THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA PRESENTED BY PROF. CHARLES A. KOFOID AND MRS. PRUDENCE W. KOFOID V PRINCIPLES OF PHYSIOLOGY; DESIGNED FOR THE USE OF SCHOOLS, ACADEMIES, COLLEGES, AND THE GENERAL READER. COMPRISING A FAMILIAR EXPLAIATIOI STRUCTURE AND FUNCTIONS OF THE ORGANS OF MAN, ILLUSTRATED BY COMPARATIVE REFERENCE TO THOSE OF THE INFERIOR ANIMALS. ALSO, AN ESSAY ON THE PRESERVATION OF HEALTH. WITH FOURTEEN QUARTO PLATES AND OVER EIGHTY ENGRAVINGS ON WOOD, MAKING IN ALL NEARLY TWO HUNDRED FIGURES. BY J. C. COMSTOCK AND B. N. COMINGS, M. D. NEW. YORK: PRATT, WOODFORD & CO. HARTFORD: E. C. KELLOGG. 1851. ENTERED, ACCORDING TO ACT OF CONGRESS, IN THE YEAR 1850, BY KELLOGG & COM STOCK, IN THE CLERK'S OFFICE OF THE DISTRICT COURT OF CONNECTICUT. HARTFORD: SILAS ANDRUS & SON, STEREOTYPERS PREFACE. THE Science of Physiology, as well as that of Zoology, in its different departments, has for several years formed a prominent branch of study in the best European systems of Education. In France, especially, great attention has been paid to these subjects, and the most distinguished scientific men of that country have not thought it beneath them to write treatises expressly for elementary instruction and for popular use. In the United States, however, until recently, Physiology has not been included in the course of study pursued in Colleges, and has been introduced into comparatively few of our Academies and Common Schools. In most Institutions it has been looked upon as a matter of secondary importance, rather than as a Science of great practical interest and value to every individual. Medical Schools, alone, have been provided with ample facilities for investigating the structure of the human body; and a knowledge of the functions of the various organs, as well as the proper means of preserving them in health, has been regarded as the peculiar prerogative of professed physicians, and as a mystery too deep and too difficult for the comprehension of others. Happily, this state of things is now beginning to be changed. It has been discovered that there are few portions of the Science which any young person of ordinary intelligence cannot be made to understand and to explain. The knowledge already diffused is creating a demand for more, and Physiology will soon take its appropriate place as a necessary branch of popular education. This Science has, indeed, peculiar claims to the attention of every teacher who would be successful in his profession, and of every person in any way interested in the education of the young. There is no species of knowledge more valuable to him who has the eare of young persons, than that which leads him to appreciate the mutual dependence of the physical and intellectual powers. For want of such knowledge, the ill-judged efforts of many well-meaning teachers have been productive of the most serious evils to their pupils. When those physiological facts which lie at the foundation of mental development shall be fully understood by teachers, then only will their success become perfect, both in imparting information to their pupils, and in fitting them for the arduous duties of life. Keeping these considerations in view, in the preparation of this work, the principles of the Science have been presented as fully and clear/y as the limits of the work would seem to allow; the object having been to bring them separately before the mind, without following them into all the details of their various applications. When the student has once gained a competent knowledge of the principles of Physiology, he will not be slow to perceive their bearing upon the practical training of the body, and their importance to the welfare of mankind. Brief comparative descriptions of the structure of some of the lower orders of animals have been introduced, for the purpose of giving variety and interest to the study, and of furnishing the teacher with practical demonstrations of the relative value of each organ in the system. Few persons ever enjoy an opportunity of actually examining any of the internal organs of man, while many of the lower animals are easily accessible to investigation; and nothing is more interesting, or better calculated to give clear ideas of our own structure, than the examination of that of these animals. A distinct chapter has been assigned to the subject of Hygiene, or the Preservation of Health. It is supposed that this subject may be studied in this way better than in detached portions, in connexion with the structure and functions of separate organs. The main object of this chapter is, to show that the duration of human life corresponds very nearly with the comparative physical condition of families, communities, and nations; and that each individual is the guardian of his own health and happiness, and may, to a very great extent, avoid or induce disease, in proportion as he obeys or disobeys the laws of his own organization. The most important rules of Hygiene are stated, with familiar instruction as to the best means of preserving health. It will be observed, by the reader who has made progress in physiological studies, that many of the engravings, as also parts of several chapters, have been copied from the admirable " Animal Physiology" of Dr. Carpenter. In addition to this and other works of the same author, those of Wilson, Warren, Pereira, Bourgery, Aitken, Roget, Kirkes and Paget, Wyman, and other standard writers, have been consulted. The beautiful "Microscopic Anatomy" of Dr. Hassall has also furnished several figures. The quarto, or atlas-form, has been adopted in deference to the advice of experienced teachers. This form gives opportunity to make the plates large and complete, the full-length figures being eight inches in height. The wood-cuts were executed by Mr. S. H. Clark, of Hartford; and it is believed that both these and the plates are not inferior to any similar illustrations ever published in this country. To Professor Brocklesby, of Trinity College, Hartford; to Dr. J. L. Comstock, of Hartford; and also to several excellent teachers, thanks are due for many judicious suggestions, and for other favors received during the preparation of the work. HARTFORD, April, 1851: M361258 HINTS TO TEACHERS. FROM our own experience, we can confidently assure teachers that no branch of education can be made more attractive to the young than Physiology. But the inte. jst which pupils take in any study, is to a very great extent a reflex influence from the teacher's own mind. When a teacher attempts to instruct a class with a mind uninformed or uninterested in the subject of the recitation, the fact will invariably be made manifest in his eye, his features, or his voice; the impressions will be com- municated to his pupils with telegraphic despatch, and will lead their minds to act in sympathy with his. But when the teacher enters upon his duties with a zeal kindled by actual knowledge, he will not fail to witness a corresponding enthusiasm in his pupils, provided the study is adapted to their minds. With what success we have labored to adapt this book to the mind of the young, it is for the public, and especially for teachers, to judge. We have placed questions at the foot of each page, for the aid of such teachers as have not enjoyed opportunities to become thoroughly familiar with the principles of Physiology; but' we would recommend to all, as far as practicable, to make then- own questions, and not allow their pupils to feel that they are to study only for the answers; nor should they be required to give the answers in the language of the author. We trust it will be found that almost any scholar of twelve or fourteen years, can be interested and profited by this study. It has been our object not to write all that can be written on the science, but to give only the most important principles ; believing that a few truths, fully appreciated, are more valuable, and exert a better influence on the mind, than a mass of information which produces confusion, and is soon forgotten. But it is hoped that teachers will be able to avail themselves of lectures and of other works, which will carry their knowledge to a higher perfection than can be attained by the study of any one book. Every author has his own peculiar way of presenting truth, and each one will be likely to excel in discussing some one subject with more fullness or force than others. In Physiology, above all sciences, the teacher can find an almost infinite variety of illustrations, if he consults the numerous authors who have written on the subject. Much interest may be added to the study by examining before the class the organs of the inferior animals. The ingenuity of the teacher will also suggest other experiments that will serve to impress the lesson upon the mind of the learner. The form which primary cells assume when pressing on each other, may be shown by a very simple experiment with soap-bubbles. If a six or eight-ounce vial be filled with water saturated with soap, and then inverted, as the air rushes in to take the place of the water, the vial will be filled with cells of every variety of form and size. Membrane, muscle, cartilage, bone, nervous matter, &c., and the organs that are formed from these, may all be demonstrated by specimens obtained from the butcher. All specimens of this character should be macerated in water, long enough to render them perfectly clean, before they are presented to the class, and should then be accompanied with a napkin and a tray, to prevent soiling the table and books. A sharp knife of almost any description, with a pair of sharp-pointed scissors and a pair of forceps, are all the instruments that are necessary for physiological demonstration. By the use of these, almost any organ may be prepared for exhibition to the class. The lungs and heart may be taken from almost any of our domestic animals, unless the lungs of a horse or an ox should be found too large for convenience. By inserting a stop-cock- into the trachea, the lungs may be inflated to their full capacity of distention. The air may then be allowed to escape, and the lungs be again inflated, performing thereby a kind of artificial respiration. A very beautiful preparation, to show the distribution of the blood-vessels over the membrane of the lung, together with the membrane itself, may be prepared from the lungs of a turtle. Procure a turtle at least six or eight inches in length, cut off the head, and immediately saw the lower shell from the upper at the sides, and then with a knife dissect off the muscles from the shell. When the shell is removed, almost the first object that arrests the attention is the heart, still performing its functions with as much apparent regularity as if nothing had happened, which it will continue to do for several hours, though its action grows more and more feeble. On either side of the heart, and partly under the scapulae, the lungs may be seen in a collapsed state. A quill should be inserted into the trachea, and the lungs inflated, for the purpose of rendering then" position more apparent, when they can be easily removed with the scissors, though it must be remarked that the slightest scratch spoils the preparation. As soon as the lungs are removed, they should be inflated to their extremest capacity, a ligature applied to the trachea, and then hung in the air to dry. In half an hour you have an exceedingly handsome and useful preparation, that may be preserved for any length of time. A heart for examination should have the pericardium separated from the base about half the distance round, so that it may be held in place or turned off at pleasure. To show the valves, the heart may be cut directly across, at half the distance from the base to the centre, taking care not to cut the little cords that attach the tricuspid and bicuspid valves to the walls of each ventricle. To show the semi-lunar valves, the aorta and pulmonary artery should be cut off, according to the size of the specimen, from half an inch to an inch above their origin. The remaining portions of the heart can then be examined without difficulty. A very interesting experiment, to illustrate the importance of ventilation, may be made by lowering a lighted taper or candle, attached to a flexible wire, into an open-mouthed glass jar, holding one or more pints. In a very few minutes, carbonic acid will be formed sufficient to extinguish the taper. That it is carbonic acid may be ascertained by pouring into the jar lime-water, which immediately becomes turbid from the formation of carbonate of lime. That carbonic acid is also produced by respiration, may be made to appear by filling and inverting the jar in water; then pass the end of a tube (or, if nothing better be at hand, an open straw) under the jar, and breathe through the tube into the jar till the water is entirely displaced. The hand or a piece of glass or of brown paper must now be put snugly on the mouth of the jar, which is to be inverted, and placed on the table. On removing the hand, plunge the taper into the respired air, and it will be instantly extinguished. The necessity for a supply of oxygen may be shown by inverting the jar, and passing up the lighted taper: as fast as the oxygen is consumed, the nitrogen, which is lighter than atmospheric air, occupies the upper portion of the jar, causing the taper to grow dim or be extinguished, according to the position in which it is held. For the purpose of illustrating the subject of digestion, the teeth and stomachs of the ntfentia, the graminivorous, the carnivorous, and the omnivorous animals, can be easily obtained. The stomachs should be thoroughly washed, and inflated with air, or stuffed with cotton, hay, or hair. To obtain the brain of any animal, the skull must be sawed through completely round the head, and, if possible, its membranes must not be ruptured till the brain is removed from the cranium. The brain should then be rinsed with water, and immersed in alcohol for a few hours, or it may be preserved in alcohol as long as desired. A great variety of interesting experiments, illustrative of the reflex functions of the spinal cord, may be performed on decapitated frogs or turtles. The ear and eye both require the skill of an anatomist for their dissection. The structure of the tympanum of the ear, however, may be shown very easily by removing the skin from the head of a fowl or a bird, and cutting off with a strong pair of scissors the bone around the external ear till the tympanum is distinctly seen, supported by the stapes. If about one-fourth of the head be now removed, the whole internal structure of the ear may be seen, with the auditory nerve, the vestibule, and cochlea. It is very easy to cut open the eye, so as to exhibit the aqueous and vitreous humors, the crystalline lens, the iris, the pigmentum nigrum, and the sclerotic coat, but the retina requires more care. To show the retina, take the fresh eye of an ox, clip off the muscular and adipose substance about it, then describe with the knife a circle about the entrance of the optic nerve that shall include one-fourth of the ball of the eye; holding the eye in the thumb and fingers of the left hand, gradu- ally and carefully cut through the sclerotic to the choroid coat, at a single point in the circle already described, and then complete the dissection in water by carefully insinuating the sharp point of the scissors between the coats, and clipping round the circle. The slight attachment of the choroid to the sclerotic may now be separated with the back of the knife till the detached portion of the sclerotic can be clipped off, leaving the nerve entire. The dissection is now complete; and, if well performed, inverted images of objects can be seen on the retina as in life. The above demonstrations, though so simple as to be easily performed by almost any one, will add much interest to the study. A teacher who has enjoyed the advantages of a dissecting-room, will of course have learned a more scientific way of demonstrating than we have described for the uninitiated. CONTENTS. INTRODUCTION. PAOB. DEFINITIONS, General Remarks, &c 7 Characteristics of Organic and Inorganic Matter, 7 Distinctions between Animals and Plants, 8 CHAPTER I. Chemical Composition of Animal Bodies 9 CHAPTER II. Structural Composition of the Human Body, 9 Vital Properties of the Organs and Tissues, 10 CHAPTER III. Circulation of the Blood, 11 CHAPTER IV. Respiration, 19 Animal Heat, 23 CHAPTER V. Digestion, ' 24 Mastication, ...24 Insalivation 26 Deglutition, ............26 Chymification, 26 The Alimentary Canal, 28 CHAPTER VI. Absorption, 31 CHAPTER VII. Nutrition and Growth 33 CHAPTER VIII. Secretion, CHAPTER IX. The Skin 35 CHAPTER X. The Nervous System, 39 Comparative View of the Nervous System in different groups of animals, . 40 Nervous System of Man Oerebro-spinal Nervous System Spinal Cord, 43 The Brain, ... .44 PAGE. Functions of the Spinal Cord, 45 Functions of the Medulla Oblongata, 45 Functions of the Cerebellum, 45 Functions of the Cerebrum, 46 Connexion of the Mind with the Body, 46 CHAPTER XI. Sensation and the Senses, 60 Sense of Touch, 50 Sense of Taste 51 Sense of Smell, ...........52 Sense of Hearing, . . . . . . . . . . .56 Sense of Sight, 57 CHAPTER XII. Motion, 62 Apparatus of Movement in General, 63 Description of the Motor Apparatus of Man 67 Attitudes of the Body, and various kinds of Locomotion, ... 74 CHAPTER XIII. The Voice, 80 CHAPTER XIV. Instinct and Intelligence, 84 CHAPTER XV. Hygiene, or the Preservation of Health, . Causes of Premature Disease and Mortality, . . Hygiene of the Respiratory Organs, . Consumption, Hygiene of the Digestive Organs, . . . Diet of Children, ....... Diet of Adults, Diseases of the Digestive Organs, .... Hygiene of the Nervous System, . . . Hygiene of the Muscles, ..... Bathing, The Use of Medicine, Change of Form, GLOS S ART. Explanation of the Scientific Terms employed in this work, 90 , 91 93 93 95 96 96 96 100 101 102 104 106 LIST OF ILLUSTRATIONS. PLATES: PLATE 1. Comparative View of the Organs of Circulation, " 2. Organs of Circulation. Heart and Lungs of Man, . " 3. Organs of Respiration, ..... 4. Organs of Digestion, OPPOSITE PAGE . 14 18 . 22 30 6. Structure of the Skin, 38 6. The Brain and Spinal Cord, 42 7. The Nerves 48 PLATE OPPOSITE PAGE 8. Organs of Touch, Taste, Smell, and Hearing, . . .54 9. Organ of Vision. The Eye, 60 10. Organs of Motion. The Bones, 66 11. Organs of Motion. The Bones, 72 12. Organs of Motion. The Muscles 78- 13. Organs of Motion. The Muscles, 82 14. Change of Form, 104 ENGRAVINGS ON WOOD. PAOE 9 43. 44. 45. 46. 47. 49. 50. 51. 52. 53. 54. 55. 56. 57. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 9 . 10 12 . 12 12 . 12 12 . 12 . . . 15 . 16 12. Valves of a Vein, . . . . 19 .... 19 .... 19 20 . 20 . . . .20 .... 25 . 25 22 Jaw and Teeth of Rabbit, .... 25 . 25 25 ... 25 25 29 Section of Mouth and Throat, . . . , . . . .26 .... 31 37. Structure of the Parotid Gland, . , 43 P1OE Brain of Fish, 43 Brain of Bird, ...........43 Portion of Spinal Cord, 43 Skull of European, 46 Skull of Negro, 46 Capricorn Beetle, 51 Vertical Section of Nasal Cavity, . 52 Bones of the Ear, .......... 56 Cavity of the Tympanum, 66 Diagram of Refraction of Light, 57 " " " .......57 Interior of the Eye, 58 Section of the Eye, ..........58 Diagram of Refraction of Light, 61 Diagram showing the Operation of Muscles in producing motion, . 64 it U u u a. u u u u " . 67 " " . 67 . . 67 " . 67 . . 67 . 67 75 75 . . . .75 79 . . . . . 80 83 83 . 83 Leg of the Deer, Leg of the Horse, The Kangaroo, Tail of Whale, The Chimpanzee, The Human Laryr Section of the Lar Front View of the Larynx of a Bird, . . . . .'. . . . . 84 Section of the same, 84 Ant-lion, ...... .'.... .85 Larva of Ant-lion, 85 Pit-fall of Ant-lion, 85 Nest of Mygale, 96 Xylocopa, ..86 Nest of the Xylocopa, 86 Nest of the Baya, 86 Nest of the Tailor-bird 87 Cell of the Bee, 88 PRINCIPLES OF PHYSIOLOGY. INTRODUCTION. 1. ALL bodies in which the phenomena of life have been observed, are formed of diverse mutually adapted parts, or organs. Such bodies are therefore called organisms, or organized bodies. Their composition and structure, being peculiar, are named organic, and constitute their organization. While alive, also, they manifest cer- tain peculiar vital properties and modes of action. 2. The various races of organized beings may be studied in two ways. By the aid of the knife they may be separated into the vari- ous organs of which they are composed; by the aid of the micro- scope, the various tissues which enter into the composition of these organs may be examined ; and by the aid of chemistry, the ultimate elements of which these tissues consist may be determined. This mode of investigation is applied chiefly to the dead body, and the science derived from it is called Anatomy. 3. Anatomy (signifying literally a cutting up) therefore, since it teaches us only such facts as are found to exist in the lifeless body, gives but an imperfect idea of the purposes for which living structures were made. To acquii'e a more complete knowledge, we must adopt a different method of investigation, and observe the various phenom- ena which organized beings display during their life. \Ye must investigate the means by which these phenomena are produced, and determine, if possible, the laws by which the vital processes are car- ried on. The knowledge thus obtained constitutes the science of Physiology. 4. Thus, Anatomy makes us acquainted with the organs or instru- ments of life, their number, size, situation, form, color, texture, com- position, and relation to each other. Physiology teaches the uses and purposes of these organs. Anatomy makes known the means; Physiology the ends. To study Anatomy without Physiology, would be like gaining a knowledge of the separate parts of a steam-engine, without learning its mode of action or the purposes to which it can be applied. And to study Physiology without Anatomy, would be like observing the progress of a train of rail-road cars, without know- ing the means by which they are propelled. In this work, therefore, such anatomical description will be introduced, as will suffice for the better understanding and illustration of the phenomena which it is the office of Physiology to examine. 5. The two sciences, ANATOMY and PHYSIOLOGY, in their most extended application, take cognizance of all organized beings, whether animal or vegetable. These sciences are, however, naturally divided into several branches, according to the particular departments of the organic world to which they are applied. Thus we have the pri- mary branches of ANIMAL PHYSIOLOGY and VEGETABLE PHYSIOLOGY, each of which is confined exclusively to the great natural department which gives it its title. Animal Physiology is again divided into COMPARATIVE PHYSIOLOGY, and HUMAN PHYSIOLOGY, the first of which is limited in its application to the lower orders of animals, and the latter to man. The same divisions are made of the science of Anatomy. 6. Notwithstanding these divisions, however, there is, strictly speaking, no Physiology peculiar to any single being or class of beings, as Man for example ; and to study the phenomena exhibited by one individual would give us as imperfect a knowledge of Physiology as a person would obtain of Astronomy by studying the motions of a single star. For if we examine any one organ and the function it performs, in a certain given animal, and then refer to any other animal, we shall usually find a repetition of that organ, or some modification of it, in that other animal. And this law is so general 1. What are organized bodies? Why are they so called? 2. What is the first way in which organized bodies may be studied ? What is the science derived from this mode of investigation ? 3. What does the word Anatomy signify? Does it give a perfect idea of organized beings? How is a more perfect knowledge obtained? What science does this knowledge constitute? 4. What does Anatomy teach us? What does Physiology teach us? How is the difference between those sciences illustrated? 5. Into what branches are Anatomy and Physiology divided? Define these different branches. 6. What is said of the utility of studying comparative Physiology? as to apply to nearly the whole series of animated beings. The essential process of respiration, for instance, is the same in all ani- mals, and some form of the lung, the organ by which that process is carried on, greatly varied to be sure, and sometimes scarcely recog- nizable, is found in nearly every group of breathing organisms, from the insect to the man. By a comparative view of the various com- plications of any particular organ, as found in the different groups, we arrive at what is essential to constitute such an organ, and what is the relative importance of its function to the maintenance of life. 7. It is the design of this work, then, to teach the principles of Human Physiology, and to illustrate that science by reference to Comparative and Vegetable Physiology, whenever such illustration will add to the interest or assist in the comprehension of the main subject. CHARACTERISTICS OF ORGANIC AND INORGANIC MATTER. 8. Matter is presented to our senses in an almost infinite variety of fowns, compounds, and organizations. The earth itself is an aggregation of compounds, each possessing properties and charac- teristics peculiar to itself. The surface of the earth, including the water and the air, teems with myriads of organisms, each differing from the other in the combination of its elementary parts. Yet, when subjected to the examination of the chemist, it is found that there are only fifty-five individual elements out of which all this endless variety of* matter is composed. An explanation of this great truth in nature is found in the fact that the variety which we behold exists in the inherent power of these few elements to develope new properties and new phenomena with each new combination. Thus carbon and .oxygen form carbonic acid (a poisonous gas) ; oxygen and hydrogen form water; oxygen and nitrogen form the air we breathe; nitrogen and hydrogen form ammonia; carbonic acid and ammonia form carbonate of ammonia. Here, then, are four elements forming five different combinations, and each compound is wholly unlike the others. 9. Now, if a piece of granite is broken from the rock which has withstood the waste of centuries, it is found to retain all the charac- ters and properties which it possessed at its creation. If it is divided and subdivided, or even ground to powder, the same is true of every individual particle. But if you cut a rose from its stem, or amputate the arm of a man or the leg of a dog, they no longer possess the same properties which belonged to them while forming a part of the plant or the animal. They become withered and decomposed, their appear- ance and texture become changed, and they finally decay and are lost to the senses. The granite exists independently of any particular shape or any determinate organization. Its structure and compo- sition, unless when some foreign decomposing agent is applied to it, are always the same. The rose, on the other hand, requires certain conditions, such as warmth, moisture, and light, and connexion with its parent stem, to maintain its existence ; and the arm requires the circulation of the blood through its veins and arteries, and the purification of that blood by respiration, for the same purpose. The granite, also, has no particular part which is different from any other part, or which performs any separate office. The plant and the animal, on the contrary, possess certain organs or instruments, such as roots and leaves in the one, and lungs and stomach in the other, by which those acts necessary to their existence are performed. Here then is suggested the idea of the two great divisions of matter. Hence matter as it exists in animals and vegetables is called organic or organized, and in minerals inorganic or unorganized. (j 7. What is the design of this work? 8. Of how many individual elements is matter composed ? What is the reason of the great variety of forms which matter assumes ? How is this illustrated ? 9. Does any change take place in a piece of granite, when it is broken from the rock? What changes take place in a part of a plant or an animal, when separated ? Explain the reason of this difference. How then does organic matter differ from inorganic, and what divisions are founded on this difference ? DISTINCTIONS BETWEEN ANIMALS AND PLANTS. 10. Nevertheless, the same elements which exist in organized structures, are found in the more simple combinations of unorganized matter. No new or distinctive element, then, can characterize either plants or animals. The cause of the distinctive characteristics of these two great departments of animated existence must, therefore, be found in chemical combinations dissimilar to those which enter into the composition of inorganic matter. Chemical analysis proves this inference to be correct. 11. In the inorganic world, most substances are either in the elemental state, or are formed by the union of only two elements. In plants the most abundant substances are compounds of three elements, carbon, hydrogen, and oxygen, which, by combining with each other in different proportions, form starch, sugar, and other vegetable productions. In animals, the most abundant substances are composed of four or five elements. Albumen, fibrine, gelatine, for instance, are compounds of oxygen, hydrogen, nitrogen, carbon, and sulphur. The number of combinations of elements in plants is greater than in inorganic compounds, and the number in animals is greater than in either. It is also true that in both classes there is a great number of compounds formed from a few elements, and a large number of atoms or equivalents of each element unite to form one equivalent or atom of the compound. Now, it is a general rule that the greater the number of atoms that unite to form a compound, the less is the stability of that compound. Hence it is that inorganic compounds are decomposed only by the most powerful agents, and are consequently enabled to retain their original character for an almost unlimited period of time. Vegetable compounds, on the other hand, are decomposed with comparative ease, and animal compounds are held together with such comparatively slight affinities that they are said to decompose spontaneously, the presence of atmospheric air being the only condition requisite for the occurrence of that phenomenon. 12. All organized bodies have a limited period of life, which varies with every species. In some this period is confined to a single day in many plants to a single summer ; while some animals live more than a century, and some trees, as the oak and olive, are supposed sometimes to live a thousand years. In all, the perpetuity of the species is provided for by the power of reproduction, which is pos- sessed by both animals and plants. The life of each individual is maintained during its appointed period, by a process called nutrition, which consists in imbibing external substances, and appropriating them to its own use. Vegetables receive their nutriment from the earth and air around them, and interpose the new particles between those which already form their substance. Animals obtain their nutriment from the organized substances contained in plants and in other animals, and adapt it to the maintenance of their own life by the action of certain organs with which they are furnished for that purpose. 13. Thus each of the great departments of nature contributes to sustain the other. The unorganized substances maintain the life and growth of the vegetable, which in its turn affords food and nutriment to the animal ; and both vegetable and animal, after fulfilling their destined period of existence, die, decompose, and give back to the inorganic world the elements which they derived from it. Matter is thus constantly undergoing a series of changes reproduction and decomposition, life and death. Every particle in the vast world of matter is contributing its aid to maintain the laws of nature, in perfect obedience to the mandates of the Great Designer, who is thus exhibiting to us the inexhaustible resources of that Wisdom which can develop, from means apparently so limited, such an infinite variety of forms and structures, all mutually nourishing and sustain- ing each other. So true is this, that no part of the earth is so barren, no desert is so parched, that it may not be inhabited by some form of life, and even the hardest rock holds in its smallest crevice some- thing for the support of vegetable existence. There is no organized 10. Are the same elements found in organic and inorganic matter ? What (hen is the cause of the distinctive characteristics of organized matter ? How is this proved to be true? 11. Of how many elements are inorganic substances composed ? Of how many elements are the most abundant substances in plants composed ? In animals ? What example is given of this ? What is the general rule as to the stability of a compound sub- stance ? What then is the reason that vegetable and animal substances are more easily decomposed than minerals? What is the condition requisite for the decomposition of animal substances? 12. What is said of the period of life of organized bodies? What examples are given ? How are the species perpetuated ? How is the life of each individual maintained ? How do vegetables receive their nutriment ? How do animals obtain theirs 1 13. How do the organic and inorganic worlds sustain each other ? What changes is matter constantly undergoing? What remarks are made on this subject? being that may not in some way afford nutriment to other organisms. No plant is so poisonous as not to be fed upon by some species of insect, or so compact in its structure as to be able to resist the attacks of some little devourer. Of the almost innumerable species of insects, some prey upon other insect forms, some upon the larger animals; some live upon decayed wood, and some burrow into the hardest substance of the most lofty trees. Some feed upon the root, some upon the foliage, and others draw nectar from the delicate flowers. All these, in their turn, become food for animals of higher organization, and the latter satiate their voracious appetite upon each other. Some become victims while the life-blood is yet warm in its vessels, and others die of natural causes, and around them gathers an innumerable throng of eaters. No animal is so large that it is not soon consumed by the insatiate host, and none so offensive that there is not some fouler form to prey upon its remains. And the rule has no exception, not even in him who in the height of his pride and power exalts himself above the inferior creation ; " For when all is past, it is humbling to tread O'er the weltering field of the tombless dead, And see worms of the earth and fowls of the air, Beasts of the forest, all gathering there ; All regarding Man as their prey, All rejoicing in his decay." DISTINCTIONS BETWEEN ANIMALS AND PLANTS. 14. In the higher and more perfect forms of animal and vegetable life, there can be no difficulty in distinguishing between an animal and a plant. But when we examine some of the lower orders, it is by no means easy to determine where animal life ends and vegetable life begins. Indeed, there seems to be an intermediate group, some- times called Plant-animals, as to the nature of which naturalists themselves have hardly been able to agree. The Sponge, for instance, has been repeatedly transferred from the animal to the vegetable kingdom, and back again, as new properties have been discovered in it, until we are yet somewhat doubtful to which it rightfully belongs. 15. Passing by this ambiguous class of organisms, we shall find that the two properties of animals, by which they are always to be dis- tinguished from plants, are sensation and voluntary motion. Some vegetables, it is true, possess a certain kind of motion. The leaves of the sensitive-plant fold together, and droop from the stem, at a touch of the finger, but this movement is owing to a physical change produced in the leaf, and by no means to any feeling or sensibility such as induces a man to withdraw his hand when it comes in con- tact with fire. The plant manifests no consciousness, and therefore cannot be said to possess sensibility. Sensibility and the power of spontaneous motion are therefore called the functions of animal life, and every being which is conscious, in however slight a degree, of its own condition, and of the circumstances affecting it, must be regarded as an animal. Another mark of distinction peculiar to animals, is that they always possess a stomach or digestive cavity of some kind. Since vegetables obtain their food directly from the earth and air, in the form of a liquid or a gas, they have no need of a cavity in which to receive and prepare it. Animals, on the other hand, since they live upon organized substances, require a cavity in which to dissolve and reduce their food to a fluid form, so as to absorb its elements into their tissues. In vegetables, again, the circulation is much more simple than in animals. They have no heart or special organ for the distribution of their sap. These distinctions may be summed up as follows : ANIMALS. Motions voluntary. Possessing sensation. With a digestive cavity. Nourished by organic matter. Consume organic elements, such as starch, sugar, &c. PLANTS. Motions involuntary. Without sensation. Without a digestive cavity. Nourished by inorganic matter. Produce organic elements, such as starch, sugar, &c. 14. What is said of the difficulty in distinguishing plants from animals ? What of the Sponge? 15. What are the two distinguishing properties of animals? What is said of the motion of plants? What other peculiarity do animals possess? Why do animals need a stomach, and vegetables not? How does the circulation in vegetables differ from that in animals? Give the summary of lhe?e distinctions. COMPOSITION OF ANIMAL BODIES. CHAPTER I. CHEMICAL COMPOSITION OF ANIMAL BODIES. 16. THE animal body is composed of fluids and solids. The quantity of fluids far exceeds that of solids, though it is a difficult matter to form an entirely correct estimate of their relative propor- tions. By exposure to a process of evaporation, a body weighing one hundred and fifty pounds may be reduced to twelve or fifteen. Perfectly dry mummies are sometimes found to weigh only seven or eight pounds. The amount of fluid which can be drawn directly from the body is only about thirty pounds. The fluids vary exceed- ingly at different periods of life. In youth they are very abundant, making the form plump and round. In old age they are greatly diminished in quantity, leaving the form shrunken and wrinkled. Animal fluids are of two kinds, formative and secreted. The formative fluids contain in solution the materials for the formation of the solid tissues. The secreted fluids are separated from the tissues and the blood, and are the medium through which all the waste particles are carried out of the system. The chemical elements are not essentially different in the solids and the fluids, both being alternately converted into each other. 17. Seventeen out of the fifty-five elementary substances of which all known matter is composed, may be obtained by analysis from the human body. These are oxygen, hydrogen, nitrogen, carbon, sulphur, phosphorus, silicon, chlorine, fluorine, potassium, sodium, calcium, magnesium, iron, and sometimes manganesium, aluminum, and copper. 18. Oxygen, hydrogen, nitrogen, and carbon, are called essential elements, because they exist in nearly all animal substances, and form the largest part of ail. The remaining thirteen are less constant, and occur only in small quantities, and in combination with the essential elements. 19. The simplest of the compounds naturally formed in the body are called Proximate Principles, because they are supposed to be in order of simplicity nearest the elements. They are composed of at least three of the elements. The most essential of the proximate principles are Albumen, Fibrine, Caseine, Gelatine, and Hcematosine. 20. Albumen, composed of oxygen, hydrogen, nitrogen, carbon, and sulphur, is found solid in the brain, spinal cord, and nerves, and in the mucous membrane; fluid in the serum of the blood, and in Ivmph and chvle. The white of eggs is almost entirely composed of it. It is perfectly colorless when pure, and is coagulated or hardened by heat and by acids. 21. Fibrine, composed of the same elements, is the most abundant of animal substances. It is the basis of the muscles, and is found in the chyle, lymph, and blood. It coagulates spontaneously. 22. Caseine has many properties in common with albumen and fibrine. It exists in the greatest abundance in milk, and is the basis of cheese. 23. Gelatine is the chief constituent of the cellular or areolar tissue, skin, tendons, ligaments, cartilages, and bones. It is obtained by boiling either of these substances, and allowing the solution to cool. Glue is an impure gelatine. 24. Heematine or Hcematosine is the red coloring matter of the blood. The chief characters of the animal proximate principles are as fol- lows: They all contain carbon, oxygen, and hydrogen, and most of them also nitrogen. They are all decomposed by a red heat, and are for the most part exceedingly prone to putrefaction. 25. There is a class of substances called secondary compounds, which are formed from the elements of the tissues, to be excreted from the system by particular organs. Some of these are urea, uric acid, cholestrine, pepsine, sugar of milk, and lactic acid. The most important animal compounds have thus been enumerated, and others of less abundance and importance are omitted. 16. Of what is the animal body composed ? Which of these parts exists in the greatest abundance ? How is this proved ? Is the proportionate quantity of the fluids the same at all periods of life ? What effects are produced by this variance ? What are the two kinds of animal fluid ? Define them. Is the chemical composition of the fluids different from that of the solids? 17. How many of the elementary substances are found in the human body? What are they? 18. What are the essential elements, and why are they so called? 19. What are the Proximate Principles? Of how many elements are they composed ? Which are the most essential of them? 20. Describe albumen and its composition. 21. What is fibrine? 22. Where is caseine most abundantly found! 23. What is gelatine, and how is it obtained? 24. What is hBematosine? What are the chief characters of animal prox- imate principles? 25. What are some of the secondary compounds? i CHAPTER II. STRUCTURAL COMPOSITION OF THE HUMAN BODY. 26. CERTAIN primary forms of the solids may be observed, which, by their various modifications and modes of combination, make up the tissues and organs of the human body. The simplest and most minute of these primary forms are granules or molecules. They are particles of various sizes, from immeasurable minuteness to the ten thousandth of an inch in diameter. Granules are found floating in milk, chyle, and other animal fluids, and imbedded in most of the tissues. 27. Nuclei, or corpuscles, are found connected with cells, or tissues formed from cells. They are the germ or centre around which cells are formed. 28. Cells, primary or elementary cells, are minute bubbles, vesi- cles, or scales, larger than the nuclei, and usually filled with liquid contents. Most of the cells in the human body contain nuclei. 29. The natural shape of the cell is oval or spheroidal, but they often become flattened or many-sided by pressure. Their contents are various, according to their position, office, and age. The cells perform a highly important process in the animal economy, since they form all the tissues of the body, either directly by their depo- sition, or by elaborating the fluids from which the tissues are derived. They are concerned not only in the function of nutrition, in the development and restoration of parts, but in absorption and secretion. They are developed into tissues in various ways. In some, the cell- membranes become elongated, and are folded and divided into threads or filaments of exceeding fineness. Several elongated cells are joined end to end, the partitions at each end being removed, when they constitute tubules or little tubes. fly. 1. A. Corpuscles of human blood, magnified five hundred diameters, a. Particles collected in a columnar form. B. Red particles of ihe mood of the common fowl. b. A particle seen edgeways. By the farther development of tissue, other forms are produced, such as, 30. Filaments, or fibrils, which are exceedingly delicate threads, composed of minute particles, and usually arranged in parallel bun- dles, or fasciculi. 31. Fibres are larger than filaments or fibrils, but are similar to them in other respects. Fig. 2. Fasrkruli and fibres of cellular tissue. A. White fibrous element, with cell-nuclei visible in it B. Yellow fibrous element, showing its branching fibrils. C. Finer fibrils of the yellow element. 26. What are granules or molecules, and where are they found ? 27. What are nuclei, or corpuscles ? 28. What are cells ? 29. What is their shape and office ? (low ore tubnl^ constituted? What is represented in Fig. 1? 30. What is a filament? 31. Wlnt i* a fibre ? Describe Fig. 2. 10 VITAL PROPERTIES OF THEORGANS AND TISSUES. 32. A tissue is formed by the union of one or more of these pri- mary structures. 33. An organ is any solid part of the body. It is an instrument by which some function is performed. Thus the heart is an organ, and its function is to propel the blood through the vessels. 34. An apparatus consists of a number of different organs, arranged for the performance of some one office. Thus the digestive apparatus consists of the teeth, mouth, stomach, intestines, &c. 35. A system is a connected series of similar parts, such as the muscular system, the nervous system. 36. The different organs of the body are made up of one or more tissues or textures, according as they are of a more simple or a more complex structure. These tissues are made up of the elementary particles already described, and each of them performs the same function, in whatever part of the body it exists. Most of the organs are composed of a variety of these tissues, which are spread into membranes, collected into cords, or hollowed out into canals, and by their diversity of combination, figure, and color, they produce all the varieties of structure and function possessed by different organs. Some anatomists consider that in every animal there are as many different tissues as there are varieties of solid parts, and describe skin, bone, muscle, nerve, and every other solid, as a distinct tissue. 37. The number of tissues in the human body is variously stated. The following list seems to include all those which are considered to be ascertained by the latest writers: 1. Corpuscular tissue. 2. Epidermoid tissue. 3. Pigmentary tissue. 4. Adipose tissue. 5. Cellular tissue. 6. Fibrous tissue. 7. Elastic tissue. 8. Cartilaginous tissue. 9. Osseous tissue. 10. Muscular tissue. 11. Nervous tissue. 12. Vascular tissue. 13. Serous tissue. 14. Mucous tissue. 15. Dermoid tissue. 16. Glandular tissue. 17. Refracting tissue. 18. Petrous tissue. Found in the blood, lymph, and chyle. In the skin, hair, and nails. [coat. In a coat of the eye, called the choroid The fatty parts of the body. The primary organization of the cells. Composed of connected fibres. Constitutes the middle coat of arteries. Found in the cartilage or gristle. Found in the bones. Belonging to the muscles. That of the nerves. [atics. Composing the arteries, veins, and lymph- Belonging to those membranes which secrete a fluid called serum. That of those parts which secrete mucus. Found in one layer of the skin. That which forms the glands. That of the cornea or lens of the eye. That of the enamel or ivory of the teeth. 38. The cellular or areolar tissue or membrane is regarded as the primary form of all the others. It is formed by the crossing or inter- lacing of minute fibres, interwoven in every direction so as to leave innumerable small spaces, which communicate with each other, as is shown by filling them with air or water. When the lung and the surrounding cellular tissue is pierced, so as to allow the external air to enter, the whole body may become inflated with air. The watery portion of the blood is sometimes effused into this membrane, causing the disease called dropsy. When the finger is pressed on a dropsical limb, a hollow or indentation is produced by pressing the fluid out of this tissue. 39. The cellular tissue is the most extensively diffused of all the elementary forms of organization. It is to be found in every part of the system, except in the compact portions of bone, teeth, and car- tilage. One of its chief uses is to connect together organs and parts of organs, which require a certain degree of motion on each other. It possesses great power of extensibility and elasticity, but neither contractility nor sensibility. 40. Most of the varieties of this membrane will be described together with the organs in which it is found ; since these organs, 32. How is a tissue formed ? 33. What is an organ? 34. Of what does nn apparatus consist! Give an example. 35. What is a system ? 36. How are the different organs made up ? Of what are the tissues composed ? What is the number of the tissues, as here laid down ? What is the first in the list, and where is it found! The second? &c. 38. How is the cellular tissue or membrane formed ! How is it shown that the cells of this membrane communicate with each other? What is the reason of the indentation made by preying upon a dropsied limb? What does Fig. 3 represent? 39. In what parts of the Fystem is the cellular tissue found? What is one of its chief uses? 40. What is this mem- hrane called when it incloses organs not exposed to the air! Why is it so called? What ;- it called in the mouth and stomach! Why is it so called? When it forms an outer covering, what is it called? or its use in connection with them, give it the various names assigned to it in different positions. Thus, when this membrane incloses those organs not exposed to the air, it is called serous membrane, on account of the fluid called serum, secreted in it. In the lining of the nose, mouth, stomach, and other parts, it is called mucous mem- brane, from a secretion called mucus which is forced out from numer- ous glands beneath its surface. When it forms a covering or envelop to the body, it receives the name of dermoid membrane, or skin. 41. The cellular membrane of itself possesses no sensibility whatever, but when such a provision is necessary, nerves are distributed through it. In all those cavities not exposed to the air, when it forms an envelop for each of the organs, and a lining for the cavity in which they are contained, its use seems to be to suspend the organs, and cause them to glide upon each other with the greatest possible ease. Here it is not supplied with nerves. But in the eye, nose, mouth, lungs, stomach, and other parts, nerves are distributed for the purpose of making these organs sensitive to the presence of such sub- stances as come in contact with its surface. In the skin, the distribution of nerves is most abundant, causing the sense of touch. rig. 3. , A magnified representation of a portion of ureolar tissue. VITAL PROPERTIES OF THE ORGANS AND TISSUES. 42. Some of the actions of living bodies indicate the operation of other properties and forces besides those which can be referred to their mere chemical and mechanical organization. These properties produce phenomena peculiar to living beings, and are therefore called vital properties. The powers which they exert in maintaining the actions of living beings are called vital forces. The actions themselves produced by these properties as digestion, absorption, secretion, and circulation are vital processes, and the state in which these processes are displayed is life. 43. The most general property of living bodies is shown in their ability to form themselves out of materials dissimilar to them. As, for example, when a plant grows by appropriating to itself the elements of water, carbonic acid, and ammonia; or when an animal eats vegetables, and its blood and all its various organs are formed out of the materials of its food. 44. This power of self-formation is displayed in three modes, development, growth, and assimilation, or maintenance. 45. Development is that process by which each tissue or organ is formed. 46. Growth is the increase of a part already formed, by the addi- tion of similar materials to those of which that part already consists. It is the process by which parts increase in weight and size: 47. Assimilation, or maintenance, is that process which supports living bodies in their natural condition, notwithstanding the changes to which they are liable from external influences and from their own natural decay. This result is effected by the continual formation of new particles in the place of those which are worn out and removed. 48. Contractility is another vital property. It consists in the power which the tissues of the muscles and of some other parts possess, during life, of contracting or shortening themselves in a peculiar manner. These contractions are produced by various changes, such as the contact of foreign matters, electricity, variations of temperature, &c. These, and whatever other applications will give rise to these contractions, are called stimuli. 49. The power of conducting or transmitting the impressions produced by stimuli is another vital property. This property belongs 41. How is sensibility given to this membrane ? In what positions is it destitute of nerves? In what position is it provided with nerves? 42. What are vital properties? What are vital forces! What are vital processes? 43. How is the most general property of living bodies shown? What examples are given? 44. In what modes is the power of self- formation displayed! 45. What is development? 46. What is growth? 47. What is assimilation? 48. What is contractility? How are contractions of the tissues produced ? What are stimuli? 49. What is another vital property! To what system does this property belong! How is the impression produced by a stimulus conducted! CIRCULATION OF THE BLOOD. 11 to the nervous system alone. When a stimulus of any kind is applied to a nerve, the impression is conducted by the nerve through- out its whole length, with unmeasurable rapidity, in the same way as electricity is conducted along a wire. 50. The property of irritability belongs also to the muscles and other contractile tissues. It includes both the capacity of receiving a direct stimulus and the power of contracting in consequence thereof. 51. When an impression is conveyed from any part of the body, along a nerve, to the brain, the mind may take cognizance of it; what the mind thus becomes conscious of, is called a sensation; and the act of the mind in noticing it, a perception. All parts through the nerves of which such sensations may be derived are called sen- sible or sensitive parts. CHAPTER III. CIRCULATION OF THE BLOOD. 52. NUTRITION and growth are characteristics of all living beings. Growth, however, is only the result of nutrition, which is essential to both animal and vegetable life. In both animals and vegetables the process of nutrition is carried on by means of a circulating fluid. In plants, this fluid is the sap, which is absorbed directly from the earth or air. In animals, this fluid is called blood. The appearance and character of blood are not the same in different groups of ani- mals. The blood of insects, and that of other lower orders, is white or colorless. In mammalia, birds, reptiles, and fishes, it is of a dark purple color when drawn from a vein, and of a bright scarlet when it comes from an artery. It emits an odor similar to that which arises from the breath or skin of the animal from which it is taken. Thus the strong scent of the pig or the cat, and the peculiar milky smell of the cow, can easily be discovered in their blood. 53. When the blood of man, or of an animal of the higher order, is examined by the microscope, as it flows in the vessels of a living part, it is seen to consist of two distinct substances a colorless fluid, called liquor sanguinis (liquid of the blood), and numerous minute red particles which give the blood its color. These blood- corpuscles or blood-discs (29, fig. 1) are little round cells flattened like a piece of money. These discs in human blood are from 1 -2800th to 1 -4000th of an inch in diameter. In birds, reptiles, and fishes, they are generally much larger. 54. In a short time after blood is drawn from its vessels, it begins to clot or coagulate and become jelly-like, and the fluid changes into a solid. Soon drops of transparent yellowish liquid, called serum, begin to ooze from the surface of the solid clot. The serum con- tinues to increase, until the clot, much diminished in size, floats in it. The reason of these changes is this: the liquor sanguinis, or liquid part of the blood, consists of serum, holding fibrine in solution. The peculiarity of fibrine ( 21) is to coagulate or become solid spontaneously. When, therefore, the blood is drawn from the vessels, the fibrine begins to coagulate, contracts more and more, presses out the serum, and retains the blood-corpuscles. If the air is excluded from the blood, coagulation takes place more slowly. The serum or liquid part of the blood, which remains after the coagulation of the fibrine, is composed of several substances (the prin- cipal one of which is albumen) dissolved in about nine times their weight in water. The water of the blood is one of its most import- ant constituents, and always forms by far the greater proportion of its bulk, one thousand parts of blood containing from seven hundred to seven hundred and ninety parts of water. It is subject to frequent variation in quantity, according to the period since eating, the amount of bodily exercise, the state of the atmosphere, and many other circumstances. An uniform proportion of water is, however, on the whole, maintained; because all those things which tend to 50. What is irritability ? 51. What is a sensation? What is a perception ? What are called sensitive parts? 52. How is the process of nutrition carried on? What is the circu- lating fluid of plants called? that of animals? What is the color of the blood of insects? of the higher orders of animals? What is said of its odor? 53. Describe the micro- scopical appearance of blood. Describe the blood-discs. 54. What changes take place in the blood after it is drawn from the vessels? What is the reason of these changes? What are the principal constituents of serum? What proportion of water is contained in the blood ? How is an uniform proportion of water maintained ? What conditions of the blood depend on its water? What are the average proportions of the principal constituents of the blood? diminish the proportion of water in the blood, such as active exercise and the eating of salt food, also excite thirst, and the water drank to relieve the sensation of thirst goes to restore the original proportion. On the other hand, when an excess of water is drank, it is quickly thrown off" from the blood in more copious excretions of sweat and other fluids. The water also maintains the liquid state of the blood, and gives it its proper degree of adhesion to the vessels, through which it ought to flow with the least possible resistance from friction. On tlm water also depends the active absorption by the small blood-vessels, into which no fluid will quickly penetrate, unless its density is less than that of the blood. Indeed, the organic life of the several parts of the body is in direct proportion to the quantity of water which they contain. This is illustrated by the blood itself, which is more active in its movements and operations than any other constituent of the system. Haematine, or hffimatosine, the coloring principle of the blood, contains nearly seven per cent, of iron, and it is owing to the fact that this proportion is diminished in certain diseases, that iron is prescribed as a remedy. The average proportions of the principal constituents of the blood, in one thousand parts, are: Water, - Red corpuscles, - Albumen of the serum, Saline matters, - Fatty and other matters, - Fibrine, ... 784. 131. 70. 6.03 6.77 2.2 1000. 55. In the chemical analvsis of the blood, we have a remarkable illustration of its fitness to fulfill its most important purpose, that of renovating the tissues. Thus the elements discovered by chemistry in the dried blood of the ox, are combined 'so exactly in the same proportions as the same elements are in the flesh of the animal, that the elementary composition of the blood and flesh may be considered identical. 56. The office of the blood, then, is to carry nutriment to each of the tissues. At one point it furnishes the elements of bone, at another those of muscle, at another those of the brain, and so on. The blood also takes up and carries off, through appropriate organs, all waste particles ; thus maintaining in the body a continuous round of organ- ization and decomposition, of growth and decay a perpetual change and interchange of particles new and old. 57. It has been supposed that the human organism was subject to a complete renovation once in every seven years. It is, however, impossible to determine how frequently the whole of the refuse matter of the body is thrown off" and replaced by new particles; though it is certain that a perpetual and active change is constantly at work to keep the delicate and complicated machinery of our bodies in repair. 58. In order to bring the particles of the blood into immediate contact with the tissues, so that its renovating materials may be absorbed by them, some apparatus is necessary, and we shall soon see how admirably this condition is fulfilled. The organs by means of which the blood is thus carried from one part of the body to the other, constitute the circulatory apparatus, and the course of the blood through these organs is called its circulation. 59. The circulatory apparatus is more or less perfect and compli- cated, according as the animal in which it exists has a higher or lower position in the scale of beings, and the quantity and quality of the blood depend upon the same conditions. In the lower orders of animals, where the cavity in which digestion is performed extends through the whole system, so that every part absorbs the nutriment it requires through its own walls, and where the external surface is so constructed as to admit of the free absorption of oxygen from the atmosphere, no necessity exists for the transmission of the fluids from one part to another, and the same cavity, which is simply a sac, or several sacs united together, serves the double purpose of a digestive and a circulating canal. 55. What is said of the fitness of the blood to renovate the tissues? 56. What, then, is the office of the blood? 57. How often has it been supposed that the whole system undergoes renovation? Is it possible to determine this with certainty? 58. What is the circulatory apparatus? What is circulation? 59. What is said of the circulatory appa- ratus of the lower orders of animals? 12 CIRCULATION OF THE BLOOD. 60. The Hydra, or fresh-water polype, rep- resented in fig. 4, attaches itself to a stick or other floating substance by a kind of sucker at its lower extremity, and stretches out its seven arms in search of food. In the centre, sur- rounded by these arms, is the mouth of the animal, which leads directly to the stomach or general cavity of the body, in which the food drawn down by the arms is digested and absorbed by the surrounding tissues. Some parts of the nutriment form currents to the arms, and the portions of food not capable of digestion are thrown out at the mouth, thus forming feeble channels of circulation. The internal sac of this animal is only a prolongation of its external covering, and is so much like the ***' 4 '~ T wate? Pof*' r Fresh latter in structure, that the animal may be turned inside out, and still continue to perform its functions as perfectly as before. 61. As we ascend in the scale of the animal kingdom, we next find that in insects there is a central pulsating vessel running along the back. As different portions of this vessel successively contract, the circulating fluid is forced through numerous valvular openings at various points, throughout its length. At the head, this dorsal vessel (fig. 5), as it is called, divides into two branches, which descend on each side, and again unite to form a long trunk on the under side of the body. This last trunk joins the dorsal vessel at the tail or posterior part of the insect. This kind of circulating system is more fully represented in plate 1, fig. ]. The pulsating canal of the insect seems to be an approximation to a heart with a single cavity. 62. In fishes we find a distinct heart, with two cavities, an auricle or reservoir, and a ventricle or propelling organ. The auricle receives the blood as it is returned from the general circulation, and empties it into the ventricle, which, by contracting, propels it into the gills, where it is exposed to the action of SAL VESSEL OF THK .1 . J-/Y- i .1 , .1 mi i i i i, the abdomen ; , the air dittused through the water. 1 he blood -a..* P ding S to' the head; d,d, absorbs a portion of oxygen from the air, and vessels communicating with the i s then collected into a single large trunk, organs of respiration. , . , ,. ., . ,. which distributes it to all the parts. Alter circulating through the system, the blood again returns to the auricle, is again exposed to the action of the air, and again distributed. A fig. 5. SflD Fie. 6. CIRCULATION IN FlHS. d, heart ; &, auricle ; e, ventricle ; is doubled on itself so as to bring its two internal surfaces in contact, may be illustrated by pushing the fist into a partially inflated bladder, so that the inner surface of one half the bladder may be doubled against that of the Fig. a DIAORAM or THE PERICARDIUM. other half, the hand being in contact n, a, auricles ; r, r, ventricles : pp. pericar- , . . i , ~ , - ouum. only with half the outer surface. This will be more clearly compre- hended by reference to fig. 8. 66. The heart itself is described by anatomists as a powerful, hollow, and very peculiarly constructed muscle, or rather a combina- tion of muscles, the fibres of which are intricately interwoven with each other. It is divided in the direction of its length by a strong vertical partition which completely separates it into two halves, which have no communication with each other. Another partition, placed crosswise, separates the two upper portions, or auricles, from the two lower, or ventricles, but is perforated by orifices through which the auricle and ventricle of the same side communicate with each other. 67. The course through which the blood moves in the human circulation maybe briefly described, as follows: Commencing, we will suppose, at the left ventricle of the heart, blood is impelled into the aorta or great artery leading from the heart, and along its success- ive branches, the systemic arteries, through which all the organs of the body, except the finer textures of the lungs, derive all their blood. Through these arteries it is conveyed into the systemic capillaries, those minute vessels which lie intermediately between the arteries and veins of every part, and in which the blood is brought most nearly into contact with the very substance of the organs. From these it passes into the systemic veins, through the main trunks of which, the vence cavce, it flows into the right auricle, and thence into gacngvce the right ventricle of the heart. This com- c. left auricle ; rf, left ventricle ; pletes what is called the systemic or greater n'ar?'a?i^r*^pu"taonaf^ P v'e 1 insT circulation, OT systemic or general part of the circulation. From the right ventricle the blood is sent through the pulmonary artery into the lungs, where it passes through the branches of that artery to the capillaries of the 64. What is the difference between the heart of a fish and that of a man ? Where is the human heart situated, and what is its form ? At what part of the chest are the beatings of the heart felt? 65. What is the pericardium ? Describe the method in which it incloses the heart. 66. Describe the heart. What are the auricles? What are the ventricles? 67. Describe the course of the blood in the systemic circulation. What is the name of the great artery leading from the left ventricle ? What are its branches called, and what is their use? Describe the pulmonic circulation. Explain fig. 9. cl. PLATE I . COMPARATIVE VIEW OF THE ORGANS OF CIRCULATION, FIGURE 1. CIRCULATION IN THE INSECT. a, The dorsal vessel, divided into valvular partitions, by the successive contraction of which, the blood is propelled forward. b, b, Canals which carry the blood to the head. c, c, Canals passing backward for the supply of the body, and returning the blood to the posterior end of the dorsal vessel. The course of the circulation is indicated by the direction of the arrows. FIGURE 2. CIRCULATING APPARATUS OF THE LOBSTER. In the lobster and crab, the heart has but a single cavity, and the veins are indistinct, consisting merely of irregular channels excavated in the tissues. a, The heart ; b, c, Arteries which go to the head and to the antenna or feelers ; d, The hepatic artery, or artery of the liver ; e, f, Arteries which supply the thorax and abdomen. After the blood has been propelled through these arteries by the heart, it passes into the great vein, g, g, from all parts of the body. Thence it passes to*the gills, A, where it is exposed to the action ef the air, and is then returned to the heart by the branchial veins, i, which correspond to the pulmonary veins of Man. FIGURE 3. CIRCULATING APPARATUS OF FISH. a, The auricle, having a single cavity, to which the blood is sent from all parts of the body. 6, The ventricle, which receives the blood from the auricle, and propels it through the arterial bulb, c, into the branchial artery, d. The branchial artery is sub-divided into the arteries of the gills, e, in which the blood is aerated. /, /, The dorsal artery, or aorta, which receives the aerated blood from the gills, and distributes it to all parts of the body. g, The vena cava, or great hollow vein, which conveys the blood back again to the auricle, A, Vena portae, that branch of the vena cava which conveys the blood from the abdominal organs, i, The intestine, k, The kidneys. FIGURE 4. CIRCULATING APPARATUS OF LIZARD. a, Left auricle. 6, Right auricle. One of these receives the venous blood from the system, and the other receives the arterialized blood from the lungs. c, .The single ventricle, which receives the blood from both auricles, and transmits it partly into the lungs and partly into the aorta. d, d, Arches of the aorta, e, Carotid artery, which distributes the blood to the head. /, Pulmonary vein, which conveys the blood from the lungs to the heart. g, Brachial artery, which goes to the fore-legs. A, A, Pulmonary artery, in which the blood is submitted to the influence of air in the lungs. i, The, lungs, j, The stomach, k, Vena portae. I, Intestines, m, Ventral aorta, or that portion of the aorta contained in the abdomen, n, Kidneys, o, Liver and vena poTta3. p, Inferior vena cava, which conveys the blood from all the lower parts of the body to the heart, q, Superior vena eava, through which the blood of the upper parts of the body is sent to the heart. FIGURE . GENERAL VIEW OF THE CIRCULATING APPARATUS OF MAN. The course and relative positions of the principal arteries and veins of the systemic circulation, are shown in this figure. The pulmonary circulation is illustrated in PI. 2 and 3. The arteries commence from the great arterial trunk called the aorta, and their branches are distributed to all parts of the system. The venous branches, which accompany the arteries, unite into two great veins, the superior and inferior vena cava, which convey the blood back to the heart. a, The left ventricle of the heart. 6, The right auricle, c, The superior vena cava. d, The root of the pulmonary artery, e, e, The aorta, which is seen arching backward over the heart, and passing downward into the abdomen, where it divides into its two great branches, the iliac arteries, through which the blood passes to the lower extremities. /, The inferior vena cava, which accompanies the descending aorta and its branches, and returns the blood from the lower extremities. PRINCIPAL DIVISIONS OF THE AORTA AND VENA CAVA. It should be remembered that most of the branches which spring from the great artery and vein, are double that is, each right branch has a corresponding one at the left side so that there are, for instance, the right and the left carotid arteries, the right and the left jugular veins, &c. From the arch of the aorta are sent off those arteries which are distributed to the head and arms. The principal ones among these are named as follows : /, The carotid artery, which ascends in the side of the neck, and divides into the temporal artery, g, which is distributed in the temple, and the facial artery, A, which supplies the face ; and also sends a branch, called the internal carotid, to the parts within the skull. t, The sub-clavian artery, lying beneath the clavicle or collar-bone. That part of the continuation of this artery which passes through the axilla or arm-pit, is called the axillary artery, j; that which lies in the upper arm, the brachial artery, k; and in the fore-arm, it divides into the radial and ulnar arteries, I, m, which are distributed to the hand and ringers in the manner indicated in the figure. The principal branches of the descending aorta are named as follows: The iliac artery, n, which, on passing into the thigh, becomes the femoral artery, o, and in the leg divides into the tibial and peroneal arteries, p, q, which form numerous branches for the supply of the leg and foot. Before dividing into, the iliac arteries, the descending aorta gives off several important branches ; as the coeliac artery, from which the stomach and liver are supplied ; the renal artery, which goes to the kidneys, and the mesenteric artery, to the intestines ; besides many other sub-divisions in various parts of its course. The branches of the vena cava generally accompany those of the aorta in their distribution, as shown in the figure, and are often called by the same names. The principal divisions of the ascending vena cava are: The jugular vein, r, which accompanies the carotid artery. The sub-clavian vein, i, which accompanies the artery of the same name, and receives the blood from the arm and hand. The inferior vena cava, like the aorta, divides into two great branches, the iliac veins, t, the sub-divisions of which accompany those of the arteries, and are caUed by the same names. PL.l. CIRCULATION OF THE BLOOD. 15 lungs, in which it is exposed to the action of the air. From the pulmonary capillaries the blood enters, in converging streams, the pulmonary veins, which carry it to the left auricle, whence, after having thus completed the lesser or pulmonary circulation, it passes siirain into the left ventricle, whence, in the case here supposed, it started on its course. The human circulation is illustrated by fig. 9. 68. The blood in the left ventricle is pure or arterial blood ; that is, it is charged with oxygen in greater proportion than with carbonic acid, and it also contains materials for the supply of the organs. So it remains in all the systemic nrteries ; but when it reaches the systemic capillaries, it parts with portions of those materials, and its oxygen is, in great measure, consumed by uniting with other substances which enter the blood-vessels from the refuse matter of the tissues. The blood thus becomes impure or venous, and in this state it passes through the systemic veins, the right side of the heart, and the pulmonary arteries ; but while in these arteries, by the action of the air in the lungs, it emits carbonic acid and water, and absorbs oxygen, again becomes purified or arterial, and so passes to the left ventricle. 69. A subordinate kind of circulation is inserted in the liver, and is called the portal circulation. The veins belonging to the organs of digestion, form a common trunk, the vena portce, which, instead of joining at once with the other trunks of the systemic veins, enters the substance of the liver. There the vena port*, branching like an artery, carries its share of the blood into capillaries, through which it passes into the hepatic veins (veins of the liver), and then goes through the largest branches of these veins into the vena cava inferior, one of the two main trunks of the systemic venous system, where the portal circulation terminates by mingling its blood with that of the vena cava, which has nearly reached the end of the systemic circulation. 70. The two halves of the heart are similar to each other, except that the walls of the left side are thicker and stronger than those of the right, and the capacity of the left cavities is therefore less than that of the right. The design of the difference in the thickness of the walls would seem to be this : the right ventricle sends its con- tents only to the lungs, in immediate proximity to itself, and therefore does not require the same power of contraction and dilatation as the left ventricle, which propels the blood to all parts of the system. The walls of the auricles, also, are much thinner than those of the ventricles, and instead of being firm and muscular, like the latter, are yielding and flabby. The reason is obvious: the auricles per- form only the office of receiving the blood from the vessels, and transmitting it to the ventricles, while the ventricles, as already men- tioned, constitute the propelling organ. 71. The structure and office of the valvular apparatus of the heart are exceedingly important and interesting. Between the right auricle and ventricle are three triangular folds of membrane, called tricuspid valves, which open freely into the ventricle, but close perfectly when it is filled. A set of little muscular cords are attached at one end to the edges of the valve, and at the other to the walls of the ventricle, and prevent the valves from being forced up into the auricle by the pressure of the blood during the contraction of the ventricle. This pressure itself holds them together, and they prevent a drop of blood from passing back again from the ventricle into the auricle. Behind the tricuspid valve, leading upward, is the orifice of the pulmonary artery, through which the blood is sent to the lungs. 72. The mitral valves, between the left auricle and. ventricle, receive their name from a fancied resemblance in shape to a bishop's mitre. Their structure and mode of operation are similar to those of the tricuspid valves. 73. There are also three valves to guard the entrance of the aorta, in the left ventricle, and three similar ones belonging to the pulmo- nary artery, in the right ventricle. These six valves are named semilunar, from the half-moon shape of their folds of membrane. They are attached to the walls of the artery at its exit from the ven- tricle. The semiluaar valves sometimes become ossified, so as not to close together perfectly. In this case they permit a part of the blood to flow back or regurgitate into the ventricle, by which the fj 68. Describe the method in which arterial blood becomes venous. Describe the change from venous to arterial blood. 69. Describe the portal circulation. 70. What is the differ- ence between the right and left sides of the heart ? Why is the left side stronger than the right? Why are the auricles thinner and looser than the ventricles? 71. Where are the tricnspid valves? Describe their structure. 72. Describe the mitral valves? 73. Where are the semilunar valves situated? Describe them. What is the use of the valves? When they become ossified, what is the consequence? >What is represented in tig. 10? Explain this figure. . 10. PKCTIOS or RIGHT SIDE or THE HEART. circulation is materially disturbed, and sudden death is occasionally the consequence. A section of one side of the heart, showing the position of the tricuspid valves, and the semilunar valves of the pul- monary artery, is represented in fig. 10. By referring to plate 2, fig. 3, an idea of the direction of the current of the blood through the heart may be obtained. In fig. 10, the right auricle, a, receives its blood from the two venae cavee, e, e, and transmits it to the ventricle, b, by the orifice, c. On each side of this orifice are the mem- branous folds, which are kept in their places by the tendinous cords, d. Now, when the blood is passing from a to b, these folds yield to the current, but when the cavity, b, is filled with blood, and begins to contract, the blood presses against their under sides, and makes them close together, so as to prevent the blood from passing out of the ventricle at any point except into f, the pulmonary artery. At the orifice of this artery are situated the semilunar valves, g, which open upwards to allow the blood to pass through ; but as soon as the ventricle begins to dilate again, so as to give the blood, which has already passed, a tendency to return, the blood presses on the upper side of the semilunar valves, which instantly close again, and prevent it flowing back. 74. The average quantity of blood in a healthy man of middle age and size, is estimated at about twenty-eight pounds, and the heart of such a person contracts about seventy-five times in each minute. At each contraction the heart is supposed to empty itself of about two ounces of blood; so that in every three minutes twenty-eight pounds and two ounces of blood pass through the heart, a quantity equal to the entire weight of the blood in the system. How often all the blood of the system actually flows through the heart, we can- not, however, determine with certainty, since the circulation in the immediate vicinity of the heart is performed much sooner than in the remote extremities. The foregoing calculation is not considered, however, so satisfac- tory as the results of experiments made by physiologists to ascertain the rapidity with which poisons introduced into the blood are trans- mitted from one part of the system to another. It is considered nearly certain that such substances move with the blood and at the same rate. Taking this for granted, it would appear, from such experiments, that in man the blood completes its entire circuit in less than a minute. 75. The frequency of the heart's action gradually diminishes from the commencement to the end of life. Thus, during the first year, the average number of pulsations in a minute is from one hundred and thirty to one hundred and fifteen ; about the seventh year, ninety to eighty-five ; in middle age, seventy-five to seventy; in old age, sixty-five to fifty. In the female, the heart beats more frequently than in the male. The action of the heart is also accelerated or retarded by vari- ous circumstances. It is quicker after eating than before ; it is slower during sleep than when awake ; diseases of different kinds both quicken and retard it ; it is slower in the evening than in the morning, and in the sitting than in the standing position. 76. Some of the older physiologists estimated the force of the heart's contraction at the enormous amount of sixty pounds. Later investigations have clearly proved that its actual force is only about four and one-quarter pounds, and that this force is sufficient to pro- duce the required result. Dr. Sharpey found that by an artificial force of four pounds and three ounces, water could be propelled through the arteries of a dead body, to the capillaries, and back again through the veins to the heart. Poiseuille, a French physician, by inserting in an artery one end of a bent tube filled with mercury, and allowing the blood to flow into the tube, calculated from the weight of the column of mercury supported by the blood, that the force with which the blood is impelled into the human aorta is about 74. What is the average quantity of blood in a man? How often does this quantity pass through the heart? Judging from the rapidity with which poisons pass from one part of the body to another, how long is the blood in completing its circuit? 75. What is the average number of the pulsations of the heart in the first year of life ? in the seventh year? i n middle age? in old age? What circumstances accelerate or retard the heart's action ? 76. What is proved to be the actual force of the heart's contraction ! What was Dr. Sharpey's experiment ? what Poiseuille's ? What amount of blood passes through the heart hourly ? what daily ? with what force ! Ifi CIRCULATION OF THE BLOOD. four pounds four ounces, avoirdupois. Other physiologists, by differ- ent experiments, have arrived at nearly the same results. Taking, then, the force of the heart at four and one-quarter pounds, and sup- posing that two ounces of blood are expelled at each contraction of the ventricles, the heart sends forth five hundred and sixty pounds ot blood every hour, with a propelling power of about nineteen thousand pounds, or nine and a half tons, or thirteen thousand five hundred pounds daily, with a force of four hundred and fifty-nine thousand pounds. 77. The heart is thus an organ of most untiring energy. It main- tains, through the vessels, a perpetual transfer of nutritive fluid from one part of the system to another, furnishing to every organ and tissue the material for its growth and preservation ; and it takes away such portions of matter as are no longer of use, thereby securing a constant action and reaction between the solids and fluids of the body. 78. The arteries, veins, and capillaries, are the channels of com- munication between the heart and the various parts of the body, through which all these changes take place. Every artery has five coats. The external or resisting coat forms a strong, tough invest- ment, which appears principally designed to strengthen the walls of the artery, and to guard against its excessive distension by the force of the heart's action. The internal arterial coat consists of a very thin and brittle membrane, which possesses little elasticity, and is thrown into folds or wrinkles, when the artery contracts. The internal surface of this coat, which is next to the blood, is lined with a delicate layer of epithelium, which forms a smooth and polished surface, along which the blood may flow with the smallest possible amount of resistance from friction. The elastic coat lies directly beneath the external coat, and is composed of fibres of yellow elastic tissue. Beneath this last, and between it and the internal coat, lies the muscular coat, which is made up of fibres similar to those of the muscles. 79. The external coat gives to the artery its power of resistance to the force of the heart. The elastic coat equalizes the flow of the blood through the vessels. If the heart impelled the blood through an inelastic canal, like a tube of metal or glass, then, at each impulse of the heart, a quantity of blood must be ejected with a jerk from the other end of the tube, and, during the intervals between the con- tractions of the ventricles, it would cease to flow. To prevent this is the object of the elastic coat. When the ventricle contracts and forces its two ounces of blood into the artery, the latter yields to the increased pressure, and is distended. As soon as the ventricle ceases to act, the elasticity of the arterial coat causes the vessel to contract again, and resume its former calibre, the force of its contraction being employed in continuing the propulsion of the ventricle. The elasticity of the arteries also enables them to adapt themselves to all the different movements of the body. 80. There is, besides, a contractile power in arteries, distinct from their elasticity. This is considered to be identical with the con- tractility of the muscles. When an artery is cut or torn off, the divided ends soon contract, so as sometimes to close it perfectly. This contraction may be increased by the application of cold or of stimulating substances, or by simply pricking or irritating the cut ends of the artery. Fatal haemorrhage is thus often prevented. 81. The jetting movement of the blood, at each successive con- traction of the ventricle, being propagated through the artery, is the cause of the pulse felt by placing the finger on an artery. It is pro- duced by the elongation and dilatation of the part under the finger, occasioned by the increased pressure upon its walls at the moment when the heart's contraction forces an additional quantity of blood through the aorta and its branches. 82. There is an interesting peculiarity in the distribution of the arteries, by which circulation is provided for in case of an obstruction of any considerable trunk. The branches given off at different points have frequent communications or anastomoses with each other, so that the blood may pass from the upper to the lower part of a large artery, by means of these lateral canals, even when its pas- sage through the main trunk is completely prevented. These anasto- 77. What is said of the energy of the heart? 78. How many coats have the arteries ? Describe them. 79. What is the use of the external coat? What of the elastic coat? What would be the effect, if the blood were impelled through an inelastic tube? Describe the operation of the elastic coat. 80. Describe the contractile power of the arteries. 81. What is Ihe cause of ihe pulse? 82. How is circulation effected in case of the obstruction of an artery? moses are very numerous in the arteries of the limbs, and particularly about the joints. 83. Where a particular organ requires a large amount of blood, and yet may be injured by too rapid a supply, as in the case of the brain, there is a provision for the purpose. Thus, in grazing animals, as the horse, whose heads are held near the ground in feeding, the arteries which supply the brain suddenly divide, as they enter the skull, into a great number of branches, forming a complex network of anastomosing vessels, which again unite into trunks as before. The effect of these numerous divisions is, to produce a slow and equable flow of the blood, which would otherwise inish with force through the main trunks, every time the head was brought lower than the heart. In man, the blood is prevented from being too rapidly impelled to the brain, by the firmness of the bony canals through which the arteries enter the cranium, and their tortuous course. This will be seen, by reference to plate 2, fig. 4. 84. In the whale, and some other diving animals which breathe air, the vessels under the ribs, springing from the aorta, are enor- mously dilated, and twisted into a thousand convolutions ; thus form- ing reservoirs of arterial blood by which the capillary vessels are supplied, while the animal is under water. The corresponding veins of such animals are also capable of great distention. Were it not for these peculiarities, their circulation would cease with their breathing, during their long stay beneath the water. 85. In tracing the arteries toward their extremities, we find them gradually diminishing in size, and finally terminating in a series of vessels bywhich the blood is transmitted from them to the veins. These vessels are called capillaries, or hair-like vessels. They are so small as to require a microscope for their examination, their most common diameter in man being about l-3000th of an inch. They are intimately interwoven with all the tissues, and are so minutely distributed as to render it impossible to puncture any part of the body without wounding some of their delicate divisions. Their office is to bring the blood so near the active elements of the tissues that some of its fluid part may be imbibed by them. They thus serve the important purpose of a medium through which the essential functions of nutrition, growth, and secretion, are accomplished. In them the arterial blood parts with those properties by which it affords nourishment to the organs, and, becoming venous, is transmitted through them to the veins, which carry it back again to the heart. The motion of the blood in the capillaries may be seen, with a microscope, in the web of a frog's foot, or any other transparent part of a living animal. Fig. 11 shows a magnified representation of these ves- sels in the foot of a frog. Plate 2, fig. 5, displays their arrangement in the human intestine. It is to the con- traction of these vessels that the paleness observed in the face, during the effects of fear, is owing, and it is their dilatation which produces blushing. In the first case, the blood is withdrawn from them ; and in the second, it is driven more forcibly into them, by the action of the heart. 86. The structure of the veins does not differ essentially from that of the arteries. Their contractile power is, however, less, since they possess no complete elastic coat. They are larger and more numerous than the arteries; their collective capacity is greater, and the velocity of the blood in them is con- sequently less. The limbs are furnished with two sets of veins; one set lying directly beneath the skin, the other deep-seated and accompanying the arteries. .When the muscles are exercised, the blood in the superficial veins is made to flow with greater rapidity. The office of the veins is to carry back the blood to the heart, after it has passed through the arteries and capillaries. Their chief influence in the circulation is effected by means of the valves with which they are abundantly furnished, particularly at points where they are sub- 83. What is the peculiar arrangement of the arteries of the head ? What is the effect of this arrangement? 84. How is the circulation of the whale maintained when under water? 85. How do the arteries terminate? Describe the capillaries. What is their office? What does fig. 11 represent? How are paleness and blushing produced? 86. What is said of the structure of the veins? How is the existence of their valves easily shown 1 fig. H.-^3iPILI.ARIKS IN FROO'S FOOT MAGNIFIED. PLATE II. ORGANS OF CIRCULATION. HEART AND LUNGS. FIGURE 1. FRONT VIEW OF 1IEART AND LUNGS. BOTH organs are stripped of their envelopes, the pleura and pericardium. The right lung is drawn aside, so as to uncover the heart and large vessels. The left lung is deeply dissected, to show the distribution and mode of ramification of the air-tubes and blood-vessels. fl, The larynx, b, The trachea. The right lung is somewhat shorter than the left, and is divided into three lobes, c, d, e, while the left lung has but two lobes, /, g. The surface of the lobes is sub-divided into lobules, by the intersection of great numbers of depressed lines. k, Right auricle of the heart. ', Right ventricle, j, Left auricle, k, Left ventricle. /, The aorta, m, The pulmonary artery, n, Left pulmonary veins. These veins are four in number, two for each lung ; and they return to the heart the blood which has been conveyed into the lungs by the pulmonary artery. The division of the pulmonary artery into right and left branches, cannot be seen in this figure, being hidden by the aorta, o, The superior vena cava. p, Root of the right innominate artery, springing from the arch of the aorta, q, Root of the left sub-clavian artery, r, Root of the left carotid artery. FIGURE 2. BACK VIEW OF THE HEART AND LUNGS. a, Larynx. 6, Trachea. c, Right bronchus. d, Left bronchus. e, Left auricle of the heart. /. Left ventricle. g, Right pulmonary veins. h, Left pulmonary veins. i, Left pulmonary artery, j, Section of the aorta, k, Trunks of the brachio-cephalic veins (those which belong to the arms and head). Z, The opening of the inferior vena cava. The sub-divisions of the pulmonary arteries and veins, and of the air-tubes or bronchi, are seen accompanying each other in the left lung in bolh figures. FIGURE 3. AN OUTLINE SECTION OF THE HEART; Showing its cavities and the course of the blood through it. The fgure and explanation from Wilson's Anatomy. a, The right auricle. 6, The entrance of the superior vena cava. c, The entrance of the inferior vena cava. d, Opening of the coronary vein, partly closed by its valve. e, The auriculo-ventricular opening (from the right auricle into the right ventricle). /, The cavity of the right ventricle, g, The tricuspid valve, attached by tendinous cords to the walls of the ventricle. h, The pulmonary artery, guarded at its commencement by three semi-lunar valves, i, The right pulmonary artery, passing beneath the arch and behind the ascending aorta, j, The left pulmonary artery, crossing in front of the descending aorta. The arrows mark the course of the venous blood through the right side of the heart. Entering the right auricle by the superior and inferior vena; cavffi, it passes through the auriculo-ventricular opening into the ventricle, and thence through the pulmonary artery to the lungs, k, The left auricle. /, The openings of the four pulmonary veins, m, The auriculo-ventricular opening. n, The left ventricle, o, The mitral valve, attached by its tendinous cords to fleshy columns which project from the walls. of the ventricle, p, The commencement and course of the aorta behind the pulmonary artery, marked by an arrow. The entrance of the vessel is guarded by three semi-lunar valves, q, The arch of the aorta. The comparative thickness of the two ventricles is shown in the diagram. The course of the pure blood through the left side of the heart is marked by arrows. The blood is brought from the lungs by the four pulmonary veins into the left auricle, and passes through the auriculo-ventricular opeqing into the left ventricle, whence it is conveyed by the aorta to every part of the body^ FIGURE 4. ARTERIES OF THE HEAD. This figure is intended to show the tortuous disposition of the arteries, after they have entered the skull. It also shows some of the venous sinuses of the head, which are canals lying between the dura-mater (see CHAPTER IX.) and the bone, into which the blood is poured by the veins of the brain. The inside of the base of the skull is represented, the upper portion being removed as far as the orbit of the eye. a, Vertebral arteries, uniting at It into a single vessel, called the basilar artery, which divides in the form of a T at c, and gives off several branches, d, The internal carotid arteries, each of which sends a branch of communication to the basilar, and divides into several other branches; one of which, the ophthalmic artery, e, is distributed to the eye. The internal carotid and the vertebral artery each makes no less than five successive curves, in order to break the shock of the column of blood. / The superior longitudinal sinus, g, h, Right and left lateral sinuses. FIGURE 6. THE CAPILLARIES. This figure represents a highly-magnified view of the arrangement of the capillaries between the branches of the arteries and veins, as displayed in an intestinal villus ( 161). a, Arteries. 6, Veins. FIGURES 6, 7, 8. VIEWS OF THE HEART. The heart is here represented in a vertical position, instead of being inclined, as it actually is. The figures are half the size of the adult heart. FIODRE 6. Front View of the Heart. a, Right auricle, b, Right ventricle, c, Left auricle, d, Left ventricle. <, Aorta. /, Pulmonary artery, g, Superior vena cava. A, A, Coronary arteries and veins, the nutrient vessels of the heart. FIGURE 7. Sack View of the Heart. The references are the same, except ', Orifice of the descending vena cava. j, Right pulmonary veins, k, Left pulmonary veins. FIGURE 8, gives a view of the cavities of the heart, and their septa or partitions. The, letters have the same references as in figures 6 and 7. FIGURE 9. PLAN OF THE, CIRCULATION. The object of this figure is to give, at a single view, an idea of the entire circulation, both systemic and pulmonary. The arrows mark the course of the blood. The references are the same as in fig. 3. The capillary vessels of the systemic circulation are represented at , , and those of the pulmonary circulation at t, I. PL.'.' Fig. 3. - ' . ' l/fice. of (JtfLifnc. Const <*f GQIU RESPIRATION. 19 Fif. 12. VALVES OF A VEIN. < vein hud open ; ft, A, ft, valves. jected to pressure from the muscles between which they run. These valves are represented in fig. 12. Thev are similar in construction to the semilunar valves of the arteries already described ; but their free margins are turned in an opposite direction, namely, toward the heart, so as to stop any backward movement of the blood. While the blood is flowing in its proper direction, they lie close to the sides of the vein ; but at the least motion of the blood backward, they close together completely. The existence of these valves is readily demonstrated by pressing a finger on one of the veins of the back of the hand, near the wrist, and drawing it along the vein toward the knuckles. The vein, between the point where the nearest pair of valves is situated, rmd that where the finger remains, will disappear; since the blood is moved backward in it, and the valves prevent it from being refilled. As soon as the pressure is withdrawn, the blood will be seen to rush to its place again, with great rapidity, and often with a very per- ceptible shock. CHAPTER IV. RESPIRATION. 87. RESPIRATION is that function in living bodies by which the circulating fluid is brought under the influence of atmospheric air, during which the fluid exposed to the air, and the air itself, mutually act on each other. 'Some modification of this function is equally necessary for the maintenance of both animal and vegetable life. The ascending sap of plants cannot be perfected without being exposed to the chemical action of air in their leaves. In animals, the blood, in the capillary circulation, parts with those elements which render it fit for sustaining life; and perpetual renovation of its vital properties, by the purifying influence of the atmosphere, is indis- pensable to the performance of its functions. In every class of ani- mals, therefore, there is a constant provision made for bringing every portion of their nutritive fluids into contact with air. 88. Now, we find that in plants the process of respiration consists in exposing the sap in the leaves to the action of air. The sap decomposes the carbonic acid which exists in the air, sets free its oxygen, and absorbs its carbon, which is transformed into the various tissues of the plant. In animals, the reverse is the case. A supply of oxygen is necessary to their nutrition, and too great a quantity of carbonic acid is deleterious to them. By means of their respira- tion, the blood absorbs the oxygen of the air, and sets free its car- bonic acid. Thus the two great kingdoms of nature mutually furnish to each other those elements which are necessary for the support of both. Plants purify the air for the use of animals, and maintain in it a supply of that element without which animals cannot exist. Ani- mals, on the other hand, in parting with that which is useless and even deleterious to themselves, furnish the essential element for the growth of all vegetable structures. 89. The organs of respiration are primarily divided into two groups, one of which performs its functions in the air, and the other in the water, and the respiratory apparatus of each group is modified according to these conditions of life. In some of the most simply organized animals, no particular part is exclusively adapted to this function, but it is effected through all those portions of the body which are in contact with the element in which the animal lives, and from which it derives its oxygen. Although this cutaneous respira- tion, as it is called, or breathing through the skin, belongs in its full extent only to some of the lowest orders of animated beings, it is yet worthy of remark, that it exists in some degree in nearly all the higher orders, and even in man. In many of the cold-blooded ani- 87. Define respiration. What is said of the necessity of this function? 88. What is the process of respiration in plants? What is that process in animals? How do animals and vegetables then mutually sustain each other? 89. How are the organs of respiration divided? What is said of the respiration of the lower orders of animals? What is this respiration called? What example is given of the importance of cutaneous respiration? Describe the respiratory organs of the serpula. Fig. 13. Tai SERPULA. mals this kind of breathing is of equal importance with that effected by means of the special organs of respiration. For example, the frog will live longer with his mouth and nostrils completely closed, than when his skin is coated with a substance through which the air cannot penetrate. In the lower classes of aquatic animals, the res- piratory organs are simply prolongations of the cutaneous covering into fringes, as in the serpula, a kind of marine worm, fig. 13. 90. As we ascend in the scale of organiza- tion, the respiratory apparatus becomes more perfect and more highly developed, and the animal correspondingly more active. In fishes, respiration is effected by means of a series of arches attached to the head, each of which is supplied with a vast number of thin elongated plates, collectively forming fringes or gills. Each arch is provided with an artery contain- ing blood charged with carbonic acid, branches of which terminate in a capillary net-work on the little fibres which compose the gills. Circulating across these, the blood becomes exposed to the action of the air, and is again collected into another vessel by which it is distributed in the general circulation. The air respired by all animals of this class is obtained from the water in its passage through the plates or gills. 91. In air-breathinganimals we also find two different forms of the respiratory organs. It will be recollected that the cir- culatory apparatus of insects is exceedingly simple, being little else than a pulsating tube. The reason will become obvi- ous, when we consider that in the insect the air circulates through spiral tubes to meet the nutritive fluids, which therefore (as is very evident) does not need a complicated apparatus. The abdomen of an insect is found to be made up of a series of joints, each of which is composed of two plates, one at the upper and one at the lower side. At the edges of each joint, where the two plates meet, there is an opening called the stigma, through which the air passes into the body. These little stigmata are found to commu- nicate internally with tubes in which are little spiral threads. These tubes extend through the entire system to the head, antennae, legs, and even toes ; f i x, orrv ; rlff j,; r tn ovprv nart i g carrying air t( ' every pan of the animal, and bringing it This arrangement is seen in g. 14. RESPIRATORY APPARATUS or INSECT. n, head ; , first pair of legs; c, origin of wing; d,d, stigmata; e, air-tubes or trachea , /, in contact with the circulating tubes. fig. 14. 92. The simplest condition necessary for respiration is, therefore, a membrane supplied with the circulating fluid on one side, and the air on the other. In the various groups of the lower animals, the same principle is so modified as to adapt it to their various modes of life. Thus, in the snail (fig. 15, page 20), which presents the simplest form of a true lung, the organ consists of a respiratory sac, with a blood-vessel branching on its side. In reptiles, we find still another modification of this primitive form. In the frog, for example, the lung consists of two bags, having on their walls small cells on which 690. How is respiration effected in fishes? Whence do fishes obtain air to breathe? 91 Describe the respiratory apparatus of an insect. Explain fig. 14. 92. What is the simplest kind of respiratory apparatus? Describe the lung of the snail, frog, &c. Explain {; jj _ Describe the respiratory apparatus of birds. What is said of the activity of res- piration in birds? Point out the parts of the ostrich in fig. 16. 20 RESPIRATION. the vessels are distributed- In serpents the same condition is found, except that the lung is very long, corresponding to the form of the animal. In birds we find a still higher and more complicated appa- ratus. The lungs, instead of being simple sacs, are divided into numerous cells. The air passes ^^^^ not only into the lungs, but f .. -h.*. dKHSg^ through them into large sacs along the abdomen, and even into the bones. Respiration in birds is more active than in any other class, and their movements are correspondingly more active. It is not considered certain, how- ever, that the large amount of air contained in- the sacs and bones of birds is exclusively de- Fif. 15. INTERIOR or A SNAIL. a, the heart; t, c ,: r , ri ,,J f,,- fl,,, ,->rrIinivi7 niirnnooc hirge blood-vessel branching over the sac, c; d, Signed IOr U16 Ordinal y purpo. artery which conveys the blood to the general o f respiration. It also serves tO system ; e, part of the stomach : /, the liver. . *, , . , i i , give the bird more lightness and buoyancy in its serial flights. The lungs of the ostrich are shown in fig. 16. 93. In man, the lung does not differ in principle from that of the lowest reptile, but it is infinitely more com- plex. The air cells in the human lung are variously estimated at from 18,000,000 to 600,000,000 in number. A single one of these cells is repre- sented in fig. 17. It is a single sac, on whose surface an artery is distrib- uted in minute capillary vessels which terminate in veins that carry the blood back to the heart. Now, the most perfect lung is nothing more than a collection of these cells, with blood- vessels correspondingly increased in size and number. 94. The human lungs are confined to the thorax or upper part of the trunk, which is separated from the abdomen by the diaphragm, a muscu- lar partition. They are covered by a serous membrane called the pleura, which also lines the thorax, being reflected from one surface to the other precisely in the same manner as the pericardium. Thus the pleura of the outer surface of the lung is constantly in contact with that which forms the inner surface of the thorax. They are both kept moist by a fluid which they secrete, and there- fore glide on each other with the least possible friction. 95. The lungs are supplied with air through the trachea or windpipe. At the upper part of the trachea is a complicated apparatus, which will hereafter be described, by which the entrance and exit of the air are regulated. The windpipe itself is composed of about branching over it. eighteen cartilaginous rings, connected together so as to form a tube, capable of resisting the pressure of the atmos- phere and of maintaining a uniform size. On entering the thorax, the trachea divides into two trunks, called bronchi, one of which goes to the right and the other to the left lung. As soon as they enter the lungs, they branch off into numerous divisions and subdivisions, their ultimate extremities terminating in air-cells. In like manner, the pulmonary artery sends minute capillary divisions of itself to each of these air-cells, from each of which there issues a pulmonary vein. The whole substance of the lungs is thus a con- geries of air and blood vessels. This arrangement is shown in plate 3, fig. 4. In fig. 18, is shown the trachea, with the larynx at its upper part. The left bronchus penetrates the lung of that side, and the right bronchus is subdivided into its bronchial tubes, the substance of the right lung being removed. 96. The pulmonary arteries bring to each cell a portion of the 93. What is the number of cells in the human lung? Describe one of these cells. 94. Describe the situation of the human lungs. What is the pleura? What is the use of the pleura! 95. How are the lungs supplied with air? Describe the construction of the trachea. Turn to plate 3, fig. 3, and point out the different parts. Describe the trachea and its divisions, and the corresponding divisions of the pulmonary artery. 96. What changes does the blood undergo in the lungs? Fig. 16. LUNGS OF THE OSTRICH. a, the heart ; J, the stomach ; e, c, the intestines ; rf, the trachea or windpipe ; e, e, the lungs; /, /, /, air-cells, in which are also seen the orifices of the tubes through which these air-cells communicate with the lungs. Fig. 18. a, the larynx ; 6, the trachea c, bronchial tubes ; d, the left lung. (I dark purple venous blood. Carbonic acid and vapor, as before men- tioned, are set free from the blood into the cell, and a portion of oxygen is absorbed by it from the air, changing its color to the bright scarlet of the arterial blood. The blood is thus reno- vated, and fitted for the purposes of sustain- ing animal life. 97. The constant renewal of the air in the lungs is provided for in man by a pecu- liar and simple mechanism. The thorax is a closed cavity, bounded above and at the sides by the ribs, and below by the dia- phragm, a great muscular partition which separates the thorax from the abdomen or lower part of the trunk. " The whole of this cavity," to quote the admirable de- scription of Dr. Carpenter, "with the ex- ception of the space occupied by the heart and its large vessels (and also by the gullet, which runs down in front of the spine), is constantly filled up by the lungs. Now, the size of this cavity may be made to vary considerably, in the first place, by the movement of the diaphragm; and in the second, by that of the ribs." 98. " The diaphragm, in a state of rest or relaxation, forms a high arch, which rises into the interior of the chest, as at a, fig. 19 ; but, when it con tracts, it becomes much flatter (though always retaining some degree of convexity upward), and thus adds con- siderably to the capacity of the lower part of the chest. The underside of the dia- phragm is in contact with the liver and stomach, which, to a certain degree, rise and fall with it. It is obvious that, when the diaphragm descends, these organs, with the abdominal viscera in general, must be pushed downward ; and as there can be no yielding in that direction, the abdomen is made to bulge forward, when the breath is drawn in. On the other hand, when the contraction of the dia- phragm ceases, the abdominal muscles press back the contents of the abdomen, force up the liver and stomach against the under side of the diaphragm, and cause it to rise to its former height. 99. "The play of the ribs is rather more complicated. These bones (to the number of twelve on each side, in man) are attached at one end, by a movable joint, to the spinal column b, while at the other they are connected with the sternum or breast-bone, by an elastic cartilage. Now, each rib, in the empty state of the chest, curves downward in a considerable "degree; and it may be elevated by a set of muscles, of which the highest, d, are attached to the spine and to the first rib, while others, e, e, e, called intercostals, pass from one rib to another. When the breath is drawn in, the first rib is raised by the contraction of the muscle d, and all the other ribs, which hang, as it were, from the first, would, of course, be raised to the same degree. But each of them is raised a little more than the one above it, by the contraction of its own intercostal muscle, and thus the lower ribs are raised much more than the upper ones. 100. " Now, by the raising of the ribs, they are brought more nearly into a horizontal line, as are also their cartilages ; and as the com- bined length of ribs and cartilages is the greater the nearer they approach a straight line, it follows that the raising of the ribs must throw them farther out at the sides, and increase the projection of the sternum in front thus considerably enlarging the capacity of the chest in these directions." This will be more fully understood, by 97. Describe the thorax. 98. Describe the movements of the diaphragm in respiration. 99. Describe the play of the ribs in respiration. Explain fig. 19. 100. Explain the effect of raising the ribs in increasing the capacity of the chest. Refer to plate3, figs, 5, 6. How do the ribs fall again? How may this description be better understood? fig. 19. HCMAK THORAX. PLATE III. ORGANS OF RESPIRATION. FIGURE 1. FRONT VIEW OF THE CAVITY OF TEE THORAX. a, i, c, Lobes of the right lung, d, e, Lobes of the left lung. /, /, The diaphragm, clothed with the pleurae, g, g, Section of the ribs, h, Appendage to the breast-bone, called the ensiform cartilage. i, The heart, covered by the pericardium, the left lung being drawn aside to display it. Js, k, Internal jugular veins. 1, I, Carotid arteries, m, Larynx. FIGURE 2. POSTERIOR VIEW OF THE CAVITY OF THE THORAX. a, The larynx, t, The trachea, c, c, The right and left bronchi, d, The aorta, e, The heart. /, /, The diaphragm. FIGURE I. THE LARYNX, TRACHEA, AND BRONCHI. a, The larynx, o, The trachea, c, c, Bronchi, d, d, d, e, e, e, Outlines of the lungs. /, /, /, &.C., Bronchial tubes. These tubes continue to ramify, decreasing in size, unlil they can only be distinguished by the microscope, g, g, g, Lymphatic vessels and ganglia. FIGURE 4. A PORTION OF THE TISSUE OF THE LUNGS; , Showing the Blood-vessels, Capillaries, and Air-tubes, magnified fifty diameters. A vein, a, is represented ramifying with an artery, b, around the intricate air-cells, c. c. FIGURE 5. LATERAL VIEW OF THE OUTLINES OF THE THORAX AND ABDOMEN. This figure is intended to show the respeclive positions of the diaphragm and the walls of the chest and abdomen, in inspiration and expiration. The dotted lines, a, a, indicate the contour of the front of the chest and abdomen, when the chest is filled with air after inspiration, b, b, The line of the diaphragm, when it is contracted and flattened in inspiration, pressing down the abdominal contents, and causing the abdomen to project. c, c, The iine of the chest and abdomen, after the air is expired. d, d, The arch of the diaphragm, when relaxed in expiralion, rising into the interior of the thorax, and drawing inward and downward its point of attachment to the front of the body, e. FIGURE 6. FRONT VIEW OF THE OUTLINES OF THE THORAX AND ABDOMEN. a, a, a, a, Outlines of the thorax and abdomen after expiration. b, b, Outline of the diaphragm when relaxed after expiration. c, c, c, c, Contour of the chest and abdomen in inspiration, d, d, d, Outline of the diaphragm in inspiration. FIGURE 7. THE DIAPHRAGM. This figure shows a sketch of the diaphragm when separated from the body. a, The right vault of the diaphragm, which is higher than the left vault. b, b, The right and left crura or pillars of the diaphragm, by which it is attached to the spinal column. ri,, ill. i.j . i Fiji. 4 Fi. 3. Fig. 2. fig 7 Fig (i. ANIMAL HEAT. 23 reference to plate 3, figs. 5, 6. When the movement of inspiration is finished, the ribs fall again partly by their own weight, partly by the elasticity of their cartilages, and partly by the contraction of the abdominal muscles attached to their lower border. For a full under- standing of this description, the reader should examine the move- ments of his own chest, or that of another person, by placing the fingers upon different points of the ribs, and watching their changes of position during the drawing in and the expulsion of the breath. 101. Now, the cavity of the thorax is itself perfectly closed, so that if it were not for the expansion of the lungs, a void or vacuum would be left, when the diaphragm is drawn down and the ribs are elevated. The air around presses in to fill the vacuum, but this it can only do by entering the lungs through the windpipe, and inflating them (or blowing them out), so as to increase their size in proportion to the increase of the space they have to fill. In this manner the lungs are made constantly to fill the cavity of the chest, however great be the increase of the latter. 102. The number of inspirations varies according to the age, the state of the nervous system, and other circumstances. Infants and young persons breathe more rapidly than adults, and a person in a state of mental excitement faster than one in a state of tranquillity. Under ordinary circumstances, the inspirations of an adult are from fourteen to eighteen in a minute. The average quantity of air taken in at each inspiration is supposed to be about twenty cubic inches. Thus, reckoning sixteen inspirations in each minute, about twenty thousand cubic inches of air pass through the'lungs in an hour, making two hundred and sixty-six and one-third cubic feet in twenty-four hours. The air which has passed through the lungs contains about one twenty-sixth part of carbonic acid, and thus about ten cubic feet of tha,t gas, containing nearly six ounces of solid carbon, are thrown off in a day. At the same time, about twenty- seven ounces of oxygen are absorbed into the blood. The quantity of watery vapor exhaled from the lungs in twenty-four hours is from six to twenty-seven ounces. 103. Carbonic acid ordinarily exists in the atmosphere, in the pro- portion of one part in one thousand, and when this proportion is increased to one part in one hundred, its deleterious effects are man- ifested by head-ache, oppression, languor, and fainting. Now, since a man produces from his lungs about ten cubic feet of carbonic acid in a day, if he were inclosed in a room ten feet square and ten feet high, which would contain one thousand cubic feet of air, he would, in twenty-four hours, communicate to its atmosphere sufficient car- bonic acid to raise its proportion to one part in one hundred, provided there is no influx of air from the outside. Of course, this effect is never completely produced in fact, since no chamber is so closely constructed as to prevent some interchange of its air with that of the outside. But the same effect is often produced in a shorter time, in a badly ventilated room, by a collection of a large number of persons. 104. It is evident that if the chamber of ten feet square were to be occupied by twelve persons for two hours, the same effect would be produced as that occasioned by one person in twenty-four hours. The effect, indeed, would be greater, since the quantity of carbon is produced twelve times more rapidly, and the outer air has only one-twelfth part of the time to flow in. If, then, twelve hundred persons remain in a church or assembly-room for two hours, they will produce one thousand cubic feet of carbonic acid in that time. If the building be eighty feet long, fifty broad, and twenty-five high, it will contain one hundred thousand cubic feet of air. And thus an amount of carbonic acid equal to l-100th part of the whole con- tents will be diffused through such a building in two hours, if no provision be made for ventilating it. And the injurious effects will be the greater, if there is an additional consumption of oxygen pro- duced by the burning of lights. 105. Hence we see the great importance of providing for free ventilation, wherever large assemblages of persons are collected together, even in buildings of the largest size. The weariness, drow- siness, restlessness, and head-ache, experienced while attending a crowded assembly, may be mostly traced to this cause. In schools, factories, and other places, where a large number of persons remain 101. How are the lungs made constantly to fill the cavity of the chest. Why does the air rush through the wind-pipe'! 102. What is said of the number of inspirations? What calculation is made as to the quantity of air which passes through the lungs ? How much carbonic acid and carbon are exhaled in a day ? How much oxygen is absorbed ? How much watery vapor is exhaled ? 103. What is said of carbonic acid and its effects? 104. Repeat what is said of the effects of respiration in a large assembly, &c. 105. What is fcaid of the importance of ventilation. together during many successive hours, too much attention cannot be paid to the subject of ventilation; especially, since the smaller the, room, the greater will be the pm/iorl'mii of carbonic acid in its atmos- phere, after having been breathed by a number of persons for any given time. In the narrow and ill-arranged dwellings of the poor. lack of ventilation is productive of more sickness than even an insuf- ficient supply of food and clothing, and is especially favorable to the spread of contagious diseases. 106. When we consider the constant and important labor per- formed by the lungs, their delicate structure, their liability to injury from sudden changes of temperature, from great physical exertion, and from excessive mental excitement, it is hardly a matter of wonder that so many of the human race die of diseases of this organ. One of the most fertile causes of injury to the lungs is neglect of their most important assistant, the skin. The skin affords great relief to the lungs, in aiding in the excretion of vapor and other waste matter which pass off in the form of perspiration. If, then, the surface of the skin is allowed .to become foul, from lack of cleanliness, its pores become obstructed, and the waste matter which would otherwise be excreted, is thrown back into the blood, and carried to the lungs, which are obliged to undergo more severe labor in throwing it off. If they are not able to do this completely, the noxious particles must remain in the system, the blood must be imperfectly purified, and disease must inevitably be the consequence. The exceeding import- ance of exciting a healthy action of the skin, by bathing, and regular washing and rubbing, cannot, therefore, be overrated. 107. The lungs are also liable to injury from sudden changes of temperature. If the surface of the body is suddenly chilled, perspir- ation is checked, the capillary vessels are contracted, and a larger amount of blood than the lungs can properly act upon is thrown into them at once. Hence the necessity of protecting the body by clothing sufficient and proper to prevent its feeling in too great a degree the rapid changes to which our climate is liable. Hence, also, is per- ceived the impropriety of changing too suddenly, or at too early a season, the warm woolen clothing of winter for the thinner habili- ments of the hot season. It is better to be somewhat incommoded by the heat of flannels in the spring, than to run the risk of injury to the lungs by taking off the flannel, as is too often the custom, on the first warm 'day. A uniform protection against variations of temper- ature should be, as nearly as possible, adopted. That is, the body should at all times be so clothed that it can endure the sudden changes of each season with impunity. ANIMAL HEAT. 108. CLOSELY allied with the process of respiration is the power of maintaining animal heat, a certain amount of which is necessary to vital action in all animals. The ability to produce heat, and thereby to resist the influence of cold, varies exceedingly in different orders of animals. 109. Those animals which possess the most perfect development of the respiratory organs are capable of retaining their activity in the coldest extremes of temperature, and of generating heat sufficient to resist the severest climate. In the slug and snail ( 92), whose respiratory apparatus is exceedingly simple, the temperature is very little above that of the surrounding air. The temperature of fishes ( 62), which perform their respiration by means of gills, is almost wholly dependent on that of the water, rising or falling with the warmth or cold of the sea, river, or lake, which they inhabit. In reptiles ( 63), there is a farther development of the lung, and a cor- responding increased power of maintaining a uniform temperature. When the warmth of the air is between 44 and 50, the body of a reptile will be several degrees warmer. 110. In some cold-blooded animals a most remarkable provision exists for preserving life when the temperature of the body is reduced below the freezing point. As the cold increases, the organs become more and more inactive, until at length all the animal functions cease, and torpidity ensues. Some species of insects and fishes may be frozen solid like ice, and yet retain life, and again become active 106. How are the lungs liable to injury ? How does the skin assist the lungs ? What are the consequences of neglect of the skin ? 107. What are the consequences of sudden check of perspiration ? What is said of the importance of properly protecting the body against changes of temperature? 108. What is said of the ability of animals to produce heat 1 109. In what animals is this ability greatest ? What is said of the temperature of the snail? of the fish? of the reptile? 110. What is said of freezing insects and fishes ? 24 DIGESTION MASTICATION. when exposed to a proper degree of warmth. The freezing appears to produce no chemical change either in the tissues or the fluids ; it merely suspends the operation of their affinities ..until, by a return of warmth, they are excited to action. 111. In birds and the mammalia, whose nervous and respiratory systems are perfectly developed, an uniformly elevated temperature is sustained. Respiration is more active in birds than in any other class of animals, and their animal heat is consequently greater, ranging from 100 to 112. In man, at adult age, it may be esti- mated at from 98 to 100, and in children as high as 102. In fevers, the temperature is frequently several degrees higher; in scarlet and typhus fevers as high as 106 or 107. In health, it is about one degree and a half lower during sleep than when awake. Hence, when asleep, we always require additional clothing. 112. In the young of all animals the temperature is always a few degrees higher than in the adult, and respiration is correspondingly more frequent ; but the sensibility to cold is much greater, and the power of resisting it much less. Hence, when young animals are exposed to a low temperature, without proper protection, the animal heat rapidly diminishes ; and if artificial heat be not afforded, death ensues. 113. A comparative view of the process of respiration, in con- nection with the development of animal heat, very clearly shows that the lungs, under the influence of the nervous system, are the organs by which is maintained that degree of warmth necessary to all vital action. Nearly all the blood passes through the lungs in one minute ( 74), or, at most, in three minutes. The various tissues, together with the food received into the system, supply the blood with a cer- tain quantity of carbon. When the blood reaches the innumerable air-cells in the human lung, and the carbon which it parts with there unites with a portion of the oxygen of the air contained in the cell (96), one hundred and thirty-five degrees of heat are produced by the union of each ounce of carbon. 114. Artificial heat is produced by causing the carbon of the wood or coal to unite with the oxygen of the atmosphere. In the fire, as well as in the lungs, the amount of heat is in direct proportion to the amount of oxygen and carbon combined with each other ; and the carbonic acid, which is the product of this combination, is also set free in the same proportion. That is, the quantity of carbonic acid is large or small, according to the amount of the respiration or of the fire. 115. The lungs, however, must not be regarded as the only source of animal heat. For it is probable that some degree of heat is pro- duced by the combining of portions of blood with the tissues ; and there is reason to believe that certain chemical changes, resembling that effected in the lungs, are constantly taking place through all the tissues of the body, whereby a small amount of heat may be evolved. 110. Some very interesting phenomena have been observed in certain warm-blooded animals which hybernate or spend the winter in a semi-torpid state. One of these hybernating animals, common in New England, is a species of marmot, usually called woodchuck. This animal enters his den or burrow about the first of October, closing the entrance after him with hard-packed earth. There he remains until about the first of the next April, most of the time enjoying a very profound sleep, and breathing just often enough to keep himself comfortably warm. During the five months which he remains in this condition, his whole number of respirations does not exceed those of eight or ten days of activity in the summer months. 117. All hybernating animals become enormously fat in the autumn, before they begin their winter's nap. This fat, which contains a very large proportion of carbon, is gradually absorbed and combined with the oxygen in the lungs, in order to maintain the necessary temper- ature, and the animal comes forth from his hiding-place, in the spring, greatly emaciated, and with a most excellent appetite. During the long period of his sleep, digestion and the other animal functions not necessary to respiration have been entirely suspended ; and the small supply of oxygen, indispensable to his diminished respiration, has been obtained from the air which penetrates through the earth 111. What is said of the temperature of birds? of that of man? Why do we need more clothing when asleep than when awake? 112. What is said of young animals? 113. By what organs is animal heat principally maintained? Describe the chemical action by which heat is generated in the lungs. 114. How is artificial heat produced? How is the amount of heat and of carbonic acid proportioned ? 115. What are other sources of animal heat besides the lungs? 116. Describe the hybernation of the woodchuck. 117. What becomes of the fat of hybernating animals! Whence is their oxygen derived ? What is said as to the reason for the habit of hybernating? to his burrow. These animals possess no peculiarity of structure by which they are particularly suited to this mode of life. They seem rather to have an instinctive impulse to adopt this economical method of passing the winter, because, during that season, their usual supply of food can no longer be obtained. Indeed, some other animals, whose habits do not commonly lead them to hybernate, have the power of prolonging life in a similar manner; and the same species which hybernate in a cold climate will not do so in a warmer one. This habit of sleeping through the winter seems, therefore, by a wise provision of the Creator, to be given to certain animals, in order to supply the failure of all other means of maintaining life. CHAPTER V. DIGESTION. , 118. CHANGE is the characteristic of life or vital action. No being can be regarded as alive which is not undergoing some change per- ceptible to the senses. Plants which have ceased to grow, extend, or multiply themselves, are considered as destitute of life. Every animal, however simple its structure, however obscure its move- ments, must be performing some action, and passing through some change, or it is no longer a living being. Indeed, life, in every form, is but a series of changes, a constant succession of waste and repair, more or less apparent and active, according to the structure and necessities of each individual organism. In the higher order of animals, this phenomenon is too evident to require demonstration. The particles which constitute the body, with its assemblage of organs, become successively inert and useless, and must be thrown off for the reception of others, which, after fulfilling their office, in their turn give place to new particles. In man, these mutations of- matter are carried on with an activity and energy truly surprising. It is the object of digestion to prepare the materials for this essential process of vital action. 119. Digestion may, then, be defined as that process by which those parts of food which may be employed in the formation and repair of the tissues, or in the production of heat, are made fit to be absorbed and added to the blood. Plants ( 15) take up their nutritive elements from the air and earth, in a state already adapted to their use. But all animal organizations require a certain degree of prep- aration of their aliment, and for this purpose are supplied with organs more or less complex. In the polype ( 60), a single sac or cavity, in which to dissolve its food, is all that is required. In the higher orders, some mechanical division of the food is necessary in order to prepare it for solution, and, accordingly, instead of the single digestive sac of the polype, they have a cavity (the mouth), provided with a grinding or masticating apparatus (the teeth). Another cavity (the stomach) receives the masticated food from the mouth, and reduces it to a proper state for transmission to a third cavity (the intestines), whence it is taken up and poured into the blood. MASTICATION. 120. The apparatus for dividing the food mechanically is formed according to the condition of the food which the animal requires, and the rapidity with which the process of dissolving that food is carried on. If the process is to be performed rapidly, the food requires just that preparation which would be made by a chemist wishing to dissolve a given substance in the shortest period of time. He would not throw it into his solution in a single mass, but would divide it into as many portions as possible, in order to expose the largest amount of surface to the action of the dissolving fluid. Thus, in those animals which grind, chew, or masticate their food most per- fectly, the process of digestion is performed in a few hours, while in those which swallow their prey whole, as the snake, several days are required for the purpose. 118. What is said of change as essential to life? What changes take place in the par- ticles of the body? What is the object of digestion? 119. Define digestion. How do the digestive organs of the higher orders of animals differ from the single sac of the polype ? 120. How is the masticating apparatus formed ? If the food is to be dissolved rapidly, what ^reparation does it require? What animals digest most rapidly? what most slowly? MASTICATION. 25 121. In most of the higher animals, mastication is performed by means of teeth, implanted in the jaws, and so arranged as to act against each other, with a cutting, grinding, or crushing power, according to the nature of the food on which they operate. In man, each tooth is originally developed in the interior of a small mem- branous sac called the dental capsule, which is lodged in the jaw- bone, as seen in fig. 20, which represents half the lower jaw of a very young infant. Inside of this sac, which is abundantly furnished with blood-vessels, is a' little bud-like protuberance, b, fig. 21, to Fitr. 20. DEVELOPMENT OF TEETH. a, the gum ; 4, the lower jaw ; c, dental capsules. which are sent numerous minute nerves and vessels, c. This little bud is called the pulp, and is gradually converted into the ivory of the tooth, which, in man, constitutes nearly its whole structure. The ivory begins first to be formed at the highest parts of the cap- sule, d, d. The upper portion or crown of the tooth becomes incrusted with enamel, the hardest substance found in the human body. The fang or root is covered with a substance closely resembling bone, and called the cortical substance. 122. Finally, the whole of the pulp is changed to ivory, except a small portion in the cavity of the tooth, which retains a small nerve and artery, and is frequently laid open by the decay of the outer wall. It is by the action of the air upon the nerve, in this case, that tooth-ache is usually produced. As the root of the tooth becomes developed, the crown is gradually pushed upward through the gum, or, in common language, the tooth is cut. While the tooth is thus becoming developed, the bone of the jaw hardens, closes round the root of the tooth, and forms its socket. When it is once complete, the tooth acquires no further growth, and does not seem to possess the power of repairing injuries occasioned by disease or accident. 123. But the teeth of some ani- mals never cease to grow, and their central cavity is always filled with pulp. Additional matter is constantly being formed at their base, and the whole tooth is thus pushed upward. This is the case with the tusks of the elephant, and also with the gnawing teeth of such animals as the rabbit, the squirrel, the rat, and others (fig. 22). Notwithstanding the continual wearing away of these teeth, by contact with hard substances, they are yet kept up to their proper level by their constant growth from below. 124. Fig. 23 represents the section of a human tooth as seen under the microscope. The ivory radiates in minute wavy tubes from the centre or pulp-cavity. The enamel forms a crust on the whole surface of the crown of the tooth. It is formed of a multitude of six- sided columns, packed closely against each other, one of their extremities resting upon the ivory, and the other constituting the surface of the tooth. 125. In man, and most of the other mammalia, there are three kinds of teeth, adapted to different purposes. *;#. ^.-SECTION OF The first have a thin cutting edge, and are intended "entarc^vi~ s i m ply to divide the food; these are called incisor teeth. Others have a conical form, and especially in carniv- orous animals project beyond the first. They are called canine or dog-teeth, and are not adapted to cut the food, but, by being fixed deeply in it, enable the animal to pull it asunder. The teeth of the third kind are adapted, by means of their flattened surface, to bruise and grind the food, and are called molar, or mill-like teeth. The molars are farther divided, according to their number of fangs 121. How is mastication performed ? Describe the development of the teeth in man. Explain figs. 20 and 21. 122. What farther changes take place in the formation of teeth? What causes tooth-ache? Describe the cutting of teeth. 123 Describe the teeth of gnowing animals. 124. Explain fig. 23. 125. What are the three kinds of teeth, and what are their uses? Fig. 22. JAW AND TEETH OF RABBIT. or roots, into bicuspid or small molars, and grinders. The different forms of these teeth, as found in the human jaw, are represented in fig. 24. fig. 24. HUMAN TEETH. a, incisors; 4, canine tooth ; e, bicuspid teeth ; d, molars. 126. The manner in which these different teeth are implanted in the jaw, is in accordance with their several uses. The incisors, whose action tends rather to force them more deeply into their sockets, than to draw them forth, have but a single root or fang, of no great length. The canine teeth, which are often liable to con- siderable strain, are much more deeply set into the jaw than the incisors, especially when they are long and large, as in the cat-tribe. The molars, whose action requires great firmness, have two, three, or four spreading roots, which fix them firmly to the jaw, and pre- vent them from being forced out of their sockets. 127. The correspondence between the arrangement and form of the teeth of any animal, and the nature of the food on which that animal feeds, is so exact, that, by simply inspecting its teeth, the com- parative anatomist can usually determine the general structure and habits of the animal to which they belong. Thus, in those which feed entirely on animal flesh, the molar teeth are so compressed as to form cutting edges, which work against each other like a pair of shears (fig. 25). In animals which live on insects, these teeth are raised into conical points, which lock into corresponding depressions in the teeth of the opposite jaw (fig. 26). When the animal lives principally on soft fruits, these teeth are simply raised into rounded elevations (fig. 27) ; and when they are destined to grind harder vegetable substances, their surface is flat and roughened, as in those of the horse, cow, and elephant. In the latter case, this rough sur- face is preserved by the arrangement of the enamel, which, instead of covering the crown of the tooth, is disposed in upright plates, the space between which is filled up by ivory. The ivory, being softer than the enamel, is worn down sooner, and thus the plates of enamel are constantly left projecting, so as to form a surface admirably adapted to the grinding action of the tooth. This is shown in fig. 28. Fig. 25. TEETH OF CARNIVOROUS ANIMAL. g. 26. TEETH OF INSECT-EATING ANIMAL. Fig. 27. TEETH OF FRU- OIVOROUS ANIMAL. Fig.28. TEETH OF HERBIVOROUS ANIMAL. 128. In man, the teeth which are first formed are called milk- teeth, and are twenty in number four incisors in the front of each 126. What is said of the manner in which the different kinds of teeth are implanted in the jaw 1. and why are they so implanted ? 127. What is said of the correspondence between the arrangement of the teeth and the structure of the animal ? Describe the teeth of the carnivorous, insectivorous, frugivorous, and herbivorous animals. 128. What are the first human teeth called ? What is their number ? When do they fall out ? What is the number of the permanent teeth ? What is the wisdom-tooth 2 INS ALIVATION DEGLUTITION C H Y M I FI C AT ION. jaw, and two canine teeth ami four molars on each side. All these teeth fall out at from six to eight years of age, and are gradually replaced by the permanent teeth, which are thirty-two in number, sixteen in each jaw; namely, four incisors, two canines, four bicus- pids, and six true molars. The last great* grinder does not make its appearance until long after the rest, whence it is called dens sapientia; or wisdom-tooth. 129. It is principally by means of the muscles which move the lower jaw, that the teeth are brought into action. The extent and nature of the movements of the lower jaw differ in different animals, according to the operation necessary to be performed on their food. In carnivorous quadrupeds, the jaw has only a hinge-like action, opening and shutting, and the sharpness of the molar teeth render it a powerful cutting instrument. On the other hand, the jaw of a grass- eater, as an ox, or a horse, possesses a great degree of lateral motion, by which the food is ground between the rough surfaces of the teeth. In the gnawing animals, again, as the squirrel, the lower jaw has no power of moving from side to side, but is drawn rapidly backward and forward, and the ridges of the molar teeth being arranged cross- wise (fig. 22), they act as a powerful file, by which the hardest nut- shell is quickly rasped into pulp. 130. Since no particular class of substances forms the entire food of man, that is to say, since he does not exclusively eat flesh, like the tiger, nor vegetables, like the ox, we may naturally look for a com- bination of these different modes of action in his jaw. We find, accordingly, that the jaw of man possesses a moderate degree of motion upward and downward, from side to side, and forward and backward ; thus being adapted to the mastication of all the endless variety of articles upon which the teeth are called to operate. As the principal force is required in shutting the jaw, the muscles employed for this purpose are large and powerful. They are attached to the skull in the region of the temple, where their action may be observed during mastication, and whence they are called temporal muscles. INSALIVATION. 131. The act of insalivation, or the blending of saliva with the food, during mastication, is of great importance in preparing for digestion. The saliva is secreted in three pairs of glands, through canals leading from which it is poured into the mouth. Two pairs of these are situated beneath the tongue, and are called the sub-lingual and sub-maxillary glands. Another larger pair, the parotid glands, are situated, one at each side, in the cheek, just below the ear. The quantity of saliva secreted is subject to great variation. When the tongue and masticating muscles are at rest, only enough saliva is secreted to keep the mouth moist. But the flow is much accelerated when the movements of mastication commence, and when food is taken into the mouth. The sight or thought of food also causes an increased flow of saliva, making the "mouth water," as it is termed. 132. The purpose of the saliva is to make the food soft and moist, so that it may be reduced to a pulpy mass, which is easily swallowed. It is also supposed to perform some chemical part in preparing the food for digestion, since experiments have shown that food is more readily acted on by the stomach, when impregnated with saliva, than when moistened by water. 133. "If the preliminary operations of mastication and insalivation be not thoroughly performed, the stomach has to do the work of preparation, as well as to accomplish digestion. Thus it becomes over- worked, and manifests its fatigue by not being able to discharge its own proper duty. Thus the digestive function is seriously impaired, and the general health becomes deranged in consequence. A malady of this kind is very prevalent in the United States, and is almost universally attributed by medical men in part, at least to the general habit of very rapid eating, or rather bolting, the meals. There is another evil attendant on this practice. Much more food is swallowed than is necessary to supply the wants of the system ; for the sense of hunger is not so readily abated by food which has 129. How are the teeth brought into action ? How do the movements of the lower jaw differ in different animals ? Describe the action of the jaw in carnivorous, herbivorous, and gnawing animals. 130. Why does the jaw of man combine these different modes of action? What are the temporal muscles ? 131. What is the act of insalivation ? What is said of its importance? How is saliva secreted ? What are the names of the salivary plands, and where are they situated? How is the quantity of saliva increased? 132. What is the purpose of the saliva? 133. What is said of the evil effects of imperfect mas- tication and insalivation? not been prepared for digestion ; and thus the feeling of satiety is not produced until the stomach has already received a larger supply than it is well able to dispose of." DEGLUTITION. 134. When mastication is completed, the food is transmitted in successive portions to the stomach, by the act of deglutition or swallowing. The food being collected at the back part of the tongue into a ball or bolus, is pressed against a sort of movable curtain, called the vail of the palate, which hangs from the sides of the palate, so as to touch the tongue by its lower border. This partition opens, and allows the food to pass into a sort of funnel, called the pharynx, formed by the expansion of the top of the oesophagus or gullet, which leads to the stomach. This pharynx communicates above with the nostrils, and in front with the larynx or organ of the voice, which forms the upper part of the trachea. 135. The trachea or windpipe, being situated in front of the oesophagus, the alimentary ball must pass over the glottis or aperture of the windpipe. In order to prevent any of the food entering the windpipe, the larynx, in the very act of swallowing, is drawn beneath the base of the tongue; and this action presses down a little valve, called the epiglottis, upon the aperture, so as, in general, effectually to close it against the entrance of any solid or fluid particles. But it sometimes happens that if the breath be drawn in at the same instant, a particle of the food, or a drop of liquid, finds its way into the glottis, "passing the wrong way," as it is termed. In this case, violent coughing is excited, in order to drive the particle upward, and prevent it from falling to the lower part of the windpipe. The relative position of the oesophagus, windpipe, pharynx, &c. are shown in fig. 29. 136. After the morsel has passed by the glottis, the pharynx rises up to re- ceive it, contracts, and forces it into the oesophagus, every part of which, as it is dilated by the morsel, is also stimulated to contract; and thus, by a succession of contractions, the food is carried through the whole length of the oesophagus into the stomach. The undulatory movement produced by these contractions is readily observed in the oesophagus of a horse, while he is drinking. CHYMIFICATION. 137. After passing through the oesophagus, the food arrives at the stomach, a membranous bag, placed across the upper part of the abdomen. The form of the stomach varies much, according to the nature of the food to be digested. In carnivorous animals, those of the cat-tribe, for example, whose food is flesh, which is easily dis- solved, the stomach is small, and appears like a mere enlargement of the digestive tube. In herbivorous animals, on the cwntrary, whose food, being difficult of digestion, is delayed a long time in the stom- ach, that organ is very large, and frequently forms several bags or sacs. 138. By the degree in which the stomach bulges out, as it were, to the left side of the digestive tube, we can judge of the nature of the food, on which the animal lives. Thus, for example, in man, the large end of the stomach (fig. 31, and plate 4, fig. 1), situated on the left side, is moderately developed, showing that he is adapted to a mixed diet of animal and vegetable food. In purely carnivorous animals, the large end of the stomach is nearly wanting, while in those which feed solely on grass and grain, it is enormously developed. 134. What is deglutition? Explain the action of the vail of the palate in swallowing. 135. How is the food prevented from entering the windpipe? What is the consequence when solid or fluid particles accidentally pass into the windpipe? Refer to fig. 29, and describe the relative positions of the trachea, oesophagus, &c. 136. How is the food carried from the pharynx to the stomach? Where may the contraction of the eesophasus be observed? 137. What is the stomach ? How does the form of the stomach vary ? What is said of the stomach of carnivorous animals? of that of herbivorous animals \ 138. How can we judge of the food on which an animal lives? Give examples. -e. Fig. 29. SECTION op MOUTH AND THROAT. a, vail of tho palute; f>, pharynx ; c, oesophagus ; d, Irachoii ; c, thyroid gland ; /, larynx ; g , sali- vary glanda ; A, tongue. CIIYMIFICATION. 27 (Esophagus, Cardia SdStom Intestine Fig. 30. STOMACH OF THE 139. The most complex stomach is that of those animals which ruminate, or chew the cud. It possesses no less than four distinct cavities, as shown in fig. 30, which represents the stomach of a sheep. The food first enters the paunch or first stomach, and is there satu- rated with the fluid se- creted by the walls of the stomach. It is next pass- ed into the second cav- ity, which is called the reticular or honey-comb stomach, and which com- municates directly with the oesophagus. The flu- ids, when swallowed, are passed at once into this stomach, without enter- 4th sto'm. M stom. istsiom. ing the first, and are ab- sorbed into the system through the large surface presented by the folds of its lining membrane. .140. In the case of the camel, this reticular stomach contains that curious arrangement of water-cells by which the animal is enabled to retain a supply of water for several days. In the camel, the cells of the honey-comb stomach are much larger and deeper than in the sheep, and each of them may be closed at the top, by the drawing together of the sides of its orifice. 141. In this second stomach, the food transmitted from the first is rolled up into balls, which are returned at intervals through the oesophagus to the mouth, to undergo mastication and insalivation. During this "chewing of the cud," the animal seems the picture of quiet enjoyment. After being well masticated, the food is again swallowed, and then returns into the third, and fourth stomachs, successively, there to pass through the final process of digestion. 142. In birds, which do not masticate their food, some modification of the stomach is necessary, in order to reduce it. Accordingly, we find that in those birds which live on grain, the food is first moistened and softened in the crop or craw, just as it is in the paunch of the sheep. It then passes through the second stomach (in which the gastric juice is secreted) into the gizzard, which is a very' strong hollow muscle, with a hard tendinous lining. In this the food is thoroughly triturated, the action of the gizzard being assisted, in many birds, as the common fowl, for instance, by pieces of gravel, swallowed for the purpose. In the rapacious birds, which live only on flesh or fish, the assistance of the gizzard is not required, since their food is easily digested, and is sufficiently divided into pieces, with the help of their beaks and claws. Their gizzard is conse- quently thin, and the cavities of the stomach are almost united into one. The digestive apparatus of the fowl is represented in plate 4, fig. 6. 143. In many insects, a powerful gizzard is also found, and is placed above the digestive stomach, instead of below it, as in the bird. In plate 4, fig. 7, is represented the digestive apparatus of a beetle. At a, is the head, with the jaws and antennae. From' this the gullet passes downward, and is dilated into a crop at b, below which is the gizzard, c, leading into the true digestive stomach, d, which is surrounded by great numbers of little bags or follicles, in which the gastric juice is secreted. Into this also open the long vessels, e, which in insects constitute the only rudiment of a liver. 144. In man, the stomach is an oblong, membranous bag, placed in the upper part of the abdomen, just below and partially behind the short ribs, and extending across from the left to the right side. Its capacity varies greatly. When distended, after a full meal, or after swallowing large quantities of drink, it may contain as much as two quarts. At other times, when quite empty, it may be contracted so as to hold not more than one pint. Perhaps its average capacity, in an adult person of moderate habits, may be reckoned at one quart. 145. The stomach has two openings; one, at the left end, called the cardiac orifice, because it is near the heart, leads upward into the oesophagus, and through it the food passes into the stomach. $ 139. Describe the stomach of the sheep, and the passage of the food through its two first parts. 140. How is the camel enabled to retain a large supply of water in its stomach 1 141. Describe the operation of "chewing the cud." 142. How is the absence of teeth compensated in the bird ? Describe the operation of the gizzard. Refer to plate 4, fig. 6. 143. Describe the digestive apparatus of the beetle. Refer to plate 4, fig. 7. 144. Describe the stomach of man. What is its capacity? 145. What are the openings of the stomach called ? Explain fig. 31. The other, at the right or small end, is called the pyloric orifice, and through it the partially digested food passes into the upper por- tion of the small intestine. The position of these openings, and the shape of the stomach may be seen by reference to fig. 31. 146. The stomach pos- sesses three coats, each of which performs a separate office in the work of diges- tion. These are the outer or peritoneal, the middle or muscular, and the inner or mucous coats. The perito- neal coat is the same with the external coat of all the organs which are not ex posed to the air. It lines the abdomen, and envelops in a sac all the organs found in the abdominal cavity, and sustains them in their re- Fif. 31. A SECTION or THE STOMAPH. n, the opsophasus ; SpeCtive plaCCS. This COat, I, the cardiac orifice: c, the great end of the stomach ;<<, Us 1:1 f L npr j., r j: llm lesser or pyloric end ; /', lhu pyloric oriflce; e, the lesser 11Ke B P cl curve;/, the Kroiilt-r curve; g, the rusic or wrinklca of pours Ollt a Constant the mucous lnrMil,r:mi>; A, the pylorus; i, ;, the dui.de- T " . , nui r lir-t purtiim .f the sumll inti'Stilie; *, Iliu duct tlOH Of flUKl, Which lubriCatCS !Hed"etum. lhe b '" pou d "" o its surface, and prevents fric- tion between the organs. 147. The muscular coat is composed of muscular fibres, such as we see in lean meat. These fibres possess great power of contraction and dilatation; drawing. themselves up and being stretched out again, like india-rubber, without injury. Some of these fibres wind around the stomach in the form of rings ; others run lengthwise from one end of the stomach to the other. Thus a double muscular coat is formed, of considerable thickness, and possessed of the power of con- tracting and dilating in every direction. By this alternate contract- ing and relaxing of its fibres, a great variety of motion is produced during the process of digestion, causing the food to be rolled about and moved successively over every portion of the inner or mucous coat. 148. The mucous membrane (fig. 32) lines the inside of the stomach with a thick, soft, and loose investment. It is not elastic, like the other coats, but is drawn into folds when the stomach is contracted, and spreads out smoothly when it is dilated. This coat secretes a mucous or slimy matter, which protects the stomach from being unduly irritated by its con- tents, and it also pours out from numerous little Fig. 32. Mrcoc of the stomach" follicles or glands the gastric juice, in which the tubes open at the bottom of each. magnified, showing the food is dissolved. cells, with the mouths of . ,_ m, A . , , I*ITI fi, 149. 1 he gastric glands or follicles are of tubular form, ranging from 1-500 to 1-300 of an inch in diam- eter. Their form is shown in fig. 33. When there is no food in the stomach, these glands are at rest, and the gastric juice is not poured out. Immediately on the introduction of food, they commence secreting actively, and an acid fluid exudes in drops, which run down the walls of the stomach, and soak into the sub- stances within it. The gastric juice is a very pow- erful solvent of all proper food of every kind, whether animal or vegetable. It is peculiar to the living stom- ach, and is indispensable to digestion. 150. The nature of the gastric fluid was first accu- rately ascertained by Dr. Beaumont, of the United States' army. He made many exceedingly interesting experiments, which have thrown great light on the whole process of digestion, and some of the results of which will be hereafter mentioned. A Canadian, named St. Martin, was wounded by a gun-shot, which produced an opening into his stomach near the cardiac orifice. The borders of the wound healed, and a valve or flap of membrane closed the opening from the inside, but could be pushed back with the finger. Through this means Dr. Beaumont 146. What are the three coats of the stomach? Describe the peritoneal coat. 147. Describe the muscular coat, and its action. 148. Describe the mucous coat, and its secretions. 149. What is said of the gastric glands or follicles? When are they at rest, and when do they secrete the gastric fluid? 150. How was Dr. Beaumont enabled to make his experiments? How does he describe the gastric juice ? Ftp. 33. ONE or THE TUBULAR FOLLICLES OF THE STOMACH, MAGNIFIED. 28 THE ALIMENTARY CANAL. was enabled to draw oft' portions of the gastric fluid, and to watch the entire operation of the stomach in digestion. He describes the gastric juice as "a clear, transparent iluid, inodorous, a little saltish, and very perceptibly acid. It is powerfully antiseptic, checking the putrefaction of meat ; and effectually restorative of healthy action, when applied to old fetid sores and foul ulcerating surfaces." 151. The amount of gastric fluid secreted at one time corresponds with the quantity of food which is then needed by the body as nour- ishment. It begins to flow as soon as the first mouthful of food is received into the stomach, and continues to be poured out until the body requires no more aliment. If more food be taken than is suffi- cient for the wants of the system, it will remain undigested, and become a source of oppression and irritation; or, beins; mixed with that previously received into the stomach, the digestion of the whole is retarded. 152. The sense of hunger is felt when there is a demand for a fresh supply of nourishment, and the stomach is ready to form its secretion. If the food be then swallowed no faster than the gastric fluid is prepared to be mixed with it, hunger or the desire for food will cease when the secretion ceases, or when just food enough has been taken. But if the food be swallowed twice as fast as it can be supplied with gastric juice, the sense of hunger will continue till twice as much is taken as is actually required. Hence, those per- sons who live to eat, should eat as fast as possible ; and those who eat to live, should eat no faster than the gastric juice is secreted, and they will then be sure to eat no more than nature requires. The consequences of rapid eating are insatiable appetite, nervous irrita- bility, and dyspepsia or disease of the stomach. Rapid eating is a prevailing sin among the American people, and the practice cannot be too severely censured. 153. When the proper kind of food has been taken in proper quantities, the fibres ofrfhe muscular coat of the stomach alternately contract, pressing the mass of food forward and backward, and from side to side, exposing every part of it to the action of the gastric juice, until its solution is complete. This process lasts from two to five hours, or even longer, according to the kind of food, and the thoroughness with which it has been masticated. The agitation of the food is assisted by the action of the respiratory organs, in alter* nately raising and depressing the diaphragm, whenever the air is inhaled or expired from the lungs. 154. "The readiness with which the gastric fluid acts upon the several articles of food, is, in some measure, determined by the state of division, and of the tenderness and moisture of the substance pre- sented to it. By minute division of the food, the extent of surface with which the digestive fluid can come in contact is increased, and its action proportionally accelerated." Hence the importance of thorough mastication. " Tender and moist substances offer less resistance to the action of the gastric juice than tough, hard, and dry ones do, because they may be thoroughly penetrated with it, and thus be attacked by it, not only at the surface, but at every part at once." 155. "The readiness with which a substance is acted upon by the gastric juice does not, however, necessarily imply the degree of its nutritive property ; for a substance may be nutritious, yet, on account of its toughness or other qualities, hard to digest; and many soft, easily digested substances contain, comparatively, a small amount of nutriment. But for a substance to be nutritive, it must be capable of being assimilated to the blood; and to find its way into the blood, it must, if insoluble, be digestible by the gastric fluid or some other secretion in the intestinal canal. There is, therefore, thus far, a necessary connexion between the digestibility of a substance and its power of affording nutriment." 156. When the digestion in the stomach is completed, the gastric juice and the food are thoroughly mixed and converted into a thick pulpy mass, called chyme. Portions of chyme, as fast as they are formed, are sent through the pyloric orifice of the stomach into the $ 151. To what does the amount of gastric juice secreted correspond ? If, then, more food than necessary be taken, what are the consequences? 152. When is hunger felt? Why. does hunger cease or continue, according as a proper or improper quantity of food is taken ? What is said of rapid eating, and its consequences? 153. Describe the muscular action of the stomach on the food. How long does it continue ? How is the agitation of the food assisted? 154. How is the readiness with which the gastric juice acts, deter- mined? Why are tender and moist substances most readily acted upon? 155. Is the nutntive quality of a substance always proportioned to its easiness of digestion ? Explain this. 156. What is chyme ? Describe the passage of chyme into the duodenum. Describe the pylorie sphincter, and its action. Refertoplate 4,and point out the duodenum, pylorus, &c. duodenum or first portion of the alimentary canal (fig. 31, and plate 4, figs. 1, 2). Around the pyloric orifice, a"t the inside, is a thick band of muscular fibres, forming the kind of valve called sphincter. When digestion is not going on, this sphincter, as well as a similar one at the cardiac orifice, is firmly closed; and even in the first part of gastric digestion, the pyloric orifice is so completely closed, that none of the contents of the stomach can escape. But towards the termination of the digestive process, the pylorus offers less resistance. First, it yields, to allow the successively digested portions to go through it, and afterward, it allows the passage of even undigested portions. The latter are thus always retained in the stomach much longer than those portions which are properly digested, and produce the irritability and suffering so commonly felt after eating indigest- ible substances. 157. There are many circumstances, besides the nature of the food, which affect the process of chymification. Only a sufficient quantity should be taken to fairly fill the stomach, and not to distend it. Suf- ficient time should elapse, after each meal, to allow the stomach to become quite empty, before a fresh supply is taken. This interval may be generally stated at five or six hours, though it varies with the kind of food, and the condition of the stomach and system, gen- erally. Gentle exercise, both previous and subsequent to the meal, is favorable to digestion, while excessive exertion, whether bodily or mental, retards it. A quiet and tranquil state of mind is also essen- tial to quick and proper digestion. No man should eat his dinner in a passion. Drinks, taken in large quantities, materially interfere with the process of digestion. The juices secreted by the stomach itself are sufficient for the solution of most articles of diet ; and whenever a superabundance of liquid is swallowed, the first effort of the stomach is to get rid of it. This it does, according to Dr. Beaumont's observations, by absorbing the liquid at once, and without change, through the blood-vessels in its mucous coat. The digestion of the thicker material cannot, then, be commenced until the liquid is dis- posed of. 158. A table was constructed by Dr. Beaumont, showing the periods required for the digestion of all the usual articles of food in St. Martin's stomach. Among the substances most quickly digested were rice and tripe, both of which were converted into chyme in an hour; eggs, trout, salmon, venison, and apples, were digested in an hour and a half; tapioca, barley, milk, liver, several kinds of fish, in two hours; turkey, lamb, potatoes, pig, in two hours and a half; beef and mutton required from three hours to three and a half, and veal a still longer time ; fowls and mutton required the same length of time. Animal substances were generally chymified more rapidly than vege- tables. THE ALIMENTARY CANAL. 159. After undergoing the process of digestion in the stomach, the food is farther acted upon, and undergoes other changes, in the intes- tinal or alimentary canal. This canal is divided into two portions, named, from their difference in diameter, the small and the large intestines, which are separated from each other by a muscular valvu- lar structure, the ilio-co3cal valve. This distinction is much less marked in carnivorous animals than in those which feed on vegeta- bles, and the length of the whole canal differs greatly in the two orders. In the tiger, for instance, the intestines are about three times the length of the body ; while in the sheep they are about twenty- eight times longer than the body, or seven times as long as those of the tiger, in proportion to the size of the animal. In those animals which live on a mixed diet, the intestines are of a medium length; thus in man, they measure about six times the length of the body. 160. The small intestine, for convenience of description, has been divided into three portions: the duodenum, into which opens the pyloric orifice of the stomach, and which is eight or ten inches long; the jejunum, which is continuous with the duodenum, and constitutes about two-fifths of the rest of the small intestine; and the ilium, which constitutes the remaining three-fifths. The large intestine is 157. Mention some of the circumstances which affect the process of chymification. What are the effects of exercise, of tranquillity, of an excess of liquid? 158. Mention the different periods required for the digestion of various articles, according to Dr. Beaumont. 149. How is the intestinal canal divided? What is said of the difference in the length of the intestines in the tiger and in the sheep? What is the length of the human intestines? 160. What are the three divisions of the small intestine? What are those of the large intestine ? How are the intestines sustained in their places ? What is the omentum 1 Turn to plate 4, and point out the large and small intestines, &c. PLATE IV. ORGANS OF DIGESTION, FIGURE 1 GENERAL VIEW OF THE DIGESTIVE ORGANS OF MAN. THIS figure is intended to give a general idea of the forms and relative positions of the organs of digestion. a, The oesophagus, b, The stomach, c, The duodenum, d, d, d, Convolutions of the small intestine, e, The caecum. /, Appendix of the coecum. g, Opening of the small into the large intestine, h, The ascending colon. , i, Transverse arch of the colon, j, The descending colon, k, The liver. 1, The gall-bladder, m, The pancreas, mostly covered by the stomach, o, The spleen. In this figure, the liver is raised up and the transverse arch of the colon drawn down, in order to show parts which they cover when in their natural situation. FIGURE 2 THE PANCREAS. This figure is given to show more clearly the situation and connexions of the pancreas. a, The pancreas. 6, The duodenum, c, The gall-bladder, d, Duct of the gall-bladder, which communicates with the hepatic duct, e, which leads from the liver. /, Duct of the pancreas, which opens into the common bile-duct, g, through which the combined secretions of the pancreas and the liver are poured into the duodenum. FIGURE S. SPHINCTER OF THE PYLORUS. The figure represents this sphincter (see 156), in its closed state. The central orifice dilates by the contraction of the band of fibres around it, whenever it is necessary for food to pass through it. FIGURE 4. INNER COAT OF THE STOMACH; Showing its Ruga or Folds when in a contracted state. FIGURE f. GENERAL ASPECT OF THE ABDOMINAL VISCERA. In this figure, the anterior walls of the abdomen are removed, so as to show the organs in their natural positions. The small intestine is removed. a, The liver, situated beneath the right arch of the diaphragm. 6, The stomach, c, Epiploa, or floating folds of the peritoneum, d, Summit of the gall-bladder. e, e, Large intestine, showing all its curves. FIGURE 6. DIGESTIVE APPARATUS OF FOWL. a, (Esophagus, b, Crop, c, Second stomach, in which the gastric juke is secreted, d, Gizzard, e, Liver. /, Gall-bladder, g, Bile-ducts, h, Pancreas, i, Duodenum. k, Large intestine with its two coeca, I. FIGURE 7. DIGESTIVE APPARATUS OF BEETLE. a, The head, jaws, &c. 6, The crop, c, The gizzard, d, The true digestive stomach, surrounded by its follicles, e, The long vessels, which constitute a rudimentary liver. FIGURES 8, 9. THE LIVER. FIGURE 8, represents the inferior face of the liver, a, The right or greater lobe, b, The left or smaller lobe, c, Groove which lodges the umbilical vein, d, Hepatic vena portse. e, Hepatic artery. /, Inferior vena cava. g, Gall-bladder. tiGURE 9, represents a section of the liver, showing the ramifications of the vessels. The hepatic vena portse is a division of the abdominal vena portte, into which empty all the veins of the digestive organs. It sends branches into all parts of the liver. After these branches have reached the capillary state, they are succeeded by the roots of the hepatic veins, which unite into three large veins which empty into the inferior vena cava. FIGURE 10. THE CHYLE-VESSELS AND THORACIC DUCT. a, A portion of the small intestine, b, b, Origins of lacteals. c, Mesentery, d, Mesenteric glands, e, Lymphatic vessels. /, Thoracic duct, g, Aorta, a, Thoracic duct, curving downward and forward, to empty its chyle at the junctions of the left jugular and sub-clavian veins. I,. IV. -'i . ". t < Kg. 7. Fiq.10 Fig 9. senting to J 53) ; since changes in the character of the former usually correspond very closely with changes in the character of either the whole mass of blood, or of that in the part from which the lymph is examined." 174 During their course, both lacteals and - , . ., , . , lymphatics are frequently interrupted by superficial layer;*, deep layer. wreaths or agglomerations, formed by the con- volutions of their tubes, interwoven with minute blood-vessels, and resembling small glands. Those of the lacteals, as they pass between the folds of the mesentery, are called mesenteric glands. Those of the lymphatics are also called lymphatic ganglions, and are very abundant in the arm-pit, groin, and sides of the neck, where they sometimes become inflamed and swollen into painful lumps or tumors. 175. The lacteals and most of the lymphatics unite in forming a main trunk, called the thoracic duct, which constitutes their common reservoir. Its diameter is about that of a large quill. It is situated in front of the spinal column, and after passing upward to the height of the collar-bone, it suddenly turns forward and downward, and empties its contents into the stream of venous blood proceeding to the heart, through an orifice provided with a valve, at the angle formed by the junction of the internal jugular with the sub-clavian vein. By the union of these two veins is formed the left branch of the descending vena-cava. Plate 4, fig. 10, represents the course of the lacteals and lymphatics, and of the thoracic duct. 176. When the process of digestion is not going on, the fluid con- tained in the lacteals is clear and transparent, and exactly resembles lymph ; but, during absorption from the chyme, it becomes milky, and acquires the other characters of chyle. The whiteness and opacity of chyle are owing to the presence of innumerable particles of oily or fatty matter. The fluid in which they float is albuminous. As the chyle passes upward through the mesenteric glands, and into the thoracic duct, cells, called chyle-corpuscles, become developed in it, and it acquires the power of coagulating spontaneously, forming a clot like that of the blood (54), without the red corpuscles; thus showing that it contains fibrine. 177. Lymph is usually clear, transparent, and either colorless, or having a pale yellow tinge. It usually contains, when in the smaller vessels, no corpuscles or particles of any kind; but as it advances towards the thoracic duct, and passes through the lymphatic glands, corpuscles are developed, and it becomes like chyle, spontaneously coagulable, from the formation of fibrine. It thus appears that the essential characters of chyle and lymph are very nearly the same, and differ only in the greater quantity of fatty matter contained in the chyle. 178. Upon analyzing lymph and chyle, it is found that they contain 173. What is said of the nature of the substances absorbed by the lymphatics? From what is the lymph probably derived? 174. Describe the lymphatic and mesenteric glands? 175. What is the thoracic duct? Describe its situation and course. Refer to plate 4, fig 10. 176. What changes take place in the fluid of the lacteals? To what is the whiteness of chyle owing? What changes does it pass through in its course? 177. Describe lymph, nd its changes. 178. How, then, are lymph and chyle adapted to replenish the blood? What is said of the quantity of lyrnph, compared with that of the blood ? essentially the same constituents as are found in the blood. What- ever difference there is between them and the blood, gradually diminishes as they pass through the thoracic duct and approach the place where they are to be mingled with the blood. In quality, therefore", they are adapted to replenish the blood, ahd their quantify appears ample for this purpose. From experiments upon animals, it has been estimated that in some cases, as in that of the cat, the quantity of lymph which daily passes through the thoracic duct is equal to the amount of blood at any time contained in the body. 179. The lymphatic and lacteal vessels not only contain, but move, the lymph and chyle. Their valves are turned towards the heart, like those of the veins, and are usually arranged in pairs ; so that all muscular and other external pressure accelerates the flow of the lymph as it does that of the blood in the veins. In reptiles, and some birds, this movement is assisted by muscular sacs, called lymph- hearts. Into each of these cavities several lymphatics open, and from each leads a single vein which conveys the lymph directly into the blood. These hearts have a distinct pulsation, though more slow and feeble than that of the blood-heart. 180. In addition to their principal office, that of taking up a part of the worn out particles of matter, and again fitting them to the purposes of nutrition, the lymphatics also occasionally absorb sub- stances which are placed in contact with the skin. This is some- times called accidental absorption ; and when the substance absorbed is of an acrid or poisonous nature, its passage may be traced by the red streak caused by inflammation of the lymphatic vessels. Unless arrested, the poison, in such cases, may spread rapidly, and even cause death. Liability to such results is greatly increased, when the poisonous matter comes in contact with a puncture or scratch in the skin, which exposes the absorbent vessels. The lives of many emi- nent medical men have been lost in this way, in consequence of a slight wound of the hand, while opening the bodies of persons who have died of particular diseases. 181. The function of absorbing from the skin, however, is princi- pally effected by the blood-vessels, and especially by the veins. Unlike the lymphatics and lacteals, the blood-vessels seem to exercise no choice in the selection of the materials absorbed by them. They take up every substance, whether gaseous or liquid, or even minutely divided solids, without regard to quality, provided it can penetrate their walls and mix with the blood. The less dense the fluid to be absorbed, however, the more rapid is its absorption. 182. When the amount of fluid in the body has been greatly reduced, as after long fasting, absorption of fluid through the skin may take place in considerable quantity. A patient of Dr. Currie, who was was entirely unable to receive food into his stomach, was kept alive for several weeks by being immersed in a bath of milk and water. His weight did not diminish, and Dr. Currie estimates that from one to two pints of fluid must have passed daily through his skin. Numerous instances have occurred where shipwrecked sailors, suffering from thirst, have found themselves greatly relieved by dip- ping their clothes in the sea, and putting them on while wet. Even the moisture ordinarily contained in the atmosphere may be absorbed so rapidly as sensibly to increase the weight of the body, and it seems that this absorbing power is peculiarly excited by drinking a small quantity of wine or spirits, or some hot fluid. Thus it is related that a boy, who had been nearly starved, in order to reduce his weight for riding a match, increased nearly thirty ounces within an hour, though he had drank only half a glass of wine during that period. A similar fact is mentioned, where the increase was still greater, and was produced by drinking a cup of tea. These instances will indicate the reasons why it is more dangerous when fasting, than after a full meal, to be exposed to the miasma contained in the air of the apartment of a person laboring under a contagious disease. Depression of mind, anxiety and fear, as well as every thing which tends to lessen the tone and vigor of the system, and to diminish the action of the heart and arteries, is also found to promote the activity of absorption. Hence, experience proves that during the prevalence of an epidemic which is propagated by an impure state of the atmosphere, those persons who most dread the . How is the chyle moved in its vessels? Describe the lymph-heart? 180. What is accidental absorption? What is said of the absorption of poisons? 181. How is absorp- tion from the skin principally effected? How do the blood-vessels differ from the lymph- atics and lacteals in the selection of materials for absorption? 182. When is absorption through the skin most active? Relate the illustrations given of this. How is the absorption of moisture peculiarly excited ? Give the illustrations. What is said of other causes which promote absorption through the skin, and of their influence in infectious diseases! NUTRITION AND GROWTH. 33 disease are most liable to contract it; while, on the contrary, hope and confidence are among the surest preservatives. From this, also, we can understand how of two individuals who are exposed to con- tagion under exactly the same circumstances, one escapes all evil consequences, while the other dies. CHAPTER VII. NUTRITION AND GROWTH. 183. NUTRITION is that vital process by which organs and tissues already formed are maintained in the same general conditions of shape, size, and composition, which they have obtained by develop- ment and growth. It is by this process that an adult person, in health, preserves, through a series of years, the same general outline of features, the same size and form, and, perhaps, even the same weight; although, during all this time, the several portions of his body are continually changing; their particles decaying and being removed, and then replaced by new ones, which, in their turn, also die and pass away. 184. Not only is the whole body thus preserved in its identity, but every organ and every part exactly maintains its form and compo- sition. The loss consequent on the waste and natural decay of the particles is repaired by the introduction of fresh nutritive principles ; and each elementary particle, in whatever tissue or organ it is found, seems to be capable of attracting materials from the blood, and causing them to assume its own peculiarities of structure. Thus, from apparently the same materials, the muscles form muscular sub- stance, the nerves nervous substance, the bones bony substance, and so on. 185. The source of nutrition is the blood, from which each tissue draws those materials which it requires. The blood is distributed in the substance of the tissues by means of the capillary vessels ( 85), and each tissue is supplied with a greater or less quantity of blood, in proportion to the degree of minuteness with which the capillaries ramify in it. Since the capillaries do not open directly into any tissue, no fluid can escape from them for the purpose of nourishment, except by passing through their walls. Hence it is supposed that none of the cells which float in the blood can be deposited in the tissues ; but that the liquor sanguinis ( 53) furnishes the elements of nutrition, since it readily permeates the walls of the capillaries, so as to arrive at the parts to which it is to be applied. 186. Some of the tissues, however, are not traversed by blood- vessels, but derive their nutriment by absorbing the liquor sanguinis which is brought into their neighborhood. This is the case, for instance, with the layer of cartilage which covers the ends of many of the bones, and which obtains the little nourishment it requires from the vessels which surround it. The changes which such tissues undergo are slow and limited, and there is, consequently, no necessity for an active circulation through them; while the delicate cellular or areolar tissue ( 39), which is constantly passing through rapid and extensive changes, is, in general, most abundantly supplied with capillaries which traverse it in every direction. 187. As the liquor sanguinis is withdrawn from the blood, it is continually being reformed from the food; but if it be not supplied in sufficient quantity by-the latter, the nutrition of the body cannot take place with its proper energy. The same result happens, if its fibrine ( 54) be not properly elaborated. The tissues are imperfectly nourished ; and the strength of the body and the vigor of the mind become, consequently, alike impaired. " This imperfect elaboration," says Dr. Carpenter, " seems to be the essential condition of one of the most destructive diseases to which the human frame is liable that commonly known as consumption. This is, however, but one out of several diseases which may result from the same state of consti- 183. Define nutrition. What are the effects of this process on the frame ? 184. Explain the process still farther. 185. What is the source of nutrition? How is the blood distrib- uted to the tissues, and in what proportion? Since the capillaries have no orifices, how must the nutritive fluid escape from them? What part of the blood, then, is supposed to furnish the elements of nutrition? 186. What is said of those tissues which are not traversed by blood-vessels? What of the difference between the changes undergone by such tissues and those which take place in the areolar tissue? 187. How is the liquor sangninix reformed ? If it is not sufficiently furnished, or if the fibrine is not properly elaborated, what are the consequences ? What is said of the causes of consumption ? 9 tution. If the fibrine of the blood be imperfectly elaborated, it is less fit to undergo organization ; and, consequently, instead of being con- verted into living tissues, part of it is deposited as an uimni:miz:d>le mass, in the state known to the medical man as tubercle." Such depositions in the lungs produce all the dreadful symptoms of con- sumption, and their result is almost always fatal. The means by which tubercular disease is most likely to be arrested in its first stages, therefore, is by invigorating the system by good food, active exercise, pure air, warm clothing, and cheerful occupations. 188. A remarkable example of the nutritive process is seen in the closing of a wound or the union of a broken bone. The neighboring vessels pour out their liquor sanguinis, or coagulable, lymph, as it is called by the surgeon ; this fills up the open space, and when it coagulates, it forms a connecting medium between the separate parts. The lymph soon begins to show a regular organization ; fibres and cells appear in it ; some of the cells break down into vessels, which form connexions with those in the nearest living part; the blood begins to circulate through the newly-forming tissue; and, in time, such change takes place in it as connects its several portions into fabrics resembling those with which they are connected whether bone, nerve, or skin, until the union is complete. 189. Besides the impairment and change of composition to which all parts are subject in the discharge of their natural functions, they are all liable to decay and degeneration of their particles, even while their natural actions are not called forth. It may be proved that every particle of the body is formed for a certain period of existence in the ordinary conditions of active life ; at the end of which period, if not previously destroyed by outward force or exercise, it degener- ates and is absorbed, or dies and is cast out. 190. The hair and teeth are the simplest examples of this. An eye-lash, which naturally falls out, or can be drawn out without pain, is one that has lived its natural time, and has died and been sepa- rated from the living parts. Its death is spontaneous, independent of exercise or any mechanical external force the natural termination of a certain period of life; and before it dies, provision has been made for its successor. In the case of the milk-tooth ( 128), its crown dies as the new tooth comes, and is cast out like the dead hair; while its fang, with its nerves, vessels, and pulp, degenerates and is absorbed. 191. Similar to these processes is that which constitutes the ordi- nary nutrition of a part. Each elementary particle of the body, after attaining its perfect state of development and growth, remains in that state for a time ; then dies or degenerates, and is cast out or absorbed, to make way for its successor. The length of life enjoyed by some, if not all, the parts, appears to be fixed and determinate. Thus the milk-teeth, just mentioned, fall out at a certain average age ; and of the yearly moulting and change of plumage of birds, and shedding of the antlers and hair of beasts, the only explanation is, that these sev- eral organs have their appointed times of living, at the end of which they are cast off" and replaced by others, which, in their turn, are to pass through the same changes. 192. Thus we see that from the first development of life in the infant, to the extinction of life in old age, the body is continually changing. The man is composed of different particles from those which constituted the child ; the matter which forms the dead body is not the same which formed it while a living body a few years before. Many, even, of those particles which entered into the com- position of our frames yesterday, do not belong to them to-day. The rapidity with which these changes are effected depends on a great variety of causes ; on age, constitution, the state of health, and the texture of the individual parts. In the earlier periods of life, nutrition goes on rapidly, speedily producing the renewal of parts ; but, as life advances, the changes occur more slowly. The soft parts, and those organs in which absorption and deposition are carried on with activity, are renewed more often than those of firmer texture and less active circulation. 193. In order that the process of nutrition may be perfectly accom- plished, several conditions are necessary. Among these, one of the most important is a right state and composition of the blood, from 188. What is a remarkable example of the nutritive process ? Describe it. 189. What is said of natural decay? 190. What examples are given ? Describe the death of a hair, and of a tooth. 191. Describe the process of nutrition. What is said of the fixed period of life enjoyed by some organs? 192. What is said of the constant changes of particles in the body? What are the circumstances on which the rapidity of these changes depends? 193. What are some of the conditions of healthy nutrition ? How is the effect of the mind through the nervous system on nutrition illustrated? 84 GROWTH SECRETION. which the materials of nutrition are derived. If the blood be impure, the effects of the improper material contained in it will be manifested by a. general derangement of the tissues to which it is sent. A certain influence of the nervous system is another of these condi- tions. Sufficient proof of influence on nutrition, through the nervous system, is shown by the effect of the imagination in the production or cure of organic diseases. One, among numerous illustrations of this, is the case mentioned by Dr. Warren, of Boston, in which a large tumor on the neck of a female was absorbed, and entirely disap- peared, after the application of a dead person's hand; which remedy the patient had adopted, through her conviction of its efficacy. 194. Another condition necessary to healthy nutrition, is a healthy state of the part to be nourished. This seems proved, by the very nature of the process. Since nutrition consists in the formation of new parts like those already existing, unless the latter be healthy, the former cannot be so. So long as a part is healthy, and the other . conditions of healthy nutrition exist, it preserves its healthy state. But, according to the same law, if the structure of a part be diseased or altered, the diseased state is likely to be perpetuated. In certain chronic diseases, therefore, the impure state of the blood is main- tained, notwithstanding all diversities of diet. The period in which an alteration of structure may be maintained by nutrition is not, however, unlimited ; for there is a general tendency to return to the perfect state. Thus, after the body has undergone the alteration produced by vaccination, or by small-pox, it may return, after a time, to its original state; and the person may require re-vaccination, or may have the small-pox a second time. GROWTH. 195. Growth consists in the increase of a part in bulk and weight, by the addition of particles similar to its own, but more than suffi- cient to replace those which it loses by waste or natural decav. The mode and conditions of growth are similar to those of nutrition, from which it differs only in degree. When parts have reached their ordinary size, they generally retain, through the adult period of life, nearly the same dimensions. But when a full-grown part is called upon to exercise its usual function in an unusual degree, the demand is met by a corresponding increase or growth of the part. Thus the skin increases in thickness in parts subjected to an unusual amount of pressure or friction, as in the sole of the foot, generally, or in the palms of the hands of those accustomed to rough manual labor. This is also the cause of the enlargement and increased hardness of the muscles, when actively exercised. Such a growth is a healthy process; but a part may grow by the deposit of unhealthy material in it, through an excess of nutrition ; thus diminishing, instead of increasing, its fitness for its natural office. This is often seen in the undue deposition of fat, by means of which the body is rendered cor- pulent and unwieldy. Another example is that of the enlarged liver, so frequently the result of an intemperate use of alcoholic liquors. This kind of growth is called hypertrophy; that is, excess of nutrition! CHAPTER VIII. SECRETION. 196. SECRETION is the process by which materials are separated from the blood and from the organs in which they are formed, for the purpose either of serving some office in the animal economy, or of being discharged from the body as useless substances. In the first case, both the materials separated, and the processes by which they are separated, are termed secretions; in the latter case, they are named excretions. Thus, by the process of secretion, bile is formed m the liver; and the bile thus formed is the secretion of the liver Most of the secretions consist of substances which do not previously exisi m the blood, but require special organs for their formation. Ihe excretions, on the contrary, usually consist of substances which exist, ready-formed, in the blood, and are merely abstracted from it 197. How so many different gaseous, fluid, and solid substances are formed out of the same material, the blood, no laws hitherto ascertained have enabled us to explain. The same power of forma- tion exists in plants as well as in animals. By the sap of different plants, and in which, from whatever plant derived, no chemical analysis has detected the slightest difference, we see the most oppo- site and varying products elaborated. Thus the sap of the poppy produces the narcotic opium ; that of the cherry-laurel, the deadly pjussic acid ; that of the olive, its oil. Acids are obtained from some, alkalies from others ; sweet juices, nutritive principles, oils, and resins, from others. Even different parts of the same plant furnish different secretions. The oil of the olive, for instance, is found in the fleshy part of its fruit; and yet no trace of this oil can be detected in the sap of the vessels of any other part of the tree. So in animals, the nutritious milk is secreted in one organ, the bitter bile in another; and, as yet, the most careful examination of the blood has thrown no light on the reasons why, by the same process and out of the same fluid, these different secretions are formed. These are secrets which man cannot penetrate, mysteries which, for the present, at least, are hidden from his gaze; it is only the construction of the machinery by which the secretions are effected, and the properties which they possess, that he can investigate. 198. All the varieties of secreting apparatus, both in the animal and in the plant, contain cells ( 29), by which the process of secretion is performed. These cells are arranged in different methods, accord- ing to the structure of the organ in which they are found. They have the power of sepaiating the peculiar secretion from the blood of the membrane with which they are connected, and are continually bursting, and pouring forth their contents. At parts where it is necessary that the secretion should be particularly abundant, the secreting surface is increased by great numbers of little bags or bottles, called follicles (fig. 35, c), lined with cells. 199. The two principal divisions of the secreting apparatus are membranes and glands. The principal secreting membranes are the serous and mucous membranes, and the skin. The serous mem- branes form closed sacs, and exist wherever the free surfaces of viscera come in contact with each other. Their chief purpose is to furnish a smooth, moist surface, to facilitate the movements of the inclosed organ, and to prevent the injurious effects of friction; as in the case of the pericardium ( 65), and the outer coat of the stomach ( 146). A peculiar variety of serous membrane is also found at the ends of the bones, which secretes a thick fluid, called synovia, by which the joints are lubricated. The mucous membranes line all those passages by which internal parts communicate with the exterior. They are soft and velvety, and abundantly supplied with vessels. They line the whole digestive apparatus, from the mouth through the intestines; and the whole respiratory apparatus, from the noselo the minute divisions of the bronchi. They are covered with a delicate cuticle, called epithelium, which contains the secreting cells, and which renders the surface smooth. 200. The secreting glands are the organs to which the office of secretion is especially ascribed, since they appear to be occupied by this office alone. They are divided into different classes, according to the manner in which the cells are arranged. Sometimes they are in the form of simple tubes, as the gastric glands of the stomach ( 149), or the sweat-glands of the skin, which will be hereafter described. Sometimes the gland is composed of an aggregation of small lobules, the cells of which open by minute ducts which con- verge and unite into larger and larger ducts, and at last into a common trunk, through which their united contents are carried to the point where their presence is needed. The structure of such a gland may be compared, in ar- rangement, to a bunch of grapes, as is represented in fig. 37. Ex- amples of the latter kind are 197. What is said of the formation of so many different secretions out of the some materials? 198. By what is the process of secretion performed? What is the character and operation of secreting cells? What are follicles? 199. What are the principal divisions of the secret ing organs? What the principal secreting membranes? Describe the serous membranes the mucous membranes. 200. What are secreting glands, and thci. varieties? Give examples of tubular and agglomerated glands. Fig. 37. STRUCTURB or THK PAROTID GLAND. THE SKIN. 35 the tonsils, the lachrymal gland which secretes the tears, the salivary glands, the pancreas, and the liver. In all these glands the blood is distributed by capillary vessels. 201. Those glands which secrete substances, the excretion of which is necessary for the purification of the blood, as the kidneys, which secrete the urine, generally discharge their secretions as soon as they are formed. Others, the secretions of which are only needed under particular circumstances, retain them until those circum- stances occur. Thus, the lachrymal glands retain the tears, the salivary glands the saliva, until these secretions are called forth by their appropriate stimuli. The liver, as we have already seen ( 164), has a reservoir, the gall-bladder, in which the bile is stored until needed for use. In the cow, the udder?, in like manner, perform the office of a receptacle for the milk. 202. The quantity and character of the secretions are influenced by variations in the quantity of blood, and of the peculiar materials for any secretion which it contains, and by the condition of the nervous system. An increase in the quantity of blood which passes through a gland generally increases its secretions. Through the nerves, various emotions of the mind influence the secretions. Thus the thought of savory food increases the flow of saliva, and grief pro- duces a flow of tears. In the case of .milk, not only the quantity but the quality of the secretion is influenced. The more tranquil the temper and mind of the mother, the better adapted will her milk be for the nourishment of her offspring ; and, on the other hand, there are several well-attested cases, where some violent passion of the mother has so changed the quality of her milk as to have instantly rendered it an absolute poison, which has caused the sudden death of her child. 203. The office of excretion is especially performed by the lungs, the liver, the kidneys, and the skin. The lungs, as we have seen ( 102), throw off carbon and hydrogen, in the form of carbonic acid and vapor. The liver separates the same elements from the blood in the form of a peculiar fatty matter ; the kidneys, in the form of urine ; and the skin, in that of sweat. If these excretions, or either of them, be checked, they speedily accumulate in the blood, and sometimes lead to the most deleterious results. The biliary and urinary matters, when retained in the blood, produce the effect of narcotic poisons upon the brain, and as their amount is increased, these effects increase in intensity until death ensues. Whenever the liver does not properly perform its function, head-ache, languor, and nausea, are certain to come on. The great importance of the func- tions of secretion, and the necessity of carefully attending to them, cannot be too strongly enforced. In the next chapter, excretion by the skin will be specially considered. CHAPTER IX. THE SKIN. 204. THE skin is one of the most important organs of the body, and a proper discharge of its various functions is indispensable to health, and even to life. As an external envelop, it protects and preserves the deeper tissues ; as a sensitive organ, it is the seat of the sense of touch, through which we become acquainted with the char- acters of external objects; as an organ of excretion and absorption, it is next in importance to tha liver and the kidneys. The office of the skin as a sensitive organ will be considered in tha* part of this work which treats of the sense of touch; this chapter will be con- fined to its general structure, and its functions of excretion and absorption. 205. The skin consists of two principal layers, the cutis vera, corium, or true skin ; and the epidermis, cuticle, or scarf skin. The corium is a firm, elastic membrane, composed of innumerable fibres, interwoven in every direction, and traversed by a vast number of 201. Explain the difference between glands, as relates to the discharge of their secre- tion. Mention examples. 202. How are the quantity and character of the excretions influenced? Give the instances of saliva, tears, and milk. 203. What organs specially perform excretion, and how? What is the result, if the materials to be excreted are retained in the blood 1 204. What is said of the importance of the skin ? What are its different functions ? 205. What are the two layers of the skin? Describe the cutis vera. What organs lie beneath the true skin ? What are the papillae? 7 blood-vessels, lymphatics, and nerves. The fibres intercept spaces or areolae, which, at its inner surface, are large and generally filled with fat; but at its outer surface, are nearly or quite obliterated. Within and beneath the corium lie perspiratory glands, hair follicles, and sebaceous or oil glands, and its outer surface is studded with minute conical elevations, called papillce. 206. The papillae are arranged in different modes, and are more densely set and more prominent in some parts of the body than in others. Thus they are readily seen in the palm of the hand, where each raised line is composed of a double row of them ; and on the ends of the fingers, where they are disposed in concentric curves. In each papilla there is a net-work of minute blood-vessels, surround- ing the extremity of a filament of a sensitive nerve. (5233). 207. The epidermis or outer skin, is a semi-transparent membrane, the thickness of which, on different portions of the body, is in pro- portion to the friction or pressure to which it is exposed ; and the more it is subjected to friction or pressure, within certain limits, the thicker and more horny does it become. Thus, on the palm of the hand and the sole of the foot, it is thicker than any where else. Its principal office is to protect the sensitive corium from injury. In itself, the epidermis is insensible, and parts of it may be removed without pain. 208. " The epidermis consists of a vast number of minute cells, which are formed in layers upon the external surface of the true skin. In man, and most other soft-skinned animals, the outer layers of this epidermis are continually being worn off, and new layers are as constantly being formed from within. Thus each layer is gradu- ally pushed from within outward ; and its cells undergo a consider- able change in their form, being nearly globular when first produced, and being gradually flattened into scales, by the loss of their con- tained fluids, as they approach the outer surface." 209. "It is in the latter condition that they are pressed together into that continuous membrane, which is raised by the fluid poured out from the surface of the true skin, where a blister has been ap- plied, or a hot body has touched it. Where this membrane has been removed, some more delicate layers are seen upon the surface of the true skin, which consist of cells having more of the globular form, and not holding firmly together. These soft layers were formerly regarded as a distinct tissue, to which the name of rete mucosum (mucous network) was given ; but this is now known to consist sim- ply of the last-formed portions of the epidermis. It was supposed to be the peculiar seat of the coloring matter of the skin ; this does not form a distinct layer, however, but is contained in pigment cells, which lie among the ordinary cells of the epidermis, and are conse- quently diffused through its whole thickness." The coloring matter contained in the pigment cells of the cuticle produces those varied tints which distinguish the complexions of different individuals and races. 210. From the translucence of the cuticle, the color of the parts beneath is seen through it, though the tints are modified by its not being perfectly transparent. When the surface of the body is exposed to a high degree of irritation, as by contact with fire, the vessels which pour out the lymph from which the cuticle is derived, are stimulated to increased action, and their fluid is effused so rapidly that it has no time to coagulate or be organized into cells. In this case it causes a separation between the cuticle and the true skin, and a blister is raised. When moderate irritation is applied, as that caused by friction or pressure, the increased amount of fluid has time to become organized, and the cuticle is rendered more thick and tough. It thus affords protection to the sensitive corium, at just those parts where the latter most needs protection. 211. The perspiratory glands are imbedded in the fatty layer beneath the skin, through which their ducts pass, twisting in a spiral manner, and opening at the surface of the cuticle, by a little valve or flap of which their orifice is covered. These glands are abundantly distributed over all parts of the body, and are especially numerous in the palm of the hand, where they have been estimated to amount to 3,528 in each superficial square inch. Their total number in the whole body has been reckoned at 7,000,000. 6 206. Describe the arrangement and construction of the papillae. 207. Describe the epidermis. Where is it thickest, and why? 207. Describe the construction and mode of renewal of the epidermis. 208. What are the pigment cells? What do they con- tain? 210. How is a blister raised? What is the effect of friction on the cuticle? 211. Describe the perspiratory glands. Where are they most abundant, and what is their number? THE SKIN. 212. The tubes of each of these glands being about one-quarter of an inch long, "it follows," says Erasmus Wilson, "that in a square inch of skin on the palm of the hand, there exists a length of tube equal to 882 inches, or 73 feet. Surely such an amount of drainage as seventy-three feet in every square inch of skin assuming this to be the average for the whole body is something wonderful; and the thought naturally intrudes itself, What if this drainage be obstructed ? 213. "To obtain an estimate of the length of tube of the perspira- tory system of the whole surface of the body, I think that 2800 might be taken as a fair average of the number of pores in the square inch, and consequently 700, the number of inches in length. Now the number of square inches of surface in a man of ordinary height and bulk is 2500; the number of pores, therefore, 7,000,000; and the number of inches of perspiratory tube is 1,750,000; that is 145,833 feet, or 48,611 yards, or nearly twenty-eight miles." 214. The perspiratory glands constantly exhale fluid, which is usually formed so gradually that it is carried off" by evaporation, as fast as it reaches the surface. In this case, it is called insensible perspiration. But during exercise, or exposure to heat, this secretion being poured forth in greater abundance than the air can dissolve and carry off, accumulates on the surface in drops, and is then named sensible perspiration. If the air be dry and warm, so as to carry off the fluid readily, the amount of insensible perspiration may be very great ; but, on a damp day, when the air is already too much loaded with moisture to receive the additional quantity, the exhaled fluid will remain on the skin, and the uncomfortable effect produced by this is often experienced by a person wearing a water-proof garment, which keeps in the perspiration almost as effectually as it keeps out the rain. 215. "The purpose of this watery exhalation, and of its increase under a high temperature," says Dr. Carpenter, "is evidently to keep the heat of the body, as near as possible, to a uniform standard. By the evaporation of fluids from the surface of the skin, a considerable quantity of heat is withdrawn from it. The greater the amount of heat applied to the body, the more fluid is poured out by the perspir- atory glands ; and as the air can carry it off more readily, in propor- tion to its own heat, the evaporation becomes more rapid, and its cooling effect more powerful. 216. "It is in this manner that the body is rendered capable of sustaining very high degrees of external heat, without suffering injury." Thus, the workmen of Sir F. Chantrey, the sculptor, were in the habit of going into a furnace in which his moulds were dried, when the thermometer stood at 350 , and the floor of the furnace was red- hot; and Chabert, the 'Fire-King,' accustomed himself to enter an oven, at the temperature of 400 or 600. The capability of many persons, employed in glass-blowing and iron-founding, to resist a very high degree of heat, is well known. 217. The quantity of watery vapor exhaled from the skin has been carefully estimated. M. Seguin inclosed himself in an air-tight bag, with a mouth-piece glued to his lips, so as to ascertain the difference between the amount of exhalation from the lungs and that from the skin. He found, by a long series of experiments made in this way, that the average daily amount of cutaneous exhalation was about two and a half pounds. 218. The existence of this exhalation may be easily tested by placing the naked hand and arm within a glass cylinder, or a wide- mouthed glass jar, winding a cloth around the arm at the mouth of the jar, so that the arm may not touch the glass. The inner surface of the jar will soon become dim with vapor, and the exhalation will collect into drops, and run down as a fluid. This exhalation is composed of carbonic acid and water, together with other matters, which, when the perspiration becomes sensible, are deposited on the skin, and mixed with the sebaceous secretion. The importance of the excretory function of the skin, in carrying off such substances as would be injurious, if allowed to remain in the blood, and the benefit of removing such portions of the 212. What is Wilson's estimate of the aggregate length of the perspiratory tubes in a square inch of skin 1 What are his remarks on the subject? 213. What is Wilson's esti- mate of the whole length of the perspiratory tubes in the body ? How does he arrive at this conclusion ? 214. What is insensible perspiration ? What is sensible perspiration? What is the reason of the discomfort produced by wearing a water-proof coat? 215. What is the purpose of the perspiration, and of its increase under a high temperature? 216. How is the cooling effect of the evaporation of perspiration shown. Mention the examples. 217. What was M. Seguin's experiment, and what is the amount of cutaneous exhalation, according to his estimate? 218. How may the existence of this exhalation be tested ? Of what is this exhalation composed ? What is said of the importance of the excretory function of the skin ? perspiration as may collect on the surface, impeding the pores and preventing the free action of the perspiratory glands, will be enlarged upon hereafter. 219. The fact, that the absorption of fluids takes place through the skin, even when covered by its cuticle, though long disputed, may now be considered as well established. In the lower animals, experiment has shown that such absorption may go on very rapidly. Not only frogs, whose skin is thin, but lizards, whose cuticle is thicker than that of man, after having lost weight by being kept in a dry atmosphere, quickly recovered their weight and plumpness, when immersed in water ; and when only the tail and lower parts of the body of the lizard were immersed, the absorbed water was dis- tributed through the system." 220. In the case of man, it has been ascertained by Dr. Madden, in ten experiments upon himself, that there was an average absorp- tion, by the surface of the body, of four drachms, one scruple, and three grains, in half an hour. We have seen (182) that life may be sustained for a considerable period, by means of the introduction of nutritious substances into the system, through the agency of this function of the skin. 221. Besides the perspiration, the skin secretes a peculiar fatty substance, and for this purpose is provided with another set of glands, called sebaceous glands. They are most abundant in those parts most largely supplied with hair, as the scalp and face, and are thickly distributed in the nose, lips, and ears. They are entirely absent from the palms of the hands, and the soles of the feet. Their ducts open either on the surface of the skin, or directly into the follicles of the hair. The fatty secretion of these glands seems to keep the skin smooth and supple, preventing it from drying up and cracking, and also from the effects of moisture. The contents of these glands, in the nose and forehead, often remain in their ducts, and become hard- ened, so that, when pressed out, they assume the cylindrical shape of the duct, and are then called worms. It has, in fact, been discov- ered that the ducts of the sebaceous glands are inhabited by a minute, insect, closely resembling the cheese-mite. Along the margin of the eyelids there is a row of these glands, like a string of beads. They furnish a thick fluid, which keeps the eye- lids sufficiently slippery and smooth, and prevents the tears from trickling over the margin. Occasionally, one of these glands becomes inflamed, and produces what is called a stye. In the passages of the ears, another set of these glands is arranged in clusters, and secretes the ear-wax. This secretion, being of a resinous nature, serves to facilitate the transmission of sound, while, at the same time, it repels moisture, and its disagreeable odor and taste make it a good means of preventing the entrance of insects. 222. The hair and nails are appendages of the skin. Each hair originates in a small follicle or bag, the lower part of which is seated in the sub-cutaneous fat. Sebaceous glands open into these follicles, and furnish the oily material with which the hair is anointe3 and softened. The coloring matter of the hair is secreted by cells within the follicle, and, through a deficiency of this secretion, the hair becomes gray, and finally white. Each hair is formed of a compact outer wall, with a porous and cellular centre. The uses of the hair are various. In many parts of the body it serves for ornament; on the head, it protects the brain from injury, and maintains an even temperature of that important organ; in the nose and ears, it prevents the passage of insects or other foreign bodies ; on the eyebrows and eyelashes, it shades and guards the eye. The nails are inserted between folds of the skin, which is reflected backward to the root of the nail, and then passes forward beneath its concave surface, to which it adheres. Their structure, like that of the scales, feathers, hoofs, and horns, of different animals, is similar to that of the epidermis, to which they belong. The use of the nails is to strengthen and support the ends of the fingers and toes ; they afford a support to the fingers in grasping, and are par- ticularly useful in taking hold of minute objects. Plate 5 is to be referred to, and studied, in connexion with the physiology of the skin. 219. How is it shown that cutaneous absorption takes place? 220. What is Dr. Madden's estimate of the average amount of absorption by the surface of the body? 221. What are the sebaceous glands ! Where are they most abundant? What is the use of their secretion? What is said of the contents of these glands remaining in their ducts? By what are these ducts inhabited ? How are these glands arranged in the eyelids ? What is the use of their secretion ? What is the cause of a stye ? What is the use of the ear-wax? 222. Describe the origin, structure, and uses of the hair? What is said of the nails? PLATE V. THE SKIN. FIGURE 1 A PORTION OF THE EPIDERMIS OF THE PALM OF THE HAND, MAGNIFIED BY A SIMPLE LENS, Showing the direction of the rugic or wrinkles, and the arrangement of the apertures of the sudoriferous glands. FIGURE 2. A PORTION OF THE SAME, MAGNIFIED ONE HUNDRED DIAMETERS. FIGURE 1. A TRANSVERSE SECTION OF THE RIDGES OF THE EPIDERMIS OF THE PALM OF THE HAND, Showing a side view of the apertures of the sudoriferous glands, their spiral ducts, the thickness of the epidermis in that situation, its composition of layers of cells, and its mode of connexion with the true skin. FIGURE 4. A PORTION OF THE EPIDERMIS FROM THE BACK OF THE HAND, Showing the disposition of the folds in that situation, the arrangement of the papilte, the disposition of the hairs, and the apertures of the sudoriferous and sebaceous glands, magnified with a simple lens. FIGURE fi. A PIECE OF THE SAME, MAGNIFIED ONE HUNDRED DIAMETERS, Showing that each line is a furrow or groove, a provision which allows of very great extension of the epidermis. FIGURE 6. A SQUARE OF CUTICLE, SEEN UPON ITS INTERNAL SURFACE. a, The sulci, or depressions, which correspond with the ridges of the external surface, b, The ridges which correspond with the furrows of the external surface. FIGURE 7. THE UNDER SURFACE OF THE EPIDERMIS, Showing the pigment-cells, which contain the coloring matter of the skin. These cells are seen to be collected principally in the furrows between the papillae. FIGURE 8. THE PIGMENT-CELLS OF THE NEGRO, Showing that his darker complexion is owing to the darkness of the color of the pigment contained in these cells. FIGURE 9. A SECTION OF THE SSIN, Showing the hair follicles and sebaceous glands, a, a, Sebaceous glands, opening into the hair follicle, i, A hair, with its follicle, c, surrounded by fat-cells. FIGURE 10. A SECTION OF ALL THE LAYERS OF THE SKIN. a, The epidermis, b, c, The two layers of the cutis vera. d, A sweat-gland, surrounded by cells of fet, and Bending its spiral duct upward through all the layers of the skin to the surface. FIGURE 11, A SECTION OF THE THUMB, Showing the manner in which th nail is inserted between the folds of the skin. a, The last bone of the thumb, 6, The cutiele reflected upon the root of the nail. e, The nail, d, The cuticle of the point of the thumb, continuous with that at the inner svrfaee of the nail. PL.V. Fig.l. .Fig. 5. PiQ 7. Fig 2. Fio.10. Fig. 4. F16.6. l.irli di' Kellogg THE NERVOUS SYSTEM. 39 CHAPTER X. THE NERVOUS SYSTEM. 223. The functions of organic life have been considered in the former chapters, and \ve have seen that the immediate object of those functions is to form living organized tissue out of the materials taken into the body as food. We also find that the whole animal structure is formed for motion, and that every act of motion involves a decay or waste of the fabric by which it is executed. An energetic perform- ance of the nutritive actions is required, therefore, in the more active animals, simply to make good the loss which in this way takes place. Thus we may say that the ultimate purpose of the organic life of animals is "to construct and to maintain, in a state fit for use, the mechanism which is to serve as the instrument of their animal func- tions; enabling them to receive sensation, and to execute spontaneous movements, in accordance with their emotions, instincts, or will." 224. "This mechanism consists of two kinds of structure, the Nervous and the Muscular; which have entirely different offices to perform. The Nervous system is that which is the actual instrument of the mind. Through its means, the individual becomes conscious of what is passing around him; its operations are connected, in a manner we are totally unable to explain, with all his thoughts, feelings, desires, reasonings, and determinations ; and it communicates the influence of these to his muscles, exciting them to the operations which he desires or determines to execute. But, of itself, it cannot produce any move- ment, or give rise to any action ; any more than the expansive force of steam could set a mill in motion, without the machinery of the steam engine for it to act upon. The Muscular system is the apparatus by which the movements of the body are immediately accomplished ; and these it effects by the peculiar property it possesses of contracting, upon the application of certain stimuli, of which Nervous Agency is the most powerful." 225. In its combined action, the nervous system may be regarded as the instrument of sensation, of voluntary motion, and of mind. In the lowest orders of animals, it serves no higher purpose than to render the individual conscious of the presence of foreign, bodies, and to give it power voluntarily to receive, by appropriate organs, whatever may contribute to nourishment, and to avoid or reject whatever may be injurious or useless. At each step in its progressive development, it becomes the medium of the manifestation of higher powers and facul- ties, until in the human species it attains its perfection, and becomes the instrument of those phenomena of mind and thought which dignify and ennoble man. Forming, as it does, the bond of union between the physical and intellectual conditions of the constitution of man, its study is equally interesting and important, whether in connexion with the science of Physiology, or with that of Mental Philosophy. Upon no branch of Physiology has so much been written, or have so many contradictory'opinions been advanced, as that which comprises the functions of the nervous system ; and the most subtle and powerful intellects have as yet failed to elucidate many of the mysteries con- nected with it. In this work, therefore, controverted points are as much as possible avoided, and an attempt will only be made to give a clear and concise account of it and its functions. GENERAL STRUCTURE AND ACTIONS OF THE NERVOUS SYSTEM. 226. The nervous system consists of two kinds of structure ; the white or fibrous, and the gray or vesicular. The nerves themselves, or the cords through which the nervous agency is communicated, are entirely composed of the white substance ; while the vesicular or gray substance is usually mingled with the fibrous structure, and collected into masses, as in the brain, spinal cord,*^and ganglia or knots of nervous matter ; and these masses constitute what are called nervous centres, because they are considered the organs in which the nervous force may be generated. 223. What is the immediate ohject of the functions of organic life? What is the con- sequence of every act of motion ? Why is the energetic performance of the nutritive actions required? What, then, may we say, is the ultimate purpose of organic life? 224. Of what does this mechanism consist? What is the nervous system, and what is said ot its opera- tions? Can this system, of itself, produce any movement? How are the movements of the body accomplished ? 225. What is said of the combined action of the nervous system ? What of its function in the lower animals! What of its progressive development, the importance of its study, &c. 7 226. Of what two kinds of structure does the nervous tissue consist? Of which of these are the nerves themselves composed? Where is the gray ubstance found ? What are nervous centres, and why are they so called ? 227. The white tissue consists of minute fibres, which under the microscope are found to be little tubes, transparent like glass, and con- taining a clear oil-like or pulpy substance, which may be squeezed from their ends when cut off. Each of these tubules proceeds without interruption from the nervous centre with which the nerve is con- nected, to the point at which it terminates, without uniting with any other. To form a nerve, these tubules are arranged in parallel or interlacing bundles or fasciculi, and these bundles are connected by fibro-cellular tissue, which also forms a sheath, called neurilemma, for the nerve. 228. The office of this kind of nervous structure is two-fold ; first, to convey impressions made upon the extremities of the nerves to the brain, or other nervous centre, whereby the mind becomes conscious of external objects ; and second, to convey impressions from the brain to the ultimate distributions of the nerves. The nerves are, therefore, divided into two classes; nerves of sensation and nerves of motion. 229. Thus, if the hand be accidentally placed in contact with a hot iron, the feeling of pain is communicated to the brain by the nerves of sensation, and the impulse to remove the hand is immediately returned from the brain by the nerves of motion. There is, however, no apparent difference in the structure of the nerve-fibres. Indeed, fibres of sensation and of motion may be included in the same bundle, in their distribution, though each set has a distinct and separate origin from the nervous centre. Nerves have no power of originating impressions. Those of sensation are stimulated by external agents, and those of motion by the will or some other force generated in the nervous centres. 230. Nervous force travels along a nerve like the electric fluid along a wire, without the lapse of an appreciable period of time in its passage. Each nerve-fibre can carry only one kind of impression. A nerve of motion can convey only motor impulses; a sensitive fibre can transmit only such as may produce sensation, if propagated to the brain. Moreover, the fibres of a nerve of special sense, as those of the eye or ear (optic or auditory nerves) can convey only such impressions as may produce a peculiar sensation, as that of light or sound. 231. The gray or vesicular nervous substance is very different from the other, both in its structure and functions. It is composed of great numbers of vesicles or nerve-corpuscles, lying in the interstices of a net-work of minute blood-vessels. It is found in the interior of the ganglia or nervous centres of all animals, and in the brain and spinal cord of those which possess a vertebral column or back-bone. In the brain, it is often called the cortical (bark-like) substance, because it forms a thin outer layer, inclosing the white substance, of which the greater part of the brain is composed. 232. "The precise manner in which this structure acts, we are unable to define ; but this much is tolerably certain: that, in the gray matter of the nervous centres, all those changes originate which are propagated by the motor-fibres to the muscles; and that these changes depend upon the continual supply of arterial blood. If this supply be cut off" by failure of the heart's action (as in ordinary fainting), or by pressure on the vessels which carry blood to the head, immediate insensibility, with loss of all power of motion, is the result." 233. The fibres of motor nerves, which are distributed to the mus- cles, spread forth from their trunks into branches which anastomose with each other, forming a kind of net-work through the muscle. But the nerves of sensation, which start from the skin, and convey impressions to the brain, originate in minute elevations or papillae on the surface of the corium or true skin, and in their infinite ramifica- tions is seated the sense of touch. In every one of these papilla? (see plate 5) is a net- work of minute vessels, interspersed with cells, which thus forms a sort of gray substance around the extremity of the nerve- fibre, and the influence of which is carried by the sensitive nerves to the brain, just as the influence of the brain is carried through the motor nerves to the muscles. 227. Describe the white tissue, and the manner in which it forms the nerves. 228. What is the office of the white tissue? How are nerves divided? 229. Give the example of the distinct actions of the two classes of nerves. What is said of the difference in the structure of the nerve-fibres? How are the nerves of sensation stimulated? How are those of motion stimulated? Can nerves originate impressions? 230. What is said of nervous force, and of the peculiar impressions conveyed by each class of nerves? 231. What is said of the gray nerve-substance? Where is it found? What is it called in the brain ? 232. What changes originate in it, and upon what do they depend? What is the consequence, if the supply of blood be stopped ? 233. What is said of the distribution of motor-fibres? What of the origin of the sensitive fibres? Describe the structure of the papillae. 40 THE NERVOUS SYSTEM. 234. "Hence we find that, before the mind of an individual can become conscious of what is passing around him, a change or impres- sion must be effected by external objects upon the origins of the sen- sory nerves in the papillae ; this impression must be conducted by the nervous trunk to the sensorium (brain or other centre of perception), and there it becomes a sensation. On the other hand, before the mind can direct the body to perform any movement, an emotion, or an act of the will, must produce a change at the origin of the motor nerves in the brain, and this change is conducted along the motor trunks to the muscles, where it excites $ contraction adapted to the required purpose." 235. It is by actions of this kind that the nervous system ministers to the animal functions ( 15). But there is another set of move- ments, controlled by it, which are concerned in the maintenance of organic life ; such, for instance, as those of Deglutition and Respira- tion. "These movements require the same double set of nervous fibres, and the same kind of nervous centre, containing gray matter; but they do not require that sensation should intervene, or (in other words), that the individual should become conscious of the impression in which they originate, in order that the muscles may be excited to contraction. Movements of this class are termed reflex; from the peculiar action of the nervous centre in throwing back, or reflecting, along the motor nerves which pass from it, the impression which it receives from the fibres that pass towards it." 236. " There can never be more than a single centre of sensation in any animal ; for if there were two, or more, there must be two sets of feelings, and consequently two distinct individual minds. But there may be many centres of reflex action, having different purposes, because connected with different functions. In the lower classes of animals, these centres are often very numerous; and the actions to which they minister constitute a great part of the movements per- formed by them. But as we ascend the scale, we find that these centres of reflex action are less important, in comparison with the organ by which the mind operates ; and the body is more influenced by the latter than by the former." COMPARATIVE VIEW OF THE NERVOUS SYSTEM IN DIFFERENT GROUPS OF ANIMALS. 237. To illustrate the comparative importance of the different por- tions of the nervous system, it will be proper to begin with its simplest forms, and proceed gradually to the more complicated. As we thus examine the peculiarities of this system in the different groups of ani- mals, we shall the better understand the mutual relations of its parts to each other, and shall be enabled to trace the gradual development of the powers connected with it. Nature has, in this way, furnished us with living demonstrations of the uses and functions of the complex parts which make up our own nervous system. 238. One of the simplest forms of the nervous system is shown by fig. 39. The animal to which it belongs is the Ascidia, one of the lowest of the class Mollusca, of which the slug and snail are higher members. In this and similar animals, we find only a single ganglion, or nervous centre. At a is seen the orifice by which water enters for supplying the stomach with food ; and at b, that through which it passes out again. Between these orifices is the single ganglion, c, which sends filaments to both orifices, and also over the surface of the envelop or mantle, d. These animals are mostly fixed to one spot during their whole exist- ence. The continual entrance and exit of the cur- Kg 39 -NERVOUS SYS- ren . ts f water constitute the only phenomena of life TEH of "AlcimA. YS which they exhibit, except when the current draws in any injurious substance. The mantle then con- tracts, causing a jet of water to issue from each orifice, and throwing off the offending material. This little animal has no eyes or other organ of special sense. The small tentacula or feelers, at the upper orifice, are the only parts which seem to be peculiarly sensitive ; and the irritation caused by the contact of a hard substance with these, or with the general surface of the body, causes an instinctive con- 234. Describe the mode in which sensation and movement are produced through the IHTVCS. 235. How do the movements concerned in organic life differ from those of animal life ! What are such movements called, and why ? 236. What farther remarks are made in tfgnri in centres of sensation and of reflex action? 237. What is said of the importance of studying the nervous system in various animals! 238. Describe the nervous system of the Ascidia, and the phenomena of life exhibited by that animal. traction of the mantle, for the purpose of getting rid of the irritating cause. This contraction can only be performed by the aid of a nervous system, which has the power of receiving impressions, and of exciting the most distant parts of the body to act in accordance with them. 239. In those animals which inhabit bivalve shells, the powers of respiration, sen- sation, and voluntary motion, are evinced, and the nervous system is correspondingly developed. The nervous system of the Pecten, or Scallop-shell, is shown in fig. 40. At B is a large ganglion, which distributes branches to the gills and mantle, and regu- lates the respiratory movements. Another ganglion, c, is connected with the thick fleshy part on which the animal crawls, and which is called its foot. Near the sides of . the oesophagus, e, are situated two other Kg. 40-NEavo, -, S*.T , PEC,- &?&*> A > A > *e nerves of which are dis- TKX. A, A, [flinsiinofthehead; n> tnbuted to the sensitive tentacula which f guard the mouth. These two cephalic ganglia, or ganglia of the head, evidently correspond to the brain of higher animals, being the instruments of sensation and voluntary power ; and they exert a controlling direction over the movements of the animal, while the pedal and branchial ganglia (those of the foot and of respiration) minister to the reflex actions of the organs supplied by them. 240. In animals of higher orders, the ganglia are more numerous, as the variety of functions to be performed becomes greater ; and in proportion as we ascend the scale, the cephalic ganglia become more and more developed, and more and more elevated above the oesopha- gus, until they finally meet on the central line above it, and, in the more perfect animals, take their place in the top of the head, over- looking, as it were, all the other organs. 241. The nervous system of Insects, whose actions are generally' energetic and rapid, and in which the ap- paratus of movement (wings and legs) is highly developed, presents a marked differ- ence from that of the Mollusca, which are usually inert and sluggish. It consists of a large ganglion in the head, analogous to the brain of vertebrated animals, and a chain of ganglia, one for each segment of the body, united by a double cord, as in fig. 41. In the larva or caterpillar, before it is changed into the perfect insect, the nerves arising from the ganglia are chiefly distributed to the muscles of the legs, and the ganglia are only repetitions of each other, and are of nearly uniform size; but, in the perfect insect, the ratus of locomotion, are confined to the tho- rax, and the segments of the abdomen have no legs. Accordingly, the ganglia of the thorax, in the perfect insect, are found to be very much increased in size, and sometimes concentrated in one mass, while those of the abdomen are much smaller. 242. This difference is shown in fig. 42 (page 43), where A repre- sents the nervous system of the caterpillar from which is produced a species of Sphinx or hawk-moth ; and B, that of the moth itself. In B, the cephalic ganglion is much larger than that of A, since it has become connected with more perfect eyes and other organs of sense. The ganglia which supply the wings and legs (those next below that of the head) are also greatly enlarged and concentrated, while the abdominal ganglia are diminished, and some of them are wanting. The whole chain is also considerably shortened, the body of the moth not being as long as that of the caterpillar. 243. The chain of ganglia in insects is found to consist of two dis- tinct tracts. One is composed of nerve-fibres only, and passes from the cephalic ganglion over the surface of all the other ganglia, giving 239. Describe the nervous system of the Pecten. Explain fig. 40. What .is said of the cephalic, pedal, and branchial ganglia? 240. What is said of the progressive development of the ganglia, especially those of the head ? 241. Describe the nervous system of insects. How does the nervous system of the caterpillar differ from that of the perfect insect, and what is the reason for this difference ? 242. Explain this difference farther, by referring to fig. 42. 243. Explain the arrangement of the ganglia in insects, and why the motions of a headless insect may be performed without sensation. PLATE VI. THE BRAIN AND SPINAL CORD. FIGURE 1. POSTERIOR VIEW OP THE BRAIN AND SPINAL CORD, ISOLATED FROM THE SKELETON. a, D, Right and left hemispheres of the brain, the left hemisphere covered by the arachnoid and pia-mater; the right hemisphere naked, displaying the convolutions e, d, Right and left lobes of the cerebellum, e, The medulla oblongata. /, /, The spinal cord, covered by the pia-mater on the right side, and showing th< origins of the spinal nerves on the left. At /, /, are seen the two enlargements of the cord, in the neck and the loins, g to g, The eight cervical pairs of nerves A to A, The twelve dorsal pairs; i to i, The five lumbar pairs; i, Ts, The six sacral pairs. /, Lateral column of the cord, m, Posterior column (251). FIGURE 2. A SECTION OF THE BRAIN AND SPINAL CORD, INCLOSED IN THE SKULL AND VERTEBRAL COLUMN. a, The cerebrum, b. The cerebellum, c, c, The spinal cord. The vertebra are cut through, so as to display a lateral view of the cord. FIGURE I. THE CEREBELLUM. A, Anterior view. B, Posterior view. FIGURE 4. BASE OF THE BRAIN. In this figure are shown the vessels of the brain, with their complicated windings, and also the origins of some of the cerebral nerves, a, a, The anterior lobes of the cerebrum, b, b, The middle lobes, c, c, The posterior lobes, the left one being nearly covered by the cerebellum ; the right one exposed by the removal of a portion of the cerebellum, d, The cerebellum. , The medulla oblongata. /, /, &c., Arteries and veins, g, g, &c., Origins of nerves. FIGURE . A HORIZONTAL SECTION OF THE BRAIN, SHOWING ITS INTERIOR. a, a, &c., The cineritious or gray substance at the outside of the brain, following the convolutions, b, b, The white or medullary substance at the inside of the brain. c, c, The lateral and middle ventricles. FIGURE 6. A VERTICAL SECTION OF THE BRAIN (161), SHOWING THE ORIGINS OF ITS NERVES. a, a, The cerebrum, with its convolutions. i, The cerebellum, displaying the arbor-vitm upon its section, c, The medulla oblongata. d, The corpus callosum, a band of fibres which connects the two hemispheres of the brain, e, The eye. /, The first pair of nerves, g, The second pair. A, The third pair, i, The fourth pair. ;', The fifth pair, k, The sixth pair. I, The seventh pair, m, The eighth pair, n, The ninth pair, o, The tenth pair, p, The eleventh pair, q, The twelfth pair, r, Spinal nerves. 1'lj.VI. IBlfi^ItSf JIRD gmiS Fig. 3 .'. i>r,tiuy > THE NERVOUS SYSTEM. Fifr, -12. NERVOPS SYSTEM op Si'iiiNX-LiorsTRl. A, that nf the caterpillar; fl. that uf tlio perfect insect. branches to the nerves which proceed from each ; while the other tract connects the ganglia themselves. Thus each segment of the insect has a distinct nervous connexion with its own ganglion, and a sympathetic connexion with the others, extending to the cephalic ganglion, which seems to have a controlling influence over all the rest, and alone to possess the faculty of sensation. Hence the motions produced by the ganglia of the trunk, when the head of the insect is removed, although they may seem to indicate sensation, are found to be only rejlez in their nature a certain irritation producing a certain movement, without choice, and probably with- out consciousness on the part of the animal. 244. Thus, if the head of a Centipede be cut off, while it is in motion, the body will continue to move as before ; or, if the body be divided as many times as it has segments, each portion will still continue to move ; but all conscious- ness seems to be lost; for if the decapitated body come in contact with an obstacle equal to its own height, it remains fixed against it, the legs moving as before, but without change of direction, or the ability of surmounting the obstacle being no longer subject to the will of the animal, but performing reflex move- ments by the influence of their own ganglia. 245. In Vertebrated animals (those which possess a back-bone or spinal column), the ganglia are no longer scattered through the body, but are united into one continuous mass ; and this mass, constituting the brain and spinal cord, is inclosed within the skull and vertebral column in such a manner as to be protected from the injuries to which it would otherwise be liable. The brain, in the higher Vertebrata, consists of a principal mass, called the cerebrum, which occupies all the front and upper part of the skull, and is divided into two hemispheres, or halves, by a membranous partition, running from back to front; and of a smaller mass, called the cerebellum, situated beneath the cerebrum, at the back part of the skull. At the base of the cerebrum are two ganglia, the olfactive and the optic, which belong to the nerves of smell and sight. All these parts in the human brain will be particu- larly described. 246. In the brain of a Fish, fig. 43, A the parts just mentioned are beginning to acquire considerable development. The cerebral hemispheres, ch, the optic ganglia, op, the cerebellum, ce, are plainly to be distinguished, and their relative sizes are in proportion to the intelligence of the animal. Thus the ganglia which control the sense of sight are very large, since this sense is pos- sessed, in a very high degree, by fishes ; while the cerebral hemispheres, to which belong more especially the manifesta- tions of will and thought, are corres- pondingly smaller. The cerebellum, fYir 43 A, brain of a ccd ; B, brain of the i L l_ L 11 shark; ul, olfactory ganglia ; ^opiiial cord, also, which, aS WC shall 866, IS COnnCCt- ed with the powers of motion, is large, as we should expect to find it, in animals possessed of the power of rapid movement ; and is larger in the active and rapacious shark than in the less ener- getic cod. The spinal cord is large, and is divided at the top, so as to form an opening between its two halves. In man, as we shall see, this opening be- comes entirely closed. 247. In birds, the brain has made a considerable advance towards what it finally becomes in man. . The cerebral hemispheres, a, fig. 44, are greatly increased in size, and cover in the olfactory ganglia entirely, and the optic ganglia, b, partly. The cerebellum, c, is also 244. How is it shown, in the case of the Centipede, thnt consciousness depends on the cephalic ganglia? 245. How are the ganglia united in Vertebrated animals? Describe th* 1 brain and spinal cord. 246. Explain fig. 42. Describe the progressive development of the brain in fishes. 247. Describe the further advances of the brain in birds. much more developed, as we should expect from the variety of move- ments performed by birds, but is as yet undivided into lobes. The spinal cord, d, is still of considerable size, and is much enlarged at the points from which originate the nerves of the wings and legs. As might be inferred from these improvements in the structure of the brain, the intelligence of birds is vastly superior to that of the ani- mals previously considered; and they are the first in the ascending scale which are capable of education or training. Did the limits of this work permit, we might thus go on to trace the progressive devel- opment of the brain through individual species of birds and mammalia, and to show at each step a corresponding advance in intelligence, as we approximate to man. 248. In man, the cerebral developments are greatly increased, while the spinal cord is diminished in size. The surface of the brain is not smooth, but divided by furrows into a series of convolutions, by which the surface over which the blood comes in relation with the nervous matter is greatly increased, thus adding to the activity of the organ. The two hemispheres are more closely connected with each other by fibres running across from each side. The cerebellum is also divided into two hemispheres. NERVOUS SYSTEM OF MAN. 249. The Nervous System of Man, and of the higher animals, consists of two portions, the cerebro-spinal, and the sympathetic or ganglionic. The Cerebro-spinal System is composed of the brain and spinal cord, with the nerves which proceed from them to the skin and muscles. It has been called the nervous system of animal life, because it is that by which sensations are received, and voluntary motions executed, and with which the mind is more immediately connected. The Sympathetic System is connected with the nutritive processes alone, and, from its influence over the organs of the thorax and abdo- men, has been called the nervous system of organic life. It consists of a chain of ganglia, communicating with each other by nervous cords, extending from the head along each side of the back-bone. Nerves proceed from it to the organs of digestion and secretion, and to the heart and blood-vessels. i CEREBRO-SPINAL NERVOUS SYSTEM. SPINAL CORD. 250. The spinal cord is a long, irregularly cylindrical column of nerve-substance, surrounded by a membranous envelop, and inclosed within a bony canal formed by the vertebrae or pieces of the back- bone. It extends from the head to the first or second vertebra of the loins, and is composed of both white and gray nervous matter. The white matter constitutes its greater portion, and is situated externally, while the gray matter occupies its central part. The spinal cord con- sists of two symmetrical halves united in the middle line by a set of converging fibres, called a commissure, but separated anteriorly and posteriorly by a vertical fissure ; the posterior fissure being deeper, but less wide and distinct, than the anterior. Each lateral half is marked by two longitudinal furrows, which divide it into three columns or tracts, called anterior, lateral, and posterior. There is a considerable enlargement of the cord, in the lower part of the neck, at the part from which arise the large nerves which supply the upper extremities ; and a similar enlargement is also found in the loins, at the origin of the nerves which go to the lower extremities. 251 . The number of nerves given off by the spinal Fi/?.i6. PORTIOIIOFTHI cord, is thirty-One on each side: eight pairs in the i^urf^"^ region of the neck; twelve in that of the back, cor- -a, spinal cord; i,po8- res ponding to the twelve pairs of ribs; five in the tenor root; c, ganglion; r & . . .r ur*u 5. How are the pray and white nervous substances disposed in the cerebrum ? How may the formation of the convolutions be illustrated ? What is the effect of ihis arrange- ment ? What is said of the quantity of blood sent to the brnin ? 256. Describe the ven- tricles of the brain, and refer to them in the plate. 257. Describe the position and structure of ihe cerebellum. The different parts of the spinal cord and brain are represented in plate 6, which is to be referred to and studied in connexion with this chapter. 258. The membranes which surround and protect the brain are three in number, and are called the Dura mater, the Pia mater, and the Arachnoid. These membranes are also prolonged downwards, so as to form a tubular sheath to the spinal cord. The dura rnater (hard mother) was so called by the old anatomists, because they sup- posed it to be the origin or mother of all the hard, firm membranes of the body, and the pia mater (tender mother) was also thus named by them as being the origin of the soft membranes. 259. The Dura mater is a strong, dense, fibrous membrane, which forms the external envelop of the brain, and is in contact with the bones of the skull, to which it strongly adheres. It is separated into two layers, the internal of which is doubled on itself so as to form two remarkable processes. The one, from its resemblance, in shape, to a sickle, is called falx cerebri, and is interposed between the two hemis- pheres of the brain, so that when the head rests on one side, the upper- most hemisphere is prevented from pressing upon the lower. The other process is called tentorium cerebelli, and is extended over the cerebellum, so as to prevent the pressure of the cerebrum upon the latter, when the head is in an erect position. There is also a smaller process, called falx cerebelli, which separates the two hemispheres of the cerebellum. 260. The Arachnoid (spider's web) membrane, so called from its extreme thinness, lies beneath the dura mater, and is spread over the entire surface of the brain. It corresponds in its use to the serous membranes of the heart and other organs, and serves to keep the oppo- site surfaces of the dura mater and the pia mater, between which it lies, moist and smooth. 261. The Pia mater is a loose, transparent web, in which a multi- tude of blood-vessels cross each other in every possible direction. Minute branches of these vessels, in immense numbers, penetrate the brain, to which they afford nutriment, at the same time that they serve as a means of attachment between the brain and the membrane. The ' pia mater follows all the convolutions and enters all the cavities of the brain, and is also prolonged over the spinal cord and the nerves, con- stituting one of the most important membranes of the body. 262. Twelve pairs of nerves spring from the brain, and are named as follows: 1st pair, Olfactory nerves; 2d, Optic nerves; 3d, Motores oculorum; 4th, Patheticus; 5th, Trifacial nerves ; 6th, Abducentes ; 7th, Portio dura, or facial nerve; 8th, Portio mollis, or auditory nerve; 9th, Glosso-pharyngeal nerve; 10th, Pneumo-gastric nerve; llth, Hypo-glossal nerve; 12th, Spinal accessory nerve. 263. The first pair is distributed to the inner membrane of the nose, and transmits to the brain the impression produced by odors; the second pair is distributed to the retina of the eyes, and in like manner conveys the impression of sight ; the third, fourth, and sixth pairs are nerves of motion only, and are distributed to the muscles which move the eyes. The fifth pair is for the most part a nerve of sensation only. It divides into three branches ; the first of which, called the ophthalmic nerve, passes into the orbit or cavity in which the eye is lodged, and is then distributed in the forehead and temples; the second branch, or superior maxillary, supplies the cheeks, nose, and upper lip, with sensitive filaments ; the third, or inferior maxillary, which, like the spinal nerves, has also a motor root, gives the power of moving to the masticating muscles, and gives sensibility to the parts about the mouth. 264. The seventh pair is the general motor nerve of the muscles of the face ; the eighth pair is the nerve of hearing, and is distributed to the internal ear ; the ninth pair supplies the back of the mouth and pharynx, and is concerned in the act of swallowing ; the tenth pair originates from the medulla oblongata, and supplies the lungs and air- passages, as also the heart and stomach ; the eleventh pair gives motion to the tongue ; the twelfth pair is concerned in respiration. All these nerves are supposed to contain two sets of filaments one communi- cating with the cerebral hemispheres, and the other with the spinal cord. Plate 6, fig. 6, represents a section of the brain, showing the arrangement of these nerves, That of the nerves proceeding from the spinal cord is seen in plate 7, fig 1. 258. What are the membranes of the brain? Why were the dura and pia mater so called? 25!). Describe the dura mater. What are its processes called, and what are their uses? 260. Why is the arachnoid so called? What is its use and position? 261. Describe Ihe structure of the pia mater, and its mode of attachment to the brain. 262. How many pairs of nerves spring from the brain? What are they called ! 263. What are the distri- bution and offices of the six first pnirs? 264. What of the six last pairs? What two sew of filaments do these nerves contain ? Refer to plate 6, fig. G, and point out these nerves. THE NERVOUS SYSTEM. FUNCTIONS OF THE CEREBRO-SPINAL SYSTEM IN MAN. OF THE SPINAL CORD. 205. Some fibres of the nerves which pass off from the spinal cord terminate in its central gray matter, while others are continued through its outer white substance to the brain. The first set of fibres constitutes, properly, the spinal portion of these nerves, and the second, the cerebral portion ; and it is not to be doubted that, although united in the same sheath, the two sets are as distinct as if they were sepa- rately distributed. The cerebral fibres carry impressions to the brain, where they are felt as sensations; and by other fibres of the same nerves, which pass from the brain to the muscles, the muscles are acted upon by the will, and voluntary motion is produced. 206. But the brain is not at all times in a state of activity. During sleep, for instance, it is completely torpid, and ordinary consciousness is entirely gone. Yet all those movements which are necessary to life go on as usual ; the breathing continues liquid poured into the mouth is swallowed the body changes its position. The same things take place in an animal whose cerebrum has been removed. A pigeon, for example, has been kept alive for several months in that condition; Tunning, when pushed; flying, when thrown into the air; drinking, when its beak was plunged in water; swallowing food, when placed in its mouth; but at all other times, when left to itself, appearing in a profound sleep. 267. From these facts, it is evident that the brain is not to be regarded as the only . centre of nervous power, nor as essential to the life of the body, and that the spinal cord must possess a great degree of independent power. This power differs from that of the brain, chiefly in the fact that it is exerted without the concurrence of the judgment and will; and the movements produced by it are rather like those of an automaton, set in motion by pulling certain wires, or touching certain springs. Thus the motions of a decapitated animal are never spontaneous, but are always excited by a stimulus of some kind. 268. As to the question whether these motions can take place without any feeling or sensation, the better opinion seems to be, that they do not depend upon sensation, but upon a property peculiar to the spinal cord, by which certain impressions necessarily excite motions of an involuntary character. This seems to be nearly proved by the fact, that such motions take place not only when the brain has been removed, and the spinal-cord has been left entire, but even when the spinal-cord has itself been cut into several portions. Thus, if the head of a frog be cut off, and the spinal cord divided in the middle, so tint the fore-legs remain connected with the upper part, and the hind- legs with the lower, each pair of legs will separately move when irritated, but the two pairs will not act together, as they do when the spinal cord is undivided. Or, when the spinal cord has been divided, without removal of the brain, the upper limbs remain under the con- trol of sensation, judgment, or will, while the lower may be excited to movement, though they are completely paralyzed to the will. Similar phenomena have been evinced in cases of injury to -the spinal cord in man. 269. A good example of this reflex function of the spinal cord is also found in the act of deglutition. When any substance reaches the back part of the trnoat, there is an involuntary effort to swallow, caused by irritation of the glosso-pharyngeal nerve, which is conveyed to the spinal cord, and the muscles concerned in the act of swallowing are excited by a returning influence through the pneu'mo-gastric ii "rve. In the same manner it is supposed that different portions of the bowels are excited to contract and force along their contents. Thus the mind is relieved from any effort of will, so far as the opera- tion of many important organs is concerned, and nature carries on its work unseen and unfelt. Were it otherwise, it is to be feared that many persons would waste their lives in anxious watching of the pro- cesses of digestion and nutrition. 265. What are the two different sets of filires found in the nerves of the spinal cord ? How do they terminate! how are they united? nnd what is the. operation of the cerebral fibres? 266. What is said of the continuance of life when the brain is torpid, and when it has been removed ? 267. What is made evident by these facts? How does the power of the f piiuil-cord differ from that of the brain ? What do the movements produced by it resemble ? 268. Do such movements depend upon sensation ? How is it proved that they do not ? 2C!il. \Vhnt is an example of I he reflex action of the spinal cord? What is fnrthir said on this subject '. 12 FUNCTIONS OF THE MEDULLA OBLONGATA. 270. The medulla oblongata differs from the rest of the spinal cord in its functions, mainly in the importance and extent of the actions governed by it. Like the cord, it is a conductor of nervous impres- sions ; but it has a wider extent of function, since all impressions which pass to and fro from the brain and spinal cord must be transmitted through it. Motor impressions are transmitted through its anterior columns and sensitive impressions through its posterior columns. Thus, if one of its anterior columns be divided, the animal will lose the power of motion in one-half the body, while its sensation will remain unimpaired. 271. The functions of the medulla oblongata, as a nervous centre, are more immediately important to the maintenance of life than those of any other part of the system. The nervous force necessary for deglutition and respiration is generated in this organ. It has been proved by repeated experiments, that the entire brain may be cut away, in successive portions, and life yet continue for a considerable period, and the respiratory movements be uninterrupted. Life may also continue when the spinal cord is in like manner cut away from below upward as high as the phrenic nerve, which commences in the throat. In some of the Amphibia (tortoises, frogs, &c.), the brain and spinal cord have both been thus removed, and still respiration and life continued, so long as the medulla oblongata was untouched. But a very slight wound at its central portion will produce suffocation and sudden death. 272. The power of reflection is more apparent in this than in any other portion of the nervous centres. By this power the respiratory movements are carried on. Thus the application of stimuli to many parts of the body, the nerves of which transmit impressions to the medulla, will cause respiratory movements, by reflection through the nerves which proceed from the medulla to the muscles concerned in respiration. This accounts for the "catching for breath" produced by dashing cold water in the face. Convulsive movements are also produced through the agency of this part of the cord. In those con- vulsions which result from "the teething of children, in lock-jaw, and other like diseases, death usually ensues through suffocation, the mus- cles of respiration becoming so fixed that the air cannot be breathed. FUNCTIONS OF THE CEREBELLUM. 273. Much discussion has taken place in regard to the functions of the cerebellum. It seems to be now agreed, however, by the most intelligent physiologists, that it is the organ which is more especially connected with motion. Numerous experiments have been made on living animals, all of which go to show that, after the removal of the cerebellum, the power of executing those movements necessary to locomotion, is lost. The faculties of volition and sensation remain, but the power to walk, fly, or even stand, is uniformly lost, from the inability to combine the action of the muscles in groups. 274. From facts of this character, it seems to be. most probable that the function of the cerebellum is to harmonize and regulate the actions of the voluntary muscles. In accordance with this theory, we find that those animals which possess variety of motions, or muscular actions, in the highest degree, are endowed with a cerebellum cor- respondingly large. In animals of the cat tribe, which use their limbs for seizing their prey, and which are capable of great muscular exertion, the cerebel- lum is larger than in those whose limbs are subservient to locomotion only. In birds, the variety of whose movements is still greater, it is larger than in most of the mammalia. It acquires its highest develop- ment in man; as might be expected, from the muscular combinations necessary to maintain the erect position, anJ to perform the intricate and varied movements of the human hand. The influence of each half of the cerebellum is directed to muscles on the opposite side of the body, and it would appear that, for the right ordering of movements, the actions of its two halves must be always mutually balanced and adjusted. 270, 271. Explain the functions of the medulla oblongata. What has been proved by experiment? 272. What is said of the power of reflection in the medulla ! 273. Expla,.i the functions of the cerebellum. What experiments are mentioned ? 274. What deduction. is drawn from such facts ? What is said of the size of the cerebellum in cats, birds, &.c. and of the direction of its influence? 40 THE NERVOUS SYS T KM. FUNCTIONS OF THE CEREBRUM. 275. The cerebrum is the organ through which the phenomena of thought and intelligence are manifested. By its means, we reason upon the ideas excited by sensations, we judge and decide upon our course of action, and put that decision into practice, by issuing a mandate which is conveyed by the nervous trunks to the muscles. 276. It is a common, but erroneous idea, that reason is peculiar to man; and that the actions of the lower classes of animals are due to instinct alone. There can be no doubt that reasoning processes, exactly resembling those of man, are performed by many animals ; such, for instance, as the dog, the horse, and the elephant. We must admit that an animal reasons, when it profits by experience, and obvi- ously adapts its actions to the end it desires to gain, especially when it departs from its natural instincts to do this. The presence of intelli- gence is also perceived in the differences of character found in various individuals of the same species. Thus, some dogs are stupid, others sagacious, some ill-tempered, others good-tempered ; just as there are stupid men and intelligent men, ill-tempered men and good-tempered men. But the actions of insects seem to be wholly instinctive; so that we observe no difference of temper or capacity in them. 277. Birds, however, which resemble insects in many of their instinctive tendencies, exhibit a remarkable distinction in their actions. In escaping from danger, in obtaining food, and in constructing their habitations, the actions of birds, like those of insects, are instinctive. But, in adapting their operations to peculiar circumstances, birds dis- play a variety and fertility of resource not to be found in insects. Birds also learn by experience, and may be educated ; while insects observe perfect uniformity in all their actions. 278. The relative amount of intelligence in different animals bears a pretty constant proportion to the size and developments of the cerebral hemispheres. Size alone, however, does not produce all the difference. In ascending from the lower to the higher animals, a marked advance in the complexity of the brain is observed. The convolutions become more and more prominent, giving a proportion- ally increased surface for the entrance of the blood-vessels, and an equally increased amount of the gray matter, which seems to be the real centre of all the operations of the organ. Still, the size of the cerebrum, compared with that of the spinal cord and the ganglia at its top, usually affords a tolerably correct measure of the intelligence of the animal. The same rule holds good in comparing different men, if due allowance be made for the comparative activity of their general functions, or, in other words, for their differences in temperament. 279. Thus, two men, whose brains are of the same size, may differ widely in mental vigor, because the general system of one performs its functions more actively and energetically than that of the other. For the same reason, a man of small brain, but whose general habit is active, may have a more powerful intellect than another of much larger brain, but whose system is sluggish and inert. But of two men alike in temperament, and having the same general configuration of head, it cannot be doubted that the one with the larger brain will sur- pass the other. It is a striking fact, that almost all those persons who have been eminent for their acquirements, or for the influence of their talents over their fellow-men, have had large brains. This was the case, for example, with Newton, Byron, Cuvier, Cromwell, and Napo- leon. The average weight of the brain is about three pounds two ounces. That of Cuvier weighed four pounds eleven and one-half ounces ; and those of Byron and Cromwell are said (though the fact is doubtful) to have weighed nearly six pounds. In idiots, on the contrary, the brain is usually very small, in some instances weighing only one and one-half pounds. 280. The size of the brain may be pretty correctly estimated by the facial angle. This angle is obtained by drawing a horizontal line (c d, figs. 46 and 47) from the entrance of the ear to the floor of the cavity of the nose ; and a second line (a b) from the most prominent $ 275. What are the functions of the cerebrum ? 276. Is reason peculiar to man ? What is said of the reasoning powers of animals? How is the presence of intelligence indicated! 277. What is said of the difference, in respect to intelligence, between birds and insects? 278. What is said of the relative proportion between the amount of intelli- gence and the size and conformation of the cerebrum? 279. Why do men, having brains of the same size, differ in intellectual power? What other facts does the same reason account for? Of two men alike in temperament, which will surpass the other! What class of persons have had large brains ? What instances are given ? 280. How may the size of the brain be estimated! How is the facial angle formed, and how does it indicate the proportion which the brain bears to the face ! part of the forehead to the front of the upper jaw, so as to intersect the other. This line will evidently be more inclined to the former, and the angle formed by the two will be more acute, in proportion as the face is more projecting and the forehead more retreating; while it will be nearer a right angle, if the forehead be prominent, and the muzzle project but little. Hence the facial angle will indicate the proportion which the brain bears to the face. Fig. 48. SKULL or EUROPEAN. ff. 47. SKULL OF NEGRO. 281. This angle is more open in man than in any other animal, and it varies greatly in the various races of men. The difference between the facial angle of the European and American head, and that of the negro, is seen in figs. 46 and 47. In the one it is about 80 = , and in the other about 70. In monkeys, it varies from about 65 to 30, and as we descend still lower, it becomes still more acute. CONNEXION OF THE MIND WITH THE BODY. 282. The brain, as appears from what has already been said, is the connecting link between the mind and the body. All the organs of the body may be said to be the agents of the mind, inasmuch as the mind manifests itself through them all. Thus the hand, the eye, the muscles which control the features, are all made to express more or less forcibly the varied emotions of the mind. The mind, too, con- ' trols, to a certain extent, the operations of the various functions of the body, to which it gives a shape and form in correspondence with itself. 283. In this view, we may regard the entire body as an assemblage of organs for the manifestation of mind. But the influence of the mind upon the body is reciprocated by that of the body upon the mind. If the body can be made to suffer from the condition of the mind, the mind itself is not less affected by the condition of the body. A single illustration will suffice to show this fact. Melancholy or depression of spirits, from any cause whatever, will often produce disease or derangement of the liver; and, on the other hand, a derange- ment of the liver will almost always induce melancholy, though no other cause exists. 284. But the intellect is not limited, in its influence, to the organic and animal functions; it also stamps itself upon every lineament and feature of the man. Thus, as we ascend in the scale of being, intelli- gence, as it becomes more highly developed, is marked by a more perfect organization of the brain and nervous system. The nervous power in man is at its highest point, as is shown by the very liability to disease or derangement of the vital functions, arising from the sym- pathy between the physical and intellectual powers. The mere animal has no desires to gratify but those of appetite. These satiated, he has no anxiety for the future, no repinings for the past ; he is not wasted by care nor harassed by toil and disappointment. Man, on the con- trary, is the constant subject of exciting and depressing influences of functional and organic derangements, growing out of his complex nature. Ever restless, never satisfied, he is constantly on the rack of physical and mental torture. He consequently obtains the highest perfection and excellence, when he obeys most perfectly the laws of his nature, and retains the physical and the intellectual in the most com- plete harmony of development and action. 285. Hence in the education of the young, the powers of the body and the mind should be cultivated together. The mind should not be excited to such a degree as to overtask the brain, nor so neglected as to leave the latter without a proper degree of exercise. One extreme 281. What is said of the difference of this angle in different races? 282, 283. What is said of the brain, as connecting mind and body? What of the mutual effect of mind and body? What is the illustration ! 284. What is said of the nervous power of man, com- pared with that of the animal ? How is this difference shown ? 285. What inferences aro drawn froir the preceding sections? What is said of " precocious children?" PLATE VII. THE NERVES. FIGURE i. POSTERIOR VIEW OF THE PRINCIPAL NERVES. THE general arrangement of the nervous centres and the distribution of the nervous trunks, are shown in this figure. The spinal column is laid open, so as to display the cord, with the nerves which pass from it. The muscles of the left side and limbs are dissected, to show the' course of the principal nerves. a, The hemispheres of the cerebrum. 4, The lobes of the cerebellum, c, The spinal cord. d, The facial nerve, the principal motor nerve of the face. , The brachial plexus, a net-work of nerves, originating by several roots from the spinal cord, and going to supply the arm. From this plexus proceed /, the scapular nerve ; g, the median nerve ; A, the ulnar nerve ; i, the musculo-spiral nerve ; j, the intercosto-humeral (nerve of Wrisberg). From the spinal cord proceed the intercostal nerves, k, k, running between the ribs ; the nerves forming the lumbar plexus, 1, from which the front of the leg is supplied ; those forming the sacral plexus, TO, which supplies the back of the leg. The chief branch of the sacral plexus is the great sciatic nerve, n, which divides into the tibial nerve, o, the peroneal or fibular nerve, p, and many other branches. The nerves seen on the right side of the figure, are the ramifications of the sub-cutaneous nerves, Drenching beneath the skin, in which they are finally lost. Some of the superficial veins are also represented. FIGURE 2. PRIMITIVE NERVE TUBULES (227), GEEATLY MAGNIFIED. a, A perfectly fresh tubule, with a single dark outline, i, A tubule or fibre, examined some time after death, showing a double outline, in consequence of the coagulation of the fluid contained in the tube ; the outer line being that of the sheath of the fibre ; the inner, that of the margin of the coagulated contents. 7t<-e f fhf 7)ffnff fottrt cf i am <-./., THE NERVOUS SYSTEM. 4!) wastes the vital energies, while yet immature; the other degrades heaven-born powers to a level with the brute. The injudicious course, so often pursued, of stimulating the intellect of what is called "a pre- cocious child," too often results in misery to the sufferer itself, and in disappointment to its unwise friends. Such children rarely fulfill the promise of their earliest years, and the anxious parentusually sees the brilliancy of their youth fade into dullness and disease in after life. 280. Not much less injurious is the mere training of the body with- out a suitable exercise of the intellect. The mind must have occu- pation as well as the body. Just as the muscles increase in strength and firmness, by a proper degree of constant and regular exertion of their powers, so does the mind expand and improve by a healthy and sound action of the brain. Of the whole train of nervous diseases, there is none more dreadful than those which result from a lack of some absorbing mental pursuit. Those persons who "have nothing to do," are the most miserable of mortals. By nothing interested or amused with no high aim, no grand object in life they squander, in the most frivolous and superficial diversions, that existence which was given them for nobler and more exalted purposes. Let the fashionable idler think of Brummel in his mad-house and his filth, and be warned. 287. But the importance of mental culture is apt also to be forgot- ten by a better and worthier class. The Farmer, the Merchant, the Mechanic, are too liable to confine their attention simply to their routine of daily labor. They do not always think of the happiness produced by a cultivated and enlightened mind, or even of the advan- tage which such a mind gives them over others in their own voca- tions. The most successful men in any business, or even in any profession, have not been those who have exerted themselves to know only what is necessary to that one calling. Most great merchants have had a knowledge of other books besides their Ledgers of other sciences besides that of their Interest tables. Elihu Burritt is none the worse blacksmith because he speaks ten languages ; nor Talfourd the worse lawyer because he wrote "Ion." These are extreme instances, to be sure, and men seldom excel in more than one pursuit ; but yet a general cultivation of the mental powers is none the less essential to complete success in any thing. 288. On the other hand, he who over-taxes his brain with long- continued and unvarying mental exertion in any one pursuit, is sure to find the tone of his mind become relaxed, the energies of his frame paralyzed ; and he fails of success through the very means which he takes to secure it. Thus variety is a necessary ingredient in well- disciplined mental effort. When the attention becomes fatigued, and the mind refuses to act with clearness and precision, as often happens in a favorite and absorbing occupation, change should-be immediately resorted to. The mind should be withdrawn from the exciting topic, and applied to another. 289. It has hitherto been, and probably always will be, impossible to determine the mode in which the mental principle operates in its connexion with the brain. But it appears that one of the cerebral hemispheres is sufficient for all operations of the mind, except the highest intellectual acts ; for numerous cases have occurred where no loss of mental vigor ensued, though one hemisphere of the cerebrum was so injured or disorganized as to be incapable of performing its functions. The remaining hemisphere, in these cases, was sufficient to perform the office of both : but the mind does not seem, in any of these cases, to have been tested in very high intellectual exercises; so that it is not certain that one hemisphere will suffice for these. 290. In ordinary sensation and effort of the will, the impressions are carried across from one hemisphere to the other ; so that, if either hemisphere be injured, the effects produced, whether in loss of sensa- tion or of voluntary motion, are observed on the side of the body opposite to that on which the injury to the brain is received. Thus, if the left lobe of the optic nerve be divided, the sight of the right eye will be lost. 291. Most physiologists are of opinion that each mental faculty has a special portion of the brain appropriated to it as its proper organ. 286. What is said of bodily training without the exercise of intellect? What of those who "have nothing to do?" 287. What is said of the importance of mental culture in men of business? 288. What is said of the necessity of variety in mental effort ? 289. Is it known how the mind is connected with the brain ? Are both cerebral hemispheres neces- sary to the operations of the mind? How is this proved? Is it certain that one hemis- phere is sufficient for the highest intellectual effort? 290. What is said of the transference of sensation from one hemisphere to the other ? If the right side of the brain is injured, on which fide of the body are the effects produced ? How is this illustrated in the case of the optic nerve? 291. What is said of the theory of the division of the brnin into separate organs? What of the truth of the system of phrenology as founded on this theory? 13 The evidence in support of this opinion is too lengthy to be intro- duced here. It may be found in the works of phrenologists, who have so adapted and illustrated it, that the theory of a plurality of organs in the cerebrum is often regarded as peculiar to phrenology, and as so c^mtially connected with it, that this theory Cannot be maintained, unless that system is true. "But it is plain, that all the system of phrenology built upon the theory may be false, and the theory itself may be true ; for the schools of Gall and Spurzheim assume not only this theory, but also that they have determined all the primitive facul- ties of which the mind consists; that is, all the faculties to which special organs must be assigned, and the places of all those organs in the cerebral hemispheres and cerebellum. Possibly this may be a system of error, founded on a true theory : the cerebrum may have many organs, and the mind as many faculties; but what are the facul- ties which require separate organs, and where those organs are, may be subjects of which only the first or most general knowledge is yet attained." 292. The brain has been called the organ of the mind ; but this must be taken with some restriction. There is little doubt of the truth that the brain is the organ of those parts of the mind which have to do with the things of sense ; but it is by no means certain that the cerebral hemispheres, or any other parts of the brain, are, " in any meaning of the term, the organs of those parts or powers of the mind which are occupied with things above the senses. The Reason or Spirit of man which has knowledge of divine truths, and the con- science, with its natural discernment of right and wrong, cannot be proved to have any connexion with the brain. In the complex life we live, they are, indeed, often exercised in questions in which the intellect, or some other lower mental faculty is also concerned ; and, in all such cases, men's actions are determined as good o.r bad, accord- ing to the degree in which they are guided by the higher or by the lower faculties. But the reason and the conscience must be exer- cised independently of the brain, when they are engaged in the con- templation of things which have not been learned through the senses, or through any intellectual consideration of sensible things. All that a man feels in himself, and can observe in others, of the subjects in which his reason and his conscience are most naturally engaged ; of the mode in which they are exercised, and the disturbance to which they are liable by the perceptions or ideas of sensible things ; of the manner and sources of their instruction ; of their natural superiority and supremacy over all the other faculties of the mind ; and of his consciousness of responsibility for their use : all teach him that these faculties are wholly different, not in degree only, nor as different mem- bers of one order, but in kind and very nature, from all else of which he is composed ; all, if rightly considered, must incline him to receive and hold fast the clearer truth which Revelation has given of the nature and destinies of the Spirit to which these, his highest faculties, belong." 293. And it is here that we are most forcibly impressed with the superiority of man over all other created things. His reason and his conscience, apart from any other faculties, naturally induce in him the belief in some unseen but powerful Being, whose favor he seeks, whose anger he deprecates, and whose spiritual existence he desires to share. "This desire, which seems to have been implanted by the Creator in the mind of man, is one of the chief natural arguments for the immortality of the soul, since it could scarcely be supposed that such a desire should have been implanted by our beneficent Maker, if it were not in some way to be gratified. By the Immortal Soul, the existence of which is thus guessed by man, * * * he is connected with beings of a higher orde.r, among whom Intelligence exists, unre- strained in its exercise by the imperfections of the bodily frame, with which it is here connected, and by which it here operates." 294. "Such views tend to show us the true nobility of Man's rational and moral nature; and the mode in which he may most effect- ually fulfill the ends for which his Creator designed him. We learn from them the evil of yielding to those merely animal tendencies those ' fleshly lusts which war against the soul' that are characteristic of beings so far below him in the scale of existence, and tend to degrade him to their level ; and the dignity of those pursuits which, by exercising his intellect, and by expanding and strengthening his loftier moral feelings, raise him towards beings of a higher and purer 292. What remarks are made concerning the connexion of Reason and Conscience with the brain? When must these faculties be exercised independently of the brain ? To what should man be inclined by these considerations? 293, 294. Repeat the remarks made in application of this subject to the Immortality of the Soul and a Soiritual existence. 50 SENSATION AND THE SENSES. order. But even the loftiest powers and highest aspirations of which he is at present capable, may be regarded as but the germs or rudi- ments of those more exalted faculties which the human mind shall possess, when, purified from the dross of earthly passions, and expanded into the comprehension of the whole scheme of Creation, the soul of man shall reflect, without shade or diminution, the full effulgence of the Love and Power of its Maker." CHAPTER XI. SENSATION AND THE SENSES. 295. THERE are two kinds of sensation, named common and special. Common or general sensation is the consequence of that sensibility which exists in nearly every part of the body, and is manifested when- ever any part is touched or stimulated. It is by this that we feel those impressions made upon our bodies by the objects around us, or by actions taking place within, which produce the various modifications of pain, the sense of contact or resistance, the sense of variations of temperature, and others of a similar character. 296. Since impressions made upon the sensory nerves are depend- ent upon the action of the blood-vessels ( 232, 233), it is obvious that no parts destitute of the latter can receive such impressions, or, in common language, can possess sensibility. Accordingly we find that the hair, nails, teeth, cartilages, bones, and other parts whose sub- stance contains few or no vessels, are either completely incapable of receiving painful impressions, or are possessed of a very dull sensibility to them. On the other hand, the skin, and other parts, which usually receive such impressions, are abundantly furnished with blood-vessels. 297. It does not necessarily follow, however, that parts should be sensible in a degree proportioned to the amount of blood they contain ; since this blood may be sent to them for other purposes. Thus, it is a condition necessary to the action of muscles, that they should be copiously supplied with blood ; but they are not acutely sensible ; and glands, also, the substance of which has very little sensibility, receive a large amount of blood for their peculiar purposes. 298. Besides this common sensibility, there are certain parts of the body which are capable of receiving impressions of a peculiar or spe- cial kind, such as sounds or odors; and the sensation produced by these is called special sensation. The varieties of this sensation are perceived by the five senses, Touch, Taste, Smell, Hearing, and Sight. The nerves which convey these special impressions are not able to receive those of a common kind ; thus the eye, however well fitted for would not feel the touch of the finger, if it were not supplied seeing with branches from the Fifth pair, as well as by the optic nerve. Nor can any nerve of special sensation be affected by impressions that are adapted to operate on another ; thus the ear cannot distinguish the slightest difference between a luminous and a dark object ; nor can the eye distinguish a sounding body from a silent one, except by seeing its vibrations. SENSE OF TOUCH. 299. By the sense of Touch is usually understood that modification of the common sensibility of the body, of which the surface of the skin is the especial seat. In some animals, as in Man, nearly the whole exterior of the body is endowed with it, in no inconsiderable degree ; but in others, as in most Birds and Reptiles, and many Fishes, the greater part of the body is so covered by hairs, scales, or bony plates, as to be nearly insensible ; and the faculty is restricted to particular portions of the surface, which often possess it in a very high degree. The sensory impressions, by which we receive the sensation of touch, 295. What are the two kinds of sensation? What is common sensation? What impressions are felt by this kind of sensation ? 296, 297. What is said of the blood-vessels as connected with sensibility? Are all parts, however, sensible in proportion to the amount of blood they contain? What is said of the muscles and glands in this connexion? 298. What is special sensation? How are the varieties of this sensation perceived? What are the names of the senses? Can the nerves which convey special impressions also receive common impressions? How is this illustrated in the case of the eye? Can any nerve of special sensation receive an impression adapted to operate on another nerve of the same kind? How is this illustrated? 299. What is understood by the sense of Touch? What parts of the body possess this sense in man? In other animals? How are the impressions made by which the sensation of touch is received ? are made by the objects themselves upon the nerves which are dis tributed to the skin ( 206). 300. The especial organ of touch in Man, is the hand, which is peculiarly fitted for this purpose by the great sensibility of its skin, especially at the extremities of the fingers, the variety of movements of which it is capable, the power of opposing the thumb to the rest of the hand, and the flexibility of the fingers. In most other Mammalia, the lips and tongue are employed as the chief organs of touch ; in the Elephant, this sense is evidently possessed very acutely by the little finger-like projection at the end of its trunk ; and in the Bat, it seems to be diffused over the whole membrane of which the wings are formed. It has been found that bats, when deprived of sight, and, as far as possible, of hearing and smelling also, still flew about with equal certainty and safety, avoiding every obstacle, flying through passages only just large enough to admit them, avoiding threads stretched in every direction across the apartment, and passing through places pre- viously unknown, with the most unerring accuracy. Hence some- naturalists were inclined to attribute to the bat the possession of a sixth sense unknown to man. All these facts can be accounted for, however, on the supposition that the wings of the bat possess a high degree of sensibility in their delicate membrane, which receives impressions from the pulses of the air, produced by the act of flying, and modified by the neighborhood of solid bodies. 301. The only idea communicated to our minds by the sense of Touch, when this is exercised in its simplest form, is that of resist- ance ; and we cannot form a notion of the size or shape of an object, nor of the nature of its surface, by feeling it, unless we move the object over our hand or other sensory organ, or the latter over the former. By the various degrees of resistance which we encounter, we estimate the hardness or softness of the body ; and by the impressions made upon the papilla? ( 206), when they are moved over its surface, we form an idea of its smoothness or roughness. It is by attention to the muscular movements we execute, in passing our hands or fingers over its surface, that we acquire our ideas of its size and figure ; and hence we perceive that the sense of touch, without the power of moving the tactile organ over the object, would have been of comparatively little use. 302. This sense is capable of improvement to a remarkable degree; as we see in persons who have become more dependent upon it, in consequence of the loss of their sight. This doubtless results, in part, from the increased attention which is given to the sensations; and, partly from the greater acuteness or impressibility of the organ itself, arising from the use of it. Of the wonderful delicacy reached by this sense, from its cultivation by the blind, many singular instances are recorded. 303. The sense of temperature is of a different character from the common sensation of touch ; and either may be lost, without the other being affected. It is rather of a comparative than a positive kind ; that is, we form our estimate of temperature rather by comparing it with that to which our body (or that part of it employed to test the heat or cold) has been previously exposed, than by the knowledge which we derive through the sensation, of the actual degree of heat or cold to which the organ is exposed. Thus, if we plunge one hand into a basin of hot water, and the other into cold, and then transfer both hands to a basin of tepid water, this will feel cold to the hand which has been previously accustomed to the heat, and warm to the other. 304. In the same manner, the temperature of the city of Quito, which is situated half way up a lofty mountain, one of the Andes, is felt tp be chilly by a person who has ascended from the burning plains below, while it seems intensely hot to another who has descended from the snowrcapped summit above ; the residents in the town at the same time regarding it as moderate neither hot nor cold. It is a curious circumstance, that a weak impression made on a large surface seems more powerful than a stronger impression made upon a small surface ; 300. What is the especial organ of touch in Man? How is the hand fitted for this purpose? What are the chief organs of touch in other Mammalia? What in the Elephant ? What is said of the sense of touch in the Bat? 301. What is the first idea communi- cated by this sense ? How must we form a notion of the size and shape of an object ? How of its roughness or smoothness ? Why would this sense be imperfect without the power of moving the organ of touch ? 302. What is said of the improvement of this sense ? From what does this improvement result? 303. How does the sense of temperature differ from the common sensation of touch ? How do we form an estimate of temperature ? What illustration is given? 304. As another illustration, what is said of the temperature of Quito ? How does extent of surface affect the power of an impression ? What fact does this account for ? SENSATION AND THE SENSES. 51 thus, if the fore-finger of one hand be immersed in water at 104, and the whole of the other hand be plunged in water at 102, the cooler water will be thought the warmer ; whence the well-known fact, that water, in which a finger can be held, will scald the whole hand. 305. Besides the hand, the lips and tongue also are delicate instru- ments of touch, and are probably the first employed for that purpose. Thus, an infant, having learned, by application to its mother's breast, to receive and cultivate impressions of touch in the lips and tongue, continues for some time to rely with greater confidence on the evi- dence obtained from this, than from any other source, and accordingly persists in carrying every object to its mouth, in order to test it, until its hands and fingers become sufficiently educated and manageable to serve as substitutes. 306. In most insects, the special organs of touch are prolongations from the portion of the head near the mouth. These are called antenna or feelers; they are often of great length, and present an extraordinary variety in their forms, usually contain a great number of joints, and are very flexible. This flexibility enables them to be turned towards any object which the insect wishes to examine ; and when it is walking, we see them constantly applied to the surfaces of the bodies which it approaches, in a manner which leaves no doubt that they are used as organs of touch, just as a blind man uses a stick in feeling his way. By the antennae, also, in- Kf. 49,-CAKucoR,. BIETL..-B, *, antennas. sects communicate with each other. Almost every person must have observed the action of these organs, where two bees meet each other out of their hive. They seem to reconnoitre one another for some time, by moving their antennae; and often keep up these movements for a considerable period, as if carrying on a close conversation. SENSE OF TASTE. 307. The sense of taste, like that of touch, is excited by the direct contact of particular substances with certain parts of the body ; but it is of a much more refined nature than touch; inasmuch as it com- municates to us a knowledge of properties which that sense would not reveal to us. All substances, however, do not make an impression upon the organ of taste. Some have a strong savor, others a slight one, and others, again, are altogether insipid. The cause of these differences is not understood ; but it may be remarked, that, in gen- eral, bodies which cannot be dissolved in water have no savor, but that most of those which are soluble have a more or less strong taste. Their solubility, in fact, seems to be one of the conditions requisite for their action on the organ of taste ; for, when that organ is com- pletely dry, it does not receive any sensation from solid bodies brought into contact with it, though the same bodies may have the most pow- erful taste, if reduced to a fluid form. 308. The chief purpose of the sense of taste is to direct animals in the choice of their food, and hence its organ is always placed at the entrance of the digestive canal. In higher animals, the tongue is the principal seat of taste ; but the lips, the inside of the cheeks, and the upper part of the throat, are also capable of receiving the impressions of certain savors. The mucous membrane which covers the tongue is copiously supplied with blood-vessels, and is thickly set, especially upon its upper surface, and towards the tip, with papillae, resembling in structure those of the skin, but larger. PI. 8. fig. 3. 309. The tongue itself is made up of muscular substance, which accomplishes the various movements required in the acts of mastica- tion, and in the production of articulate sounds. It is supplied with nerves from the third division of the fifth pair, from the glosso-pharyn- 305. What other parts, besides the hand, possess delicacy of touch? Why does an infant habitually carry objects to its mouth ? 306. What are the special organs of touch in insects? Describe the antennae, and their uses. 307. How does the sense of taste com- pare with that of touch? What is said of the differences of savor in different substances? Is the cause of these differences known ? What class of bodies generally possess savor, and what do not? What condition is requisite for the action of bodies on the organ of taste ? How is this shown ? 308. What is the chief purpose of the sense of taste ? Where is the organ of taste always situated ? What parts are the principal seat of taste ? Describe the membrane and papillae of the tongue. 309. Describe the structure and nerves of the tongue. What effect have savory substances on the papillae? geal, and from the hypo-glossal. The last is the motor nerve of the tongue ; the first is the one chiefly concerned in the conveyance of sensoiy impressions from the front and sides of the tongue ; and the other (the glosso-pharyngeal) seems to have for its office the convey- ance of those impressions from the back of the tongue which excite the muscles of swallowing to action, as well as those which produce the sensation of nausea, and excite the act of vomiting. The papill.T are, for the most part, if not entirely, supplied from the fifth pair ; and the branch of this pair which proceeds to the tongue, is known as the lingual nerve. When the papillae are called into action by the con- tact of substances having a pleasant savor, they become dilated and lengthened, and rise up from the surface of the mucous membrane. In this manner is produced the roughness which is felt on the surface of the tongue, or the inside of the cheek, when a piece of sugar has .lain there for awhile. 310. The tongue presents nearly the same structure among the mammalia, in general, as in man ; but, in birds, it is usually cartilagin- ous or horny in its texture, and destitute of nervous papillae ; so that their sense of taste cannot be very acute. Several of them use their tongues for other purposes; the wood-pecker, for instance, whose tongue is sharp and barbed, transfixes insects with it, and the parrot uses it to keep steady the nut or seed which is being crushed between the mandibles. In some reptiles, the tongue is large and fleshy; in others, as the serpent tribe, it is slender and forked, and possessed of great quickness of motion. In the frog and chameleon, it serves as an organ of prehension, and is darted out with great rapidity to catch the insect-food of the animal. The tongue of fishes is generally in a mere rudimentary state, and is fixed in the throat, and often covered with teeth. The tongue of the bee forms a little tube, through which it draws up the juices of flowers. 311. A most important function of the sense of taste is that of directing animals in the choice of their food. Most of the lower ani- mals will instinctively reject articles of food that would be pernicious to them. Even the voracious monkey will seldom touch poisonous fruits, though their taste be agreeable; and animals, whose digestive apparatus is adapted to one kind of food, will reject all others. As a general rule, it may be stated that substances, the taste of which is agreeable to us, are useful and wholesome articles of food, and that those which are nauseous and disgusting are injurious. But there are many signal exceptions to this rule ; as, for instance, both the taste and smell of Prussic acid are very agreeable, and yet it is one of the most deadly of known poisons. 312. Much of the perfection of the sense of taste is often due to the sapid substance being also possessed of odor, and exciting the simul- taneous action of the sense of smell. Of this, any one may convince himself, by closing the nostrils, and breathing through the mouth only, while holding in the mouth, or even rubbing between the tongue and the palate, some aromatic substance ; when its taste will be scarcely recognized, though it is immediately perceived when the nasal pas- sages are again opened, and the effluvia received into them. Frequent and continued repetition of the same taste renders the perception of it less and less distinct, just as a color becomes more and more dull and indistinct, the longer the eye is fixed upon it. This is exemplified in the foolish experiment, sometimes practised, of giving a person, blind- folded, brandy, rum, gin, or several different kinds of wines, in rapid succession. In a short time, the most experienced taster becomes entirely incapable of distinguishing between the liquors. 313. No sense is more influenced by habit, than taste. Many sub- stances that are exceedingly disgusting at first, finally become not only less repugnant, but even highly grateful to the taste. Fashion, or necessity, may at first have been the cause of their being taken ; but habit causes them to be eagerly sought after. Innumerable instances of these acquired tastes are observed in all ages, in every country, and in every state of. society. Thus, the most celebrated sauce of anti- quity was made from the half-putrid intestines of fish ; a rotten egg, especially if it contains a chick, is highly esteemed among the Siamese : raw fish is devoured with great eagerness by the South Sea Islanders ; and the civilized epicure prefers his game and venison in what he calls a "ripe" condition. 310. What is said of the tongue of birds? Of reptiles? Of fishes? Of the bee? 311. What is said of the sense of taste in connexion with the choice of food ? What general rule is stated? What exception to this rule is mentioned ? 312. What is said of the dependence of the sense of taste upon that of smell? How may this be shown ? What is said of repe- tition of the same taste ? How is this exemplified ? 313. What is said of the influence of habit on taste? Give the examples of acquired tastes. 52 SENSATION AND THE SENSES. 314. The susceptibility of the organs of taste to pleasurable sensa- tions depends very much on the state of the stomach, even in health. With whatever enjoyment we partake of a favorite dish when we first sit down with a good appetite, the relish diminishes as hunger is appeased; and if, notwithstanding, we persist in eating, nausea and disgust at length supervene, and the glutton is compelled to desist. This consent subsisting between the stomach and the organ of taste is an important and wise provision, informing the individual when a suf- ficiency of food has been taken. SENSE OP SMELL, 315. By the sense of smell we are enabled to perceive scents or odors, which are the finely divided particles of odoriferous bodies. These odorous particles must be exceedingly minute, for many sub- stances do not seem to lose weight by freely imparting their scent to an unlimited quantity of air. Thus the experiment has been tried of keeping a grain of musk freely exposed to the air of a room, the doors and windows of which were constantly open, for a period of ten years ; the air, thus continually changed, was completely impreg- nated with the odor of musk ; and yet, at the end of that time, the particle was not found to have suffered any perceptible diminution in weight. 316. The most advantageous position of the organ of smell is evi- dently at the commencement of the respiratory passages ; so that, by its peculiar sensibility, the air which is received into the lungs may be tested, as it were. In all the air-breathing Vertebrata this organ con- sists of a pair of cavities called the nasal fossce, situated between the mouth and the orbits of the eyes. They open upon the front of the face by two orifices or nostrils, and into the pharynx by two other orifices. The two cavities are separated from each other by a vertical partition, situated on the middle line ; their sides are formed by the various bones of the face, and by the cartilages of the nose. These cartilages are elastic, and readily admit of motion and modification of shape in the nose, while at the same time they contribute to its form, as characteristic of an individual or a race. In some animals, they are largely developed, as in the Elephant, where they form the trunk or proboscis. Several small muscles are attached to the cartilages, by which the nostrils are dilated or contracted. The nasal cartilages are represented in plate 8, fig. 5. 317. The interior of the nasal fossae is lined by a delicate mucous membrane, the pituitary or schneiderian, on which the olfactory nerves are distributed, and the extent of this is increased, by its being folded over certain projections from the walls of each cavity, which are termed spongy bones. In man, these are three in number. Prolongations of this membrane are carried also into cavities hollowed out in the neighboring bones, called sinuses (fig. 50). The frontal sinuses, i, are situated between the eye- brows ; and the sphenoidal sinuses, k, are placed farther back. There is also a large cavity excavated in the bone of the upper jaw, on either side. The membrane which lines these is kept moist by its own secre- tions. A vertical section across the middle of the nasal cavities, which exhibits some of these parts more clearly, is given in fig. 6, plate 8. 318. The mechanism of the sense of smell is very simple. When air, charged with odorous particles, passes over the membrane that lines the nose, some of these particles are delayed by the mucous secretion that covers it, and act upon the delicate sensory extremities of the olfactory nerve, with which it is thickly set. The highest part of the nasal cavity appears to be that which possesses the most acute sensibility to odors ; and hence it is that when we snuff the air so as to direct it into the upper part of the nose, instead of allowing it to pass simply along the lower portion from the nostrils to the posterior 314. What is said of the connexion between the organs of taste and the stomach? 315. What do we perceive by the sense of smell? What is said of the minuteness of odorous particles? By what experiment is this illustrated? 316. Why should the organ of smell occupy the position which it does? Describe the construction, divisions, &c., of the nose. 317. Describe the interior conformation of the nasal cavities, the pituitary mem- brane, spongy bones, sinuses, &.C. 318. Explain the mechanism of the sense pf smell. Why are odors perceived more acutely when we snuff up the'air ? Fiff. 50. VERTICAL SECTION OF THE NASAL CAVITY. a, mouth; i, nostril; c, posterior opening ; d, e, passages be- tween the spongy bones ; /, g, A, spongy bones ; t, frontal sinuses ; k, sphenoidal sinuses ; /, vail of the palate. orifices, we perceive delicate odors which would otherwise have escaped us. 319. Besides the olfactory nerve, the mucous membrane of the nose receives branches of the fifth pair. This latter nerve endows it with common sensibility, and also conveys the impressions produced by acid or pungent vapors, which act upon it in the same way as the cor- responding fluids do upon the tongue. Vapors of this kind, such as the fumes of nitric acid, or of ammonia, are felt by the irritation which they produce, rather than smelt; and the impression they occasion gives rise to the reflex action of sneezing, by which they are driven from the nose. Hence sneezing will be caused by an irritating substance, such as snuff, after the olfactory nerve is divided, if the branches of the fifth pair be entire. 320. Animals do not all equally perceive the same odors. Carniv- orous animals have the power of detecting most accurately, by the smell, the peculiarities of animal matters, and of tracking other ani- mals by the scent, but have no apparent sensibility to the odors of plants and flowers. Herbivorous animals are peculiarly sensitive to the latter, but their sensibility to animal odors is generally much less acute. Some herbivorous animals, however, possess great accuracy and delicacy of smell, even in regard to animal odors. Thus the deer and antelope frequently escape the hunter, who cannot advance suffi- ciently near to shoot them, except by stealing upon them in a direction contrary to that of the wind. In these Animals the sense of smell serves as the chief means by which they are warned of the presence of their enemies. One special purpose why herbivorous animals should possess an acute susceptibility to the odors of plants, undoubtedly is, to guide them in the selection of their food from among a variety of plants, some of which are highly deleterious, and by which they might otherwise be readily poisoned. 321. The wonderful perfection in which this sense exists in the dog is well known. The certainty with which he detects the footsteps of animals, long after they have passed the facility with which he traces the progress of his master through crowded streets, recognizing the emanations which his foot has left, among all the diversity and multi- tude of odorous particles is truly astonishing. In like manner, the deer-hound pertinaciously pursues his victim, and follows its traces through the herd of its fellows, among which it vainly seeks for protection. 322. Birds, in general, do not appear to possess the sense of smell in as great perfection as quadrupeds. It has been, indeed, supposed that vultures and other rapacious birds are guided to their prey, from immense distances, by the scent ; but it is well proved, by Audubon and other naturalists, that these birds are guided by sight, and not by smell. In the case of the Fish-hawk, which suddenly turns and plunges upon its scaly victim from a great height in the air, the sense of smell cannot be supposed to give the least direction to the motions of the bird. In fishes, the nasal cavities have no communication with the gullet; the lining membrane is beautifully plaited, and abundantly covered by mucus. The sense of smell in fishes has, however, been denied by some physiologists, and it does not seem probable that they possess it in a high degree. Many insects are able to distinguish odorous substances at considerable distances. By this sense, the common flesh-fly is guided to the putrid meat on which its eggs are deposited. 323. Like all the senses, that of smell is greatly improved in acute- ness by education. In the blind, especially, it becomes, next to that of touch, the sense on which they place the greatest reliance, and by which they distinguish individuals from each other. The Indians of Peru, according to Humboldt, can ascertain, by the smell, in the mid- dle of the night, whether a visitor be an European, an American, an Indian, or a negro. Sometimes the smell becomes morbidly sensitive. There are those who shudder at the scent of cheese, or faint at that of a cat. Cloquet mentions the case of an eminent French physician, who was exceedingly annoyed, during an illness in which his smell became remarkably acute, by the odor of copper; and, after careful search, the source of his annoyance was found to be a brass pin which had fallen among the bed-clothes. 319. What is the office of the nasal branches of the fifth pair of nerves? How is the action of sneezing produced. 320. What is said of the peculiarities of smell in different animals? In the deer and antelope ? Why should herbivorous animals be acutely sensible to the odors of plants? 321. What is said of the delicacy of smell in the dog? 322. What of the smell of birds? Of fishes? Of insects? 323. How is the sense of smell improved? What is said of this sense in the blind? In the Indian? What instances are given of a morbid sensitiveness of smell ? PLATE VIII. ORGANS OF SENSE FIGURE 1. SPECIAL ORGANS OF TOUCH. A, Papillae of the skin, magnified, containing expansions of the nerves and of the capillary blood-vessels. B, The spiral arrangement of the papillae at the ends of the fingers, in which the sense of touch is especially seated. (See PLATE V.) FIGURES 2, 3, 4. ORGANS OF TASTE. THE TONGUE AND SALIVARY GLANDS. FIGURE 2. a, The tongue. b, c, d, t, Muscles of the tongue. /, The parotid gland, g, Duct of the parotid gland, through which the saliva is poured into the mou' h, Sub-maxillary gland, t, Sub-lingual gland, j, Vessels of the tongue, k, k, Nerves of the tongue. THE D O R S U M, OR UPPER SURFACE OF THE TONGUE. FIGURE 3. a, The epiglottis. 6, The root of the tongue, c, c, Mucous glands, covering the root of the tongue, d, d, d, The large papillae, arranged in two obliq lines, meeting at the middle of the root of the tongue. Spreading over the rest of the surface are seen the small papillae, in great numbers and different shapes and sizes. THE INFERIOR SURFACE OF THE TONGUE. FIOTORK 4. This figure represents the lower side of the tongue laid open, so as to show the distribution of the nerves, a, The hyoid-bone, to which the base of the long is attached. I, b, The stylo-glossus muscles, reaching from the tongue to the styloid processes of the temporal-bone, c, c. Their action is to draw t tongue backward, d, The hypo-glossal nerve, e, The lingual nerve. Minute filaments of these nerves are spread throughout the tongue and in its papill FIGURES S, 6. ORGAN OF SMELL: THE NOSE AND N A S A L C A V I T I E S. FIGURE 5. a, Nasal bones. 6, Ascending processes of the superior maxillary-bone, articulating with the nasal-bone, c, d, e. Cartilages of the nose. VERTICAL SECTION ACROSS THE NASAL CAVITIES. Fietrai 6. a, b, c, The superior, middle, and inferior spongy bones, d, The vomer,- which forms the vertical partition between the cavities, e. Roof of the mouth. FIGURES 7, 8. ORGAN OF HEARING: VERTICAL SECTION OF THE ORGAN OF HEARING. Floras 7. a, I, c, The external ear. d, Entrance to the auditory canal. /, Auditory canal, e, e, Petrous portion of the temporal-bone, in which the internal ear excavated, g, Membrane of the tympanum, h, Cavity of the tympanum, the chain of ear-bones being removed. (For the relative positions of the bones, see fig. 52, page Of)-) *> Cells excavated in the temporal-bone, j, Opening from the ear-cavity into these cells. On the side of the cari opposite the membrana tympani, are seen the fenestra ovalis and rotunda, which open into the vestibule. I, The vestibule, k, The Eustachian tub m, Semi-circular canals, n, Cochlea, o, Auditory nerve, p, Canal, by which the carotid artery enters the skull, q. Part of the glenoid fossa, whii receives the head of the lower-jaw, r, Styloid process of the temporal-bone. A MAGNIFIED REPRESENTATION OF THE LABYRINTH, LAID OPEN SO AS TO DISPLAY ITS CAVITY. 8. I, The vestibule, n, The cochlea, q, The partition by which the cochlea is divided into two parts, o, The fenestra ovalis. p. The fenestra rotund r, r, r. The semi-circular canals. , Portion of the temporal-bone. ..VIII. Fig 2. ' Hartford Conn. SENSATION AND THE SENSES. 55 SENSE OF HEARING. 324. By this sense, we become acquainted with the sounds pro- duced by bodies in a certain state of vibration. The vibrations of the sounding body are communicated to the air, in which they produce, in every direction, a series of undulations or waves, by which the sound is conveyed to a distance. These undulations spread more widely as they become more distant from the sounding body, just like the rippling circles produced on the surface of water when a stone is thrown into it; and, in proportion as they spread, they become less powerful. This is the reason why Sound becomes less intense, as the sounding body is more distant. 325. Although air is the usual conducting medium for the sonorous undulations, liquids or solids may answer the same purpose. Thus, if a person hold his head under water while two stones are struck together, also under water, at a considerable distance, he will hear the sound produced by the blows with extreme distinctness, and even with painful force. Or, if the ear be laid against one end of a long piece of timber, while a scratch of a pin be made, or a watch be laid, upon the other end, even the faint sounds made by them will be heard dis- tinctly. That a medium of some kind is necessary to convey the sonorous vibrations, is proved by the fact, that, if a bell be made to ring in the receiver of an air-pump from which the air is exhausted, no sound is heard, though the ringing immediately becomes audible when the air is allowed to enter. 326. It is a fact of much importance in regard to the action of the organ of hearing, that sonorous vibrations which have been excited, and are being transmitted in a medium of one kind, are not imparted with the same readiness to others. The following conclusions have been drawn from experimental inquiries on this subject: 1. Vibrations excited in solid bodies may be transmitted to water, without much loss of their intensity; although not with the same readiness with which they would be communicated to another solid. 2. On the other hand, vibrations excited in water lose some of their intensity in being propagated to solids ; but they are returned, as it were, by these solids to the liquid, so that the sound is more loudly heard in the neighborhood of the solids than it would otherwise have been. 3. The sonorous vibrations of solid bodies are much more weak- ened by transmission to air; and those of air make but little impres- sion on solids. 4. Lastly, sonorous vibrations in water are transmitted but feebly to air ; and those which are taking place in air are with difficulty com- municated to water; but the communication is rendered much more easy by the interposition of a membrane between the air and the water. 327. The Auditory nerve, or nerve of Hearing, is adapted to receive and transmit to the brain the sonorous undulations produced in the surrounding medium by vibrating bodies. Now, it is obvious that it may be affected by these in various ways, especially in animals that inhabit the water. The vibrations excited in the liquid will be trans- mitted to the solid parts of the head, and thence to the nerve con- tained in it, without much interruption ; and this, independently of any special apparatus of hearing. Indeed, the simplest form of this appa- ratus is only designed to give increased effect to the vibrations thus excited in the solid parts of the head ; for it consists merely of a cavity excavated in their thickness, filled with fluid, and lined by a mem- brane on which the nerve of hearing is minutely distributed. This is the form of the organ of hearing in those of the Mollusca which pos- sess any such organ. 328. Of the degree in which sonorous vibrations may be communi- cated to our own auditory nerves through the solid parts of the skull, we may easily satisfy ourselves, by closing the ears carefully, and placing any part of the head against a solid body which communicates with the one in vibration. In this manner we may hear the sounds produced by the latter, with considerable distinctness, though accom- panied by an unpleasant jarring. A deaf gentleman was once agree- ably surprised to find that when smoking his pipe, with the bowl resting $ 324. How are sounds produced? Describe the effect produced upon the air by the vibra- tions of sounding bodies. 325. Is air the only conducting medium ? How is it shown that water and wood conduct sounds ? How is it proved that a medium of some kind is necessary for this purpose? 326. What important fact is stated in regard to the action of the organ of hearing ! What four conclusions are drawn from experiments on this subject ? 327. What is the use of the auditory nerve? How do animals hear, which inhabit the water? What is the simplest form of the organ of hearing ? 328. How is it shown that sounds may be communicated through the solid parts of the skull? Repeat the instance of the deaf gen- tleman, &c. on his daughter's piano-forte, he could distinctly hear the music she was producing from it; and many deaf persons may be made to hear conversation, by holding a piece of stick between their own teeth, and placing it against the teeth of the person speaking. 329. In animals which have the organ of hearing constructed upon the simple plan just mentioned, the force of the vibrations of the fluid contained in the cavity is increased by several minute stony concre- tions suspended in it, which act according to the second principle stated in 326. They are termed otolith.es or ear-stones ; and some traces of them may be found even in Man and the higher animals. They may be found in the head of a boiled fish, where they are hard and brittle as porcelain, and they frequently attract attention at table, from their curiously smooth and polished appearance. 330. It appears, then, that a cavity excavated in the solid walls of the head, covered in externally by a membrane, having the auditory nerve distributed upon its walls, and filled with fluid, is the simplest form of the organ of hearing, and may be regarded as including all that is essential to the exercise of this function. On the other hand, in Man and the higher Vertebrata, we find a very complex structure, adapted to render the faculty much more perfect; by receiving impressions which make us aware, not only of the presence of a sound- ing body, but of its nature and direction, and of the pitch and peculiar quality of the sound ; and also, it is probable, by taking cognizance of sounds much fainter than those which would be perceptible to the lower animals. Yet, even in the most complicated forms of the organ of hearing, we shall find that the essential part is still the same with that which forms the whole organ in the lower tribes; and that the faculty is still possessed, though in an inferior degree, when by disease or injury the accessory parts are prevented from acting. 331. The organ of Hearing, in Man, may be divided into three parts, the external, middle, and internal ear The former is the fibro- cartilaginous appendage placed on the outside of the head, to receive and collect the sounds which are to be transmitted to the interior. The two latter divisions are excavated in a bone of remarkable solid- ity, the petrous (stony) portion of the temporal-bone. The use of the different hollows and elevations on the surface of the external ear of man is, probably, to direct the sonorous undulations towards the entrance of the canal which leads to the middle ear. The form of the external ear, in Quadrupeds, evidently adapts it to this purpose; and there are several which, like the Horse, have the power of changing its direction, by muscular action, in such a manner as to enable it to catch most advantageously the faintest sounds from any quarter. This is especially the case with timorous animals, such as the Hare or the Deer, which also have very large external ears. 332. The canal, d, plate 8, fig. 7, into which the external ear col- lects the sonorous vibrations, passes inward, until it is terminated by a membrane stretched across it, which is called the membrana tympani, or membrane of the drum of the ear, g. This forms the outside wall of a cavity excavated in the petrous portion of the temporal-bone, the inner wall of which is bony, and forms the partition between the mid- dle and internal ear. The cavity of the tympanum, constituting the middle ear, is not one of the essential parts of the organ; for nothing analogous to it exists in fishes, nor in the lower reptiles. It contains air, and communicates with the back of the nasal cavity by a canal termed the Eustachian tube, k. It is the partial or complete closure of this tube, by swelling of its lining, or by the viscid secretion from it, that occasions the slight deafness common among those who are suffering from colds. 333. "Within the cavity of the tympanum there is a very curious apparatus of small bones and muscles, which seems to establish a con- nexion between the membrane of the drum and the small membrane covering the entrance to the internal ear. These bones are four in number, and are termed the malleus or hammer, a, fig. 51 ; the incus or anvil, 6; the os orbiculare, a minute globular bone, c; and the stapes or stirrup-bone, d. These bones are connected together in the manner represented in fig. 52, where b represents the membrana tym- pani; c, one of the long processes of the malleus, which is attached to the membrane ; d, the head of the malleus, which articulates with the 329. How is the acuteness of the simple organ of hearing increased? On what princi- ple? What is said of the otolithes or ear-stones? 330. What, then, appears to be the simplest form of the organ of hearing? In what respects is the ear of Man superior to this? What is said of the essential part of the organ of hearing? 331. Describe the divisions of the organ of hearing in Man. What is said of the uses of the external ear? 332 Explain the construction, and point out in the plate the parts, of the middle ear. What is the occasion of deafness in a person who has a cold? 333. Describe the bones of the ear and their relative position, &c., in fig. 52. 56 SENSATION AND THE SENSES. incus ; e, the other long process of the malleus, which is acted on by the minute muscle,/; g, the incus, of which one leg is in contact with the wall of the cavity, while the other is connected with the orbicular- bone, A ; , the stapes, the upper end of which joins the orbicular- bone, and the lower, which is of an oval form, is attached to the membrane that covers the entrance to the internal ear; and k is a small muscle which acts upon the stapes in such a manner as to regulate its movements. Fig, 51. BONES OF THE EAR. Fig. 52. CAVITY OF THE TYMPANUM, WITH THE BONES IN THEIR PLACES. 334. The use of this apparatus is evidently to receive the sonorous vibrations from the air, and to transmit them to the membrane forming the entrance to the internal or essential part of the organ of hearing, in such a manner that the sonorous vibrations excited in the latter may be much more powerful than they would be if the air acted immediately upon it. The usual state of the membrane of the tympa- num appears to be rather lax or slack, and, when in this condition, it vibrates in accordance with grave or deep tones. By the action of a small muscle lodged within the Eustachian tube, it may be tightened so as to vibrate in accordance with sharper or higher tones ; but it will then be less able to receive the impressions of deeper sounds. 335. This state may be artificially produced, by holding the breath and forcing air into the Eustachian tubes, so as to make the membrane bulge out by pressure from within ; or, by exhausting the cavity, by an effort at inspiration with the mouth and nostrils closed, which will cause the membrane to be pressed inwards by the external air. In either case, the hearing is immediately found to be indistinct ; but it will be observed, that the experimenter thus renders himself deaf to grave sounds ; while acute sounds are heard even more distinctly than before. If the sound be so acute that the membrana tympani will not vibrate in unison with it, the individual will not hear it, although it may be loud. It has been observed that some persons cannot hear the very shrill tones produced by particular insects, or even by birds, which are distinctly audible to others. 336. The internal ear is composed of various cavities that commu- nicate with each other. Of these, the vestibule (I, plate 8, fig. 7) may be regarded as the centre ; from it pass ofF, on one side, the three semi-circular canals, m ; and on the other, the cochlea, n. The vesti- bule is the part which corresponds with the simple cavity that consti- tutes the whole organ of hearing in the lower animals, and the canals and cochlea may be regarded as extensions of it for particular purposes. It communicates with the tympanum, by a small orifice in the bony wall that separates them, called the fenestra ovalis, or oval window. This orifice is closed by a membrane, to which the lower end of the stapes is attached. The three semi-circular canals are passages exca- vated in the solid bone, and lined by a continuation of the same mem- brane that lines the vestibule ; each passes off" from the vestibule and returns to it again. 337. The cochlea, n, or snail-shell, also is a cavity excavated in the hard bone, and lined by a continuation of the same membrane; its form is almost precisely that of the interior of a snail-shell (whence its name), being a spiral canal, which makes about two turns and a half around a central pillar. This canal is divided into two, however, by a partition running along its whole length ; which partition is partly formed by a very thin lamina of bone, and partly, in the living state, by a delicate membrane. The two passages do not communicate with each other, except at the top or centre ; at their lower end, corres- 334. What is the use of this apparatus? What is said of the action of the membrane of the tympanum? 335. How may this membrane be tightened artificially, and what is the consequence of doing so? Why are some persons deaf to certain sounds? 336,337. Describe the parts of the internal ear. Refer to plate 8, fig. 7, in connexion with the description. ponding to the mouth of the snail-shell, they terminate differently; for while one freely opens into the vestibule, the other communicates with the cavity of the tympanum by an aperture termed the fenestra rotunda, or round window, which is closed by a membrane. Thus the internal ear communicates with the cavity of the tympanum by two minute orifices only the fenestra avails, and the fenestra rotunda both of them closed by membranes, against the former of which the stapes abuts, while the latter is free. A magnified view of a section of the semi-circular canals, and the cochlea, is given in plate 8, fig. 8. 338. The whole internal ear is lined by a delicate membrane, on which the auditory nerve is very minutely distributed ; and terminates in papilla, which are especially visible on the partition between the two passages of the cochlea. The cavities are completely filled with fluid, which is set in vibration by the movements of the stapes, com- municated through the membrane of the fenestra ovalis ; and these vibrations are probably rendered more free by the existence of the second aperture, the fenestra rotunda. It is by the influence of these, undulations upon the expanded fibrils of the auditory nerve, that the sensation of sound is produced; but in what way the different parts of the labyrinth, as this complex series of cavities is not inaptly called, contributes to the performance of this function, is not yet known. In all but the lowest Fishes, these semi-circular canals exist; but there is no vestige of a cochlea. In the true Reptile's, a rudiment of the cochlea may be generally discovered. In Birds, this cavity is more completely formed, though the passage is not spiral, but is nearly straight ; it is divided, however, like the cochlea of Man, by a membranous partition on which the nerve is spread out. 339. In almost every instance in which the semi-circular canals exist, they are three in number, and lie in three different directions, corresponding to those of the bottom and two adjoining sides of a cube ; hence it has been supposed, and with much probability, that they assist in producing the idea of the direction of sounds. It has also been supposed that the cochlea is the organ by which we judge of the pitch of sounds ; and this would seem to be not improbable, especially when we compare the development of the cochlea, in different animals; with the variety in the pitch of the sounds which it is important that they should hear distinctly, especially the voices of their own kind. The compass of the voice that is, the distance between its highest and lowest tones is much greater in Mammalia than in Birds, as is also the length of the cochlea. In Reptiles, which have little true vocal power, the cochlea is reduced to its lowest form, and in Fishes it disappears altogether. 340. That the vestibule, and the passages proceeding from it, con- stitute, even in man, the essential part of the organ of hearing, is evident from the fact, that when, as not unfrequently happens, the membrana tympani has been destroyed by disease, and the chain of bones has been lost, the faculty is not by any means lost, though it is deadened. In this state, the vibrations of the air must act at once upon the membrane of the fenestra ovalis, as in the lower animals which possess no external or middle ear, instead of striking the mem- brane of the tympanum, and being transmitted along the chain of bones. 341. It has been stated ( 324) that the sensation of hearing is pro- duced by the successive undulations or vibrations communicated to the ear, from the sounding body, by the air, or by a liquid .or solid medium. This is the case with all continued sounds or tones; but single momentary sounds such as those produced by the discharge of a pistol, the blow of a hammer, or the ticking of a watch make their impression on the ear by a single shock. All continuous tones are, in fact, caused by a succession of such shocks, communicated to the ear with sufficient rapidity for the interval between them not to be dis- tinguished. Thus, if a tight string be caused to vibrate, by pulling or striking it, there is occasioned, not one vibration only, but a long suc- cession, every one of which gives a new impulse to the air, and pro- duces a new impression on the organ of hearing. 342. These vibrations we can see, when they are sufficiently exten- sive, and we can always feel them, by placing a finger on the string. In the same manner, the vibrations of a bell, or a tuning-fork, continue long after the first blow; and these, though we cannot see them, may 338. Describe the membrane of the internal ear. What is said of the fluid contained in the cavities, and of its undulations ? What of the modifications of this part in fishes, reptiles, and birds? 339. What is snid of the semi-circular canals, and of their use? What of the use of the cochlea ? 340. How is it shown that the vestibule is the essential part of the organ of hearing? 341. What is said of the production of continuous and momentary sounds? How is this illustrated by the vibration of a string? 342. What is said of seeing and feeling vibrations? How are continuous tones otherwise produced ? Upon what does the pitch of these sounds depend ? SENSATION AND THE SENSES. 57 be readily felt by the finger. Tt is, in fact, in their power of continu- ing to vibrate after they have been struck, that the peculiarity of these resonant bodies consists. In other instances in which continuous tones are produced, the vibrations are kept up by the continued appli- cation of the original cause, and cease as soon as it is withdrawn ; this is the case, for instance, in the string of the violin, when set in motion by the bow, and in the flute and organ-pipe, when caused to sound by the passage of air through them. In all these cases, then, the continued tones are due to a succession of impulses given by the sounding body to the air; and, according to the rapidity with which the impulses succeed each other, will be the pitch of the sound. 343. The strength or loudness of musical tones depends upon the force and extent of the vibrations communicated by the sounding body to the air. Thus, when the middle of a tight string is drawn iar out of the straight line, and then let go, a loud sound is produced, and we can see that the space through which the string passes from side to side is considerable. As the extent of the vibrations of the string dimin- ishes, the sound becomes less powerful ; and when we can no longer see the vibrations, but only feel them, the sound is faint. The length of the undulations in the air corresponds with that of the vibrations in the sounding body, and consequently they will strike upon the tympa- num with more or less force, as these are longer or shorter. The cause of the differences in the quality of musical tones such differ- ences, for instance, as are perceived between the tones of a flute, a violin, and a trumpet is unknown ; but they probably depend upon the different form of the vibrations. 314. The faculty of hearing, like that of sight, may be very much increased in acuteness, by cultivation ; but this increase depends rather upon the habit of attention to the faintest impressions made upon the organ, than upon any change in the organ itself. Thus, the watchful Indian recognizes footsteps, and can even distinguish the tread of a friend from that of a foe ; while his white companion, who lives among the busy hum of cities,. is unconscious of such slight^ound. Yet the latter may be a musician, capable of distinguishing the tones of all the different instruments in a large orchestra, of following any one of them through the part which it performs, and of detecting the least discord in the blended effects of the whole effects which would be to his colored friend only a confused mass of sound. In the same manner, a person who has lived much in the country is able to distinguish the note of every species of bird which lends its voice to the general con- cert of Nature; while the inhabitant of a town hears only an assem- blage of shrill sounds, which may impart to him a disagreeable rather than a pleasurable sensation. 345. Of the direction and distance of sounds, our ideas are for the most part formed by habit. Of the former we probably judge, in great degree, by the relative intensity of the impressions received by the two ears ; though we may form some notion of it by a single organ ( 339). Of the distance, we judge by the intensity of the sound, comparing it with that which we know the same body to produce when nearer to us. The ear may be deceived in this respect as well as the eye ; thus the effect of a full band, at a distance, may be given by the subdued tones of a concealed orchestra close to us ; and the Ventriloquist pro- duces his deceptions by imitating, as closely as possible, not the sounds themselves, but the manner in which they would strike one's ear. SENSE OF SIGHT. 346. By the faculty of Sight we are made acquainted, in the first place, with the presence of light ; and, by the medium of that agent, we take cognizance of the form of surrounding bodies, their color, size, and position. As to the nature of Light itself, there is a difference of opinion; some philosophers believing that it is propagated by rays consisting of actual particles emitted from the luminous body ; while others consider it more probable that it is transmitted by means of vibrations or undulations, analogous to those by which sound is propa- gated. An exposition of these theories, and the arguments and facts upon which they are based, may be found in works on the science of Astronomy. In the present connexion, it is of little consequence which doctrine is true; since we have to do alone with the laws that 343. Upon what does the strength and loudness of a tone depend ? How is this shown by the vibration of a string? What is said of the quality of musical tones? 344. What is said of the cultivation of the faculty of hearing ? How is this illustrated ? 345. What is said of our ideas of the direction and distance of sounds? What of the deception of the ear in this respect? 346. What is the office ot the faculty of sight? What is said of the nature of light ? For what physiological purpose is a knowledge of the laws of light requisite ? 15 regulate the transmission of the rays of light, or their passage through dif- ferent substances, which laws are the same under both theories. Some knowledge of these laws is requisite to a proper understanding of the beautiful action of the eye, and a short account of them will be here given. 347. The rays of light uniformly travel in straight lines, as long as they traverse the same medium (air, water, glass, or any other trans- parent body) without obstruction. In passing from a single luminous point into space, they diverge, or separate, in such a manner as to cover a larger and larger surface as they advance; while, at the same time, the intensity of the light is diminished. But when a ray passes from one medium into another of a different density, as from air into water, or from water into air, it is bent out of its straight course, or refracted; except when it happens to pass in a direction perpendicular to the plane which separates the two transparent bodies. 348. This may be proved by he following simple experiment: Place a piece of money (a, fig. 53) at the bottom of a cup or basin, and stand in such a position that the side of the vessel will just hide the coin from the eye. Then let another person fill the basin with water, the eye of the observer being kept in the same position as before. As the water is poured in, the coin will become visible, appearing to rise with the water. The effect of the water is to bend the ray of light coming from the coin, so as to make it meet the eye below the point which the ray would have reached, if it had proceeded in a straight line. Thus the eye of the observer, situated at the end of the line a c, could not see the coin in a straight line, because rays passing in that line would be interrupted by the side of the basin; but it receives the ray which was passing in the direction a d, and which was bent downwards at the moment of quitting the water. Kg. ss. If, however, the eye had been placed directly over the coin, so that the ray, passing from the latter to it, would "have emerged in a direction perpendicular to the surface of the water, no change in the apparent place of the object would have been made by pouring in the water, since a ray which passes from one medium to another, in a direction perpendicular to the surface which separates them, is not refracted. Those rays which pass out most nearly in this direction are refracted least, while those which pass out most nearly in the horizontal direction, are refracted most. 349. The general law of refraction, then, is, that rays passing from a denser to a rarer medium are refracted /rom the perpendicular; the degree of refraction being less in proportion as they are near the per- pendicular. On the other hand, when rays pass from a rarer into a denser medium, they are bent towards the perpendicular; and this in a greater or less degree, according as their direction is more distant from, or nearer to, the perpendicular. 350. When the surface which separates the two media is not flat, t but is convex or concave, a very important alteration is produced in the direction of the rays that fall upon it. Thus, let us suppose that three diverging rays, issuing from a point, , fig. 54, tra- verse the air and strike upon a convex surface of glass, b, b. The central ray, a c, falls in a direction perpendicular Fig. 54. to the surface of the glass at that point, and passes on unchanged in its course. But the ray, a d, falls upon the surface very obliquely ; and consequently, in entering the glass, it will be bent towards the line e, which is perpendicular to the surface, and will pass onwards in the direction _f. In the same manner, the ray, a g, will be refracted in the direction i. Hence these rays, now converging, would meet each other again, if sufficiently pro- longed ; and the point at which they meet is called the/ocws. To this point all the other rays which strike the convex surface, at a moderate distance from the central ray, will be also conducted. 347. When do rays of light pass in straight lines? What is understood by the refrac- tion of light! 348. Explain fig. 53, and show why the coin seems to be raised up by pouring in water. If the eye had been placed directly over the coin, would the place of the latter have appeared to be changed ? Why not? What rays are refracted least, and what most? 349. What, then, is the general law of refraction? 350. Explain fig. 54, and the operation of a convex lens upon rays of light. What is the focus of a lens? 58 SENSATION AND THE SENSES. 351. On the other hand, if the surface of the glass on which the ray falls be concave instead of convex, the diverging rays which fall- upon it will be made to diverge still more. In both cases, it is easy to understand that the change of direction of the rays will be greater, as the curvature of the lens is more considerable. Thus, a convex lens has a longer or shorter focus (that is, it brings the rays to a focus at a greater or less distance from itself), according as the curvature of its surface is greater or less. The rays issuing from every point in an object, and falling upon a convex lens, are brought to a point or focus on the other side of the lens ; and thus a distinct image or picture is formed upon any screen placed at the proper distance to receive it, every point in that image being the representative of the correspond- ing point in the object. But this image, according to the laws of optics, will be inverted. ,1, ttjr.55. INTERIOR or TUB EYE. t'ig. 56. SECTION or THE EYE. e, cornea ; *, sclerotic ; s, portion of the sclerotic turned back to show the subjacent parts ; cA, choroid ; r, retina; , optic nerve; ca, anterior chamber; t, iris; p, pupil; cr, crystalline lens; cp, ciliary pro- e; v, vitreous humor; b, b, conjunctiva. 352. Now, the Eye, in its most perfect form such as it possesses in Man and the higher animals is an optical instrument of wonderful completeness, designed to form an exact picture of surrounding objects upon the expanded surface of the optic nerve, by which the impression is conveyed to the brain. The form of the eye is nearly globular. The walls of the sphere are composed of three coats ; while in its inte- rior are found three humors of a more or less fluid character. The outer coat, named the sclerotic (s, figs. 55, 56) is tough and fibrous, and is destined to support and protect the delicate parts which it con- tains. It covers the whole globe of the eye, except a part of the ante- rior portion, where it gives place to the transparent cornea, c. The cornea is set into a circular groove in the anterior margin of the sclerotica, in the same manner as a watch-glass is inserted into the case. The cornea is more convex than the rest of the globe, and pro- jects forward beyond the circumference of the sclerotica. Beneath the sclerotica lies the second coat, called the choroid, ch, which is a delicate structure, consisting of blood-vessels and nerves. It has a deep black color, owing to its being covered with a layer of pig- mentum nigrum, or black pigment, 353. The cornea becomes continuous, at the front of the eye, with the iris or colored portion, i, which forms a kind of curtain, which hangs behind the cornea. The surface of the iris being nearly flat, a space is left between it and the cornea, like that between the dial- plate and glass of a watch. This space is called the anterior cham- ber of the eye. The iris is perforated in its middle by an aperture, termed the pupil, p. In Man, this aperture is round ; but in animals whose range of vision is required to extend widely in a horizontal direction, such as the sheep, the cow, and other grazing animals, it is lengthened horizontally, so as to admit the rays which come from either side. In the cat-tribe, on the contrary, which live among trees and high places, and which have to watch their prey m situations either above or below them, the pupil is elongated vertically. 354. By the relaxation and contraction of certain fibres in the iris, the size of the pupil is changed, according to the degree of light to which the eye is exposed. Thus the pupil contracts in a strong light, so as to exclude the rays which would be superfluous, and to prevent too many from falling upon the expansion of the optic nerve ; while in a faint light it dilates, so as to admit as many rays as possible. If 351. What is the effect of a concave glass upon the rays of light? How is the change of direction of the rays proportioned ? Upon what does the length of the focus of a convex lens depend? How is an image formed by a convex lens? 352. What is said of the eye as an optical instrument ? How many coats has the eye ? How many humors? Describe the sclerotic coat, and refer to figs. 55, 56. How is the cornea inserted into the sclerotic? Describe the choroid-coat. 353. Describe the iris The anterior chamber The pupil, and its modifications in different animals. 354. How is the size of the pupil varied, and what is the use of this variation in size 1 we notice the pupil of a man who is looking towards the mid-day sun, we shall see that it is contracted to a small round speck. Under the same circumstances, the pupil of a sheep would be contracted into a horizontal slit, and that of a cat into a vertical one. 355. The blackness of the pupil is owing to the pigment or black lining of the eye being seen through it. In many quadrupeds, the black pigment is replaced; in a portion of the internal surface of the choroid, by a bluish layer of bright metallic lustre, and from this the light is brilliantly reflected, when the eye is seen in certain directions. This flashing of the eye, as it is often called, is apparent in cats and dogs, in an obscure light. The back of the iris is always covered with black pigment, the use of which is, to absorb the rays that would otherwise be reflected from one side of the interior of the eye to another, rendering the image indistinct. 356. Beneath the choroid-coat is situated the third layer, r, of which the wall of the eye is composed. This is a delicate film, chiefly con- sisting of nervous fibres which spread out from the optic nerve, n, and . it is called the retina. It extends nearly as far as the iris, but is defi- cient in front of the eye. 357. The cavity of the globe is occupied by three humors, of differ- ent consistence, called the aqueous, vitreous, and crystalline. The aqueous humor is nearly pure water, being nothing else than the serum of the blood much diluted. It occupies the anterior chamber, c a, and a small space behind the iris, in front of the crystalline lens. The vitreous humor, v, resembles thin jelly in consistence, and occupies the greater part of the globe behind the iris. The crystalline, cr, is of much firmer consistence, resembling very thick jelly or soft gristle. It has the form of a double convex lens, the posterior surface of which is more convex than the anterior, and hence it is usually known as the crystalline lens. It is suspended in its place by a set of little bands proceeding from the choroid coat, and called the ciliary processes, c p. 358. The cornea is covered externally by a membrane termed the conjunctiva, b, b. *This membrane is perfectly transparent where it covers the cornea, of which it seems like the outer layer. The front of the sclerotic is also covered by it, and it is there semi-opaque, and ' of a whitish color. The conjunctiva, however, does not pass back over the globe of the eye; but bends forward again, as seen at b b, so as to form the lining of the eyelids, at the edge of which it becomes continuous with the skin. Thus the smooth surfaces of the eye, and of the under-side of the lids, are both formed by this membrane, the mucous secretion from which serves to diminish the friction of one upon the other. 359. The smallest particle of any hard substance which may be interposed between these surfaces, produces great irritation. It can- not pass far backward, however, on account of the bend of the mem- brane at b b; and thus it may be always removed, if loose, with little difficulty. The lower lid can be drawn down so as to expose the membrane as far as this bend, and thus any loose particle upon its surface can be detected and removed; but the upper lid being longer, cannot be drawn out sufficiently for this purpose, and it is necessary to evert it, or turn it inside out. Much suffering may be avoided by a knowledge of the mode of performing this simple operation. Nothing more is requisite than to close the upper lid, without force ; next to press upon its upper part with a pencil, bodkin, knitting-needle, or other hard body of small diameter, and then, taking hold of the eye- lashes, to draw the lower edge of the lid forward and upward. By a dexterous movement of this kind, the lid can be everted without pain, and any offending particle can be readily removed from the lining membrane. 360. The globe of the eye is moved by six muscles, which are lodged within the bony cavity or orbit, hollowed out in the skull. All these muscles, except one, originate at the back of the orbit, and are inserted into the sclerotic-coat, near its front. Four of them are termed recti, or straight muscles. One of them, the superior rectus, d (plate 9, figs. 1 and 2), is inserted at the upper part of the eye, and, by its contraction, rolls the globe upward. Another, the inferior rectus, e, produces a corresponding movement downward. A third, 355. To what is the blackness of the pupil owing? What is the reason of the glisten- ing of a cat's eyes in the dark ? What is the use of the black pigment? 356. Describe the retina. 357. What are the humors of the eye? Describe them separately-. 358. What is the conjunctiva? How is it formed, and what is the use of its secretion ? 359. What is said of the effect of a hard substance between the surfaces of this membrane, and how may it be removed? 360. How many muscles move the eye? What are their names, situ- ations, and mode of action? Particularly describe the superior oblique. Refer to plate 9 and point out these muscles. PLATE IX. ORGAN OF VISION.--THE EYE. FIGURE 1. THE RIGHT EYE, WITH ITS MUSCLES. DISPLAYED in the cavity of the orbit, on the vertical plane of a section corresponding to the middle of the arch of the eyebrow: that is, as if the right side of the head were removed, as far as the middle of the right eyebrow, leaving the eye in its place, to be seen from the right. FIGURE 2. BOTH EYES, WITH THEIR MUSCLES, As they appear upon a horizontal section through the orbits, immediately above the eyes. As if the upper part of the head were removed, as far down as the top of the eyes, the observer looking at them from above. They are represented in this position, in order to show clearly the situation and action of the superior oblique muscle, and also the crossing or decussation of the optic nerve. The letters of reference indicate the same parts in both figures. a, The optic nerve, s, figure 2, The chiasma or commissure of the optic nerve, whence each nerve extends forward and outward, passes into the eye at t, and becomes continuous with the retina. 6, The common oculo-motory nerve, which is distributed to five of the muscles of the eye. k, Trunk of the fifth pair, a branch of which constitutes the ophthalmic nerve, and gives sensibility to the different parts of the eye. The distribution of this nerve is seen in the left eye in figure 2. I, Artery of the eye. c, The elevator muscle of the upper eyelid, d, The superior rectus, or elevator of the eye. e, The -inferior rectus, or depressor of the eye. /, The internal rectus. g, The external rectus. A, figure 2, The superior oblique muscle, m, Its pulley. , figure 1, The inferior oblique muscle. In figure 2, parts of several of these muscles are removed, in order to display the others distinctly. FIGURE 3. THE LACHRYMAL APPARATUS. a. The lachrymal gland, b, b, The lachrymal ducts, which collect the tears, and transmit them to the lachrymal sac, c, whence they pass into the cavity of the nose. FIGURE 4. THE LEFT EYEBALL, SHOWING THE POSTERIOR SURFACE OF THE RETINA. a, A small transparent spot, situated near the optic nerve, called the foramen of Soemmering, surrounded by a yellow halo, the limbtis luteus. b, The optic nerve, cut off at its entrance into the retina, c, The central artery of the retina, d, d, Ramifications of the artery on the inner wall of the retina, seen through the two outer layers. FIGURE 5. THE FIBRES OF THE IRIS, DETACHED. a, The pupil, b, The circular fibres at the central margin of the iris, by the action of which the pupil is diminished or enlarged, c, c, The radiating fibres, which proce'ed from the external border of the circular fibres. FIGURE 6. A PORTION OF THE PIGMENT MEMBRANE OF THE CHOROID COAT HIGHLY MAGNIFIED. It is seen to consist of regular six-sided plates, the tissue of which is filled with grains of coloring matter. FIGURE 7. THE GLOBE OF THE EYE, MAGNIFIED, AND SEEN IN FRONT. This figure represents tne second or choroid coat, the sclerotica being removed, a, The pupil. 6, The iris, c, e, The choroid membrane, d, d, The ciliary nerves running from every part of the circumference towards the iris, e, e, The ciliary arteries. FIGURE 8. THE GLOBE OF THE EYE, SEEN IN THE SAME VIEW. The iris is removed, in order to display the ciliary processes, and their position in regard to the lens. The ciliary nerves and arteries are the same as in fig. 7. FIGURE 9. THE ANTERIOR HALF OF THE GLOBE OF THE EYE. SEEN FROM BEHIND. The lens and the vitreons humor are removed, a, The pupil. b, 6, The posterior surface of the iris, called, from its dark grape-like color, uvea. c, c, The ciliary processes, d, d, The internal surface of the choroid. e, e, The cut edges of the choroid and sclerotic. ^f FIGURE 10. THE POSTERIOR HALF OF THE GLOBE OP THE EYE, SHOWING ITS INTERNAL SURFACE. a, a, The cut edges of the sclerotic, choroid, and retina, b, b, The internal surface of the retina, c, The foramen of Soemmering. d, The optic nerve. , Branches ol the central artery of the retina. FIGURE 11 . A, Front View of the Crystalline Lens. B, Side View of the Lens. a. Its anterior and least convexity. 6, Its posterior and greatest convexity. FIGURE 12. THE CRYSTALLINE LENS, AFTER BEING IMMERSED IN BOILING WATER. The lines on the surface show its division into three parts. FIGURE IS. THE THREE SEGMENTS OF THE CRYSTALLINE LENS. The faces of the segments show the concentric layers of which it is composed (like the coats of an onion). The nucleus, or central portion of the lens, is seen on one of the segments, and on the other two are corresponding depressions. FIGURE 14. COMPOUND EYES OF THE BEE, HIGHLY MAGNIFIED, SHOWING THE DIVISION INTO FACETS, AND ALSO THE CONICAL SHAPE OF EACH SEPARATE PORTION. A, Facets still more highly magnified. B, The same with hairs growing between them. 1M.. IX (THE EYE.) fig 2 t ftset>niiaff to . trt >f(wyrrss l in fAf \-far lf>SO. lyfirltogy KvrxJfwA' rntfie (7srJfJ Offkft cf'UuKsCrtct (tmrt 'of' ' ( SENSATION AND THE SENSES. 61 the internal rectus, f, rolls the globe inward, or towards the nose; while a fourth, the external rectus, g, turns it outward. Besides these, there is a remarkable muscle, the superior oblique, h, fig. 2, in con- nexion with which there is a very curious and beautiful contrivance. It originates at the back of the orbit, proceeds forward, and, when it reaches the front, its tendon passes through a little cartilaginous pulley, m, and then turns backward to be inserted into the sclerotic-coat, at a point considerably behind the pulley. The direction of its action is thus changed, just like that of a rope which is passed through a block in the rigging of a ship, and, by drawing downward upon which, a sail is raised. This muscle rolls the eye downward and inward. The inferior oblique, i, fig. 1, arises from the lower border of the orbit. It turns the eye upward and outward. 361. The eyebrows, eyelids, and eyelashes, serve in various ways for the protection of the eyes. In Birds and Reptiles, there is a third eyelid, which is drawn across the eye by a muscle that passes through a loop in it. This nictitating membrane, as it is called, is semi- transparent, and it seems to protect the eye from the too powerful rays of light, without preventing the power of vision. Beneath the upper eyelid, in the upper and outer portion of the orbit, is situated the lachrymal gland, a, plate 9, fig. 1. This gland is continually pouring out a watery secretion, the lachrymal fluid or tears, which moistens the globe of the eye, and keeps it free from impurities. This secretion is increased by mental emotion, or by irritation of the eye, If the edges of the lids be carefully examined, there will be seen upon each of them, close to the inner corner, a minute spot, which is the entrance to a small canal, called the lachrymal duct, b, b. The two ducts, one commencing at the corner of each lid, soon unite into a single canal, which swells out into a sort of reservoir, the lachrymal sac, c, that lies upon the side of the upper part of the nose, and from this a canal passes down into the cavity of the nose. By this appa- ratus, the fluid poured by the lachrymal glands over the eye is drawn off, after washing its surface, and carried into the nose, where it is got rid of by the current of air which passes through the nostrils in breath- ing. The lachrymal fluid, when the eyelids are closed, passes along the gutter formed by the meeting of their edges. 362. The structure of the eye, and the general actions of the parts connected with it, having thus been described, some of the phenomena of vision will be briefly noticed. 363. The rays of Light which diverge from the several points of any object, and fall upon the front of the cornea, are refracted by its convex surface, while passing through it into the eye, and are made to converge slightly. They are brought more closely together by the crystalline lens, which they reach, after passing through the pupil ; and the refracting influence of the lens, together with that produced by the vitreous humor, is such as to cause the rays that issued from each point to meet in a focus" on the retina. As every point is thus represented in its proper position relative to others (except that those which were above are now below, and vice versa), a complete inverted image or picture of the object is formed upon the retina. This is shown in fig. 57; where, for the sake of convenience, two rays only are represented as issuing from each of the two extremities of an object, a, c. These rays cross each other in the middle of the eye, those from a being brought to a focus at b, and those from c at d; and as all the other rays are refracted in the same manner, a com- plete inverted picture of the object is formed at the back of the eye. 364. This fact may be seen, by taking the eye of a rabbit, cleansing it of the fat and muscles which adhere to its back part, and bringing a candle in front of it ; when, by looking at the back, the transparency of the sclerotic-coat will allow the inverted image of the candle to be distinctly seen upon the retina. Or, if the eye of a sheep or an ox be taken (the coats of which are thicker than those of the rabbit, and will not allow the passage of light), a part of the sclerotic and choroid coats should be cut out at the back part, and the portion of the retina thus left bare should be covered with a piece of tissue paper, for the purpose of keeping in the vitreous humor. A similar image of all the 361. What is the use of the eyebrows, eyelashes, and eyelids? Describe the third eye- lid of Birds and Reptiles, and state its use? Describe the situation and secretion of the lachrymal gland, and its uses. How is the lachrymal fluid carried off? Refer to plate 9, and point out the parts of the lachrymal apparatus. 363. Explain the manner in which an inverted image of an object is formed upon the retina. 364. How may the picture upon the retina of a rabbit or a sheep be observed? 16 Fig. 57. objects in front will be seen through the tissue paper. In performing these experiments, the eye should be perfectly fresh, as the cornea and humors soon lose their transparency, and the picture becomes indistinct. 365. The black pigment, which is situated immediately behind the retina that is, in contact with its external surface is destined to absorb the rays of light as soon as they have passed through the retina, so as to prevent them from being reflected from one part of the eye to the other, which would cause great confusion and indistinctness in the picture. Hence it is that, in those individuals, both among man and the lower animals, in whose eyes this pigment is deficient, vision is extremely imperfect. Such individuals are termed Albinos, and their eyes present a very peculiar appearance. The color of the iris is a bright red, owing to the great number of minute blood-vessels. The choroid-coat, seen through the. pupil, has exactly the same aspect; so that, the pupil is not readily distinguished. During the day, the vision of these Albinos is very indistinct, and the glare of light is painful to them ; and it is only when twilight comes on. that they can see clearly and without discomfort. 366. By the perfect construction of the eye, the defects to which ordinary optical instruments are liable, are avoided. A full explana- tion of the causes of these defects, and of the methods in which they are corrected by the structure of the eye, belongs to the science of optics. It may be mentioned, however, that one of these, called spherical aberration, results from the fact that those rays which fall upon the outer parts of an ordinary convex lens, meet in a focus nearer the lens than those which fall in the centre. In optical instru- ments, this is remedied by using only the central portion of the lens; thus cutting off a great portion of the light, and rendering the image dark. But, in the eye, the difficulty is completely avoided by the adjustment of the unequal curvatures of the cornea, and of the two surfaces of the crystalline lens, in such a manner that the rays are brought to the same focus. Chromatic aberration, or that defect which causes the edges of an object seen through a microscope to appear colored with different tints, is also corrected by the combina- tion of the lenses of the eye, the different density of the humors of which is adapted exactly to the purpose. Indeed, the first idea of uniting glasses of different kinds, so as to produce what is called an achromatic lens, of the kind now employed in the best instruments, was taken from the eye. The eye also possesses the power of adjust- ing itself perfectly to the vision of objects situated at different dis- tances an arrangement which is imitated in optical instruments only in the most bungling manner. The mode in which this adjustment of the eye is effected is not yet explained. 267. -It frequently happens, in advanced life especially, that the con- vexity of the cornea and the refracting power of the humors diminish ; and, in such cases, the eye is no longer able to accommodate itself to near objects, the rays from which are not brought to a focus by the time they reach the retina, and the individual is said to be long-sighted. This defect may be remedied by the use of convex glasses. On the contrary, there are other cases, in which the cornea is too convex, and the rays from distant objects are brought to a focus before they reach the retina. This is the cause of near-sightedness, which is remedied by using double concave glasses. 368. In the choice of spectacles or eye-glasses, for these purposes, particular care should be taken that they are not so powerful as to produce a strained feeling in the eye. Convex glasses should never be used of such power as to magnify objects, nor concave glasses of such power as to diminish them ; the proper degree of strength in both kinds of glasses being only that which will enable the eye to see with distinctness. The evil influence of using spectacles of too high power is not confined to the straining of the eyes. That feeling soon ceases ; but it is because the eye adapts itself to the glasses, and thus under- goes a change which might otherwise have been deferred for years. 369. The facility with which distant or minute objects may be per- ceived, is much influenced by the degree of attention directed to them. Thus, a person sees a balloon, or a faint star, in a clear sky, as long as his attention is fixed upon it ; but, if he withdraw his eye for a moment, he is unable to find the object again until it is pointed out to 365. What is the use of the black pigment ? What is the cause of the imperfect vision of Albinos? 366. What defects, to which optical instruments are liable, are avoided by the construction of the eye? Explain the mode in which spherical and chromatic aberra- tion are corrected? What is said of the adaptation of the eye to distances? 367. What is the cause of long-sightedness? What of near-sightedness? How may these defects be remedied? 368. What is said of the choice of spectacles? 369. What is said of the attention, as connected with sight? What of the combination of touch with eight? Describe the case of the boy whose sight was restored. A JN I M A L MOTION. him. Sailors, from long habit of using their eyes in search of distant objects, are able to descry land, or a far-off sail, when, to the eye of the landsman, the horizon appears vacant. The sense of sight is much assisted by that of touch, and the combination of the two senses, by which the child learns to judge of the nature of objects, is the result of experience. This is shown by cases where a young person, having been blind from birth, is restored to sight. In one very interesting instance of this kind, a boy of twelve years of age, after his sight became tolerably correct, saw every thing fiat, as on a picture, and it was some time before he was able, by sight alone, to judge of the real forms, characters, and distances of objects around him. He was well acquainted with a dog and a cat, by feeling, but he could not remem- ber their respective characters when he saw them. Being thus puz- zled, one day, he took up the cat, and felt her attentively ; at the same time, looking steadfastly at her, and then setting her down again, he said, "So, puss, I shall know you another time." 370. The eyes of Insects are of an entirely different kind from that of Man. On each side of the head of a fly, or bee, is situated a brown projecting hemispherical body, the surface of which, under the micro- scope, is seen to be divided into an immense number of six-sided facets, which vary in number in different insects. In the eye of a butterfly, seventeen thousand have been counted ; in that of a species of beetle, twenty-five thousand ; in that of the common house-fly, four thousand. Each of these facets forms the front or cornea of a distinct eye, which is conical in its form, the apex of the cone being directed towards the expansion of the optic nerve within the hemispherical body. No two of these facets have the same direction; so that a separate eye, as it were, is provided for every point to be viewed, and thus the lack of power to move the eye is compensated. The compound eye of the bee is represented in plate 9, fig. 5. 371. There are many interesting topics connected with Vision, which the space allotted to that subject will not allow to be intro- duced here. Among these are, the science of Chromatics, or the Phi- losophy of Colors; the causes of single vision, or of our seeing only one object, while pictures of it are formed upon both retina? ; the rea- sons why we see objects in an erect position, while their images on the retina are inverted. The latter question, in particular, has given rise to great discussion. The student, who wishes to pursue the investigation of these and kindred subjects, should consult some of the numerous excellent treatises on the science of Optics. CHAPTER XII. MOTION. STRUCTURE AND ACTION OF MUSCULAR TISSUE. ' 372. THE different modifications of the faculty of Sensation, which have been described in the preceding chapter, enable Man and other animals to become acquainted with what is going on around them. But they possess another faculty, that of spontaneous movement, which serves the double purpose of enabling them to act upon the inanimate world around them, and of communicating to each other their feelings and ideas. 373. The movements of animals are effected by the action of a peculiar property, called contractility. This property consists in the power which certain parts of the body possess, of contracting suddenly when excited to do so, and of afterwards lengthening again by relax- ation or slackening. In the higher animals, this property is nearly restricted to a peculiar tissue, called muscular fibre. In Vertebrated animals, the muscles, which are the active instruments of all their movements, form the greater part of the mass of the body, and con- stitute what is commonly known as the flesh or meat. 374. Every muscle is formed by the union of a number of bundles, which are united together by means of areolar tissue, and are them- selves composed of bundles still more minute, united in a similar man- ner. These, again, may be separated in the same way ; and at last we come to the primitive fibres of which this tissue is composed. 370. Explain the structure of the compound eye of insects. How is lack of motion in these eyes compensated? 372. What is the use of the faculty of Sensation ? What of that of spontaneous movement? 373. How are the movements of animals effected ! In what does the property of contractility consist? To what tissue is this property restricted in the higher animals 1 374. How are the muscles formed ? What is the diameter of the mus- cular fibrils 1 Each of these primitive fibres consists of a delicate membranous tube, inclosing a great number of fibrilla;, or extremely minute fibrils, which are not capable of further division. The diameter of these fibrils is nearly the same in all animals, being about l-10,000th of an inch. 375. Under the influence of certain exciting causes, or stimuli, muscular fibres suddenly and forcibly contract. Their two ends approach each other, and they swell out in the middle, to a corre- sponding degree. This causes a like change in the bundles which are made up of these fibres ; and thus the whole muscle, when short- ened by the drawing together of its two ends, is greatly enlarged in diameter, especially towards its middle. This may be readily observed by bending the fore-arm upon the arm, as when the hand is carried to the mouth, and feeling the fleshy mass in front of the arm, between the shoulder and the elbow. The actual bulk of the muscle is not changed by the act of contraction, for its enlargement in diameter is exactly equivalent to the shortening of the distance between its extremities. 376. The extremities of muscles are usually attached to bones, which their contraction causes to move one upon the other. One of these attachments is culled the origin, and the other the insertion of the muscle. The origin is at that part which is the most fixed, and the insertion at that part which is moved by the contraction. Thus, the muscle just mentioned has its origin at the shoulder, which is fixed, or nearly so, and its insertion in the bones of the fore-arm, which is moved by it. Muscles are seldom directly attached to bones, but are usually united with them by means of tendons or sinews. These ten- dons are sometimes cylindrical, tapering from the muscle to the point of their insertion ; and sometimes they are broad flat bands. Exam- ples of the first kind are some of those which move the hand and fingers ; they arise from the muscles which form the fleshy part of the fore arm, and may be felt at the wrist as hard, round cords. 377. Muscles are commonly divided into voluntary and involuntary, according as they act in obedience to the will, or are not under its dominion. There are, however, many muscles whose actions are usually of an involuntary character, but which are capable, to some extent, of being controlled by the will. Thus, the muscles concerned in breathing continue to act, even when the brain has been removed ; and, in the most profound sleep, when the will can have no influence over them ; but yet, these same muscles are made use of in speaking and singing, which are voluntary actions. On the other hand, many- other muscles, the actions of which are usually voluntary, are some- times made to act independently of, or even contrary to, the will. Such are those muscles which are excited by the emotions, and are called into play in laughing, crying, sobbing, &c. We often have the strongest desire to check these actions, on account of the unfitness of the time or place for their manifestation, and yet we are unable to do so. 378. The vigorous action of the muscular structure is dependent upon several causes. In the first place, it requires an active nutrition of the muscles themselves. Firm, plump, and high-colored muscles act with greater force than those which are pale and flabby, even though the size of the latter may be greater. Again, in all the most active animals, a constant supply of oxygen is requisite for muscular vigor. This is conveyed tp the muscles, in Birds and Mammalia, by the blood (56); in Insects, on the other hand, it actually enters the muscular tissue, in the state of atmospheric air (91). The energy of muscular contraction also depends, in a great degree, upon the power of the stimulus which is transmitted to the muscles through the nervous system. This is frequently observed in instances of great nervous excitement ; as when a person is under the influence of violent passion or insanity. A delicate female is thus often a match for three or four strong men, and can even break cords and bands that would hold the most powerful man, in his ordinary state. 379. The sense of fatigue, which comes on after prolonged mus- cular exertion, is really dependent upon a change in the brain, though usually referred by us to the muscles which have been exercised. This fatigue is felt after voluntary motions only, for the very same 375. What is the effect of contraction upon the length and diameter of a muscle? How may this change be observed? 376. Describe the attachments of muscles. At what part is the origin, and at what part the insertion, of a muscle? Where is the origin, and where the insertion, of the large muscle of the arm? What are tendons? What examples are given of tendons of the cylindrical form? 377. How are muscles divided? What is said of involuntary muscles sometimes becoming voluntary, and vice versa? 378. Upon what causes does the vigorous action of muscles depend? How are they supplied with oxygen? What instances are given of great muscular power under nervous excitement? 379. On what does the sense of fatigue depend? Why is fatigue felt only after voluntary motions? Why do Birds and Insects perform long flights without fatigue? ANIMAL MOTION. muscles may be kept in rejlrx action (235, &c.) for a much longer time, without any fatigue being experienced. Thus, \ve never tec] tired of breathing; :md yet a forced voluntary action of the mu of respiration soon causes fatigue. The voluntary use of the muscles of our limbs, in walking or running, soon causes weariness; but sim- ilar muscles are used, in Birds and Insects, for very prolonged flights, without apparent fatigue: and. since the actions of night maybe per- formed after the brain has been removed (243, 266, &c.), these actions may be considered as ot a reflex character, and the absence of fatigue is" thus accounted for. 380. The energy of muscular contraction appears to be greater in Insects, in proportion to their size, than in any other animals. Thus, a Flea will leap sixty times its own length, and move as many times its own weight. The same muscular energy in a man of six feet high, and weighing one hundred and seventy pounds, would enable him to leap to the distance of three hundred and sixty feet, or more than twenty rods, and to move a weight of over ten thousand pounds. A species of Beetles can support a weight equal to at least five hundred limes that of its own body; and another, by the power of its jaws, has been known to gnaw a hole of an inch in diameter in the side of an iron canister in which it had been confined. The rapidity of the movements of Insects is also so great, that the vibrations of the wings, in some species, have been calculated at several hundred, and, in some of the smaller insects, at many thousand, in a second o'f time. THE APPARATUS OP MOVEMENT IN GENERAL. 381. The organs by which are produced the locomotion ( change of place) of animals, their attitudes, and other important actions of a mechanical nature, may be divided into active and passive. The active organs are those which have peculiar vital powers within them- selves, and which exert these powers by giving motion to other parts. To this class, therefore, belong the muscles. The passive organs, on the other hand, are those which perform no action of themselves, which have no power but that of yielding a simply mechanical sup- port, and which, consequently, perform no movements but such as they are made to do by the muscles. Of this kind ai~e the hard parts which form the skeleton or solid frame-work of the body. 382. In the lower tribes of animals, the muscles are all inserted in the soft and flexible membrane which covers the body. By acting upon this membrane, they can change the form of the body in such a manner as to cause it to move, either altogether or in part. This is the case, for example, with the Earth-worm and the Leech, which are furnished with two sets of muscular fibres, one running along the body, and the other passing around it in rings. By the contraction of the former, the two ends are drawn together, so that the body is short- ened; while, by that of the latter, its diameter is lessened, so that it is necessarily lengthened. By these two movements, which take place alternately, the progression of the animal is accomplished; and, by varying the contractions of one part or another, almost any form and direction can be given to this soft and flexible body. 3S3. But in the higher animals, the apparatus of movement con- sists, not only of muscles, but of a frame- work of solid pieces, which serves to augment the precision, force, and extent of the movements, while, at the same time, it determines the general form of the body, and protects the viscera against external forces. This solid frame- work or skeleton is sometimes internal, as in Man and the other ver- tebrated animals, and sometimes external ; forming a shell, as in the lobster and other similar animals. 384. The bones, of which the skeleton of Man is composed, are the hardest parts of the body. They exist, at first, in the state of car- tilage or gristle, which is gradually converted into bone by the deposi- tion of phosphate and carbonate of lime. This process, which is called ossification (or bone-making), commences at certain places in the interior of the cartilage, called points or centres of ossification, and spreads from them to the outer surfaces. 380. What is said of the energy of muscular contraction in insects? What examples are given? What if the muscular energy of a man were equal to that of a flea, in propor- tion to his size? What is said of the rapidity of the movements of insects ? 381. Into what two classes are the organs of locomotion divided ? How do the active differ from the passive organs ! 382. Where are the muscles inserted in the lower animals ? Describe the mode in which the movements of the worm and leech are effected. 383. In the higher animals, of what does the apparatus of movement consist? How does the skeleton of man differ from that of the lobster, in position? 384. In what state do the bones of man originally exist? How is cartilage converted into bone ? 385. In their perfect state, the mixture of earthy and animal matter in the bones renders them hard, without being brittle; and touaih, without being pliable. The earthy matter gives them their strength and firmness, and the animal matter their vitality, growth, and nutri- tion. The animal matter may be separated by soaking a slender bone in a diluted acid, which dissolves the lime, and leaves the bone of the same size and shape as before, but so soft and flexible that, if a long bone, it may be tied in a knot. The earthy portion may also be obtained separately, by exposing the bone to the action of a hot fire, which completely burns out the animal matter, leaving the bone white and brittle, so that it may be easily broken, and even crumbled, into fragments. 386. The bones do not arrive at their perfect state until about the twentieth year. In the bones of children, it has been estimated that the earthy matter constitutes one-half; in adults, four-fifths ; and in old persons, seven-eighths. The bones of young children are, therefore, yielding and flexible. This is a wise provision against the accidents to which children are liable. If they had perfect bones, every fall might break them ; but, as it is, their bones bend to the shock, and again recover their shape, so that a child may fall from a considerable height, without permanent injury, while a similar accident would frac- ture the bones of an adult. In old age, on the contrary, the opposite is the case. The bones become unyielding and brittle, and are exceed- ingly liable to be fractured. A simple slip-down, which a man of thirty would scarcely feel, often breaks the leg of one of seventy. When, as is sometimes the case, the muscles of an old person retain a power and vigor disproportionate to the strength of their bones, the latter may even be fractured by a severe muscular exertion. Thus, there is a well-attested instance of an aged dandy, who broke his arm in attempting to pull on a tight glove. 387. The inside of the bones is generally porous or spongy, being composed of a great number of small cells. This arrangement increases their lightness, while it does not diminish their strength. In the long bones, as those of the thigh and arm, the two finds are usually spongy, while the middle portion, or shaft, is hollow, and contains marrow. The cvlindrical shape of these bones, however, renders them as strong as if they were solid. This fact is owing to the mechanical principle on which they are constructed. The form of the arch is that which best resists pressure, and since the hollow cylindrical shape is formed by two arches set together, it follows that it is best adapted to sustain a heavy weight. This principle may be illustrated, by rolling up a sheet of paper in the form of a cylinder, holding it at both ends, and hanging over the middle a string with a weight attached. It will now be found capable of sustaining a considerable weight ; but if the paper- roll be flattened, so as to destroy its cylindrical form, the same weight will at once break it down in the middle. 388. The different portions of the skeleton are articulated, or united by joints to each other, in such a manner that they can move with more or less freedom. These joints or articulations are of several kinds. Those bones of the skull which surround and protect the brain are very firmly united by what are termed sutures (or seams), which are formed by the interlocking of the jagged edges of one bone into corresponding notches of the adjoining one. So firmly are the bones united in this manner, in the adult, that it is difficult to separate them without breaking away some of the projecting parts. In the skull of the infant, however, the bones are only united by a membranous sub- stance ; and there is a point at the top of the head which is not cov- ered by a bony layer until some months after birth. In other articu- lations, where little or no motion is required, the adjacent surfaces of the bones are united by an intervening layer of cartilage, and also by fibrous membranes which inclose the articulations. The vertebrae, or pieces of the back-bone, are joined in this manner. 389. The proper movable articulations, by which the limbs are con- nected with the trunk, and the different parts of the limbs to each other, are those usually called joints. In these, the surfaces of the adjacent bones are not united in any other way than by the ligaments 385. What are the respective uses of the earthy and animal portions of bones? How may the two portions be separated from each other? 386. When do the bones arrive at their perfect state? What is the proportion of earthy matter in the bones of children? In adults? In old persons? What good purpose is served by the flexibility of the bones of children? What is said of the liability of the bones of old persons to be fractured? 387. What is said of the structure of bones? How is the strength of the cylindrical form of long bones illustrated ? 388. What is said of the articulations ? How are the bones of the skull united? What is said of the firmness of the sutures? What of the infant's skull? How arc the vertebrae united? 389. What are usually called joints? How are they joints formed? What is synovia? ANIMAL MOTION. and muscles which surround them, and they have a free gliding move- ment over each other. They are covered by cartilage, which does not pass from one bone to the other, but forms a thin layer over the end of each, and presents a very smooth surface, which greatly facili- tates the motion of the joint. These surfaces are kept moist by a fluid, termed synovia, secreted by a serous membrane, which forms a closed sac or capsule over them. 390. The beautiful smoothness of the surfaces of the joints, and the manner in which the bones are held together by the muscles and liga- ments, is well seen by examining the knuckle-joint at the lower end of a leg of mutton (before being cooked), and the other joint which con- nects it with the bones at the top. These two joints are examples of the two principal varieties of freely-movable articulations the hinge- joint, and the ball-and-socket joint. In the first of these, the surfaces of the bones are so formed that the movement, though free as regards its extent, is very limited in its direction, being restricted to a back- ward and forward action in the same line, just like that of a common hinge. In the second, the end of one bone is formed into a rounded head or ball ; and this is received into a corresponding socket or cup in the other, the edge of which is usually deepened by cartilage. In this manner, the bone which carries the ball is enabled to move upon the other in any direction, unless checked in other ways. 391. Of the hinge-joint, we have examples in the elbow, the knee, and the joints of the fingers and toes. Of the perfect ball-and-socket joint, we have, in Man, only two examples the shoulder, and the hip. In the former, the socket is much shallower than in the latter; and the motions of the arm are, consequently, more extensive than those of the thigh. The wrist and the ankle-joint are of an intermediate character; the former more resembling the ball-and-socket, and the latter the hinge-joint. 392. All these joints are more or less subject to dislocation, by violence of different kinds. This takes place by the slipping away from each other of the two surfaces, which ought to be in contact. Thus the head of the humerus or arm-bone may slip over the edge of its socket, so as to lie entirely on the outside of it ; and this, in con- sequence of the shallowness of the cup, happens not unfrequently. The head of the thigh-bone, also, may slip out of its socket; but this accident is more rare, on account of the deepness of its cup. The elbow and knee-joints, as also those of the wrists, ankles, fingers, and toes, may be dislocated by the slipping of one surface on the other, either forward, backward, to one side, or to the other. One of the most common dislocations is that of the thumb, the lowest articulation of which has rather the character of the ball-and-socket, with a very shallow cup, than of the hinge-joint. But in proportion to the liability of any joint to dislocation, is usually the ease with which it may be brought into place again. 393. The action of any muscle in producing a change in the posi- tion of a movable bone on which it acts, is determined, in the first place, by the nature of the movement of which the bone is capable ; and in the second, by the direction in which the power of the muscle is applied to it. The contraction of a straight muscle which is attached to a fixed point at one end, and to a movable point at the other, will obviously tend to draw the latter towards the former. Thus, the muscles which bend the fingers, lie in the palm of the hand and on the corresponding side of the fore-arm; while those that straighten the fingers are situated on the opposite side. But we often find that the direction of a muscle's action is changed, by the passing of its tendon through a pulley-like groove or loop; so that it draws the movable bone in a direction different from that of its fixed attach- ment. This is the case, for example, with some of the muscles that bend the toes ; these being situated in the calf of the leg, would draw the toes upward, were it not that their tendons are carried beneath the bones of the heel, working in smooth pulley-like channels hol- lowed out in them ; hence, when the muscle contracts, the tendons draw the ends of the toes towards the heel, and thus bend them. 394. We generally find that even movements of a simple character 390. Where may the construction of the joints be well seen? What is said of the two joints in the leg of mutton? Describe the hinge-joint, and the ball-and-socket joint. 391. What examples are mentioned of these two joints? How does the shoulder-joint differ from that of the hip ? 392. How does dislocation take place in the different joints ? What is one of the most common dislocations? 393. How is the action of a muscle on a bone determined ? What is the obvious action of a straight muscle 7 How is the direction of a muscle's action often changed? What examples are given of this change of direction? Describe the action of the muscles which bend the toes. 394. What is said of the com- bined action of muscles? Describe the action of the various muscles exerted in bending the wrist. are performed by the combined action of several muscles; of which some may be considered as the principal, and others as assistants. Those which are principals in one movement may become assistants in another; and vice versa. Thus, if we wish to bend the wrist directly downward upon the fore-arm, we put in action, not only certain muscles whose tendency would be to produce this movement, but others which, acting by themselves, would produce a different motion. One of these would draw the wrist towards the thumb-side of the fore-arm ; and the other towards the little-finger side ; and they become the principal muscles in these movements respectively ; but when they act together, their several tendencies to draw the wrist to the opposite sides counterbalance one another, and they simply assist the principal muscles in bending the wrist downward upon the fore-arm. 395. Almost every muscle has its antagonist, which performs an action precisely opposite to its own. Thus, by one set of muscles, termed flexors, the joints are bent ; by a contrary set, the extensors, they are straightened. One set of muscles draws the arm or leg away from the central line of the body; another draws the limbs inward. One set, again, closes the jaws; and another opens them. In short, we shall find that probably every muscle in the human body has its antagonist in another muscle, or in some part of it. But we find an economy of muscular substance in some of the lower animals, where parts are to be usually kept in a particular position, and this is only to be changed occasionally and for a short time. Thus the valves of a clam, or an oyster, are kept apart, not by a muscle, but by an elastic ligament; but they are closed, when the animal is alarmed, and wishes to protect itself, by muscular action. In the same man- ner, the sharp claws of the Cat tribe are usually drawn in by an elastic ligament, that their points may not be worn away by rubbing against the ground ; while they are forced outward by the action of a muscle provided for the purpose, when the animal desires to fasten them into its prey. 396. We commonly find that, in order to preserve the necessary form of the animal body, muscles are applied at a great mechanical disadvantage, as regards the exercise of their power; that is, a much larger force is employed than would suffice, if differently applied, to overcome the resistance. But we generally find that in this, as in other forms of lever action, what is lost in power is gained in time ; and thus a very slight change in the length of a muscle is sufficient to produce a considerable movement. 397. The first source of disadvantage results from the direction in which ihe muscle is attached to the bone. This is rarely at right angles to it; and, conse- quently, a considerable part of the power is lost. Thus, if the muscle m (fig. 59), whose force we shall suppose equal to 10, is fixed at right angles to the bone, I, whose ex- tremity, a, is movable upon the point of sup- port, r; its force of contraction will be most advantageously applied to overcome the resistance, and will draw the bone from the position a b into the direction a c, making it traverse a space which we shall also represent by 10. But if this muscle act obliquely on -the bone, in the direction of the line n b for example, it will be quite otherwise ; for it will then tend to draw the bone in the direction b n, and, con- sequently to make it approach the articular surface, r. But as this bears upon an immovable socket, and as the bone can move in no other way than by turning upon the point r, as upon a pivot, the con- traction of the muscle to the same amount as before will carry the bone no further than into the direction a d; three-quarters of the force employed will thus be lost, and the resulting effect will be no more than one-fourth of that which the same power, applied perpen- dicularly to the bone, would have produced. 398. Now, in the animal body, we usually find that the muscles are inserted so obliquely, that their power is applied at a great disadvan- tage ; but this disadvantage is rendered much less than it would have otherwise been, by a very simple contrivance the very enlargement 395. What are antagonist muscles? What is said of flexor and extensor muscles? How are the valves of a clam kept apart, and how closed ? How are the claws of a cat drawn in and forced out? 39fi. What is said of the loss of power in consequence of the mode in which muscles are usually applied? 397. Explain fig. 59, and explain why a muscle fixed at right angles to a bone will exert a greater force than if fixed obliquely. 398. How is the oblique insertion of the muscles rendered less disadvantageous than it would otherwise have been? Kxplain this by reference to figures 60, fil. PLATE X. ORGANS OF MOTION.- -THE BONES. FIGURE 1. FRONT VIEW OF THE HUMAN SKELETON. AT the right half of this figure, the bones are represented in their natural connexions, and divested, of all covering. At the left side, the joints are covered by their ligaments. The outer lines show the form of the body when covered with flesh. THE HEAD. a, The frontal-bone. 6, The parietal-bone, c, The temporal-bone, e, The superior maxillary-bone. /, The malar-bone, g, The nasal bones. A The vomer. t The inferior maxillary-bone, k, The orbits. 1, 1, Sutures. THE TRUNK. 1, 1, The spinal column. 2, The sternum. 3,3, The ribs. 4, The os innominatum, or haunch-bone. 5, The sacrum. THE SUPERIOR EXTREMITIES. 6, The clavicle. 7, Acromion process of the scapula, which articulates with the clavicle. 8, The humerus. 9, The elbow-joint. 10, The radius. 11, The ulna 12, Bones of the carpus. 13, Bones of the metacarpus. 14, The phalanges. THE INFERIOR EXTREMITIES. 15, The hip-joint. 16, The femur, or thigh-bone. 17, The patella. 18, The tibia. 19, The fibula. 20, Bones of the tarsus. 21, Bones of the metatarsus. 22, Bones of the toes. On the left side of the figure, I indicates the ligaments of the shoulder, m, Ligaments of the elbow, n, Ligaments of the wrist. , Ligaments of the hip-joint o, Ligaments of the knee, and tendon of the extensor muscle of the leg. p, Ligaments of the ankle, q, Large vein and artery of the arm. r, Large vein and artery of the leg. FIGURE 2. SKELETON OF THE CAMEL. The black ground in this and the two following figures shows the outline of the form when clothed with flesh. a, The skull, i, The cervical vertebrae, c, The dorsal vertebra. The spinous processes of these vertebrae are much longer and larger than those of man, for the purpose of giving attachment to the strong muscles and ligaments by which the heavy neck and head are supported. d, The lumbar vertebrae, e, The sacrum. /, The caudal vertebrae, which compose the tail, g, The ribs, h, The scapula, i, The humerus. j, The fore-arm. In this part, corresponding to the fore-arm of man, the two bones are united into a single one. k, Bones of the wrist or carpus. I, Bones of the metacarpus or hand, m, The phalanges, n, The femur, or thigh-bone, united to the bones of the pelvis, o, The patella, or knee-pan, p, The tibia; the two bones of the leg being united into one. q, Bones of the tarsus or ankle. r, Bones of the foot and toes. In animals which do not possess fingers, the bones of the fore-arm, wrist, and hand are always few in number. Thus, in the camel, and in all herbivorous quadrupeds, the fore-arm has only one bone ; the wrist, about four ; the hand, only one, with sometimes the rudiment of another ; while some species have two toes, and others only a single one. ( 441, 446, &c.) FIGURE 3. SKELETON OF THE YULTURE. ve, Cervical vertebras, fifteen in number, i, Dorsal and lumbar vertebrae. r, The sacrum, vq, Vertebrae of the tail, st, The breast-bone, or sternum, el, The clavicle. h, The humerus. o, The two bones of the fore-arm, ea, Bones of the wrist, imperfectly developed, ph, Bones of the hand and fingers. /, The thigh-bone, t, The two bones of the leg. ta, The shank or ankle bones. (See 454, 455.) FIGURE 4. SKELETON OF THE PERCH. a, b, First and second dorsal fins, c, The caudal or tail-fin, d, The anal-fin, e, One of the ventral fins, which correspond to the legs. /, One of the pectoral fins, which are analogous to the arms. ( 450 and following.) The spinous processes of the vertebrae are long, and are connected with another set of bones, by which they are continued upward, so as to form the frame-work of the fins which arise from the back. The whole number of vertebrae is forty-two, of which twenty-one are dorsal, and twenty-one caudal or coccygeal. The number of pairs of ribs is the same as that of the dorsal vertebra. ( 416, 422.) I'l, X ANIMAL MOTION. Fig. 61. fig. 62. of the bones at the joints, which is necessary to give them the required extent of surface for working over each other. Thus, let r and o (fig. 60) be two bones connected by a joint; and let the muscle m, which moves the, lower bone upon the upper, be attached r . to the former, at i. Now, as this muscle acts almost pre- cisely in the line of the bones themselves, almost all its power will be expended in drawing the lower bone against the upper. But by the enlargement of the ends of the bones, as seen in fig. 61, the direction of the tendon of the muscle m, is so changed, near its insertion, i, that the con- traction of the muscle will cause the lower bone to turn upon the upper one with comparatively little loss of power. In the knee we find a still greater change of direction effected, by the interposition of a movable bone, the patella or knee- pan, in the substance of the tendon. 399. But the advantage or disadvantage with which the muscles act upon the bones depends, in great degree, upon the distance^ their point of attachment from that of the point of sup- port on which the bone moves, and from the point at which the resist- ance is applied. Every bone acted on by muscles may be regarded as a lever, having its fulcrum or point of support in the joint, its power where the muscle is attached to it, and its weight where the resistance is to ne overcome ; and the distances of the fulcrum from the power and the weight respectively, are termed the two arms of the lever. Now, the relative length of these two arms governs the force which is necessary to overcome a given resistance. Thus, in the Steelyard (fig. 62), the beam is divided into two arms, of unequal length, at the a point of support or fulcrum, a ; at the end of o the short arm, r, hangs the body whose down- J ward pressure we wish to determine ; and on r*""? 1 - i"" i ~3>' the other, p, there slides a weight, which will balance a greater or less amount of pressure at the opposite extremity, r, according as it is made to hang from a point which is more distant from the fulcrum or nearer to it; that is, according as the length of the power-arm of the lever is increased or diminished, that of the weight-arm remaining the same. 400. Now, in order that there may be an equilibrium, or balancing between the power and the weight, it is necessary that they should be inversely proportional to the lengths of their respective arms; that is, the power multiplied by the length of its arm, should be always equal to the weight multiplied by the length of its arm. Thus, to balance a certain resistance, r (fig. 63), equal to 10, and applied at the end of a lever, a b, whose length we shall call 20, it is necessary that a force, p, applied at the same point, and consequently at the same distance from the fulcrum, a, should also be equal to 10; but, if the power be applied at the point, c, which is at only half the distance from the fulcrum, a, it must be doubled in amount, or equal to 20 since it must be sufficient, when multiplied by its distance, 10, from the fulcrum, to make 200, which is the pro- duct of the resistance, 10, and its distance from the fulcrum, 20 ; and, in like manner, if the power be applied at d, where its distance from the fulcrum is only 2, its amount must be 100, in order that its pro- duct, with the distance at which it is applied, may be equal to 200. Hence, when a muscle is applied near the ful- crum, while the resistance is at a distance from it, its force must be proportionably greater. 401. But this " arrangement greatly increases the rapidity of the motion, which is the consequence of the muscu- lar action. For, let us suppose that the muscle, p (fig. 64), acts upon the lever, $399. Upon what circumstances does the advantage or disadvantage with which the muscles act, depend ? Regarding the bone as a lerer, where would be its fulcrum, its power, its w^ght? What are-the two arms of the lever? Explain, by reference to fig. 62, how the relative length of these two arms governs the force necessary to overcome a given resist- ance ? 400. What is necessary, in order that the power and weight may balance each other? Explain this principle by reference to fig. 63. What deduction is drawn from this in regard to a muscle? 401. Explain fig. 64, and show how, in the case of the muscles, what is lost in power is gained in time. Vig. 63. a Si- Fig. 65. ar,'m such a manner that its point of insertion, c, traverses a space equal to 5 in one second ; the extremity, r, of the lever will traverse a space equal to 25 in the same time, its distance from the fulcrum, a, being five times as great as that of the point c from the fulcrum. Hence, although, to raise a given weight at r, a power more than five times its amount must be applied at c, that power will raise the weight through a space five times as great as that through wliieh itself passes in the same time. Thus, what is lost in power is gained in time; and the shortening of a muscle, small in amount, but effected with sufficient power, causes the raising of a weight through a considerable space. 402. We shall find that this is the case in regard to most of the mus- cular actions in the ani- mal economy. Thus the fore-arm is bent upon the arm by a muscle, d (fig. 65), which arises from the top of the latter, and which is inserted at e, at a short distance from the elbow-joint. Hence its contraction to a very slight extent will raise the hand through a considerable space ; but a proportional increase in its power will be required to overcome any resisting force in the hand. The arm is straightened again by an antagonist muscle,/, which lies on the back of the arm, and which is attached to a short projection made by one of the bones of the fore-arm behind the elbow. This muscle also acts at a similar disadvantage in regard to power, and advantage in point of time ; in consequence of its point of attachment being so near to the fulcrum. DESCRIPTION OP THE MOTOR APPARATUS OF MAN. 403. The skeleton of Man (plates 10, 11) is formed by the union of about two hundred and fifty bones, and is divided, like the body, into head, trunk, and members. The bones of these parts will now be separately described. 404. The Head is composed of two parts, the cranium or skull, and the face. The cranium is a bony case, of oval form, occupying the upper and back part of the head, and serving for the protection of the brain, which is lodged in its cavity. Tts walls are made up of eight bones : the frontal, a, in the region of the forehead ; the two parietal bones, b, b, which occupy the top and sides of the skull ; the two tem- poral bones, c,c, which form the walls of the temporal region; the occipital-bone, d, at the back of the head ; and the sphenoid, and the ethmoid, which assist in forming the floor of the cavity. 405. These bones are firmly united to each other by sutures, the character of which varies in different parts of the cranium, so that they are the better able to resist external violence. Thus, a blow upon the top of the arch formed by the parietal bones, will tend to separate them from each other and from the frontal-bone, and to force asunder their lower borders. Both these effects are resisted, by the peculiarity of the suture which unites different parts of the parietal- bone to its neighbors. For, at the top of the skull, the bones are firmly held together by the interlocking of the projections of each ; while the lower edge of the parietal-bone is prevented from being driven out- ward by the overlapping edge of the temporal bones, which form, as it were, a buttress to the arch. This same contrivance prevents the temporal-bone from being driven inward by a blow on the side of the head, as it might have otherwise been. 406. In the base or floor of the cavity of the cranium are seen a number of apertures, which serve for the passage of the blood-vessels that supply the brain, and of the nerves that issue from it. One of these apertures, much larger than the rest, and situated in the occip- ital-bone, serves for the passage of the spinal cord; and on each side of this aperture there is a large bony projection from the under sur- face, termed the condyle, by which the skull rests upon the vertebral column, and is enabled to move forward or backward upon it. The $ 402. Illustrate this principle still farther, by referring to fig. 65, and explaining the mode in which the fore-arm is bent and straightened. 403. Of how many bones is the humnn skeleton composed, and how is it divided? 404. What are the two parts of the head >. Describe the cranium. Give the names and situations of the cranial bones. 405. How are these bones united, and how are they enabled to resist the effects of blows ? 40fi. Describe the apertures of the cranium, the condyles, and the manner in which the head ia balanced and kept upright on its pivot. Where is thie joint situated in other animals] GS ANIMAL MOTION. head is nearly balanced upon this pivot; nevertheless, the portion situated in front of the joint is more heavy than that which is situated behind it, and is, in consequence, not altogether counterpoised by the latter. Hence the muscles, which, arising from the back, and being attached to the occipital-bone, tend to draw the head backward, and thus to keep it upright, are more numerous and powerful than those which are situated in front of the vertebral column, and tend to draw the head downward and forward ; and when the former are relaxed, as in a person sleeping upright, the head has a tendency to fall for- ward upon the chest. In no other animal is this joint situated so far forward as in Man. As we descend the scale, we find it nearer and nearer to the back of the skull ; and, consequently, the whole weight of the head bears, not directly upon the spine, but upon the muscles and ligaments by which it is attached to the vertebral column. 407. On each side of the base of the cranium we observe a large rounded projection, termed the mastoid. To this projection (which we feel behind the lower part of the ear) is attached, on either side, a powerful muscle, which passes downward and towards the central line ; so that the two muscles nearly meet at the bottom of the neck, where they are attached to the upper edge of the breast-bone. These muscles, acting together, serve to draw the head forward ; but either of them, acting separately, will turn it to one side or the other. In front of these two projections of the skull we notice the opening of the external ear, which, like the different chambers of the internal ear, is excavated in a portion of the temporal-bone, which is termed petrous, from its very dense and stony character. 408. The face is formed by the union of fourteen bones; and pre- sents five large cavities, which serve for the lodgment and protection of the organs of sight, smell, and taste. All the bones of the face, with the exception of the lower jaw, are completely immovable, and are firmly united to each other and to the bones of the cranium. The two principal are the superior maxillary, e, which form nearly the whole of the upper jaw, and which are connected with the frontal- bone, in such a manner as to contribute to the formation of the orbital cavities in which the eye is lodged, and of the nasal cavities which form the interior of the nose ; they also constitute the front of the roof of the mouth ; on the sides of the face, they articulate with the malar or cheek bones,/; and behind, with the palate bones, which form the back part of the roof of the mouth. These, in their turn, are united to the sphenoid. 409. The orbits, as we have already seen (360), are two deep cavities, of a conical form the base of the cone being directed for- ward, and the apex, or point of it, backward; the roof of these cavi- ties is formed by a portion of the frontal-bone, and their floor chiefly by the superior maxillary. Their inside wall is formed by the ethmoid- bone, and by the small bone termed the lachrymal, in which is the canal for the passage of the tears into the nose (361) ; and the out- side wall is composed of part of the cheek-bone, and of a portion of the sphenoid ; the latter also bounding the cavity at its deepest part, and containing the apertures which serve for the passage of the optic and other nerves that enter the orbit from the cranium. In the roof of the orbit, on its outer side, there is a broad shallow pit or depres- sion, in which the lachrymal gland is lodged. 410. The greater part of the nose is formed by cartilages; so that, in the bony skull, the anterior opening of the nasal cavity is very large ; and the bony portion of the nose, formed by the two small bones termed nasal, projects but slightly. The nasal cavity, divided in the middle by a vertical partition, into two fossce, or excavations, is very extensive; at the upper part, it is hollowed out in the ethmoid- bone, the whole interior of which is made up of large cells ; its floor is formed by the arch of the palate, which separates it from the mouth ; behind, it extends as far as the back of the mouth, and communicates with the pharynx by two apertures termed the posterior nares. The partition between the fossa3 is formed at the upper part by a plate that projects downward from the ethmoid-bone, and at the lower by a dis- tinct bone called the vomer (or ploughshare), from its peculiar form ; to the front edge of this last is attached a cartilage, which continues the partition forward into the soft projecting portion of the nose. 411. It is through the thin horizontal plate of the ethmoid-bone, $407. Where is the mastoid process, and what muscles are attached to it? 408. How many bones compose the face 1 How many cavities has the face, and what do they con- tain ? Describe the superior maxillary, and the malar bones. 409. Describe the formation of the orbits. 410. Describe the nasal fossffi. How is the partition between them formed? Refer to plate 10. 411. Describe the cribriform plate of the ethmoid. Why is it so called? What are the sinuses, and the spongy-bones? which separates the nasal cavity from that of the skull, that the olfac- tory nerves make their way out from the former into the latter; they descend in numerous branches, for the passage of which, through the roof of the nose, this plate is perforated by a number of small aper- tures, which give it a sieve-like aspect; whence it is called the cribri- form plate (from the Latin, cribrum, a sieve) of the ethmoid. The cavity of the nose is further extended, by its connexion with the sinuses or excavations that exist in the lower part of the frontal-bone (where they sometimes cause a considerable projection above the eyes), in the superior maxillary bones, and in the sphenoid. The sur- face of the mucous membrane which lines it, and on which its nerves are distributed ( 317), is also extended, by being carried over a set of bones, termed spongy bones, which hang, as it were, from the side- walls of the cavity. 412. It is in the superior maxillary-bone that all the teeth of the upper jaw are implanted in Man; but, in the infant, this bone is com- posed of several pieces ; and one of these .pieces, termed the inter- tnaxillary-bone, remains permanently separate in most fjf the lower animals. The lower jaw of adult man, also, is composed but of a single piece; though this is divided in the infant on the central line, and the two halves remain separate in many of the lower animals. This bone has a general resemblance, in form, to a horse-shoe with its extremities turned up considerably. It is articulated with the tem- poral bones by a condyle or projecting head, with which each of these extremities is furnished ; and this head is received into what is termed the glenoid cavity on the under side of the temporal-bone. In front of the condyle is another projection, or process, termed the coronoid, which serves for the attachment of one of the principal muscles that raise the jaw. These muscles are all attached near the angle of the jaw (or the point at which it bends upward), and they consequently act at a small distance from its fulcrum, while the resistance is applied at the furthest point. We are continually reminded of the loss of mechanical power which results from this, by our inability to exercise the same force with our front teeth, that we can employ with the back. Thus, when we wish to crack a nut, or to crush any hard substance between the teeth, we almost instinctively carry it to the back of the jaws, so as to be nearer the joint, and thus to receive more of the power of the muscle. 413. The general arrangement of the chief muscles of the face is seen in plate 12. The largest is the temporal muscle, the fibres of which arise from an extensive surface of the parietal and temporal bones, and then converge or approach each other, passing under the bony arch or zygoma (which is partly formed by a process from the temporal-bone, and partly by the malar or cheek-bone), to be attached to the coronoid process of the lower jaw. This muscle is of extraor- dinary power in those beasts of prey which lift and drag heavy car- cases in their jaws; and in those which (like the hya?na) obtain their support by crushing the bones which others have left. It is assisted by the masseter muscle, which passes from the zygomatic arch and cheek-bone to the angle of the lower jaw, and also by other muscles. Besides these, the figure shows the ring-like muscle or sphincter which surrounds the opening of the eye, and serves, by its contraction, to close the lids ; and also the similar muscle which surrounds the mouth and draws together the lips. The antagonists ta these are several small muscles, which form the fleshy part of the face, and pro- duce the various changes by which its expression is given. These muscles are more numerous in Man and the Monkey tribe than in any other animals. 414. Besides the twenty-two bones of which the skull is properly composed, we may reckon as belonging to it the four small bones which form part of the apparatus of hearing ( 333) ; and also the hyoid-bone, which lies at the root of the tongue and at the top of the larynx. This last bone, in Man and the Mammalia generally, is con- nected with other parts of the skeleton only by ligaments and mus- cles; but in Birds it is connected with the temporal-bone, on each side, by a set of bony pieces jointed together like links in a chain. 415. The most important part of the Trunk, and even of the whole skeleton that which serves to sustain the rest, and which varies the least in the different classes of Vertebrated animals is the spinal or vertebral column, commonly called the back-bone. In Man, it con- sists of thirty-three pieces, called vertebrce, which are arranged into 412. What is said of the superior maxillary -bone? Describe the lower jaw-bone, its condyles, and the muscles which move it. 413. Refer to plate 12, and point out and describe the muscles of the face. 414. What other bones belong to the skull? 415. What number of vertebra compose the spinal column? How are they divided? ANIMAL MOTION. five divisions: 1. The cervical vertebras, or vertebrae of the neck, of which there are seven; 2. The dorsal vertebrae, or vertebrae of the back, of which there are twelve; 3. The lumbar vertebras, or verte- brae of the loins, of which there are five ; 4. The sacral vertebrae, of which also there are five; and 5. The coccygeal vertebrae, of which there are four.* All these vertebrae are separate at the time of birth; but the five sacral vertebrae are soon afterwards united into one piece, forming the bone which is termed the sacrum; and the coccygeal vertebrae are also commonly united into one piece, the coccyx, which is not unfrequently united, in old age, to the sacrum. In old persons, too, it is not uncommon for the lumbar vertebrae to be united together by bony matter deposited in their cartilages and ligaments. 416. The dorsal vertebrae are distinguished from the cervical and lumbar, as being those to which the ribs are attached. It is remark- able, that the number of the cervical vertebrae should be the same in all the Mammalia: the long-necked Giraffe having only seven; and the Whale, whose head seems to be joined to its body without the intervention of any neck, also having seven cervical vertebrae, although they are almost as thin as a sheet of paper. It is owing to the small number of joints in its neck, that the movements of the head of the Giraffe are far less graceful than those of the Swan and other long-necked Birds, in whom the number of cervical vertebrae is much greater. The following table shows the number of vertebrae in ani- mals of different groups : Mammalia. Cervical. Dorsal. Lumbar. Sacral. Coccygeal. Total. Man, - 7 12 5 5 4 33 Long-tailed Monkey, 7 12 7 3 31 60 Lion, - 7 13 7 3 26 56 Long-tailed Opossum, 7 16 6 2 36 64 Long-tailed Ant-Eater, 7 16 3 6 40 72 Elephant, 7 20 3 4 27 61 Giraffe, - 7 14 5 4 18 48 Whale, - 7 15 9 1 27 59 Birds. Vulture, - - - 15 7 13 6 41 Swallow, 13 7 10 7 37 Turkey, - 14 7 15 6 42 Ostrich, - 18 9 19 9 55 Crane, - - - 17 10 15 6 48 Swan, - 23 11 1C 8 58 Beptiles. Tortoise, 9 10 3 20 42 Monitor (Lizard), 6 21 2 2 115 146 Python (Bon), - 320 102 422 Rattle-Snake, 171 36 207 Land Salamander, 1 14 1 26 44 Fishes. Perch, --- 21 21 42 Mackerel, \. 15 16 31 Salmon, 34 22 56 Cod, 19 34 53 Conger Eel, 60 102 162 Electric Eel, 236 Shark, - 95 270 365 We see, from the above table, that it is by the multiplication of the coccygeal vertebrae, that the tail is prolonged in those animals which possess it. In fact, it is only in Man, and in those of the Ape tribe which approach nearest to him, that the number is as low as four. 417. The essential character of the vertebra; consists in being per- forated by an aperture, which, when several vertebrae are united together, forms a continuous tube or canal for the lodgment of the spinal cord. This character is usually lost, however, in the coccygeal vertebrae; which are so much contracted and simplified, as to contain no aperture. The purpose of the division of the spinal column into so large a number of separate bones, is obviously to allow of con- siderable freedom of motion, by a slight change of place in the indi- vidual parts; while any sudden -bend, which would be injurious to the spinal cord, is avoided. Each vertebra consists of a solid body (plate 11, fig. 4), which is situated in front of the spinal canal, in Man, but below it, in animals whose back has a horizontal position, and which serves to give solidity to the structure and of processes or pro- 41 6. IIW are the dorsal vertebra! distinguished from the others ? What is remarkable in regard to the cervical vertebras! Why are the movements of the head of the Giraffe more awkward than those of the Swan? State the number of vertebra? in some of the animals in the table. How is the tail of animals prolonged? 417. In what does the essen tial character of vertebra consist? Which vertebras possess no aperture? Why is the spinal column divided into so large a number of bones? Describe the parts of each ver- tebar, referring to plate 11, figs. 4, 5. How are the bodies of the vertebras united to each other ' 18 jections, that serve to form the spinal canal, and to unite the vertebrae to each other. In Man, and other warm-blooded animals, the two surfaces of the body are nearly flat, and are parallel to each other; and they are united to the corresponding surfaces of the neighboring vertebrae by a disc of fibro-cartilage, which extends through the whole spare that intervenes between them, and which, being firmly adherent to both, prevents them from being far separated from each other. 418. But, in Reptiles and Fishes, a different plan is adopted. In the animals of the former class, particularly in Serpents, we find one surface of each vertebra convex or projecting, and the other concave or hollowed out; and the convex surface of each vertebra fits into the concave surface of the next, in such a manner that the whole spinal column becomes a series of ball-and-socket joints, and is thus endowed with that flexibility which is essential to the peculiar move- ments of these animals. In Fishes, both surfaces are concave; and between each vertebra there is interposed a bag containing fluid, and having two convex surfaces, over which those of the vertebrae can freely play. Extreme facility of movement is thus given to the spinal column ; but its strength is proportionally diminished. It is to be remembered, however, that strength is not required in the bony frame-work of animals whose bodies, instead of being supported upon four fixed points, are buoyed up in every part by a liquid of nearly the same density with themselves. The extreme flexibility of the spine of Fishes enables them to propel their bodies by the movements of the hinder portion and tail, from side to side ; their members, or pectoral and ventral fins (plate 10, fig. 4) being but little used, except for influencing the direction of their motion. And thus we see that in fishes, as in the Leech and Earth-worm, the propulsion of the body being accomplished by the movements of the trunk itself, its skeleton (internal in the one case, external in the other) is left in the soft con- dition, which it has in all at an early period ; while in the higher classes Birds and Insects, for example it undergoes great consoli- dation, its various pieces being so knit together as to make the trunk almost immovable; the extremities being so developed, and being furnished with muscles so powerful, that the function of locomotion is entirely committed to them. 419. This knitting together is partly accomplished by means of projections or processes from the several vertebrae, which are united to one another by muscles and ligaments. Of these processes there are seven in Man from each vertebra. One of these, termed the spinous process (b, plate 11, fig. 4), projects directly backward ; and thus is formed the prominent ridge on the back, in which the ends of these projections can be distinguished. The spinous processes serve, in Man, to give attachment to the muscles, by which the trunk and head are kept erect; in Animals whose spine is horizontal (plate 10, fig. 2, as the Camel), they are generally much longer, in order to give firm attachment to the muscles and ligaments which support the head; and in Fishes they are greatly prolonged, in order to increase the sur- face, by the stroke of which from side to side the body is propelled through the water. On each side of the vertebra is a process, which is called transverse; this serves for the attachment of the ribs to the vertebra. And lastly, from the upper and under side of each verte- bra, two articulating processes project, which lock against each other in such a manner as to prevent the movements of the vertebrae from being carried to an injurious extent. These processes are peculiarly long in Birds, where they almost completely check the movements of the dorsal vertebrae ; thereby giving to the trunk that firmness which is required for the attachment of the muscles of the wings. The por- tions of bone which pass backward from the body to the transverse processes, and form the side wall of the spinal canal, are called the arches of the vertebrae. These are the parts first formed. On the under edge of each there is a notch which corresponds with one in the upper side of the next; in such a manner that, when two verte- bras are placed together, a complete foramen or aperture is formed, which serves for the passage of the nerves that are given off' from the spinal cord. 420. The vertebral column of Man is disposed in a double curve, 418. How are they united in Reptiles, and how does this mode of union increase the flexibility of those animals! How is the vertebral column of Fishes constructed, and why is its strength sacrificed to its flexibility? Why is the skeleton left soft in the lower ani- mals, and why is it consolidated in the higher? 419. Describe the processes of the verte- bras, and their uses. Why are the spinous processes longer in the Camel than in Man? Why are they prolonged in Fishes? Where are the articulating processes situated, and what is their use? What peculiarity do they possess in Birds! Describe the arches and foramina of the vertebras. 420. Why is the spinal column of Man disposed in a double curve ! Why is a man taller in the morning than at night? 70 ANIMAL MOTION. as seen in plate 6, the effect of this is to diminish the shock that would be produced by a sudden jar such as when a man jumps from a height upon his feet. If the vertebral column had been quite straight, this jar would have been propagated directly upward from the pelvis to the head, and would have produced very injurious effects upon the brain ; but by means of the double curvature, and the elasticity of the ligaments, &c., which hold together the vertebrae, it is chiefly expended in increasing, for a moment, the curves of the spine, which thus acts the part of a spring. The constant pressure of the head and upper part of the trunk has a tendency to increase these curves permanently, and thus to diminish the height of the body. The elasticity of the intervertebral substance, however, causes it to recover, during the time when the body is in the horizontal posture, the form it had lost by pressure in the upright position ; and thus a man is taller, by half an inch or more, when he rises in the morning, than he was when he lay down the night before. 421. The first vertebra of the neck, termed the atlas, is much more movable than the rest, and differs considerably from them in its form. It is destitute of body; but it has a broad smooth surface on either side, on which rest the condyles of the occipital-bone of the skull, in such a manner that the head is free to nod backward and forward. The atlas itself turns upon a sort of pivot, formed by an upward pro- jection from the next vertebra, which is termed the axis ; this pro- jection, called, from its form, the processus dentatus (or tooth-like process), occupies the place of the body of the atlas ; and, by the rotation of the atlas around it, the movements of the head from side to side are accomplished. Wherever great freedom of motion is per- mitted, displacement or dislocation is necessarily more easy; and, accordingly, we find that the atlas and axis can be more easily sepa- rated from each other than can any other two vertebrae. This dislo- cation may be produced by violence of different kinds; thus, if the head be suddenly forced forward, while the neck is held back, the tooth of the axis may be caused to press against the spinal cord, and thus to interrupt or completely check its functions. Or, again, if the weight of the body be suspended from the head, and especially if it be thrown upon it with a jerk, the two vertebrae are likely to be drag- ged asunder, and the spinal cord to be stretched or broken. This is sometimes the immediate cause of death in hanging; and it has not unfrequently occurred when children have been held in the air, by the hands applied to the head a thing often done in play, but of which the extreme danger should prevent its ever being practised. Any serious injury of the spinal cord in this region must be immediately fatal, for the reason formerly stated (272), that it causes the suspension of the motions of respiration. 422. The number of the ribs which are attached to the bodies and transverse processes of the dorsal vertebrae, is, in the human species, twelve on each side.* The number in different animals may be judged of by that of the dorsal vertebrae in the table already given (416); since it is the attachment of the ribs that makes the essential difference between the dorsal vertebras and the cervical or lumbar. The other extremity of each rib is connected with a cartilage, which is a sort of continuation of it; in Birds, the cartilages of the ribs are ossified or converted into bone. The cartilages of the first seven ribs (in Man), which are termed the true ribs, are united to the sternum or breast-bone, which forms the front wall of the thorax. The car- tilages of the five lower ribs are not directly connected with this, and they are hence called false ribs; those of three of them, however, are connected with the cartilage of the seventh rib; and the other two ribs, being completely unattached at their anterior ends, are termed floating ribs. The sternum or breast-bone is flat, and of simple form in Man; but it is much larger in many other animals. In those which have need of great strength in the upper limbs, such as Birds. Bats, and Moles, it is not only increased in breadth, but is furnished with a projecting keel or ridge, for the attachment of powerful muscles. In the Turtle tribe, on the contrary, it is very much extended on the sides, so as to afford, with the ribs, a complete protection to the con- tained parts. 423. We have next to consider the structure of the members or 421. Describe the atlas. The axis, and its tooth-like process. How may the atlas and axis be separated, and what are the effects of this dislocation? What is said of the danger of raising children by the head? 422. What is the number of the ribs in Man? How are they divided, and how attached? Describe the sternum. How does this bone differ in form in different animals, and why? 423. What is said of the structure of the memliers? What of the frame-work by which they are connected with the trunk? * It is scarcely nucesaury to state, that the common notion respecting the deficiency of a rib on one side of the body of Man is :i popular error. appendages, which are attached to this central frame-work. These are spoken of as superior and inferior, when treating of Man, whose erect posture places one pair above the other. But when the ordi- nary Quadrupeds are alluded to, they are termed anterior and poste- rior, one pair being in front of the o'ther. Each member consists of a set of movable bones, which serve as levers; but* the socket in which the first of these works, is formed by a bony frame- work, which is connected more or less closely with the spinal column. This frame- work, in the upper extremity, consists of the Scapula or blade-bone, and the Clavicle or collar-bone. In the lower extremity, it is formed by a set of bones, the union of which with the sacrum completes the Pelvis or basin, which is found at the bottom of the spinal column. 424. The Scapula is a large flat bone, which occupies the upper and external* part of the back. Its form is somewhat triangular; and at its upper and external angle, is a broad but shallow cavity, destined to receive the head of the humerus or arm-bone. Above this cavity is a large projection, termed the acromion process, which is united by. ligaments, &c., with the external end of the clavicle, and thus forms the bony eminence that we feel at the top of the shoulder. A little internally to this we find another process, the coracoid, which only serves in Man for the attachment of certain muscles, but which in Birds is developed into a distinct bone. The back surface of the scapula is divided into two by a projecting ridge or keel, which gives a more extensive and firmer attachment to the muscles that arise from it. The scapula is never deficient in animals that possess a superior extremity, though sometimes it is very narrow. The muscles attached to it are chiefly those which draw the arm upward, and which turn it on its axis. In Man, their actions are very numerous and varied; but in animals that only use their extremities for giving motion to the body, the muscular apparatus is much simpler, and the scapula is nar- rower. This is particularly the case in Birds, the raising of whose wings in flight is an action that requires very little power, though for their depression or pulling-down, great muscular force is needed. 425. The Clavicle is a rounded bone, attached at one extremity to the acromion process of the scapula, and at the other to the top of the' sternum. Its principal use is to keep the shoulders separate ; and we accordingly find it strongest in those animals, the actions of whose superior extremities tend to draw them together ; while it is compara- tively weak, or altogether deficient, in the animals, the actions of whose limbs naturally tend to keep them asunder. Thus, in Birds, the violent drawing down of whose wings, in flight, would tend to bring the shoulders together, if they were not prevented, there is not only a strong clavicle, but usually a second bone, having a similar function ; while, in the horse and other animals, the bearing of whose weight on their fore-legs would tend rather to separate the shoulders than to bring them together, the clavicle is deficient. 426. The scapula is connected with the central frame- work of the skeleton by various muscles which pass towards it from the spinal column and ribs ; and which serve alike to fix it, and to assist in sus- taining the weight which it sometimes has to bear. In Man. these are numerous, and their actions are various; since the scapula is left very movable in him, that the actions of the arm may be more free. In Quadrupeds, it is generally more fixed; and the trunk is slung from it, as it were, by a muscle (the serratus magnus) of moderate thickness in Man,*but in these animals of great strength, which passes from the scapula to be attached to the ribs. 427. The superior or anterior member itself is divided into three principal portions the arm, fore-arm, and hand. The arm is sup- ported by a single long and cylindrical bone, which is called the humerus ; this has a large rounded head, which is received into the glenoid cavity of the scapula; and, at the bottom, it assists in forming the hinge-joint of the elbow. The muscles which move it are for the most part attached to its upper third ; and the chief of them are the pectoralis major, which rises from the sternum and cartilages of the ribs, and consequently draws the arm forward, inward, and down- ward (when it is raised) ; the latissimus dorsi, which rises from the spinal column and hinder part of the ribs, and consequently draws the arm backward, inward, and downward; and the deltoid, which arises from the upper edge of the clavicle, and from the ridge of the scapula, 424. Describe the scapula and its processes? What is said of the muscles attached to it? 425. Describe the clavicle and its uses. 42fi. What is said of llie muscles which con- nect the scapula with the trunk ? 427. What are the three portions of the superior mem- ber? Describe the bone of the arm, and the muscles attached to it What are the actions of the pectoralis major, the latissimus dorsi, and the deltoid muscles? * The term eitemal is continually used, in Anatomy, to describe the parts furthest removed from the central or median line of the body. PLATE XI. ORGANS OF MOTION. --THE BONES. FIGURE 1. POSTERIOR VIEW OF THE HUMAN SKELETON. THE HEAD. a. The frontal-bone. 6, b, The parietal bones, c. The left temporal-bone, d, The occipital-bone. THE TRUNK. 1, 1, 1, Spinous processes of the vertebrae. 1, t, t, Transverse processes of the vertebrae. 2, 2, The ribs. 3, The os innominatum, showing its three parts, viz: n, the ilium; 6, the ischium; c, the os pubis. 4, The sacrum. 5, The coccyx. THE SUPERIOR EXTREMITIES. 6, The scapula. 7, Acromion process of the scapula. 8, The clavicle. 9, Head of the Humerus placed in the glenoid cavity of the scapula. 10, Shaft of the humerus. 11, 12, Internal and external condyles of the humerus. 13, The olecranon process of the ulna, which articulates with the pulley-like surface at the lower end of the humerus, forming the elbow-joint. 14, Shaft of the ulna. 15, Shaft of the radius. 16, Lower extremity of the radius, which articulates with the bones of the carpus. 17, Bones of the carpus. The first row consists of four bones, viz: a, the scaphoid-bone; 4, the semi-lunar-bone; c, the cuneiform-bone; d, the pisiform-bone. The second row consists of four bones, viz: e, The trapezium. /, The trapezoid-bone. g, The os magnum. A, The unciform-bone. 18, 18, Bones of the metacarpus 19, 19 20, 20, 21, 21, First, second, and third ranges of finger bones. THE INFERIOR EXTREMITIES. 22, Head of the femur, placed in the acetabulum or cotyloid cavity of the haunch-bone. 23, 24, Projections called trochanter major and minor, to which the muscles of the hip are attached. 25, Shaft of the femur. 26, 27, External and internal condyles of the femur. 28, Upper extremity of the tibia, which articulates with the femur. 29, Shaft of the tibia. 30, The internal malleolus, a projection of the tibia which forms the inner ankle. 31, The fibula. 32. External malleolus, a projec- tion of the fibula which forms the outer ankle. 33, Bones of the tarsus. 34, Bones of the metatarsus. 35, Bones of the toes. The ligaments of the various joints are seen on the left side of this figure. FIGURE 2. GROUP OF BONES. The bones here represented are so situated that they cannot be distinctly seen in the large figure, a, The sphenoid-bone. 6, The malar-bone. c, The ethnoid bone. d, One of the spongy bones, e, The vomer. /, The lachrymal-bone, g, The nasal bones. A, The palate-bone. FIGURE 3. THE OCCIPITAL-BONE, DETACHED FROM THE SKULL, o, The foramen magnum or occipital hole, which gives passage to the spinal cord, b, b, Condyles, which rest upon the vertebral column. (See 406). FIGURE 4. A DORSAL VERTEBRA. o, The body of the vertebra. 6, The spinal foramen, c, c, Articulating processes, d, d, Transverse processes, e, Spinous processes. FIGURE fi. A LUMBAR VERTEBBA, TO SHOW THE GREATER THICKNESS AND STRENGTH OF ITS BODY AND PROCESSES. FIGURE 6. THE LOWER JAW. a, The condyle, which articulates with the temporal-bone. 6, The coronoid process, c, The ramus. ( 412). FIGURE 7. THE BONES OF THE FOOT, SEEN UPON THE UPPER SURFACE. a, The os calcis, or heel-bone, b, The astragalus, which articulates with the lower end of the tibia, c, The cuboid-bone, d, The scaphoid-bone, e, e, e, Cuneiform, bones. FIGURE 8. THE KNEE-JOINT, WITH ITS INTERNAL LIGAMENTS, THE PATELLA BEING REMOVED. a, a, The condyles of the femur, covered with cartilage, b, b, The two semi-lunar cartilages, which form cup-shaped depressions for the reception for the condyles. c, The anterior crucial ligament which passes from the tibia to the femur. FIGURE 9. A SECTION OF THE HIP-JOINT. a, The head of the femur. 6, b, The capsular ligament, embracing the cavity of the hip-bone and the head of the femur, and keeping both bones firmly together, e, A round ligament attached to the inside of the cavity and to the head of the femur. FIGURES 10 AND 11. These figures are designed to show the minute structure of bone. FIGUBE 10 represents a cross-section of bone, highly magnified, a, a, Orifices of small tubes, called, from their discoverer, Haversian canals. They usually run in the direction of the length of the bone, the membrane lining the hollow of which is prolonged into their canals. This membrane contains innumerable small blood-vessels, and the interior of the bone is thus supplied with blood. Around each of the Haversian canals are seen concentric circles of small dark spots, which are found to be flattened cavities or bone-cells, from which proceed numerous minute tubules. These open into the sides of the Haversian canals, and communicate from one bone-cell to another, thus transmitting the nourishment with which they are supplied by the blood-vessels throughout the substance of the bone. FIGURE 11, represents a longitudinal section of bone, highly magnified, showing the Haversian canals seen lengthwise, their connexion with each other, and the direction of thp bone-cells. These figures are from Hassall's Microscopic Anatomy. !'L XI Ulhc.fi Entxnxi :ifnnbn.74. FRONT VIEW OFTiia LARYNX. The interior wall is marked by the lines o, n, b,b; /i, inferior litramrnt* of the glottis, or vocal cords ; Is, superior ligaments. The other references as before. Fig. 7il. HUMAN LARYNX, VIEWED SIDEWAYS. k, hy oid-bone ; I point of origin of muscles of the tongue ; (, thyroid cartilage ; a. pro- jection in front, commonly called Adam's Apple ; e, cri- coid cartilage : tr, trachea ; 0, posterior side of the la- rynx, in contact with the oesophagus. 463. To these arytenoid cartilages are attached two ligaments, of elastic fibrous substance, which pass forward, to be attached to the front of the thyroid cartilage, where they meet in the same point. These are the instruments concerned in the production of sound, and also in the regulation of the aperture by which air passes into the trachea; and they are usually termed the vocal cords or ligaments. By the meeting of these ligaments in front, and their separation behind, the aperture has the form of a V; but it may be narrowed, by the drawing together of the arytenoid cartilages, until the two vocal liga- ments touch each other along their whole length, and the aperture is completely closed. In this manner, the amount of air permitted to pass through the larynx is regulated, and a protection is afforded against the entrance of solid substances. An additional guard is afforded, by the doubling of the lining membrane in such a manner as to form a second pair of folds (I, s, fig. 74) above the preceding; and over the space between these (which is much wider than that between the vocal cords) there is a valve-like flap, the epiglottis (e, fig. 73), which is pushed down upon it in the act of swallowing, so as to prevent the entrance of solid or fluid particles into the space beneath, which is called the glottis. From the causes formerly men- tioned ( 135), such particles are occasionally drawn into the glottis; and they excite, by a reflex action, an involuntary and extremely violent cough, which tends to expel them again. Sometimes, how- ever, solid bodies of no inconsiderable size find a lodgment in the wide spaces (v, fig. 74) between the upper and lower pair of ligaments, which are termed the ventricles of the larynx ; and occasionally they pass through the opening between the vocal cords, which is termed the rima glottidis, or fissure of the glottis, into the windpipe. 464. In the ordinary acts of inspiration and expiration, the aryte- noid cartilages are wide apart, so that the aperture is as large as pos- sible; but for the production of vocal sounds, it is necessary that the aperture should be narrowed, and that the flat sides, rather than the edges, of the vocal ligaments, should be opposed to one another. This is accomplished by a peculiar movement of the arytenoid cartilages, occasioned by the contraction of certain muscles. When this is the case, the air, in passing through the larynx, sets these ligaments in vibration, in a manner very much resembling that in which the reed of a hautboy, or clarionet, or the tongue of an accordion, is set in vibration by the current of air that is made to pass beneath it. The rapidity of the vibrations, and consequently the pitch of the sound ( 342), depends on the degree of tension or tightness of the vocal lig- aments; and this is regulated by muscles which act upon the thyroid and arytenoid cartilages. If the thyroid cartilage be drawn forward, and the arytenoid cartilages backward, the two ends of the vocal cords will be further separated from each other, and they will con- sequently be tightened ; by the contrary movements, they will be relaxed. The power which the will possesses, of determining, with the 463. Describe the vocal ligaments. What is their use? How are foreign bodies pre- vented from entering the glottis? Point out all the parts of the vocal apparatus in the figures, and in plate 9. 464. How is the size of the aperture of the larynx varied, and for what purpose? To what is the vibration of the vocal cords compared? On what does the pitch of the voice depend, and how is it regulated? What is said of the power of the will in determining the tension of the vocal cords ? How is this illustrated ? Give the calcu- lations as to the variation of length of the vocal cord in musical intervals? What remark- able instance of ability to sound intervals is given ? most perfect precision, the exact degree of tension which these liga- ments shall receive, is extremely remarkable. Their average length in the male, in a state of repose, is estimated at about 73-100ths of an inch; ^yhile, in the state of greatest tension, it is about 93-100ths; the difference is, therefore, about 20-lOOths, or l-5th of an inch. In the female glottis, the average dimensions are about 51-100ths, and 63-100ths, respectively; so that the difference is only 12-100ths, or less than l-8th of an inch. Now, the natural compass of the voice (or distance between its highest and lowest notes), in most persons who have cultivated the vocal organ, may be stated at about two octaves, or twenty-four semi-tones. Within each semi-tone, a singer of ordinary capability could produce at. least ten distinct intervals ; so that, for the total number of intervals, two hundred and forty is a very moderate estimate. There must, therefore, be at least two hundred and forty different states of tension of the vocal cords, every one of which can be at once determined by the will; and the whole varia- tion in their length being not more than l-5th of an inch, even in Man, the variation required to pass from one interval to another will not be more than l-1200th of an inch; and yet this estimate is much below that which might be truly made, from the performance of a practised vocalist. It is said that the celebrated Madame Mara was able to sound one hundred different intervals between each tone. The compass of her voice was at least twenty tones ; so that the total number of intervals was two thousand, all comprised within an extreme variation of l-8th of an inch : hence it may be said that she was able to determine the contractions of her vocal muscles to the 1- 16,000th of an inch. 465. It is on account of the greater length of the vocal cords, that the pitch of the voice is much lower in man than in woman ; but this difference does not arise until the end of the period of childhood the size of the larynx being about the same in the boy and girl, up to the age of fourteen or fifteen years, but then undergoing a rapid increase in the former, while it remains nearly stationary in the latter. Hence it is that boys, as well as girls and women, sing treble ; while men sing tenor, which is about an octave lower than the treble ; or bass, which is lower still. The cause of the variations of timbre or quality in different voices, is not certainly known; but it appears to be due, in part, to differences in the degree of flexibility and smoothness in the cartilages of the larynx. In women and children these cartilages are usually soft and flexible, and their voices clear and smooth ; while in men, and in women whose voices have a masculine roughness, the cartilages are harder, and are sometimes almost completely ossified. The loudness of the voice depends, in part, upon the force with which the air is expelled from the lungs ; but the variations, in this respect, which exist among different individuals, are due to the degree in which its resonance is increased by the vibration of the other parts of the larynx, and of the neighboring cavities. In the howling mon- keys of South America, there are several pouches which open from the larynx, and are destined to increase the volume of tone that issues from it ; one of these is excavated in the substance of the hyoid-bone itself, which is very greatly enlarged ; and to this bony drum seems due the mournful plaintiveness which characterizes the tone of these animals. Although the largest of American monkeys, these howlers are inconsiderable in size; yet their voices are louder than the roaring of lions, being distinctly audible at the distance of two miles ; and, when a number are congregated together, the effect is terrific. 466. In Birds, the situation of the vocal organ is very different. The trachea opens into the pharynx, as in reptiles, by a mere slit, the borders of which have no other movement than that of approach- ing one another, so as to close the aperture when necessary. But at the lower extremity of the trachea, just where it subdivides into the bronchial tubes, there is a sort of larynx or vocal organ, which is of very complex construction, especially in the singing-birds. The external surface of this larynx is represented in fig. 75; its mus- cles, m, m , being left in their places on one side, and removed on the other. At t t, is seen the trachea ; at the lower extremity of which, t', is a sort of bony drum, /, divided at its lower part by a partition of the same material (o, fig. 75), which is surmounted by a semi-lunar mem- brane (c, fig. 76). This drum communicates below with the two 4f>5. What is the reason of the difference in pitch between male and female voices? When does this difference arise, and what is said of it? What is the reason of variations in quality of voice ? Upon what does the loudness of the voice depend ? What is said of the voice of the howling monkeys? How is the loudness of their voice increased? 466 How does the larynx of the Bird differ from that of Man ? Refer to figures 75, 76, and explain the construction of the vocal organs in birds. 84 INSTINCT AND INTELLIGENCE. ff. 75. LARYNX or A ROOK. ''if. 76. VERTICAL SUCTION OF THK SAME. bronchial tubes, & ft', each of which has its own glottis and vocal cords ; the inner lip of one of these is seen at a (fig. 76) ; and at me is shown a drum-like membrane, forming the inner wall of the bronchial tube, which probably increases the resonance of the voice. These parts are acted on by several mus- cles, the number of which varies according to the compass and flexi- bility of the voice in the different species ; being very considerable in the most esteemed of the singing-birds, and being reduced to a small amount in those which have no vocal powers. In some, indeed, they are altogether absent ; and the state of the glottis can be influenced only by those muscles which raise and lower the whole trachea. 467. The vocal sounds produced by the action of the larynx are of very different characters ; and may be distinguished into the cry, the song, and the ordinary or acquired voice. The cry is general! v a sharp sound, having little modulation, or accuracy of pitch, and being usually disagreeable in its timbre or quality. It is that by which animals express their unpleasing emotions, especially pain or terror; and the human infant, like many of the lower animals, can utter no other sound. In song, by the regulation of the vocal cords, definite and sustained musical tones are produced, which can be changed or modulated at the will of . 499. What is said of the mortality of children in St. Kilda, anil how is it accounted for'! What other examples of mortality are given, what were their causes, and by what remedies was the amount of their mortality reduced I 23 increased attention to the physiological conditions requisite for health. In twenty years, iVoin 17I10 to 1750. out of every one hundred chil- dren horn, seventy-four, or nearly three, out of four, died before they Were li\e \ears old. In the next twenty years, the number was reduced to sixty-three in one hundred, or less than two-thirds. Betweenl770 and 1790, it was only lifts -one in one hundred, or little more thai) one-half. From 1700 to 1810, it was farther reduced to for! y -one in one hundred, or little more than two- fifths. And between 1810 and 1830, it was no more than thirty-two in one hundred, or less than one-third. 500. Attention to the laws of life proves efficacious not only in diminishing mortality, but in promoting health. A remarkable < in point, is that of the Orphan Asylum in Albany, New York, which was opened in the end of 18-29, with about seventy children, which number was subsequently increased to eighty. "During the first three years, when an imperfect mode of management was in operation, from torn- to six children were constantly on the sick-list; the physician was in regular attendance twice or thrice a week: and the deaths, for the three years, amounted to between thirty and forty, or about one in every month. At the end of this time, an improved system of management and diet was adopted. The sick-room was entirely vacated, and the services of a physician no longer needed, and for more than two years no case of sickness or death took place. It is also true, that, since the new regimen has been fully adopted, there has been a remarkable increase of health, strength, activity, vivacity, cheerfulness, and contentment, among the children. The change of temper is also very great; they have become less turbulent, irritable, peevish, and discontented, and far more gentle and kind to each other. 501. Every improvement that has taken place in regard to the physical condition of both infants and adults has been attended by a corresponding increase in the duration of life. Historical facts abund- antly prove that the term of human life has undergone an increase since the last century. Tables, computed as the basis of life insurance seventy or eighty years ago, are found to have underrated the present duration of life very considerably. But the abodes of squalid poverty are still marked by a fearful mortality of the inmates; showing that filth, destitution, and wretchedness, court diseases. In France, the number of deaths among the poor is more than twice as great, in pro- portion to the whole number, as among those in easy circumstances. In England and Scotland, no more than one in fifty-eight, out of the whole population, now die everv year ; while, in Germany, the average is one in forty-five; in France, one in thirty-nine; in Turkey, one in thirty ; in the Roman States, one in twenty-eight. In Franklin county, the most favorable to longevity of all the counties of Massachusetts, the average duration of life is about thirty-nine years ; in the city of Boston, it is a little less than twenty-two and three-quarter years. Among foreigners, who constitute the poorest class in Boston, the average duration of life is only 13 5-10th years, or about one-third the average in Franklin county. Statistical facts are not wanting to prove that the scale of human life every where rises or falls with the physical condition of the people. The intelligent physician, as he goes the round of his daily calls in all grades of families, is constantly reminded of the contrast in health and happiness between those who observe an intelligent obedience to the rules of health, and those who live in careless ignorance of the laws of their own being, and are regardless of personal filth and exposure. 502. The natural period of man's life has been stated at seventy years; and this is, perhaps, near the true estimate; since almost all who die previous to that age are the victims of violence or disease. Thus, about one in thirty in those localities most favorable to lon- gevity live out the appointed period of life, and go to their rest in a good old age ; while twenty-nine-thirtieths are wasted by disease, or fall victims to accidental injury. In the cities, one-fourth of all that are born, die within the first year, and one-half during the first four or five years. In the country, the mortality of children is less, one- half surviving the first six years. The lowest rate of mortality is found among the convicts of our best regulated state-prisons. In the New Hampshire state-prison, where great attention has been paid to the diet of the prisoners, and ventilation of their apartments, the rate of mortality for thirty-six years is only one in seventy-nine; showing been 500. What is said of the sickness in the Orphan Asylum at Albany, and what have en the effects of an improved system of regimen and diet 1 501. What other facts are given to show that the increase of the duration of life is dependent upon improvement in physical condition? 502, 503. What further statements are made in regard to this subject 1 Where is the lowest rate of mortality said to be found? What is the reason of this! THE PRESERVATION OF HEALTH. that the systematic care of their physical condition, in regard to diet, labor, and ventilation of their apartments, is some compensation for the privations of this most unfortunate class of persons. 503. Nothing is more certain than that communities are healthy and long-lived, just in proportion as their localities, occupations, and nabits of life are favorable to these conditions. Individuals, in like manner, are blessed with health and long life, according to'the vigil- ance and care they exercise over their physical habits. Health and life are blessings lent us only on certain conditions, or during obe- dience to those physical laws which are written on the bones, mus- cles, nerves, and all the organs of the body. Laws of our organization are more binding on us than the decalogue; since physical sin is with- out pardon or atonement; impaired health, and shortened life, are the irrevocable penalties of their violation. 504. Again, a very large proportion of those diseases that prove fatal, arise from avoidable causes. Nearly all acute or inflammatory diseases, and many chronic diseases, can be traced to some known cause that might have been avoided by a more intelligent care of the health. The age, too, at which a majority of the deaths occur, can- not be accounted for on any natural principles with which we are acquainted. No law of our nature has yet been discovered which would seem to indicate a necessity that one-half of all that are born in this country should fall victims of disease during the first five or six years. Indeed, every physician knows that, in the best-regulated families with which he is acquainted, no such mortality exists; and yet it is equally true, that even the best-regulated families have not reached any thing like perfection in observing the laws of health. CAUSES OF PREMATURE DISEASE AND MORTALITY. 505. It is not possible, in the space proper to allot to this subject, to give any thing like a full account of the causes that operate to impair health and bring on a premature death. Many are of a gen- eral character, that exhaust and enfeeble the vital powers; such as insufficient supply of warmth, food, and air. Improper management in respect to these three great essentials of vitality prevails in all grades and classes of society. Among the affluent, the fashions of society forbid the use, in many instances, of proper clothing to pro- tect the body against the inclemencies of a changeable climate; and the food is too great in quantity, or rich in quality; while, among the poor, still more suffering is induced by an insufficient supply of these necessaries. Insufficient ventilation of sleeping apartments, and too much confinement in doors; excessive labor and fatigue; or too con- stant occupation of the mind in study all exert a depressing influ- ence on the vital powers, and render the body susceptible to the attacks of disease, from the slightest causes. In persons that habit- ually suffer from bad air, improper clothing, or insufficient or improper nourishment, the blood becomes impoverisjjed, or circulates imper- fectly; and the functions of one or more organs become deranged, and disease speedily ensues. 506. Improper treatment of infants and children is a most prolific source of disease and premature death. It commences with the first hours of existence, follows the young child with unrelenting severity, and hurries off" to a premature grave the very large proportion of one- fourth during the first year, and one-half in five or six years after birth ( 500). A mortality so fearful and unaccountable on any natural principles, seems to call for special consideration. The first act of attention the young infant receives is to be clothed according to the prevailing fashion. Fortunately for the young of other ani- mals, nature clothes them in garments adapted to their condition; but the young of the human species require artificial clothing, and this is too often made to please the eye, rather than to meet the wants of nature. When dressed according to the American fashion, it exposes to the admiration of friends, and the cold of a changeable climate, the breast, neck, and arms; and, in some instances, portions of the lower limbs are unprotected. Through the neck and breast, the lungs and respiratory passages are exposed to the almost direct action of the changes of the weather. The blood that circulates through the hands and arms is very superficial, nearly all the vessels lying just under the skin, and possessing but very slight natural pro- 504. Do most diseases arise from accountable causes? Is there any necessity for the death of so many children ? 505. What remarks are made concerning the general causes of disease? Mention some of these causes. 506. What is said of improper treatment of infants and children? What of the fashionable mode of dressing children? Explain the effect of this mode of dressing on the health. tection against the influence of cold. Children thus dressed, are seldom without cold hands and arms. Hence, almost the entire mass of blood flowing from these parts is thoroughly chilled before it returns to the heart, where it mingles with the blood from the gen- eral circulation, and is thrown into the lungs to be again raised to its proper temperature by them, while their own heat is diminished by the external action of cold. Fortunately, however, the blood goes the round of the circulation so rapidly (74), that an equilibrium is constantly maintained, and no very perceptible difference of tem- perature would be observed, except in the hands and arms; still, the true operation of this fashion of dressing children, on the blood, is in principle just as described. Indeed, there is not an equal extent of the entire surface of the unfortunate victim of fashion, that might not be left exposed in better accordance with its physical organization. In any other part of the body, the circulation is better protected, and would suffer less from exposure. 507. A much larger proportion of deaths among children occur during the winter months than any others of the year; showing that cold destroys more than any other cause. No period of life is so susceptible to the influence of cold, and at no period is the power of resisting it so feeble. Yet there is such a passion among Ameri- can women to see the plump breast and round graceful arms of the young child, that multitudes fall victims to these vain fashions, notwithstanding every intelligent physician disapproves of the prac- tice. A child, thus imperfectly protected against cold, will contract bronchitis, croup, inflammation of the lungs, or diseases of the digestive organs, from the most trivial exposure. A current of air at the sides of the closed window or door, so trifling as not to be noticed by an adult, is sufficient to induce a fatal disease in an exposed infant. Each winter makes known to our experience instances of severe inflammation of the respiratory passages, induced by the slightest exposure, in spite of the most watchful and anxious care in all things except one of the most essential the dress. 508. Another fruitful source of mischief in the management of children, is excessive and improper food. Instead of allowing them only the aliment nature has designed, they are fed on a mixed diet of whatever they can be induced to swallow. An apology for depriving the young infant of the nutriment which an All-wise Creator has provided for it in the mother's milk, is found in disinclination, arising from false views of a mother's highest duty, or actual inability, from feeble health. Not less than three-fourths of the American women reared in our cities are incapacitated for discharging the duties of a mother, by impaired health. Their offspring inherit more or less of the same frailty of constitution, and are then put on an artificial system of diet, characterized by an ignorant and blind disregard of all true physiological principles, with the almost certain result of swelling the fearful bill of infantile mortality. Among the poor, children suffer from actual neglect, from undue exposure, and from bad or insufficient food. Among the affluent, they are too often committed to the care of hireling nurses, to be over-fed, or to be con- stantly surfeited with candies and luxuriant food. Among the middle , classes of society, the mother is neither above nor below the care of her own child ; and, as we might anticipate, the rate of the mortality of children is much lower in these than in any other class-. 509. Among adults, the causes that induce disease are not unlike those that affect children. In this respect, ."the child is father to the man." Those habits that render the child weak in health, and liable to disease, continue to act as direct agents, or as predisposing causes of disease. Those adults who have escaped the perils of childhood with vigorous health, too often waste it by excesses in sensual indulgence, or in bodily or mental labor, through the natural enthu- siasm of impulsive youth. Thus, the strength which, carefully hus- banded and sustained, might have kept the body and mind in activity and enjoyment up to the allotted period of "three score years and ten," is too frequently dissipated in early manhood. 510. Still, young men are exempted from evils that fall heavily on the other sex. While at school, the boy is allowed to follow the bent of his inclinations, during the intervals of study, and to seek in play that exercise which his nature imperiously demands. By this means, his body and its organs attain a certain development, though not as 507. Mention other evil consequences produced by improper dressing of children. 508. What is another source of disease in children? What is said of feeding children with improper food! 509. How may the health of adults be injured? 510. What is said of the difference between the management of boys and that of girls, as regards exercise? What is the nature of the exercise usually permitted to girls, and why is it insufficient? THE PRESERVATION OF HEALTH. perfect as he might enjoy, if his education was conducted in strict accordance with physiological principles. The young miss of the boarding-school, on the other hand, as soon as she ceases to be a little girl, is discouraged from active exercise, as unbecoming her sex, and is taught to pass her leisure hours in a quiet, lady-like manner, at home, or in her room. If away at school, the only exercise she is permitted to take is a sedate, measured walk up and down the street, or around the square, once and occasionally twice a-day. At boarding- school, she is permitted still less of unrestrained and healthful exercise. The delicacy and refinement of her sex here become so great, that the restless activity of youth must be kept in check by the power of the matron or teacher, and her walk be taken in a solemn procession of her mates, two by two. A walk in a procession may afford some exercise to the limbs principally concerned in locomotion, but it is of very little benefit to the other organs of the body, while it certainly must fail to unbend and relax the mind. 511. Exercise, to benefit the young, must call into action all the muscles of the body as well as of the limbs; and it should also be of a character corresponding with the vivacity of youth something into which the feelings and spirit can enter with enthusiasm and delight. An hour's exercise on a tread-mill, or in a walk, though better than none at all, does not meet the wants of the growing young of either sex. Hence a most marked difference can be seen between the coun- tenances of those young ladies who are compelled to be "lady-like," and those who are permitted the unbounded and unrestrained pleasure of acting like little girls. By reference to tables of mortality, we find, on comparing the proportion of deaths among the two sexes, that the number of females who die between the ages of fifteen and thirty is greater than at any other period of life, and than that of males at the same period. Dr. Warren, of Boston, in a very excellent treatise on Preservation of Health, says, "In the course of my observations, I have been able to satisfy myself that about half the young females, brought up as they are at present, undergo some visible and obvious change of structure; and, of the remainder, a large number are the subjects of great and permanent deviations; while not a few entirely lose their health, from the manner in which they are reared." A physician and professor, who is now unquestionably the highest med- ical authority in the city of New York, gave it as his opinion, to his class, that three-fourths of the young ladies in the highest ranks of society are more or less deformed in structure, and impaired in health. 512. A late and most judicious French writer, speaking of lateral curvature of the spine, tells us that out of twenty young girls who have attained the age of fifteen years, there are not two who do not present very manifest traces of that species of deformity. The causes of spinal distortion, in general, are whatever weakens the constitution. Want of exercise, and improper dress, are the more immediate causes. It is a very remarkable fact, that where there is one case of lateral curvature of the spine among males, there will be, on an average, as many as nine among females. And it is equally worthy of thoughtful consideration, that the number of young ladies, between fifteen and twenty-five, who enjoy full and uninterrupted health, is very small, compared with the other sex; and yet it will be found that girls are quite as healthy as boys, so long as they are allowed the same amount of liberal exercise. The education of females, as now generally con- ducted, is a most prolific source of serious physical evil, terminating in premature death, or in feeble health during a life of impaired usefulness. 513. A love of life is inherent in each individual of the race; and no other earthly object can ever become more dear to the natural feelings of a sane mind, than the preservation of life. In consistency with this inborn desire to live, the preservation of health would seem a proper object of care and watchful thought on the part of each individual, and an object, too, in regard to which each would always possess the deepest solicitude to be fully and rightfully informed. Strange as it may seem, directly the opposite of what we would naturally expect, is true. There is no subject that pertains to our welfare, in reference to which we act so ignorantly, or with so little apparent thought and care. Most men seem to live in utter disregard of the laws of health, as if reckless of its preservation. Indifference 511. Whnt kind of exercise is beneficial to the young? Give the remarks of Dr. Warren, and of a New York physician, on the consequences of improper training of young ladies? 512. What are the remarks of a French writer on distortion of the spine among females? What remarkable fact is stated, and to what cause is it attributed ! How may girls continue as healthy as boys? 513. What considerations are mentioned why the preservation of health would seem to be nn object of solicitude ? What reasons are given for the carelessness of men in regard to their health? to health is, therefore, one of the most prominent causes of premature mortality. It has its origin in most minds, cither in the belief thnt a certain number of days or a certain length of life is preordained to each individual as his appointed period, which can neither be increased nor diminished by his own conduct; or, in other cases, there is so little apparent and immediate connexion between the violation and the penalty of physical laws, that they cannot be made to realize that a vicious habit can injure them, so long as they are not actually diseased in other words, they can see no connexion between debil- itating habits and wasted health, unless they experience a severe pain, or are thrown upon a bed of sickness, the instant they indulge in that which is injurious. 514. For instance, a student, who has been accustomed to much exercise, becomes unusually interested in his studies, and omits his daily exercise, without the least consciousness that his body is suffer- ing, till, by and by, he becomes debilitated and feeble, and finally a confirmed invalid a miserable sufferer throughout life. What is lost can seldom be regained. As time past never returns, so the vigor and energy of youth ays never restored to a worn-out constitution. The transgressor is overtaken too late to be saved by repentance. The causes of disease are for a time latent it may be unseen and unfelt. Its seeds are sown in the tissues and fluids of the body, stealthily, and often-times are not discovered till they have so far ger- minated as to be difficult of eradication. 515. Some of the general causes that produce disease have thus been mentioned; others will be considered in connexion with the means of preserving particular organs in health. The means of pro- moting health are general and special. General means, are those that improve the health of all the organs, such as exercise and bathing, which tend to invigorate the whole system, instead of confining their influence to particular organs. In a certain sense, all means are general; since whatever favorably affects a part, so far exerts a salu- tary influence on the whole body. But certain means have reference mainly to particular organs, and may therefore be considered as special means of preserving health. The organs of respiration and digestion, and the nervous system, have more to do with health than any others, and therefore require special consideration ; while those of absorption; secretion, and excretion, are more general in their influence. HYGIENE OF THE RESPIRATORY ORGANS. 516. The function of respiration consists in exposing the blood or circulating fluid to the action of atmospheric air (CHAP. iv.). It has for its object the purifying of the blood, and the generating of a certain amount of animal heat, which is produced by combining the oxygen taken into the lungs with the carbon and hydrogen in th tissues and fluids of the body. Nearly all our food contains more or less carbon, which is laid in store as the natural fuel to supply warmth to the body ; and were it not for the existence of this constant demand for carbon, it would accumulate in large and injurious excess. By the process of respiration, carbon combines with oxygen to form car- bonic acid, which is thrown from the lungs at each expiration. This work of purifying the blood, and producing heat, requires constant action of the lungs. The perfect performance of their function is more essential to vitality than that of any other organ, since the purity of the nutritive fluid depends on it. To insure health of the respiratory organs, the most important conditions are, that the lungs should at all times be free and unrestrained in their actions ; that the blood itself should be presented to the lungs in as healthy a condition as possible, and that the temperature of all parts of the body should be nearly the same at all times. 517. To a reflecting mind, no argument would seem necessary to prove the importance of preserving the action of the respiratory organs free and unrestrained. It has been seen ( 100, &c.), that the capacity of the lungs is increased and diminished by the alternate depression and elevation of the walls of the chest, together with the contraction and relaxation of the muscles of the abdomen. What- ever, therefore, constrains the free play of the walls of the abdomen 514. What illustration is given of ill-health produced without consciousness of the subject ? What other remarks are made? 515. Give the distinction between general and special means of preserving health. What organs are most important in this connexion ? 516. Review the process of respiration. What conditions are mentioned as essential to the healthy action of the lungs? 517. Explain the play of the lungs in respiration, and show why tight dresses are injurious. What remarks are made in regard to stays, &c. Whnt other effects of compression of the lungs are mentioned? 92 THE PRESERVATION OF HEALTH. and the chest, interferes so much with the natural process of respir- ation. It is to be hoped that stays and corsets are now known only in the history of a less enlightened age. But a wasp-like waist still commands admiration; and though public sentiment has con- signed to obscurity those mechanical instruments of disease, yet their place has been supplied by the thickly-set row of hooks and eyes. Any dress that will not hold together with very slight fastenings, and allow the same free expansion of the chest which nature permits, should be discarded as at war with health. Nor should any portion of a lady's or a gentleman's dress be so tight about the waist as to support its own weight. It does not belong to Physiology to teach the proper style of dress, but the principles of the science should be known by our mantua-makers and tailors; and the model of Grecian beauty should occupy a place in their diagrams of the latest fashions. Public lecturers and the press have exposed the evils of titrht dressing, and some reform has followed; yet the great mass of our ladies, and many gentlemen, are still suffering from this plain violation of nature's laws Probably nine-tenths of our well-educated females, above twenty years of age, have suffered fro^ deformity or weakness of the spine. How many of this number are afflicted with disease of the lungs, induced by constant compression of these vital organs, can never be known. Just in proportion as the lungs are constrained in their action does the 7'espiration become more frequent wearing out and exhausting the power of these organs, in the same manner that a piece of machinery will wear out faster when it is propelled beyond its appropriate velocity. It is owing to the diminished capacity of the respiratory organs, that persons tightly dressed can take only moderate exercise without inducing that hurried respiration called panting. The lungs seem struggling with intense effort to perform their office, while the whole system is suffering from the imperfect oxygenation of the blood. The extremities are habitually cold, and the countenance pale and sickly, from a feeble circulation of the vital fluid, and the subject becomes an invalid for life, or an early victim of disease. 518. The health of the lungs also depends, to a very great extent, on the purity of the blood. It should not be overcharged with carbon, or with any other impure matter. When the skin, the liver, or the kidneys, fail to perform their functions properly, the blood becomes loaded with injurious elements, tending to disease. It has been esti- mated (217, &c.) that the average amount of daily exhalation from the skin of a healthy man is about two and a half pounds, while that from the lungs is only one pound. Much of the exhalation from the skin consists of watery vapor, but it also contains carbonic acid, with some other elements injurious to the lungs. The excretions of the liver also afford the lungs important aid in purifying the blood from carbonaceous matter, and imperfect performance of its functions is sometimes a cause of pulmonary disease. The action of the kidneys is of importance, as well as that of the skin and liver, and is indis- pensable to that pure state of the blood which the health of the lungs requires. It is also necessary that the blood should abound with highly nutritive elements. Persons who are deficient in the red blood-discs, or in the nutritious portions of the blood, are exceedingly liable to scrofulous and tubercular diseases to consumption, and to the acute diseases of the respiratory organs. The symptoms that indicate that low state of the blood, so often the forerunner of fatal disease of the respiratory organs, are pearly whiteness of the sclerotic coat of the eye, a pale countenance, with or without a flushed cheek, and habitual coldness of the extremities. 519. The maintenance of animal heat is the most important and the most trying work of the lungs, in all temperate climates; since they are almost daily subjected to some vicissitude or change. The air inhaled, often varies several degrees in a few hours, and, in some instances, in -a few minutes ; the individual being at one moment in a room at summer heat, and the next in the open air at a temperature below zero. The delicate membrane that forms the six hundred millions of air cells in the human lungs, though admirably adapted to its office, is nevertheless subjected to a severe trial by the sudden changes of the temperature to which it is exposed. The amount of heat necessary to maintain a uniform temperature of the body is equally variable. No organ of the Body enjoys so little uniformity in 518. What condition of the blood is necessary to the health of the lungs? What effect has action of the excretory organs on the blood? What are the effects of a deficiency of nutritive matter in the blood? 519. What is said of the importance of maintaining a uni- form temperature of the body? How is this result to be effected > Whnt kinds of clothing ore best adapted to this purpose, and why ? the performance of its functions as the lungs, and no other is so liable to disease. But it is equally true, that diseases of this most delicate origin diminish just in proportion as the temperature becomes more uniform prevailing most during those months and in those localities where the weather is most variable. It may, therefore be regarded as a law of our organization, that the body should maintain as uni- form a temperature as possible. In New England, during most of the year, we can live up to this law only by the use of artificial heat and clothing. Artificial heat, as produced hv fires, is far more likely to be too high than too low for the welfare of all such as enjoy the means of procuring this comfort; because we cannot at all times be within doors, and the change, on going out, is proportionate to the degree of heat within. Clothing is, therefore, the most uniform and the most reliable protection against cold. The cause of the sensation of cold in the body, or any portion of it, is the conducting of the natural heat from the body to the surrounding medium. This takes place at all times when the temperature of the body is higher than that of the air. The object of clothing is not to create heat around the body, but to preserve that which is constantly forming. The best protection, therefore, is that kind of clothing which is the worst conductor of heat. Linen is a good conductor of heat, and may, therefore, be worn in warm weather, when we wish to reduce the temperature of the body, instead of maintaining it; but woolen and furs are very poor conductors of heat, and form the best articles of dress in cold weather. 520. The amount of clothing must depend upon the season of the year, the exposure, an'd the capacity of the individual for producing heat. In the young, and the aged, this faculty is at its lowest power, and they require more clothing than those in the middle and prime of life. Those who are subjected to an external exposure, require more than those who remain in-doors. The clothing should at all times be sufficient to preserve a comfortable warmth of the whole body during any ordinary exposure, and should be increased in pro- portion to the length and severity of the exposure. The extremities are the most exposed to the influence of cold, and require the best protection, though they are apparently the least cared for by a majority of persons. Any one who is accustomed to riding, may know, by experience, that the rest of the body is warm enough, so long as the hands and feet can be kept warm. Many persons bundle up the neck and throat to sweating point, while the extremities hardly enjoy a moderate protection. Such persons are the most likely to be afflicted with colds. The neck requires no more clothing than any other part ; and the effect of shawls and fur collars is to invite an unusual supply of blood, which is driven away when the extra appliances are removed, giving rise to sudden chill and congestion, followed by inflam- mation. Garments that fit the person loosely, atl'onl more warmth than such as are called "a snug fit;" for the reason that there will be a space of air left between the clothing and the body, through which the heat will be conducted from the body less rapidly, on the same principle that the warmth of a room is preserved by using double windows, with a stratum of air between them. Any one who has been caught out on a cold day, in a pair of tight boots, will need no argument to prove the correctness of this assertion. 521. The function of respiration has been described as- consisting mainly in exposing the circulating fluid to the action of air. We regard a due supply of air as one of the conditions of respiration, and not a mere rule of Hygiene a law of our existence, not a law of experience. That we consume a certain amount of air at each inspiration, and throw out a corresponding amount of carbonic acid, and that this carbonic acid is poisonous and destructive to human life, are facts of universal application ( 103 and 104). Life cannot be sustained for any considerable time witho'ut air; nor can it be continued in carbonic acid gas. An obvious law follows that every apartment we occupy, whether private or public, should be provided with some reliable means for conveying off the poisonous gas, and supplying the pure air. When we consider the delicate structure of the lungs, and the absolute necessity of efficient ventilation, we are not surprised, that from one-fourth to one-third of our race are scourged with lung diseases ; since little or no care has been taken to obey this imperious law of animal existence. The dwellings of our 520. What circumstances should regulate the amount of clothing? Why do the extremities require protection ? Why is too much covering on the neck injurious? Why do loose garments afford more warmth than tight ones? 521. What conditions and general facts in regard to respiration are stated? What follows from these? What is said of the usual deficiency of ventilation? What is the proper mode of ventilating a room? THE PRESERVATION OF HEALTH. fathers that hardy race which irave character to our republic were ((instructed with reference to a liberal supply of this vital element. The massive chimney in the centre of the house, with its wide lire- places, constituted a noble ventilator. At the present day, we have small chimneys, closed as perfectly as art can close them, by the use of air-tiuht stoves: excluding wholly the entrance of the pure air of heaven, and permitting only such gases to escape as have been formed by the burning fuel. In some instances, to place a more effectual prohibition on all possible ventilation, double windows and double doors are adopted as it ill imitation of the hybernating habits of the marmot (11(>). who shuts himself into his den for the winter, and then gives himself up to the full influence of the narcotic poison exhaled from his own lungs. If a state of torpidity for the winter be the object aimed at by close chimneys, air-tight stoves, double win- dows, and double doors, then these contrivances are admirably adapted to the end proposed; but, for any other object of animal economy, their influence is highly pernicious. Every room should have an aperture for the entrance of pure air, and another near the top for the exit of the gases given off by respiration; and no family or com- munity which neglects these conditions of health ought to expect exemption from disease. 522. The respiratory organs are liable to several distinct diseases ^hich receive their names from the part affected. Thus we have pneumonia, or inflammation of the substance of the lungs ; bronchitis, or inflammation of the air-tubes or bronchi; croup, laryngitis, &c. all of which are acute diseases, and are usually caused by cold. The lungs are also subject to a disease of a chronic character, which is the cause of more deaths than any other known disease. This is called Phthisis pulmnnahs, or pulmonary consumption. A disease so peculiar in its character, and so fatal to our race* seems worthy of a special description, though it is not our purpose to enter the lists with those who claim to have made some wonderful discovery, or to have found out some infallible remedy for its cure. Multitudes of such pretenders have arisen, but consumption still marches on to its fearful work of death, regardless of new theories and new systems of practice. CONSUMPTION. 523. By consumption is meant the wasting of the body, from the effect of a disorganizing process in the lungs. It is induced by a variety of causes, the most important of which are hereditary pre- disposition, imperfect nutrition, and exposure to cold. The heredi- tary origin of consumption is established by the concurrent testimony of almost all writers on the subject; though it is believed by many, that the development of the disease may be prevented by proper care, or by removal to a climate where it does not prevail, at an early age, before the consumptive predisposition has commenced its work. Improper or insufficient nourishment confinement in impure air neglect of cleanliness exhausting mental labor, or bodily fatigue exposure to wet and cold may interfere with the process of perfect nutrition of the body, or of some of its tissues, and predispose it to consumption ( 188). Confirmation of this is observed in the lower animals. The lungs of cows confined in close stables in the city become tuberculous ; and rabbits may be rendered so, by confinement in a close place, with bad food, for only a few weeks. 524. Consumption prevails more extensively in temperate than in hot or very cold climates. Particular localities, especially such as are exposed to damp and bleak winds, and sudden changes of temper- ature, are most favorable to its development; while, in places which enjoy the greatest uniformity of temperature, it prevails least. In New England, and the middle states, from one-fourth to one-third of all who survive the hazardous period of childhood, die of consumption. The period between the twentieth and thirtieth year appears to be ^that which is most liable to its attacks; and it is the conviction that many of those into whose hands this work will fall, are approaching that period of life, which has induced us to describe some of the symptoms of this fearful disease. 525. No disease with which we are acquainted is more insidious, or more deceptive, in its progress. In, perhaps, a majority of cases it becomes permanently seated and incurable, before its subject is made aware of its presence. In its first stages, it is seldom attended , r >22. Mention some of the diseases to which the lungs are liable. What is said of Consumption? 523. What are some of the causes of Consumption ? 524. In what coun- tries and localities, and at what age, does it most prevail? 525. Describe some of the symptoms of Consumption. 24 with much pain; or, with pain alxmt the shoulders and chest, so rilling as to create no alarm, or to be referred to some other than the true cause. Xor is it always preceded by a cough. Tubercles, which may be regarded as the germs of the disease ( 187), are often leposited before the couirh commences, or while the cough is not sufficient to attract attention. The first symptom, however, which :>lainly indicates the true nature of the disease, is usually a dry, lacking cough, accompanied, or soon followed, by paleness of the countenance, ami general debility. These steal on. with little apparent eason tor apprehension, till tubercular deposition has so far taken lace, that the disease runs a rapid course to a fatal termination. In other cases, the disease follows one or more severe colds, or inflam- nations of the lungs, or a fever accompanied by pectoral symptoms. In oth*- cases still, the patients have been out of health, or in a debili- tated State, before the commencement of any cough. In most cases, there is but little pain or suffering till the very last stages of the disease, and the unfortunate victim is seldom aware of its progress, xcept as it is learned by an increasing weakness and inability to ndure accustomed labor or exercise. The subject of the disease, for the most part, is cheerful and full of hope, and unconscious of the destructifc; process that is wasting the vital powers. A kind of bal- ance seems to be maintained between all the functions, that secures xemption from suffering. As the available portion of the lungs is diminished, the mass of blood that has to pass through them becomes less and less, and the wasting of the body and the failing of the strength seem to keep pace with the decay of the lungs. Thus the descent is easy, and the parting of the last filament of life very gentle. 526. Again and again has the discovery of a new and infallible remedy for consumption been announced, and almost every medical journal contains some account of the beneficial effects of some new course of treatment. Although a few instances have undoubtedly occurred where persons clearly marked as its victims have escaped, yet the great majority who have the disease once seated, fall before it, in spite of all that science or art can do. The only reliable course is in preventing or avoiding its grasp. Where strong predisposition exists, it can be avoided only by the most rigid observance of the laws of health, and an early removal to a climate where it does not prevail. Exemption can be purchased only by the most constant and persevering vigilance not in the use of any panacea retailed at the shops, but in the avoidance of all debilitating causes; in the use of a generous and wholesome diet, and of all those means that invig- orate and nourish the body. . HYGIENE OF THE DIGESTIVE ORGANS. 527. The function of the digestive organs (CHAP, v.) is to prepare the appropriate elements for the nutrition and growth of the body. The food of man, for the most part, undergoes some process of cooking, and then of mechanical sub-division, before it is received into the digestive apparatus. The first process of digestion consists in its further mechanical subdivision by chewing, or mastication. In the act of mastication, it becomes mixed with saliva, the natural secretion of the glands of the mouth. The solubility of the food is influenced, to a great extent, by the perfection of this process. If it is thoroughly mingled with the saliva, the second process is performed with more facility; but if the food is swallowed very rapidly, insali- vation is imperfectly performed, and the process of digestion is more difficult. Americans, it is said by foreign writers, eat faster, if not more, than any other nation. This is unquestionably trufe of most of those persons who eat at hotels and boarding-houses, and who are engaged in active employments. The excitement of business impels men to hurry through their meals in the least possible amount of time, just as though time spent at the table was lost or wasted to no good purpose. In private families, and by men not devoted to some absorb- ing mercantile occupation, there is a more reasonable time devoted to supply the body with the nutriment it requires. Men who allow themselves but fifteen minutes at the dinner table, must expect to make a liberal allowance annually for the services of a physician ; or if they should chance, by reason of a strong constitution, to escape for a few years, they may rest assured that their digestive organs will ultimately demand back, with interest, too, every hour they have been cheated out of at meals. Dyspepsia or indigestion will sooner or later follow 526. What precautions should be taken to avoid this disease ?> Is there any infallible medicine for its cure? 527. Give a summary of the processes of digestion. What is said of rapid eating, and its consequences? THE PRESERVATION OF HEALTH. the habit of rapid eating. Thirty minutes is the shortest time that should ever be allowed for taking a full meal. 528. When the rneal is finished, the stomach should remain undis- turbed till the process of digestion is completed. The interval proper to be observed between the meals, depends on circumstances. In the adult, the healthy stomach requires from one to three or four hours for the process of digestion. Some kinds of food are digested much quicker than others; but hunger, or the desire for more food, is not usually experienced till some time after the stomach has disposed of its contents. If fresh food is received into the stomach before that of the previous meal has been digested, the process of digestion is disturbed. In this country, where the meals are composed of a mixed diet, from five to six hours is the proper interval to be observed. In children, all the functions are much more active, and they accord- ingly require nourishment more frequently. For children under seven years of age, about three hours is an average time of rest between the meals. Infants sometimes seem to need food as often as every two hours; while older children can often wait four, without inconve- nience. Eating oftener than this is decidedly injurious. 529. The practice of allowing children fruit, candies, cake, &c., at any or at all hours, is always pernicious, and product!^ of evil. Such children are seldom healthy, and never pleasant and agreeable in their disposition. If they must be indulged in luxuries of this sort, let it be at regular hours. All the processes of digestion and absorption are carried on according to uniform systematic laws. Any interference with the processes of nature disturbs the whole economy. Irregular feeding not only disturbs and interferes with digestion, but it is a source of unpleasant and injurious irritation to the nervous system. We have often observed that clerks in grocery stores, who are allowed to be constantly nibbling at all sorts of articles, very soon become dyspeptic, or uneasy and restless in their habits, and ill- natured in their disposition. A druggist, also, who contracts the habit of tasting every prescription of medicine which he puts up, will very soon destroy the tone of his stomach, and will require a special pre- scription for himself. Children who need candy or sugar to keep them in good humor by day, will require an opiate to keep them asleep at night. 530. The true rule for the number and intervals of our meals, is "to proportion them to the real wants of the system as modified by age, sex, health, and manner of life, and as indicated by the true returns of appetite." The frequency with which different animals require their food bears a very close proportion to the frequency of their respiration ; and, in the case of man, this proportion will also apply. Persons who take much exercise in the open air, and whose respira- tion is thereby accelerated, require food, and have a proper appetite for it, much more often than those who are indolent and sedentary. On the same principle, birds, whose respiration is very rapid ( 92), will die of starvation in three days; while a serpent will live without food as many months. Carnivorous animals, such as the lion, tiger, cat, &c., thrive the best with only one meal a day, and their health will suffer if they are fed oftener. The law of health and of nature is, that the meals should be at regular intervals, and at about the same hour on each successive day. The number of the meals should be proportioned to the wants and circumstances of the individual. 531. Breakfast should always be at an early hour, and soon after rising. It is well known that the system is more susceptible of infec- tion, miasma, and other causes of contagious or epidemic diseases, in the morning, than at any other hour of the day; unless, perhaps, when the system'nas been exhausted by fatigue ( 182). Naval and military commanders have found, by experience, that, in unhealthy climates, their sailors and soldiers are much less susceptible to noxious influ- ences after they have been fortified by a comfortable breakfast. In countries where ague prevails, the proportion of sick among those who are exposed before getting any thing to eat, is infinitely greater than among those who have breakfasted before exposure. In chil- dren, or in persons who are at all delicate, much exercise in the morning, before eating, produces exhaustion and languor, and often unfits the stomach for receiving and digesting food properly. 528. What is said of the proper intervals to be observed between the meals? Why may chiWren eat more often than adults? 529. What is said of allowing children to eat candies, &,c., at all hours? What ore the effects of the habit of irregular eating? What chirrs of persons are mentioned as liable to this habit? 530. What is the true rule for the frequency of our meals ? What proportion exists between the frequency of eating and of respiration! What examples are given of this proportion ? 531 When should breakfast be taken, and whv ? 532. The advice sometimes given to invalids, to take a long walk or ride before breakfast, is not in accordance with the principles of Physiology, or the teachings of experience. Still, there are some persons who not only appear to suffer no injury from active exercise before breakfast, but are apparently benefited by it. Although many persons who have been accustomed to much active exercise in the morning, before eating, enjoy excellent health, yet it is in spite of the morning exercise, not in consequence of it. During sleep, at night, the absorbents become comparatively quiet and inactive, but com- mence their operations with renewed vigor as soon as the other powers are fairly awake, and an almost immediate demand for a fresh supply of nutriment is felt throughout the whole system. When this demand is not satisfied, prostration of the nervous energy of the digestive organs very soon follows, with sympathetic exhaustion of all the powers. We are acquainted with an enterprising farmer, possessed of what is sometimes called "an iron constitution," who has been the means of ruining the health of six sons in succession,- by requiring them to work several hours before breakfast, under the delusive idea that in this way they would be strengthened and invig- orated. There can be no objection, however, to early rising, pro- vided it be followed by an early breakfast. Indeed, "early to bed and early to rise" is unquestionably the best practice for the preservation of health, under ordinary circumstances. 533. Dinner should be taken about six hours after breakfast. If the dinner hour is later, there should be a regular hour for luncheon; otherwise a luncheon should not be indulged in, except by children, and those who take active exercise in the open air. Children imper- atively require some nourishment between the usual hours of meals, but it should invariably be taken at about the same hour every day. The dinner, as it is the principal meal, should occupy a longer time, and be followed by at least one hour's rest from any active exercise. The supper should be a light meal, taken at least two or more hours before retiring to bed, except in special cases. If a person has been obliged to undergo great exposure or severe labor in the evening, and has become truly hungry, some light nourishment is proper, even just before bed-time ; since an empty stomach will sometimes prove a most disagreeable preventive of sleep. But the indulgence of the appetite in late suppers is one of the worst kinds of dissipation a practice most pernicious to health, and a fruitful source of disease. 534. The kind of food proper for each individual must of necessity depend on as great a variety of circumstances as there are conditions of life. The climate, the season of the year, the age, the tempera- ment, the occupation, and the habits of life all require certain mod- ifications of diet. If we examine the structure of the human body at different ages, and in different individuals, we shall observe marked difference in the relative proportions of the elements of which it is composed. In the man of mature years, the muscular system pre- dominates, and the body is remarkable for the compactness of its fibre, which adapts it for feats of strength .and activity. In the child, the fluids are most abundant, producing a full, soft, and rounded form. In advanced age, the soft tissues have greatly diminished, and the whole frame is dry and wasted. So, in different individuals of the same age, we observe dissimilarity of structure quite as conspicuous. One possesses large and powerful muscles, by which he is enabled to perform great physical labor; another has a "lean and hungry look," his muscles are smaller and weaker, and he pursues the sedentary life of a student; another has an excess of the lymphatic system, and is "fat and sleek-headed," and is only fitted for a life of comfort and ease. In short, there are almost as many different developments of the various tissues as there are individuals. The young, the aged the laborer, the idler, the man of ease, and the student each requires a diet adapted to his own peculiarities. The same is true of all the various grades and classes of men; and they can no more exchange diet with each other, than the young and the aged can exchange habits and feelings. ' Modifications of diet are not less required by residence in different climates. The inhabitants of the Polar regions live, to a great extent, upon that kind of food which is called calorific or heat-producing, such as animal fats and oils. On the other hand, those of the tropical regions abstain almost entirely from such arti- 532. What is said of the propriety of severe exercise before breakfast ? Why do we demand nutriment on rising in the morning ; and what are the consequences, if the demand be not satisfied? What other remarks are made on this subject? 533. What is said of dinner? What of supper? 534. Is the same kind of diet adapted to every individual? What peculiarities of age and temperament are mentioned, and what is the result of these peculiarities in regard to diet? What is said of climate in this respect'! THE PRESERVATION OF HEALTH. 95 cles, and confine themselves to fruits and vegetables. It would be impossible to live, in Hindustan, on the seal-fat and whale-oil of the | Greenlander; or, in Greenland, on the plantain and rice of the Hindoo, j The same principle should govern, and does govern, to a great degree, j (lie diet of the inhabitants of temperate countries. In winter, we eat larger quantities of meat than in summer; and in summer, more fruit ! and vegetables than in winter. No person could, in summer, indulge ! in the same diet which would be highly conducive to health in winter, without overloading the system with carbonaceous matter, and indu- , cing congestive and inflammatory diseases. 535. The value of any particular article of food depends mainly on the facility with which it can be converted into the organized tissues of the body. In this process of assimilation, as it is called ( 47), the nutriment undergoes digestion, absorption, and secretion. Of the facility with which different kinds of food are absorbed and secreted, we have but little practical knowledge. A variety of circumstances, however, affect the facility with which different articles of food are digested. Some kinds of food are naturally more difficult of digestion than others. This is especially the case with all oily or fatty sub- stances. Tenderness of fibre renders the digestive process more easy ; and, therefore, all those circumstances that affect the texture of flesh have an influence upon its digestibility. Violent muscular exertion, immediately previous to the death of the animal, renders the flesh more easy of digestion. The flesh of young animals, though more soluble and. tender than that of the adult, is not so easily digested. Vegetables are generally more slowly digested than meat. Minute division facilitates digestion; hence, if food is perfectly mas- ticated, the process of digestion will be more rapid than otherwise. 536. The art of cooking has as much to do with the digestibility of food as any circumstances belonging to the food itself, or the man- ner in which it is received into the stomach. The immediate object of cooking, as practiced by all civilized nations, is the gratification of the palate, the promotion of digestion being a secondary object. Cooking, for the most part, produces no chemical change in the con- stitution of food ; it simply destroys its organization, and softens its texture. The process of frying, however, has an effect upon all ani- mal fat or oils, which is exceedingly unfavorable to digestion. For the same reason, melted butter, buttered toast, butter-cakes, pastry, marrow and suet puddings, are all difficult of digestion, and "lie heavy on the stomach," as it is termed. Butter, and all fat substances, if used as food, should not be subjected to any process by which they become melted and their fixed oils set free. The whole process of pastry-cooking is at war with digestion. Articles of food that are naturally easy of digestion, become the most obnoxious to the digest- ive organs, by being compounded together. Thus eggs, fresh butter, bread, and sugar, are each very wholesome, and readily digested, when eaten separately; but when the eggs and butter are combined with the flour and sugar, to form cake, the compound may almost defy the powers of the stomach. Eggs, too, when slightly boiled, will not offend the most delicate stomachs; but, when fried hard in animal fat or butter, they are exceedingly difficult of digestion by the most vigor- ous. All compound food, or such as is formed by cooking several simple articles of diet in combination, is found more or less indigesti- ble, according to the richness of the compound. DIET OF CHILDREN. 537. We have already seen that the kind of food proper for each individual depends on as great a variety of circumstances as there are conditions of life. No specifications of diet can, therefore, be made applicable to every person. There should, however, be a marked difference between the diet of children and that of adults. Previous to dentition, the only diet proper for the infant is such as nature has provided the mother's milk. When the young infant must be deprived of this provision for its wants, through inability or disinclination of the mother, a very close approximation to its natural diet may be made from the top of cow's milk that has stood two or three hours, combined with two parts of water and a small quantity of sugar. This mixture contains the same chemical elements, in nearly the same I) 535. What circun^jtances affect facility of digestion 1 536. What is said of cooking as affecting the digestibilily of food? What articles are mentioned as difficult of digestion ? How should fnt substances be used? What is said of pastry-cooking, and of the impro- priety of artificial compounds for diet? 537. What is the proper food of infants previous to dentition ? How may this natural food be imitated? When the teeth begin to appear, what is the proper diet ? What rules are given in regard to eating meat? proportions, as the mother's milk. It is found to be a very safe and wholesome diet for the young infant. No oilier diet has been so fully authorized, either by chemical analysis, or by the observations of experience. This, or the mother's milk, is the only food that should be allowed previous to dentition; and a vast amount of infantile suf- fering would be saved by its rigid adoption. When the teeth begin to appear, something more is required. Farinaceous food, or such as is prepared from wheat, rice,