " matter " and 
 " spirit." We shall not argue that this means that there is a 
 dualism, believing with Bergson that matter and spirit are only 
 tendencies that are contrasted in some way or other, or opposite 
 directions, perhaps, of the same series of changes. Assuming, 
 then, the existence of mind as something apart from the organic 
 activities in which it is expressed, we may regard it as legitimate 
 to investigate its working, while disregarding the " link with 
 body." At any rate, it is " up to " science to take that road 
 provisionally and to see to where it leads. 
 
186 THE MECHANISM OF LIFE 
 
 Now quite a considerable fraction of all the " 
 that have ever been written deal with the question, Have we 
 ideas that exist prior to ordinary experience ? We regard this ] 
 question as having been finally resolved by Kant in the Critique ] 
 of Pure Reason, even if it had not been answered in the affirma- j 
 tive by the methods of science. There are " ideas " or " cate- 
 gories " that are " innate " in the sense that they are in the] 
 human mind before the latter attains experience of the outer \ 
 world. Perhaps it is more correct to say, with Driesch, that! 
 they exist and are awakened by experience. Evidently some- j 
 thing is there that makes use of or co-ordinates experience, but 
 from the point of view of this book we prefer to think about; 
 modes of mental operation rather than " innate ideas." 
 
 This point of view is actually forced upon us, since we must ; 
 consider not only the human mind, but also that of the animals 
 other than man. Between the generalised intellect of the highest 
 kind, which was analysed by Kant, and that of the higher animals 
 other than man there exist, even now, almost all possible grada- 
 tions. Further, the gap between the higher mammals and] 
 primitive man has been filled, almost in our own day, by the] 
 discovery of extinct human races, and below these forerunners 
 of modern man are the lower mammals, the arthropods and other] 
 animals, in which we may easily trace the workings of incipient! 
 intelligence. We must assume a series of transitions between 
 man and the lower animals both on the physical and the psychical - 
 sides, and we must assume, provisionally at all events, that a] 
 " Critique of Comparative Reason " may yet be made, and that 
 until it is made the analysis of the human understanding will] 
 always be defective. 
 
 A theory of knowledge and a theory of life, says Bergson, are, 
 not to be distinguished. What does this mean ? We note, first; 
 of all, that theory of life and theory of evolution are one and the 
 same thing. No form of life stands apart and is stable, for, 
 obviously, every one man, the anthropoid apes, the lower 
 mammals, the arthropods are phases in an evolutionary flux. 
 There must have been continuity at all times in this flux, for 
 evolution has proceeded by little steps, or changes, which are 
 usually so small as to appear insensible to ordinary observation. 
 Every one of these little steps is an adaptation some change of 
 functioning whereby the organism becomes the more able to act 
 upon the outer world. The variation (or, better, mutation) may 
 
THE MEANING OF PERCEPTION 187 
 
 be some nutritional change, a change of bodily form or colouring 
 rendering the animal less conspicuous to its enemies, a sharpened 
 sensation, an increased power of locomotion, a new bodily 
 weapon such as stronger claws or teeth, a new response to the 
 usual stimuli. It may be one of very many kinds of change, but. 
 it /.? always something quite neiv. Some time or other it occurs for 
 the first time in the evolutionary process, and, having occurred, 
 it becomes a habit. 
 
 Reflect upon the discussions of the previous chapters, and it 
 will be seen that we cannot make any clear distinctions anywhere 
 between the " highest " forms of behaviour that is, the intel- 
 ligently chosen responses to new situations in which we utilise 
 memory and appear to act spontaneously and the lowest forms, 
 such as those in which an organism acts tropistically. The 
 freely chosen and spontaneous action may become a motor habit, 
 something incorporated in the " make-up " of the animal that 
 evolved it, but so also does the tropism. Between these ex- 
 tremes, the freely chosen activity that involves the utilisation 
 of memory, reflection, and reasoning, and the very simple 
 tropisms there are all gradations reflex actions, instincts, auto- 
 matic and habitual behaviour, etc. Now can we say that the 
 intelligent action occurring for the first time, and thus accom- 
 panied by or involving perception, is radically different from the 
 tropism, and that when the latter first occurs as a mutation it 
 also is not accompanied by perception 1 
 
 Just because of the continuity in organic form and behaviour 
 that evolution shows us we seem obliged to assume that whenever 
 an animal does something quite new there is perception, which 
 has something in common with that which we call perception in 
 ourselves when we do novel actions. The mutation means that 
 the animal enters into a new relationship with the outer world, 
 and just because it does so it becomes aware of some external 
 things in different aspects from those in which they formerly 
 occurred in its experience. Obviously this appreciation of its 
 new relations with the other world is knowledge, which can, 
 therefore, only be attained when there is action that presents 
 novelty. This, then, is what may be understood by Bergson's 
 saying that theory of life is also theory of knowledge. 
 
 In the new action there is deliberation and choice, the " turn- 
 ing the situation over in the mind," reflection in short, the 
 application of pure memory (but not motor-habit memory, for 
 
188 THE MECHANISM OF LIFE 
 
 the activity is to be a new one, and cannot therefore make use of 
 any established cerebral and nervous paths). The memory is 
 applied in a number of ways, and if we may compare mental 
 operations with certain modes of animal acting, we might think 
 about them as beginning by a method of trial and error. The 
 memories of past stimuli and their responses are tried in one way 
 or other until a way is discovered that is likely to be successful. 
 The cerebral paths are then discovered or made, and the response 
 is effected. The mental operation that was employed and which 
 succeeded is thus an acquired one, and we must suppose that it 
 is transmitted by heredity. 
 
 These transmitted mental operations whereby pure memory is 
 utilised in the elaboration of new activities are the Kantian 
 categories. 
 
 Apparently, then, we think (that is, employ the categories) 
 because we have acted, or because we are about to do so. That 
 means that we ought to show that all thinking, all intellectual 
 constructions, are actual, or nascent, or virtual ways in which 
 we are acting, or might act upon the outer world. It would not 
 be difficult to show that this is really true, that Newton, for 
 instance, in deducing the law of gravitation, was virtually acting 
 upon the earth-moon system. He could not, of course, move the 
 moon in its orbit with a certain velocity, nor could he pull it 
 towards him, but he could produce accelerations in the velocity 
 of an apple, and he could exert such muscular force as would 
 counteract the acceleration of the latter body when it was free 
 to fall towards the earth. Now, by " producing " this personal 
 power of acting he could imagine himself moving the moon in its 
 orbit, and at the same time pulling it towards the earth in such 
 a way that the observed movements of our satellite were as they 
 would have been if the action had been a real instead of a virtual 
 one. Something like this occurs whenever we think, even in the 
 most abstract way: we do not actually do things, but we " set the 
 points," so to speak, in order that certain actions might occur if 
 it were to become practicable to effect them. 
 
 
 Space and Time.* 
 
 There are, said Kant, the categories of quantity, quality, 
 relation, and modality; these are ways in which we think about 
 the objects of sensible experience. There are also the " pure 
 
 * See also Appendix II. 
 
THE MEANING OF PERCEPTION 
 
 189 
 
 intuitions " of space and time. Now it is useful to consider the 
 latter ideas for a moment, for this will enable us to see that the 
 categories have another aspect they are mental constraints. 
 What, then, is abstract space ? Space, we may say for the 
 moment, has three dimensions: in whatever way a body may 
 move we can describe its position at any moment by placing it 
 in a " frame of reference." It is distant so much above or below 
 a plane (1) which stretches out horizontally to an indefinite 
 extent; it is also distant so much to the right or left from 
 another plane (2), which is at right angles to the first one, and is 
 also indefinite in extent; and, lastly, it is so much in front or 
 
 FIG. 49. THE THREE CO-ORDINATE PLANES OF EUCLIDIAN SPACE. 
 
 behind a third plane (3), which is perpendicular to the first two. 
 We suppose, in making this three-dimensional frames of reference, 
 that we are situated at the point where all three planes intersect 
 each other. Plainly, then, if we move we carry the frame of 
 reference with us, and the position of the body we are observing 
 changes relatively to the three co-ordinates. 
 
 Thus the motions of all bodies outside our own are relative to 
 our motions. Conceivably we might move in such a way as to 
 keep the frame of reference (which we carry with us) in the same 
 position relatively to a body which would be moving if we were 
 at rest, but which would appear to be at rest relatively to our 
 moving frame of reference. Motions of external objects, then, 
 
100 THE MECHANISM OF LIFE 
 
 can be exactly compensated by the opposite kinds of motions of 
 our own body.* 
 
 That means that our three-dimensional space is merely al 
 description of our three degrees of freedom of mobility. If we were ' 
 immovable both in body and eyes, fitted tightly into a solid, 
 transparent universe, we could not have the conception of space,! 
 although we could perceive phenomena to succeed each other 
 there might, for instance, be successive light and darkness. If 
 we imagine ourselves fitted closely into an indefinitely long 
 drain-pipe, we should have freedom of mobility in our dimen- 
 sion (xox) ; if we had to crawl wedged in between two immensely l 
 stretched out plates of glass, we should have freedom of mobility 
 in two dimensions (xox and yoy). If we could move everywhere 
 between the two plates of glass, the latter w r ould be parallel to 
 each other ; note, however, that we need not think about the 
 drain-pipe being straight, nor of the plates of glass being flat 
 both might be curved, and still our space would have one or twoj 
 dimensions. Now remove the plates of glass so that while we 
 are still able to move backwards and forwards, and from side 'to 
 side, we can also gravitate upwards or downwards. 
 
 In the latter case we have three degrees of mobility, and our 
 space becomes three-dimensional (xox, yoy y and zoz). As yet 
 our freedom of mobility is not quite the same in all three dimen- 
 sions. We can move forwards and backwards and from side to 
 side on smooth ice with equal facility, though we always have 
 the " pure intuition " of backwards and forwards (because w 
 can only see in front), and we always have that of right and left 
 (because the functioning of the nervous system is bilaterally 
 asymmetrical). But we can with difficulty raise ourselves above 
 the level of the ice, and we do not usually fall through it. There- 
 fore the intuition of movement up and down is not nearly so 
 intense as are those of movement in the other dimensions. So, 
 also, we judge distances on the horizontal more accurately than 
 we do in the vertical dimensions, and we are afraid to look down 
 through " empty space " from a height that is, we have little 
 control of our mobility in that way, and so we are afraid to fall. 
 And so also non-Euclidean geometries are less familiar and 
 more difficult to us because they express less easily our intuitions 
 of freedom of movement. The four-dimensional geometry is still 
 more difficult, because most people do not appear yet to have 
 
 * That is to say, all motions would be relative if we did not have the 
 absolute intuition of motion of our own body. 
 
THE MEANING OF PERCEPTION 191 
 
 any intuition corresponding with freedom of mobility in a fourth 
 dimension. Clearly in developing four-dimensional gecmetiy 
 mathematicians are trying to remove a mental constraint from 
 their space intuitions. 
 
 Our notion of space is not, then, a " form of our sensibility " so 
 much as an intuition, or feeling of our freedom to move in the 
 ways that are possible to us. " Abstract space " is not an 
 absolute something which we contemplate, but it is just the way 
 in which we describe our mobility of body assuming, of course, 
 that this has become equally well developed in three dimensions. 
 Neither is time something absolute which we live through. 
 When we speak about it and measure it we do nothing more than 
 observe certain simultaneously occurring events. Last night, 
 when the hands of the clock pointed to twelve midnight, a certain 
 star crossed the meridian, and to-night, after the hour hand of tht 
 clock has twice turned through 360, the same star again crosses 
 the meridian. The time here is a space measurement, but there 
 is something else. We had to live through that space measure- 
 ment, if one may say so, and our intuition of time past is our 
 mental duration. If we had no memory, all that we should know 
 would be the appearances of the star on the cross wires of the 
 telescope simultaneously with the positions of the hands of the 
 clock indicating twelve midnight. But we remembered the lapse 
 of twenty-four hours because, during that period, our perceptions 
 have endured as memory. 
 
 This matter, however, we consider in the following chapter. 
 Meanwhile, we have suggested to the reader an interpretation of 
 the Kantian categories, which seems to be that forced upon us 
 by the study of biology. These mental operations are not " pure 
 conceptions of the understanding," the latter being thought 
 about as something sitting still and apart from the body, so to 
 speak, and contemplating the universe through the sense organs. 
 They are nascent or virtual actions of the body, each of which 
 changes in some way the system which is made up of the body 
 and the universe. In thinking and employing the categories of, 
 say, quantity and substance, we are really acting schematically. 
 Let an analogy make this clear : a man who moves a little lever, 
 and so actuates a hydraulic ram, is not really lifting an enormous 
 weight, nor does a signalman move the trains that pass over a com- 
 plicated system of junctions. So the categories of the understand- 
 ing are the ways in which we turn stimuli (the " sensible objects of 
 perception ") into the bodily mechanisms that we will to respond. 
 
CHAPTER XI 
 ON THE NATURE OF LIFE 
 
 IN our own time, as at the middle of the seventeenth century, 
 speculative biology has come to an impasse. Now, a word more 
 about the Cartesian physiology. Descartes, as we have seen,* 
 made a mechanical theory of the animal body, explaining all the 
 ordinary functioning that is, the motions of the muscles and! 
 fluids and the activities of the glands by means of the physics 
 known to him. He saw very clearly that there was little 
 difference between the anatomical structure of man and that of 
 the other mammals, and so he was compelled to hold that in 
 both cases the animal body was an automatic machine, not 
 essentially different from those that had been constructed by 
 expert mechanicians, but, of course, immensely more complicated. 
 Nevertheless, there were profound differences between the 
 behaviour of man and that of the lower animals, and to account 
 for this he supposed that the human body possessed a " sensitive 
 soul " which was not contained in the bodies of other animals. 
 
 It would be difficult for a biologist to support this latter 
 assumption in these days, because we are now thoroughly con- 
 vinced that an evolutionary process has occurred, and that there 
 must, therefore, be absolute continuity between the human and 
 animal minds. We have also studied the psychology and 
 behaviour of the lower animals with a degree of care, and an 
 intuitive understanding, of which we can find no trace in the 
 biology of the Continental schools of the seventeenth century. 
 Further, the study of development shows that precisely the ?ame 
 processes occur during the train of events along which the mature 
 animal body comes from the unfertilised egg, whether we in- 
 vestigate man, or the higher mammal, or the fowl, or the fish. 
 Therefore we cannot now place a " sensitive soul " in the human 
 body and deny its presence in that of the lower animal. 
 
 In the seventeenth century, then, there was an absolute dis- 
 tinction between man and the lower animals. The former was 
 dominated or controlled by the soul, which was an immaterial, 
 
 * In Chapter IX. 
 192 
 
ON THE NATURE OF LIFE 193 1 
 
 non-energetic agency, and therefore something that was quite 
 outside the scope of physical investigation. This was Cartesian 
 vitalism, an attitude that persisted throughout the greater part 
 of the eighteenth century. 
 
 But with the development of modern chemistry and physics 
 biology again applied itself to the investigation of modes of 
 functioning, and, with every new advance made by physical 
 science, our knowledge of the ways in which the animal body 
 worked became more and more intimate. Now it will be clear 
 to the reader, from what we have said in Chapter IX., that all 
 that has been attained by the application of physical and chemical 
 methods has been a great refinement of our analysis of organic 
 activities, and not at all an explanation of animal behaviour. 
 Just the same things may be said about hypotheses of develop- 
 ment and evolution. The best known of such Weismann's 
 theory of the germ plasm might have become a physico- 
 chemical one, but there is every indication that it has now quite 
 broken down, so that, as yet, we have no explanation of heredity, 
 which involves only the concepts of physics and chemistry. We 
 have, it is true, an analysis which is gradually becoming more 
 minute, but a little reflection will convince the reader that this 
 also is not an explanation. 
 
 Within the last generation there has been a recrudescence of 
 vitalism " neo- vitalism " it is now called, being obviously some- 
 thing that seems to be different from the Cartesian speculations 
 about the sensitive soul. At its best this is seen in the " psy- 
 choids " and " entelechies " of Driesch and others, concepts 
 which are applicable to living things only, and not to chemical 
 and physical phenomena. At its worst modern vitalism is 
 exhibited in the crude and even grotesque " spiritualism " which 
 has attained such a vogue with the less resolute thinkers of our 
 own generation. This, then, is the modern impasse to which 
 biology has come. Purely physico-chemical explanations of life 
 are not satisfactory, and the immaterial and non-energetic 
 agencies that are being invoked in their place can have no interest 
 for science, since they cannot be the objects of investigation. 
 
 Now it may help the reader in trying to apprehend this situa- 
 tion if he notes that physical science itself is apparently con- 
 fronted with a somewhat similar impasse. Trace the history of 
 physics during the last two centuries, and it will be seen that its 
 face is being changed all the while. There was the " classical " 
 
 13 
 
194 THE MECHANISM OF LIFE 
 
 mechanics of Newton and his successors; the rise and develop- 
 ment of atomic chemistry; the wonderful progress that followed 
 Faraday's discovery of the laws of electrical induction; and then] 
 the thermo-dynamic theory elaborated by Carnot, Joule, Rankine, >. 
 Kelvin, and Gibbs. Then came the electro-magnetic theory of j 
 Clerk Maxwell and those who followed him, and, in our own time, 
 the French and English work on radio-activity and the new physics] 
 built up by J. J. Thomson, Planck. Einstein, and Nernst. This 
 latter theory, which is still in the making, is obviously the bridge 
 that will lead us across the gulf between matter and the physics 
 of the ether. Apparently, we have yet to find whether or not] 
 physical science has said the last word in its effort to explain life. 
 
 The Laws of Thermo-dynamics. Go back now to our discus- 
 sion of the laws of thermo-dynamics. The first law, we have seen, ' 
 is really a kind of mental postulate, or convention. We make 
 up our minds that there is something that is permaiient that can 
 neither be created nor annihilated in a.11 the changes that occur 
 in nature. This something we call energy. If it appears to arise 
 out of nothing, or to vanish into nothing, we simply do not 
 believe that apparent result, and we proceed to invent poten- 
 tialities that will account for the appearances or disappearances. 
 Usually we are successful, and we find that our hypotheses of i 
 potential energies work, and we are so led to further discoveries. 
 Then we say that the things we are investigating are real ones, 
 since they are conserved. Or we may find (when we try to 
 investigate spooks) that our hypotheses astral bodies, higher 
 planes of being, telepathy, and the like do not work. They 
 mislead us. The test of their validity is that they should enable 
 us to predict, and we find no good evidence that they do so. 
 Therefore, being useless to us, and rather a nuisance, we say that 
 the phenomena of spiritualism are unreal. 
 
 That is the first law that something is conserved so that the 
 total quantity of it in our universe remains constant. It is not so 
 much a physical as an a priori " law," or mode of our mentality. 
 
 The second law is quite different, inasmuch as there is nothing 
 at all a priori about it. It merely describes, in the most com- 
 prehensive way possible, our experience of the universe. It tells 
 us how things happen: that water runs downhill; that a red-hot 
 poker taken from the fire cools down to the temperature of the 
 room; that a cigarette burns away, leaving behind it some ash, 
 water vapour, and carbonic acid gas, and evolving heat as it 
 
ON THE NATURE OF LIFE 195 
 
 burns. These things seem to us to be so inevitable that we find 
 it difficult to imagine them happening in the opposite way. It 
 appears to be ridiculous to think of water running uphill of itself; 
 of a table rising up of itself from the floor ; of a cold poker becom- 
 ing red-hot by exposure to the air; or of the ash, the water 
 vapour, and the carbonic acid combining together (with absorp- 
 tion of heat) to form a cigarette. But why should not these 
 things happen ? Puzzle over this, and we find we can give 
 no reason other than that no one has seen them happen. 
 
 Nevertheless, there is no logical reason why they should not 
 happen, and we can easily imagine them to do so. We can even 
 picture them or imagine ourselves so situated that they appear 
 to happen. Take the case of the red-hot poker: it is red-hot 
 because its molecules are moving so rapidly that their atoms 
 radiate and so give off visible vibrations. While they glow they 
 are vibrating much more quickly than are the molecules of air 
 which come into contact with them, and so they accelerate the 
 velocity of the latter just as a quickly moving billiard ball would 
 accelerate the motion of one that is moving less rapidly when the 
 two collide. There is, therefore, an excess of kinetic energy in 
 the motions of the molecules of the poker compared with that 
 in the molecules of the air in the room, and so the excess becomes 
 levelled down, so to speak. When the poker has cooled to the 
 room temperature all the energy which it contained is still in 
 existence, but it is now uniformly distributed between the metal 
 and the air which came in contact with the latter instead of 
 being concentrated in the poker. 
 
 Imagine that we could actually see the molecules and their 
 motions, and that we could photograph them so as to make a 
 kinematographic record. Working the latter, we should then 
 have a picture of the transfer of molecular motions from the hot 
 metal to the cold air, but if we were to work the record backward 
 just the same motions and. molecules would be thrown upon the 
 screen, but in the reverse order. W^e should first see molecules 
 of air and metal moving at certain rates in equilibrium with 
 each other, and we should then see the air molecules begin to 
 move more slowly and to give up some of their kinetic energy 
 to the molecules of the poker, which would move more and more 
 rapidly, until they become red-hot and give off visible radiation. 
 Or imagine a record of a cigarette smoker to be worked back- 
 wards ; we should then seem to see the smoke and ash solidify to 
 
196 THE MECHANISM OF LIFE 
 
 form the cigarette, and the latter to grow to its original, unlit, 
 unsmoked size. If the molecules were visible, we should see those 
 of the ash, the water, the carbonic acid gas, and the particles of 
 smoke combine together to make up the molecules of the cellulose 
 of the tobacco and paper. We should see energy taken up from 
 the air and become concentrated, so to speak, in the glowing 
 cigarette end, and then pass into the potential, chemical state in 
 the tobacco and paper. 
 
 Or, again, take the famous illustrations of Einstein's theory of 
 relativity. A man looking down from a stationary balloon 
 might have seen the explosion of a mine on the Menin Ridge, but 
 suppose that, during the few seconds of that occurrence, the; 
 balloon had been moving away from the earth with the velocity 
 of light; then our observer would have seen nothing happen, 
 although someone in a stationary balloon would have seen the 
 earth, stones, smoke, etc., thrown up into the air. Suppose that 
 the observer had witnessed the explosion from his balloon, and 
 that the latter had then immediately started to move away from 
 the earth with a velocity greater than that of light; then he 
 would (assuming he had a telescope powerful enough) have seen 
 the whole train of events proceed backwards. Smoke, stones, 
 dust, and earth would appear to come together from nowhere 
 and coalesce to form the solid ground. 
 
 Apparently, then, there is no logical reason why any physical 
 change should not go in either direction. Water runs downhill, 
 but it might run uphill. (Note that we can make it go uphill 
 by forcing it through a closed pipe, but what we are to imagine is 
 it running uphill itself. We can spend energy in forcing it up, 
 and then we find that we have spent more power than can be 
 recovered by allowing the water to run downhill again and work 
 a pump.) We can think of, or imagine, or picture, the water 
 going uphill of itself, and we cannot find any reason why it 
 should not except that it does not. 
 
 In the abstract, therefore, all physical changes are reversible, 
 but it happens that we are living in an universe in which they 
 are irreversible. This is our experience, and we can generalise it 
 in several ways. The most comprehensive way of doing so is to 
 say that in all things that happen a certain mathematical func- 
 v tion, called entropy, increases in quantity. What is meant by 
 entropy we shall explain presently. Note, in the meantime, that 
 the necessary condition that any phenomenon or physical change 
 
ON THE NATURE OF LIFE 197 
 
 may happen in our universe is that there must be available 
 lergy, and that this must transform into the inavailable form, 
 [n other words, energy must be concentrated somewhere, and 
 when a phenomenon occurs it becomes dissipated, or spread out. 
 Having become dissipated or spread out, nothing more can happen, 
 for this energy has lost its capacity for doing work. The second 
 law of thermo-dynamics states that in all natural changes energy IX 
 becomes dissipated, or entropy increases. 
 
 The Condition of the Universe. Next, we must consider this 
 problem, Is the universe finite or infinite in space and duration ? 
 Or we may put the question more clearly. If we were able to 
 travel away from our earth into outer space, like Richter's 
 Dreamer, should we at length enter regions where there were no 
 longer any stars ? Or if we could, like Mr. Wells's Time Traveller, 
 go indefinitely far back into duration, should we come to a time 
 when there were no earth and stars ? It may seem foolish to 
 suggest these questions, but, as a matter of fact, we can, pro- 
 visionally at least, answer the first one. There does seem to be a 
 limit to the number of stars in the sky, and it seems that these 
 form a cluster, or galaxy, which is finite. If the numbers of 
 stars were really infinite, certain consequences would be the 
 result. If there were no absorption of light by dark cosmic 
 bodies the night sky would be an uniform blaze of light, and even 
 if there were absorption of light energy would still be infinite 
 in some form or other. Such is not the case, and so we conclude 
 that the stellar universe which radiates energy is a finite one.* 
 
 Is its past duration also infinite ? We can easily imagine 
 ourselves travelling further and further away from the earth 
 without limit, and passing out at last from the region of stars, 
 or, more generally, of available energy. But we cannot think of 
 a time in the past, however remote, when the universe did not 
 exist, when there was no available energy. For the quantity 
 of available energy in our universe is decreasing, and in the past 
 it must have been greater than it now is. Was there a time, then, 
 when energy came into existence from nothing ? That implies 
 creation, and, since we hold to the law of conservation as a mode 
 of our thought, we cannot, therefore, think of a time when the uni- 
 verse began. We conclude, then, that its past duration is infinite.f 
 
 * The theory of relativity helps us here. The universe that we see 
 is finite, but unbounded. Its form of space is four-dimensional and 
 spherical. See Appendix II. 
 
 t See Appendix II. 
 
198 THE MECHANISM OF LIFE 
 
 Now we take the plain result of our experience that available 
 energy is continually decreasing in quantity (or that the entropy 
 of the universe is increasing). This is the bed-rock fact of our 
 experience: that everything that happens depletes the universe 
 of its store of available energy. " Every energy transformation 
 that occurs leaves an indelible imprint somewhere on the course 
 of events in the universe considered as a whole." There was an 
 original quantity of available energy in our universe, and this 
 is becoming exhausted as entropy increases. Now this quantity 
 being finite, and duration being infinite, it follows that the time 
 must come when there will be no available energy left (entropy 
 having attained its maximum), and so there must occur a com- 
 plete cessation of all energy transformations, or phenomena. 
 
 But if past time is infinite, why has not this physical death of 
 the universe already occurred ? No matter how great a lapse 
 of duration is required, we can still imagine this to be possible. 
 Here, then, we have our physical impasse. The second law, 
 solidly based on our experience, says that entropy tends towards 
 a maximum, when universal happening must cease. Past time 
 is without limit, so that, no matter how slowly entropy is being 
 augmented, the maximum ought to be attained. But the uni- 
 verse is still the locus of energy transformations, so that entropy 
 has not attained its maximum value. 
 
 Obviously the second law cannot be universally true, although 
 
 .is true of our experience. Somewhere or other, or at some 
 time or other in the universe, it must be reversed or evaded. 
 Now since it is not logically necessary that entropy should always 
 tend towards a maximum value, we must next enquire under 
 what circumstances the second law can reverse itself; under 
 what conditions may water flow uphill of itself, or may heat flow 
 of itself from a region of lower to a region of higher temperature. 
 
 The Reversal o! the Second Law. In order that such an 
 investigation may be possible we must choose some simple, 
 mathematically manageable case. Consider, then, the physical 
 condition of a small volume of some gas, say hydrogen, at 
 ordinary temperature and barometric pressure; it consists of an 
 enormous number of molecules which are moving very rapidly. 
 and so incessantly colliding with each other. It would be rather 
 like a swarm of midges in the air provided that the insects fl-\v 
 about in straight lines instead of avoiding each other. In a 
 volume of hydrogen equal to 1 decilitre (that is, a cube of 
 
 nas 
 ^it i 
 
ON THE NATURE OF LIFE 199 
 
 rather less than 2 inches along each side), there will be 
 100x2-705xl0 19 molecules. Each of the latter is moving, but 
 the velocities are variable within certain limits, and their average 
 is about 1-65X10 6 cm. per second. No molecule can move very 
 far without colliding with some other one. and the average free 
 path of a molecule is about 0-00000182 cm. We may regard 
 them as small, spherical bodies of about 2-17xlO~ 8 cm. in 
 diameter. The average velocity of movement depends on the 
 temperature (or, rather, what we call temperature is the variable 
 velocity of the molecules; the higher the latter the higher is the 
 temperature, and vice versa). 
 
 Evidently the molecules will be moving in every conceivable 
 direction, and so they must collide with each other in all sorts of 
 possible ways. Usually the collisions will take place at some 
 ansjle to the directions of movement; sometimes a rapidly moving 
 
 \ (Z)* * f3*f 
 
 \x 
 
 
 FIG. 50. DIAGRAM OF CHANCE COLLISIONS BETWEEN MOLECULES 
 
 IN A GAS. 
 
 molecule will overtake a more slowly moving one travelling in 
 the same direction, and sometimes two, which are moving in the 
 same direction, but towards each other, will collide " end on." 
 Being perfectly elastic bodies, no energy will be dissipated in 
 such collisions. After the collision the directions and velocities 
 will be changed in ways that are easily worked out from the 
 well-known theorems of mechanics. The reader may easily 
 construct these results by making use of vector diagrams, such 
 as those of Fig. 50. 
 
 Now by certain mathematical formulae (deduced by Clerk 
 Maxwell and others from the theorems of probability) the relative 
 frequencies with which encounters between pairs of molecules 
 moving in all possible directions, and with all the possible 
 velocities lying between the upper and lower limits, can be 
 calculated. An encounter in any possible way (such as in the 
 Cases 1 to 4 of Fig. 50) is equally likely to occur. But there 
 
200 THE MECHANISM OF LIFE 
 
 is one case which is of particular interest to us, that of two 
 molecules moving end on with the same velocity (Case 4), and 
 at any moment a certain very small fraction of all the pairs of 
 molecules must collide in such a way. It is possible that all, 
 or |, or J, or T ^, and so on, of the total number may collide 
 end on, but it is much more probable that y^ will collide end 
 on than will }, and so on. 
 
 It is possible, we say, that all the molecules will at the same 
 moment collide in pairs, and end on, as in Case 4, Fig. 50, and 
 the probability that this sort of encounter may simultaneously 
 occur throughout the whole volume of gas can be calculated. 
 Now, though possible, the chance of this occurrence is almost 
 incredibly small; small as it is, however, we must consider it in 
 speculative reasoning. 
 
 Imagine now our decilitre of hydrogen enclosed in a box 
 divided into two equal parts by a partition, and imagine the walls 
 of the box and the partition to be made of some material abso- 
 lutely impermeable to heat. Let there be a hole in the partition 
 closed by a valve which is also a non-conductor of heat. Let 
 the gas in the right-hand side have a temperature of 20 C., and 
 that on the left-hand side a temperature of 10 C. 
 
 In such a case the system contains available energy. There is 
 the same mass of gas in either division, but that on the right has 
 a temperature of 20 C., and so its pressure is higher than the gas 
 in the other division. If the valve is now opened the gas at 
 high pressure will rush through the aperture, and it can do work 
 (say, by turning a small propeller) while the pressure is being 
 equalised. But when the latter condition has been attained 
 the system, in itself, can do no more work. Its total energy is 
 still the same, but there is no difference in intensity, and so no 
 transformation (with regard to pressure change) can occur.* 
 
 Consider, also, what happens while the temperatures and 
 pressures are being equalised. The gas in the right-hand division 
 is at a temperature 20 C., and contains a certain quantity of 
 heat, Q'. After the transformation has occurred, the whole mass 
 of gas attains the average temperature 15 C., because Q units 
 of heat have now flowed from a region originally at 20 C. to a 
 region originally at 10 C. Now replace ordinary temperatures 
 by absolute ones that is, add 273 C. to each of the former; 
 
 * For simplicity we neglect here the conditions under which the trans- 
 formation must occur to render the calculation that follows applicable. 
 
ON THE NATURE OF LIFE 201 
 
 Q units of heat, then, have flowed from a mass at 293 (or 273+20) 
 to another mass at 283 C. (or 273+10). When heat flows from 
 a body at high temperature to another body at low temperature, 
 the entropy of the former is diminished and that of the latter is 
 increased. Change of entropy is measured by the simple ex- 
 pression -, Q being the quantity oJ^e^^wMcluDJBes-aiid JT_ 
 being the absolute temperature_. The entropy of the gas at 293 
 
 is therefore diminished by the amount 
 
 293 
 
 But at the same time a mass of gas. originally at 10 C. (or 
 283 Abs.) becomes raised in temperature to 15 C. (or 288 Abs.) 
 as the result of Q units of heat entering it, and so we get the 
 change due to the receipt of Q units of heat by the colder gas as 
 
 . Now the total entropy change due to the Q units of heat 
 
 288 
 
 lost by the hot gas, and the Q units gained by the cold gas is 
 + - . Obviously this expression is positive, and so we 
 
 get our result that entropy increases as the result of the mixture 
 of a hot with a cold gas. 
 
 Let, then, all this occur in the case of our specified decilitre 
 of hydrogen. The mixture of gases takes place in a few seconds, 
 and the resulting gas, at a temperature of 15 C., is stable and 
 homogeneous as regards temperature. No further work (with 
 regard to its temperature) can be done by it. 
 
 But while that is the case, it is still possible that the state of 
 all the molecules in the gas may become such that at a given 
 moment the latter become disposed, purely by " chance," in pairs, 
 the members of which are approaching each other with equal 
 velocity in the same straight line. When they collide their 
 velocities will be the same as before, but the directions in which 
 all the molecules are moving will become simultaneously reversed. 
 
 A very important result flows from this. It can be shown 
 (though the proof cannot be given here) that when every mole- 
 cule completely reverses its direction of motion, the velocity 
 remaining the same, the gas will retrace its past history. That 
 means that the total quantity of gas at a temperature of 15 C. 
 (or 288 Abs.) will separate into two portions one at a tem- 
 perature of 10 (or 283 Abs.), and the other at a temperature of 
 20 C. (or 293 Abs.). Therefore Q units of heat will flow from 
 
202 THE MECHANISM OF LIFE 
 
 a region of 288 to a region at 293, and entropy will decrease 
 
 by the amount . At the same time Q units of heat will enter 
 288 
 
 a region at a temperature of 293, and entropy will increase by 
 
 the amount . Therefore the total change is }-- , and 
 
 293 288 293 
 
 obviously this expression is negative. Now we get the result 
 that when, by reason of the reversal of the direction of motion 
 of all the molecules of the gas, the latter segregates into regions 
 at different temperatures entropy decreases. 
 
 We have now obtained a result of considerable interest. It 
 has been shown that it is theoretically possible that a gas origi- 
 nally at an uniform temperature can, of itself, separate into two 
 equal portions, one of which is at a higher temperature than is 
 the other. Heat can flow of itself from a region of lower to a 
 region of higher temperature. A gas which in virtue of its 
 temperature alone possesses no available energy can pass, of 
 itself, into a condition in which it does possess available energy. 
 A system can, of itself, decrease its entropy. All this is very 
 surprising, for these statements mean the same thing that is 
 expressed by saying that water can, of itself, flow uphill ! 
 
 They mean that we can state the second law of thermo- 
 dynamics as follows: The entropy of the universe may tend to 
 a maximum or a minimum. 
 
 But we must consider the probability of this reversal of the 
 sign of the second law of thermo-dynamics, for our results merely 
 give a theoretical possibility. The condition that it may occur 
 in such a case as we have investigated above is that at the same 
 moment all the molecules of the gas are arranged in pairs, and 
 the members of each pair are approaching each other end on and 
 with the same velocities. Now this condition is only one of an 
 incredibly great number that may exist in the gas, and the 
 probability that it may exist is the ratio, unity to a very large 
 numerical value. Perhaps the reader will best appreciate what 
 is this probability if we put it in Boltzmann's own way. In order 
 that we may observe this reversal of the second law in a decilitre 
 of gas, we might have to wait for a number of centuries repre- 
 sented by unity followed by 10,000 millions of zeros ! It is as 
 probable that it may occur as it is probable that every house in 
 London may catch fire accidentally and independently of all the 
 others on the same day, or that every grown-up person in London 
 
ON THE NATURE OF LIFE 203 
 
 may commit suicide (also indepeadently of all the others) on the 
 same day. Now an insurance company taking risks of houses 
 being burned down by accident, or of people committing suicide, 
 calculates its possible liability from the application of the 
 theorems of probability, and it would safely ignore such risks as 
 those we have just mentioned. 
 
 The latter are incredibly small. The chance that a decilitre 
 of gas will separate into two portions of different temperature is 
 also incredibly small, but it is not zero. In the ordinary affairs 
 of life we neglect small risks, and say they are " practically zero," 
 or " infinitesimal," but when we apply such chances in specula- 
 tions concerning the origin and fate of the universe we must not 
 dismiss them unless we are sure that they are really insignificant 
 in the conditions. Now we are not going to extend Boltzmann's 
 results obtained from a study of the kinetic theory of gases to 
 the whole universe that is, we must not suppose that a reversal 
 of the second law depends on the collision end on of every mole- 
 cule. All that we suggest is that it is possible in some way 
 other that entropy may decrease in our universe instead of 
 increasing. This is a logical possibility, and, given certain 
 arbitrary conditions in a very limited system, it is a probability, 
 the numerical value of which can be estimated. Further, we are 
 compelled to postulate that somewhere or other, or some time 
 or other, the second law of thermo-dynamics must reverse itself 
 that is, some time or somewhere entropy must decrease, or have 
 decreased, in our universe, otherwise we shall be compelled (as 
 Sir William Thomson was) to postulate a beginning, or creation. 
 
 Disentropic Phases in the Universe. Neglecting, in the mean- 
 time, the numerical value of the probability that entropy may 
 decrease (or that unavailable universal energy may become 
 available), we. may now proceed to consider the possible history 
 of our universe, taking as our " conceptual model " the changes 
 that occur in a small volume of gas left to itself. In the latter, 
 then, incredibly great periods of time may pass, and during these 
 the molecules of our gas are moving and colliding in a haphazard 
 fashion. Nothing happens in the system considered as a whole; 
 it does no work, although the total quantity of energy contained 
 in it is conserved. But some time or other, if we wait long 
 enough, the system changes, and its original, segregated condition 
 becomes restored. This condition in which the gas system 
 contains available energy and can do work lasts for an in- 
 
204 THE MECHANISM OF LIFE 
 
 finitesimal period of time, and the " normal " condition of 
 maximum entropy is again attained. 
 
 The visible universe that is, our galactic system of radiating 
 stars has, we have reasons for believing, definite boundaries. 
 If we could travel out from our sun in any direction for about 
 30,000 light years* (that is, 30,000x365x24x60x60 seconds X 
 186,000 miles) it is possible that we should reach those boun- 
 daries. But beyond this we can still imagine ourselves travelling 
 on indefinitely far.f In this outer space there would, however, 
 be no radiating cosmic bodies, though we might suppose that 
 there are dark, cold suns and satellites, and cold, cosmic dust. 
 Then, after incredibly further voyaging, we might encounter 
 other galactic universes. This means that our picture of the 
 entire universe is one in which the normal condition is physical 
 death the complete cessation of all happening entropy having 
 attained its maximum. But here and there in the whole uni- 
 verse, and occupying regions that are of infinitesimal extent, 
 there are individual universes, of which our galactic system is 
 one. The normal condition of the entire scheme of things is 
 that to which we see all physical changes tending, the complete 
 dissipation of energy. But now and then, and for periods of 
 time that are infinitesimal in duration, infinitesimally small 
 regions of the entire universe blaze up, so to speak, the second 
 law becoming reversed and entropy becoming decreased. After 
 this has occurred, the individual universe then much more 
 slowly sinks back again to the normal condition that in which 
 entropy tends slowly towards its maximum and to which physical 
 death is the limit. The universe, then, in which we are living 
 is an incredibly small fraction of all that exists, and its genesis 
 as an individual, physically active universe was an occurrence 
 essentially similar to that of our gas model. Some time in the 
 past the second law became reversed, and available energy was 
 restored. Then this became slowly dissipated, so that the phase 
 in which we are living is the more probable one that which 
 tends always towards complete degradation of energy. 
 
 In this way we avoid the physical impasse to which we are 
 brought when we assume the universal validity of the first and 
 second laws of thermo-dynamics. Our whole universe becomes a 
 
 * A light-year is about six billions of miles. 
 
 f Not on the theory of relativity. We cannot go beyond the boundary 
 of the universe, for outside the latter there is no space. See Appendix II. 
 
ON THE NATURE OF LIFE 205 
 
 cyclic order, such that the most probable phases are those in 
 which entropy tends towards its maximum value, and the least 
 probable ones are those in which entropy tends towards its 
 minimum value. As such it is a permanent universe, self- 
 sufficient, without beginning and without end. Throughout its 
 greater extent nothing happens, and this condition we call the 
 normal one. Here and there, however, and for infinitesimally 
 small periods of duration, there is physical activity, and this 
 condition we call the abnormal one. The probability that any- 
 where and at any time there is such physical activity is of the 
 same order of magnitude as that calculated by Boltzmann for 
 the reversal of direction of motion of all the molecules contained 
 in a small volume of gas. 
 
 A Digression on Orders of Magnitude and Duration. The 
 
 principal difficulty in appreciating the force of such an argument 
 as the above one lies in the reluctance with which we apprehend 
 extremely small and extremely large magnitudes. We refer all 
 measures, space, and time to those spatial and temporal values 
 that limit our bodily activities, and if something is very great 
 compared with these we are apt to say that it is " infinite," 
 while if it is very small we say that it is " practically zero." 
 Now a magnitude that we can estimate is always finite, no 
 matter how big or how small it may be, and greatness and small- 
 ness are always relative to something or other. The same 
 magnitude may be extremely small compared with some other 
 one, but extremely great when compared with yet another. 
 Thus the radius of the earth (4,000 miles) is to us a fairly familiar 
 magnitude, being a distance that we might traverse during a 
 few weeks by means of a steamship. A micron, however that 
 is> TTiVo millimetre, the microscopist's unit of length is a 
 magnitude that is about I/ 6378X10 8 of the earth's radius, and 
 we may call it an infinitesimal of the first order of smallness.* 
 But, again, a molecule of hydrogen is about 2-17X10' 8 mm. 
 in diameter, so that it is an infinitesimal of the first order when 
 compared with a micron, but of the second order when compared 
 with the radius of the earth. f Starting now with our standard 
 magnitude that is, the earth's radius we may compare it with 
 others that are " infinitely great "; thus it is about one-millionth 
 
 * It is about i -billionth part of the earth's radius. 
 
 2-17 
 t 2-17 x 10 - 9 mm. = mm., say 1 thousand millionth mm. 
 
206 THE MECHANISM OF LIFE 
 
 of the radius of the orbit of Neptune that is, the limit of our 
 solar system and so it is itself an infinitesimal. But the diameter 
 of the solar system is also an infinitesimal when compared with 
 the distance of the nearest fixed star (about 3J light years, or 
 3-25x6 billions of miles). Now let the reader calculate for him- 
 self this series of magnitudes in terms of the same unit (say a 
 kilometre), and he ought to have little difficulty in seeing that 
 any one, however small or great, may have significance with 
 regard to some other one. 
 
 Consider now intervals of time. It is said that an ordinary 
 person can easily realise, or feel, the lapse of duration represented 
 by A second of astronomical time, and certainly one second may 
 in some circumstances be a rather prolonged period. Now, 
 A second is about r roWv~ i niUu n rth of an ordinary lifetime (that 
 is, it is about 10 ~ n lifetime), and thus, with respect to the lapse 
 of duration which we may call the standard one, 5 V second is an 
 infinitesimal of the first order. But in looking at a red light for 
 that length of time we receive some 400 billions of ether vibrations 
 of a certain length that is to say, 4xl0 14 separate events or 
 things that actually happen. One of these events, therefore, has 
 a duration of 2 X 10 ~ 10 seconds, and this we call an infinitesimal 
 of the second order with respect to the lapse of a lifetime. 
 
 Probably life itself has existed on the earth for 1,000 million 
 years, so that even a very long lifetime is only about Vo -millionth 
 of the whole life-period of the globe. But the latter (10 7 years) 
 must itself be only an infinitesimal fraction of the period of time 
 during which our solar system has been in existence. 
 
 The Meaning of Duration. For men and women this period 
 of gV second of astronomical time is to be regarded as an unitary 
 lapse of duration: it is the smallest period for which one may 
 exist as a conscious, sentient being. While it passes, a star 
 moves through an arc of about 1-5 seconds that is, a shift in 
 the sky which can easily be measured by refined astronomical 
 methods. In looking at red light for the same period one 
 gathers up into perception some 400 billions of ether vibrations, 
 each of which has a wave-length of 8xlO~ 4 cm. In listening 
 to the note on the piano which is three octaves above the middle 
 C one combines together about forty-one separate vibrations of 
 the atmosphere to make a sound of a certain quality. Our 
 rhythm of duration (in Bergson's phrase) is therefore such that 
 we synthesise these motions, or changes, to make unitary, 
 
ON THE NATURE OF LIFE 207 
 
 individualised perceptions. We see the motion of the star through 
 1 -5 seconds of arc, we see light of a particular colour, and hear 
 sound of a particular pitch. Each of these perceptions is there- 
 fore a synthesis of external events effected during the same small 
 fraction of an individual lifetime. Our sensory mechanisms are 
 such that we can make these syntheses, but not some others ; thus 
 there are ether vibrations which are less rapid than those corre- 
 sponding to the red of the spectrum, but we cannot see them, 
 though we can feel them as heat. Below these, again, are others 
 which are much slower, and which we can neither see as light nor 
 feel as heat, though we can detect them by the receiver of a wire- 
 less telegraphic apparatus. Above the violet of the spectrum are 
 vibrations which are so rapid that we cannot see them, though we 
 can make them act upon a photographic plate. Other animals are 
 certainly different from what we are in respect of these matters; 
 thus a dog can certainly see light of lower wave-length than we can, V 
 and some insects can hear sounds which are too acute for our ears. 
 
 The reader will easily see, then, that rhythm of duration varies. 
 Obviously it varies even in ourselves ; thus the familiar experience 
 that time passes more quickly when one is fifty years old than 
 when one is ten means that the later ones have fewer events in 
 them that is, the periods of our duration which correspond 
 with a revolution of the earth round the sun do not gather up so 
 much of other external events when one is old as when one is 
 young. To say that young people have a " fuller " life is not 
 figurative, but is strictly scientific language. 
 
 Now think of how the rhythm of duration varies in different 
 animals. It is probable that the imagines (the fully meta- 
 morphosed insects) of some species of Ephemerida3 which live 
 for only a few hours can appreciate, or feel as distinct lapses of 
 duration, very much smaller intervals of astronomical time than 
 we do. Suppose, for instance, that a period of ^STTOOTJZF second 
 can be felt by them (in which period they would still receive 
 some 8,000 millions of vibrations when looking at red light) ; then 
 their lifetime of a few hours would contain as much (that is, have 
 the same duration) as ours of seventy years. Suppose, again, 
 that some very long-lived animals, such as crocodiles, have 
 unitary periods of duration that are much greater than ours; 
 then their lives would be no longer, for they would contain no 
 more. Note, however, that in such cases the universes con- 
 structed from perception would be different ones. 
 
208 THE MECHANISM OF LIFE 
 
 Carry such speculations to their limits. Imagine minds which, 
 just as we synthesise in perception billions of ether vibrations to 
 make visible radiation, might synthesise hundreds of lunar 
 revolutions or solar ones in a single perception. Or in such a 
 mind all the changes that make up the origin, growth, and dot-ay 
 of a solar system might be gathered up as a single perception. 
 As the rhythm of duration thus successively slows down the 
 existence in tune (with respect to astronomical, periodic events) 
 would become more and more prolonged, and in the limit it 
 would be without end, or immortal. But note that for such an 
 immortal mind all the details of the universe, and its changes, 
 that exist for us would be unknown, inasmuch as they would be 
 synthesised in other perceptions which can have no meaning for us. 
 
 To summarise, then, the very important results that we have 
 now obtained: 
 
 If we regard the laws of conservation of energy and augmenta- 
 tion of entropy as of universal validity, we come to an impasse. 
 For the latter law tells us that the changes that occur in the 
 universe tend towards absolute degradation of energy, and there- 
 fore towards cessation of all physical phenomena. But since there 
 can be no limit to past tune, this ultimate degradation ought to 
 have already occurred, and we know that it has not occurred. 
 
 Therefore the second law is not of universal application. AVe 
 must postulate that it may be reversed, so that, in certain circum- 
 stances, energy that has become unavailable may again become 
 available. In other words, we are compelled to think about the 
 universe that we know as a mechanism that has been started, and 
 is now running down. There must be some means of winding it up. 
 
 We can imagine no logical reason why unavailable energy 
 should not pass into the available foim; all that we can say is 
 that, in our experience, this transformation does not occur. 
 Then, since absolute reliance upon our experience of the universe 
 brings us to an impasse, we must conclude that this experience 
 cannot be inclusive of all that happens. 
 
 There must be two directions taken by energy transformations. 
 In one of these energy is continually degraded and entropy 
 becomes augmented. That such changes occur is highly probable 
 so probable that we know by actual observation of no other 
 direction of change. In the other direction available energy 
 becomes restored, and entropy diminishes. That these changes 
 
ON THE NATURE OF LIFE 209 
 
 occur is highly improbable so improbable that we have no 
 experience of them, though we can imagine them occurring. 
 
 This means that, in addition to the physics that we know, 
 another and a transcendental physics is conceivable. 
 
 The Improbability of Life in the Universe. It follows from 
 what has been said that it is extremely improbable that any- 
 where in the entire universe energy is available for the production 
 of physical phenomena. It is easy to show that such is the 
 condition that we know, and that even in OUT individual, galactic 
 universe, where an initial store of energy is exhausting itself, 
 this availability is improbable. 
 
 If we were able to travel away from the sun in every direction- 
 and with the speed of light, we should reach the boundaries of 
 our solar system in about four hours. Then we should travel 
 outwards into cosmic space for over three years and a quarter 
 before coming to the nearest fixed star. This sphere of space, 
 containing only our solar system (so far as we know), is not, 
 however, empty, but is " full " of ether. The latter is the 
 " substance " of all energetic changes, but we must regard it as 
 inert, or physically inactive. Nothing happens in it of itself. 
 It can be temporarily strained or modified in some way, as when 
 potential energy is locked up in it, or withdrawn from it, or 
 when radiant gravity or gravitational energy is transmitted 
 across it. It is permanently modified in localised regions as 
 physical gravitating matter, which may or may not be the locus 
 of energy transformations. We must think about it as some- 
 thing real, but physically dead or inert. What fraction of this 
 universal substance is, then, physically active ? 
 
 Calculating the volume of this sphere of three and a half light 
 years in radius, we find that it is about 240 X 10 35 cubic miles. In 
 it practically the only cosmic body that contains available energy 
 is our sun, and this has a diameter of (say) 870,000 miles. Find- 
 ing its volume, we get the value 15X10 16 cubic miles. It is 
 now easy to find that our sun occupies about 1/2 X 10 21 th of the 
 part of the stellar universe that we have in imagination ex- 
 plored. The probability, then, that we should find physical 
 activity anywhere in the region of space within three and a half 
 light years from our sun is something like 1 in 2,000 trillions. 
 
 Now consider the probability that living matter exists in this 
 physically active fraction of the cosmic space which immediately 
 surrounds'us. We can hardly think of it as existent in our sun, 
 
 14 
 
210 THE MECHANISM OF LIFE 
 
 nor in most of the planets that surround the latter (unless we 
 extend the definition of life in a way that we do not contemplate 
 just yet). Even upon our own earth living substance forms only 
 a surface film of incredible tenuity when compared with the 
 dimensions of the planet. It is probably most dense in shallow 
 seas near the land, but even there the mass of organised matter 
 is only a few parts per million of water. In oceanic regions, both 
 at the surface and in the depths, the density of life is much less, i 
 and on the land (when we take account of polar, mountainous, and 
 desert areas) the density is still less. All that is, of course, only 
 the surface of our planet ; in its depths life is very nearly absent. 
 
 From the idealistic standpoint life (in strictness, our own life, 
 or mind) is all that is, for the universe and all the things that 
 science places there are only mental constructions. 
 
 Man, says Pascal, is only a reed, but he is a thinking reed. 
 He is among the least and most fragile of things. But he is also, 
 among the greatest of things ; he is, indeed, greater than all other 
 things, for he can comprehend the universe. 
 
 Life is like Pascal's reed. Anywhere in the universe it is 
 highly improbable that energy exists in such a form as to givej 
 rise to physical changes. Even in this fraction of the universe,] 
 where there is physical change it is also highly improbable that 
 among these activities there are some that exhibit what we call 
 the phenomena of life. In other words, the chance that life exists 
 anywhere in our universe is an infinitesimal of the second order, 
 
 The Physical Nature of Life. If we would try to explain life 
 we must, first of all, be very clear about what we mean by anj 
 " explanation." Consider the motions of the bodies that make 
 up our solar system: the planets revolve round a central sun in 
 elliptical orbits, and the satellites revolve round the planets ; thd 
 sun, planets, and satellites rotate on their own axes, and that 
 latter process, or rotate, while still retaining their inclination to 
 the plane of the ecliptic; the ellipticity of each orbit clumps 
 periodically; the various bodies " nutate " ; each of them perturb^ 
 all the others, and so on through a host of motions. To describe 
 all the latter would be very difficult, and the description would 
 be very hard to understand for anyone who is not an astronomer. 
 But assume that each body has a certain mass, that it had aj 
 certain initial velocity, and that it attracts every other body with 
 a force that depends on the various masses and on the squared 
 of the distances between the bodies. Then all the motions can 
 
ON THE NATURE OF LIFE 211 
 
 be seen to be consequences of the assumptions that we have just 
 made. 
 
 We start with certain very simple concepts, mass and time 
 and space and the law of gravitation, which is a relation between 
 mass and space and time. Given these concepts and a know- 
 ledge of the velocities of the various bodies, and we can then 
 deduce all their movements, present, past, and future. Every- 
 thing that happens in the motions of the solar system happens 
 because the various bodies have mass and attract each other in 
 a certain way. Therefore we " explain " these very complicated 
 movements by the concepts of mass and gravitation. The latter 
 are simple or irreducible that is, we cannot (so far) explain them 
 by supposing them to be the consequences of something still 
 simpler and more general than mass and gravity. 
 
 Knowing the positions of the planetary bodies at any moment, 
 as well as their velocities, knowing also their masses and assum- 
 ing that the law of gravitation holds good in all circumstances, 
 we can then find what will be their positions at any future time 
 and what were their positions at any past time. These pre- 
 dictions and retrospects generally are successful, and when they 
 fail (as they do in rare instances) astronomers assume other simple 
 concepts (as in the theory of relativity), and then their calcula- 
 tions work out true to what can be observed. Thus we have a 
 planetary theory, which is the explanation of a host of complex 
 events by a very simple hypothesis that the cosmic bodies have / 
 mass and attract each other. This theory is verified, inasmuch 
 as it enables us successfully to predict forwards or backwards. 
 
 Now if we had a theory of life we should also have certain very 
 simple, irreducible concepts, and it would be the case that all 
 organic phenomena growth, reproduction, assimilation, ex- 
 cretion, behaviour, adaptation, evolution, and so on would be 
 the inevitable consequences of these fundamental concepts, just 
 as solar and lunar eclipses, tides, seasons, etc., are the inevitable 
 consequences of the ways in which the heavenly bodies move, 
 which are, again, the consequences of the law of gravitation. 
 Knowing these fundamental factors, we should be able to 
 " explain " life. We should be able to say what an animal will 
 do at any future time, and what it did at any past time. We 
 ought to be able to trace out the past history of the species to 
 which it belongs, as well as its future evolutionary history, just 
 because we know what the animal now is and what are the 
 concepts by means of which we " explain " its activities. 
 
212 THE MECHANISM OF LIFE 
 
 Now we cannot do any of these things. We cannot predict 
 what an individual man or woman will do at any time in the i 
 future, though we can sometimes say what a population will do. 
 (The reader must note this distinction between individual and 
 statistical effects; it is most important in speculative reasoning.) 
 We cannot say, merely from a knowledge of its structure and 
 behaviour, what was the past of the species to which an animal 
 / , belongs, and what is going to be its future. Let there be no 
 misunderstanding here: we do know a great deal about the j 
 phylogenies (or lines of descent) of some races of animals, but 
 that is because these evolutionary careers have left historical 
 records. Thus we know (or at least we believe) that the existing 
 one-toed horse has descended from a three-toed species which 
 had, in its turn, come down from a five-toed form, but this is 
 because we have fossil remains of the three- and five-toed horses, 
 because the living animal has vestiges of the second and fourth 
 toes, while the skeleton of the three-toed horse has vestiges of the 
 first and fifth digits. Evolutionary careers have therefore left 
 records in the form of fossil remains and vestigial structures, and 
 we are sometimes successful in reading these and so in tracing 
 out a line of descent. Obviously our success is not due to any 
 process of true deduction from a few fundamental concepts, as i 
 is the astronomer's when he calculates backwards to find when 
 eclipses occurred. Eclipses leave no records on the faces of sun, 
 moon, and earth like the evolution of the horse has done. 
 
 And so the astronomer can calculate forward or predict, while 
 the biologist cannot do so, since the successes of the latter depend 
 upon records which cannot exist in the cases of events which 
 have not yet happened. Therefore we cannot predict what will 
 be the future evolutionary history of the horse or of any other 
 race of existing animals or at least no biologist has yet risked 
 his reputation by making such an attempt ! 
 
 Now we may as well admit that this argument may not appear 
 to possess much force in itself. It may be that biology cannot 
 predict (and so supply the necessary test of the validity of its 
 theories) just because it has not yet attained the necessary 
 knowledge. No doubt Copernicus could not have predicted the 
 times at which equinoctial spring tides would occur at any 
 particular place in the North Sea, for his knowledge of planetary 
 theory was imperfect, and he had not obtained the concept of 
 gravitation. So it may be that biology cannot yet predict just 
 
ON THE NATURE OF LIFE 213 
 
 because its knowledge is incomplete and its concepts are not yet 
 formulated. This latter conclusion is, however, all that we seek to 
 make here biology, not having attained the concepts that will 
 enable it successfully to predict, has not yet been " explained." 
 
 Now let the reader refer back to our discussions of Chapter VIII. 
 It is very clear that organic behaviour (in the cases of the higher 
 animals, at all events) is indeterminate: it cannot be predicted. 
 When anyone says that physical determinism must hold in organic 
 as well as in inorganic functioning, he means that it ought to 
 hold because the concepts by means of which we " explain " life 
 ought to be the same as those by which we explain inorganic 
 happening. It may be that they are the same, and that by-and- 
 by biological predictions will be successful; but the plain fact is 
 that, so far as we know, much of the behaviour of the higher 
 animal is spontaneous, and cannot be predicted, while no satis- 
 factory logical proof can be given of determinism in its application 
 to organic acting. 
 
 We are not going to argue here that, because physiological 
 investigation discloses no activities in the animal body other than 
 physical and chemical ones, organic happening is necessarily the 
 same thing as inorganic happening. It is quite true that the 
 physiologist finds nothing in the functioning of an organ but 
 physical and chemical reactions, for, because of his methods, these 
 are all that he could possibly expect to find. When he studies the 
 submaxillary gland, say, he finds that a saliva, a liquid of a 
 certain chemical composition, is secreted and poured into the 
 mouth. He finds that this secretion occurs when food has 
 entered the mouth, and when the nerve endings there have been 
 stimulated chemically by the substances of the foodstuff. A 
 reflex occurs and the gland functions. The nature and abund- 
 ance of the saliva depend on various factors blood-pressure, 
 osmosis, hydrostatic pressure, chemical reactions in the cells, 
 etc. and investigation has taught us very much as to the ways 
 in which these various factors act. We have dealt in some detail 
 with this typical example of animal activity in the Appendix, 
 and we may leave the reader to study it further in the textbooks. 
 He will see, however, that all that has been obtained by in- 
 vestigation is a description of the manner of functioning of the 
 gland; there are nervous impulses, changes in the calibre of the 
 bloodvessels, changes in the pressure of the circulating blood, 
 changes in the pressure of the salivary liquid contained in the duct 
 
214 THE MECHANISM OF LIFE 
 
 of the gland, osmotic changes, chemical reactions, and so on. These 
 tilings do not explain the secretion of saliva : they only describe it. 
 An explanation would place beneath blood-pressure, chemical 
 reactions, etc., some simple irreducible concepts, just as in the 
 planetary theory we place beneath the complex motions of planets 
 and satellites (which correspond logically to the molecular move- 
 ments occurring in the tissues of the gland) the law of gravitation. 
 
 It is, perhaps, not easy for the non-professional reader to 
 appreciate this point. It seems natural to suppose that, because 
 chemical and physical reactions are all that we can study scien- 
 tifically in the living animal body, these things explain life. They 
 only describe life: they are the physical expressions of the activities 
 of the organism. Investigation itself is not primarily speculative, 
 but is rather something useful. Just as the submaxillary gland 
 and all the other parts of the body that enable it to act 
 function so as to produce a liquid that digests certain foodstuffs, 
 so the mental mechanism that we call the categories of the under- 
 standing functions so as to do something that (like the production 
 of a ferment) is useful to the organism. In the higher races of 
 man it has become speculative also, but even there it is pre- 
 dominantly practical, and (as Bergson shows) is therefore 
 hampered when its aims are purely speculative. 
 
 Let the reader reflect on all this, and he will certainly see that 
 physiology has never explained life activities, but has only 
 described them, and that indeed is the glory of the science, for 
 the description has given us (or will yet give us) the mastery over 
 inimical nature. Let him examine the attitude of the " ordi- 
 nary " man that is (say), some nine-tenths of all those who are 
 capable by prolonged study of following the methods and TOM ills 
 of scientific investigation. This ordinary man will almost 
 certainly ask, What is it for? He will expect that investiga- 
 tion ought to have some useful result, and he will probably be 
 unable to conceal his disappointment (or even contempt) when 
 he learns that the result is only an increase in abstract knowledge ! 
 
 Life activity therefore expresses itself in physical and chemical 
 phenomena, but physical and chemical reactions do not " ex-j 
 plain " life that is the result to which we seem to be led. 
 
 Life and Energy. In seeking for the explanation we Lave, 
 therefore, to find some concept which will be special to the' 
 phenomena which we call vital ones, and which need not be 
 required when we investigate and explain inorganic happenings. 
 
ON THE NATURE OF LIFE 215 
 
 It must have the " property " of reality that is, it must not be 
 in contradiction to the law of conservation for it is to be a 
 means of investigation. Thus we are compelled to reject any of 
 those concepts upon which the theories of spooks are based. It 
 must work that is, it must lead to the discovery of further 
 workable results or hypotheses and it must therefore avoid the 
 reproach (addressed originally by Bacon to certain philosophies), 
 and extended to Driesch's " psychoids " by a well-known 
 cytologist,- that it is like the vestal virgins, dedicated to God 
 and barren ! Now it ought to be clear to the reader at this stage 
 in what direction we are seeking for our concept. We have 
 throughout this book kept the notion of energy in the forefront 
 in order that we might lead up to our thesis that the concept \ 
 which is special to the organism is one which involves the reversal v 
 of the second law of thermo-dynamics. We must not be troubled 
 by the strangeness of such a concept or by the appearance of 
 paradox that goes with it. It is not more strange than some 
 notions in mathematics that have, nevertheless, been fruitful of 
 result than, for instances, the non-Euclidean postulate that more 
 than one line can be drawn through a point and still be parallel 
 to some other line not going through the point ; that the sum of 
 the angles of a triangle may be less than two right angles ; that 
 a negative number may have a square root. The test is that, 
 like the notions just given as instances, ours shall have pragmatic 
 value may work. 
 
 So, again, we contrast the inorganic and organic physico- 
 chemical systems. 
 
 In the former, anything at all that happens of itself happens 
 because energy transformations take a certain direction. This 
 direction is that which leads to a decrease of the differences of 
 intensity in different parts of the system. Available energy is 
 concentrated in this part of the system rather than that, and if 
 it can be levelled down, so to speak, something will happen. If 
 it cannot be levelled down, being equally distributed everywhere, 
 nothing will happen. If water is accumulated in a reservoir at 
 a higher level than that in a lake, it can flow down into the latter, 
 turn a mill-wheel as it flows, and so produce various phenomena. 
 If it is all contained in the lake, if there is no difference of level, 
 there can be no flow, and no energy available for the production 
 of physical phenomena. 
 
 This, then, is the concept by which we " explain " the fact that 
 
216 THE MECHANISM OF LIFE 
 
 physical phenomena do occur; there are in the universe 
 differences of intensity of energy, and these differences tend, of 
 themselves, to become levelled down. As the process of reduc- 
 tion of intensity difference takes place, energy transformations 
 occur and entropy increases. But, as we have seen, the concept 
 of the second law proves inadequate as an explanation in the 
 universal sense, and we are compelled, even in the treatment of 
 inorganic, cosmic systems, to postulate another concept, that of 
 the second law of thermo-dynamics reversed in sign. Somewhere 
 or some time entropy must decrease instead of increasing. 
 
 If this reversal were to happen, the results would be unex- 
 pected, fantastic, and paradoxical. But a physicist would not 
 be incredulous. He would, probably, seek to be very sure that 
 lie was not mistaken, and that his observations were trust- 
 / worthy; then, having satisfied himself upon these points, he 
 would accept the result. But, as a matter of fact, the reversal 
 does not occur, and every energy transformation that the physi- 
 cists and chemists can observe and investigate happen because, 
 in this part of the universe known to us, entropy always tends to 
 increase. Therefore the concept which he utilises in order to 
 explain physical happenings is this energy differences tend, of 
 themselves, to become abolished. 
 
 Now it is quite clear that this concept (which is, of course, only 
 another way of stating the second law) does not fully explain 
 organic happening, for in such processes energy differences do 
 not tend, of themselves, to become abolished. Again, we must 
 be very precise in our statement of this result; we do not mean 
 to say that the animal physico-chemical system, or body, does 
 not " obey " the second law. We recall here the caution sug- 
 gested in the first part of Chapter II., that we must not seek for 
 absolute distinctions anywhere in nature, since these are logical 
 constructions only. There are no such things as mathematical 
 points, straight lines, or planes, for the points and lines and 
 planes which we observe and measure are only approximations. 
 Let a very small spot become smaller and smaller, and " in the 
 limit " it becomes a mathematical point, and so on. Logically, 
 then, we can construct a conceptual world in which there are 
 absolute distinctions between inorganic and organic, and between 
 processes which tend to entropy increase and others which tend 
 to entropy decrease, but wJuit we do actually observe are only the 
 tendencies which, in reasoning, we carry towards their limits. 
 
ON THE NATURE OF LIFE 217 
 
 In the activities of life as a whole, what we observe, then, is 
 the tendency for the perpetuation of differences of energy 
 intensity. In purely physical happenings energy tends to 
 become unavailable, but organic changes set themselves against 
 this tendency. When sunlight falls upon desert sand, rocks, 
 raw soil, and upon the surface of the sea, its energy of radiation 
 transforms into heat. Sand and stones become warm, and when 
 the sunlight is withdrawn this heat becomes radiated away into 
 outer space, is dissipated, and is, for us at least, for ever irre- 
 coverable. When it falls on the surface of the sea it heats up 
 the water, which then evaporates, rises up into the atmosphere, 
 is distributed in winds, and is precipitated in rain, etc., returning 
 ultimately to the ocean from which it came. In these changes 
 the motions of the winds and water become transformed by 
 friction into waste heat, which, as before, is radiated away and 
 lost. Sunlight, which is energy of high intensity, thus, of itself, 
 becomes degraded or levelled down, entropy increasing in all the 
 transformations that occur. 
 
 Let, however,the sunlight fall upon green vegetation, and some- 
 thing very different occurs. Its energy transforms into chemical 
 changes, as the result of which water and carbonic acid (sub- 
 stances which are fully degraded and have no free or available 
 energy) become combined together, with increase of available 
 energy, to form carbohydrate. Trace the entropy change, and it 
 will be found that this is positive, and has a certain value when 
 solar radiation transforms wholly into waste, low-temperature 
 heat. Trace it again when sunlight is absorbed by the green 
 plant and transforms into the potential chemical energy of 
 carbohydrate, and it will be found that the increase of entropy 
 is now much less than it was. In the first case the solar energy 
 is for ever lost to this world, but in the second it becomes fixed, or 
 stored up, as the chemical energy of wood, vegetation, oil, or coal. 
 
 Vegetable life (that is, the predominant form in which life 
 exists on our world), then, has for its tendency the storing up of 
 available chemical energy. The latter becomes locked up, so 
 to speak, in the form of starches, celluloses, proteids, and oils of 
 fruits, seeds, woody tissues, etc. The vegetative processes of 
 reproduction are the most powerful in the animate world, so that 
 on every available place on the surface of the earth, in swamps 
 and in the shallow water of seas and lakes and ponds, plant life 
 spreads and accumulates to the greatest extent possible. Even 
 
218 THE MECHANISM OF LIFE 
 
 when, by reason of overcrowding and the absence of the neces- 
 sary mineral food substances, this continued multiplication is no 
 longer possible, dead vegetable substance in the form of woody 
 tissues, mould, seeds, leaves, oil, etc., accumulate and form 
 deposits of material of high calorific value. Some of these, in 
 the form of coal, and perhaps oil, remain throughout long periods 
 of geological time in a permanent, utilisable form. 
 
 The fraction of the solar energy that is absorbed by green 
 plants, and is afterwards dissipated as waste, irrecoverable heat, 
 is very small. For these organisms are, in general, immobile, 
 and so their energy is not transformed into mechanical work, 
 which would become dissipated by friction into waste, low- 
 temperature heat. But in the animal the latter kinds of energy 
 transformations occur to a far greater extent than they do 
 among the plants. The animal is characteristically a machine 
 for the conversion of potential chemical energy into movement 
 of body and limbs, and this movement inevitably leads to friction, 
 and thus transforms into heat which becomes dissipated. Here 
 we are not considering the processes of reproduction ; if we were, 
 we should see that the tendency is, even in the animal kingdom, 
 for the indefinite multiplication of every species, and for the 
 distribution of the individuals over as wide an area of the surface 
 of the earth as possible. All animals, even the most slowly 
 breeding ones, are enormously prolific, and there seems to be no 
 limit to the numbers to which species may theoretically attain. 
 There is, of course, a practical limit which depends upon the 
 quantity of vegetable food substance available, which depends 
 again upon the area of land and sea which can be occupied by green 
 plants, and upon the quantities of the ultimate foodstuffs (chiefly 
 mineral compounds of nitrogen) that are available for the plants. 
 
 If there were (say, as the results of volcanic activity) a con- 
 tinued supply of these indispensable food substances (nitrates., 
 nitrites, ammonia compounds, carbonic acid gas, and some other 
 mineral salts), there would seem to be no theoretical reason \\ liy 
 the quantity of available energy upon the earth in the form of 
 the organic substance of plants and animals should not steadily 
 accumulate. As it is, some substances which are the results of 
 organic activity do tend to accumulate; these are peat, lignite, 
 coal, perhaps oil, carbonate of lime in the form of coral reefs ;m<l 
 other limestone formations, silica in the form of diatom, and 
 radiolarian oozes and deposits, etc. Animal substances (fats, 
 

 ON THE NATURE OF LIFE 219 
 
 carbohydrates, and proteids) do not, in general, accumulate, 
 since they are destroyed by bacteria, when they transform 
 chemically into mineral compounds, which again become avail- 
 able as the foodstuffs of plants. 
 
 Evidently, then (and the thesis could be supported to a much 
 greater degree than our limits of space allow), the general 
 tendency of life upon the earth is towards the accumulation of 
 available energy in the form of chemical compounds of high 
 calorific value. The process is restricted mainly by animal and 
 bacterial life, and by the paucity on the earth of mineral 
 nitrogenous compounds. But it is clear that on a lifeless planet 
 the enormous store of available energy contained in the incident 
 solar radiation would at once be dissipated, whereas on one which 
 is the seat of life the energy so received tends to be accumulated. 
 
 The feral animal, as a rule, disappears and leaves no addition 
 to the earth's store of available energy, except in the somewhat 
 rare cases where its tissues become partially converted into 
 bituminous substances. Of all the myriads of animals that have 
 existed on the earth in past geological times, this remains here 
 and there some oily or bituminous shales and other rocks, and 
 for the rest " ashes and a skeleton and a name, or not even a 
 name." Throughout their lifetimes the chemical energy taken 
 into the bodies of these animals in the shape of foodstuffs has 
 been, in the main, transformed into mechanical energy, which 
 dissipates into heat that becomes radiated away into space. 
 Some of it goes to form the bodies of new individuals which reach 
 maturity, and then cease to grow, but the fraction which is 
 absorbed in reproductive processes (and therefore adds to the 
 mass of animal life, or accumulated available energy) is much 
 smaller than it is among plant organisms. Two tendencies 
 restrict this accumulation in the animal kingdom: (1) Death and 
 putrefactive decomposition by bacteria; and (2) the greater 
 mobility of animals, which carries with it dissipation of energy. 
 The former, however, do not seem to be inevitable processes, for 
 death is something that might have been averted (and is, indeed, 
 averted in the case of certain protozoa), while conditions in which 
 putrefactive decomposition need not occur are easily conceivable, 
 and may, indeed, be realised. 
 
 The tendencies of vegetable metabolism, therefore, make for the 
 accumulation of available energy, but the opposite is the case 
 with animal metabolism. Now this is mainly because the 
 
220 THE MECHANISM OF LIFE 
 
 mechanical energy of almost all animals that now exist on the 
 earth becomes frittered away uselessly in random, misdirected 
 movements which end in friction and dissipation as waste heat. 
 But let these movements be directed and planned, and the energy 
 that would otherwise be degraded tends to accumulate. The 
 tendency to utilise the activities of the animal sensori-motor 
 M>tein in retarding the natural degradation of cosmic energy 
 may easily be traced even in the lower animals, and it operates 
 in the highest degree in human activities. 
 
 Even among the lower forms of life we see it, however, in 
 everything that is called an adaptation that is, in every device 
 which gives an animal greater power over inimical nature. The 
 change of coat colour from brown to white in an arctic mammal 
 as the winter approaches is clearly such an attempt. This is the 
 real meaning of adaptation, and one might discuss it at length if 
 it did not constitute the greater part of biological science. 
 
 Obviously, adaptation attains a maximum in the human 
 animal when the latter develops and perfects the use of tools. 
 We do not distinguish between the bodily weapon and the tool 
 or machine adapted or manufactured from inorganic materials: 
 these are, in their effects, as much adaptations as is the change 
 of coat colour from brown to white, or the conversion of a fold 
 of skin into a flying membrane in some squirrels. The ball- 
 bearings in the hub of a bicycle, or the use of grease in the axle- 
 boxes of a railway waggon, are adaptations ; they would not exist 
 apart from life ; they are the indications of life, and their effect is 
 to retard the dissipation of mechanical energy into waste heat by 
 minimising friction. We cannot think of any inorganic, physico- 
 chemical system tfiafc does this ; the essence of any such system 
 that is, of any mechanism that " goes " of itself, or of any 
 aggregation of chemical substances that interact with each 
 other is that the system tends as rapidly as possible towards 
 a condition of equilibrium in which the mechanism ceases to 
 " go," and the chemical substances cease to interact. 
 
 Here we get our first concept by which we " explain " physical 
 happening: when water runs downhill; when combustible sub- 
 stances burn and apparently disappear into the air; when coal 
 burns in a fire and generates heat; when proteids and carbo- 
 hydrates decompose into water, carbonic acid, and mineral 
 nitrogenous salts; when a hot poker becomes cold, and so on 
 when these things happen, energy becomes degraded or entropy 
 increases, and it is because entropy increases that they do 
 
ON THE NATURE OF LIFE 221 
 
 happen. So water does not rraf uphill ; a piece of brick put into 
 the fire does not generate heat; and water and carbonic acid do 
 not, of themselves, combine to form carbohydrate, because in 
 such processes entropy would necessarily increase. Our inor- 
 ganic concept is, therefore, the entropy of the universe tends 
 towards a maximum value. 
 
 On the other hand, things do happen in the course of which 
 energy is not degraded, but, on the contrary, becomes elevated, 
 so to speak. Water and carbonic acid react together and utilise 
 the degrading energy of sunlight, producing carbohydrate. 
 Proteid, carbohydrate, and fat, when aggregated in the form of 
 living protoplasm, do not decompose, but increase in quantity 
 by reproduction. In these energy transformations entropy 
 decreases, and unavailable energy becomes available.* Some 
 adaptations among wild animals clearly tend to reduce the waste 
 of available energy (as when change of coat colour or the accumu- 
 lation of blubber beneath the skin minimises the loss of heat), 
 and all adaptations have the same general tendency. Human 
 operations may lead to degradation (say by war, by the cutting 
 down of forests, the extinction of seals and whales, or the de- 
 pletion of coal resources), but they may also have the opposite 
 tendency (as in afforestation, the cultivation of the land, the 
 breeding of domestic animals, the increase of populations, and 
 the invention of antifriction machinery). A man may expend 
 his energy in rolling stones downhill, when he dissipates potential 
 energy in useless friction; or he may by the same expenditure of 
 energy carry stones uphill, so that they are in a position to roll 
 down again, thus accumulating potential energy. In such 
 " vital " processes and tendencies available energy accumulates 
 and entropy decreases. 
 
 Therefore the concept by which we explain truly inorganic 
 happening fails to explain organic happening, for in the latter 
 the inorganic concept is reversed. 
 
 In living processes the increase o! entropy is retarded this is 
 our " vital " concept. 
 
 * The reader must not misunderstand this statement. Starch accumu- 
 lates in the green leaf exposed to sunlight, but the whole system is the 
 green leaf + the C0 2 and OH 2 + the degrading sunlight. In the system 
 thus denned entropy increases very slowly. The system is one in which 
 there are coupled energy transformations, (1) the degrading sunlight: and 
 (2) the photo-synthetic process. If there were no coupling, the solar 
 energy would degrade, with a maximum entropy increase; if there is a 
 coupling, the entropy increase becomes minimal. The coupling is always 
 the mark of life activity. 
 
APPENDIX I 
 A METAPHYSICAL DISCUSSION 
 
 THE argument of Chapter X. may be summarised as follows : 
 
 " Sensation" is a purely physical process, beginning with the 
 stimulation of a sense organ and ending with the physical affec- 
 tion of a nerve centre. In it are involved only molecular dis- 
 placements of the substance of nerve fibres and cells. It is not 
 accompanied by changes in consciousness. 
 
 " Pure perception " is the prolongation of this physical process 
 of sensation into appropriate actions. Beginning in the animal 
 body with the stimulation of a sensory organ, it ends with a 
 motor action (that is, the co-ordinated movements of a system 
 of muscles), or a glandular change (that is, a process of secretion, 
 or other form of metabolism). It is unaccompanied by any 
 change of consciousness when it is " pure." That is, of course, 
 a limiting case exemplified by typical reflexes, most habitual, 
 visceral responses (respiration, the heart -beat, peristalsis, 
 " mechanical," " habitual," " automatic," and instinctive re- 
 sponses). In such cases consciousness may be absent or very dim. 
 
 " Perception " simply means a chosen response one that is 
 accompanied by some degree of deliberation, and exhibits some 
 degree of freedom or novelty. We are compelled to postulate 
 this indeterminism of response in admitting the continued evolu- 
 tion of form and functioning, and we are convinced of it if we 
 reflect upon our conduct and the motives that sway us. Such 
 deliberated, chosen, and free response is again the limiting case, 
 because habit; physical, legal, and moral restraints; heredity 
 and convention, determine largely what we shall do in any 
 circumstances. But not altogether. There is some degree of 
 true freedom of acting. 
 
 The greater the degree of indeterminism that follows upon a 
 sensation, so much greater is the pitch of consciousness. In 
 strong and sustained mental effort there is just this intensely felt 
 deliberation. The latter may end in a choice (when we take 
 some course of action), or it may end in si solution (as when one, 
 works out some physical or mathematical problem) that is, it 
 ends in an actual or virtual action. Then one feels the strong t 
 possible mental satisfaction. 
 
A METAPHYSICAL DISCUSSION 223 
 
 Or our mental struggle may Ite equivocal, so that there is no 
 choice or solution, or at least none that is satisfactory. Then 
 we are certainly impressed with the sense of failure and, it may 
 be, of acute dissatisfaction. 
 
 Or, again, there may be persistent sensation that fails to result 
 in appropriate response, and then we experience pain. Between 
 our consciousness that we call mental dissatisfaction resulting 
 from the failure to find a solution and the consciousness that we 
 call "physical" pain there is no clear and essential distinction. 
 But in pain what we describe as " physical " consciousness attains 
 its maximum of intensity, fading away only when, at last, a 
 response (or healing process, or reaction) occurs, or when 
 there is partial or complete destruction of the receptors and 
 neural tracts that are involved in the physical process of 
 sensation. 
 
 Life Intuition. Clearly there is something in the living animal 
 that is additional to, or, at least, is not merely the physical 
 process of, sensation and response. Subject the latter to analysis, 
 and we find a mechanism, receptor organ, afferent nervous tract, 
 central nervous connections and tracts, efferent nervous- tract, 
 and effector organ. This may be set in motion, so to speak, and 
 evoke nothing but processes that are certainly energetic ones, 
 although the behaviour that we can thus observe may be appro- 
 priate, purposeful, and complete. But it may set in motion 
 again, and then it may evoke response only after deliberation 
 and more or less acute consciousness. Something, then, is 
 operative which is not merely physical sensation and physically 
 actual or virtual response. This may be the exercise of pure 
 memory, the disciplined deliberation that we call reasoning, 
 " mental " and " physical " pain, pleasure, the unsustained and 
 desultory efforts of reverie and " day-dreaming," and so on. It 
 would be as foolish to deny this as to attempt to characterise it 
 all in terms of energy. 
 
 Just what to call it we do not know. " Spirit " will be a term 
 that is objectionable to many; mind is only one aspect of the 
 thing that we mean; consciousness is inadequate, for we almost 
 seem compelled to postulate " subconscious " mentality; Berg- 
 son's " vital impetus " indicates a passage of some kind or other, 
 and is not just the term we require. In the meantime, at all 
 events, we may refer to it as simply " life intuition," suggesting 
 nothing more than what is meant here in the context. We deal, 
 then, with something that is expressed in the functioning and 
 behaviour of the animal body. 
 
224 THE MECHANISM OF LIFE 
 
 Intuition and the Law of Conservation. When the animal 
 dies it does not cease to exist, because its molecules fall into new 
 chemical configurations and its energy is conserved even if it is 
 dissipated. But we cannot apply the law of conservation to 
 life intuition, because the law itself is only a mode of operation, 
 or a category, of mind which is the intuition itself. And the 
 application of the law of conservation is necessarily restricted 
 to entities that are measurable, while, obviously, mind and the 
 qualities of mind and " feeling " are not, in themselves, capable 
 of measurement they are not " in " space and time. It is not 
 a perception that we measure in applying the " psycho-physical " 
 law of Weber and Fechner, but the strength of the stimulus that 
 precedes a change of consciousness, while the mathematical 
 relation between the change of consciousness and the energetic 
 change in the stimulus proves to be a spurious one when sub- 
 mitted to analysis. Intuition is, therefore, non-energetic, non- 
 measurable, and is not conserved. All that we know about it is 
 itself it is us. It manifests or expresses itself in animal 
 behaviour and functioning, and so, when the animal body dies 
 or becomes unable to function we must believe that life 
 intuition ceases to exist. Or, at the very least, we have no 
 reason for believing that it can survive the dissolution of the body. 
 
 Of course, the latter, and therefore the life that it expresses, 
 is immortal in a certain sense. In reproduction a part of the 
 body becomes detached and grows to form a new individual, 
 which again reproduces, and so on indefinitely. Since the ability 
 of expression in functioning and behaviour is thus transmitted 
 through an endless series of generations, something in the life 
 intuition is immortal, but undoubtedly something is abolished 
 in the act of reproduction. This is the intuition of continuous, 
 personal identity, which is lost, or at least does not pass from 
 individual to individual. That which we call memory comes 
 into existence with the individual, and ceases to exist when the 
 latter dies. With memory there is associated, in some way, 
 mdetermmation of response to sensation, and thus the possibility 
 of evolutionary change. And, of course, with this indeterrnina- 
 tion goes what we call motive, praise and blame, personal respon- 
 sibility, and " sin." All these cease with the death of the 
 individual body, some of the activities of which are their ex- 
 pression. If the expression of the whole intuition does not 
 persist, then, can we say that the whole intuition itself is con- 
 served ? When we say that life intuition is, but expresses itself 
 in functioning and behaviour, we make the statement because of 
 
A METAPHYSICAL DISCUSSION 225 
 
 our own personal intuition and its expression in our activities, 
 and then we assume that activities in other individual bodies 
 similar to ours are also the expressions of individual life intuitions. 
 
 So, in an individual, human career, we seem to see the gradual 
 cessation of intuition : first, the loss of creative mental power at 
 a relatively early period of life ; then the slackening of the feelings 
 of initiative and responsibility, and a progressive stagnation of in- 
 tellect with increasing domination of habit. Then follow the loss of 
 reproductive power and senile decay. What remains of life becomes 
 concentrated in the effort to give bodily metabolism the peculiar 
 " vital " directions, and little by little even that fails and ceases. 
 
 What is transmitted in the act of reproduction is the vital 
 impetus; in reproduction a life intuition dissociates, one part 
 remaining as the parental body, and the other part becoming the 
 offspring. All the powers of life that manifest themselves in the 
 functioning and actions characteristic of the parental organism 
 are conveyed, in reproduction, to the offspring. What is not 
 conveyed is the memory of the former and, in general, the new 
 form of acting which it acquires in the course of its individual 
 experience. Just as death ends the individuality and personal 
 continuity of the parental organism, so in giving birth to a new 
 individual that personality dissociates itself. 
 
 The Working Out of Intuition. The life intuition of one 
 individual is incommunicable, as such, to another. No process 
 of instruction conveys the knowledge of life in itself; what is 
 conveyed is the materialisation of that life intuition. If there is 
 any way in which feeling and thought can be transmitted from 
 one organism to another otherwise than by an action, we do not 
 know of it. What we are told by modern " spiritualism " as to 
 the possibility of " telepathy" and the like does not come within 
 the purview of science, and, at the very best, those results are 
 grossly materialistic ones, and are their own refutation; it is not 
 " spirit " that the " seances " disclose to us, but matter. 
 
 And the individual and personal life intuition is not trans- 
 mitted from parent to offspring. Heredity is the communication 
 of motor and chemical habit. Form, the idiosyncrasies of acting, 
 moies of functioning these are transmitted. It is the generalised 
 intuition that passes over in reproduction. 
 
 In the interpretation of the thoughts and intuitions of other 
 people, and in the interpretation of our own thought and intuition, 
 what we have to do with are the physico-chemical expressions of 
 those thoughts and intuitions. Life manifests itself " objec- 
 tively " in no other way. 
 
 15 
 
226 THE MECHANISM OF LIFE 
 
 The appearance of the sky at dawn . may give one a feeling of 
 strong mental satisfaction. Now to describe that appearance in . 
 terms of form, colour, and intensity of illumination, and to 
 explain it by reference to sunlight falling obliquely through an j 
 atmosphere laden with water vapour and dust, may seem to 
 many to be an imperfect and sordid interpretation of a very 
 beautiful phenomenon. But that is all that we can do. If we 
 speak of the " dawn in russet mantle clad," do we express our 
 intuition in a manner that is essentially different ? Obviously 
 not. In either case the materialism is patent. 
 
 The same kind of mental satisfaction may be experienced when 
 we listen to music. If we describe this in terms of rhythm, 
 pitch, and the relations between sounds heard simultaneously 
 and sounds heard consecutively, we may be told that the descrip- 
 tion is clumsy and inadequate. But the analysts are no more 
 successful. " Thus Fate knocks at the door," said Czerny. 
 "It is the song of the Yellow Hammer," said Beethoven, with 
 reference to the same melodic phrase. Necessarily, the analogy 
 or description, whatever it may be, makes use of materialism. 
 
 Sometimes one is seized with a feeling of apprehension, and 
 even quite irrational fear, in walking along a lonely road at 
 midnight. The feeling itself is quite indescribable, and it, like 
 aesthetic ones, is private to the individual that experiences it.' 
 But try to describe it: one listens intently; moves as silently as 
 possible; the body is held tensely and in a posture of preparation; 
 heart-beat and respiration respond to the slightest stimuli. 
 Obviously we translate the emotion of fear in materialistic 
 terms. 
 
 Our feeling of pleasure on looking at a dawn or sunset, or in 
 listening to music, and the dread that may possess one in a 
 strange situation are certainly experiences sui generis. But our 
 intellectual, communicable description of them is, and must be, 
 a materialistic one. " Sunset and evening star " we know to be 
 an atmosphere of a certain constitution penetrated by the radia- 
 tion from cosmic bodies. The C minor symphony, as we hear it, 
 is an exceedingly complex series of displacements of the molecules 
 of the atmosphere. Irrational fear is the emotional, afferent 
 reflexes from a body that is thrown into a state of preparation to 
 resist something material. And so on. Seek to understand and 
 express intellectual, emotional, or other intuitive experience, and 
 we find that we can do so only in terms of matter and energy. 
 Life passes into materialistic phenomena. Or, at least, that is our 
 " first approximation." 
 
A METAPHYSICAL DISCUSSION 227 
 
 The Description oi Nature. Now we go a step further. Our 
 life intuition, disappearing as such, inserts matter and energy 
 into nature. Or, rather, it does not create matter, so much 
 as pass over into it. That is what we should have said twenty 
 years ago, but can we say it now ? Obviously not. 
 
 For we do not really observe matter and energy in nature. 
 We observe space-time coincidences, and from these we infer' 
 space and time measurements. It is not matter, nor even energy, 
 that we infer it is only space-time measurements or intervals. 
 That is our " second approximation," but we can go further still, 
 thanks to the modern, generalised theory of relativity. That 
 which we know as " nature " is not matter and energy, nor even 
 space and time, but the relations between space-time coincidences. 
 
 What is the "weather" ? Intuitively it is a feeling of dis- 
 comfort, but we " know " it and describe it, and act upon it (or 
 are acted upon by it, when the " sign " of the action is changed) 
 in strictly materialistic ways: it is rain and wind and a low 
 temperature and mist. Looking into the matter more closely, 
 we find, for instance, that the low barometer is one of the terms 
 in our description, and the quantity of water in the rain gauge is 
 another. But these are space measurements, for the low baro- 
 meter is a column of mercury (a linear dimension) of a certain 
 magnitude, and the rain-gauge observation is a volume of water 
 (a cubic dimension). The temperature is also a linear dimension 
 the length of a mercury, capillary column. Coupled with these 
 observations there are time ones: we read barometer, rain gauge, 
 and thermometer at a certain instant when the hands of the 
 clock had moved through a certain arc (since the last twelve noon 
 or midnight). Obviously the time is a space measurement. 
 
 So, also, the material water, nitrogen, and oxygen in the 
 environment are time and space measurements. Whatever they 
 feel to us that is, whatever our intuition of them may be we 
 describe them as molecules which are made up of atoms which 
 we regard as electrons in movement. And of the electrons we can 
 know nothing but space measurements : a positive nucleus with 
 several electrons and a surrounding swarm of negative ones; 
 relative velocities and stresses, repulsions and attractions. The 
 " actual " molecule, or atom, or electron, moves; thus we have, 
 coupled with positions, instants in time. 
 
 That is, our knowledge of nature is space and time measure- 
 ment, and, indeed, we can see no distinction between the measure- 
 ment of space and that of time. We do not know space and time 
 intellectually, for space is the intuitive knowledge of our freedom 
 
28 THE MECHANISM OF LIFE 
 
 of mobility, and time is our duration as conscious, remembering 
 individuals. What we know and deal with intellectually are 
 points in space and instants in time. There is nothing between 
 the space points but a mathematical relation and the intuition 
 of a possible actual or virtual bodily movement, and between the 
 time instants there is a similar relation and duration, which is, 
 perhaps, the " stuff " of our intuition. 
 
 Proceed a little further, and note that in "reading the baro- 
 meter ' ' we observe a space-time coincidence. There is an instant 
 at which the top of the mercury column coincides with a mark 
 on the adjoining scale. That instant itself is a coincidence of 
 the hand of the clock with a mark on the scale or dial. There is, 
 therefore, a double space coincidence. In the barometric reading 
 we define a point y l in reference to another point = / , the scale 
 zero. In the time measurement we find an arc by defining a 
 point x, y in reference to the " zero " of the dial its centre. This 
 latter point we, however, call t, the time. Thus our measure- 
 ments are defined by reference to some co-ordinate system, and 
 their statement is always an arbitrary or conventional one, and 
 depends on the choice of the scale zeros or co-ordinate systems. 
 
 Proceed with such an analysis in relation to any transforma- 
 tion, or event, or object, or phenomenon whatever. The 
 " elements " of our knowledge the perceptions with which we 
 construct the universe are space-time coincidences. In any such 
 perception we generally observe the coincidences of four points 
 (three space points and one time instant), x l7 y^ z ]7 ^, with other 
 four, #,, y 2 , z 2 , t. z . Let us state this in a quite dogmatic way (for 
 it can be amply demonstrated, if necessary) all the data of our 
 knowledge of nature are space-time coincidences and the relations 
 of such. 
 
 Nature a System of Relations. We do not deal with even the 
 space and time points, for these have no meaning except with 
 relation to a " frame of reference," or co-ordinate system. The 
 latter is always an arbitrary (though convenient) one, and obvi- 
 ously the position of any point depends on the zero from which 
 it is measured. We take as an illustration (for a clear under- 
 standing of the matter is very desirable) the trajectory of a 
 material body falling freely in vacuo. 
 
 We do not deal with " spaces " and " times " here, but with 
 " differentials," ds and dt. The symbol ds is not an infinitesi- 
 mally small space, but the limit to a space that becomes smaller 
 and smaller without ever becoming zero. It can be so small that 
 the error involved in regarding it as zero will be less than any 
 
A METAPHYSICAL DISCUSSION 229 
 
 standard error, no matter how^ small this is taken to be. So 
 also with the symbol dt : it is not an infinitesimally small dura- 
 tion, but the limit to a duration which always diminishes. Our 
 falling body, therefore, is at certain points s , s^ s 2 > etc. when 
 the time is ? , ^, Z 2 ? etc. The s-points coincide with the ^-points, 
 the former being read from a measuring rod, and the latter from 
 a clock. Representing these coincidences graphically, we get the 
 following " one-to-one correspondence " of s-point with -point. 
 
 Obviously we might remove the origins, or zeros, of the time and 
 space scales further to the left without altering the character of the 
 diagram: t = 1 second might just as well be t = 10, and s 100 feet 
 might as obviously be t = 1,000 the inclinations of the dotted 
 lines showing the coincidences would not be changed. 
 
 
 >OO SOO -4-OO ^OO 600 ' -7OO 
 
 / 77 Q. //near space cL ' m e " -S" / on 
 
 FIG. 51. THE UPPER POINTS REPRESENT OBSERVED TIMES, THE LOWER 
 ONES THE CORRESPONDING SPACE. 
 
 What, then, we obtain from such an analysis is the relation 
 between s-points (which have no magnitudes) and ^-points (which 
 have no duration). In other words, it is a naked relation that we 
 obtain, and nothing else there are no space or time regarded as 
 entities. Space and time are not entities, as we have seen already. 
 
 The relation is the ratio between the differentials ds and dt\ 
 
 thus - ds 
 
 and this is what we find when we observe the fall of a stone in 
 wcuo. Such differential equations are the " la^s of nature." 
 As a rule, however, we try to avoid using the space-time equa- 
 tions in their differential forms, and we express them, when 
 possible, otherwise. Thus the above relation is more easily 
 recognisable in its integral shape 
 
 We have now reached the conclusion to which we have been 
 tending. Nature, as we know it intellectually that is, the Nature 
 of Science is a system of relations, and nothing more. It is, 
 however, our practice to speak of these more fundamental rela- 
 tions as " matter," " fields of force," " energy," " atoms," 
 " electrons," " radiation," and so on, all these things " occurring " 
 
230 THE MECHANISM OF LIFE 
 
 in space and time. It would be highly inconvenient in the 
 practical affairs of life to do otherwise, and it would be clumsy 
 even in scientific investigations. Just now, however, we are not 
 concerned with practice, but with the attempt to search into the 
 meaning of things. 
 
 These meanings we can grasp in a fumbling kind of way. We 
 postulate relata between which we make differential equations or 
 relations. But the relata are only space-time coincidences, 
 ds's and dt's, and these have no " reality " apart from co-ordinate 
 frames to which we refer them, and the choice of such co-ordinate 
 frames is quite arbitrary. Something, however, is an absolute 
 element of our knowledge, and this is the relation itself which 
 exists, and is invariant even if the frames of reference vary. 
 
 Obviously, then, " our knowledge of nature is a knowledge of 
 form and not of content,"* and what that content is we do not 
 know. We are assuming (though the assumption cannot be 
 proved " scientifically ") that there is a content of nature some- 
 thing apart and independent of the intuition of life. The latter 
 works itself out in a description of the manner in which it acts upon 
 the content, but it does not describe the content. That is the 
 ordinary, " natural " way in which we think about things here 
 is life, in us as well as in a multitude of other animals, and there 
 is an " environment " upon which life acts. That assumption 
 is contained in everything that an animal does, and it is implicit 
 in all our civilisations, and so one cannot but make it a part of 
 our philosophy. We know imperfectly how we act upon nature 
 (the environment), but we do not know what it is that we act 
 upon. The elements of our scientific knowledge are the forms 
 of the actions, but not the stuff acted upon. 
 
 The " Passage of Nature."t Assuming, then, this nature that 
 exists independently of our form and power of action upon it, 
 we note that it " passes " that it is a progress or career. It has 
 a tendency in a particular direction, that tendency being described 
 by the second law of energetics. This law we must regard as the 
 fundamental thing in our experience, the concept that is un- 
 shaken by any recent development of scientific theory and specu- 
 lation. And, of course, its fundamental character suggests that 
 the law is in us in our mode of acting upon nature. 
 
 First, we note the phenomena of radio-activity. Certain ex- 
 ceptional substances the atoms of uranium, radium, actinium, 
 
 * Eddington, Space, Time and Gravitation, Cambridge University Press, 
 1920, p. 200. 
 
 t A. S. Whitehead, The Concept of Nature, Clarendon Press, 1920. 
 
A METAPHYSICAL DISCUSSION 231 
 
 thorium, polonium, etc. disintegrate, giving off large quantities 
 of radiant energy. We know that this energy is not created in 
 the atom, but is a store that diminishes by reason of its emission. 
 Therefore, the radio-activity of a disintegrating atom comes to 
 an end sooner or later. Uranium passes into radium, and 
 radium, after a long series of intermediate steps, passes into lead. 
 The atomic energy of lead is bound, since this substance shows 
 no sign of radio-activity. 
 
 Now, the great majority of the chemical atoms are in the same 
 condition as lead, and we know that the few that are still capable 
 of emitting radiant energy will pass into the condition of lead, 
 when their energy will exist entirely in the bound state. Extra- 
 polating the process far into the future, we predict the time when 
 all chemical atoms in the earth, sun, and the luminous stars, will 
 have ceased to emit energy that is, the substance of the cosmic 
 bodies will have passed into the inert material stage. So, also, 
 extrapolating the process far back into the past, we may be sure 
 that we pass through stages in which the rate of emission of 
 energy from radio-active elements was progressively greater and 
 greater, and was at a maximum at some remote stage in the 
 evolution of our universe. In that stage the substance of the 
 latter existed in the pre-material state. With regard, then, to 
 this generation of available energy by radio-activity, the passage 
 of nature is one from a pre-material to an inert material stage ; 
 from the condition of free energy to that of bound energy. 
 
 This, doubtless, accounts for the greater fraction, or perhaps 
 all, of the energy that takes part in universal transformations, or 
 physical phenomena. But consider further the free or available 
 energy emitted by radio-active bodies, or free energy in general, 
 apart from any question as to its origin. This energy degrades. 
 Whatever its nature, and whatever the nature of the trans- 
 formations which it undergoes, it ultimately becomes heat at low 
 temperature. Further, this heat tends always to become equally 
 distributed everywhere throughout the universe, so that the time 
 will come when there will no longer be any difference of 
 intensity, or potential, in the unique form (heat) in which energy 
 will then exist. 
 
 That will be the condition of universal physical death (" Warm- 
 Todt," in Clausius' term). 
 
 The universe, therefore, tends from a condition of physical 
 activity to one of physical death. It runs down. Free energy 
 becomes completely dissipated, and unbound energy becomes 
 bound. From a condition of minimum entropy the universe 
 
232 THE MECHANISM OF LIFE 
 
 attains a condition of maximum entropy. From a pre-material 
 state it passes into an inert material state. 
 
 This is the tendency, or direction, of the passage of nature. 
 
 The Relativity of the Passage. Now one thinks about the 
 life passage in just the same way. If we are right in our inter- 
 pretation of life intuition as something indescribable (though 
 perfectly well "known" to us) which "runs down" in some 
 way, expressing itself in inert material manifestations, then the 
 life passage exhibits the same tendency, or direction, as does the 
 natural passage.* In Bergson's terminology it defends. Note 
 that all recent work interprets " matter," just as Descartes did, 
 as extension, and note that the quality of life, mind, memory, 
 intuition (call it what one likes), is that space measurements 
 cannot be made with regard to it. There is, therefore, some 
 quality in life that we express as intensitiveness, meaning the 
 opposite of extension. By detending this quality becomes 
 something extensive, and so capable of space measurements, and 
 therefore of physical investigation. That which Bergson puts 
 in this way we have tried to put in another way, but the idea 
 involved is the same one. 
 
 Life, then, passes always into the inert material state, and the 
 passage is our intellectual description of it. 
 
 Assume, now (if for no other reason than to see where the 
 assumption leads), that there is a stuff of nature, an environ- 
 ment of life, something other than life which exists as well as 
 life. Assume that our superficial investigation of nature tells us 
 that this environment does not remain the same, but passes into 
 an inert material state. Life, acting on this stuff of nature, 
 retards the progress of the latter towards the inert material state 
 (when life could no longer utilise it). It retards the augmentation 
 of entropy, but does not (cannot, in fact) prevent the ultimate 
 attainment of maximum entropy. As the temperature of the 
 sun falls life must (as we know it) become the less and less able 
 to persist, and must ultimately cease. That is our ordinary 
 conception of the relation of life to its environment. 
 
 But it seems (since one has been compelled to think in another 
 way by the stimulus of the modern relativity theory) that we 
 may just as easily regard nature as something that is at rest, and 
 which does not pass, and hold that the apparent passage is due 
 to the passage of life. If the atmosphere is at rest, and if one 
 
 * Remember the (easily ignored) fact, that of living substance we literally 
 know nothing. We study the behaviour only of a living organism. When- 
 ever we study organic substance, it is necessarily dead, inert material that 
 \vc investigate. 
 
A METAPHYSICAL DISCUSSION 233 
 
 moves quickly through it, one sets up an apparent current of air, 
 and this current may apparently blow in either of two opposite 
 directions, according to the way one moves. It would not be 
 difficult to devise conditions in which it would be impossible for 
 a man to ascertain whether he was at rest and the air were blowing 
 past him, or whether he was being carried through air which was 
 at rest. 
 
 So, instead of the environing nature changing as we have 
 suggested, it may be that it is " at rest " ; that it is all there, so to 
 speak, though locally different; and that life changes, or moves, 
 so that it encounters the local modifications, just as a man in a 
 railway carriage is carried through a "changing" landscape 
 which is " really " at rest. 
 
 There is an apparently formidable difficulty to our thinking 
 anything of the kind : we seem to be convinced that nature passes 
 whether we are there or not. We believe that the sun shone 
 and its energy became degraded before we were born, and 
 that it will shine and dissipate its heat after we are dead. 
 Certainly these processes occur while we are asleep and uncon- 
 scious (though we are still there in those conditions, it should 
 be noted). 
 
 It is worth considering whether we are not there also before we 
 were born, and will be there after we are dead; and to those that 
 accept the doctrine of personal immortality the apparent objec- 
 tion we have suggested ought not, of course, to exist. 
 
 Now, we do suggest that we were there, and shall be there, 
 before and after individual life; indeed, that is really the case. 
 We (that is, our life) actually and really existed, in the strictest 
 scientific sense, before our individual lifetime as a fragment of 
 germ plasm in the parental body, and in the grand-parental 
 body before that, and so on indefinitely. And, potentially at 
 least, all the future generations of life are contained in us in our 
 actual, physical bodies. Therefore, life is continuous, unitary, 
 and always there, just as the hypothetical environment is there 
 simultaneously that is the plainest and most easily grasped 
 result of biological science. 
 
 We ignore it just because we are obsessed by the notion of 
 individual forms, individual bodies, species, genera, families, and 
 so on. But the forms are surely continuous, flowing into and 
 out from each other in an evolutionary process. 
 
 And we are also obsessed by the notion of our personal con- 
 tinuous memories, which are most certainly discontinuous 
 careers, begininng with the awakening of the " categories of the 
 
234 THE MECHANISM OF LIFE 
 
 understanding " by experience, and ending with the death of the 
 matured " somatoplasm " (not necessarily the "body," note, 
 for the latter was both somatoplasm and germ plasm). This 
 remembered experience, with the indeterminism that is asso- 
 ciated with it, and the morality, immorality, virtue, and " sin " 
 that it is also associated with in virtue of its indeterminism, is 
 discontinuous, comes into existence and ceases to exist. We 
 have no reason to say that it is conserved, and the very notion 
 of conservation is inapplicable to it. 
 
 Yet that personality is only a little of life, and the greater 
 fraction of the latter is continuous, and all the life of the world is 
 one. That is just what one means by reproduction and heredity. 
 What one means by evolution (or transformism) is that life 
 changes, that it undergoes passage. 
 
 There is, of course, the other difficulty, felt by those who 
 profess " solipsism " : there is, literally, nothing in nature but the 
 thinking mind, or rather the thought. That, we hold, is negatived 
 by every life action. It is absurd to ' ' common sense ' ' (and surely 
 no philosophy can disregard that !). It is a pretence in anyone 
 who merely says or writes that he believes it (for in speaking and 
 writing he assumes that there are other minds that listen to his 
 words or read his writings), and we even suggest that there is a 
 kind of intellectual dishonesty in talking about it. 
 
 So the objection fails, for life is always and everywhere there 
 (for we cannot say that thought is in space; a man is in a room, 
 but is his mind there ? Obviously not when he thinks of the 
 seaside holiday or of his boyhood). Therefore, life being always 
 and everywhere, we are free to suggest that its passage and that 
 of environing nature are relative to each other, and that we 
 cannot say " which is which." 
 
 But, again, it seems quite possible to hold that this question 
 of the relativity of the life passage and the environmental nature 
 passage is one that has no meaning. We do not know what is 
 the stuff of nature, since all that we discover by investigation of 
 any kind is a system of naked relations. There may be an homo- 
 geneous stuff (as Bergson says), and it may be that the foim of 
 this is just what our possibilities of action make it. Atoms, and 
 electrons, and energy, are just the ways, it may be, in which our 
 life intuition cuts up this homogeneous stuff. The latter may 
 be the actual stuff of our consciousness, so that there is, then, 
 nothing in the universe but this: the stuff of life which continually 
 
SPACE IN MODERN THEORY OF RELATIVITY 235 
 
 appears to itself to degrade itself into inert-material that is, into 
 nothing, for the qualities of inertia are negative ones. Here it is 
 obvious that our discussion is becoming very frankly metaphysical , 
 and must stop. 
 
 APPENDIX II 
 SPACE IN THE MODERN THEORY OF RELATIVITY 
 
 SOME of the leading ideas in the theory of relativity are not at 
 all difficult to grasp, and they are of extreme importance. 
 
 First, then, we note that the Euclidean system of geometry is 
 insufficient for speculation upon the nature of the universe. It 
 is based largely on the parallel postulate that no more than one 
 straight line can be drawn through a point that lies outside a 
 given straight line and still be parallel to that given straight line. 
 Euclid assumed this, but could not prove it, and no one since 
 him has been more successful. We can assume the contrary 
 that more than one straight line can be drawn parallel to a given 
 straight line. Then we can prove a number of propositions that 
 seem to be absurd, but are really quite consistent and free from 
 contradiction. The non-Euclidean geometries describe the 
 ordinary things that we see quite satisfactorily, but they are not 
 so easily worked as the classic system, so we use the latter in our 
 everyday affairs. 
 
 Second. The Euclidean geometry of three dimensions does 
 not describe events. A thing is not simply there, so to speak 
 it always happens. A molecule of water is not really a thing it 
 is things in motion (electrons). Therefore, we need the idea of 
 time in describing nature. We say that a thing is somewhere 
 it is so much distant from the plane of X, so much from the plane 
 of Y, and so much from the plane of Z. Its space description 
 involves three variable numbers X, Y, and Z but since its 
 description also requires a statement of the time at which it 
 happens, we want a fourth variable, T. (We want the when as 
 well as the where.) 
 
 Thus our universe must, at least, be four-dimensional. Things 
 are really and actually "specified" by four variables: X, Y, 
 Z, T. This is the space-time continuum of Minckowski, adopted 
 by Einstein. 
 
 Third. The four-dimensional (but still Euclidean) geometry 
 is insufficient. The latter is to be thought about as a three- 
 dimensional one of three co-ordinate planes^, Y, Z which are 
 
23(5 THE MECHANISM OF LIFE 
 
 continued somehow in a straight line, T. Now this does not 
 describe the universe when we take account of relative positions 
 and motions. Somehow or other the co-ordinates must be 
 curved, and to allow for this we have to introduce a fifth dimen- 
 sion. This is " quite all right," and the mathematics of a five- 
 dimensional geometry are as straightforward (though immensely 
 more difficult) as are those of three dimensions. The trouble is 
 that we cannot visualise five dimensions. Still, the mathe- 
 matical results are there a fact of very great significance, for an 
 abstract result of this kind always means possible action. 
 Given any abstruse mathematical result, and we may be pretty 
 certain that by-and-by it will mean something " real " that is,, 
 something that practical science, and by-and-by even industry > 
 will do. 
 
 SD we live in a non-Euclidean universe of four dimensions 
 which is "in" a continuance of a fifth dimension. Leave it at 
 that and see what are the consequences. 
 
 The consequences (for our speculations of Chapter XL) are that 
 space cosmic space, that is is " finite but unbounded." Think 
 about this by analogy: the surface of the earth to us is finite,, 
 but unbounded. One can go on travelling over it in " straight "" 
 lines (geodesies) without being brought up anywhere ; there 
 would be no end to our journeying if we could live for ever. We 
 might come back to the same place from which we started, though 
 we should be travelling in a " straight " line, but even then we 
 could still go on in the same direction. 
 
 Now extend that to cosmic space, and suppose that it, too, is- 
 finite and unbounded. We are stepping out of non- Euclidean,, 
 two-dimensional, curved space into non-Euclidean, four-dimen- 
 sional, curved space. This means that the universe is finite but 
 unbounded. We can go on through it with the velocity of light 
 for ever, and always in the same direction. There is no end ta 
 our journey. Perhaps we might come back to the starting-place, 
 perhaps not; if we did, we should still be facing the same way. 
 It means that the galactic universe is finite (and Einstein even 
 tells us how big it is). Outside it there is nothing, not even 
 space. Light travels in what we call curved lines, and never 
 goes outside the universe from which it originated. 
 
 The notion is of immense speculative importance. 
 
 Suppose, though, that the time dimension is also curved t 
 Einstein does not seem to have worked out that. Perhaps the 
 reader may see to what extraordinary speculations this may 
 lead. 
 
THE SUBMAXILLARY GLAND 
 
 237 
 
 APPENDIX III 
 
 THE SUBMAXILLARY GLAND AN INSTANCE OF 
 ORGANIC FUNCTIONING 
 
 THREE pairs of glands open into the mouths of mammalian 
 animals: the sublingual, submaxillary, and parotid salivary 
 glands. Their function is to secrete saliva. Saliva consists 
 mostly of water which contains some mineral salts, some soluble 
 proteids, some mucin, and, in some animals, an enzyme called 
 ptyalin, which has the power of dissolving starch and converting 
 the latter into sugar. Saliva mixes with the food, so that the 
 latter can be worked up into a " bolus," which is then swallowed; 
 it cleans the mouth in that it removes dirt in the " spittle," and 
 it may be a digestive ferment, or enzyme. 
 
 FIG. 52. A DIAGRAM OF A SALIVARY GLAND WITH ITS VESSELS, 
 NERVES, AND DUCT. 
 
 The gland itself consists of a very great number of alveoli 
 which are connected with ductules, which unite to form the duct 
 which opens into the mouth. The saliva is formed, or secreted, 
 in the cells which constitute the walls of the alveoli. Leaving 
 the details for a moment, note the structures connected with the 
 gland. 
 
 An artery conveys blood to it. The latter circulates round 
 the alveoli, and leaves the gland by means of a vein. Lymphatic 
 vessels are also connected, and through these a watery fluid, 
 called lymph, leaves the gland and finally enters the circulating 
 blood of the body. 
 
 A ducb leaves the gland, and through this the saliva that is 
 secreted in the alveoli reaches the mouth. 
 
238 
 
 THE MECHANISM OF LIFE 
 
 Two nerves enter the gland. One of these comes from the 
 medulla, and is called the chorda tympani. The other comes 
 from the sympathetic nervous system. These are efferent nerves 
 conveying stimuli to the gland from the central nervous system. 
 
 The duct' divides into (or, rather, is formed by the union of) a 
 great number of smaller ductules or tubes. The ends of these 
 tubes may be thought about as swelling out to form bulbar 
 enlargements. The walls of the ductules are thin, but those of 
 the enlargements, or alveoli, are thick, being made up of large 
 cells. It is in these cells that the actual formation of saliva 
 occurs. 
 
 The artery which enters the gland divides up into a great 
 number of smaller vessels, or arterioles, and the latter again 
 
 FIG. 53. 
 
 A, Diagrammatic section through an alveolus (or secreting unit) of the 
 gland, with its capillary circulation; B, diagram of the terminations 
 of a secretory nerve among the cells of an alveolus. 
 
 break up into capillaries. The capillaries are distributed over 
 the alveoli, as shown in Fig. 53. After circulating through the 
 capillaries, the blood leaves the gland by means of the vein. 
 
 The nerves similarly break up into smaller branches. Some 
 of these divide further into very fine fibrils, which are distributed 
 as a fine network over the alveoli on the outside, or even in 
 between, the cells. Other branches of both nerves break up into 
 fibres which end in the muscles of the walls of the artery and 
 arterioles. 
 
 Such is the structure. How does it function ? 
 
 Generally saliva flows into the mouth when savoury food 
 substances stimulate the gland to act. This is a reflex, and the 
 afferent nerves are the gustatory and olfactory ones that is, 
 those conveying the taste and smell stimuli to the brain. But 
 this is not all by any means, for if meat be shown to a hungry 
 
THE SUBMAXILLARY GLAND 239 
 
 dog there will be a reflex flow of saliva from the mouth, even 
 though the animal does not s-mdl the food. The afferent paths 
 are now the optic nerves. But, again, this is not all, for " when 
 the dog realises that he is being played with . . . the psychical 
 secretion of saliva ceases." And yet again, it is said that the 
 thought, or memory, of savoury food may cause a secretion of 
 saliva in ourselves. 
 
 When small pebbles are put into a dog's mouth, the animal 
 shifts them about with his tongue and then spits them out. 
 When the same substance, crushed into sand, are put in his 
 mouth, thin watery saliva is secreted, and the sand is washed out. 
 So, also, with dry biscuits. When meat is put in his mouth, 
 thick, viscid saliva is secreted. 
 
 Thus, although the secretion of saliva is a reflex, it is one that 
 is controlled and modified (or even arrested) in a variety of ways. 
 There is no simple, invariable, mechanical response. 
 
 How is the control effected ? 
 
 It was thought at one time that the control was effected by 
 the change of calibre of the arterioles. The saliva comes ulti- 
 mately from the blood that circulates through the gland. Now 
 the fibres of the chorda tympani dilate the vessels and permit a 
 more abundant flow of blood through the gland. Accordingly, 
 there is a greater flow of saliva. The fibres of the sympathetic 
 contract the arterioles, and so a decreased flow of blood takes 
 place and there is a decreased flow of saliva. The inference 
 appeared to be that the secretion of saliva was a mere filtration 
 from the blood, and depended on the pressure of the latter the 
 greater the pressure, the greater the flow. But the saliva has not 
 the same composition as the liquid part of the blood, so there is 
 more than mere filtration to be accounted for. Further, when 
 the gland is poisoned with atropin, the dilating action of the 
 chorda tympani is not affected; yet when the latter nerve is 
 stimulated in such a poisoned gland, there is no increased flow of 
 saliva. On the other hand, when it is poisoned with pilocarpin 
 there is an increased flow. 
 
 The explanation of these experiments is that there are secre- 
 tory nerve fibres in addition to those that dilate or contract the 
 arterioles. Poisoning these secretory fibres in one way or 
 another affects the secretion of saliva. We see that there are 
 nerve fibres coming into connection with the cells of the alveoli. 
 
 It is, then, these secretory nerve fibres, acting directly on the 
 cells, that stimulate the latter to secrete the liquid. The other 
 nerve fibres which act on the bloodvessels control the supply of 
 
240 THE MECHANISM OF LIFE 
 
 blood to the cells as the circumstances require. How do the 
 secretory fibres act on the cells ? When the latter function, it 
 can actually be seen that granules of some material taken from 
 the blood have been formed and deposited in their substance. 
 The stimuli of the secretory fibres seem to cause these granules 
 to disintegrate, absorb water, and swell, finally discharging their 
 contents into the cavity of the alveolus. This is the thing, then, 
 that has to be " explained." 
 
 What is the substance of the granules ? How is it taken from 
 the blood ? How is it liberated into the alveolus when the cell 
 is stimulated ? . How is it that the nature of the substance varies 
 according to the kind of food ? How can the sight of food 
 produce the same effect as the taste or smell of food ? How can 
 the memory of food do the same thing as the taste of food that 
 is, how does an immaterial, non-energetic stimulus produce the 
 same response as an energetic, material one ? 
 
 It will be seen that what we have attained so far is a rather 
 imperfect description of the mechanism of control of the secretion 
 of saliva. So far as the act of secretion itself is concerned, 
 physiology has succeeded in substituting a biochemical descrip- 
 tion for a mechanical one, but even the biochemical description 
 involves at the present time hypotheses that have still to be 
 verified. 
 

 INDEX 
 
 ABSORPTION, 61, 62 
 
 Acceleration, 35 
 
 Acquirements of behaviour, 151 
 
 Acting, functional, 180; indetermin- 
 ism of, 180; unconscious, 178 
 
 Action, creative, 181; difficult, 178; 
 forms of, 178; habitual, 13, 180; and 
 psychical processes, 178; studied 
 objectively, ISO 
 
 Activation of enzymes, 58 
 
 Activity, individualistic, 8; spon- 
 taneous, 9, 182 
 
 Adaptations, 149; concealing, 150; 
 functional, 150; and intelligence, 
 151 
 
 Aggression, organs of, 3, 11 
 
 Air, fixed, 157; sacs in birds, 2; 
 vesicles in lungs, diagram, 67 
 
 Algae, 9 
 
 Alimentary canal, 163 
 
 Altruism, 142 
 
 Amino-acids, 59, 62 
 
 Ammonia in nature, 79 
 
 Analysis of activities, 12, 193; of 
 sensation, 174 
 
 Anatomy, 20; comparative, 2; and 
 function, 125 
 
 Animal automata, 13, 153, 178, 192; 
 behaviour, 192; combustion, 56; 
 definition of, 4; and energy, 156; 
 feral, 142; saprozoic, 84; sedentary, 
 9; spirits, 155; structure, 1; and 
 vegetable, 85; warm-blooded, 1 
 
 Animate engine, 81; mechanism, 69 
 
 Anthropoids, brain of, 127 
 
 Antiseptics, 78 
 
 Appendages and locomotion, 12 
 
 Aqueduct of Sylvius, 97 
 
 Arteries, muscles of, 22 
 
 Articulations in forelimb, 18; in 
 skeleton, 14 
 
 Asphyxiation, 6 
 
 Assimilation, 57, 63 
 
 Association paths, diagram, 103 
 
 Atmosphere, composition of, 82 
 
 Atomic weight, 36 
 
 Atoms, 33, 36; indivisibility of, 36; 
 rearrangements of, 167 
 
 Auditory organ, 108; ossicles, 12; 
 sensation, 114; sense and equili- 
 brium, 114; tracts in brain, 114 
 
 Autolysis of flesh, 78 
 
 Automata, artificial, 153; decerebrate, 
 
 141 ; Organic, 13, 153, 178, 192 
 Available energy, 38 ; decrease of, 197; 
 
 in universe, 209; waste of, 197 
 Axons of nerves, diagram, 26 
 
 Bacteria, action of, 219; fermentative, 
 78; nitrifying, 79; oceanic, 80; 
 pathogenic, 77; putrefactive, 78 
 Barnacles, taxis in, 135 
 Becoming, principle of, 49 
 Behaviour, animal, 9, 12, 149; grading 
 of, 187; measurement of, 171; pre- 
 diction of, 172 
 
 Bell and nervous impulses, 164 
 Bergson and duration, 206; .and in- 
 telligence, 214; and matter, 185; 
 and spirit, 185 
 Biology, the modern impasse, 193; 
 
 utilitarian, 160 
 Black and combustion, 151 
 Blood, arterial and venous, 67; circu- 
 lation of, 63; corpuscles, diagrams, 
 68; gases of, 68; plasma of, 69; 
 venous, 70 
 
 Blood flow, regulation of, S8 
 Blood, vascular svstem, diagram, 63, 
 
 64 
 
 Bloodvessels, 61 ; calibre of, 22 
 Bodily action, scheme of, 130 
 Boltzmann and entropy, 202 
 Bones, fixed and movable, 22 
 Brain, 94; alternative paths in, 183; 
 development diagrams, 95 ; diagrams 
 of, 98, 100; earthworm, 92; evolu- 
 tionary stages in, 105; fish, 96; 
 general scheme of, 89; lower, 103, 
 139, 146; of mammal, 96; vesicles 
 of, 94; working of, 148 
 Burrowing animals, 11 
 
 Calorie, 37; definition of, 40, 44 
 
 Calorific values, 158 
 
 Calorimeter, 40, 44 
 
 Cancer, 7 
 
 Capacity factors of energy, 50 
 
 Capacity for doing work, 37, 33, 42 
 
 Capillaries, 61 
 
 Carbohydrates, 58, 60; decomposition 
 
 of, 79 
 241 16 
 
242 
 
 THE MECHANISM OF LIFE 
 
 Carbonic acid in lungs, 67; in nature, 
 84 
 
 Carnivorous animals, 84 
 
 Carnot, 194; and the steam engine, 157 
 
 Catalysis, 158 
 
 Categories of locomotion, 11 
 
 Categories of the mind, 54, 185, 188; 
 biological view of, 191 ; are mental 
 constraints, 189; mental operators, 
 188; and virtual action, 191 
 
 Cause and effect, 179 
 
 Cerebellum, 95, 123; connections of, 
 diagram, 124; connections of, 102; 
 importance of, 125; injuries to, 124; 
 and standard movements, 148; 
 structure of, diagram, 26 
 
 Cerebral hemispheres, 94; functioning 
 of, 125 
 
 Cerebral physiology, 133 
 
 Chemical combustion, 48 
 
 Chemical configurations, 34 
 
 Chemical energy, 48 
 
 Chemical reactions, 51 
 
 Chemical reactions, types of, 167; 
 endothermic, 83; and energy trans- 
 formations, 167 
 
 Chemistry of eighteenth century, 157 
 
 Chemistry and modern biology, 193 
 
 Chlorophyll, 82 
 
 Chyle, 154 
 
 Circulation in body, 152; 
 
 Circulation of the blood, diagram, 65; 
 discovery of, 66; in fish, 64; 
 
 Civilisations,industrial and pastoral, 86 
 
 Clausius, 231 
 
 Claw- like organs, 8 
 
 Coal and plant life, 86 
 
 Cochlea, 115 
 
 Coherence, 31 
 
 Cold, sensation of, 109 
 
 Colloids, 158 
 
 Colour change in animals, 156; nature 
 of, 33 
 
 Combustion in animal bodj r , 44, 56; 
 nature of, 39, 40; theory of, 157 
 
 Commissures in cortex, 128 ; in nervous 
 system, 92 
 
 Compensatory transformations, 83 
 
 Compounds, chemical, 34 
 
 Consciousness and colloids, 184; and 
 cortex cerebri, 110; and energy, 133, 
 184; not measurable, 184; and 
 sensation, 110, 176 
 
 Conservation, law of, 46; and action, 
 54; a convention, 179, 194: a law 
 of thought, 46, 53; qualifications 
 of, 52 
 
 Consumption by animals, 85 
 
 Convolutions of brain, 103, 128 
 
 Co-ordination of activities, 87, 182; 
 and cortex cerebri, 129, 140; mus- 
 cular, 123 
 
 Cord, spinal, tracts in, 11 1 
 
 Corpora striata, 95; quadrigemina, 95 
 Corpuscles of blood, diagram, 68 
 Correspondences, one to one, 229 
 Cortex cerebri, 143; and cerebellum, 
 129; connections of, 102, 127; 
 and control of body, 120; diagram 
 of, 128; in evolution, 127, 144; 
 and learned actions, 149; localisa- 
 tion of function in, 127; motor area 
 of, 128; prefrontal area, 127; sen- 
 sory area of, 129; and spontaneous 
 actions, 149; structure, diagram, 
 126; and volition, 105 
 Corti, organ of, 108, 115 
 Cosmic order and behaviour, 150 
 Cosmic space and matter, 209; finite, 
 
 236 
 
 Cranial nerves, 89, 113 
 Cranium, the vertebrate, 16 
 Crura of brain, 128 
 Crustacea, locomotion in, 11 
 Crystal growth, 171 
 Crystalloids, 158 
 
 Death, meaning of, 225 
 
 Death, universal, 231 
 
 Decerebrate bird, 140; dog, 141; 
 fish, 140; frog, 139; mammal, 141 
 
 Decussation of nervous tracts, 100 
 
 Defensive organs, 3, 11 
 
 Degeneration of nerve fibres, 165 
 
 Dendrites of nerves, diagram, 26 
 
 Density and temperature, 31 
 
 Descartes and animal heat, 155; cos- 
 mogony of, 161; and Harvey, 66; 
 and mechanism. 153; and modern 
 vitalism, 193; and physical specula- 
 tions, 161 ; physiology of , 154 
 
 Detension, 232 
 
 Determinism, physical, 134, 172; 
 and free-will, 179; in organic re- 
 sponse, 173; in spinal frog, 136 
 
 Dietaries, energy of, 76 
 
 Differentials, 228 
 
 Differentiation of structure, 2 
 
 Digestion, 57, 58, 61 
 
 Digestive juices, 58 
 
 Dimensions of space, 189; of time and 
 space, 235 
 
 Dinosaurs, skeleton of, 15 
 
 Discourse on method, 160 
 
 Disease, biology of, 6; epidemic, 77 
 
 Disentropic phases, 203 
 
 Disharmonies of functioning, 6, 7, 74 
 
 Dissipation, law of, 97,217; life retards, 
 221; in nervous pystem, 133 
 
 Dog, scratch reflex of, 119 
 
 Dreams. 171> 
 
 Driesch and logical categories, ISO 
 
 Ductless glands, 99 
 
 Duration, ni'-aiiingof, 206: not infinite, 
 197; orders i.f. :.'<>.">: and perception, 
 207; rhythms of, 206; and time, 207 
 
INDEX 
 
 243 
 
 Dynamics, 46 
 
 Dyspepsia and secretion, 7 
 
 Ear, internal, 103 
 
 Effort, bodily, 35 
 
 Einstein, 194, 235 
 
 Electricity and heat, 44; measure- 
 ment of, 37 
 
 Electrons, 37; and modern cos- 
 mogony, 162 
 
 Elements, Cartesian, 161 ; chemical, 36 
 
 Embryo brain, diagram, 96 
 
 Emotion, absent in decerebrate ani- 
 mals, 141 ; nature of, 25 
 
 Endothermic reactions, 83 
 
 Enererj', abstract, 45; available, 38; 
 bodily sources of , 69 ; bound, 53 , 23 1 ; 
 
 ' conservation of, 46; definition of, 
 46, 52; dissipation of, 51, 86, 231; 
 electric, 38; equations, 45; factors 
 of, 50; of food, 69; forms of, 38; 
 gravitational, 38; history of, 157; 
 kinetic, 39; leakage of, 45; loss by 
 friction, 41; muscular, 38; radiant, 
 40; of space, 209; of state, 48; of 
 steam, 39; unavailable, 42 
 
 Energy transformations, 39, 41, 42; 
 alternative, 108; and becoming, 49; 
 compensatory, 83; conditions of, 
 215; coupled, 221; examples of, 49; 
 irreversible, 42; quantitative, 44; 
 release of, 30; reversible, 42; sign of, 
 51 ; and work, 51 
 
 Engine, animate, 56; inanimate, 55 
 
 Entelechy, 193 
 
 Enterokinase, 58 
 
 Entropy, 52, 196; and becoming, 216, 
 220; changes, 200, 202; increase of, 
 221 ; retarded by life, 220 
 
 P^nvironment, action on, 55; of life, 
 55: stimuli of, 13 
 
 Enzymes, action of, 62; nature of, 58 
 
 Epiphenomena, 184 
 
 Equations, differential, 168, 171, 
 229 
 
 Equilibrium and auditory sensation, 
 114; mechanical, 40 
 
 Erepsin, 61 
 
 Ether of space, 162; seat of potential 
 energy, 48 
 
 Evolution and adaptations, 186; cos- 
 mic, 181 ; creative process, 181 ; 
 historic proofs, 212; and structure, 
 2; unpredictable, 211 
 
 Excretion, 70; vicarious, 6 
 
 Exercise, physiology of, 87 
 
 Existence, real and unreal, 52 
 
 Exoskeletons, 15 
 
 Experience and acting, 182; and 
 adaptations, 151 ; in reflexes, 118 
 
 Explanations, scientific, 210 
 
 Extension, 232 
 
 Extensor muscles, 21 
 
 .Eye a camera, 108; cyclopean, 99; 
 of invertebrates, 2 ; of vertebrates, 2 
 Eyelids, movements of, 23 
 
 Fabricius, 66 
 Fats, 58 
 
 Fats, decomposition of, 79 
 Fatty acids, 60 
 Fear, analysis of, 226 
 Ferments, digestive, 58 
 Fibres, muscular, 23 
 Fischer, 59, 159 
 
 ! Fish, bloodvessels of, 64; brain, 
 diagram, 97; cerebellum of, 95; 
 locomotion of, 11 
 i Flagellates, 11 
 ! Flexor muscles, 21 
 i Food, composition of, 58; distribution 
 of, 61; and energy, 158; inedible 
 parts of, 57; of plants, 81 ; prepara- 
 tion of, 57 ; substances, 57 
 Foodstuffs, categories of, 58 
 i Food substances, ultimate, 218 
 | Foot, movements of, 20 
 i Forearm, diagram, 21 ; movements of, 
 
 18,21; muscles of, 21 
 l Forebrain, 94 
 1 Forelimb, diagram, 19; of mammals, 
 
 18 
 
 i Foster and vitalism, 160 
 j Freedom, degrees of, 18, 19 
 Free-will, 179 
 Friction and heat, 43 
 Frog, decerebrate, 139 
 Functioning, organic, 213 
 
 ; Galen, physiology of, 154 
 ! Ganglia, *in brain, diagram, 101; 
 medullary, 100; nervous, 91; seg- 
 mental, 91, 92; structure of cells of, 
 26 
 
 i Gases, heat transference in , 200 ; kinetic 
 theory of, 198; in lungs, 67 
 
 Gastric glands, 58 
 
 Genetics, 8 
 
 : Geometry, 235; Euclidean, 90; four- 
 dimensional, 190; non-Euclidean, 
 215,235 
 
 Gibbs, 194 
 
 Gills of fish, 64 
 
 Gland action, 70, 74; control, 239; 
 ductless, 74; nature of, 58; pineal, 
 74; pituitary, 74; structure of, 238; 
 thyroid, 74 
 
 Goitre, exophthalmic, 7 
 
 Golz, 141 
 
 Graham and dialysis, 158 
 
 Gravitation energy, 35; an explana- 
 tion, 211 
 
 Green plants, 87 
 
 Grey matter of brain, 91, 128 ; structure 
 of, 126 
 
244 
 
 THE MECHANISM OF LIFE 
 
 Growth, directive. 135; movements, 9; 
 
 organic, 171; orientation of, 9 
 Guano, origin of, 80 
 
 Habits, 179; variable, 173 
 
 Haemoglobin of blood, 69 
 
 Hallucinations, 179; and energy, 52 
 
 Hand, articulation of, 18 
 
 Harmony of activities, 6 
 
 Harvey and circulation of blood, 66, 154 
 
 Head, structure of, 94 
 
 Heat of body, 70; economy of, 150; 
 engines, 69; and entropy,' 201; flow 
 of , 43 ; a form of energy, 3 1 ; measure- 
 ment of, 37 ; mechanical equivalent 
 of, 43 
 
 Hearing analysed, 174; organ of, 114 
 
 Heart-beat, regulation of, 88 
 
 Heart of vertebrates, 2; action of, 65; 
 inhibition of, 142; innate heat of, 
 154; muscles of, 22 
 
 Heliotropism, 135, 183 
 
 Hemiplegia in man, 137 
 
 Hemispheres, cerebral, 98; connections 
 of, 102; functions of, 125; removal 
 of, effects, 139 
 
 Herbivorous animals, 84 
 
 Herd instinct, 142 
 
 Heredity, not explained, 193; what is 
 transmitted, 225 
 
 Hibernation, 150 
 
 Horse-power, 37, 44 
 
 Hunger, 6 
 
 Hydraulics of body, 156 
 
 Idealism, 176 
 
 Immortality, physical, 208 
 
 Inanimate engine, 55 
 
 Inco-ordination, 6 
 
 Indeterminism, 222 
 
 Indeterminism of response, 138; 
 
 evolution of, 139 
 Individualism, 142 
 Inertia and mass, 35 
 Inevitability of response, 137 
 Infusoria, 9, 11 
 Inhibition, 119, 141; of conduct, 143; 
 
 of heart, 142 ; and shock, 142 
 Insanity and herd instinct, 142 
 Insects, locomotion of, 11 
 Instinct, herd, 142 
 Integration of activities, 4, 88; of 
 
 functioning, 74 
 Intelligence and adaptations, 151; 
 
 and spontaneity, 144 
 Intensity factors of energy, 50 
 Intestine, muscles of, 22 
 Intuition not communicated, 225; 
 
 not conserved, 224 
 Irrevereibility, physical, 195 
 Irritability, 69; the basis of sensation, 
 
 106 
 Isolated systems, 45 
 
 Joints, ball-and-socket, 21 ; and de- 
 grees of freedom, 19; elbow and 
 knee, 20; Joule and theory of heat, 
 42, 158, 194; and ligaments, 20 
 
 Kant and a priori ideas, 54, 186; and 
 space and time, 188 
 
 Kelvin and mechanism, 4, 194 
 
 Kidney, diagram of, 72, 73, 76; func- 
 tioning of, 6 
 
 Kinetic energy, 39, 47; and vis viva, 
 154 
 
 Knowledge, elements of, 230; theory 
 of, 186 
 
 Labyrinth, auditory, 114 
 
 Lavoisier, 157 
 
 Law of conservation and intuition, 
 224; qualifications of, 52 
 
 Laws, scientific, 170; of thermody- 
 namics, 54 
 
 Legal inhibitions, 143 
 
 Lemniscus tracts, 100, 111 
 
 Levers in the skeleton, 20 
 
 Life, balance of, 84; concept of, 221 ; 
 continuity of, 233; degradation to 
 materiality, 226; duration of, 207; 
 energy, cycle of, 57; and energy, 2 14; 
 improbability of, 209; and indeter- 
 minism, 213; intuition, 223; mani- 
 festations of, 225; passage into 
 material, 233; physical nature of, 
 210; and senile decay, 225; sub- 
 stance, 233; tendencies of, 216 
 
 Light and plant growth, 82, 135 
 
 Limb appendages, 18 
 
 Limb girdles. 17 
 
 Limbs, modifications of, 17; skeleton 
 of, 17 
 
 Liver of vertebrates, 03, 04 
 
 Locomotion in animals, 11; and cere- 
 bellum, 125; ciliary, 11 ; in Crustacea, 
 2; in earth worm, *1 5; in jelly-fishes, 
 15; in molluscs, 15; in starfishes, 2; 
 in vertebrates, 2, 17; in water, 11 
 
 Lodge and matter, 162; and waves, 
 168 
 
 Loeb, 159; and consciousness, 184 
 
 Lungs in man, 65; structure, 67; 
 in vertebrates, 2 
 
 Lymphatic vessels, 61 
 
 Magnitude, orders of, 205 
 
 Malpighi, 66 
 
 Mass, nature of, 34 
 
 Materialism, 226 
 
 Materiality, 31; analysis of, 227; and 
 
 energy, 37 
 Matter, conservation of , .")_' : Descartes' 
 
 theory of, 161 ; and extension, 161; 
 
 theory of, 36; universal, 36 
 Maxwell, J. C., and Descartes, 101; 
 
 and gas theory, 199 
 
INDEX 
 
 245 
 
 Measurements, microscopic, 206 
 Meat, tainting of, 78 
 Mechanical energy, unit of, 43 
 Mechanical models or theories, 4 
 Mechanics, the Cartesian, 160; classi- 
 cal, 193 
 
 Mechanism, the animal, 4, 152 
 Mechanism, physico-chemical, 158 
 Mechanisms, automatic, 24 
 Mechanisms, bodily, integrating, 148; 
 
 of joints, 20; sensori-motor, 116; 
 
 non-skeletal, 22 
 Mechanistic conception of life, 152, 
 
 160; insufficient, 162 
 Medical research, 160 
 Membranes, animal, 61 ; synaptic, 
 
 108 
 Memory not conserved, 225; a factor 
 
 in action, 178; and habit, 182; 
 
 and morality, 224; a multiplicity 
 
 in unity, 183 ; pure, 182 ; how stored, 
 
 183 
 
 Mental satisfaction, analysis of, 226 
 Mentality, subconscious, 223 
 Metabolism, 55, 56, 70; animal, 76; 
 
 plant, 81, 219 
 Micro-organisms, activity of, 79; 
 
 occurrence of, 77 
 Mid-brain, 104; and cord, 122, 146; 
 
 and sensation, 122; and sensory 
 
 tracts, 111, 115 
 Minckowski, 235 
 Mind, continuity of, 192 
 Mobility in animals, 9; and bodily 
 
 environment, 55; freedom of, 190; 
 
 inorganic, 10; organic, 10; patterns 
 
 of, 11; in plants, 9 
 Molecules, 31, 33; collisions among, 
 
 199; dimensions of, 199; energy of, 
 
 48; velocities of, 199; weight of, 35 
 Molluscs, locomotion in, 11 
 Motion, cosmic, 10; Newton's laws of, 
 
 47; relative to the body, 189 
 Motor actions, complexity of, 118; 
 
 organs, 20 
 Motor habits, 182; inherited, 183; 
 
 mechanism of, 182; nerves of, 117 
 Movements, adaptive, 10; apparatus 
 
 of, 13: control of, 121 
 Moulting in Crustacea, 15 
 Muscle-nerve preparation, 135, 172 
 Muscle tissues, 23 
 Muscles, antagonistic, 21, 22, 25, 29, 
 
 117, 119; contraction of, 21 ; control 
 
 of, 121; disintegration of, 70; 
 
 extensor, 21 ; fibres of, 23; flexor,21 ; 
 
 of intestine, 22; involuntary, 24; 
 
 receptors of, 121, 165; and skeleton, 
 
 14; striped, 23; trunk, 123; un- 
 
 striped, 23; voluntary, 23 
 Muscular power, 38, 70 
 Muscular sensation, nerves of, 119 
 Mutations, 187 
 
 Nature, its form and content, L'^O; 
 laws of, 229; materialistic descrip- 
 tion of, 227; passage of, 230; as 
 space-time data, 227; stuff of, 233 
 
 Neanderthal man, 99 
 
 Nebular hypothesis, 162 
 
 Neovitalism, 193 
 
 Nerast, 194 
 
 Nerve fibres, 26, 89; secretory, 239 
 
 Nerve-muscle preparations, 117 
 
 Nerves, 89; afferent, 25; Cartesian 
 cells, 26; centres, 87; conception of, 
 155; cranial, 89: dendrites of, 
 27; efferent, 25; and energy, 133; 
 respiratory, 88; sciatic, 27; spinal, 
 90; sympathetic, 88; terminations, 
 106, 107 
 
 Nerve tracts, 100; ascending, 100; 
 association, 103; commissnral, 103; 
 projection, 103; pyramidal, 104; 
 sensory, 100 
 
 Nervous arborisations, 27 
 
 Nervous co-ordination, 88 
 
 Nervous impulse, 166; Cartesian con- 
 ception of, 164; and energy, 131; 
 model of, 131; paths of, 112 
 
 Nervous releases, 131 
 
 Nervous system, 90, 130, 145; central, 
 91; development of, 94; divisions 
 of, 89; of earthworm, 91; and hi- 
 de terminism, 174; mechanism of, 
 25; peripheral, 91; primitive, 147; 
 scheme of, 26;structure of, 26, 91 
 
 Neurones, 27 
 
 Newton, 188 
 
 Nitrates in nature, 79, 84 
 
 Nitrification, 79 
 
 Nitrogen, fixation of, 85 
 
 Nitrogenous minerals, 80; residues, 80 
 
 Noumena, 176 
 
 Nutrition, organs of, 2 
 
 Nutritive materials, 69 
 
 Objectivity, 176 
 
 Oceanic bacteria, 80 
 
 Oil and animal life, 86 
 
 Olfactory paths, 113 
 
 Optic radiations, 116: thalami, 95^ 
 
 tracts, 115 
 Organic and inorganic concepts, 215; 
 
 movements, 10; reactions, 170 
 Organisms, remains of, 219; and the 
 
 State, 7 
 
 Ossicles, auditory, 108 
 Oxygen in the lungs, 67 
 Oyster, shell of, 14 
 
 Pain, mental, 223; physical, 223 
 Pallium, cerebral, 96 
 Pancreatic gland, 58 
 Paralysis, motor, 93; sensory, 93 
 Parasites, animal, 9 
 
THE MECHANISM OF LIFE 
 
 Parthenogenesis, artificial, 159 
 
 Pascal's reed, 210 
 
 Passage of nature, 230; its relativity 
 
 to life, 232 
 Pasteurisation, 78 
 Paths in nervous system, 101, 147 
 Patterns of locomotion. 11 
 Pelvis, 17, 19 
 Pepsin, 58 
 Perception, 24, 222; and dissipation, 
 
 of energy, 184; and indeterminism, 
 
 181; kinds of, 178; nature of, 176; 
 
 persistence of, 185; pure, 178; a 
 
 sensori-motor process, 178 
 Personality, 234 
 Phagocytes, 69, 77 
 Photosynthesis, 82 
 Phototaxis, 136 
 Physico-chemistry of life, 214 
 Physiology an analysis, 164, 166; 
 
 Cartesian, 192; and medicine, 166; 
 
 the modern impasse, 193; and 
 
 mind, 185 
 Pineal gland, 99 
 
 Pituitary gland, 99; and goitre, 7 
 Planck, *162, 194 
 Plants, characters of, 8; and entropy, 
 
 217; motions of, 9 
 Pleasure, analysis of, 226 
 Plexus, limb, 93; solar, 90 
 Poison glands, 12 
 Postures, animal, 11 
 Potential energy, 47 
 Prediction in science, 212; the test of 
 
 theory, 173 
 
 Preservation of meat, 78 
 Priestley, 157 
 
 Probability and entropy, 203 
 Production by plants, 85 
 Proprioceptors, 121 
 Proteids, chemistry of, 59; foodstuffs, 
 
 58; putrefaction of, 79; residues, 77 
 Psychoids, 215 
 Psycho-physics, 175 
 Ptyalin, 58 
 
 Pulmonary circulation, 65 
 Putrefaction, 78 
 Pyramidal cells, 128 
 Pyramidal tracts, 104, 129; evolution 
 
 of, 129 
 Pyramids, decussation of, 104 
 
 Radicles, chemical, 59 
 
 Radioactivity, 53, 86, 230 
 
 Radium, 53 
 
 Rankine, 194 
 
 Rational dynamics, 46 
 
 Reactions, coupled, 83 
 
 Real and unreal existence, 179 
 
 Reality and law of conservation, 53, 
 
 215 
 
 Reason, evolution of, 186 
 Receptors, 24; cold, 109; connections 
 
 of, 102; distance, 24; equilibrium, 
 25, 109; gustatory, 25; heat, 25; 
 joints, 110; muscle, 110, 112, 119, 
 124; olfactory, 109; touch, 25; 
 vision, 107 
 
 Reference, frames of, 189 
 
 Reflex action and cerebellum, 120; 
 control of, 120; and cortex, 120; 
 craniospinal, 121; diagram, 117; 
 gustatory, 238; inhibition of, 138; 
 in normal animal, 138; olfactory, 
 238; predictable, 172; schematic, 
 120; simple, 120; scratch, 119 
 
 Reflex arcs, diagram, 29, 120; units 
 of nervous system, 130 
 
 Regulations of activities. 136 
 
 Relations, 228 
 
 Relativity, 235; and Descartes, 161; 
 and a finite universe, 197; and re- 
 versibility, 196 
 
 Relays in nervous system, 100 
 
 Representations, mental, 176 
 
 Reproduction, 3 
 
 Respiration, automatic, 87; and 
 carbonic acid, 87; gaseous inter- 
 change, 67; mechanism, 17, 65; 
 movements, 68; regulation, 87; 
 theory of, 157 
 
 Responses, adaptive, 110; animal, 10, 
 13; and heredity, 150; indeter- 
 mined, 138, 175; and stimuli, 139; 
 unpredictable, 173 
 
 Restlessness and decerebration, 141 
 
 Retina, 107, 115 
 
 Reversibility, physical, 43, 195 
 
 Ribs, skeleton of, 16 
 
 Rolandic fissure, 104 
 
 Roots of plants, 9 
 
 Rotifers, locomotion in, 11 
 
 Rumford and heat, 157 
 
 Russell, 185 
 
 Rutherford, 53 
 
 Salivary gland, 58 
 
 Saltpetre, 79 
 
 Saprozoic nutrition, 84 
 
 Science, method of, 166, 170; and 
 
 utility, 214 
 
 Scratch reflex, 137, 138 
 Second law of thermodynamics, 194; 
 
 reversal of, 198, 216 
 Secretion, Cartesian theory of, 155; 
 
 chemistry of, 239; psychical, 239 
 Segments in nervous system, 92 
 Sensation, 24, 106, 222; and action, 
 
 133; chemical, 108; complex, 116; 
 
 disordered, 52; exteroceptive, 
 
 general, 110; gustatory, 25; and 
 
 knowledge, 133; muscular, 109; 
 
 nature of, 174; olfactory, 108; 
 
 paths of, 111; and perception, 175; 
 
 phvsical, 176; proprioceptive, 24; 
 
 tactile, 25; visual, 107; 
 
INDEX 
 
 247 
 
 Sense organs, stimulation of, 24, 107; 
 accessory, 108 
 
 Sensibility, general, 1 1 1 
 
 Sensori- motor activity, 28 ; mechanism, 
 117; system, 13, 145; 
 
 Sensory mechanisms, 24 
 
 Servetus, 66 
 
 Sherrington, 165 
 
 Shock, nervous, 142 
 
 Skeleton, 152; absence of, 15; analysis 
 of, 19; axial, 16; of birds, 16; con- 
 nections of, 14; in Crustacea, 15; 
 internal, 15; joints in, 16; and 
 mechanics, 14; of mollusca, 14; 
 muscles and, 20; plan of, 13; size 
 of, 16; of snakes, 16; of sponges, 14 
 
 Skull, structure of, 16 
 
 Smell, 33; associations of, 114; deli- 
 cacy of, 109; sensation of, 113 
 
 Snaring devices, 12; 
 
 Social complexes, 7 
 
 Socialisation of activities, 7 
 
 Solar radiation, 69, 86 
 
 Soul, rational, 154; sensitive, 192 
 
 Sound, analysis of, 116; waves, 168 
 
 Space, abstract, 191; Euclidean, 189; 
 a form of sensibility, 191; measure- 
 ment of, 169; non-Euclidean, 235 
 
 Space time, 228, 235 
 
 Spermatozoa, 11 
 
 Spinal animal, 118 
 
 Spinal cord, 92, 93; autonomy of, 94; 
 connections of, 110 
 
 Spinal frog, 136, 172 
 
 Spinal nerves, 93; roots of, 93, 165; 
 sensory, 111 
 
 Spinal reflex, 136, 137 
 
 Spontaneous actions, 105 
 
 Spooks and energy, 52 
 
 Spirits, animal, 155; natural, 154 
 
 Spiritualism, 193 
 
 Starch in foods, 60; in plants, 83 
 
 State and organism, 75 
 
 Statistical and individual concepts, 
 212 
 
 Steam engine, 42, 55 
 
 Stearic acid, 60 
 
 Sterilisation, 78, 79 
 
 Stimuli of environment, 13; intrinsic, 
 140; physical, 13; and sensation, 106 
 
 Stimulus and response, model of, 132, 
 134 
 
 Structure of animals, 2; Descartes' 
 analysis of, 156; and function, 3; 
 modifications of, 12 
 
 Sweating, regulation of, 88 
 
 Swimmerets in Crustacea, 18 
 
 Swimming, 11 
 
 Subjectivity, 53, 176 
 
 Submaxillary gland, 237 
 
 Sugars, chemistry of, 60 
 
 Suggestion, mass, 180 
 
 Sunlight, energy of, 218 
 
 Sympathetic nervous system, 88, 90, 
 
 165 
 
 Synapses, 27 
 Syntheses, organic, 159 
 Systems, physical, 45 
 
 Taste, 33, 108 
 
 Taxis in animals, 135 
 
 Techniques, origin of, 182 
 
 Telepathy, 225 
 
 Temperature, 31 
 
 Tendencies, logical, 216 
 
 Tendons, 21 
 
 Texture, 32 
 
 Theory, astronomical, 210 
 
 Thermodynamics, 194; history of, 157; 
 
 laws of, 52; and life, 69; and 
 
 mechanism, 69 
 Thinking and action, 188 
 Thomson, Sir J. J., 53, 194 
 Thomson, Sir W., 203 
 Thorax, movements of, 17 
 Threshold of sensation, 106 
 Thumb, mobility of, 19 
 Time, astronomical, 191; dimension, 
 
 236; measurement of, 191, 206; 
 
 a reversible series, 196 
 Touch organs, Il8 
 Toxins of bacteria, 77 
 Tracts, nervous, 99 
 Transformations, releasing, 48 
 Transformism, 234 
 Trial and error, 188 
 Tropisms, 9, 13, 135, 180 
 Trypsin, 58 
 
 Unavailable energy, 42 
 
 Universe, condition of, 97; cycles in, 
 205; dimensions of, 204; and dissi- 
 pation, 204; finite, 197, 236; in 
 passage, 231; running down, 203; 
 structure of, 162 
 
 Urea, chemistry of, 59; excretion of, 
 70, 76; nitrification of, 84 
 
 Urine, 73 
 
 Variability and determinism, 181 
 Variations, organic, 181 
 Vascular system, 152 
 Vegetative processes, 217 
 Veins, 63 
 
 Ventricles of brain, 97 
 Vertebrae, articulations of, 16 
 Vertebrates, locomotion of, 2; skele- 
 ton of , 17 
 Vestal virgins, 215 
 Vestibular sensation, 1 14, 124 
 Vicarious functioning, 144 
 Vision, organs of, 108 
 Vis viva, 55 
 
 Vital impetus, 223, 225 
 Vivisection, 160 
 
248 
 
 THE MECHANISM OF LIFE 
 
 Volition and cortex cerebri, 140; and 
 
 motion, 118 
 Voluntary muscles, 23 
 Vortices, Cartesian, 162 
 Vries, de, 150 
 
 Walking, 11; nervous mechanism, 123 
 
 Waller and nerves, 165 
 
 War psychoses, 143 
 
 Waste matter of body, 70 
 
 Watt, 157 
 
 Waves, 168, 169 
 
 Weather, analysis of, -'21 
 
 Weber, 224 
 
 Weight, 34; and mass, 36 
 
 Weismann and germ plasm, 193 
 
 White matter, 91 
 
 Winking, 2,8, 29 
 
 Wohler, 159 
 
 Work, 37; capacity for doing, 37 
 
 Working substance, 42; in life, 56, 77 
 
 Wrist, 18 
 
 Yeasts, 78 
 
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