ilwiwr? m?m THE PRINCIPLES PSYCHOLOGY, HERBERT SPENCER. o YOL. I. THIRD EDITION. (FOURTH THOUSAND.) WILLIAMS AND NOBGrATE, 14, HENRIETTA STREET, COVENT GARDEN, LONDON | and 20, SOUTH FREDERICK STREET, EDINBURGH. 1881 . - The Right of Translation is Reserved* Vv'. J. JOHNSON, PRINTER, 121, NLEKT STREET, LONDON, H.C. * I So Sp 3 ml V • I PREFACE TO THE THIRD EDITION. to <> ■> * A This third edition of the Principles of Psychology differs from preceding editions, mainly by containing anew division; which occurs in the second volume, under the title —“ Part VIII., Congruities.” By the preface to the first edition, it will be seen that I had originally intended to write, under the title of “ Summary and Conclusion/'’ a division having for its purpose “ to bring the several lines of argument to a focus : ” believing that “ the harmony that may be shown to subsist between the doctrines elaborated in the respective divisions, is a strong confirmation of their truth." When I began to prepare the second and greatly enlarged edition, I looked forward to fulfilling this intention, which disturbed health had before obliged me to abandon. Eventually, however, I left the additional part unwritten—partly because the work had already become too bulky, and partly because I thought that the harmonies I proposed to point out were so conspicuous that all readers would perceive them. This last reason proved to be ill-grounded. Far from re¬ cognizing the harmonies which I thought conspicuous, sundry critics have enlarged on the incongruities. In a review pub¬ lished in the Academy for April 1, 1873, Mr. Henry Sidgwick speaks of “ the mazy inconsistency of his [my] metaphysical results.” Similarly, a writer in the Spectator for the 21st of June, 1873, asserts that “Mr. Spencer's system has the incurable defect of fundamental incoherence.''’ Prof. Green, also, in two articles which appeared in the Contemporary Review for December, 1877, and March, 1878, devotes much space to showing, as he thinks, that my views are not coherent. Thus I find it, if not necessary, at any rate desirable, to fulfil my first intention. Vi PREFACE TO THE THIRD EDITION. It is a common remark that where party feeling, political or theological, runs high, one who, believing that neither side is wholly right or wholly wrong, declines to commit himself entirely to either, is usually looked upon by both with suspicion, if not with aversion. And it is curious to see how, analogously, in a controversy so remote in its issues from human interests as that between Realism and Idealism, the enunciation of a view which recognizes an element of truth in each, seems to beget antipathy rather than sympathy. The adherents of either doctrine, believing that it is entirely true or entirely false, are averse to a conciliation which requires any sacrifice. Surrender of a part of their doctrine is almost as offensive to their amour propre as surrender of the whole; and the proposer of it i3 censured all round. Recognizing, thus, the disfavour with which both Realists and Idealists naturally regard that Transfigured Realism which accepts from each a moiety of his doctrine but rejects the rest, I scarcely expect that where they before discovered only incongruities, this new division will show them that there are congruities. I can do no more than exhibit these, in the belief that they will be apparent to all who have not yet committed themselves to one side or the other. I may add that inability to recognize these congruities admits of two interpretations, conveniently sug¬ gested by a simile which I have before employed in another relation. Taken at different angles from the same object, the two photographs placed in a stereoscope, when first viewed, not unfrequently form a confused double image ; but after persevering contemplation, most observers find them suddenly unite into a single clear representation of the object. Meanwhile, there are some eyes which to the last fail in combining them; and to which they continuously appear conflicting and confused. London, October , 1880. PREFACE TO THE SECOND EDITION. Words are somewhat strained in their meanings by calling that a Second Edition, of which the new portion greatly ex¬ ceeds the old portion in amount; as happens with this volume, and as will happen with its successor. Of the five Parts here bound together, the two that have previously ap¬ peared cover 217 pages; while the three that now appear for the first time cover 425 pages. Nevertheless, the fact that sundry of the cardinal ideas contained in this work were enunciated many years ago, must not be lost sight of. When, in 1855, the First Edition of The Principles of Psychology was issued, it had to en¬ counter a public opinion almost universally adverse. The Doctrine of Evolution everywhere implied in it, was at that time ridiculed in the world at large, and frowned upon even in the scientific world. Naturally, therefore, the work, passed over, or treated with but small respect, by re¬ viewers, received scarcely any attention; and its contents remained unknown save to the select few. The great change of attitude towards the Doctrine of Evolution in general, which has taken place during the last ten years, has made the Doctrine of Mental Evolution seem less unacceptable ; and one result has been that the leading conceptions set forth in the First Edition of this work, have of late obtained, considerable currency. In France, some of them have been made known incidentally by the treatise of M. Taine, Be l’Intelligence; and the lucid exposition of Prof. Ribot in vi PREFACE TO THE SECOND EDITION. his Psychologie Anglaise Gontemporaine, has presented them all in a systematic form. In England, they have spread through various channels. Among these I may more espe¬ cially name The Physiology and Pathology of the Mind, by Dr. Maudsley, the first division of which work is pervaded by them. As most of those who will read this Second Edition of the Principles of Psychology, never saw the First Edition, and cannot now get access to it; and as, in Parts III. and IV., they will meet with ideas that have been already made, in the ways indicated, more or less familiar to them; it is needful that I should state these facts to prevent misapprehensions. Part V., which closes this volume, is the Part referred to in the final paragraph of the Preface to the First Edition, as omitted for the reasons given. In now fulfilling the half¬ promise there made, eventually to add it to the rest, I have the satisfaction of feeling that during the fifteen years that have elapsed, the hypothesis set forth in it has assumed a much higher development. The successive instalments of the System of Philosophy which compose this volume, were issued to the subscribers at the following dates :—No. 20 (pp. 1—80) in June 1868; No. 21 (pp. 81—160) in October 1868 ; No. 22 (pp. 161— 240) in July 1869; No. 23 (241—336) in December 1869 ; No. 24 (pp. 337—416) in January 1870 ; No. 25 (pp. 417— 496) in March 1870 ; and No. 26 (pp. 497—635) in December 1870. The long delays in the issues of these instal¬ ments, have arisen in part from disturbances of health that have from time to time compelled me to desist from work, and in part from the continuous attention taken in arranging and superintending a systematic collection of materials for the Principles of Sociology, presently to be commenced. I have reason to hope that neither of these causes will operate so seriously in delaying the issue of the numbers which are to compose the second volume. London, December 1870. PREFACE. The four parts of which this work consists,* though intimately related to each other as different views of the same great aggregate of phenomena, are yet, in the main, severally in¬ dependent and complete in themselves. The particular serial arrangement in which they should be presented, has consequently been in great measure a question of general expediency; and while the order I have chosen is one which seems, on the whole, the most advantageous, it is not one which all readers are bound to follow. A brief characterization of each part, will enable every one to de¬ cide for himself which he may best commence with.-f- The General Analysis (of which the essential portion was originally published in the Westminster Review for October, 1853, under the title of “The Universal Postulate,” and re¬ appears here with additional arguments and explanations) is an inquiry concerning the basis of our intelligence. Its object is to ascertain the fundamental peculiarity of all modes of consciousness constituting knowledge proper— knowledge of the highest validity. The Special Analysis has for its aim, to resolve each species of cognition into its components. Commencing with the most involved ones, it seeks by successive decomposi- * The number of parts is now greatly increased : this volume contains five, and the second volume will contain four. t The order has now been wholly changed : the two parts immediately named as coming first, being relegated to the second volume. Vlll PREFACE. tions to reduce cognitions of every order to those of the simplest kind; and so, finally to make apparent the com¬ mon nature of all thought, and disclose its ultimate con¬ stituents. While these analytical parts deal with the phenomena of intelligence subjectively, and, as a necessary consequence, are confined to human intelligence; the synthetical parts deal with the phenomena of intelligence objectively, and so include not human intelligence only, but intelligence under every form. The General Synthesis, setting out with an abstract state¬ ment of the relation subsisting between every living organ¬ ism and the external world, and arguing that all vital actions whatever, mental and bodily, must be expressible in terms of this relation; proceeds to formulate, in such terms, the successive phases of progressing Life, considered apart from our conventional classifications of them.* And the Special Synthesis, after exhibiting that gradual differentiation of the psychical from the physical life which accompanies the evolution of Life in general, goes on to de¬ velop, in its npplication to psychical life in particular, the doctrine which the previous part sets forth : describing the nature and genesis of the different modes of Intelligence, in terms of the relation which obtains between inner and outer phenomena. As may be supposed, the analytical divisions are much less readable than the synthetical ones. Hence, while all who are accustomed to studies of an abstract character are recommended to follow the order in which the parts stand, as being that most conducive to a clear understanding of the system in its ensemble ; those who are unfamiliar with mental philosophy may, perhaps, more advantageously begin with Parts III. and IV. : returning to Parts I. and II. should they feel sufficiently interested to do so. A portion of the Part thus described, is now embodied in The Principle* of Biolor/y. PREFACE. IX Respecting the execution of the work, I may say that in sundry ways it falls much short of my wishes. There are places in which the argument is incompletely carried out; places in which, from inadequate explanation, there is an appa¬ rent incongruity between the statements there made and those made elsewhere; snd there are, I fear, places where the form of expression is not so precise as it should be. Add to which, that in treating under several separate aspects a subject so extensive, I have perhaps erred in attempting too much; and have so devoted neither thought enough nor space enough to any one of the several aspects under which the subject is presented. While, however, I am conscious that the work contains many more imperfections than it would have done had its scope been more limited and its elaboration longer, I would excuse the issue of it in its present form on several grounds : partly on the ground that it is almost useless to wait until any organized body of thought has reached its full develop¬ ment, which it never does in the course of a single life ; partly on the ground that it is next to impossible for the writer of a work like this, to dispense with the aid of candid criticism ; but chiefly on the ground that the general truths enunciated, being, as I believe, both new and im¬ portant, it seemed to me undesirable to delay their publica¬ tion with the view of by and by presenting them in a more finished guise. For the somewhat abrupt termination of the work, my apology must be, that disturbed health has obliged me to desist from writing a “ Summary and Conclusion/' in which I purposed to bring the several lines of argument to a focus. I greatly regret this; not only because the harmony that may be shown to subsist between the doctrines elabor¬ ated in the respective divisions, is a strong confirmation of their truth; but because, in the absence of explanation, some misunderstanding may arise concerning the implica* X PREFACE. tions—ontological and other—which many will think manifest. It may be well further to say that, originally, I had in¬ tended to add a fifth division, which should include sundry deductions and speculations that could not properly be em¬ bodied in the other divisions. But before being compelled to do so, I had decided, that as this fifth division was not strictly necessary ; and as certain of the suggestions contained in it might prejudice some against the doctrines developed in the others; it would be better to withhold it— at any rate for the present. July , 1855. CONTENTS OF VOL I PART I.—THE DATA OF PSYCHOLOGY. CHAP. page. I.-THE NERVOUS SYSTEM ... ... 3 II. —THE STRUCTURE OP THE NERVOUS SYSTEM ... 15 III. -THE FUNCTIONS OP THE NERVOUS SYSTEM ... 46 IV. THE CONDITIONS ESSENTIAL TO NERVOUS ACTION... 68 V. NERVOUS STIMULATION AND NERVOUS DISCHARGE... 79 VI.-2ESTHO-PHYSIOLOGY ... ... ... 97 VII.—-THE SCOPE OP PSYCHOLOGY... ... ... 129 PART II.—THE INDUCTIONS OF PSYCHOLOGY. I. -THE SUBSTANCE OP MIND ... • • • • • • 145 II.- —THE COMPOSITION OF MIND ... • • • • * • 163 III. -THE RELATIVITY OF FEELINGS • • • • • • 193 IV. -THE RELATIVITY OF RELATIONS BETWEEN FEELINGS 210 V. -THE REVIVABILITY OF FEELINGS • • • . . . 228 VI. -THE REVIVABIT.ITY OF RELATIONS BETWEEN FEEL- INGS ... . . • ... 240 VII.- —THE ASSOCIABILITY OF FEELINGS • • • ... 250 VIII. —THE ASSOCIABILITY OF RELATIONS BETWEEN FEEL- INGS ... • • • • • • 259 IX.- —PLEASURES AND PAINS • • • • • • 272 PART III.—GENERAL SYNTHESIS. I.-LIFE AND MIND AS CORRESPONDENCE ... ... 291 II.-THE CORRESPONDENCE AS DIRECT AND HOMOGENEOUS 295 III.-THE CORRESPONDENCE AS DIRECT BUT HETERO¬ GENEOUS ... ... ... ... 300 304 IV.-THE CORRESPONDENCE AS EXTENDING IN SPACE CONTENTS. CHAP. PAGE V.-THE CORRESPONDENCE AS EXTENDING IN TIME ... 320 VI. -THE CORRESPONDENCE AS INCREASING IN SPECIALITY 329 VII. -THE CORRESPONDENCE AS INCREASING IN GENE¬ RALITY ... ... ... ... 342 VIII.-THE CORRESPONDENCE AS INCREASING IN COM¬ PLEXITY ... ... ... ... 350 IX. -THE CO-ORDINATION OP CORRESPONDENCES ... 370 X. -THE INTEGRATION OP CORRESPONDENCES ... 377 XI. -THE CORRESPONDENCES IN THEIR TOTALITY ... 385 PART IY.—SPECIAL SYNTHESIS. 1.-THE NATURE OF INTELLIGENCE ... ... 395 II.-THE LAW OP INTELLIGENCE... ... ... 407 III. -THE GROWTH OP INTELLIGENCE ... ... 418 IV. -REFLEX ACTION ... ... ... ... 427 V. -INSTINCT ... ... ... ... 432 VI. -MEMORY ... ... ... ... 444 VII. -REASON... ... ... ... ... 453 VIII. -THE FEELINGS ... ... ... ... 472 IX.-THE WILL ... ... ... ... 495 PART V.—PHYSICAL SYNTHESIS. I.-A FURTHER INTERPRETATION NEEDED ... ... 507 II.-THE GENESIS OF NERVES ... ... ... 511 III. —THE GENESIS OF SIMPLE NERVOUS SYSTEMS ... 521 IV. —THE GENESIS OF COMPOUND NERVOUS SYSTEMS ... 532 V.-THE GENESIS OF DOUBLY-COMPOUND NERVOUS SYSTEMS ... ... ... ... 542 VI. -FUNCTIONS AS RELATED TO THESE STRUCTURES ... 559 VII. -PSYCHICAL LAWS AS THUS INTERPRETED ... 577 VIII.-EVIDENCE FROM NORMAL VARIATIONS ... ... 585 IX.-EVIDENCE FROM ABNORMAL VARIATIONS ... 604 X.—RESULTS ... ... ... ... 614 APPENDIX. ON THE ACTIONS OF ANAESTHETICS AND NARCOTICS ... 629 CONSCIOUSNESS UNDER CHLOROFORM ... ... 636 PART I. THE DATA OF PSYCHOLOGY. ' CHAPTER I. THE NERVOUS SYSTEM. § 1. The lowest animal and the highest animal present nc? contrast more striking than that between the small self-mo-- bility of the one and the great self-mobility of the other. A monad passing, apparently with some rapidity, across the • field of the microscope, really advances with extreme slow¬ ness : its velocity, nnexaggerated by combined lenses, being about that of the minute-hand of a watch. The parts of a disturbed sea-anemone draw themselves together with « a speed which, though immensely greater than that of a monad through the water, is insignificant as measured by the" speed of most terrestrial and aerial creatures. Comparing:' the movements of Protozoa, or of Zoophytes, with those of Birds that keep pace with railway trains or those Mammals that gallop a mile in a minute, their locomotive powers seem scarcely appreciable. Masses being supposed equal, the quantity of motion generated in the last case approaches a million times that generated in the first. Contrasts of this kind exist within each great division of the animal kingdom, as well as in the animal kingdom taken as a whole. The sub-kingdom Annulosa shows us an immense difference between the slow crawling of worms and quick flight of insects. Among Mollusks the sluggish¬ ness of the Tunicata is no less marked than the activity of the Cephalopoda. And between the inferior or water- 13 2 4 THE DATA OF PSYCHOLOGY. breathing Vertebrata and superior or air-breatliing Yerte- bratci } there is an equally conspicuous unlikeness in energy of movement. This self-mobility which by its greater amount generally distinguishes higher animals from lower, and, indeed, enters largely into our conceptions of higher and lower, is displayed in several ways. We see it in the changes of attitude that are made without moving the body from place to place. We see it in the transference of the body as a whole through space : considering this transference apart from external resistances overcome. And we see it in the over¬ coming of resistances—both those of media and those due to gravity. All these, however, are manifestations of one ability—the ability to generate a force which either shows itself as momentum or would generate momentum but for a counterbalancing force. And it is in this general form that we are here concerned with this ability. We have to contemplate the inferior animals as being generators of very small quantities of actual or potential motion, and the higher animals as being generators of relatively-immense quantities of actual or potential motion. § 2. With what internal differences are these differences of external manifestation connected ? No doubt with several. An active organism contains various appliances no one of which can be spared without greatly diminishing, or quite destroying, its activity. If the alimentary system be incapacitated, there must pre¬ sently result a decreased power of generating motion, from lack of the materials whence motion is obtained; and hence the fact, conspicuous throughout the animal kingdom, that along with much locomotive activity there goes a developed apparatus for taking up nutriment. It is manifest, too, that there cannot be great self-mobility unless the absorbed materials are efficiently distributed to the organs which transform insensible motion into sensible motion ; and thus THE NERVOUS SYSTEM. 5 it happens that as we ascend from creatures which movo little to creatures which move muclr, we meet with a more and more evolved vascular system. Similarly with the organs for separating from the blood the substances that have yielded up their contained motion. If the blood be¬ comes choked with inert matter, there necessarily results a decreased genesis of motion; and therefore,, as we see on comparing inactive with active animals, the exaltation of activity is accompanied by the development of depurating structures. Still clearer is it that the production of much motion, and the resistance to those forces which antagonize motion, imply parts capable of bearing great strains— masses of dense tissue such as in vertebrate animals form bones, and in invertebrate animals form dermal frame¬ works ; and, accordingly, as we ascend from creatures that are inert to creatures that are vivacious, we advance from weak to strong skeletons, internal or external. Above all it is self-evident that along with locomotive activity there must exist those contractile organs which are the imme¬ diate movers of the limbs and consequently of the body; and hence the direct connection between absence of mus¬ cular fibres and extremely-small self-mobility, and the direct connection between development of the muscles and much self-mobility—connections so direct as to make it at first sight seem that the genesis of motion varies as the mus¬ cular development. Remotely dependent, however, as the genesis of motion is on digestive, vascular, respiratory, and other structures; and immediately dependent as it is on contractile struc¬ tures; its most important dependence remains to be named. For all of these appliances taken together can do nothing* of themselves. The muscles are but instruments, which remain passive until their power is evoked by the structure which uses them; and the quantity of motion they then give out varies according to the demand made by this exciting and controlling structure. In other words, th& 6 THE DATA OF PSYCHOLOGY. initiator or primary generator of motion is the Nervous System. Where there is extremely little power of generat¬ ing motion, as among the Protozoa and the inferior Coelen- terata, there is no nervous system. Where activity begins to show itself a nervous system begins to be visible. And where the power of self-movement is great, the nervous system is comparatively well developed. Though the mus¬ cular system also becomes larger and better organized; yet the quantity of motion produced is fundamentally related to the degree of nervous development. Not, indeed, that it is so related with anything like uniformity: this we shall presently see that it cannot be. But it is so related more uniformly than in any other way. A few typical contrasts will show this. § 3. The absence of measurements renders detailed com¬ parison among the various classes of Mollusca unsatisfactory. On putting side by side the extreme terms, however, we find an unquestionable difference in the proportion between the nervous system and the rest of the body. The sedentary Ascidians, which do little in the way of moving beyond occasionally contracting themselves, severally possess only a single small ganglion with its fibres; but Cephalopods of the dibranchiate order, which are active creatures that dart through the water fast enough to catch fish, contain masses of nerve-tissue that bear much larger ratios to their total masses. It is with annulose animals as with molluscous animals— we have no definite estimates of the sizes of their nervous systems; and hence can bring in evidence only the marked differences. As before, the extreme forms supply these. The sluggish annuloid types, when contrasted with the energetic kinds of Annulosa, present decided deficiencies of nerve-substance; and even between such less-remote orders as the tubicolous Annelids, leading stationary lives, and the decapodous Crustaceans, leading active lives, a kindred THE NERVOUS SYSTEM. 7 difference may be safely asserted. There are also, in some annulose types contrasts between the nervous system in the inactive and active stages of the same individual. The feebly- moving caterpillar has but a small nervous system; the butterfly, with its power of vigorous flight, has a relatively large one; and during the intermediate pupa-state, in which the organization is being adapted to this more viva¬ cious life, a rapid growth of the nervous system may be traced. It is in the Vertebrccta , however, that the most striking evidence meets us. According to Leuret, the average ratio of the brain to the body is—in fishes, 1 to 5,668 ; in rep¬ tiles, 1 to 1,321 ; in birds, 1 to 212; and in mammals, 1 to 186. Now though these can be but rude approximations, since there are great differences within each class, and since the ratio of the brain to the body is not the ratio of the whole nervous system to the body; yet the relations they indicate are substantially true. Were the weight of the spinal cord and the nerves added to that of the brain in each case, the strengths of the contrasts would be consider¬ ably diminished; but the contrasts would still be strong. And with them there go the strong contrasts between the activities in the respective classes—the Fishes that swim in a medium of their own specific gravity; the Reptiles of which the higher have to support the weights of their bodies as they move about over the land, but cannot do this for long together; the Birds and Mammals that are in constant locomotion, often at high velocities. Here, too, the alleged connection is rendered the clearer by the approximate uniformity of the relative amount of muscle. The weight of muscle in a fish forms something like as large a part of the total weight as it does in a reptile— perhaps a larger part; and a reptile is scarcely if at all inferior to a bird or a mammal in the proportion of contrac¬ tile tissue it possesses. Hence it becomes manifest that indispensable as is contractile tissue to the genesis of motion, its quantity does nob determine the quantity of 3 THE DATA OF PSYCHOLOGY. motion generated. Whereas, notwithstanding the many complicating circumstances, a general relation between quantity of nerve and quantity of motion is traceable. There are special cases which illustrate this relation. I may name one—the case of the Porpoise. A Porpoise's brain exceeds greatly in size the brains of other Mammals that have bodies commensurate with its own, except that of Man and, perhaps, that of the Gorilla. Such a structure in a creature leading so simple a life, is a serious difficulty in the way of current interpretations ; but is quite in harmony with the interpretation here given. Porpoises accompanying a steam-vessel, gambolling and making ex¬ cursions on either side without apparent effort, prove, by keeping up so high a velocity through so dense a medium, that their motor energies are enormous. § 4. A closer examination of the facts soon reveals the insufficiency of the foregoing generalization. Deep as is the connection between nervous development and locomotive activity, further comparisons show that it is complicated with some other connection scarcely less radical. If, other things being equal, the quantity of motion generated varied directly as the quantity of nerve-tissue, then, in creatures constitutionally alike or but little dissimilar, a tolerably constant ratio would exist between the mass of the nervous system and the mass of the body : supposing the body, whether large or small, to be carried from place to place with equal velocity. The ratio is far from constant however. A horse gallops much faster than a man runs; and a horse in ordinary work daily moves his body through a space greater than that through which a man moves his body, or greater than that transposition of his body which a man's daily labour is equivalent to. Hence were there a simple relation between amount of nerve-tissue and amount of mo¬ tion evolved, ahorse, which weighs some seven times as much THE NERVOUS SYSTEM. 9 as a man, should have a nervous system at least seven times as heavy. Instead of this it has a lighter nervous system. Its brain weighs but one pound seven ounces; and were its spinal cord added, the total weight would probably not exceed two pounds. But a man's brain and spinal cord weigh between three and four pounds. Thus the horse's cerebro-spinal axis is but one-tenth of what it should be, were this relation the only one. Still clearer is the proof that there is some other relation, when we avoid modifying causes, by comparing animals of the same genus, or species, but of different sizes. The varieties of dogs supply good illustra¬ tions. A newfoundland and a spaniel are alike in organiza¬ tion, food, temperature, respiration, &c.; and they are approximately alike in their powers of locomotion: the advantage being on the side of the larger of the two. Were genesis of motion measured by quantity of nerve- tissue, a newfoundland's cerebro-spinal axis should, there¬ fore, exceed in size that of a spaniel as much as a newfound¬ land's body exceeds in size that of a spaniel. But it by no means does so. While considerably larger absolutely, it is much smaller relatively. Consequently, we must say that though the nervous system is the initiator of motion, and though there is evidently some relation between degree of nervous develop¬ ment and degree of motor energy; yet this relation is involved with, and obscured by, another. Let us re-examine the facts in search of it. § 5. In what other way than in relative feebleness, do the motions of inferior creatures differ from those of superior creatures ? They differ in relative simplicity. Animals that are but little evolved perform actions which, besides being slow, are few in kind and severally uniform in com¬ position. Animals that are much evolved perform actions which, besides being rapid, are numerous in kind and seve¬ rally involved in composition. The movements in the one 10 THE DATA OE PSYCHOLOGY. case are small and homogeneous, and in the other case great and heterogeneous. Each sub-kingdom of animals exemplifies this second general relation, as much as it does the first. Humble Mollusks, like the fixed Tunicatct, display scarcely any energies beyond those required to contract their bodies when disturbed and afterwards to unfold them. But in the highly-organized Cuttle-fishes, besides the rapid, quickly- varied, and well-adjusted movements exhibited in the pur¬ suit and Capture of prey, wrn have the numerous and com¬ bined movements of the suckered arms, used not only for prehension but occasionally for travelling over solid surfaces. The Annulosa, including with them the Annuloida , supply a like general contrast. Between the uniform, little-varied motions of a Nemertine worm, and the multiform, variously- combined motions of the Crab or the Spider, the difference is paralleled by the difference in nervous evolution. And a like structural contrast accompanies the contrast between the few simple actions of the caterpillar and the numerous complex actions of the butterfly. But that heterogeneity of movement increases along with relative size of the nervous system, is best shown by com¬ parisons among vertebrate animals. Progressing by alter¬ nate contractions of its lateral muscles, and opening its jaws to take in food and water, the Fish adds to these little else but those undulations of the fins and tail that serve to balance and turn it. A Reptile, using its limbs in the water or on land or both, performs muscular actions considerably more varied and more combined; but still, actions that are directed to comparatively few ends. An ordinary Mamma] exhibits in the chase and destruction of prey, in the making of burrows, in the rearing of young, in the laying up of food, a greater variety of actions that are severally more compound. On arriving at the higher Mammals, ending with Man, we meet with motions that are almost countless in their kinds, that are severally composed of many minor THE NERVOUS SYSTEM. 11 motions accurately adjusted in their relative quantities and successions, and that are themselves compounded into courses of action directed to multiform objects. And with each such increment of complexity in the motor functions throughout the Vertebrata , there goes an increment of nervous endowment. This, then, is the secondary connection which traverses and complicates the primary connection. We saw that were there no other relation than that between quantity of nerve-tissue and quantity of motion generated, a Horse should have a far larger nervous system than a Man, instead of having a smaller one. But finding that there is also a relation between quantity of nerve-tissue and complexity of motion, we are led to expect an excep¬ tionally large nervous system in Man; and are enabled to understand why he has a larger one than a Horse has. More obvious, because not involved with irrelevant differences, is the interpretation thus yielded of the general rule, already illustrated in the case of the Dogs, that in each natural group or order of Mammals, the nervous systems do not in¬ crease in the same ratio as the bodies. We will glance at another illustration of this, supplied by the Primates: specially instructive because of the significant exception it contains, and specially interesting because that exception is furnished by mankind. The small monkeys have relatively very large brains— larger relatively than the brains of their congeners, in¬ cluding even the highest. This connection, parallel to that presented in the spaniel and the newfoundland, has a parallel explanation. The movements of the little Capuchin monkey are approximately as varied and complex as those of the great Gorilla; and hence, in so far as nervous evolution is related to heterogeneity of motion, the Capuchin should have a nervous system differing but little in size from that of the Gorilla. But since there is also a relation between quantity of nerve and quantity of motion generated, the Gorilla's 12 THE DATA OP PSYCHOLOGY. nervous system must be absolutely greater tbougb relatively smaller: wbicb we find it to be. Between the Gorilla and Man, however, there exists a converse contrast. Heavier than a Man, and moving about in the trees, a Gorilla pro¬ bably generates daily as mucb motion as a savage, or as a civilized labourer; and were it the sole function of nerve- tissue to originate motion, should have at least as large a nervous system. But the nervous system of Man is twice as heavy. Here, therefore, all other relations being sub¬ stantially the same, and the physiological processes being approximately alike in the two cases, the relative largeness of the human nervous system stands clearly related to the relatively-enormous complexity of human actions—a com¬ plexity shown partly in the more compound simultaneous movements, but mainly in the combination of successive movements, simple and compound, directed to remote ends. § 6. This double relation must still be taken as ap¬ proximate only. Seeing as we did at the outset that the genesis of motion depends on many physiological conditions, of which each is separately variable, it is manifest that the fundamental connections we have traced must have sundry minor irregularities. Without treating of these in detail, it may be well to instance one—that due to differ¬ ence of bodily temperature. Birds as a class are more active than Mammals as a class; and though many Mammals go through motions more heterogeneous than those of Birds, yet the inferior Mammals can scarcely be said to exceed Birds in the heterogeneity of their motions. Nevertheless, the nervous systems of Birds are relatively somewhat smaller than the nervous systems of Mammals. The explana¬ tion is that Birds have a higher blood-heat with its accom¬ panying more active respiration—both implying a greater rate of molecular change. And a greater rate of molecular change enables a smaller nervous system to generate an THE NERVOUS SYSTEM. 13 amount of motion which, would require a larger nervous system if the rate of molecular change were less. A further qualifying fact to he here named is that, all other things being equal, the power of a nervous system does not vary exactly as its mass. For reasons that will hereafter appear, its efficiency as a motor agent increases in a somewhat higher ratio than the quantity of matter it contains. But after all modifying causes have been allowed for, there remain substantiallv intact the fundamental rela- «/ tions set forth—namely, that wherever much motion is evolved, a relatively-large nervous system exists; that wherever the motion evolved though not great in quantity is heterogeneous in kind, a relatively-large nervous system exists; and that wherever the evolved motion is both great in quantity and heterogeneous in kind, the largest nervous systems exist. § 7. It is with deliberate intention that I have set out with this unfamiliar and, as many will think, somewhat strange presentation of the facts. My reasons for doing so are several. One of them is that we are here primarily concerned with psychological phenomena as phenomena of Evolution; and, under their objective aspect, these, reduced to their lowest terms, are incidents in the continuous re-distribution of Matter and Motion. Hence the first question respecting the nervous system as studied from our point of view is— what are the leading facts it presents as expressed in terms of Matter and Motion ? Another reason is that, apart from any doctrine of Evolu¬ tion, true conclusions respecting psychical phenomena must be based on the facts exhibited throughout organic nature; and that the above statement does literally nothing else than express these facts—expresses, too, all that direct induction can tell us respecting their essential relations. 14 THE DATA OP PSYCHOLOGY. The actions of all organic beings, including those of our own species, are known to us only as motions. Shutting out our inferential interpretations, the leaps and doublings of the escaping prey in common with the variously-adapted and rapidly-changed actions of the pursuer, are, to our per¬ ceptions, nothing but movements combined in particular ways; and so too are the changes of expression, tones of voice, and verbal articulations of our fellow-beings, on which we put such hidden implications. As, then, science requires us to distinguish the facts as actually presented from the suppositions we ordinarily join with them, it is needful to exhibit, in all its nakedness, this primordial relation between the external motions and their internal originator. Yet a further reason for setting out thus, is that we so escape from pre-conceptions. Those who bring with them to the investigation of psychical phenomena, the hypotheses that have descended to us from the past, are almost sure to be more or less biassed thereby. While intending to avoid assumptions they are in great danger of having their con¬ clusions vitiated, if not by some ancient or mediaeval idea under its overt form, yet by corollaries from it that have unobtrusively embodied themselves in unsuspected pos¬ tulates. As we shall presently see, even physiologists have been in some cases thus misled. Hence, then, without at all calling in question the truth of those other and quite different interpretations of nervous phenomena that are tacitly expressed in ordinary language, it is proper for us here to ignore them. Before studying the facts from a psychological point of view, we have first to study them from a physiological point of view. The pri¬ mary truth disclosed by the facts as so studied, is the universality of this relation between the degree of nervous evolution and the quantity and heterogeneity of the pro¬ duced motion. We now pass to the secondary truths similarly disclosed. CHAPTER II. THE STRUCTURE OF THE NERVOUS SYSTEM, § 8. An outline of nervous structure must precede a detailed account of it; and tlie essential facts to be indi¬ cated in an outline may be brought most clearly into view by comparing with one another the nervous systems possessed by different types, and by different grades of the same type. We will limit our comparisons to the three superior sub¬ kingdoms of animals. A minute nodule with diverging threads constitutes the rudimentary nervous system, as existing in the lowest Mollusk. In the Lamellibranchs several such minute nodules, or ganglia, are distributed, usually in pairs, in different parts of the body; and beyond the free fibres which they severally give off to neighbouring organs, there are fibres by which they are connected together. Gastero- pods, considerably higher in organization and activity, have nervous centres among which a considerable heterogeneity is produced by the greater size of some than of others. And besides a local integration of paired ganglia into single bi¬ lob ed ganglia, there is an advance in general integration, shown by a clustering of the more important ganglia about the head. The Cephalopods, and especially the dibranchiate division of them, in which the molluscous type reaches its highest, show us, carried still further, that integration of the nervous system due to simple growth, joined with that 16 THE DATA OP PSYCHOLOGY. integration due to concentration and coalescence of indepen¬ dent centres; and they also show us the differentiations involved by their changes of size,, form, and distribution. A delicate cord running from end to end of the body, and giving off lateral fibres in pairs, constitutes the nervous system in the lower Annulosa. When from limbless Annelids we pass to the Articulate types, composed of seg¬ ments bearing limbs, we find the nervous system formed of a series of centres, each sending fibres to the different organs of its own segment, and all of them united by a thick cord of fibres with a fused cluster of similar centres in the head. In the higher Articulata there is an increased relative size of the nervous centres as compared with their connecting structures; an actual approach of the chief nervous centres to one another, both longitudinally and laterally; and a final coalescence of them. This integration disclosed by comparisons of lower and higher types, may also be observed in progress during the development of the individual insect or the individual crustacean. And along with advancing growth, consolidation, and combination of nervous structures, there may be traced an increasing un¬ likeness, both among the central masses themselves, among their connecting cords, and among their divergent fibres. Such traits of evolution are exhibited under another form in the vertebrate sub-kingdom. Its lowest known member* the Amjphioxus , has a simple cranio-spinal axis, the anterior extremity of which is not made appreciably different from the rest by development of distinct cerebral ganglia, and which gives off lateral nerves that have but minor dis¬ similarities. The cyclostome Fishes, possessed of cerebral ganglia that are tolerably manifest, lead us to the ordinary fishes, in which these ganglia, individually much larger, form a cluster of masses, or rudimentary brain. Here, however, though in contact, they preserve a serial arrangement: their aggregation is little more than THE STRUCTURE OF THE NERVOUS SYSTEM. 17 that of close linear succession. But in the highest fishes certain of them which have greatly increased, overlap the others; and tend so to form a more compact, as well as a larger, aggregate. Superior Reptiles and Birds display this relative increase of certain of the clustered ganglia, and con¬ sequent obscuration of the rest, in a greater degree. It is carried still further in the inferior Mammals. From them upwards, the leading change of nervous structure is an augmentation of the two largest pairs of these aggregated nervous centres. In Man one pair has become so enormous that the others are most of them hidden by it, and nearly merged in it. Along with this direct inte¬ gration there goes on the indirect integration constituted by more intimate and multiplied connections. These are both longitudinal and transverse. While in the Amphioxiis, the cranio-spinal axis contains but a small proportion of the nerve-fibres which, running longitudinally, serve to unite its different parts ; in a superior vertebrate animal, such uniting nerve-fibres are among the chief com¬ ponents of the cranio-spinal axis. And, similarly, while the lateral halves of the cerebrum are but slightly connected in Birds, and have connections that are relatively deficient in the inferior Mammals, they become, in the highest Mammals, joined together by a thick mass formed of innumerable fibres. Meanwhile there have been arising diffe¬ rentiations no less conspicuous. Beyond that general one due to development of the anterior end of the cranio¬ spinal axis into cerebral ganglia ; and the further one of like nature which results from the relatively - enor¬ mous growth of some of these ; other differentiations have been constituted by the local unlikenesses of structure simultaneously established. As they enlarge, the greater ganglia are rendered externally dissimilar from the rest by the formation of folds or convolutions; and their internal parts severally acquire distinctive characters. The same thing holds of the peripheral nervous system. Pairs of c id THE DATA OF PSYCHOLOGY. nerves that were originally almost uniform, are rendered multiform by the much greater growth of some than of others, and by the inner differences that accompany these outer differences. This cursory survey of the nervous system under the various forms it presents throughout the animal kingdom, suffices to show how its evolution conforms to the laws of evolution in general. We are also shown by it what here more immediately concerns us—that while the rudimentary nervous system, consisting of a few threads and minute centres, is very much scattered, its increase of relative size . and increase of complexity, go hand in hand with increased concentration and increased multiplicity and variety of con¬ nections. Carrying with us this general conception, let us now study its structure more closely : considering, at first, not any particular forms of it but its universal form. § 9. The nervous system is composed of two tissues, which both differ considerably from those composing the rest of the organism. They are usually distinguished from one another by their colours as grey and white, and by their minute struc¬ tures as vesicular and fibrous. Chemical analyses have not at present thrown more than a flickering light on the consti- -iution of nerve-matter in general, or on the constitution of one kind of nerve-matter as contrasted with the other. All that can be asserted with safety is, that each kind contains pliosphatic fats and protein-substances; but that these com- -ponents are both differently distributed and in different states in the two tissues. Let us see what we are told about them by the microscope, aided by chemical re-agents. Where their evolution can be traced, the vesicles or cor¬ puscles of the grey tissue appear to take their rise out of a nitrogenous protoplasm, full of granules and containing nuclei. Round these nuclei the protoplasm aggregates into spheroidal masses, which, becoming severally inclosed in delicate membranes (in many cases inferred rather than seen) THE STRUCTURE OF THE NERVOUS SYSTEM. 19 are so made into nerve-cells. Tlie protein-sub stance, thus forming alike the chief contents of the nerve-cells and the chief part of their matrix, is, though coagulated, soft. The granules imbedded in it, both within and without the cells, consist of fatty matter. And on comparing together nerve- cells in different stages, there are seen differences in the colours of the granules, indicating a progressive meta¬ morphosis. To complete a general idea of the grey tissue, it must be added that the more developed of these nucle¬ ated cells, or nerve-corpuscles, give off processes, usually branched, that vary in number and degree of ramification; that among the corpuscles and their branches are dis¬ tributed the terminations of nerve-fibres; and that while in some nervous centres it is common for these fibres to run directly into the cells or to be continuous with certain of the processes, in other nervous centres the connections between fibres and cells are rarely if ever direct, but where they exist, are made through the remote sub-divisions of branches given off by both. When we pass to the white or fibrous tissue, we meet with matters that at first sight appear as distinct from the others in nature as in mode of arrangement. The fibres prove to be minute tubes. Within the extremely delicate membrane of which each tube is formed, there is a medullary substance or pulp, which is viscid like oil, has a pearly lustre, and consists of albuminous and fatty substances. But unlike as the contents of the nerve-tubes and the nerve-cells thus appear to be, a careful scrutiny discloses between them an essential kinship. For imbedded in the pulp which fills the tube or sheath, there lies a delicate fibre, or e< axis-cylinder,” which is composed of a protein-substance. Though chemically similar to the protein-substance contained in the cells of the vesicles, this is physically different; since, besides being comparatively firm or solid, it is uniform and con¬ tinuous, instead of having its continuity broken by fat granules. That this central thread of protein-substance is c 2 20 THE DATA OF PSYCHOLOGY. tlie essential nerve,, to wliicli the sheath of medullary matter with its surrounding membranous sheath are but acces¬ sories,, there are several proofs. One is that in the lower animals, as well as in the embryos of the higher, no me¬ dullary sheaths exist: the nerve consists of the axis-cylinder and its protecting membrane, without any pulp lying between them. Another proof is that at the peripheral ter¬ minations of nerves, even in superior animals, the medullary sheath commonly, if not always, stops short; while the cen¬ tral thread, covered by the outermost membrane, continues further, and ends in delicate ramifications not inclosed in distinguishable sheaths. And a further proof is that where a nerve-fibre unites with a nerve-cell, the medullary sheath ceases before arriving at the place of union; while the axis- cylinder joins the contents of the cell, and its protecting membrane becomes continuous with the cell-wall, where this exists. Hence concluding, as we are warranted in doing, that the axis-cylinder is its essential part, we see that the matter of nerve-fibre has much in common with the matter of nerve-vesicle : the differences between them ap¬ pearing to be mainly that, in the nerve-vesicle, the protein- substance contains more water, is mingled with fat- granules, and forms part of an obviously unstable mass; whereas in the nerve-tube the protein-substance is denser, and is distinctly marked off from the fatty compounds that surround it: so presenting an arrangement that is relatively stable. What is the meaning of this difference ? Before seeking an answer we must remember that compound substances undergo two fundamentally different kinds of metamorphosis —one in which the components are some or all of them dis¬ sociated and distributed through surrounding space, either apart or in new combinations; and one in which the com¬ ponents, instead of being dissociated, are merely re-ar¬ ranged, so as to alter the perceptible properties of the mass without destroying its physical continuity. The first THE STRUCTURE OF THE NERVOUS SYSTEM. 21 we call decomposition; tlie second isomeric transforma¬ tion. These forms of change are further distinguished in this, that the one is usually accompanied by a great dissipation of motion, whereas the motion given out or taken up along with the other is relatively insignificant. There is yet a third contrast. After decomposition the separated components cannot be readily made to resume their previous relations : often it is impossible to combine them again; and in most other cases it is difficult to do this. But in many instances of isomeric transformation, resump¬ tion of the original form may be produced by a very moderate change of conditions. Now the two kinds of molecular change thus strongly contrasted, are the two kinds of molecular change which we have reason to suspect are undergone by the two forms of nervous matter. While the protein-substance mingled with fat-granules in the vesicles, is habitually decomposed; the protein-substance forming the axes of the nerve-fibres is habitually changed from one of its isomeric states to another. Such, at least, is the assumption here made, in conformity with the conclusion drawn in the Principles of Biology (§ 302); where it was argued that the propagation of mole¬ cular disturbances from one place in an organism to another, tends so to modify the mingled colloidal substances as to produce, between the two places, a form of colloid that undergoes isomeric transformation when disturbed, and com¬ municates the disturbance in undergoing the transformation; and where it was argued that this easily-transformable colloid, having had such a change set up at one end of it and passed on to the other, giving out in the process some molecular motion and consequently falling in temperature, immediately re-absorbs from the adjacent tissues permeated by blood, an amount of molecular motion equal to that which was lost: thereupon resuming its previous isomeric state, and its fitness for again propagating a wave of transformation. Much as there is here of hypothesis, the indirect evidence 22 THE DATA OP PSYCHOLOGY. makes it probable that if this is not tbe true interpretation, tlie true interpretation is analogous to it. That tbe matter contained in tbe vesicles is tbe seat of destructive molecular changes, with accompanying disengagement of motion, while tbe matter contained in tbe tubes is the seat of changes which, of whatever special nature, do not involve much destructive decomposition and disengagement ol motion, are beliefs for which we have several warrants;— among others, the following. The grey tissue contains far more water than the white tissue : the propor¬ tion of solids to water being about 12 per cent, in the grey tissue, while in the white tissue it is some 25 per cent. Now abundance of water facilitates molecular change, and habitually characterizes parts in which the rate of molecular change is high. Hence the implication is that the grey matter undergoes metamorphosis with much greater rapidity than the white. Stronger evidence is afforded by the fact that the grey or vesicular substance has a vascularity immensely exceeding that of the white or fibrous sub¬ stance. On comparing the net-works of blood vessels that permeate the two, the difference is conspicuous; and it is much greater than at first appears. An estimate based on measurements, proves that a given bulk of the one contains about five times as many capillaries as an equal bulk of the other.* Now since these minute canals that bring and take * The drawing on which this estimate is based, is contained in the Manual of Human Histology , by A. Kolliker : translated and edited by George Busk, F.R.S., and Thomas Huxley, F.R.S. The estimate is easily made. A number of equi-distant parallel lines being drawn transversely through the two net-works, the number of places at which one of these lines crosses blood-vessels within a given length (say an inch) is counted, and the like being done with an equal length of each of the other parallel lines traversing the same net¬ work, there is obtained, by taking an average, the number of vessels usually met with in a specified distance. The like process is then gone through with lines of the same length traversing the other net-work. These averages do not, however, truly express the comparative numbers of such intersections in the Wo net-works ; since the meshes of the one are unlike those of the other in shape. Hence it is needful to draw an equal number of parallel longitu- THE STRUCTURE OF THE NERVOUS SYSTEM. 23 away materials, must be numerous in proportion as com¬ position and decomposition are quick; we may infer a great difference between tbe rates of destructive change in the two tissues. Another contrast supports this con¬ clusion no less strongly. The unstable granular protoplasm contained in the corpuscles, is shielded from adjacent dis¬ turbing forces by a membrane which, even where thickest, is so delicate that its existence can be demonstrated only by the help of re-agents; and which in many corpuscles cannot be made visible at all. Hence between the matter contained in these corpuscles, or vesicles, and the streams of blood that run among them so abundantly, are interposed little else than the delicate walls of the capillary blood-vessels; and thus the disturbing substances brought by each capillary, can pass with the least possible hindrance into the unstably-arranged contents of the neighbouring vesicles. Quite otherwise is it with the relations of the blood to the contents of nerve-tubes. The wall of each nerve-tube is thick enough to make it easily demonstrated; and between it and the central thread of essential matter, comes the coat of nerve-medulla. Through these barriers the disturbing agents, carried among the nerve-tubes by sparingly-distributed capillaries, cannot readily pass ; and the essential nerve-thread is prevented from having molecular changes set up in it at places between its two extremes. This protection suffices so long as the disturbing agents remain normal in their amounts; but when they become excessive, as they do if the blood-vessels become congested, local changes in the nerve-threads are caused : whence one kind of neuralgia. It should be added that by. - dinal lines ; and to repeat with them this process of averaging. By taking the means between the resulting numbers and the previous numbers, we get a cor¬ rect representation of the relative frequencies with which the vessels occur ir. space of one dimension. To ascertain their relative frequencies in space of three dimensions, or in solid tissue, it is of course needful simply to cube the two numbers so arrived at. 24 THE DATA OF PSYCHOLOGY. this sheathing of nerve-medulla, the essential nerve-threads, besides being shielded against disturbances from neighbour¬ ing currents of blood, are shielded against disturbances from nerve-threads in the same bundle. Were “ axis-cylinders ” lying in lateral contact not thus coated, a molecular change propagated through one would set up molecular changes in its neighbours; as, in fact, it does in an early stage of ataxy, characterized by loss of the medullary sheaths. Hence, too, the explanation of that normal absence of me¬ dullary sheaths which sundry nervous structures show us. For among the Invertebrate, in which this normal absence occurs, the fibres contained in the same bundle have nothing like those many and varied distinctions which they have in the higher animals: they have termini of which the structures and functions are much less differentiated. Similarly with those bundles of grey or non-medullated fibres, contained in the sympathetic system of vertebrate animals; for these bundles, serving to establish relations among the viscera, each of which is much less divided into parts that act independently, there needs no such perfect in¬ sulation of the nerve-fibres. And the like holds even in cer¬ tain portions of the peripheral cerebro-spinal system ; as the olfactory expansion, which consists of an extensive plexus of non-medullated fibres, and which has the peculiarity that different parts of its area are not acted upon sepa¬ rately. The evidences, direct and indirect, thus justify us in concluding that the nervous system consists of one kind of matter under different forms and conditions. In the grey tissue this matter exists in masses containing cor¬ puscles, which are soft and have granules dispersed through them, and which, besides being thus unstably composed, are placed so as to be liable to disturbance in the greatest possible degree. In the white tissue this matter is collected together in extremely slender threads, that are denser, that are uniform in texture, and that are shielded in an unusual THE STRUCTURE OF THE NERVOUS SYSTLLT. 25 manner from disturbing forces, except at tbeir two extre¬ mities. And the implication on which we henceforth pro¬ ceed is, that the masses, unstably constituted and con¬ ditioned, are seats of destructive molecular changes, and disengagement of motion; while the stably constituted and conditioned threads, are the seats of molecular changes that are not destructive, and are probably isomeric. § 10. Nerve-tubes with their contained protein-threads, and nerve-cells with their contained and surrounding masses of changing protein-substance, are the histologic elements of which the nervous system is built up; and we have now to ask in what way they are put together. We will begin with the peripheral termina¬ tions of the nerve-tubes ; or rather, with those of them which lie on the outer surface. Suppose the skin, including those introverted portions of it which form the receptive areas of the special senses, to be marked all over in such a way as to form a net-work. Suppose the meshes of this net-work to vary extremely in their sizes; so that while in some places they are as large as those of a fishing-net, they are in other places not large enough to admit the point of a needle. Or, to speak specifically, suppose that on the middle of the back the meshes are some inches in diameter, and that being equally large over the middles of the fore-arms, and the middles of the thighs, they diminish to 2 inches and less over the neck and breast, to 1^ inches at the extremities of the legs, to 1J inches on the backs of the hands, to less than an inch on the forehead, to less than half-an-inch over the cheeks and over the palms of the hands, to a quarter of an inch and less over the fingers, to a twelfth of an inch at the inner tips of the fingers, and at the tip of the tongue to one twenty-fourth of an inch in diameter; and sup¬ pose, further, that over the back of that dermal sac which forms the eye, these meshes are so small that a microscope 26 THE DATA OF PSYCHOLOGY. is required to distinguish t-liem. Haying imagined such a net-work of which the meshes, irregularly polygonal in their outlines, are thus wide over parts of the surface that have but little variety of converse with the external world, and become smaller in proportion as the surfaces have multiplied and variable contacts with things ; we shall have gained an approximate idea of the relations among the separate local areas in which there arise independent nerves. To complete the conception, however, something else must be supposed. The large meshes we must represent as marked out by very broad lines—say a quarter of an inch broad where the meshes are largest. We must imagine them narrowing as the meshes become smaller; until, when we come to the meshes over the surface of the retina, the dividing lines have dwindled to the thick¬ ness of a gossamer thread. And now let us conceive that within each of these areas, large or small as it may happen, there exists a plexus of fibres, formed of the essential nerve substance, that are continuous with one another, but have no connection with the fibres occupying adjacent areas. Not, indeed, that we must conceive any sharp limitation of the space occupied by each plexus. We must assume that the line separating two areas, here very broad and here very narrow, covers a space into which fibres from both the areas run, without joining one another. Hence the area belong¬ ing to each independent plexus, is the internal area of the mesh, plus the space occupied by its circumscribing broad or narrow line; and the breadth of the line represents the extent to which adjacent areas overlap. Such, then, are the peripheral expansions of those nerves which are liable to be acted on by external forces. Here each monopolizes a relatively-great tract of the surface, and here an extremely minute one. Each is an independent agent— each is capable of having a change set up in it without changes being set up in its neighbours. The skin is, as it were, occupied all over with separate feelers, that are here THE STRUCTURE OE THE NERVOUS SYSTEM. 27 widely scattered, here clustered, and here crowded together as closely as maintenance of their individualities will allow. From the nerve-plexus occupying one of these areas, there takes its rise the central fibre, or axis-cylinder, of a nerve- tube. Coated with its medulla and inclosing sheath, it takes its way from the surface inwards, and, proceeding without any branch or junction, eventually reaches a mass of grey matter with imbedded vesicles—a nerve-centre or ganglion. Into the substance of this the essential nerve- thread runs, becoming divested of its medullary sheath; and where the structure is least involved, the essential nerve-fibre frequently if not always ends in a nerve-vesicle. In such simple, and what we may call typical, centres, there branches out from some other part of the nerve-vesicle, another nerve-fibre which, similarly inclosed in its double sheath, pursues an outward course, ordinarily along the same general route as the first, until, reaching the same part of the body, it buries itself in a bundle of muscular fibres amid which its ramifications end. Thus we have as the elements of what is called a nervous arc—1, a peri¬ pheral expansion, placed where it is liable to be disturbed by an external agent, and so formed as to be most easily disturbed; 2, a connected fibre capable of being readily affected by disturbances at this outer end, but shielded from disturbances elsewhere; 3, at, or near, the inner end of this fibre, a corpuscle of unstably-arranged substance, apt to give out much molecular motion when disturbed; 4, a second fibre diverging from the corpuscle, or its neigh¬ bourhood, and subject to disturbance from the molecular motion disengaged near its origin, but protected from other influences ; 5, at the remote extremity of this second fibre, a subdivided termination amid a substance that contracts greatly when disturbed, and which, in contracting, moves the part of the body in which the first fibre took its rise. Fig. 1 is a diagram representing these elements of a nervous arc: A being the first, or, as it is called, afferent nerve. 28 THE DATA OP PSYCHOLOGY. with its peripheral expansion a-, B being the nerve-cor¬ puscle or ganglion-cell; and C the second, or efferent, nerve, with its termination c. Fig. l ri&S. This arrangement of parts is perpetually repeated through¬ out the nervous system; and if we generalize the concep¬ tion somewhat by supposing that the commencement a is not necessarily external, but may be on an inner surface, or within an organ, while the termination c is not necessarily in a muscle but may be in a gland; we shall have a concep¬ tion that is, in a certain sense, universally applicable. I say in a certain sense, because, until another element is added, the conception is incomplete. These coupled nerves, with the ganglion-cell acting as a direct or indirect link between them, recurring everywhere in substantially the same relations, appear to form a compound structure out of which the nervous system is built—its unit of composi¬ tion. But this is not so. By multiplication of such arcs we may get a multitude of separate nervous agencies, but not a nervous system. To produce a nervous system there needs an element connecting each such nervous arc with the rest—there needs a third fibre running from the ganglion-cell, or its neighbourhood, to some place where other communicating fibres come; and where, by direct or indirect junctions, actual or approximate, the primary couples of nerves may be brought into relation. That is, there requires what we may call a centripetal nerve.* In * The words centripetal and centrifugal are occasionally used in nerve- physiology as the equivalents of afferent and efferent. But as afferent and efferent are by far the most generally adopted, and are also the most descrip¬ tive, it seems to me that the word centripetal may with advantage have this more special meaning given to it; and centrifugal the correlative meaning. THE STRUCTURE OE THE NERVOUS SYSTEM. 29 Fig. 2 is shown, diagrammatic ally, the relation in which this stands to the others. A centripetal nerve being added, there results what we may fairly regard as the unit of com¬ position of the nervous system. We shall have presently to recognize certain fibres which this conception does not include. But they are not essential; for a nervous system is possible without them. Let us, then, taking this as our unit of composition, consider the general method after which a nervous system is constructed. § 11. The fibres represented in the above diagrams, do not ordinarily pursue their respective courses by themselves : thev proceed in company, as shown in Fig. 3. The afferent nerves arising at a , in separate but adjacent areas on the skin, or in other organs recipient of external impressions, converge; and, while maintaining their separate indivi¬ dualities, become united into a bundle inclosed in a sheath. Other sheathed bundles of fibres from other clustered areas in the same region, presently join them, and run along with them in a compound bundle, until they eventually reach the mass of imbedded nerve-vesicles constituting a ganglion or nervous centre B. Similarly the efferent nerves which have their roots in this ganglion, issue from it as a bundle, which, commonly inclosed in the same general sheath as the afferent nerves, goes back to the part of the body whence these arose; and secondary bundles of these efferent nerves, diverging and re-diverging from one another as they enter this part, as at c, finally become lost in its various muscles. 30 THE DATA OP PSYCHOLOGY. In like manner tke centripetal fibres cl , originating in this ganglion, take tkeir common course, joined perhaps by other fibres originating elsewhere, towards a ganglion E, that is larger and has more numerous connections. Of course the clustered lines and spotted circles in Fig. 3, are entirely diagrammatic—give no idea of the separate nerves and bundles and ganglia as they actually exist; but merely of the relations in which they stand to one another. It should be added that the more central ganglion, to which converge other bundles of centripetal nerves (together with some afferent nerves that pass through inferior ganglia without stopping) may itself be subordinate to a still superior, or still more central, ganglion. To this it gives off what may be called superior centripetal nerves ; and other nerves of the same or of a lower order being brought to it, this highest ganglion becomes a place where there are established communications among all the subordinate and sub-subordi¬ nate ganglia, with their afferent and efferent fibres. One further kind of connection exists. The immense majority of animals, have their parts symmetrically arranged —sometimes radially but more frequently bi-laterally. For the corresponding parts there are habitually corresponding ganglia; and the connections that remain to be named are those between these corresponding ganglia, or ganglia which belong to the same grade. Such connections consist of what are called commissural fibres. They are indicated at b } where they transversely join the structure shown in detail, with the answering structure belonging to the other side of the body. The word commissural is, indeed, sometimes used in a wider sense: including fibres that unite ganglia of different grades. But since the great majority of the fibres called commissural are those which join duplicate ganglia, or else ganglia that occupy like relations in the hierarchy, it will, I think, conduce to clearness to restrict its application to these : leaving the word centripetal for fibres which con¬ nect ganglia of lower orders with those of higher orders. I THE STRUCTURE OF THE NERVOUS SYSTEM. 31 The commissures thus bringing into relation the members of each pair of centres,, inferior or superior, and so linking the two halves of the nervous system, complete the nervous communications throughout the organism. This description, purposely generalized with a view of exhibiting the principles of nervous organization, apart from any particular type, may be fitly supplemented by the description of a special structure that illustrates them. Each sucker on any arm of a cuttle-fish, has a ganglion seated beneath it. To this descend the afferent nerves that are affected by touching the sucker; and from it ascend the efferent nerves distributed to the muscular fibres of the sucker. These form a local nervous system that is ex¬ perimentally proved to have a certain completeness in itself. But now from the ganglion underneath each sucker, fibres run along the arm, in company with fibres from all similar ganglia in the arm; and this bundle of centripetal fibres eventually reaches a ganglion at the base of the arm. Each arm, similarly constructed, thus has a chief nervous centre in which the fibres from all its minor nervous centres are brought into communication. Further, all round the ring formed by the united base of the arms, there runs an an¬ nular commissure connecting these superior ganglia. And then from each of them is given off a bundle of fibres that proceed centripetally to a still higher centre—the cephalic ganglion ; where, consequently, nerves from all the arms are brought into direct communication with one another, and also into communication with nerves arriving from ganglia in other parts of the body. Omitting details and qualifica¬ tions, not essential to such a conception as concerns us here^ we thus see that in nervous structure there is a centraliza¬ tion and re-centralizafcion, that is carried far in proportion as the organization is high. § 12. We may be sure that along with a principle of ar¬ rangement among connecting structures, there goes some 82 THE DATA OF PSYCHOLOGY. principle of composition in tlie centres that are connected; which are not simply places for the meeting of fibres, but places in which there exist agents liable to be acted on by the in-coming fibres and capable of acting on the out-going fibres. Respecting the principle of composition, our reason¬ ings must be mainly hypothetical; but they will, I think, prove of some worth, by leading us to conclusions that har¬ monize with observation, so far as this carries us. In ascending from the lowest to the highest types of the nervous system, we see that the distribution and combi¬ nation of nerve-fibres are so modified, as to make possible an increasing multiplicity, variety, and complexity of relations among different parts of the organism. What kind of mo¬ dification does this necessitate at places where the nerve- fibres are put in communication ? Without assuming that two fibres which bring two parts of the organism into rela¬ tion, are always united at their central extremities by an intermediate nerve-corpuscle, it may be safely assumed that continuity between their central extremities must be effected either by a nerve-corpuscle or by some less-defined portion of grey substance ; and it is clear that in proportion to the number of different connections to be established among the nerves coming to any ganglion, must be the number of the more or less independent portions of grey substance re¬ quired to establish them. Let us consider the implications. Suppose that a and b, Rig. 4, are two points in the or¬ ganism. To join the nerves proceeding from them, there needs only the single ganglion-cell A. Similarly, to bring into nervous relation the points c and d } the single ganglion¬ cell B suffices. So long as A and B remain unconnected, these two simple relations are the only possible ones among the points a , b, c, d. But now assume that from A and B there run fibres to the centre C—not a single fibre from each, but two fibres, one of which in each case proceeds from a or b , and from c or d. This being so, there may be formed at C, eleven simple and compound relations: these four THE STRUCTURE OF THE NERVOUS SYSTEM. 33 points can be arranged into six groups of two, a b, a c, a d, be, b d, c d ; into four groups of three, b a c,b a d, a c d, cb d; and into one group of four, abed. Hence, supposing tlie centre C to be made up of the independent cells, or portions of grey substance, severally serving to link the members of a group into a separate combination, there must be at least eleven such. If, again, from this centre C, we assume that there ^un adequately numerous fibres to the higher centre F, and that this is also duly connected through the centres D and E, with the points e, f } g , h; then the possible number of groups, simple and compound, that may be formed at F, will amount to 247 ; and to unite the members of each group so that it may be inde¬ pendent of the rest, there must be at least 247 connect- ing links at the centre F. Without pursuing the calcu¬ lation, it will be manifest that as these points in the or¬ ganism increase in number, and as the clusters of them that are to be brought into relation become larger and more various, the central elements through which their relations are established must grow multitudinous. An in¬ adequate conception, however, is thus reached; for we have considered only the requisites for forming among these points, the greatest number of different groups, simple and compound; ignoring the different orders in which the D 34 THE DATA OF PSYCHOLOGY. members of eacb group may be combined. Two things can be arranged in succession in only 2 different ways; three things can be arranged in 6 different ways; four things in 24 ways; five things in 120 ways ; six things in 720 ways; seven things in 5,040 ways; and so on in a progression increasing with enormous rapidity. Assuming, then, that at the centre F, certain points, a, b. c, d , e, are to be com¬ bined, not in this succession only, but in all possible suc¬ cessions, there will require 120 different links of connec¬ tion for this one group of five points only. These links, whether separate vesicles or less-differentiated portions of grey matter, must occupy a considerable space; and sup¬ posing they are aggregated near those pre-existing cells or links which they have to re-combine in various orders, there may result a protuberance from the centre F, as shown at G. If we suppose that instead of a group of five, a group of six is to have its members thus variously combined; or if instead of one group to be so dealt with, there are many; this lateral outgrowth may become relatively very large. And since its vesicles, or portions of grey matter, will be much more bulky than the fibres running from them to the members of groups which they combine, there may be expected to arise, as at H, a lateral centre attached to the original centre, F, by a pedicle of fibres. Of course these diagrams and numbers are intended to convey nothing but a general idea of the principle of com¬ position of nerve centres—not to represent any actual com¬ position. It would be an absurd assumption that among a number of points in the body, there have to be formed as many unlike groups as are theoretically possible ; and it is not to be supposed that the members of any group need ever to be combined in as many different orders as they might be combined. But while, on the one hand, the above description greatly over-states the accumulation of nerve- vesicles, or their equivalents, implied by such correlations as are actually required among a given number of points in the THE STRUCTURE OF THE NERVOUS SYSTEM. 35 organism ; it immensely under-states tlie number of points to be so correlated, as well as tbe number, and variety, and complexity, of the groups into which they are to be com¬ bined. The places from which afferent nerves proceed, as well as the places to which efferent nerves proceed, are multitudinous. Very large groups of such places have their members put in simultaneous communication. The different groups so formed are innumerable. And extremely varied relations of succession are established among members* of the same group ; as well as among different groups. Hence we are safe in asserting that along with an increasing multi* plicity and heterogeneity of nervous connections, there must, go increasing massiveness of the nervous centres, or accu-. mulations of vesicular matter. , One further corollary deserves noting. Each vesicle, on- each portion of grey matter that establishes a continuity between the central termini of fibres, is not merely a con¬ necting link : it is also a reservoir of molecular motion, which it gives out when disturbed. Hence, if the composition of nerve-centres is determined as above indicated, it follows that in proportion to the number, extensiveness, and com¬ plexity, of the relations, simultaneous and successive, that are formed among different parts of the organism, will be. the quantity of molecular motion which the nerve-centres; are capable of disengaging. § 13. As a datum for Psychology of the most general kind, the foregoing description of nervous structure might suffice. But having to deal chiefly with that more special Psychology distinguished as human, it will be proper to add some account of the human nervous system. A few facts of moment respecting its peripheral parts, may be set down before we study its central parts. At the surface of the body, where the extremities of nerve-fibres are so placed as to be most easily disturbed, we generally find what may be called multipliers of distur- d 2 THE DATA OF PSYCHOLOGY. 3 ‘) bances. Sundry appliances which appear to have nothing in common, have the common function of concentrating, on the ends of nerves, the actions of external agents. That this is the effect produced by the lenses of the eyes, is a familiar fact. It is a less familiar fact that certain otolites and minute rods or fibres, immersed in a liquid contained in the internal ear, serve to transform the less sensible vibrations communicated to this liquid, into the more sensible vibrations of solid masses, and to bring these directly to bear on the nerve-terminations. So, too, is it over the integument; or, at any rate, over the parts of it subject to many and varied contacts. Though men have not, like many inferior Mam¬ mals, the well-developed tactual multipliers called mbrissce (known in a cat as “ whiskers”), each of which is a lever that intensifies a slight touch at the outer end into a strong pressure of the imbedded end upon an adjacent nerve-fibre; yet every one of the short hairs on a man's skin acts in the same way. And then, in addition to these, there are, at places where the contacts with objects are perpetual, and where hairs do not grow, certain multipliers below the sur¬ face—small dense bodies named corjmscula tactus , round each of which* a nerve-fibre ramifies, and each of which, when moved by the touch of a foreign body, gives to its attached nerve-fibre a greater pressure than this would receive were the surrounding substance homogeneous : a fact which will be understood on remembering the effect of pressure on the skin when some small hard body, as a thorn, is imbedded in it. So much for the instruments that are external to the peripheral expansions of the nerves, and serve to exag¬ gerate the effects of incident forces. We may now con¬ template these peripheral expansions themselves, as being adapted to receive these exaggerated incident forces. In the first place, the ultimate nerve-fibrillae, ramifying where they are most exposed to disturbances, consist of nerve-protoplasm unprotected by medullary sheaths and not THE STRUCTURE OP THE NERVOUS SYSTEM. 37 even covered by membranous sbeatbs. In fact, they appear to consist of matter like that contained in nerve-vesicles, but without the fat-granules; and may be regarded as, like it, more unstable than the matter composing the central fibres of the fully-differentiated nerve-tubes. To this general character of the nerve-terminations, have to be added the more special characters of the terminations exposed to special forces. The delicate pale fibres which form a layer on the surface of the retina, are not directly affected by the rays of light concentrated upon them; but these rays, pass¬ ing through them, fall on a layer of closely packed, but quite separate, little bodies which are the true sensitive structures; and then the minute nerve-fibrillae that run from these to the stratum of retinal nerve-fibres, pass on their way into a layer of nerve-vesicles, with which we may presume they have connections. That is to say, this peripheral expansion of the nerve on which visual images fall, contains numerous small portions of the highly-unstable nerve-matter, ready to change, and ready to give out molecular motion in changing. It is thus, too, with those terminal ramifications of the auditory nerve, on which sonorous vibrations are concentrated. And there is an analogous peculiarity in the immensely-expanded extremity of the olfactory nerve. Here, over a large tract covered by mucous membrane, is a thick plexus of the grey un¬ sheathed fibres; and among them are distributed both nerve-vesicles and granular grey substance, such as that out of which the vesicles arise in the nervous centres. The significance of these structural peculiarities we shall see hereafter. For the present we need only note the distri¬ bution of them. Over the skin, which is conversant with forces of a relatively-considerable intensity—mechanical impacts, pressures, tensions,—we do not find that the nerve- terminations contain deposits of the peculiarly unstable nerve-substance. But we find such deposits where tlio incident forces are extremely feeble, or fall on excessively 38 THE DATA OF PSYCHOLOGY. small areas, or botli. The quantity of matter which, floating as faint odour through the air, reaches the end of the ol¬ factory nerve, is infinitesimal. Such luminiferous undula¬ tions as are allowed, during a momentary glance, to fall on one of the minute areas of the retina, are equivalent to a mechanical force inappreciable by our measures, if not in¬ expressible by our figures. Similarly with those atmo¬ spheric waves which, produced by the church-bell a mile away, and weakening as they spread in all directions, are conveyed to the minute otolites and rods of the inner ear, to be by them impressed on the auditory nerves. And in these places it is that we find peripheral deposits of the specially-unstable nerve-substance. § 14. Arising from these variously-specialized peripheral structures, the afferent nerves, collected into their bundles and compound bundles, run inwards to the spinal cord; out of which issue the corresponding bundles of efferent nerves. In one sense the spinal cord may be regarded as a con¬ tinuous nervous centre; and, in another sense, as a series of partial^-independent nervous centres. Each pair of trunk nerves with its segment of the spinal cord, has a certain degree of individuality; and those segments into which enter the pairs of massive nerves from the limbs, have individualities considerably pronounced; since it is experi¬ mentally proved that when severed from the rest they are not incapacitated. The tract of grey matter in the spinal cord to which the afferent nerves of a limb come, and from which the efferent nerves issue, is practically the ganglionic centre of that limb, having very much of automatic inde¬ pendence ; and being joined by commissural fibres to a like centre belonging to the fellow limb, it forms with this an automatic pair. So that, remembering how the entire cranio-spinal axis is originally one and continuous, and that its anterior part has been differentiated and developed into quite distinct centres, we may say that its posterior part, the STRUCTURE OP THE NERVOUS SYSTEM. 39 spinal cord, lias also been so differentiated, tbougb to a much smaller extent. To this conception two additions must be made. Beyond the internal tracts of grey or vesicular matter, and the bundles of nerve-fibres that enter into and issue from them laterally; and beyond the tranverse commissural fibres which connect the corresponding lateral portions of grey matter or partially-differentiated pairs of nervous centres; there are longitudinal commissural fibres, joining these successive pairs of nervous centres with one another, and serving to integrate the series of pairs in the same way that the members of each pair are integrated. And then, along with these fibres that unite nervous centres of the same order, there are what we found it desirable to distinguish as centripetal fibres, running from the relatively- inferior nervous centres to the relatively-superior ones ; with centrifugal fibres running back. Of these relatively-superior nervous centres, we have first to notice the medulla oblongata; including those parts of the pons Varolii which are woven into it, and similarly arise out of the fourth ventricle. This is the enlarged termination of the spinal cord, lying within the skull. Distinguished as it is from lower parts of the spinal cord by its greater massiveness, it is much more distinguished by the multiplicity and variety of its peripheral connections. While the successive segments of the spinal cord proper, have pairs of afferent and efferent nerves which are limited in their distributions to particular regions of the body ; and while even such an entire group of these segments as occupy the lumbar region, have relations only with the legs and the lower part of the body; the medulla oblongata , by the intermediation of centripetal fibres, is brought into relation not only with the lower part of the body and its limbs, but with the upper part of the body and its limbs ; and not only with these, but also with sundry of the parts which we know as the organs.of the special senses ; and not only with these., but also with the more important 40 THE DATA OF PSYCHOLOGY. viscera. The auditory nerves and the nerves of taste go directly into it, and though the optic nerves do not, yet from the centres to which theyrun there are fibres communica¬ ting with it; from its laterally-appended parts arise the nerves of the eye-muscles and the facial nerves; and the pneumogastric nerves, given off from its posterior part, put it in communication with the larynx, the lungs, the heart, the liver, and the stomach. Respecting its connections, direct and indirect, much remains to be ascertained; but what is known justifies the conclusion that the medulla oblongata, including the structures that are adnate, is a portion of the originally-uniform cerebro-spinal axis, which has been differentiated into a centre of a higher order than those behind it, or those at the base of the mass in front of it—higher in the sense that it has become that portion of the axis in which centripetal fibres running from the posterior ganglia, and from some, if not all, of the anterior ganglia, called by some sensory, are brought into relation with one another—a centre through which these local centres are united into one system. Passing over with a mere recognition the anterior ganglia just named, the exact relations of which are ill-understood, but some of which comparative morphology proves to be portions of the front end of the cerebro-spinal axis that have become differentiated into ganglia of the first order, receiving those special external stimuli to which the front end of the body is exposed; there remain only to be noticed the two great bi-lobed ganglia, which in Man form the chief mass of the brain—the cerebellum and the cerebrum. Phy¬ siologists and anatomists are agreed in regarding these as centres of a still higher order. Anatomical proof of their superiority, as being the seats of still higher centralization, is very incomplete ; for the difficulty of tracing the courses of all the nerve-fibres that enter into and issue from them, has hitherto been insuperable. But their connections with the subjacent minor centres and with the medulla oblongata, THE STRUCTURE OF THE NERVOUS SYSTEM. 41 are sucli as to make it certain tkat through the intermedia¬ tion of these, they communicate with the whole peripheral nervous system ; and are places in which centripetal fibres from centres of both the first and second orders, joined, possibly, with some simply afferent fibres, are brought into various relations : relations, however, that most likely differ in their natures from those established in inferior centres— differ, perhaps, as those supposed to be formed in the centre H, Fig. 4, differ from those formed in the centre F. Among the facts of fundamental significance with which we are here concerned, one other may be named. This concerns the histological structures of nervous centres. In automatic ganglia, the direct union of nerve-fibres with nerve- cells is habitual. Throughout the spinal cord the “ axis-cylin¬ ders 33 may not unfrequently be traced running into the vesicles. But in the higher nerve-centres direct connections are much less readily made out; and it is question¬ able whether in the highest they occur at all. In the grey substance of the cerebrum, the delicate nerve-fibres which, divested of medullary sheaths, run among the im¬ bedded corpuscles, do not directly unite with them; or if it is too much to say that there are no such unions, we may say that they are rare. Such communications as exist are apparently between the branched terminations of the fibres and the ramified processes of the corpuscles. Thus at the one extreme, simple, clear, and complete connections are the rule; and at the other extreme, involved, vague, and incomplete connections. § 15. Some account must be given of certain remaining nervous structures, with which Psychology is indirectly concerned. Thus far we have dealt only with the fibres and centres that stand passively and actively related to the ex¬ ternal world; but there are fibres and centres that stand related to those internal organs which make possible the continuance of relations to the external world. 42 THE DATA OF PSYCHOLOGY. The first to claim attention are the vaso-motor nerves. Bun¬ dles of these issue from the spinal cord, and, joined by bundles of nerves arising from the sympathetic system, accompany the main arteries : dividing and subdividing wherever these do, so as to supply fibres to all their branches down to the most minute. The vaso-motor nerves form, in fact, an additional series of efferent nerves. The nervous arc under its lowest form, consists of the afferent nerve with its peripheral ex¬ tremity exposed to external actions, the ganglion-corpuscle to which its central extremity runs, and the efferent nerve thence issuing to end in some muscle. But as we-have seen, the unit of composition of the developed nervous system, includes a centripetal fibre, running from the first or subordinate centre to a higher centre; and here we have to add, as an habitual element of this unit of composition in its complete form, a vaso-motor fibre, running to that part of the body in which the two ends of the nervous arc lie, and bringing the blood-vessels of that part into relation with the other parts of the apparatus. The cerebro-spinal nervous system, besides having these direct communications with the muscular walls of the tubes which bring blood, both to itself and to the muscles it sends fibres to, is also put into relation with other parts on which it is equally, though less immediately, dependent— the viscera. These have, indeed, a nervous system of their own, possessing apparently a considerable degree of inde¬ pendence—the sympathetic system; and one all-important viscus, the heart, has a nervous system that is demonstrably independent. The morphological interpretation of the vis¬ ceral nervous system is not settled; but whether it has a separate origin, or belongs to the periphery of the cerebro¬ spinal system, the undoubted fact is that the cerebro-spinal system, through the nerves running from it into the trunks of the sympathetic, communicates with all these vital organs; and that even the heart, complete as is its local THE STRUCTURE OF THE NERVOUS SYSTEM. 43 nervous system, is, by tlie vagus or pneumogastric nerve, integrated with, the cerebro-spinal system. A more particular account of tliese and sundry structures of tlie same class is not necessary here. The general fact of significance for us, is, that the brain and spinal cord which through their afferent nerves are put in relation with the actions of the external world, and which through their efferent nerves are put in relation with the structures that react on the external world, are also put in relation with the organs immediately or remotely instrumental in supply¬ ing them with nutriment, and removing the effete matters resulting from their activities. § 16. In the foregoing description I have endeavoured to include all that Psychology needs. Many conspicuous traits of nervous structure which some will think ought to be set down, are really altogether irrelevant. That in the spinal cord the grey matter is placed internally, while in the cerebrum it forms an outside stratum, is a fact of moment in anatomy, but one which throws no light on the science of mind. Knowledge of the truth that the posterior roots of the spinal nerves are afferent, while the anterior are efferent, is all-important to the pathologist; but to the psychologist it is quite unimportant, since this arrangement might have been reversed without the principles of nervous structure being in the least changed; and it is with these principles only that the psychologist is concerned. The leading facts embodying these principles may be summed up thus :— The three great sub-kingdoms of animals in which the nervous system becomes considerably evolved, show us that along with the relatively-increased massiveness distinguish¬ ing the higher types of the nervous system, there goes that other kind of integration implied by increase of structural combination. There is multiplication and enlargement of the parts that unite local nervous centres with general nervous centres. Very frequently there is an approach or 44 THE DATA OF PSYCHOLOGY. clustering of nervous centres that were previously far apart. And there is both a relative and an absolute increase in those centres which have the most multiplied relations with local centres, and through them with all parts of the body. The nervous system is made up of threads inclosed in sheaths, and corpuscles imbedded in protoplasm; of which the threads, united into bundles, constitute almost the whole of the peripheral parts, while the corpuscles with their matrix are found chiefly in the central parts. Having at its outer ex¬ tremity a plexus of highly-unstable matter, a nerve-thread, consisting as we conclude of less unstable matter but matter isomerically transformed with ease, runs inwards, surrounded by substances that shield it from lateral distur¬ bance. Eventually it reaches a mass of highly unstable matter, so conditioned as to undergo decomposition with the greatest facility; and from the place where this lies there run other like fibres to other masses of unstable matter, of the same kind, or of a different kind, or both—here to a portion of substance that contracts when disturbed, and here to a superior centre containing more of the easily-de¬ composed nerve-substance. These threads, afferent, efferent, and centripetal, with their connecting corpuscle or portion of grey matter, we regard as forming the unit of composition of the nervous system. Such units are variously grouped and combined. Each local ganglion is a place where many afferent and many efferent nerves are connected by many portions of the un¬ stable nerve-matter, capable of suddenly giving out much molecular motion. Each superior ganglion is a place where centripetal and centrifugal fibres from such local or inferior ganglia, are similarly connected by similar matter. And so with still higher ganglia in their relations to these. From which principle of combination it results that the possibili¬ ties of different compound relations increase as fast as the centralization progresses. We saw, however, that this establishment of more THE STRUCTURE OF THE NERVOUS SYSTEM. 45 numerous, more involved, and more varied relations among the parts of the organism, implies not simply this grouping of fibres and this arrangement of centres; but also a mul¬ tiplication of the nerve-corpuscles, or portions of grey matter, occupying their centres. And we found it to follow that where the compound relations formed are among many points, or where the points are to be combined in many orders, or both, great accumulations of grey matter are needed: an important corollary being that the quantity of this matter capable of giving out much motion, increases in proportion as the combinations formed become large and heterogeneous. Passing to the special nervous structure related to that special Psychology of chief importance to us, we saw that the spinal cord is a series of partly dependent, partly independent, double nerve-centres; each concerned with a particular portion of the trunk or a particular limb, to the skin, muscles, and vessels of which it sends nerves. The enlarged cephalic extremity of the spinal cord, the medulla oblongata , is a centre connected by centripetal fibres with these partially-differentiated inferior centres; and receiving, as it also does, directly or indirectly, nerves from the special sense-organs, the medulla oblongata is a centre where the local centres concerned with nearly all parts of the body, are brought into communication. We saw, lastly, that the two great bi-lobed masses overlying the medulla oblongata and the sensory ganglia, with which they are in¬ timately connected, may be regarded as centres in which these compound connections are united into connections still more compound, still more various, and still more numerous. One further fact which it remained for us to note, was that while the more important nervous structures are those which bring the parts that are acted upon by the outer world, into relation with the parts that react upon it, there are also nervous structures that bring all these into relation with the vital organs: so serving to unite the parts which expend, with the parts which accumulate and distribute. CHAPTER III. THE FUNCTIONS OF THE NERVOUS SYSTEM. § 17. When, at the outset, we inquired wliat are the manifestations with which the nervous system is associated, we necessarily, in drawing a conclusion, asserted in general terms the part performed by the nervous system. And though in the chapter just ended the sole aim has been to describe nerve-threads, nerve-cells, nerve-trunks, nerve- centres, and the ways in which they are put together; yet the ends subserved have unavoidably, from time to time, come into view. Structure and function are in our thoughts so intimately related, that it is scarcely possible to give a rational account of the one without some tacit reference to the other. Here, however, function is to be our special topic. Having seen how the nervous system is constructed, we have now to see how it works. The proposition with which the first chapter ended was that nervous evolution varies partly as the quantity of motion generated in the organism, and partly as the com¬ plexity of this motion. Here the initial inquiry must be, how the nervous system serves as at once the agent by which motions are liberated and the agent by which motions are co-ordinated. Three things have to be explained :—1. What are the causes which on appropriate occasions determine the nervous system to set up motion ? 2. By what process THE FUNCTIONS OF THE NERVOUS SYSTEM. 47 does it liberate tbe insensible motion locked up in certain tissues, and cause its transformation into sensible motion ? 3. How does it adjust sensible motions into those com¬ binations, simultaneous and successive, needful for efficient action on the external world ? These questions cover the whole of its functions; or, at any rate, all those of its functions with which we are directly concerned. We have to interpret its passive function as a receiver of disturbances that set it going; its active function as a liberator of motion; and its active function as a distributor or appor- tioner of the motion liberated. Probably it will be thought that there is here introduced a function distinct from those before named. It seems that the receiving of disturbances, or stimuli, can be included neither under the head of disengaging motions nor under the head of co-ordinating motions. But on reducing the facts to their lowest terms, and to those terms which Physiology proper can alone recognize, the difficulty disappears. For all nervous stimuli are motions, molar or molecular; and the function of co-ordinating motions comprehends not simply the combining and apportioning of the motions ex¬ pended, but also the combining of the motions received, and the adjustment of the one set into harmony with the other. A moment's thought justifies this proposition. The stimuli to the nerves of touch are sensible motions of the imbedding tissue, caused either by the impacts of external moving bodies or by motions of the organism which bring it against external bodies, fixed and moving. The auditory nerve receives the motions conveyed to it from masses of matter that are vibrating. Those minute agents that termi¬ nate the nerves of the retina are acted on by luminiferous undulations—motions of the ethereal medium which produce motions among their molecules. So, too, the nerves excited by sapid and odorous substances, are, in fact, excited by the molecular movements these substances cause in their extre¬ mities by chemically changing them. Thus, speaking not 48 THE DATA OP PSYCHOLOGY. figuratively but literally, an afferent fibre of whatever kind is a recipient of motion given to its molecules: either by molar motion, as when a blow is received; or by the motion of other molecules, as when there is contact with a chemically-active body; or by those ethereal molecular motions which constitute radiant heat and light. It will be well to consider more fully this sub-division of nervous functions, and the reasons for here proceeding upon it. § 18. Physiology is an objective science; and is limited to such data as can be reached by observations made on sensible objects. It cannot, therefore, properly appropriate subjective data; or data wholly inaccessible to external observations. Without questioning the truth of the as¬ sumed correlation between the changes which, physically considered, are disturbances of nerves, and those which, psychically considered, are feelings; it may be safely affirmed that Physiology, which is an interpretation of the physical processes that go on in organisms, in terms known to physical science, ceases to be Physiology when it imports into its interpretations a psychical factor—a factor which no physical research whatever can disclose, or identify, or get the remotest glimpse of. The relations between nerve- actions and mental states form a distinct subject, to be dealt with presently. Here we are treating of nerve-ac¬ tions on their physiological side, and must ignore their psychological side. Doing this, we have no alternative but to formulate them in terms of motion. And having recognized the primary division to be that between the liberation of motions and the co-ordination of motions, we find that this last division must be sub-divided. It includes, first, the co-ordination of the motions received with one another; and, second, the co-ordination of the motions expended with the motions received, and with one another* Hence results a generalized TIIE FUNCTIONS OF THE NERVOUS SYSTEM. 49 idea of nervous functions, as divisible into recipio-^iotor, libcro-motor, and dirigo-motor . It must be admitted that in tlieir higher forms, these functions are so entangled that a tripartite division of them is difficult, if not impossible. To the simplest types of nervous structure, the classification is easily applied: each afferent nerve is a recipio-motor agent; each ganglion is a libero-motor agent; each efferent nerve is a dirigo-motor agent. But in complex nervous systems, formed of inferior and superior centres connected by parts containing nerves that are centripetal, centrifugal, and commissural, there arise corresponding secondary functions which greatly obscure the primary functions. It remains true that all the afferent nerves are receivers of motions, and that all the efferent nerves are directors of motions; and it remains true that the vesicles and portions of grey substance throughout the centres are liberators of motions; but of the fibres largely composing these centres we must say that their functions are both receptive and directive. Nevertheless, we shall be considerably helped by thinking of the afferent nerves as recipio-motor and the efferent nerves as dirigo-motor ; while we think of the nervous centres as composed of libero-motor elements along with elements that perform both the other functions. This general conception has now to be made specific. In dealing with functions we will follow the same order as we did in dealing with structures—we will consider first the offices of the different kinds of nervous matter. § 19. The grey substance and the white substance—or, to speak more strictly, the nitrogenous matter in and around the vesicles and the nitrogenous matter occupying the centres of the nerve tubes—have not absolutely distinct duties. Certain simple animals yield evidence that in the rudimentary nervous system, there is no such structural differentiation and consequently no such functional differ- E 50 THE DATA OF PSYCHOLOGY. v entiation; and there is proof that even in the highest animals the differentiation is incomplete. On the one hand the vesicular substance, having for its chief office to give out molecular motion when disturbed, has also a considerable power of conveying or conducting molecular motion. When the fibrous parts of the spinal cord have been cut, it is found that if the central columns of grey matter remain uncut, or if there remains even a narrow link to maintain the continuity of the grey matter, disturb- . ance is still communicated through it to the brain: not, in¬ deed, disturbance of any special kind, but disturbance of the most general kind. True, it does not follow that such dis¬ turbance passes along the grey matter from end to end. Throughout the whole length of the spinal cord, nerve-fibres divested of their medullary sheaths enter into and afterwards issue from the grey matter; and, again protected by their sheaths, proceed upwards to the brain in the surrounding white matter. Very likely these take up and convey molecular disturbances set up in the grey matter imbedding them. But even this implies that disturbances are propagated to some extent through the grey matter; and the argument requires ,no more. Conversely, it is found that the matter forming the “ axis- cylinder/'’ or essential nerve-thread, can do something more than transmit molecular motion. It has a certain power of simultaneously giving out molecular motion: so sharing the property of the vesicular matter. When a nerve is irritated not far above its termination in a muscle, the effect is but small. If the irritation is at a point further removed from the muscle, the effect is greater. And the effect increases as the length of nerve through which the •disturbance is conveyed increases. From this we must infer that besides the molecular motion received and transferred, there is molecular motion liberated in the nerve-fibre itself. Not that this molecular motion, like that which the vesicular matter yields up, implies an equivalent decomposition. Pro- THE FUNCTIONS OF THE NERVOUS SYSTEM. 51 bably it is a concomitant of the isomeric transformation propagated through a disturbed nerve, and serving to con¬ vey the disturbance. Some such accompanying result is to be inferred, cv priori, if the conduction is effected by isomeric transformation, or by any kind of molecular re-arrangement. When the molecules of a mass change from one form of combination to another, either absorption or liberation of motion is sure to occur. That there cannot in this case be absorption of motion is manifest; since that would involve a proportionate resistance to the transfer—the amount of force or motion received by the extremity of the nerve, would quickly be used up in transforming the adjacent part of the nerve, and the change would travel but a little way. Being thus obliged to infer that motion is liberated, we at once see whence nerve-fibre derives the power to increase the disturbance it conveys; since each portion, while passing on the wave of molecular motion, adds the molecular motion given out during its own transformation. This action may be rudely symbolized by the transfer of sensible motion along a row of bricks on end, so placed that each in falling knocks over its neighbour. For if instead of bricks which stand on tolerably broad ends and require some force to overturn them, we suppose bricks that are delicately balanced on narrow ends.; and if we further suppose them so constituted that they do not dissipate motion by per¬ cussion or friction; we shall see that the motion transmitted will accumulate. Each brick, besides the motion it re¬ ceives, will pass on to the next the motion which it has itself gained in falling. The general truths to be carried with us are, that in its primordial undifferentiated state, nerve-matter unites the properties of giving out molecular motion and convey¬ ing molecular motion ; but that with the advance of evolution, it becomes specialized into two kinds, of which the one, collected together in masses, has mainly the function of giving out motion, though it can still to some extent con- E 2 THE DATA OF PSYCHOLOGY. V> 'J ml duct it, while the other, collected together in threads, has mainly the function of conducting motion, though, it can slid to some extent give it out. § 20. The co-operation of these differentiated kinds of nerve-substance, having differentiated functions, is seen in its simplest form where they are combined into what was before described as the unit of composition of the nervous system. An afferent nerve, changed by a touch at its outer end, and traversed by a wave of isomeric transformation that gathers strength as it goes, communicates this wave to the comparatively large mass of unstable matter connected with its inner end. The shock of molecular disturbance, im¬ mensely increased by the decomposition set up in this unstable matter constituting a ganglion-corpuscle or its matrix, diffuses itself around, but takes mainly the shape of a relatively-powerful wave of isomeric transformation along the efferent nerve. And the efferent nerve being distributed at its other end among the fibres of a muscle, this powerful wave sets up in them an isomeric transformation of another kind, resulting in contraction (Principles of Biology, § 303). The belief that these are the offices of the respective parts, is borne cut by those peculiarities of structure which were described as occurring in the afferent fibres of certain special sense-organs. We saw that the outer ends of the optic- nerve, the auditory nerve, and the olfactory nerve, are alike characterized by the presence of vesicular matter; and that while in this they differ from the outer ends of the nervee of touch, they also differ in being excessively sensitive. If grey matter, or the matter of vesicles, has the function of immensely multiplying any molecular motion it receives, and passing on the augmented wave of change along connected fibres, we at once have a satisfactory explanation of these peculiar peripheral structures. Take as an example the retina. One of the minute cones in its sensitive layer, measuring not j^r 0 th of an inch in diameter, has its com- THE FUNCTIONS OF THE NERVOUS SYSTEM. 53 ponent matter changed by the etlierial vibrations emanating from a candle in a cottage-window at a great distance. The infinitesimal impact received from so faint a ray, may well be supposed insufficient to send through a considerable length of afferent nerve,, an adequately-rapid wave of molecular change; but this wave, after passing through an extremely delicate fibril less than ^ of an inch in length, comes to a layer of ganglion-corpuscles, with one of which we may presume that it unites. In this the minute disturbance sets up destructive molecular change—unlocks a considerable amount of molecular motion; and thus greatly augmented, the wave of transformation traverses the remainder of the afferent nerve without that loss of time that would result had it to gain strength by a series of increments, starting from an infinitesimal first term. How such appliances for multiplying action co-operate in these cases where the initial action is excessively minute, may be illustrated by certain artificial appli¬ ances that co-operate in an analogous manner. A man with a hair-trigger pistol in his hand, puts its muzzle to the end of a train that runs to a powder-magazine. The slightest pressure on the trigger liberates a spring, and this drives down the hammer. Here is something like the external multiplier which, as we have seen, habitually intensifies the action that falls on the end of an afferent nerve. The propelled hammer explodes the unstable detonating powder in the cap ; thus playing a part com¬ parable to that of the concentrated pencil of light, which causes decomposition in one of the minute sensitive rods or cones of the retina. The explosion of the cap explodes the powder in the pistol: a change that may symbolize the setting up of decomposition in an adjacent ganglion-cell by a disturbed retinal element. The flash from the mouth of the pistol fires the train, which, carrying the flame onwards, blows up the magazine; and this serves to illustrate the action of the partially-decomposed ganglion-cell which pro- 54 THE DATA OF PSYCHOLOGY. pagates a sliock through the afferent nerve to a large de¬ posit of unstable matter in the optic centre, where an immense amount of molecular motion is thereupon disengaged. The joint action of an afferent fibre, its centrally-seated ganglion-corpuscle, and the connected efferent fibre, is com¬ monly known as a reflex action. The name indicates the general truth that the disturbance in travelling from its place of origin to the place where its effect is seen, passes through a point at which its course is bent or reflected; and in so far as it describes this very general trait the term is a good one. But if the foregoing interpretation be correct, the term is in other respects objectionable. On the one hand, it implies as essential what is non-essential. That the wave of disturbance makes a sudden turn at one part of its course, is a fact of no intrinsic moment—is merely a con¬ comitant of the fact that the nerves it traverses have to be put in communication with other nerves, and that points of junction imply angles. On the other hand, it leaves out of sight the fact that one of these points of junction from which the wave of disturbance is said to be reflected, is a place at which it is greatly augmented; and that this augmentation of the wave is the all-important office of the matter lying at the point of junction. § 21. Remembering that bundles of such afferent nerves are joined to bundles of such efferent nerves, by clusters of such corpuscles imbedded in the grey matter of a ganglion, and that bundles of centripetal nerves proceed thence to higher ganglia; we have next to consider the functions of these structures as wholes. A nervous centre, even of an inferior order, is not simply a place where afferent nerves are severally linked with their corresponding efferent nerves, by corpuscles or portions of grey matter that multiply and pass on disturbances; nor is the only further office it serves that of sending to higher ganglia., portions of these disturbances; but it is also a THE FUNCTIONS OF THE NERVOUS SYSTEM. 55 place where more involved communications are effected. For in all ganglia save,, perhaps, the very simplest, the corpuscles or vesicles give off processes more or less numerous, and usually more or less branched; and these branched processes, spreading through the matrix of grey matter, may be assumed to propagate in various directions, and various degrees, the disturbance set up in the corpuscle. This diffusion of liberated molecular motion has two im¬ plications. First, the number and complexity of the cor¬ related changes produced by the original change, increase with the multiplication and variety of these processes and their connexions. And, second, along with increase in the number of correlated changes, there goes increase in the total quantity of molecular motion given out, directly or indirectly. Fully to understand the importance of this last implica¬ tion, it is needful to refer back to Fig. 4, and to the accom¬ panying description of the way in which a nervous centre that serves to establish the various possible relations among different points in an organism, must contain a large accu¬ mulation of these connecting and multiplying links; and where it was shown how immense must become the accumu¬ lation of vesicular matter in a centre that has the office of establishing relations among these many parts in various orders. For it will be seen that as fast as the connexions become numerous and complex, so fast will enlarge the crowds of these connecting corpuscles and multipliers of disturbance which simultaneously come into action. And hence the quantity of molecular motion evolved in the nervous centres will become great in proportion as the nervous relations increase in integration and heterogeneity. When we see how the arrangements for liberating and multiplying motion, described under their simple form in the last section, are thus compounded—when, recurring to our simile, we see how the first central magazine of force exploded, communicates with other larger magazines, and THE DATA OF PSYCHOLOGY. 5tf these again with still larger, which are subsequently ex¬ ploded; we shall be at no loss to understand how the slightest impression on one of the recipio-motor nerves, may evoke from the libero-motor centres a relatively-in- commensurable amount of force, which, discharged along the dirigo-motor nerves, may generate violent muscular contractions. So that, to take a case, a slight sound may produce a convulsive start of the whole body; or an un¬ expected motion of some adjacent object, infinitesimal as is the modification it produces in the retina, may neverthe¬ less cause an involuntary jump and scream. § 22. In treating nervous functions in general, I have unawares ended with illustrations from the nervous func¬ tions of human beings: so coming to the division of the subject on which we have next to enter. For the brief account given in the last chapter of the special nervous structures with which we are most concerned, must here be supplemented by a brief account of their special functions. If we leave out such afferent and efferent fibres as pass through the spinal cord to and from the encephalon, and also those centripetal and centrifugal fibres which connect its various parts with the encephalon, we may regard the partly dependent and partly independent centres composing the spinal cord, as being co-ordinators of the actions performed by the skin and muscles of the trunk and limbs. A large proportion of these actions, including many of considerable complexity, the spinal cord is able to co-ordinate without aid from the higher centres; and some of the partially- differentiated centres composing the spinal cord, are able to effect simple co-ordinations without aid from the rest. We will glance at these simple co-ordinations first. If a patient paralyzed by some injury of the spinal cord that has left the lumbar enlargement intact, has his foot touched, the leg is quickly withdrawn ; not only without a cerebral act, but even without his brain being in any way affected. THE FUNCTIONS OF THE NERVOUS SYSTEM. 57 unless indirectly by tbe shaking of the bed. Thus the branched corpuscles and fibres contained at that point in the cord with which the afferent and efferent nerves of the leg are connected, have at once the function of giving out, when the disturbance is communicated to them, the requisite quantity of molecular motion, and of so directing this to the respective muscles of the leg, as to cause the appropriate movement. More involved co-ordi¬ nations are effected by the co-operation of several such centres, or portions of the grey substance, contained in adjacent parts of the spinal cord. In the human subject demonstration of this is not easy; but it is shown by ex¬ periments on inferior Vertebrata. A decapitated frog that has its side irritated, will bring the hind foot of that side to the spot, and move it so as to displace the irritating object. Even something further is done. If a scalpel be applied to the skin between the hind legs, these act jointly in such a manner as to push away the scalpel. The explana¬ tion is that by commissural fibres, transverse and longi¬ tudinal, the disturbances conveyed to particular centres, are communicated to sundry adjacent centres; and through their efferent nerves these direct and appor¬ tion the multiplied disturbances among a great variety of muscles. How such definite co-ordinations as these are effected by such an apparatus, we shall better understand on remembering that the relations between positions on the skin and the movements needed to bring the extremities to touch them, are tolerably constant. A frog's hind foot can reach a given point on the frog's side, only by one particular muscular adjustment; or, at any rate, by a muscular adjustment that varies within narrow limits. And since in all frogs, generation after generation, the pro¬ portions of parts, and therefore the relations of muscular adjustments to given positions, remain practically the same; it becomes comprehensible how, through the organized nervous connections that arise, a touch at any point may 58 THE DATA OF PSYCHOLOGY. cause the combined contractions needful to bring the end of the limb to that point. It should be observed here, that the conception of these acts of the spinal cord as co-ordinations of motions, is incomplete so long as the only motions contemplated are those of the muscles. Under the head of motions must be included the disturbances con¬ veyed along the afferent nerves; for the muscular motions are so adjusted that their joint results have special rela¬ tions to these received disturbances. The co-ordination is between the recipio-motor acts and the dirigo-motor acts. We may, then, regard the spinal cord as a centre of co-ordinations which, though some of them have considerable complexity, are yet relatively simple—simple, inasmuch as the disturbances received from the skin are much alike from all parts; simple, inasmuch as each muscular adjustment is mainly of a fixed or invariable kind; and simple, inasmuch as the component acts of the co¬ ordinated group are practically simultaneous. That enlarged and differentiated part of the spinal cord called the medulla oblongata, including the root-portion of the pons Varolii , adnate with it and structurally so entangled that the two cannot be demarcated, we may roughly distinguish as a centre of compound co-ordination. It receives directly the auditory impressions, the impressions of taste, and, indirectly through the corpora quadrigemina , is affected by visual impressions : meanwhile sending impulses to the various muscles of the eyes, the face, the jaws, and the mouth. By it the movements of all four limbs are com¬ bined in joint acts; and by simultaneously regulating them, it makes the head and jaws co-operate with the limbs. The various impressions and muscular motions implied by the act of swallowing, it brings into due relation. Receiving the respiratory stimulus, it emits the stimuli to those muscles which enlarge and diminish the thoracic cavity, so causing inspiration and expiration; and, as a consequence, it is the centre which, disturbed by the more violent irritations THE FUNCTIONS OF THE NERVOUS SYSTEM. 59 of the respiratory surface, sends out to tlie respiratory muscles those more violent impulses which cause coughing and sneezing: to which may be added, as actions belonging to the same class, crying and yawning. Lastly, through the pneumogastric nerve, it controls the action of the heart, and the actions of other viscera. Thus it is a centre to which come, in some cases directly but in most cases in¬ directly, impressions from all parts of the external surface, as well as from the mucous lining of the mouth, oesophagus, and lungs; and to which there also come, directly or indirectly, impressions received through the higher senses. At the same time the minor centres severally commanding groups of muscles, are by it put in relation with one another; and their respective simple actions so combined as to constitute compound actions. In short it has recipio-motor relations with all the parts that hold con¬ verse with the external world, while it has dirigo-motor relations with all the parts that react on the external world; and its function is that of adjusting the complex movements in obedience to the complex stimuli. This is not all. Being the centre which initiates and directs involved and extensive bodily actions, entailing rapid expenditure, it is the centre in which the demand for materials is indicated; and hence it becomes the regulator of the circulation, of the aeration of the blood, and of the visceral actions generally. Clearly, then, its co-ordinations are compound in comparison with those of the spinal cord—compound, because the impressions which afferent and centripetal nerves bring to it, are not only more numerous but also more heterogeneous; compound, because the impulses which it sends out are also more numerous and more heterogeneous; and compound, because it brings more involved acts into correspondence with more involved stimuli. The functions of the two still higher centres, the cere - helium and the cerebrum, have now to be defined in terms of the same nature. How shall we express them ? Both of these great bi-lobed masses arise as buds out of the GO THE DATA OF PSYCHOLOGY. originally almost-uniform cranio-spinal axis; and as tliey enlarge, their distal portions grow more massive than their proximal portions, so that they end in being pedunculated. Each of them thus bears to the medulla oblongata , a relation like that which the superior ganglion H, in the diagrammatic Fig. 4, bears to the inferior ganglion F ; and we may not unreasonably infer that their functions are analogous to those hypothetically assigned to the ganglion H. That is to say, we may regard them as organs of doubjy-compound co¬ ordination—organs which have for their common function, the re-combining into larger groups, and into countless different orders, the already-complex impressions received by the medulla oblongata; and which have the further function of so arranging the already-complex motor im¬ pulses issuing from the medulla oblongata , as to form those far more involved aggregate actions, simultaneous and successive, which, being adjusted to these involved im¬ pressions, achieve remote ends. The general truth of this definition may, I think, be safely assumed; since it is simply a statement in other terms, of what, in ordinary language, is called intelligent action; which habitually characterizes vertebrate animals in proportion as these centres are largely developed. Thus much being granted, there arises the further question—what are the respective parts played by these two great organs in this doubly-com¬ pound co-ordination ? Much difference of opinion has long existed, and still exists, respecting the particular offices of these supreme ganglia; and especially respecting the office of the cerebellum. Without committing myself to it as anything more than a hypothesis, I will here venture to suggest a not improbable interpretation. The common function of the two being that of co-ordinating in larger groups and in various orders, the impressions and acts co-ordinated in the lower centres, we may fitly ask—• are there any fundamentally distinct kinds of order in which impressions and acts may be co-ordinated ? The obvious THE FUNCTIONS OF THE NERVOUS SYSTEM. Cl answer is, that there are tlie two fundamentally distinct orders of Co-existence and Sequence. All phenomena are presented to us either as existing simultaneously or as existing successively. If, then, these two highest nervous centres, which together perform the general function of doubly-compound co-ordination, take separate parts of this function, as, from their separateness, we must conclude that they do; we can scarcely make a more reasonable assump¬ tion than that the respective orders in which they co¬ ordinate compound impressions and acts, answer to the re¬ spective orders in which phenomena are conditioned. In brief, the hypothesis thus reached a priori, is that the cerebellum is an organ of doubly-compound co-ordination in space; while the cerebrum is an organ of doubly-com¬ pound co-ordination in time. The a posteriori evidence, so far as I have examined it, appears congruous, both with this view of the general function of these centres, and with this view of their respective special functions. There is complete harmony between the hypothesis and the seemingly-strange facts that these centres may be partially destroyed without causing obvious incapacity, and that they may be wholly removed without destroying the ability to co-ordinate the less complex impressions and acts. As¬ suming, as we may fairly do, that the cells and fibres which subserve the more involved co-ordinations, are successively added at the surfaces of these centres as they develop, it is inferable that the superficial parts may be sliced off with the least-appreciable effects on the actions; and that the effects on the actions will become conspicuous in proportion as the slices destroy the parts nearer to the lower centres: and these are the results established by experiment. Besides finding, as the hypothesis leads us to expect, that these nervous masses are relatively large in all creatures capable of adjusting their involved and continuous actions to complex and distant environing agencies; we also trace some relation between the development of each and the 62 THE DATA OF PSYCHOLOGY. peculiar capacities of the species. There is, for instance, the fact that the cerebellum is unusually developed in birds of prey, which have to co-ordinate with great accuracy the relations of distance, direction, and complex form, as well as very precisely to co-ordinate the involved movements ap¬ propriate to these involved impressions. And there is, on the other hand, the fact that the cerebrum predominates in creatures showing, like ourselves, the power of adapting, throughout long periods, concatenated compound actions to concatenated compound impressions.* Of course this classification of the functions of the nervous centres, as co-ordinations that are simple, compound, and doubly compound, must be taken as merely approximate. No definite divisions can be made. The functions arise through increasing complications; and these general con¬ trasts become conspicuous only when we look at the facts in their main outlines. Here, however, where the * Let me here draw attention to papers in the Medical Times and Gazette , for December 14 and December 21, 1867, in which Dr. Hughlings Jackson has published some facts and inferences that quite harmonize with these interpre¬ tations, in so far as the common function of the great nervous centres is concerned. It should be remarked that the above-proposed definitions, are, to a con¬ siderable extent, coincident with current conceptions. The cerebrum is generally recognized as the chief organ of mind ; and mind, in its ordinary acceptation, means more especially a comparatively intricate co-ordination in time —the consciousness of a creature “looking before and after,” and using past experiences to regulate future conduct. In like manner the function ascribed to the cerebellum in the foregoing paragraph, partially agrees with that which M. Flourens inferred from his experiments. It differs, however, in two respects. It implies that the cerebellum is not an organ for the co-ordi¬ nation of motions only, or of synchronous motions only; but that it is also an organ for the co-ordination of simultaneous impressions, and for the co-ordi¬ nation of the synchronous motions in adaptation to the simultaneous impres¬ sions. And it further implies that not all simultaneous impressions and adapted synchronous motions are co-ordinated by the cerebellum ; but only the doubly-compound ones, which have for their external correlatives the intricate combinations of attributes that distinguish objects from one another, and the more multiplied and varied localizations of objects in the space that extends beyond the immediate limits and reach of the organism. THE FUNCTIONS OF THE NERVOUS SYSTEM. G 3 object is to give an idea of the principles of nervous func¬ tion in its successive stages of evolution, detailed qualifica¬ tions do not concern us. § 23. A few words are due to the functions of tliat sub¬ ordinate nervous apparatus, tlie structure of which we glanced at in the last chapter—the nervous apparatus pre¬ siding over the vital processes. It will suffice if we take the functions of the vaso-motor division of it as exemplifying the whole. Each vaso-motor nerve, having roots in both the cerebro¬ spinal system and the sympathetic system, conveys to all branches of the artery it accompanies, the impulses arising from the activities of the great nervous centres and muscles, as well as from the activities of the viscera. Probably the ordinary amount of disturbance propagated along each vaso¬ motor nerve, simply excites the muscular coats of the adja¬ cent artery sufficiently to maintain its due elasticity. But stronger disturbances produce marked alterations of its calibre: those brought by the sympathetic fibres being said to cause contraction; and those brought by fibres from the cerebro-spinal system being said to cause dilatation. Some of these changes have relation to actions going on in the part itself; and others to actions going on in the chief vital organs, or in the body as a whole. But all of them show us that by means of the vaso-motor nervous system, the blood-vessels are so regulated as to subserve general and local needs. One further fact belong¬ ing to this class may be added; partly because of its in¬ trinsic interest, and partly because it illustrates certain supplementary nervous functions not hitherto named. We have already seen that, among its many duties, the medulla oblongata controls, through the medium of the pneumogastric nerve, the action of the heart. So long as the disturbance conveyed to the medulla oblongata, either from the periphery of the nervous system or from its great Gi THE DATA OP PSYCHOLOGY. centres, does not exceed a moderate amount, the resulting waves of molecular change sent by it through the pneumo- gastric, do not interfere with the heart's action—perhaps en¬ force it. But when the medulla is excessively disturbed, the increased quantity of stimulus it sends, either diminishes the action of the heart, or stops it altogether: so causing arrest of the circulation and consequent insensibility. Noting, as we pass, that this is one of the most remarkable forms of that co-ordination which the nervous system everywhere effects, since the arrangement is such that when the nervous system becomes abnormally active, and its chief centres sur¬ charged with blood, they themselves arrest the organ which propels blood to them; we have to ask how it happens that in this case the propagation of disturbance through a nerve checks action instead of causing it. The reply is that in addition to the systems of nerves which excite action, there is found to exist a system of nerves which diminish action— inhibitory nerves as they are called. Through these it is alleged that the brain controls the spinal cord—restraining those reflex movements which, when connection with the brain is cut off, become so much more marked. And through one of these it is concluded that the medulla oblongata reins in the heart, when the cerebral irritation is excessive. Be this as it may, the facts named illustrate the way in which the nervous system, while it co-ordinates the external actions, also co-ordinates those internal actions which make them possible. The reader has but to conceive that through other systems of nerves, other organs which absorb, secrete, excrete, &c., are similarly controlled, and he will under¬ stand sufficiently for present purposes, how demand for materials and supply of materials are harmonized. § 24. In summing up the functions of the nervous system PuS thus formulated in terms of motion, it will be useful to observe the greater comprehensiveness of view we obtain by excluding the ordinary implications. THE FUNCTIONS OF THE NERVOUS SYSTEM. 0 £> When one part of a Zoophyte is touched, the contraction set up in that part slowly diffuses itself through the whole body. Two things are here to be noted. There is a propa¬ gation of disturbance through the nerveless sarcode of which the creature is composed; for distant parts are eventually affected. There is also an increase of disturb¬ ance ; for in successive moments the mass of tissue under¬ going change is greater. Thus the relatively-homogeneous substance of these simple animals, exhibits the two essential phenomena exhibited by the nervous system in all phases of its development: there is propagation of molecular motion, and there is a simultaneous augmentation of this mole¬ cular motion. Such essential phenomena grow more con¬ spicuous as the nervous system develops, partly because the changes set up become limited to narrow lines and small masses, and partly because the matter of which these are formed becomes distinguished by an increased degree of the general instability. Since, then, the functions of the nervous system as expressed in terms of molecular motion, are functions exhibited in a vague way by the undifferentiated tissue from which the nervous system insensibly arises; it is clear that by so expressing them we include alike their lowest and their highest forms, which we cannot otherwise do. Moreover, only in these terms can there be given an adequate definition of fully - developed nervous functions. If we admit any subjective element, our definition becomes inapplicable to all those nervous actions which have no sub¬ jective accompaniments—which go on without feelings; and a conception of nervous functions which excludes those of organic life, cannot be a complete conception. On the other hand, the definition of nervous functions as consist¬ ing in the conveyance and multiplication of molecular motions, holds in all cases. It includes equally the conduc¬ tion of an impression made on a nerve of sense, and the excitement of chemical metamorphoses in a gland. F 66 THE DATA OF PSYCHOLOGY. The subdivision of this general function under the above- proposed heads of recipio-motor , liber o-motor, and dirigo- motor , has also the advantage of greater comprehensive¬ ness. No word at present in use expresses the office which afferent nerves have in common, more specifically than the word afferent itself expresses it. Whether disturbance of its outer end produces in an afferent nerve a change causing a reflex contraction, or whether it produces a change causing what we call a sensation, is a circumstance of secondary import; as is proved by the fact that by use the last may become the first. The essential thing common to the two, is that molecular motion is propagated from periphery to centre. So, too, is it with the liber o-motor functions. Whether, as in the ganglia of the sympathetic, the multipli¬ cation of communicated disturbance has no subjective con¬ comitant, or whether, as in the cerebrum , it has a subjective concomitant, there is in both cases a liberation of molecular motion; and this, being the common character of the changes in nerve-centres, must determine the definition of their common function. In like manner, all efferent nerves, whether conveying disturbances that set up contractions in muscles, or cause constrictions of arteries, or excite chemical transformations in glands, serve to direct the waves of molecular motion—waves that are intrinsically alike in nature, though the results produced by them in the organs to which they are carried differ so widely, and though they are now associated with consciousness and now are not. A more special view of the functions thus classed, dis¬ closes two essential facts. Considered as an agent for generating movements, we see that the nervous system acts by liberation of successively-larger amounts of molecular motion in the centres successively disturbed. A very small change at the outer end of an afferent nerve, sets up a relatively-large quantity of change in some adjacent un¬ stable nerve-matter; whence the change, thus increased, is propagated to some internal ganglion; to be passed on by THE FUNCTIONS OF THE NERVOUS SYSTEM. G7 it immensely multiplied as before; until there is unlocked an amount of disturbance capable of causing muscular con¬ tractions throughout the whole body. Meanwhile these centres in which molecular motion is liberated, are also the centres in which it is co-ordinated ; and the successively higher and larger centres which evolve successively larger quantities of molecular motion, are also centres in which successively more complex co-ordinadions are effected. Whence follows the general result that along with each further development of the nervous system, enabling it to make all parts of the body work together more efficiently in simultaneous and successive actions, there goes an increased power of evolving the energy re¬ quired for such larger aggregates of actions. These principles w'e found to be well exemplified in the case which most nearly concerns us. It is needless to re-state the results so recently arrived at. One remark, however, may be added. In the functions of the suc- eessively-higher vertebrate centres, reaching their climax in the human being, we see well exemplified the law of development of functions in general (First Principles, Part II. § 142). This progress from co-ordinations that are small and simple to those that are larger and compound, and to those that are still larger and doubly compound, is one of the best instances of that progressive integration of motions, simultaneously becoming more heterogeneous and more definite, which characterizes Evolution under all its forms. CHAPTER IV., THE CONDITIONS ESSENTIAL TO NERVOUS ACTION. § 25. Of these, the first in order is continuity of nerve- substance. Disturbance is not conveyed from end to end of a nerve that has been cut in two; and section of a nerve- centre similarly prevents the transfer of an impulse from one of the dissevered parts to the other. The requisite continuity is not simply the continuity of unbroken contact: there must be continuity of molecular 'Cohesion. Placing in apposition the two ends of a divided nerve, does not re-establish nervous communication. Even when, after a cut, the surrounding flesh has been healed, it is long before the sundered nerve-threads re-nnite so com¬ pletely that they transmit stimuli as well as before. Further, there must be no destruction of continuity by molecular disorganization. Without division of a nerve, and without injury of its sheath, there may result from disease a change which incapacitates the nerve-fibres—an atrophy, or a breaking-up by decomposition: the result being a de¬ rangement of those lines of peculiar nitrogenous molecules which receive and pass on the waves of disturbance. § 26. Nerve-structures, whether peripheral or central, permanently disabled as they are by actual discontinuity, either molar or molecular, are temporarily disabled by dis¬ continuity of molecular equilibrium. Pressure is capable of THE CONDITIONS ESSENTIAL TO NERVOUS ACTION. GO producing re-arrangement of particles, even in substances that are simple and comparatively bard; as is shown by its power of altering the direction of diamagnetic polarity in metals. We may therefore expect that in substances of com¬ plex composition and little cohesion, pressure will readily cause the particles to change their relative positions. Hence there is no difficulty in understanding why nerve-substance, having a balanced molecular structure such that it is ever ready to pass when disturbed from one of its isomeric states to the other, may be so modified by pressure, even when small, as to be incapacitated for undergoing these alternate molecular re-arrangements. Be this as it may, however, the fact is that one of the conditions to nervous action is ab¬ sence of much pressure. In the case of nerve-trunks, demonstration of this general truth is easy. A ligature round a nerve prevents a dis¬ turbance set up at one end of it from producing any effect at the other end. Partial results of this nature are familiar- By external pressure on a limb, the conducting power of the nerves affected is much diminished. That pressure on the centrally-seated tracts of fibres,, hinders or arrests their actions, is shown in every case of paralysis. By a clot of blood that has escaped from a ruptured vessel, or by a quantity of lymph that has oozed through the walls of capillaries over-distended, bundles of fibres at the base of the brain, or in the spinal cord, are un¬ duly squeezed; and if afferent or centripetal fibres they cease to bring disturbances from the periphery, while if efferent or centrifugal fibres they cease to convey impulses to the muscles. The like is true of nerve-centres as wholes. Indeed pres¬ sure appears to be a greater hindrance to their actions than to the actions of nerve-trunks. That certain forms of the- abnormal arrest of nervous action called coma, are due to excessive congestion of the blood-vessels of the encephalon, seems possible; but as some question this interpretation wc 70 THE DATA OP PSYCHOLOGY. cannot safely base an inference on it. There is, however, one conclusive piece of evidence. A fracture of the skull that causes indentation over a considerable area, and leaves the bone intruding on the space previously filled by the brain, stops the functions of the brain—disturbances sent to it call forth no appropriate co-ordinated motions, and, indeed, no motions at all. But when, by means of a trephine, the depressed portion of bone is cut out, the brain, relieved from pressure, at once resumes its duties. Further support is yielded by what may be regarded as converse evidence. If excess of pressure arrests nerve- action, and if the normal amount of pressure allows the normal amount of nerve-action; then it is inferable that nerve-disturbances will pass with undue facility if the pres¬ sure is deficient. Now as the brain is contained in an al¬ most-closed chamber which cannot collapse, it follows that if the cerebral blood-vessels are rapidly drained, the masses of nerve-fibres amid which they ramify, being subject to less pressure than usual, will allow waves of molecular change to pass with unusual facility; and ordinary impressions pro¬ pagated to the centres, will produce extraordinary motor impulses. Hence the seemingly-anomalous fact that great loss of blood, or great local anaemia caused by stoppage of a cerebral artery, causes convulsions. Such a result may be anticipated as the first result, before innutri¬ tion begins to tell; though innutrition will afterwards cause prostration or paralysis. And this is the order in which the phenomena actually occur. The like appears true of the peripheral nervous system. The afferent nerves of individuals who, though otherwise healthy, have lax tissues, are often unduly impressible. And there are instances of undue local impressibility which, I think, admit of this interpretation. It has been found that an arm rendered anaemic by unnatural constriction of its arteries, thereby reduced in temperature and beginning to atrophy, may nevertheless have its afferent nerves affected by electrio THE CONDITIONS ESSENTIAL TO NERVOUS ACTION. 71 discharges in an unusual degree.* Deficient pressure on the nerve-trunks appears a possible cause of this otherwise strange result. § 27. Proof that heat kept above a certain level is a con¬ dition to the maintenance of nervous action, is difficult to disentangle from proof that the maintenance of nervous action depends on a due circulation of blood; for the one condition is usually but a concomitant of the other. Never¬ theless, there is reason to infer that a supply of free mole¬ cular motion is requisite, apart from a supply of nutritive materials. The general fact that cold-blooded animals are com¬ paratively inactive, admits of the interpretation that their low temperature is due to their inactivity, as well as to the interpretation that their inactivity is a consequence of their low temperature; for the two act and react in such a way that neither can properly be called the cause of the other. But reptiles which remain quiescent in cold weather, and become active when they are warmed by the summer’s sun, yield us good evidence. Though it may be alleged that their greater activity arises from accelerated circulation and aeration of the blood, yet as the heart and lungs are set going by their respective nervous centres, we must infer that the warming of these nervous centres by external heat, is the initial change in these animals that have but little power of producing heat by their own actions. In support of this interpretation may be cited the converse fact. When active creatures, capable under ordinary conditions of generating enough heat within themselves, are exposed to conditions under which they lose heat faster than they make it, their nervous actions decrease, and they eventually cease to move. In hybernating mammals we see * I am indebted for this fact to Dr. Bastian, who observed it in one of his own patients. 72 THE DATA OF PSYCHOLOGY. an annual recurrence of this relation of cause and effect; and in mammals tliat do not hybernate, as in ourselves, it equally holds that prolonged exposure to extreme cold de¬ presses nervous action, causing strong tendency to sleep, and that death results if the bodily temperature is allowed to fall still lower. That local loss of heat when carried far, is followed h 7 local inaction of the nerves, is shown by the fact that parts of the body that have been greatly cooled down, naturally or artificially, may be pricked or cut or pinched without any of the usual disturbances being conveyed to the nerve- centres. It is true that where the refrigeration is extreme, there is usually a partial deprivation of blood; but there is evidence that when this is not the case—when, indeed, the blood-vessels are congested, as in red hands on a winter's day, loss of heat entails decrease of nerve-function. That the like holds of the respective centres, is shown by the use of cold as a therapeutic agent: ice to the head being pre¬ scribed when there is excessive cerebral action, and ice to the spine being a means of diminishing reflex excitability. It is worth remarking that this dependence of nervous action on supply of heat, yields indirect support to the views set forth in foregoing chapters. If, as was inferred, the disturbance conveyed along a nerve-thread is an isomeric change, during which some molecular motion is yielded up by each molecule as it passes on the accumulated wave to its neighbour—if resumption of the previous isomeric state implies an equivalent absorption of molecular motion from surrounding matter; then, in proportion to the heat of surrounding matter, will be the rapidity with which the nerve-fibre, resuming this previous isomeric state, becomes fit to transmit another wave of molecular change. § 28. That nerves and nerve-centres act only so long as they are furnished with those materials which the blood¬ vessels bring them, is a familiar truth. The quantity of THE CONDITIONS ESSENTIAL TO NERVOUS ACTION. blood present in any part, and tlie rapidity with which fresh blood is propelled to the part, both affect the degree of nervous activity in the part. General depletion is a cause of nervous inaction: if the total quantity of blood in the body is much diminished, the great nervous centres are the first organs to feel the change. Temporary loss of blood produces fainting, or sudden arrest of cerebral action; and permanent deficiency of blood is accompanied by debility, which implies a decreased nervous discharge. Supposing that no blood has been lost, in¬ sensibility nevertheless instantly results if the heart ceases to supply the brain with fresh blood in place of the blood it contains. Or if there is chronic feebleness of the heart's action, there is proportionate diminution of nervous power. Where the total quantity of blood is adequate and the heart is not in fault, local nervous function may still be hindered by local anaemia, resulting from aneurism in an artery, or from what is called an embolism—a plugging up of an artery with coagulated blood. Thus paralysis is caused by embolism of the cerebral blood-vessel which supplies the highest part of the motor tract. The converse facts similarly imply this same general relation. When, other conditions being normal, a nerve-centre is highly charged with arterial blood, it responds with unusual rapidity to the disturbances it receives; and evolves more than ordinary amounts of force, shown in secondary nervous changes, or in muscular motions, or both. Supposing, again, that there is no hyperasmia of a nervous centre, it will still happen that if the heart propels blood to it with unusual rapidity, its libero-motor function will be exalted. At the periphery of the nervous system, like variations of efficiency follow like variations of circulation. A reduction in the quantity of blood present, caused by constriction of the vessels, is probably one cause of the decreased nervous impressibility in a part that is exposed to cold ; and to the same cause is perhaps to be ascribed some of the comparative THE DATA OF PSYCHOLOGY. n frluggishness with which tlie muscles of the part respond to motor impulses. If instead of local lack of blood there is retardation or stoppage of the local current of blood, the nerves of the part similarly become incapacitated in a pro¬ portionate degree: instance the blindness that results from blocking up the central retinal artery; or instance the gradual disappearance of impressibility in a region of the skin that has had its supplying blood-vessel tied. Con¬ versely, excess of blood around the peripheral nerve-fibres, causes unusual excitability of them. A gentle touch on the skin in its normal state, sends through the afferent nerves a disturbance so small as to call forth from the central organs scarcely any response; but where the skin is highly inflamed, a like touch affects them so much that the disturbance, when reflected from the central organs, produces a start of the whole body. If in addition to local excess in the quantity of blood there is an accelerated flow of blood, a still greater exaltation of local nervous action follows. It is a familiar truth that, other things remaining the same, an inflamed part is made more irritable by anything which increases the action of the heart. § 29. Nervous action depends not alone on the quantity of blood supplied but also on its quality—on the proportion of the needful elements contained by it. General rather than special warrant must suffice for this proposition. Little is known about variations in the consti¬ tution of the blood; and still less about the relations between these and variations of nervous activity. That a blood greatly impoverished, as in dropsical persons (whose tissues become infiltrated because the thin serum passes too easily through the walls of the capillaries), is accompanied by ener¬ vation, is pretty clear; and we can scarcely be wrong in concluding that a blood rich in the constituents of nerve- substance, renders possible a great evolution of nerve-force. But there is indirect evidence serving to enforce the scanty THE CONDITIONS ESSENTIAL TO NERVOUS ACTION. 75 direct evidence. For we have abundant proof that by adding certain matters to the blood, unusual amounts of nervous action may be evoked. Alcohol, nitrous oxide, the vegeto- alkalies, and other stimulants, are not, indeed, components of nerve-substance; nor is there any reason to suppose that they can serve in place of components. Probably their im¬ mediate influence is that of setting up or facilitating the change of nerve-substance, and so causing unusual disen¬ gagement of molecular motion. But by showing that the supply of particular substances to the nervous system exalts nervous activity, they make it more manifest that nervous activity must partly depend on the supply of substances which re-build nerve-tissue as fast as action disintegrates it. We must not omit a further qualitative character of a positive kind. The blood must contain oxygen. What is the special action of oxygen—whether it is a direct disinte- grant of the tissues, including nerve-tissue; or whether it simply facilitates by its presence molecular disintegrations otherwise caused; or whether it serves merely to combine with, and carry away, the products of such disintegrations otherwise caused ; are questions about which there are differences of opinion. But there can be no difference of opinion as to the necessity for an oxygenated blood. And opinions can scarcely differ respecting the general relation that exists between the degree of oxygenation and the de¬ gree of nervous activity. § 30. While, for the maintenance of nervous action, it is requisite that certain matters shall be present in the blood, it is also requisite that certain other matters shall be absent; or, to speak strictly, that they shall be present in but small proportions. These are the compounds resulting from de¬ composition of the tissues—the nervous tissue included. The two most important are carbonic acid and urea. If the exhalation of carbonic acid by the lungs is greatly retarded, lethargy ensues: disturbances at the periphery 76 THE DATA OF PSYCHOLOGY. of the nervous system fail to call forth the usual responses. If the exhalation is completely arrested, complete insensi¬ bility is soon produced; followed quickly by arrest of the inferior nervous functions, and consequently of all other functions. And these effects arise still more rapidly if there is an absorption of carbonic acid through the lungs, instead of an arrested excretion of the carbonic acid internally generated. In an analogous but less rapid manner, a decrease and final stoppage of nervous action is caused by an accumula¬ tion in the blood of urea, or of those nitrogenous products represented by it. If the kidneys fail to perform their func¬ tion, or if the waste nitrogenous products which they have separated from the blood are prevented from escaping out of the body, and are re-absorbed; there results a nervous in- action, ending presently in coma and finally in death. § 31. Such, stated as fully as is needful here, are the con¬ ditions essential to nervous action. Qualifications have been passed over; and much evidence has been omitted. In summing up these leading facts which alone concern the psychologist, we may with advantage observe how they harmonize with the general views of nerve-structure and nerve-function set forth in foregoing chapters. All these pre-requisites to nervous action obviously admit of being grouped as pre-requisites to the genesis of molecular motion, and pre-requisites to the conveyance of molecular motion. That molecular motion may be disengaged there must be decomposition; and, therefore, for the discharge of molecular motion to be maintained, decomposition must be facilitated. The quantity of waste being a measure of the quantity of force evolved, it follows that the nervous system requires a good supply, and quick exchange, of blood; since in the blood are brought the matters that favour disintegration. Similarly with respiration, considered as a process of THE CONDITIONS ESSENTIAL TO NERVOUS ACTION. 77 absorbing that oxygen which directly or indirectly aids the metamorphosis. And so likewise with the excre¬ tion of those waste products which hinder the metamor¬ phosis. But perpetual waste must be met by perpetual repair. If its action is to continue, nervous tissue must be re-composed as fast as it is decomposed. Hence the reason why there is needed a blood that is rich in nerve- constituents. Hence the fact that abundant blood must be present wherever there is much nervous action. And hence, also, the necessity for an efficient circulation to replace by fresh blood, the blood that has been used. Equally well do the several conditions essential to the transmission of nervous disturbance, conform to the hypo¬ thesis that the disturbance transmitted is a wave of isomeric change. For if it is, we at once see why there must be not merely contact-continuity of nerve-fibre, but molecular continuity. We are helped to understand how pressure, by deranging that delicate molecular balance which makes possible the alternation of isomeric states, may prevent the passage of nervous discharges. And we are supplied with an explanation of the fact that the presence of free molecular motion or heat, is needful to enable a nerve con¬ tinually to resume its fitness for conveying a wave of change. Before closing the chapter it should be pointed out that these many conditions essential to nervous action, are never all fulfilled at one time in the same degree, but are usually fulfilled in various degrees and combinations ; and that by now conspiring and now conflicting, they produce results that are complicated and often perplexing. Thus, for in¬ stance, substances which directly stimulate the nervous system, are usually substances which retard the exchange of gases in the lungs, and by so doing depress the state of the nervous system; and these conflicting actions, diffe¬ rent in their proportions in different individuals, and in the same individual at different times, often work opposite effects, or work first one effect and then the other. Again, 78 THE DATA OF PSYCHOLOGY. richness of blood, by facilitating high nutrition of nerve- centres, conduces to nervous activity. Yet there is a plethoric state which is not nervously active; and starva¬ tion, with its greatly impoverished blood, has a phase at which delirium sets in, in consequence of the unduly rapid disintegration of the nerve-centres. Analogous incongruities, too numerous to specify here, continually occur. This en¬ tanglement of the conditions must be borne in mind and allowed for in each case* CHAPTER V. NERVOUS STIMULATION AND NERVOUS DISCHARGE. § 32. Every agent capable of altering tlie molecular state of a nerve, causes the nerve to produce the particular change which it habitually produces/ Experiments prove that each nerve is made to work the same kind of effect bv stimuli of •/ all orders; or, to speak strictly, it is found that the effect is of the same kind wherever its kind renders it accessible to observation. Thus, if an exposed end of a nerve which goes to a muscle is roughly touched, the muscle contracts. If it is eroded by an alkali or an acid, the muscle contracts. If it is galvanized, the muscle contracts. If it is suddenly heated, still the muscle contracts. Similarly with a vaso-motor nerve. No matter whether the disturbing agent be me¬ chanical, chemical, thermal, or electric, there results at the peripheral extremity a like change in the state of the ad¬ jacent arteries. An allied truth is that whether a nerve be irritated at the end which normally receives the disturbance, or whether it be irritated at some place between this and the organ acted upon by it, the effects wrought are alike—in nature, at least, if not in degree. As already said, the quantity of change set up increases with the length of the nerve through which the impulse is transmitted. But the quality of this 80 THE DATA OF PSYCHOLOGY. change remains identical be the stimulus applied at a near point or a remote point. This last truth, equally with the first, harmonizes with the supposition on which we have thus far proceeded. If the disturbance that travels along a nerve is a wave of isomeric transformation, the kind of effect produced by the wave at the place it eventually reaches, will be the same whatever stimulus set it up, or wherever it commenced. § 33. Nerve is not capable of continuous stimulation or continuous discharge. Persistent action of whatever kind on a nerve-termination or the cut end of a nerve, does not produce a persistent effect on the connected nerve-centre, or on the connected peripheral organ. Supposing the nerve supplying some muscle has been dis¬ sected out and cut in two ; then, if the exposed part be sud¬ denly pressed the muscle will suddenly contract; but main¬ tenance of the pressure will not cause maintenance of the contraction. Or if this nerve is made part of an electric circuit, then, at the moment of completing the circuit, the muscle will contract; but its contraction is only momentary, and the subsequent continuance of the current works no visible effect. To keep up muscular contraction, it is requi¬ site to send through the nerve a quick succession of separate disturbances. If the nerve forms part of an electric circuit in which there is an apparatus for breaking and completing the circuit; then, at each completion of the circuit, the muscle contracts ; and when the alternate breaks and com¬ pletions follow one another very rapidly, the contraction of the muscle becomes practically persistent. This truth is demonstrable by experiment on a dead frog, and also by experiment on the living human subject. A man who grasps the two metallic cylinders forming the poles of a magneto-electric machine, cannot leave hold of the cylinders when the intermittent current is passed through his arms. The like result occurs when the disturbances are mechanical NERVOUS STIMULATION AND NERVOUS DISCHARGE. 81 instead of electric. If tlie cut end of a motor nerve is sub¬ ject to a rapid series of taps, the muscle it supplies is thrown into a state of tetanus. The fact that the so-called nerve-current consists of successive pulses, is one of great significance. We shall find hereafter that it has many important corollaries. For the present it will suffice to observe how entirely congruous it is with the hypothesis on w T hich we have thus far pro¬ ceeded. If a nervous disturbance travels as a wave of mole¬ cular change—if this wave is such that the molecules of nerve-substance fall from one of their isomeric states to the other; then, having fallen in passing on and increasing the pulse or shock, they remain incapable of doing anything more until they have resumed their previous isomeric state. Hence the very nature of the process necessitates the inter¬ mittent character of nerve-action. § 34. The transmission of a disturbance through a nerve takes an appreciable time. The rate of transmission, as measured by Professor Helmholtz, has been found to vary from about 28 yards per second to 32 yards per second. Difference of constitution is doubtless the cause of this variation—a variation to which is due that individual pecu¬ liarity recognized by astronomers in what they call “ the personal equation.” This peculiarity affords yet another confirmation of the belief that a nervous discharge is a wave of isomeric trans¬ formation. If the disturbance propagated through any series of molecules is one that does not permanently change their relative positions; then the transfer of the disturbance may be excessively rapid, because the amount of molecular momentum to be generated is excessively minute. But if the molecules have to be transposed—if, as in isomeric transfor¬ mation, the components of each compound molecule have their relative positions altered; then the quantity of molecular momentum generated must be comparatively very large; o 82 THE DATA OF PSYCHOLOGY. and as the genesis of this momentum takes place m eacn molecule before the next is affected, the transfer of the disturbance must be greatly retarded.* * Perhaps too much has already been said respecting the nature of nerve-action. But before finally leaving the subject, I must add some important illustrative facts that have come to my knowledge while writing this chapter. They are contained in a paper by Mr. Gore, published in the Transactions of the Royal Society for 1858, describing the allotropic .changes undergone by electro-deposited antimony. Antimony so depo¬ sited assumes, according to the conditions, two forms—a dark, amorphous, or unstable form ; and a grey, crystalline, or stable form. When a mass of the amorphous antimony is disturbed at one end, there begins a change into the crystalline antimony, which spreads almost instantly throughout - the whole mass, with great evolution of heat. A slight tap with a hard substance suffices to initiate this transformation. Touching one of the angles with a hot body equally produces the effect. And it is also produced by an electric spark. A temperature below that of boiling water sufficed to cause the change ; and Mr. Gore found that in proportion as the whole mass was raised towards this temperature, the metamorphosis, more easily set up, travelled with greater rapidity. When a copper wire was coated with a film of this amorphous antimony, the allotropic change progressed along it at a rate varying from 12 to 30 feet in a minute. Absorption of the evolved heat by the copper wire, was found to be the cause of retardation when the change advanced slowly ; whence it follows that were none of the disengaged heat allowed to escape, the wave of change would travel much faster. A further significant fact is that when this transformation was propagated through some amorphous anti¬ mony that had been previously reduced to powder, part of it was oxidized—the disturbance caused by the allotropic change initiated chemical change, in parts of the substance that were favourably circumstanced for chemical change. See then the parallelism. We have the transformation set up indifferently, as in nerve, by mechanical force, heat, electricity. We have it facilitated, as in nerve, by raised temperature. We have it travelling from end to end of a mass with a velocity which, though far less than that of the nerve-wave, is still considerable. And we have allotropic change initiating chemical change, just as we concluded that isomeric change in a nerve-fibre sets up chemical change in a nerve-vesicle. Let me not omit an interpretation of nerve-structure, which is suggested by Mr. Gore’s experience that the transmission of the allotropic change is rapid in proportion as the evolved heat is retained. In developed animals, nerve-fibres are surrounded by sheaths of medullary matter ; and we saw reasons for concluding that this medullary matter is an insulator. Now t has recently been discovered that white or fibrous nerve-tissue, is chemically distinguished from grey or vesicular nerve-tissue, by the NERVOUS STIMULATION AND NERVOUS DISCHARGE. 83 § 35. Allied with the fact that a nerve-disturbance takes an appreciable time to travel from periphery to centre or from centre to periphery, is the fact that the effect produced presence in large quantity of a substance called protagon ; and this sub¬ stance proves to be of excessively complex composition—has a molecule more highly compounded than any other known molecule. But in pro¬ portion as molecules become complex and large, the masses formed of them become bad conductors of molecular motion. It is inferable, then, that the essential nerve-fibre is imbedded in a substance especially distin¬ guished by inability to absorb the molecular motion disengaged during the isomeric change of the nerve-fibre. I have hitherto passed over without remark, the hypothesis at one time current, and still surviving in some minds, that the nervous force is either electricity or some form of force allied to it. In addition to the many foregoing reasons for adopting another hypothesis, it may be well to set down here the reasons for rejecting this. The highest rate of the nervous discharge is some 32 yards per second. The electric discharge travels at the rate of 2S0,000 miles per second. The one velocity is thus neai'ly 16,000,000 times the other. That a force allied to the electric should have a velocity so enormously different, seems very unlikely. Again, an electric current, so long as its source is unexhausted and the circuit unbroken, is a continuous current ; but the nerve-current is not continuous. Hence if the nerve-force is of a kind allied to the electric, its mode of alliance is quite exceptional; for the other allied forces, heat, light, and magnetism, are not intermittent. Once more, nervous transmission is facilitated by heat; whereas heat is an obstacle to electric transmission, and diminishes or destroys magnetic action. The fact is that but for the accidental observation of Galvani, the suspicion that the nerve-force is electric or quasi-electric, would probably never have been entertained ; and it should have been abandoned as soon as it was found that other disturbing agents, physical and chemical, work just the same effects. The conception has, indeed, been kept alive by the discovery that electricity is generated by certain fishes. But the supposed support is wholly imaginary. If because the Torpedo evolves electricity by the help of nerves ramifying through its electric organ, it is inferred that the nerve-force is electricity ; it may in like manner be inferred that the nerve-force is sensible motion, because it generates sensible motion in muscles. But, it may be asked, do not the experiments of Du Bois-Beymond yield support to the hypothesis ? A very doubtful support I think. The phenomena he describes may well be merely incidental accompaniments of actions that are in themselves neither electric nor quasi-electric. The truth that both molar and molecular changes in the distribution of matter habitually destroy the electric equilibrium, would be a sufficient general justification for this belief. But there is a special justification. Direct j>roof exists that the G 2 THE DATA OF PSYCHOLOGY. 84 ' at the centre or at the periphery lasts an appreciable time. That muscular contraction is continuous though the stimulus is intermittent, goes to show this. The genesis of molecular modification in muscle by the molecular modi¬ fication in the nerve-fibres permeating it, has a duration that bridges the interval between each pulse of stimulus and the next. We have no direct proof that a like con¬ tinuity of state results from the successive waves propa¬ gated to a nerve-centre; for the actions of which nerve- centres are the seats are not objectively perceptible. But we shall presently find abundant indirect proof that these changes also last for measurable periods. This general truth, like its predecessor, may be regarded as a corollary from what has gone before. The transforma¬ tions classed as chemical take time, equally with those classed as isomeric. It is true that explosions due to chemical action are what we call instantaneous (a descrip¬ tion of them which is not, however, scientifically accurate; as may be perceived when the matter exploded is of con- particular kind of molecular change we have supposed to take place in nerve, and in muscle, is adequate to produce the phenomena observed by Du Bois-Reymoud. Mr. Gore found that if a copper-wire, coated with amorphous antimony, formed part of an electric circuit, it happened that whenever the allotropic change propagated along the antimony stopped, the galvanometer-needle was deflected. Now, since during the maintenance of a muscular contraction, nerve-pulses are continually arriving and ceasing, and the muscular fibres (never all in action together) are at every instant some of them contracting and some relaxing, it follows that there will be a succession of stoppages of isomeric changes. Consequently there will be a maintenance of deflection in the galvanometer if a contracted muscle forms part of the electric circuit. [Since this note has been in type, I have referred to the lecture delivered by M. Du Bois-Reymond at the Royal Institution on April 13, 1866, “On the Time required for the Transmission of Volition and Sensation through the Nerves f for the purpose of verifying one of the statements above made; and I find that at the close of this lecture he goes a long way towards abandoning his hypothesis. Though he says “ it would be rash, as the matter stands, entirely to dismiss the notion of electricity being con¬ cerned and though he sketches out a theory of nerve-composition such as makes it conceivable that an electric disturbance might travel along a nerve NERVOUS STIMULATION AND NERVOUS DISCHARGE. 85 siderable bulk). But explosions occur only in those excep¬ tional cases where the elements concerned are either, as in detonating compounds, distributed among one another molecularly, or, as in gunpowder, with minute intimacy. In ordinary cases, where sensible masses of the elements concerned are external to one another, the chemical actions, limited to the surfaces of contact, proceed with compara¬ tive slowness. Now the granular protoplasm contained in and around nerve-vesicles, forms, with its permeating liquids and the blood in adjacent capillaries, a mass of which the components are but imperfectly interfused; and therefore a chemical change cannot pass through it in¬ stantly. Hence between the reception of a pulse of mole¬ cular motion by a nerve-centre, and the emission of a gush of molecular motion, or discharge, some little time must elapse. § 36. If a nerve-centre that receives a stimulus through an incoming nerve, undergoes a chemical change and sends at the observed rate (basing this, however, on the gratuitous assump¬ tion that the molecules of nerve-matter have north and south poles) ; yet he admits that much evidence points another way. He says that “to identify it (the nervous agent) with the electric current as it circulates in a telegraph-wire must appear hopeless, even if a circuit, such as would be necessary for the supposed nerve current to circulate in, were anatomically demonstrated. Thrs to the other arguments against this view of the nervous agent—that the resistance of the nerve-tubes would be far too great for any battery to send an available current through them—that the physiological insulation of the nerve-tubes from each other would be im¬ possible to explain—that the effect of ligature or of cutting the nerve and causing its ends to meet again, would be equally obscure—to these argu¬ ments, unanswerable as they are in themselves, the researches sketched in this lecture have added corroborative evidence of the highest order. What we have termed the nervous agent, if we look upon its very small velocity, in all probability is some internal motion, perhaps even some chemical change, of the substance itself contained in the nerve-tubes, spreading along the tubes, according to the speaker’s experiments, both ways from any point where the equilibrium has been disturbed ; being capable of an almost infinite number of variations or gradations, and of so peculiar a character as to require the unimpaired condition of the nervous structure.”] 86 THE DATA OP PSYCHOLOGY. a discharge along ont-going nerves,, it thereupon becomes less capable of emitting such discharges in response to such stimuli. The quantity of molecular motion locked up in a nerve-centre, is measured by the contained quantity of unstable nerve-matter; and decomposition of that part of the unstable nerve-matter which was most favourably placed for being acted on, leaves not only a diminished quantity but a quantity that is less favourably placed for being acted on—leaves, therefore, a decreased readiness to undergo change when disturbed, as well as a decreased stock of molecular motion to be liberated. Consequently, other things remaining the same, every excitation of a nerve-centre reduces, for a time, its impressibility and its energy. This temporary enfeeblement of a nerve-centre, when caused by moderate action, is inconspicuous. The disinte¬ grated mass quickly re-integrates itself from the materials brought by the blood. But if the stimulation and con¬ sequent discharge are violent, or if stimulations and dis¬ charges are repeated very rapidly, then repair falls so far in arrear of waste that partial or entire incapacity of the nerve-centre results. All its unstable substance within easy reach of in-coming disturbances has been decomposed; leaving such part only of its unstable substance as is most removed from disturbances, and can be affected only by excessive ones. A well-known experiment on the vaso¬ motor system of a frog, may be cited in illustration. If a frog’s foot be placed under a microscope, and so adjusted that the arteries ramifying through the transparent mem¬ brane between the toes are brought into view, then, if a powerful irritant be applied to this membrane, the first re- suit observed is that these arteries are violently constricted —the strong impression conveyed to the vaso-motor centres there liberates an excessive discharge along the fibres supplying these arteries, causing spasmodic contractions of their muscular coats. The second result is that these NERVOUS STIMULATION AND NERVOUS DISCHARGE. 87 arteries dilate : losing tlieir normal contractility they become distended with blood, and the part is, as we say, congested. That this is due to extreme prostration or tem¬ porary paralysis of the vaso-motor centre, has been clearly proved; for if the nerve-trunk containing the vaso-motor fibres be dissected out and artificially irritated, the dilated arteries instantly contract. How a nerve-centre may be prostrated by a rapid succession of moderate stimuli and . discharges, instead of by one violent stimulus and discharge, is shown by the familiar effect of friction on the human skin. A single moderate rub causes only a slight reflex action on its vessels, and leaves the vaso-motor apparatus ready to act afresh with no apparent diminution of power. But a series of rubs is followed by temporary congestion of the vessels : it is some little time before the vaso-motor centre regains its full control over them. And if the skin be continuously chafed, the excessive waste and debility of the vaso-motor centre entail that enduring redness called congestion. Those parts of the nervous system con¬ cerned in muscular action, daily illustrate the same general relation. Fatigue is a state in which the ability to generate motion has been greatly diminished by long-continued genesis of motion; and every tired horse shows, by the small response he makes to a cut of the whip, that a more violent impulse must be propagated to the nerve-centres to cause the ordinary evolution of nervous energy. Irregularities in the manifestation of this truth, are due to that entanglement of the conditions which was indicated at the close of the last chapter. It frequently happens, for instance, that after performing its function for some time, a nerve-centre responds to the demands on it better than at first—a fact apparently at variance with the foregoing con¬ clusion. But this conclusion supposes all the circumstances to have remained the same; and in such cases they have not remained the same. There has been an exaltation of the heart's action, or a local increase in the quantity of blood, or 83 THE DATA OE PSYCHOLOGY. a more rapid aeration of blood, or all of tliese. When every appliance which furthers the disintegration and re-integra¬ tion of a nerve-centre, has been brought into full play, both waste and repair go on faster; and there result greater impressibility and energy than when the previously-unused centre contained but little blood languidly circulating. § 37. Were Life uniform in its rate—were terrestrial con¬ ditions such that actions of all kinds could be performed as readily at one time as at another, repair and waste of all organs, including nervous organs, would have to keep an ap- proximately-even pace, one with the other. But the alterna¬ tion of day and night entails an alternation of greater and less facility for actions ; and there has resulted in organisms an adapted alternation in the relative rates of waste and repair. The adaptation is manifestly due to survival of the fittest. An animal so constituted that waste and repair were balanced from moment to moment throughout the twenty-four hours, would, other things equal, be overcome by an enemy or -competitor that could evolve greater energy during the hours when light facilitates action, at the expense of being less energetic during the hours of darkness and concealment. Hence there has necessarily established itself that rhythmical variation in nervous activity, which we see in sleep and waking. Let us observe how these are interpretable, the one as a state of the nervous centres in which waste has got -considerably in excess of repair, and the other as a state in which repair has made up for previous excess of waste. Confining ourselves to persons whose functional rhythms have not been deranged by undue excitements, we see that after some sixteen or eighteen hours of sustained impressi¬ bility and energy, there is a diminished readiness to respond to stimuli that fall on the eyes, ears, and surface of the body at large; and presently this becomes so pronounced that loud sounds and the irritations produced by strained atti¬ tudes, fail to evoke movements. When great exertion has NERVOUS STIMULATION AND NERVOUS DISCHARGE. 89 been gone through, or when previous intervals of sleep have been omitted, the decrease of impressibility is such that tickling the nostrils or pinching the skin does nothing more than cause, perhaps, a reflex start. This change, so marked and often so rapidly established, seems greater than the alleged cause can produce; but it is fully accounted for when we include an indirect effect of this cause. The waste of the nerve-centres having become such that the stimuli received from the external world no longer suffice to call forth from them adequate discharges, there results a di¬ minished impulse to those internal organs which subserve nervous activity, including, more especially, the heart. Consequently the nerve-centres, already working feebly, are supplied with less blood and begin to work more feebly— respond still less to impressions, and discharge still less to the heart. And so the two act and re-act until there is reached this state of profound unimpressibility and inactivity. Between this state and the waking state, the essential distinction is a great reduction of waste. Certainly in some nervous centres and probably in all, waste does not abso¬ lutely cease: there continue those emissions of force which keep up the vital processes ; and it is, I think, unlikely that there is ever an entire stoppage of those changes which take place in the highest centres. But the rate of waste falls so low that the rate of repair exceeds it. It is not that during the period of activity waste goes on without repair, while during the period of inactivity repair goes on without waste; for the two always go on together. Very possibly—probably even—repair is as rapid during the day as during the night: perhaps even more rapid; for the blood is on the average richer and circulates faster. But during the day the loss is greater than the gain, whereas during the night the gain is diminished by scarcely any loss. Hence results accumula¬ tion : there is a restoration of the nerve-tissue to its state of integrity. In the course of some hours this restoration begins to 90 THE DATA OF PSYCHOLOGY. show its effects in returning impressibility. While in sleepiness we see a decreasing readiness to respond to ex¬ ternal stimuli, the approach to a waking state is character¬ ized by an increasing readiness to respond to external stimuli. Throughout the period of quiescence the afferent nerves remain subject to incident forces. The pressure of the body on the bed affects some of them, and others are affected by the touch of the bed-clothes ; degrees of heat a little above or below the average, act on others ; and yet others receive sonorous vibrations constantly occurring. But whereas sleep results because the centres worn by action become less and less sensitive to these stimuli, waking results because the centres repaired during rest become more and more sensitive to them. The strains of muscles and liga¬ ments which during the first part of the night fail to cause changes of attitude, cause such changes towards morn¬ ing. The amount of light that traverses the eye-lids pre¬ sently suffices to call forth movements. Some slight noise which, hours before, would have had no effect, now produces a start. Even in the absence of external stimuli (which, however, can never be absent) there are the stimuli from the viscera, and especially from the alimentary canal: an empty stomach eventually sends to the cerebro-spinal system enough disturbance to end the quiescent state. The longer repair goes on unopposed by appreciable waste, the greater must become the instability of the nerve-centres, and the greater their readiness to act; so that there must at length come a time when the slightest impressions will produce motions. Such impressions, however slight, are necessary antecedents. The re-integrated nerve-centres do not resume their activity until an impulse arriving from the periphery overthrows some of their molecules. Evidence of this is furnished to most every morning. On awakening from refreshing sleep, there commonly occurs an involuntary stretching of the muscles of the whole body; showing an immense undirected motor discharge. But this is not the NERVOUS STIMULATION AND NERVOUS DISCHARGE. 91 initial fact. No one awakes to find liimself then and there stretching; which might happen were the discharge spon¬ taneous. It comes after those stronger disturbances that are propagated to the centres,, as soon as some slight distur¬ bance has led to the slight movements that accompany waking. A trifling sound causes opening of the eyes and a turn of the head. Thereupon follow vivid impressions through the eyes, through the skin that rubs against the bed-clothes, and through the muscles that set up the movements. And a relatively-large aggregate of stimuli being sent from the periphery, there results this relatively- large gush of motor excitement. On pursuing the argument we may understand why the energies continue to rise for some time after awaking. We saw that when once sleepiness has commenced, it increases because in proportion as the nervous centres fail in their discharges, the heart, losing part of its stimulus, begins to flag, and that the flagging of the heart leads to a greater inertness of the nerve-centres, which re-acts as before. Conversely, it will here be manifest that when the nerve-centres, repaired by sleep, become again ready for discharging with vigour, there take place an action and re¬ action which have the opposite effect. The pulsations on awak¬ ing are comparatively feeble. As soon as stimuli begin to be received through the sensory organs, and the discharges of the nerve-centres are renewed, the heart comes in for its share of these and acts more vigorously. By so doing it supplies the nerve-centres with more blood in quicker gushes. A greater nervous discharge is thereby made possible, which again, among other results, exalts the heart's action. And so the mutual aid goes on: the greatest nervous vigour being reached when the vascular activity has been still further raised by a meal, and the blood has been enriched by the absorbed materials. § 38. As implied by much that has gone before, and as 92 THE DATA OF PSYCHOLOGY. especially implied by the last section, nervous stimulation and nervous discharge have always both special and general results. Beyond the primary and definite effect wrought on a particular part by a particular impression, there are in every case secondary and indefinite effects diffused through the whole nervous system, and by it through the body at large. It was pointed ont (§§ 10,11) that the simplest nerve-centre puts in relation not afferent and efferent fibres alone; but that through other fibres, commissural and centripetal, con¬ nections are made between it and other nerve-centres of the same grade and of a higher grade. Further, we saw that when such a nerve-centre is excited through an afferent nerve, the disengaged molecular motion does not escape wholly along one or more efferent nerves; but that part of it, propagated to higher centres, there sets up supplemen¬ tary changes. The diffusion does not stop here—remoter parts are reached; and thus the disturbance of a single nerve-fibre, if at all considerable, reverberates throughout the entire nervous system, and affects all the functions con¬ trolled by it. Digging a pin into the foot may cause a con¬ vulsive contraction not of the leg-muscles only, but of many other muscles throughout the body. At the same time it may alter the rate of pulsation, and send waves of constriction along the arteries. The excreting structures of the skin may be so affected that a burst of perspiration results ; and the actions going on throughout the alimentary canal may be deranged. Such reverberations, which become conspicuous when the disturbances are decided, take place also when they are slight. A more vivid light, causing as it does stronger pulses of change through the optic nerve, increases the rate of respiration ; and doubtless the other vital functions are simultaneously exalted. So that each nervous impression, beyond a direct response in the shape of increased action from one or more organs, calls forth an indirect response in the shape of increased action of the organism as a whole. NERVOUS STIMULATION AND NERVOUS DISCHARGE. .93 Remembering that every instant the disturbance ttius echoing throughout all passages of the nervous system is not solitary, but that there are many such disturbances, here arising from pressure there from touch, in this place pro¬ duced by sound and in that by light, at one part by muscular strain and at another by heat or cold; it will be manifest that, besides the few distinct waves of nervous change working their distinct effects, there are multitudinous indistinct waves, secondary and tertiary, travelling in all directions working their indistinct effects. § 39. Since such reflected and re-reflected disturbances everywhere act as stimuli, we must regard the entire nervous system as at all times discharging itself. The unstable molecules of its centres, exposed to this confused reverberation, are liable to be decomposed wherever a concurrence of small waves makes the local agitation con¬ siderable ; and the molecular motion thereupon disengaged, adds to the centrifugal gush perpetually going on. Rightly to conceive nervous action then, we must think of the con¬ spicuous emissions of force from parts of the nervous system that are strongly disturbed, as standing out from a vague back-ground of inconspicuous emissions from the whole nervous system, which is slightly disturbed. To this general nervous disturbance with its consequent general discharge, is probably due a certain general action of the motor organs. No muscles are ever in a state of absolute rest. What we distinguish as muscular motion is produced by the greater contraction of some muscles than of others. The others, however, are all slightly contracted; and would severally produce motion were they not balanced or out-balanced by their antagonist muscles. This per¬ vading activity of the muscles is called their tonic state. And while we regard particular contractions as the results of particular nervous discharges, we have good reasons for concluding that this universal contraction is the result 94 THE DATA OF PSYCHOLOGY. of the universal nervous discharge. Here are a few of them. Sleep, as above explained, implies dimi¬ nished nervous discharge, special and general. A dimi¬ nution of the general discharge ought, then, to be shown in a decrease of the tonic contraction. It is so shown. Falling asleep is accompanied by muscular relaxation: though previously the attitude was such that no effort seemed requisite to maintain it ; yet that there was some muscular strain, and that it has suddenly become less, is proved by the sliding down of a limb, or of the head, to a more stable position. Certain dis¬ orders, as palsy, yield further proof. The flexors and extensors which, when duly contracted, serve by their balanced antagonism to hold a limb steady, cease to do this when the general nervous discharge is not great enough to keep them and all other muscles braced up: in default of sufficient stimulus for both, now one set and now the other fails to put the due check on its opponent. That such shakings are so caused, we see clearly in persons debilitated by over-stimulation; for in them this symptom may be temporarily mitigated, or almost cured, by temporarily in¬ creasing the general nervous discharge. The drunkard who early in the day cannot lift his glass without spilling the contents, is able to do this after his brain has been excited by the usual doses of alcohol. Of course it is not the muscles alone on which this con¬ tinuous centrifugal gush is expended. Through the inter¬ mediation of nerves connecting the cerebro-spinal system with the sympathetic system, the viscera receive their share of it. Hence the overflow of nervous energy which, without special solicitations, diffuses itself throughout the motor structures, giving elasticity to the step, and producing the concave bend of the back, the opened-out shoulders, the raised head, &c., has, for its simultaneous results, an accelerated circulation, an invigorated digestion, and an exaltation of the vital processes at large. NERVOUS STIMULATION AND NERVOUS DISCHARGE. 95 § 40. Briefly reviewed from a somewhat different stand¬ point, the following are the leading facts which it concerns us to remember. Nervous stimulations and discharges consist of waves of molecular change, that chase one another rapidly through nerve-fibres. The stimulus or discharge formed of such waves, arises at some place where unstable nerve-substance has been disturbed; and is the same no matter what agent caused the disturbance. The successive waves severally travel with a velocity which, though considerable compared with ordinary sensible motions, is extremely slow compared with other kinds of transmitted molecular motions. And each set of waves, while itself caused by the decomposition of unstable nerve-matter, is a means of decomposing other unstable nerve-matter : so generating further and often stronger sets of waves, which similarly chase one another into many and distant parts of the nervous system. There is a triple rhythm in these nervous stimulations and discharges—each form of rhythm being due to the greater or less incapacity for action which an action produces. We have seen that every wave of isomeric transformation passing along a nerve-fibre, entails on it a momentary unfitness to convey another wave; and that it recovers its fitness only when its lost molecular motion has been replaced and its unstable state thus restored. We have also seen that any portion of grey matter in a nerve-centre, which having been disturbed and partially decomposed has emitted a shock of molecular change, is proportionately incapacitated ; and that it recovers its original ability only as fast as it re-integrates itself from the materials brought by the blood. And then there comes the further rhythm constituted by the alterna¬ tions of sleep and waking—a rhythm having the same origin as the last, and being supplementary to it. The remaining truth which w r e have contemplated is that each special stimulation and the special discharge produced by it, do not together form the whole of every nervous act; THE DATA OF PSYCHOLOGY. 00 lout that there is always an accompanying general stimula¬ tion and general discharge. Every part of the nervous system is every instant traversed by waves of mole¬ cular change—here strong and here feeble. There is a universal reverberation of secondary waves induced by the stronger primary waves, now arising in this place and now in that; and each nervous act thus helps to excite the general vital processes while it achieves some particular vital process. The recognition of this fact discloses a much closer kinship between the functions of the nervous system and the organic functions at large, than appears on the sur¬ face. Though unlike the pulses of the blood in many respects, these pulses of molecular motion are like them in being perpetually generated and diffused throughout the body; and they are also like them in this, that the cen¬ tripetal waves are comparatively feeble while the centrifugal waves are comparatively strong. To which analogies must be added the no less striking one, that the performance of its office by every part of the body, down even to the smallest, just as much depends on the local gushes of nervous energy as it depends on the local gushes of blood. CHAPTER VI. ^ESTIIO-PH YSIOLOGY. * § 41. Throughout the foregoing chapters nervous pheno¬ mena have been formulated in terms of Matter and Motion. If from time to time the phrases used have tacitly referred to another aspect of nervous phenomena, the tacit references have formed no parts of the propositions set down; but have * This new word will possibly be condemned as not legitimately com¬ pounded. The objection that the root from which its prefix is derived, is shorn of its fair proportions, admits, I am told, of a satisfactory answer : from the proximate root, appeal may be made to the original root, which, following the Greek method of forming derivatives, would admit of the required modification. But to the criticism that the word involves the . logical inconsistency of uniting a verb with a noun, there is no such suffi¬ cient answer. Nevertheless, I deliberately adopt jEstho-physiology in pre¬ ference to the more cumbrous and cacophonous JZsthesi-physiology. A progressive integration by which the originally-distinct and numerous parts of compound words become fused together, blurred, and some of them lost, is one of the essential processes in the development of language. If man¬ kind had refrained from the obliteration and disfigurement of roots, and parts of roots, language would have continued wholly inadequate for all but its simplest functions. Omitting those formed by onomatopoeia, the best words are those from which long use has worn away all, or nearly all, traces of their origin. We may as well, therefore, begin with abbreviated and modified words when we have to coin them ; instead of leaving time to bring about the needful shortening and shaping. Those who, dealing with words as counters, see that their convenience as counters is the chief con¬ sideration, will probably coincide in this view ; though I suppose it will be wholly disapproved by those who regard words not as counters but as money. H 08 THE DATA OF PSYCHOLOGY. been due to lack of fit words—words free from unfit associa¬ tions. As already said, the nervous system can be known only as a structure that undergoes and initiates either visible changes, or changes that are representable in terms furnished by the visible world. And thus far we have limited ourselves to generalizing the phenomena which it thus presents to ns objectively. Now, however, we turn to a totally-distinct aspect of our subject. There lies before us a class of facts absolutely without any perceptible or conceivable community of nature with the facts that have occupied us. The truths here to be set down are truths of which the very elements are unknown to physical science. Objective observation and analysis fail us; and subjective observation and analysis must supple¬ ment them. In other words, we have to treat of nervous phenomena as phenomena of consciousness. The changes which, re¬ garded as modes of the Non-Ego , have been expressed in terms of motion, have now, regarded as modes of the Ego , to be expressed in terms of feeling. Having contemplated these changes on their outsides, we have to contemplate them from their insides. To speak with exactness, indeed, it cannot be said that we have so to contemplate these changes; for this expression implies that these changes can be simultaneously seen by more than one, which is not true. Kigorously limiting the proposition to that which is alone possible, it amounts to this :—I have to describe the laws of relation between the states of feeling occurring in my own consciousness, and the physical affections of that nervous system which I conclude I possess; and the reader has to observe whether in himself there exist parallel rela¬ tions between such known states of consciousness and such supposed nervous affections. This will perhaps be thought a needlessly roundabout, if not a sceptical, statement; but it is in fact not roundabout enough. It does not bring sufficiently into view the re- .&STH0-PHYSI0L0GY. 99 motely-inferential cliaracter of the belief that feeling and nervous action are correlated. Before proceeding on this belief, let us observe bow indirect is the path wbicb leads to it.—1. Each individual is absolutely incapable of knowing any feelings but bis own. That there exist other sensations and emotions, is a conclusion implying, in the first place, the reasonings through which he identifies certain objects as bodies of like nature with his own body; and implying, in the second place, the further reasonings which convince him that along with the external actions of these bodies, there go internal states of consciousness like those accom¬ panying such external actions of his own body. 2. This conclusion that there exist beings like himself, and that under like conditions they experience like feelings, even supposing it entirely true (and it is not entirely true, for many facts unite to prove that under like conditions both the quantities and the qualities of sensations and emotions in different individuals differ considerably), by no means implies that what he knows under its subjective aspect as feeling, is, under its objective aspect, nervous action. The average observer has no direct evidence that these other like beings have nervous systems, any more than that he has himself a nervous system; and he has no direct evidence in the one case anv more than in the other, that nervous excitations are the causes of feelings. Experi¬ mental physiologists and pathologists only have proofs; and even their proofs are mostly indirect. The experiments which yield them are usually made on beings of another and much inferior order. The contractions of muscles and arteries, caused by irritating nerve-trunks in frogs, the convulsive movements, and sometimes the sounds, made by birds and mammals whose nerve-centres are variously in¬ jured—these are the phenomena from which it is inferred that the human nervous system is the seat of the human feelings, and that these feelings are the correlatives of its excitations: the only important verifications of the inference being those 100 THE DATA OF PSYCHOLOGY. obtained during surgical operations where nerve-trunks are cut through, and those furnished by post mortem examina¬ tions of morbid nervous structures in the bodies of those who when alive displayed abnormal excesses or defects of feel¬ ing. 3. And then, having learnt at second hand, through the remotely-inferential interpretation of verbal signs, that in now one and now another of the bodies he recognizes as like his own there has been found a nervous system, and that the stimulations of this produce those manifestations which in himself accompany feelings, the reader imagines a nervous system contained in his own body, and concludes that his sensations and emotions are due to the disturbances which the outer world sets up at its periphery, and arouses by indirect processes in its centres. Such, stated as briefly as possible, is the long and involved series of steps by which alone the connection between nervous action and feeling can be established. Nevertheless, the evidence of this connection is so largo in amount, presents such a congruity under so great a variety of circumstances, and is so continually confirmed by the correct anticipations to which it leads, that we can entertain nothing more than a theoretical doubt of its truth. Here accepting the belief, alike popular and scientific, that all the human beings known objectively have feelings like those which each knows subjectively; and accepting also the belief, originating with science but now diffused through the general mind, that feelings arc the concomitants of nervous changes; we will proceed to consider the relation between feelings and nervous changes under its leading aspects. A. £ 42. And first let us observe that the circumstances conducive to the one are identical with the circumstances conducive to the other. The conditions which we before found essential to the production of nervous action, we shall now find essential to the- production of feeling. We may pass i^STEO-PHYSIOLOGY. 101 over tlie evidences briefly, as being many of them the inner aspects of phenomena already observed under their outer aspects. That without continuity of nerve-fibre between periphery and centre, a disturbance at the one causes no feeling at the other, is proved to every one who has cut himself deeply: for a long time the part that has had its nervous com¬ munication destroyed, remains numb. This experience, usually very limited in each person, is borne out by the testimony of those seriously injured; and especially by the testimony of those whose sensations over large parts of their bodies have ceased, and who, after death, are found to have lesions in the conducting structures of the nervous centres. The hindrance or prevention of feeling by pressure, is illustrated by the numbness of a limb so placed that its whole weight, and perhaps the weight of another limb lying over it, comes on the edge say of a table; so that great stress is borne by some portion of the chief nerve- trunk. Local ana 3 sthesia thus caused in strong persons, is caused still more readily in feeble persons; who, on awaking, not unfrequently find complete insensibility of the parts that have been pressed against the bed during sleep. Ability to feel depends on the maintenance of a certain temperature. This also is a general truth of which some proof is furnished to every individual by his own experience —or, at any rate, to every individual inhabiting a climate where the winters frost suffices greatly to chill the ex¬ tremities. Evidence much stronger but indirect, is given him by those who have undergone surgical operations in parts deprived of feeling by freezing mixtures or by ether- spray. Loss of local sensibility from local cold, ordinarily not very manifest unless the cold is great, becomes manifest when the cold is slight if the circumstances supply a delicate test. This is interestingly shown among compositors. The air of a printing-office has to be kept very warm, even at 102 THE DATA OF PSYCHOLOGY. the expense of unhealthy closeness; otherwise the fingers of the compositors cease to lay hold of, and manipulate, the types with the requisite nicety and speed. Few persons have immediate experience of the fact that defect of blood in a part causes defective sensibility of that part; but all persons have immediate experience of the local exaltation of sensibility that accompanies local excess of blood. The inflamed neighbourhood of a wound, or even the surface of a pimple, yields to consciousness when touched, an amount of feeling far greater than is yielded by a part of the skin supplied with the ordinary amount of blood. Special organs of touch show us well the increased sensitiveness thus caused. When one of those sacs con¬ taining the bulbs of the small hairs scattered over the skin, is congested, the rubbing of the clothes against the hair growing from it, especially if it has been broken short, pro¬ duces an unbearable smart. Among evidences yielded by the other senses, a familiar one is the intolerance of light that g’oes along with inflammation of the eyes. And there is an unfamiliar one particularly worth noting, because it exhibits the effect due to increased quantity of blood apart from increased temperature. The observation may be made wdien taking a hot bath. Let the water be above blood-heat—say 100° Fh. After remaining quiet for a time until equally heated all over, stand up and rub one portion of the body with a flesh-brush until it is red. Pause for a few moments, and lie down again in the water. It will then be perceived that to the reddened part the water seems much hotter than it does to any other part.* That degree of feeling is affected by quality of blood as well as by quantity, is a truth not easily discerned within the * This fact yields proof, if there needs any, that the nerves which appreciate temperature are not the nerves of touch. Violent friction must produce a momentary incapacity of the nerves of touch; and this in¬ capacity would be shown in a decreased, instead of an increased, appreciation of temperature, were they the agents concerned. 5CSTH0-FHYGI0L0GY. 103 experiences of each individual, if attention is limited to those variations of feeling that accompany naturally-produced variations in quality of blood. For such variations cannot be identified with precision; and they arise so slowly that the concomitant mental states cannot be brought into close con¬ tiguity, so as to bring out their contrasts clearly. But by making certain artificial additions to the blood, every one gets proof of the connection between its quality and the genesis of feeling. The effects of stimulants on conscious¬ ness are mostly traced in the intensification of those in¬ ternally-initiated feelings with which we shall deal pre¬ sently; but they may also sometimes be traced in the intensification of the externally - initiated feelings. In nervous subjects, ordinary impressions on the senses are often rendered abnormally acute by tonics. When under the influence of opium, music that was previously unenjoyed may be greatly enjoyed; and it is a well-known result of hashish to give an excessive vividness to the sensations. How, contrariwise, there are substances which, when added to the blood, render sentiency less vivid, is shown by other facts similarly reached. We have sedative medicines—medicines that diminish the amounts of painful consciousness caused by irritations at the periphery of the nervous system. And wo have agents of the same class called anaesthetics, which, in a still greater degree, hinder the genesis of feelings by the actions that usu¬ ally generate them. These effects so caused, help us to understand the stupor produced by the natural anaesthetics, carbonic acid and urea; and prove that some variations in degree of feeling are determined by variations in the activities of excreting organs. § 43. Now that we have noted how feelings and nervous changes arc facilitated or hindered by the same conditions, we may go on to collate them in detail. Let us begin by distin- 104 THE DATA OP PSYCHOLOGY. guisliing those nervous changes which are accompanied by feelings from those which are not. For, as we noted in passing, several classes of them have objective aspects only —do not present inner faces to consciousness ; and others have subjective aspects in early life but cease to have them in adult life. Chief among the nervous changes that have no identiti- able subjective aspects, are those occurring in the visceral nervous system. So long as they are normal in their amounts, the stimulations and discharges of which the sympathetic is the seat, go on without sensations; and even when abnormal, the resulting discomfort or pain is probably not due to them but to disturbance of those cerebro-spinal fibres which accompany the sympathetic through all its branch¬ ings. Similarly with the local ganglia and fibres of the heart. Ordinarily there is no consciousness of the heart's action; and even when the pulsations are violent, the modifications of consciousness do not arise from the state of the heart's nervous system, but from disturbance of cerebro-spinal nerves caused by the bounds of the heart against adjacent structures. The like holds with the vaso-motor nerves. Under ordinary conditions these regulate the diameters of the arteries without our knowing anything about it; and though where, as in a blush, great dilatation of the vessels lias been produced, we are made aware of their action, yet wc are made aware of it indirectly, through the local change in the quantity of blood and the consequent effect on the nerves that appreciate temperature. The majority of stimulations and discharges occurring in the spinal cord, have subjective accompaniments. These, however, are not localized at those points in the spinal cord where the essential nervous changes take place; as is proved by the fact that when some lesion of the spinal cord which has not injured its lower part, has cut off com¬ munication with the brain, the reflex acts performed by this lower part are unconscious. Proceeding upon the inference .ESTHO-PHYSIOLOGY. 105 before drawn (§21) that when a wave of disturbance brought by an afferent nerve to a spinal centre, liberates a quantity of molecular motion, a portion of it, not discharged along the efferent nerves, is propagated through a centri¬ petal nerve to a higher centre, we may conclude that it is this portion which comes, in the higher centre, to have a subjective aspect as a sensation : being there joined with other sensations and feelings of other orders into a chain of states of consciousness, out of which no sensation is ever known to exist. For recognition of a sensation as such or such, necessitates the bringing of it into relation with the continuous series of sentient states, from some of which, simultaneously experienced, it is dissociated by perceived unlikeness, and with others of which, previously ex¬ perienced, it is associated by perceived likeness; and the implied comparisons of sentient states are impossible unless the correlative nervous changes are put in connexion at one place. It does not follow, as it at first seems to do, that feelings are never located in the inferior nervous centres. On the contrary, it may well be that in lower types the homologues of these inferior centres are the seats of consciousness. The true implication is that in any case the seat of consciousness is that nervous centre to which, mediately or immediately, the most heterogeneous impres¬ sions are brought; and it is not improbable that in the course of nervdus evolution, centres that were once the highest are supplanted by others in which co-ordination is carried a stage further, and which thereupon become the places of feeling, while the centres before predominant become automatic. Quite congruous with this conception is the above-named fact, that certain nervous changes which have subjective sides early in life cease to have them later in life. Many acts performed by the child slowly and consciously, the adult performs rapidly and unconsciously. Every step taken during the first efforts to walk has ite accompanying 100 THE DATA OP PSYCHOLOGY. distinct feelings; but eventually, the successive steps are made while consciousness is wholly or almost wholly occu¬ pied with other feelings. Still better is the illustration furnished by speech. Each muscular adjustment of the vocal organs and each articulate sound made, have, in childhood, concomitant sentient states that are vivid, and, for the moment, all-absorbing. Gradually, however, these be¬ come less dominant in consciousness ; until at maturity there is entire oblivion of the one, and sometimes partial oblivion of the other: witness the not unfrequent verbal mistakes unconsciously made in the heat of discussion. Now facts of this kind, countless in number and of many varieties, are explicable if we regard feelings as the subjective sides of such nervous changes only, as are brought to the general centre of nervous connections. When we remember that early in life each inferior ganglion, or cluster of co¬ operating inferior ganglia, is imperfectly organized, and the connections among its fibres incomplete; we shall see that if there comes to it a disturbance, the gush of molecular motion liberated, not having in the incompletely-connected commissural and efferent fibres, adequate channels of escape, will part of it escape along a centripetal fibre to a higher centre, so awakening a feeling. And it will mani¬ festly happen that the approach to automatic action of the lower centre, will be an approach to a state in which the liberated molecular motion, having in the efferent fibres fully-opened channels of emission, will little or none of it be forced into centripetal fibres, and will so awaken little or no feeling. It is a corollary from this interpretation, that all gradations will exist between wholly unconscious nervous actions and wholly conscious ones; since there will be all gradations in the relative amounts of the disturb¬ ances which take their courses along centripetal fibres. It obviously follows, too, that in adult life a nervous action may or may not have an identifiable subjective aspect, according as it is strong or weak; since, if there comes ^STHO-PHYSIOLOGY. 107 to a finished ganglion constructed as described, a feeble disturbance, the whole of the small quantity of molecular motion liberated may be drafted off by the efferent fibres; whereas, if the disturbance is great, the disengaged molecular motion, being more than can find its way along the efferent fibres, will some of it take a centripetal course and cause a subjective change. § 44. A kindred aspect of this correlation presents itself when we contemplate feeling as occupying time. A sub¬ jective state becomes recognizable as such, only when it has an appreciable duration: it must fill some space in the series of states, otherwise it is not known as present. This general truth harmonizes with a general truth before pointed out respecting nervous action, as well as with the above interpretation. The observed fact that time is taken in the transit of a nerve-wave, is not to the point; for this transit has no concomitant subjective state. But the inferred fact that the change set up in a nerve-centre must take time, and a more considerable time (§ 35), is relevant; for what is objectively a change in a superior nerve-centre is subjectively a feeling, and the duration of it under the one aspect measures the duration of it under the other. That feeling persists after the force arousing it ceases, is not proved by the lengthened sensation produced by a moderate blow on the skin, or by that which follows dip¬ ping the hand into hot-water, or by those which the palate and the nostrils experience from pungent substances mo¬ mentarily applied; for though in such cases the external action of the exciting agency is brief, the local changes it sets up, lasting some time, continue for some time to dis¬ turb the local nerve-fibres. But good evidence is supplied by impressions on the retina. To quote the words of Professor Huxley :—“ A flash of lightning is, practically, instantaneous, but the sensation of light produced by that 108 THE DATA OP PSYCHOLOGY. flash endures for an appreciable period. It is found, in fact, that a luminous impression lasts for about one-eighth of a second; whence it follows, that if any two luminous impressions are separated by a less interval, they are not distinguished from one another. For this reason a f Catherine-wheel/ or a lighted stick turned round very rapidly by the hand, appears as a circle of fire; and the spokes of a coach-wheel at speed are not separately visible, but only appear as a sort of opacity, or film, within the tire of the wheel.” As above said, this general truth that feeling implies time, harmonizes with the interpretation given in the pre¬ ceding section; and supplies a further elucidation of the relation between conscious and unconscious nervous action. For manifestly, in proportion as nervous co-ordinations become more automatic they become more rapid; and for this reason also, cease to present such conspicuous subjective aspects. Returning to the inferior ganglion, or cluster of co-operating ganglia, above described, it will be obvious that a state in which the local or¬ ganization is incomplete, and the various afferent and commissural fibres not brought into definite relations with vesicles, and through them with efferent fibres, must be a state in which the molecular motion liberated by an in¬ coming shock of change, will pass through the imperfectly differentiated structure with comparative slowness ; and there will therefore be an appreciable time during which centripetal fibres may receive disturbance. But as fast as the local connections of fibres and cells become complete, the gush of molecular motion, following the completely- formed channels, will escape rapidly; and the period during which excitement of the centripetal fibres may take place will be abridged. The concomitant subjective state will therefore be rendered shorter by the same change that renders it feebler. J?oTH0-l'HYS10L0GY. 100 § 45. The fact that each feeling lasts an appreciable time, introduces us to the allied fact that each feeling produces a greater or less incapacity for a similar feeling, which also lasts an appreciable time. This, too, is the subjective side of a phenomenon before noticed under its objective side (§ 36). For as the duration of a feeling answers to the duration of the molecular disintegration in a disturbed nerve-centre; so the subsequent interval of diminished ability to feel, answers to the interval during which the disintegrated nerve-centre is re-integrating itself. Let us observe how among sensations ot all kinds we may trace conformity to this law. An illustration is supplied by the sense of touch. If the fingers be repeatedly swept rapidly over something covered by numerous small prominences, as the papillated surface of an ordinary counterpane, a peculiar feeling of numbness in them results : the objects touched the moment after seem smoother than usual; implying that the small irregularities on them produce less vivid impressions. That the sensation of muscular tension undergoes a variation similarly caused, everyone knows. After carrying a very heavy body in the hand for some time, a small body held in the same hand appears to have lost its weight; showing that the nerve-centre which is the seat of the sensation has been, for the moment, rendered obtuse. How the gustatory faculty is exhausted for a time by a strong taste, daily experience teaches. When sugar or honey has just been eaten, things that are but slightly sweetened seem to have no sweetness. While the palate is still hot with a curry, an unflavoured dish seems insipid; and a glass of liqueur is fatal to the appreciation of a choice wine. Even more marked is that incapacity of the sense of smell caused in like manner. The intensity of the pleasurable feeling given by a rose held to the nostrils, rapidly diminishes ; and when the sniffs have been continued for some time, scarcely any scent can be per- 110 THE DATA OP PSYCHOLOGY. ceived. A few minutes* rest partially restores the impressi¬ bility; but a long interval must elapse before the odour is enjoyed as keenly as at first. This quick exhaustion, producing in such cases some disappointment, has its correlative advantage when the smells are disagreeable. Very soon these become much less perceptible; and to those living in it a stench gives scarcely any annoy¬ ance. The feelings generated by sonorous vibra¬ tions rarely show us this variation in a marked degree; being, as they commonly are, too short to leave much nervous prostration. A strong taste, or odour, or sensation of muscular tension, is due to an action on the nerves that is maintained for a considerable time; but the actions to which are due those loud sounds required to cause tempo¬ rary unimpressibility, are mostly very brief. Illustrations are to be expected only in special cases ; and in these we find them. The bang of a cannon is described as deafening by those who are close to the cannon when it is fired, be¬ cause they are rendered for a time partially deaf to ordi¬ nary sounds. On men engaged in artillery-practice, the repeated explosions entail a dulness of hearing that lasts for hours ; and this dulness of hearing becomes per¬ manent in those who are permanently occupied in such practice. Numerous and very conclusive proofs are supplied by the feelings we receive from light. There are two classes of them : those showing us a variable sensi¬ bility to light in general, as contrasted with darkness; and those showing us a variable sensibility to each kind of light —each colour. Under the one head the reader may first be reminded of the experience that on going out of broad sun¬ shine into a dimly-lighted place, it is impossible to discern the surrounding objects : only after a time do they become faintly visible, and a considerable interval elapses before they are seen with clearness. Disabilities similarly caused are dis¬ closed, when, instead of acting on the retinas as wholes, we act differently on their different parts. Hence what are called iESTHO-PHYSIOLOGY. Ill negative images. If, after gazing for some moments at an object presenting strong contrasts of light and dark parts, the eyes are turned towards a shaded space, containing no¬ thing conspicuous, there will be perceived a transient image of the object, in which the light and dark parts are re¬ versed. The interpretation of this fact is that those por¬ tions of each retina on which strong light had fallen, to¬ gether with the answering portions of the optic centres, having undergone the most change with correspond¬ ing production of the most feeling, are the next in¬ stant less capable of undergoing change and evolving feeling than the portions on which feeble light had fallen; and hence, when they are together exposed to the same feeble light, the unexhausted parts appreciate it more than the exhausted parts, and a negative image results. The cases of the second class are the well-known phenomena of subjective complementary colours. After looking intently at a surface of bright red, an adjacent surface of white seems to have a greenish tint. The explanation is obvious. Those nervous elements changed by the rays which produce in us the sensation of redness, having been partially inca¬ pacitated, the red rays contained in the white light cause less of their appropriate effect than usual; while the blue and yellow rays causing their usual effects, and therefore relatively-predominant effects, a sensation of greenness arises. This decrease in the susceptibility to a feeling of any kind, which immediately follows a feeling of that kind, is not a constant decrease. It is a decrease that varies greatly in degree ; and from its variation we may derive further instructive evidence. Other things equal, it is small or great according to the great or small constitutional vigour. One of these disabilities lasts for a scarcely appreciable time when the vital activities are high; and lasts for a time that becomes longer and longer as the vital activities flag. Abundant proof of this is 112 THE DATA OF PSYCHOLOGY. furnished by the negative images just described. In youth these are scarcely if at all to be observed: only when an extremely-vivid retinal impression has been pro¬ duced, as by looking at the Sun, is the negative image perceptible. But in middle life and afterwards, especially in debilitated persons, negative images of ordinary objects are very commonly perceived, and often have considerable durations.* Feeling being the subjective correlate of that which we know objectively as nervous action, these facts are obvious corollaries from facts set down in the last chapter. We there saw that the excitement of a nerve- centre involves waste; and that restoration of the nerve- centre to a state of equal susceptibilit}^ can be effected only by repair. Hence the return of fitness for what is objectively stimulation and subjectively feeling, will vary in quickness according to the rate of repair. When the blood is rich and rapidly circulated, the partial disability will bo but momentary; and, unless the sensation has been intense, will be inappreciable. But along with failing nutri¬ tion of the tissues, the disability will become marked and its duration longer. In further illustration of this, I may name the fact that negative images are most conspicuous on awaking in the morning, when the circulation is slow. The sense of hearing yields parallel evi¬ dence ; though evidence of which the parallelism is not * This change comes on so gradually that very few remark it ; and the usual supposition is that negative images are much the same at all ages and in all persons. I am able, however, to give personal testimony to the con¬ trary. When about twenty years of age, my attention was drawn by my father to a case in which the circumstances were favourable for perceiving the negative image, and in which he perceived it clearly. To me it was invisible ; and I well remember his remark, that I should begin to see such, images as I became older. He was right. I now see them distinctly ; and, moreover, I observe that they are most distinct at times of least vigour. It is worth while inquiring how far this change affects the appreciation of the chromatic harmonies. It seems inferable that the harmonies of complementary colours become more perceptible as life ad¬ vances. 2ESTH0-PHYSI0L0GY. 113 immediately obvious. Persons on whom old age or debility brings deafness, frequently describe themselves as having no difficulty in hearing sounds, but as being unable to disentangle and identify words when they are indistinctly or rapidly uttered. Supposing that in such cases the nervous structures concerned suffer from faulty nutrition, we have an explanation of this peculiarity. For if each of the successive sounds entails waste of the auditory centres, and leaves them less sensitive to like sounds, it must fol¬ low that, when re-integration is slow, the like sounds im¬ mediately afterwards received will produce less than their due amounts of sensation. These defects of sensation will show themselves most in a comparative deadness to those delicate consonantal modifications by which words are mainly distinguished from one another—the utterances listened to will seem a series of vowel-sounds joined by blurred consonants. Hence the reason why persons thus affected, ask those who address them to articulate slowly and clearly. The confusion of impressions produced by rapid speech on auditory centres thus debilitated, may be conceived by supposing debilitated optic centres to be similarly treated. If a person in whom the negative images are strong, has a series of objects passed before his eyes so fast that he can have only a momentary glance at each (to parallel the momentary opportunity which the ears have of identifying each successive articulation) ; then it will manifestly happen that the negative image of each object will interfere with, and confuse, the positive image of the next; and such a person will therefore not identify the successive objects so readily as one whose optic centres are repaired with normal speed. As confirming the be¬ lief that this defect of hearing is so caused, I may add that it frequently co-exists with the defect of vision to which I have compared it; and also that the one, like the other, is most marked early in the day, and is diminished by whatever invigorates the circulation. i 114 THE DATA OF PSYCHOLOGY. § 46. Another class of correlations demands a passing notice. Up to this point, the feelings considered have been subjective aspects of those changes which objectively are nervous stimulations. We have now to consider certain other feelings which are the inner faces of what on their outer faces are nervous discharges. Having traced pretty fully the concomitance of sentient states and recipio-motor acts, it will suffice to trace briefly the concomitance of sentient states and dirigo-motor acts. Certain inferior dirig o-motor acts are unconscious; but omitting these, the law is that with each muscular contraction there goes a sensation more or less definite. This is not a sen¬ sation indirectly produced through the nerves proceeding in¬ wards from the skin, some of which are nearly always disturbed by each bodily motion; but it is a sensation directly pro¬ duced, either by the discharge itself or by the state of the muscle or muscles excited. It is most clearly distinguished when, without touching anything and without moving, a leg or arm is held out at right angles to the body. Vague as are feelings of this class in comparison with most feelings accompanying nervous stimulations, and much less numerous as are the varieties of quality among them, they are nevertheless so far definite and different that we can, to a certain extent, recognize the separate feeling be¬ longing to each separate contraction. We are aware with¬ out looking at it, and without touching anything with it, which finger has been bent by the discharge sent to its flexor muscles; and, by the particular combination of feel¬ ings accompanying the act, the placing of a limb in a given attitude is present to consciousness without aid from the eyes or hands. I say we can to a certain extent recognize the changes we thus set up; because the differences among the sensations of muscular tension soon lose much of their distinctness. It is a curious fact that when a limb has been held for some time in any position, especially if the position is one involving but little strain, the subjective state asso- iESTHO-PHYSIOLOGY. 115 ciated with the nervous discharge to its muscles, becomes so indefinite that the attitude of the limb is unknown, if there does not happen to be a recollection of it. Besides the connection between what we know objectively as a particular motor act, and subjectively as a particular feel¬ ing of muscular tension, there is a connection between the accompanying motor excitement propagated throughout the muscular system, and a certain diffused feeling of which it is the seat. How along with each special nervous discharge there goes a general nervous discharge, we saw in the last chapter; and here we recur to the relation only to observe that there is a parallel relation between the concomitant states of consciousness. Thus the vivid sensation caused by putting the foot into scalding water, does not lead only to the muscular contractions and muscular feelings which accompany the sudden withdrawal of the leg, but also to contractions of countless other muscles throughout the body, and a feeling called a shock or start. Nor are these subjective states, special and general, that accompany special and general discharges to the muscles, the only subjective states that accompany discharges. As before pointed out, the vascular system and the alimentary system receive their shares of each discharge—very appre¬ ciable when it is intense, and probably in ho case wanting; and these, too, present inner aspects to consciousness. Some¬ times, indeed, the feelings that go along with discharges into the vaso-motor and sympathetic nerves, are the pre¬ dominant ones; as instance the thrill diffused through the body by certain acute creaking sounds said to “ set the teeth on edgeor the nausea produced by particular kinds of disagreeable odours. § 47. Are these correlations between nervous actions and the concomitant feelings quantitative ? Is there such con¬ nection between a physical change in the nervous system and the nsychical change accompanying it, that we may i 2 no THE DATA OF PSYCHOLOGY. regard the one as an equivalent of the other, in the same sense as we regard so much heat as the equivalent of so much motion ? The reader will perhaps expect an affirma¬ tive answer; hut if an affirmative answer is to he given, it must be given in a greatly-qualified form. On remembering that many nervous actions are always unconscious; on also remembering that various objective states of the nervous system which have associated sub¬ jective states early in life, cease to have them later in life; and on remembering, further, that at the same period of life a change set up in an afferent nerve may cause an appreciable feeling, or may not cause it, according as the attention is free or occupied; we shall see that the con¬ nection between feelings and nervous changes is con¬ ditioned in a very complex way, and that if they are quantitatively related it can be only within the narrow limits implied by the complex conditions. If be¬ tween a purely voluntary act and a purely automatic act there are gradations—if, at the one extreme, feeling is a conspicuous accompaniment, and, at the other extreme, ceases to be an accompaniment; then, clearly, in the in¬ termediate phases the amount of feeling must bear a vary¬ ing ratio to the amount of nervous change which the act implies. Again, if we assume that what is present to consciousness as a sensation of given strength, is the correlate of a proportionate molecular disturbance in all the nervous structures concerned, how shall we interpret the sensations distinguished as subjective ? In sundry ab¬ normal states, strong feelings of cold or heat are felt throughout the body, though its actual temperature has remained unaltered. As in any case of this kind the total nervous change cannot have been the same as if the skin had fallen or risen in temperature to the degree ordinarily required to produce- the feeling, we cannot say that there is a quantitative equivalence between the amount of nervous change and the amount of feeling. The disagreeable smell ^STHO-PHYSIOLOGY. 117 which, on the approach of a fit, the epileptic patient fre¬ quently complains of, affords a yet better illustration. Here the outer ends of the afferent nerves being undis¬ turbed, and only certain central structures irritated, the quantity of nervous action is not the same as if the sensation had been generated by an actual smell. More conspicuously still do we see the variability of this rela¬ tion, when we compare the feelings called efforts with the discharges and muscular strains produced by them under different conditions. If the psychical force known as effort were transformable into a constant quantity of physical force, then, in any two cases, equal efforts should produce equal contractions. But the} 7 do not. Great exertion in a child fails to evolve from its motor organs the dynamic effect which a small exertion evolves from those of a man. Any one who is fatigued finds that an in tenser feeling of strain is requisite to generate a given degree of muscular tension, than when he is fresh. And those pros¬ trated by illness show us that immense expenditures of feel¬ ing are needed to perform acts which, during health, need scarcely appreciable expenditures of feeling. Doubtless these differences are partly due to differences in the muscles ; which, when undeveloped or when wasted, are excited to smaller amounts of tension by equal amounts of discharge. But we must regard them as partly due to the imperfect development, or the worn state, of the interme¬ diate motor centres and efferent nerves, in which a given feeling excites a smaller molecular disturbance than when they are finished in structure and in complete repair—a conclusion enforced by the familiar experience that purely nervous acts, as those of thought, require unusual efforts when the brain is tired. This variability of the quantitative relation between nervous actions and psychical states, is equally seen when we limit our comparisons to those nervous actions and psychical states which occur in the same individual under 118 THE DATA OF PSYCHOLOGY. the same bodily conditions. To show that unlike but equally intense sensations may be produced by peripheral disturbances widely unlike in their amounts, providing they arise in different external sense-organs, is scarcely possible without comparing the amounts of the incident forces,- and this 'we cannot properly do, since we are here confining our attention to correlations within the organism. We are similarly debarred from going at length into the quantita¬ tive contrasts between the muscular tensions produced by the same feeling of effort, according as the muscles excited are large or small; for we cannot well establish these con¬ trasts without measuring the muscular tensions by the external actions they are equivalent to. There is, however, one class of appropriate cases—those in which irritations arising within the organism, set up sensations that cause undirected motor discharges. Violent toothache, for ex¬ ample, is due to waves of molecular change sent through one or two minute afferent nerve-fibres ; but the bodilv contortions show us that the feeling so produced, suffices to send waves of molecular change through various large bundles of efferent nerve-fibres, and to contract numerous muscles with much force. To which of these disturbances, centripetal or centrifugal, is the feeling equivalent ? We cannot say to both, for one is many times the other in amount; and we have no reason to say that it is equivalent to one rather than to the other: the rational inference being' that it is not equivalent to either. To understand the real relations between objective and subjective changes in the nervous system, we need but to recall certain of the conclusions reached in preceding chapters. The essential principle of nervous organiza¬ tion we have seen to be that the small amounts of mo¬ tion received, liberate larger amounts, and these again still larger amounts. A disturbance in the end of an afferent nerve is multiplied as it traverses the nerve, and the degree of multiplication varies with tho length of the AESTHO-PHYSIOLOGY. 119 nerve; it is mucli more multiplied in the first ganglion reached, and increases further in traversing the centripetal nerve; it is again multiplied in the superior centre, to be afterwards augmented in its subsequent centrifugal course ; and it is once more multiplied, probably in a far greater degree, in the contractile substance of the excited muscles. Hence the accompanying feeling, which is the subjective aspect of this disturbance at one of its intermediate stages, can be a quantitative equivalent neither of the initial nervous change nor of the terminal nervous change. Moreover, since the multiplication varies in degree, being much greater in the organs of the higher senses than in those of the lower, it follows that the ratio between the amount of feeling and the amount of initial change is far from constant; and the evidence clearly indicates a like inconstancy of the ratio between the amount of feeling and the amount of terminal change, according as one or other muscle or set of muscles is made to act. How then can there be any quantitative relation, it will be asked. If there is no equivalence between a disturbance set up at the periphery and the produced feeling, and no equivalence between the produced feeling and the motor discharge that follows—if the feeling does not even bear the same ratio to either the initial or the terminal nervous change in different cases; what quantitative relation can there be ? The reply is simple. There is a quantitative relation between nervous change and feeling when all other things remain the same; and there is a quantitative relation between feeling and resulting contraction when all other things remain the same. Supposing every condition to continue unaltered, then the stimulus conveyed through a given nerve to a given centre, will evoke a feeling that increases and decreases in something like the same pro¬ portion as the stimulus increases and decreases; and, sup¬ posing a given muscle to be contracted, then the amount of its contraction will bear a tolerably constant ratio to 120 THE DATA OF PSYCHOLOGY. the feeling of effort that accompanies tlie contraction of it. The nature of these correlations may best be expressed by numbers. If, coming through a given afferent nerve, a disturbance represented by 1 generates a feeling repre¬ sented by 5, then disturbance 2 will generate feeling 10, and disturbance 5 feeling 25; and if, acting through a given efferent nerve, feeling 5 results in muscular tension 60, feeling 10 will result in muscular tension 120. But to complete this numerical expression of the facts we must suppose these ratios to vary with every set of afferent nerves and every set of efferent nerves. If we say that 1 to 5 represents the ratio of disturbance to feeling in the sense of touch, then to represent it in the sense of hearing will need, say, 1 to 100, and in the sense of sight perhaps 1 to 1,000 ; and similarly with the ratios throughout the motor apparatus, according as the muscles are large or small. In brief, then, the quantitative correlation of feeling' and nervous change, holds true only within narrow limits. We have good reason to conclude that at the particular place in a superior nervous centre where, in some mys¬ terious way, an objective change or nervous action causes a subjective change or feeling, there exists a quantitative equivalence between the two : the amount of sensation is proportionate to the amount of molecular transformation that takes place in the vesicular substance affected. But there is no fixed, or even approximate, quantitative relation between this amount of molecular transformation in the sentient centre, and the peripheral disturbance originally causing it, or the disturbance of the motor apparatus which it may eventually cause. § 48. The feelings called sensations have alone been con¬ sidered thus far; leaving out of view the feelings dis¬ tinguished as emotions. Much less definite as they are, and not capable of being made at will the objects of ob- ^STHO-PHYSIOLOGY. 121 servation arid experiment, tlie emotions are more difficult to deal with. But having discerned certain general laws to which the simpler feelings conform, we may now ask whether, so far as we can see, they are conformed to by the more complex feelings. We shall find that they are. The conditions essential to the one are essential to the other. Emotions, like sensations, may be increased or decreased in intensity by altering either the quantity or the quality of the blood. That general abundance of blood is a cause of emotional exaltation, though tolerably certain, is not easily proved; but there is sufficient evidence of the converse fact that, other things equal, depletion is a cause of apathy. The effect of local abundance of blood is undoubted : there is no question that, within limits, the amount of emotion varies as the amount of blood supplied to the great nervous centres. That nervous stimulants in¬ tensify the emotions, or, as we say, raise the spirits, is even more manifest than that they make the sensations keener. And it is a familiar truth that sedatives diminish what is distinguished as moral pain, in the same way that they diminish pain arising in the trunk or limbs. That a feeling lasts an appreciable time, is no less true of an emotion than of a sensation : indeed the per¬ sistence is relatively conspicuous. The state of conscious¬ ness produced by a flash of lightning, is so brief as to seem instantaneous : only by the help of artificial tests are sensations of this kind found to have measurable durations. But no such tests are needed to prove that emotions con¬ tinue through appreciable periods. Even a simple emotion, as of anger or fear, does not reach its full strength the moment the cause presents itself; and after the cause is removed it takes some time to die away. When hereafter we deal with the origin of emotions, and recognize the fact that they are of far more involved natures than sensa¬ tions, and imply the co-operation of extremely intricate 122 THE DATA OF PSYCHOLOGY. nervous structures, we shall understand liow this greater duration is necessitated. That an emotion, like a sensation, leaves behind it a tem¬ porary incapacity, is also true; and as the emotion produced by a momentary cause lasts longer than a sensation pro¬ duced by a momentary cause, so does the partial incapacity for a like emotion last longer than the partial incapacity for a like sensation. Passions of all kinds come in gushes or bursts. That they often continue for hours and days, is true; but they are never uniform throughout hours and days. Be it in grief, or joy, or tenderness, there is always a succession of rises and falls of intensity—a paroxysm of violent feeling with an interval of feeling less violent, followed by another paroxysm. And then, after a succession of these comparatively-quick alternations, there comes a calm — a period during which the waves of emotion are feebler: succeeded, it may be, by another series of stronger waves. As in the case of the sensations so in the case of the emotions, this follows from the fact that what is objectively a nervous action and subjectively a feeling, involves waste of the nervous structures concerned. The centres which are the seats of emotions undergo dis¬ integration in the genesis of emotions ; and, other things remaining equal, thereupon become less capable of genera¬ ting emotions until they are re-integrated. I say, other things remaining equal, because the rise of an emotion brings blood to the parts implicated, and so long as the afflux is increasing the intensity of the emotion may in¬ crease, notwithstanding the waste that has taken place; but the several conditions on which activity depends having become constant, a diminished capacity for emotion inevit¬ ably follows each gush of emotion. That daily rises and falls of strength, consequent on daily periodicities of waste and repair, occur in the emotions as in the sensations, is also tolerably mani¬ fest. Cultivated people, mostly leading lives that exercise 5CSTH0-PHYSI0L0GY. 123 their brains too much and their muscles too little, and placed in social conditions that commonly bring the strongest excitements towards the close of the day, are subject to an abnormal periodicity. But those whose lives conform best to the laws of health, exhibit early in the day a general joyousness and emotional vivacity greater than they do towards its close, when approaching sleepi¬ ness is shown by a flagging interest in the things and actions around. These complex feelings that are centrally initiated are also like the simple feelings that are peripherally initiated, in having general discharges as well as special discharges : indeed their general discharges are the more conspicuous of the two. A sensation is often visibly followed only by local movement: unless very strong its effect on the organ¬ ism as a whole is unobtrusive. But an emotion, besides the more obvious changes it w T orks in the muscles of the face, habitually works changes, external and internal, throughout the body at large. The respiration, the circula¬ tion, the digestion, as well as the attitudes and movements, are influenced by it even when moderate; and everyone knows how strong passions, pleasurable or painful, pro¬ foundly disturb the whole system. § 49. Nothing has yet been said about the most con¬ spicuous and most important distinction existing among the feelings. Every feeling, besides its minor variations of intensity, exists under two strongly-contrasted degrees of intensity. There is a vivid form of it which we call an actual feeling, and there is a faint form of it which we call an ideal feeling. What is the nature of this difference as interpreted from our present stand-point ? When studying nerve-structure, we saw that, in addition to connections formed by grey matter between the central ends of afferent and efferent nerves, these have connec¬ tions with centripetal and commissural nerves, which 124 THE DATA OF PSYCHOLOGY. are again connected with more distant nerves. And when studying nerve - function, we saw that a disturb¬ ance set up by an afferent nerve in its ganglion, does not affect exclusively the efferent nerve, but that part of it, conveyed through centripetal and commissural nerves, affects other centres, and these again others, until it has reverberated throughout the entire nervous system. What follows ? These reverberations are feeble disturb ances. And every centre, liable as it is to be strongly disturbed through its afferent or centripetal nerve, is liable also to be feebly disturbed by these reverberations arriving through other nerves. What then must happen with each of the libero-motor elements composing those higher centres in which nervous changes become changes of consciousness ? When it is affected through the direct and fully-opened route, by that peripheral impression to which it stands organically related, it evolves much molecular motion, becomes an active propagator of disturbances throughout the nervous system, and is the seat of what we call a real feeling; but when it is affected by these secondary waves diffused from other strongly excited parts, it becomes, as compared with them (or with itself under the previous condition) a generator of but little molecular motion, and is the seat of that faint feeling which we distinguish as ideal. In brief, those vivid states of consciousness which we know as sensations, accompany direct and therefore strong excitations of nerve- centres ; while the faint states of consciousness which wo know as remembered sensations, or ideas of sensations, accompany indirect, and therefore weak, excitations of the same nerve-centres. That the contrast of intensity between the effects of direct and indirect excitations, though it holds generally, does not hold without exception, is a fact quite reconcilable with this interpretation. For, on the one hand, a direct excitation may be very feeble ; while, on the other hand, through a concurrence of diffused disturbances, an indirect 2ESTH0-PHYSI0L0GY. 125 excitation may rise to considerable strength. Hence,, oc¬ casionally, an ideal feeling will become almost or quite equal in vividness to a real feeling. Especially may this happen when the nerve-centre concerned is surcharged with blood; since a small disturbance may then set up in it an amount of change equal to that which a great disturbance produces when only the ordinary quantity of blood is pre¬ sent. And it is a matter of observation that congested nerve-centres are those in which indirectly-excited feelings reach an intensity scarcely less than that of directly-excited feelings. When we pass from the feelings called sensations, of which the strong forms are peripherally initiated, to the feelings called emotions, of which the strong forms are centrally initiated, we find the difference between the strong and the weak forms by no means so great; so that, in fact, ideal emotion passes into actual emotion without any line of de¬ marcation. Obviously this is what might be anticipated. For whether ideal or actual, emotion is an accompaniment of an indirect excitation : it is not an immediate result of peripheral impressions, either simple or combined; but a mediate or remote result of them. Hence, all emo¬ tions, vivid and faint, being the subjective aspects of ob¬ jective nervous changes that are produced indirectly, are distinguishable only according to the degree of indirectness of the excitation, and this admits of insensible gradations. § 50. One more general truth must be set down to com¬ plete the outline. The foregoing inferences joined with some contained in the last chapter, introduce us to it. In §§ 36, 37, it was pointed out that nerve-centres dis¬ integrated by action, are perpetually re-integrating them¬ selves, and again becoming fit for action. We saw that repair partially makes up for waste from instant to instant, and that the arrears of repair are made up daily during that period of quiescence when waste almost ceases. We 126 THE DATA OF PSYCHOLOGY. further saw that the restoration of a nerve-centre to its state of integrity, is not only the filling up of its quantum of de¬ composable matter, but is also the replacing of molecules most exposed to disturbance, and consequently the produc¬ tion of a comparatively-unstable state. And we saw how, after a period of profound repose, there thus arises a con¬ dition of the nerve-centres such that very slight stimuli cause nervous discharges. This law applies not generally only, but specially to each nerve-centre and each of its component parts. In propor¬ tion as any part of a nerve-centre has been for a long time unused—in proportion, that is, as repair of it has gone on day after day and night after night unhindered by appre¬ ciable waste, it must be brought to a state of more than ordinary instability—a state of excessive readiness to de¬ compose and discharge. What must happen ? In common with all other parts, it is exposed to these reverberations which from instant to instant fill the nervous system. Its extreme instability must render it unusually sensitive to these reverberations—unusually ready to undergo change, to yield up molecular motion, and to become the seat of the concomitant ideal feeling. Besides a great liability to the ideal feeling this same condition must entail a great strength of it; and so while the instability continues, a strong ideal feeling will be perpetually aroused. As, however, the nerve-centre in which such secondary mole¬ cular changes and accompanying ideal feelings are thus set up, is somewhat wasted by them, it follows that after they have gone on for a considerable period the instability of the centre will be diminished: it will no longer be so easily de¬ composed by indirect disturbances, and the feeling will not be produced. Here we have the interpretation of what are called desires. Desires are ideal feelings that arise when the real feelings to which they correspond have not been ex¬ perienced for some time. They are then liable to be excited ^ToTHO-PHTSIOLOGY. 127 \y various of tlie indirect disturbances reflected from part to part of the nervous system. They are usually vivid and persistent in proportion to the previous period of rest— more vivid and more persistent than ideal feelings of the same kind under ordinary conditions. But after a pro¬ longed period during which they continue to arise and almost monopolize consciousness, they become feebler and finally die away. § 51. Such are the leading truths of AEstho-Physiology, set forth with as much fulness as is here requisite. Sensa¬ tion and emotion in their relations to nervous action, have been dealt with generally; and whatever has been said of special sensations and special emotions has been said merely to illustrate a law which holds among all the rest. The concomitants, subjective and objective, of each particular kind of sensation and each particular kind of emotion, I here pass over. They may be studied to great advantage in the works of Professor Bain on The Senses and the Intellect, and The Emotions and the Will; in which lie has given an elaborate account of the connection between each particular feeling, simple or complex, and its various physical accompaniments. To these works I must com¬ mend the reader who wishes to trace out these minor cor¬ relations. As data for the present treatise, the only facts needful to be carried with us are those set forth in the preceding sections. They may be summed up thus. Feeling of whatever kind is directly known by each person in no other place than his own consciousness. That feelings exist in the world beyond consciousness, is a belief reached only through an involved combination of infer¬ ences. That alike in human and inferior beings, feelings are accompaniments of changes in the peculiar structure known as the nervous system, is also an indirectly- established belief. And that the feelings alone cognizable by any individual are products of the action of his own 128 THE DATA OP PSYCHOLOGY. nervous system, which he has never seen and on which ho can try no experiments, is a belief only to be arrived at through a further chain of reasoning. Nevertheless, the evidence, though so indirect, is so extensive, so varied, and so congruous, that we may accept the conclusion without hesitation. The conclusion having been accepted—provisionally if not permanently—its validity is shown by leading us to anticipate truly, in one set of cases after another, the par¬ ticular subjective phenomena that accompany particular objective phenomena. We have seen that the several circumstances which facilitate or hinder nervous action, are also circumstances which facilitate or hinder feeling. We have seen that as nervous action occupies appreciable time, so feeling occupies appreciable time. We have seen that each feeling leaves a partial incapacity for a like feel¬ ing, as each nervous action leaves a partial incapacity for a like nervous action. We have seen that, other things equal, the intensities of feelings vary as the intensities of the correlative nervous actions. We have seen that the difference between direct and indirect nervous disturbances, corresponds to the difference between the vivid feelings we call real and the faint feelings we call ideal. And we have seen that certain more special objective phenomena which nervous actions present, have answering subjective phenomena in the forms of feeling we distinguish as desires. Thus, impossible as it is to get immediate proof that feeling and nervous action are the inner and outer faces of the same change, yet the hypothesis that they are so harmonizes with all the observed facts; and, as elsewhere shown (First Principles , § 40) no other verification is pos¬ sible for us than that which results from the establishment of complete congruity among our experiences. CHAPTER VII. THE SCOPE OF PSYCHOLOGY. § 52. We may now enter on our special topic. Tlius far we have been ocoupied with the data of Psychology, and not with Psychology properly so-called. Here leaving the foundations we pass to the superstructure. Not a few readers will be surprised by the assertion that none of the truths we have been contemplating are psycho¬ logical truths. Since the anatomy and physiology of the nervous system have occupied so much attention, and since it has been growing manifest that there is a fundamental connection between nervous changes and psychical states, there has arisen a confusion between the phenomena which underlie Psychology and the phenomena of Psychology itself. In reality, all the facts ascertained by those who have made nerve-structure and nerve-function their studies, are facts of a simpler order than those rightly termed psychological; though they are facts entering into the composition of psychological facts. Most will admit without hesitation that the first five chapters of this part consist of propositions which are exclu¬ sively morphological and physiological. In them the struc¬ ture of the nervous system, its functions, the conditions to its action, &c., have been dealt with purely as physical phenomena—phenomena as purely physical as the absorption K 130 THE DATA OF PSYCHOLOGY. of nutriment or the circulation of the blood. Whatever implications may have arisen from the use of words that carry with them indirect meanings, the direct meanings of all the propositions set down have nowhere implied con¬ sciousness or feeling; and, ignoring consciousness or feeling, they have left out that which is tacitly or avowedly contained in every proposition of Psychology. It will probably be thought, however, that at any rate truths belonging to Psychology proper are to be found in the last chapter. Dealing as the last chapter does with the connections between nervous changes and feelings, it necessarily becomes, by including a psychical element, a part of psychical science. To this the rejoinder is that, though it can scarcely be excluded absolutely from the body of this science, yet it does not strictly fall within that body. ^Estho- physiology has a position that is entirely unique. It belongs neither to the objective world nor the subjective world; but taking a term from each, occupies itself with the correlation of the two. It may with as much propriety be included in the domain of physical science as in the domain of psy¬ chical science; and must be left where it stands, as the link between them. Perhaps this explanation will increase rather than decrease the surprise produced by the assertion that was to be justi¬ fied. To clear up the confusion, we must examine more carefully the distinction between the truths which are strictly psychological and those which merely enter into the composition of psychological truths. § 53. Throughout the preceding chapters, including even the last, every proposition set down has expressed some relation of phenomena occurring within the limits of the organism. The subject-matter has been the character of a structure; or the effect which a disturbance set up in one place has in causing motion in another; or the connection between the physical state of the whole or a part of the THE SCOPE OF PSYCHOLOGY. 131 organism, and some general or local nervous process; or the variable intensity of an action in a nerve-centre as determined by a preceding like action; or the interdepen¬ dence of internal physical changes and internal psychical changes. That is to say, the attention has everywhere been directed exclusively to co-existenc6s and sequences of which the body alone is the sphere. Distinct or tacit reference has, indeed, frequently been made to some external force. Either a disturbing agent lying beyond the limits of the organism has been referred to in general terms, or, for illustration's sake, this or the other kind of disturbing agent has been named. But such references, vague or distinct,, have been made merely because it was needful to suppose something by which an organic change was set up; not because this something had to be included in the proposi¬ tion set down, which in every case formulated an internal relation only. The entanglement of phenomena is such, that we can never cut off absolutely from all others the par¬ ticular phenomena we are dealing with; but, because we presuppose these other phenomena, it does not follow that the science to which they pertain forms part of the science with which we are specially occupied. For instance, it is impossible to describe, or think of, a chemical experiment that discloses some chemical relation, without making dis¬ tinct or tacit references to physical relations—the pouring and mixture of liquids, the ascent of bubbles of disengaged gas, the falling of a precipitate ; but it is not therefore held that we are including physics in our chemistry. Similarly, it must be admitted that though the foregoing chapters have tacitly assumed environing forces, yet this assumption has been simply incidental to the study of internal co¬ existences and sequences. Now so long as we state facts of which all the terms lio within the organism, our facts are morphological or phy* siological and in no degree psychological. Even though the relation with which we are dealing is that between a k 2 132 THE DATA OF PSYCHOLOGY. nervous change and a feeling, it is still not a psychological relation so long as the feeling is regarded merely as con¬ nected with the nervous change, and not as connected with some existence lying outside the organism. As certainly as the man who demonstrates by dissection the articulations of the bones, and the man who, by a sphygmograph, delineates the varying motions of the heart, are respectively studying morphology and physiology; so certainly is the man who examines nervous structure and experiments on nervous function, a student of these same sciences, if he considers the inner correlations only and does not simultaneously consider the answering outer correlations. For that which distinguishes Psychology from the sciences on which it rests, is, that each of its propositions takes ac¬ count both of the connected internal phenomena and of the connected external phenomena to which they refer. In a phy¬ siological proposition an inner relation is the essential sub¬ ject of thought; but in a psychological proposition an outer relation is joined with it as a co-essential subject of thought. A relation in the environment rises into co-ordinate im¬ portance with a relation in the organism. The thing con¬ templated is now a totally different thing. It is not the connection between the internal phenomena, nor is it the connection between the external phenomena; but it is the connection between these two connections. A psychological proposition is necessarily compounded of two propositions, of which one concerns the subject and the other concerns the object; and cannot be expressed without the four terms which these two propositions imply. The distinction may be best explained by symbols. Suppose that A and B are two related manifestations in the environment—say, the colour and taste of a fruit; then, so long as we contemplate their relation by itself, or as associated with other external phenomena, we are occupied with a portion of physical science. Now suppose that a and b are the sensations pro¬ duced in the organism by this peculiar light which the fruit THE SCOPE OP PSYCHOLOGY. 133 reflects, and by tbe chemical action of its juice on the palate; then, so long as we study the action of the light on the retina and optic centres, and consider how the juice sets up in other centres a nervous change known as sweetness, we are occupied with facts belonging to the sciences of Physiology and iEstho-physiology. But we pass into the domain of Psychology the moment we inquire how there comes to exist within the organism a relation between a and b that in some way or other corresponds to the relation between A and B. Pyschology is exclusively concerned with this connection between (A B) and (a b )—has to investigate its nature, its origin, its meaning, &c. A momenPs introspection will now make it clear to the reader, that he cannot frame any psychological conception without thus looking at internal co-existences and se¬ quences in their adjustments to external co-existences and sequences. If he studies the simplest act of per¬ ception, as that of localizing a touch in some part of Ins skin, the indispensable terms of his inquiry are:—on the one hand a thing (1) and a position (2), both of which he regards as objective; and on the other hand a sensation (3), and a state of consciousness constituting his apprehen¬ sion of position (4), both of which he regards as subjective. Again, to cite an example from the opposite extreme, if he takes for his problem one of his involved sentiments, as that of justice, he cannot represent to himself this sentiment, or give any meaning to its name, without calling to mind actions and relations supposed to exist in the environment: neither this nor any other emotion can be aroused in consciousness even vaguely, without positing something beyond consciousness to which it refers. And when, instead of studying Psychology subjectively, he studies it objectively in the acts of other beings, he similarly finds himself incapable of stirring a step with¬ out thinking of inner correlations in their references to outer correlations. 134 THE DATA OF PSYCHOLOGY. § 54. It is contended by some that Psychology is a part of Biology, and should be merged in it; and those who hold this view will possibly .answer the above argument by saying that in many cases the non-psychological part of Biology also takes into account phenomena in the environ¬ ment, and even definite connections among these pheno¬ mena. The life of every organism is a continuous adaptation of its inner actions to outer actions ; and a complete interpretation of the inner actions involves recognition of the outer actions. The annual production of leaves, flowers, and seeds by plants, is adjusted to the annual changes of the seasons; and there is in animals an adjust¬ ment between external changes in temperature and abund¬ ance, and internal production of ova. Moreover, there are many special relations of structure and function in plants and animals, that have reference to special relations of struc¬ ture and function in surrounding plants and animals: in¬ stance those arrangements of the sexual organs that fit particular phaenogams for being fertilized by the particular insects that visit them. But true as is this conception of Life (and having based the Principles of Biology on it I am not likely to question or to undervalue it), I nevertheless hold the distinction above drawn to be substantially valid. For through¬ out Biology proper, the environment and its correlated phenomena are either but tacitly recognized, or, if overtly and definitely recognized, are so but occasionally; while the organism and its correlated phenomena practically monopolize the attention. But in Psychology, the cor¬ related phenomena of the environment are at every step avowedly and distinctly recognized ; and are as essential to every psychological idea as are the correlated phe¬ nomena of the organism. Let us observe the contrast as exemplified. We study digestion. Digestion implies food. Food implies neighbouring plants or animals. But this implication scarcely enters into our study of digestion. THE SCOPE OP PSYCHOLOGY. 135 unless we ask the quite special question—how the digestive organs become fitted to the materials they have to act upon? Again, when we interpret respiration we take for granted a surrounding oxygenated medium. And yet to show how far the two may be separated, we need only remember that the phenomena of respiration may be very well traced out in one who breathes a bladder of gas artificially obtained from peroxide of manganese or chlorate of potash. Once more, if, in following out the life-history of a plant, we have to note the adaptation of its hooked seeds to the woolly fleece of the animal which accidentally carries them off and disperses them, this distinct reference to specially-connected phenomena in the environment, occurs either but once in an account of the plant’s life, or only at long intervals. In fact, we may say that the great mass of purely biological pheno¬ mena may be displayed for some time by an organism de¬ tached from its medium, as by a fish out of water. Now observe how different it is with psychological phenomena. We cannot explain a single act of a fish as it moves about in the water, without taking into account its relations to neighbouring objects distinguished by specially-related attributes. The instinctive proceedings of the insect, equally with those which in higher creatures we call in¬ telligent, we are unable even to express without referring to things around. In brief, then, the propositions of Biology, when they imply the environment at all, imply almost exclusively its few general and constant phenomena, which, because of their generality and constancy, may be left out of con¬ sideration; whereas the propositions of Psychology refer to its multitudinous, special, and ever-varying phenomena, which, because of their speciality and changeability, can¬ not be left out of consideration. § 55. The admission that Psychology is not demarcated from Biology by a sharp line, will perhaps be construed 136 THE DATA OP PSYCHOLOGY. into the admission that it cannot rightly he regarded as a distinct science. But those who so construe the admission, misconceive the natures of the relations among the sciences. They assume that there exist objectively those clear separa¬ tions which the needs of classification lead us to make subjectively. Whereas the fact is, that beyond the divisions between the three fundamental orders of the sciences, Ab¬ stract, Abstract-concrete, and Concrete, there exist ob¬ jectively no clear separations at all: there are only differ¬ ent groups of phenomena broadly contrasted but shading off one into another. To those who accept the doctrine of Evolution, this scarcely needs saying; for Evolution being a universal process, one and continuous throughout all forms of existence, there can be no break—no change from one group of concrete phenomena to another without a bridge of intermediate phenomena. It will be well here, however, to show by illustrations that the simpler concrete sciences are separable from one another only in the same way that Psychology is separable from Biology. Astronomy and Geology are regarded as distinct. But Geology is nothing more than a chapter continuing in detail one part of a history that was once wholly astrono¬ mic ; and even now, many of its leading facts belong as much to the older part of the history as to the younger. Not only do we trace back the Earth to a time when its astronomic attributes were uncomplicated by those geo¬ logic ones that have gradually arisen as it cooled; not only in the solar heat, causing the aerial, marine, and fluvial currents which work most geologic changes, are we com¬ pelled to recognize an astronomic force ; but in the tidal wave we have a phenomenon as much astronomic as geo¬ logic, and as much geologic as astronomic. Even he who arbitrarily excludes from astronomy everything but the molar motions throughout the Solar System (so ignoring the radiant light and heat by which alone the Sun and planets are known to us) does not escape this difficulty; THE SCOPE OF PSYCHOLOGY. 137 for the motion of the tidal wave is a molar motion gene¬ rated by forces such as generate all other molar motions exhibited by the Solar System; and yet it is at the same time a motion of matter on the Earth’s surface, not dis¬ tinguishable from those other motions of matter which constitute geological changes, many of which, indeed, are concomitants of it. The separation between Bio¬ logy and Geology once seemed impassable; and to many seems so now. But every day brings new reasons for believing that the one group of phenomena has grown out of the other. Organisms are highly-differentiated portions of the matter forming the Earth’s crust and its gaseous envelope; and their differentiation from the rest has arisen, like other differentiations, by degrees. The chasm between the inorganic and the organic is being filled up. On the one hand, some four or five thousand compounds once regarded as exclusively organic, have now been produced artificially from inorganic matter; and chemists do not doubt their ability so to produce the highest forms of organic matter. On the other hand, the microscope has traced down organisms to simpler and simpler forms until, in the Protogenes of Professor Haeckel, there has been reached a type distinguishable from a fragment of albumen only by its finely-granular character. Thus the distinction between Biology and Psychology has the same justification as the distinctions between the con¬ crete sciences below them. Theoretically, all the concrete sciences are adjoining tracts of one science, which has for its subject-matter, the continuous transformation which the Universe undergoes. Practically, however, they are dis¬ tinguishable as successively more specialized parts of the total science—parts further specialized by the introduction of additional factors. The Astronomy of the Solar System is a specialized part of that general Astronomy which includes our whole Sidereal System; and becomes specialized by taking into account the revolutions and rotations of planets i 38 THE DATA OF PSYCHOLOGY. and satellites. Geology (or rather Geogeny let us call it, that we may include all those mineralogical and meteorological changes which the word Geology, as now used, recognizes but tacitly) is a specialized part of this special Astronomy; and becomes specialized by joining with the effects of the Earth's molar motions, the effects of continuous decrease in its internal molecular motion, and the effects of the molecular motion radiated from the Sun. Biology is a specialized part of Geogeny, dealing with peculiar aggregates of peculiar chemical compounds formed of the Earth's superficial ele¬ ments—aggregates which, while exposed to these same general forces molar and molecular, also exert certain general actions and reactions on one another. And Psy¬ chology is a specialized part of Biology, limited in its appli¬ cation to the higher division of these peculiar aggregates, and occupying itself exclusive^ with those special actions and reactions which they display, from instant to instant, in their converse with the special objects, animate and in¬ animate, amid which they move. But this introduction of additional factors, which differ¬ entiates each more special science from the more general science including it, fails in every case to differentiate it absolutely; because the introduction of the additional factors is gradual. It is so not with the Concrete Sciences alone, but even with the Abstract-concrete Sciences, which at first sight seem sharply demarcated; as, for instance. Physics and Chemistry. Physics, dealing with changes in the distribution of matter and motion considered apart from unlikenesses of quality in the matter, is obliged to include in its inquiries all the molecular integrations and disintegrations caused by alterations of temperature—the meltings and evaporations which increase of heat produces, as well as the condensations and crystallizations which follow decrease of heat. Among other molecular transformations resulting from losses and gains of molecular motion, are those known as alio tropic—transformations which, without THE SCOPE OF PSYCHOLOGY. 139 appreciably altering tlie degrees of integration, leave tlie molecules so re-arranged that they exhibit new properties of the order we call chemical; as is shown by their changed affinities for the molecules of other substances, and by their changed effects on our nerves of sense. Must we class such molecular transformations as physical phenomena, because in each case the molecules concerned are all of one kind ? If so, what are we to say of isomeric transformations, which all chemists recognize as of essentially the same nature ? In these, molecules of different kinds are concerned. And if, because they show us a re-distribution of heterogeneous molecules instead of homogeneous ones, we put them in the category of chemical phenomena, we arbitrarily dissociate two fundamentally-similar classes of facts. Perhaps it will be replied that in isomeric transformations the molecules are homogeneous, relatively to the re-distribution they undergo; that each of them, retaining its individuality unchanged, comports itself towards the rest as though it were a simple molecule; that nothing more takes place than a re-grouping of these unchanged molecules ; and that there is thus an absence of what constitutes a truly chemical change—union or disunion of unlike molecules. The reply is plausible, but it is easily disposed of. For there are transformations of this nature in which such unions and disunions occur. A colloid compound in passing from one of its isomeric forms to another, very generally parts with some of its contained water, or takes up additional water. Does this make the change a chemical one ? Then we must relegate to the domain of Physics that isomerism which is not accompanied by loss or gain of water, and include in the domain of Che¬ mistry that isomerism which is so accompanied—a very artificial disunion of the sciences, to which I think neither Physicists nor Chemists will agree. Nevertheless, un¬ decided as is the line which separates them, we are not prevented from recognizing the broad distinction between Molecular Physics and Chemistry. The new factor which TIIE DATA OF PSYCHOLOGY. uo differentiates Chemistry from Molecular Physics, is the heterogeneity of the molecules with whose re-distributions it deals. And the contrast hence resulting is too strongly marked to be obliterated by transitional cases. In this way it is, then, that the conspicuous presence of additional factors differentiates Psychology from Biology proper; although in Biology proper these factors make an occasional appearance. The contrast between the two is no more destroyed by such community as exists, than is the contrast between night and day destroyed by the occurrence of a dawn which belongs as much to one as to the other. § 56. A far more radical distinction remains to be drawn. While, under its objective aspect, Psychology is to be classed as one of the concrete sciences which successively decrease in scope as they increase in speciality; under its subjective aspect. Psychology is a totally unique science, independent of, and antithetically opposed to, all other sciences whatever. The thoughts and feelings which constitute a consciousness, and are absolutely inaccessible to any but the possessor of that consciousness, form an existence that has no place among the existences with which the rest of the sciences deal. Though accumulated observations and experiments have led us by a very indirect series of inferences (§ 41) to the belief that mind and nervous action are the subjective and objective faces of the same thing, we remain utterly incapable of seeing, and even of imagining, how the two are related. Mind still continues to us a something without any kinship to other things; and from the science which discovers by in¬ trospection the laws of this something, there is no passage by transitional steps to the sciences which discover the laws of these other things. Following M. Comte, there are a few who assert that a subjective Psychology is impossible; and to such the above paragraph will, I suppose, be meaningless. But whoever recognizes a subjective Psychology, and admits, as he must. THE SCOPE OF PSYCHOLOGY. 141 that without it there can be no objective Psychology, there¬ upon finds himself obliged to assign a quite special rank, not to the first only, but, by implication, to the second. To those who see that the essential conceptions on which Psy¬ chology in general proceeds, are furnished by subjective Psychology—to those who see that such words as feelings, ideas, memories, volitions, have acquired their several mean¬ ings through self-analysis, and that the distinctions we make between sensations and emotions, or between auto¬ matic acts and voluntary acts, can be established only by comparisons among, and classifications of, our mental states; it will be manifest that objective Psychology can have no existence as such, without borrowing its data from sub¬ jective Psychology. And thus perceiving that, until it acknowledges its indebtedness to subjective Psychology, objective Psychology cannot legitimately use any terms that imply consciousness, but must limit itself to nervous co¬ ordinations considered as physical only; they will see that even objective Psychology contains an element which differ¬ entiates it from the rest of the special concrete sciences more than any of these are differentiated from one another. The claims of Psychology to rank as a distinct science, are thus not smaller but greater than those of any other science. If its phenomena are contemplated objectively, merely as nervo-muscular adjustments by which the higher organisms from moment to moment adapt their actions to environing co-existences and sequences, its degree of speciality, even then, entitles it to a separate place. The moment the element of feeling, or consciousness, is used to interpret nervo-muscular adjustments as thus exhibited in the living beings around, objective Psychology acquires an additional, and quite exceptional, distinction. And it is further distinguished in being linked by this common . element of consciousness, to the totally-independent science of subjective Psychology—the two forming together a double science which, as a whole, is quite sui generis. 142 THE DATA OF PSYCHOLOGY. § 57. So understanding its scope, we are now prepared to enter on tlie study of Psychology proper. The foregoing discussion serves not unfitly to introduce the several divi¬ sions into which the entire subject falls. First come the Inductions of Psychology ; under which title we will deal with the leading empirical generaliza¬ tions—presenting them, however, under an aspect some¬ what different from the usual one. And the truths in¬ ductively reached will, when possible, be elucidated deduc¬ tively, by affiliating them on the truths of Neuro-physiology and iEstho-physiology set down in the foregoing chapters. We will next pass to Objective Psychology; of which three divisions may conveniently be made. In the first, or General Synthesis, we will trace throughout the animal kingdom, the progress in these perpetual adjustments of special inner actions to special outer actions, which accom¬ panies increasing evolution of the nervous system—omitting, so far as may be, the element of consciousness. In the second, or Special Synthesis, we will consider this same progress more closely, with the view of delineating and formulating it in terms that imply consciousness. And in the third, or Physical Synthesis, an endeavour will be made to show how, by an ultimate principle of nervous action, this progress is explicable as part of Evolution in general. Turning then to Subjective Psychology, the natures of particular modes of consciousness, as ascertained by in¬ trospection, will first be treated under the head of Special Analysis. And then, under the head of General Analysis, we will enter upon the ultimate question of the relation between Thought and Things. Two remaining divisions will be devoted, the one to a comparison between the results reached in the preceding divisions, with a view of showing their congruity, and the other to a series of corollaries constituting that special part of Human Psychology on which Sociology must be based. PART II. THE INDUCTIONS OF PSYCHOLOGY. CHAPTER I. THE SUBSTANCE OF MIND. § 58. To write a chapter for the purpose of showing that nothing is known, or can be known, of the subject which the title of the chapter indicates, will be thought strange. It is, however, in this case needful—needful because, in the ab¬ sence of explanation, much that has gone before, and much that will come hereafter, may be misinterpreted; and needful also because we have to distinguish between that absolute ignorance and that partial knowledge which may be asserted according as we give one or other meaning to the terms used. For if by the phrase “ substance of Mind,” is to be under¬ stood Mind as qualitatively differentiated in each portion that is separable by introspection but seems homogeneous and un- decomposable»; then we do know something about the sub¬ stance of Mind, and may eventually know more. Assuming an underlying something, it is possible in some cases to see, and in the rest to conceive, how these multitudinous modifi¬ cations of it arise. But if the phrase is taken to mean the underlying something of which these distinguishable portions are formed, or of which they are modifications ; then we know nothing about it, and never can know anything about it. It is not enough to say that such knowledge is beyond the grasp of human intelligence as it now exists; for no amount of that which we call intelligence, however tran¬ scendent, can grasp such knowledge. L 146 THE INDUCTIONS OF PSYCHOLOGY. These two propositions will need a good deal of elucida¬ tion. It will be most convenient to deal first with the last of them. § 59. To meet all imaginable possibilities, let us set out with the doctrine of Hume, that impressions and ideas are the only things known to exist, and that Mind is merely a name for the sum of them. In this case, the expression