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With engravings. Ifc )A<°jXo - i>Qj , x . jo, mm m ANIMAL PHYSIOLOGY / / jf-jr t » C tMh REGISTRAR QE THE UNIVERSITY OE LONDON. Htfo (Kbition, THOROUGHLY REVISED, AND PARTLY RE-WRITTEN. LONDON : H. G. BOHN, YORK STREET, COVENT GARDEN. 1859. O'T’ o 1947 London: It. CLAY, PRINTER, BREAD STREET HILL. T/ S^U TO SIE JAMES CLAEK, BAET. M.D. F.E.S. PHYSICIAN IN ORDINARY TO THE QUEEN AND TO PRINCE ALBERT, ETC. ETC. My dear Sir James, I cannot more appropriately inscribe this Treatise, haying for its object the general diffusion of sound Physio¬ logical knowledge, than to one whose Professional eminence is founded on his enlightened application of it to the pre¬ vention and cure of Disease, and who has ever been the consistent advocate of Liberal Education. > The grateful sense I entertain of many acts of personal ■> kindness, makes me feel additional pleasure in paying this humble tribute. ■ Believe me to remain, My dear Sir James, i Your obliged Friend and Servant, WILLIAM B. CARPENTER. > University Hall, London, January, 1859. PREFACE, The issue of the present Volume may be considered as an attempt to supply what the Author has long considered to be a deficiency in the literature of this country, — that, namely, of an Educational Treatise on Animal Physiology, which should at the same time communicate to its readers the facts of greatest importance as regards their practical bearing, and present these in such a form as to place the learner in possession of the essential 'principles of Physiological Science. The Author has followed the general plan of the Treatise on Animal Physiology contributed by Professor Milne-Edwards, one of the most eminent Naturalists in France (in which country it is not thought beneath the dignity of men of the highest scientific reputation to write elementary books for the instruction of the beginner), to the “ Cours Elementaire d’Histoire Naturelle ” adopted by the French Government as the text-book of instruction in the Colleges connected with the University of Paris, which requires from every Candidate for its Degree of “ Bachelor of Sciences ” a competent know¬ ledge both of Animal and of Vegetable Physiology. He has also had at his disposal the admirable series of Illustrations prepared for that work, which, as a whole, are unsurpassed either in beauty or in exactness. In carrying-out this plan, however, the Author has entirely followed his own judgment ; and has made so much more use b VI PREFACE. of his own materials than of those supplied by the treatise of Professor Milne-Edwards, that the work may be regarded as almost entirely original. The present Edition, too, has undergone very considerable modifications ; the first chapter, which now contains a complete outline of the Elementary Tissues of the Animal Body, and the last, in which a com¬ prehensive sketch is given of the principal phenomena of Reproduction and Development throughout the Animal' Kingdom, having been entirely re-written and illustrated with numerous additional figures. In order to make room for the large amount of new matter now introduced (not less than one-fifth of the entire volume), the second chapter, contain¬ ing a General View of the Animal Kingdom, has been much abridged ; — a change the Author has the less regretted being obliged to make, since there are now before the public several excellent Elementary Treatises on Zoology, which had no existence at the time this volume originally appeared. Everyone who desires to see the study of Physiology duly appreciated as a branch of General Education, must feel gratified at the progress which has been made of late years in the public recognition of its value. The University of London led the way, by the introduction of Animal Physiology into the programme of study to which all Candidates for its Degree of Bachelor of Arts are required to conform. The Universi¬ ties of Oxford and Cambridge have since admitted it as one of the subjects which Candidates may select for their Bachelor of Arts Examination, and in which they may obtain Honours. And in many of the large Public Educational Institutions with which this country is now so abundantly furnished, it forms a part of the regular course of instruction. It has been the Authors steady aim, not merely to adapt his treatise to the wants of those who wish to acquire a general knowledge of the principal facts and doctrines of Physiological Science, but also to render it suitable to that PREFACE. Vll which he considers a far more important purpose of the study, — namely, the culture and discipline of the Mind itself Having been satisfied, by no inconsiderable experience of different modes of Education, that Natural Science, if judiciously taught, is second in value to no other subject as an educational means , and that it may be made to call forth a more varied and wholesome exercise of the mental powers than almost any other taken singly, — he has kept this purpose constantly in view ; and he trusts that the experience of intelligent In¬ structors will be found so far to concur with his own, that the study of Physiology may be still more generally introduced into Popular Education. It can only be by the general diffu¬ sion of sound information on this subject, that the Public Mind can be led to understand the difference between Rational Medicine, and that Empiricism which now presents itself under so many different forms ; that it can appreciate the true value of measures of Sanitary Reform, the efficiency of which must depend upon the amount of support they receive from an intelligent public opinion ; and that it can be preserved from those Epidemic Delusions, whose preva¬ lence, from time to time, is not less injurious to the minds of which they lay hold, than is that of Epidemic Diseases to the bodies of those who suffer from them. He has only further to add that, whilst keeping in view the most important practical applications of the Science of Physiology, he has not thought it desirable to pursue these too far; since they constitute the details of the Art of pre¬ serving Health, which is founded upon it, and which may be much better studied in a distinct form, when this outline of the Science has been mastered. And, for the same reason, he has adverted but slightly to those inferences respecting the Infinite Power, Wisdom, and Goodness, of the Great Eirst Cause, which are more obvious, although, perhaps, not really more clear and valid, in this Science, than in any other. Vlll PREFACE. Believing, as he does, that such inferences are more satisfac¬ torily based upon the general manifestations of Law and Order, than upon individual instances of Design, he has thought it the legitimate object of this treatise to lay the foundation for them, by developing, so far as might be, the Principles of Physiology, — leaving it to special treatises on Natural Theology, to build-up the applications. University Hall, London, Jan. 1859. CONTENTS. - - PAGE INTRODUCTION . 1 CHAPTER I. On the Vital Operations of Animals, and the Instruments BY WHICH THEY ARE PERFORMED . 17 CHEMICAL CONSTITUTION OF THE ANIMAL BODY ... 31 STRUCTURE OF THE PRIMARY TISSUES . 36 CHAPTER II. General View of the Animal Kingdom . 84 Vertebrata . 84 MAMMALS . 89 BIRDS . 92 REPTILES . 93 BATRACHIA . 97 FISHES . 100 Articulata . . 102 INSECTS . 104 ARACHNID A . 105 CRUSTACEA . 106 CIRRHIPEDA . . 109 MYRIAPODA . , ... 110 ANNELIDA . ib. ENTOZOA . Ill Mollusca . 112 CEPHALOPODA . 116 PTEROPODA . 117 GASTEROPODA . . 118 CONCHIFERA . ib. TUNICATA . 1 21 POLYZOA . 122 X CONTENTS. CHAPTER II. — Continued . PAGE Radiata . 123 ECHINODERMATA . 125 ACALEPM . 128 POLYPIFERA . 129 Protozoa . 135 rhizopoda ................ 136 INFUSORIA . 139 PORIFERA . 149 CHAPTER III. Nature and Sources of Animal Food ....... , 142 CHAPTER IY. Digestion and Absorption . 162 prehension of food . 163 mastication . . 166 IN SALIVATION . . 176 DEGLUTITION . 178 DIGESTIVE APPARATUS . 181 GASTRIC DIGESTION : — CHYMIFICATION . 188 INTESTINAL DIGESTION : — CHYLIFICATION . .193 DEFECATION . 195 ABSORPTION OF NUTRITIVE MATERIAL ....... 196 SANGUIFICATION . . ...199 CHAPTER V. Of the Blood, and its Circulation . . 202 PROPERTIES OF THE BLOOD . 203 CIRCULATION OF THE BLOOD . 216 CIRCULATING APPARATUS OF THE HIGHER ANIMALS . . 222 FORCES THAT MOVE THE BLOOD . 232 COURSE OF THE BLOOD IN THE DIFFERENT CLASSES OF ANIMALS . 240 CONTENTS. XI CHAPTER VI PAGE Op Respiration . 258 NATURE OF THE CHANGES ESSENTIALLY CONSTITUTING RESPIRATION . 259 STRUCTURE AND ACTIONS OP THE RESPIRATORY APPARATUS 265 CHAPTER VII. Op Excretion and Secretion . 292 GENERAL PURPOSES OP THE EXCRETING PROCESSES . . lb. NATURE OP THE SECRETING PROCESS.— STRUCTURE OP THE SECRETING ORGANS . 298 5 CHARACTERS OP PARTICULAR SECRETIONS . 304 CHAPTER VIII. General Review op the Nutritive Operations. — Formation op the Tissues . 316 GENERAL REVIEW OP THE NUTRITIVE OPERATIONS . . lb. FORMATION OP THE TISSUES . 317 REPAIR OP INJURIES . 323 CHAPTER IX. On the Evolution of Light, Heat, and Electricity by Animals . 327 animal luminousness . ib. animal heat . 332 animal electricity . 340 CHAPTER X. Functions of the Nervous System . 345 STRUCTURE AND ACTIONS OP THE NERVOUS SYSTEM IN THE PRINCIPAL CLASSES OP ANIMALS . 350 FUNCTIONS OP THE SPINAL CORD.— REFLEX ACTION . . 374 FUNCTIONS OP THE GANGLIA OP SPECIAL SENSE. — CON¬ SENSUAL ACTIONS . 380 FUNCTION OP THE CEREBELLUM. — COMBINATION OF MUS¬ CULAR ACTIONS . 384 FUNCTION OF THE CEREBRUM.-- INTELLIGENCE AND WILL . 385 Xll CONTENTS. CHAPTER XI. PAGE On Sensation, and the Organs of the Senses . 387 SENSE OF TOUCH . 390 SENSE OF TASTE . 395 SENSE OF SMELL . 398 SENSE OF HEARING . 401 SENSE OF SIGHT . 413 CHAPTER XII. Of Animal Motion, and its Instruments . 443 CONTRACTILE TISSUES.— MUSCULAR CONTRACTILITY . . 444 APPLICATIONS OF MUSCULAR POWER. — BONES AND JOINTS . 453 MOTOR APPARATUS OF MAN. — SKELETON AND MUSCLES . 464 OF THE ATTITUDES OF THE BODY, AND THE VARIOUS KINDS OF LOCOMOTION . 489 CHAPTER XIII. Of the Production of Sounds : Voice and Speech .... 513 CHAPTER XIV. Of Instinct and Intelligence . 525 MANIFESTATIONS OF INTELLIGENCE . 546 CHAPTER XV. Of Reproduction . * . 552 GEMMIPAROUS OR NON-SEXUAL REPRODUCTION .... 553 SEXUAL REPRODUCTION, OR GENERATION . 557 ANIMAL PHYSIOLOGY. INTRODUCTION. The importance of the study of Animal Physiology, as a branch of General Education, can scarcely he over-estimated ; and it is remarkable that it is not more generally appreciated. It might have been supposed that curiosity alone would have led the mind of Man to the eager study of those wonderful actions by which his body is constructed and maintained ; and that a knowledge of those laws, the observance of which is necessary for the due performance of these actions, — in other words, for the maintenance of his health , — would have been an object of universal pursuit. That it has not hitherto been so, may he attributed to several causes. The very familiarity of the occurrences is one of these. We are much more apt to seek for explanations of phenomena that rarely present themselves, than of those which, we daily witness. The Comet excites the world’s curiosity, whilst the movements of the sun, moon, and planets are regarded as things of course. We almost daily see vast numbers of animals of different tribes, in active life around us ; their origin, growth, movements, decline, death, and reproduction, are continually taking place under our eyes ; and there seems to common apprehension nothing to explain, where everything is so apparent. And of Man too, the ordinary vital actions are so familiar, that the study of their conditions appears superfluous. To he horn, to grow, to he subject to occasional disease, to decline, to die, is his lot in common with other animals ; and whafc know¬ ledge can avail (it may he asked) to avert the doom imposed on him by his Creator h 2 INTRODUCTION. In reply to this it is sufficient to state, that millions annually perish from a neglect of the conditions which Divine wisdom has appointed as requisite for the preservation of the body from fatal disease ; and that millions more are constantly suffering various degrees of pain and weakness, that might have been prevented by a simple attention to those principles which it is the province of Physiology to unfold. Prom the moment of his birth, the infant is so completely subjected to the in¬ fluence of the circumstances in which he is placed, that the future development of his frame may be said to be governed by them ; and thus it depends, in great part, upon the care with which he is tended, and the knowledge by which that care is guided, whether he shall grow up in health and vigour of body and mind ; or shall become weakly, fretful, and self- willed, a source of constant discomfort to himself and to others ; or shall form one of that vast proportion, whose lot it is to be removed from this world before infancy has ex¬ panded into childhood. The due supply of warmth, food, and air are the principal points then to be attended to ; and on every one of these the greatest errors of management prevail. Thousands and tens of thousands of infants annually perish during the few first days of infancy, from exposure to cold, which their feeble frames are not yet able to resist ; and at a later period, when the infant has greater power of sustain¬ ing its own temperature, and is consequently not so liable to suffer from this cause, the seeds of future disease are sown, by inattention to the simple physiological principles, which should regulate its clothing in accordance with the cold or heat of the atmosphere around. Nor is less injury done by inattention to the due regulation of the diet, as to the quan¬ tity and quality of the food, and the times at which it should be given ; the rules for which, simple and easy as they are, are continually transgressed through ignorance or carelessness. And, lastly, one of the most fertile sources of infantile dis¬ ease, is the want of a due supply of pure and wholesome air; the effects of which are sure to manifest themselves in some way or other, though often obscurely and at a remote period. It is physiologically impossible for human beings to grow up in a sound and healthy state of body and mind, in the midst of a close, ill-ventilated atmosphere. Those that are least able to resist its baneful influence, are carried off by the dis- INTRODUCTION. 3 eases of infancy and childhood ; and those whose native vigour of constitution enables them to struggle through these, become the victims, in later years, of diseases which cut short their term of life, or deprive them of a large part of that enjoyment which health alone can bring. Nor is the effect of these injurious causes confined to infancy, though most strikingly manifested at that period. “ The child is father to the man,” in body as well as in mind ; but the vigorous health of the adult is too often wasted and destroyed by excesses, whether in sensual indulgence, in bodily labour, or in mental exertion, to which the very feeling of buoyancy and energy often acts as the incentive ; and the strength which, carefully husbanded and sustained, might have kept the body and mind in activity and enjoyment to the full amount of its allotted period of “ threescore years and ten,” is too frequently dissipated in early manhood. Or, again, the want of the necessary conditions for the support of life, — the warmth, food, and air, on which the body depends for its continued sustenance, no less than for its early deve¬ lopment, — may cause its early dissolution, even where the individual is guiltless of having impaired its vigour by his own transgressions. These statements are not theoretical merely : they are based upon facts drawn from observations carried oil upon the most extensive scale. Wherever we find those conditions, which the Physiologist asserts to be most favourable to the preservation of the health of the body, most completely fulfilled, there do sickness and mortality least prevail. A few facts will place this subject in a striking light. “ The average mortality of infants among rich and poor in this country (and with little variation throughout Europe) is about one in every four and a-half before the end of the first year of existence. So directly, however, is infant life influenced by good or bad management, that, about a century ago, the workhouses of London presented the astounding result of twenty-three deaths in every twenty- four infants under the age of one year. Eor a long time this frightful devastation was allowed to go on, ns beyond the reach of human remedy. But when at last an improved system of management was adopted in consequence of a parliamentary inquiry having taken place, the proportion of deaths was speedily reduced from 2,600 to 450 in a year. Here, then,-, b 2 4 INTRODUCTION. was a total of 2,150 instances of loss of life, occurring yearly in a single institution, chargeable, not against any unalterable decrees of Providence, as some are disposed to contend as an excuse for tbeir own negligence ; but against tbe ignorance, indifference, or cruelty of man. And wbat a lesson of vigi¬ lance and inquiry’ ought not such occurrences to convey, when, even now, with all our boasted improvements, every tenth infant still 'perishes within a month of its hirih ! ” 1 Tbe effect of attention to cleanliness and ventilation in tbe reduction of an excessive infantile mortality, has been equally shown in tbe experience of tbe Dublin Lying-in Hospital. At tbe conclusion of 1782, it was found that out of 17,650 infants born alive, no fewer than 2,944, or one in every six , bad died within tbe first fortnight. By tbe more efficient ventilation of tbe wards, tbe proportion of deaths during tbe first fortnight was at once reduced to 419 out of 8,033, or but little more than one in twenty ; and it has subsequently been still further diminished. In tbe island of St. Hilda, tbe most northern of tbe Heb¬ rides, according to tbe statement of a gentleman who visited it in 1838, as many as eight out of every ten children die between tbe eighth and twelfth day of tbeir existence ; in consequence of which terrible mortality, tbe population of tbe island is diminishing rather than increasing. This is due, not to anything injurious in tbe position or atmosphere of tbe island ; for its a air is good, and tbe water excellent : ” but to tbe “ filth in which the inhabitants live, and the noxious effluvia which pervade their houses.” The huts are small, low- roofed, and without windows ; and are used during the winter as stores for the collection of manure, which is carefully laid out upon the floor, and trodden under foot, till it accumulates to the depth of several feet. The clergyman, who lives exactly as those around him do, in every respect, except as regards the condition of his house, has reared a family of four children, all of whom are well and healthy ; whereas, accord¬ ing to the average mortality around him, at least three out of the four would have been dead within the first fortnight. It is not a little remarkable that a recent sanitary inquiry carried out by order of the Danish government, into the con- 1 Dr. A. Combe on the Physiological and Moral Management of Infancy. INTRODUCTION. 5 dition of the Icelandic population, should ha, ye disclosed the existence of almost precisely similar habits of life among them, with almost precisely the same results. The dwellings of the great bulk of the peasantry seem as if constructed for the express purpose of poisoning the air which they contain. They are small and low, without any direct provision for ventilation, the door serving alike as window and chimney ; the walls and roof let in the rain, which the floor, chiefly composed of hardened sheep’s-dung, sucks up ; the same room generally serves for all the uses of the whole “family, and not only for the human part of it, but frequently also for the sheep, which are thus housed during the severest part of the winter. The fuel employed in this country chiefly con¬ sists of cow-dung and sheep’s-dung, caked and dried ; and near the sea-coast, of the bones and refuse of fish and sea- fowl ; producing a stench, which to those unaccustomed to it is completely insupportable. In addition to this, the people are noted for their extreme want of personal cleanliness ; the same garments (chiefly of black flannel) being worn for months without having even been taken off at night. Although the Icelanders enjoy an almost complete exemption from many diseases (such as consumption) which are very fatal elsewhere, and the number of births is fully equal to the usual average, the population of the island does not increase, and in some parts actually diminishes. This result is in great measure due, as at St. Hilda, to the very high rate of infantile mortality ; a large proportion of, all the infants born being carried off before they are a fortnight old. It is in the little island of Westmannoe, and the opposite parts of the coast of Iceland, where the bird-fuel is used all the year round, instead of (as elsewhere) during a few months only, that the rate is the highest ; the average mortality for many years having been sixty-four out of every hundred, or nearly two out of three , of all the infants born in these localities. But it is yet more remarkable that the immediate cause of the high rate of infantile mortality should have been pre¬ cisely the same in the Workhouses of London, the Lying-in Hospital of Dublin, and the close filthy huts of the peasantry of Iceland and St. Hilda ; for it was almost entirely referrible to one single disease, “ Trismus nascentium,” or, “ Lock-jaw of the Hew-born ; ” and this disease has diminished in exact 6 INTRODUCTION. proportion to the improvement of the places it previously infested, in respect to ventilation and cleanliness. Thus, it is so rare for a case of it now to occur in London, that many practitioners of large experience have never seen the disease. In the Dublin Lying-in Hospital, the number of deaths from it has been reduced to three or four yearly. And there can¬ not be a reasonable* doubt, that, by due attention to the same conditions, it might be exterminated from Iceland and from St. Hilda. There is scarcely, in fact, a disease incident to humanity, which is more completely preventable than this ; and yet the annual sacrifice of life which it formerly caused in our own country alone, might have been reckoned by tens of thousands. Although the peculiar susceptibiltty of the constitution of children, gives to foul air and other causes of disease a much more destructive influence over them, than the. like causes have over persons more advanced in life, yet it is now well ascertained that the rate of mortality among different classes of the community varies in a degree which bears a very close relation to the nature of the conditions under which they live. Thus, whilst the annual average number of deaths in the whole of England and Wales is about. 22 out of every thousand persons living, there are localities in which the annual average exceeds 50 in a thousand, and others in which it falls as low as 11 in a thousand. And it is not a little remarkable, that the difference is almost entirely referable to the mortality produced by Fevers and allied diseases, which, as experience has now fully demonstrated, are absolutely preventible by due attention to the ordinary conditions of health. As the population of England and Wales may at present be estimated at about twenty millions, and its actual mortality at about 440,000, what maybe termed its inevitable mortality — arising from diseases that would not be directly affected by sanitary improvements — would be only one half, or 220,000 ; so that the same number of lives may be considered to be annually sacrificed by the public neglect of the means of pre¬ serving them, “—the deaths from typhus alone being no fewer than 50,000. But as it is scarcely to be supposed that every part of our population could be placed in conditions as favour¬ able as those which prevail where the rate of mortality is the lowest, we may take 13 per thousand as the average to INTRODUCTION. 7 which it may be safely affirmed, on the basis of actual expe¬ rience, that the annual mortality may be reduced, by such efficient sanitary measures as render the dwellings of the mass of the population fit for human habitation ; this would give an annual mortality for England and Wales of 260,000, showing a saying of 180,000 lives annually in that one por¬ tion of the British empire. And it must be remembered that this amount of mortality represents a vastly greater amount of sickness, since, for every death, there are numerous cases of severe illness ; so that it would be scarcely too much to affirm that at least a million out of the whole number of such cases annually occurring, are preventible, like the 180,000 deaths, by adequate provisions for the supply of pure air and water, and by efficient sewerage for the removal of decomposing matters. It cannot be doubted that, even in a mere pecuniary point of view, the expense of such arrange¬ ments would be amply compensated by the prevention of a vast amount of .that loss of productive labour of various kinds, which is at present due to disease ; and, considered on the large scale, as a question of social economy, the import¬ ance of sanitary legislation can scarcely be over-rated. But much cannot be expected to be done in this direction, until such an intelligent 'public opinion shall have been created, by the general diffusion of sound physiological information, as shall be sufficiently forcible to bear down the self-interested opposition of those, who do not see that the value of their property will be permanently increased at least in proportion to the amount of money judiciously expended upon it. A more remarkable illustration of what is to be effected by sanitary improvements can scarcely be adduced, than that which is presented by the comparison between the locality termed “the Potteries,” in the immediate vicinity of Ken¬ sington, and the “ Model Lodging-houses,” which have been erected in various parts of the Metropolis. The site of the group of dwellings constituting the former is far from being insalubrious in itself, and rows of handsome houses are rising up in its immediate neighbourhood; but the condition of these dwellings is most filthy. A few years ago, as many as 3,000 pigs were kept in this locality (the number has since been somewhat diminished) ; and the boiling of fat and other offal, which is carried on by some of the pig-feeders, some- 8 INTRODUCTION. times taints the air for a mile round. Very few of the tene¬ ments have any water-supply ; the wells are useless, or worse than useless, through the contamination of their water with putrescent liquid which filters down into them ; and the drainage of the dwellings both for men and pigs is almost entirely superficial, being chiefly discharged into a stagnant piece of water called the “ Ocean,” which is covered with a filthy slime and bubbles with poisonous gases, and very commonly has dead dogs or cats floating on its surface. It is difficult to conceive anything more horribly offensive than the rears of some of the houses, whose yards are filled with ordure and other filth collected for manure, which is here stored for weeks, or even months, until an opportunity occurs for selling it. And even the public ways are generally covered with black putrescent mire. How, during ten months of the year 1852, when no epidemic prevailed, as many as forty deaths occurred in the Potteries, out of a population of about one thousand, — the mortality being thus at the rate of 48 per thousand annually ; and no fewer than four-fifths of these deaths occurred at, or beneath, five years of age. In the first ten months of 1849, when cholera was prevalent, the number of deaths was fifty, or about one in twenty of the whole population, twenty- one of these being due to cholera and diarrhoea, and twenty-nine to typhus and other diseases. — On the other hand, in the whole population of the “ Model Lodging-houses,” amounting to 1,343, only seven deaths took place in the whole twelve months of 1852, or at the rate of scarcely more than 5 per thousand; and although they contain a large proportion of children, yet only half the number of deaths occurred below ten years old. During the prevalence of the cholera-epidemic, no cases of that disease occurred among them, although it was raging in their various neighbourhoods ; and from the time that their drainage has been rendered thoroughly efficient, no case of fever has presented itself among their inmates. The experience of Cholera- epidemics is peculiarly valuable, on account of the marked tendency of this disease to search out and expose defects, which have continued to produce other diseases year after year, without having been suspected as the causes of them. The greatest severity in each visita¬ tion has shown itself in identical localities, provided those INTRODUCTION. 9 remained in the same font state as at first ; whilst new loca¬ lities have been affected, just in proportion to the degree in which they have participated in the same conditions ; and those originally attacked have escaped, wherever they had adopted the requisite means of purification. Thus, at Newcastle-on- Tyne and Gateshead, the first outbreak occurred in the very same streets, and even in the same houses, in the three visi¬ tations of 1831, 1848, and 1853. An outbreak which occurred in 1853, at Luton, in Bedfordshire, — -vhere, out of a population of 126 persons, inhabiting twenty-five houses, no fewer than fifty-four attacks of choleraic disease, fifteen of them fatal, took place within three weeks, — was most dis¬ tinctly traceable to defect of sewerage, which had been pre¬ viously manifesting its malign influence on the general health of the town. And the fearful pestilence which devastated the neighbourhood of Golden Square (London) in the autumn of 1854, was no less distinctly traceable to the contamination of the pump-water by the bursting of a sewer into the well. On the other hand, Exeter and Nottingham, which suffered severely in the first epidemic, escaped comparatively un¬ harmed in the subsequent visitations ; and this result is plainly due to the sanitary improvements which had been made in the interval. In 1832 there perished of the epide¬ mic in Exeter, as many as 402, out of a population of 28,000, or no fewer than one in seventy ; and a vast amount of suffering, with a heavy expense, was entailed upon the town. In 1848-9, on the other hand, out of a population of about 32,600, there were but 44 deaths, or less than one in seven hundred ; and upwards of one-half of these occurred in a single parish, that lies very low, and in the midst of putrid exhalations from the city drains. In Nottingham, with a population of 50,000, there were 296 fatal cases of cholera in 1832, nearly all of these being in the lower part of the town, which was ill-drained, extremely filthy, and densely popu¬ lated ; but in 1848-9, though the population had increased to 58,000, the number of deaths from cholera was no more than 18, all of these occurring in localities, which, in spite of what had been done, retained much of their previous filth. The foregoing are only samples of a vast number of e^ses which might be adduced, in proof of the absolute preventi- bility of Cholera, and of other diseases of the same class. It 10 INTRODUCTION. may be well to subjoin a few additional facts, derived from the cholera-experience of 1848-9, which, from its general diffusion, tested, in a very remarkable degree, the relative healthfulness of different provincial towns, and of different metropolitan districts. Thus, among the whole population of the ten towns of Exeter, Derby, Cheltenham, Leicester, Nottingham, Eochdale, Norwich, Preston, Halifax, and Bir¬ mingham, amounting to 657,000, there were no more than 238 deaths from cholera ; whilst, in an equal population inhabiting the towns of Newcastle-under-Lyne, Plymouth, Brighton, Merthyr Tydvil, Portsea, Tynemouth, Wigan, Hull, Wolverhampton, and Leeds, the number of deaths was no fewer than 10,415, on forty-three tunes as great. So again, in twenty-five Metropolitan districts, chiefly on the north side of the Thames, having a total population of about 310,000, the number of deaths from cholera was only 389 ; whilst in twenty-two districts, almost entirely on the south side of the river, the number of deaths, out of a population of almost exactly the same amount, was 5,932, or more than twelve times as great. In no instance is there the least difficulty in accounting for these contrasts. They all point to the same general conclusion; that, namely, of the immense influence which is exercised over human health by the purity of the air that is breathed, and of the water that is drunk ; and it is because these two conditions are in a great degree capable of public regulation, that legislative interference has so much in its power, and is so imperatively called for by the interests of humanity, which speak solemnly and distinctly to all who claim the rights of property in the foul “ plague-spots ” which deface our country, of their bounden duty to render them not unfit for human occupation. But although the magnitude of the evils resulting from the neglect of the conditions of Public Health, gives to this sub¬ ject the first claim on our consideration, yet it is not the less important that every individual should acquire as much knowledge of the constitution of his body, and of the right means of keeping it in working order, as will save him from seriously damaging either himself or other people by his ignorance of such matters. It is less than ten years since a fearful sacrifice of life occurred among the deck-passengers on board the Irish steamer f Londonderry,” who were ordered INTRODUCTION. 11 below by the Captain on account of the stormy character of the weather, and on whom the hatches were closed down, although the cabin which was crowded by them had scarcely any other means of ventilation. Out of 150 of these unfor¬ tunates, no fewer than 70 died of suffocation before the morning, — a catastrophe only second to that which occurred in the “Black Hole of Calcutta,” in which 123 out of 146 died during one night’s confinement in a room eighteen feet square, provided with only two small windows. Yet the Captain of the “ Londonderry ” was acquitted of all blame ; since he had done what seemed to him best for the welfare of his passengers, the result being due simply to his astound¬ ing ignorance of the fact that men cannot live without having air to breathe. Hot a year passes without the occurrence of numerous deaths from the like cause ; and yet these are really insignificant, when compared with the vast amount of disease which is constantly attributable to inattention, on the part of individuals, to those simple means of securing an adequate supply of air which are within the reach of every one. And when we bear in mind that the respiratory func¬ tion is only one of the processes whose due performance has to be provided for, and that the regulation of the food and drink, of the excretions, of clothing and temperature, of exercise (bodily and mental) and repose, and of the repro¬ ductive functions, all fall within rules which it is the pro¬ vince of Physiology to prescribe, we see how vain it is to expect that the body can be maintained in health, without some acquaintance with that science, or at least with the rules which it lays down. Por, although it is quite true that man has within himself certain instincts which afford him a considerable measure of guidance in all these particulars, — hunger and thirst, for example, leading him to take the sustenance which his body requires, weariness tempting him to needed repose, and so on, — yet it is no less certain that in a state of artificial civilisation these instincts are so often over¬ borne by acquired tastes, or by the pressure of other circum¬ stances, that they cannot alone be safely relied on. Hence it is all the more important that the rules for preserving health should be based on an intelligent knowledge of Physiological principles ; otherwise, like the natural instincts, they are likely to be put aside as occasion prompts ; whereas, in proportion as 12 INTRODUCTION. the individual is possessed of their rationale , will he be likely to shape his conduct in accordance with them. The general principles of Physiological science, again, will be likely to be thoroughly apprehended, in proportion as they are based on an extended recognition of the phenomena which they comprehend. Every physiologist is now satisfied that the life or vital actions of no one species of animal can be correctly understood, unless compared with those of other tribes of different conformation. Hence, for the student of physiology to confine himself to the observation of what takes place in Man alone, would be as absurd as for the astro¬ nomer to restrict himself to the observation of a single planet, or for the chemist to endeavour to determine the properties of a metal by the study of those of that one only. There is not a single species of animal, that does not present us with a set of facts which we should never learn but by observing it ; and many of the facts ascertained by the observation of the simplest and most common animals, throw great light upon the great object of all our inquiries, the Physiology of Man. Eor though in him are combined, in a most wonderful and unequalled manner, the various faculties which separately exhibit themselves in various other animals, he is not the most favourable subject for observing their action ; for the obvious reason that his machinery (so to speak) is rendered too complex, on account of the multitude of operations it has to perform : so that we often have to look to tjie lowest and simplest animals for the explanation of what is obscure in man, their actions being less numerous, and the conditions which they require being more easily ascertained. The diffusion of Animal life is only one degree less exten¬ sive than that of vegetable existence. As animals cannot, like plants, obtain their support directly from the elements around, they cannot maintain life, where life of some kind has not preceded them. But vegetation of the humblest character is often sufficient to maintain animals of the highest class. Thus the lichen that grows beneath the snows of Lapland, is, for many months in the year, the only food of the rein-deer ; and thus contributes to the support of human races, which depend almost solely upon this useful animal for their existence. Ho extremes of temperature in our atmo¬ sphere seem inconsistent with animal life. In the little pools INTRODUCTION. 13 formed by tbe temporary influence of the sun upon the sur¬ face of the arctic snows, animalcules have been found in a state of activity ; and the ocean of those inhospitable regions is tenanted, not only by the whales and other monsters which we think of as their chief inhabitants, whose massive forms are only to be encountered “ few and far between,” but by the shoals of smaller fishes and inferior animals of various kinds upon which they feed, and through vast fleets of which the mariner sails for many miles together. On the other hand, even the hottest and most arid portions of the sandy deserts of Africa and Asia are inhabited by animals of various kinds, provided that vegetables can find sustenance there. The humble and toilsome ants make these their food, and become in turn the prey of the cunning ant- lion and of the agile lizard ; and these tyrants are in their turn kept under by the voracity of the birds which are adapted to prey upon them. The waters of the tropical ocean never acquire any high temperature, owing to the constant interchange which is taking place between them and those of colder regions ; but in the hot springs of various parts of the world, we have examples of the compatibility of even the heat of almost boiling water with the preservation of animal life. Thus in a hot spring at Manilla which raises the ther¬ mometer to 187°, and in another in Barbary whose usual tem¬ perature is 172°, fishes have been seen to flourish. Bishes have been thrown up in very hot water from the crater of a volcano, which, from their lively condition, was apparently their natural residence. Small caterpillars have been found in hot springs of the temperature of 205°; and small black beetles, which died when placed in cold water, in the hot sulphur baths of Albano. Intestinal worms within the body of a carp have been seen alive after the boiling of the fish for eating ; and the inhabitants of some little snail- shells, which seemed to have been dried up within them, have been caused to revive by placing the shells in hot water for the purpose of cleaning them. The lofty heights of the atmosphere, and the dark and rayless depths of the ocean, are tenanted by animals of beautiful organisation and wonderful powers. Vast flights of butterflies, the emblems of summer and sunshine, may some¬ times be seen above the highest peaks of the Alps, almost 14 INTRODUCTION. touching with their fragile wings the hard surface of the never-melting snow. The gigantic condor or vulture of the Andes has been seen to soar on its widely-expanded wings far above the highest peak of Chimborazo, where the baro¬ meter would have sunk below ten inches. The existence of marine fishes has been ascertained at a depth of from 500 to 600 fathoms ; and in the deep recesses of those caverns in Styria and Carniola, which are inhabited by the curious Proteus (Zool. § 532), numerous species of insects are found, all of which, however, like the Proteus, are blind. Having thus glanced at some of those facts which demon¬ strate the practical importance of the study of Physiology, and having indicated ’ the mode in which that study should be pursued, it remains to offer a few observations upon its value with reference to the culture and discipline of the mind itself. One of its great advantages is, that it not only calls forth, in a degree second to no other, both the observing and the reasoning powers ; but that it offers so much that is attractive by its novelty to those who enter upon it seriously, and make it an object of regular pursuit. For it affords abundant opportunities, even to the beginner, of adding to the common stock of information respecting the structure and habits of the vast number of living beings that people our globe. The immense variety of the objects which come under the investigation of the physiologist, so far from discouraging the learner, should have the effect of stimulating his exertions, by opening to him new fields for productive cultivation. Of by far the larger part of the organised crea¬ tion, little is certainly known. Of no single species, — of none of our commonest native animals, — not even of Man himself, — can our knowledge be regarded as anything but im¬ perfect. Of the meanest and simplest forms of animal life, we know perhaps even less than we do of the more elevated and complex ; and it cannot be doubted that phenomena of the most surprising nature yet remain to be discovered by patient observation of their actions. It was not until very recently, that the existence of a most extraordinary series of metamor¬ phoses, more wonderful than those of the insect, has been discovered in the jelly-fish of our seas, in the barnacles that INTRODUCTION. 15 attach themselves to floating pieces of timber, and in the crabs, lobsters, and shrimps of our shores. The very best accounts we have, of the structure, habits, and economy of the lower tribes of animals, have been furnished to us by individuals who did not think it beneath them to devote many years 'to the study of a single species ; and as there are very few which have been thus fully investigated, there is ample opportunity for every one to suit his own taste in the choice of an object. And none but those who have tried the experiment, can form an estimate of the pleasure which the study of Eature is capable of affording to its votaries. There is a simple pleasure in the acquisition of knowledge, worth to many far more than the acquisition of wealth. There is a pleasure in looking in upon its growing stores, and watching the expan¬ sion of the mind which embraces it, far above that which the miser feels in the grovelling contemplation of his hard-sought pelf. There is a pleasure in making it useful to others, com¬ parable at least to that which the man of generous benevo¬ lence feels in ministering to their relief with his purse or his sympathy. There is a pleasure in the contemplation of beauty and harmony, wherever presented to us. And are not all these pleasures increased, when we are made aware, — as in the study of Eature we soon become, — that the sources of them are never-ending, and that our enjoyment of them becomes more intense in proportion to the comprehensiveness of our knowledge ? And does not the feeling that we are not look¬ ing upon the inventions or contrivances of a skilful human artificer, but studying the wonders of a Creative Design infinitely more skilful, immeasurably heighten all these sources of gratification? If it is not every one who can feel all these motives, cannot every one feel the force of some ? ‘There is certainly no science which more constantly and forcibly brings before the mind the power, the wisdom, and the goodness of the Creator. Tor whilst the Astronomer has to seek for the proofs of these attributes in the motions and adjustments of a universe, whose nearest member is at a distance which imagination can scarcely realize, the Physio¬ logist finds them in the meanest worm that we tread beneath our feet, or in the humblest zoophyte dashed by the waves 16 INTRODUCTION. upon our shores, no less than in the gigantic whale, or massive elephant. And the wonderful diversity which exists amongst the several tribes of animals, presents us with a continual variety in the mode in which these adjustments are made, that prevents us from ever growing weary in the search. Eut it is not only in affording us such interesting objects of regular study, that the bounty of Nature is exhibited. Perhaps it is even more keenly felt by the mind which, harassed by the cares of the world, or vexed by its disap¬ pointments, or fatigued by severer studies, seeks refuge in her calm retirement, and allows her sober gladness to exert its cheering and tranquillizing influence on the spirit. “ With tender ministrations, thou, 0 Nature, Healest thy wandering and distracted child ; Thou pourest. on him thy soft influences, Thy sunny hues, fair forms, and breathing sweets, The melody of woods, and winds, and waters.- — Till he relent, and can no more endure To be a jarring and dissonant thing Amidst the general voice and minstrelsy, — But bursting into tears wins back his way. His angry spirit healed and harmonized By the benignant touch of love and mercy.” Coleridge,, DISTINCTIVE CHARACTERS OF ORGANIZED BODIES. 17 CHAPTEE I. OF THE VITAL OPERATIONS OF ANIMALS, AND THE INSTRUMENTS BY WHICH THEY ARE PERFORMED. 1. Living beings, whether belonging to the Animal or to the Vegetable kingdom, are distinguished from the masses of inert matter of which the Mineral kingdom is made up, by peculiarities of form and size, of structure, of elementary composition, and of actions. — "Wherever a definite form is exhibited by Mineral substances, it is bounded by plane surfaces, straight lines, and angles, and is the effect of the process of crystallization, in which particles of like nature arrange themselves on a determinate plan, so as to produce a regular aggregation ; and there is, probably, no Inorganic element or combination which is not capable of assuming such a form, if placed in circumstances adapted to the manifestation of its tendency to do so. The number of different crystalline forms is by no means large y and as many substances crystal¬ lize in several dissimilar forms, whilst crystals resembling one another in form often have a great diversity of composition, there is no constant correspondence between the crystalline forms and the essential nature of the greater number of mineral substances. If that peculiar arrangement of the molecules which constitutes crystallization should be wanting, so that simple cohesive attraction is exercised in bringing them together, without any general control over their direc¬ tion, an indefinite or shapeless figure is the result. With this indefiniteness of form, there is an absence of any limit whatever in regard to size : a crystal may go on increasing continuously, so long as there is new material supplied ; but this new material is deposited upon its surface merely, and its addition involves no interstitial change ; the older particles, which were first deposited, and which continue to form the nucleus of the crystal, remaining just as they were. In Or¬ ganized bodies, on the other hand, we meet with convex surfaces and rounded outlines, and with a general absence of angularity ; and the simplest grades, both of Animal and of 18 DISTINCTIVE CHARACTERS OF ORGANIZED BODIES. Vegetable life, present themselves under a shape which ap¬ proaches more or less closely to the globular. From the highest to the lowest, each species has a certain characteristic form, by which it is distinguished ; this form, however, often presents marked diversities at different periods of life, and it is also liable to vary within certain limits among the individuals of which the species is composed. The size of Organized structures, like their form, is restrained within tolerably definite limits, which may nevertheless vary to a certain extent among the individuals of the same species. These limits are most obvious in the higher animals, whilst they seem almost to disappear among certain members both of the Animal and the Vegetable kingdoms, which tend to increase themselves almost indefinitely by a process of gemmation or budding, so as to produce aggregations of enormous size. Such aggregations, however, being formed by the repetition of similar parts, which can maintain their existence when detached from one another, may, in some sense, be regarded as clusters of distinct organisms, rather than as single individuals. Such is the case, for example, with the wide-spreading forest-tree, and with those enormous masses of coral of which reefs and islands are composed in the Polynesian Archipelago. For every separate leaf-bud of the tree, like every single polype of the coral, if detached from its stock, can, under favourable circumstances, perform all the functions of life, and can develop itself into a new fabric resembling that from which it was separated. 2. The differences between Organized and Inorganic bodies, in regard to their structure, are much more important than those which relate to their external configuration. . Every particle of a mineral substance, in which there has not been a mere mixture of components, exhibits the same properties as those possessed by the whole ; the minutest atom of car¬ bonate of lime, for instance, has all the properties of a crystal of calc-spar, were it as large as a mountain. Hence it is the essential nature of an Inorganic body that each of its particles possesses a separate individuality, and has no relation but that of juxtaposition to the other particles associated with itself in one mass. — The Organized structure, on the other hand, receives its designation from being made up of a greater or less number of dissimilar parts or organs ; each of these DISTINCTIVE CHARACTERS OP ORGANIZED BODIES. 19 being tlie instrument of some special action or function, which, it performs under certain conditions ; and the concurrence of all these actions being necessary to the maintenance of the .structure in its normal or regular state. Hence there is a relation of mutual dependence among the parts of an Organized fabric, which is quite distinct from that of mere proximity ; and this relation is most intimate, not in the case of those beings which have the greatest multiplication of parts, but among those in which there is the greatest dissimilarity among the actions of the several organs. Thus it has been just shown that among Plants and Zoophytes, a small fraction of an organism may live independently of the rest; the necessary condition being that it shall either itself contain all the organs essential to life, or shall be capable of pro¬ ducing them, — as when the leaf-bud develops rootlets for its nutrition. This “vegetative repetition,” and consequent capacity of sustaining the loss of large portions of the fabric, still shows itself in animals much higher in the scale than Zoophytes ; thus it is not uncommon to meet with Star-fish in which not only one or two, out of the five similar arms, but even three or four, have been lost, without the destruction of the animal’s life ; and this is the more remarkable, as these arms are not simply members for locomotion or prehension, but are really divisions of the body, containing prolongations of the stomach. In like manner, many of the Worm tribes, whose bodies show a longitudinal repetition of similar parts, can lose a large number of their joints without sustaining any considerable damage. In the bodies of the higher animals, however, where there are few or no such repetitions (save in the two lateral halves of the body), and where there is, consequently, a greater diversity in character and function between the different organs, the mutual dependence of their actions upon one another is much more intimate, and the loss of a single part is much more likely to endanger the existence of the whole. Such structures are said to be more highly organized than those of the lower classes ; the principle of “ division of labour ” being carried much further in them, a much greater variety of objects being attained, and a much higher perfection in the accomplishment of them being thus provided for. Thus the individuality of a plant or a zoo¬ phyte may be said to reside in each of its multiplied parts ; c 2 20 DISTINCTIVE CHARACTERS OF ORGANIZED BODIES: whilst that of one of the higher animals resides in the smd of all its organs. 3. The very simplest Organized fabric is further dis¬ tinguished from Inorganic bodies by marked differences in regard to intimate structure and consistence. Inorganic sub¬ stances can scarcely be regarded as possessing a structure, since their perfection consists in their homogeneousness and their solidity. It is the essential character of Organized fabrics, on the other hand, that they are formed by a com¬ bination of solid and liquid components, so intimately combined and arranged as to impart a heterogeneous cha¬ racter to almost every portion of their substance ; and in all the parts which are most actively concerned in the vital operations, softness of texture seems an essential condition, — those parts only being so consolidated as to acquire anything comparable to the density of mineral bodies, which are destined to possess the simply physical property of resistance , so as to be subservient either to support, to protection, or to mechanical movement. A comparison between the pulpy portion of the leaves of Plants and the heartwood of the stem, between the membranous tissues of the Coral-polypes and the stony masses which they form, between the firm shell of the Crab or the Oyster and the substance of the included body, or between the solid bones of Man and the flesh which clothes them, will serve to illustrate this principle. It is in such solidified portions of the Organized fabric, that the greatest resemblance exists to Inorganic bodies; but even these portions all pass through the condition of soft tissue, the consolidation of which is effected by the deposit of some hardening material (generally carbonate or phosphate of lime), in its interstices. — It is by the reaction which is continually taking place between the solid and the liquid parts of Organized structures, that their integrity is maintained. For we shall find it to be a result of their peculiar composition, that they are prone to continual decay ; and this decay would speedily destroy them altogether, if it were not compensated by new formation. The materials for their reproduction must always be presented to the tissues in a liquid state, and all the dead and decomposing matter must be reduced to the same form, in order that it may be carried off ; so that the intermingling or mutual penetration of solids and liquids, in DISTINCTIVE CHARACTERS OF ORGANIZED BODIES. 21 the minutest parts of Organized bodies, is a necessary condition of their existence. 4. Organized structures are further distinguished from In¬ organic masses by the peculiarity of their chemical constitution. This peculiarity does not consist, however, in the presence of any elementary substances which are not found elsewhere ; for all the elements of which Organized bodies are composed, exist abundantly in the world around. This, indeed, is a necessary consequence of the mode in which they are built up ; for that which the parent communicates in giving origin to a new being, is not the structure itself, but the capacity to form that structure from materials supplied to it ; and it is by progressively converting these materials to its own use, that the germ develops itself into the complete fabric. — ISTow out of about seventy simple or elementary substances which are known to occur in the Mineral world, not above twenty present themselves as constituents of Vegetable and Animal fabrics ; and many of these occur there in extremely minute proportion. Some of them, indeed, appear to be introduced merely to answer certain chemical or mechanical purposes ; and the composition of the parts which possess the highest vital endowments is extremely uniform. They are nearly all formed at the expense of certain “ organic compounds,” which are made up of the four elementary substances, oxygen, hy¬ drogen, carbon, and nitrogen ; and these elements appear to be united, — not as in the case of inorganic compounds, two by two, or after the binary method, — but all four together, so as to form a compound atom of great complexity. Thus common nitre is regarded as a binary compound of nitric acid and potass, since it can be decomposed into those two con¬ stituents and can be re-formed by their union ; and in the same manner, its nitric acid is a binary compound of nitrogen and oxygen, whilst its potass is a binary compound of potassium and oxygen. But neither albumen nor gelatine, which are the principal materials of the animal tissues; can be resolved into any two other substances, by the union of which it can be re-formed ; and when once it has been decomposed by che¬ mical agencies, no means known to the chemist can reproduce it. Albumen can, in fact, be generated only by the living Plant, at the expense of the carbon, hydrogen, oxygen, and nitrogen, which it draws from the elements around; and 22 DISTINCTIVE CHARACTERS OF LIVING ORGANISMS. gelatine can only be formed in the animal body by a meta¬ morphosis of the albumen which it derives from the Plant. The peculiar mode in which the elements of these substances are held together, renders them very prone to decomposition ; so that Organized bodies, when no longer alive, rapidly pass into decay, unless they are secluded from the contact of oxygen, or are kept at a very low temperature. Such decay, however, is continually taking place during life, and would make itself obvious if its products were not carried out of the system as fast as they are generated within it. It essentially consists in the resolution of the four principal components of organic compounds — carbon, hydrogen, oxy¬ gen, and nitrogen, in combination with oxygen drawn from the atmosphere — into the three binary compounds, water, carbonic acid, and ammonia, which thus restore to the In¬ organic world the original materials of Organized fabrics, in the very forms from which those materials were first derived by the agency of the growing Plant. (See Veget. Physiol.) 5. It is, however, by their peculiar actions , that living Organisms are most completely differentiated from the inert bodies of which the Mineral kingdom is composed. There can be no doubt that of many of the changes which take place during the life of an Organized being, a large proportion (especially in the Animal kingdom) are effected by the direct agency of physical and chemical forces ; and there is no reason to believe that these forces have any other operation in the living body, than they would have out of it under similar circumstances. Thus the propulsion of the blood by the heart, through the large vessels, is a purely mechanical phenomenon ; as is also the movement of the limbs by the lever-action of the forces brought to bear on their bones. So, again, the digestive operations which take place in the stomach are of a purely chemical, nature ; and the interchange of gases between the air and the blood, which takes place in the act of respiration, must be regarded in the same light. — But after every possible allowance has been made for the operation of physical and chemical forces in the living or¬ ganism, there still remain a large number of phenomena which cannot be in the least explained by them, and which must be regarded as the result of an agency that differs from these as they differ from each other ; and this agency, which DISTINCTIVE CHARACTERS OF LIVING ORGANISMS. 23 is recognised by the effects it produces — in the same manner as we recognise heat or electricity by their effects — may be conveniently designated vital force.1 Thus, to revert to our previous illustrations, the mechanical power employed in the propulsion of the blood, or in the movements of the limbs, is evolved by muscular contraction, a phenomenon altogether peculiar to the living muscle ; and the muscle derives its pro¬ perty of contractility from the previous development of its peculiar tissue in the act of nutrition. So the solvent fluids by which the digestion of food is accomplished, are separated from the blood by an act of secretion, which can only be performed by a glandular apparatus in the living walls of the alimentary canal. And the materials for the nutrition of the muscular tissue, and for the secretion of the digestive solvent, as of all the other acts of nutrition and secretion which are continually going on in the living body, are derived from the blood, — a liquid which possesses properties very different (as we shall hereafter see) from any mere mixture of chemical compounds, and which is prepared by actions totally beyond the power of the chemist to imitate, — * the laboratory of the living organism being requisite for their performance. 6. The whole assemblage of vital actions which is per¬ formed by the living Animal, may be arranged under two principal groups ; one of them consisting of those which are directly concerned in the development and maintenance of its Organized fabric ; the other including all those by which it is brought into conscious relation with the world around. The former group includes the acts of digestion, absorption, and assimilation, by which the nutritive materials are pre¬ pared for becoming part of the living fabric ; the circulation of the assimilated materials through the body ; their conver¬ sion, by the act of nutrition, into the solid textures ; the formation of various secretions, having various purposes to serve in the economy ; the removal, by the acts of respiration 1 The Author has elsewhere given his reasons for the belief, that Vital force bears the same “correlation” to the Physical and Chemical forces, as the latter bear to each other; but the discussion of this sub¬ ject is not suited to an elementary treatise ; and the essential peculiarity of the manifestations of vital force in the phenomena of life, requires that it should be treated as belonging to a distinct category. 24 DISTINCTIVE CHARACTERS OP ANIMALS. and excretion, of the effete matters with which the blood be¬ comes charged by the decomposition continually going on in the body ; the maintenance of animal-heat by the same process ; and the act of reproduction, whereby the race is perpetuated, in spite of the limited duration of the individual. The fore- . going, which are for the most part common to the Animal and the Plant, are termed Organic Functions , or Functions of Vegetative Life. But, in addition to these, it is the character¬ istic of Animals generally, that they are sensible to impressions made by surrounding objects, so that they possess some con¬ sciousness of what is going on about them ; and that they also possess the power of re-acting on those objects by movements of their own, so as to change either their own places, or the places of surrounding objects in relation to them¬ selves. These two functions, sensibility and the power of spontaneous motion, being peculiar to animals, are distin¬ guished as Animal Functions, or Functions of Animal Life. In the higher animals, they are the most important and charac¬ teristic phenomena of their existence ; so that it would seem as if the whole assemblage of organic functions had no other destination in them, than to build up and keep in order the apparatus by which the functions of animal life are performed. But this state of things is entirely reversed among those • lower tribes of animals which border most closely on the Vegetable kingdom ; for we find that among such, the mani¬ festations of sensibility and power of spontaneous movement are so feeble, that it may be doubted whether these attributes are really present in them ; and even in higher orders, there are many in which the proper animal powers are in such a low grade of development, that they appear as if they were destined merely to minister to the organic functions. 7. Thus, although the characteristic difference between the Animal and the Vegetable kingdom, taking each as a whole, may be truly said to consist in the possession by the former of endowments which do not exist in the latter, this does not express the essential difference between Animals and Plants ; since, while there are many tribes among the former in which the proper animal powers are reduced to so low a degree as to prevent it from being certainly affirmed that they are present at all, there are many tribes among the lower plants which exhibit a power of spontaneous movement fully as DISTINCTIVE CHARACTERS OF ANIMALS. 25 great as tliat which exists among the lowest animals ; so that no positive line can be drawn between the two kingdoms on the basis of this distinction alone. There is another very important physiological difference, however, between the two kingdoms, which seems to afford an adequate means of settling the true place of those tribes whose position would otherwise be doubtful. This lies in the nature of their food, and the source from which it is obtained. Tor although it .is now known that the primary tissues of plants are originally formed of the same albuminous material as are those of animals (the cellulose layers which constitute the great bulk of the vegetable fabric being a subsequent deposit), yet this material is generated in the Plant by the combination of the elements which it obtains from the carbonic acid, water, and ammonia of the soil or of the atmosphere ; whilst the Animal is destitute of all power of thus forming it for itself, and is hence entirely dependent upon the plant for its sup¬ plies of nutriment. Thus, whilst the very humblest forms of Yegetation, in common with the highest, are found to have the power of decomposing carbonic acid under the influence of sunlight, setting free its oxygen and retaining its car¬ bon, the humblest forms of Animal life, in common with the highest, derive their nutriment either directly from plants, or from the bodies of other animals which have sub¬ sisted on vegetable food, whilst they produce a converse change in the atmosphere by their respiration, absorbing from it oxygen, and giving forth to it carbonic acid. This criterion will serve, it is believed, to distinguish the very lowest forms of Animal life from those humble forms of Yege¬ tation which they most closely resemble in the simplicity of their organization (§ 128); and its application will generally be found to be very easy. There is now no longer any doubt that a large proportion of the beings formerly ranked as Animalcules, are really to be regarded as Plants, notwithstand¬ ing that they possess a power of active and apparently spontaneous movement, far greater than that of many unques¬ tionable animals. And generally it may be said that the presence of a bright-green or bright-red colour in any of these simple organisms, where it is not derived from coloured sub¬ stances taken in as food, affords a strong probability of their vegetable character; these colours being produced in the 26 DISTINCTIVE CHARACTERS OP ANIMALS. course of that series of chemical changes, by which, under the influence of light, the living plant can unite, inorganic elements into organic compounds. 8. Hot only do Animals differ from Plants in the nature and sources of their aliment, hut also in the mode in which ' it is taken into their bodies ; and this difference is related alike to the character of the food of animals, and to the general conditions of animal existence. For the Plant extends its roots through the soil in search of liquid, and spreads out its leaves to the air for the purpose of imbibing some of its gaseous ingredients. But the Animal could not so exist, and be at the same time endowed with the power of moving from place to place ; nor could it appropriate solid nutriment, if it were not provided with some peculiar means of receiving and preparing this. For these purposes, animals (with few exceptions) are provided with an internal cavity or stomach into which the food is received from time to time, in which it can be carried about in the general movements of the body, and within which it can be prepared for being received by absorption into the current of nutrient liquid which circu¬ lates through the body. This stomach is nothing else than a bag formed by the prolongation of the external covering of the body into its interior (§ 36); its cavity receives the food introduced into it by the mouth ; its walls pour out or secrete a fluid which acts upon the food in such a manner as to dis¬ solve it ; and through its walls are absorbed those portions of the food which are fit to be employed as nutriment, while the remainder is cast forth from the cavity, either by the aperture which first admitted it, or by a distinct orifice. The exceptional cases, in which no stomach exists, chiefly occur in one particular tribe of animals, the Entozoa (§ 105), which live either in the intestinal canal or in the substance of the tissues of other animals, and which are supported by the nutrient juices of these ; such an organ obviously not being required by creatures which have no power of locomotion, and which can imbibe liquids already prepared for their use, through the whole of the soft surface of their bodies. But there is a large tribe of very simple animals, the Rhizopoda (§ 129), in which, notwithstanding the absence of any regular stomach, the food is,, received into the very substance of the; jelly-like particle of which the body consists ; a mouth and DISTINCTIVE CHARACTERS OP ANIMALS. 27 stomach, being extemporized, as it were, on each occasion that aliment is ingested ; and an anal orifice being extemporized in like manner, when the indigestible residue has to be cast forth. All true Animalcules (§ 133) have a proper mouth, into which food is drawn by the current created by the cilia (§ 45) wherewith it is fringed ; and this mouth leads to the general cavity of the body, within which the food is subjected to the digestive process, hi Zoophytes (§ 121) which possess a proper stomach, this organ forms so large a part of the animal, that its entire body may be almost said to consist of the stomach and of the prehensile appendages by which it draws in its food. Eut in all the higher tribes, the stomach, with the alimentary canal proceeding from it, are suspended freely within the general cavity of the body ; and we shall find that the space that surrounds these viscera is extremely important in the economy of all but vertebrated animals, as being a sort of reservoir into which the nutrient materials prepared by the digestive process first transude, and from which it is carried into the remoter parts of the system. In vertebrated animals, this cavity — called in them the peritoneal cavity, from its being lined with a serous membrane (§ 28), termed the peritoneum— As not subservient to the same pur¬ poses ; the nutrient materials being taken up from the walls of the digestive cavity, both by the blood-vessels and by special absorbents, and being by them carried into the current of the circulation. It is obvious that until they have found their way, through one or other of these channels, into the general system, the nutrient materials introduced as food into the stomach of an animal are not within its body, properly so called, any more than a fluid is within a plant when it bathes the exterior of its roots, or within an entozoon when in con¬ tact with the soft surface of its integument. In each case, the absorption of the fluid is first requisite ; and it is with this that its application to the requirements of the living body really commences. 9. Eut further, when we compare together, not the lowest, but the highest members of the Vegetable and Animal king¬ doms respectively — those in which their respective attributes are most characteristically displayed, — we find that they present such differences as to render it quite impossible to confound the one with the other. Although it is easy even 28 DISTINCTIVE CHARACTERS OF ANIMALS. for the scientific naturalist to mistake a Protophyte (or one of the simplest forms of vegetation) for an Animalcule, and although Zoophytes are continually ranked in the popular mind with the Plants they so much resemble in form, no one is in any danger of confounding the Oak and the Elephant, the Palm and the Whale. Eor among the higher Animals, not only the principal organs, hut the greater part of their elementary parts or tissues, are formed upon a plan entirely different from that which prevails in Plants. All the arrangements of their organism or corporeal edifice are made for the pur¬ pose of enabling them to perform, in the most advantageous manner possible, those peculiar functions with which they have been endowed, — to receive sensations, — to feel, think, and will, — and to move in accordance with the directions of the instinct or the judgment. Eor these purposes we find a peculiar apparatus, termed the Nervous system, adapted. .This apparatus consists of a vast number of fibres, spread out over the surface of the body, and especially collected in certain parts, called Organs of Sense (such as the eye, nose, ear, tongue, lips, and points of the fingers). These have the peculiar property of receiving impressions which are made upon their extremities, and of conveying them to the central masses of nervous matter (known in the higher animals as the Brain and Spinal Cord ), by the instrumentality of which they are communicated to the mind. 10. From the .Nervous centres, other cords proceed to the various Muscles , by which the body is moved. These muscles, commonly known as “ flesh,” are composed of a tissue which has the power of contracting suddenly and forcibly, when peculiar stimuli are applied to it. In this respect, it bears a resemblance to the contractile tissues by which the move¬ ments of plants are produced (Veget. Phys. § 390) ; but it differs from them in being thrown into action, not only by stimuli that are applied directly to itself, but by an influence conveyed through the nervous system. Thus, in an annual recently dead, we may excite any muscles to contraction, by sending a current of electricity into the nerves supplying them ; and in a living animal we may do the same by simply touching those nerves. But the stimulus which these nerves ordinarily convey, originates in an act of the mind , which is connected in some mysterious and inscrutable manner with DISTINCTIVE CHARACTERS OF ANIMALS. 29 the central masses of the nervous system. Thus/ we desire to perform a certain movement or set of movements ; this desire leads to an act of volition or will; and the will causes a certain force or motor impulse to issue from the brain and travel along the nerves, so as to produce the desired motion, by exciting contractions in the muscles that perform it. Or, again, a certain sensation calls forth an emotion, which prompts a certain muscular movement, and may even cause it to take place against the will, — as when a strong sense of the ludicrous produces laughter, in spite of our desire (owing to the unfitness of the time and place) to restrain it ; for the emotion, like the act of volition, produces a change in the nervous centres, which causes a motor impulse to travel along the nerves, and thus calls the muscles into contraction. And it seems to be in the same manner that those instinctive actions are produced, which, although few in adult Man when compared with those resulting from his will, predominate in his infant state, and through the whole life of the lower animals (Chap. xiv.). We shall also find that the nervous and muscular systems of animals are concerned in a class of actions with which the mind has no necessary connexion; these autom¬ atic actions, such as those of swallowing (§195) and breathing (§ 340), having for their object to assist in the performance of the organic functions, and to protect the body from danger. 11. In the higher Animals, then, the presence of this Nervo-Muscular apparatus is an essential and obvious dis¬ tinction between their structure and that of Plants ; and we find that it constitutes a large part of the bulk of the body. Thus the whole interior of the skull of Man is occupied by his brain ; his limbs are composed of the muscles, and of the bones which support them and which are put in motion by them ; and it is only in the interior of his trunk, that we find organs corresponding with those which form the entire fabric of the Plant. These organs of Nutrition have for their main pur¬ pose, to supply the wants of the organs of animal life ; every exercise of which is accompanied by a certain decay or wear of their structure, and which consequently require to be con¬ tinually nourished and repaired, by the materials provided by what may be termed the vegetative organs. But in the lower of tribes of Animals, we do not find the animal functions to possess this predominance. In fact, among the 30 DISTINCTIVE CHARACTERS OF ANIMALS. many which, are fixed to one spot during nearly their whole lives, and which grow and extend themselves like plants, the movements of the body are but few in number, and trifling as to their variety; these movements are only destined to assist in the performance of the organic functions, as by bringing food to the mouth, and water to the respiratory organs ; and the nervo-muscuiar apparatus by which they are effected, bears so small a proportion to the organs of nutrition, as to seem like a mere appendage to them, and is sometimes altogether undiscoverable. This is the case, for example, in the lowest kinds of shell-fish, such as the Oyster, and in the Coral-polypes. 12. Hence we perceive, as we descend the Animal scale, a nearer and nearer approach to the character of Plants ; and this we shall find to be the case, not only in the general arrangement of the organs, but also in the nature of the elementary tissues of which these are composed. Por in the higher animals, the whole organism is constructed in such a manner as to admit a free motion in its individual parts. The different portions of the skeleton or hard framework are connected with each other by flexible ligaments, which are adapted to resist a very powerful strain; the muscles are attached to these by fibrous cords or tendons, which, also, can support a vast weight ; and the several muscles and other parts, which need to be mutually connected, but also require a certain power of moving independently of one another, are bound together by a very elastic loosely-arranged tissue, consisting of fibres crossing and interlacing in every direction, the interstices between which are filled with fluid. How to these fibrous tissues, there is nothing analogous in plants, because no freedom of motion is required, or even permitted, among their parts ; and we find them bearing a less and less proportion to the whole, as we descend the animal scale. On the other hand, we find the various forms of true cellular tissue, such as predominate in plants (V eget. Phys. Chap, hi.), becoming more and more abundant, as we pass from the highest to the lowest animals, and having more and more important duties to fulfil. But even in the highest Animals, as will hereafter appear, they are the im¬ mediate instruments of the most important among the organic functions, just as they are in Plants. CHEMICAL CONSTITUENTS : - ALBUMEN. 31 Chemical Constitution of the Animal Body. 13. Ey far tlie larger proportion of tire Animal fabric is formed at the expense of the substance termed A lbumen ; the composition and properties of which, therefore, claim onr first attention. The fundamental importance of albumen in the animal economy, is shown by the fact that it constitutes, with fat, and a small proportion of certain mineral ingredients, the whole of that mass of nutrient material stored up in the eggs of oviparous animals, which, being appropriated by the germ to the building up of its fabric, is converted by it into the bones, muscles, nerves, tendons, ligaments, glands, mem¬ branes, &c. of the embryo. We find it also constituting a large proportion of the solid matter of the blood and other nutrient fluids of the adult animal ; and it is the fundamental form to which the various azotized substances employed as food (§ 153) — such as animal flesh, or the gluten of bread — are first reduced by the act of digestion. It is composed of 49 carbon, 36 hydrogen, 14 oxygen, 6 nitrogen, with a minute proportion of sulphur ; it is generally blended, also, with more or less of fatty matter, and with saline and earthy substances. 14. Albumen may exist in two states, — the soluble and insoluble. In the animal fluids it exists in its soluble form ; and is united (as an acid to its base) with about 1^ per cent, of soda, forming an albuminate of soda. It is not altered by being dried at a low temperature, but still retains its power of being completely dissolved in water. When a considerable quantity of it exists in a fluid (as in the white of the egg), it gives to it a glairy tenacious character ; but it is nearly tasteless. When such a fluid is exposed to a tempe¬ rature of about 150°, a coagulation or ‘setting’ takes place, as in the familiar process of boiling an egg. Eut if the albumen be present in smaller quantity, the fluid does not form a consistent mass, but only becomes turbid ; and this only after being boiled. Albumen which has been dried at a low temperature, however, may be heated to the boiling point of water, without passing into the insoluble condition ; a fact which is of peculiar interest in relation to the power which the Tardigrada (Zool. § 841) possess, of sustaining a very high temperature without the loss of their vitality, when 32 CHEMICAL CONSTITUENTS * - ALBUMEN, CASEIN. their bodies have been completely dried np in , the first instance. 'No trace of organization can be detected in coagulated albumen, which seems to be composed only of a mass, of granules; and in this respect it differs in an im¬ portant degree from fibrin — ns we shall presently see. Albumen may also be made to coagulate readily by the action of acids, especially the nitric (aqua-fortis) ; so that a very small quantity of it may be detected in water, by the tur¬ bidity produced by adding to it a drop or two of nitric acid, and then heating it. 'Now, when thus coagulated, albumen cannot be dissolved again by any ordinary process ; but its solution may be accomplished by rubbing it in a mortar with a caustic alkali, potass or soda. From this solution it may be precipitated again on the addition of an acid in sufficient quantity to neutralise the alkali. Albumen is distinguished, then, by its peculiar property of coagulating on the applica¬ tion of heat, or on being treated with certain acids. 15. Nearly allied to albumen is the substance termed Casein, which replaces it in milk ; and this is specially worthy of notice here, because it is the sole form in which the young Mammal receives albuminous nourishment during the period of suckling, in which it draws its sustenance from its parent. Like albumen, this substance may exist in two forms, the soluble, and the insoluble or coagulated ; and the presence of a small quantity of free alkali seems essential to its continuance in the soluble form. Casein differs from albumen, however, in this, that it does not coagulate by heat, and that it is precipitated from its solution by organic acids, such as the acetic and lactic, which have no coagulating action on albumen. It is further remarkable for the facility with which its coagulation is effected by the contact of certain animal membranes ; as we see when a small piece of rennet (which is the dried stomach of the calf) is put into a large pan of milk in the process of cheese-making, the ‘ curd’ which then separates being composed of casein entangling the oily particles of the milk. In the coagulated state, casein differs but very little from albumen, and is readily converted into it by the gastric fluid. It is remarkable for its power of dissolving the earthy phosphates, as much as 6 per cent, of phosphate of lime being usually obtainable from it ; and it is in this combination, that the large quantity of bone-earth CHEMICAL CONSTITUENTS : — CASEIN, SYNTONIN, FIBRIN. 33' required for tlie consolidation of the skeleton of the young animal, is introduced into its system. A substance resembling casein is obtainable from the serum of the blood, especially in pregnant females ; and also from the serous fluid which occupies the interstices of the tissues. It is found, also, mingled with albumen, in the yolk of the egg, forming a compound which (before its true character was known) has been distinguished as vitellin. How as all the liquids con¬ taining casein have it for their special function to supply formative materials to rapidly-growing tissues, we may with much probability regard it as still more closely related to them than is albumen itself. It differs from albumen but little, if at all, in the ultimate proportions of its elements (§ 1 3). 16. The substance of which muscles are composed, has been commonly considered to be Fibrin (§ 17) ; but it differs essentially from fibrin in its properties, and is now dis¬ tinguished as Syntonin . Its chief peculiarity is its solubility in very dilute muriatic acid (1 part to 100 of water), and its precipitation in the form of a jelly wdien the acid is neutra¬ lised ; this jelly treated with dilute alkalies forms a solution which coagulates by heat ; and thus it seems to be reduced nearly to the condition of albumen. This is, in fact, very much what takes place in the act of digestion of flesh-meat ; the muscle-substance being first dissolved by the muriatic or other acid of the gastric fluid, and the solution being then rendered alkaline by the mixture of bile and other secretions in the small intestine. 17. In the blood and other nutrient fluids of the animal body, there is found a substance which is so closely related to albumen in its ultimate chemical composition, as not to be dis¬ tinguishable from it with any certainty ; but which, though fluid whilst circulating in the living vessels, coagulates spon¬ taneously after having been for a short time withdrawn from them, the coagulum or clot being distinguished from that of albumen or by the fibrillar arrangement of its particles, ? which indicates an incipient organization. This substance, termed Fibrin , may be obtained in a separate form, by stirring fresh-drawn blood with a stick, to which it adheres in threads. In this condition it possesses the softness and elasticity which characterise the flesh of animals, and con¬ tains about three-fourths of its weight - of water. It may be D 34 CHEMICAL CONSTITUENTS I — FIBRIN. deprived of this water by drying, and then becomes a bard and brittle substance ; but, like dried flesh, it imbibes water again when moistened, and recovers its original softness and elasticity. From the recent experiments of Dr. Eichardson, it appears that the coagulation of blood-fibrin depends upon the escape of ammonia, being accelerated by such conditions as favour the liberation of this gas, and retarded or prevented by such as cause its retention in the liquid; whilst, even after the clot has been formed, it may be dissolved by ammonia, forming again when that gas is set free. Fibrin differs from syntonin or muscle-substance in not being dis¬ solved by very dilute muriatic acid, but being merely caused to swell up into a gelatinous mass, which contracts again when more acid is added. It combines with the earthy phosphates, of which as much as 2\ per cent, is sometimes found in the ash left by its combustion. 18. There can be no doubt that fibrin is formed in the blood and in the other fluids in which it presents itself, at the expense of albumen. What is its precise destination, cannot as yet be clearly specified ; but there are several circumstances which point to the conclusion that it is to be regarded as a transitional stage in the metamorphosis of albumen into the simple fibrous tissues (§ 23.) Thus, when the ordinary clot of blood is examined microscopically, it is found to consist, not, like an albuminous coagulum, of a homogeneous mass of granules, but of a network of im¬ perfectly-formed fibres, enclosing the red corpuscles in its interstices. A much more distinct network of the same kind, may be seen in the colourless coagulum formed by the liquid which may be skimmed off the surface of the blood drawn from persons suffering under any severe inflammation’; such blood coagulates slowly, and its red corpuscles and the fluid in which they float have an unusual tendency to separate from each other ; and the fibrin previously dissolved in the latter sets into definite fibres, which continue for some days to increase in firmness. It is a liquid of the same kind, charged with fibrin in a peculiarly “ plastic ” condition, that is poured forth for the formation of new tissue when the repa¬ rative processes are at work for the healing of a wound or the reunion of divided parts ; and it is by a plug of coagulated fibrin, which gradually comes to present a more and more CHEMICAL CONSTITUENTS : — FIBRIN, GELATIN. 35 distinctly fibrous structure, tliat tbe mouths of divided blood¬ vessels are closed up, when tbe flow of blood from tliem spontaneously stops. In all sucb cases, tbe fibrous network, if formed out of connexion witb a living body, passes after a time into decay ; but if it be formed in apposition witb living parts, blood-vessels gradually extend into it from these, its nutrition is maintained and improved, and it progressively comes to present tbe ordinary characters of tbe simple fibrous tissues (§ 22). 19. Although tbe tissues most actively concerned in carrying on tbe vital operations, retain for tbe most part tbe composition of albumen, yet that very large proportion of tbe fabric of tbe higher animals whose offices are essentially mechanical, has a very different chemical constitution. • If we boil down either their bones, their skin, or their internal membranes, we shall get a considerable quantity of the sub¬ stance scientifically termed Gelatin , familiarly glue. Though consisting of the same elements as albumen, its composition is simpler, because these elements are united in smaller propor¬ tions ; the atom or combining equivalent of gelatin being made up of 13 Carbon, 10 Hydrogen, 5 Oxygen, 2 Nitrogen. The distinctive character of gelatin consists in its sohibility in warm water, its coagulation on cooling into a uniform jelly which can be liquefied again by warmth, and its formation of a peculiar insoluble compound with tannin. Gelatin is very sparingly soluble in cold water, though made to swell up and soften by prolonged contact with it. A solution of only one part of gelatin in 100 of hot water is sufficiently strong for the whole to form a consistent jelly on cooling. The re¬ action of gelatin with tannin is so decided, that the presence of only one part in 5000 of water is at once detected by infusion of galls ; and it is in this action that the process of tanning consists, — the gelatinous fibre of the skin, which would speedily pass into decay, being converted into a com¬ paratively unchangeable substance. The different tissues which have gelatin for their base, yield it to boiling water with different degrees of facility ; this diversity apparently depending in some degree upon the definiteness of their organization. Thus the “ sound 55 or air-bladder of the cod, sturgeon, and other fish, which, when dried and cut into strips, is known as isinglass, is very readily acted on ; the d 2 36 CHEMICAL CONSTITUENTS : — GELATIN, CHONDRIN. same is the case with the animal substance of bones from which the earthy matter has been removed ; and in each case the fibrous texture of the living tissue is but very imperfectly developed. For the extraction of gelatin from the skin, the ligaments, the tendons, and various internal membranes, whose fibrous texture is more pronounced (§ 29), a much longer action of boiling water is required. 20. A peculiar modification of gelatin, which presents itself in Cartilage (or gristle), is distinguished as Chondrin. This requires longer boiling than gelatin for its solution in water; as is seen when a knuckle of veal or of mutton is cooked, the tendons and ligaments about the joint being almost reduced to pulp, whilst the cartilages are scarcely at all softened. The essential properties of chondrin are nearly the same as those of gelatin, and its composition seems nearly identical; but it is thrown down from its solution by muriatic and acetic acids' and some other reagents, which do not disturb a solution of gelatin. 21. It is not yet fully known how the material of the gelatinous tissues is produced in the animal body. There can be no doubt of its being producible from albumen ; since we find it in large proportion in the tissues of animals that have never received gelatin into their bodies in any shape. And although carnivorous animals will receive it as part of their aliment, yet there is strong reason to believe that the gelatin which is thus supplied to them does not really serve to nourish their bodies, but that it is speedily decomposed and got rid of (§ 159). It may be considered as quite certain that the albuminous tissues cannot be formed by the meta¬ morphosis of gelatin ; whilst conversely, looking to the fact that in the egg and in milk no gelatin is provided for the young animal, although the gelatinous tissues form a yet larger proportion of its body than they do in the adult, we seem entitled to question whether it is possible that these tissues can be formed in any other way than at the expense of the albuminous constituents of the blood. Structure of the Primary Tissues . 22. In considering the structure of the “ primary tissues,” of which the various organs of animals are composed, it will be convenient first to treat of those which are subservient PRIMARY TISSUES S - SIMPLE FIBROUS TISSUES. 37 merely to the physical actions of the framework ; as, for ex¬ ample, by holding its parts together, by communicating motion, or by giving them mechanical support and protection. — The several parts of the body, even to the very minute divisions of its organs, are held together by what may be termed, in contradistinction to Muscular and Nervous fibre, the simple fibrous tissues ; and these are merely endowed, like ordinary cords, with the power of resisting tension or strain, either without themselves yielding to it at all, or with a certain amount of elasticity, which enables them first to yield to a certain degree, and then to recover their previous state. These two qualities are characteristic of two distinct forms of simple fibrous tissue, the white and the yellow . 23. The White fibrous tissue presents itself under various forms, being sometimes composed of fibres so minute as to be scarcely distinguishable, but more commonly presenting itself under the aspect of flattened bands, which are but imperfectly divided into fibres, and have more or less of a wavy aspect (fig. 1). This tissue is resolved, by long boiling, into gelatine ; and when treated with acetic acid, it swells up and becomes *** l-white fibrous tissue. transparent, by which peculiarity it can be readily dis¬ tinguished from the other kind, to be next described. The Yellow fibrous tissue presents itself in the form of long, separate, clearly defined fibres, which sometimes branch, and which break short off when overstrained, their extremi¬ ties being disposed to curl up (fig. 2). They are, for the most part, between 1-5, 000th and 1-1 0,000th of an inch in diameter ; but they are often met with both larger and smaller. This kind of tissue un¬ dergoes but very little change from long boiling, and it is 38 PRIMARY TISSUES : — AREOLAR TISSUE. not acted on by acetic acid. It is but little prone to decom¬ position, and will exhibit its peculiar elasticity long after it has been separated from the body, provided it be kept moist. — These two forms of tissue exist separately in certain parts of the fabric, but they are much more frequently combined ; and the proportion of the yellow elastic tissue which exists in any such combination, may be readily determined under the microscope by the use of acetic acid, which renders all the white fibrous structure so transparent, that the yellow fibres are seen completely isolated in the midst of it. 24. One of the tissues which is composed of such an admixture of white and yellow (or non-elastic and elastic) fibres, is the one which was formerly called “cellular,” but which is now more correctly designated as Areolar } This is composed of a mesh-work of fibres, and of bands of fibrous membrane, which are interwoven in such a manner as to leave very numerous interstices and cavities amongst them, having a tolerably free communication with each other (fig. 3). These Fig. 3. — Portion of Areolar Tissue. cavities are filled during life with a serous fluid ;1 2 and it is a necessary result of the communication between them, that if an accumulation of this fluid takes place to an undue extent, 1 From the Latin areola , a small open space. 2 A fluid resembling the serum of the blood, diluted with water (§ 236). PRIMARY TISSUES : - AREOLAR TISSUE. 39 as in dropsy, it descends by gravity to the lowest situation. Hence, the legs swell more frequently than any other parts. In its natural state, this tissue possesses considerable elas¬ ticity ; hence, when we press upon any soft part, and force out the fluid beneath into the tissue around, the original state returns as soon as the pressure is removed. But in dropsy, it appears as if the elasticity of the fibres were impaired or destroyed by their being over-stretched ; for when we press with the finger upon a dropsical part, a pit remains for some time after the finger has been removed. 25. This Areolar tissue is diffused through almost the whole fabric of the adult animal, and enters into the compo¬ sition of almost every organ. It binds together the minute parts of which the muscles are composed ; it lies amongst the muscles themselves, connecting them together, but yet per¬ mitting them sufficient freedom of motion ; it exists in large amount between the muscles and the skin ; it forms sheaths to the blood-vessels and nerves, and so connects them with the muscles that they shall not be strained or suddenly bent by the movements of the latter ; and it enters into the struc¬ ture of ahnost every one of the organs which are contained in the cavity of the trunk, uniting its parts to each other, and keeping the whole in its place. But it is a great mistake to assert, as it was formerly common to do, that it penetrates the harder organs, such as bone, teeth, and cartilage. Its purpose obviously is to allow a certain amount of motion among the parts it unites ; and we find that the more free this motion is required to be, the larger is the proportion borne by the yellow or elastic fibres, to the white or non-elastic. 26. Although the Areolar tissue contains a very large number of blood-vessels and nerves, yet it does so merely because it furnishes the bed or channel in which they are conducted to the parts where they are really wanted. Its own vitality is low, and its sensibility very slight. It is quickly reproduced after injury ; and it is by its means that losses of substance are repaired in tissues of a more elaborate kind, which are not so easily regenerated. 27. The continuity or connectedness of this tissue over the whole surface of the body, admits air to pass readily from one part to another ; and the inflation or blowing-up of its cavities with air, which has sometimes happened accidentally, . 40 PRIMARY TISSUES : — SEROUS MEMBRANES. and lias sometimes been purposely effected, does not produce any disorder in the general functions of the body. In blow¬ ing the nose violently, some part of the membrane lining its cavity has occasionally given way, so as to allow air to pass into the areolar tissue of the face, and especially into that contained in the eyelids, which is particularly loose ; an enor¬ mous swelling of these parts then takes place, presenting a very frightful appearance, but not attended with the least danger, and subsiding of itself in a few days. This swelling presents a character to the touch quite different from that which would be occasioned by a similar distension with liquid; for it gives somewhat of the crackling feel that is occasioned by pressing on a blown bladder. A similar inflation of the areolar tissue of the body has sometimes occurred from the formation of an aperture, by disease or injury, in the walls of the lungs or air-passages, and the consequent escape of air during the act of breathing : in one remarkable case of this kind, the skin of the whole body was so tightly distended with air as to resemble a drum. It is intentionally practised by butchers, who “ blow up ” the areolar tissue of their veal, in order to increase its plumpness of aspect; and the in¬ flation of the areolar tissue of the head, in the living state, has been sometimes practised by impostors, in order to excite commiseration. 28. Fibres and shreds of fibro-membrane, resembling those of which areolar tissue is composed, may be so interwoven as to form a continuous sheet of membrane, having a smooth and glistening surface ; and in this manner are produced the Serous Membranes that line the different cavities in which the viscera (or organs contained within the skull, the chest, and the abdomen) are lodged. The peculiar manner in which these membranes are arranged, will be explained hereafter (§ 43). One of their surfaces is always free or. unattached, whilst the other is in contact with the outer wall of the cavity ; and from the free surface, which is covered with a layer of flattened epithelium-cells (fig. 10), a serous fluid is exhaled, which adds to its smoothness. It is by an accumula¬ tion of this fluid, that dropsies of the cavities are produced, — such as water on the brain, or in the chest. 29. By the union of fibres of a stronger kind, those firmer * tissues are produced, which are employed wherever a greater FIBROUS MEMBRANES AND LIGAMENTS. 41 strain has to be borne. This is the case with the Ligaments , which bind together the bones at the joints, the Tendons , by which the muscles are usually attached to the bones, and the tough Fibrous Membranes that envelope and protect many of the most important viscera. In these any considerable amount of elasticity would be misplaced ; and we conse¬ quently find that they are chiefly or entirely composed of the white fibrous tissue. Whenever an elastic ligament is re¬ quired, however, we find the white replaced by yellow. One of the best examples of this is seen in the ligament of the neck of many quadrupeds, commonly known as the paxy- waxy ; which is given to the large herbivorous quadrupeds, such as the ox, to assist them in supporting their heavy heads with as little exertion as possible ; whilst carnivorous quadrupeds, such as the lion and tiger, are endowed with it to give them additional power of carrying away heavy bur¬ dens in their mouths. In Man we scarcely find a trace of it. This yellow fibrous tissue is found, moreover, in the walls of the arteries (§ 248), to which it gives their peculiar elas¬ ticity; and it also forms the vocal cords of the larynx (§ 681). It is by the same kind of elastic ligament that the claws of the Feline tribe are drawn back into their sheaths when not in use, being projected (when required) by muscular action; and that the two pieces of the shell of Bivalve Mollusks are united at the hinge, and are at the same time kept apart for the admission of water between them, except when, the animal forcibly draws them together by its adductor muscle (§ H3). 30. All these fibrous tissues, then, are concerned in actions purely mechanical ; and there is nothing in their properties which is so distinct from those of inorganic substances, as to require to be considered as vital. We may consider them, therefore, as among the lowest forms of animal tissue ; and accordingly we find that, when the higher forms degenerate or waste away, these appear in their place. Such a degene¬ ration may take place simply from want of use. Thus if, from palsy or want of power of the nerves, the muscles of the legs are disused for several years, they will lose their peculiar property of contractility (§ 5 ) ; and it will be found that scarcely any true muscular structure remains, but that it is replaced by some form of fibrous tissue. Or again, if the 42 BASEMENT MEMBRANE : — CELLS. front of tlie eye be so injured by accident or disease, that light cannot pass through it to make its impression on the nerve, that nerve, being thrown into disuse, will gradually degenerate into fibrous tissue. Moreover, this change may take place as a part of the regular actions of life j for there are certain organs in the young animal previous to birth, which are not required afterwards ; and these degenerate in like manner, gradually wasting away, and leaving only traces behind them, - — tubes shrivelling into fibrous ligaments, and glandular structures remaining only as areolar tissue. 31. Along every free surface of the body, both external and internal, is spread out a delicate structureless layer, which is termed the Basement or Brimary Membrane. This forms the outer layer of the True Skin, lying between it and the Epidermis or scarf-skin (§ 37) j in the same manner it underlies the Epithelial layer of the Mucous membranes which line the open cavities of the body (§ 39), and of the Serous membranes which line its closed cavities (§ 43) ; and it occupies the same position in the walls of the blood- vessels, gland-ducts, and other tubes. It is difficult to sepa¬ rate it, in any of these parts, from the tissues with which it is in contact ; and its characters may be well studied by dis¬ solving the calcareous part of an oyster or mussel-shell in dilute acid, when it will be found that layers of a thin trans¬ parent membrane are left, which have been thrown off at each act of shell-formation, from the surface of the mantle. This elementary membrane, like that which forms the walls of cells (§ 32), is remarkable for the readiness with which it is permeated by fluid, al¬ though no visible pores can be seen in it. 32. A considerable part of the fabric of even the highest Animal is formed, like the entire organism of the Plant, of Cells , either unchanged or in some way metamorphosed. A cell is a minute bag or vesicle, formed of a structure¬ less membrane, and having its cavity filled with fluid of some kind. In some part of its interior, most commonly adhering cells; their mode of multiplication. 43 to its wall, there is usually to be observed a solid collection of granular matter, which is termed the nucleus (fig. 4, a a). The typical form of the cell is globular or oval (fig. 5 ) ; but when a number of cells are in contact with each other, and are pressed together, their sides become flattened; so that when they are cut across no intervals are seen between them, but their walls are everywhere in contact (fig. 6), just as in Fig. 5. Rounded Cells in Cartilage of Bat’s Ear. Fig. 6. Polygonal Cells from Car¬ tilage in Mouse’s Ear. the section of a vegetable pith. The chemical composition of the nucleus differs from that of the cell- wall ; for whilst the latter is dissolved by acetic acid, the former (like the yellow elastic tissue, with which its substance appears to have some relationship) is unchanged by it. When the formation of a cell is complete, and it is not destined to reproduce its kind, the nucleus frequently disappears ; this is the case, for example, with the red corpuscles of the blood of Mammalia (§ 229), and also with Fat-cells (§ 46). 33. blew cells may originate in one of two very distinct modes ; either from a pre-existing cell, or by an entirely new production in the midst of an organizable fluid or blastema . The most remarkable example of the first process is presented in the early development of the germ, which entirely consists of an aggregation of cells, every one of 'which undergoes successive subdivisions into two, so that the total number in the germ-mass is repeatedly doubled (Chap. xv.). The same method of multiplication by binary subdivision may be seen to continue throughout life in Cartilage-cells (§ 47), the growth of which almost exactly repeats the history of the growth of the lowest forms of Sea- weeds. The process of sub¬ division seems to commence in the nucleus, which begins to separate itself into two equal parts, and each of these draws 44 MULTIPLICATION AND NEW PRODUCTION OF CELLS. around it a portion of tlie contents of tlie cell ; so that the cell- wall, which is at first merely doubled inwards by a sort of hour-glass contraction, at last forms a complete partition between the two halves of the original cavity. The process may be repeated either in the same or in a transverse direc¬ tion, so as to produce four cells, which may be either arranged in a single line OOOO or may form a cluster gg ; and another subdivision of each cell will, of course, again double the entire number. In other cases, however, the nucleus appears to break up at once into several fragments, each of which may draw around it a portion of the contents of the parent-cell, which becomes invested by a cell- wall of its own ; and thus the cavity of the parent-cell may at once become filled with a whole brood of young cells, without any successive subdivision. Generally speaking, the former method seems to prevail in structures which, like Cartilage, have a com¬ paratively per man ent destination ; whilst the latter is followed in cases in which the cells thus formed are destined only for a transitory existence. This is the case especially in Can¬ cerous structures, which are particularly distinguished by their proneness to the rapid production of cells within cells. 34. The production of new cells in the midst of an or- ganizable blastema or formative fluid, such as is poured out from the blood for the reparation of an injury, is a very different process. This blastema, when first effused, is an apparently homogeneous semi-fluid substance ; as it solidifies, however, it becomes dimly shaded by minute dots, and as it is acquiring further consistence, some of these dots seem to aggregate, so as to form little round or oval clusters, bearing a strong resemblance to cell-nuclei. These bodies appear to be the centres of the further changes which take place in the blastema ; for if it be about to undergo development into a fibrous tissue (§ 18), they seem to be the centres from which the fibrillation spreads ; whilst, if a cellular structure is to be generated, it is from them that the cells take their origin. The first stage of the latter process appears to consist in the accumulation of the substance which the cell is to include, about each nucleus, and around this the cell-membrane is subsequently developed. It is in this mode that the de¬ velopment of new structures, for the filling up of losses of substance, is provided for; and it appears, from recent ISOLATED CELLS OF ANIMAL FLUIDS. 45 inquiries, that the blastema will resolve itself into fibres or into cells, according as the wound is completely secluded from the air, or is exposed to it. It is under the former condition that losses of substance are most rapidly and most completely repaired ; whilst it is under the latter that inflammation is most likely to arise, in consequence of the bad effect pro¬ duced by the contact of air with the raw surface ; the process of healing, when thus interfered with, going on less favourably as well as more slowly. 35. The very simplest and most independent condition of the animal Cell, is probably to be found in the nutritive fluids of the body ; in which we meet with floating cells that are completely isolated from each other, and which are conse¬ quently just as self-sustaining as are the separate vesicles of the Yeast-plant, of the Eed Snow, or of other simple cellular Plants. These cells are of two classes. In the blood of animals generally, and in the chyle and lymph of Yertebrata, we find a larger or smaller proportion of colourless corpuscles, which are usually nearly spherical in form, and which exhibit various stages of development into cells, being sometimes little else than collections of granules, without any distinct enveloping membrane, whilst, in other instances, there is a distinct cell-wall, cell-cavity, and nucleus. These bodies, if watched under a sufficiently powerful microscope, may often be seen to undergo very curious changes of form, resembling those of the Amoeba (§ 129). Besides the foregoing, however, the blood of Yertebrated animals contains a far larger pro¬ portion of red corpuscles, which are flattened disks, sometimes circular but more commonly oval, having pellucid and colour¬ less walls, but having their cavities filled with a peculiar coloured fluid. As these will be more fully described here¬ after (§ 229), it is not requisite to do more than notice them here as constituting a most important part of the animal organism, probably not less than a twelfth part of the entire weight of Man and the higher animals, being thus composed of nothing else than these isolated cells.1 36. Next in independence to the cells or corpuscles float¬ ing in the animal fluids, are those which cover the free 1 The entire weight of the blood of Man seems to be about one-sixth part of that of the body ; and the moist corpuscles constitute about half the entire weight of the blood. 46 SKIN AND MUCOUS MEMBRANES. membranous surfaces of the body, and which form the Epidermis, or superficial layer of the skin, and the Epithelium of the internal membranes. And it will be convenient here to consider the entire structure of the Skin, the Mucous Membranes, and the Serous Membranes, which are complex fabrics, chiefly made up of the elementary tissues already described. — These membranes may each be considered as composed of three principal parts, namely, the superficial layer or layers of cells, the basement-membrane whereon the cells lie, and the subjacent texture covered by this, which consists of fibrous tissue compactly interwoven and traversed by blood-vessels, nerves, absorbents, and also containing glands of various kinds. The Skin and Mucous Membrane may, in fact, be regarded as belonging to one and the same type ; for they are continuous with each other wherever one of the open cavities of the body communicates with the surface, as at the mouth, nostrils, and anus ; and in the Hydra (§ 121) it has been experimentally found that the membranous layer covering the body may be made to change places with that which lines the stomach, without any sensible disturbance in the functions of either. The difference between the two essentially consists in this; that the Skin, being destined especially for the reception of sensations, and for the protection of the soft parts beneath, is more copiously furnished with nerves than with blood-vessels, and has its surface covered by a firm, dry cuticle ; whilst the Mucous Membrane, ministering especially to the organic functions, is comparatively little supplied with nerves, but is abundantly furnished with blood-vessels, and in certain parts with absorbents, whilst its cellular layer is soft and easily permeable by liquids. Both in the skin and in mucous membrane we find a multitude of minute glands, for the separation of particular fluids from the blood ; the nature of these differs with the locality. 37. The fibrous mesh-work of the Cutis or True-Skin is con¬ tinuous with that of the Areolar tissue which lies immediately beneath it ; so that the two textures are not separated one from the other by any definite boundary (as the examination of a vertical section (fig. 7) clearly proves), but are dis¬ tinguishable only by the compactness of the one, as contrasted with the looseness of the other. The outer surface of the Cutis usually presents numerous minute elevations or papillce STRUCTURE OP THE SKIN. 47 (fig. 7, i i)y which, are commonly arranged in rows ; of these, some are organs of touch, being furnished with sensory nerves that end upon a peculiar cushion-like organ in their interior (§ 490) ; hut into others no nerves can he traced, so that, as these are copiously supplied with hlood- vessels, it is pro¬ bable that they minister to the -nutrition of the epidermis. / Fig. 7.— Vertical Section oe the Skin, Showing the different structures which it contains. A, Epidermis ; a a, its outer surface ; a — b, its horny layer ; b — c its inner soft layer, dipping down into the hollow between the papillae ; B, Cutis ; d, arterial twig supplying its vascular papillae ; e e, perspiratory glandulae ; f, cluster of fat-cells ; g g, perspiratory duct, traversing the true skin ; h, its continuation through the epidermis ; i i, tactile papillae, with their nerves. This is the more probable from the fact that we find these vascular papillae very large and full of blood-vessels in the interior of corns, -warts, and other such productions, formed by a “ hypertrophy ” or over-nutrition of the epidermis in particular spots ; and also in situations in which the ordinary epidermis is very thick, as it is on the black pads of the foot of the dog or cat. And a highly vascular structure of the STRUCTURE OF THE SKIN. 48 same kind is found in the matrix or receptacle of the growing roots of nails, hoofs, horns, &c. which are only modified forms of epidermis. Imbedded in the substance of the cutis we find, in most situations, the perspiratory glands (fig. 7, ee), by which the watery fluid that is continually being exhaled from the skin, is separated from the blood (§ 371) ; these send forth their secretion by canals (g h)y which traverse the epidermis in a corkscrew-like manner, and then open upon its surface by oblique valvular orifices. In the Cutis, also, are lodged the hair-follicles (§ 38), which are really pits or depressions of its surface, with a vascular papilla at the bottom of each, supplying nutriment for the abundant development of the cells in which the hair originates, as will be presently described. Wherever the hair-fol¬ licles occur, there do we also find sebaceous follicles (fig. 8, a a) ; these are peculiar glandulse, secreting fatty matter, which is poured into the hair- canal, so as to come through it to the surface of the epidermis ; and the use of this secretion, which is particularly abundant in the dark skins of the natives of warm climates, is to pre¬ vent the cuticle and the hair from being too much dried up by exposure to air. — The surface of the Cutis is covered by a layer of basement-mem¬ brane (§ 31), which is not traversed either by blood-vessels, nerves, or absorbents ; so that none of these pass into the epidermis which lies on its outer side. 38. The Epidermis , otherwise Fi g termed the cuticle , or “ scarf-skin,” is „ _ -jnr composed of numerous layers of nu- Thin Section of the Human r J Scalp ; — a a, sebaceous glands ; cleated Cells j Ot which We find tllOSe b, a hair, with its follicle c. jmmecqate contact with the base¬ ment-membrane to be nearly spherical ; those a little removed from it to be rendered polygonal by the mutual pressure of their sides; those nearer the outer surface to be flattened, and this in an increased degree, as we pass from within STRUCTURE OF THE SKIN: — EPIDERMIS. 49 outwards, until we arrive at layers composed entirely of dry flat scales, which show hut little indication of ever having been cells. There is no doubt, however, that all these forms are but different stages of the existence of one and the same set of epidermic cells ; these taking their origin in the formative fluid exuded on the surface of the basement-mem¬ brane, and being progressively carried towards the surface by the . successive development of new layers beneath them, whilst the layers above them are thrown off, or are worn away ; and at the same time undergoing a change of form, in the first instance from mutual pressure, and afterwards from the loss of their contained fluid. At the same time they are rendered more firm in texture, by the formation of a homy secretion in their interior ; so that ' the outer layers of epi¬ dermis form a consistent membrane, which is raised from the surface of the Cutis when fluid infiltrates between them (as when the hand has been long soaked in water), or is poured out by the vessels of the latter (as when a blister is applied) ; whilst the soft internal layers remain in contact with the basement-membrane.— The number of layers varies greatly in different parts, being usually found to be greatest where there is most pressure or friction, as if the irritation deter¬ mined an increased supply of blood to the spot, and thus favoured an augmented development of epidermic cells. Thus, on the soles of the feet, particularly at the heel and the ball of the great toe, the Epidermis is extremely thick ; and the palms of the hands of the labouring man are distinguished by the horny hardness of their thick cuticle. — It was formerly supposed that a special layer of a soft spongy tissue, termed the rete mucosum, intervenes between the Cutis and the Epidermis ; and that this was the special seat of the colour of the skin in the dark races. It is now well ascertained, however, that this supposed rete con¬ sists of nothing else than the newly-forming soft layers of the true • epidermis ; and that the colouring matter is diffused through the epidermic cells, so as to tinge the entire thickness of the cuticle, although its presence is particularly obvious in the deeper layers.— The Nails may be considered as nothing more than an altered form of Epidermis ; when examined near their origin, they are found to consist of cells which gradually dry into scales that remain E 50 EPIDERMIC APPENDAGES : - NAILS, HAIR, &C. coherent ; and when thin sections are treated by a dilute solution of soda, these scales swell out again (as do also those of the cuticle) into globular cells. A new production is continually taking place in the groove of the skin in which the root of the nail is imbedded, and also from the whole of the surface beneath it ; the former adds to the length of the nail ; the latter to its thickness. — The structure of Hairs is essentially the same. The base of each is formed of a “ bulb,” which consists of a mass of epidermic cells developed from the vascular papilla at the bottom of the hair follicle (fig. 8, c) ; and as this narrows into the “ shaft ” of the hair, a difference shows itself between the cortical or outer layer, and the medullary or pith-like substance of the interior. The former, which is continuous with the outer layers of the epi¬ dermis, is composed of flattened scales, arranged in an imbri¬ cated (tile-like) manner, so that the surface of the hair is usually marked by transverse jagged lines ; the latter consists of cells which frequently retain their spheroidal form, like the inner layers of the epidermis ; but in the human hair these cells are elongated into fibres. It is very seldom that there is any canal in the interior of the Hair, although irregular spaces are not unfrequently left by the drying-up of the fluid con¬ tents of the cells. The structure of Quills is essentially the same as that of hairs on a large’ scale ; and we there see the difference very distinctly marked between the cortical portion which forms the “barrel” of the quill, and the medullary portion which forms the white pith-like substance of the stem of the feather. The Scales , where they are really epi¬ dermic appendages, as is the case in serpents and lizards, are formed upon the same pattern ; and we have a good example of the detachment of the entire epidermis at once (reminding us of the casting of the shell of the crab and lobster) in the “ sloughing ” of the snake. 39. The Mucous Membranes form a sort of internal skin, lining those cavities of the body which open on its surface ; and the elements of which they are composed are essentially the same, though combined and arranged in a different manner, in accordance with their difference of function. The principal part of the thickness of every ordinary mucous membrane is made up, as in the skin, by the consolidation of areolar tissue, the fibres of which are continuous with those MUCOUS MEMBRANES : - EPITHELIUM. 51 of the ordinary areolar tissue on which the membrane rests ; this layer is copiously furnished with blood-vessels, but it is seldom supplied with many nerves. Thus the mucous mem¬ brane lining the stomach possesses in health so little sensi¬ bility, that we are not aware of the contact of the substances taken in as food, unless they are of an acrid character, or of a temperature very different from . that of the body ; and though the mucous membrane lining the air-passages is very susceptible of certain kinds of irritation, yet it has but little ordinary sensibility in the state of health, except near the entrance to the windpipe. The large supply of blood which these membranes receive, has reference to their active partici¬ pation in the functions of secretion and absorption. One secretion is common to all, that of the mucus by which they are covered ; this serves to protect them from the irritation that would otherwise be produced by the contact of solid or liquid substances, or even of air, with their free surfaces; and we see the results of its deficiency, in the inflammation which attacks the membrane, sometimes proceeding to its entire destruction, when from any cause the secretion is checked, as it sometimes is by injuries of the nerves sup¬ plying the part. 40. In every mucous membrane, as in the skin, the fibrous texture is bounded on the free surface by basement-mem¬ brane, beyond which no blood-vessels pass. And the surface of the basement-membrane is covered by cells, arranged either in a single layer or in multiple layers, constituting the Epithelium. This, although answering to the Epidermis in structure and position, has a very different character ; for its cells neither dry up nor become horny ; nor do they adhere in such a manner as to form a continuous membrane, except in the interior of the mouth and oesophagus (gullet), where the epithelium is endowed with somewhat of the firmness of cuticle, in order to resist the abrading contact of hard substances. The epithelium cells of mucous membranes are commonly somewhat flattened ; but in some situations, as on the villi of the intestinal canal (fig. 9, d\ they have more of a cylindrical, or rather conical shape, their smaller extremities being in contact with the basement-membrane. The epi¬ thelial cells are frequently cast off, like the epidermic, espe¬ cially from the parts that are most concerned in secretion ; e 2 52 MUCOUS MEMBRANES : - EPITHELIUM. and they are as continually replaced by newly-formed cells, which are produced on the surface of the basement-mem¬ brane, at the expense of the fluid that transudes through it from the blood-vessels copiously distributed to its under surface. 41. Mucous membrane may either exist in the condition of a simple expanded surface, or may have a much more complex arrangement, by which its surface is greatly increased. The simple mucous membrane, such as that which hues the nose and air-passages, is found, for the most part, where no ab¬ sorption has to be performed, and where only a moderate amount of secretion is necessary. Eut where it is to absorb as well as to secrete, it is usually involuted or folded upon itself, in such a manner as to form a series of little projec¬ tions, and also a number of minute pits (fig. 9). These pro- Fig. 9.— Diagram representing the Mucous Membrane op the Intestinal Canal. a a, absorbent vessels ; b 5, basement membrane ; c c, epithelium-cells of level surface of membrane ; d d, cylindrical epithelium-cells of villus ; e e> secreting cells of follicle. jections sometimes have the form of long folds ; in other instances they are narrow filaments, crowded together so as almost to resemble the pile of velvet. In either case, the absorbent surface is vastly increased ; but chiefly so by these filaments, which are termed villi , and act as so many little rootlets. On the other hand, it is in the pits or follicles , that the production of the fluid which is to be separated or secreted from the blood, chiefly takes place. — hTot only are the flat expanded surfaces of the mucous membrane covered with epithelium cells, but the villi also are sheathed by them; and the secreting follicles are lined by the same. STRUCTURE OF GLANDS. - SEROUS MEMBRANES. 5Z The cells covering the villi (fig. 9, d) perform the important function of selecting and absorbing certain nutritions ele¬ ments of the food, which they communicate to the absorbent vessels in the interior of the villi. On the other hand, the epithelium-cells of the follicles ( e ) seem to be the real agents in the secreting process ; drawing from the blood, as materials for their own growth, certain elements contained in it ; and falling off, when mature, so as to discharge these substances as the product of secretion, giving place to a fresh crop or generation of cells, which go through a series of changes precisely similar to the preceding. 42. Now these follicles are the simplest types or examples of all the Glandular structures, by which certain products are separated from the blood, some to be cast forth from the body as unfit to be retained in it, and some to answer particular purposes in the system. In all of them the structure ulti¬ mately consists of such follicles, sometimes swollen into rounded vesicles, and sometimes extended into 'long and narrow tubes. Each follicle, vesicle, or tube, is composed of a layer of basement-membrane, lined with epithelium-cells, and surrounded on the outside with minutely distributed blood-vessels ; and it seems to be by the peculiar powers of these cells, that the products of the secreting action, whether bile, saliva, fatty matter, or gastric fluid, are formed (see Chap. vii.). — Hence we see that the act of Secretion is, in animals as in plants, really performed by cells. It is neces¬ sary to bear in mind, however, that a simple transudation of the watery parts of the blood may take place without any proper secreting action, in the dead as in the living body ; it is in this manner that the serous fluid of areolar tissue and serous membrane is poured out, and that the watery portion of the urine is separated. 43. The Serous Membranes which line the closed cavities of the body, though composed of the same elements as the skin and mucous membranes, have a much simpler structure, and can scarcely be said to minister directly to any important vital function. The tissue of which Serous membrane is principally composed, scarcely differs, except in its greater density, from the laxer areolar tissue whereby the membrane is attached to the walls which it covers like plaster ; it is but sparingly supplied either with blood-vessels or absorbents; and 54 ARRANGEMENT OF SEROUS MEMBRANES. Fig. 10. Pavement Epithe it contains very few nerves. The smooth surface of the mem¬ brane forms one unbroken plane, being neither raised into villi, nor depressed into follicles ; and its basement-membrane is covered with a single layer of flat epithelium-cells, which are closely applied to it and to each other, like the pieces of a pavement (fig. 10). It is with such a membrane that every one of those great cavities is lined, which contains important viscera ; and it is also continued on to the outer surface of these viscera, so as to afford them an external coating* over every part save that by which they are attached. Thus the heart is suspended freely, by the large vessels proceeding from lium Cells oe Serous its summit, within a bag or sac of fibrous Membrane. membrane peculiar to itself, which is termed the pericardium. The cavity of this bag is completely lined by the serous membrane (fig. 11, p' ), which closely embraces the vessels, and which then bends down over the surface of the heart, so as to enclose it in the envelope p. Hence it will be seen that this membrane, whilst -including the heart, and allowing it to s communicate with its vessels, iP forms a completely shut sac; and it may be likened to a common double cotton or woollen night-cap, which has a similar cavity between its Fig. H.-DlAGRAM OP THE PERICARDIUM. tW° ^ S> ^ a a , auricles ; v v, ventricles ; b, pulmonary really On the Outside 01 artery; c, aorta; ppf, pericardium. this> whilst geeming to be within the envelope. The two layers of the pericardium, though separated in the diagram for the sake of distinctness, are really in mutual contact, save when separated by the in¬ terposition of fluid poured out in disease. Each of the lungs, in like manner, is suspended in a closed sac of its own, termed the pleura; and the surface of the lung is covered by a serous membrane, which is reflected over the wall of the pleural cavity. .?< SEROUS AND SYNOVIAL MEMBRANES. 55 A similar arrangement exists in the great cavity of the ab¬ domen ; but the number and the complex relations of the viscera which this contains, give to the disposition of its serous membrane, termed the peritoneum , a peculiar complica¬ tion. The cavity of the skull also is lined by a serous mem¬ brane, termed the arachnoid , and this is prolonged over the surface of the brain, and enters its lateral ventricles (§ 458). The chief purpose of these membranes appears to be to faci¬ litate the movements of the included organs, by forming smooth surfaces which shall freely glide over each other ; this * is evidently of great importance, where such constantly- moving organs as the heart and lungs are concerned. Their surfaces are kept constantly moist with a serous fluid which exudes from the blood ; but in the state of health this fluid does not accumulate in their cavities, being absorbed as fast as it is poured out. Various forms of dropsy, however, — such as “water on the brain,” “water on the chest,” and “ ascites,” or dropsy of the abdomen — are the result of the increased outpouring of fluid into the serous cavities of the arachnoid, the pericardium, the pleura, and the peritoneum respectively. 44. Nearly allied to the Serous mem¬ branes are the Synovial , which form closed sacs in the interior of joints, covering the ends of the cartilages, and then lining the fibrous capsule which passes from one bone to the other. The mode of their arrange¬ ment will be understood from the accom¬ panying diagram ; in which a a represent the extremities of the two bones which are jointed together, b b the layers of car¬ tilage with which they are severally covered, Diagram of the struc- and the dotted line c c the synovial mem- TURE °F A °INT* i if ? , i a a, extremities of the brane, which is seen to form the sac or tones, covered with bag c' c', whilst at the points CCCC it is cartilage ; ^Mayerof reflected upon the cartilages of the joints, vered with synovial In point of fact, however, the Synovial Sanyer of’sy™- membrane is not ordinarily traceable as a yiai membrane form- distinct layer over the surface of these lng synovia capsue. cartilages, but seems to have become incorporated with them. ; for though in the embryo its presence may be distinctly proved. Fig. 12. 56 SYNOVIAL MEMBRANES.— CILIATED EPITHELIUM: by tbe continuity of its blood-vessels over the entire car¬ tilage, yet these are found to retreat gradually as the joint is brought into use, until at last they only form a circle round the border of the cartilage. Some of the Synovial mem¬ branes, as that of the knee-joint, are furnished with little fringe -like projections, somewhat resembling the villi of mucous membranes (§ 41) ; these are extremely vascular, and are furnished with an epithelium which very readily falls off; and there is a strong probability that they are concerned in the secretion of the synovial fluid, which is much denser than the ordinary serous transudation, having from 6 to 8 per cent, of additional albumen, and presenting a glairy appearance like that of white of egg. It is interesting to see that the same purpose may thus be served by the extension of the membrane in either direction, either out¬ wards into a villous filament, or inwards into a follicle ; the function being determined in each case rather by the attributes of the cells, and by the supply of blood, than by the form which the secreting surface may happen to present. 45. The cells of Epithelium, whether flattened or cylindrical, are observed to be furnished in particular situations with a fringe of delicate filaments, which are termed cilia . These, although of extreme minuteness, are organs of great importance in the animal economy, on account of the extra¬ ordinary motor powers with which they are endowed. The form of the cilia is usually a little flattened, and' tapering gradually from the base to the point. Their size is Big. 13.— Ciliated Epithelium , , \ n 1 , cells; as seen sideways at a, extremely variable ; the largest that fyd. in transverse section at B; bave been observed being about their cilia are seen at b, their . , . , n nuclei at c; at a is shown one of 1 -500th of an inch m length, and these cells unusually elongated. the smallest 1-13, 000th. When in motion, each filament appears to bend from its root to its point, returning again to its original state, like the stalks of CILIARY MOVEMENT. 57 corn when depressed "by the wind; and if a number be affected in succession with this motion, the appearance of progressive waves following one another is produced, as when a corn-field is agitated by repeated gusts. When the ciliary motion is taking place in full activity, however, nothing can be distinguished save the whirl of particles in the surround¬ ing liquid; and it is only when the rate of movement slackens, that the shape and size of the individual filaments, * and the manner in which their stroke is made, can be made out. The motion of the cilia is not only quite independent (in all the higher animals at least) of the will of the animal, but is also independent even of the life of the rest of the body ; being seen to continue after the death of the animal, and even going on with perfect regularity in parts separated from the body. Thus, isolated epithelium-cells have been seen to swim about actively in water, by the agency of their cilia, for some hours after their detachment from the mucous membrane of the nose ; and the regular movement of cilia has been noticed fifteen days after death, in the body of a tortoise in which putrefaction was already far advanced. In the gills of the Eiver Mussel, which are amongst the best objects for the study of this most curious phenomenon, the movement endures with similar pertinacity. — The purpose of this remarkable agency is obviously to propel fluids over the surfaces which are furnished with cilia. We find it taking the most important share in the functions of life among the lowest classes of animals. Thus, in Animalcules of various kinds, the cilia are the sole instruments, not merely for the production of those currents in the water which may bring them the requisite supplies of air and food, but also for pro¬ pelling their own bodies through the liquid. In most Zoophytes, and in the inferior Mollusks, which pass their lives with little or no change from one spot to another, the motion of the cilia lining the alimentary canal and clothing the gills (where such have a special existence), draws into the mouth the minute currents which serve as food, and also renews the layer of water in contact with the respiratory surface. The gills of Fishes are not furnished with cilia, another provision being ^made by muscular action for conti¬ nually driving fresh streams of water over them ; but the motion may be very well seen upon the gills of the young 58 CILIA.— FAT CELLS. Tadpole or larva of the Water Newt, which hang down as fringes on either side of the neck. In the higher air-breathing animals, the function of the cilia is much more limited. They clothe the mucous membrane which lines the air-passages ; and their function appears to he, in that and other cases, to prevent the accumulation of the secretion with which the membrane is kept moist, by keeping up a continual onward - movement of it towards the outlet of the passage. In some other cases, however, we find the ducts of secreting organs furnished with cilia, whose action is obviously to assist in carrying the products of secretion towards their outlet. 46. Passing on, now, to those tissues of animals of which cells constitute the permanent components, instead of being successively thrown .off and replaced as they are in the Epidermis and Epithelium, we may first notice the Adipose tissue, or Fat , in which the oily and fatty matters of the body are for the most part contained. This tissue is composed of minute cells or vesicles (fig. 14), having no communication with each other, but lying side by side in the meshes of the areolar tissue, which serves to hold them together, and through which also the blood¬ vessels find their way to them. From the fluid in these vessels, the fatty matter is separated in the first place by the secreting action of the cells ; and it is prevented from making its way through the very thin walls of the cells, by the simple expedient of keeping these constantly moist with a watery fluid, the blood.1 The blood-vessels have also the power of taking back the fatty matter again into the circulation, when it is wanted for other purposes in the economy. These deposits of fatty matter answer several important objects. They often assist the action of moving parts, by giving them support without interfering with their free motions j thus the eye rests on a sort of cushion of fat, on which it can freely turn, and through which the muscles 1 Thus oil will nob pass into blotting-paper, if this have been previously moistened with water. FAT. - CARTILAGE. 59 pass that keep it in play. It also affords, by its power of re¬ sisting the passage of beat, a warm covering to animals that are destined to live in cold climates ; and it is in these that we find it accumulated' to the largest amount. Further, being deposited when nourishment is abundant, it serves as a store of combustive material, which may be taken back into the system, and made use of in time of need. The causes which peculiarly contribute to the production of fat, will be considered hereafter (§ 162). 47. Another tissue of which cells form the principal part, is that termed Cartilage or gristle. Its simplest state is that of a mass of firm substance, composed of chondrin (§20), through which are scat¬ tered a number of cells, at a greater or less distance from one another. In the simple cellular cartilages, such as those which cover the ends of ’the bones where they glide over one another so as to form moveable joints, no trace of structure can be seen in the intervening substance. Fig. is. — Section op But in cartilages which have to resist not _ . Caiitila®e> only pressure but also extension or strain, ded in intercellular sub- we find the space between the cells partly stance- occupied by fibres, which resemble those of ligaments ; and such are termed fibre- cartilages. . They are found in Man be¬ tween the vertebrae of which the spinal column is made up (§ 71); and also uniting the bones of the pelvis (§ 645). Sometimes, where elasticity is required, the fibres are those of the yellow fibrous tissue (§ 23) ; this is the case with the cartilage which forms the external ear. Cartilage is not penetrated by blood-vessels, at least in its natural state. The blood is brought to its surface by a set of vessels which bulge out into dilatations or swellings upon it, so that a large quan¬ tity of fluid comes into the immediate neighbourhood of the cartilage, being only separated from it by the thin walls of the vessels ; and it appears that this fluid, or so much of it as is required, is absorbed by the nearest cells, and trans¬ mitted by them to the cells in the interior, so that the whole substance is nourished. This is precisely the mode in which the interior of the large sea- weeds (whose tissue consists of cells imbedded in a gelatinous substance, and therefore bears 60 CARTILAGE. — BONE. a close resemblance to animal cartilage) obtains its nourish-, ment from the surrounding fluid. 48. The permanent Cartilages seem to undergo very little change from time to time. Their wear is slow ; and, being purely mechanical, it is confined to the surface. It is replaced by the materials absorbed from the blood, which are employed in the development of new cells, — sometimes within the old ones, sometimes in the space between them. When a portion of cartilage has been destroyed, however, by disease or injury, it is not renewed by true cartilaginous structure, but by what seems a condensed areolar tissue. Although cartilage does not usually contain vessels, yet these may be rapidly deve¬ loped in its substance, by a process which will be described hereafter (§ 393), when it becomes inflamed. This may be often seen to take place. The front of the eye is formed by a transparent lamina of a substance somewhat resembling cartilage, which bulges like a watch-glass : this, which is termed the cornea (§533), is properly nourished only by vessels that bring blood to its edge, where it is connected with the tough membrane that forms the white of the eye. But when the cornea becomes inflamed, minute vessels may be seen to spread over it, proceeding from its circular edge towards its centre ; and at last some of these often become of considerable size. Under proper treatment, however, these vessels gradually shrink and disappear ; and the cornea becomes nearly as transparent as before. 49. Many parts exist in the state of Cartilage in the young animal, which are afterwards to become Bone; and it has been commonly believed that all bone has its origin in a cartilaginous structure. This, however, is not the fact, as will be presently shown. Before attempting to explain the formation of Bone, it will be desirable to describe its structure. When we cut through a fully formed bone, such as that of the thigh, we find that the shaft or elongated portion is a hollow cylinder ; of which the walls are formed by what appears to be solid bone ; whilst the interior is filled, in the living state, by an oily substance laid up in cells, and termed marrow. Towards the extremities, however, the struc¬ ture of bone is very different. The outside wall becomes thinner ; and the interior, instead of forming one large cavity, is divided into a vast number of small chambers, like those STRUCTURE OP BONE. 61 of areolar tissue, by tbin bony partitions, which cross each other in every direction, -forming what is called the “ cancel¬ lated ” structure. These chambers or cancelli are filled with marrow, like the central cavity, with which they communi¬ cate. In the flat bones, moreover, — such as those of the head — we find that the two surfaces are composed of dense plates of bone, like that which, forms the shaft of the long bones ; but that between them there is a layer of cancellated structure, filled in like manner with marrow. But when we examine with the microscope a thin section of even the densest bony matter, we find it traversed by a network of minute canals, continuous with the central cavity. These canals usually run, in the shafts of long bones, in the direction of their length ; and are con¬ nected, every here and there, by cross branches (fig. 16). They are termed the Haversian canals, after the name of their disco¬ verer, Havers. — The lining mem¬ brane of the large central cavity is copiously supplied with blood¬ vessels ; and this sends off pro¬ longations into the cancelli at the extremities of the bone, and Fig* ^.-diagram representing into the Haversian canals. Ihus the shaft of a long bone. blood is conveyed into the in- ® transverse ^ectionf'L/c, "surface terior of the bone ; but no vessels can be traced absolutely into its texture, so that all the spaces which lie between the Haversian canals are as destitute of vessels as is healthy cartilage. These spaces are provided with nutriment by the following very remarkable arrangement. 50. When we eut across the shaft of a long bone, and examine a thin section with a microscope, we of course see the open extremities of the Haversian canals (fig. 17, a) ; just as we *see the cut ends of the ducts and vessels of wood, when we make a transverse section of a stem. Around each of these apertures, the bony matter is arranged in concentric rings, which are marked out and divided seen in longitudinal section ; i, Ha¬ versian systems cut across, each having an Haversian canal in its centre; g v, Haversian systems cut longitudinally ; l , lamellae near the surface of bone, destitute of Haver¬ sian systems. 62 STRUCTURE OF BONE. by circles of little dark spots; and when these spots are examined with a higher magnifying power, it is seen that they are small flattened cavities, from which proceed a number of extremely minute tubules (A). These tubules pass out Fig. 17. — Transverse Section of Bone. Showing the concentric rings round a a , the Haversian canals. At A are seen some of the cavities with^their radiating tubes, more highly magnified. from the two flat sides of each cavity ; one set passes inwards, towards the centre of the ring, and the other outwards, to¬ wards the ring that next surrounds them. These minute tubuli, which are far smaller than the smallest blood-vessels, may thus be traced into every part of the substance of the bone ; and those proceeding from different rings are so con¬ nected with each other, that a communication is- established between the innermost and the outermost circles. The tubuli which open upon the sides of the Haversian canals, are thus enabled to take up the nourishment with which they are COMPOSITION OF BONE. 63 supplied by the blood-vessels, and to transmit it to the outer circles, or those furthest removed from those vessels ; and in this manner, a much more active nutrition takes place in bone than that which is performed in cartilage. It has been proved by various experiments, that the substance of bone is undergoing continual change ; and it is owing to the comparative activity of its nutritive processes, that bone is so readily and perfectly repaired, when it has been broken by violence or has been injured by disease. 51. But the peculiarity of Bone consists, not so much in this remarkable arrangement of its organic structure, as in its solidity and firmness. This is given to it by the union of a large quantity of mineral matter with the organic substance of its tissue. The mineral matter of bones consists almost entirely of two compounds of Lime ; the carbonate , with which we are familiar in the form of limestone and chalk ; and the phosphate, which is seldom found as an ingredient of rocks or soils, except where it has been derived from animal remains. The latter greatly predominates, at least in the bones of the higher animals. We may easily separate the animal and the mineral portions of the bony tissue. If we soak a small bone for some time in muriatic acid much diluted with water, the compounds of lime are entirely removed from it, and the organic substance remains ; the latter is now quite flexible, and almost transparent, so that the distribution of its vessels (if they have been pre¬ viously injected with colouring matter) may be distinctly seen. On the other hand, if we subject a bone to strong heat, the animal portion will be burnt out, and the earthy matter will remain. The form of the bone will be still retained ; but the cohesion between the earthy particles is so slight, that the least touch will break them asunder. Thus we see that the hardness of bone, or power of' resisting pres¬ sure, is given by the earthy matter; whilst its tenacity , or power of holding together, depends upon the animal portion. Although the animal substance which remains after the solu¬ tion of the mineral matter, has been commonly described as Cartilage, yet it is not so in reality ; for it consists not of chondrin, but of gelatin ; and instead of being made up of an aggregation of cells united by an intervening substance, it may be torn into layers of an indistinctly-fibrous matting. In fact, 04 COMPOSITION AND DEVELOPMENT OF BONE. it corresponds closely with, the white fibrous tissue (§ 23), both in structure and composition ; and so far from this view of its nature being inconsistent with the history of tbe formation of bone, it will be found to be in entire harmony with it. The proportion which the mineral bears to tbe animal substance of bone is very constant, when the proper - osseous tissue alone is taken into account ; being almost exactly two of the former to one of the latter, or 66f per cent, to 33^ per cent. Eut when the composition of entire bones , including the contents of the Haversian canals and cancelli, is compared, the proportion of mineral to animal matter is found to vary greatly in different classes of animals, in the same animal at different ages, and even in different bones of the same individual ; the mineral matter predominating in bones of a compact texture, and the animal in those whose substance is more spongy. 52. In the first development of the embryo, a sort of mould of cartilage is laid down for the greater part of the bones • though, in the case of the fiat bones, this mould is generally limited to the central portion, the place of their marginal part being occupied by a fibrous membrane only. The process of ossification, or bone-formation, commences with the deposit of calcareous matter in the intercellular substance of the cartilage, so as to form a sort of network, in the interspaces of which are seen the remains of the cartilage-cells. The tissue thus formed can scarcely be considered as true bone, for it contains neither lacunae nor canaliculi. Eefore long, however, it undergoes very important changes; for many of the partitions are removed, so that the minute chambers which they separated coalesce into larger ones ; and thus are formed the cancelli of the spongy substance, and the Haversian canals of the more compact. These are at first much larger than they are subsequently to become ; for they are gradually narrowed by deposits of true bony tissue, which successively take place upon their interior walls, at the expense of the materials supplied by the blood brought thither by their contained vessels ; and it is by this forma¬ tion of concentric layers around the cavities of the Haversian canals, that the appearance of concentric rings is produced, which we have just seen to be presented by transverse sec¬ tions of long bones. In old bones the Haversian canals are so nearly filled by these deposits, that there is barely room DEVELOPMENT OF BONE : OSSIFICATION. 65 for the blood-vessels to pass along them. And it is through their complete blocking up, by a continuance of the same growth, that the supply of blood is cut off from the interior of the bone which forms the antlers of the deer, so that they die and fall off ; their shedding and renewal being an annual process.1 — Whilst the formation of the Haversian canals and cancelli is being effected by the partial removal of the first formed partitions, a complete cavity is formed in the centre of the shaft of every long bone (at least in Mammals and Birds), by the entire removal of the solid tissue. This cavity is at first not much larger than one of the Haversian canals ; but as the bone grows in diameter by additions to the exterior of its shaft, so is the cavity in its interior augmented by the removal (by absorption) of the first-formed bone ; and this double process continues until the bone has attained its full diameter. The formation of new bone on the exterior of the shaft seems to be the result of the consolidation- of the fibrous tissue of the periosteum (or membrane covering the bone) by calcareous deposit ; the lacunae being probably the cavities of cells which were entangled in the fibres, and the canaliculi being outgrowths from these; and new fibrous tissue being formed on the outside of the periosteum, to replace that which has been taken into the bone. Thus it comes to pass, that after a time none of the bone first formed in its cartilaginous mould any longer remains, the whole of it having been removed by absorption ; since the central cavity of the perfect bone is much larger than the entire carti¬ laginous shaft in which it originated. And thus it also comes to pass, that (as gelatin is the basis of fibrous tissue) bones yield gelatin, not chondrin, upon being long boiled. — The increase of the shaft in length, however, is the result of a different process. In all bones of any considerable dimen¬ sions, the process of ossification commences in more than one point at a time. In the long bones, there are usually three such points; one for the shaft, and the others for the two 1 It is commonly stated that the death of the antlers is due to the formation of a bony ring at their base, which cuts off the supply of blood from the (i velvet ” which covers them ; but though this may con¬ tribute to produce the effect, it is by no means the sole cause, as the interior of the antlers is supplied with blood from the vessels of the bone from which they sprout, and not from those of the “ velvet ” only. F 66 DEVELOPMENT OF BONE : OSSIFICATION. extremities. Long after the ossification of the shaft and of the extremities has been completed, these parts remain sepa¬ rated from each other by the interposition of a thin layer of nnconsolidated cartilage ; so that, although the bone appears firm and complete, its three portions fall apart, if it be macerated sufficiently long in water for the cartilage to decay. Now it is by the progressive consolidation of the cartilage at these two junctions, and by the continual forma¬ tion of new cartilage as the old is taken into the bone, that the length of the shaft continues to increase up to adult age ; and then, its full size having been attained, the whole thickness of the intervening layer of cartilage is replaced by bone, so that the shaft and extremities become firmly con¬ solidated. — The general history of the formation of the flat bones is nearly the same. In these, when they are large, or have projecting out-growths, there are several centres of ossi¬ fication ; and although the first ossification takes place in the substance of cartilage, yet the subsequent growth seems to be effected mainly by the consolidation of fibrous mem¬ brane. 53. The foregoing description applies chiefly to those higher and more complete forms of Bone, which are found in Birds and Mammals. In Beptiles and Fishes, the process of ossification is stopped short, as it were, at an early period ; and thus the texture of their bones resembles that which we find the skeleton to present in the earlier life of the higher animals. — The long bones of Beptiles (with one remarkable exception in the Pterodactylus , § 669, which is adapted to the life of a Bird) have no one central cavity, but are pene¬ trated by numerous large Haversian canals, like those of very young bone ; and various pieces remain separate in them throughout life, which, originating in distinct centres of ossi¬ fication, subsequently coalesce in Birds and Mammals. This permanent separation is still more remarkable in the bones of Fishes ; and it is consequently in them that we can best study the real composition of the skeleton, — every piece which originates in a distinct centre of ossification, being, in the eye of the philosophical anatomist, a separate bone. Further, there is a large group of Fishes in which the skeleton retains the cartilaginous character through life ; a certain quantity of mineral matter being deposited in the BONES OP FISHES : — TEETH. 67 cartilage, but its conversion into true bony structure never taking place. In a few, not even a firm cartilage is produced; and all the trace of a skeleton is a cylinder formed of hex¬ agonal cells, resembling those of the pith of plants, which takes the place that is generally occupied by the “ bodies ” of the vertebrae (§ 71). Such a cylinder, which is termed the chorda dorsalis , precedes the formation of the vertebral column in other vertebrated animals (§ 757). In the curious Amphioxus (Zool. § 642), even this is wanting ; and the only rudiment of the bony skeleton is to be found in the fibrous sheath that surrounds the nervous centres, and sends off prolongations between the successive transverse bands of muscles, which are attached to these, as they are in other fishes to the ribs and the spines of the vertebrae. 54. In connexion with the structure of Bone, it will be convenient to describe that of Teeth, although the general description of the form and development of these organs will be more appropriately given in connexion with the account of their instrumental uses (§§ 181—183). The principal part of the substance of all teeth is made up of a solid tissue, which has been appropriately called Dentine. Of this sub¬ stance, one variety, which is peculiarly close in texture, and susceptible of a high polish, is familiarly known as %vory. The more perfect forms of dentine, such as present them¬ selves in Man and the Mammalia generally, consist of a. hard transparent substance formed by the union of animal matter and calcareous salts (chiefly phos¬ phate of lime), in the proportion of about 28 of the former to 72 of the latter; the mineral matter thus bearing a somewhat larger ratio to the organic, than it" does dn bone. This dentinal substance is traversed by minute tubuli of about 1-1 0,000th of an inch in diameter, which appear as dark fines, generally very close to¬ gether : these pass in a radiating Portion of Dentine (highly magni- ^ n jV , -i fied), showing its tubular structure. manner from the central cavity h 5 of the tooth, diverging from each other as they approach its exterior; but when seen in only a small part of their f 2 Fig. 18. 68 STRUCTURE OF TEETH. course, they appear to be nearly parallel (fig. 18), though usually more or less wavy. They occasionally divide into two branches, which continue to run, at a little distance from one another, in the same parallel direction ; and they also frequently give off small lateral branches, wliich again send off smaller ones. In some animals the tubuli may be traced at their extremities into minute cavities analogous to the lacunee of bone ; and the lateral branchlets also occasionally terminate in similar cavities. Thus the whole tooth may be likened, in some degree, to a single Haversian system in bone ; the central cavity, which is lined by a vascular mem¬ brane, representing the Haversian canal, while the radiating tubuli of the former correspond with the radiating canaliculi of the latter ; the chief difference lying in the absence of lacunae along the course of the radiating tubes. In a large proportion of Fishes, however, there is no single central cavity, but the whole tooth is traversed by a system of medullary canals, not only resembling the Haversian, but actually con¬ tinuous with those of the bone on which the tooth is im¬ planted; and as each of these is the centre of a distinct system of radiating tubuli, the resemblance of their dentine to bone . is very close. A somewhat similar condition of the dentine (obviously a lower or less specialized form of this substance) presents itself in certain Eep tiles and Mammals. — In the Teeth of Man and most other Mammals, and in those of many Eep tiles and some Fishes, we find two other sub¬ stances, one of them harder and the other softer than dentine. The former, which is called Enamel , consists of long pris¬ matic cells, which pass from one surface to the other of the thin layer formed by this substance over the crown, or sometimes in the interior of the tooth (§ 182). These prisms are usually hex¬ agonal in form, as is seen in Fig. 19. less wavy. In teeth which have to sustain an extraordinary amount of compression (as is especially the case with those of TEETH. - MUSCLE AND NERVE. 69 the Rodentia), the enamel-prisms cross and interlace with one another, in such a manner as to prevent that separation which would readily occur if the direction of all of them were the same. Of all the tissues of the animal body, the Enamel is the most remarkable for the predominance of mineral ingredients ; these amount to no fewer than 98 parts in 100, leaving when removed only 2 per cent, of organic matter. The softer component of Teeth, known as the Cementum , or Crusta 'petrosa , possesses the essential characters of true bone ; but when only a thin layer of it is present, we do not find it traversed by medullary canals, its system of lacunae and canaliculi being then in relation to the nearest vascular surface, — as is the case also with very thin laminae of ordiuary bone, such as we find in the scapula (blade-bone) of a Mouse. 55. We come, lastly, to the two tissues which are of the highest importance in the Animal fabric, and to which all the rest are merely subsidiary ; namely, the Muscular and the Nervous. It is through the instrumentality of these, that all the actions are performed which essentially constitute Animal life ; for the nervous apparatus is the medium by which the consciousness of the individual is affected by what takes place around him, or within his own body, and by which, in his turn, he originates movements in his body, and through it in things external to it ; whilst the muscles are, so to speak, the servants of the nerves, doing , with a force of their own, the work which the nerves direct. The relation between the two may be likened to that of the rider and his horse, or of the engine-driver and his locomotive ; for the nerves can put forth no motor power by themselves ; whilst, on the other hand, the muscles (with certain excep¬ tions) remain inert except when stimulated to contract by the agency of the nerves. The muscles use the tendons and the framework of bones, joints, &c., for the mechanical appli¬ cation of their power, as will be shown hereafter (Chap, xil); but these parts of the fabric have not the slightest power of originating motion by themselves. Hence, all Animal Force takes its rise in one or other of these two tissues ; and w^e shall find that the special purpose of the whole apparatus of Organic life, is, by providing materials for their nutrition and renovation, to build them up in the first instance, and then 70 STRUCTURE OF MUSCLE. to keep them in working order. For every development of animal force involves a change of state of the Nervo-mus- cular substance : a certain amount of it ceasing to exist as living tissue, and passing into the condition of dead matter ; and its elements resolving themselves, under the influence of the free oxygen brought to them by the blood, into new combi¬ nations, which are carried forth from the body as quickly as possible. Consequently, if the Eervo-muscular tissues be not renewed as rapidly as they are used up, their powers must speedily fail from the progressive loss of their substance. In this particular they are on a different footing from the other elementary parts of the organism ; for although each of these seems to have a certain term of life, the length of which is in some degree related inversely to its functional activity, — those which live the fastest having the. shortest individual duration, and vice versd , — there are none which are called upon to give forth their whole vital energy in one effort, and which may thus have their existence as parts of the living organism terminated at any moment by a demand for their peculiar power. 56. Muscular Fibre presents itself under two forms, which are ordinarily very distinct from each other ; although it is probable that they may ultimately prove to be but modifi¬ cations of one and the same. The first, which is known as the striated fibre, is that of which all those muscles are com¬ posed, which constitute what is commonly designated as “flesh” or the “lean” of meat. If any “joint” of meat be even cursorily examined, it will be seen that its whole substance is made up of distinct masses, held loosely together by areolar tissue ; and these masses, which are known as “ muscles,” are easily isolated from each other by dissection. Every such Muscle is formed by the union of a number of bundles, having a generally parallel arrangement, which are closely bound together by areolar tissue, and are themselves composed of bundles still more minute, united in a similar manner. These, again, may be separated in the same way ; and at last we come to the 'primitive fibres of which this tissue is composed. Each of these primitive fibres termi¬ nates at either extremity in tendinous fibre, which unites with other fibres to form the tendinous cords or bands, that are attached to the points of the skeleton which the muscle STRIATED MUSCULAR FIBRE. 71 has to bring together. The muscular fibre itself consists of a delicate membranous tube, enclosing a great number of fibrillce , or extremely minute fibrils, which are not capable of further division (fig. 20). The peculiar transverse marking Fig. 20. — Striated Muscular Fibre separating into Fibrill.se. or striation by which this form of muscular fibre is characterised, is found, when the fibre is separated into its fibrillse, to be due to the peculiar markings which every fibril presents. These markings, consisting of alternate light and dark spaces, give to the fibril a beaded appearance ; but this is only an optical deception, since its form is in reality cylindrical, or nearly so. It is easy to see how the correspondence of the light and dark spaces respectively, throughout the whole bundle of the fibril , will give rise to the banded appearance which the entire fibre presents. The form and diameter of the fibres vary considerably, both in different tribes, and in different parts of the same animal. In the higher classes, their form usually approaches a cylinder ; but the parts which press against one another are somewhat flattened, so that it is more or less prismatic. In Insects, on the other hand, the fibrillse are arranged in flat bands, so that the fibre often consists of but a single layer of them. The diameter of the fibres in Man averages about 1-40 0th of an inch, and does not differ very widely in either direction; in the cold-blooded Vertebrata, however, the average size is greater, and the extremes are also wider ; the diameter of the fibres varying in the Frog from l-100th to l-1000th of an inch, and in the Skate from l-65th to l-300th of an inch. The diameter of the fibrils is nearly the same in all classes, seldom departing much from 1-1 0,000th of an inch ; and the average distance of the dark striae from each other is nearly the same. 7 2 NON-STRIATED MUSCULAR FIBRE. 57. The other form of Muscular Fibre, which, from the absence of transverse striation, is distinguished as smooth or non-striated , is found not in large masses, but in thin layers, forming part of the wall of various hollow organs, such as the stomach and intestinal canal, the bladder, the principal gland- ducts, and the larger blood-vessels. In all these situations it is so exclusively concerned in the performance of the vege¬ tative or nutritive functions, and it is so entirely withdrawn from the influence of the will, that it has been frequently designated as “ the muscular fibre of organic life the striated fibre, of which the voluntary muscles are composed, being distinguished as the “muscular fibre of animal life/5 But these designations are not by any means consistent with the facts of the case ; for in a large proportion of the Molluscous classes, the muscles of animal life are composed of non- striated fibre, whilst the heart of Man and of other Verte- brata, though a muscle of organic life, is made up of striated fibre. In fact, the employment of the one or of the other kind of fibre would seem to be chiefly determined by the kind of contraction which is required from it (§ 59). The non-striated fibres are arranged, like those of the other muscles, in a parallel manner into bands or bundles; but these bundles, instead of being them¬ selves grouped into larger ones having a like parallel arrangement, are gene¬ rally interwoven into a kin d of network, having no fixed points of attachment. The form of the individual fibres is much more variable than that of the striated kind, being often very much flattened out ; and hence their general dimensions cannot well be estimated. By macerating a portion of this kind of tissue in dilute nitric acid, each fibre may be resolved into bundles of long spindle-shaped bodies, which, contain¬ ing elongated staff-shaped nuclei (fig. 21), may be regarded as cells, al¬ though it is difficult to distinguish their walls from their contents. This form of muscular tissue is commonly mingled with a large quantity of the ordinary fibrous A B c Fig. 21. A, Portion of a band composed of non-striated muscular fibre, showing, a a , the spindle-shaped cells, and, b b, the elongated nuclei ; B, a single cell isolated, and more highly magnified ; C. a similar cell treated with acetic acid. MUSCULAR CONTRACTION. 73 structure ; and we find it dispersed in small quantity through the latter in the skin, to which (especially in particular regions) it gives a contractility that is manifested under the influence of cold or of mental emotions, and thus produces that general roughness and rigidity of the surface which is known as cutis anserina, or “ goose’s skin.” 58. Under the influence of certain exciting causes, or stimuli (Chap, xn.), striated muscular fibres suddenly and for¬ cibly contract. Their two ends approach one another, and their striae become closer ; but they bulge out in the middle to a corresponding degree. This causes a like change in the bundles which are made up of these fibres ; and thus the whole muscle, when shortened by the drawing together of its two ends, is greatly enlarged in diameter, especially towards its middle. Of this any one may convince himself, by bending his fore¬ arm upon the arm (as when the hand is brought to the mouth), and feeling the fleshy mass upon the front of the latter. The muscle, in fact, does not in the least degree change its own bulk in the act of contraction ; for its enlarge¬ ment in diameter is exactly equivalent to the shortening of the distance between its extremities. The contraction of a muscular fibre is ordinarily followed, after a short interval, by its relaxation ; of this we have a remarkable illustration in the contractions excited by the electric stimulus. But relax¬ ation of individual fibres is not incompatible with the con¬ tinuance of the state of contraction of the muscle as a whole. Bor it appears that when an ordinary muscle is thrown into contraction, all its fibres do not usually contract together, but only a small part of them ; and that, as long as its contraction is maintained so as to exert a constant force, a continual in¬ terchange is taking place in the action of the fibres by which this is kept up — those which have been shortened becoming slack, and being replaced (as it w^ere) by others, which pass into the contracted state for a time, and then relax again, being succeeded by another set. Now as the ends of those fibres which are actually in a relaxed condition, are brought near together by the contraction of the rest, the fibre is thrown out of the straight line, and assumes a wavy or zigzag form, which was formerly supposed to be the state of con¬ traction, but is now known to be otherwise. This peculiar arrangement -gives place to the straight form, either when the 74 MUSCULAR CONTRACTION. - NERVOUS TISSUE. fibre passes into the state of contraction, or when, by the relaxation of the whole muscle, its ends are separated again to their full extent. 59. Now the alternate contraction and relaxation, which is thus made to produce a continued contraction in ordinary muscles, elsewhere occasions a different effect. Thus in the heart, all the fibres of the ventricles seem to contract to¬ gether and all to relax together, — those of the auricles contract¬ ing whilst the others are relaxing, and vice vend; — and in this way the alternate contractions and dilatations of that most important organ are continually kept up. Again, in the muscular coat of the intestinal canal, we observe the contraction of each part to be almost immediately followed by its relaxation ; but the peculiarity of its movement is, that the contraction is pro¬ pagated on (as it were) to the succeeding part, which in its turn contracts and then relaxes, producing the same action in the part that follows it, — and so on along the whole canal. This peristaltic motion (§ 215), as it is called, is obviously adapted to propel the contents of the intestinal tube from one ex¬ tremity of it to the other ; just as the peculiar action of the heart is adapted to receive and propel the blood alternately, or as the mode of contraction of the ordinary muscles enables them to keep up a continued strain for a great length of time. It is much less rapid and energetic than the action of the heart ; for it is the characteristic of the non-striated fibre, that its contraction follows much less closely on the application of the stimulus, and is much less rapidly succeeded by relaxa¬ tion, than that of the striated fibre. 60. The Nervous tissue consists of two distinct structures, of one of which the trunks of the nerves are entirely made up, whilst the other enters largely into the composition of the ganglia or centres of action (§ 61). The former, termed the white or fibrous tissue, consists of straight fibres, lying side by side, and bound together by areolar tissue into bundles (fig. 22) ; these, again, are united with others into a larger group ; and by the union of a considerable number of such groups, the nervous trunks are formed, which are dis¬ tributed through the body, especially to the skin and muscles. Nervous Fibre, like muscular, presents itself in the higher animals under two forms, of which one may be considered as more completely developed than the other ; these are known STRUCTURE OF TUBULAR NERVE-FIBRES. 75 as the tubular and the gelatinous. The “ tubular ” fibres are so named because each possesses a distinct tubular sheath of a delicate structureless membrane (fig. 22, a), which encloses the proper nerve-substance, and isolates it completely from the Fig. 22.— Structure of Nerve-Tubes. Tubular Nerve-fibres ; A, from a nerve-trunk; B, from the substance of the brain. blood-vessels and other surrounding structures ; this tube does not either branch or unite with others, and there is reason to believe it to be continuous from the origin to the termination of the nerve-trunk. Within the tube is a hollow cylinder of a material known (after its discoverer) as the “ white substance of Schwann and this encloses a sort of central pith, which is transparent and semi-fluid in the living state, but undergoes a kind of coagulation into a granular sub¬ stance after death, and under the influence of chemical re-agents. There is reason to believe that this central pith or “ axis-cylinder ” is the essential component of the nervous fibre, and that the hollow cylinder which surrounds it serves only to isolate it more completely; for we not unfrequentlv see the former to be alone continued, both the tubular sheath and the white substance stopping short ; and this at either extremity of the fibre, where it separates itself from those with which it is bound up in the nerve-trunk. The proper form of the fibre seems always to be truly cylindrical ; though 76 TUBULAR AND GELATINOUS NERVE-FIBRES. it is very liable to be altered by manipulation, a small excess of pressure in one part forcing the contents of the tube towards some other where they are more free to distend it, and thus producing a swelling. The greater delicacy of the tubular sheath in the fibrous substance of the brain and spinal cord, renders its fibres peculiarly susceptible of this kind of alteration, so that they often present under the microscope a somewhat beaded appearance (fig. 22, b) ; when carefully examined, however, without any previous disturb¬ ance, these fibres are found to be as cylindrical as those of the nerve- trunks. The diameter of the nerve- tubules is usually between 1 -2000th and 1 -4000th of an inch ; but it may be somewhat greater or considerably less than this average. They are larger in the .nerve-trunks than they are near their central termination in the brain ; and it is a remark¬ able circumstance that the fibres of the nerves of “ special sense” are considerably smaller than the average in every part of their course. — The “ gelatinous” fibres cannot be shown to consist of the same variety of parts as the preceding ; for neither the tubular sheath nor the white substance of Schwann can be distinguished in them. They are flattened, soft, and apparently homogeneous, sometimes showing a dis¬ position to split into very delicate fibrillse ; being of a yellowish-grey colour, they are sometimes designated the grey fibres. Their diameter averages between the 1 -4000th and l-6000th of an inch. As these “gelatinous” fibres form a considerable proportion of the trunks of the Sympa¬ thetic system of nerves (§ 461), they have been supposed to belong properly to it, and to minister exclusively to the organic functions, like the non-striated muscular fibre (§ 57); but 'Hhere is no doubt that this is an incorrect notion, and that even the fibres of the ordinary nerve- trunks may present the “ gelatinous ” aspect, probably from incompleteness of development. 61. In the central organs of the Nervous system — namely, the brain and spinal cord of the Yertebrata, the ganglia or knot-like swellings on the nervous cords which take their place in the lower animals, and similar ganglia belonging to the Sympathetic system — we find a form of nervous tissue altogether distinct from the preceding ; which, from its con¬ sisting of large cells or vesicles, is generally known as the VESICULAR OR GANGLIONIC NERVE-SUBSTANCE. 77 vesicular. These nerve-vesicles, sometimes known as gan¬ glion-globules, may be regarded as originally spherical, or nearly so, in form (fig. 23, a ); but they often present one or more prolonged extensions ; and as these when single re¬ semble tails, and when multiple are like the rays proceeding from a star, the cells are said in the first case to be “caudate,” Eig. 23. — Vesicular Nerve-substance. A, combination of Ganglion-cells (of which one is shown separately at a , more highly magnified), and Nerve-fibres in the grey substance of the brain, which is also traversed by a capillary vessel, b; B B, Ganglionic cells with caudate pro¬ longations. and in the second to be stellate (b). These prolongations have been traced into continuity, in some instances, with the axis-cylinders of nerve- tubes, whilst in other cases they seem to unite with those proceeding from other vesicles. It is not by any means certain, however, that the nerve-tubes thus connect themselves with the nerve-vesicles in all instances ; since it frequently appears as if the former passed in among the latter, without coming into direct continuity with them. Sometimes a ganglion-cell seems to lie in the course of a tubular fibre, which enlarges to envelope it, and then con¬ tracts again to its former dimensions. There can be no reasonable doubt, however, that in some way or other the nerve-fibres and the nerve-vesicles come into some kind of communication in the ganglionic centres. The vesicles are 78 STRUCTURE OP GANGLIA. — NERVOUS ACTION. Fig. 24.— Thin slice of filled with, a finely-granular substance, which extends into their prolongations; and in the warm-blooded Vertebrata they contain pigment-granules, which give them a reddish or yellowish-brown colour; so that the aggregations of vesicular substance which we find in the larger nervous centres, are distinguishable by their greyish hue. This “ grey matter, ”as it is frequently called, is disposed on the surface of the brain; but it occupies the interior of the spinal cord, and holds the same position in the smaller ganglionic centres (fig. 24). It is not only, however, in the central organs that nerve- vesicles are found ; for they present themselves also in certain situa¬ tions at the other extremities of the nerve- I" E Ganglia fibres. Thus we find a large proportion System, showing the of the retina (§ 535), which is commonly fibresge amongstnegIn- described as a mere expansion of the optic giionic ceils. nerve, to be composed of nerve-vesicles that are scarcely distinguishable from those of the brain; and it is probable that the ultimate branches of other sensory nerves have some such termination. "Wherever we meet with vesicular substance, we find it imbedded in a minute net¬ work of blood-vessels; and a copious supply of oxygenated blood is requisite to the due performance of its actions. 62. There can be no doubt that the special office of the Her YQ-fibres is to convey the influence of the changes which are effected in one part of the system, to other and remote parts ; just as the wires of a galvanic battery conduct the electric influence from the instrument which excites it, to some distant point where it is to be applied to some use. The effects of such changes in the state of the Hervous System are propagated in two opposite directions ; — the im¬ pressions made upon the skin and other parts possessed of sensibility, being conveyed towards a portion of the nervous centres called the sensorium, and there giving rise to sensa¬ tions ; — and the influence of the emotions or volitions to which these sensations give rise (§ 7), being propagated from the central organs to the muscles, which they excite to con¬ traction. And by the discoveries of Sir C. Bell, hereafter to ACTIONS OF NERVOUS SYSTEM. 79 be described it has been fully proved that these opposite changes are conducted by two different sets of fibres ; — one conveying to the central organs those which originate in the circumference; — and the other conveying to the circum¬ ference those which originate in the centre (§ 451). The transmission of these changes is completely interrupted by division of the nervous trunk, or by pressure upon it ; and it sometimes happens that one set of conducting fibres is thus affected, whilst the functions of the other are not impaired; so that a limb may retain its sensibility and yet be totally destitute of the power of motion, or may be completely obedient to the will though totally destitute of sensibility. In Yertebrated animals, we find some nerves in which there is only one set of fibres, so that the trunk is only sensory or only motor (§ 459); but in general, the two sets are bound up together in the same sheath. 63. The motor fibres may be considered as originating in the vesicular substance of the central organs, and as termi¬ nating in the muscles ; the power which is generated in the former being conveyed by their means to the apparatus through which it operates to produce mechanical motion. When the nerve-trunks reach the muscles, they divide into branches which penetrate their substance, and these again subdivide and ramify minutely, so that at last the fibres may often be observed running singly, passing amongst the muscular fibres, but not appearing to penetrate their tubular sheaths. These terminal fibres seem often to double back upon themselves, so as to form loops, either re-entering the branch from which they issued, or connecting themselves with some neighbour¬ ing branch ; so that the ultimate distribution of the motor nerves in the muscular substance, is a sort of jplexus or net¬ work. The sensory fibres, on the other hand, may be con¬ sidered as originating in the sensory surfaces, such as the skin, the interior of the nose, the lining membrane of the cavities of the internal ear, the retina of the eye, &c. ; and as passing towards the central organs, conveying to these the impressions they have received, which impressions may either affect the consciousness, or may excite respondent move¬ ments, or may act in both modes, through the instrumentality of the vesicular substance to which they are transmitted. The immediate dependence of the functional activity of this $0 SIMPLIFICATION OF STRUCTUBE IN LOWEST ANIMALS. substance upon tbe supply of blood which it receives, is shown by the fact, that if this supply be temporarily cut off, either by failure of the heart’s action (as in fainting), or by pressure on the blood-vessels which convey it, immediate insensibility, with loss of all power of motion, is the result. And the same is the case with regard to the organs of sense ; for if the circulation through them be interrupted, no sensory impression can be made upon the nerve-fibres which originate in them, as we see when the movement of blood in a limb is suspended by pressure upon its artery. 64. The foregoing constitute the principal tissues among the higher animals, in which the principle of division of labour is most fully carried out, every component part having its own peculiar structure and its own special action. As we de¬ scend in the scale, we find these distinctions less and less obvious, so that when we come down to Zoophytes (§ 121), we meet with but little differentiation either in the textures or in the actions of the several parts of the body ; the whole sub¬ stance of these animals being composed of a tissue, which very closely resembles that which is first formed in higher animals for the reparation of wounds, having the appearance of a solidified blastema (§ 34), with nuclear particles, in various phases of development into cells and fibres, more or less thickly scattered through it ; and this substance being everywhere contractile, and everywhere (at least in many instances) equally capable of participating in the func¬ tions of nutrition and reproduction. And when we pass still lower, to that simplest type of animal life, which is pre¬ sented to us in the Bhizopods (§ 129), we do not meet with even this amount of definite structure, but find the entire sub¬ stance of their bodies composed of an apparently homogeneous jelly, which, like the more organized tissue of the Zoophytes, is everywhere contractile, and which has also the power of performing every operation required for its growth and main¬ tenance as a living being. In such creatures there is not the slightest vestige of a Nervous s}^stem ; and it remains a question whether, in consequence of this deficiency, they are altogether destitute of consciousness, or whether this endowment is dif¬ fused, as it were, through the whole substance of their bodies. 65. Every component part of the fabric must be regarded INDEPENDENT VITALITY OF PARTS OF ORGANISM. Si as having a life of its own, which it maintains by drawing to itself the nutrient material supplied by the circulating cur¬ rent ; but as the continuance of its vital activity is dependent upon the continuance of its nutrition, the life of no tissue can be prolonged for any considerable period after the circu¬ lation has ceased. But after the movement of the blood has come to an end, though the body as a whole is dead , its parts may remain alive for a certain time, and may perform their functions, so long as they are supplied with the necessary materials. Thus, various secretions, the growth of hair, and muscular movements, have been observed to take place in dead bodies. But they cannot continue, because the neces¬ sary Conditions are withheld by the stoppage of the circu¬ lation, — a function which thus binds, as it were, into one whole the scattered elements, and causes the different opera¬ tions to minister one to another. As every component part has an independent life, so has it a limited duration, quite irrespective of that of the organism as a whole. Thus the cells which float separately in the blood, seem to be con¬ tinually undergoing change,- — dying, and giving place to new ones. We have seen that the cells of the epidermis and of some parts of the epithelium are being constantly thrown off and renewed. The duration of the cells of fat and cartilage appears to be much greater; in fact, we have no precise knowledge of their term of life. That of the bony tissue is probably greater still ; yet there is adequate evidence that it is by no means indeterminate. But that of the muscular and nervous tissues seems to depend almost entirely on the use that is made of them. Thus we may justly say, — how¬ ever startling the assertion may seem, — that death and decay are continually going on in every living animal body, and are essential to the activity of its functions. 66. Many animals are reduced to a state of apparent deqth by dryness, by cold, or by exclusion of* the air. A curious example of the first kind is furnished by the Tardigrada (Zoology, § 841) ; some species of which may not only be completely dried up, but may even be exposed in that state to a temperature much exceeding that of boiling water,, without losing the power of recovery when moistened. A similar power of revival after being dried up: is possessed by the common Wheel Animalcule , and probably also by the G 82 SUSPENDED ANIMATION. eggs of many minute Entomostracous Crustacea (Zoology, §§ 883, 931). It is unquestionable that many Fishes, especially those of fresh- water lakes, will revive on being thawed after having been completely frozen; and the same has been ascer¬ tained of certain Caterpillars. The Snail, when retiring for the winter, seals the orifice of its shell with an impervious lid ; and in this cavity it may remain shut up for years, until re-excited to activity by warmth and moisture. Animals in such states of torpidity strongly resemble seeds that are pre¬ vented from germinating, apparently for unlimited periods, by being kept at a moderate temperature, and excluded from the influence of air and moisture, which, with adequate warmth, would call them into active growth, but which, at a lower temperature, would occasion their decomposition. There are no positive facts which enable us to say how long Animals may remain in a parallel condition ; but there seems no reason why it might not be indefinitely prolonged. 67. The death of the body, then, does not consist in the mere suspension of its vital activity; for so long as that activity may be renewed when the requisite conditions are supplied, so Jong must the organism be considered as alive , however death-like its condition may seem. Among warm¬ blooded animals, such a suspension, if complete, cannot be endured for more than a very brief period, without the extinction of life ; for the substance of their tissues is so prone to decomposition, that it speedily passes into decay unless prevented from doing so either by a reduction of tem¬ perature, or by complete drying-up, or by entire seclusion from air; and although each of these methods, practised upon animal substances already dead, may prevent the occur¬ rence of decomposition for almost unlimited periods, yet neither can be applied to the living tissues of any of the higher animals, without occasioning the entire loss of their vitality, as we see (in regard to cold) in the loss of members by “ frost-bite.” Such parts die} because not only is their vital activity suspended, but their vital properties are annihilated. Their death, however, does not necessarily involve that of the organism as a whole ; since the stoppage of their function may not disarrange the general train of vital operations, or their duty can be discharged by other organs. And among many of the lower animals, we find that there is a provision DECAY CONSTANT DURING LIFE. 83 for their replacement by ordinary acts of growth ; and that even when the body has been so severely injured that the organic functions are seriously disturbed for a time (as when a Hydra is divided into two or more pieces, § 122), the vitality of the individual parts is sufficiently enduring, and their reparative powers sufficiently energetic, to enable them to reproduce all that is wanting for the completion of the organism, and for the renewal of its ordinary actions. Among the higher animals, the death of the organism at large may be said to take place when the circulation finally ceases ; since, as we have just seen, every individual part must ere long lose its peculiar functional activity, and the entire body be subject to decay. 68. From what has been stated, it will be seen that Life cannot be regarded as a condition in which decay is resisted; for an incessant decay is taking place in every living organism as a necessary condition of its vital activity, being only checked when that activity is itself suspended. But it is a condition in which, by the wonderful harmony and mutual adaptation of the operations of the different parts, the repa¬ rative action of the Organic Functions is made to countervail the destructive action involved in the exercise of the Animal Faculties ; whilst the latter, in their turn, serve to furnish the conditions requisite for the maintenance of the former. So long as all these actions go on with regularity and com¬ pleteness, so long the whole body lives ; but if any one of the more important among them be interrupted, the stoppage of the whole is the result. This relation of mutual dependence is most intimate in the higher animals ; in which, by the differentia¬ tion of the several tissues and organs, and the specialization of their functions, the division of labour is carried to its greatest extent, so that no part can entirely fulfil the duty of any other. On the other hand, it is among those lowest forms of animal life, in which there is the greatest multipli¬ cation of similar parts, and the greatest diffusion of the same endowments amongst them all, that we find the dependence of the several parts of the organism upon each other to be the slightest, and severe injuries to be tolerated with the least general disturbance. 84 PRINCIPAL TYPES OF ANIMAL STRUCTURE. CHAPTEK II. GENERAL VIEW OF THE ANIMAL KINGDOM. 69. When we examine the Animal Kingdom as a whole, it is easy to distinguish in it four general plans or types of struc¬ ture, by which, with almost infinite variations in detail, the formation of the several beings that compose it has been guided. As specimens of these four plans or types, we may name four animals which are familiar to almost every one, — the Dog, the Lobster , the Snail , and the Star-fish . The dif¬ ferences by which these types are distinguished, are mani¬ fested in the arrangement of the different organs of the body ; and particularly in the form of the nervous system and its instruments. It has been already stated (§ 4 ) that the power of feeling , and of spontaneous motion , is that which peculiarly distinguishes the Annual from the Plant ; and as these powers are possessed in very different degrees, and exercised in very different modes, by the various tribes of animals, — whilst the operations' of - nutrition are performed, as in plants, in a much more uniform manner, — they afford us a satisfactory means of separating these tribes from one another. Eor the nervous system is the organ to which these powers are due ; and we find it presenting forms so different in the four great divisions already alluded to, that we can at once distinguish them by this alone, even where (as sometimes happens) there may be such a blending, in a particular animal, of the general characters of two of them, as to lead us to hesitate in assigning its precise place in the animal kingdom. 7 0. The highest of these four divisions is that denominated Vertebrata, or Vertebrated Animals; it receives its name from the structure characteristic of it, — the possession of a jointed back-bone or vertebral column, — which will be pre¬ sently described. This is the group to which Man belongs ; and all the animals it contains bear a greater or less resem blance to him in structure. We notice in regard to their external form, that they are alike on the two sides of their body; every part having its fellow on the other side. This VERTEBRATED TYPE OF STRUCTURE. 85 u bi-lateral symmetry ” extends to the arrangement of those internal parts which are connected with the functions of animal life ; namely, the nervous system, the organs of sense, and the muscular apparatus. But it does not always extend to the organs of nutrition, which are unequally disposed on the two sides : thus, in Man, the heart and stomach are on the left side, and the liver on the right, while the lungs are much larger on the right side than on the left. But in many of the lower Yertebrata, there is an almost perfect symmetry in the disposition of these organs, as there is also in the early embryo of those in which this symmetry is subsequently departed from ; so that it may be truly said that this symmetry is cha¬ racteristic of the Vertebrate type, although for special purposes it is frequently superseded. Fig. 25. — Skeleton of the Ostrich. 71. In all Yertebrated animals, the skeleton is chiefly internal (fig. 25); and consists of bones, which are capable of 86 VERTEBRAL COLUMN. growing, and of being reproduced after injury, like any other part of the living tissue ; being copiously supplied with blood¬ vessels, which penetrate into their interior. These bones give support, and afford points of attachment, to the soft parts, in the limbs (where they exist) as well as in the trunk ; but the former are not unfrequently wanting, as in Serpents : and we must look in the trunk, therefore, for that peculiar arrange¬ ment which is characteristic of this division of the Animal Kingdom. The back-bone, as it is commonly termed, is found in all Vertebrated animals ; though in a few among them (the lowest Fishes) it is very imperfect (§ 53). It consists of several pieces jointed together, so as to possess great flexibility; whilst they are so firmly connected by ligaments, that they cannot easily be torn asunder or displaced. The number of these pieces varies considerably ; in Man there are only 33 ; in some long-tailed Mammals there are more than 7 0 ; but in many Serpents there are several hundred. Each of them is termed a vertebra; and the whole structure, composed of the Fig. 26.— VERTE-united vertebrae, is termed the vertebral column bral qlumn. 26). The ordinary character of the vertebrae is, that each is perforated by an aperture, which, united to the corresponding apertures of those above and below it, forms a continuous canal ; and in this canal, one of the most im¬ portant parts of the nervous system, the spinal cord (commonly but erroneously termed the spinal marrow), is contained. The solid portion of the vertebra (fig. 27, a) is termed its body; and the projections, b and c, are termed its processes , the former spinous , the latter transverse. The row a of spinous processes forms the ridge which we Fi VertebraGLE Passing down the back; it is seen on the right-hand side of fig. 26. To the transverse processes the ribs are attached. The vertebral column is ex¬ panded (as it were) at its upper extremity, to form the skull ; in the large cavity which it contains, the brain is lodged ; and its bones are so arranged as to give protection to the organs of sense also. At the opposite extremity we see it contracted into the tail; which is composed of a series of vertebrae NERVOUS SYSTEM OF VERTEBRATA. 87 resembling those of the back, but simpler in their form, and not possessing a cavity for the spinal cord. We commonly find that in those animals in which the sknll is very large, the tail is short ; and that where the tail is very long or powerful, the head is small. Thus in man and in the apes, the head is large, and there is no external appearance of a tail ; but there are some very imperfect vertebrae at the lower end of the spinal column, which constitute the rudiment of it. In the long-tailed monkeys and in the kan¬ garoo (whose tail is like a third hind¬ leg), the head is comparatively small. But this rule does not hold good uni¬ versally. 72. The Nervous system of Verte- brated animals consists of a Brain and Spinal Cord (fig. 28), which are lodged within the skull and vertebral column ; and of nervous trunks proceeding from these, which are distributed to all parts of the body. The Brain is not (as commonly reputed) a single organ, but is composed of a number of ganglionic masses, differing considerably in their functions. Thus each of the nerves of special sense (smell, sight, hearing, and taste) has its own proper centre ; and there is another of considerable size, which seems to perform the same office in regard to common sensation. These are found in Yertebrata generally ; and their proportionate size corresponds with the relative development and ac¬ tivity of the several organs of sense with which they are connected. The bulk of the brain of Man, however, is Fis'28^d A0T/ manSpINA1 made up by two large masses of nervous matter, which are known as the Cerebral Hemispheres ; these, as will be shown hereafter (chap, x.), are so small in the brains of Fishes as to be scarcely distinguishable ; and their relative size 88 NERVOUS SYSTEM OF VERTEBRATA. and complexity of structure increase as we ascend the scale, in pretty close accordance with the increase of the intelligence or reasoning faculty. There is also another large nervous mass, called the Cerebellum ; the function of which seems to consist in the regulation of the more complex movements. The Spinal Cord is made up of a longitudinal succession of independent centres, of which one corresponds with each of the vertebral segments of the body. 73. The distinguishing feature of the Nervous system in Yertebrata is, that its several centres are thus united into one large mass, instead of forming a number of separate small masses or ganglia , as we shall find that they do in the lower classes of animals : and that it is inclosed in the bony casing which has been described as peculiarly destined for its pro¬ tection, instead of being enveloped with all the other organs in a hard covering, as in the Lobster, or of being entirely destitute of protection, as in the Slug. That it should receive this peculiar protection is quite necessary, in consequence of the much higher development which it attains, and the much greater importance which it possesses, in this division of the animal kingdom, than in any other. In all but the very lowest Yertebrata, all five kinds of sensation exist ; — namely, sight, hearing, smell, taste, and touch. We find in this group more intelligence than in any other ; that is to say, the animals composing it act more with a designed adaptation of means to ends ; instead of being impelled by a blind instinct to perform actions of whose objects they are not aware. And we find, by observing and comparing the structure and actions of the dif¬ ferent groups, that the intelligence gains upon the instinct, as we ascend from the lowest Fishes towards Man, in whom the intelligence is at its highest; whilst we observe a similar increase in the proportion which the brain bears to the rest of the nervous system. Hence we conclude, that the brain is the organ of intelligence , or of the reasoning faculties. 74. The general arrangement of the other organs in Yerte- brated animals, is shown in fig. 29. At m is seen the mouth, forming the entrance to the digestive cavity, of which the termination is at the opposite extremity of the body ; i, i, is the intestinal canal, and l , the liver : these organs occupy the part of the body which is called the abdomen or belly. The mouth also opens, however, into the windpipe, or trachea, t , GENERAL STRUCTURE OF VERTEBRATA. 89 which conducts air into the lungs, p ; these organs, with the heart, h, are contained in the portion of the trunk called the b si Fig. 29.— Diagram, showing the position of the principal Organs in Vertebrata. thorax , or chest. At b is seen the position of the brain ; and at s that of the spinal cord. 75. The foregoing characters apply, with greater or less modification as to details, to the classes of Mammals (com¬ monly termed Quadrupeds), Birds, Reptiles, and Fishes ; and these further agree in the following points, all of which, therefore, enter into our idea of a Yertebrated animal. The number of limbs or members never exceeds four ; and of these, two, or even all four, may be absent. In all the classes just named, four is the general number; and the absence of two or more is the exception. Thus in Mammals, we find all four present in every tribe save that of Whales, which want the hinder pair ; though the upper or anterior pair may take the form of arms, wings, legs, or fins, accord¬ ing to the element which the animal is formed to inhabit. In Birds we find the posterior pair invariably present in the form of legs ; whilst the anterior pair, though almost always developed into wings, is absent in a few instances. In Reptiles we find considerable variety ; all four members are present in the Turtle tribe, and in most Lizards, as well as in the Frog tribe ; but they are entirely absent in the whole tribe of Ser¬ pents ; and there are Lizards which have only one pair. And in Fishes, we usually find two pairs, constituting the pectoral and ventral fins ; but one or both pairs are sometimes absent, as in the Eel, Lamprey, &c. We have further to remark, in regard to the general characters of Yertebrated animals, that, 90 CLASSES OF VERTEBRATA : — MAMMALS. with one exception, they have all red blood (§ 226) ; and that they possess a complex apparatus for circulating this through the body. 76. The four principal modifications under which the Yer- tebrated type presents itself, constituting the classes of Mam¬ mals, Birds, Reptiles, and Fishes, are respectively character¬ ised by the mode in which the principal functions of life are performed in each.* Thus there are some Yertebrated animals which produce their young alive, and which nourish them afterwards by suckling ; while the greater part rear them from eggs which contain a store of nutritive matter, and do not afford them any further nourishment from their own bodies. Again, some breathe air; whilst others live con¬ stantly in water, and have no direct communication with the atmosphere. Some, moreover, have the power of keeping up a high temperature, so that their bodies always feel warm to the touch ; whilst the temperature of others varies with that of the atmosphere, so that their bodies give a feeling of coldness : the former are termed warm-blooded — the latter cold-blooded. There is a like difference in their mode of life ; some of them being destined to live on the surface of the earth, whilst others are chiefly inhabitants of the air, and others again are the tenants of the ocean. 77. Mammals are distinguished from all other Yertebrata by the first of the characters just adverted to ; being the only animals that produce their young alive, and nourish them afterwards by suckling. Like Birds and Reptiles, they breathe air by means of lungs ; and, in common with Birds, they are warm-blooded and have a complete double circula¬ tion of their blood, carried on by a heart with four cavities. They are for the most part quadruped (that is, four-footed), and are destined to live upon the surface of the earth ; but Man, and the Apes that approach nearest to him, are biped , having the power of walking on two limbs, and of using the others for different purposes ; whilst the Bat tribe have the two arms converted into wings, which enable them to fly through the air like birds (for which the older naturalists mistook * Many Zoologists range the Frogs and their allies in a separate class, under the name of Amphibia; but when looked at from a physiological point of view, the author does not see that they require to be separated from the true Reptiles. GENERAL STRUCTURE OF MAMMALS. 91 them); and the Whale tribe are adapted in their general form to lead the life of fishes (among which they are still commonly ranked by persons ignorant of natural history). 1ST otwithstanding these marked differences in external form, there is a great correspondence as to internal structure ; for bats and whales, as well as ordinary quadrupeds, produce their young alive, and suckle them afterwards ; they are also warm-blooded, breathing air, and having an active circulation. The bodies of Mammals are, for the most part, more or less completely covered with hair, which serves to keep in their warmth ; and this is seldom absent, except in such as inhabit warm climates and do not require this provision. In the Whales, the same end is answered by the thick layer of oil in the substance of the skin, constituting the blubber ; and Man is left to form a protective covering for his body by the exer¬ cise of his own ingenuity. The general arrangement of the Sub-maxillary Gland Parotid Gland Windpipe « Pharynx (Esophagus Colon Caecum. Small Intestines Fig. SO. — Interior, of a Monkey. internal organs of Mammals will be seen from the accom¬ panying figure of the body of a Monkey, laid open in such. 92 GENERAL STRUCTURE OF BIRDS. a manner as to exhibit the chief of them. The cavity of the trunk is completely divided, by the muscular partition termed the diaphragm , into two portions — the thorax , con¬ taining the heart and lungs ; and the abdomen , containing the digestive apparatus. It is chiefly by the alternate con¬ traction and relaxation of this muscle, that the act of breathing is performed in Mammals, as will be explained hereafter (§ 331). 78. In Birds there is a much closer conformity to one general plan than we find among Mammals. The covering of feathers, by which we ordinarily distinguish the members of this class, prevails universally ; and there is no wide depar¬ ture from the typical form. This class belongs to the oviparous division of the Yertebrata ; the young being reared from eggs. But it is distinguished from Beptiles, which are also oviparous and air-breathing, by being warm-blooded; and by having a very energetic instead of a very slow circu¬ lation. The warmth of the maternal body, moreover, is im¬ parted to the egg in the act of incubation ; and without the heat thus communicated (unless it be supplied from some other source) the embryo cannot be developed. The covering of feathers is given, not only to keep in the heat of the body, which is even greater than that of Mammals, but also to afford the required surface for the wings, on which the Bird is supported and propelled through the air. The feathered portion of the wings is stretched out upon the bones which answer to those of our arm, and is moved by its muscles. The wings are very small, or are entirely absent, in the Ostrich and a few other birds, which present the nearest approach to the Mammalia in their internal structure ; and these cannot rise from the ground, but run swiftly along it, by means of their powerful legs. In the Penguin, also, the wings are small ; and they are used as fins, by the assistance of which this bird, which can neither walk nor fly with rapidity, can swim very quickly through the water. 79. Generally speaking, Birds are characterized by the extraordinary power of motion which they possess, and by the great acuteness of the sense of sight, by which their movements are chiefly directed. They are also remarkable for their instinctive actions, which are chiefly related to their care of their young, for whom they usually construct a protective GENERAL STRUCTURE OF BIRDS. 93 nest. The nutritive functions are performed with extra¬ ordinary activity in Birds, that the means may be supplied for the maintenance of their locomotive activity. Their blood is particularly rich in red particles, and its heat is usually considerably above that of Mammals. Its circulation is very energetically carried on ; and although the lungs themselves are constructed upon a type inferior to that of Mammals, and the mechanism of respiration is less complete, yet, by an extension of the respiratory organs through the whole fabric, the aeration of the blood is carried on with unequalled energy (§ 326). 80. The arrangement of the organs contained in the cavity of the trunk of Birds differs from that which has been described in Mammals, chiefly in this, — that there is usually no diaphragm to separate the chest from the abdomen, and that although the lungs them¬ selves are confined to the upper part of this cavity, they are con¬ nected with a series of air-sacs which are distributed through the whole of it. In the accom¬ panying figure, which repre¬ sents the internal organs of the Ostrich, the heart is seen at a, the stomach at b, and the in¬ testinal tube at c. The windpipe, d, opens into the lungs, e, which are themselves small, and are attached to the ribs, instead of lying freely in the cavity of the Fig. si.— lungs of the Ostrich. chest ; but the space they would otherwise have occupied is filled up by the large air-cells, /,/, which communicate freely with the lungs and with each other, and which even occupy a large part of the cavity of the abdomen, as seen in the figure. 81. In the class of Beptiles we find a variety of form so remarkable, that, if we were influenced by this alone, we should scarcely regard the animals it contains as belonging to the heart; b, the stomach ; c c, the intestines ; d, the trachea ; e, the lungs; ///, air-cells, in which are also seen the tubes by which these air- cells communicate with the lungs. 94 GENERAL STRUCTURE OP REPTILES. the same group ; yet the structure of the internal organs, on which classification is founded, is essentially alike in all; and their physiological condition presents no important dis¬ similarity. Tour obviously different tribes, Turtles. , Lizards. , Serpents , and Frogs , are brought together by the following characters. They are all oviparous , in this respect agreeing with Birds and Fishes ; but they are cold-blooded, and have not a complete apparatus for the double circulation of the blood, in which respect they differ from Eirds; and they breathe air by means of lungs, instead of breathing water by gills, in which respect they differ from Fishes. But by the lowest group, that of Frogs and their allies, this class is united to that of Fishes in a most remarkable manner ; for these animals in their young state breathe by gills, and lead the life of a fish ; and some of them retain their gills during the whole of life, even after the lungs are developed (§ 87). The first three of the tribes just mentioned un¬ dergo no such change : and they further agree in this, that they breathe air during the whole of their lives, coming forth from the egg in the same condition as that in which they are subsequently to live, and also in having their bodies covered with horny scales or plates, whilst the skin of the Frog tribe is soft and unprotected. 82. The class of Reptiles presents a marked contrast to that of Eirds, in the comparative slowness and feebleness of its movements, the dulness of its sensibility, and the in¬ activity of its organic functions. As there is no fixed tempe¬ rature to be maintained, one important source of demand for food is withdrawn; and when not excited to activity by external warmth, these animals may pass long periods without fresh supplies of food. Their blood is very poor in red corpuscles, and its circulation is comparatively languid. A reduction of the temperature of their bodies to within a few degrees of freezing point, induces complete torpidity, which continues until they are roused by a renewal of warmth. 83. The Turtle tribe is peculiarly distinguished by the inclosure of the body in a bony covering; of which the upper arched portion (termed the carapace) is formed by the coalescence of the ribs with a set of bony plates deve¬ loped in the substance of the skin; whilst the lower flat plate (termed the plastron ), which is often incomplete, is STRUCTURE OF TURTLES AND LIZARDS. 95 formed by an expansion of the sternum or breast-bone, which is spread out sideways, instead of being raised into a project¬ ing keel as in Birds. The carapace and plastron are covered with large horny plates, variously arranged in the dif¬ ferent species, and constituting what is commonly called tortoise-shell. These plates are often very beautifully disposed, forming a kind of tesselated pavement ; as in the common Tortoise (fig. 32), which is often preserved alive in our gardens. 84. In the tribe of Lizards, , the body has no such covering ; but these animals, having more activity than the tortoises (which are proverbially slow), are enabled Flg* 32-“Tortoise- to make their escape from danger, whilst the latter are obliged to trust to their bony casing for protection from it. In their general form, Lizards approach Mammals, being four-footed, and living for the most part on land ; but they differ from them not only in their essential reptilian characters, but also in several others of less consequence. Their bodies are usually covered with scales, which lap over one another like the tiles of a roof ; but in the Crocodile tribe, many parts of Fig. 33. — Crocodile. the surface are covered with large knotted horny plates, that meet at their edges like the scales of tortoise-shell, and afford an almost impenetrable covering. Although some of the Lizard tribe spend a large part of their time in water, they all breathe air ; but, as their respiration is very inactive, they can remain for long periods beneath the surface, without being obliged to come up to breathe. 85. The tribe of Serpents may be regarded as lizards with¬ out feet ; their spinal column is immensely prolonged ; and 96 STRUCTURE OF SERPENTS. their ribs are also very numerous ; and they are able to crawl upon the points of these, using them almost as Centipedes do their legs (fig. 42). But in general the movement of their Fig. 34.— Anatomy oe a Coluber bodies is accomplished by their being drawn-up into folds, and then straightened so as to project the head. The pro¬ longed form of the body in Serpents occasions a curious variation in the arrangement of the principal organs, which is shown in the accompanying figure. The oesophagus or STRUCTURE OF SERPENTS AND BATRACHIA. 97 gullet, ce, which leads from the mouth to the stomach, is a long and very wide canal, being even larger than the stomach at its commencement ; a portion of it is removed at ce', in order to show the heart, &c., which would otherwise be con¬ cealed by it. The stomach, i, is long and narrow ; and the intestinal tube, i', after making a few turns or convolutions, passes backwards in a straight line, to terminate in the cloaca, cl, which opens externally by the orifice, an. The liver, /, is also much lengthened. From the mouth also proceeds the long windpipe, t t, which conveys air to the lungs, or rather to the single lung; for the lung on the left side, p', is scarcely at all developed, whilst that on the right, p, extends along a great part of the body. At o is seen the ovarium, in which the eggs, o' o', are produced ; and this also is very much lengthened, extending from the cloaca a good way up the body, so as nearly to meet the lung. The other references are to the parts of the heart, and the principal vessels ; the structure and arrangement of which will be explained here¬ after (§ 284). 86. The Batrachia, or animals of the Frog tribe, are readily distinguished from all the preceding, by their soft naked skins ; even when the form of the body, as in the com¬ mon Salamander or Water-Newt, resembles that of the lizards. They are also remarkable for the metamorphosis which they undergo in the early part of their lives : for they come forth from the egg in a condition which is, in all essential particu¬ lars, that of a fish, and undergo a gradual series of changes, by which their form and structure become assimilated to those of the true reptiles. This change is most complete in the Frogs and Toads ; the early form of which is known as the tadpole . The principal stages of this change are represented in figs. 35 to 39 ; in which, however, the relative sizes are not preserved, the tadpoles being much larger in proportion (for the sake of displaying their form and the gradual development of their legs) than the complete frog. Soon after the young tadpole has come forth from the egg, it pre¬ sents the form which is shown in fig. 35 ; its head and trunk are large, and the latter is prolonged into a flattened tail, by which the little animal swims freely through the water. There is not the least appearance of limbs or mem¬ bers. It breathes by gills, which are long fringes, hanging H 98 METAMORPHOSIS OF BATRACHIA. loosely in the water on either side of the head. At a later period, however, these gills, which are merely temporary, disappear ; and the breathing is carried on by another set, which are situated behind the head, and are covered in by a fold of skin ; the water gains access to these by passing through the mouth, exactly as in Fishes. The form is then that which is represented in fig. 36. In a short time after¬ wards, the animal still breathing by its gills, the hind-legs begin to sprout forth, as it were, at the base of the tail ; this stage is shown in fig. 37. At a still later period, the fore¬ legs begin to be developed, as seen in fig. 38 ; and from that time they are nourished at the expense of the tail, which gradually disappears, as seen in fig. 39, a, b. During this period, other important changes are taking place in the inte¬ rior of the body ; the chief of which are the development of the lungs and the gradual disuse of the gills, so that the animal becomes fitted to live on land and breathe air, and is no longer capable of remaining long under water without coming to the surface to respire. 87. The metamorphosis in other members of the group is PERENNIBRANCHIATE BATRACHIA. 99 less complete than in the Frog, being checked at a less advanced stage. Thus in the common Water-Newt, the tail is retained during the whole of life, and the animal continues to be an inhabitant of the water, though breathing air alone. There are some very curious animals, however, in which the change is stopped, as it were, at a much earlier period, so that the gills also are retained ; and in these, the lungs are suffi¬ ciently developed to enable the animals to breathe air, so that they can live either on land or in water. Such Batrachia are scientifically known as perennibrandiiate , this term express¬ ing the persistency of their gills. In fig. 40 is represented Fig. 40. — Axolotl. an animal of this kind, the Axolotl, which inhabits some of the lakes of Mexico. And in fig. 41 is shown the form of a still more remarkable animal, the Lepidosiren , or .mud-fish, recently Fig. 41. — Lepidosiren. brought from th.3 rivers of Africa, the metamorphosis of which appears to be checked at a still earlier period, so that it is very difficult to decide whether it should be regarded as h 2 100 STRUCTURE OF FISHES. a Fish, or as a Keptile, so complete is the mixture of charac¬ ters which it presents. 88. The class of Fishes is distinguished from all other Vertebrata, by the adaptation of the animals composing it to breathe by means of water in their adult state, so as to be capable of hying in that element only. Like Keptiles, they are oviparous and cold-blooded ; and in these characters they differ completely from the Whales and other Mammals, which are, like them, inhabitants of the great deep, but which are warm-blooded, viviparous, and air-breathing animals. There is a simple external character, by which the members of the two classes may be at once distinguished. The animals of the Whale tribe are, like fishes, chiefly propelled through the water by means of a flattened tail ; but in the former the tail is flattened horizontally, so that its downward stroke may serve to bring the animal to the surface to breathe ; whilst in Fishes it is flattened vertically, that its strokes from side to side may simply propel the fish through the water. A flattening or compression of the body is seen more or less in almost all fishes, and is intimately connected with the nature of their motion through the element they inhabit ; as it serves the double purpose of diminishing the resistance which is offered to their progress, and of increasing the extent of the oar-like surface, by the lateral stroke of which the body is propelled forwards (Chap. xn.). This stroke is given by- a series of muscles of great power, which pass from the prolonged extensions of one vertebra to those of another, and altogether make up the principal part of the bulk of the animal. The fins which represent the limbs are not so much used in propelling the Fish, as in changing its direction either laterally or vertically. Thus in the lowest group of the Yertebrated series, the act of motion is chiefly performed by the vertebral column itself, instead of being committed to the limbs, as in Mammals, Birds, and most Keptiles. The larger number of Fishes swim with great activity ; and their lives may be said to be passed in seeking their subsistence and in flying from their enemies. 89. Fishes are for the most part very voracious, and their food consists in great part of the members of their own class. In seeking it, they appear to be chiefly guided by the sight ; for their eyes are usually large and highly developed, while STRUCTURE OF FISHES. 101 the other organs of sense are formed upon a very inferior type. They swallow it without much division in the mouth; but it seems to undergo rapid digestion. The blood of some Fish, whose muscular activity is peculiarly great, is rich in red corpuscles, and of a temperature not much lower than that of Mammals ; but, generally speaking, it contains much less solid matter than that of the warm-blooded Yertebrata, and its temperature follows that of the surrounding medium. 90. Although Fishes breathe by gills instead of by lungs, these gills are connected with the mouth, so that the water which passes over them is received into it, in the same man¬ ner as the air is in the higher Yertebrata. This is a character which distinguishes the position of the gills of fishes from that of the corresponding organs of any of the inferior tribes. They are lodged in a cavity on each side of the throat ; and this cavity opens outwardly, either by one large valve-like aperture on either side, or by several; through these apertures the streams of water which have been taken in by the mouth, and forced over the gills by the action of its muscles, make their exit. 9 1 . All Fishes are oviparous ; and the number of eggs which they produce is generally prodigious. It is very seldom that after the eggs have been deposited and fertilized, the parents take any further concern in regard to them ; though there are a few instances in which a kind of nest is made, and others in which the egg is retained and hatched within the body, so that the young comes forth alive. This last is the case with the Sharks and Kays, which, notwithstanding that their skeleton is cartilaginous, are higher than Fishes generally in several other parts of their organization. 92. All the animals which are destitute of a vertebral column are called Invertehrata ; and this division into the Yertebrated and Invertebrated groups was formerly regarded as the first step in the classification of the animal kingdom. But it was pointed out by Cuvier, that in the Invertebrated division are comprehended three groups, of which the mem¬ bers differ as much from one another as they do from Yerte¬ brated animals; and that each of these ought, therefore, to rank with the first, as a primary division. This is evident to those who are but slightly acquainted with the structure of the animals already named (§ 69) as characteristic speci- 102 GENERAL STRUCTURE OF ARTICULATA. mens of these divisions ; and it will become more apparent as we proceed. 93. In the second division, that of Articulata, or Articu¬ lated (jointed) animals, we find a conformation very different from that which has been just described. The exterior of the body is still perfectly symme¬ trical, as in the Yertebrata ; and the interior is even more symmetrical ; for the organs that represent the heart and lungs are equally dis¬ posed on the two sides of the central line of the body. But the skeleton, instead of being internal, is external; and is composed of a series of pieces jointed together, which form a casing that in¬ cludes the whole body. In general, these pieces are very similar to each other ; so that the whole body appears like the repetition of a number of similar parts, as we see in the Centipede (fig. 42). The limbs are usually very numerous, where they exist at all ; and they have a jointed cover¬ ing, like that of the body. But in the lower tribes of this group, such as Leeches and Worms , the limbs or members are but slightly developed, or are altogether absent ; and in the highest, which approach most nearly to the Yertebrata in their general organization, the number of members is much reduced, — although it is never less than six. The hard matter of which the external skeleton is composed, undergoes little or no change when it is once fully formed ; and, in order to accommodate it to the increasing size of the animal, this covering is thrown off and renewed at intervals during the period of growth. 94. The nervous system consists of a series of separate ganglia , which are arranged in a cord or chain along the central line of the body. There is usually a pair of large ganglia in the head, bearing a resemblance (in their peculiar connexion with the eyes) to the ganglionic centres of the optic nerves in Yertebrata; and there is commonly one for each segment or division of the body, from which the nerves pass to supply its muscles, as they do from the spinal cord of Yertebrata. The cord which connects these ganglia is double, Fig. 42. Centipede. GENERAL STRUCTURE OF ARTICULATA. 103 and the ganglia themselves are composed of two halves, which have little connexion with each other. The chain thus formed (fig. 43) passes along the under-side of the trunk of the animal (as seen at g , fig. 44), not on what seems its hack ; and by the presence of this double chain of ganglia an Articulated animal may be distin¬ guished, even when, in its general structure, it should seem to belong to the group of Mollusca (§ 102). 95. The general arrangement of the organs in the Articulata is shown in the accompanying figure of a Cray¬ fish. The mouth, situated on a pro¬ jecting head, opens into s, the stomach, from which passes backwards the in- XT ... -i.-i .., , • , lin Fig. 43. — Nervous System of testmal tube, i, ^, to terminate at the an Insect. opposite extremity of the body. The upper part of the tube is surrounded by the liver, /, which is here very large. In the head are seen the ganglia, c; and along the under- side of the body is seen the chain of ganglia, s f h i \ \ \ \ c 9 Fig. 44.— Diagram showing the position of the principal Organs in the Articulata. g. The blood is nearly colourless, and is usually impelled through the body not by a single organ or heart, but by a succession of contractile cavities, one for each segment, which open into one other longitudinally, forming what is known as the dorsal vessel ; in the Cray-fish and its allies, however, one part of this, h , is specially enlarged, so as in great degree to serve as a heart for the system generally. The respiratory organs are not connected with the mouth ; and are not usually 104 STRUCTURE OF INSECTS. restricted to one part of the body, but are diffused either on its outside or through its substance. 96. The organs of sense, in this group, are less numerous than in Yertebrata, and are inferior in perfection; those of sight are the most developed, and are formed upon a very peculiar plan (§ 573); but all organs of special sense appear wanting in the lowest tribes. Yet we find that the muscular power is very great ; for the animals of this group, taken as a whole, can move faster in proportion to their size, and possess greater strength, than those of any other. We observe, too, that with little or no intelligence, they are prompted to the most remarkable actions by instinct alone. They seem to act like machines, doing as they are prompted, without choice, or knowledge of the end to be gained ; and consequently the dif¬ ferent individuals of the same species have not that difference of capacity and of disposition, which we see in animals whose endowments are higher. 97. In the highest division of the Articulated series, we easily recognise, as forms quite distinct from each other, the Insects , the Spiders , the Crustaceous animals (crabs, lobsters, &c.), and the Centipedes. The class of Insects is distinguished, for the most part, by the presence of wings ; but to this there are exceptions. It includes those of the higher Articulata, which breathe air by means of air-tubes distributed through the body (§ 320), which have no more than six legs, and whose body, in its perfect form at least, manifests a division into three distinct parts — the head , thorax , and abdomen (fig. 45). To the thorax alone are attached the six legs, as well as the wings ; and its cavity is principally occupied by the muscles that move them : the abdomen contains the organs of digestion and reproduction, as in vertebrated animals. In the greater part of this class, the young animal comes forth from the egg in a condition very different from that which it is ultimately to possess; and it undergoes a complete meta¬ morphosis , the larva which the egg produces bearing a close resemblance in form to the lower Articulata, and only attain¬ ing the condition of the imago or perfect insect by passing again into a state of inactivity, during which the store of nutriment wdiich it has acquired is applied to the development of new organs. This pupa or chrysalis condition may be considered as a sort of postponed completion of the embryonic STRUCTURE OP INSECTS AND ARACHNID A. 10 5 life, which, was interrupted at a very early period. In some tribes, however, the general form is the same from the first, and the wings are the only parts deficient ; these gradually Antennas Eyes 1st pair of Legs 1st pair of Wings 2nd pair of Legs 2nd pair of Wings 3rd pair of Legs Tibia Tarsus Fig. 45.— Skeleton of an Insect. make their appearance, and the insect is then complete. Such is the case with the Grasshopper and Cricket; and a change of this kind is termed an incomplete metamorphosis. 98. The animals of the class Arachnid a, which includes the spiders , scorpions , and mites , are, like Insects, articulated, breathing air, and possessing legs, but the number of these legs is never less than eight ; there is an entire absence of wings, and the head is united with the thorax, so that the body seems to be formed of two principal divisions, — the cephalo-thorax (as it is termed), and the abdomen. In fig. 46 we have a representation of the arrangement of the parts con- 106 STRUCTURE OF ARACHNID A AND CRUSTACEA. tained in these cavities. At c t is seen the cephalo-thorax opened from below, and giving attachment to the legs ; at m is shown the place of the mandibles or jaws ; at p is seen one ct pa ab pa s p pa t a l s ma o f Fig. 46.— Anatomy of Spider. of the palpi, which are appendages to the mouth ; pa is the foremost leg ; t , the large nervous mass, from which the legs are supplied; a , the collection of ganglia supplying the abdomen ; a b, the abdomen ; p a , the respiratory chambers ; s s , the stigmata or openings into these ; l, the leaf-like folds within them (§ 323); m $, the muscles of the abdomen ; a n, the termination of the intestine ; /, the spinnerets ; o, the ovaries ; and o r, the opening of the oviduct. 99. The class of Crustacea, of which the Crab , Lobster , and Cray-fish are the best-known forms, differs from both the preceding, in being adapted to breathe by means of gills, and thus to reside in or near water, instead of inhabiting the air. Moreover, the body is inclosed in a hard covering, which generally contains a good deal of carbonate of lime, and which is thrown off at regular intervals. This covering also incloses the members, which are never less than ten in number, and are frequently more numerous. There is great variety of form among the animals of this group, which is altogether one of great interest. — In the Crab tribe, the head, thorax, and abdomen are all drawn together, as it were, into one mass ; and the general arrangement of the organs it con¬ tains is exhibited in the succeeding figure, which shows them nearly as they are found to lie, when the upper part of the shell, or carapace , is removed. At t there is left a portion of STRUCTURE OF CRUSTACEA. ior the membrane which lines the carapace and covers in the viscera. On the central line, at c, is seen the heart, which in the Crustacea is large and powerful in its action ; from it there passes forwards the artery a o, which supplies the eyes fo e m ao t Fig. 47.— Anatomy of a Crab. and the front of the body ; whilst the artery a a passes to the lower and hinder parts ; at b are seen the gills of the left side in their natural position ; whilst at V are seen those of the right side, turned back to show their under-surface, and to disclose the lower portion of the shell, fl. Ate is seen the stomach, situated close behind the mouth; and at m are pointed out its powerful muscles, by the action of which the food is ground down. The bulky ovary is seen on either side of the stomach ; and the space between this and the edge of the shell is occupied by the very large liver, / o. 100. In most of the Crustacea, however, the body is more prolonged. In some, as the Lobster , there is an indication of a division of the body into three parts, representing the head, thorax, and abdomen of insects ; whilst in others, as the Sand- 108 STRUCTURE AND METAMORPHOSIS OF CRUSTACEA. hopper , the rings or segments are almost as similar to each other as they are in the centipede tribe. There is no class in which we find the same parts exhibiting so great a variety of forms, and rendered subservient to so many uses. Thus in the Crab and Lobster the members of the first pair are not used for walking, but form the claws or arms by which the food is seized ; in the Cray-fish, these members may be used either as legs or claws ; whilst in the Sand-hopper, they closely resemble the other legs. And the jaws of the higher Crustacea, of which there are several pairs, are really meta¬ morphosed legs ; as may be seen by comparing them with the corresponding appendages of the Limulus or king-crab, the first joints of which act as jaws, whilst the remaining portions of these members serve either as legs for locomotion, or as claws for prehension. 101. Most of the Crustacea, like insects, come forth from the egg in a state very different from their adult form ; and afterwards undergo a scries of changes, which are in some instances so remarkable as to approach the complete metamorphosis of insects, and which end in the production of the complete form. An early form of the common crab, at a time when it is of the minute size indicated on the scroll, is shown in fig. 48. The immature Crustacea of different tribes bear much more resemblance to each other, than do the forms into which they are ulti¬ mately to be developed ; and the dif¬ ferences they afterwards present are chiefly due to a variety in the amount of growth which the different parts undergo. 102. It is one of the most remarkable results of modern zoological research, that in immediate connexion with the class of Crustacea, if not as actual members of it, we have to place a group of animals which were for some time asso¬ ciated with the Mollusca ; their bodies being inclosed ,in shells, which do not fit closely around them, nor give more than a general protection to their members. This group is STRUCTURE OP CIRRHIPEDA. 109 the Barnacle tribe, forming the class Cirrhipeda, or tendril¬ footed animals. They agree with the lower Mollusca, in being fixed to one spot during all but the earliest period of their lives ; the shell being sometimes attached by a long membranous or leathery tube, as that of the Barnacle (fig. 49) ; and sometimes being itself fixed on the surface of Fig. 49. — Shell of Fig. 50. -Body of the Barnacle. the Barnacle. a rock, or on another shell, as is that of the Balanus or acorn-shell. In both cases, the form and structure of the animal are essentially the same. When taken from the shell (in which it lies doubled up, as it were) and spread out, its articulated nature is evidenced by its division into segments, and by the regularity of the arrangement of their tendril-like appendages. These are not formed like legs, since they could be made no use of, the animal being incapable of moving from place to place ; but they serve to produce currents in the surrounding water, by which food is brought to the mouth, and the blood is submitted to the influence of a fresh supply of air. The nervous system of this group is formed precisely upon the plan of that of the Articulata generally (§ 94) : and if any doubt could have remained as to its true place in the series, it is removed by the knowledge of the fact, that the animals composing it bear a strong resemblance in their early condition to some of the lower Crustacea, possessing eyes and legs, and swimming freely about; and that they attain their adult form by passing 110 STRUCTURE OF MYRIAPOD A AND ANNELIDA. through a series of metamorphoses, in which they lose their eyes and legs, and become fixed for the remainder of their lives. 103. We now pass hack to another class of the higher group of Articulata, adapted to breathe air and to inhabit the land, — the Myriapoda or Centipede tribe (fig. 42). Both these names are derived from the great number of legs possessed by these animals, which often amount to 60 pairs or even more. In this class we see a more perfect equality of the segments or divisions of the body than in any others among the higher Articulata; and the similarity is scarcely less complete in the internal arrangement, than it is in the external form. In its lower tribes (fig. 51), the legs are so Fig. 51.— Iulus. weak as scarcely to be able to sustain the body, which moves, therefore, partly in the manner of that of a worm. The animals of this class undergo no proper metamorphosis ; but there is a considerable addition to the number of their seg¬ ments and legs after they have come forth from the egg. 104. We now pass to the lower division of Articulata, in which the body possesses no jointed members ; and the animals belonging to this group are for the most part included in the class of Annelida, the Leech and Worm tribe. We here find the body enveloped, — not in a hard casing, formed of distinct pieces united by a flexible membrane, — but in a skin which is altogether flexible, and which gives little indication of a division into segments. This class includes several distinct tribes, which all agree, however, in the long worm-like form of the body, and in the similarity of the different ganglia oi their nervous system. The Earth-worm and its allies are adapted to live on land and to breathe air ; but the greater number of Annelids are purely. aquatic ; and these breathe by gills, which form tufts that are disposed on various parts of the body. In the Nereis, or Sea-centipede (fig. 52), these STRUCTURE OF ANNELIDA AND ENTOZOA. Ill tufts are arranged regularly on the several segments, and the animal can swim by the motion that it gives them ; besides these, it has a kind of bristle-shaped appendage, that seems Fig. 52. — Nereis. like a rudimentary leg, which assists it in crawling. But there are others of these marine-worms, that form a tubular shell, in which they reside during the greatest part of their lives ; and in these the gills, if disposed along the body, would have been removed from the access of water ; they are therefore arranged round the head, often forming (as in the Serpulce , fig. 145) tufts of great brilliancy and elegance. 105. Below the Annelida are other worm-like tribes of yet greater simplicity of conformation, but still presenting the same general plan of structure. Of one of these the common Leech may be taken as an example ; of another, the Tape - Fig. 53.— Tape-worm. worm (fig. 53). This last belongs to a group termed Entozoa, from the circumstance that they inhabit the bodies of other animals. They are remarkable for the very low development of their digestive apparatus, — their nourishment being appa- 112 GENERAL STRUCTURE OP MOLLUSCA. rently imbibed through the whole surface of their bodies from the juices in the midst of which they live ; whilst, on the other hand, their reproductive apparatus is enormously developed, the multiplied segments of the Tape-worm (for example) containing this alone, and the head (as it is com¬ monly termed, though really the body) being able to repro¬ duce these to an indefinite extent after they have been thrown off. The group of Eotipera, or Wheel- Animalcules, which is one of great interest to the Microscopist, also belongs to this lower section of the Articulated sub-kingdom. 106. The general character of the animals composing the group or division Mollusca, is, in many respects, the very opposite of that which prevails in the Articulated animals. The body is soft (whence the name of the group is derived), neither possessing an internal skeleton, nor any proper ex¬ ternal skeleton. In some of the most characteristic specimens of the group, such as the Slug , there is no hard frame- work or skeleton whatever, the body being alike destitute of support and protection. In most Mollusks, however, the body has the power of forming a shelly covering, which serves for its protection ; but this does not give any assistance in its movements by affording fixed points for the attachment of the muscles ; in fact, when the animal puts itself in motion, it is obliged to make its locomotive organs project beyond the shell. We must not regard the shell as an essential part of the Molluscous animal ; because there are many tribes entirely destitute of it; and also because some of the Articulata have the power of forming a shell (§ 102), which bears a close resem¬ blance to that produced by the animals of this group. Hot un- Fig. 54. — Testacella. frequently we see that, of two animals whose general structure is almost exactly the same, — as that of the Snail and Slug,— STRUCTURE OF MOLLUSCA. 113 one possesses a shell into which it can withdraw its whole body for the sake of protection, whilst the other has none; and several intermediate forms exist, in which the shell bears a larger or smaller proportion to the body, sometimes being able to contain nearly the whole of it, and sometimes being a mere rudiment, as in the Testacella (fig. 54). 107. The external form of the body of Molhisks is subject to great variation ; and generally has a good deal to do with the degree in which the organs of sense and the instruments of motion are developed in the particular animal. Tor these are almost always symmetrical, being arranged with equality on the two sides of a middle line ; whilst the rest of the body, containing the organs of nutrition, is often unequal on the two sides. But in the lower Mollusca, which have little or no power of moving from place to place, even this degree of symmetry is altogether lost. Tew of the Mollusca have any powers of active movement ; in fact, the term sluggish¬ ness, derived from a characteristic member of the group, very well expresses their general habit. The Gasteropods, which may be regarded as the types of the whole series, crawl upon a fleshy disk, by the successive contractions and relaxations of which they advance slowly along the surface over which they move ; this kind of action is easily studied, by causing a Snail or Slug to crawl- upon a piece of glass, and by looking through this at the under side of its foot. Hence, there is a great contrast between the inertness of the Mollusca, and the high activity of the Articulata. This contrast shows itself in the structure of their bodies ; for whilst the chief part of the interior of an Insect is made up of the muscles which move its legs and wings, — the apparatus of nutrition being small,— the chief part of the bulk of a Slug or Snail is given by its very complex apparatus for nutrition — there being no other muscles (except some small ones connected with the mouth and head) than the fleshy disk already mentioned. The blood of the Mollusca is nearly colourless, as it is in the Articulata ; but the organ by which it is circulated through the body is much more powerful and complete, bearing more resemblance to the heart of Yertebrated animals. The skin is usually thick and spongy in its texture ; having muscular fibres inter¬ woven in its substance, so that it can contract or extend itself in any part ; and having the power of exuding shelly matter i 114 STRUCTURE OF MOLLUSCA. from its surface, in those species which form such a pro¬ tection. This envelope, which is called the mantle, , is very loosely applied round the parts which it contains; and it frequently extends itself into folds or duplicatures, which wrap round the gills, and sometimes meet and adhere so as to inclose them within a cavity of their own. In the Cuttle¬ fish, the water within this cavity is renewed from time to time by the muscular movements of its walls ; but usually a current of fluid is kept up over the surface of the gills, by the action of the cilia (§ 45) with which they are covered. d Fig. 55.— Anatomy of Turbo Pica. 108. The accompanying figure of the interior of a Turbo will show the very large size of the digestive apparatus, and of the other organs of nutrition. The muscular disk or foot is seen at p; and this carries the operculum o, which serves to close the mouth of the shell when the body of the animal is drawn within it. At t is shown the proboscis, on either side of which are the tentacula or feelers, ta , bearing the eyes at y. Just behind the tentacula is seen the large cephalic ganglion, sending nerves to the eyes ; and behind this again are the salivary glands. The mantle, m, is opened and folded back to show the respiratory cavity, in which lie the gills STRUCTURE OP MOLLUSCA. 115 b h; to this cavity, water has access by means of a wide slit, of which the edge, /, of the mantle forms one part of the border, whilst at d is seen a fringed membrane that forms another part. At c is seen the heart, which receives the blood from the gills by v 5, the branchial vein, and then transmits it to the body generally ; at e, far np in the spire, are the stomach and liver ; at a, the anal orifice of the intes¬ tine within the branchial cavity, and at ov the oviduct, which opens in the same situation. 109. Thus it is seen that, — whilst the body of an Articu¬ lated animal may be compared to that of a man in whom the apparatus of nutrition (contained in the chest and abdomen) is of the smallest possible size, but whose limbs are strong, and his movements agile,- — the body of a Mollusk resembles that of a man “ whose god is his belly,55 his digestive appa¬ ratus becoming enormously developed, whilst his limbs are feeble, and his movements heavy. Such varieties, in a greater or less degree, are continually presenting themselves to our notice. 110. The nervous system of the Mollusca generally consists of a single ganglion or pair of ganglia, which are placed in the head, or (when that is deficient) in the neighbourhood of the mouth ; and of two or more separate ganglia, which are found in different parts of the body, and are connected with the preceding by nervous cords. The former correspond to those con¬ tained in the head of Insects ; but of the latter, one only is connected with the foot or organ of motion, the remainder having for their function to regulate the action of the gills, and to perform other movements connected with the operations of nutrition. In fig. 56 is represented one of the simpler forms of this nervous system, — that of the Pecten or Scallop-shell ; a a are the ganglia near the mouth, Fig- 56.— Nervous system of from which the organs of sense are supplied ; b is the ganglion connected with the gills ; and c is that from which power is given to the foot. The i 2 ’ 116 • GENERAL STRUCTURE OF MOLLUSCA. — CEPHALOPODS. two first lie wide apart, but are connected by an arched band that passes oyer the gullet, e. The organs of sense in the higher forms of Mollusca are more developed than those of motion. They serve to direct the animal to its food, and to warn it of danger ; but there seems an absence, in all save the highest species, of that ready and acute sensibility which is so remarkable in the preceding groups ; and the variety of impressions which they can receive appears to be but small. In no instance has a special organ of smell been certainly dis¬ covered ; the organ of hearing is always imperfect, and fre¬ quently absent altogether ; and the eyes are very often wanting. In the lower Mollusca there are no certain indications of the existence of any organs of special sense ; and there is probably but a limited amount of general sensibility. 111. As the Articulata are divided into two subordinate groups, according to the presence or absence of articulated limbs or members, so may we arrange the Mollusca in two subdivisions, according to the presence or absence of a dis¬ tinct head , that is, a projecting part of the body, containing the mouth or entrance to the digestive cavity, and also bearing the organs of sense which guide the animal in the discovery and selection of its food. In the higher Mollusca, there is a distinct head, furnished with eyes, and sometimes with im¬ perfect ears ; but in the lower, the entrance to the digestive cavity or stomach is buried deep among other parts, and is guarded by no other organs of sense than the tentacula or sensitive lips. These are termed acephalous , or headless Mollusca : and among the lowest of them (§ 114), we meet with composite fabrics, formed by the process of multiplica¬ tion by budding, which was formerly regarded as peculiar to Zoophytes, — The highest group of Mollusca, in regard to the approach of several parts of its structure to that of Verte- brated animals, is the class of Cephalopoda, or Cuttle-fish kibe : which receives its name from the peculiar arrangement of the arms or feet around the mouth, which is the cha¬ racteristic of its members (fig. 57). The common Cuttle-fish and its allies are destitute of any external protection; but they usually have a flat shell, commonly known as the cuttle¬ fish bone, inclosed in a fold of the mantle, and lying along the back. In the Calamary , this is horny in its texture, and is sufficiently flexible to offer no resistance to the action of STRUCTURE OP CEPHALOPODS AND PTEROPODS. 117 the fin-like tail. By which the animal is propelled through the water very much in the manner of a fish. The Pearly Nautilus is the only type now existing of an inferior order of Fig. 57.— Calamary. Cephalopods, which approaches the Gasteropods in many parts of its organization. The "body is inclosed in the last chamber of a shell (usually spiral in form), the cavity of which is divided by numerous transverse partitions ; and such shells, the fossilized remains of very numerous forms of this group that existed in the ancient seas, con¬ stitute the nautilites , ammonites, belemnites , &c., which abound in . many rocks (fig. 58). The Cuttle¬ fish are animals of considerable activity; their mouth is furnished with a horny beak, strongly resem¬ bling that of the parrot ; and their arms are provided with a series of very curiously constructed suckers, by the action of which they can take a very firm hold of anything which they desire to grasp. 112. The class of Pteropoda, or wing-footed Mollusks, consists of but few species, and the animals which it contains are all of them of small size ; but the individuals are often very numerous, whole fleets of them being sometimes seen covering the ocean, especially in the Arctic and Antarctic regions, where they constitute one of the principal articles of food to the Whale. The general form of the body usually differs but little from that represented in fig. 59. Fig. 59.— Hyal-ea. 118 STRUCTURE OP GASTEROPODS AND BIVALVES. On either side, a little beliind the head, the mantle is extended into a fin-like expansion, by the aid of which the animal can swim through the water. The hinder part of the body is usually inclosed, more or less completely, in a shell, which is commonly of extreme thinness and delicacy. The head is not furnished with long arms, to grasp the food ; but it has a number of minute sucking disks, by which it can lay firm hold of whatever it attacks : whilst its powerful rasp-like tongue is set to work upon it. — The class Gasteropoda con¬ tains those animals which, like the Snail and Slug, crawl upon a fleshy disk on the under side of their bodies ; and the number of distinct forms which it includes is very large. The greater part of them are inhabitants of the sea-shore, rivers, lakes, &c. ; some have the power of swimming freely through the open sea; and the proportion of those that breathe air and live on land, is comparatively small. The general structure of the animals of this group has been already described (§ 108). Some of them form shells, whilst others are destitute of them. The shells are composed of a single piece, or are univalve , except in one tribe ; and they have usually more or less of a spiral formation (fig. 60). The animals of this class all possess a distinct head ; and this is generally furnished with eyes, as well as with tentacula. They have often a powerful masticating ap¬ paratus, and are voracious in their habits ; Pig. 60.— shell some of them feed upon vegetable matter, others OF Paludina. npon animals> 113. The Acephalous Mollusca are divided into two groups, — those which form shells, and those which do not. The former are termed Conchifera, or shell-bearing animals ; and this class includes all the Mollusca that form a shell composed of two parts or valves fitted together (which shell is termed bivalve ), as well as some others whose general structure is the same, but whose shell is formed in several pieces, or multivalve. The two valves of a bivalve shell (fig. 61) are connected by a hinge, where they are united by a ligament, which, by its elasticity, keeps them apart while it holds them together. This is their usual condition when the animal is alive ; and in this manner the water which is required for their respiration, and also to convey their supply of food, has free access to the internal STRUCTURE OF CONCHIFERA, OR BIVALVES. 119 parts. But when any alarm or irritation causes the animal to close its shell, it does so by means of a muscle (sometimes Fig. 61. — Shell of Tridacne. single, sometimes double), which stretches across from one valve to the other, and which, by contracting, draws them together. Each valve is lined by an extended fold or lobe of the mantle. In the higher tribes of the class, these lobes are united along their edges, leaving apertures for the ingress and egress of water (which are sometimes prolonged into tubes, fig. 150), and another for the foot. But in the Oyster and its allies, which have no foot, or a very small one, the mantle-lobes are quite disunited. The accompanying diagram (fig. 62) gives a general idea of the arrangement of the organs in one of the higher acephalous Mollusca, the Mactra , which is among those having two muscles for the drawing together of the valves. The upper end, as represented in this figure, is that which is considered as the anterior end or front of the animal, being that nearest which the mouth lies ; and the posterior extremity (the lowest in the figure) is that at which the intestinal canal terminates, and at which the respiratory tubes are formed. Hear the anterior muscle, we find the mouth, or entrance to the stomach ; it is furnished with four riband-shaped tentacula, of which one is seen in the figure ; and these seem to possess peculiar sensitiveness. Hear the mouth lie the anterior ganglia of the nervous system, which represent the brain of higher animals ; and these are connected by long cords with the posterior ganglion, which lies near the posterior muscle. The stomach, intes¬ tines, and liver occupy the central portion of the cavity of the shell ; and the intestinal tube is seen to pass backwards, 120 STRUCTURE OF CONCHIFERA OR BIVALVES. terminating near one of tlie canals or siphons, which also carries out the water that has been taken in through the other for the purposes of respiration. The figure also shows the large fleshy foot, by which this animal can move itself along Fig. 62. — Anatomy of Mactra. the ground, or bore into sand or mud. The heart and circu¬ lating system are less complete than in the Gasteropoda ; but are far higher in character (as are most of the other parts STRUCTURE OF TUNICATA. 121 of the nutritive apparatus) than the corresponding parts in Articulated animals, in which the apparatus for locomotion so much predominates. 114. The group”'of Acephalous Mollusks which are desti¬ tute of the power*of forming a shell, includes two classes, of which one does not depart widely from the general Molluscan type, whilst the other presents ‘so strong a general resem¬ blance to Zoophytes, that until recently it has been universally ranked with it. The first of these classes receives its name Tunicata from the circumstance that the mantle, instead of secreting a shell, is very commonly condensed into a tough leathery or cartilaginous tunic. Many of these animals live separately, and have the power of freely moving through the water. Others are associated in compound masses, of which, however, the individuals are not connected by any internal union. But others form really composite structures, like those of Zoophytes (§ 124) ; each individual being able to live by itself alone, but being connected by a stem and vessels with the rest. The general structure of the individuals is the same, however, in the single and in the composite animals of this class, and may be understood from the accom- Fig. 63. — Social Ascidians. panying figure (fig. 63). The cavity of the mantle possesses, as in the former instance, two orifices ; by one of which, 6, a current of water is continually entering, whilst by the other, a, it is as continually flowing out. These orifices lead into a large chamber, the lining of which, folded in various ways, constitutes the gills ; and at the bottom of this chamber lie the stomach, e, and the intestinal canal, i, which terminates near the aperture for the exit of the water. All these parts 1 22 STRUCTURE OF TUNICATA AND POLYZOA. are covered with cilia , by the action of which a continual stream is made to flow over the gills and to enter the stomach ; and the minute particles which the water brings with it, and which are adapted to serve as food, are retained and digested in the stomach. Even these animals, fixed to one spot during all but the early part of their lives, and presenting but very slight indications of sensibility, possess a regular heart and system of vessels ; and these vessels form part of the stem, t, by which the compound species are connected. A single nervous ganglion is found between the two orifices ; this seems to receive sensory fibres from tentacula situated around the oral orifice, and to transmit motor filaments to the mus¬ cular coat which underlies the outer tunic, so that any irrita¬ tion applied to the former occasions a contraction of the latter, which tends to expel the offending particle. — This class is one of particular interest to the naturalist, since we see in it the tendency to the formation of compound struc¬ tures, by a process resembling that of the budding of plants, which is essentially characteristic of Zoophytes ; this ten¬ dency, however, is more fully manifested in the succeeding class. 115. The animals forming the class Polyzoa (more com¬ monly known as Bryozoa) are seldom or never found solitary ; since, in consequence of their universal tendency to multiply by gemmation, they form clusters or colonies of various kinds. The body of each individual is inclosed in a sheath or “ cell,” which is sometimes horny, sometimes calcareous; and the composite skeleton formed by the aggregation of these, which has sometimes a branching or leaf-like form, but sometimes possess the compactness of a stony coral, is known as the “ polyzoary.” In their general structure the animals of this class possess considerable analogy to the Tunicata ; but the Molluscan type presents itself under a more degraded aspect, no vestige of a heart or of blood-vessels being here dis¬ cernible, and the general structure being so simplified as to manifest no great degree of elevation above that of Polypes. The typical structure of these animals may be understood from that of the BowerbanJda (fig. 64), which is one of those whose cells are not in contact with each other, but grow forth at intervals from a creeping stem. The mouth, a, is situated in the midst of a circle of arms fringed with cilia ; these STRUCTURE OF POLYZOA AND RADIATA. 123 arms do not serve, however, like those of polypes, to grasp the food ; but the vibration of their cilia produces a powerful %. J I current which brings both food and oxygen. The mouth leads by a large funnel-shaped oesophagus or gullet, to a gizzard, b ; in which the particles of food that enter it are ground down, by the action of its muscular walls and of the tooth-like processes that line it. Below this gizzard is the true digestive stomach, c, around which the rudiment of a liver may be traced ; and from this stomach there passes upwards an intestinal tube, which terminates by a distinct orifice at d, on the outside of the circle of arms. The digestive apparatus is evi¬ dently formed, therefore, upon a much higher plan in these animals than it is in the true polypes, which have no true anal orifice. The Molluscan character of these animals is further shown by the presence of a single nervous ganglion, situated between the two orifices, as in the Tunicata ; this acts upon a complex a, oesophagus ; b, giz- apparatus of muscles, by which the animal ^orifice^f^ntes- can be cither drawn into its cell or projected tine* forth from it, with great rapidity. 116. The fourth subdivision, that of Radiata, includes those animals which have the parts of the body arranged in a circular manner around a common centre, so as to present a radiated or rayed aspect. This arrangement is well seen in the common Star-fisk (fig. 65), which has five such rays, all having a precisely similar structure, and thus repeating each other in every respect. The mouth of this animal is in the centre ; and it opens into a stomach, which occupies the cen¬ tral disk, and sends prolongations into the rays. The nervous system is, in like manner, composed of a repetition of similar parts. A plan of it is seen in fig. 66 ; where a shows the position of the mouth, which is surrounded by a ring or nervous cord, having five ganglia, corresponding to the five arms. From each of these ganglia proceeds a branch along its arm, terminating in a little organ at its extremity, which is believed to be an imperfectly-developed eye. FTo other organs of special sense can be detected in any of these ani- Fig. 64. Bowerbankia. 124 STRUCTURE OP RADIATA. mals ; and it is only in a few that even these imperfect eyes can he discovered. In the inferior Badiata, not the slightest Fig. 65. — Shell of Star Fish. traces of a nervous system have yet been discovered; and it is very doubtful whether any such structure exists in them. It is only among the higher Badiata that any locomotive power exists ; and this is usually so feeble that the animals remain in the same locality during the greater portion of their lives. Generally speaking, there is a period in the history of each species, in which there is a more active movement, that serves to prevent the accumulation of indi¬ viduals in one spot ; but this move¬ ment is of a purely automatic character, rather resembling that of the “ zoospores ” of plants, than the intentional change of place of the higher animals. Fig. 66. — Nervous System of Star Fish. STRUCTURE OF RADIATA. - ECHINODERMATA. 125 117. The circular arrangement of the organs of Radiated animals is a striking point of resemblance to the Vegetable kingdom ; and it has frequently caused mistakes to be made in regard to the Sea- Anemones, and other large polypes, which, when their mouths are open and their arms spread out, look so much like the blossoms of some of the Com¬ posite tribe of plants, as to have received the name of animal flowers . Rut there is yet a stronger analogy between the lower members of the Radiated group and the Vegetable kingdom ; for among the former, as in the latter, we find a union of many individuals, which are capable of existing separately, into one compound structure, having a plant-like form. This is the nature of the stem of Coral (fig. 76) ; which is, in fact, the skeleton of one of these compound systems, consisting of a number of polypes united by a jelly- like flesh ; just as the woody stem of a tree is the skeleton that supports a vast number of buds, each of which is capable of living by itself. This aggregation results from the in¬ definite multiplication of parts by the process of gemmation or budding, and from the persistence of the connexion between these parts, notwithstanding that, if separated, they can maintain an independent existence. To the tree¬ like fabrics thus produced, the name Zoophytes (animal plants) is commonly given ; and ordinary observers often find it difficult to get rid of the idea of their vegetable origin. The animals that formed them are, of course, fixed to one spot during all but the earliest periods of life ; and the amount of movement which they perform, for the pur¬ pose of obtaining and securing their food, is very little greater than that which is witnessed in the Sensitive plant and Venus’s fly-trap. 118. The class of Eohinodermata receives its name from the prickly character of its covering, which is evident enough in the Echinus or Sea-Urchin, and in the Star-fish; but there are other animals, sufficiently resembling these in general structure to be united in the same class, which have a body entirely soft,- — namely, the Holothurice (fig. 67), commonly termed Sea- Cucumbers. This class ranks as the highest among the Radiata, in regard to general complexity of struc¬ ture. The skeleton of the Sea-Urchin, Star-fish, and other animals resembling them, is a box-like shell or “ test,” formed 126 STRUCTURE OF ECHINODERMATA. of a great number of pieces, very regularly arranged and united together (fig. 69, e) ; but these pieces are for the most Fig. 67. — Holothuria. part only repetitions of one another ; and the different portions have not that variety of uses which we see in higher animals. With the exception of the tribe of Encrinites or lily-like animals (fig. 68), of which there are very few now existing, but which were very abundant in former ages, all the animals belonging to this class are unattached, and are capa¬ ble of moving freely from place to place. Their motions are very sluggish, however, and are princi¬ pally effected by means of an im¬ mense number of minute tubular feet (fig. 68, c ), furnished with suckers at their extremities, which can be projected from almost any part of the body. These are seen in rows on the under side of each arm of the Star-fish; they are put forth through rows of very minute aper¬ tures in the shell of the Sea- Urchin (commonly termed the Sea- Fig. 68.— Encrinite. STRUCTURE OF ECHINODERMATA. 127 Egg) ; and they are also arranged in rows on the surface of the body of the Holothuria, as seen in fig. 67. 119. The radiated arrangement is very evident in the whole bodies of the Star-Fish (fig. 65), and Echinus or Sea- Urchin (fig. 69); but in the Holothuria (fig. 67) it is nearly confined to the parts about the mouth; which, however, exhibit it so completely, that such an animal cannot be mis¬ taken for one of the Articulated series, even though, as some¬ times happens, the body is prolonged into a worm-like form. The digestive apparatus in this class has usually a high degree of complexity, as will be seen by the accompany¬ ing figure (fig. 69), which shows the interior of an Echinus , whose globular shell has been sawn across its equator, so as 128 STRUCTURE QE ECHINODERMATA AND ACALEPH^. to allow of the separation of its two halves. The mouth, situated at one of the poles of the shell, is surrounded by a very curious apparatus of jaws and teeth (fig. 69), which forms what is termed the “ lantern from the mouth com¬ mences the long narrow oesophagus, m, that leads to the stomach, w, which is merely a dilated portion of the alimen¬ tary tube ; the continuation of this, o, q, r, forms the intestinal canal, which winds once round the shell, and then doubles back and winds in the opposite direction, terminating at the anal orifice, s, which is situated at the opposite pole. The intestine is held in its place by a double fold of” the mem¬ brane lining the shell, resembling the mesentery of higher animals ; the blood is distributed over this membrane, to be exposed to the aerating influence of the water admitted into the cavity of the shell ; and the water is kept in movement by the cilia with which the membrane is clothed. Round the anus, s , are seen the five branching ovaries, each of which discharges its contents by a distinct orifice. The circulating apparatus is imperfect, the blood not being impelled by a distinct heart ; still, however, it moves in great part of its course through proper vessels, and not through mere chan¬ nels in the tissue. — In the Star-fish , however, the body is very much flattened ; and the stomach, instead of having a separate intestinal tube with a distinct orifice, is a mere bag with a single aperture, which serves both to take in food and to cast forth the indigestible remains. This character will be found to prevail among all the inferior Radiata. 120. The radiated structure is also well seen in the greater number of animals forming the group of Acalephje, or Sea* Nettles . Their bodies are entirely soft and jelly-like ; contain¬ ing so small a quantity of solid matter, that, when upon being taken out of the water their fluid drains away, there is scarcely anything left ; hence they are commonly termed Jelly-Fish . They derive their other name of Sea-hTettles from the stinging power which most of them possess. They are formed to float freely in the water; but they do not in general possess any means of actively propelling themselves through it. The radiated arrangement is very regularly pre* served in some of this group, whilst it is less evident in others. The accompanying figure (fig. 7 0) represents one of the Medusa tribe, as seen floating in water. The umbrella- STRUCTURE OF ACALEPHiE. 129 shaped disc above contains the stomach, which is placed in the centre, and which opens by a single orifice or month, directed downwards. Around the stomach are four chambers, in which the eggs are prepared. The mouth is surrounded by four large tentacula, which bring to it the necessary Fig. 70. — Pelagia. food; and other tentacula are seen, in this species, to be hanging from the edge of the disc. In the edge of this disc, the nutritious fluid, which flows in channels prolonged from the stomach and excavated out of the soft tissues, seems to be exposed to the influence of the surrounding water; but nothing like a heart or a regular circulation exists. — Recent discoveries in regard to the developmental history of the Medusce and their allies, have rendered it very doubtful whether the Acalephce should continue to take rank as a dis¬ tinct class ; since many of them constitute only a particular phase in the life of the Hydroid Zoophytes (§ 124). 121. The class of Polypifera, or coral-forming animals, commonly known as Zoophytes, includes two principal tribes, which differ from one another in structure to such a degree as to K 130 STRUCTURE OF HYDRA. require separate notice. The group of Hydrozoa , or Hydroid Zoophytes, so named from the little Hydra , or fresh- water polype, which may be regarded as its type, will be first described on account of its near con¬ nexion with the preceding. The Hydra (fig. 71) is a solitary polype, not at all uncommon in ponds or other collections of fresh water, where it is found attached to aquatic plants, or to floating sticks, straws, &c., by means of a kind of sucker at its lower extremity, stretching out its tentacles in search of its food, which consists of minute aquatic worms and insects. These are securely laid hold of by one or more of the tentacles, and are Fig. 71.— Hydra, or Fresh-water drawn into the mouth, a , which ’f L 5 P£ leads to the stomach or general cavity of the body, in which they are digested, and from the walls of which the nutritious portions are absorbed, the portions of the food which are not capable of being digested being cast out through the mouth. 122. The Hydra multiplies in two ways ; namely, by gem¬ mation or budding, and by a proper generative process. Little bud-like processes are developed from various parts of the walls of the stomach, which gradually assume the form of the parent, possessing a mouth surrounded by tentacles, and a digestive cavity which is at first in connexion with that of the parent ; the communication is gradually cut off, how¬ ever, by the closure of the canal of the footstalk of the young polype ; and ere long the footstalk itself separates, and the young polype henceforth leads an entirely independent life. Hot unfrequently, however, the young polype itself puts forth buds before its separation ; and as many as nineteen young Hydrae, in different stages of development, have been seen to be thus connected with one and the same stock. Another very curious endowment of the Hydra depends upon the same facility of developing the whole structure from any part of it ; 131 HYDRA, AND HYDROID ZOOPHYTES. for into whatever number of parts its body may be cut up, each, under favourable circumstances, can give origin to a new and entire polype, so that thirty or forty individuals may thus be produced by the division of one. 123. The proper generative process, here reduced to its utmost simplicity, consists in the development of a germ- cell and of sperm- cells in the substance of the wall of the stomach, the former being produced near the footstalk, the latter just beneath the arms. The egg which is evolved from the former, being fertilized by the products set free from the latter, gives origin to a young Hydra, which resembles its parent. The two reproductive processes, however, are performed under very different conditions ; for whilst multiplication by gemmation is favoured by warmth and a copious sup¬ ply of food, the true gene¬ rative process seems to be brought about by a lower¬ ing of the temperature, and to have for its object the perpetuation of the race through the winter, the egg being capable of enduring a degree of cold which would be fatal to the polype itself. 124. The group of Hy- drozoa is for the most part made up of composite fabrics more or less resembling the Campanularia (fig. 72 ), which may be likened to a Hydra whose buds do not detach themselves, but re¬ main in connexion with the stock that produced them ; the whole plant-like struc- Fig. 72.— Campanularia. ture, moreover, being strengthened by the consolidation of its external layer into a horny sheath, which retains its form after the destruction of the soft parts. Thus each comes to consist of a stem and branches, on the sides or ends of which k 2 132 REPRODUCTION OF HYDROID ZOOPHYTES. are a number of little cells or bell-shaped chambers, with their mouths upwards, every one of them containing a polype that bears a strong resemblance to the Hydra. Each of these polypes is capable of living independently of the rest, obtains its nourishment by means of its own arms, and digests it in its own stomach ; but all are connected by a canal that passes along the stem and branches, in which a kind of circulation takes place, that strongly reminds us of that of the compound Tunicata (§ 114). This plant-like structure extends itself by budding ; new branches are formed from those previously existing ; and these are enlarged at a certain point into cells, in which after a time polypes make their appearance. 125. Besides the cells containing the polypes, however, we find capsules in which are evolved buds of a different nature, that form within themselves the generative products. These buds in some instances assume the form of Medusas, and, becoming detached from the stalk that put them forth, swim about freely, living upon food obtained by themselves, and setting free either sperm-cells or germ-cells, by the concur¬ rence of whose contents eggs are formed, from which new polype-growths arise. In other instances the Medusoid bodies give forth their generative products, without ever leaving the capsules in which they were themselves developed. And in other cases, again, it does not seem that any Medusoid form intervenes at all, the germ-cells and sperm-cells being evolved from the Zoophytic structure itself. But since it is also known that even the most characteristic Medusan forms are evolved as buds from a Zoophytic stock (Chap, xv.), and since those composite forms of Acalephse whose structure has until lately been most obscure, turn out to be, as regards their essential characters, Hydrozoa organized for floating, there seems to be no longer any sufficient ground for ranking the Acalephse as a separate class. 126. It is not, however, by animals of this very simple structure, that the massive stony fabrics are built up, which constitute the coral islands of the Pacific Ocean, and of which a large portion of our limestone rocks seems to be composed. These are constructed by animals belonging to the group of Anthozoa , and formed upon the same general plan with the Sea- Anemone, — a plan which is higher than that of the Hydra, inasmuch as we find the interior of the body containing other STRUCTURE OF ANTHOZOA : - SEA- ANEMONE. 13S cavities around the stomach, which are destined to pre¬ pare the generative products. In fig. 73, we have a repre¬ sentation of the Sea- Anemone, as seen from above ; showing its mouth in the centre, surrounded by its numerous radi¬ ating tentacula ; these are often brightly coloured, and give to the animal the appearance of a beautiful flower. In fig. 74, a similar animal is represented, cut open to show its interior. Fig. 73.— Sea-Anemone, seen from above. Fig. 74.— Section of Sea-Anemone. «, cavity of stomach ; bf surrounding chambers. The mouth is seen to open into a rounded stomach, a , which has no other orifice outwards ; and round this stomach there is a series of radiating membranous partitions, which divide the space intervening between it and the outer covering of the body into numerous chambers, b. Within these chambers, and attached to their partition- walls, are found the bodies which are commonly designated ovaries, but which contain sperm- cells or germ- cells according to the sex. It is doubtful whether these two products are ever formed by the same individual, as they are in the Hydra. The Sea- Anemone does not usually multiply itself by budding, though some species do so ; but large numbers of young are produced from the eggs, which are fertilized and partly developed whilst still within the ovarian chambers, and these make their way into the stomach through an aperture at its deepest point, and finally escape by the mouth. 127. The Sea- Anemone itself, like the Hydra, is a solitary animal, capable of shifting its place at will; and it forms no stony skeleton or support. But there are other animals of the same general structure, which have the power of depositing stony matter in the membrane of their base or foot, and in the membranous partitions between the chambers ; and this stony deposit forms a Coral or Madrepore , such as is shown 134 ANTHOZOA : — STONY CORALS. in the accompanying figure (fig. 75). The particular arrange¬ ment of the radiating plates of the Madrepore (shown at the top of each stem) is the result of the form of the soft structures by which it was deposited; and wherever we see a structure of this kind in coral, whether upon a large or a small scale, we may infer that it was formed by an animal nearly allied in structure to the Sea- Anemone. Of the stone depositing coral- animals, a large number are often associated in a com¬ pound structure, as in fig. 76; this consists of a stony tree-like stem and branches ; but instead of the soft ani- Fig. 75.— Caryophyllia. ma]. matter being contained in its interior, as in the Hydrozoa, it usually forms a kind of flesh Fig. 76. — Stem of Coral. that clothes the surface, and connects together the different STRUCTURE OF PROTOZOA. 135 polypes; and new branches, are formed either by the sub¬ division of the polypes, or by gemmation from the connecting substance. 128. When we pass from Zoophytes to animals of still simpler organization, we lose all trace of definite symmetry, and find ourselves amid forms which cannot be referred to any particular plan of growth. These, moreover, are for the most part distinguished by an extreme simplicity of structure ; no such differentiation of parts exhibiting itself among them, as is shown in the “ organs ” of even the simplest Zoophyte or Worm. Hence they are appropriately designated Protozoa. They may, in fact, be considered as essentially consisting of homogeneous particles of a jelly-like substance, to which the name of Sarcode has been given ; and the chief modification this undergoes, consists in the consolidation of certain parts of it by the deposit of horny, calcareous, or siliceous matter, so as to form a skeleton. This may take place on the outer surface only, so as to form shells very like those of Mollusks in miniature, as we see among Foraminifera (fig. 78) ; or it may occur in the midst of the fleshy substance, so as to form an internal network, such as presents itself in the Sponge. The endowments of the “ sarcode ” are very extra¬ ordinary ; and will be best understood by observation of the life-history of one of those simplest Protozoa, in which the whole body consists of but a minute particle of it. Fig. 77. — Riiizopoda : — A, Amceba ; B, Actinophrys. 129. Such an example is afforded by the Amoeba (fig. 77 a), — a creature frequently to be met with in great abundance in fresh and stagnant waters, vegetable infusions, &c. Its 136 RHIZOPODA : - AM(EBA j ACTINOPHRYS. organization is so low, that there is not even that distinct differentiation into containing and contained parts which is necessary to constitute a cell (§ 32) ; for although the super¬ ficial layer of the sarcode possesses more consistence than the interior, it is nevertheless obvious that it has not the tenacity of a membrane, since (as will be presently seen) it does not oppose the passage of solid particles into the interior. How¬ ever inert this creature may seem when first glanced at, its possession of vital activity is soon made apparent by the movements which it executes and the changes of form it undergoes; these being, in fact, parts of one and the same set of actions. For the shapeless mass puts forth one or more finger-like prolongations, which are simply extensions of its gelatinous substance in those particular directions ; and a continuation of the same action, first distending the prolongation, and then, as it were, carrying the whole body into it, causes the entire mass to change its place. After a short time another prolongation is put forth, either in the same or in some different direction ; and the body is again absorbed into it, so as to shift its place still more. It is by means of this movement that the creature obtains its supplies of food ; for when, in the course of its progress, it meets with a particle appropriate for its nutriment, its gelatinous body spreads itself over this, so as to envelope it completely ; and the substance (sometimes animal, sometimes vegetable), thus taken into this extemporized stomach, undergoes a sort 01 digestion therein, the nutrient material passing into the sub¬ stance of the sarcode, and any indigestible portion making its way to the surface, from some part of which it is (as it were) finally squeezed out. 130. Many other forms of this group, which has received the designation of Ehizopoda , have less power of moving from place to place, but obtain their food by a modification of the same arrangement : of this we have an example in Actinophrys (fig. 77 b). The body being stationary, its gelatinous substance extends itself into long filaments, termed pseudopodia : these often divide themselves again like the roots of a tree (whence the designation of the group), so as to form threads of ex¬ treme tenuity; and sometimes these threads meet again and coalesce, so as to form a sort of irregular network. When any minute animal or vegetable organism happens to come in contact RHIZOPODA : - FORAMINIFERA. 137 with one of these threads, it is usually held by adhesion to it, and the filament forthwith begins to retract itself; as it shortens, the surrounding filaments also apply themselves to the captive particle, bending their points together, so as gra¬ dually to inclose it, and themselves retracting until the prey is brought to the surface of the body; and the substance of the threads being itself drawn into that of the body, the entrapped particle is embedded along with this, and under¬ goes digestion in the surrounding sarcode, any indigestible particle being subsequently extruded from the surface of the body, just as in the Amoeba. The reproduction of these creatures, so far as is yet known, is effected by self-division, like that of the Infusoria (§ 135); but there is reason to believe that a “ conjugation/’ or reunion of two individuals, sometimes occurs, and that this is to be looked on as repre¬ senting the sexual propagation of higher animals. Fig. 78. — Foraminifera. A, Oolina; B, C, Nodosaria; D, Cristellaria ; E, Polystomella ; F, Dendritina, G, Globigerina ; H, Textularia; I, Quinqueloculina. 131. This Ehizopod type of animal life is manifested in two groups of great interest, which are characterised by the possession of hard shells, formed by the consolidation of the external layer of sarcode. The Foraminifera have calcareous shells, which often bear a strong resemblance to those of Nautili, &c. in miniature (fig. 7 8), but which really have an entirely different relation to the animals that form them. For whilst the Nautilus occupies only the last or outer chamber of its shell, the chambers previously formed 138 FORAMINIFERA AND POLYCYSTINA. being empty and deserted, each chamber of the Rotalia , or any other Eoraminiferous shell, is occupied by a segment of sarcode, which is to a great degree independent of the rest, and is only connected with those on either side of it by delicate threads of the same substance ; and the extension of the shell is due to the formation of an additional segment of sarcode on the outside of the last-formed chamber. Each segment has usually the power of putting forth its own “ pseudopodia ” through minute apertures in the shell, and thus of drawing in its own nourishment through these ; but even when (as sometimes happens) these food-collecting threads are put forth from the last chamber alone, the nutri¬ ment there obtained is transmitted to the segments within by percolation through the substance of the sarcode, and not through any tubular canal. — The accumulation of the shells of Eoraminifera in some parts of the existing sea-bottom is very remarkable ; and similar accumulations in past ages have formed no unimportant part of the crust of the earth — a large part of the Chalk-formation having had its origin in them, as well as nearly the whole of the ISummulitic limestone by which it was succeeded. 132. Eut animals whose essen- a b tial structure seems to be nearly the same, may form siliceous in¬ stead of calcareous shells ; and thus are produced those beautiful organisms, known under the name of Polycystina (fig. 7 9), which are occasionally found in the existing seas, but whose re¬ mains are met with under a far greater variety of forms in certain of the newer marine deposits. There is not in these the same tendency to form composite structures by the multiplication of segments, as in the Eoraminifera ; but the complication of the individual form is often much greater. Yet, however complex the form, the essential composition of these crea¬ tures seems to retain the same attribute of simplicity, which cannot be conceived capable of further reduction. Fig. 79. — Polycystina. A, Podocyrtis; B, Rhopalocanium. INFUSOEY ANIMALCULES. 139 133. The Animalcules to which the name of Infusoria may be properly restricted (the Eotifera , or Wheel- Animal¬ cules, § 105, whose organization is much higher, together with many organisms whose true nature is vegetable, being ex¬ cluded), present an advance upon the simplicity of the Bhizo- poda in this, — that whilst their bodies consist for the most part of sarcode, and present scarcely anything that can be termed a distinction of organs, their external surface is con¬ densed into a membrane too firm to admit either of indefinite extension into pseudopodia, or of the passage of alimentary particles through it ; and consequently the form of the body, although not insusceptible of being temporarily changed by pressure, possesses a considerable degree of constancy for each species (fig. 80). A mouth, or definite aperture for the in¬ gestion of food, is provided; with an additional orifice in some instances, through which indigestible or effete matters may be discharged from the interior. Into this mouth, ali- i. Monads; n. Trachelis anas; in. Enchelis, discharging fsecal matter , iv. Para- mcecium ; v. Kolpoda ; vi. Trachelis fasciolaris. mentary particles are drawn by the agency of the cilia with which some part of the surface of the body is provided; these cilia being always so disposed as to serve at the same time for the general locomotion of the animalcule, and for the production of currents that shall bring food to its interior. 134. Although most Infusoria move freely through the water in which they live, yet certain kinds of them attach themselves by footstalks to marine plants or other floating bodies, during at least a part of their lives ; and in this con¬ dition bear no slight resemblance to Zoophytes, though of far simpler organization. It is in these sessile forms that the agency of the cilia in creating currents which bring food to 140 INFUSORIA. — PORIFERA OR SPONGES. the month, becomes most conspicuous. The alimentary par¬ ticles introduced into the mouth commonly have to pass down a short canal before they enter the general cavity of the body ; and within this cavity a number of minute par¬ ticles are commonly aggregated into a sort of little pellet, as may be well seen when Infusoria are fed with carmine or indigo. One after another of these pellets being thus intro¬ duced into the interior, which is occupied by a soft sarcode, each seems to push onwards its predecessors ; and a kind of circulation is thus occasioned in the contents of the cavity. The pellets that first entered make their way out after a time (their nutritive materials having been yielded up), generally by a distinct anal orifice, sometimes, however, by any part of the surface indifferently, and sometimes by the mouth. 135. The multiplication of Infusoria ordinarily takes place by spontaneous fission, precisely after the manner of the multiplication of ordinary cells (§ 33). This process, under favourable circumstances, may be performed with such rapidity, that, according to the computation of Ehrenberg, no fewer than 268 millions might be produced in a month by the repeated subdivision of a single Paramecium. Sometimes, instead of undergoing subdivision into two equal parts, the Animalcule puts forth a bud, which gradually increases, and then detaches itself from the parent stock. Whether any¬ thing equivalent to the sexual generation of higher animals occurs among Infusoria, is yet uncertain; but recent re¬ searches afford a probability in the affirmative. 136. In the tribe of Porifera, or Sponges , we seem to have the connecting link between Protozoa and Zoophytes. Eor their animality does not lie so much in the individual particles, as in those aggregations which begin to shadow forth that distinction into organs which is carried out more completely among Zoophytes : and there is a large section of the last-named group, in which the polypes are connected together, not by a regular stony or horny stem, but by a sponge-like mass ; while the extension of the fabric is provided for -by the budding out of this spongy portion of it, the orifices of whose canals after a time become furnished with polype-mouths. The true Sponge (fig. 81) consists of a fleshy substance, composed of an aggregation of particles of sarcode, supported upon a skeleton which usually consists of a net- PORIFERA OR SPONGES. 141 work of horny fibres, strengthened by spicules of mineral matter, sometimes calcareous, but more commonly siliceous. The entire mass is traversed by a great number of canals, which may be said to commence in the small pores upon its surface, and which discharge themselves into the wide canals that terminate in the large orifices, or vents, that usually pro¬ ject more or less from the surface of the Sponge. Through this sys¬ tem of canals, there is continually taking place, during the living state of the animal, a circulation of water, which is drawn in from without through the minute pores, then passes into the large canals, and is ejected in a constant stream from the vents. The immediate cause of this movement seems to lie in the vibration of cilia so extremely minute that their existence can only be detected by the most careful micro¬ scopic examination. Its purpose is evidently to convey to the animal the nutriment which it requires, and to carry off the matter which it has to reject. No distinct indications of sensation, or of power of locomotion, have been seen in the Sponge : but changes in the form of its projecting vents may be seen to take place from time to time, if it be watched sufficiently long. 137. The reproduction of the Sponge is commonly effected by the budding forth of little particles of sarcode, from the layer which lines the larger canals ; these become furnished with cilia, and, when detached and carried out by the current that issues from the vents, swim freely about for some time ; so as, before fixing themselves and beginning to develope into Sponges, to spread the race through distant localities. But it appears that Sponges are also reproduced by a true sexual process ; “ sperm-cells ” and u germ-cells ” being pro¬ duced (as in the Hydra, § 123) in different parts of the organism, and a true embryo taking its origin in the action of the contents of the former upon those of the latter. 142 NATURE AND SOURCES OP ANIMAL FOOD. 138. We thus conclude our general survey of the Animal Kingdom ; which, it is hoped, will he found to answer the purpose for which it was designed, — that of giving such an amount of preparatory knowledge respecting the principal types of animal structure, as may enable even the beginner to comprehend what will hereafter be stated of their physiological actions. It has not been attempted to observe any proportion in the notice of these several types ; the higher forms having been slightly passed over, because the details of their vital phenomena will constitute the principal subject of the follow¬ ing pages; whilst some among the lower have been more fully treated, because the ordinary reader cannot be expected to have even that outline-acquaintance with their nature and actions, which he can scarcely help possessing in the case of animals that are familiar to him. CHAPTER III. NATURE AND SOURCES OF ANIMAL FOOD. 139. Before we examine the nature of the process by which the food of animals is prepared for absorption into their bodies, it will be desirable to consider the characters of the aliment itself, and the purposes to which it is to be appro¬ priated. The term food or aliment may be applied to all those substances which, when introduced into the living body, serve as materials for its growth, or for the repair of the losses which it is continually sustaining (§ 55). When animals are deprived of these materials, we see their bodies progressively diminishing in bulk, their strength decreases, and death at last takes place, after sufferings more or less prolonged. In warm-blooded animals, however, a yet more urgent demand for food is created by the requirements of the heat-producing process ; and many substances are fitted to supply this, which cannot serve for the nourishment of the tissues. 140. The demand of the body for food is made known by a peculiar sensation, which has its seat in the stomach, namely, hunger. It is increased by mental and bodily exercise, and NATURE AND SOURCES OF ANIMAL FOOD. 143 by everything which augments the general energy of the system ; whilst, on the contrary, everything which tends to retard the operations of life, such as bodily and mental inac¬ tivity, sleep, or depression of spirits, tends also to render the demand for food less imperious. Thus, cold-blooded animals, particularly Eeptiles, can sustain a very prolonged abstinence, when the general activity of their functions is kept down by a low temperature; and hybernating Mammals, which pass the winter in a state of torpidity, require no food during the continuance of their lethargy. Eut with this exception, warm-blooded animals require a constant supply of nutriment, not merely for the maintenance of their proper heat, but also for the repair of the waste resulting from that continuous activity which the uniform temperature of their own bodies enables them to keep up. This is the case with Man and the Mammalia generally, and still more with Eirds, whose temperature is higher, and whose movements are more active and energetic. It is also more the case with young animals than with adults ; since in the former the changes in the tissues, in consequence of the increase they are undergoing, take place with much more rapidity than in the latter, the bulk of whose bodies remains stationary. Hence, if children, young persons, and adults be shut up together, and deprived of food, the younger will usually perish first, and the adults will survive the longest. The Italian poet Dante has given a terrible picture of such an occurrence, in his history of the imprisonment of Count Ugolino and his children. 141. The difference in the demand for food between the young growing animal and that which has arrived at maturity, is very remarkable in the case of Insects. There are no animals more voracious than the larva or caterpillar; and there are none that can sustain abstinence, with little dimi¬ nution of their activity, better than the imago or perfect insect. The larvae of the Flesh-fly, produced from the eggs laid in carrion, are said to increase in weight 200 times in the course of 24 hours ; and their voracity is so great as to have caused Linnaeus to assert, that three individuals and their immediate progeny (each female giving birth to at least 20,000 young, and a few days sufficing for the production of a third generation) would devour the carcase of a horse with greater celerity than a lion. The larva of the Silk-worm 144 NATURE AND SOURCES OF ANIMAL FOOD. weighs, when hatched, about 1-1 00th of a grain ; previously to its first metamorphosis it increases to 95 grains, or 9,500 times its original weight. The comparative weight of the full-grown caterpillar of the Goat-moth to that of the young one just crept out of the egg, is said to be as 72,000 to 1. Tor this enormous increase a very constant supply of material is necessary, and many larvae perish if left unsupplied with food for a single day. On the other hand, a black beetle (Melasoma) has been known to live seven months, pinned down to a board ; and another beetle (Scarabaeus) has been kept three years without food, — and this without manifesting any inconvenience or loss of activity. There are many perfect insects which never eat after their last change, but die as soon as they have performed their part in the propagation of the race. 142. The nature of the food of animals is as various as the conformation of their different tribes. It always consists, however, of substances that have previously undergone organ¬ ization. There are some apparent exceptions to this, in the case of animals which seem to derive their support, in part at least, from mineral matter. Thus, the Spatangus (an animal allied to the Echinus, § 119) fills its stomach with sand ; but it really derives its nourishment from the minute animals which this contains. The Earthworm and some kinds of Beetles are known to swallow earth ; but only to obtain from it the remains of vegetable matter that are mixed with it. By some races of Man, too, what seems to be mineral matter is mixed with other articles of food, and is said to be nutri¬ tious ; this may be beneficial, in part, by giving bulk to the aliment, and thus exciting the action of the stomach (§ 205); but it has been found, in one case at least, that the supposed earth consists of the remains of animalcules, and contains no inconsiderable portion of organic matter. 143. There are many instances in which, no obvious sup¬ plies of food being afforded, the mode of sustenance is obscure ; and it has been frequently supposed that, in such cases, the animals are sustained by air and water alone. But it will always be found that, where food is taken in no other way, a supply of the microscopic forms of animal or vegetable life is introduced by ciliary action (§ 45); and it is on these, indeed, that a large proportion of the lower forms of aquatic animals depend entirely for their support. NATURE OF THE FOOD OF ANIMALS. 145 144. The first division of aliments is naturally into those which are derived from- the Animal and Vegetable kingdoms respectively. Wherever plants exist, we find animals adapted to make use of the nutritious products they furnish, and to restrain their luxuriance within due limits. Thus among Mammals, the Dugong (an animal having the general form and structure of the whale, hut adapted to a vegetable diet) browses upon the sea-weeds that grow beneath the surface of the tropical ocean ; the Hippopotamus roots up with his tusks the plants growing in the beds of the African rivers, and fills his huge paunch, not only with these, but with the decaying vegetable matter which he finds in the same situa¬ tion; the Antelopes, Deer, Oxen, and other Euminants, crop the herbage of the plains and meadows ; the Giraffe is enabled by his enormous height to feed upon the tender shoots which are above the reach of ordinary quadrupeds ; the Sloths, living entirely in trees, and hanging from their branches, strip them completely of their leaves ; the Squirrels extract the kernels of the hard nuts and seeds ; the Monkeys devour the soft pulpy fruits ; the Boar grubs up the roots and seeds buried under the soil ; the Beindeer subsists during a large part of the year upon a lichen that grows beneath the snow; and the Chamois finds a sufficient supply in the scanty vegetation of Alpine heights. Hot less is this the case among Birds ; but in the classes of Reptiles and Fishes, the number of vegetable-feeders, and consequently the variety of their food, is much less. 145. Among Insects, a very large proportion derive their food entirely from Plants, and many from particular tribes of plants only; so that, if from any cause these should fail, the race may for a time disappear. There is probably not a species of plant which does not furnish nutriment for one or more tribes of insects, either in their larva state or their per¬ fect condition ; and in this manner it is prevented from mul¬ tiplying to the exclusion of others. Thus, on the Oak no less than two hundred kinds of caterpillars have been estimated to feed ; and the Hettle, which scarcely any beast will touch, supports fifty different species of insects,— but for which check it would speedily annihilate all the plants in its neigh¬ bourhood. The habits and economy of the different races existing on the same plant, are as various as their structure. L 146 VORACITY OF INSECTS. Some feed only upon the outside of the leaves ; some upon the internal tissue ; others upon the flowers or on the fruit ; ' a few will eat nothing but the hark ; while many derive their nourishment only from the woody substance of the trunk. 146. The excessive multiplication of certain tribes of Insects has sometimes had the effect of devastating an entire country. Thus the “ plague of locusts ” is not unfrequently repeated in tropical countries, and is dreaded by the inhabi¬ tants even more than an earthquake. These insects are of such extreme voracity that no green thing escapes them; and when their numbers are so increased that they fly in masses which look like dark clouds, and cover the ground where they alight for miles together, it may be easily con¬ ceived that the devastation they create must produce incal¬ culable injury. The north of Africa and the west of Asia are the countries most infested by these pests. It is related by Augustin, that a plague, induced partly by the famine they had created, and partly by the stench occasioned by their dead bodies, carried off 800,000 inhabitants from the kingdom of hTumidia and the adjacent parts. They occasionally attack the south of Europe. It is recorded that Italy was devastated by them in the year 591 ; and that a prodigious number both of men and beasts perished from similar causes, — no less than 30,000 persons in the kingdom of Yenice alone. These tremendous swarms usually advance towards the sea ; and being there checked, and having completely exhausted the country behind them, they themselves die of famine, or are blown into the sea by a gale. In 1784 and 1797, they de¬ vastated Southern Africa; and it is stated by Mr. Barrow (in his Travels in that country) that they covered a surface of 2,000 square miles; that, when cast into the sea by a strong wind from the north-east, and washed upon the beach, they formed a line fifty miles long, and produced a barrier along the coast three or four feet high ; and that, when the wind again changed, the stench created by the putrefaction of their bodies was perceived at a distance of 150 miles inland. A similar event occurred in the Barbary States in 1799, and was followed, as in the other cases, by a plague. 147. We have occasionally an example of similar devasta¬ tion in our own country, though on a smaller scale. Thus, a few years ago, the turnip-crops of some parts of England VORACITY OF INSECTS. 147 were almost entirely destroyed by the larvae of an insect called the “ turnip -fly.” The parent insects were seen buzzing over the fields, and depositing their eggs in the plants, which they do not themselves employ as food ; and in a few days all the soft portions of the leaves were destroyed, and nothing but the skeletons and stalks were left. — Some kinds of timber occasionally suffer to no less an extent from the devastations of insects, whose operations are confined to the wood, and do not manifest themselves externally, until the tree is seen to languish and at last to die. The pine-forests of the Hartz mountains in Germany have been several times almost de¬ stroyed by the ravages of a single species of beetle, less than a quarter of an inch in length. The eggs are deposited beneath the bark; and the larvae, when hatched, devour the sap- wood and inner bark (the parts most concerned in the func¬ tions of vegetation) in their neighbourhood. It was estimated that, in the year 1783, a million and a half of pines were destroyed by this insect in the Hartz alone ; and other forests in Germany were suffering at the same time. The wonder is increased, when it is stated that as many as 80,000 larvse are sometimes found on a single tree. 148. But every class in the Animal Kingdom has its car¬ nivorous tribes, which are adapted to restrain the too rapid increase of the vegetable-feeders (by which a scarcity of their food would soon be created), or to remove from the earth the decomposing bodies that might otherwise be a source of dis¬ ease or annoyance. The herbivorous races, being for the most part very prolific, would very rapidly increase to such an extent as to produce an absolute famine, if not kept in check by the races appointed to limit their multiplication. Thus, the myriads of Insects which, find their subsistence in our forest-trees, if allowed to increase without restraint, would soon destroy the life that supports them, and must then all perish together ; but another tribe (that of the insectivorous Birds, as the woodpecker) is adapted to derive its subsistence from them, and thus to keep their numbers within salutary bounds. Their occasional multiplication to the enormous extent mentioned in the preceding paragraphs, is probably due in general to the absence of the races that should keep them in check. This may occur from accidental causes, or may be produced by the interference of Mam Thus, a set of. l 2 148 BALANCE AMONG DIFFERENT RACES. ignorant farmers have imagined that a neighbouring rookery was injurious to them, because they saw the rooks hovering over the newly-sown corn-fields, and seeming to pick the grains out of the ground ; and having extirpated the rookery, they have found in the course of a year or two that they have done themselves an immense injury, — the roots of their corn and grasses being devoured by the grubs of cockchafers and other insects, the multiplication of which was before prevented by the rooks, whose natural food they are. 149. On the other hand, by an intelligent application of this principle, the excessive multiplication of insects has been prevented where it had already commenced. Thus, no means of extirpating the larvae of the turnip-fly was found so suc¬ cessful, as turning into the fields a number of ducks, which quickly removed them from the plants. And in the island of Mauritius, the increase of locusts, which had been accidentally introduced there, and which were becoming quite a pest, was checked by the introduction from India of a species of bird, the grakle, which feeds upon them. 150. Of the carnivorous tribes themselves, however* the increase might be so great as to destroy all the sources of their food, were it not that they are kept in check by others, larger and more powerful than themselves, which, not being prolific, are not likely ever to gain too great a power. Thus, among birds, the eagles, falcons, and hawks rear only two or three young every year, whilst many of the smaller birds produce and bring up four or five times that number. — The following is a curious instance of the system of checks and counter¬ checks, by which the “balance of power” is maintained amongst the different races. A particular species of moth having the fir-cone assigned to it for the deposition of its eggs, the young caterpillars, coming out of the shell, consume the cone and superfluous seed ; but, lest the destruction should be too great, another insect of the ichneumon kind lays its eggs in the caterpillar, inserting its long tail in the openings of the cone until it touches the included insect, its own body being too large to enter. Thus it fixes upon the caterpillar its minute egg, which., when hatched, destroys it. 151. The peculiarity of the agency of Insects, in the economy of nature, has been justly remarked to consist in their power of very rapid multiplication, in order to accomplish a VARIATIONS IN POWER OF ABSTINENCE. 149 certain object, and then in their as rapidly dying off. In this re¬ spect they resemble the Fungi among plants. (Botany, § 7 89.) 152. There are great variations in the degree of power possessed by animals of different species to sustain abstinence from food, which appear to be related to their respective habits of life ; such as most easily obtain a constant supply of food being immediately dependent upon it, and vice versd. Thus, among the larvae of Insects, those that feed upon vege¬ tables or dead animal matter (in the neighbourhood of which their eggs are usually deposited by the parent) speedily die if placed out of reach of their aliment ; whilst those that lie in wait for living prey, the supply of which is uncertain, are able to endure a protracted abstinence, even to the extent of ten weeks, without injury. Again, carnivorous Birds and Mam¬ mals are generally able to exist for some time without food ; their natural habits leading them to glut themselves upon the carcase of the animal they have destroyed, in such a manner as to prevent them from requiring any new supply for some time : thus the wild cat has been kept twenty days without food, the dog has lived for thirty-six days in the same circum¬ stances, and the eagle for a similar period. But some herbi¬ vorous animals, such as the camel and the antelope, whose habits are such as to keep them out of the reach of food for several days together, are able to endure a similar abstinence ; whilst among the insectivorous Mammals, which naturally take food often, and but little at a time, the power of absti¬ nence is much less, — the mole, for instance, perishing in confinement, if not fed once a day, or even more frequently. 153. We have next to consider the different substances used as food, in regard to their chemical composition ; and to inquire for what purposes in the nutrition of the body they are respectively destined. The Vegetable tissues are chiefly made up of the three components, oxygen, hydrogen, and carbon ; the oxygen and hydrogen having the same proportions as in water. Their composition being thus nearly the same as’ that of starch, gum, and sugar (into which, indeed, they may for the most part be converted by a simple chemical process), alimentary substances of this kind form a natural group to which we may give the name of Saccharine (sugary). — But in many vegetable substances used as food, there is a considerable quantity of oily matter, stored up in cells ; and the same kind 150 ORGANIC CONSTITUENTS OF ANIMAL FOOD. of matter constitutes the principal part of the fat of animals. Of these oily and fatty matters, also, the chemical elements, oxygen, hydrogen, and carbon, are the only ingredients ; but they are combined in proportions different from the last, the two latter predominating considerably. Hence they consti¬ tute another group of alimentary materials, to which the term Oleaginous maybe given. — Lastly, most Vegetables con¬ tain, in greater or less amount, certain compounds which consist of the four elements, oxygen, hydrogen, carbon, and nitrogen, of which the animal tissues are composed. These compounds exist most largely in the corn-grains, and also in the seeds of the pea and bean tribe ; but there are few vege¬ table substances used as food by animals, that do not contain them in some small amount. The gluten of wheat, the legu- min of peas, and other vegetable substances of this kind, together with the flesh of animals, the composition of which (§ 13) is identical with theirs, are united into a third group, to which the name Albuminous is given. — We cannot pro¬ perly include in this group, however, the gelatinous portions of the animal tissues, which exist largely in gristle, bone, the skin, and other parts ; because gelatin (the substance that forms glue), though it agrees with albumen in being made up of the four ingredients just named, differs from it extremely in the proportions of those elements (§ 19) ; so that, although there is good reason to believe that gelatin may be formed out of albumen, it does not seem that any albuminous compound can be formed out of gelatin. Hence we must consider the gelatinous compounds separately. 154. Of these four groups, the last two are distinguished as azotized compounds, or substances that contain azote or nitro¬ gen ; whilst the first two are spoken of as non- azotized, being destitute of this element. The distinction is a very important one ; and must be kept steadily in view in considering the ulti¬ mate destination of each kind of food. It is obvious from what has'been already stated as to the composition of the animal tis¬ sues (§§ 1 3 — 21), that azotized compounds must supply the chief materials for their nutrition and re-formation. The non-azotized substances must be for the most part destined, unless converted into azotized compounds within the living body, either to be simply deposited in its interstices, or to be thrown off from it again without ever actually forming part of its organised structure. DESTINATION OF NON-AZOTIZED ALIMENTS. 151 155. JSTow, in regard to tire non-azotized, or the saccharine and oleaginous groups of alimentary substances, it appears to be an established fact, that none of the higher animals can be permanently supported upon them alone. Thus, dogs that have been fed on sugar and starch only, do not survive long ; and it is evident, before their death, that their tissues are gradually undergoing decay. It has been thought that such results might be partly explained upon the fact, that animals fed upon one simple substance soon become disgusted with it, and will even refuse it altogether ; but the experiments have been repeated with a combination of various non-azotized sub¬ stances, and the same result has occurred. Still it is too much to affirm, as some have done, that these substances do not con¬ tribute in any degree to the nutrition of the animal tissues; since there is ample evidence that the presence of fatty matter in the blood is a condition essential to the production of newly forming tissue ; and we find that either oleaginous substances, or substances belonging to the saccharine group which can be readily converted into fat within the body, constitute an im¬ portant part of the food of Man, and of animals generally.1 156. That such a conversion can take place, has been de¬ monstrated by experiments carefully conducted upon bees, which have been found to generate wax when fed upon sugar only ; and also upon cows, which give off in their milk so much larger a quantity of butter than can be produced at the expense of the fat contained in their food, that there is no other mode of accounting for its presence, than by regarding it as generated from the starchy portion of their diet. And the fattening power of starchy and saccharine articles of diet is well known to breeders of cattle ; though the articles which contain oily matter in addition seem to possess a higher value in this respect. 157. But if these non-azotized compounds, which exist so largely in the food of herbivorous animals, are not destined to form any other permanent part of the animal organism than the oleaginous contents of the fat-cells (§ 46), the ques¬ tion again arises, — what becomes of them ? It is not enough 1 The value of cod-liver oil, which is now so extensively used in the treatment of diseases of imperfect nutrition, seems to depend upon the readiness with which it can be digested and assimilated, so as to furnish the supply of fat required by the formative processes. 152 DESTINATION OF NON-AZOTIZED ALIMENTS. to say that they are deposited as fat ; since it is only when a large quantity of them is taken in, that there is any in¬ crease in the quantity of fat already in the body. We shall hereafter see that they are used up in the process of respira¬ tion, one great object of which is, to produce a certain amount of heat, sufficient to keep up the temperature of the body, in warm-blooded animals, to a high standard. We might almost say with truth, that a great part of the oleaginous and sac¬ charine principles is burned within the body, for this pur¬ pose. The process will be hereafter considered more in detail (§§ 412, 413) ; and at present we need only stop to remark upon the adaptation between the food provided for animals in different climates, and the amount of heat which it is necessary for them to produce. Thus the bears, and seals, and whales, from which the Esquimaux and the Green¬ lander derive their support, have an enormous quantity of fat in their massive bodies : this fat is as much esteemed as an article of food amongst these people, as it would be thought repulsive by the inhabitants of southern climates ; and by the large quantity of it they consume, they are able to support the bitterness of an Arctic winter, without appearing to suffer more from the extreme cold than do the residents in more temperate climes during their winter. On the other hand, the antelopes, deer, and wild cattle, which form a large pro¬ portion of the animal food of savage or half-cultivated nations inhabiting tropical regions, possess very little fat ; and the comparatively small supply of carbon and hydrogen, of which the combustion is required to keep up the bodily temperature of the inhabitants of those regions, is derived from the flesh of these animals, in the manner that will be presently explained. 158. The application of the substances forming the albu¬ minous group, to the support of the animal body, by affording the materials for the nutrition and re-formation of its tissues, needs little explanation. The proportions of the four ingre¬ dients of which they are all composed, are so nearly the same, that no essential difference appears to exist among them ; and it is a matter of little consequence, except as far as the gra¬ tification of the palate is concerned, whether we feed upon the flesh of animals (syntonin, § 16), upon the white of egg (albumen, § 13), the curd of milk (casein, § 15), the grain of wheat (gluten), or the seed of the pea (legumin). All these DESTINATION OF NON-AZOTIZED ALIMENTS. 153 substances are reduced in the stomach to the form of albumen ; which is the raw material out of which the various fabrics of the body are constructed. But the rule holds good with re¬ gard to these also, that by being made to feed constantly on the same substance, — boiled white of egg for instance, or meat deprived of the principle that gives it flavour, — an animal may be effectually starved ; its disgust at the food being such, that even if it be swallowed it is not digested. It is very interest¬ ing to remark that, in the only instance in which Mature has provided a single article of food for the support of the animal body, she has mingled articles from all the three preceding groups. This is the case in Milk ; wdiich contains a consider¬ able quantity of the albuminous substance, casein , that forms its curd ; a good deal of oily matter, the butter ; and no in¬ considerable amount of sugar , which is dissolved in the whey. The proportions of these vary in different Mammalia, being related as it would seem to the habits of the young animal thus sustained, while they depend in part upon the nature of the food supplied to the animal that forms the milk ; but the three substances are thus combined in every instance. 159. But although the greater part of the organised tis¬ sues of animals have a composition nearly allied to that of albumen, many of them also contain a large quantity of gelatin (§ 19). It seems certain that this gelatin may be pro¬ duced out of albuminous substances ; since in animals that are supported on these alone, the nutrition of the gelatinous tissues does not seem to be impaired. But it appears equally certain, that gelatin cannot be applied to the nutrition of the albuminous tissues. Many series of experiments have been made on this subject, with a view of determining how far gelatin-soup made from crushed bones (such as that which long constituted a principal article of diet in the hospitals of Paris) is adequate for the support of the body in health. The result of these has been uniformly the same, — namely, •that although gelatin may be advantageously mixed with albumen, fibrin, gluten, &c., and those other ingredients which exist in meat-soup and bread, yet that, when taken alone, it has little (if any) more power of sustaining life, than sugar or starch possesses. Although it might have been thought likely that gelatin employed as food might be applied within the body to the nutrition of its gelatinous tissues, yet there 154 SOURCES OF DEMAND FOR ALIMENT. is strong reason to believe that these, like the albuminous, are formed at the expense of the albuminous matter of the blood, and that gelatin thus introduced undergoes a rapid decomposition, yielding up a considerable part of its carbon and hydrogen to the combustive process, which is the only function to which it affords any substantial 'pabulum. Con¬ sequently the current idea regarding the nutritive value of jellies of various kinds, has little or no real foundation. 160. It has been already stated (§68) that all the living tissues of the body are continually undergoing a sort of death and decay ; and that they do this the more rapidly, in pro¬ portion as they are called upon for the discharge of their functions. The need of material capable of replacing that which has been lost, is consequently the chief source of the constant demand for aliment. Even in young, actively growing animals, the quantity required for the increase of their bodies constitutes but a very small proportion of that which is taken in ; of the remainder, a part is at once re¬ jected as indigestible ; and the rest is appropriated to the repair of the waste which is continually going on. This waste is much greater in young animals than in adults ; for all their vital processes are more actively and energetically performed : their movements are quicker in proportion to their size ; and injuries are more speedily repaired. To remove the products of this decomposition is the special object of the various pro¬ cesses of excretion ; and among these, the respiration , by which a large quantity of carbon and hydrogen is carried off in the form of carbonic acid and water, is of the most constant importance, on account of the heat which it thus enables the animal body to maintain. This temperature, in Carnivorous animals, appears to be sufficiently kept up by the combustion of the carbon and hydrogen set free by the decay (or metamorphosis, as it may be termed) of their tis¬ sues ; but this combustion goes on with much more rapidity, in consequence of their almost unceasing activity, than it does in the Herbivorous animals, which lead comparatively inac¬ tive lives. Every one who has visited a menagerie must have noticed the continual restlessness of the Tigers, Leopards, Hyenas, &c., which keep pacing from one end of their narrow cages to the other ; and it would seem as if this restlessness were a natural instinct, impelling them to use muscular exer- NUTRITION OF CARNIVOROUS ANIMALS. 1 55 tion sufficient for the metamorphosis of an adequate amount of tissue, that enough carbon and hydrogen may be set free for the support of the respiratory process. And we see a cor¬ responding activity in the Human hunters of the swift-footed antelope and agile deer, which answers a similar purpose ; and which is remarkably contrasted with the stupid inertness of the inhabitants of the frigid zone, that is only occasionally interrupted by the necessity of securing the supplies of food afforded by the massive tenants of their seas. 161. The nutrition of the Carnivorous races may, then, be thus described. The bodies of the animals upon which they feed, contain flesh, fat, &c., in nearly the same proportion as their own ; and all, or nearly all, the aliment they consume, goes to supply the waste in the fabric of their own bodies, being converted into its various forms of tissue. After having remained in this condition for a certain time, varying ac¬ cording to the use that is made of them, these tissues un¬ dergo another metamorphosis, which ends in restoring them to the condition of inorganic matter ; and thus give back to the mineral world the materials which were drawn from it by plants. Of these materials, part are burned off, as it were, within the body, by union with the oxygen of the air taken in through the lungs, from which organs they are discharged in the form of carbonic acid and water : the remainder are carried off in the liquid form by other channels. Hence we may briefly express the destination of their food in the following manner : — Carbonic acid and water, thrown off by respiration. Urea and biliary matter, &c., thrown off by other excretions. 162. But in regard to the Herbivorous animals, the case is different. They perspire much more abundantly, and their temperature is thus continually kept down (§ 372). They consequently require a more active combustion, to de¬ velop sufficient bodily heat ; and the materials for this are supplied, as we have seen, by the non-azotized constituents of their food, rather than by the metamorphosis of their own tissues, which takes place with much less rapidity than in the carnivorous tribes. Hence we may thus express the Food consisting 'v of albuminous I and other com- { pounds J converted into r Living } < organised \ ( tissue, j and this metamorphosed into 1 56 NUTRITION OF HERBIVORA AND OF MAN. destination of this part of their food ; that of the albuminous matters, here much smaller in amount, being the same as in the preceding case : — Starch, oil, and 'j partly C Fatty and 'J but chiefly ( Carbonic acid and water, other non -azo- > converted -j other animal > thrown off < disengaged by the respi- tized compounds j into t tissues, j directly as (. ratory process. The proportion of the food deposited as fat, will depend in part upon the surplus which remains, after the necessary sup¬ ply of materials has been afforded to the respiratory process. Hence, the same quantity of food being taken, the quantity of fat will be increased by causes that check the perspiration, and otherwise prevent the temperature of the body from being lowered, so that there is need of less combustion within the body to keep up its heat. This is consistent with the teach¬ ings of experience respecting the fattening of cattle ; for it is well known that this may be accomplished much sooner, if the animals are shut up in a warm dwelling and are covered with cloths, than if they are freely exposed in the open air. 163. Now the condition of Man may be regarded as inter¬ mediate between these two extremes. The construction of his digestive apparatus, as well as his own instinctive pro¬ pensities, point to a mixed diet as that which is best suited to his wants. It does not appear that a diet composed of ordinary vegetables only, is favourable to the full develop¬ ment of either his bodily or his mental powers ; but this cannot be said in regard to a diet of which the corn-grains furnish the chief ingredient, since the gluten they contain appears to be as well* adapted for the nutrition of the animal tissues, as is the flesh of animals. On the other hand, a diet composed of animal flesh alone is the least economical that can be conceived ; for, since the greatest demand for food is created in him (taking a man of average habits in regard to activity and to the climate under which he lives) by the ne¬ cessity for a supply of carbon and hydrogen to support his respiration, this want may be most advantageously fulfilled by the employment of a certain quantity of non-azotized food, in which these ingredients predominate. Thus it has been calculated that, since fifteen pounds of flesh contain no more carbon than four pounds of starch, a savage with one animal and an equal weight of starch, could support life for the same length of time during which another restricted to animal COMPOSITION OF ARTICLES OF HUMAN FOOD. 157 food would require five such, animals, in order to procure the carbon necessary for respiration. Hence we see the immense advantage as to economy of food, which a fixed agricultural population possess over the wandering tribes of hunters which still people a large part both of the Old and Hew Continents. 164. The following Table exhibits the proportions of albu¬ minous, starchy or saccharine, fatty, and saline substances, contained in various articles ordinarily used as food by Man ; together with the proportion which water bears in each case to the solid constituents of the food, which becomes a most important element of consideration when the nutritive value of different kinds of food is compared : — Substances, 100 parts. j Water. Albumi¬ nous sub¬ stances. Starch, Sugar, &c. Is Pm Salts. Carboni¬ ferous.* Nitroge¬ nous. Total nutri¬ ment. Human Milk . 89 3.5 4.2 3.0 0.2 11.4 3.5 14.9 Cow’s Milk . . 86 4.5 5.0 4.1 0.7 14.8 4.5 19.3 Skimmed Milk . 87 4.5 5.0 2.7 0.7 11.5 4.5 16.0 Butter Milk . 87 4.5 5.0 0.5 0.7 6.0 4.5 10.5 Beef and Mutton . 73 19.0 5.0 2.0 12.0 19.0 31.0 Veal . 77 19 0 1.0 0.6 2.4 19.0 21.4 Poultry . 74 21.0 3.0 1.2 7.2 21.0 28 2 Bacon . 20 0.8 7 0.0 1.3 168.0 0.8 168.8 Cheese (Cheddar) . 36 29.0 30.0 4.5 72.0 29.0 101.0 „ (Skimmed) . 44 45.0 6.0 5.0 14.4 45.0 59.4 Butter . 15 83.0 2.0 199.0 ... 199.0 Eggs . 74 14.0 10.5 1.5 25.0 14.0 39.0 White of Egg . 78 20.0 1.6 20.0 20.0 Yolk of Egg . . 52 16.0 30.0 1.3 72.0 16.0 88.0 White Fish . * . 79 19.0 1.0 1.2 2.4 19.0 21.4 Salmon . 78 17.0 4.0 1.4 9.6 17.0 26.6 Eel . 80 10.0 8.0 1.3 19.2 10.0 29.2 Wheat Flour . 15 11.0 70.0 2.0 1.7 74.8 11.0 85 8 Barley-meal . 15 10.0 70.0 2.4 2.0 75.8 10.0 85.8 Oat-meal . 15 12.0 62.0 6.0 3.0 76.4 12.0 88.4 Rye-meal . 15 9.0 66.0 2.0 1.8 70.8 9.0 79.8 Indian-meal . 14 9.0 65.0 8.0 1.7 84.2 9.0 93.2 Rice . 14 7.0 76.0 0.3 0.3 76.7 7.0 83.7 Haricots . 19 23.0 45.0 3.0 3.6 52.2 23.0 75.2 Peas . . 13 22.0 58.0 2.0 3.0 62.8 22.0 84.8 Beans . 14 24.0 44.0 1.4 3.6 47.4 24.0 71.4 Lentils . 14 29.0 44.0 1.5 2 3 47.6 29.0 76.6 Wheat-bread . 44 9.0 49.0 1.0 2.3 51.4 9.0 60.4 Rye-bread . 48 5.3 46.0 1.0 1.4 48.4 5.3 53.7 Potatoes . 74 20 23.0 0.2 0.7 23.5 2.0 25.5 Green Vegetables . 86 2.0 4.0 0.5 0.7 5.0 2.0 7.0 Arrow-root . 18 82.0 82.0 82.0 * The value of the Fat is stated in this column according to its heating equivalent of starch, which is larger in the ratio of 2*4 to 1. Hence, in the last column, the proportion of nutriment in aliments containing fat, comes to be greater than the weight of their solids would indicate. ECONOMY OF HUMAN DIET. 158 Those articles of food in which the nitrogenous compounds predominate, are especially fitted for the maintenance of the solid fabric of the body ; whilst those in which the carbon¬ aceous compounds are in largest excess, are those which are most effective as supplying materials for the combustive pro¬ cess. Conspicuous among the former are the various kinds of animal flesh, as also the white of eggs ; whilst among the latter the most noticeable are bacon and butter, rice and potatoes, the former consisting almost wholly of fat, the latter being chiefly composed of starch. Of all single articles of food, good wheaten bread, in which the proportion of nitro¬ genous to carbonaceous components is about as 5.7 to 1, seems to be the one best suited to the ordinary wants of Man ; but this acquires much additional value from the con¬ current use of a moderate amount of fatty matter in the form of butter. 165. If the more highly azotized forms of food be em¬ ployed exclusively, a great excess of them must be consumed to supply the carbon needed for respiration ; whilst if the more carbonaceous kinds of food be used as the sole susten¬ ance, unless the quantity ingested be large enough to afford the requisite supply of azotized material for the maintenance of the tissues, their nutrition must be imperfectly effected, and the strength must fail. Hot only in the instance just cited, but in a variety of others, the instincts of mankind have led to such a combination of different articles of diet, as includes in their appropriate proportions the albuminous, the saccharine, and the oleaginous principles. Thus with meat we eat potatoes ; and with the white meats which are deficient in fat, we eat bacon. We use melted butter with most kinds of fish, or fry them in oil ; whilst the herring, the salmon, and the eel, are usually fat enough in themselves, and are dressed and eaten alone. A similar adjustment is made when we mix eggs and butter with sago, tapioca, and rice ; when we add oil and the yolk of an egg to salad ; when we boil rice with milk, and combine cheese with maccaroni. Bacon and greens, and pork and pease-pudding, again, are combinations founded in taste, which approve themselves to the judgment ; as is also the Irish dish termed kolcannon, con¬ sisting of potatoes and cabbage, with a little bacon or fat pork. So are the mixture so common in Ireland and Alsace, of butter- ECONOMY OF HUMAN DIET. 159 milk or curdled-milk with potatoes ; and the combination of rice and fat, which is the staple of the diet of many Eastern nations. Even the morsel of butter or the bit of cheese which the English labourer eats with his hard-earned bread, are not matters of luxury, but have a positive importance ; and the existence of these tastes and habits shows how by long experience Man has at last learned to adjust the com¬ position of his food, so as best to maintain the health and vigour of his body. With a difference of requirement comes a difference of tastes. Thus men who are going through a very laborious course of exertion, prefer meat to bread or vegetables, feeling it to be more sustaining to their strength. On the other hand, those who are continuously exposed to the severity of an Arctic winter, eat with relish large masses of fat, on which they would look with disgust under other circumstances. The quantity of work which a man can do, and his power of sustaining extreme cold, both depend in great part, as has now been abundantly proved, upon the adequacy of the sustenance he takes : the demand, in the first case, being for albuminous material to supply the waste of his tissues ; whilst in the second it is for combustive material suitable to generate heat in large measure, — a purpose which is far more efficiently answered by oleaginous substances, than by those of a starchy or saccharine nature. Experience fur¬ ther shows that the healthy condition of the blood of Man can only be maintained by the use of fresh vegetables as part of his ordinary diet. When these are withdrawn for any length of time, the disease known as Scurvy is certain to appear, unless lemon -juice or some other efficacious anti¬ scorbutic be employed as a substitute. This is a fact of the utmost importance in provisioning ships for long voyages ; the tendency to scurvy being increased by confinement and insufficient ventilation, and by the exclusive use of salt provisions. 166. Besides these organic substances, there are certain Mineral ingredients, which may be said to constitute a part of the food of Animals ; being necessary to their support, in the same manner as other mineral substances are necessary to the support of Plants. Of this kind are common salt, and also phosphorus, sulphur, lime, and iron, either in combina¬ tion or separate. — The uses of Salt are very numerous and 160 MINERAL INGREDIENTS OF FOOD. important. It exists largely in the blood, and in the various animal fluids which are secreted from it ; and it is also an essential ingredient of most of the solid tissues. Its presence obviously tends to prevent that spontaneous decomposition to which organic substances are liable. Phosphorus is chiefly required to be united with fatty matter, to serve as the material of the nervous tissue ; and to be combined with oxygen and lime, to form the bone-earth by which the bones are consolidated. Sulphur exists in small quantities in several animal tissues ; but its part seems by no means so important as that performed by phosphorus. Lime is required for the consolidation of the bones, and for the production of the shells and other hard parts that form the skeletons of the Invertebrata. Of the limestone rocks of which a great part of the erust of our globe is composed, a very large proportion is made up of the remains of animals that formerly existed in the ocean. Thus some almost entirely consist of masses of Coral, others of beds of Shells, and others of the coverings of minute Toraminifera (§131). To these mineral ingredients wre may also add Iron, which is a very important element in the red blood of Yertebrated animals. 167. These substances are contained, more or less abun¬ dantly, in most articles generally used as food ; and where they are deficient, the animal suffers in consequence, if they be not supplied in any other way. Common Salt exists, in no inconsiderable quantity, in the flesh and fluids of animals, in milk, and in the egg : it is not so abundant, however, in plants ; and the deficiency is usually supplied to herbi¬ vorous animals by some other means. Thus salt is purposely mingled with the food of domesticated animals ; and in most parts of the world inhabited by wild cattle, there are spots where it exists in the soil, and to which they resort to obtain it ; such are the “ buffalo-licks” of North America. Phos¬ phorus exists also in the yolk and white of the egg, and in milk, — the substances on which the young animal subsists during the period of its most rapid growth ; it abounds not only in many animal substances used as food, but also (in the state of phosphate of lime or bone- earth) in the seeds of many plants, especially the grasses ; and in smaller quantities it is found in the ashes of almost every plant. When flesh, bread, fruit, and husks of grain, are used as the chief articles MINERAL INGREDIENTS OF ANIMAL FOOD. 161 of food, more phosphorus is taken into the body than it requires ; and the excess has to be carried out in the excre¬ tions. Sulphur is derived alike from vegetable and animal substances. It exists in flesh, eggs, and milk ; also in the azotized compounds of plants ; and (in the form of sulphate of lime) in most of the river and spring water that we drink. Iron is found in the yolk of egg, and in milk, as well as in animal flesh ; it also exists, in small quantities, in most vegetable substances used as food by man, — such as potatoes, cabbage, peas, cucumbers, mustard, &c. ; and probably in most articles from which other animals derive their support. 168. Lime is one of the most universally diffused of all mineral bodies ; there being very few animal or vegetable substances in which it does not exist. It is most commonly taken in, among the higher animals, combined with phos¬ phoric acid, so as to form bone-earth, in which state it exists largely in the seeds of most grasses. A considerable quantity of lime exists, moreover, in the state of carbonate and sul¬ phate, in all hard water. 169. When an unusual demand exists for lime, however, for a particular purpose, an increased supply must be afforded. Thus a hen preparing to lay, is impelled by her instinct to eat chalk, mortar, or some other substance containing the car¬ bonate of lime which is required for the consolidation of the shell ; and if this be withheld, the egg is soft, its covering being composed of animal matter alone, not consolidated by the deposit of earthy particles. The thickness of the shells of aquatic Mollusks depends greatly upon the quantity of lime in the surrounding water. Those which inhabit the sea, find in its waters as much as they require ; but those that dwell in fresh- water lakes, which contain but a small quan¬ tity of lime, form very thin shells ; whilst, on the other hand, those that inhabit lakes in which, from peculiar local causes, the water is loaded with calcareous matter, form shells of remarkable thickness. 170. The mode in which the Crustacea, whose calcareous shell is periodically thrown off (§ 99), are able to renew it with rapidity, is very curious. There is laid up in the walls of their stomachs a considerable supply of calcareous matter, in little concretions, which are commonly known as “ crabs’ eyes.” When the shell is cast, this matter is taken up by M 162 DIGESTION AND ABSOEPTION. the blood, and is thrown out from the surface, mingled with animal matter. This hardens in a day or two, and the new covering is complete. The concretions in the stomach are then found to have disappeared ; but they are gradually replaced, before the supply of lime they contain is again required. CHAPTEB IV DIGESTION AND ABSORPTION. 171. Having now considered the nature of the food of Animals, and the sources from which it is obtained, we have next to consider the process by which the aliment is received into their bodies, and prepared to form a part of their own fabric. This process, termed Digestion , is naturally divided, among the higher animals at least, into various stages. In the first place, there is the prehension or laying hold of the aliment, and its introduction into the mouth or entrance to the digestive cavity. In the mouth it usually undergoes a preparation ; which consists partly in its being cut, ground, or crushed, by mechanical action, into minute pieces ; and partly in the working-up of these pieces with a fluid that is poured into the mouth, — the saliva. These two processes are termed mastication and insalivation ; similar processes are performed, in some animals, in a part of the digestive tube intermediate between the mouth and the stomach, and even in the latter itself. The stomach is usually situated at some distance from the mouth, and is connected with a tube called the oesophagus or gullet ; and the passage of the food into this, constituting the act of swallowing, is termed deglutition . The food, having arrived in the stomach, is acted-upon by a peculiar fluid which it contains, and much of its alimentary portion is dissolved, so that a pulpy mass is formed which is termed chyme ; hence this process, which is the first stage of digestion properly so called, is termed chymification or the manufacture of chyme. The chyme, which passes into the intestines, is further acted- on by secretions that are poured into them ; and a certain nutritive combination of albuminous PREHENSION OF FOOD. 163 and fatty matters, termed chyle , is separated from the matters that are to be thrown off : this process, which is the second stage of true digestion, is termed chylification. The rejected portions of the food, with secretions poured into the alimen¬ tary canal, find their way out through the intestinal tube ; and are voided at its terminal orifice by the act of defecation. And lastly, the nutritive materials are taken up by absorption into vessels that are distributed upon the walls of the diges¬ tive cavity, and undergo a gradual change, by which they are converted into blood. These two processes are called absorp¬ tion and sanguification (or manufacture of blood). Each of the foregoing stages will now be separately considered. Prehension of Food. 172. The introduction of aliment within the entrance to the digestive cavity is accomplished in various methods in dif¬ ferent animals. In the Mammalia in general, the aperture of whose mouth is guarded by fleshy lips, these, with the jaws and teeth, are the chief instruments of this operation. But in Man and the Monkey tribe the division of labour is carried further ; the food being laid hold of by the anterior members, or hands, and by them carried to the mouth. Where the hand has the power of grasping, and especially where the thumb can be opposed to the fingers, the action of a single member is sufficient ; but there are several animals which, like the Squirrel, use both limbs conjointly to hold their food, the extremity not having itself the power of grasp¬ ing. The Ant-eaters, Woodpeckers, Chameleons, and other insect-eating animals, obtain their food by means of a long extensible tongue ; this either serving to transfix the insect, or being covered with a viscid saliva which glues it to the surface. The Giraffe uses its long tongue to lay hold of the young shoots on which it browses ; and the Elephant employs its trunk, which is nothing else than a prolonged nose, for every kind of prehension (fig. 82). Many of the Invertebrata are furnished with little appendages round their mouths by which the food is conveyed into them ; such are the palpi m 2 Fig. 82 Head of Elephant. 164 RECEPTION OF SOLID AND LIQUID ALIMENT. of Insects, of which a pair is attached to each jaw (fig. 84); the tentacula of Mollusks, which are sometimes extremely prolonged, as in the Cuttle-fish tribe (fig. 85) ; and the similar organs of the polypes (fig. 71). 173. The reception of liquids is accomplished in two ways. Sometimes the liquid is made to fall into the mouth, simply by its own weight (fig. 86) ; in other instances it is drawn or pumped up into this cavity, — either by the expansion of the chest, which causes a rush of air towards the lungs, — or by the movement of the tongue, which, being drawn back like a piston, produces the action of sucking. Some of the lower animals are destined to be entirely supported by liquids which they find in plants, or which they draw from the bodies of other animals whereon they live as parasites. This is the case with many Insects ; and their mouth, instead of present¬ ing the ordinary structure, is formed into a sort of tube or trunk, very much extended, through which the juices are drawn up according to the wants of the animal. Such a conformation exists in the butterfly and moth tribe, whose SUCTION OF LIQUIDS. 165 trunk, when not in use, is coiled up in a spiral beneath the head ; as is shown in fig. 87, representing the head of a Fig. 86. — Chimpanzee drinking. Butterfly, a, of which the eye is seen at c, the base of the antennae at b , the palpi at e, and the trunk at d. In some of the Fly tribe, the trunk attains a length several times greater than that of the body, as shown in fig. 88, representing a dipterous (two-winged) insect from the Cape of Good Hope, which sucks the juices of a single kind of flower, the length of whose tube just equals that of its long proboscis. 166 DEVELOPMENT OF TEETH. Mastication. 174. The act of Mastication , or the mechanical division of the alimentary matter, is effected in most of the higher animals, by the Teeth ; which are implanted in the jaws, and are so fixed as to act against one another, with a cutting, crushing, or « grinding power, according to the nature of the food on which they have to operate. The manner in which they are formed is worthy of note. In Man, who may be taken as a fair example, each tooth is developed in _ _ the interior of a little mem- Fig. 89. — Development op Teeth. , , . , . , a, the gum ; b, the lower jaw ; c, angle of the branOUS Sac, Which IS lodged jaw ; d, dental capsules. in tlie thickness of the jaw¬ bone ; as seen in the accompanying figure, which represents half the lower jaw of a very young infant, from which the outside has been removed. This sac, which is named the dental capsule (a, fig. 90), is composed of two membranes, abundantly furnished with blood-vessels ; and it encloses in its interior a little bud-like protuberance, b, in which ramify a great number of nervous filaments and minute vessels, c. The matter composing this little body, which is termed the pulp, is gradually converted into the dentine (§ 54) of the tooth, d d which in Man constitutes nearly its whole v ^ structure ; this conversion takes place first at its highest points, d, d. The crown or upper portion of the tooth receives a b — covering of enamel (§ 54). Gradually the process of conversion extends more and more to the interior of the pulp ; and at last the whole is changed into dentine, with the exception of a small portion that still remains, occupying what is termed the cavity of the tooth, which is frequently laid open by decay of its external wall. The fang of the tooth, which is the part last formed, receives an envelope of cementum (§ 54), which invests it up to the part at which the enamel begins. As the Fig. 90. — Dental Capsule. DEVELOPMENT OF TEETH. 167 root of the tooth is developed, the crown is gradually pushed upwards, so as to press against the upper portion of the capsule and the gum by which this is covered. These parts yield slowly to the pressure ; and the tooth makes its way to the surface ; or, in common language, is cut. 175. The process of “cutting teeth” is usually not a severe one in the healthy and well-managed infant ; but it occasions the death of vast numbers of children who are injudiciously treated ; and it is especially fatal to those who have a ten¬ dency to disease of the nervous system. The irritation caused by the pressure of the tooth against the gum, is liable to excite, in such cases, convulsive actions of various kinds, on the principles hereafter to be explained (§ 473); and, as the removal of the source of irritation is of the most urgent importance, the lancing of the gums, — doing that in an instant which the pressure of the tooth might not accomplish for days, — is a measure of most obvious utility ; however unnecessary it may seem, in ordinary cases, to in¬ terfere with the course of nature. But it is of the utmost importance at the same time to bring the nervous system into a less excitable condition ; and no measure is commonly more efficacious in this respect, than removal into a fresh and pure atmosphere. 176. At the same time that the development of the tooth is thus taking place, the bone of the jaw is becoming hardened, and closes round its root, forming a complete socket. This partly interrupts the passage of vessels and nerves to the tooth, which, when once fully formed, seems to acquire no further growth, and to possess but little power of repairing injuries occasioned by disease or accident. Hence a tooth which is broken or decayed, is not restored as a bone would be. Still, however, its root or fang is penetrated by a small nerve and artery, which are distributed to the membrane that lines the cavity ; and it is to the action of air upon the former, when the cavity is laid open by decay, that the pain of tooth-ache is chiefly due. The remedies which are most effectual in removing this pain, such as kreosote, nitric acid, or a heated wire, are those which destroy the vital power of the nerve. 177. But there are teeth, in many animals, which never cease to grow, and in which the central cavity is always filled 168 TEETH OF RODENTS. - MOTION OF JAWS. with pulp. Such have no proper root ; for additional matter is being continually formed at their base, and thus the whole tooth is pushed upwards. This is the case with the Elephant’s tusks ; and also with the large teeth that occupy the front of the jaw in Eabbits, Squirrels, Eats, and other gnawing ani¬ mals (fig. 91). The upper edges of these teeth are being con¬ stantly worn away by use : and they are kept up to their proper level by the growth of the tooth from below. But it Fig. 91.— Jaw and Teeth of Rabbit. sometimes happens that one of these teeth is broken off ; and the one opposite to it in the other jaw is then thrown into dis¬ use. It continues, however, to grow up from below ; but, not being worn down at the top, its length increases greatly, so that it may become a source of great inconvenience to the animal. 178. The teeth are but passive instruments in the act of mastication. They are put in movement by the jaws in which they are fixed ; and these are made to act against each other by various muscles. The upper jaw is usually fixed to the head ; and has not, therefore, any power of moving inde¬ pendently of it. But the lower jaw is connected with the skull by a regular joint on either side ; and is so moved by the muscles attached to it, as to cut, crush, or grind the food, according to the nature of the teeth. 179. There is considerable variety, in different animals, as to the extent of motion which the lower jaw possesses. In the purely Carnivorous quadrupeds, it has merely a hinge-like action, that of opening and shutting ; and by the sharpness of the edges of the molar teeth, it is thus rendered a powerful cutting instrument. But in the Herbivorous animals, which have to grind or triturate their food between the roughened surfaces of their molars, such a limited motion would be of no avail ; and we accordingly notice, if we watch an ox or a horse whilst masticating its food, that the lower jaw has con¬ siderable power of motion from side to side. On the other hand, in the Eodents, or gnawing animals furnished with MOVEMENTS OF LOWER JAW. 169 two large front teeth, the lower jaw has no power of moving from side to side, but is rapidly drawn backwards and for¬ wards ; and, as the ridges of the molar teeth are arranged in the opposite direction, they become very powerful filing in¬ struments, by -which the toughest vegetable substances are quickly reduced. 180. In the Human jaw, there is a moderate power of motion in all these different directions ; and it is furnished with all the muscles by which they are effected in the different animals that perform them ; but these are not so large or strong. The most powerful of the muscles of the lower jaw, in all animals, is that by which it is drawn up against the upper, so as to close the mouth. This arises from the side of the skull in the region of the temple, and is hence called the temporal muscle. It covers at its origin a large surface of bone ; but its fibres approach one another as they descend, and pass under a bony arch (which may be felt between the cheek and the ear), to attach themselves to a process or projection of the lower jaw (a, fig. 92), about an inch in front of the joint. As the distance from the ful¬ crum of the point a, at which the power is applied, is thus much less than that of the front of the jaw b, where chiefly the resistance is encountered, the powrer of the muscle is applied at a mechanical disadvantage ; and, to overcome a given resist¬ ance, the muscle must itself be several times more powerful. Thus the Tiger and Lion, which can lift and carry away the bodies of animals weighing several hundred pounds, must possess temporal muscles that shall contract with a force of two thousand, or even more. 181. In Man, as in most of the other Mammalia, there are three kinds of teeth, adapted for different purposes. The first terminate in a thin cutting edge, and are intended simply to divide the food introduced into the mouth ; these are termed incisor teeth (fig. 93). Others have more of a conical form,, Fig. 92. — Human Skull. 170 DIFFERENT KINDS OF TEETH. and in many animals (especially those of carnivorous habits) project far beyond the former ; they are adapted not to cut the food, but, by being deeply fixed in it, to enable the animal to tear it asunder : these are termed canine teeth. The teeth of the third kind have large irregular flattened surfaces, and are adapted to bruise and grind the food ; these are called molar (or mill-like) teeth. The manner in which these different teeth are implanted in the jaw, varies with the form of their crowns, and is in accordance with their several uses. The incisors, whose action tends as much to bury them in their sockets as to draw them forth, have but a single root or fang of no great length. The canine teeth, on which there is often considerable strain, penetrate the jaw more deeply than the incisors ; especially when they are large and Molars. Bicuspid. Canine. Incisors. Fig. 93. — Human Teeth. long, as in the Cat tribe (fig. 94). And the molars, whose action requires great firmness, have two, three, or even four roots or fangs, which spread out from each other ; and these at the same time increase the solidity of their attachment to the jaw, and prevent the teeth from being forced into their sockets by any amount of pressure. 182. The arrangement of the dental apparatus varies, in different Mammalia, according to the nature of the aliment on which they are destined to feed ; and this correspondence is so exact, that the anatomist can generally determine by the simple inspection of the teeth of an animal, not only the nature of its food, but the general structure of the body, and even its ordinary habits. Thus, in those that feed exclusively on animal flesh, the molar teeth are so compressed as to form DIFFERENT KINDS OF TEETH. 171 cutting edges, which work against each other like the blades of a pair of scissors (fig. 94) ; whilst in animals that live on insects, these teeth are raised into conical points, which lock Fig. 94. — Teeth of Carnivorous Animai. Teeth of Insectivorous Animal. into corresponding depressions in the teeth of the opposite jaw (fig. 95). When the nourishment of the animal con¬ sists principally of soft fruits, these teeth are simply raised into rounded elevations (figs. 97, 98) ; and when they are Fig. 96. Teeth of Herbivorous Animal. Fig. 97. —Teeth of Frugivorous Animal. destined to grind harder vegetable substances, they are termi¬ nated by a large flat and roughened surface (figs. 96, 99). The roughness of this surface is maintained by the peculiar arrangement of the three substances of which the tooth is composed. The enamel, instead of covering its crown, is arranged in upright plates, which are dispersed through the tooth ; and the space between them is filled up by plates of ivory and of cementum (§ 54). These last, being softer than the enamel, are worn down the soonest ; and thus the plates of enamel are left constantly projecting, so as to form a rough surface admirably adapted to the grinding action which the tooth is destined to perform. The mode in which these plates are disposed, affords a most characteristic distinction between the two species of Elephant at present existing, 172 DIFFERENT KINDS OF TEETH. namely, the African and the Indian ; as also between each of these and the great extinct species known as the Mam¬ moth (fig, 99). In the great gnawing teeth of the Eabbit, Fig. 98. — Molar Tooth of Mastodon. Fig. 99. — Molar Teeth of Elephants. 1 , African Elephant ; 2, 1 ndian Elephant ; 3, Mammoth. &c., the front surface only is covered with enamel ; and as this is worn away more slowly than the ivory, it stands up as a sharp edge (fig. 91), which is always retained, however much the tooth may be worn away. 183. Of all the teeth, the molars may be regarded as the most useful. They are seldom absent in the Mammalia ; and their office is usually essential to the proper digestion of the food. Animal flesh (the most easily digested of all substances) needs but to be cut in small pieces ; but the hard envelopes of beetles and other insects must be broken up ; and the tough woody structure of the grasses, and the dense coverings of the seeds and fruits on which the herbivorous animals are supported, must be ground down. The incisors and canines Fig. loo. Skull of Boar. are ch{efly employed among Carnivorous animals for the purpose of seizing their living prey, and are never deficient in them ; but they are less re¬ quired in Herbivorous animals ; and either or both kinds are not unfrequently deficient. Sometimes, however, they are not SUCCESSION OF TEETH.— WHALEBONE. 173 only present in the latter, but are largely developed, serving as weapons of attack and defence ; as in the Boar (fig. 100). 184. In the Mammalia in general, as in Man, the teeth are not much developed at the time of birth, that they may not interfere with the act of sucking; and they do not make their appearance above the gum, until the time approaches when the young animal has to prepare its own food, instead of simply receiving that which has been prepared by its parent. The teeth which are first formed are destined to be shed after a certain period, and to be replaced by others. They are called milk-teeth ; and in Man they are twenty in number,— namely, four incisors in the front of each jaw, and two canines and four molars on each side. These begin to fall out at about the age of seven years ; previously to which, however, the first of the permanent molars appears above the gum, behind those of the first set. The incisors and canines of the first set are replaced by incisors and canines respec¬ tively ; but the molars of the first set are replaced by teeth like small molars, having only two fangs ; these are called false molars, or, more properly, bicuspid teeth (fig. 93). The second of the true molars does not make its appearance until all the milk-teeth have been shed ; since it is only then that the jaw becomes long enough to hold any additional teeth. The third does not usually come up until the growth of the jaw is completed ; and as this time corresponds with that at which the mind as well as the body is matured, they are commonly known as wise or wisdom teeth. There are then thirty- two teeth in all, or sixteen in each jaw ; — namely, four incisors, two canines, four bicuspid, and six true molars. — In extreme old age, these teeth fall out like those of the first set ; but they are not replaced by others, and their sockets are gradually obliterated. 185. There are a few Mammalia which do not possess teeth. This is the case with the common Whale, in which they are replaced by an entirely different structure. From the upper jaw (fig. 102) there hang down into the mouth a number of plates of a fibrous substance (fig. 101), to which we give the name of whalebone , though its is really analogous to the gum of other animals. The fibres of these plates are separate at their free extremities, and are matted (as it were) together, so as to form a kind of sieve. Through this sieve the Whale 174 ABSENCE OF TEETH IN WHALE, ANT-EATER, ETC. draws water in enormous quantities, whenever it is in want of food ; and in this manner it strains out, as it were, the minute gelatinous animals upon which it lives, from the water of the seas it inhabits. The water thus taken in is expelled from the nostrils or blow-holes, which are situated at the top Fig. 101.— Whalebone. of the head. Most of the Whale tribe have short fringes of this kind in the roof of the mouth ; but in none, except the Balcena , or Greenland Whale, is it long enough to make it worth separating ; all the other species having teeth, either in one or both jaws. — It is a curious fact, that the rudiments of teeth may be discovered in both jaws of the young Greenland whale, although they are never to be developed. And the rudiments of incisor teeth in the upper jaw, and of canine teeth in both jaws, may also be discovered in the young of the Kuminant quadrupeds (oxen, sheep, &c.), though they never show themselves above the gum. 186. The Ant-eaters, also, are destitute of teeth, and usually obtain their food by means of their long extensible tongues, which are covered with a viscid saliva ; this being pushed into the midst of an ant-hill, and then drawn into the mouth, brings into it a large number of these insects, which are sufficiently bruised between the toothless jaws (fig. 103). Lastly, may be mentioned as a curious exception to the general rules respecting the teeth of Mammalia, the remarkable Orni¬ thorhyncus of New Holland (Zoology, § 317), which feeds, Fig. 103.— Skull of the Ant-eater. TEETH OF REPTILES AND FISHES. 17 5 like the duck, upon the water-insects, shell-fish, and aquatic plants, that it obtains from the mud, into which it is continu¬ ally plunging its singular bill; and its jaws, entirely destitute of teeth, are furnished with horny ridges, by which it can in some degree masticate its food. 187. Among Birds, there is an entire absence of teeth; and the mechanical division and the reduction of food is per¬ formed in the stomach, in the manner hereafter to be men¬ tioned (§ 200). The mouths of almost all Beptiles, excepting the Turtle tribe, are furnished with numerous teeth (fig. 104) ; but these are not adapted for much variety of purposes, being principally destined to prevent the escape of the prey which the animals have Secured ; Fi^* 104- — Head of Gavial. (Crocodile and their shape is conse¬ quently nearly uniform, being for the most part simply conical. There are some Lizards, however, which are herbivo¬ rous ; and these have large rough teeth, somewhat resembling the molars of Mammalia. The Iguanodon, an animal of this tribe, attained a gigantic size in past ages of the world. 188. In Fishes, the teeth are commonly very numerous (fig. 105) , but they have for their object only to separate and retain Fig. 105. — Head of Shark. their food ; and there is little variety in their form. Fre¬ quently they have no bony attachment, being only held by the gum, as in the Shark ; and they are consequently often torn away, but they are as readily replaced. Sometimes, how- 176 MASTICATING INSTRUMENTS OF INVERTEBRATA. ever, the tooth seems like a continuation of the hone of the jaw, not being in any way separated from it, and the tubular structure of the latter being continued into it without any interruption. The teeth of fishes are often set, not only upon the proper jaw-bones, but upon the surface of the palate, and even in the 'pharynx or swallow. 189. In the Invertebrata there are generally no proper teeth ; in the Articulated and sometimes in the Molluscous series, however, we meet with firm horny jaws, which are often furnished with projections that answer the same pur¬ pose ; and in most Gasteropods we find a very curious organ, commonly designated as the tongue , more correctly the palate , the surface of which is beset with innumerable tooth¬ like points (fig. 106), by whose rasping action the food is reduced. These teeth present great varieties of form and arrangement in the different genera and species of this group ; and these varieties appear to hear some relation to the nature of the food on which the animals respectively live. It is remark¬ able that in an animal so low in the scale as the Echinus or Sea-Urchin (§ 1 19), a very com¬ plex dental apparatus should exist. This consists of five long hard teeth, which surround the mouth ; and these are fixed in a framework which is worked by a powerful set of muscles, and thus serve effectually to grind down the food. Fig.106. — Dental Organ of Nerita. Insalivation. 190. The act of mastication is connected with another; which is also of great importance in preparing for the sub¬ sequent process of digestion. This is the blending of the saliva with the food, during its reduction between the teeth, — an act which is termed insalivation. The saliva is separated from the blood, by glands which are situated in the neigh- SECRETION OF SALIVA, AND ITS USES. 177 bourhood of tbe mouth ; of these there are three pair in Man, two beneath the tongue (fig. 107), and one in the cheek, each pouring-in its secretion by a separate canal. The salivary fluid is principally composed of water, in which a small quantity of animal matter and some saline substances (chiefly common salt) are dissolved ; the whole amount of these, however, is not more than 1 part in 100. The secretion of saliva is not constantly going on ; but the fluid is formed as it is wanted. The stimulus by which the gland is set in action may be simply the motion of the jaws ; thus, on first waking in the morning, the mouth is usually dry, but it is soon rendered moist by the movements wThich take place in speaking. The contact of solid substances with the membrane lining the mouth appears also to excite the flow; hence dryness of the mouth may often be remedied for a time, when no water is at hand, by taking a pebble into its interior, and moving this from side to side. There are certain substances, however, whose presence in the mouth has a special influence in provoking an increased secretion of saliva ; and every one knows, too, that the simple idea of savoury food will excite an increased flow, making the “ mouth water ” as it is popularly termed. These are instances of the power of the nervous system, through which such impressions are conveyed, over the act of secretion. 191. In the case of farinaceous or starchy food, the admix¬ ture of saliva occasions the commencement of that chemical change in which its digestion consists, namely, its conversion into sugar ; but in general, the benefit derived from this pro¬ cess of insalivation is just that which is obtained by the chemist, when he bruises in a mortar, with a small quantity of fluid, the substances he is about to dissolve in a larger amount of the same. If the preliminary operations of masti¬ cation and insalivation be neglected, the stomach has to do the whole of the work of preparation, as well as to accomplish the digestion ; thus more is thrown upon it than it is adapted to bear ; it becomes over- worked, and manifests its fatigue by not being able to discharge even 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 N 178 DEGLUTITION OR SWALLOWING. general habit of very rapidly eating or rather “ bolting ” the meals. There is another evil attendant on this practice, — that 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 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. Imperfect mastication of the food is very apt to occur, in persons who are losing their teeth by old age or decay; and where these are not replaced by artificial means, the next best remedy is to cut the food into very small por¬ tions, before it is taken into the mouth, and to masticate it there as thoroughly as possible. Deglutition. 192. In the Mammalia, the cavity of the mouth is guarded behind by a sort of moveable curtain, which is known as the veil of the palate (fig. 107) ; and this hangs down during Veil of the palate Pharynx Nose Tongue Salivary glands Os hyoides Larynx _ _ _ Thyroid gland (Esophagus lilt ■ Trachea Fig. 107. — Perpendicular Section of the Mouth and Throat. mastication, in such a manner as to prevent any of the food from passing backwards. This partition, which does not exist DEGLUTITION OR SWALLOWING. 179 in Birds and other animals that do not masticate their food, hangs from the arch and sides of the palate, so as to touch the tongue by its lower border ; but it can be lifted in such a manner as to give the food free passage beneath it, into the top of the gullet. When mastication is completed, the food is collected on the back of the tongue into a kind of ball ; and this, being carried backwards by the action of its muscles, presses against the partition just mentioned, and causes it to open. The food thus passes into a sort of funnel, formed by the expansion of the top of the oesophagus or gullet ; this cavity, termed the pharynx , communicates above with the nostrils, and in front with the larynx , which is at the top of the trachea or windpipe. The oesophagus is a long and narrow tube, which descends from the pharynx to the stomach, lying just in front of the vertebral column, and behind the heart and lungs. It is surrounded by muscular fibres, disposed in various ways ; by the action of which the food that has once passed into the pharynx is propelled downwards to the stomach. 193. But in order to reach this tube, the alimentary ball must pass over the glottis or aperture of the larynx. With a viewr to prevent its falling-in, the larynx is drawn, in the very act of swallowing, beneath the base of the tongue ; and this action presses down a little valve-like flap, the epiglottis , upon the aperture, so as in general effectually to prevent any solid or fluid particles from entering it. But it sometimes happens that, if the breath be drawn-in at the moment ot swallowing, a small particle of the food, or a drop of fluid, is drawn into the glottis ; and this action (commonly termed “ passing the wrong way,”) excites a violent coughing, the object of which is to drive up the particle, and to prevent it from finding its way into the lower part of the windpipe. It may also happen that a larger substance may slip backwards, by its own weight, into the glottis, when there was no intention of swallowing, and when the larynx was conse¬ quently not drawn forwards beneath the tongue. The presence of such a substance in the windpipe excites a violent and fre¬ quently almost suffocating cough (§ 342) ; the effect of which is sometimes to drive it up through the glottis, and thus to get rid of the source of irritation. 194. The act of swallowing is itself involuntary, and may n 2 180 MOVEMENTS OF DEGLUTITION. be even made to take place against tbe will. This may seem contrary to every one’s daily experience ; but it is nevertheless true. The movement by which the food is carried back, beneath the arch of the palate, into the pharynx, is effected by the will j but when the food has arrived there, it is laid hold of, as it were, by the muscles of the pharynx, and is then carried down involuntarily. It has several times happened, that a feather, with which the back of the mouth was being tickled to excite vomiting, having been introduced rather too far, has been thus grasped by the pharynx, and has been swallowed. Moreover, we cannot perform the act of swallowing, without carrying something backwards upon the tongue ; and it is the contact of this something , even if it be only a little saliva, with the membrane lining the pharynx, that produces the muscular movement in question. 195. This action is one of the kind now denominated reflex (§ 430). It is produced through the nervous system ; for if the nerves supplying the part be divided, it will not take place. But it does not depend upon the Brain ; for it may be performed after the brain has been removed, or when its power has been destroyed by a blow. It is caused by the conveyance to the top of the Spinal Cord, of the impression made on the lining of the pharynx ; this impression, brought thither through one set of nerves, excites in the spinal cord a motor impulse ; which, being transmitted thence through another set of nerves, calls the muscles into action. 196. This action is, therefore, necessarily connected with the impression, so long as this portion of the spinal cord, and the nerves proceeding from it, are capable of performing their functions : and it is one of those to which we may give the name of instinctive , to distinguish it from those which are effected by an effort of the Will, intentionally directed to accomplish a certain purpose. It may even take place without the animal being aware of the contact of any substance to be swallowed with the lining of the pharynx ; for there is good reason to believe that when the brain has been destroyed, or paralyzed by a blow, all sensibility is destroyed ; and we have also sufficient reason to consider it as suspended in profound sleep or apoplexy, in which states swallowing is still per¬ formed. In the severest cases of apoplexy, however, the power of swallowing is lost ; and this is a symptom of great DEGLUTITION - DIGESTIVE APPARATUS. 181 danger, since it shows that not the brain alone, but the upper part of the spinal cord, is suffering from the pressure ; and that the movements of respiration, which depend upon a similar action of the nervous system (§ 340), will probably soon cease, so that death must ensue. Digestive Apparatus. 197. The food, thus propelled downwards by the action of the muscles of the pharynx and of the oesophagus (gullet), Large Intestine - Spleen — Colon Small Intestine -- Colon Small Intestine Rectum Fig. 108.— Digestive Apparatus or Man. arrives, in Man and the Mammalia, at the stomach ; which is a large membranous bag, placed across the upper part of the 182 FORM OF THE STOMACH. abdomen (fig. 108). The form of this stomach varies much, according to the nature of the aliment to be digested. Where the food is animal flesh, which is easily dissolved, the stomach is small, and appears like a mere enlargement of the alimentary tube ; this is the case in the Cat tribe, for example. In Her¬ bivorous animals, on the contrary, the stomach is very large, the food being delayed there a long time on account of the difficulty with which it is digested ; and the principal part of its cavity is not a simple enlargement of the alimentary tube, but a bag or sac that bulges out, as it were, on the left side of that canal. By the degree of this bulging, we can judge of the nature of the food on which the animal is destined to live. Thus in Man (fig. 108), the large end of the stomach, situated on the left side (the right side of the figure as we look at it), is moderately developed; showing, as we might expect from the form of his teeth, as well as from his natural tastes, that he is adapted for a diet in which animal and vegetable food are mixed. In the purely carnivorous tribes, this large end of the stomach is almost deficient ; whilst in the herbivorous races, it is enormously developed, and some¬ times forms a distinct pouch. (Esophagus Cardia 3d Stom. Intestine Pylorus 4th Stom. 2d Stom. 1st Stom. Fig. 109.— Stomachs of the Sheep. 198. The most complex form of the stomach among Mam¬ mals, is that which we find in the animals that ruminate or chew the cud. It possesses, in fact, no less than four distinct cavities, through all of which the food has to pass during the STOMACH OF RUMINANTS. 183 process of digestion. The external appearance of the stomach of the Sheep is seen in fig. 109 ; and its interior is displayed in fig. 110. The food of the Ruminant animals is not chewed by them before it is first swallowed. In their wild state, they are peculiarly exposed to the attacks of their car¬ nivorous enemies, when they come down from their rocky heights to browse upon the rich pastures of the valleys. If they were then obliged to masticate every mouthful, they would be subjected to long- continued danger at every meal ; but, by the curious construction of the digestive apparatus, this is spared to them ; for they are enabled to swallow their food as fast as they can crop it, and afterwards to return it to their mouths, so as to masticate it at their leisure, when they have retreated to a place of safety. The crude unmasticated food, which is brought-down by the oesophagus, first enters the large cavity on the left side, which is commonly termed the paunch. It is there soaked, as it were, in the fluid secreted (Esophagus Groove Manyplies Reed Intestine Honeycomb Paunch Fig. 110. — Section of the Stomachs of the Sheep. by its walls ; and is then transmitted to the second cavity, which, from the sort of network produced by the irregular folding of its lining membrane, is called the reticulum or honey-comb stomach. This stomach also has a direct commu¬ nication with the oesophagus, and appears destined especially to receive the fluid that is swallowed; for this passes im¬ mediately into it, without going into the first stomach at all. The folds of its lining membrane present a large surface, through which fluid may be absorbed into the system. It is 184 ACT OF RUMINATION. here that we find the curious arrangement of water-cells in the stomach of the Camel, by which that animal is enabled to retain a supply of water for several days. These cells corre¬ spond with the little pits which are seen in the honey-comb stomach of the Sheep, but are much deeper, and their orifices may be closed by the action of a set of muscular fibres which pass in every direction round each, so as to form a net- work including these orifices in its meshes. 199. After the food has been macerated in the fluids of the first and second stomachs, it is returned to the mouth by a reversed peristaltic action of the oesophagus, which brings it up as a succession of globular pellets, that are formed by compression in a sort of mould at the lower end of the oeso¬ phagus. These pellets are subjected within the mouth to mastication and insalivation ; and the food is then ready for the real process of digestion. It is this mastication which is commonly known as the “ chewing of the cud ; ” and the animal, whilst performing it, seems the very picture of placid enjoyment. When again swallowed, the food is directed, by a peculiar valvular groove at the bottom of the oesophagus, into the third stomach, commonly termed the many plies, from the peculiar manner in which its lining membrane is arranged. This presents a number of folds, lying nearly close to one another, like the leaves of a book, but all directed, by their free edges, towards the centre of the tube, — a narrow fold intervening between each pair of broad ones. The food has, therefore, to pass over a large surface, before it can reach the outlet of the cavity ; and this leads to the fourth stomach, commonly termed the reed. This is the seat of the true digestive process, the gastric juice (§ 204) being formed here only ; and it is from this that the rennet is taken, which is used in making cheese to cause the milk to coagulate or curdle. In the sucking animal, the milk passes directly into this fourth stomach, without entering either the first or second stomachs, and without being delayed in the third, the folds of which adhere together so as to form a narrow undivided tube. The paunch is at that time comparatively small, being of less size than the reed ; and its dimensions increase, as soon as the young animal begins to distend it by swallowing solid vegetable matter. 200. In the digestive apparatus of .Birds, we find a con- DIGESTIVE APPARATUS OF BIRDS. 185 siderable modification of form, resulting from the fact that, as these animals do not masticate their food, they require some (Esophagus Ventriculus) SuccenturiatusJ Gizzard Pancreas Duodenum - Caeca, - M Large Intestine Large Intestine Ureter Oviduct Cloaca Anus ^ Fig. 111. — Digestive Apparatus of Fowl. other means of reducing it. This means is provided for them in their stomach. In the tribes whose food is of such a nature as to require being moistened before it is rubbed down, and especially in those which feed upon grains, the oesopha¬ gus has a pouch-like dilatation, termed the crop or craw (fig. Ill) j in this it is retained, and exposed to the action 186 TRITURATING ACTION OF GIZZARD. of fluid secreted by its walls, just as it is in the paunch of ruminant quadrupeds. This crop is of enormous size in some of the grcmivorous (grain-eating) birds, such as the Turkey. The second stomach (or ventriculus succenturiatus) is the one in which the gastric juice is secreted ; but this is seldom large enough to retain the food, which passes-on through it to the gizzard , a hollow muscle, furnished with a hard tendi¬ nous lining. In the granivorous birds this is extremely strong and thick ; and pieces of gravel are swallowed by them, which, being worked-up with the food by the action of the gizzard, assist in its reduction. In the rapacious flesh- or fish-eating birds, however, no such assistance is required, the food being easy of solution ; the walls of their gizzard are thin, possessing but few tendinous fibres ; and the three cavities of the stomach are almost united into one. 201. Various experiments have been made to test the mechanical powers of the gizzard of Birds. Balls of glass which they were made to swallow with their food, were soon ground to powder ; and the points of needles and of lancets, fixed in a ball of lead, were blunted and broken-off by the power of the gizzard, whilst its own internal coat did not appear to be in the least injured. On the other hand it has been ascertained, that grain enclosed in metal balls which protected it from the mechanical action of the gizzard, but which were perforated so as to afford the gastric fluid free access to their contents, was not in the least digested ; so that the utility, and even the necessity of this operation, become evident. 202. As there are few animals, save the Mammalia, that perform any proper mastication in their mouths, the grinding down of their food (where it is of such a nature as to require it) must be performed in the stomach ; and accordingly we find many tribes, belonging to different divisions of the animal kingdom, in which a gizzard, or something analogous to it, exists. It is possessed by almost all Cephalopods , and by many of the Gasteropoda In the walls of the stomach of some of these last, there is a considerable amount of mineral matter deposited, intermixed with the hard tendinous fibres of which they chiefly consist. A powerful gizzard is also found in many Insects , but here it is placed above the diges¬ tive stomach (fig. 112, c). The accompanying figure exhibits REDUCING APPARATUS OP INSECTS, ETC. 187 the alimentary canal of a Beetle, from its commencement to its termination. At a is seen the head, hearing the jaws, &c. ; from this the gullet passes straight backwards, and is dilated into a crop at b , below which is the gizzard, c. This opens at its lower end into the true digestive stomach, d; which is surrounded by an immense number of little follicles or bags, by which the secretion of the gastric juice is effected (§ 204). Into the lower end of this, the long vessels, e , open, which constitute in Insects ths only rudiment of a liver (§ 358). In many of the Crustacea , the walls of the stomach are beset with re¬ gular rows of teeth, which are moved by the action of powerful muscles. These teeth are cast or shed at the same time with the shell. In the Wheel- Animalcules, the place of the gizzard is occupied by a curious pair of jaws, armed with teeth ; by the working of which, the food is effectually crushed. In the Bryozoa , a gizzard exists between the oesopha¬ gus and the true digestive stomach ; and the stomach itself is surrounded by the little follicles which secrete the bile, and pour it into that cavity (§ 115). 203. In animals which subsist exclusively on flesh, how¬ ever, no such complicated apparatus exists. Thus in Serpents (fig. 34), the stomach is but a slight dilatation of the alimen- Fig. 112.— Digestive Apparatus of Beetle. 188 DIGESTIVE APPARATUS— GASTRIC DIGESTION. tary tube ; and it is not easy to say where it commences and terminates. In Spiders and Scorpions , too, which live upon the juices they suck from other animals, the alimentary tube is very simple ; and it is scarcely dilated into a proper sto¬ mach. And in most of the Eadiated classes, we find the stomach to possess only one orifice, through which the undi¬ gested residue of the food is cast out, as well as fresh sup¬ plies taken in. But this stomach is not always a simple bag ; thus in the Star-fish it sends prolongations into the rays, the use of which is at present un¬ determined. There are certain animals in which no digestive cavity exists : their sustenance being derived either from the juices prepared by other animals, in whose tissues or cavities they are im¬ bedded, and being introduced by absorp¬ tion through the whole surface, as is the case in the lower Entozoa (fig. 53) ; or from particles which are drawn into the midst of the soft gelatinous substance of their bodies, and undergo a sort of diges tion there, as is the case with the Khizo - poda (§ 129). Gastric Digestion : — Chymification. 204. The food which has been re¬ duced in the mouth by the action of the teeth, or in the stomach itself by the movement of its own tendinous walls, is prepared for the real process of digestion; by which it is converted into a fluid, and thus made fit to be truly received into the system, by being absorbed into its vessels. The chief agent in the digestive process is a fluid termed the Gastric’ Follicles* gastric juice, which is secreted or sepa- as seen in a vertical section rated from the blood by a vast number three* diameters "at^and of bags or follicles (fig. 113), im- twenty diameters at b. bedded in the walls of the stomach. When the cavity is empty, this fluid is secreted in very small quantities ; but, like the salivary secretion, it is poured out SENSE OF HUNGER — SECRETION OF GASTRIC JUICE. 189 in abundance when the lining membrane is stimulated by the contact of food, especially solid food. Only a limited quantity is secreted at any one time ; and this quantity is just that which is sufficient to dissolve food enough for the supply of the natural wants of the system. The contact of any solid substances with the interior of the stomach, is suffi¬ cient to produce a flow of this fluid into its cavity ; but the secretion soon ceases if the substance be not of an alimentary nature. 205. The sense of hunger appears due to the distension of the blood-vessels of the stomach, which takes place in pre¬ paration for the secretion of the gastric fluid. This deter¬ mination of blood towards the stomach seems to occur when¬ ever the body needs a fresh supply of nourishment ; and it ceases as soon as a sufficient amount of gastric fluid has been drawn off. Hence it is, that hunger is relieved by eating ; and hence it is, also, that hunger is for a time relieved by taking solid substances into the stomach, even though they contain no nourishing matter. It is from having experienced this, that savage nations are in the habit of mixing indiges¬ tible solid matter with the fluids that sometimes constitute their principal articles of food. Thus the Kamschatdales mix earth or saw-dust with the train-oil on which alone they are frequently reduced to live ; and the Yeddahs, or wild hunters of Ceylon, mix the pounded fibres of soft or decayed wood with the honey on which they feed when meat is not to be had. One of them being asked the reason of the practice, replied, “ I cannot tell you, but I know that the belly must be fUled.” It has been found by experiment, that soups and other forms of liquid aliment are not alone fit for the support of the system, even though they may contain a large amount of nutritious matter ; and the medical man well knows, that many persons have stomachs too weak and irritable to retain “ slops” (as they are commonly termed), who can yet digest solid food of a simple kind. All these instances show, that the contact of a solid substance with the walls of the stomach, is the proper stimulus or excitement to the secretion of the gastric fluid. 206. This fluid, when poured upon the food, is thoroughly mixed-up with it by a peculiar movement of the walls of the stomach, which is continually bringing fresh portions of the 190 PROPERTIES OF GASTRIC JUICE. alimentary mass into contact with its sides, so that the whole is after a time equally exposed to the influence of the gastric secretion. If this movement were not to take place, only the outside of the mass would he digested, and the central portion would remain but little affected. 207. The nature of the gastric fluid, and the mode of its operation upon the food, have been studied by withdrawing a portion of it from the stomach, and by observing its pro¬ perties and actions out of the body. A sufficient quantity for this purpose cannot be easily procured. Spallanzani, an Italian physiologist of the last century, contrived to obtain it, by causing birds and other animals to swallow sponges to which pieces of thread were attached ; these, when they had remained long enough in the stomach to cause a secretion of the gastric juice, were drawn up again ; and the fluid they had absorbed was pressed out into vessels, in which its pro¬ perties could be examined. More recently, however, an advantageous opportunity has presented itself for obtaining supplies of gastric fluid in a less objectionable manner. A young man, named Alexis St. Martin, received a very severe wound in his left side, by the bursting of a gun ; and al¬ though this wound laid open the cavity of his stomach, he recovered his health completely, and subsequently married and had a family. There remained, however, an aperture in his stomach, which would not close up ; and through this orifice, which was usually covered by a bandage, the contents of the stomach could be drawn out. The gastric juice was obtained by introducing an India-rubber tube into the sto¬ mach when it was empty, and by moving it about within the cavity ; the contact of the tube then excited the follicles to secretion (on the principle already mentioned, § 204) ; and the fluid thus poured into the stomach was drawn off through the tube. 208. The Gastric Juice is very like saliva in its appearance, but it is distinctly acid to the taste ; and it is found, by chemical examination, to contain a considerable quantity of muriatic acid * in an uncombined state. Besides this, it con¬ tains a considerable quantity of a peculiar animal substance which seems like altered albumen, and which has been desig¬ nated pepsin; as well as other ingredients of less importance. * Muriatic acid is commonly known as spirit of salt. ACTION OF GASTRIC JUICE. 191 This fluid possesses the power of dissolving albuminous sub¬ stances of various kinds, when these are submitted to its action at the constant temperature of 100° (which is about that of the stomach), and are frequently shaken-up with it. The solution appears to be in all respects as perfect as that which naturally takes place in the stomach, but requires a longer time. It does not seem, however, that the gastric juice has a special solvent power for any other than albuminous substances. Gelatinous and saccharine matters are taken-up by it, as by other watery fluids ; but neither starchy nor oleaginous substances undergo any other change by its action, than consists in the separation of their particles by the solu¬ tion of the membranes and fibres which held them together. There is every reason to believe that what is true of artificial is true of natural digestion ; and that so far from the whole operation being performed in the stomach, as was formerly supposed, gastric digestion is limited to the solution of the albuminous, gelatinous, and saccharine constituents of the food. 209. With regard to the precise mode in which the gastric fluid acts in dissolving albuminous substances, there is yet some uncertainty ; although there can be no longer any rea¬ sonable doubt, that the operation is of a purely chemical nature. An artificial gastric fluid, capable of effecting all that can be done by that which is secreted in the living stomach, may be made, by macerating (or soaking) a portion of the membrane lining the stomach of a pig, or of the fourth stomach of a calf (even after it has been washed and dried) in water, which dissolves a portion of the pepsin ; and by then acidulating this solution with muriatic or acetic acid. It has been proved that both the acid and the pepsin are essential to the process of solution ; for the acidulated fluid without the animal matter acts extremely slowly upon pieces of meat, hard-boiled egg, &c., submitted to it ; and water in which the stomach has been macerated, but which contains no acid, will not act at all. But the acidulated water alone will readily dissolve the substances just mentioned, at a higher temperature ; and thus it appears that the acid is the real sol¬ vent ; and that the pepsin has for its office to produce some change in the albuminous substances, by which they are more readily dissolved The recent inquiries of Liebig and other 192 GASTRIC DIGESTION : CHYMIFICATION. Chemists, render it probable that this change is of the nature of fermentation. 210. It is a fact of great practical importance, that a cer¬ tain quantity of the gastric fluid can act only upon a limited amount of alimentary matter ; so that, if more food be taken into the stomach than the gastric fluid can dissolve, it remains there undigested. How it has been already mentioned, that the quantity of the gastric fluid secreted at any one time, is proportional, not to the amount of food in the stomach, but to the wants of the system ; so that, if more food be swal¬ lowed than is required to repair the waste of the body, it lies for some time unchanged in the stomach, and becomes a source of irritation which prevents the due discharge of its functions ; and the evil goes on increasing with every addi¬ tion to the contents of the cavity. This may not be felt by the individual at the time ; but it leaves permanent effects, which manifest themselves sooner or later in derangement of the general health. The habit of taking more food than is really necessary, and of irritating the stomach by stimulating substances or fluids (such as pepper, mustard, spirits, &c.), is a fertile source of disease. The injurious effects of these are manifested by the thirst which is the consequence of their use, and which is a call (as it were) on the part of the stomach, to prevent their irritating action by diluting them with water. 211. By the solution of its albuminous portion, and the separation of its other component particles, the food is re¬ duced in the stomach to a kind of pulp, which is termed chyme. The consistence of this will of course vary accord¬ ing to the nature of the food, and the quantity of fluid in the stomach ; but in general it is grayish, semi-fluid, and uniform throughout. When the food has been of a rich character, the aspect of the chyme resembles that of cream ; but when the food has consisted of farinaceous substances (rice, potatoes, &c.), the chyme is more like gruel. At the point where the stomach opens into the intestinal canal, which is called the 'pylorus , there is a kind of valve, which permits the chyme to pass as fast as it is formed, but closes against the portions of the food which are yet solid and undigested ; and thus the chyme escapes from the stomach in successive waves, slowly at flrst, but afterwards more rapidly, as the digestive process approaches its completion. INTESTINAL DIGESTION. 193 Intestinal Digestion ; Ghylification . 212. The process of digestion is by no means completed in the stomach ; for much of the matter which escapes from it in the chyme, is destined to undergo a further change whilst passing through the intestinal canal ; especially in the her¬ bivorous tribes, whose food, being less digestible than that of the carnivorous races, requires to be longer delayed in the intestinal canal, in order that it may yield up its nutritious portion. Hence we find this canal of enormous extent in most animals whose food is vegetable, being in the Sheep about twenty-eight times the length of the body ; in the purely carnivorous animals, on the other hand, it is compara¬ tively short, being in the Lion only about three times the length of the body, while in the Serpent it runs almost straight from one extremity to the other ; and in animals which live on a mixed diet, it is of medium length, being in Man about six times as long as his body. The intes¬ tinal tube is usually distinguished into the small and the large intestine ; of which the small is the first portion, and the large the second. The former, as shown in fig. 108, is disposed in a convoluted or twisted manner, so that a great extent of it may be packed within a small compass ; it usually forms about three- fourths of the whole length of the canal. It is held in its place by a serous membrane termed the 'peritoneum , which forms an immense number of folds that suspend it (as it were) from the vertebral column ; but these still allow it a considerable power of movement. 213. Soon after passing from the stomach into .the intes¬ tinal canal, the food is mingled with three secretions, which have an important influence on the changes it is further to undergo ; these are the Bile, the Pancreatic fluid, and the In¬ testinal juice. The two former are prepared by two large glan¬ dular masses, the Liver and the Pancreas (or sweetbread), which, in all the higher animals, are completely detached from the alimentary canal, and send their secretions into it through special ducts ; the latter, like the gastric juice, is formed in little follicles lodged in the wall of the canal itself. The peculiar matter which forms the chief solid constituent of bile, is essentially a soap formed by the union of two resinoid acids, with soda as a base (§ 364). The composition of the o 194 BILIARY, PANCREATIC, AND INTESTINAL SECRETIONS. pancreatic fluid closely corresponds with that of saliva, which it much resembles in appearance. The intestinal juice , like the gastric, is a nearly colourless, somewhat viscid fluid, con¬ taining an organic compound not far removed from albumen ; but it differs from the gastric juice in being alkaline instead of acid. The relative offices of these three fluids have not yet been determined with certainty ; but there appears good reason to believe : (1) that the bile, by its alkalinity, neutralizes the acidity which the chyme derives from the gastric juice, and that this neutralization favours the metamorphosis of starch into sugar, which has been almost suspended in the stomach ; (2) that the bile aids the pancreatic fluid in re¬ ducing the oleaginous particles to the condition of an emul¬ sion, , that is, in bringing them into a state of very minute division, in which they remain suspended in the albuminous solution ; (3) that the pancreatic fluid aids the salivary mat¬ ter which was swallowed with the food, in the transforma¬ tion of starch into sugar ; (4) that the intestinal juice has a solvent power for albuminous substances which is scarcely inferior to that of the gastric juice, with a power of converting starch into sugar which is scarcely inferior to that of saliva or pancreatic fluid. The fluid of the Small Intestine, com¬ pounded of the salivary, gastric, intestinal, biliary, and pan¬ creatic secretions, appears to possess a far greater digestive power than that of the stomach, being capable of dissolving, or at any rate of reducing to an absorbable condition, nutri¬ tious substances of every class. This process goes on during the passage of the alimentary mass along the small intes¬ tine ; and the nutritious materials are progressively with¬ drawn by absorption, partly into the blood-vessels, which appear to receive whatever are in a state of perfect solution (§ 218), and partly into the lacteal absorbents, which take up nothing but that peculiar emulsion of albumen and fatty matter which is termed chyle (§. 222). 214. At the extremity of the Small Intestine, there is a kind of pouch, called the coecum ; which in some animals seems almost like a second stomach, and which is furnished with one or more little appendages, termed coeca * This is very small in Man, and does not seem to perform any important * The word coecum is used in Anatomy to denote a tube closed at one extremity. PERISTALTIC MOVEMENT - DEFECATION. 195 function ; but in most herbivorous animals it is larger (as in the Monkey, fig. 30) ; and it is found to secrete an acid fluid, which resembles the gastric juice, and which may have for its office to perform a second digestion upon the sub¬ stances which have escaped the first. These coeca are some¬ times very large in the intestinal canal of Birds (fig. 111). — From the coecum, the Large Intestine ascends as high as the liver, crosses the upper part of the abdomen, and then descends again, as shown in fig. 108 ; this portion is termed the colon ; and it terminates in the rectum , which forms the extremity of the intestinal tube. 215. The alimentary mass is propelled along the first part of the intestinal canal, — and the residue left after the absorp¬ tion of the nutritive materials is carried along the continua¬ tion of it, — by the contraction of its muscular coat, producing what is termed the 'peristaltic motion of the bowels. The fibres of this muscular coat are chiefly arranged in a ring-like manner around the tube ; so that, when they contract, they narrow the diameter of the tube. They are stimulated to contract by the contact of the solid or liquid matter passing through it (Chap, xn.) ; and their contraction forces this matter onwards, into the succeeding portion of the tube. This con¬ tracts in its turn, so as to propel its contents further ; and thus the mass is gradually driven from one extremity of the canal to the other. The peristaltic movement does not seem to depend (as do the contractions of the muscles concerned in swallowing, § 195) upon the nervous system ; for it will take place after the intestinal tube has been completely separated from the principal nervous centres ; and also after the death of the animal, if this have been produced by a sudden cause. Thus, if a Babbit be killed by a smart blow at the top of the neck, and the abdomen be immediately opened, the peristaltic movement will be seen in vigorous action, especially if the animal have eaten a full meal an hour or two previously. Defecation. 216. In passing through the large intestine, the undigested residue is still more completely deprived of the nutritive matter it may contain ; and its fluid portion is absorbed, so that it becomes more solid. It is allowed to accumulate in the rectum, until its bulk occasions inconvenient pressure upon o 2 196 DEFECATION - LACTEAL ABSORPTION. the surrounding parts ; and it is kept-in by a circular muscle or sphincter , which surrounds the outlet of the alimentary canal. But when the accumulation has taken place beyond this amount, it excites a reflex action (§ 195) in the muscles that surround the abdomen; and these make pressure suf¬ ficient to overcome the resistance of the sphincter, and to force out the contents of the rectum. Absorption of Nutritive Material . 217. We have only now to inquire into the mode, by which the nutritive matter extracted from the food is taken-up from the alimentary canal and applied to the nutrition of the body. In all Yertebrated animals, there exists a special set of vessels termed Absorbents; of which those forming one division, Thoracic Mesenteric Aorta Duct Glands Fig. 114.— Chyle-vessels. known as Lacteals , from the milk-like character of their con¬ tents, originate in the numberless villi or minute projections with which the mucous membrane that lines the small intes¬ tine is covered (§ 41). During the act of digestion, the ABSORPTION BY LACTEALS AND BLOOD-VESSELS. 197 epithelium-cells, which clothe the extremity of each villus (fig. 9), become distended with an opalescent fluid, the chyle (§ 222), which they select from the contents of the small intestine ; and this is subsequently given up by them to a lacteal tube, which, without any open mouth, eommences in the midst of each villus. The vessels which thus originate, unite into minute trunks, and these again into larger ones ; and these pass between the two layers of the mesentery (or fold of peritoneum by which the intestines are suspended, § 212) towards the lower part of the spinal column : where they deliver their contents into a sort of reservoir, which thus becomes the receptacle for all the chyle that has been collected from the alimentary canal (fig. 114). In traversing the me¬ sentery, the lacteals of the higher animals pass through little knot-like bodies of a peculiar nature, which are called mesen¬ teric glands. These appear to afford the means for the per¬ formance, within a more concentrated space, of the assimi¬ lating action which is carried on during the passage of the chyle through the lacteal system ; for in Eeptiles, in which these glands do not exist, the absorbent vessels are much more extended and spread out than they are in Birds and Mammals. 218. Near the surface of each of these villi, moreover, lies a minute network of Blood-vessels ; and there is now no longer any doubt that these receive, by simple imbibition,45, any substances, whether alimentary or otherwise, which exist in a state of perfect solution in the contents of the intestinal canal. For a great variety of such substances have been detected, by chemical analysis, in the blood which is returned from the walls of the intestines by the mesenteric veins ; whilst it is seldom that anything is found in the lacteals, save the proper constituents of chyle. It is through this channel that poisonous substances are taken into the circula¬ tion ; and these may be absorbed from the walls of the stomach (on which there are no villi or lacteals), without ever passing from it into the intestinal tube. Hence it is a great * That tendency — called Endosmose —which thinner liquids have to pass-towards and mix-with such as are more viscid, even through an intervening membrane, seems to be the physical cause (as experi¬ ment indicates) of this imbibition ; which is greatly promoted by the movement of blood in the vessels. 198 ABSORPTION BY LYMPHATICS. mistake to characterise the lacteals (with the lymphatics) as Absorbents in any exclusive sense ; the fact being that their function is limited to a special selective absorption, whilst the more general action is performed by the blood-vessels. 219. But the reservoir above-mentioned receives, not only the lacteal vessels that bring nutritious matter from the intes¬ tinal tube, but also lymphatics , which are absorbent vessels of similar character, that originate in every part of the body. These, also, pass through a set of (so-called) glands, in their way towards this receptacle ; and the structure of these glands, of which many are seated in the neck, some in the arm-pit, others in the groin, &c., is exactly the same as that of the mesenteric glands. The fluid they convey, which resembles very dilute liquor sanguinis (§ 229), seems evidently destined to be again applied to the purposes of nutrition. There is some obscurity as to its source ; but it seems probable that it may partly consist of the residual fluid, which, having escaped from the blood-vessels into the tissues, and having furnished the latter with the materials of their nutrition, is now to be returned to the former ; and partly of those par¬ ticles of the body, which, though they have lost their vitality in the course of the change it is continually undergoing, have not undergone a degree of decay that unfits them for serving, like the dead bodies of other animals, as a material for reconstruction by the organizing process. The lymphatics, being copiously distributed in the true Skin, absorb substances which are introduced into its tissue ; and if these substances be of an irritating nature, they may occasion an inflammatory action in the absorbents and their glands. Thus when poisoned wounds in the hand have been received, as in opening the bodies of men or animals that have died of particular diseases, the effect is usually manifested at first by heat and pain in the arm, along which the inflamed absorbents can be traced as hard cords ; and the glands in the arm-pit swell and become tender. 220. The lymphatics do not appear destined, however, to absorb from the surface of the skin ; this function being per¬ formed by the blood-vessels which are distributed abundantly in its substance. It is a fact now well established, that when the quantity of fluid in the body has been greatly reduced, absorption of water through the skin may take place to a ABSORPTION THROUGH SKIN - THORACIC DUCT. 199 considerable amount. Thus there is a case recorded by Dr. Currie, of a patient who suffered under obstruction of the gullet, of such a kind that no nutriment, either solid or fluid, could be received into the stomach ; and who was supported for some weeks by immersion of his body in milk and water, and by the introduction of nutritive liquids into the lower end of the intestine. During this time, his weight did not diminish ; and it was calculated by Dr. Currie, that from one to two pints of fluid must have been daily absorbed through the skin. The patient’s thirst, which had been very trouble¬ some previously to the adoption of this plan, was removed by the bath, in which he experienced the most refreshing sensa¬ tions. — It is well known that shipwrecked sailors and others, who are suffering from thirst owing to the want of fresh water, find it greatly alleviated, or altogether relieved, by dipping their clothes into the sea, and putting them on whilst still wet. 221. From the receptacle into which the chyle, and a con¬ siderable proportion of the contents of the lymphatics, are delivered, a tube passes upwards in front of the spine (fig. 114) ; and this tube, called the Thoracic Duct, conveys these nutritious fluids to the point where they are to be delivered into the current of blood. This delivery takes place at 4he angle where two great veins unite, — a point at which there is less resistance than in any other part of their walls. These veins are the Jugular, which brings the blood from the neck, and the Subclavian, which conveys it from the arm, of the right side (fig. 122) ; on the left side there is a smaller duct, which receives some of the lymphatics of the left side, and opens into the blood-vessels at a corresponding point between the left jugular and subclavian veins. Sanguification . 222. The Chyle of Yertebrated animals, as taken-up by the lacteals, may be regarded as blood in an early stage of its formation, with a large excess of fatty matter. It contains about 90 parts of water in 100 ; about 3| parts of albumen, and the same of fat ; and about 3 parts of other animal and saline matter. Its appearance and characters differ, according to the part of the lacteal system from which it is drawn. If obtained near the surface of the intestines, before it has passed 200 PROPERTIES OF CHYLE - SANGUIFICATION. through the glands, it is entirely destitute of that power of spontaneously coagulating, or dotting , which is so remarkable in blood : and when examined with a microscope, it is seen to present a number of oily globules of various sizes ; together with an immense number of very minute particles or mole¬ cules, which also seem of a fatty nature ; and to these last, whose diameter is between 1-24, 000th and 1-36, 000th of an inch, the milky whiteness which characterises chyle appears principally due. But the chyle drawn from the lacteals, after they have passed through the mesenteric glands, possesses the power of coagulating slightly ; hence it would seem that some of its albumen has undergone a transformation into fibrin (§ 17). At the same time, a great increase is observed in the number of certain floating corpuscles, which are occa¬ sionally to be noticed in the first chyle, but which are very abundant in the fluid drawn from the glands and from the lacteals that have passed through them ; of these, which bear a strong resemblance to the colourless corpuscles of the blood (§ 234), the average diameter is about 1-4, 600th of an inch. — By the time that the chyle reaches the central receptacle, its power of coagulating has still further increased ; so that its resemblance to blood, except in regard to colour, is much stronger. The proportion of fibrin and albumen which it contains, is much greater than that which existed in the first chyle, whilst the amount of oily matter is less. 223. There can be little doubt that the change which the chyle undergoes in its passage through the lacteals, is partly due to the influence of the living walls of these vessels upon the fluid in contact with them, and partly to that of the colourless corpuscles which float in the fluid, and which form the principal constituents of the absorbent glands. The whole apparatus, indeed, may be looked upon as one great Assimi¬ lating Gland, having for its function to make blood out of crude nutriment ; provided-for in the higher Yertebrata by the convolution of the lacteals in the mesenteric glands, and in the lower, by the simple extension of the vessels themselves. It is probable that, by being brought into very close neighbourhood with the blood in these glands, the chyle may be made to undergo some further change ; although, as each fluid is con¬ tained in its own tubes, which do not communicate, there can be no proper intermixture. ASSIMILATING GLANDS - ABSORPTION IN INVERTEBRATA. 201 224. There are certain glandular bodies, disposed in various parts of the system, which seem to discharge a similar office ; withdrawing the raw material (so to speak) from the general current of the circulation, and returning it again in a state of higher elaboration. Such are the Spleen, the Thyroid and Thymus glands, and the Supra-Kenal capsules. Besides these, the Liver probably exerts an assimilating action upon the crude materials which are made to pass through its substance, almost immediately after having been received into the blood-current, and before they are allowed to pass into the general circula¬ tion ; the whole of the blood returned by the gastric and mesenteric veins from the walls of the alimentary canal, being conveved through the liver by the portal system, in its way to the heart (§ 267). 225. In the Invertebrated animals, neither lacteals nor lymphatics exist; and the blood-vessels, whose absorbent powers are to a certain extent restricted in the higher animals, have to perform the functions of these. There are animals, however, which are destitute not only of lacteal and lymphatic vessels, but even of blood-vessels ; and in these, as in the Cellular Plants, there is but little transmission of fluid from one part of the body to the other ; for every portion, both of the internal surface (or lining of the stomach), and of the external surface which is bathed in the surrounding fluid (for most of these animals are aquatic), seems equally to possess the power of absorption ; and the parts to whose nourishment the fluid thus received into the body is to be appropriated, are in the immediate neighbourhood of those which have absorbed it. This is the case, for example, in the Hydra and Sea- Anemone, and, more or less, in all the Polypes ; as well as in the lower "Worms. Between these, therefore, and the Cellular Plants, a remarkable analogy exists in regard to the mode in which the nutriment is absorbed and applied ; the difference being, that the Animal possesses a digestive cavity, lined by an inward extension of the external surface, which does not exist in Plants (§ 8). And it is upon the walls of this cavity, that the absorbent vessels of the higher Animals (whether lacteals or blood-vessels) are distributed, collecting the nourishment in contact with them ; just as the roots of a Plant, spread through the soil, draw up that which it contains. But among those lowest animals in which the digestive cavity altogether 202 OF THE BLOOD, AND ITS CIRCULATION. disappears (§ 203), the function of absorption is not in any way limited ; since every part seems to have the power of re¬ ceiving from without, and of assimilating to its own substance, the nutrient materials which it needs. CHAPTER Y. OF THE BLOOD, AND ITS CIRCULATION. 226. The processes that have been already explained, have for their object to prepare the nutritious fluid, which supplies the materials for the growth of the several parts of the body, and which is conveyed through them by the apparatus to be presently described. In Man and the higher animals, this fluid, which is known as the Blood , has a red colour, and con¬ tains a large quantity of solid matter. The redness of the blood has been mentioned as a distinctive character of the Verte- brated classes (§7 5) ; it exists in Mammalia, Birds, Reptiles, and Fishes, and in these alone. In the Molluscous classes, as also in most of the Articulated, the nutritious fluid is nearly colourless ; and it will hereafter appear that this fluid bears, in some respects, a stronger resemblance to the chyle and lymph of the Vertebrata, than to their blood (§ 234). There is an apparent exception in the case of certain marine Worms, the fluid circulating in whose vessels has a reddish hue ; this does not depend, however, upon the presence of any red par¬ ticles, such as are characteristic of the blood of Vertebrata (§ 229), but upon a reddish tinge in the fluid itself, which does not seem altogether to answer to the character of blood (§ 294). 227. The blood of all the higher animals exists in two different states. When it is drawn from a slight scratch or other wound of the skin, it is of a bright red hue ; whilst that which is drawn in bleeding from the arm, is of a dark purple. The former is termed arterial blood, because it is contained, for the most part, in the tubes which are called Arteries, and which are conveying it from the heart to the tissues it has to nourish. The latter is called venous blood, because it is drawn from the Veins, by which it is returned from the tissues to the heart, after having performed its part in them. Hence it VENOUS AND ARTERIAL BLOOD. 203 is evident that this change of character has been produced during the passage of the blood through the tissues ; and so important is the alteration, that the blood which has been subjected to it is not fit to pass again into the arteries of the body, until it has been renewed by exposure to air in the Lungs. In their vessels, the contrary change — of which the nature will be presently explained (§ 253) — is effected, the dark hue of venous blood giving place to the bright red of the arterial fluid ; this is again changed during the passage of the blood through the body, to be again restored in the lungs. The same is the case in regard to Fishes, whose gills perform the same function as the lungs of air-breathing Vertebrata. And among the Invertebrated classes, although the deteriora¬ tion of the blood in its passage through the body is not made manifest by any change of colour, yet its renewal by exposure to air in the respiratory organs is not less requisite. 228. Hence the continual movement of the> blood is neces¬ sary for two purposes in particular; — -first, to convey the nutritive materials from the place where they are received and prepared, to that in which they are appropriated, and thus to afford to every organ a constant supply of the materials which it requires ; — and, second, to carry this fluid, at regular intervals, to certain organs by whose instrumentality it may be exposed to the influence of the air, so as to regain the qualities it has lost, and part with what it has taken-up to its prejudice. But there are many other objects fulfilled by it, which will unfold themselves as we proceed. Properties of the Blood. 229. When the circulating blood of a red-blooded animal is examined with a microscope, it is seen to consist of two distinct parts ; — a clear and nearly colourless fluid, to which the name of liquor sanguinis (or liquor of the blood) is given ; and of an immense number of rounded particles floating in this fluid, which are often termed the globules of the blood. The shape and size of these particles are, for the most part, very uniform in animals of the same species ; but in no instance are they globular ; and it is better, therefore, to term them corpuscles. In Man and most other Mammals, they are nearly flat discs, resembling pieces of money, but usually exhibiting a slight depression towards the centre (fig. 115). 204 BLOOD-DISCS OF MAN AND MAMMALS. No nucleus can be distinguished in them, but they present a dark central spot, which is an optical effect of their bi-concave form; and this spot may be made to disappear by the addition Fig. 115.— Red Corpuscles of Human Blood. Seen separately at a , aa showing the front view, b the profile or edge view, and * a three-quarter view; at b united with each other so as to form columns like pibs of money; at c in a state of alteration such as exposure to air will produce; d shows a colourless corpuscle, or lymph-glohule. of water to the liquid in which they are suspended, the discs first becoming flat, then bulging-out on either side, and at last swelling so as to burst. The reason of this will be pre¬ sently explained* (§231). In Man and Mammals generally, the diameter of these blood-discs varies from about 1-2 800th to 1 -4000th of an inch ; but in the small Musk-deer , it is less than 1-1 2,000th. In the Camel tribe, the discs are oval, as in the lower Yertebrata. 230. In Birds, Beptiles, and Fishes, the blood-particles present some curious differences from those of Mammalia. In the first place, they are much larger ; their form, also, is oval instead of being round ; and instead of being depressed in the centre, they bulge-out on each side. This bulging is Fig. 116. — Blood Corpuscles of Pigeon. At a are seen the red corpuscles 6, and the colourless, or lymph globules c, c; at u, a red corpuscle treated with acetic acid; and at c, the same treated with water, so as to render the nucleus more distinct. evidently occasioned by the presence of a nucleus which is more solid than the rest ; the nucleus, however, is not so well BLOOD-DISCS OF BIRDS AND REPTILES. 205 seen in the corpuscles of circulating or of freshly-drawn blood, as it is in that of blood which has been drawn for some little time ; and it is best brought into view by treating the blood either with water or with acetic acid. The long diameter of the oval discs of Birds (fig. 116) varies from about 1-1 7 00th to 1-240 0th of an inch ; and the short diameter from about Fig. 117.-— Blood Corpuscles of Frog. At a are seen the red corpuscles a, b, and the colourless corpuscle c ; at b, a red corpuscle treated with acetic acid. 1 -300th to 1 -4800th. Thus the discs, though much longer than those of Man, are not in general much broader. In Reptiles, Fig. 118. — Blood Corpuscles of Proteus. o, bt red corpuscles ; a*, corpuscle showing the nucleus ; c, colourless corpuscle.; d, red corpuscle treated with water. 206 BLOOD-DISCS OP KEPTILES AND FISHES. there is considerable diversity as to the size of the discs ; but the largest particles are found in the group of Amphibia , and especially in those species which retain their gills through life. The oval discs of Frogs (fig. 117) have a long diameter of about 1-1 000th of an inch, and a transverse diameter of about 1-1 800th. Those of the perennibranchiate Amphibia (§ 87) may even be distinguished by the naked eye ; those of the Siren having a long diameter of about 1 -435th of an inch, whilst in the Proteus (fig. 118) the long diameter is stated occasionally to reach 1 -337th of an inch. In Fishes, also, the size of the blood-discs is variable ; they are sometimes smaller (fig. 119), though generally larger, than those of the Frog ; but they never approach those of the last-named remarkable ani¬ mals. Hence the great size of the Fig. 119. — Blood Corpuscles of blood-discs of the Curious Lcpido- Roach* siren (fig. 41) is strongly indicative corptcie of the Reptilian affinities of that treated with water. species. 231. It is by observing the large blood-discs of the Frog, and still better those of the Proteus and Siren, that we can obtain the best information as to their structure. They are evidently flattened cells , having an envelope or cell-wall, which consists of an extremely delicate membrane, and which con¬ tains a fluid. The nucleus consists of an assemblage of minute granules, which seem adherent to each other and to the wall of the cell ; and it corresponds, in all essential particulars, to the nuclei of the cells of other Animal tissues (§ 32). The fluid contained in the cells has a red colour.; and it is to this that the peculiar hue of the blood of Yertebrata is owing. When we are looking at a single layer of blood-discs, how¬ ever, their red colour is not apparent, but they have rather a yellowish tint; and it is only when we look through a number at once, that the characteristic hue is seen. The fluid is of about the same density as that in which the par¬ ticles float ; and thus neither will have a tendency to pass towards the other. But, if we dilute the liquor sanguinis with water, the fluid outside the cells will have a tendency to pass towards their interior, according to the law of Endos- mose. The cells will in consequence be first distended, and STRUCTURE AND COMPOSITION OF RED CORPUSCLES. 207 will then burst ; and their contents will be diffused through the surrounding fluid, whilst their membranous walls will subside to the bottom. On the other hand, if the liquor sanguinis be rendered denser than the fluid in the blood- discs, as by the admixture of gum or syrup, the latter will pass towards it, and the cells will become still more flattened, and more or less completely emptied. The flexibility and elasticity of the blood-discs are well seen, in watching (with a microscope) its flow through the minute vessels ; for if one of them meets with an accidental obstruction to its progress, its form becomes accommodated to that of the space left for it to pass, and it makes its way through a very small aperture, recovering its usual form immediately afterwards. 232. The Red Corpuscles differ considerably in chemical composition from the liquid in which they float. Of the solid residue obtained by drying, about one-eighth is formed by their cell-walls, the remainder being yielded by the cell- contents. The latter portion seems to consist chiefly of a mixture of two components, which have been named globulin and hcematin. The former is a colourless substance, nearly allied to albumen in composition, but differing from it in some of its reactions ; its most characteristic peculiarity, how¬ ever, being its power of crystallizing. Its crystals, the form of which varies in different animals, are usually tinged deeply with hsematin, from which they cannot easily be freed. The composition of hsematin, to which alone the colour of the red corpuscles (and consequently of the whole mass of the blood) is due, is notably different from that of the albuminoid compounds ; the proportion of carbon to the other components being much greater, and a definite quantity of iron being an essential part of it. This iron, in a certain state of oxidation, has been supposed to be the source of the red colour ; but such is certainly not the case ; and this hue must be, like the colours of Plants, a peculiar attribute of the organic compound which presents it. — Besides their globulin and hsematin, the red corpuscles contain a certain proportion of fatty and mineral matters. The former, which are united with phos¬ phorus, are of a kind which are scarcely traceable in the liquor sanguinis ; and the latter are remarkable as having potass for their principal base, whilst the base of the salts of the liquor sanguinis is chiefly soda . Hence it appears that 208 PROPORTION OF RED CORPUSCLES. the Bed Corpuscles draw into themselves nearly the whole of the iron, phosphorus, and potass, which the chyle pours into the circulating current ; and that they modify a large pro¬ portion of the solid matter of the blood, that which they con¬ tain being notably different in composition from that of the liquor sanguinis, which does not differ, save in the proportion of its components, from the liquid portion of Chyle or Lymph. 233. The proportion of Bed Corpuscles to the whole mass of the blood varies greatly in different animals, and even in different states of the same animal. It is greatest in those which have the highest muscular vigour and activity, and which consume the largest quantity of oxygen by respiration ; hence these particles are rather more numerous in the blood of Birds than in that of Mammals, and far more abundant in these last than in Beptiles or Fishes. Again, they are more numerous in Men of ruddy complexion, strong pulse, and active habits, than in those of pale skins, languid circu¬ lation, and comparatively feeble powers. In a healthy Man they seem to constitute about half the mass of the circulating blood ; but they contain as much as three-fourths of its solid matter, the proportion of dry corpuscles being about 150 in 1000 parts of blood, whilst that of the other solid matters is about 50. A very marked decrease occasionally presents itself in disease ; the proportion of dry corpuscles being some¬ times reduced as low as 27. When too abundant, they pro¬ duce what is known as the plethoric condition of the body, in which haemorrhage from the bursting of a blood-vessel is liable to occur. Their number is effectually reduced by bleed¬ ing ; and the aspect of those who have suffered from extreme loss of blood, gives sufficient evidence that the deficiency is not made-up for a long period. The most effectual means of restoration, in cases where the proportion of blood-corpuscles is too low, is a highly nutritious diet, with the administration of iron as a medicine ; for this substance seems to have the power of hastening the reproduction of the corpuscles, being itself an essential ingredient in their contents ; and there are facts which show its remarkable power of increasing their amount in proportion to the mass of the blood. 234. It appears that the red corpuscles, like other cells, have a certain allotted term of life ; and as they are con¬ tinually dying, they must be as continually reproduced. The COLOURLESS CORPUSCLES — USES OF RED CORPUSCLES. 209 mode in which this reproduction is effected has not yet been •clearly made out ; but there is strong reason to believe that the 'red corpuscles are developed from the corpuscles of the chyle and lymph (§ 222) which are continually being poured into the circulating current, and of which isolated examples, known as the white or colourless corpuscles, are met with in every drop of blood that is examined under the microscope. The size of these is pretty much the same in all Yertebrata, their diameter being usually about 1 -3000th of an inch. In the blood of Man and the Mammalia in general (fig. 115, d) they are not easily distinguished from the red particles ; their diameter being nearly the same, wdiile the colour of single discs of the two kinds is not very dissimilar. But in the lower Yertebrata, whose blood has large oval red particles, the differ¬ ence between the two kinds is very obvious ; and the resem¬ blance which the colourless globules (c, figs. 116-119) bear to those of the chyle and lymph, is very striking. Similar colour¬ less particles exist, to a variable amount, in the nutritive fluid of Invertebrated animals ; so that in this, as in some other respects, that fluid bears a stronger resemblance to the chyle and lymph of the Yertebrata, than it does to their blood, which is characterised by the presence of the red particles. 235. Physiologists are now generally agreed, that one of the functions of the Eed Corpuscles is to convey oxygen from the lungs to the tissues and organs through which the blood circulates, and to bring back the carbonic acid which is set free in these, so as to deliver it at the lungs. Por although it is certain that the liquor sanguinis can also convey these gases, yet experiment shows that the red corpuscles can take up, bulk for bulk, a much larger proportion of them ; and that the blood which is richest in these particles is, therefore, most fit to serve as the medium for the transmission between the respiratory organs and the body at large. How it is in the nervo-muscular apparatus that there is the greatest demand for oxygen; for this apparatus is not capable of vigorous action, unless oxygen be freely supplied to it. The quantity of this it requires, however, depends upon the exercise of its powers ; for when at rest, it needs little or no more than is made use of by the other tissues ; but whilst in activity, it needs a greatly-increased supply. The quantity of oxygen which the animal takes-in by its lungs, and the amount of p 210 USES OF RED CORPUSCLES - LIQUOR SANGUINIS. carbonic acid which it gives-off by the same channel, vary, therefore, with the muscular exertion it makes. This variation is most easily observed and measured in Insects ; and it is found in them to be enormous (§ 308). As, however, the blood of the Invertebrata does not contain these red particles, to which so important a function has been assigned, it may be asked, how the conveyance of oxygen to their tissues is provided for. The reply is very simple. In Insects , and other Arti- culata which have active powers of motion, the air is con¬ veyed to the tissues, not through the medium of the blood, but directly through air-tubes which convey it to every part of the body (§ 321). And in the Molluscous classes, as among the Crustacea also, the nervo-muscular system forms so subordinate a part of the general mass of the body, and its movements are so sluggish, that the quantity of oxygen which the fluid part of the blood conveys to them, is sufficient for their need. 236. Of the properties of the Liquor Sanguinis , whilst it is circulating in the vessels, the microscope tells us nothing ; since it constantly remains in the state of a transparent fluid. Eut if the blood be withdrawn from the living body, it soon undergoes a very curious and important change. A large portion of it passes into the solid state, forming the crassa- mentum or clot ; whilst there remains a transparent liquid of a yellowish hue, which is termed the serum . It is evident that the clot contains all the red particles ; but it is easily proved that its coagulation is not due to them. For the blood of a Frog, or of any other animal having blood-discs suffi¬ ciently large, may be caused to pass through filtering-paper, which will retain and collect its blood-discs, allowing the liquor sanguinis to flow through it ; and this fluid will coagu¬ late just as completely as if these particles were retained in it. Again, in certain conditions of the blood (generally result¬ ing from disease), even when the coagulation is allowed to take place in the ordinary manner, the fibrin and the red particles separate from one another, — the latter gradually subsiding, whilst the former are left at the surface ; and the upper part of the clot is then nearly colourless, exhibiting what is commonly known as the huffy coat or crust ; whilst the lower part of it includes the red particles, and has a very deep colour. The huffy coat, being composed almost exclu- LIQUOR SANGUINIS - COAGULATION. 211 sively of the fibrous network, is very firm in its texture, being sometimes almost leathery in its character ; whilst the lower part of the clot, which is chiefly composed of the red particles, loosely bound together by scattered fibres, is very soft, and easily broken asunder. This effect may be also produced, by acting on healthy blood with certain substances which retard its coagulation, such as a strong solution of Glauber’s salt ; for if sufficient time is allowed, the red par¬ ticles will subside in consequence of their greater specific gravity, leaving a colourless layer of fibrin above them. — It is of the liquor sanguinis , in a concentrated form, that those exudations consist, which are poured out from the blood for the repair of injuries, and which pass spontaneously into the condition of a simple form of tissue (§ 393). 237. When a very thin slice of the clot is examined with a microscope, it is found to be made up of a net- work of an imperfectly fibrous character, interlacing in every direction, and including the blood-discs in its meshes. These fibres are produced by the spontaneous change in the fibrin of the blood, from the fluid to the solid form. So long as the blood is circulating in the vessels of the living body, so long does its fibrin remain dissolved in the watery part of it ; but so soon as it is withdrawn from these, and is allowed to remain at rest, it undergoes this remarkable change. If fresh-drawn blood be continually stirred with a stick or beaten with twigs, the fibrin coagulates in irregular strings, which adhere to the stick or twigs ; and it does not then include the red particles, which are left behind in the fluid. In this manner it may be completely separated from the other elements of the blood, which have not in themselves the least tendency to coagulate spontaneously. Although forming a large proportion of the substance of the clot, the fibrin, when dried, does not consti tute more than from 2 to 3 parts by weight in 1000 of blood. This proportion is augmented to 6, 8, or even 10 parts, in severe inflammatory diseases. 238. When the fibrin and the red particles have both been separated from the blood, there remains a fluid, the serum . in which a good deal of albumen is dissolved, together whn fatty matter, and other organic substances ; with the addition of saline matter, of which a considerable proportion is chloride of sodium, or common salt. The proportion which the solid. p 2 212 SERUM - USES OF BLOOD. matter of the serum bears to the whole mass of blood, in health, is about 5 3 parts in 1000 ; and of these about 40 parts are albumen, 8 parts saline matter, and 5 parts fat, with certain ill-defined substances, of which some appear to be organic compounds that are undergoing metamorphosis into solid tissues, whilst others are the products of the decay of the tissues, which are being progressively withdrawn and eliminated by the excretory organs. 239. The influence of the Blood as a whole upon the animal as well as on the nutritive functions, is easily proved. When an animal is bled largely, it is gradually weakened as the flow proceeds, and at last it seems to lose all consciousness and power of movement. If allowed to remain in this con¬ dition, it seldom or never recovers of itself. But if we inject into its veins, by small quantities at a time, blood similar to that which it has lost, the apparent corpse becomes as it were reanimated, and all its functions are completely re-established. The importance of the red particles is manifestly seen in the effect of this remarkable operation, which is called the trans¬ fusion of blood ; for if, instead of blood freshly obtained from another living animal, we inject serum without these particles, the effect is but little greater than if so much water were introduced, and the animal dies of the haemorrhage. By this operation, practised on the Human subject, many valuable lives have been saved, that would otherwise have been de¬ stroyed by loss of blood. Again, if, by mechanical means, as by tying the principal blood-vessel going to any organ, we cause a permanent diminution to any considerable extent, in the quantity of blood with which it is supplied, a decrease in its size is soon apparent, and it may even shrink almost to nothing. On the other hand, we observe that, the more active the function of a part, the larger is the quantity of blood with which it is supplied. Thus, when the antlers of the Stag, which fall off every year, are being renewed, the arteries that supply the parts of the skull from which they spring, are greatly increased in size ; but they shrink again, as soon as the growth of the horns is completed for that year. A similar increase takes place among animals that suckle their young, in the size of the arteries that supply the mammary glands, by which the milk is formed ; and these also shrink, when this liquid is no longer required. USES OF SEPARATE CONSTITUENTS OF BLOOD. 213 240. The following appear to be the chief uses of the principal constituents of the Blood, considered separately, in the general economy The fibrin is the material which is most assimilated to the condition of the solid tissues, having the power of passing from the liquid state into a low and simple form of organization. It was formerly supposed to be the nutritive material at the expense of which the solid tissues generally are immediately produced ; the muscular substance, in particular, being regarded as chemically identical with it. But there is now good reason to think that the greater part of the tissues form themselves at the expense of the albumen of the serum and perhaps of the globulin of the red corpuscles ; and that the purpose of the fibrin is chiefly to give origin to those simple forms of fibrous or connective substance, the production of which is the first step in the reparation of injuries. Were it not for its power of coagula¬ tion, the slightest cut or scratch might become fatal, from the gradual draining-away of the blood ; and such, in fact, has actually happened, in cases of disease in which the fibrin is deficient. The presence of fibrin also gives a degree of vis¬ cidity to the blood, which, as experiment proves, favours (instead of resisting, as might have been expected) its passage through capillary tubes ; and thus, when there is a deficiency in this ingredient, local stagnations and obstructions in the circulation of the blood are very liable to occur. The albumen of the blood may be considered, like that of the egg, as the raw material, at the expense of which (in combination -with fat) every other organic compound in the body is generated. It is, as we have seen, the substance to which all the tissue- forming elements of the food are reduced in the process of digestion ; and in this condition it seems to be continually appropriated by the acts of self-formation that are taking place, with varying rapidity, throughout the body, just as the albumen of the egg is appropriated by the self-formative operations of the embryo. There is strong reason to believe that a large proportion of the solid tissues regenerate themselves by the direct appropriation of this material ; and if (as has been already stated to be probable) the simple fibrous tissues find their material in the fibrin, and the muscular substance in the globulin of the red corpuscles, it is from the albumen that these substances are themselves elaborated, both of them 214 USES OF SEPARATE CONSTITUENTS OF BLOOD. being, as it were, in process of organization. The albumen of the hlood further serves to supply the albuminoid matters which are required as constituents of various secretions, espe¬ cially those which are concerned in the digestive process, as the saliva, the gastric juice, and the pancreatic fluid. A large amount is daily drawn-off for the production of the peculiar ferments contained in these secretions, whose action upon the food is necessary for its reduction to the form in which alone it can he received into the circulating current. Hence the making of new blood involves a considerable ex¬ penditure of the old. 241. The liquid in which the fibrin and albumen are dis¬ solved, has a considerable power of absorbing gases ; and this is greatly increased by the presence of the saline matters which it holds in solution. Hence the liquor sanguinis not only sustains the nutrition of the body, but can also serve, to a considerable extent, as a medium of communication between the lungs and the tissues. In this kind of activity, however, it is completely surpassed by the red corpuscles (§ 235). Independently of their use in ministering to the function of Respiration, there seems reason to believe that the red cor¬ puscles are also subservient to that of Nutrition ; for a certain conformity which exists between the organic and mineral sub¬ stances they contain (§ 232), and the composition of Muscle and ISTerve, taken in connexion with the manifest relation between their number and the activity of the Hervo-muscular apparatus, makes it probable that they have it for their especial office to prepare the materials which are to be used in its pro¬ duction and renewal of those tissues. The saline matter of the blood has many important offices : thus it furnishes the mineral ingredients which are requisite for the production of the tissues and secretions ; it helps to preserve the organic substances from decomposition ; and, in conjunction with the albumen, it keeps up the density of the serum to the point at which it is equi¬ valent to that of the contents of the red corpuscles, without which balance the condition of the latter would be seriously impaired (§ 231). Finally, the fatty matters of the blood are subservient to two very important functions — the maintenance of heat, and the formation of tissue. They maintain the combustive process, whenever there is a deficiency of more readily combustible material j and they also take part with ASSIMILATING AND SELF-PURIFYING POWER OF BLOOD. 215 albumen in the formation of all new tissue, its nuclear par¬ ticles being always found to include fat-granules. 242. The presence of a due proportion of the foregoing substances in the blood is an essential condition of health ; and we find it provided-for in the marvellous power which the blood, like any solid tissue, seems to have of making itself from the materials supplied to it, and of getting rid of what is superfluous or unsuitable. Thus an excess of albuminous matter in the food does not seem to produce more than a very limited increase in the quantity of albumen in the blood, the surplus being made to undergo changes within the body, which issue in its being removed by the excretory organs. An excess of any of the saline compounds is very speedily strained off (as it were) into the urine. And an excess of fatty matters is drawn off either by the formation of fat as a tissue, or by the augmented activity of the liver in producing bile. This conservative power is still more remarkably shown in the completeness with which the poisons that are generated in the body by the decay of its tissues, and which are received into the current of the circulation for the purpose of being conveyed to the several excreting organs, are drawn off from it, so as to leave the blood pure. Thus, carbonic acid is being continually produced in such large quantities, that its accu¬ mulation in the blood, even for five minutes, would be fatal ; yet by the aerating process to which the lungs are subservient, it is got rid of as fast as formed, so that the blood is restored to its previous purity. In like manner, the urea, which is one of the products of the wear and tear of the muscles consequent upon their use, is so perfectly and constantly eliminated by the kidneys, that its detection in the circulating current is a matter of difficulty, although we know that it must always be passing through this. 243. Thus the circulating current may be likened to a tidal river running through the midst of a large town, and supplying it with the water needed for the drink of its human and other inhabitants, as well as with that which is required for the various manufacturing and cleansing opera¬ tions carried on within its precincts ; the same stream also receives the drainage of the town, and consequently becomes charged with the products of animal and vegetable decompo¬ sition, and the foul refuse of manufactories ; and as the flow 216 CIRCULATION OF THE BLOOD. of the tide brings back a large proportion of what is carried down at ebb, the waters speedily become so contaminated with hurtful and offensive matters as to be unfit for use, unless means be provided for getting rid of these as fast as they are poured in. The perfection with which this requirement is fulfilled in the Animal body, while it excites our admiration, should also incite us to imitation, so far as the art of Man can hope to imitate the works of the Divine Artificer. Circulation of the Blood. 244. In some of the lower tribes of Animals, the blood appears to circulate in channels which are merely excavated in the substance of their tissues and organs. But among all the Yertebrata, and even in most of the Invertebrated classes, the movement of the blood takes place in a very complicated apparatus, which is composed, 1st, of a system of tubes or canals which serve to convey it through every part of the structure, and 2d, of a special organ for the purpose of giving motion to that liquid. These canals are known as the blood-vessels ; and this special organ is the heart. 245. The Heart is the centre of the circulating apparatus. It is a kind of fleshy bag, communicating with the blood¬ vessels : and it alternately dilates to receive the blood, which is conveyed to it by one set of these ; and then contracts so as to force it out into another set of tubes. In this manner a continual current is kept up. All but the lowest animals have a heart, or something which represents it. Such an organ exists, not merely among all the Vertebrated classes, but in all the Mollusca, and in the higher Articulata. But, as will presently appear, there is a great diversity in its form, and in the complexity of its construction ; for whilst, in its simplest condition, it possesses but one cavity, communicating with both sets of vessels, — it contains, in its highest forms, four different chambers, each of which has its own peculiar function. 246. The two sets of blood-vessels just adverted-to are, 1st, the Arteries, which convey the blood from the heart into the several parts and organs of the body ; and 2d, the Veins , which collect the blood that has been distributed through these, and return it to the heart. The Arterial system, as it issues from the heart, consists of one or more large trunks, which divide into branches, very much in the manner of the, DISTRIBUTION OF ARTERIES AND VEINS'. 217 stem of a tree ; these branches again subdivide into others more numerous but smaller, and these again into twigs still more numerous and more minute ; until almost every portion of the body is so penetrated with them, that not even a trifling scratch, cut, or prick, can be made, without wounding some one of these small divisions (fig. 120). — The Yenous system presents a corresponding distribution, but it is destined for an opposite purpose ; and we must regard it as commencing in the tissues by the minuter canals, which run together like the Fig. 120. — Distribution of the smaller Blood-Vessels in the Membrane BETWEEN TWO OF THE TOES OF THE HIND FOOT OF THE COMMON FROG ; a veins ; b b, arteries. little rivulets that form the origin of a mighty river, or like the smallest fibres of which the roots of a tree are made up. The larger canals thus formed gradually unite with each other as they approach the heart, towards which they all tend, just as the various tributary streams pour their contents into one prin¬ cipal channel : and at last all the veins empty into the heart, by one or two large trunks, the blood which they have conveyed from the several parts of the body ; just as all the tributaries which have arisen over a wide extent of country, pour into the ocean the water they have collected, by one mouth which is thus common to all of them. 247. Although the number of the Arterial branches increases 218 AREAS OF ARTERIAL TRUNKS AND BRANCHES. so vastly, as we proceed from their origin towards their termi¬ nation, yet their capacity does not, at least in any considerable degree ; — that is, the first or main trunk will allow as much fluid to pass through it in a certain time, as will the whole of the first set of branches into which it divides, or the still more numerous subordinate branches into which these diverge. Or, to put this fact in another form, if we cut across the main trunk, and compare the area , or space included within its circular walls, with the sum of the areas of all the branches it supplies at a certain distance — say a foot — from the heart, we shall find them precisely equal ; and the same will hold good, if the comparison be made with the sum of the areas of the more numerous but smaller branches at a greater distance from the main trunk. It is quite true that, when an artery divides into branches, the combined size of these seems to be greater than that of the trunk ; but this is only because the compa¬ rison is made, not between the areas of their circles, but their diameters . Thus, an artery of 10T lines in diameter, may divide into three branches, two of them having a diameter of 7 lines, and the third a diameter of 2 lines ; — and yet these will convey no more blood than the .single trunk. For, according to a simple rule in geometry, the areas of circles are to each other as the squares of their diameters. The area of the trunk is expressed, therefore, by the square of 10T, which is almost exactly 102. The area of each of the two large branches, in like manner, is expressed by the number 49, which is the square of 7 ; and that of the smaller one by 4, the square of 2 ; and the sum of these (49+49+4) is ex¬ actly 102, making the combined areas of the branches the same as that of the trunk. In like manner, one of the branches of 7 lines diameter might subdivide into two branches of a little less than 5 lines each ; for, as the square of 5 is 25, and twice that number is equal to 50, the combined areas of the two branches of 5 lines each, exceed by very little the area of the trunk of 7 lines. — Hence it results, that the pressure of the blood upon the walls of the arteries will be everywhere almost exactly the same ; — a conclusion which is confirmed by experiment. 248. There are certain differences in the structure and dis¬ tribution of the Arteries and Veins, which it is desirable to mention. The Arteries receive the blood pressed out from STRUCTURE AND DISTRIBUTION OF ARTERIES AND VEINS. 219 the heart, and must be strong enough to resist the force of its contraction ; otherwise, as there is a considerable impediment to its onward flow, produced by the minuteness of the tubes through which it has to pass, and the friction to which it is subjected against their sides, their walls would give way, and they would burst. They have, accordingly, a tough elastic fibrous coat, which contains also more or less of non-striated muscular fibre. On the other hand, the Veins receive the blood after the heart’s power over it has been almost ex¬ pended in forcing it through the capillary system, and when it is consequently moving much more slowly. They are very large in proportion to the arteries ; so that, if we were to cut across a limb at any place, and to estimate the respective areas of all the veins and arteries, we should find that of the veins two or three times as great as that of the arteries. Hence the pressure on their walls is much less ; and their strength does not require to be so great. Accordingly we find their walls much thinner, and the tough elastic fibrous coat almost entirely wanting. 249. The difference in the force with which the blood presses on the walls of the arteries and veins, is seen when these vessels are wounded. If a small incision be made into an artery, the blood spouts from it to a great distance ; but if a similar incision be made in a vein, the blood merely flows out, unless we stop its passage to the heart, by making pres¬ sure on the vein above the orifice, as in ordinary blood-letting (§ 277). Hence much greater pressure is requisite to check bleeding from an artery, than to stop bleeding from a vein ; and it frequently happens that no amount of pressure can prevent the continued drain of blood from the former, so that it becomes necessary to stop the flow of blood through the artery altogether, by tying a ligature tightly round it. 250. The Arteries are for the most part so distributed, that their trunks lie at a considerable distance from the surface of the body, so as to be secluded from injury ; and they are often specially protected by particular arrangements of the bony parts. Of the Veins, on the other hand, a large proportion lie near the surface, and they are consequently more liable to be injured ; but, for the reason just stated, wounds in them are of comparatively little consequence. 251. The ultimate ramifications of the Arteries are conti- 220 CAPILLARY BLOOD-VESSELS. nuous with, the commencing twigs of the Venous system. The communication is established by means of a set of extremely minute vessels, which are termed Capillaries * These capil¬ laries form a network, which is to be found in almost every part of the body (fig. 121). It is in them alone that the blood ministers to the opera¬ tions of nutrition and secre¬ tion. Even the walls of the larger blood-vessels are inca¬ pable of directly imbibing nourishment from the blood which passes through them ; but are supplied with minute branches, which proceed from neighbouring trunks, and form a capillary network in their substance. The diameter of the capillaries must of course bear a certain proportion to that of the blood-discs which have to pass through them : in Man they are commonly from about 1 - 2500th to 1-1 600th of an inch in dia¬ meter. In the true capilla¬ ries, it would seem that only one row or file of these particles can pass at a time ; but we frequently see vessels passing across from the arteries to the veins, which will admit several rows. There seems, however, to be a considerable difference in the diameter of the same capillary at different times ; a change sometimes taking place from causes which are not yet understood, f The rate at which the blood moves * From the Latin capilla, hair ; so named on account of their being, like hairs, of very minute size. Their diameter is really, however, far less than that of ordinary hairs. f The circulation of the blood in the Frog’s foot, the tail of the Tadpole, the gills of the larva of the Water-Newt,, the yolk-bag of embryo Fish, and other appropriate subjects for the observation, is one of the most beautiful and interesting spectacles that the Microscope can open to us. Details of the various modes of exhibiting it will be found in the Author’s treatise on “ The Microscope and its Revelations,” Chap, xviii. Fig. 121. — Portion of the Membrane BETWEEN THE TOES OF THE HIND foot of the Frog, more highly magnified than in fig. 120, showing the network of Capillaries that traverses it ; a, small venous trunk ; b b, branches communicating with the capillaries ; c, intervening tissue covered with epi¬ thelium cells. RESPIRATORY CIRCULATION. 221 through the capillaries of a warm-blooded animal, has been determined by microscopic examination to be about 3-100ths of an inch per second. From the comparison of this rate with that of the flow of blood through the larger arteries, which has been found by experiment to be nearly 12 inches per second, it appears that the area of the capillary system must be nearly four hundred times as great as that of the vessels which supply it with blood. 252. Thus the Arterial and Yenous systems communicate with each other at their opposite extremities ; their large trunks through the medium of the heart ; and their ultimate subdivisions through the capillaries. Hence we may consider this double apparatus of vessels as forming a complete circle, through which the blood flows in an uninterrupted stream, returning continually to its point of departure ; and the term circulation is therefore strictly applicable. 253. But the conveyance of the nutritive fluid to the several organs of the body, for their support and maintenance, is not the only object to which its circulation has to minister. It is requisite that the blood should be continually exposed to the influence of the air, by which it may get rid of the carbonic acid with which it has become charged during its circulation in the system, and may take-in a fresh supply of oxygen to replace that which has been withdrawn from it. In order to effect this exposure, the blood is conveyed to a particular organ, in which it is made to pass through a special set of capillary vessels, that bring it into almost immediate contact with air. In the lower tribes, in which this aeration is (from various causes hereafter to be explained) much less constantly necessary than in the higher, we find the respiratory organ supplied by a branch from the general circulation ; and the blood which has passed through it, and which has been sub¬ jected to the invigorating influence of the air, is mingled in the heart with that which has been deteriorated by circulating through the system, which is again supplied with this mixed, half-aerated blood. But in the highest classes, there is a dis¬ tinct circle of vessels subservient to the respiratory function : namely, an arterial trunk issuing directly from the heart, and subdividing into branches which terminate in the capillary system of the respiratory organ ; a set of capillaries, in which the aeration of the blood takes place ; and a system of veins 222 OTHER USES OF THE CIRCULATION. which, collects the blood from these, and returns it to the heart. This circuit of the blood is sometimes called the lesser circulation; to distinguish it from that which it makes through the general system, which is called the greater circulation . 254. Although carbonic acid is one of the chief impurities with which the blood becomes charged during its circulation, in consequence of the changes of composition which are con¬ tinually taking place in the living body, it is by no means the only one ; and other organs are provided, besides the lungs, for removing the noxious matters from the current of the cir¬ culation as fast as they are introduced into it. Thus, in the course of its movement through the general system, the blood is made to pass through the liver, the kidneys, and the skin, each of which has its special purifying office ; these organs, however, have no such special circulation of their own as the respiratory apparatus of higher animals possesses, though the liver, as we shall hereafter see (§ 267), is peculiarly supplied by a sort of offset from the general circulation, so that the blood from which its secretion is formed is venous instead of arterial, like that transmitted to the lungs. 255. The course which the blood takes, and the structure of the apparatus which is subservient to its movement, differ very greatly in the several classes of animals. The chief of these differences will be pointed out hereafter ; and it will be preferable to commence with the highest and most complex form of the circulating system, such as we find in Man, that it may serve as a standard of comparison with which the rest may be contrasted. Circulating Apparatus of tlie Higher Animals. 256. In Man, and those animals which approach him most nearly in structure, the heart is situated between the lungs in the cavity of the chest, which is termed by anatomists the thorax. Its form is somewhat conical ; the lower extremity tapering almost to a point, and the upper part being much larger. The lower end is quite unattached, and points rather forwards and to the left ; during the contraction of the heart, it is tilted forwards, and strikes against the walls of the chest, between (in Man) the fifth and sixth ribs. It is from the large or upper extremity that the great vessels arise ; and CIRCULATING APPARATUS IN MAN. 223 these, being attached to the neighbouring parts, serve to suspend the heart, as it were, in a cavity in which its movements may take place freely. This cavity is lined by a smooth serous membrane (§ 43), which, near its top, is vj ac t ac vj ar vc vr a vl Fig. 122. — Lungs, Heart, and principal vessels op Man. ar, right auricle; vr, right ventricle; vl, left ventricle; a, aorta; vc, vena cava; ac, carotid arteries ; vj, jugular veins ; as, subclavian artery ; vs, subclavian veins ; t, trachea. reflected downwards over the vessels, and covers the whole outer surface of the heart. Hence as the surface of the heart, and the lining of the cavity in which it works, are alike smooth, and are kept moist (in health) with a fluid secreted for the purpose, there is as little interruption as possible from friction in the working of this important machine. 257. The heart may be described as a hollow muscle, which, in Birds and Mammalia, as in Man, is divided into four distinct chambers. This division is effected by a strong vertical partition, that divides the entire heart into two halves, which are almost exactly similar to each other, excepting in the greater thickness of the walls on the left side ; and each of these halves (which do not communicate with one another) is again subdivided by a transverse partition, into two cavities, 224 STRUCTURE OF THE HEART. of which, the upper one is termed the auricle , and the lower the ventricle . Thus we have the right and left auricles, and the right and left ventricles. Each auricle communicates with its corresponding ventricle, by an aperture in the Superior Pulm. Pulmonary vena cava art. Aorta artery — y Pulmonary veins Pulmonary veins , Right auricle - Tricuspid valves - ~J Inferior vena cava * — ~l Right ventricle Left ventricle Partition Aorta Fig. 123. — Ideal Section op the Human Heart. transverse partition, which is guarded by a valve. The walls of the ventricles are much thicker than those of the auricles ; and for this evident reason, — that the ventricles have to propel the blood, by their contraction, through a system of remote vessels ; whilst the auricles have only to transmit the fluid that has been poured into them by the veins, into the ventricles, which dilate themselves to receive it. The difference in the thickness of the walls of the left and the right ventricles is explainable on the same principle ; for the left ventricle has to send the blood, by its contractile power, through the remotest parts of the body; whilst the right has only to transmit it through the lungs, which, being much nearer, require a far less amount of force for the circulation of the blood through them. 258. The arterial system of the greater circulation entirely springs from one large trunk, which is called the aorta (see figs. 122-124); this originates in the left ventricle, and is the only vessel which passes forth from that cavity. It first ascends towards the bottom of the neck ; then forms what is termed the arch , a sudden curve, which gives it a downward arterial system of man. 225 Femoral artery direction ; and then descends along the front of the column, behind the heart, as far as the Temporal artery Carotid art. Aorta Renal artery Iliac artery Anterior ttibial artery Posterior tibial artery Peroneal artery Art. of foot Fig. 124. — Arterial Q of Man- 226 ARTERIAL SYSTEM OF MAN. trunk, where it divides into two great branches, which proceed to the lower extremities. From the arch of the aorta are given off the arteries which supply the head and upper extremities. These are, the two carotids , which ascend on either side of the neck; and the two subclavian, which pass outwards beneath the clavicles, so as to arrive at the arms, — becoming successively in their course the axillary and brachial arteries, as they pass through the axilla or arm-pit, and along the arm. The subclavian and carotid arteries of the right side arise together from the aorta, in Man, by a common trunk ; but this arrangement varies much in different Mammals. Thus in the Elephant, the two carotids arise by a common trunk, — the two subclavians separately. In some of the Whale tribe, all four are separate. In the Bat, the subclavian and carotid of the left side arise from a common trunk, like those of the right. And in those Ruminating animals which possess a long neck, all four arteries come off from the aorta together, by a large trunk, which first gives off the subclavians on either side, and then divides into the carotids. All these varieties occasionally present themselves in Man; — a fact of no small interest. 259. The descending aorta , in its progress along the trunk, gives-off several important branches; — as the coeliac , from which the stomach, liver, and spleen are supplied ; the renal, to the kidneys ; and the mesenteric, to the intestines. It divides at last into the two iliac arteries ; which, after giving off branches for the supply of the lower bowels, pass into the thighs, where they become the femoral arteries ; and these again subdivide into branches for the supply of the leg. 260. For the sake of comparison, a figure of the arterial system of a Bird is introduced ; from which it will be seen that by far the larger proportion of its blood is distributed to its upper extremities. In Man, the descending aorta is evidently the continuation of the aortic arch ; and the parts which it supplies receive far more blood than the head and upper extremities, — the locomotion of biped man being per¬ formed almost entirely by his lower limbs. In Quadrupeds, which require nearly as much strength in their fore feet as in their hind, the subclavian arteries bear a larger proportion to the iliac. But in Birds, the function of locomotion is almost entirely performed by the wings ; and their powerful- muscles, ARTERIAL SYSTEM OF BIRD. 227 which constitute the mass of flesh lying on the breast, are supplied with blood by the arteries of the upper extremities, which here possess a manifest predominance. The aorta, soon after its origin, subdivides into three large branches ; of which the first two (one on either side giving-off the subclavian and carotid arteries) convey the blood to the head, the wings, and Lingual artery External carotid artery Ischiatic artery Aorta Renal Subclavian artery Mammary artery Femoral artery Sacral artery Cloaca Fig. 125. — Arterial System of Bird. the muscles lying on the thorax ; whilst the middle one curves backwards and downwards, and becomes the descending aorta. hTow that which is here the continuation of the great side Q 2 228 DISTRIBUTION OF ARTERIES. branch, is neither the carotid nor the subclavian, both of which are subordinate branches given-off from it; but it is the trunk which distributes the blood to the muscles of the breast, and which in Man is a subordinate branch of the sub¬ clavian artery (the mammary). The descending aorta is seen to lose itself almost entirely in supplying the viscera of the trunk ; so that the branches into which it divides at last for the supply of the legs, are very small. These limbs, in birds, are usually required only for the support of the body at times of rest, and are seldom much concerned in locomotion ; so that they possess little muscular power, and require but a small supply of blood. 261. It is very interesting to trace such differences in the arrangement of the vascular system, corresponding with vari¬ ations in the general plan of structure, yet not exhibiting any actual departure from the general type. Thus, there is probably not a single large artery in Man, to which a corre¬ sponding branch might not be found in the Bird ; on the other hand, there is perhaps not a single large artery in the Bird, to which there is not an analogous branch in Man. The chief difference consists in the relative sizes of the seve¬ ral trunks ; and these correspond closely with the amount of tissue they have respectively to supply. Here, then, we have one example, out of many that might be adverted-to, of that Unity of Design which we see everywhere prevalent through¬ out nature ; manifesting itself in the close conformity of a great number of apparently-different structures to one general plan, whilst there is, at the same time, an almost infinite variety in the details. 262. There is a very interesting peculiarity in the distribu¬ tion of the arteries, by which the due circulation of blood in their branches is provided for, even though there should be an obstruction in the main trunk. The branches which are given-off from it at different points, have frequent communica¬ tions or anastomoses with each other ; so that blood may pass from an upper part of a main artery into the lower, by means of these lateral communications, even though its flow through the trunk itself should be completely stopped. 263. These anastomoses are very numerous in the arteries of the limbs, and particularly about the joints ; and it is well that they are so ; for, by relying on the maintenance of the ANASTOMOSES OF ARTERIES - ANEURISM. 229 circulation through them, the Surgeon is often able to save a limb, or even a life, which would otherwise be sacrificed. Arteries are liable to a peculiar disease, termed aneurism which consists in a thinning-away, or rupture, of the tough fibrous coat, and a great dilatation of the other coats, so that a pulsating tumour is formed. This change takes place most frequently at the bend of the thigh, the ham, the shoulder, and the elbow ; where the artery, in the working of these joints, often has to undergo sudden twists. The result of the disease would be generally fatal, in consequence of the gradual thinning-away of the walls of the tumour, which at last bursts, allowing the blood to escape from the arterial trunk with such rapidity as, if unchecked, to cause almost instantar neous death. In order to prevent this, the surgeon ties the artery at some little distance above the aneurism, — that is, he puts a thread round it, which is drawn so tight as to prevent the passage of any blood to the aneurism. The circulation in the lower part of the limb is at first retarded ; its temperature falls ; and it becomes more or less insensible. But after the lapse of a few hours, the circulation becomes quite vigorous, the pulsations strong, the temperature rises, and the numb¬ ness passes off ; and as the main trunk still continues com¬ pletely obstructed, this can only have been brought about by the flow of blood through the anastomoses, which must in that short period have undergone considerable enlargement. Examination of the vessels after death shows that this has been actually the case. Even the aorta has thus been tied in dogs, without causing death ; the anastomoses of the branches given-off from its upper part, with those proceeding from the lower, being sufficient to maintain the circulation in the latter, when the current through the main trunk is obstructed. 264. A very complex series of anastomoses, forming a com¬ plete network of large tubes, is found in several situations, where it seems desirable that the flow of blood to a particular organ should be retarded, whilst a large amount is to be allowed to pass through. Thus in animals which keep their heads near the ground for some time together, as in grazing, the arteries which supply the brain suddenly divide, on their entrance within the skull, into a great number of branches, by the anastomoses of which a complex network is formed ; and from this network, by the reunion of its small vessels. 230 PECULIARITIES OF DISTRIBUTION OF ARTERIES. originate the trunks which supply the brain in the usual manner. The object of this apparatus appears to be, to pre¬ vent the influence of gravitation from causing a too great rush of blood towards the brain, when the head is in a depending position ; for the rapidity of its flow will be checked, as soon as it enters the network, and is distributed through its numerous canals. A similar conformation is found in the blood-vessels of the limbs of the Sloth, and of some other animals which resemble that animal in the sluggishness of their movements ; and its object is probably to prevent the muscles from receiving too rapid a supply of blood, which would give them what (for these animals) would be an undue energy of action ; whilst, by the very same delay, their power of acting is greatly prolonged, — as we find it to be in Beptiles, whose circulation is languid (§ 284). 265. In the Whale tribe, and some other diving animals that breathe air, we find a curious distribution of the blood¬ vessels, which has reference to their peculiar habits. The intercostal arteries (which are sent-off from the aorta to the spaces between the ribs on each side) are enormously dilated, and are twisted into thousands of convolutions, which are bound together into a mass by elastic tissue. This mass, which is of considerable bulk, lies at the back of the chest, along both sides of the vertebral column ; and it serves as a reservoir, in which a great quantity of arterial blood may be retained. The veins also have very large dilatations, which are capable of being distended, so as to hold a considerable amount of venous blood ; and thus, while the animal is pre¬ vented from breathing by its submersion in the water, the circulation through the capillaries of the system is sustained, by the passage of the blood stored up (as it were) in the arterial system, into the venous reservoirs. If this provision did not exist, the whole circulation would come to a stand, in consequence of the obstruction it meets with in the lungs, when the breathing is stopped. 266. With regard to the Venous system , there is little to be added to what has been already stated (§§ 248-250) as to its general character and distribution. The large proportion which its capacity bears to that of the arterial system, is shown by the fact, that every main artery is accompanied by a vein (fre¬ quently by two) considerably larger than itself ; and that the DISTRIBUTION OF VEINS — PORTAL SYSTEM. 231 superficial veins, which, lie just beneath the skin, are capable of conveying at least as much more. The veins of the body in general unite in two large trunks, the superior and inferior vena cava ; which meet as they enter the right auricle of the heart (fig. 123). The superior vena cava is formed by, the union of the veins which return the blood from the neck (the jugulars ) with those which convey it from . the arms (the subclavians), as shown in fig. 122 ; and the inferior cava (y c, fig. 122) receives the blood from the trunk, the organs contained in the abdomen, and the lower extremities. 267. There is, however, an important peculiarity in the distribution of the veins of the Intestines, which should not pass unnoticed. Instead of delivering their blood at once into the inferior vena cava, these veins unite into a trunk, called the Vena Portae (fig. 1 34), which enters the liver and subdivides into branches, whence a capillary network proceeds that per¬ meates the whole of its mass. It is from the venous blood, as it traverses this network, that the secretion of bile is formed ; and the blood which is brought by the hepatic artery serves chiefly to nourish the liver, — no bile being formed from it, until it has become venous. The blood is carried-off from this double set of capillaries by the hepatic vein, which conveys it into the inferior vena cava. In Tishes, not only the blood of the intestines, but that of the tail and posterior part of the body, enters this “ portal ;; system, which is distributed to their kidneys as well as to their liver. Thus all the blood which flows through the portal system, has to go through two sets of capillaries, between each period of its leaving the heart by the aorta, and its return to it by the vena cava. 268. We have yet to notice the lesser circulation, which is confined to the Lungs only. The venous blood which is returned to the heart by the vense caves, enters the right auricle, and thence passes into the right ventricle. By the contraction of this last cavity, it is expelled through the pul¬ monary artery (fig. 123), which soon divides into two main trunks that proceed to the right and left lungs respectively. The right trunk again subdivides into three principal branches, which are distributed to the three lobes or divisions of the right lung ; whilst the left divides into two branches, which are in like manner distributed to the two lobes of the left lung. The capillaries, into which these branches ultimately 232 LESSER CIRCULATION — FORCES THAT MOVE THE BLOOD. subdivide, are distributed upon the walls of tbe air-cells (fig. 162)* and tbe character of the blood is in them converted, by exposure to the air, from the dark venous to the bright arte¬ rial. From this capillary network the pulmonary veins arise ; and the branches of these unite into trunks, of which two proceed from each lung, to empty themselves into the left auricle (fig. 123). This auricle delivers the blood, now arte- rialized or aerated (§ 253), into the left ventricle, whence the aorta arises ; and by the contraction of this cavity, it is delivered through that vessel to the system at large.— It will be observed that the vessel which proceeds from the heart to the lungs is called the pulmonary artery , although it carries dark or venous blood. This is because it conveys the blood from the heart towards the capillaries. And, for a similar reason, the vessels which return the blood from the capillaries to the heart are termed pulmonary veins, although they carry red or arterial blood. Forces that move the Blood. 269. The mechanical action, by which the blood is caused to circulate in the vessels, is easily comprehended. The cavi¬ ties of the heart, as already explained (§ 245), contract and dilate alternately, by the alternate shortening and relaxation of the muscular fibres that form their walls (Chap, xn.) ; and the force of their contraction is sufficient to propel the blood through the vessels which proceed from them. The two ventricles contract at the same moment ; the auricles contract during the relaxation of the ventricles, and relax whilst the ventricles are contracting. The series of movements is there¬ fore as follows : — The auricles being full of the blood which they have received from the venae cavae and pulmonary veins, discharge it by their contraction into the ventricles, which have just before emptied themselves into the aorta and pul¬ monary artery, and which now dilate to receive it. When filled by the contraction of the auricles, the ventricles contract in their turn, so as to propel their blood into the great vessels proceeding from them ; and whilst they are doing this, the auricles again dilate to receive the blood from the venous system, after which the whole process goes-on as before. It is when the ventricles contract, that we feel the heat of the heart, which is caused by the striking of its lower extremity MECHANISM OF THE HEART. 233 against the walls of the chest ; and it is by the same action that the pulse in the arteries is produced (§ 27 6\ 270. The combined actions of each auricle and its ventri¬ cle, may be illustrated by an apparatus like that repre¬ sented in fig. 126. It con¬ sists of two pumps, a and 6, of which the pistons move up and down alternately ; and these are connected with a pipe c f, in which there are two valves d and e, opening in the direction of the arrow. The portion c of the pipe represents the venous trunk by which the blood enters the heart ; the pump a represents the auricle, and the raising of its piston enables the fluid to enter and fill it. When its piston is lowered, its fluid is forced through the valve d into the pump b {which represents the ventricle), whose piston rises at the same time to receive it ; and when this piston is lowered in its turn, the fluid (being prevented from returning into a by the closure of the valve d) is propelled through the valve e into the pipe f which may represent an arterial tube ; whilst at the same time a fresh supply of blood is received into the pump a by the raising of its piston. 271. The number of contractions of the heart ordinarily taking place in an adult man, is from 60 to 70 per minute. It is usually rather greater in women ; and in children it is far higher, being from 130 to 140 in the new-born infant, and gradually diminishing during the period of infancy and child¬ hood. It is rather greater in the standing than in the sitting posture, and in sitting than in lying down : it is increased by exercise, especially by ascending a steep hill or going upstairs, and also by any mental emotion. It is important to remember these facts, in reference to the management of those who are suffering under diseases of the heart or of the lungs, which prevent the ready passage of the blood through these organs ; for if more blood be brought to the heart by the great veins, than it can propel through the pulmonary arteries, a feeling of 234 VALVES OF THE HEART. very great distress is experienced ; and there may be even danger of rapture of the heart or large vessels, or of sudden cessation of the heart’s action, causing instant death. Such persons ought, therefore, carefully to refrain from any violent muscular movement, and also to avoid giving way to strong mental emotions. — In syncope or fainting, the heart’s action is so weakened as to be scarcely perceptible, though it does not entirely cease ; and this state may be brought on by several causes which make a strong impression on the nervous system, such as violent mental emotion (whether joy, or grief, or terror), sudden loss of blood, and the like. 272. The blood which has been received by each ventricle from its auricle, is prevented from being driven back into the latter, on the contraction of the former, by a valve that guards the aperture through which it entered. This valve consists of a membranous fold, surrounding the borders of the aperture, and so connected with the neighbouring parts, as to yield when the blood passes from the auricle into the ventricle, but to be tightened so as completely to close the aperture when the blood presses in the contrary direction. The manner in which these valves act will be seen from fig. 127, which is a section of the right auricle with its ventricle. The auricle, a , receives its blood from the two venae cavae, e, e ; and transmits it into the ventricle, b , by the orifice, c. On either side of this orifice are seen the membranous folds, which are kept in their places by the tendinous cords, d. a Now when the blood is passing from a to b , these folds yield to the current ; but when the cavity b is filled and begins to contract, the blood presses against their under sides, so as to make them close against each other, as far as they are permitted to do by the tendinous cords. In this manner the aperture is completely shut, and no blood can flow back. A valve of this land exists on each side of the heart; but there is a slight difference between the forms of the two, whence they have received different names. That on the right side has three pointed divisions, to which the tendinous cords are attached, and it is hence called the tricuspid valve; - 9 Fig. 127. — Section- op one SIDE OF THE HEART. SEMILUNAR VALVES. 235 whilst that on the left side has only two, so as to bear some resemblance to a bishop’s mitre, whence it is called the mitral valve. 273. The aorta and pulmonary artery are in like manner furnished with valves, which prevent the blood that has been forced into them by the contraction of the ventricles, from returning into those cavities when they begin to dilate again. These valves, however, are formed upon a different plan, and more resemble those of the veins, which will be presently described. They consist of three little pocket-shaped folds of the lining membrane of these arteries (similar to those at b b , fig. 128), which are pressed flat against the walls of those tubes when the blood is forced into them ; but as soon as they are filled, and the ventricles begin to dilate, so that the blood has a tendency to return, it presses upon the upper side of these pockets, and fills them out against one another, in such a manner as completely to close the entrance into the ven¬ tricle. The three little pocket-shaped folds, however, would not close the centre of the aperture, were it not that each of them has a little projection from its most prominent part, which meets with those of the others, and effects the requi¬ site end. The situation of these valves (which are termed semilunar from their half-moon shape) is seen at g , fig. 127, / being the pulmonary artery. 274. The amount of blood sent-out from either ventricle at each contraction, in a middle-sized man, seldom exceeds 3 ounces ; but the whole quantity of blood contained in the body is not less than 18 lbs. : hence, it would require 96 contractions of the heart to propel the whole of this blood through the body, and these (at the ordinary rapidity) would occupy about 1-J minute. It has been calculated, from recent experiments, that the usual force of the heart in man would sustain a column of blood about 7 feet 2 inches high, the weight of which would be about 4 lbs. 3 oz. on every square inch. The backward pressure of this column upon the walls of the heart, or in other words, the force which they have to overcome in propelling the blood into the aorta, is estimated at about 13 lbs. 275. From the mode in which the blood is forced into the arterial system by a series of interrupted impulses, it might be supposed that its course would be a succession of distinct 236 EQUALIZING ACTION OF ARTERIES : — PULSE. jets ; but this is prevented, so that the current is reduced to an equable stream by the time it reaches the capillaries, through the elasticity of the walls of the arteries. In order to comprehend how this acts, we may suppose a forcing-pump (§ 270) to propel its fluid, not into a hard unyielding tube of iron or lead, but into an elastic tube of india-rubber. The effect of each stroke of the pump will be partly expended in distending the tube, so as to make it contain an additional quantity of water ; and the suddenness of the jet at its oppo¬ site extremity will be diminished. In the interval of the stroke, the elasticity of the wall of the tube will cause it to con¬ tract again, and to force-out the added portion of its contents ; this it will not have completed by the time that the action of the pump is renewed ; and in this manner, instead of an inter¬ rupted jet at the mouth of the tube, we shall have a continuous flow, which, if the tube be long enough, will become quite equable.* It is precisely in this manner that the elasticity of the arteries influences the flow of blood through them, by converting the interrupted impulses which the heart com¬ municates to it, into a continued force of movement. In the large arteries, these impulses are very evident ; in the smaller branches they are less so, but they still manifest themselves by the jerking in the stream of blood proceeding from a wound in one of these vessels ; whilst in the capillaries, the influence of the heart’s interrupted impulses cannot usually be seen at .all, the streams that pass through them being perfectly equable. 276. The phenomenon which we call the pulse , is nothing else than the change in the condition of the artery occasioned by the increased pressure of the fluid upon its walls, at the moment when the heart’s contraction forces an additional quantity of blood into the arterial system. By the frequency and force af this change, we can judge of the power with which the blood is being propelled. But the pulse can only be well distinguished, when we can compress the artery against some resisting body, so that there is a partial obstruc¬ tion to the flow of blood through it, which causes the disten¬ sion to be more powerful ; the most convenient artery for this * The same effect is obtained in an ordinary fire- or garden-engine, by the interposition of an air-vessel, in which the elasticity of com¬ pressed air is substituted for that of the wall of the pipe. PULSE : — WOUNDS OF ARTERIES. 237 purpose is the radial artery (fig. 124) at the wrist ; but the carotid artery in the neck, and. the temporal artery in the temple, may be felt, when it is desired to know the force of the circulation in the head; as may the arteries supplying other parts, when we wish to gain information respecting the organs they supply. An increased action in the organ, whether this be due to inflammation, or to a state of unusual activity of its function, causes an increase of size in the artery which supplies it ; and thus the pulsation may be unusually strong in a particular trunk, when the heart/ s action and the general circulation are not in a state of excitement. For instance, a whitlow on the thumb will occasion its artery to beat almost as powerfully as the radial artery usually does ; and excessive activity of the mind, prolonged for some hours, greatly increases the force of the pulsations in the carotid arteries, from which the brain is chiefly supplied. 277. When an artery is wounded, there is often great difficulty in controlling the flow of blood ; for pressure can seldom be effectually applied in the situation of the wound ; and the surgeon is generally obliged to tie the vessel above the orifice. As a temporary expedient, the loss of blood may be prevented by making firm pressure upon the artery above the wounded part, that is, nearer the heart ; and many valu¬ able lives have been saved by the exercise of presence of mind, guided by a little knowledge. The best means of keeping-up the requisite pressure, until the proper instrument (the tourniquet) can be applied, is to lay over the artery (the place of which may be found by its pulsation) a hard pad, made by tightly rolling or folding a piece of cloth ; this pad and the limb are then to be encircled by a bandage, by which the pressure is maintained ; and this bandage may be tightened to any required degree, by twisting it with a ruler or a piece of stick. Thus a constant pressure may be exercised upon the artery, which will be generally sufficient to control the bleeding from it. But there are, unfortunately, many cases in which pressure of this kind cannot be applied ; as for instance when the femoral artery is wounded high up in the thigh, or the carotid artery in the neck. And nothing else can then be done, but to compress the artery with the thumb, or with some round hard substance (such as the handle of an awl), until proper assistance can be obtained. 238 FLOW OF BLOOD THROUGH THE VEINS. 278. The impulse of the heart, and the elasticity of the arteries, which together propel the blood through the capillary system, continue to act upon it after it is received into the veins ; and are in fact the chief causes of its movement in them. If we interrupt the current of blood through an artery by making pressure upon it, and open the corresponding vein, the fluid will continue to flow from the latter, so long as the artery contains blood enough to be forced into the vein by its own contraction ; but as soon as it is emptied, the flow from the orifice in the vein will cease, even though the vein itself remains nearly full. If the pressure be then taken off the artery, there is an immediate renewal of the stream from the vein, which may be again checked by pressure on the artery. In the ordinary operation of bleeding, we cause the superficial veins of the arm to be distended, by tying a bandage round them above the point at which we would make the incision ; and when an aperture is made, the blood spouts forth freely, being prevented by the bandage from returning to the heart. Eut if the bandage be too tight, so that the artery also is compressed, the blood will not flow freely from the vein ; and the loosening of the bandage will then produce the desired effect. When a sufficient quantity of blood has been with¬ drawn, the bandage is removed ; and the return-flow through the veins being now unobstructed, the stream from the orifice immediately diminishes so as to be very easily checked by pressure upon it, or may even cease altogether. Fig. 128.— Vein laid open, to show its Valves. a 279. The veins contain a great number of valves , which are formed, like the semilunar valves of the aorta (§ 273), by "a doubling of their lining mem¬ brane. Their situation may be known by the little dilatations which the veins exhibit at the points where they occur ; and which are very obvious in the arm of a person not too fat, when it is encircled by a bandage that causes distension of the superficial veins. The structure of these valves is seen at b 6, FLOW OF BLOOD THROUGH THE VEINS. 239 fig. 128; they consist of pocket-like folds of the lining mem¬ brane, which allow the blood free passage as it flows towards the heart, but check its reflux into the arteries. Hence it follows, that every time pressure is made upon the veins, it will force towards the heart a portion of the blood they con¬ tain, since this cannot be driven in a contrary direction. How, from the manner in which the veins are distributed, some of them must be compressed by almost every muscular move¬ ment ; these will become refilled as soon as the muscles relax ; and they will be again pressed-on, when the movement is repeated. Hence a succession of muscular movements will act the part of a diffused heart , over the whole of the venous system, and will very much aid the flow of blood through its tubes. It is partly in this manner, that exercise increases the rapidity of the circulation. If the blood is brought to the heart by the great veins more rapidly than usual, the heart -must go through its operations more rapidly, in order to dispose of the fluid ; and if these actions be impeded, great danger of their entire cessation may exist. Hence the importance of bodily tranquillity to those affected with diseases of the heart or lungs (§ 271). 280. Besides the aid thus afforded to the venous circulation, it is probable that there is another cause of the motion of the blood in them, which is independent of the action of the heart and of the arteries. Many facts lead to the belief that a new force is produced, while the blood is flowing through the capillary vessels, — a force which may, in some instances, maintain the circulation by itself alone. Thus in many of the lower animals, it seems as if the power of the heart were so unequal to the maintenance of the circulation, that this must partly depend upon some other influence ; and even in the highest, there is evidence that the movement of blood in the capillaries may continue for a time, after the action of the heart and of the arteries has ceased to affect it.* This movement seems intimately connected with the changes to which the blood is subservient in the capillaries ; for, if these be checked, not even the heart7 s action can propel the blood through them, although no mechanical * For a full consideration of this question, see the Author’s Principles of Comparative Physiology (4th edition), §§ 247-251 ; and Principles of Human Physiology (5th edition), §§ 267-275. 240 CIRCULATION IN MAMMALS AND BIRDS. obstruction exists. Thus, when the admission of air to the lungs is prevented, the blood will not pass through the pulmonary capillaries, since it cannot undergo the change which ought to be performed there ; and it therefore accumu¬ lates in the pulmonary artery, the right side of the heart, and venous system ; and if no relief be afforded by the admission of air into the lungs, the whole circulation is thus brought to a stand. This condition, which is termed Asphyxia , occurs in drowning, hanging, and other forms of suffocation (§ 338). Course of the Blood in the different Classes of Animals . 281. The Circulation of the Blood takes place on the same general plan in all other Mammals, and in Birds, as in Man. In all the animals included in these groups, the heart is composed of two halves quite distinct from each other ; each possessing an auricle or receiving cavity, and a ventricle or propelling cavity. The course of their blood, which goes through a complete double circulation , is shown by the diagram (fig. 129). The vessels and cavities of the heart which contain venous blood are shaded ; whilst those which convey arterial blood are left white : and this distinction is kept-up in the other figures. The direction of the blood is indicated by the arrows. Every drop of blood which has passed through the capillaries of the system, is transmitted to the lungs before it is allowed again to enter the aorta ; and the whole mass of the blood passes twice through the heart, before any part of it is transmitted a second time to the vessels from which it was before returned. 282. The two sides of the heart do not possess, when that organ is perfectly formed, any communication with each other, except through the pulmonary vessels ; and thus, they might be regarded as two distinct organs, united for the sake of convenience. The right side of the heart, being placed at the origin of the pulmonary artery, and having for its office to propel the blood through the lungs so as to receive the influence of the air, may be called the respiratory heart : whilst the left side, which is placed at the origin of the aorta, and has to propel the blood to the body in general, may be called the systemic heart. The circulation would be per¬ formed precisely in the same manner, if these two organs DIFFERENT FORMS OF CIRCULATING APPARATUS. 241 Lesser circulation. Fig. 129.— Diagram of the Circulation in Mammals and Birds. were quite distinct from each other; and in fact they are almost so in the Dugong , one of the herbivorous Whales. Lesser circulation. 242 DIFFERENT FORMS OF CIRCULATING APPARATUS. Lesser circulation. Ventricle Greater circulation. Fig. 131. — Diagram of the Circulation in Fishes. (Zoology § 305). In the lower tribes of animals we shall Greater circulation. Fig. 132.— Diagram of the Circulation in Crustacea. CIRCULATION IN FCETUS AND IN REPTILES. 243 presently find that there is but a single , instead of a double , heart ; and that the organ which is absent is sometimes the systemic, and sometimes the respiratory heart. 283. Previously to birth, when the lungs are not yet dis¬ tended with air, and the aeration of the blood is provided-for in other ways, the circulation takes place on a different plan from that on which it is afterwards performed. There exists at that period an opening in the partition between the two auricles, by which they have a free communication ; and there is also a large trunk which passes from the right ventricle into the aorta. By these channels, the blood which is received from the systemic veins can pass at once into the aorta, without going through the pulmonary vessels. But when the young animal begins to breathe, these communi¬ cations are speedily obliterated ; the blood is transmitted through the pulmonary vessels to the lungs ; and the whole circulation takes place upon the plan just described. There are occasional instances, however, in which the communica¬ tion between the auricles remains open, so that the double circulation is never perfectly established ; for a portion of the blood is allowed to pass from the right to the left side of the heart, without being aerated in the lungs, so that the blood which is sent to the system contains a mixture of venous with the proper arterial fluid, — a state which will be presently seen to be natural in the Beptile. Such cases are recognised by the blueness of the skin, the lividity of the lips, and the indisposition to bodily or mental exertion. Persons affected with this malformation seldom reach adult age. 284. In the class of Reptiles, there is not a complete double circulation ; for a mixture of arterial and venous blood is sent alike to the lungs and to the general system ; and no part is supplied with the pure arterialized fluid. In general the heart contains only three cavities, — two auricles and one ventricle (fig. 133). One of the auricles receives the venous blood from the system ; whilst the other receives the arterial¬ ized blood from the lungs. Both these pour their contents into the same ventricle, where they are mingled together; and this mingled blood is transmitted, by the contraction of the ventricle, partly into the lungs, and partly into the aorta (fig. 130). In some Reptiles there is a partial division of the ventricle, so that the mixture of the arterial and venous r 2 244 CIRCULATION IN REPTILES. blood is not complete ; and whilst the blood transmitted to the lungs is chiefly that which has returned from the systemic veins, the blood which enters the aorta for the supply of the Pulmonary artery^ Pulmonary vein** Right auricle — Vena cava — Right aorta— If - - - — Ventral aorta Fig. 133.— Heart of Tortoise. system is chiefly that which has returned from the lungs in an arterialized state. Hence such animals have a circulation which approaches very closely to that of Mammals and Birds ; and it is among them that we find the greatest vigour and activity in this generally inert and sluggish class. 285. The general arrangement of the blood-vessels in Eeptiles is shown in fig. 134. It is seen that the aorta, soon after its origin, divides into three arches on either side ; and that these, after sending off branches to the head and to the lungs, reunite into a single trunk, which corresponds exactly with the aorta of the higher animals. These arches are in fact the remains of a set of vessels, which will be found to be of the highest importance in Fishes, being there subservient to the aeration of the blood : in the true Eeptiles, however, they are never concerned in this function, but they still remain, as if to show the unity of the plan on which this apparatus is formed. Precisely the same arrangement of the vessels may be seen in Birds and Mammalia, at an early stage of their development; but it afterwards undergoes* considerable changes, by the obliteration of several of the arches ; for of the four pairs which may be seen at one period, a single branch only remains on either side ; and one of these becomes the permanent arch of the aorta, whilst the other becomes the permanent pulmonary artery. Pulmonary artery Pulmonary vein Left auricle Sngle ventricle CIRCULATION IN REPTILES. 245 Arches of aorta Left auricle v Super, vena cava Ventral aorta - Pulmonary artery _ Inferior vena cava Carotid artery / * / Arches of Aorta s' / Right Auricle Ventricle ^ Pulmonary vein __ _ . Brachial artery Pulmonary artery Liver and hepatic vein Kidneys - , Ventral Aorta * . at Gastric vein Fig. 134.— Circulating Apparatus op Lizard. 286. In the class of Fishes, the circulating apparatus is still more simple. The heart only possesses two cavities, an 246 CIRCULATION IN FISHES. auricle and a ventricle. It is placed at the origin of the vessels which are concerned in the aeration of the blood ; and it receives and transmits venous blood only; hence it is analogous to the right side, or respiratory heart, of Birds and Mammalia. The venous blood, which is brought to it by the CIRCULATION IN BATRACHIA. 247 systemic veins, is transmitted by its ventricle into a trunk, which subdivides into four or five pairs of branches or arches (fig. 135). These branches run along the fringes which form the gills of the fish, and send a minute vessel into every one of their filaments (§ 312). Whilst passing through this vessel, the blood is submitted to the influence of the air diffused through the water, to which the gills are freely exposed, and is thus aerated ; and it is then collected from the several filaments and fringes, into a single large trunk, analogous to the aorta of the higher animals, by which the whole body is supplied with arterialized blood. After circu¬ lating through the system, the blood returns to the heart in a venous condition, and again goes through the same course. This course is represented in a simple form in the diagram, fig. 131 ; and it will be seen, on a little consideration, that it does not differ from that which exists in Animals with a complete double circulation, in any other essential particular than this, — that there is no systemic heart to receive the blood from the gills or aerating organs, and to convey it to the body at large. But, though all the blood must necessarily pass through the gills before it can again proceed to the body, it does not follow that the blood should be as completely aerated as in Beptiles, in whose circulation there is a mixture of venous and arterial blood ; for the exposure of the blood to the small quantity of air which is diffused through water is not nearly so effectual as its direct exposure to air. 287. There is a group of animals which forms the transition between Fishes and Beptiles ; some of them being Fishes at one part of their lives, and Beptiles at another ; whilst others remain, during their whole lives, in a condition intermediate between the two groups. Of this group (§ 86), the common Frog is a familiar example. In the Tadpole state, it is essen¬ tially a Fish, breathing by means of gills, and having its cir¬ culation upon a corresponding plan; but after it has gone through its metamorphoses, it breathes by lungs, its heart acquires an additional auricle, and the whole plan of the circu¬ lation is changed, so as to become comformable to that of the true Beptile. This process takes place, not suddenly, but by progressive stages; and as these are extremely interesting, they will now be briefly described. In fig. 136 we have a representation of the circulating apparatus of the Tadpole in 248 CHANGE OP CIRCULATION IN TADPOLE. its fish-like condition. At a is seen the large trunk which issues from the ventricle, forming a bulbous enlargement like that which is seen in the corresponding part of the Fish. From this enlargement proceed three trunks on each side, called the branchial arteries (frr1, br2, hr3), which convey the blood to the gills or branchiae; and after being aerated bypassing through their filaments, the blood is collected by the bran- 1 tot o ah vb 3 a ap av c ah 2 vb Fig. 136. — Blood-vessels op the Tadpole, in first State. chial veins (vb, vb). Of these, the first pair transmits its blood by the vessels o, o, t, (which are also formed in part by the ^econd pair) to the head and upper extremities ; whilst the greater part of the blood of the second pair, with the whole of that of the third, is discharged into the trunk c on either side. By the union of that vessel with its fellow, the trunk a v is formed ; which conveys the blood that has been aerated in the gills, to the general system, and is thus to be evidently regarded as the aorta. But we find here three small vessels (1, 2, and 3), which do not exist in the Fish; and which establish a communication between the branchial arteries and the branchial veins, in such a manner, that the blood may pass from the former into the latter, without going through the filaments of the gills. These communicating vessels are very small in the Tadpole, and scarcely any blood passes through them ; but it is chiefly by their enlargement, that the course of the blood is subsequently altered. There is also a fourth branch, ap, which proceeds to the lungs on either side ; and as these organs are not yet developed, this pulmo¬ nary artery also is at first of very small size. 288. As the metamorphosis of the other parts proceeds, CIRCULATION IN TADPOLE. 249 however, and the animal is being prepared for its new mode of life, the lungs are gradually developed, and the pulmonary arteries greatly increase in size ; whilst the gills, on the other hand, do not continue to grow with the animal, but rather shrink, from the diminished supply of blood which they receive. For, during this period, the communicating branches 1 t o t 3 ap av ap Fig. 137. — The Same, in a more advanced State. increase in size ; so that a considerable part of the blood which has been transmitted into the branchial arteries passes at once into the veins, and thence into the aorta, without being made to traverse the gills ; its aeration being partly accomplished by the lungs. This state of things is seen in fig. 137 ; where ap, ap, are the enlarged pulmonary arteries ; and where the communicating branches are seen almost to form the natural continuations of the branchial arteries. A condition of this kind exists permanently in those Batrachia which retain their gills during their whole lives, and have the lungs imperfectly developed (§ 87). When the metamorphosis is complete, the branchial vessels altogether disappear, but the arches still remain, as shown in fig. 138. The first of these arches sup¬ plies the vessels of the head, 1 1 ; which also, however, receive a branch o from the second arch. The second arch, after giving off that branch, unites with its fellow to form the aortic trank av . The third arch has completely shrivelled up. And the fourth arch or pulmonary artery has now attained its full size, and is become the sole channel through which the aeration of the blood is effected. 250 CIRCULATION IN INVERTEBRATED CLASSES. 289. Among the Invertebrated classes generally, the condi¬ tion of the circulating apparatus differs from that which prevails 0 1 1 throughout the Vertebrata, in one remarkable feature; — namely, that whereas in the latter the blood moves in every part of its course through a set of closed vessels, it meanders in the former through a set of channels or sinuses excavated in the substance of the tissues, and communicating with the “ general cavity of the body ” in the midst of which the viscera lie. Generally speaking, it is in the venous system that the greatest deficiency exists ; for the heart usually sends forth the blood by definite arterial trunks, which distribute it by its ramifications through the substance of the various parts of the body ; and it is in its course from these to the respiratory organs that it is least restrained within definite boundaries. The degree of this imperfection differs considerably in the several groups of Invertebrata ; for whilst, in the highest Mol- lusca and Articulata, the vascular system is almost as complete as in Yertebrated animals, we find it gradually becoming less and less distinct as we descend, so that in the lower forms of both series it presents itself merely as an extension of the general cavity of the body, and is not furnished with any special organ of impulsion. 290. In the greater part of the Mollusca, the circulation CIRCULATION IN MOLLUSCA. 251 takes place nearly on the same general plan as in Fishes ; the heart having, two cavities, and the whole of the blood travers¬ ing both the respiratory and the systemic vessels, between each time of its leaving the heart and returning to it again. But this heart is systemic , and not pulmonary ; for it receives the arterial blood from the gills, and transmits it to the great systemic artery ; and after the blood has been rendered venous by its passage through the tissues of the body, it enters the channels which distribute it to the gills, before being again subjected to the action of the heart. The accompanying figure (fig. 139) of the circulation in the Doris (a kind of sea slug) Fig. 139.— Circulating Apparatus of Doris. will serve to show the general distribution of the vessels in this group. The heart consists of the ventricle a , whence issues the main artery b ; and of a single or double auricle c, in which terminate the veins, d, of the branchial apparatus e. The aerated blood which these convey to the heart, is trans¬ mitted by it, through the artery h , to the system at large ; and from this it is collected, in the state of venous blood, by the sinuses which terminate in the large trunk //. By this trunk it 252 CIRCULATION IN GASTEROPODA AND CEPHALOPODA. is distributed to the gills e; and thence it returns to the heart, after having undergone aeration. Now if a second heart had been placed on the trunk f f just as it is about to subdivide for the distribution of the blood to the gills, the circulation would have been analogous to that of Birds and Mammals. There is a great variety in the position of the gills in Mollus¬ cous animals, and a corresponding variety in the situation of the heart, which is usually placed near them. In the Doris the gills are arranged in a circular manner, round the termina tion of the intestinal canal ; but in many Mollusca they form straight rows of fringes on the two sides of the body. In these last, the heart not unfrequently has two auricles ; but these are not analogous to the two auricles of Beptiles ; for each has the same function with the other — the reception of the blood from the gills of its own side. 291. There is a very interesting variety in the conformation of the heart in the Cephalopoda, or Cuttle-fish tribe ; which vb cs vc as b Fig. 140.— Circulating Apparatus of Cuttle-fish. seems to form a connecting link between the plan of the cir¬ culation that prevails among the Mollusca in general, and that CIRCULATION IN CEPHALOPODA AND CRUSTACEA. 253 which we have seen in the class of Fishes. The auricle and ventricle of the heart are separated from each other ; and whilst the latter remains in the position just described, the auricle occupies the place which the whole heart possesses in the class above. The course of the blood in these animals is shown in fig. 140 ; where c represents the ventricle or sys¬ temic heart, from which arises the aorta a, a , as, av, that supplies the body with arterial blood. The venous blood is returned through the great vein vc, covered with a curious spongy mass cs, the use of which is not known; this also receives the blood from the intestinal veins vv ; and it divides into two trunks which convey the blood to the gills or branchiae (br and hr), where it undergoes aeration. On each of these trunks is an enlargement, cb, which has the power of con¬ tracting and dilating, and thus of assisting the transmission of the blood through the arteries of the gills, a b. The blood is returned to the ventricle by the branchial veins, vb, on each of which there is another dilatation, bu, which might be regarded as analogous to the auricle of the other Mollusca, but that it is not muscular. Thus in the Cuttle-fish, the blood receives an impulse from the systemic heart, by which it is transmitted into the main artery ; and when it returns by the systemic veins, it receives another impulse from the branchial hearts, before it passes through the gills ; — an arrangement obviously analogous to that which we meet with in the highest Yertebrata. 292. In the Crab and Lobster, and other animals of the class Crustacea, the blood for the most part follows the same e f i a d b c course as in the Mollusca, excepting that the heart contains but a single cavity. The arrangement of the circulating appa- 254 CIRCULATION IN CRUSTACEA. ratus of a Lobster is seen in fig. 14 1, in which, a is the heart ; b and c, the arteries to the eyes and antennae ; d, the hepatic artery ; and e and f the arteries which supply the abdomen and thorax. The blood that has been propelled through these by the action of the heart, finds its way into the great venous sinus g g, which receives the fluid collected from all parts of the body ; from this it passes to the gills, h ; and thence it is returned to the heart by the branchial veins, i. Another view of a portion of the circulating apparatus is given in fig. 142, which represents a transverse section of it in the region b ve c f vb st ce Fig. 142.— Branchial Circulation of Lobster. of the heart, with one pair of gills. The heart is seen at c ; and from its under , side proceeds one of the arterial trunks which convey the blood to the system. Beturning thence, the blood enters the venous sinus s, which has an enlarge¬ ment at the base of each gill ; and this seems to act the part of a branchial heart, like the corresponding enlargement on the branchial vessels of the Cuttle-fish. From this cavity, it is carried by the vessel va into the branchiae b ; and after it has passed through the capillaries of the gill-filaments, it is collected by the vessels ve, which carry it to the branchial veins, vb, and thence to the heart. The general plan of the circulation in this class is shown in fig. 132. 293. In the class of Insects we find a still greater incom¬ pleteness in the system of vessels for the conveyance of blood. Arterial trunks can only be traced to a short distance from the dorsal vessel, which answers the purpose of a heart ; and the nutritive fluid which they convey is delivered into the channels or sinuses that exist among the different organs. CIRCULATION IN INSECTS. 255 Nevertheless, it has a tolerably regular circulation ; and the organ by which this movement is chiefly effected is a long tube, termed the dorsal vessel, which seems to propel it for¬ wards, whilst two principal sinuses, one on either side, convey it backwards. The dorsal vessel, seen at a, is a membranous tube lying along the back of the insect, and partly divided into several compartments by incomplete valvular partitions, which bear no inconsiderable resemblance to the valves of veins (§ 279). By the successive contraction of these different por¬ tions, the blood which entered at the posterior extremity of the dorsal vessel is gradually propelled forwards ; and when it arrives at the front of the body, it passes out by a series of canals, some of which convey it to the head, whilst others pass sideways and backwards for the supply of the body, with its appendages, the legs and wings. On returning from these parts, it re-enters the posterior end of the dorsal vessel. But, besides ministering to this general circulation, the several compartments of the dorsal vessel seem to act as independent hearts, each for its own segment ; into which they send forth blood by minute arterial trunks, and from which they receive it again by minute apertures furnished with valves. It is to be remarked that in Insects no special arrangement of vessels for the aeration of blood is required ; since this aeration is 256 CIRCULATION IN ARACHNIDA AND ANNELIDA. Fig. 144. — Dorsal Vessel of Spider. accomplished by the conveyance of air, by means of minute air- tubes, into every part of the body, however small (§321) ; a mode of respiration different from any that we notice elsewhere. — A very similar arrangement of the circulating apparatus is met with in the Spider tribe ; but as the body is not so long, the dorsal vessel is less extended in length, and is of larger diameter. This is seen in fig. 144 ; where a represents the abdomen of the animal ; ar, the large dorsal vessel or heart ; c, a trunk passing forwards to the head; and v, vessels communicating with the re¬ spiratory organs. 294. In the animals of theWorm tribe, belonging to the class Annelida, there is a general similarity in the course of the blood to that which prevails in Insects ; but as the respi¬ ration is accomplished by means of special organs, which are sometimes diffused along the entire body, and sometimes restricted to one part of it (§ 314), there is considerable variety in the provisions for submitting the blood to the influence of the air. In those which possess red blood (§ 226), this fluid can be seen to move in a closed system of vessels ; whilst a colourless fluid containing numerous corpuscles flows through a set of canals prolonged from the general cavity of the body. It may be surmised that the two principal offices to which the circulation of the blood is subservient, are here separately performed; the red non-corpusculated fluid having for its office to aerate the tissues, whilst the colourless but corpus- culated fluid serves for their nutrition. 295. A very curious departure from the normal type of the circulation presents itself in the class of Tunicata, the lowest of the Molluscous series (§ 114). The heart in these animals is much less perfectly formed than in the higher tribes ; though it still contains two cavities, one for receiving and the other for impelling the blood. The blood may be sometimes seen to flow in the direction customary among Mollusks; coming to the heart from the respiratory surface, and then going forth through an arterial trunk that conveys it into a CIRCULATION IN LOWER INVERTEBRATA. m system of channels excavated through the tissues, after passing through which it finds its way again to the respiratory sur¬ face, and thence to the heart. But after a certain duration of its flow in this direction, the current stops, and then re- com¬ mences in the contrary direction, proceeding first to the respiratory organs, and then to the system in general. It would seem as if in this, one of the lowest forms of animals possessing a distinct circulation, the central power were not yet sufficiently strong to determine the course which the fluid is to take. In the group of Bryozoa , which forms a connecting link between the Tunicata and Zoophytes, we lose all trace of a distinct circulation, which is only represented by the movement of fluid in the general cavity of the body, and in the prolongations of this cavity in the arms that surround the mouth (fig. 64). — -In the Star-fish , Sea-Urchin , and other animals belonging to the class Echinodermata, there seems to be a regular circulation of nutritious fluid, carried on through distinct vessels, but without any definite heart The only trace, indeed, of anything like a propelling organ, is an en¬ largement of one of the trunks, which pulsates with tolerable regularity ; but this would not seem to have force enough to propel the fluid through a complex system of vessels ; and the circulation seems to be carried on chiefly by some force produced in the capillaries (§ 280). 296. The circulating apparatus of higher animals is only represented in Zoophytes , Medusae. , and the lower Worms , by ramifying prolongations of the digestive cavity, which extend throughout the body, and are specially distributed to the respiratory surface, so as to subject the products of digestion to the aerating process. Thus, in the stony corals which are formed by animals constructed upon the general plan of the Sea Anemone (§ 127), the gelatinous flesh that connects the polypes is traversed by a network of canals that open freely into the sides of their visceral cavities, of which they may be regarded as extensions ; whilst in the Campanularia (fig. 72) and other composite Hydraform Zoophytes, a like communi¬ cation is established by a system of canals passing along the stem and branches, and becoming continuous with the base of each polype. In this system of canals, viewed under a sufficient magnifying power, a granular fluid may be seen to move, the direction of the flow being sometimes from the stem towards 2 58 CIRCULATION IN ZOOPHYTES AND SPONGES. the polypes, and sometimes from the polypes towards the stem ; the rapidity and constancy of these currents depending apparently on the activity of the growth of the parts towards which they are directed. In the Medusae we find the central stomach sending out prolongations towards the margin of the disk, where they frequently inosculate so as to form a net¬ work, which seems to have for its purpose to expose the product of digestion to the aerating action of the surrounding water ; and in this system of canals, also, a movement of fluid may be observed, which appears to depend upon the action of cilia in their interior. In all these cases, it is to be observed that the circulation of nutritive fluid is really effected by a modification of the digestive apparatus, instead of by an appa¬ ratus set apart for this sole purpose ; and the blending of the two functions is still more remarkably exhibited in the Sponge, the inosculating canals of which (§ 136) may be regarded alike as constituting a ramifying digestive cavity, or as a simple form of circulating apparatus. The most correct method is perhaps to consider it as representing both these systems, which are here blended (as it were) into one ; the simplicity of structure characteristic of this type not admitting of the division of labour which we meet with in higher organisms. CHAPTER VI. OF RESPIRATION. 297. We have seen that arterial blood, by its action on the living tissues, loses those qualities which rendered it fit for the maintenance of life ; and that after having undergone this change, it recovers its original properties by exposure to air. This exposure is necessary, therefore, to the continued existence of Animals in general. If we place an animal under the receiver of an air-pump, and exhaust the air either partially or completely, a great disturbance soon shows itself in its various functions ; shortly afterwards, the several actions of life cease to take place ; and a state of apparent death comes on, which speedily becomes real, if air be not re-admitted. The influence of air is not less necessary to NATURE OF RESPIRATION. 25 9 Plants than to Animals ; for they also die when excluded from it : and thus its presence may be stated to be a general condition, necessary for the continuance of the life of all organised beings. — There is, however, a marked difference in the rapidity with which the deprivation of air occasions death in different animals (§ 310). 298. At first sight it might be thought that Animals which always live beneath the surface of the water, such as Fishes, Zoophytes, and many Mollusca, are removed from the influence of the air ; and that they hence constitute an exception to this general law. But such is not the case ; for the liquid which they inhabit has the power of absorbing, and of holding dis¬ solved in it, a certain quantity of air, which they can easily separate from it, and which is sufficient for the maintenance of their life. They cannot exist in water which has been deprived of air (as by boiling, or by being placed under the exhausted receiver of an air-pump) ; for they then become insensible and die, just as do Mammalia and Birds when prevented from inhaling air in the ordinary manner. 299. The changes which result from the exposure of the blood or nutritious fluid of Animals to the air, either in the atmosphere, or through the medium of water, form a very important part of their vital actions ; and the changes them¬ selves, together with the various mechanical operations by which they are effected, constitute the function of Respiration. The nature of these changes will be first explained ; and the structure and operations of the organs by which they are performed will be afterwards described. Nature of the Changes essentially constituting Respiration . 300. Atmospheric air, it has been stated, is necessary to the continued life of all animals ; but this fluid is not com¬ posed of one element alone. By the science of Chemistry, it is shown to be a mixture of three gases possessing very different properties. Besides the watery vapour with which the atmosphere is always more or less charged, the air con¬ tains 21 parts in 100 of oxygen , and 79 parts of nitrogen or azote ; with about 1 -5000th part of carbonic acid gas. The question immediately presents itself, therefore, whether these gases have the same action on animals ; or, if their actions s 2 260 CHANGES IN AIR BY RESPIRATION. be different, to which of them specially belongs the property of thus contributing to the maintenance of life. This question may be decided by a few simple experiments. If we place a Bird or small Mammal in a jar filled with air, and cut off all communication with the atmosphere, it perishes by suffo¬ cation in a longer or shorter time ; and the air in the vessel, which has thus lost the power of maintaining life, is found by chemical analysis to have lost the greater part of its oxygen. If we then place another animal in a jar filled with nitrogen gas, it perishes almost immediately; whilst if we place a third in pure oxygen, it breathes with greater activity than in air, and shows no sign of suffocation. It is then evident, that it is to the presence of oxygen that atmospheric air owes its vivifying properties. 301. But the change produced in the atmosphere by animal respiration is not limited to this. The oxygen which disap¬ pears is replaced by carbonic acid ; which, instead of being favourable to the maintenance of life, causes the death of animals which inhale it, even in small quantities. The exhalation of this substance is an action not less general in the Animal kingdom than the absorption of oxygen ; and it is in these two changes that the act of respiration essentially consists. 302. The quantity of nitrogen or azote in the air that has been respired, varies but very little. There appears, however, to be a continual absorption of nitrogen by the blood, and as continual an exhalation of it. When the quantity exhaled and the amount absorbed are equal, or nearly so, no change manifests itself in the air which has been breathed ; when the quantity absorbed is the greater, there is a diminution in that which the respired air contains ; and when the quantity exhaled is the greater, there is a corresponding increase. An exhalation of nitrogen seems to be ordinarily taking place in warm-blooded animals, to an extent varying between l-50th and 1- 100th of the oxygen consumed; but when the same animals are partially or wholly deprived of food, an absorption of nitrogen usually occurs. 303. The differences in the character of the blood which are produced by its exposure to the air, have already been noticed (§ 227) ; and we now see that they are essentially due to the absorption of oxygen, and the setting free of carbonic CHANGES IN BLOOD BY RESPIRATION. 261 acid. These changes will take place out of the living body as well as in it ; provided that the blood can be exposed as com¬ pletely to the influence of the atmosphere. When blood is dr^wn from a vein into a basin or cup, the dark hue of the surface is usually seen to undergo a rapid alteration, so as to pre¬ sent the arterial tint ; but this is confined to the upper surface, because it alone is exposed to the influence of the atmosphere. The alteration takes place still more rapidly and completely if the blood be exposed to pure oxygen gas ; but even then it is almost confined to the surface. It is not prevented, even though the direct communication between the blood and the gas be cut off by a membranous partition, as it is in the living animal ; for if the blood be tied up in a bladder, the gas has still the power of penetrating to it, and of effecting the change in it ; and the change is manifested, not only by the alteration in the aspect of the blood, but by the disappearance of a certain quantity of oxygen, and its replacement by carbonic acid. Now if, by spreading out the blood in a very thin layer, we expose a much larger surface to the air, or if, by frequently shaking it, we continually change its surface, we render the change more complete. Eut still it is accomplished far less effectually than it is in the lungs or gills of a living animal ; for when it is passing through their capillaries, it is divided into an immense number of very minute streams, each of which is completely exposed to the influence of the air, and the combined surface of which is very great. 304. The question next arises, what becomes of the Oxygen which disappears, and what is the origin of the Carbonic acid which is thus produced by respiration 'l This question will now be considered. 305. When charcoal is burned in a vessel filled with air, the oxygen disappears, and is replaced by an equal bulk of carbonic acid : at the same time there occurs a consider¬ able disengagement of heat. During respiration, the same phenomena occur : there is always an evident relation between the quantity of oxygen employed by an animal, and the amount of carbonic acid it produces (the latter being usually somewhat less than the former) ; and, as we shall see hereafter (Chap, ix.), there is always a greater or less amount of heat produced. There exists, then, a great analogy between the principal phenomena of respiration, and those of the combus- 262 SOURCE OF CARBONIC ACID EXHALED. tion of carbon ; and this agreement in tlie results naturally leads to the belief that the causes of both are the same. — It is to be borne in mind, however, that the substitution of carbonic acid for oxygen is not the only change produced by respiration in the air ; for there is nearly always a disap¬ pearance of oxygen (to an amount sometimes equal to one- third of that exhaled in combination with carbon), which is taken into the system to be applied to other uses (§ 343). 306. It was at one time supposed that the oxygen of the inspired air combines, in the lungs themselves, with the carbon brought there in the blood ; and thus produces the carbonic acid which is expired, occasioning at the same time the development of heat. But this theory is inconsistent with experiment ; for it has been proved that the carbonic acid is not formed in the lungs, but that it is brought to them in the venous blood of the pulmonary artery ; and that their office is to disengage or get rid of it, whilst they at the same time introduce oxygen into the arterial blood. For in the first place, it can be shown that a considerable quantity of carbonic acid exists in venous blood, from which it may be removed by drawing it into a vessel filled with hydrogen or nitrogen, or by placing it under the vacuum of an air-pump ; it can also be shown that arterial blood contains a consider¬ able quantity of oxygen. Again, if Frogs, Snails, or other cold-blooded animals, be confined for some time in an atmo¬ sphere of nitrogen or hydrogen (neither of which gases in itself exerts any injurious effect upon them), they will continue for some time to give off nearly as much carbonic acid as they would have done in common air ; thus proving that the carbonic acid is not formed in the lungs by the union of carbon brought in the venous blood with the oxygen of the air, since here no oxygen was supplied ; and showing that the carbonic acid must have been brought ready-formed. This process, however, could not be continued for any great length of time, even in cold-blooded animals ; since a supply of oxygen is necessary to the performance of their various functions. And in warm-blooded animals, a constant supply of this element is so much more important, that they will die if cut off from it, even for a short time. 307. The quantity of oxygen thus taken in, and of carbonic SOURCE OF CARBONIC ACID EXHALED. 263 acid thus disengaged, bears a very regular proportion to the amount of exertion which is made during the same time. Hence it is much greater in tribes whose habits are active, than in those which are inert ; and it is also greater in any individual, during a day spent in active exercise, than it is in the same person during a day passed in repose. This obviously results from the fact, now established beyond all doubt, that every muscular contraction or production of muscular-force , and every production of nerve force by which muscular contrac¬ tion is usually called forth, involve, as their essential condition, the death and disintegration of proportionate amounts of muscular and nervous substances, which pass from the state of living tissues to that of dead matter ; and for this operation, the presence of the oxygen in arterial blood is required. This oxygen combines with part of the materials thus set free as waste (§ 55), and converts them into the products that are thrown off by the various excretions. One of the chief of these products is carbonic acid, which is carried off by the lungs in the manner already described. Thus the presence of oxygen in the blood is essential to the development of nervo-muscular force; while the elements of the blood itself are required to re-form the tissues which have been thus destroyed. 308. It is among Birds and Insects that we find the greatest quantity of carbonic acid produced, in proportion to the size of the animals ; and in both these classes we find extraordi¬ nary provisions for the energetic performance of this function (§§ 321 and 326). The greater energy of the respiration of Birds than that of Mammals, when compared with the greater number of the red corpuscles in their blood, gives an increased probability to the idea, that the red corpuscles are the chief carriers of oxygen from the lungs to the capillaries of the system, and of carbonic acid from the capillaries of the system to those of the lungs (§ 235). The energetic respiration of Insects, though these corpuscles are absent, is fully accounted for by the peculiar manner in which the air enters every part of their bodies (§321). In no case do we see the influence of muscular activity, on the amount of carbonic acid thrown off, more strongly manifested than in Insects. A humble-bee, while in. a state of great excitement after its capture, made from 110 to 120 respiratory movements in a minute; after 264: DIMINUTION OF RESPIRATION IN TORPID STATE. the lapse of an hour, they had sunk to 58 ; and they sub¬ sequently fell to 46. In the first hour of its confinement it produced about l-3rd of an inch of carbonic acid (a quantity many times greater, in proportion to its size, than that which Man would have set free in the same time) ; and during the whole twenty-four hours of the subsequent day, the insect produced a less amount than that which it then evolved in a single hour. In the Larva state, which is usually one of com¬ parative inactivity, the respiration is not much above that of the Worm tribes; and in the Chrysalis state of those which become completely inactive, the amount of respiration is still lower. 309. This chrysalis state, indeed, bears a strong resemblance to the condition of torpor in which many animals pass the winter. Eeptiles, and most Invertebrata that inhabit the land, become (to all appearance) completely inanimate when the temperature is lowered below a certain point ; yet retain the power of exhibiting all their usual actions when the temperature rises again. In this state, their circulation and respiration appear to cease entirely ; or, if these functions are carried on at all, they are performed with extreme feebleness ; and the animals may be prevented from reviving for a long time, without their vitality being permanently destroyed, if they be surrounded by an atmosphere sufficiently cold. Thus Serpents and Frogs have been kept for three years in an ice-house, and have completely revived at the end of that period. Among Mammals there are several species which pass the winter in a state of torpidity ; but this is less profound than the torpidity of cold-blooded animals, for the circulation and respiration never entirely cease, though they become very slow. There are various gradations between this condition and ordinary deep sleep. Thus some of the animals which hybernate or retire to winter quarters, lay up a supply of food in the autumn, and pass the cold season in a state differing but little from ordinary sleep, from which they occasionally awake, and satisfy their hunger. But others, like the Marmot, are inactive during the whole period, taking no food, and exhibiting scarcely any evidence of life, unless they are aroused. The consumption of oxygen and the production of carbonic acid, under such circumstances, are extremely slight, as might be anticipated from the languor of the circulation and the inactivity of the nervo-muscular TOLERANCE OF DEPRIVATION OF AIR. 265 system. But a very slight irritation is sufficient to produce respiratory movements ; the heart’s action is quickened ; and the animal for a time shows an increase of its general activity. 310. Animals will in general bear deprivation of air well or badly, according as the respiration is more or less active. Thus a warm-blooded animal usually dies, if kept beneath water for more than a few minutes ; though there are some which are enabled, by peculiar means, to sustain life much longer (§ 265). In cold-blooded animals, however, whose demand for oxygen is much less energetic, this treatment may be continued for a much longer time without the loss of life. Thus the common Water-Newt naturally passes a quarter of an hour or more beneath the surface, without coming up to breathe ; and it may be kept down for many times that period without serious injury. And, as we might expect from their peculiar condition, warm-blooded animals, when hyber- nating, may be kept beneath water for an hour or more, without apparent suffering; although the same animals, in their active state, would not survive above three minutes. There is reason to believe that a similar condition may be produced in Man, by the influence of mental emotion, or of a blow on the head, at the moment of falling into the water ; so that recovery is by no means hopeless, even though the individual may have been more than half an hour beneath the surface. Structure and Actions of the Respiratory Apparatus. 311. In animals whose organization is most simple, the act of respiration is not performed by any organ expressly set apart for it ; but it is effected by all the parts of the body that are in contact with the element in which the animal lives, and from which they derive their necessary supply of oxygen. This is the case, for example, in the lower class of Animal¬ cules, in the Polypes, Jelly-fish, Entozoa, and many other animals. Even in the higher classes, a considerable amount of respiratory action takes place through the skin, especially when it is soft and but little covered with hair, scales, &c., as in Man, and in the Erog tribe ; but we almost invariably find in them a prolongation of this membrane, specially designed to enable the blood and the air to act upon each other, and having 266 STRUCTURE OF RESPIRATORY ORGANS. its structure modified for the advantageous performance of this function. This modification consists in the peculiar vascularity of this membrane, that is, in the large number of vessels that traverse its surface ; and also in the thinness of the membrane itself, by which gases are enabled to permeate it the more readily. Moreover, we always find this membrane so arranged, that it exposes a very large surface to the air or water which comes into contact with it ; and this surface may be immensely extended, without any increase in the size of the organ. Thus the small lungs of a Babbit really expose a much larger respiratory surface to the air, than is afforded by the large air-sacs of a Turtle which are ten times their size. This is effected by the minuteness of the subdivision of the former into small cavities or air-cells, whilst the latter remain as almost undivided bags. 312. It is desirable to possess a distinct idea of the mode in which the surface is thus extended by subdivision. We may, for the purpose of explanation, compare the lung to a chamber, on the walls of which the blood is distributed, and to the interior of which the air is admitted. This chamber, for the sake of convenience of description, we shall suppose to have two long and two short sides, as at a. How if a parti¬ tion be built-up in the direction of its length, as at B, a new surface will be added, equal to that which the two sides previously exposed ; since both the surfaces of this partition are supplied with blood, and are exposed to the air. Again, if another partition be built-up across the chamber, as at c, a new surface will be added, equal to that which the ends of the chamber previously exposed. And thus, by the subdivision of the first chamber into four smaller ones, the extent of sur¬ face has been doubled. How if each of these small ones were divided in the same manner, the surface would again be doubled ; and thus, by a continual process of subdivision, the STRUCTURE OF RESPIRATORY ORGANS. 267 surface may be increased to almost any extent compatible with the free access of air to the cavities, and of blood to the walls. In the same manner, where the respiratory membrane is pro¬ longed outwardly, so as to form gills , which hang from the exterior of the body (as is the case in most aquatic animals), its surface is very much extended by disposing it in folds, and by dividing these folds into fringes of separate filaments. It has been calculated that, by this kind of arrangement, the gills of the Skate present a surface four times as great as that of the Human body. 313. The structure and arrangement of the Bespiratory organs differ, according as they are destined to come in con¬ tact with air in the state of gas, or to act upon water in which a certain amount of air is dissolved. In the former case, we usually find the respiratory membrane (which is but a pro¬ longation of the skin or general envelope) forming the wall of an internal cavity, — just in the same manner as the membrane, through which the act of absorption takes place in animals, is prolonged from the skin so as to form the wall of the digestive cavity (§ 14). Such a cavity for the reception of air into the interior of the body, exists in all air-breathing animals ; and in the Yertebrata it receives the name of lung. On the other hand, in animals that breathe by means of water, the respiratory surface is prolonged externally , so as to be evidently but an extension of the general surface, — just in the same manner as the roots of plants are prolonged into the soil around them. These prolongations, termed branchiae or gills, which may have various forms, carry the blood to meet the- surrounding water, and to be acted-on by the air it contains ; and then return it to the body in a purified condition. 314. The form and arrangement of the gills vary greatly in the different classes of aquatic animals. Sometimes they simply consist of little leaf-like appendages, which have a texture rather more delicate than that of the rest of the skin, and which receive a larger quantity of blood. In other in¬ stances, they are composed of a number of branching tufts, which are more copiously supplied with vessels. Among the Annelida we observe a great variety in the mode in which these tufts are disposed ; and this is connected with the general habits of the animal. Thus in the Serpula (fig. 145), whose body is inclosed in a tube, the tufts are disposed 268 RESPIRATORY ORGANS OF AQUATIC ANIMALS. around the head alone, and spread out widely into the sem¬ blance of a flower. In the Nereis (fig. 52) and its allies, they are set upon nearly every division of the body, and are much smaller. Their usual arrangement in these marine worms may be seen in fig. 146, which represents one of the appendages of Eunice. The tuft of gills is shown at b ] at c is seen a bristle¬ shaped filament, which may perhaps be regarded as the rudiment of a leg ; and the projections to which the letters t and ci point, also seem connected with the movements of the animal. In the Arenicola (the lob¬ worm of fishermen) we find the respiratory tufts dis¬ posed on certain segments only, and possessing more of an arborescent (tree¬ like) form (fig. 147). 315. A somewhat similar arrangement is seen in the larvae of many aquatic In¬ sects, which breathe by means of gills ; although all perfect Insects breathe air in the manner to be pre¬ sently described. In fig. 148 is represented the larva of the Ephemera (Day-fly), which breathes by means of a series of gill-tufts disposed along the abdomen, and also prolonged as a tail. In the Crustacea, we usually find the gills presenting the form of flattened leaves or plates. In the lower tribes of the class, they project from the surface of the body; but in the higher, they are inclosed within a cavity, through which a stream of water is made con- Fig. 145. Gill-tufts of Serpula. Gill-tuft of Eunice. RESPIRATORY ORGANS OF AQUATIC ANIMALS. 269 stantly to flow, by mechanism adapted for the purpose. Their form and position in the Crab are shown at b , Although these animals usually reside in the water, or only quit it occasionally, there are some species, known under the name of land-crabs, which have the power of living for some time at a distance from water. In order to prevent their gills from drying up, which would destroy their power of acting on the air, there is a kind of spongy structure in the gill-chamber, by which a fluid is secreted that keeps them constantly moist. 316. In the Mollusca we find the gills arranged in a great variety of modes. In the lowest class, the Tunicata, the respiratory membrane is merely the lining of the large chamber formed by the mantle (fig. 63), through which a stream of water is continually made to flow by ciliary action (§ 319); and this surface is sometimes extended by the folding or plaiting of the membrane. In most of the Con- chifera, however, we find four lamellae, or folds of membrane m' a i f v' Fig. H9. — Respiratory apparatus of the Oyster. *, one of the valves of the shell ; vf, its hinge ; m, one of the lobes of the mantle; m', a portion of the other lobe folded back; c, muscles of the shell; br, gills; b, mouth ; t, tentacula, or prolonged lips; /, liver; i, intestine; a, anus- co heart. V, %• 47. Fig. 148. Larva of Ephemera. 270 RESPIRATORY ORGANS OF AQUATIC ANIMALS. (hr, fig. 149), lying near the edge of the shell, and copiously supplied with blood-vessels. In the Oyster, these are freely exposed to the surrounding element, the lobes of the mantle being separated along their entire length ; but where the mantle-lobes are united along their margin, so as to shut-in the gills, there are two ori¬ fices, often prolonged into tubes (as in the Tellina, fig. 150), through one of which the water is drawn-in for the purpose of respiration, whilst through the other it passes out, as in the Tunicata. In the aquatic Gasteropoda there is scarcely any part of the body to which we do not find the gills attached in some species or other. In the naked marine species, which may be called Sea-slugs, they form, fringes which are sometimes disposed along the sides of the body, as in the Tritonia and Glaucus (figs. 151, 152), Fig. i5U. — Tellina. Fig. 152.— Glaucus. sometimes arranged in a circle around the end of the intestine, as in the Doris (fig. 153, — see also fig. 139); and are some¬ times covered-in, more or less completely, by a fold of the mantle. In most of the species that pos¬ sess shells, the gills form comb¬ like fringes, which are lodged in a cavity inclosed in the last turn of the spiral shell; and to this cavity the water is admitted, sometimes by a large opening, sometimes by a prolonged tube. In the Cephalopoda, we find Fig. 153.— Doris. RESPIRATORY ORGANS OF AQUATIC ANIMALS. 271 the gills composed of a collection of little leaf-like folds, placed on a stalk (h, fig. 154) ; they are inclosed in a cavity which is covered-in by the mantle ; and the walls of this cavity have the power of alternately dilating and contracting, so as to draw-in and expel water. It communicates with the exterior by two orifices, one of which, o, a wide slit, is for the entrance Fig. 154. — Gills op Poulp. of water • whilst the other, t, is tube-like, and serves not only to carry-off the water that has passed over the gills, but also to convey away the excrements, and the fluid ejected by the ink-bag. This is called the funnel. 317. In Fishes, the gills are composed of fringes, which are disposed in rows on each side of the throat, and are covered by the skin. The cavity in which they lie has two sets of apertures ; one communicating with the throat, and the other opening on the outside. In the Fishes with a car¬ tilaginous skeleton, we usually find as many of these external orifices as there are rows of gills ; thus in the Lamprey there are seven, as shown in the succeeding figure (a). But in Fishes with a bony skeleton, there is usually but a single large orifice on either side ; and this is covered with a large valve-like flap, which is termed the operculum or gill-cover. A continual stream of water is made to pass over the gills by the action of the mouth, which takes-in a large quantity of 272 RESPIRATORY ORGANS OP FISHES. fluid, and then forces it through, the apertures on each side of the throat, into the gill-cavities ; and from these it passes out by the other orifices just described. Fishes, in common with other animals that breathe by gills, can only respire properly Fig. 155. — Lamprey. when these are kept moist, and are so spread out as to expose their surface to the surrounding element. The act of respira¬ tion can take place when they are exposed to air , provided these conditions be fulfilled ; hut in general it happens that, when a Fish is taken out of water, its gills clog together and dry up, so that the air cannot exert any action upon them ; and the Fish actually dies of suffocation, under the very cir¬ cumstances which are necessary to the life of an air-breathing animal. 318. There are certain Fishes, however, which are provided with an apparatus for keeping the gills moist, somewhat re¬ sembling that which has been already noticed in the land- crab. The bones of the pharynx are extended and twisted in such a remarkable manner (fig. 156), as to form a number of small cavities ; these cavities the Fish can fill with water; and they form a reservoir of fluid, from which the gills may be sup¬ plied with a sufficient amount to keep them moist during some time. The gill-fila¬ ments themselves are so arranged that they do not clog together ; and by this combi¬ nation of contrivances, the species of Fish that are furnished with it can live for a long time out of water, so as to be able to journey for a considerable distance on land. Such a provision Fig. 156. — Respiratory Apparatus of Anabas. AQUATIC RESPIRATION : — USE OF CILIA. 273 is especially desirable in tropical climates, where shallow lakes are often dried-np by continued drought, so that their inhabitants must perish, if they were not thus enabled to migrate. One of the most curious of these Fishes (most of which are inhabitants of India and China) is the Anabas or climbing-perch of Tranquebar ; which climbs bushes and trees in search of its prey, a species of land-crab, by means of the spines on its back and gill-covers. — The gills of the Amphi¬ bious Reptiles, in their Tadpole state, resemble those of Fishes, and are connected with the mouth in the same manner. 319. In the respiratory actions of nearly all these animals, a very important part is performed by the cilia (§ 45) which cover the surfaces of the gills. Even in such as do not possess any special respiratory organs (§ 311), the action of the cilia is very important, in causing a constant change in the water that is in contact with their surface. Thus in Zoophytes, which are for the most part fixed to one spot, the action of the cilia produces currents in the surrounding water. On the other hand, in the actively-moving Animalcules, the same action propels their bodies rapidly through the water ; though in some of them, which occasionally fix themselves like Polypes, the action of the cilia resembles theirs. In either case there is a continual change in the layer of water which is in immediate contact with the surface ; and thus a constant supply of the air contained in the water is secured. A similar action goes-on, still more energetically, over the gill- tufts of the Annelida; and this action continues after the death of the animal, or after the tuft has been separated from it, producing evident currents in the water in which it is placed. It is by the action of the cilia alone, that the con¬ tinual current of water is kept-up through the respiratory chamber of the lower Mollusca; but this is superseded in Cephalopods and Fishes by the other means for sustaining this current which have been already noticed. Ciliary action may be well observed in the young Tadpole of the common Water-Rewt, whose gills hang freely from the neck on either side; the cilia are themselves so minute that they cannot be readily distinguished ; but the motion of the water produced by them may be at once perceived in a tolerable microscope, especially when small light particles, such as those of powdered charcoal, are diffused through it. T 274 ATMOSPHERIC RESPIRATION. 320. In animals whose blood is made to act directly upon . the air, we usually find a provision of some kind for intro¬ ducing the air into the interior of the body. The simplest arrangement is that which we meet- with in the Snail and other terrestrial Gasteropods ; and it consists merely of a cavity (p, fig. 157), resembling that in which the gills are disposed in the aquatic Mollusca, but having a free communication with h v ap p r s d f Fig. 157. — Anatomy of Snail. f, muscular disc or foot; t, tentacula; d, diaphragm separating the respiratory- cavity p from other organs, but here turned hack; s , stomach; o, ovary; ar , arterial trunk supplying the system; i, r, intestine; l , liver; h, heart; ap, vascular trunk spreading over the pulmonary cavity p ; cv, canal for excreting the viscid mucus secreted by the gland v. the external air, and having the blood minutely distributed by vessels upon its walls. — In the Myriapoda or Centipede tribe, in conformity with the general plan of Articulated structure (§ 93), we find a repetition of similar cavities along the body, one pair usually existing in each segment ; and these open externally by small apertures, which are termed spiracles. 321. In Insects, the same general arrange¬ ment is modified in the most remarkable manner. The spiracles do not open into distinct air-sacs, but into canals, which lead to two large tracheae which run along the sides of the body, and are connected by several tubes that pass across it — one usually for each segment. From these tracheae others branch off, which again subdivide into more Fig. 158. Air-tube of In¬ sect. RESPIRATION OF INSECTS. 275 minute tubes ; and, by the ramifications of these, even the minutest parts of the body are penetrated (fig. 159). These tubes are formed upon a similar plan with the air-vessels of Plants, having a spiral fibre winding inside their outer membranous coat (fig. 158) ; by the elasticity of which fibre, the tube is kept from being closed by pressure. In this manner the air is brought into contact with almost Head. Fig. 159.— Respiratory Apparatus op Insect (Nepa). every portion of the tissue, and is enabled to act most ener¬ getically upon it ; and thus the feeble circulation of these animals (§ 293) is in a great degree counterbalanced by t 2 276 RESPIRATION OF INSECTS AND SPIDERS. the extraordinary activity of their respiration. There are no animals which consume so much oxygen, in proportion to their size, as Insects do when they are in motion (§ 308) ; but when they are at rest, their respiration falls to the low standard of the tribes to which they bear the greatest general resemblance. Although, as we have seen, the . respiration of aquatic larvae is sometimes accomplished by means of gills, yet many aquatic larvae breathe air by means of tracheae ; and such are consequently obliged, like Whales and other aquatic Mammals, to come occasionally to the surface for the purpose of gaining a fresh supply of air. The larva of the Gnat, which breathes in this manner, has one of the spiracles of its tail-segment prolonged into a tube ; and it may often be seen suspended, as it were, in the water, with its head downwards, the end of this tube (t, fig. 160) being at the surface. 322. In the greater number of perfect Insects, we find the tracheae dilated at certain parts into large air-sacs (fig. 159) ; these are usually largest in Insects that sustain the longest and most powerful flight ; in some of which, as in the common Bee, they occupy a greater portion of the trunk than they do in the insect whose system of air-tubes has been just represented, — this insect, the Nepa or water-scorpion, being of aquatic habits, and seldom using its wings for flight. There can be little doubt that one use of these cavities is to diminish the specific gravity of the Insect, and thus to render it more buoyant in the atmosphere ; but it would not seem improbable that they are intended to contain a store of air for its use while on the wing, as at that time a part of the spiracles are closed. We shall find in Birds, the Insects of the Yertebrated division, a structure bearing remarkable analogy to this (§ 326). 323. In some of the Arachnida, such as the Cheese-mite , the respiration is accomplished by tracheae, as in Insects ; but in the Spiders it is performed by a different kind of apparatus. Instead of opening into a system of prolonged tubes, each spiracle leads to a little chamber, the lining membrane of which is arranged in a number of folds that lie together like the leaves of a book; and thus a large surface is exposed t Fig. 160. Larva of Gnat. RESPIRATION OF AIR-BREATHING VERTEBRATES. 277 to the air which is admitted through the spiracles. This arrangement is shown in fig. 46, l . 324. Hitherto it has been seen that the respiratory appa¬ ratus is not connected with the mouth, which in the Inverte- brated classes has the reception of food as its sole function. On this account, we cannot regard the air-sacs of Insects as bearing any real analogy to the lungs of Vertebrata. The simplest condition of the true lung is that which constitutes the air-bladder (or “ sound ”) of Fishes. This we sometimes find in the condition of a closed bag, lying along the spine ; and its use cannot be that of assisting respiration, since air is not ad¬ mitted to it from without. But in other cases we find it connected with the intestinal tube, by means of a short wide duct ; and since many Fishes, as the Loach, are known to swallow air, which is highly charged with carbonic acid when it is again expelled, it seems probable that their air-bladder / effects this change in precisely the same manner as a lung would do — the air being transmitted to it from the intestine. There are some Fishes in which the resemblance of the air- bladder to a lung is more decided, and its connexion with the function of Respiration is evidently more important. The canal by which it communicates with the alimentary canal opens into the latter above the stomach, and even, in some instances, at the back of the mouth; so that a gradual approach is seen to the arrangement which exists in air- breathing animals. In these Fishes, as in the Amphibia that retain their gills (§ 87), it would appear that the respiration is accomplished partly by the lungs, and partly by the gills ; this is the case in the curious Lepidosiren (fig. 41), which, as formerly mentioned, is regarded by some naturalists as a Fish, and by others as a Reptile. 325. The lungs of Reptiles are for the most part but little divided ; so that, although they hold a very large quantity of air, this does not act advantageously upon the blood, in con¬ sequence of the small surface over which the two are brought together (§312). In Serpents we find but a single lung, that of the other side not being developed (fig. 34) ; and this lung extends through nearly half the length of the body. Reptiles have no power of filling their lungs by a process resembling our inspiration , or drawing-in of air ; but they are obliged to swallow it, as it were, by the action of the mouth. The skin 278 RESPIRATION OF REPTILES AND BIRDS. of Frogs is of great importance in their respiration — in fact, of almost as much consequence as their lungs. The necessity for more energetic respiration increases in these animals with the temperature, every rise in which excites them to greater activity. During the winter, which they pass beneath the water in a state of torpidity, the action of the water upon their skin is sufficient to aerate their blood. When the re¬ turning warmth of spring arouses them from their inaction, they need a larger amount of respiration, and come occa¬ sionally to the surface to take-in air by their lungs. And when summer comes on, the greater heat increases their need of respiration ; and they quit their ditches and ponds, so as to allow the atmosphere to act upon their skin as well as upon their lungs. If they are prevented from doing so, they will die ; and the same result follows if the skin be smeared with grease, so that the air cannot permeate it. Moreover, if the lungs be removed, and the animal be made to breathe by its skin alone, it may live for some time, if the temperature be not high. These facts show the great importance of the skin as a respiratory organ in Frogs. 326. The respiration of Birds is more active than that of any other Yertebrata; that is, they consume more oxygen, and form more carbonic acid, in proportion to their size. RESPIRATION OF BIRDS AND MAMMALS. 279 Their lungs, however, are not so minutely subdivided as are those of Mammals ; but the surface over which the air can act upon the blood is immensely extended, by a provision which is peculiar to this class. The air introduced by the windpipe passes not only into the lungs properly so called, but into a series of large air-cells, which are disposed in various parts of the body, and which even send prolongations into the bones, especially in Birds of rapid and powerful flight, whose whole skeleton is thus traversed by air. The mode in which some of the bronchial tubes , or subdivisions of the windpipe, pass from the lungs to these air-cells, is shown in fig. 161. Sow, by this arrangement, a much larger quan¬ tity of air is taken-in at once, and a much more extensive sur¬ face is exposed to its action, than could otherwise be provided for ; and as the air which is received into the air-cells has to pass through the lungs, not only when it is taken-in, but when it is expelled again, its full influence upon the blood is secured. 327. Birds, like Beptiles, are destitute of the peculiar apparatus by which Mammals are enabled to fill their lungs with air ; but it is introduced without any effort on their parts. Bor the cavity of their trunk is almost surrounded by the ribs and breast-bone ; and the elasticity of the former keeps it generally in a state corresponding to that of our own lungs when we have taken-in a full breath. Thus the state of fullness is natural to the lungs and air-cells of Birds. When the animal wishes to renew their contents, however, it compresses the walls of the trunk, so as to diminish its cavity and to force out some of the air contained in the lungs, &c. ; and when the pressure is removed, the cavity returns to its previous size by the elasticity of its walls, and a fresh supply of air is drawn into the lungs. The air-sacs answer the same purpose in Birds as in Insects, diminishing the specific gravity of the body, by increasing its size without adding to its weight, and thus rendering it more buoyant. 328. In Man and other Mammals, the lungs are confined to the upper portion of the cavity of the trunk, termed the thorax; which is separated from the abdomen by the diaphragm , a muscular partition, whose action in respiration is very important. (An imperfect diaphragm is found in some Birds, which approach most nearly to Mammals in their general structure.) The lungs are suspended, as it were, in 280 RESPIRATORY APPARATUS OF MAMMALS. this cavity, by their summit or apex ; and are covered by a serous membrane termed the 'pleura, , which also lines the thorax, being reflected from one surface to the other precisely in the manner of the pericardium (§ 43). Thus the pleura of the outer surface of the lung is continually in contact with that which forms the inner wall of the thorax ; they are both kept moist by fluid secreted from them ; and they are so smooth, as to glide over one another with the least possible friction. The lungs themselves are very minutely subdivided ; and thus expose a vast ex¬ tent of surface in proportion to their size. The air-cells of the human lung, into which the air is conveyed by the branches of the wind-pipe, and on the walls of which the blood is distributed, do not average above the 1-1 00th of an inch in diameter. In the accompanying figure is repre¬ sented, on one side, the lung, d, presenting its natural ap¬ pearance ; and on the other, the ramifications of the air- passages or bronchial tubes, c, e, by which air is con¬ veyed into every part of the Fig. 162. — Air-tubes and Lung of Man. lungs. The trachea Or wind¬ pipe, 5, opens into the pharynx or back of the mouth, by the larynx, a. The con¬ struction of this is especially destined to produce the voice, , and will be explained under that head (Chap, xiil); but it may be here mentioned that the entrance from the pharynx into the larynx consists of a narrow slit, capable of being enlarged or closed by the separation or approximation of its lips, which form what is called the glottis . The aper¬ ture of the glottis is regulated by the muscular apparatus of the larynx; the actions of which are not under the direct control of the will, but are automatic, like those concerned in swallowing (§ 194) ; and the purpose of this provision is to RESPIRATORY APPARATUS OF MAMMALS. 281 prevent the entrance of anything injurious into the windpipe. Thus if we attempt to breathe carbonic acid gas, which would produce an immediately fatal result if introduced into the lungs, the lips of this chink immediately close together, and so prevent its entrance. The contact of liquids or of solid substances, too, usually causes the closure of the aperture, so that they are prevented from finding their way into the wind¬ pipe ; but this does not always happen, especially when the glottis is widely opened to allow the breath to be drawn- in (§ 193). 329. The larynx, trachea, and bronchial tubes, to their minutest ramifications, in all air-breathing Yertebrata, are lined by a mucous membrane continued from the back of the throat ; and this membrane, like the gills of aquatic animals, is covered with cilia, which are in continual vibration. It is obvious, however, that the purpose of this ciliary movement must be here different from that which is fulfilled by the same action on the surface of the gills (§ 319); and it probably serves to get rid of the secretion which is being continually poured out from the surface of the mucous membrane, and which, if allowed to accumulate there, would clog up the air- cells, and in time produce suffocation. The vibration of the cilia is observed to be always in one direction, — towards the outlet ; and it is in this direction, therefore, that the fluid is gradually but regularly conveyed. The ciliary movement may be seen to take place on the surface of the mucous membrane u* of the nose ; but not on that of the pharynx, where it would be continually interrupted by the passage of food. 330. The constant renewal of the air in the lungs is pro¬ vided for, in Mammals, by a peculiar mechanism, which accom¬ plishes this purpose most effectually, though itself of the most simple character. We must recollect that the thorax in this class is an entirely closed cavity. It is bounded above and at the sides by the ribs (the space between which is filled up by muscles, &c.), and below by the diaphragm, which here forms a complete partition between the thorax and abdomen. The whole of this cavity, with the exception 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, blow the size of this cavity may be made to vary considerably; — in the first place, by the 282 RESPIRATORY MOVEMENTS OF MAMMALS. movement of the diaphragm ; and in the second, by that of the ribs. 331. i. The diaphragm, in a state of rest or relaxation, forms a high arch, which rises into the interior of the chest, as at <7, fig. 163 ; but when it contracts, it becomes much flatter (though always retaining some degree of convexity upwards), and thus adds considerably to the capacity of the lower part of the chest. The under side of the diaphragm is in contact with the liver and stomach, which, to a certain i a c c a h Fig. 163. — Thorax of Man. 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 downwards ; and as there can be no yielding in that direction, the abdomen is made to bulge forwards when the breath is drawn-in. On the other hand, when the contraction of the diaphragm ceases, the abdominal muscles press back the contents of the abdomen, force up the RESPIRATORY MOVEMENTS OP MAMMALS. 283 liver and stomach against the under side of the diaphragm, and cause it to rise to its former height. 332. ii. The play of the ribs is rather more complicated. These bones, c c, and c c (to the number of twelve on each side in Man), are attached at one end by a moveable joint to the spinal column, a ; whilst at the other they are connected with the sternum (breast-bone) by an elastic cartilage. ISTow each rib, in the empty state of the chest, curves downwards in a considerable degree ; and it may be elevated by a set of muscles, of which the highest, are attached to the vertebrae of the neck and to the first rib, whilst others, e , e, e (termed intercostals), pass between the ribs. The cartilages also curve downwards in the opposite direction, from their points of attachment to the sternum. When the breath is drawn-in, the first rib is raised by the contraction of the muscles, i ; and all the other ribs, which hang, as it were, from it, would of course be raised by this action 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 very much more than the upper ones. ISTow by the raising of the ribs, they are brought more nearly into a horizontal line, as are also their cartilages ; and since the combined length of the two is greater, the nearer they approach to a straight line, it follows that the raising of the ribs must throw them further out at the sides, and increase the pro¬ jection of the sternum in front, thus considerably enlarging the capacity of the chest in these directions. When the movement of inspiration is finished, the ribs fall again, partly by their own weight, partly by the elasticity of their carti¬ lages, and partly by the contraction of the abdominal muscles which are attached to their lower border. — Tor the full under¬ standing of this description, it is desirable that the reader should examine the movements of his own or another person's chest, by placing his fingers upon different points of the ribs, and watching their changes of position during the drawing-in and the expulsion of the breath. 333. JSow 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 elevated. The atmosphere around presses to fill that vacuum; but this it can only 284 RESPIRATION IN MAN. 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 may be the increase in the latter. But if we were to make an aperture through the walls of the chest, the air would rush directly into its cavity, when the move¬ ments of inspiration are performed, and the lung of that side 1 would not be dilated. The same thing would happen if there were an aperture in the lung itself, allowing free communica¬ tion between one of the larger bronchial tubes and the cavity of the chest ; for the air, although still drawn-in by the wind¬ pipe, would pass directly into the cavity of the chest, rather than dilate the lung, which would thus become entirely useless. Such an aperture is sometimes formed as the result of disease ; and if the action of both lungs were thus prevented, death must immediately take place from suffocation. 334. The extent of the respiratory movements varies con¬ siderably ; but in general it is only such as to change about the seventh part of the air contained in the lungs. (It may be generally noticed, that every fifth or sixth inspiration in Man is longer and fuller than the rest.) Their rate varies according to age, and to the state of the nervous system; being faster in infants and in young persons than in adults ; and more rapid in states of mental excitement, or irritation of the bodily system, than in a tranquil condition. In a state of rest, from 14 to 18 inspirations take place every minute in an adult, and at each about 20 cubic inches of air are drawn-in ; but both the depth and frequency of the inspirations are con¬ siderably increased by exercise. Taking an average alterna¬ tion of activity and repose, it appears that about 360 cubic feet of air pass through the lungs every twenty-four hours, or 15 cubic feet every hour; and as the air which has once passed through the lungs contains about l-24th part of carbonic acid, about 15 cubic feet of that gas, containing nearly 8 ounces of solid carbon, are thrown-off in the course of twenty-four hours. 335. Now carbonic acid, when diffused through the atmo¬ sphere to any considerable amount, is extremely injurious to 1 Each lung has its own cavity ; the two being completely separated from each other by the pericardium (§43). IMPORTANCE OF FREE VENTILATION. 285 animal life ; for it prevents the due excretion by the lungs of that which has been formed within the body ; and the latter consequently accumulates in the blood, and exercises a very depressing influence on the action of the various organs of the body, but particularly on that of the nervous system. The usual proportion is not above 1 part in 5000 ; and when this is increased to 1 part in 100, its injurious effects begin to be felt by Man, in headache, languor, and general oppression. fSTow it is evident, from the statements in the last paragraph, that, as a man produces in twenty-four hours about 15 cubic feet of carbonic acid, if he were inclosed in a space containing 1500 cubic feet of air (such as would exist in a room 15 feet by 10, and 10 feet high), he would in twenty-four hours communicate to its atmosphere from his lungs as much as 1 part in 100 of carbonic acid, provided that no interchange takes place between the air within and the air outside the chamber. The amount would be further increased by the car¬ bonic acid thrown off by the skin, the quantity of which has not yet been determined. 336. In practice, such an occurrence is seldom likely to take place ; since in no chamber that is ever constructed, except for the sake of experiment, are the fittings so close as to prevent a certain interchange of the contained air with that on the outside. But the same injurious effect is often produced by the collection of a large number of persons for a shorter time, in a room insufficiently provided with the means of ventilation. It is evident that if twelve persons were to occupy such a chamber for two hours, they would produce the same effect with that occasioned by one person in twenty-four hours. ISTow we will suppose 1200 persons to remain in a church or assembly-room for two hours ; they will jointly produce 1500 cubic feet of carbonic acid in that time. Let the dimensions of such a building be taken at 100 feet long, 50 broad, and 30 high ; then its cubical content will be (100 x 50 x 30) 150,000 cubic feet. And thus an amount of carbonic acid, equal to 1-1 00th part of the whole, will be communicated to the air of such a building, in the short space of two hours, by the presence of 1200 people, if no pro¬ vision be made for ventilating it. And the quantity will be greatly increased, and the injurious effects will be pro- portionably greater, if there be an additional consumption of 286 IMPORTANCE OF FREE VENTILATION. oxygen, produced by the burning of gas-ligbts, lamps, or candles. 337. Hence we see tire great importance of providing for free ventilation, wherever large assemblages of persons are collected together, even in buildings that seem quite adequate in point of size to receive them ; and much of the weariness which is experienced after • attendance on crowded assemblies of any kind, may be traced to this cause. In schools, facto¬ ries, and other places where a large number of persons remain during a considerable proportion of the twenty-four hours, it is impossible to give too much attention to the subject of ventilation ; and as, the smaller the room, the larger will be the proportion of carbonic acid its atmosphere will contain, after a certain number of persons have been breathing in it for a given time, it is necessary to take the greatest precaution when several persons are collected in those narrow dwellings, in which, unfortunately, the poorer classes are compelled to reside. Even the want of food, of clothing, and of fuel, are less fertile sources of disease than insufficient ventilation; which particularly favours the spread of contagious diseases, on the one hand by keeping-in the poison, and thus concen¬ trating it upon those who expose themselves to its influence ; and, on the other, by obstructing the elimination of the waste matter from the system, the presence of which in the blood renders it peculiarly liable to be acted-on by all poisons having the nature of “ferments.” 338. When the quantity of carbonic acid in the air accu¬ mulates beyond a certain point, it speedily produces suffocation and death. This is occasioned by the obstruction to the flow of blood through the capillaries of the lungs, which takes place when it is no longer able to get rid of the carbonic acid with which it is charged, and to absorb oxygen in its stead. The general principle to which this stagnation may be referred has already been noticed (§ 280). How, as all the blood of the system, in warm-blooded animals, is sent through the lungs before it is again transmitted to the body, it follows that any such obstruction in the lungs must bring the whole circulation to a stand. The functions of the nervous system are directly dependent upon a constant supply of arterial blood (Chap, x.) ; and, accordingly, as this supply becomes progressively diminished in quantity and deteriorated in qua- SUFFOCATION FROM WANT OF RESPIRATION. 287 lity, its actions first become irregular, producing violent con¬ vulsive movements, and at last cease altogether, the animal becoming completely insensible. In this condition, which is termed Asphyxia, , the pulmonary arteries, the right side of the heart, and the large veins which empty themselves into it, are gorged with dark blood ; whilst the pulmonary veins, the left side of the heart, and the arteries of the system, are compara¬ tively empty. Hence the action of the right side of the heart comes to an end, through a loss of power in its walls, occa¬ sioned by their being over-distended ; whilst that of the left side ceases for want of the stimulus of the contact of blood, by which the muscular fibre is excited. If this state be allowed to continue, death is the consequence ; but if the carbonic acid in the lungs be replaced by pure air, the flow of blood through their capillaries recommences, — the right side of the heart is unloaded and begins to act again, — arterial blood is sent to the left side, and excites it to renewed motion, — and the same being propelled by it to all parts of the body, their several functions are restored, the nervous system re¬ covers its power of acting, and all goes on as before. These changes occur in exactly the same manner when a warm¬ blooded animal is made to breathe nitrogen or hydrogen ; since these gases do not perform that which it is the office of oxygen to effect, — the removal of carbon from the system, in the form of carbonic acid. And they also take place in a perfectly pure atmosphere, when the individual is prevented from receiving it into its lungs by an obstruction to its passage through the windpipe, such as that produced by hanging, strangulation, drowning, &c. For the air in the lungs, not being renewed, speedily becomes charged with carbonic acid, to an extent that checks the circulation through their capillaries ; and all the consequences of this follow as before. 339. The most efficient remedy in all such cases is evidently that suggested by the facts stated in the last paragraph, — the renewal of the air in the lungs. But with this, other means should be combined; and the general directions1 of Dr. Marshall Hall, with the method of producing artificial 1 The instructions, though specially intended for the resuscitation of persons apparently drowned, are applicable with slight modification to other forms of Asphyxia. 288 TREATMENT OF THE APPARENTLY- DROWNED. respiration suggested by Dr. H. R. Silvester, seem most likely to answer in practice : — Treat the patient instantly, on the spot, in the open air, exposing the face and chest to the breeze, except in severe weather. i. To clear the Throat , place the patient gently on the face, with one wrist under the forehead — (all fluids and the tongue itself then fall forwards, leaving the entrance into the windpipe free). If there be breathing, wait and wratch ; if not, or if it fail, — ii. To excite Respiration , turn the patient well and in¬ stantly on his side, and excite the nostrils with snuff, or the throat with a feather, &e., and dash cold water on the face previously rubbed warm. If there be no success, lose not a moment, but instantly, — - hi. To imitate Respiration , lay the patient on his back, with the head and shoulders slightly elevated ; then let the arms be raised and steadily extended upwards, by the sides of the head, so as to draw-up the shoulders. In this way, the ribs are drawn-up by the muscles passing to them from the shoulders, and the cavity of the chest is enlarged. If the arms be then carried-down to the sides of the body, the shoulders fall, the ribs are lowered, and the sides of the thorax approach one another, as in natural expiration, — an effect which may be increased by moderate pressure on the front and sides of the chest. By an alternation of these movements, an artificial Inspiration and Expiration will be effected, which, though imperfect, may restore life. iv. To induce Circulation and Warmth , meantime rub the limbs upwards, with firm grasping pressure and with energy, using handkerchiefs, &c. (by this measure the blood is propelled along the veins towards the heart) ; let the limbs be thus warmed and dried, and then clothed, the bystanders supplying the requisite garments ; avoiding the continuous warm bath, and the position on, or inclined-to, the back. 340. The ordinary movements of respiration belong, like those of swallowing, to the class of reflex actions (§ 430). We have seen that, in every such movement, a great number of muscles are called into play simultaneously; and this is effected by means of the stimulus which is sent to them from the spinal cord. But this stimulus does not originate there ; CAUSE OF RESPIRATORY MOVEMENTS. 289 for it is the consequence of impressions conveyed to the spinal cord, and especially to its upper end, by several nerves, — some originating in the lungs, and others in the general surface. The nerves originating in the lungs convey to the spinal cord the impression produced by the presence of venous blood in their capillaries : of this impression we are not ordinarily conscious ; but if we hold our breath for a few moments, we become aware of it ; and it speedily becomes so distressing as to force us to breathe, even though we may try to resist it by an effort of the will. The impression conveyed by the nerves of the general surface is chiefly that produced by the applica¬ tion of cold to the skin. It is this which is the cause of the first inspiration in the new-born infant ; which is not unfre- quently prevented by the seclusion of its face (the part most capable of receiving this impression) from the influence of the air. Every one knows that, when the face is dipped into water, an inspiratory movement is strongly excited; and the same happens when a glass of water is dashed over the face. This simple remedy will often put a stop to hysterical laughter, by producing a long sighing inspiration. A still stronger tendency to draw-in the breath is experienced in the first dash of water over the body in the shower-bath. The respi¬ ratory movements, in the higher Animals, are placed under the control of the will, to a certain extent, because on them depend the production of sounds, and in Man the actions of speech ; but that they are quite independent of the will, and even of sensation, is shown by the fact that they will continue after the brain has been completely removed, provided the spinal cord and its nerves are left without injury. In most of the Invertebrata they are connected with distinct ganglia, which minister to them alone. (See Chap, x.) 341. The actions of sighing , yawning , sobbing , laughing , coughing , and sneezing, are nothing else than simple modifica¬ tions of the ordinary movements of respiration, excited either by mental emotions, or by some stimulus originating in the respiratory organs themselves. Sighing is nothing more than a very long-drawn inspiration, in which a larger quantity of air than usual is made to enter the lungs. This is continually taking place in a moderate degree, as already noticed (§ 334) ; and we notice it particularly, when the attention is released after having been fixed upon an object which has excited it u 290 VARIOUS MOVEMENTS CONNECTED WITH RESPIRATION. strongly, and which has prevented our feeling the insufficiency of the ordinary respiratory movements. Hence this action is only occasionally connected with mental emotion. Yawning is a still deeper inspiration, which is accompanied by a kind of spasmodic contraction of the muscles of the jaw, and also hy a very great elevation of the ribs, in which the shoulders and arms partake. The purely involuntary character of this movement is sometimes seen in a remarkable manner in cases of palsy, in which the patient cannot raise his shoulder hy an effort of the will, but does so in the act of yawning. Never¬ theless the action may be performed by the will, though not completely ; and it is one that is particularly excited by an involuntary tendency to imitation, as every one must have experienced who has ever been in company with a set of yawners. Sobbing is the consequence of a series of short convulsive contractions of the diaphragm ; and it is usually accompanied by a closure of the glottis, so that no air really enters. In Hiccup , the same convulsive inspiratory movement occurs, the glottis closing suddenly in the midst of it ; and the sound is occasioned by the impulse of the column ol air in motion against the glottis. In Laughing , a precisely reverse action takes place ; the muscles of expiration are in convulsive movement, more or less violent, and send out the breath in a series of jerks, the glottis being open. This some¬ times goes on until the diaphragm is more arched, and the chest more completely emptied of air, than it could be by an ordinary movement of expiration. The act of Crying , though occasioned by a contrary emotion, is, so far as the respiration is concerned, very nearly the same. We all know the effect of mixed emotions in producing something “ between a laugh and a cry.” 342. The purposes of the acts of coughing and sneezing are, in both instances, to expel substances from the air-passages, which are sources of irritation there ; and this is accomplished in both by a violent expiratory effort, which sends forth a blast of air from the lungs. — Coughing occurs when the source of irritation is situated at the back of the mouth, in the trachea, or bronchial tubes. The irritation may be produced by acrid vapours, or by liquids or solids that have found their way into these passages, or by secretions which have been poured into them in unusual quantity as the result of COUGHING AND SNEEZING— -AQUEOUS EXHALATION., 291 disease ; and the latter will be the more likely to produce the effect, from the irritable state in which the lining membrane of the air-passages already is. The impression made upon this membrane is conveyed by the nerves spread out beneath its surface to the spinal cord; and the motor impulses are sent to the different muscles, which they combine in the act of coughing. This act consists, 1st, in a long inspiration, which fills the lungs ; 2d, in the closure of the glottis at the moment when expiration commences ; and 3d, in the burst¬ ing-open, as it were, of the glottis, by the violence of the expiratory movement, so that a sudden blast of air is forced up the air-passages, carrying before it anything that may offer an obstruction. — Sneezing differs from coughing in this, that the communication between the larynx and the mouth is partly or entirely closed, by the drawing-together of the sides of the veil of the palate over the back of the tongue ; so that the blast of air is directed more or less completely through the nose, in such a way as to carry-off any source of irritation that may be present there. 343. Every one is aware that the air he breathes-forth con¬ tains a large quantity of vapour : this is not perceptible in a warm atmosphere, because the watery particles remain dis¬ solved in it and do not affect its transparency ; but in a cold atmosphere they are no longer held in solution, and conse¬ quently present the appearance of fog or steam. The quantity of fluid which thus passes off is by no means trifling, — probably not less than from 16 to 20 ounces in the twenty- four hours ; a portion of it undoubtedly proceeds from the moist lining of the mouth, throat, &c., but the greater part is thrown-off by the lungs themselves. This fluid, when col¬ lected, is found to contain a good deal of decomposing organic matter, especially in cases in which the respiratory process has not been carried on with perfect freedom ; such matter being oxydized and thrown-off under other forms, when the blood is duly aerated. Various substances of an odoriferous character, which have been taken into the blood, manifest their presence in this exhalation : thus turpentine, camphor, and alcohol, communicate their odour to the breath ; and when the digestive system is out of order, the breath fre¬ quently acquires a disagreeable taint, from the reception of putrescent matters into the blood, and their exhalation through u 2 292 ABSORPTION OF VAPOUR — POISONOUS GASES. this channel— Of the water of the blood, from which this exhalation is given-off, a small part is most probably formed by the direct union of the hydrogen contained in the food (especially when this is one of its predominating components, § 153) with the oxygen absorbed. For it has been found by careful experiment, that the proportion of inspired oxygen which disappears (not being contained in the carbonic acid expired, § 305), is much greater in animals that are fed on a flesh diet, than in those living on farinaceous food. Another portion of such oxygen probably unites with the sulphur and phosphorus of the food and tissues, to form sulphuric and phosphoric acids, which are excreted through the kidneys in combination with alkaline bases (§ 367). 344. Certain gases act as violent poisons, even when respired in very small proportion. Thus, a Bird is speedily killed by breathing air which contains no more than 1-1 500th part of sulphuretted hydrogen ; and a Dog will not live long in an atmosphere containing l-800th part of this gas. The effects of carburetted hydrogen are similar ; but a larger proportion is required to destroy life. Both these gases are given-off by decomposing animal and vegetable matter ; the neighbourhood of which is consequently very injurious to health. Several cases of arsenical poisoning have occurred, from the accidental inhalation of a small quantity of arseniuretted hydrogen, the amount of arsenic contained in which must have been so minute as to be scarcely appreciable. CHAPTER VII. OF EXCRETION AND SECRETION. General Purposes of the Excreting Processes. 345. We have seen that the Blood, in the course of its circulation, not only deposits the materials that are converted into the several fabrics of which the body is composed, but also takes-up into itself the products of the decomposition which is continually going-on in its various parts ; and it is to replace this, that the constant Nutrition of the tissues is required. In order that the blood may retain its fitness foi OF EXCRETION AND SECRETION. 293 the purposes to which it is destined, it is requisite that these products should be drawn-off from the current of the circula¬ tion, as constantly as they are received into it : and this is accomplished by the various processes of Excretion , which are continually taking place in different parts of the body. The uninterrupted performance of these is even more essential to the maintenance of life, than is an uninterrupted supply of nutritive materials ; for an animal may continue to exist for some time without the latter, but it speedily dies if either of the more important excretions be checked. We have a striking instance of this in the case of the Bespiration, which may be regarded as a true function of Excretion, having for its object to set free Carbonic acid from the blood in a gaseous form, — thereby contributing to the introduction of Oxygen into the blood, for the various important actions to which that element is subservient, especially the maintenance of Animal Heat. (Chap, ix.) The effects of the suspension of the respiratory process, even for a few minutes, in a warm¬ blooded animal, have been shown (§ 338) to be certainly and speedily fatal ; and they are as certainly fatal in the end in cold-blooded animals, though a longer time is required to produce them. 346. The products of excretion are the same, as to their essential characters at least, through the whole Animal king¬ dom ; and for this it is not difficult to find a reason. It will be remembered that the ultimate elements of the Animal tissues are four in number : oxygen, hydrogen, carbon, and nitrogen ; and that the materials which make up the chief part of the fabric of different classes of animals — albumen, gelatin, fatty matter, &c. — contain these elements united in constant proportions, from whatever source we obtain them. Hence we should expect to find the products of their decom¬ position also the same ; and this is, for the most part, the case. Of these four ingredients, oxygen can never be said (in the healthy state at least) to be superfluous in the body ; for a large and constant supply of it is required, to unite with the others and carry them off in their altered conditions. Thus, unless oxygen were continually introduced into the system, for the sake of uniting with the carbon that is to be thrown off by Bespiration, that excretion must be checked ; and it is required, in like manner, for uniting with hydrogen 294 NATURE AND OBJECTS OP EXCRETORY ACTIONS. to form water, and with, compounds of nitrogen to form urea. Hence there is no need of an organ to carry off the super¬ fluous oxygen ; hut an organ to introduce it is rather required ; and this purpose, as we have seen, is answered by the Respi¬ ratory apparatus. But we find organs of excretion specially destined to carry off the carbon, hydrogen, and nitrogen, which are set free, under various forms, by the decomposition of the tissues. Thus the Respiratory organs, as we have seen, throw off carbon in the form of carbonic acid, and hydrogen which, has been in like manner united with oxygen so as to form water. The Liver has for its office partly to separate these same elements from the blood in a different form, throwing them off in the condition of a peculiar fatty matter, which consists almost entirely of carbon and hydrogen. But it has another function of no less importance in animals whose respiration is active ; for by its agency the hydro-carbonaceous matter circulating in the blood is brought into a state in which it readily combines with oxygen to form carbonic acid and water ; and thus the liver may be said to prepare the pabulum for the combustive process. Lastly, the Kidneys have for their chief object to throw off the azotized compounds which result from the decomposition of the tissues ; these contain a very large proportion of azote or nitrogen, which is united with the other elements into the crystalline compounds, urea, and uric or lithic acid, the latter of which is usually thrown off in combination with soda or ammonia. And the kidneys further serve as the channel through which soluble matters of various kinds, which have found their way into the current of the circulation, and are foreign to the composi¬ tion of the blood, are eliminated from it. 347. It is obvious that, when an animal has retained its usual weight for any length of time without change, the total weight of its excretions must be equivalent to the total weight of the solids and fluids it has taken-in. If these last have been no more in amount than was absolutely necessary for the main¬ tenance of the body during that period, all the azotized portion of the food was first appropriated to the formation of the azotized tissues ; whilst the non-azotized portion was used-up in maintaining the respiration (§ 157). Consequently, no part of the food would pass at once into the biliary and urinary excretions ; and these would have no other function than to EXCRETION OF SUPERFLUOUS AZOTIZED NUTRIMENT. 295 separate or strain-off, as it were, the products of the decompo¬ sition of the tissues formed from it, when their term of life had expired (§ 161). But it is certain that Man (as well as other animals which have in some degree learned his habits) frequently consumes much more food than is necessary for the supply of his wants ; and a little consideration will show, that the surplus must pass-off by these excretions, without ever forming part of the living fabric. For new muscular tissue is not formed in proportion to the quantity of aliment supplied, but in proportion to the demand created by the exercise of it (§ 587); consequently, if more food be taken-in than is necessary to supply that demand, no use can be made of it. We never find that a Man becomes more fleshy by eating a great deal and taking little exercise ; indeed, the very contrary result happens, his flesh giving place to fat. But let him put his muscles to regular and vigorous exercise, and eat as much as his appetite demands, and they will then increase both in strength and bulk. 348. Hence, if more azotized food be taken-in, than is required to supply the waste of the muscular and other azotized tissues, the surplus must be carried-off by the organs of excretion — chiefly, indeed almost entirely, by the Kidneys. By throwing upon them more than their proper duty, they become disordered and unable to perform their functions ; hence the materials which they ought to separate from the blood accumulate in it, and give rise to various diseases of a more or less serious character, which might have been almost certainly prevented by due regulation of the diet. The most common of these diseases among the higher classes are Gout and Gravel ; both of these may be often traced to the same cause, — the accumulation in the blood of lithic acid, which results from the decomposition of the superfluous azotized food, and which the kidneys are not able to throw-off in the proper state, that is, dissolved in water. That these diseases are, comparatively speaking, rare among the lower classes, is at once accounted-for by the fact, that they do not take-in any superfluous azotized food; — all that they consume being appropriated to the maintenance of their tissues, and the kidneys having only to discharge their proper function of removing from the blood the products of the decomposition of these. 296 EXCRETION OF SUPERFLUOUS NON-AZOTXZED NUTRIMENT. 349. Hence the radical cure of these diseases, in most persons who have a sufficiently vigorous constitution and firm resolution to adopt it, is abstinence from all azotized nutri¬ ment— whether contained in animal flesh, bread, or other articles of vegetable diet, — save such as is required to supply the wants of the system. If such abstinence he carried too far, however, it will produce injurious instead of beneficial results, weakening the fabric, and impairing the digestive powers ; and if food be employed of a kind which is liable to produce lactic acid (the acid that appears in milk, when it turns sour), much disorder may still remain, which must be avoided by using the kind of diet that is least liable to undergo this change. 350. Again, if more non-azotized food is taken into the system than can be got rid of by Eespiration, it must either be deposited as fat, or it must be separated from the blood, and carried-off by the excretion of the Liver. But if too much work be thrown upon this organ, its function becomes disordered, from its inability to separate from the blood all that it should draw-off ; the injurious substances accumulate in the blood, therefore, producing various symptoms that are known under the general term of bilious ; and to get rid of these, it becomes necessary to administer medicines (especially those of a mercurial character) which shall excite the liver to increased secretion. The constant use of these medicines has a very pernicious effect upon the constitution; and careful attention to the regulation of the diet, and especially the avoidance of a superfluity of oily or farinaceous matter, will generally answer the same end in a much better manner. 351. That the materials of the Biliary and Urinary excre¬ tions pre-exist (like the carbonic acid thrown-off by respiration) in the blood, in forms which, if not identical, are at any rate closely allied to those under which they present themselves in the bile and urine, has now been fully proved. The quantity of them present in the circulating fluid, however, is usually very small ; for the simple and obvious reason that, if the excreting organs are in a state of healthy activity, these substances are drawn-off by them from the blood, as fast as they are introduced into it. But if the excretions be checked, they speedily accumulate in the blood, to such a degree as to be easily detected by the Chemist, and also to make their presence evident by their effects upon the animal functions* NATURE AND PURPOSES OF ANIMAL SECRETIONS. 297 especially those of the nervous system. This sometimes happens in consequence of disease, and it may be imitated by experiment ; for when the trunk of the blood-vessel convey¬ ing the blood to the liver or kidney is tied, the excretion is necessarily checked, and the same results take place as when the stoppage has depended on want of secreting power. The biliary and urinary matters have the effect of narcotic poisons upon the brain ; when they have accumulated in the blood, their symptoms begin to manifest themselves ; and these symptoms increase in intensity, as the amount of the sub¬ stances becomes augmented, until death takes place. 352. Besides the Excretions, we find various Secretions elaborated in different parts of the bodies of animals, with a view not so much to the purification of their blood, as to the fulfilment of special purposes in their economy. These vary considerably in the different classes of animals ; though some of them, being concerned in functions almost universally per¬ formed, are equally general in their range. Thus we find the Salivary and Gastric fluids poured into the mouth and stomach, for the reduction and solution of the food (§§ 190 and 204) ; and the Lachrymal secretion poured out upon the surface of the eye, for the purpose of washing it from impurities (§ 541) : while the secretion of Milk for the nourishment of the young is limited to Mammals ; and poisonous secretions are formed in Serpents and Insects, for the destruction of their prey or for means of defence. Any one of these may be checked, without rendering the blood impure by the accumu¬ lation of any substances that should be drawn-off from it ; but its cessation may produce effects fully as injurious, by disordering the function to which it is subservient. Thus, if the salivary and gastric secretions were to cease, the reduction of the food could not be effected, and the animal must starve, though its stomach were filled with wholesome aliment. — It is to be observed, in regard to nearly all these secreted fluids, that they contain but a small quantity of solid matter, and that this matter seems to be formed from the albumen of the blood by a process of incipient decomposition, which gives it the character of a “ ferment.” 353. The various acts of Secretion and Excretion which are continually taking place in the living body, are, like those of Nutrition, completely removed from the influence of the will; 298 INFLUENCE OF EMOTIONS UPON SECBETIONS. but they are strongly affected by emotions of the mind. This has been already pointed out in regard to the Saliva (§ 190) ; and it is equally evident in the case of the Lachrymal secre¬ tion (§ 541). In these instances, however, the effect of the emotion is manifested upon the quantity only of the secretion ; in the case of the secretion of Milk, not only the quantity but quality is greatly influenced by the mental state of the nurse. The more even her temper, and the more free from anxiety her mind, the better adapted will be her milk for the nourish¬ ment of her offspring. There are several instances now on record, in which it has been clearly shown, that the influence of violent passions in the mother has been so strongly exerted upon the secretion of milk, as almost instantaneously to com¬ municate to it an absolutely poisonous character, which has occasioned the immediate death of the child. 1 The influence of emotional states upon the Secretions is probably communicated by the Sympathetic system of Nerves (§ 461), which is very minutely distributed, with the blood-vessels, to the several glands which form them. Nature of the Secreting Process. — Structure of the Secreting Organs. 354. 1ST otwithstanding the different characters of the pro¬ ducts of Secretion and Excretion, and the variety of the pur¬ poses to which they are destined to be applied, the mode in which they are elaborated or separated from the blood is essentially the same in all. The process is performed, in the Animal, as in the Plant, by the agency of cells; and the variety of the structure of the different Glands , or secreting organs, has reference merely to the manner in which these, their essential parts, are arranged. The simplest condition of a secreting cell, in the animal body, is that in which it exists in Adipose or fatty tissue ; which is composed, as formerly explained (§ 46), of a mass of cells, bound together by areolar tissue that allows the blood-vessels to gain access to them. Every one of these cells has the power of secreting or separating fatty matter from the blood ; and the secreted product remains stored-up in its cavity, as in Plants (Veget. 1 See the Author’s Principles of Human Physiology, chap. xv. ; and Dr. A. Combe on the Management of Infancy, chap. x. ESSENTIAL STRUCTURE OF SECRETING ORGANS. 299 Phys. § 324) — not being poured forth, as it is in most other- cases, by the subsequent bursting of the cell. 3 55. But when the secreting cells are disposed on the surface of a membrane, instead of being aggregated in a mass, it is obvious that, if they burst or dissolve-away, their contents will be poured into the cavity bounded by that membrane; and this is the mode in which secretion ordinarily takes place. Thus, the Mucous Membranes (§ 39) are covered with epithelium-cells , which are continually being cast-off, and which are replaced as constantly by a fresh crop ; and they form by their dissolution the glairy viscid substance termed mucus , which covers the whole surface of the membrane, and serves for its protection. In parts of the membrane where it is necessary that the secretion should be peculiarly abundant, we find its secreting surface greatly increased, by being prolonged into vast numbers of little pits or bags, termed follicles , which are lined with epithelium-cells, that resemble those of its general surface (see fig. 9). Such follicles are very abundant along the whole alimentary canal of Man; and the glandulse in which the Gastric and Intes¬ tinal fluids are elaborated, are almost equally simple in then- structure (§ 204). 356. Now although the most important Secretions and Ex¬ cretions are separated, in Man and the higher animals, by organs of a much more com¬ plex nature, yet in the lower we find them generated after the same simple fashion. Thus in the little Bowerbanhia (§ 115), the bile is secreted by minute follicles which are lodged in the walls of the stomach (fig. 64, c) and pour their secretion separately into its cavity, having no communi¬ cation with one another. In more complex forms of glan¬ dular structure, however, several follicles open together into a tube, which discharges the product of their secretion Bombardier Beetle. 300 ESSENTIAL STRUCTURE OF SECRETING ORGANS. (fig. 164); and thus the entire mass may he composed of numerous lobules, each having its own duct. Passing to still higher forms, we find all the ducts coalescing into a common trunk, so that the gland bears a strong resemblance to a bunch of grapes ; as is seen in fig. 1 65, which represents the structure Fig. 165. — Intimate structure of a Composite Gland (the Parotid). of the Parotid (one of the salivary glands) of Man. The main stalk is the duct into which all the others enter ; from this pass off several branches, and these again give off smaller twigs, the extremities of which enter the minute follicles in which the secretion is formed. These follicles are lined, as in their simple condition, with cells, which are the essential instru¬ ments in the production of the secretion; the fluid which they separate is poured, by the giving-way of their walls, into the small canals proceeding from the follicles, thence into the larger branches, and finally into the main trunk, by which it is carried into the situation where it is to be employed or from which it is to pass out. The Liver will be seen to possess a structure exactly resembling this, in the Fig. 166. — Portion of one of the TUBULI URINIFERI OF THE Human Kidney; Showing its lining of flattened epithe¬ lium cells. ESSENTIAL STRUCTURE OF SECRETING GLANDS. 301 Crustacea, by referring to fig. 47, fo ; and in the Mollusca it is nearly the same (figs. 157, l, and 149,/). 357. The required extent of secreting surface is not unfre- quently given, however, by the prolongation of the follicles into tubes, rather than by a great multiplication in their number. Of this we have a remarkable example in the Kidney of the higher animals (§ 368), which is entirely com¬ posed of such tubes, together with areolar tissue which binds them together, and the blood-vessels distributed amongst them. These tubes, like the follicles, are lined with epithelium-cells (fig. 166), which are the real instruments in the separation of their secreted product. 358. That there is nothing in the form of any secreting apparatus, however, which determines the peculiar nature of its secretion, is evident from this fact, — that, in glancing through the Animal series, we find the same secre¬ tion elaborated by glandular struc¬ tures of every variety of form. Thus, we have seen that the bile is secreted, in the lowest animals in which we can distinguish it, by a number of distinct follicles, as simple in their structure as are those by which the mucous secretions are formed in the highest. Again, the bile is secreted in Insects, by a small number of long tubes, which open separately into the intestinal canal just below the stomach (fig. 112); and these tubes appa¬ rently differ in no respect from those that form the urinary secretion in the same animals, which open nearer the outlet of the intestinal canal. In fact, the distinct function of the latter was not known, until it was ascertained that uric acid is to be found in them. In fig. 167, which represents the digestive apparatus of the Cockchafer, it is seen that the biliary vessels are only four in number, but are very long ; and that, for a good part of their length, Fig. 167. Alimentary Canal and HEPATIC TUBULES OF COCK¬ CHAFER. 302 ESSENTIAL STRUCTURE OF SECRETING GLANDS. they are beset with a series of short tubes opening from them, by which the extent of secreting surface is much in¬ creased. — On the other hand, although the urinary secretion is generally formed by long tubes, yet in the Mollusca it is secreted by follicles, according to the general plan of their glandular structures. 359. The secreting cells not unfrequently possess the power of elaborating a peculiar colouring matter, either separately, or along with the substances which seem more characteristic of the secretion. Thus the ink of the Cuttle-fish is in reality its urine, charged with a quantity of black matter formed in the pigment-cells (resembling those of the interior of the eye, § 533) that line its ink-bag ; and the corresponding secretion in other Mollusca is rendered purple by the same cause. The bile seems to be universally tinged with a yellow or greenish colouring matter, which may be regarded, therefore, as an essential part of the secretion ; and the urine of Mam¬ mals is also tinged by a yellow pigment, which seems related in its nature to that of the bile. In all these pigments, carbon is the predominating ingredient ; and their amount is increased when the respiratory process is insufficiently performed. 360. It appears, then, that the different secreting cells have the power of elaborating a great variety of products ; and that no essential differences can be discovered in the structure of the glands into whose composition they enter, which can account for that variety. We are entirely ignorant, therefore, of the reason why one set of cells should secrete biliary matter, another urea, another a colouring substance, and so on ; but we are as ignorant of the reason why, in the parti-coloured petal of a flower, the cells of one portion should secrete a red substance, whilst those in immediate contact with it form a yellow or blue colouring matter ; and we know as little of the cause, which occasions one set of the cells of which the embryo is composed to be converted into muscular tissue, another into cartilage, and so on. 361. One of the most curious points in the Physiology of Secretion, is the interchange which sometimes occurs in the functions of particular glands. When the operation of some one gland is checked or impaired by disease, it not unfrequently happens that another gland, or perhaps only a secreting sur¬ face, will perform its functions more or less perfectly; this RECEPTACLES FOR SECRETED PRODUCTS. 303 happens most frequently in regard to the important Excretions, as if Nature had especially provided for their continued sepa¬ ration from the blood, that its purity may be unceasingly maintained. Thus the urinary secretion has been passed off from the surfaces of the skin, stomach, intestines, and nasal cavity, and also from the mammary gland ; the colouring matter of the bile, when it accumulates in the blood (as in jaundice), is separated from it in the skin and conjunctival membrane of the eye (§ 537) ; and milk has been poured forth from pustules on the skin, and from the salivary glands, kidneys, &c. Such cases have been regarded as fabulous ; but they rest upon good authority, and they are quite consistent with physiological principles. 362. Some of the main ducts or channels, through which the glands pour forth their secretions, are provided with enlargements or receptacles, which serve to retain and store up the fluid for a time, until it may be desirable or convenient that it should be discharged. Thus, in most of the higher animals, the duct which conveys into the intestinal tube the bile secreted by the liver, is also connected with a receptacle termed the gall-bladder ; the bile, as it is secreted, passes into this, and is retained there until it is wanted for assisting in the digestive process (§ 213); when it is pressed out into the intestinal canal. It is a curious fact, that in most persons who die of starvation, the gall-bladder is found, dis¬ tended with bile ; showing that the secretion has continued, although it has not been poured into the intestine for want of the stimulus occasioned by the presence of food in the latter. In many quadrupeds, especially those of the Buminant tribe, the milk- ducts are in like manner dilated into a large re¬ ceptacle, the udder, which retains the secretion as it is formed, until the period when it is needed. In all Mammals, and in some Beptiles, Mollusks, and Insects, but not in Birds or Fishes, we find the ureters , which convey away the urinary excretion from the kidneys, dilated at their lower extremity into a Fig. 168. — Urinary Ap¬ paratus. ' a, kidneys ; b, ureters; c, bladder; d, its canal, the urethra. 304 STRUCTURE OF THE LIVER. bladder (fig. 168), which serves to retain all the fluid that is poured forth by the gland during a considerable length of time, and thus prevents the necessity for its being continually passed out of the body. Characters of Particular Secretions. 363. In nearly all animals, the Liver holds the first rank among Glands or secreting organs, in regard both to its size and to the obvious importance of its function. The principal varieties of its plan of structure in the Invertebrated classes having been already noticed (§ 356), we shall here limit ourselves to a sketch of that peculiar arrangement of its elementary parts, which presents itself in Man and other Yertebrata. The position of this organ in the abdominal cavity is shown in fig. 30. It is chiefly composed of a mass of cells of a flattened spheroidal form (fig. 169, b), the dia¬ meter of which is usually from 1 -800th to 1-1 600th of an inch ; each cell presents a distinct nucleus, which is surrounded by yellow biliary matter in a finely granular condition ; and in the midst of this there are usually one or two large fatty globules, or five or six small ones. The quantity of fat in the liver is very liable to increase, however, when there is a large amount of oily or fatty matter in the food, or when the respiratory function is not performed with sufficient activity. The hepatic cells are clustered together into lobules of irregular form, but about the average size of a millet-seed ; these lobules are disposed upon the ramifications of the hepatic vein (fig. 169, a), like leaves upon the branches of a tree ; and they are separated a a Fig. 169.— Portion of the Human Liver. from One another by the a, Showing the manner in which the substance peculiar distribution of of its lobules is disposed around the branches ,, , ,, n 1 of the hepatic vein a ; b, cells of which the the “ portal Vessels and lobules are composed, more highly magnified. hepatic ducts. The Vena Portae , it will be remembered, cohects the blood that has been distributed to the alimentary canal, and conveys STRUCTURE OF THE LITER. 305 it to tlie liver, through, which it is distributed by the sub¬ divisions of this vessel, which acts the part of an artery (§ 267). Its branches proceed to the surfaces of the lobules, amidst which they form by mutual inosculation a tolerably regular network (fig. 170, b , b, b ); and from these branches a Fig. 170. — Transverse Section of three Lobules of the Liver; Showing the passage of the ramifications of the portal vessels from the network b b bb, which surrounds the lobules, towards the centre of each lobule, near which they become continuous with the rootlets a a a of the hepatic veins. set of capillary twigs proceeds inwards towards the centre of each lobule, traversing in their course its aggregation of secreting cells. These capillaries finally terminate in the rootlets of the hepatic veins, which diverge from the centre of each lobule (fig. 170, a, a , a), and which collect the blood that has traversed its capillary system, to transmit it through larger trunks into the Vena Cava (§ 266), and thence to the heart. The liver is also supplied with arterial blood by the Hepatic artery ; but this seems to have for its function rather to nourish the solid tissues of the organ, than to supply the materials for secretion. The bile-ducts, which convey away the fluid that is elaborated by the hepatic cells, appear to form a network which surrounds the lobules, connecting them together and sending branches towards the interior of each (fig. 171). It is still doubtful, however, whether they extend through the entire substance of the lobules, and whether the x 306 STRUCTURE OF THE LIVER BILE. hepatic cells are really included within their extensions (as they are within the tubes or follicles of the liver of Inverte- brata) ; or whether the cells lie outside the bile-ducts, in immediate contact with the capillary blood-vessels that tra¬ verse the lobule, filling up the entire space not occupied by them, and transmitting the products of their secretion from one to another, until these reach the exterior of the lobule, where they find their way into the bile-ducts and are carried Fig. 171.— Transverse Section of two Lobules of the Liver; Showing the bile-ducts distended by injection ; a a, ramifications of the hepatic vein, occupying the centres of the lobules ; b b b, branches of the hepatic ducts, which are largest in the space c, between the lobules* and which pass towards the centre through d d, the substance of the lobules. off by them. — The bile may flow directly, as it is secreted, into the intestinal tube (§ 213); but if digestion be not going on, so that its presence there is not required, it regurgitates into the gall-bladder (fig. 30), which stores it up until it is needed. In this reservoir it undergoes a certain degree of concentration by the removal of its watery part. 364. Bile is a yellowish (sometimes a greenish-yellow) fluid, somewhat viscid and oily-looking, and having a very bitter taste, followed by a sweetish after-taste. It is readily miscible with water, its solution frothing like one of soap ; and it has the power, in common with soap, of dissolving oily matters ; so that ox-gall is not unfrequently used to remove grease-spots from woollen stuffs. The basis of the principal ingredient of biliary matter, which constitutes about 5 parts in 100 of the secretion, is a fatty or resinoid acid which is termed the Cholic ; this consists of 49 Carbon, 39 Hydrogen, and 9 Oxygen ; and it forms, by “ conjugation ” with glycine (a SECRETION OF BILE. 307 sugary substance that is derivable from the decomposition of gelatin and albumen) and with ta urine (a substance distinguished for the large proportion of sulphur it contains, — no less than 25 per cent), two other acids, the Glycocholic and the Tauro- ckolic , which are mingled in different proportions in the bile of different animals, both being combined with soda as a base. Bile also contains a white crystallizable fatty substance resembling spermaceti, which is termed Gliolesterin ; this consists of 36 Carbon, 3 2 Hydrogen, and 1 Oxygen ; and though its quantity in healthy bile appears to be very small, yet it occasionally increases to such an extent as to form the concretions known as “gall-stones,” which, getting into the bile-duct, are transmitted along it with great pain and diffi¬ culty, or block it up altogether. The peculiar colouring matter of bile is quite distinct from the preceding substances ; but like them it is extremely rich in carbon and hydrogen. 365. The bulk of the Liver, and the activity of the Respira- tory apparatus, seem generally to bear an inverse ratio one to the other. Thus we find in Insects, a respiratory system possessing enormous extension and activity of function, and a liver so slightly developed, that for a long time it was not recognised as such. On the other hand, in the Mollusca, we find the respiration carried-on upon a lower plan, and with far less activity; but the liver is of enormous size, often making up a large part of the bulk of the body. Moreover, in the Crustacea, which are formed upon the same general plan with Insects, but which have an aquatic and therefore less energetic respiration, we find the liver very large, as in the Mollusca. In Reptiles and Fishes, again, whose respira¬ tion and temperature are low, the liver is comparatively larger than in Birds and Mammals, in which classes the respiration is more energetic, and the blood warm. In all these in¬ stances, however, the bulk of the liver depends in great part upon the accumulation of fat in its cells ; and the secreting activity may be positively less in them, than it is in animals which have a comparatively small biliary apparatus. 366. The materials of the secretion of Bile are probably derived in part from the disintegration of the tissues, and in part more directly from the food. It is an interesting fact that the composition of bile and urine, taken together, corre¬ sponds closely with the composition of the blood ; so that it x 2 308 ASSIMILATING ACTION OF LIVER. would appear as if the nutritive materials, in their ultimate metamorphosis, resolved themselves chiefly into these two excretory products. The greater part of the biliary matter poured into the intestinal canal seems to be ordinarily re¬ absorbed with the fatty matter of the food, and to be, like it, carried out of the system through the lungs in the form of carbonic acid and water ; it being only when the bile has either been formed in excessive amount, or has been pro¬ pelled along the intestinal tube with undue activity, that it is discharged in any quantity from the rectum, as in bilious diarrhoea. — The secreting action of the Liver, however, is by no means its sole mode of influencing the composition of the blood ; for it has been shown by the recent researches of M. Bernard, that the blood which leaves the liver by the hepatic vein contains a peculiar substance of a saccharine nature,1 which does not exist in the blood brought to the organ by the portal vein. This substance appears to be elaborated by the converting power of the liver, either from materials supplied by the food, or from the products of the waste of the system ; and it seems to be specially destined as a pabulum or fuel for the combustive process, being usually eliminated from the blood in the form of carbonic acid and water during its passage through the lungs, so as not to pass into the systemic circulation unless either its quantity be un¬ usually great, or its elimination be interfered with by imperfect respiration. The liver seems also to form a peculiar fat, which is usually consumed in the same manner ; but if the respiratory process be feeble, this fat accumulates in the cells of the liver itself. 367. The TJrinary excretion has for its chief purpose to throw off those products, formed in a similar manner, which are highly charged with azote. The most important of its ingredients, in Man and the Mammalia, is the substance termed Urea , which has a crystalline form, and is very soluble in water. It contains 2 equivalents of Carbon, 4 of Hydrogen, 2 of Azote, and 2 of Oxygen ; and it will be seen, by referring to the statement formerly given of the composition of albumen 1 This substance is spoken of by M. Bernard as sugar : it has been demonstrated, however, by the recent researches of Dr. Pavy, that the liver does not form sugar, but a substance that becomes sugar almost immediately upon contact with albuminous matters. SECRETION OF URINE. 309 (§ 13) and gelatin (§ 19), that the amount of azote in propor¬ tion to that of the other elements is much greater in urea than it is in these substances, which form the materials of the animal tissues. The quantity of Urea which is daily excreted is very considerable, the average in an adult being about an ounce, and in a child of eight years old about half as much.- — There is another compound which does not usually exist in large amount in the urine of the Mammalia, but which makes up a considerable part of the solid matter of this secretion in Birds and the lower Yertebrata ; this is uric or lithic acid, which consists of 10 equivalents of Carbon, 4 of Hydrogen, 4 of Azote, and 6 of Oxygen. It is almost entirely insoluble in water, unless it be combined with soda or am¬ monia ; and in this state it ordinarily exists. When formed in too large quantity, however, it may be deposited in an insoluble form, constituting gravel (§ 348) ; and the same effect may result from the presence of some other acid, which, combining with the ammonia, precipitates or sets free the lithic acid. In the urine of herbivorous animals, uric acid is replaced by Hippuric acid, which contains a much larger proportion of carbon, its composition being 18 Carbon, 8 Hydrogen, 1 Nitrogen, and 5 Oxygen. Urine also contains a considerable quantity of Saline matter; part of which consists of what has been introduced into the system in the same form, and has to be got rid of as superfluous ; whilst another part results from the conversion of the sulphur and phosphorus of the food into sulphuric and phosphoric acids by union with atmospheric oxygen (§ 343), and from the com¬ bination of these acids with alkaline bases furnished by the food. The amount of alkaline phosphates contained in the urine may be considered as in some degree a measure of the expenditure of nervous tissue ; whilst that of alkaline sulphates has some relation to the expenditure of muscular substance. 368. The Kidney, by which the secretion of Urine is eli¬ minated from the blood, is an organ whose structure in the higher animals is very peculiar, although in the lower it is a mere aggregation of tubes or of follicles. If we make a ver¬ tical section of the kidney of Man or any of the higher Mam¬ malia (fig. 172, a), we find that it seems composed of two different substances, one surrounding the other; to the outer, a, the name of cortical (bark-like) substance has been given ; 310 STRUCTURE OF THE KIDNEY. whilst the inner, 6, is termed medullary (or pith-like). In the cortical substance, no definite arrangement can he de- A tected by the naked eye ; it chiefly consists of a very intricate network of blood-vessels, surrounding the extremities of the tubes. But in the medullary substance we can trace a regular passage of minute tubes, from the circumference to¬ wards the centre. They commence in the midst of the network of blood-vessels (b, a ), and then pass down in clusters, nearly in a straight direction, and slightly con¬ verging towards each other, until each cluster terminates in a little body, called the calyx or cup, which discharges the fluid it receives into the large cavity of the kidney, termed the pelvis or basin (a, c ). From this it is conveyed away by the ureter d, which terminates in the bladder. 369. One of the most interesting circumstances in reference to the Urinary secretion, is the very large quantity of water which, in the higher animals, is got rid of through this channel, and the means by which it is drawn off. The kidneys seem to form a kind of regulating valve, by which the quan¬ tity of water in the system is kept to its proper amount. The exhalation from the Skin is liable to sustain great variations in its amount from the temperature of the air around ; for when this is low, the exhalation is very much diminished ; and when it is high, the quantity of fluid that passes off in this manner is increased (§ 371). Hence, if there were not some other means of adjusting the quantity of fluid in the blood-vessels, it would be liable to continual and very inju¬ rious variation. This important function is performed by the kidneys, which allow such a quantity of water to pass into Fig. 172.— Structure of the Kidney of Man. A, vertical section of the kidney ; a, cortical substance ; &, tubular substance ; c, calyx and pelvis ; d, ureter. B, portion of the gland enlarged; a, extremity of the uriniferous tubes; b, straight portion; c , their termination in the calyx. MALPIGHIAN BODIES OP THE KIDNEY; 311 the urinary tubes, as may keep the pressure within the vessels very nearly at a uniform standard ; and a distinct and very curious provision exists for its separation. The extremity of many of the uriniferous tubes is made to include little knots or bunches of capillary vessels, which have extremely thin walls (fig. 173) ; and a vast number of such knots, which are termed “ Malpighian bodies,” after the name of their dis¬ coverer, are scattered through the cortical portion (§ 368) of the kidney. To these the blood brought to the organ by the renal artery is first conveyed ; and the membranes that sepa¬ rate the interior of the capil¬ lary vessels from the cavity of the uriniferous tube, being of Fig* m‘”^AEL kidney bodies of extreme thinness, water is readily able to traverse them ; and will do so in larger or smaller quantity, according as the pressure upon the walls of the capillaries is greater or less. The blood which has passed through these is next conducted to another set of capillaries, which form a network upon the part of the tube that is lined by the secreting cells ; and it is there subservient to the elaboration of the solid part of the secretion. Hence the quantity of water in the urinary secretion depends in part upon the amount exhaled from the skin, — being greatest when this is least, and vice versa, — and in part upon the quantity which has been absorbed by the vessels. The quan¬ tity of solid matter in the secretion has but little to do with this ; for it depends upon the amount of waste of the muscular and other tissues that has been occasioned by their activity (§ 367); and also upon the quantity of surplus aliment which has to be discharged through this channel, there being no other vent for it (§ 348). 370. Hext to the excretions formed by the liver and the kidneys, that of the Skin probably ranks in importance. A large quantity of watery vapour is constantly passing-off from the whole surface of Man and other soft-skinned animals; 312 EXHALATION FROM THE SKIN. and this amount is greatly increased under particular circum¬ stances. A continual evaporation takes place from the surface of the skin, wherever it is not protected by hard scales or plates ; and the amount of it will depend upon the warmth, dryness, and motion of the surrounding air, exactly as if the fluid were being evaporated from a damp cloth. Every one knows that the drying of a cloth is much more rapidly effected in a warm dry atmosphere, than in a cold moist one ; more quickly, too, in a draught of air, than in a situation where there is no current, and where the air is consequently soon charged with moisture. That all these influences affect the evaporation from the bodies of Animals, there is ample evi¬ dence derived from experiment. 371. But besides this continual evaporation, a special exhalation of fluid takes place from the vast number of minute perspiratory glands imbedded in the fatty layer just beneath the Skin (§ 37). Every one of these glandulse, when straightened out, forms a tubule about a quarter of an inch in length ; and as it has been estimated that in a square inch of surface on the palm of the hand there are no fewer than 3528 of these glandulse, the length of their tubing must be 882 inches or 7 3J feet. The average number in other parts of the body may be estimated at about 2800 per square inch ; and as the number of square inches of surface on a man of ordinary stature is about 2500, the total number of perspiratory glan¬ dulse must be not less than seven millions, and the length of their tubing nearly twenty-eight miles. The fluid which these perspiratory glands ordinarily exhale, is dissolved by the atmo¬ sphere, and carried off in the state of vapour, so as to pass away insensibly ; but they are stimulated to increased action by the exposure of the body to heat, which causes them to pour forth their secretion in greater abundance than the air can carry off, and this consequently accumulates in drops upon the surface of the skin. The amount of perspiration may be considerably increased, without its becoming sensible, if the air be warm and dry, so as to carry off, in the form of vapour, the fluid which is poured out on the skin ; but, on the other hand, a very slight increase in the ordinary amount immedi¬ ately becomes sensible on a damp day, the air being already too much loaded with moisture to carry off this additional quantity. The distinction between insensible and sensible COOLING EFFECT OF CUTANEOUS EXHALATION. 313 perspiration, is not the same, therefore, with the difference between simple evaporation and exhalation from the skin ; for a part of the latter is commonly insensible ; and the degree in which it is so depends upon the amount of fluid exhaled, and the state of the surrounding atmosphere. If the fluid thus poured forth be allowed to remain upon the surface of the skin, it produces a very oppressing effect ; most persons have experienced this, when walking in a mackintosh cloak or coat, on a damp day. The waterproof garment keeps in the perspiration, almost as effectually as it keeps out the rain ; and consequently the air within it becomes loaded with fluid, and the skin remains in a most uncomfortable as well as pre¬ judicial state of dampness. 372. The purpose of this watery exhalation, and of its increase under a high temperature, is evidently to keep the heat of the body as near as possible to a uniform standard. By the evaporation of fluid from the surface of the skin, a considerable quantity of heat is withdrawn from it, becoming latent (according to ordinary phraseology) in the change from fluid to vapour : of this we make use in applying cooling lotions to inflamed parts. The more rapid the evaporation, the greater is the amount of heat withdrawn in a given time ; hence, if we pour, on separate parts of the back of the hand, small quantities of ether, alcohol, and water, we shall find that the spot from which the ether is evaporating feels the coldest, that which was covered by the alcohol less so, whilst the part moistened with water is comparatively but little chilled. The greater the amount of heat applied to the body, then, the more fluid is poured out by the perspiratory glands; and as the air can carry it off more readily in proportion to its own heat, the evaporation becomes more rapid, and its cooling effect more powerful. It is in this manner that the body is rendered capable of sustaining very high degrees of external heat, without suffering injury. Many instances are on record, of a heat of from 250° to 280° being endured in dry air for a considerable length of time, even by persons unaccustomed to a peculiarly high temperature; and indi¬ viduals whose occupations are such as to require it, can sustain a much higher degree of heat, though perhaps not for any great length of time. Thus, the workmen of the late Sir F. Chantrey were accustomed to enter a furnace in which his 314 IMPORTANCE OF CUTANEOUS EXHALATION. moulds were dried, while the floor was red-hot, and a ther¬ mometer in the air stood at 350°; and Chabert, the “Fire- king,” was in the habit of entering an oven whose temperature was from 400° to 600°. It is possible that these feats might be easily matched by many workmen, who are habitually exposed to high temperatures; such as those employed in iron-foundries, glass-houses, and gas-works. 373. That the power of sustaining a high temperature mainly depends upon the dryness of the atmosphere, is evident from what has just been stated ; since, if the perspiration that is poured-forth upon the skin is not carried-off with sufficient rapidity, on account of the previous humidity of the air, the temperature of the body will not be sufficiently kept down. It has been found, from a considerable number of experiments, that when warm-blooded animals are placed in a hot atmos¬ phere saturated with moisture, the temperature of their bodies is gradually raised 12° or 13° above the natural standard; and that the consequence is then inevitably fatal. 374. The amount of fluid exhaled from the skin and lungs (§ 343) in twenty-four hours, probably averages about three or four pounds. The largest quantity ever noticed, except under extraordinary circumstances, was 5 lbs. ; and the smallest, 1 1 lbs. It contains a small quantity of solid animal matter, besides that of the other secretions of the skin which are mingled with it ; and there is good reason to think that this excretion is of much importance, in carrying off certain sub¬ stances which would be injurious if allowed to remain in the blood. That which is called the Hydrophatic system, proceeds upon the plan of increasing the cutaneous exhalation to a very large amount; and there seems much evidence, that certain deleterious matters, the presence of which in the blood gives rise to Gout, Eheumatism, &c., are drawn off from it more speedily and certainly in this way, than in any other. 37 5. Besides the perspiratory glands, the skin contains others, which have special functions to perform. Thus in most parts which are liable to rub against each other, we find a considerable number of sebaceous follicles (fig. 8, a a), which secrete a fatty substance that keeps the skin soft and smooth. Besides these, the skin contains other follicles in particular parts, for secreting peculiar substances ; as, for instance, those which form the cerumen , a bitter waxy substance that is MAMMARY GLAND .* - SECRETION OF MILK. 315 poured into the canal leading to the internal ear, for the pur¬ pose (it would seem) of preventing the entrance of insects. 37 6. The secretion of Milk is important, not so much to the parent who forms it, as to the offspring for whose nourish¬ ment it is destined. It does not seem to carry off from the system any injurious product of its decomposition ; for it bears a remarkable analogy to blood in the combination of substances which it contains ; nevertheless it is found that, when this secretion is once fully established, it cannot be suddenly checked, without producing considerable disturbance of the general system. The structure of the Mammary gland closely resembles that of the parotid already described (fig. 165). It consists of a number of lobules, or small divisions, closely bound together by fibrous and areolar tissue ; to each of these proceeds a branch of the milk-ducts, together with numerous blood-vessels ; and the ultimate ramifications of these ducts terminate in a multitude of little follicles, about the size (when distended with milk) of a hole pricked in paper by the point of a very fine pin. 377. The nature of the composition of Milk is made evident by the processes to which we commonly subject it. When it is allowed to stand for some time, its oleaginous part, forming the cream , rises to the top. This is still combined, however, with a certain quantity of albuminous matter, which forms a kind of envelope round each of the oil-globules ; but in the process of churning, these envelopes are broken, and the oil-globules run together into a mass, forming butter . In ordinary butter a certain quantity of albuminous matter remains, which, from its tendency to decomposition, is liable to render the butter rancid ; this may be got rid of by melting the butter at the temperature of 180°, when the albumen will fall to the bottom, leaving the butter pure and much less liable to change. In making cheese , we separate the albuminous portion, or casein, , by adding an acid which coagulates it. The buttermilk and whey left behind after the separation of the other ingredients, contain a considerable quantity of sugar, and some saline matter. The proportion of these ingredients varies in different animals ; and also in the same animal, according to the sub¬ stances upon which it is fed, and the quantity of exercise it takes. The amount of casein seems to be greatest in the milk of the Cow, Goat, and Sheep ; that of oleaginous matter in the 316 GENERAL REVIEW OF NUTRITIVE OPERATIONS. milk of the Human female ; and that of sugar in the milk of the Mare. The milk of the Cow, if a portion of its casein were removed, would resemble Human milk more nearly than any other ; and it is therefore best for the nourishment of Infants, when the latter cannot he obtained. The important influence of Mental emotion on this secretion has already been noticed (§ 353) ; and many more instances might he related, were not the ordinary facts in regard to it generally known. CHAPTER VIII. GENERAL REVIEW OP THE NUTRITIVE OPERATIONS — FORMATION OF THE TISSUES. General Review of the Nutritive Operations. 378. In the preceding Chapters (hi. to v.) those processes have been described, by which the alimentary materials that constitute the raw material of the tissues, are converted into a fluid adapted for the Nutrition of the body ; and we then (Chaps, vi. and vii.) considered those functions, by which this fluid is kept free from the impurities it acquires during its circulation through the body, and is maintained in the state which alone can adapt it to the purposes it is destined to fulfil. These purposes may be regarded as fourfold. In the first place, the Blood is destined to supply the materials of the fabric of the body; which, as it is continually undergoing decay (§ 68), requires the means of as constant a renovation. Secondly , the Blood (in most animals at least) serves to convey to the tissues the supply of oxygen which is required by them, — especially by the muscular and nervous tissues, — as a necessary stimulus to the performance of their functions. Thirdly , the Blood furnishes to the secreting organs the materials for the elaboration of the various fluids, which have special purposes to serve in the Animal economy, — such, for instance, as the Saliva, Gastric juice, Milk, &c. And lastly , the Blood takes up, in the course of its circulation, the pro¬ ducts of the waste or decomposition of the various tissues, which it conveys to the several organs, — the Lungs, Liver, FORMATION OF THE TISSUES. 317 Kidneys, &c., — destined to throw them off by Excretion. The greater number of these processes have already been treated of in more or less detail. Those included under the first head were considered, in a general form, in Chap. i. of this Treatise. Those which are comprehended under the second head have been dwelt-on in Chaps, v. and vi.; and will be again noticed, when the actions of the Nervous and Muscular tissues are described. And the varied actions which are included under the third and fourth classes, have been discussed in the two Chapters which precede the present one. We have now to enter, in more detail, into the mode in which the circulating fluid is applied to the Nutrition and Formation of the Tissues. Formation of the Tissues . 379. There is sufficient reason to believe that every living being is developed from a germ ; no organized structure being able to take its origin (as some have supposed) in a chance combination of inorganic elements. All the facts relating to the production of Eungi and Animalcules, which have been imagined to favour this doctrine, may be satisfactorily ex¬ plained in other ways (Veget. Phys. § 779 ; Zool. § 1213). Now the first structure developed from this germ, in the Animal as in the Plant, is a simple cell; and the entire fabric subsequently formed, however complex and various in struc¬ ture, may be considered as having had its origin in this cell. The cells of Animals, like those of Plants, multiply by the development of new cells within them; each of these be¬ comes in its turn the parent of others ; and thus, by a con¬ tinuance of the same process, a mass consisting of any number may be produced from a single one. It is in this manner that the first development of the Animal embryo takes place, as will be shown hereafter (Chap. xv.). A globular mass, con¬ taining a large number of cells, is formed before any diversity of parts shows itself; and it is by the subsequent development, from this mass, of different sets of cells, — of which some are changed into cartilage, others into nerve, others into muscle, others into vessels, and so on, — that the several parts of the body are ultimately formed. 380. This process of differentiation is carried to very different degrees in the development of the several classes of 318 DIFFERENTIATION OF STRUCTURE. animals ; for in some it is checked so early, that scarcely any distinction either of organs or of tissues ever shows itself; whilst in others it continues during a large proportion of the earlier period of life. It has no relation to groivth , or simple increase of size ; for this may take place by the multiplication of similar parts, giving rise to that “ vegetative repetition” which is so characteristic of the lower tribes of Animals, and which gives to many of them so strong a resemblance in general aspect to Plants ; whilst, on the other hand, the de¬ velopmental process by which higher forms of structure are evolved, sometimes takes place without any increase at all. It is in its degree of such differentiation, that what is called the lower or the higher organization of any living being essentially consists ; for whilst in the simplest forms of Animal struc¬ ture every part is similar to every other, so that all the functions of life are performed in common by each, we find in Man (whose body may be regarded as presenting the highest type or example of this differentiating process) that no two parts are precisely similar, except those on the opposite sides of the body. This fact is occasionally mani¬ fested in a very singular manner, in the symmetry of disease ; certain morbid poisons (as those of gout, and of several affec¬ tions of the skin), which have a tendency to single out par¬ ticular spots of the tissues whose nutrition they disturb, exhibiting their action in those parts of the two lateral halves of the body which precisely correspond with each other. 381. Now in the lowest grades of Animal structure, we find that the several tissues of the body can themselves appropriate from the products of digestion the nutrient materials they respectively require ; so that, for their growth and mainte¬ nance, it is sufficient that these products should be carried into their neighbourhood by extensions of the digestive cavity (§ 296). But in all' the more highly-organized animals, it appears requisite that the nutrient material should pass through an intermediate stage of preparation, which is termed assimilation (or making-like); and this is effected by their introduction into the current of the circulation, and their mixture with the pre-existing blood, which, in virtue of its own vital powers, exerts upon them a converting action, which prepares them for being appropriated by the solid tissues. 382. When once the several forms of tissue have been MUTUAL DEPENDENCE OF PARTS OF ORGANISM. 319 developed, their nutrition is kept np by the supply of their respective materials which they derive from the blood. Each tissue draws from the circulating current that which it re¬ quires ; and it is one of the most wonderful proofs of the skill with which the entire fabric has been planned and constructed, that the composition of the blood should be maintained at a nearly uniform standard, in spite of the continual change which is thus taking place in its actual components. It has been justly remarked, that each part of the body, by taking from the blood the peculiar substances which it needs for its own nutrition, does thereby act as an excretory organ, inasmuch as it removes from the blood that which, if retained in it, would be injurious to the nutrition of the body generally. 383. Hence it seems that such a mutual dependence must exist among the several parts and organs of the body, as causes the evolution of one to supply the conditions requisite for the production of another ; and this view is borne out by a great number of phenomena of very familiar occurrence, which show that a periodical change in one set of organs governs changes in others which at first sight might seem to have no relation to them. Thus the plumage of Birds, at the commencement of the breeding season, becomes (especially in the male) more highly coloured, besides being augmented by the growth of new feathers ; but when the generative organs pass into their condition of periodical inactivity, the plumage begins at once to assume a paler and more sombre hue, and many of the feathers are usually cast, their nutrition being no longer kept up. So, again, it is no uncommon occurrence among Birds, for the female, after ceasing to lay, to assume the plumage of the male, and even to acquire other character¬ istic parts, as “ spurs ” in the fowl tribe. That, in these and similar instances, the development of organs is immediately determined by the presence or absence in the blood of the appropriate pabulum for the parts in question, and that its existence depends upon changes taking place in other parts, has been rendered still more probable by the results of expe¬ riments, which show that if the ordinary changes in one set of organs be prevented by their removal, those usually taking place in the others do not occur. 384. Though all the tissues derive the materials of their development from the blood which circulates in the vessels, 320 NUTRITION OF NON-VASCULAR TISSUES. yet there is considerable variety in the mode in which the supply is afforded ; some tissues being furnished with blood much more copiously and directly than others, in consequence of the greater minuteness with which the capillaries are dis¬ tributed through their substance. There are several, indeed, into which no blood-vessels enter, in their natural state ; but which derive their nutriment by absorbing the liquor san¬ guinis that is brought into their neighbourhood. This is the case, for instance, with the Epidermis and Epithelium (§§ 38, 40) ; the cells of which are developed at the expense of the fluid which they absorb, through the basement mem¬ brane on which they lie, from the vessels of the skin or mucous membrane beneath it. In like manner, even the thick layer of Cartilage which covers the ends of most of the long bones, is destitute of blood-vessels ; and the small amount of nourishment it requires, is obtained by absorption from the vessels which surround it (§ 47). This tissue undergoes very little change from time to time ; and its growth takes place chiefly by addition of new matter to its surface ; consequently there is no necessity for any active circulation through its interior; and the transmission of nutritive fluid from one cell to another (as in the cellular tissue of Plants) is sufficient for its wants. Even in Bone, the blood-vessels are not very minutely distributed ; for although there is a close network of capillary vessels on the membrane lining the Haversian canals and the cells of the cancellated structure (§ 49), yet none of these pass into the actual substance of the bone. The simple Fibrous tissues are, for the most part, sparingly supplied with blood-vessels, as they are but little liable to decay or injury ; though the Areolar tissue serves as the bed for the reception of the vessels which are on their passage to other tissues. Thus it is by its means that blood-vessels are conveyed into the Adipose tissue ; for the ultimate elements of that tissue, namely, the fat-cells, are surrounded by capil¬ lary vessels, not entered by them. The same important pur¬ pose is answered by the areolar tissue that lies amongst the tubes which form the essential parts of the Nervous and Muscular tissues ; for these tubes are not perforated by ves¬ sels, so that their contents must be nourished by fluid absorbed through their walls. 385. In no instance that we are acquainted with, in the IMPERFECT NUTRITION : — CONSUMPTION. 321 higher animals at least, do the vessels directly pour the blood into any tissue for the purpose of nourishing it. Unless there have been an actual wound which has artificially opened the blood-vessels, no fluid can escape from them into the substance traversed by the capillaries, except by transuding the walls of the latter ; and hence it would seem impossible that any of the floating cells contained in the blood can be deposited in the tissues and contribute to their development. The Liquor Sanguinis seems, therefore, to furnish all that is wanting for this purpose ; and it readily permeates the walls of the capil¬ laries, the basement-membrane, and any other of the softer tissues, so as to arrive at the parts where it is to be applied. As it is withdrawn from the blood, it is continually being re-formed from the food ; but if it be not supplied in sufficient quantity by the latter, the tissues are imperfectly nourished, and the strength of the body and the vigour of the mind are consequently alike impaired. 386. This imperfect nutrition seems to be the essential condition of one of the most destructive diseases to which the human frame is liable, — that commonly known as Consump¬ tion ; which is, however, but one out of several diseases that may result from the same general defect of nutrition. If the liquor sanguinis be imperfectly elaborated, it is less fit to undergo organization; and, consequently, instead of being converted into living tissue, part of it is deposited, as an imperfectly organized mass, in the state known to the Medical man as Tubercle. Such deposits take place more frequently in the lungs than in any other part ; and besides impeding the circulation and respiration, they produce irritation and inflammation, in the same manner as other substances im¬ bedded in the tissues would do ; and so far from having, like many other diseases, a natural tendency to cure, this malady, if unchecked, almost certainly leads to a fatal termination. Microscopic examination of tubercular matter shows that it consists of half-formed cells, fibres, &c., together with a granu¬ lar substance which seems to be little else than coagulated albumen. The only manner in which any curative means can be brought to bear upon this terrible scourge, is by attention to the constitutional state from which it results. This is sometimes hereditary ; and is sometimes induced by insuffi¬ cient nutrition, obstructed respiration, habitual exposure to 3 22 TUBERCULAR DIATHESIS : ITS TREATMENT. cold and damp, long-continued mental depression, &c. The treatment of the Tubercular diathesis (as this state of consti¬ tution is termed) must be directed to the invigoration of the system by good food, active exercise, pure air, warm clothing, and cheerful occupation; and by the due employment of these means, at a sufficiently early period, many valuable lives may be saved which would have otherwise fallen a sacrifice. The value of cod-liver oil in the treatment of this disease, which is now a well-established fact, seems to depend upon the facility with which it is assimilated as a nutritive material. It is a remarkable fact that the inhabitants of Iceland, the greater part of whom live under conditions that might be expected to favour the development of tubercular disease, are singularly free from it ; and the source of this exemption seems to consist in the very oleaginous nature of their diet. Consumption presents itself among the inhabitants of all climates ; and the value of change to a patient who is affected with this malady, chiefly depends upon the oppor¬ tunity which it affords him for abundant exercise in the open air, without injurious exposure to cold or damp. 387. From the foregoing facts it is evident, that the opera¬ tions of Nutrition are due, on the one hand, to the indepen¬ dent properties of the several Tissues, which draw from the blood the materials of their continued growth and renewal ; and, on the other, to the properties of the Blood, which supplies them with these materials. The blood, left to itself, could form no tissue more complex than a mere fibrous net¬ work : whilst, conversely, the various tissues of the body could not draw their nourishment directly from the products of digestion, and are consequently dependent upon the blood for their supply. We may illustrate the relation between the three states, — that of aliment, blood, and organized tissue, — by comparing them with the three principal states which Cotton passes through in the progress of its manufacture, — namely, the raw cotton, spun-yarn, and woven fabric. The spun-yarn could not of itself assume that particular arrange¬ ment which is given to it by the loom ; and the loom could make nothing of the raw cotton, until it has been spun into yarn. 388. It is also evident, that the blood-vessels have no other purpose in the act of Nutrition, than to convey the circulating ACTION OF BLOOD-VESSELS IN NUTRITION. 323 fluid into the neighbourhood of the part where it is to be employed ; and the blood, or at least its organizable portion — the liquor sanguinis — must quit the vessels before it can be employed in the development of new tissue. We might illus- strate this by the distribution of water-pipes through a city ; they might pass into every house, nay, into every room, and yet the water must be drawn from the pipes before it can be applied to any required purpose. The spaces untraversed by vessels have been shown to be larger in some tissues, and smaller in others ; the distribution of the capillaries being more minute, in proportion as the nutritive actions of the part go on more energetically. Now in the embryo, even of the most complex and perfect animals, there is a period when no blood-vessels exist, the whole mass being made-up of cells, every one of which lives for itself and by itself, absorbing nutriment from a common source, and not at all dependent upon its brethren. It is only when a diversity of structure begins to show itself,— one part undergoing transformation into bone, another into muscle, and so on, — and when some portions of the fabric are cut-off from the direct supply of nourishment, — that vessels begin to show themselves. These are formed, like the ducts of Plants, by the breaking-down of the partitions between contiguous cells ; they at first seem rather like passages or channels, than tubes with walls of their own ; and this condition they retain in certain cases through life (§ 289). Repair of Injuries . 389. Every animal possesses, in a greater or less degree, the power of not merely maintaining its organized fabric in its integrity, by the renewal of the parts which are from time to time passing into decay, but also of reproducing parts of that fabric which have been lost by disease or accident. This power seems greatest among the lowest tribes of Animals ; in many of which the entire organism can be reproduced from a small portion of it, as is the case with the Hydra (§ 122), and with some species of Sea- Anemone (§ 126). In the Star -fish, a far more highly-organized animal, the regenerative power is more limited, though it is still very remarkably manifested ; for if one, two, or more of the rays be broken or cut off, they are gradually restored, provided the central disc be uninjured, y 2 324 REPARATIVE POWERS OF LOWER ANIMALS. Of certain kinds of Holothuria (fig. 67), which eject the entire mass of viscera under the influence of alarm, it has been ob¬ served that they not only continue to move about as if nothing had happened, but that, under favourable circumstances, they regenerate the whole of the digestive and reproductive appa¬ ratus thus parted- with. — H ext to Zoophytes, there are no animals in which the regenerative power is known to manifest itself so strongly as the lower members of the Articulated series, such as the inferior Entozoa and the Turbellaria (Zoo¬ logy, § 924), among which last we find the Planaria almost rivalling the Hydra, although it is an animal of much more complex structure. The common Earthworm can reproduce either the head or any portion of the body of which it may have been deprived; but it cannot be multiplied by the division of its body into two or more parts (as asserted by some), since these parts, although they continue to move for a time, soon perish. There are Worms allied to it, however, in which the regenerative power is sufficient to produce the whole body from a separated fragment; and no fewer than twenty-six have thus been made to originate by the subdivision of a single individual. In the higher Articulata, such as Crus¬ tacea, Insects, and Spiders, the reparative capacity is limited to the restoration of limbs ; and even this would seem to be seldom preserved in perfect Insects, being restricted to the larval period of their lives. Little is known of the regene¬ rative power possessed by the higher Mollusks ; but it has been affirmed that the head of the Snail may be reproduced after being cut-off, provided the cephalic ganglion be not injured, and an adequate amount of heat be supplied. 390. It is only among the cold-blooded members of the Yertebrated series, that the reparative power extends to the renewal of entire organs ; and this seems limited in Fishes to the reproduction of portions of the fins which have been lost by disease or accident. In Batrachia, however, it has been found that entire new legs, with perfect bones, nerves, muscles, &c., may be reproduced after severe loss or injury of the original members ; and even a perfect eye has been formed in place of one that had been removed. It is inte¬ resting to observe that the exercise of this reparative power essentially depends upon the temperature in which the animal is living ; the reproduction of entire members apparently REPARATIVE POWERS OF HIGHER ANIMALS. 325 requiring a higher degree of the stimulus of Heat, than does their ordinary nutrition. In Lizards , an imperfect reproduc¬ tion of the tail takes place when part of it has been broken off; but the newly-developed portion contains no perfect ver¬ tebrae, its centre being occupied by a cartilaginous column like that of the lowest fishes. — -In the warm-blooded Verte- brata generally, the power of reproduction after loss or in¬ jury seems much more limited. We do not find that entire parts or members once destroyed, are completely renewed ; though very extensive breaches of substance are often filled up. The tissues most readily reproduced are Bone, the Simple Fibres (§ 22), and the Membranes (such as the Skin, the Mucous and Serous membranes), of which these tissues form the basis. As a general rule, losses of substance in Glandular tissue, Muscle, and other parts of comparatively high organi¬ zation, do not seem to be reproduced ; but there is a curious- exception to this in the case of Nervous tissue, which, with Blood-vessels, is very readily re-formed in the new growths by which losses of substance are repaired, as we often see in the rapid skinning-over of a large superficial wound. One of the most remarkable manifestations of reparative power in the Human body, is the re-formation of an entire bone, when the original one has been destroyed by disease. The new bony matter is thrown-out, sometimes within and sometimes around the dead shaft ; and when the latter has been removed, the new structure gradually assumes the regular form, and all the attachments of muscles, ligaments, &c., become as complete as before. A much greater variety and complexity of actions are involved in this process, than in the reproduction of whole parts in the simpler animals ; though its effects do not appear so striking. It appears that, in some individuals, this regene¬ rating power is retained to a much greater degree than it is by the species at large ; thus, there is a well-authenticated instance, in which a supernumerary thumb on a bo/s hand was twice re¬ produced, after having been removed from the joint. In many cases in which the crystalline lens of the eye has been re¬ moved, in the operation for cataract, it has been afterwards regenerated ; and there is evidence that, during embryonic life, the regeneration of lost parts may take place in a degree to which we have scarcely any parallel after birth ; attempts being sometimes made at the re-formation of entire limbs, in 326 REPAIR OF LOSSES OF SUBSTANCE. place of such as are lost during the earlier periods of develop¬ ment. 391. When an entirely new structure is to be formed, — as for the closure of a wound, the union of a broken bone, or the repair of any other injury, — the process is of a kind very much resembling the first development of the entire fabric. The neighbouring vessels pour out their liquor sanguinis , which is known to the Surgeon under the name of coagulable lymph ; this fills up the open space, and forms a connecting medium between the separated parts. If this intervening layer be thin, the two sides of the wound may adhere so closely as to grow together without any perceptible interposition of new substance ; this is what is called “ healing by the first inten¬ tion.^ But if the loss of substance has been too great to allow of such adhesion, the vacant space is filled by the gradual organization of the coagulable lymph ; and this may take place in one of two very different modes, the determina¬ tion being chiefly dependent on the condition of the wound as to seclusion from air or exposure to it. 392. The former of these conditions is by far the more favourable of the two ; for the reparative material is usually developed gradually but surely into fibrous tissue, without any loss, and with very little irritation either in the part itself or in the system at large. This process seems to take place naturally in cold-blooded animals, even in open wounds ; the contact of air not having that disturbing influence in them, which it exerts in warm-blooded animals. And Mature frequently endeavours to bring it about in the superficial wounds of warm-blooded animals, by the formation of a large scab, which protects the exposed surface ; but this happens much less frequently in the Human subject than it does among the lower animals, the unnatural conditions in which a large proportion of the so-called civilised races habitually live (especially deficient purity of the air, continual excess hi diet, and the frequent abuse of stimulants) being unfavourable to it. The performance of many operations which formerly left open wounds, in such a manner that the air may be effectually excluded by a valvular fold of skin, is one of the greatest improvements in modern Surgery. 393. In an open wound, on the other hand, which is healing by the process termed “ granulation,’7 the reparative HEALING OF OPEN WOUNDS. 327 material is rapidly developed into cells, amongst which blood¬ vessels speedily extend themselves. The formation of new blood-vessels, in this and other cases, seems to commence in the giving- way of the walls of some of the previously-existing capillary loops, at particular spots, and in the escape of blood corpuscles in rows or files into the soft substance that sur¬ rounds them; thus channels or passages are excavated, which come into connexion with each other; and these channels, after a time, acquire proper walls, and become continuous with the vessels from which they originated, — to be in their turn the originators of a new series. The vitality of this new “ granulation- tissue,” however, is very low; and the part ex¬ posed to the air passes into the condition of pus (the yellow creamy fluid discharged from an open wound), which contains the same materials in a decomposing state. Thus there is a constant waste of organizable substance, the amount of which, in the case of an extensive wound, becomes a serious drain upon the system ; at the same time, there is a much greater irritative disturbance both in the part itself and in the system generally ; and the new tissue that is formed is of such low vitality that it subsequently wastes away, so as by its disap¬ pearance to leave a contracted cicatrix or scar. — The difference between the two modes of reparation now described is often one of life and death, especially in the case of large burns of the body in children. CHAPTEE IX. ON THE EVOLUTION OP LIGHT, HEAT, AND ELECTRICITY BY ANIMALS. Animal Luminousness 394. A large proportion of the lower classes of aquatic Animals possess, in a greater or less degree, the power of emitting light. The phosphorescence of the sea, which has been observed in every zone, but more remarkably between the tropics, is due to this cause. When a vessel ploughs the ocean during the night, the waves — especially those in her wake, or those which have beaten against her sides — exhibit a diffused lustre, interspersed here and there by stars or ribands of more intense brilliancy. The uniform diffused 328 LUMINOSITY OF MARINE ANIMALS. light is chiefly emitted by innumerable minute animals, which abound in the waters of the surface ; whilst the stars and ribands are due to larger animals, whose forms are thus bril¬ liantly displayed. This interesting phenomenon, when it occurs on our own coasts, is chiefly produced by incalculable multi¬ tudes of a small creature, termed the Noctilucay having a nearly globular form, and a size about equal to that of the head of a minute pin. When these cover the water, and a boat is rowed among them, every stroke of the oars produces a flash of light ; and the ripple of the water upon the shore is marked by a brilliant line. If a person walk over sands that the tide has left, his footsteps will seem as if they had been impressed by some glowing body. And if a small quantity of the water be taken up and rubbed between the hands, they will remain luminous for some time. The transparency of the little ani¬ mals to which these beautiful appearances are due, might cause them to be overlooked if they are not attentively sought ; they somewhat resemble grains of boiled sago in their aspect, but are much softer. In the general simplicity of their structure, the Noctilucce appear to correspond rather with the Rhkopoda (§ 130) than with any other group ; but they are distinguished by some remarkable peculiarities. 395. Of the larger luminous forms which are seen to float in the ocean-waters, a great proportion belong to the class Acalephce. The light emitted by these appears to be due to the peculiar chemical nature of the mucus secreted from their bodies ; for this, when removed from them, retains its pro¬ perties for some time, and may communicate them to water or milk, rendering those fluids luminous for some hours, parti¬ cularly when they are warmed and agitated. It is probably from this source, that the diffused luminosity of the sea is partly derived. The secretion appears to be increased in amount, by anything that irritates or alarms the animals ; and it is from this cause that the dashing of the waves against each other, the side of a ship, or the shore, — or the tread of the foot upon the sand, — or the compression of the animals between the fingers, — occasions a greater emission of light. But some of these causes may act, by bringing a fresh quantity of the phosphorescent secretion into contact with air, which seems necessary to maintain the kind of slow combustion on which the light depends. LUMINOSITY OF MARINE ANIMALS AND INSECTS. 329 396. But the hfoctilucse and Acalephae are by no means the only luminous animals which tenant the deep. Many Zoo¬ phytes appear to have this property in an inferior degree, and also some of the Echinodermata. Of the lowest class of Mollusks, the Tunicata, a very large proportion are luminous, especially those which float freely through the ocean, and which abound in the Mediterranean and tropical seas ; the brilliancy of some of these can scarcely be surpassed. Among some of the Conchifera, also, the phenomenon has been ob¬ served ; as well as in certain marine Annelida. Other marine animals of higher classes are possessed of similar properties ; thus, many Crustacea , especially the minuter species, are known to emit light in brilliant jets ; and the same may be said of a few Fishes : but it is probable that the luminosity attributed to many of the latter is due to the disturbance they make in the surrounding water, which excites its phospho¬ rescence in the manner just explained. In all these, the general phenomena are analogous, — the luminous matter ap¬ pearing to be a secretion from the surface of the animals, and to undergo a sort of slow combustion by combination with oxygen. Wherever it is presented by these animals, it is always most brilliant upon the surfaces concerned in respira¬ tion. The light continues for some days after death ; but ceases at the commencement of putrefaction.1 397. In the class of Insects, there are several species which have considerable luminous power ; and in these the emission of light is for the most part confined to a small part of the surface of the body, from which it issues with great brilliancy. The luminous Insects are most numerous among the Beetle tribe, and are nearly restricted to two families, the Elateridce , and the Lampyridce . The former contains about 30 luminous species, which are known as fire-flies ; these are all natives of the warmer parts of the JSTew World. Their light proceeds from two minute but brilliant points, which are situated one on each side of the front of the thorax ; and from another 1 There are certain cases, however, in which the production of Light, like that of Electricity (§ 423), appears to be a peculiar manifestation of Nervous power. There is strong reason to believe that Nerve-force may be directly metamorphosed (as it were) into these or other forms of physical and vital force, according to the principle of “ Correlation ** now generally admitted as regards the Physical Forces. 330 LUMINOSITY OF INSECTS. beneath the hinder part of the thorax, which is only seen during flight. The light proceeding from these points is sufficiently intense to allow small print to be read in the pro- foundest darkness, if the insect be held in the fingers and be moved along the lines ; and the natives of the coun¬ tries where they are found (particularly in St. Domingo, where they are abundant) use them instead of candles in their houses, and tie them to their feet and heads, when travelling at night, to give light to their path through the forest. In all the luminous species of this family, the two sexes are equally phosphorescent. 398. The family Lampyridoe contains about 200 species known to be luminous ; the greater part of these are natives of America, whilst others are widely diffused through the Old World. In most of these, the light is most strongly dis¬ played by the female, which is usually destitute of wings, so that it might be mistaken for a larva. The species of Fig. 174.— Male and Female Glow- OUr Own Country is known as WORM* the Glow-worm (fig. 174). 399. The light of the Glow-worm issues from the under surface of the last three abdominal rings. The luminous matter, which consists of little granules, is contained in minute sacs covered with a transparent horny lid ; and this exhibits a number of flattened surfaces, so contrived as to diffuse the light in the most advantageous manner. The sacs are mostly composed of a close network of finely- divided air- tubes (§ 321), which ramify through every part of the granular substance ; and it appears that the access of air through these is a necessary condition of the phosphorescence. For if the aperture of the large trachea which supplies the luminous sac be closed, the light ceases ; but if the sac be lifted from its place, without injuring the trachea, the light is not inter¬ rupted. All the luminous insects appear to have the power of extinguishing their light ; and this they probably do when alarmed by approaching danger. The sudden extinction of the light is probably due to the animals power of closing the aperture of the trachea. LUMINOSITY OF INSECTS. 331 400. There are a few other Insects not included in these families, which are reputed to possess luminous powers ; and of these the most remarkable are the Fulgorce , or Lantern- flies (fig. 175) ; of which one species inhabits Guiana, whilst another is a native of China. These are in¬ sects of very remark¬ able form, having an extraordinary proj ection upon the head ; and this is the part said to be luminous. The au¬ thority for the assertion, however, is doubtful ; and many Entomologists who have captured the insect, have denied the phosphorescent power imputed to it. But it is not impossible that the female only may possess it, and that it may only be manifested at one part of the year. One of the common English species of Centipede , which is found in dark, damp places, beneath stones, &c., is slightly luminous ; and the common Earthworm is also said to be so at the breeding season. 401. Of the particular objects of this provision in the Animal economy, little is known, and much has been con¬ jectured. It is not requisite to suppose that its purposes are always the same ; the circumstances of the different tribes which possess it being so different. The usual idea of its use in Insects, — that it enables the sexes of the nocturnal species to seek each other for the perpetuation of the race, — is pro¬ bably the correct one. The light is more brilliant at the season of the exercise of the reproductive functions, than at any other ; and is then exhibited by animals which do not manifest it at any other period. Moreover, it is well known that the male Glow-worm,*— -which ranges the air, whilst the female, being destitute of wings, is confined to the earth, — is attracted by any luminous object; as are also the Eire-flies, which may be most easily captured by carrying a torch or lantern into the open air : so that the poetical language in which this phosphorescence is described as “ the lamp of love — the pharos — the telegraph of the night, which marks by its scintillations, in the silence of the night, the spot 332 PHOSPHORESCENCE OF DECAYING ANIMAL MATTER. appointed for tlie lovers’ rendezvous,” would not seem so incor¬ rect as the ideas of poets on subjects of Natural History too frequently are. — Regarding the uses of the luminosity of the lower marine tribes, it is more difficult to form a definite idea ; since many of them are fixed to one spot during the whole of life, and in many others the sexes do not require to seek each other. 402. It not unfrequently happens, that an evolution of light takes place from the bodies of animals soon after their death, but before their decomposition has advanced far. This has been most frequently observed to proceed from the bodies of Fishes, Mollusks, and other marine tribes ; but it has been seen also to be evolved from the surface of land animals, and even from the Human body. Indeed, some well-authenticated cases have been put on record, in which a considerable amount of light was given off from the faces of living individuals, who were near their end. All animal bodies contain a considerable quantity of phosphorus (§ 166) ; and it is by no means impossible that some peculiar compound of this substance may be formed, during the early stages of decomposition, or even before death, which may, by its slow combustion, give rise to the luminous appearance. It appears that the whole substance of the body of the Fire-flies is phos¬ phorescent ; for, according to an early historian of the West Indies, “many wanton wilde fellowes ” rub their faces with the flesh of a killed fire-fly, “with purpose to meet their neighbours with a flaminge countenance.” Animal Heat 403. One of the conditions necessary for the performance of Yital action, is a certain amount of warmth ; and we have seen that the animals which alone are capable of retaining their activity in the coldest extremes of temperature, are those which have the power of generating heat within themselves, and thus of keeping-up the temperature of their bodies to a high standard. Those which do not possess a power of this kind, are either rendered completely inactive, even by a com¬ paratively moderate cold, or are altogether destroyed by it. Those which ordinarily do possess this power are destroyed even more rapidly by cold, if from any cause the production of heat within their bodies be interrupted ; for they are the TEMPERATURE OF COLD-BLOODED ANIMALS. 333 animals whose vital actions are the most varied and energetic, and in which an interruption to any one of them most speedily brings the rest to a stand. The inquiry into the amount of heat generated by different animals, and into the sources of its production, is one, therefore, of great practical importance. 404. Our knowledge of the heat evolved by the lower In- vertebrated animals is very limited ; but it is probable that in most of them the temperature of the body follows that of the element they inhabit, keeping a little above it for a time, if it be much lowered. Thus, when water containing Animalcules is frozen, they are not at once destroyed; but each lives for a time in a small uncongealed space, where the fluid seems to be kept from freezing by the heat liberated from its body. The temperature of Earthworms, Leeches, Snails, and Slugs, ascertained by introducing a thermometer into the midst of a heap of them, is usually about a degree or two above that of the atmosphere; and they also have the power of resisting for a time the influence of a degree of cold, which would otherwise immediately freeze their bodies. 405. In the cold-blooded Vertebrata, also, the heat of the body is almost entirely dependent upon that of the surround¬ ing element. Thus most Eishes are incapable of maintaining a temperature more than two or three degrees higher than that of the water in which they live ; and the warmth of their bodies consequently rises and falls with that of the sea, river, or lake they may inhabit. There are, however, a few marine Fishes which have the power of maintaining a tem¬ perature 10 or 12 degrees higher than that of the sea; and these are peculiar for the activity of their circulation, and for the deep colour of their blood, which possesses red particles (§ 229) enough to give to the muscles a dark red colour, like that of meat. The Thunny , a fish which abounds in the Mediterranean, where there are extensive fisheries for it, is one of this group. — It is to be remembered that the animals of this class are less liable to suffer from seasonal alternations of temperature, than are those which inhabit the air. In climates subject to the greatest atmospheric changes, the heat of the sea is comparatively uniform throughout the year, and that of deep lakes and rivers is but little altered. Many have the power of migrating from, situations where they might other- 334 TEMPERATURE OF FISHES AND REPTILES. wise have suffered from cold, into deep waters ; and those species which are confined to shallow lakes and ponds, and which are thus liable to he frozen during the winter, are fre¬ quently endowed with sufficient tenacity of life, to enable them to recover after a process which is fatal to animals much lower in the scale. Fishes are occasionally found imbedded in the ice of the Arctic seas ; and some of these have been known to revive when thawed. 406. In Reptiles, the power of maintaining an uniform tem¬ perature is somewhat greater; being especially shown when the external temperature is reduced very low. Thus when the air is between 60° and 70°, the body of a Reptile will be nearly of the same heat ; but when the air is between 4.0° and 50°, it may be several degrees higher. Frogs and other aquatic Keptiles have a remarkable power of sustaining a temperature above that of freezing, when the water around is not only congealed, but is cooled down far below the freezing-point. Thus in ice of 21°, the body of an edible frog has been found to be 37^°; and even in ice of 9°, the animal has maintained a temperature of 33°. In these cases, as in Animalcules, the water in immediate contact with the body remains fluid, so long as the animal can generate heat ; but at last it is congealed, and the body also is completely frozen. But it is certain that Frogs, like Fishes, may be brought to life again, after the fluids of their bodies have been so completely congealed that their limbs become quite brittle ; it is not known, however, whether this may happen with other Reptiles. It would appear that among Reptiles, as among Fishes, some of the more active species have the power of maintaining their bodies at a temperature consider¬ ably higher than that of the atmosphere ; thus in some of the more agile of the Lizard tribes, the high temperature of 86° has been noticed, when the external air was but 71°. 407. The only classes of animals in which a constantly elevated temperature is kept up, are Birds and Mammals. The bodily heat of the former varies from 100° to 112°; the first being that of the Gull, the last that of the Swallow. In general we find that the temperature is the highest in species of rapid and powerful flight ; and least in those which inhabit the earth. Birds that are- much in the water have a special provision for retaining within their bodies the heat which TEMPERATURE OF WARM-BLOODED ANIMALS. 335 would otherwise be too rapidly conducted away ; their bodies being clothed with a thick and soft down, which is rendered impenetrable to fluid by an oily secretion applied with the bill. The temperature of Mammals generally seems to range from 96° to 104°; but that of the Bat , and probably of other hybernating species, appears to be frequently much below the lower of these limits, even when the animals are in their ordinary activity. The mean or average heat of the body of Man is about 100° ; but it has been observed as low as 96Jr° when the temperature of the air was 60°, and as high as 102° when the air was at 82°. As a variation of 5^° may occur when the range of the external temperature of the air is only from 60° to 82°, it is probable that observations made in cold climates will show that the temperature of the body may be still further lowered, when that of the air around is much depressed. But it seems that, in Man, as in other animals, the lower the temperature of the air around, the greater is his power of generating heat within his body, to keep up the necessary standard; and no observations yet made indicate that the temperature of the body ever falls below 95° in health. 408. The young of warm-blooded animals have usually less power of maintaining an independent heat than adults. The embryo, whether in the egg, or within the body of the parent, is dependent upon external sources for the heat necessary to its full development. The contents of the egg, when lying under the body of its parent, are so situated, that the germ- spot (§ 7 56) is brought into the nearest neighbourhood of the source of warmth. It is not usually until some weeks after the hatching of Birds, or the birth of Mammals, that the young animals have the power of maintaining an inde¬ pendent temperature. Thus young Sparrows, taken from the nest a week after they were hatched, were found to have a temperature of from 95° to 97° ; but this fell in one hour to 66J°, the temperature of the atmosphere being at the same time 62^° ; and the rapid cooling was proved not to be due to the want of feathers alone. There are some birds, how¬ ever, which can run about and pick up their food the moment they are hatched : these come into the world in a more advanced condition than the rest, and can maintain their temperature with little or no assistance. We find the same 336 TEMPERATURE OF WARM-BLOODED ANIMALS. to be the case among Mammals. There are some species {such as the Guinea-pig) whose young are able from birth to walk and run, and to take the same food with the mother ; and these have from the first the power of maintaining a steady temperature when the air around is not very cold. But, in general, the young of Mammals are much less advanced at the time of birth, being not unfrequently born blind as well as helpless ; and they require considerable assistance, in keeping up their heat, from the parent or nurse. Thus the temperature of new-born puppies, removed from the mother, will rapidly sink to between 2° and 3° above that of the air. 409. These facts are of extreme practical importance, in regard to the treatment of the Human infant. Though not destitute of sight, at its entrance into the world, like the young of the Cat, Dog, or Babbit, it is equally helpless, and dependent upon its parent not only for support but for warmth. And as the Human body is longer in arriving at its full development than is any other, so is it necessary that this assistance should be longer afforded. This assistance is the more necessary in the case of infants bom prematurely ; and it should be kept up during the years of childhood, gradually diminishing with age. It is too frequently neglected, by those who are well able to afford it, under the erroneous idea of hardening the constitution ; and the want of it, con¬ sequent upon poverty, is one of the most fertile sources of the great mortality among children of the poorer classes. This is easily proved by the proportional number of deaths which take place in different parts of the year, at different ages. During the first month of infant life, the winter mortality is nearly double that of the summer ; though there is very little difference between the two seasons in the mortality of adults. But in old age the difference again manifests itself to the same amount as in infants ; for old persons are almost equally deficient in the power of maintaining heat; they complain that their “ blood is chill,” and suffer greatly from exposure to cold. 410. The class of Insects presents us with some very interesting phenomena. In the larva and pupa states, the temperature of the body is never more than from to 4° above that of the surrounding medium ; but, in many tribes, the temperature of the perfect Insect rises so high, when it is PRODUCTION OF HEAT BY INSECTS. 337 in a state of activity, that it might he at such times called a warm-blooded animal ; though in the states of abstinence, sleep, and inactivity, its temperature falls again nearly to that of the atmosphere. A single Humble-bee, inclosed in a phial of the capacity of 3 cubic inches, had its temperature speedily raised by violent excitement, from that of rest (2° or 3° above that of the atmosphere) to 9° above that of the external air ; and communicated to the air in the phial as much as 4° of heat within five minutes. In another similar experiment, the temperature of the air in the phial was raised nearly 6° in eight minutes. It is among the active Butterflies, and the Hymenopterous insects (Bee and Wasp tribe), which pass nearly the whole of their active condition on the wing, that we find the highest temperature ; and next to them are the more active of the Beetle tribe. Those of the latter which seldom leave the ground, have little power of producing heat. 411. The greatest manifestation of this power is shown among Insects which live in societies ; most of which belong to the order Hymenoptera. It has been seen that the body of a Humble-bee, in a state of activity, has a temperature of about 9° above that of the atmosphere ; but its nest has been found to have an ordinary temperature of from 14° to 16° above the air, and from 17° to 19° above that of the chalk bank in which it was formed. The production of heat is increased to a most extraordinary degree, when the pupae are about to eome-forth from their cells as perfect Bees, requiring a higher temperature for their complete development. This is fur¬ nished by a set of Bees termed Nurse-bees , which are seen crowding upon the cells and clinging to them, for the purpose of communicating to them their warmth; being themselves evidently very much excited, and respiring rapidly, even at the rate of 130 or 140 inspirations per minute. In one instance, the thermometer introduced amongst seven nursing- bees stood at 92|°, whilst the temperature of the external air was but 70°. In Hive-bees, whose societies are large, this process occasions a still more remarkable elevation of tempe¬ rature ; for a thermometer introduced into a hive during May has been seen to rise to 9 6° or 98°, when the range of atmospheric, temperature was between 56° and 58°. In September, when the bees are becoming stationary, the temperature of the hive is but a few degrees above that of the air. It was formerly z 338 SOURCES OF ANIMAL HEAT. supposed that Bees do not become torpid during the winter ; hut this is now known to be a mistake. Bees, like other Insects, pass the winter in a state of hybernation ; but their torpidity is never so profound as to prevent their being aroused by moderate excitement. The temperature of a hive is usually from 5° to 20° above that of the atmosphere ; being kept at or above the freezing-point, when the air is far below it. Under such circumstances, their power of generating heat is most remarkable. In one instance, the temperature of a hive, of which the inmates were aroused by tapping on its outside, was raised to 102°; whilst a thermometer in a similar hive that had not been disturbed, was only 48 |°; and the tempe¬ rature of the air was 34 J°. 412. The evolution of Heat in the Animal body may now be stated with tolerable certainty to depend for the most part on the union, by a process resembling ordinary combustion, of the carbon and hydrogen which it contains, with oxygen taken-in from the air in the process of Bespiration. It has been elsewhere shown that, even in Plants, this union, when it takes place with sufficient rapidity, is accompanied by the disengagement of a considerable amount of heat (Veget. Phys., § 381); and in all those Animals which can maintain an elevated temperature, we find a provision for this union, both in regard to the constant supply of carbon and hydrogen from the body, and to the introduction of oxygen from the air. The supply of carbon and hydrogen may be derived (as already shown, § 157), either directly from the food, a large proportion of which is thus consumed in many animals without ever forming part of the tissues of the body ; or it may be the result of the waste of the tissues, especially of the muscular, consequent upon their active employment (§ 160), and con¬ verted into a substance peculiarly adapted for combustion by the agency of the liver (§ 366). Or, again, it may be derived from the store laid-up in the system in the form of fat ; which seems destined to afford the requisite supply, when other sources fail. Thus, when food is withheld, or when dis¬ ease prevents its reception, the fat in the body rapidly diminishes ; being burnt off, as it were, to keep up the temperature of the system. This is the case, too, during hybernation; the animals which undergo this change usually accumulating a considerable amount of fat in the autumn, and SOURCES OP ANIMAL HEAT. 339 being observed to come forth from their winter quarters, with the return of spring, in a very lean condition. 413. The consumption of oxygen and the production of carbonic acid are found to bear, in every animal, a very close relation to the amount of heat liberated at the time. Thus in warm-blooded animals, the respiratory function is much more active than in the cold-blooded ; but when the former are reduced to the state of cold-blooded animals, as occurs in hybernation (§ 309), their respiration is proportionately low. On the other hand, whenever the temperature of an animal is quickly raised by any extraordinary stimulus, above that which it was previously maintaining, it is always by means of increased activity of the respiratory movements, and augmented consumption of oxygen. Thus during the incubation of Bees (§ 411), the insect, by accelerating its respiration, causes the evolution of heat and the consumption of oxygen to take place at least twenty times as rapidly as when in a state of repose. The same takes place when a hybernating animal is roused ; and it is remarkable that even extreme cold will effect this for a time ; but the animal, if exposed for too long a period to a very low temperature, will not be able to resist its influence, and will perish. 414. Although the combustion of carbon and hydrogen within the Animal body is undoubtedly the chief source of the production of heat, yet it must not be left out of view that there are other chemical changes in the system, which also con¬ tribute to it, though in a minor degree (§ 343). Of this kind are the oxidation of the sulphur and phosphorus which enter the body in the organic compounds used as food, and which, being united by a combustive process with oxygen, pass out of the system in the urine, in the form of sulphuric and phos¬ phoric acids, combined with alkaline bases (§ 367). 415. Besides all these sources, it seems probable from various considerations, that Heat may occasionally be generated, like light and electricity, by the direct agency of the Nervous system; as one of the modes of force into which nervous power may be metamorphosed. Of course, in any such gene¬ ration of heat, there must be the same consumption of nervous tissue, as would occur if its equivalent of nerve-force had been manifested. 340 PRODUCTION OP ELECTRICITY BY ANIMALS. Animal Electricity. 416. Almost all chemical changes are attended with some alteration in the electric state of the bodies concerned ; and when we consider the number and variety of these changes in the living animal body, it is not surprising that disturbances of its electric equilibrium should he continually occurring. But these, when slight, can only be detected by very refined means of observation ; and it is only when they become com siderable, that they attract notice. Some individuals exhibit electric phenomena much more frequently and powerfully than others ; and cases are occasionally recorded in the Human subject, in which there has been a most decided pro¬ duction of electricity, which manifested itself in sparks when¬ ever the individual was insulated. 417. The sparks and crackling noise, however, which are occasionally observed on pulling off articles of dress that have been worn next the skin, especially in dry weather, are partly due to the friction of these materials with the surface and with each other ; the production of electricity being greatly influ¬ enced by their nature. Thus, if a black and a white silk stocking be worn, one over the other, on the same leg, the manifesta¬ tion of electricity when they are drawn off, especially after a dry frosty day, is most decided ; but this would also be the case if they were simply rubbed together, without any con¬ nexion with the body. 418. In most animals with a soft fur, sparks may be pro¬ duced by rubbing it, especially in dry weather ; this is familiar to most persons in the case of the domestic Cat. But the electricity thus produced seems occasionally to accumulate in the animal, as in the Leyden jar, so as to produce a shock. If a cat be taken into the lap, in dry weather, and the left hand be applied to the breast, whilst with the right the back be stroked, at first only a few sparks are obtained from the hair ; but after continuing to stroke for some time, a smart shock is received, which is often felt above the wrists of both the arms. The animal evidently itself experiences the shock, for it runs off with terror, and will seldom submit itself to another experiment. 419. But there are certain animals which are capable of producing and accumulating electricity in large quantities, by ELECTRIC FISHES : - GYMNOTUS. 341 means of organs specially adapted for the purpose; and of discharging it at will, with considerable violence. It is re¬ markable that all these belong to the class of Fishes ; 1 and that they should differ alike in their general conformation, and in their geographical distribution. Thus, the two species of Torpedo, belonging to the Eay tribe, are found on most of the coasts of the Atlantic and Mediterranean ; sometimes so abundantly, as to be a staple article of food. The Gymnotus, or Electric Eel, is confined to the rivers of South America. The Malapterurus (commonly known as the Silurus) which approaches more nearly to the Salmon tribe, occurs in the Eiger, the Senegal, and the Eile; and there are two other less known Eishes, said to possess electric properties, which inhabit the Indian seas. 420. Of all these, the Gymnotus (fig. 176) is the one which possesses the electric power in the most extraordinary degree. It is an eel-like fish, having nothing remarkable in its external appearance ; its usual length is from six to eight feet, but it is said occasionally to attain the length of twenty feet. This fish will attack and paralyse horses, as well as kill small animals ; and the discharges of the larger individuals sometimes prove sufficient to deprive even Men of sense and motion. This power is employed by the fish to defend itself against its enemies ; and even, it is said, to destroy its prey (which consists of other fishes) at some distance ; the pig# m.— gymnotus. shock being conveyed by water, as a lightning-conductor conveys to the earth the effects of the electric discharge of the clouds. The first shocks are usually feeble; but as the animal becomes more irritated, their power increases. After a considerable number of power¬ ful discharges, the energy is exhausted, and is not recovered for some time ; and this circumstance is taken advantage oi in South America, both to obtain the fishes (which afford 1 Certain Insects and Mollusks have been said to possess electrical properties ; but no special electric organ has been discovered, in them. 342 ELECTRIC FISHES : — TORPEDO. excellent food), and to make the rivers they infest passable to travellers. A number of wild horses are collected in the neighbourhood, and are driven into the water ; the Gymnoti attack these, and speedily stun them, or even destroy their lives by repeated shocks ; but their own powers of defence and injury are exhausted in the same degree, and they then become an easy prey to their captors. 421. The shock of the Torpedo (fig. 177) is less powerful ; but it is sufficient to benumb the hand that touches it. From its proximity to European shores, this fish has been made the subject of observation and experiment more completely than the other ; and some curious results have been attained. It seems essential to the proper reception of the shock, that two parts of the body should be touched at the same time ; and that these two should be in different electrical states. The most energetic dis¬ charge is procured from the Torpedo, by touching its back and belly simultaneously ; the electricity of the back being posi¬ tive, and that of the belly negative. When two parts of the same surface, at an equal distance from the electric organ, are touched, no effect is produced ; but if one be further from it than the other, a discharge occurs. It has been found that, however much a Torpedo is irritated through a single point, no discharge takes place ; but the fish makes an effort to bring the border of the other surface in contact with the offending body, through which a shock is then sent. This, indeed, is probably the usual manner in which its dis¬ charge is effected. If the fish be placed between two plates of metal, the edges of which are in contact, no shock is per¬ ceived by the hands placed upon them, since the metal is a better conductor than the human body ; but if the plates be separated, and, while they are still in contact with the opposite sides of the body, the hands be applied to them, the discharge is at once rendered perceptible, and may be passed through a line formed by the moistened hands of two or more persons. In the same manner, also, a visible spark may be produced ; but this is less easily obtained from the Torpedo than from the Gymnotus. — As to the uses of the electrical organs to the STRUCTURE OF ELECTRIC APPARATUS. 343 Fishes which possess them, no definite information can be given. It is doubtful to what extent they are employed in obtaining food ; since it is known that the Gymnotus eats very few of the fishes which it kills by its discharge ; and that Torpedos kept in captivity do not seem disposed to ex¬ ercise their powers on small fishes placed in the water with them. The chief use of their electrical power appears to be, to serve as a means of defence against their enemies. 422. The electric organs of the Torpedo (fig. 178) are of flat- nl tened shape, and occupy the front and sides of the body, forming two large masses, which ex¬ tend backwards and outwards from each side of the head. They are composed of two layers of mem¬ brane, the space be¬ tween which is di¬ vided by vertical Fig. 178. — Electric Apparatus of Torpedo. partitions into hex- ' h £ a j c 7 Fig. 217. of the power-arm of the lever is increased or diminished, that of the weight-arm remaining the same. 613. Now in order that there may he 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, equal to 10, and applied at the end of a lever a b (fig. 217), whose length we shall call 20, it is neces¬ sary that a force p, ap- • plied 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 product 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 product with the distance at which it is applied may be equal to 200. Hence, when a muscle is applied near the fulcrum, while the resist¬ ance is at a distance from it, so that the bone be¬ comes a lever of the “third order,” its force must be proportionably greater. 614. But this arrange¬ ment greatly increases the rapidity of the motion which is the consequence of the muscular action. For let us suppose that the muscle p (fig. 218) acts upon the lever a r , in such a manner that its point of insertion c tra- LEVERAGE OF BONES. 463 verses 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 which 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. 615. We shall find that this is the case in regard to most of the muscular actions in the animal economy. Thus, the fore-arm (fig. 219, b, c) is bent upon the arm a by a muscle d, which arises from the top of the latter, and which is inserted at e, a short distance from the elbow-joint. Hence its con¬ traction 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 operates at a similar dis¬ advantage in regard to power, and advantage in point of time, in consequence of its point of attachment being so near to the fulcrum. In responding to its action, however, the bones of the fore-arm constitute a lever of the “ first order f the elbow- joint, which serves as the fulcrum, being now between the power and the resistance. 464 BONES OF THE SKULL. Motor Apparatus of Man : — Skeleton and Muscles. 616. Before entering npon the examination of the various movements of the lower animals, and of the means by which these are effected, it will be useful to acquire a general know¬ ledge of the structure of the Human Skeleton, and of the uses of its several parts. The skeleton, which is formed by the union of about 200 bones, is divided like the body into head, trunk, and members. The bones of these parts will now be separately described. 617. The Head is composed of two parts, the cranium or skull, and the face. The cranium (fig. 220) 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. Its walls are made-up of eight bones : the frontal / in the region of the forehead; the two parietal bones p , which occupy the top and sides of the skull; the two temporal bones t , which form the walls of the temporal region ; the occipital bone o at the back of the head ; and the sphe¬ noid s, and the ethmoid, which assist in forming the floor of the cavity. 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 neigh¬ bours ; for at the top of the skull the bones are firmly held together by the interlocking of the projections of each, whilst the lower edge of the parietal bone is prevented from being driven outwards by the overlapping edge of the temporal bones, which form, as it were, a buttress to the arch. This same contrivance a f Fig. 220.— Human Skull. /, frontal bone ; p, parietal ; t, tem¬ poral ; o, occipital ; s , sphenoid ; n , nasal ; ms, superior maxillary ; j, malar or cheek bone ; mi, in¬ ferior maxilla ; na, anterior open¬ ing of the nose ; ta, auditory aper¬ ture ; az, zygomatic arch ; a,b,c,d , lines indicating the facial angle. BONES OF THE SKULL. 465 prevents the temporal bone from being driven inwards, as it might have otherwise been, by a blow on the side of the head. 618. 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 occipital bone, gives passage to the Spinal Cord ; and on each side of this aperture there is a large bony projection from the under surface, termed the condyle , by which the skull rests on the vertebral column, and is enabled to move forwards or backwards upon it. The 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 consequently 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 backwards, and thus to keep it upright, are more numerous and powerful than those which are situated in front of the vertebral column, and which tend to draw the head downwards and forwards ; and when the former are relaxed, as in a person sleeping upright, the head has a tendency to fall forwards upon the chest. In no other animal is this joint situated so far forwards 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. 619. On each side of the base of the cranium, we observe a large rounded projection, termed the mastoid. To this pro¬ jection (which we feel behind the lower part of the ear) is, attached on either side a powerful muscle, the sterno-mastoid (23, fig. 227), which passes downwards 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 forwards ; 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 ta 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. H H 4 66 BONES OF THE FACE. 620. The face is formed by the union of fourteen bones ; and presents 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 (§ 617). The two principal are the superior maxillary (m s, fig. 220), which form nearly the whole of the upper jaw, and are connected with the frontal bone in such a manner as to contribute to the forma¬ tion 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 j ; whilst they are united behind with the palate-bones which form the back part of the roof of the mouth, and which in their turn are united to the sphenoid. 621. The orbits, as we have already seen (§ 538), are two deep cavities, of a conical form, — the base of the cone being directed forwards, and its apex or point backwards ; the roof of these cavities 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 (§ 541) ; and the outside wall is formed partly by the cheek-bone and partly by 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 depression, in which the lachrymal gland is lodged. 622. The greater part of the nose is formed by cartilages ; so that, in the bony skull, the anterior opening of the nasal cavity (n a, fig. 220) is very large ; and the bony portion of the nose, formed by the two small bones (n) termed nasal , projects but slightly. The nasal cavity, divided in the middle by a vertical partition into two fossce or excavations, is very extensive (§ 506) ; at the upper part it is hollowed-out into 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 BONES OF THE FACE. 467 as the back of the mouth, and communicates with the pharynx by two apertures termed the posterior nares (fig. 200, c). ddie partition between the fossae is formed at the upper part by a plate that projects downwards from the eth¬ moid bone, and at the lower by a distinct 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 forwards into the soft projecting portion of the nose. It is through the thin horizontal plate of the ethmoid bone, which separates the nasal cavity from that of the skull, that the olfactory 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 apertures, which give it a sieve-like aspect ; whence it is called the cribriform 1 plate of the ethmoid. 623. It is in the superior maxillary bone that all the teeth of the upper jaw are implanted in Man ; but in the embryo this bone is composed of several pieces ; and one of these pieces, termed the intermaxillary bone (im, fig. 221), remains t f o n m Fig. 221. — Skull op Horse. cc, occipital bone ; t, temporal ; /, frontal ; n, nasal ; m, superior maxillary ; im, intermaxillary; mi, inferior maxillary ; o, orbit ; i, incisor teeth; c, canines; mo, molars. permanently separate in most of 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 1 From the Latin, cribrum, a sieve. H H 2 468 BONES AND MUSCLES OF THE FACE. articulated with, the temporal bones by a condyle or projecting head with which each of these extremities is furnished ; and this head is received into what is called the glenoid 1 cavity on the under side of the temporal bone. In front of the condyle is another projection, or process , termed the coronoid (a, fig. 92), 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 upwards), and they consequently act at a small distance from its fulcrum, whilst the resistance is applied at the furthest point (§ 180). We are continually reminded of the loss of mechanical power which results .from this, by our in¬ ability 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 place it nearer the joint, where it may receive more of the power of the muscle. 624. The general arrangement of the chief muscles of the face is seen in fig. 222. The largest is the temporal muscle, t, z t 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, z (which is partly formed by a process from the temporal bone, and partly by the malar or cheek bone), to be attached to the a coronoid process of the lower jaw. This muscle is of extraordinary power in those beasts of prey which lift and drag heavy carcases in their jaws ; and in those which (like the Hyosna) obtain their support by crushing the bones that others have left. It is assisted by the masseter muscle m, 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 o, which surrounds the opening of the eye, and serves by its contraction to close the lids ; and also the similar muscle b b , which surrounds the 1 The term condyle is applied to most of the projecting surfaces of articulation, in different parts of the body ; and the term glenoid to the cavities into which these are received. Frontal bone Parietal bone Temporal bone Clavicle Pelvis 470 STRUCTURE OF THE VERTEBRAL COLUMN. mouth and draws together the lips. The antagonists to these are several small muscles which form the fleshy part of the face, and produce 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. 625. 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 (§ 516); and also the hyoid bone, which lies at the root of the tongue and at the top of the larynx (fig. 107). This last bone, in Man and the Mammalia generally, is connected with other parts of the skeleton by ligaments and muscles only ; 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. 626. 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 Verte- brated animals, — is the spinal or vertebral column. The general conformation of this has been already described (§ 71). In Man it consists of 33 vertebrae (fig. 224), which are arranged under five divisions ; — I. The Cervical vertebrae c} or vertebrae of the neck, of which there are 7 ; ii. The dorsal vertebrae d, or vertebrae of the back, of which there are 12 ; — hi. The lumbar vertebrae l , or vertebrae of the loins, of which there are 5 ; — iv. The sacral ver¬ tebrae 5, of which also there are 5 ; — and v. The coccygeal vertebrae co, of which there are 4. All these vertebrae are separate at the time of birth ; but the 5 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. 627. The dorsal vertebrae are distinguished from the cervi¬ cal and lumbar, as being those to which the ribs are attached. It is remarkable that the number of the cervical vertebrae Fig. 224. Vertebral Column. STRUCTURE OF THE VERTEBRAL COLUMN. 47 1 should be the same in all the Mammalia ; even the long¬ necked Giraffe having only seven, while the Whale, whose head seems to be joined to its body without the intervention of any neck, also has 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 which the number of cervical vertebrae is much greater. The following table shows the number of vertebrae in animals of different groups. Cervical. Dorsal. Lumbar. Sacral. Coccygeal. Total. Mammalia. 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 67 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 _ 10 7 37 Turkey . 14 7 — 15 6 42 Ostrich . 18 9 _ 19 9 55 Crane . 17 10 — 15 6 48 Swan . . . 23 11 — 16 8 58 Reptiles. Tortoise . . 9 10 _ 3 20 42 Monitor (Lizard) . . 6 21 2 2 115 146 Python (Boa) . . _ 320 _ _ 102 422 Rattle-Snake . _ 171 _ _ 36 207 Land Salamander . 1 14 _ 1 26 42 Axolotl . 2 18 — 42 62 Fishes. Perch . _ 21 — _ 21 42 Mackerel . . . . — 15 — — 16 31 Trichiurus . — 60 — — 100 160 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 multiplica¬ tion 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 of these vertebrae is as low as 4. 472 STRUCTURE AND CONNEXIONS OF VERTEBRiE. 628. It has been already noticed (§71) that an ordinary character of the vertebras consists in their being perforated by an aperture (fig. 225), which, when several vertebrae are united together, forms a continuous tube or canal for the lodgment of the spinal cord. This cha¬ racter is usually lost, however, in the coccy¬ geal 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 hones, is obviously to allow of considerable freedom of motion by a slight shifting amongst the individual parts ; whilst any such sudden bend as would be injurious to the spinal cord, is avoided. Each vertebra con¬ sists of a solid “ body ” a , 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 projections, b and c, 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 neighbouring vertebrae by a disk of fibro-cartilage (§ 47), which extends through the whole space that intervenes be¬ tween them, and which, being firmly adherent to both, prevents them from being far separated from each other. 629. But in Bep tiles 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 flexibi¬ lity which is essential to the peculiar movements 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 dimi¬ nished. It is to be remembered, however, that strength is not required in the bony framework of animals, whose bodies, MUTUAL CONNEXIONS OP VERTEBRAS. 473 instead of being supported upon foui 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 (fig. 243), being but little used except for influencing the direction of their motion. And thus we see that in the lowest Yertebrata, as in the lower Articulata (such as the Leech and Earth-worm), the propul¬ sion 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 condition which it has in all at an early period : whilst in the higher classes of both series, — Birds and Insects for example, — the extremities being so developed, and being furnished with muscles so powerful that the function of locomotion is entirely committed to them, the skeleton of the body undergoes great consolidation, its various pieces being so knit together as to make the trunk almost immovable. 630. 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 (6, fig. 225), projects directly backwards ; 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, they are generally much longer, in order to give firm attachment to the muscles and ligaments which support the head (fig. 229, vc, vd). And in Fishes they are greatly prolonged (fig. 243), so as to in¬ crease the surface 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 ( c , fig. 225) 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 vertebra, 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 474 UNION OF VERTEBRA INTO SPINAL COLUMN. completely check the movements of the dorsal vertebras ; thereby giving to the trunk that firmness which is required for the attachment of the muscles of the wings. The portions of bone which pass backwards from the body of each vertebra to its transverse processes, and thus 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 vertebrae 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 (§ 457). 631. The vertebral column of Man is disposed in a double curve, as seen in fig. 224 ; 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 upwards 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 elasti¬ city 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. 632. 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 “ con¬ dyles 99 of the occipital bone of the skull (§ 618), in such a manner that the head is free to nod backwards and forwards. The atlas itself turns upon a sort of pivot, formed by an upward projection from the next vertebra, which is termed the axis; this projection, called from its form the processus ARTICULATION OF HEAD WITH SPINAL COLUMN. — RIBS. 47 5 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 permitted, displacement or dislocation is necessarily more easy ; and accordingly we find that the atlas and axis can be more easily separated from each other, than can any other two vertebrae. This dislocation may be produced by violence of different kinds ; thus if the head be suddenly forced forwards 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 liable to be dragged 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 for¬ merly stated (§ 470), — that it causes the suspension of the motions of respiration. 633. 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.1 The number in different animals may be judged-of by that of the dorsal ver¬ tebrae in the table already given (§ 627) j since it is the attach¬ ment of the ribs that makes the essential difference between the dorsal vertebrae 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 (fig. 163). The cartilages 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 1 It is scarcely necessary here to state, that the common notion respecting the deficiency of a rib on one side of the body of Man is a popular error. 476 RIBS AND STERNUM. - BONES OF SHOULDER. connected with, the cartilage of the seventh rib ; while the other two ribs, being completely unattached at the 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 (fig. 250). 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 contained parts (§ 83). 634. We have next to consider the structure of the members or appendages which are attached to this central framework. These are spoken-of as superior and inferior , when we are treating of Man, whose erect posture places one pair above the other : but when the ordinary Quadrupeds are alluded to, they are termed anterior and posterior , one pair being in front of the other. 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 framework, which is connected more or less closely with the spinal column. This framework, 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 bason at the bottom of the spinal column (fig. 223). 635. The Scapula is a large flat bone, which occupies the upper and external1 part of the back. Its form is somewhat triangular ; and at its upper and outer 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 (§ 668). The hinder surface of the scapula is divided into two by a projecting ridge or keel, which gives a more extensive and 1 The term external is continually used in Anatomy, to describe the parts furthest removed from the central or median line of the body. MUSCLES OF THE BACK OF THE TRUNK. 477 Fig. 226 — Muscles of the Back of toe Trunk, the Superficial Layer being SHOWN ON THE LEFT SIDE, THE MIDDLE LAYER ON THE RIGHT. 1. Trapezius; 2, occipital bone; 3, spine of the scapula; 4, acromion process; 5, deltoid; 6, infraspinatus; 7, teres minor; 8, teres major; 9, latissimus dorsi; 10, its aponeurosis; 11, glutaeus magnus ; 12, space between the latissimus dorsi and external oblique; 13, rhomboideus major; 14, splenius ; 15, angularis scapulae; 16, rhomboideus minor; 17, external oblique; 18, supraspinatus ; 19, serratus magnus ; 22, inferior serratus minor ; 23, internal ob ique ; 24, crest of the iliac bone; 25, glutaeus medius ; 26, pyramidalis ; 27, gemellus superior; 28, obturator internus ; 29, gemellus inferior; 30, quadratus femoris ; 31, semi-membranosus ; 32, tuberosity of the ischium ; 33, insertion of the glutaeus maximus ; 38, long head of the triceps extensor of the arm. 478 BONES OF SHOULDER : SCAPULA AND CLAVICLE. firmer attachment to the muscles that arise from it (fig. 226, 3). 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 upwards, 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 narrower (fig. 229, o). This is particularly the case in Birds (fig. 250, o), 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. 636. The Clavicle is a rounded bone, attached at one ex¬ tremity 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 ; whilst it is comparatively weak or altogether deficient in animals, the actions of whose limbs naturally tend to keep them asunder. 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 (§ 668). In the Horse and other animals, on the contrary, the bearing of whose weight on their fore¬ legs tends rather to separate the shoulders than to bring them together, the clavicle is deficient. 637. The Scapula is connected with the central framework of the skeleton by various muscles (fig. 226), which pass towards it from the spinal column and ribs, and which serve alike to fix it, and to assist in sustaining 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, 9) of mode¬ rate thickness in Man, but in these animals of great strength, which passes from the scapula to be attached to the ribs. 638. The superior or anterior member itself is divided into three principal portions, — the arm, fore-arm, and hand. The arm is supported by a single long and cylindrical bone, which MUSCLES OF THE FRONT OF THE TRUNK. 479 is called the humerus ; this has a large rounded head, which is received into the glenoid cavity of the scapula ; whilst its lower end is rather flattened, so as to articulate with the two bones of the fore-arm in the hinge-joint of the elbow. The Fig. 227. — Muscles of the Front op the Trunk, the Superficial Layer BEING SHOWN ON THE LEFT SIDE OF THE FIGURE, THE MIDDLE LAYER ON THE RIGHT. 1, sternum; 2, sternal portion of the pectoralis major; 3, clavicular portion of the same muscle ; 4, space between the deltoid and pectoral muscles ; 5, deltoid ; 6, 6, clavicles; 7, external oblique; 8, digitations of the serratus magnus ; 9, 9, latissi- mus dorsi ; 10, aponeurosis of the external oblique ; 11, linea alba ; 12, umbilicus ; 13, pectoralis minor; 14, 14, crests of the iliac bone; 15, symphysis of the pubis ; 16, crural arches; 17, rectus abdominis; 18, inferior oblique; 20, internal inter¬ costal; 21, external intercostal; 22, infra-clavicular ; 23, coracoid process; 24, in¬ ferior border of the internal oblique and transversalis muscles; 25, short head of the biceps ; 26, long head of the biceps ; 27, biceps ; 28, sternomastoid ; 32, adductor of the thigh ; 33, rectus femoris. 480 BONES AND MUSCLES OF UPPER EXTREMITY. muscles which move it are for the most part attached to its upper third ; and the chief of them are the pectoralis major (fig. 227, 2, 3) which rises from the sternum and cartilages of the ribs, and consequently draws the arm forwards, inwards, and down¬ wards, — the latissimus dor si (fig. 226, 9), which rises from the spinal column and hinder part of the ribs, and consequently draws the arm backwards, inwards, and downwards, — and the deltoid (fig. 226, 5), which arises from the upper edge of the clavicle, and from the ridge of the scapula, and is the chief muscle concerned in raising the arm. The first of these forms the principal part of the fleshy mass upon the front of the chest, and is the muscle which is so remarkably developed in Birds. It forms also the front boundary of the axillary space, or hollow of the arm-pit, the hinder boundary of which is formed by the second muscle. This space, of which we can distinctly feel the front and back walls when we raise the arm a little from the side, contains the large vessels and nerves proceeding to the arm, and also a number of lymphatic glands (§ 219). The deltoid muscle forms the thick fleshy mass on the top of the shoulder and on the upper part of the outside of the arm. 639. In the fore-arm of Man there are two long bones, termed the Radius and the Ulna, which lie nearly parallel to each other ; the radius being on the outer or thumb side of the fore-arm, and the ulna on the inner. They are connected with one another, not only by ligaments at their extremities, but by a strong fibrous membrane that passes between their adjacent edges, along their entire length. Nevertheless they have considerable freedom of motion, not only upon the humerus, but upon each other; so as to give to the fore -arm the power of rotation on its own axis, by which either the palm or the back of the hand may be turned upwards. The ulna is connected with the humerus, at the elbow, by means of a hinge-joint, into which the radius does not enter ; but it is the radius with which the hand is connected at the wrist, by a kind of ball-and-socket joint, the ulna having no direct share in this articulation : hence, while both bones move together in bending or straightening the elbow, we can make the radius roll round the ulna, carrying the hand with it. This movement is one of very great importance, in rendering the hand capable of a great variety of uses to which it would BONES AND MUSCLES OF ARM AND HAND. 481 be otherwise inapplicable. It is only among the higher orders of Quadrupeds, however, that it can possibly be exe¬ cuted ; for in the lower, the two bones are united more or less completely into one, or are articulated in such a manner as to be incapable of rotation. 640. The fore-arm is bent upon the arm, chiefly by muscles that lie upon the front of the latter ; of these, the principal is the biceps or two-headed muscle (7, fig. 227), which arises from the coracoid process of the scapula, and from the top of the glenoid cavity, and is inserted into the radius a little in front of the elbow, forming a great part of the fleshy mass in front of the arm (fig. 219). The arm is straightened again by a large muscle, the triceps or three-headed muscle, which arises from the back of the humerus and scapula, and passes down to be inserted into a projection of the ulna behind the elbow-joint, forming the fleshy mass, at the back of the arm. The muscles which rotate the fore-arm arise from the lower end of the humerus, or from one of its own bones, and pass obliquely across to the other. 641. The Hand is anatomically divided into three portions, ■ — the carpus, metacarpus and phalanges (fig. 223). The carpus, which is the portion nearest the wrist-joint, is com¬ posed of eight small short bones, which are firmly united to each other by ligaments, but yet have a certain degree of motion permitted them ; these are arranged in two rows, of which one has a rounded, surface, and enters into the forma¬ tion of the wrist-joint ; whilst the other has a series of shal¬ low pits, to receive the rounded heads of the metacarpal bones. These last almost precisely resemble the bones of the fingers, and in the skeleton might be mistaken for their first joints ; but with the exception of that of the thumb they are all united to each other by ligaments and muscles, so as to form the compact framework which gives support to the palm of the hand. The metacarpal bone of the thumb is much more free in its movements ; and it is chiefly by an alteration in its direction, that the thumb can be opposed to the fingers. The thumb and fingers are formed by a series of small bones which are termed the phalanges; of these there are only two in the thumb, whilst there are three in the fingers. They are bent on each other chiefly by the action of the muscles that occupy the front of the fore-arm ; and they are extended or 482 MUSCLES OF THE HAND. straightened by others that lie along its back. These termi¬ nate in long tendons, which are bound down at the wrist by a fibrous band that stretches between the bony projections on either side, and is termed the annular (or ring-like) ligament (fig. 228). The tendons then spread asunder in the hand, and pass-on to be inserted into the bones of the several fingers, being reinforced by a set of small muscles that arise from the hand itself. 642. When we consider the superior extremity of Man as a whole, we remark that the several levers which are joined end-to-end to form it, diminish progressively in length. Thus the arm is longer than the fore-arm ; the latter is longer than the wrist ; and each of the pha¬ langes is longer than the one which succeeds it. The purpose of this arrangement is very evident. The nume¬ rous joints, in the neighbour¬ hood of each other, which we see towards the extremity of the limb, permit its several portions to change their place in various ways, so as to ac¬ commodate themselves to the form of the body which it is desired to grasp ; whilst the long levers formed by the arm and fore-arm, allow the place of the entire hand to be rapidly changed to a considerable extent. It is principally by the movements of the humerus upon the scapula, that the direction of the limb is given ; the bending or straight¬ ening of the limb regulates its length; whilst the move- Fig. 228. — Muscles of the Palm of the Hand (Superficial Layer) 1, anterior annular ligament of the carpus; 2, 2, extremities of the short abductor of the thumb, the intermediate body of the muscle having been removed ; 3, opposing muscle of the thumb; 4, short flexor of the thumb; 5, adductor of the thumb; 6, lower border of the same muscle ; 7,7, lumbricales; 8, one of the tendons of the deep flexor of the fingers, passing-on to insert itself in the bone of the third pha¬ lanx, after perforating the tendon of the superficial flexor; 9, tendon of the long flexor of the thumb; 10, adductor of the little finger; 11, short flexor of the little finger; 12, pisiform bone; 13, first dorsal interosseous muscle. PECULIAR ENDOWMENTS OF HUMAN HAND. 483 ments of the thumb and fingers are concerned in its particular applications. 643. The hand of Man is distinguished from the extremity of most Quadrupeds by its possession of an opposable thumb , — that is, of a finger which can be made to act in a direction opposite to that of the rest. But among the Apes and Monkeys, we find this peculiarity not only in the superior extremity, but also in the inferior ; whence these animals are said to be quadrumanous or four-handed, whilst Man is bimanous, possessing two hands only. It must not be sup¬ posed, however, that Apes and Monkeys are superior in this respect to Man ; for they possess this distinguishing character in a much less striking degree than he does. All the four extremities of Apes and Monkeys possess the power of grasp¬ ing, but they are all used also for support ; and we find that in consequence of the shortness of the thumb and great toe, the grasping power is very inferior to that which Man possesses. But of the four extremities of Man, one pair is specially adapted for support, and the other for prehension or grasping ; and this by the length and mobility of the thumb, which is capable of being brought into exact opposition to the extremities of all the fingers, whether singly or in combination. But even in those Quadrumana which most nearly approach Man, the thumb is so short and weak, and the fingers so long and slender, that their tips can scarcely be opposed to each other, and then with only a slight degree of force ; hence, although completely adapted for clinging round bodies of a certain size, — such as the small branches of trees, &c. — the extremities of the Quadrumana can neither seize very minute objects with that precision, nor support large ones with that firmness, which is essential to the dexterous performance of a variety of actions for which the hand of Man is admirably suited. Hence they may be more appropriately termed claspers than hands. 644. In many of the inferior Mammalia, whose extremities are adapted for support only, we find each row of phalanges consolidated into two bones, or even into one. This is the case, for example, in the Buminant Quadrupeds, as the Camel (fig. 229), and in the Horse (§ 652). Such an arrangement obviously increases the firmness of the limb, though it altogether deprives it of prehensile power. In other im i i 2 484 EXTREMITIES OF LOWER ANIMALS. stances, we find tlie number of bones in tbe hand increased, but all of them enclosed in one envelope, so that the fingers are not separate. This is the case with many aquatic animals 'od* Fig. 229. — Skeleton of the Camel. vc, cervical vertebrae ; vd, dorsal vertebrae ; vl, lumbar vertebrae ; vs. sacral vertebrae ; vq , caudal vertebrae; s, scapula; h , humerus; cu, ulna; ca, carpus; me, meta¬ carpus ; ph, phalanges ; fe, femur ; ro, patella ; ti, tibia ; ta, tarsus ; mt, metatarsus. - — such as the Whale tribe among Mammals, Turtles among Reptiles, and Fishes in general, — in which the hand is made to serve as a fin or paddle. In most of these, the bones of the arm are very short ; and the movements of the extremity are chiefly confined to the wrist-joint. 645. The structure of the lower extremities has a very great analogy to that of the upper ; and the principal differences to be remarked between them, are such as are necessary to give to the former more solidity at the expense of freedom of motion, and to make them organs of locomotion instead of organs of prehension. Here, too, we have a bony framework, for the purpose of connecting the limb itself with the spine ; and as the weight of the body is constantly thrown upon the BONES AND MUSCLES OF LOWER EXTREMITY. 485 lower extremities, this framework is much more firmly at¬ tached to that of the trunk, than is the case with that which supports the arms. It consists, on each side, of a hone which in the adult state is single, though at an early age it is com¬ posed of three distinct pieces ; and this is closely connected with the sacrum behind, while it meets with its fellow in front in such a manner as to form a sort of bason termed the Pelvis. The spreading sides of this, formed by the iliac hones (Fig. 213), afford support above to the viscera contained in the abdomen ; and they give attachment by both surfaces to large muscles by which the thigh-bone is moved, and by their edges to large expanded muscles that pass upwards to the ribs and sternum, and form the walls of the abdomen. Below this spreading portion, we find the articular cavity of the thigh¬ bone, which is so deep as almost to form a hemispheric cup when it is completed by its cartilaginous border. The move¬ ments of the thigh-hone are consequently more limited than those of the arm ; hut it is much less liable to displacement. 646. The thigh, like the arm, contains hut a single bone, which is named the Femur. Its upper extremity is bent at an angle ; and its rounded head is separated from the rest by a narrow portion which is termed its neck. At the point where this neck joins the shaft of the hone, there are two large projections termed trochanters , one on the outer side and the other on the inner ; these serve to give attachment to the muscles by which the thigh is moved. Of these muscles, one descends from the lumbar vertebrae, and passes-down with another that rises from the upper expanded surface of the pelvis, over the front border of the pelvis, to he attached to the smaller and interior of the projections just mentioned ; these with the assistance of other muscles raise or draw forwards the thigh, — an action which does not require in Man to he performed with any great force. The muscles which draw hack the thigh, on the other hand, arise from the under surface and hack of the pelvis, where they form a very thick fleshy mass (n, 25 , fig. 226) ; and they pass to the larger and external projection, and to a ridge which runs from it down the thigh-hone. Other muscles which arise from the lower border of the pelvis, serve to rotate the thigh upon its axis. The lower end of the thigh-bone spreads into two large condyles, , on which the principal hone of the leg moves backwards and 486 BONES AND MUSCLES OF THE LOWER EXTREMITY. forwards. The knee is a good example of a pure hinge-joint, all its movements being restricted to one plane. 647. The leg, although containing two bones like the fore¬ arm, does not in Man possess the peculiar movement which characterises it. One of these hones, called the Tibia , is much larger than the other which is called the Fibula ; and it is the former alone on which the thigh-hone rests, and which itself rests upon the foot, so that no movement of rota¬ tion is permitted in the leg. In fact, the fibula, which is a long slender hone running nearly parallel with the tibia (fig. 223), looks like a mere appendage or rudiment, and serves only for the attachment of muscles. The upper end of the tibia is broad, and has two shallow excavations, in which the condyles of the femur are received. Upon the front of the knee-joint we find a small separate bone, the patella or knee-pan ; the purpose of this is to change the direction of the tendons that come down from the front of the thigh to be attached to the tibia ; in such a manner as to enable them to act more advantageously, upon the principle formerly stated (§. 611). In the elbow-joint, this change was not required ; since the ulna projects sufficiently far backwards to afford ad¬ vantageous attachment to the tendon of the extensor muscle. — The very powerful muscles which tend to straighten the knee-joint, arise from the front of the pelvis and from the femur itself; and they form the fleshy mass of the front of the thigh. On the other hand, those which bend the knee arise from the lower border of the pelvis and from the back of the thigh-bone, and pass downwards to be inserted into the sides of the tibia and fibula a little below the knee, their tendons forming the two strong cords known as the hamstrings. The articulating surface at the lower extremity of the leg, which enters into the ankle-joint, is principally formed by the tibia ; but its outer border is formed by the fibula, which there makes a considerable projection that can be felt through the skin. — In the Quadrumana, and in a less degree in some other Mammals, the two bones of the leg resemble those of the fore-arm ; and are so articulated as to give to the foot a power of rotation corresponding with that of the hand. 648. The Foot is composed, like the hand, of three distinct portions, which are called the tarsus , metatarsus , and phalanges. There are seven bones in the tarsus, all of which are larger BONES AND MUSCLES OF THE FOOT. 487 than those of the carpus, and some of them of considerable size. The articulation with the leg is formed by one of these only, the astragalus, , which projects above the rest, and is im¬ bedded between the projecting extremity of the tibia (which forms the inner boundary of the ankle-joint) and that of the fibula. The astragalus rests on the os colds or bone of the heel, which projects considera¬ bly backwards, and is connected in front with the other bones of the tarsus. In front of the tarsus we find the metatarsus, composed of five long bones, which in man are all attached to each other, but of which one is separate in the Quadrumana, in order to give freer play to the great toe, the action of which resembles that of the thumb. The toes, like the fingers, are composed of three phalanges (with the exception of the great toe, which has only two) ; these are in Man much shorter than those of the hand, and are evidently not adapted for prehension ; but in many of the Quadrumana, their length is nearly equal to that of the fingers, and the great toe is as opposable as the thumb. The foot is far from being thus con¬ verted, however, into a perfect hand; but it becomes a very useful instrument for clasping the small branches and twigs of the trees among which these animals live. The foot of Man is distinguished from theirs, by its power of being planted flat upon the ground, and thus affording a firm basis of support. Even the Chimpanzee and the Orang, when they attempt to walk erect, rest upon Fig. 230, — Muscles of the Sole of the Foot (Middle Layer). 1, accessory of the long flexor of the toes ; 2, tendon of the long flexor issuing from its sheath ; 3, tendon of the long flexor of the great toe; 4, first lumbricalis ; 5, tendon of the superficial flexor, divided be¬ hind its perforation ; 6, short flexor of the little toe ; 7, short flexor of the great toe ; 8, portion of the oblique abductor of the great toe ; 9, poste¬ rior extremity of the fifth metatar¬ sal bone ; 1 0, sheath of the long pero¬ neal; 11, os calcis, or bone of the heel. 488 BONES AND MUSCLES OF THE FOOT. the side of the foot ; and the absence of a projecting heel causes them to he very deficient in the power of keeping the leg upright upon it. For it is to this projection that the strong muscles of the calf of the leg are fixed, by which the heel is drawn upwards or the leg drawn back upon it. Other muscles at the side and back of the leg, the direction of whose tendons is changed by a sort of pulley at the ankle-joint, aided by the muscles of the foot itself, serve to bend the toes, — an action which gives great assistance in walking, running, leaping, &c. And the toes are -straightened by an extensor muscle, which lies on the front of the leg, and of which the tendon runs under an annular ligament that encircles the ankle, and is then divided and spread - out to the toes, over the upper surface of the foot. The great toe is a very important instrument in the act of walking, since much of the spring forwards is given by the -bending of its phalanges ; and it is provided with two flexor muscles of its own. 649. On the internal side of the foot, the bones of the tarsus and metatarsus form a kind of vault or arch, which serves to lodge and protect the : vessels and nerves that descend from the leg towards the toes. This arch further serves the important purpose of deadening the shock that would otherwise be experienced every time that the foot is put to the ground ; for, by the elasticity of the ligaments which hold together the bones that compose it, a sort of spring is formed, which yields for a moment to the shock, and then recovers itself. We feel the difference which this makes, when we jump from a height upon our heels ; the jar is then propagated directly upwards from the heel to the leg, thence to the thigh, and thence to the spinal column , and if it were not from the peculiar manner in which this is con¬ structed (§ 631), a severe shock of this kind might produce fatal effects by concussion (or shaking) of the brain. In animals which walk upon four extremities, the difference of direction in which the legs are connected with the spine prevents a jar from being propagated along the latter to a similar degree. But in those which are destined to obtain their food by sudden and extensive leaps, such as the animals of the Cat tribe (the Lion, Tiger, &c.), we find an arrange¬ ment of the bones of the foot, well adapted to diminish the STANDING POSTURE EQUILIBRIUM. 489 shock produced by the sudden descent of the body upon the ground. Of the Attitudes of the body , and the various kinds of Locomotion. 650. A small number of Yerteb rated animals, — Serpents, for instance, — bear habitually on the whole length of their bodies, which rest entirely on the ground ; and their only movements are effected by undulations of the spinal column. But the rest are supported upon their extremities ; and we give the name of standing to that position in which the animal rests supported by its limbs upon the ground or on any firm horizontal basis. In maintaining this position, the extensor muscles, by which the joints are straightened, must be in continual action, since the limbs would otherwise bend beneath the weight of the body. Now as the sense of fatigue, in any set of muscles, depends in great degree upon the length of time during which they have been in continuous action, the maintenance of the standing posture for a long period is, in most animals, more fatiguing than walking ; since in the latter exercise the action of the flexors alternates with that of the extensors. 651. But this condition is not the only one essential to steadiness in the standing posture ; for in order that the body may rest firmly upon the members, it must be in equi¬ librium. It has been shown (Mechan. Philos. Chap, iv.) that equilibrium exists, — or in other words, that a body remains at rest in its position, — not only when it bears upon the whole of a broad sur¬ face, but also when it is so placed that the tenden¬ cies of its different parts to descend or gravitate towards the earth counter¬ balance each other. This is the case when its centre of gravity is supported, — that is, when a line drawn perpendicularly from that centre falls within the base. In order, then, that an animal may rest in equilibrium on its legs, it is necessary that the vertical line from its centre of gravity (or line of direction) should fall within the space 490 EQUILIBRIUM OF ANIMALS : - BASE OF SUPPORT. which its feet cover and inclose between them; and the wider this space, in proportion to the height of the centre of gravity, the more stable will the equilibrium be, since the body may be more displaced without being upset. Thus in fig. 231 the table a must be upset ; because the line of direc¬ tion e from the centre of gravity c falls outside the base of support d ; whilst the table h, although equally inclined, will not be upset but will return to its proper place, because the line of direction e from its centre of gravity c falls within its base d . Hence an animal which is supported upon four legs will stand much more firmly than one which rests on two only ; since its real base is the whole space included between its four points of support. And again, an animal is more firm when standing upon two legs, than when resting upon one only. 652. Moreover when an animal rests upon four legs, the extent of its base is but little influenced by the size of the feet ; and thus to render them broad would be to increase their weight without adding much to their use as supports. This is easily understood by comparing a quadruped to a four-legged table ; if the legs are sufficiently strong to support the weight that rests upon them, it matters little in regard to the steadiness of the table, whether they bear upon the ground by mere points or by flat surfaces ; since it is the large sur¬ face that would be enclosed by lines joining them, which consti¬ tutes the real base. Hence we find that, in most quadrupeds, the limbs only touch the ground Fig. 233. Foot of Horse. Fig. 232. Foot of Dees. by slightly-dilated extremities ; and the number of fingers is reduced more and more, without diminishing their effect as instruments of locomotion. Thus in Ruminant animals, as the Deer, the number of toes is reduced to two in each foot, as seen in fig. 232, where t represents the tibia, ta the bones of EQUILIBRIUM OF ANIMALS : — BASE OF SUPPORT. 491 the tarsus, c the hone of the metatarsus termed the canon (in which the trace of a division into two pieces can he seen), and p, pi , pt, the three phalanges of the toes, of which the last is enveloped in the hoof, which is nothing else than a large nail inclosing the whole extremity of the toe. In the Horse this consolidation is carried still further than in the Ruminants, for it has only one toe in each foot (fig. 233) ; hut we see the rudiment of an additional hone in the metatarsus 6, which is commonly termed the splint hone. 653. But when an animal is supported upon two feet only, whatever may he their degree of separation from each other, the hase of support cannot have sufficient extent, unless the extremities touch the ground hy a considerable surface. This is the case with the foot of Man, and still more with that of many Birds which habitually stand upon one leg (fig. 234). In order that an animal may hold itself in equilibrium upon a single limb, it is necessary that the foot should he placed vertically beneath the centre of gravity of the body ; and that its muscles should he so arranged as to permit it to keep this limb inflexible and immov¬ able. Man can accomplish this, for the centre of gravity of his body is at about the middle of the pelvis ; and to place this vertically over one foot, it is sufficient for him to bend himself a little from the side which is not supported. But the greater number of Qua¬ drupeds are destitute of the power of doing this ; and a large part of them cannot even raise them¬ selves on their hind legs, on account of the direction of these members relatively to the trunk ; or if they can do so for an instant, they cannot maintain themselves in this position. The reason of this is very simple. The base of support, on account of the small¬ ness of the feet, is very narrow, and the centre of gravity of Fig. 234. 492 MUSCULAR EXERTION TO MAINTAIN EQUILIBRIUM. the body is placed near the front ; hence the body must be entirely changed in its position by a violent and not sustain¬ able action of the muscles which connect it with the hind legs ; and, when thus reared up, it cannot rest with firmness on account of the narrowness of the base. 654. There are some Quadrupeds, however, which are able to raise themselves occasionally into this position ; this is the case, not only with the Quadrumana, but also with the Bear, Squirrel, and other animals whose habits require them to ascend and live among trees, — as well as in the Kangaroo, and animals constructed upon the same plan, whose peculiar organisation will be presently considered (§661). In standing upright, the muscles of the back part of the neck are kept in a contracted state, to retain the head in equilibrium on the vertebral column ; and the extensor muscles of that column must also be kept in action, to prevent it from bending forwards under the weight of the head, upper extremities, and viscera of the trunk. The whole weight of the upper part of the body is thus transmitted to the sacrum, and thence to the other bones of the pelvis, by which it is brought to bear on the femur. If left to themselves, the thigh-bones would bend beneath the pelvis, and the trunk would fall forwards ; but the contraction of their extensor muscles keeps them firm. In the same manner, the extensor muscles of the knee and ankle keep these joints from yielding beneath the weight of the body, which is thus at last transmitted to the ground. The sitting posture is less fatiguing than the stand¬ ing position, because the weight of the body is then directly transmitted from the pelvis to the base of support, so that it is not requisite for the extensor muscles of the lower limbs to keep-up a sustained action. But the lying posture is that of the most complete rest ; because the weight of every part of the body is at once transmitted to the surface on which it bears, and no muscular movement is requisite to keep it in its position. 655. This difference in muscular effort, is the cause of a well-marked variation in the pulse, according to the position in which the body is at the time. From a considerable number of observations it has been found that the average pulse of an adult man is about 81 when standing, 71 when sitting, and 66 when lying ; so that the difference between PULSE IN DIFFERENT POSTURES : - ACT OF WALKING. 493 standing and sitting is 10 beats or l-8th of the whole, whilst the difference between sitting and lying is 5 beats or 1-1 3th of the whole. In the female, the pulse is quicker in each position by from 10 to 14 beats per minute ; but the differences occasioned by position are nearly the same. It will be observed that the difference between standing and sitting is greater than that between sitting and lying ; and this closely corresponds with the relative amounts of muscular exertion required in these positions respectively. At the moment when the posture is changed, the pulse is considerably quickened, in consequence of the muscular effort required for the purpose, which acts especially on the veins, and forces the blood more rapidly back to the heart (§ 279); but this increase in rapidity is temporary only. 656. All that has been said of the positions of Vertebrated animals applies equally well to those of the Invertebrata, which like them have the body raised from the ground upon extremities. This is the case in the higher Articulata, such as Insects, Crustacea, Arachnida, and Myriapoda. But the lower Articulata crawl, like Serpents, upon the whole length of their bodies ; or, being aquatic, are buoyed-up by the element they inhabit. And among the Mollusca and Badiata, there are none that have members upon which they can be said to stand. 657 . The progressive movements by which the bodies of Man and other animals are made to change their places, are accomplished by means of the alternate contractions and extensions of those limbs, which we have hitherto considered only as supporting them in a rigid position. It is easy to see that when a joint is straightened after being bent, the two ends of the levers which form it must be separated from each other, and that motion must thus be given to the parts against which one or both of them bear. Now in the ordinary move¬ ments of progression, one of these levers bears against the ground, which is immovable ; and the whole motion produced by straightening the joint must consequently be communicated to the body. In the ordinary act of walking , one of the feet is planted in front, whilst the other is extended or carried backwards beneath the leg, by the action of the muscles of the calf aided by those of the toes (§. 648). Its length is thus increased ; and as it bears upon the resisting soil, this 494 ACT OF WALKING: — OTHER MODES OF LOCOMOTION. elongation acts through the thigh npon the pelvis, and thus carries forward the whole body. At the same time, the pelvis makes a slight turn upon the femur of the other side on which it is resting ; and the limb which was at first behind the other, is now drawn forward by a flexion of its joints, and is planted on the ground in front of the other, so as to serve for the support of the body in its turn ; whilst the other, by extending itself, gives a fresh forward impulse to the body. Thus each limb is alternately made to support the whole weight of the body, just as it would do in standing on one leg ; while at the same time the other is engaged in urging it forwards. Hence the centre of gravity must vibrate a little from side to side in the act of walking, so that it may be brought alternately over each foot ; and this movement from side to side is the more obvious, in proportion as the pelvis is wider, and the limbs more separated from each other. Hence it is more seen in women than in men, on account of the greater proportional breadth of the hips in the former. 658. In all the higher animals, as in Man, there are members which serve for locomotion ; but the nature of these movements varies greatly ; and there is a corresponding differ¬ ence in the structure of the instruments by which they are performed. The manner in which the Creator has made the same organs answer a variety of different purposes, in accord¬ ance with the habits of the animals to which they belong, is a most interesting object of study ; for we see the most varied results attained, without the least departure from the general plan which has been adopted in the construction of the various species of the same group ; and this solely by slight changes in the forms and proportions of some of the instruments whose union makes-up the entire body. The organs of locomotion in the Mammalia furnish us with obvious examples of this principle. This class includes not only the quadrupeds which run or bound along the surface of the ground, — but animals which are destined to live solely in water like fishes, — others which sometimes swim through that element and sometimes inhabit the land, — others which possess wings that enable them to fly through the air like birds,- — and others which only employ their anterior members for grasping or feeling ; yet in all these animals, these organs are constructed of the same parts. In the paddles of a Seal ACTS OF WALKING AND RUNNING. 495 (fig. 240), the wing of a Bat (fig. 251), and the fore-paw of a Squirrel or a Mole, we find the same bones as in the arm of Man (fig. 223). And even in the fore-legs of the Buminant Quadrupeds, and in those of the Solidungula , or single-toed animals (such as the Horse), we can usually perceive traces of the existence of three or four toes, whose bones are more or less completely united. 659. From what has already been stated as to the influence of the length of the levers on the quickness of the movement of the extremities (§. 614), it is easy to see that animals which have the most rapid progression must necessarily have long members ; since, the quickness with which the extensor muscles act remaining the same, the change of place in the free extremity of the lever will be greater, in proportion as that extremity is more distant from the point of insertion of the muscles that move it, and from the fulcrum on which the lever works. But in proportion to the elongation of this arm of the lever, must be the increase in the power of the muscles that move it, in order to overcome the same resistance ; according to the general principle that what is gained in velocity is lost in power. Hence, in order to endow an animal with great agility, it is only necessary to lengthen its limbs, and to render its muscles capable of exerting a proportional power. 660. We have seen that in walking , the body is sustained upon one limb (in quadrupeds , upon one pair of limbs), whilst it is pushed onwards by the other ; so that it never ceases to bear upon the ground. In running , however, the body of Man momentarily quits its support at intervals ; the foot in advance not being planted on the ground by the time that the hinder one quits it. In this action, the Ostrich and its allies probably surpass all other animals ; as they can out¬ strip the fleetest horse at full gallop, or the swiftest greyhound at its greatest speed. The awhle of Quadrupeds is a pace which resembles the walk or run of bipeds, the two legs on one side being moved together, whilst the body rests upon the other. This pace is peculiar to the Giraffe, and to horses which have been trained to execute it. The trot , however, is a step of a different and much more secure nature. The fore¬ foot of one side is raised and advanced with the hind foot on the other side ; and when these are set down, the other fore 496 ACTS OF RUNNING AND LEAPING. and hind feet are raised and advanced together. Now, if we consider the fore-feet of a horse as constituting the four angles of a parallelogram, it is easy to see that the base of support, when the feet are thus raised, will he one of its diagonals ; and as the feet are alternately advanced, the weight will alternately he thrown upon these two lines. But the centre of gravity in the horse, especially when carrying a rider, is in a point almost exactly above that at which the two diagonals cross ; so that it is always supported either by the one or the other. The gallop of greatest speed is a run performed on the same plan as the trot ; — that is, the right fore and left hind feet leave and reach the ground together, and then the left fore and right hind feet are advanced. The canter is a kind of step altogether different. The four legs strike the ground successively, the left hind foot reaching it first, the right hind foot second, the left fore foot third, and the right fore foot fourth. — The celebrated race-horse Eclipse, when galloping at liberty and with his greatest speed, passed over the space of 25 feet at each stride or leap ; this he repeated 2 1 times in a second, so as to pass over 58 feet in that time, which was at the rate of nearly 4 miles in six minutes and two seconds. But this performance was completely surpassed by that of Flying Childers, who was computed to have passed over 82 \ feet in a second, or nearly a mile in a minute. 661. In leaping , the body is projected into the air by the sudden extension of the joints, especially those of the hinder part of the body which had been previously bent ; and having traversed a greater or less distance, the body comes again to the ground and may be again projected. This is a kind of motion usually practised by many animals whose structure is expressly adapted to it. Thus among Mammals we find se¬ veral in which the hind legs are enormously elongated, for the purpose of giving greater quickness to the motion of the body ; and their muscles are developed to an extraordinary degree in order to supply the necessary force. This is the case among most of the animals of the order Rodentia, such as the Hare, Rabbit, Squirrel, &c. ; but particularly in the Jerboa or Jumping Rat, and in the Kangaroo and its allies. In these animals the fore feet, which are little used for pro¬ gression, are comparatively small; and in the last they are ACTS OF RUNNING AND LEAPING KANGAROO. 497 less than half the length of the hinder limbs (fig. 235). The feet, as well as the legs, of the Kangaroo are very long (fig. Fig. 235.— Kangaroos. 236), so as to afford (in conjunction with the tail) a firm sup¬ port to the animal when preparing to leap. Quadrupeds in Fig. 236. — Skeleton of Kangaroo. which the length of the posterior extremities greatly predomi¬ nates over that of the anterior, are observed to descend hills with difficulty at a rapid pace, since the forward inclination K K 498 ACTS OF LEAPING : — FLEA : CRICKET. of their bodies places them in continual danger of oversetting • they therefore take a zig-zag course. In ascending a hill, however, their progression is greatly favoured by the length of their posterior extremities (fig. 237). The Eabbit, when moving slowly, advances the fore-feet two or three steps alternately, the posterior limbs remaining inactive ; and the body having been lengthened by these means, the posterior legs are suddenly extended together, and then drawn for¬ wards : thus the rabbit walks with the fore and leaps with the hind pair of legs. The Frog moves in a very similar manner. 662. It is among Insects that we find the most extraordi¬ nary powers of leaping, considered with reference to the size of the animals that possess them. Thus the Flea will spring to a height equal to 200 times the length of its body. Let us imagine a Kangaroo or a Tiger doing the same ! In many of the leaping insects, the hind legs are of great length, as in the Grasshopper and Cricket tribe (fig. 238) ; and in one curious family, that of the Poduras or spring-tails, the leap is accomplished by the sudden extension of the tail, which is ordinarily bent under the body (fig. 239). A very remarkable kind of leap is LEAPING INSECTS. — SWIMMING AND FLYING. 499 executed by the Beetles of the family of Elateridce ; the larva of one species of which devours the roots of wheat, and is known under the name of the “ wire -worm whilst other species inhabiting tropical cli¬ mates, and having the power of emitting light, are termed “fire-flies” (§ 397). The legs of these insects are very short ; so that when they are laid on their backs they cannot by means of them recover their natural position. This they are enabled to do, however, by their power of jerking backwards the head and upper part of the thorax, which causes the body to be projected verti¬ cally into the air, whence it usually descends with the feet towards the ground. The leap of the Crickets, Locusts, Frog- hoppers, &c. is executed more in a horizontal direction ; and it is assisted by the wings, which bear-up the body whilst it is moving onwards through the air. In this manner a Locust can traverse 200 times its length, and a Frog-hopper 25 0 times ; which is as if a Man were to take a quarter of a mile at one leap. 663. Swimming and Flying are movements which have much resemblance to each other ; both being executed in a fluid medium, which to a certain extent buoys-up the body, which offers resistance to its progress, and which also offers something resembling a fixed point against which the mem¬ bers may act to propel it. The chief differences between them depend upon the nature of the medium ; this being liquid in the one case, and aeriform or gaseous in the other. The liquid medium affords more support to the body, and a firmer surface for the action of its propelling organs ; but at the same time it offers more resistance to its progress. The movement of a body through the atmosphere, as in flight, requires a considerable expenditure of power to keep it up ; and the yielding nature of the element prevents the propelling organs from acting against a firm surface ; but the onward movement, in consequence of the slight resistance, is easily accomplished. 664. When the feet of a Quadruped are to serve both as walking and swimming organs, the end is accomplished by K K 2 Fig. 239. — Podura. 500 ADAPTATION OF EXTREMITIES FOR SWIMMING. the spreading-out of the fingers, and their union by means of a fold of skin which is stretched over them ; as the web of a swimming Bird is stretched over its toes, so as to make an oar or paddle of sufficiently wide surface. This is the case, for example, in the Ornithorhyncus of Australia, and in the Otter of our own country. When the members are intended Fig. 240.— Skeleton of Seal. vc, cervical vertebrae , vd, dorsal vertebrae ; vl, lumbar vertebrae ; vs, sacral vertebrae; vq, caudal vertebrae ; b, pelvis ; s , sternum ; h, humerus ; r, radius ; ca, carpus ; me, metacarpus ; ph, phalanges ; o, scapula; c, ribs ; /, femur; r, patella ; t, tibia; ta, tarsus ; mt, metatarsus ; ph, phalanges. exclusively for swimming, however, they undergo more con¬ siderable modifications in structure. The parts corresponding with the arm and fore-arm are very short, and the movements of the hand are thus limited, but they can be accomplished with all the more force. But the bones of the hand are large and spread asunder, and are enclosed in a firm integument which may even cover their extremities. Sometimes the number and arrangement of these bones are precisely the same as in the hand of Man ; this we see in the Seal (fig. 240), where their extremities are furnished with separate claws that project beyond the integument. Sometimes the number of phalanges in the fingers is considerably increased, as in the Whale ; and in other instances, the fingers are replaced by a multitude of small rods of bone, enclosed within a continuous skin, such as we see in the fins of Fishes (fig. 243). 665. In the Seal, which does not depart widely in its general construction from land quadrupeds, the hind feet are formed upon the same plan as the fore ; but they are carried ADAPTATION OF EXTREMITIES FOR SWIMMING. 501 far backwards, so as almost to occupy the position of the tail. In the Whale and its allies, on the other hand, the posterior extremities are almost entirely wanting, and the tail is greatly prolonged and expanded at its extremity (fig. 241). This Fig. 241. — Skeleton of Dugong. expansion, however (which is in the horizontal direction, fig. 242), is not supported by bones, except in its centre ; but it consists internally of cartilages and tendons, which last are prolonged from a set of very powerful muscles that are at¬ tached to the spine, and give to this organ an enormous force and great variety of motion. The texture of the portion of it* by which the blow is usually given, is such that it can hardly be injured ; it is so tough that it cannot be torn, and so free from feeling, that a stroke of it against a hard substance gives no pain to the animal. If it strike a boat across the middle with its edge, the boat is cut asunder as clean and suddenly as if by one stroke of a giant axe ; whereas, if it strike with 502 PROPULSION OP WHALES AND FISHES BY TAIL. the flat surface, the boat is driven to the depth of many fa¬ thoms with the swiftness of an arrow. Hence this tail is a most efficient instrument for the propulsion of the bulky body of the Whale through the water ; and it is, in fact, its principal organ of locomotion. The paddles formed by the fore-feet are placed near the centre of gravity of the whole mass ; and thus can readily exert their peculiar action, which is that of changing the direction of the movement, and espe¬ cially of raising and lowering the body. 666. The propulsion of the body by the stroke of the tail in Whales and Fishes, is effected precisely in the same manner as the urging-forwards a boat through the water, by the Fig. 243. — Skeleton of Perch. lateral strokes of an oar at the stem, in the mode commonly termed sculling. The expansion of the Whale’s tail-fin being horizontal, its stroke is vertical, and may thus readily bring the animal to the surface of the water for occasional respira¬ tion, as well as propel it forwards ; but that of the Fish’s body and tail being vertical, its stroke is horizontal, and its action will simply be to urge the body through the water. The power of ascending and descending, as well as of changing the direction of the motion, is principally due to the side-fins, which represent the arms and legs. The direction of the surface and stroke of these side-fins varies in different species. In the Cod, Halibut, and others, their action appears to be principally directed towards keeping the body in its right position in the water; since, without such an action, the body would be liable to turn over, in consequence of the position ACTION OF THE FINS OF FISHES FLYING FISH. 503 of its centre of gravity. In other instances, the pectoral and ventral fins move in such a manner as to assist the action of the tail. In the Eays, the pectoral fins are developed to an enormous extent (fig. 244) ; and being directed horizontally, their action is vertical like that of the wings of a bird. They are furnished with a great number of joints, by which they are rendered very flexible ; and their surface may he thus increased during the down- stroke of the fin, and diminished during the ^p-stroke. If this were not done, the action of the fins in elevation would exactly counterbalance the effect of their depression ; and no movement would he produced. The great power of the pectoral fins of these Fishes seems connected with their want of an air- bladder, which causes them to require a constant exercise of force to keep them up in the water. Their propulsion forwards is chiefly accomplished, as in other Fishes, by the action of the tail. But sometimes the Kays change their position and swim sideways, making horizontal strokes with the pectoral fins (whose surface is then vertical), by which they are moved through the water, and sustaining themselves by vertical strokes of the tail, whose surface is then horizontal. 667. The structure of the organs adapted for movement in air bears great analogy to that of such expanded fins ; and there are instances in which the same instruments may serve both purposes. Thus there are Fishes which are able to quit the water, and execute leaps of considerable length, supported Fig. 245.— Flying-Fish. upon their wing-like pectoral fins. These are known as Flying -Fish (fig. 245) ; but it is not correct to speak of their1 Fig. 244.— Ray. 504 ORGANS OF FLIGHT : - FLYING FISH, PENGUIN, ETC. movement as one of flight, since it does not appear that they have any power of propelling themselves in the air; the impulse being given at the moment of their quitting the water, in the manner of a leap. From 50 to 100 yards, how¬ ever, are sometimes traversed by the Fish at one leap ; and the height to which it rises from the surface of the water is occa¬ sionally such as to carry it over the deck of a ship. On the other hand, there are several among the diving Birds which use their wings as instruments of progression beneath the water — in other words, as fins. The most remarkably constructed of all these is the Penguin (fig. 246), in which the wings are so short as to he incapable of answer¬ ing any other purpose ; but there are several species in which they Fig. 246.— Penguin. may be used as organs of flight in the air, without losing their fin-like power in the water. — There are several animals that can sustain themselves for a short time in the air, by the aid of an expanded surface formed by an extension of the skin and serving as a parachute. This is the case, for instance, with the Galeopithecus, or Flying Lemur (fig. 247), the Flying Squirrel, and the Petaurus , or Flying Phalanger (Zool. § 314), which have the skin stretched out on either side like a cloak, sup¬ ported by the anterior and pos¬ terior extremities and by the tail. By this parachute-like surface they are sustained in extensive leaps from bough to bough ; though it does not enable them to support themselves in the air for any length of time. In the Draco Volans (fig. 248), a little animal which lives among the trees of tropical forests, the body is furnished DRACO VOLANS.- — WINGS OF BIRDS. 50 5 with a wing-like appendage on either side, formed by an expansion of the skin over six lengthened ribs. These appendages serve as a kind of parachute, on which this little Fig. 248. — Draco Volans. animal, not more than a few inches long, flutters from branch to branch in search of its insect prey, or shoots, like the flying squirrel, from tree to tree. They cannot be made to strike the air, and therefore are not true wings ; but they can be folded up and extended at the will of the animal. 668. True wings , or instruments of propulsion as well as of support in the air, are found in some members of all classes of air-breathing Vertebrata; but they are especially characteristic of the class of Birds, in which the absence of them is the exception to the general rule, whilst in Mam¬ mals and Bep tiles it is their 'presence which constitutes the exception. These wings are universally formed by some modification of the anterior extremities, which renders them unfit to be used as instruments of progression on the ground ; but the nature of this modification varies considerably. In the Bird , the required extent of surface is chiefly given by the feathers ; these are supported upon an anterior member, of which the arm and fore-arm (especially the latter) constitute the largest part, the hand being contracted and consolidated. The general structure of the Bird’s skeleton, the whole of which is modified with special reference to the actions of flight, is shown in fig. 249, which represents that of the Vulture. The head is supported upon a very flexible neck, of which the vertebrae vc are often very numerous. The ver¬ tebrae of the back and loins, however, are usually few in 506 SKELETON OP BIRDS. number, and are connected together very firmly, so as to form a nearly inflexible column ; and this, again, is closely united to the sacrum vs. The vertebrae of the tail vq are few in Fig, 249. — Skeleton of Vulture. &c, cervical vertebrae; vs, sacral vertebrae; vq, caudal vertebrae; cl, clavicle; h, humerus; o, fore-arm; ca, carpus; ph, phalanges; st, sternum; f, femur; t, tibia ; ta, tarsus. nulnber, and possess little motion. The ribs are very strongly connected to each other and to the vertebrae, and are united to the sternum st by bony instead of cartilaginous prolonga¬ tions. Thus the whole bony apparatus of the trunk is very strongly knit together ; and the purpose of this is evidently to give as firm an attachment as possible to the muscles which move the wings. The sternum is raised into a high keel or ridge (as is better seen in fig. 250, 5), for the attachment of the powerful pectoral muscles which draw down the wings * and the degree of this projection is proportioned to the power of flight which the species possesses, — the sternum being flat (as in Mammals) in birds which, like the Ostrich, have the wings undeveloped. The scapula (fig. 250, o), to which are SKELETON OF BIRDS. 607 attached the muscles that raise the wings, is very narrow in Birds, in accordance with their small demand for muscular power in this direction. This narrow scapula forms one part of what is known as the “ side-bone the other part c of which is formed by a bone termed the coracoid, that is only represented in Man and other Mammals by the short coracoid process of the scapula (§ 635). The two clavi- 0 cles/ /are united together where they join the ster¬ num, to form the fork¬ like bone known as the u merry - thought,” the strength of which, like the projection of the keel of the sternum, serves to indicate the power of flight, by the degree of resistance which it is ca¬ pable of affording to the drawing-together of the shoulder -joints by the action of the pectoral muscles. The bones of the pinion consist of the humerus (fig. 249, Ji), the two bones of the fore-arm o, the bones of the wrist ca (which are here scarcely developed), and the bones of the fingers phy each joint of which shows indications of being made up of two or three separate bones united together. In no bird are these bones ever separated into distinct fingers, since they are never required for any other purpose than that of supporting the wing-feathers. — The leg is connected with the spinal column by a pelvis, of which the iliac bones are greatly lengthened and firmly attached to the spine, but which is not completed into a ring by the junction of the bones in front, as in Mam¬ mals ; such a completion would have prevented the passage of the bulky eggs deposited by these animals (§ 7 55). In the hinder extremity we find the femur or thigh-bone (principally concealed in the figure by the bones of the wing), the two bones of the leg t, which are commonly united in part of their length, the shank or ancle-bones ta , which are peculiarly Fig. 250. — Bones of the Shoulder and- Breast of Birds. o , scapula ; c, coracoid bone ; /, clavicles united' at their junction with the summit of the keel b of the sternum s, which is connected with the ribs by the ossified costal cartilages co. 508 SKELETON OF BATS AND PTERODACTYLS. Fig. 252. — Skeleton of Pterodactyle. of soft and delicate skin over a framework of bones, which must consequently be made to support it to its very edge. elongated in the wading birds, and the four separate toes, by the spread of which the body is firmly supported, though resting only on two feet. Fig. 251.— Skeleton of Bat. (References as in Fig. 229.) 669. In the Bat (fig. 251), however, the plan is very dif¬ ferent. We have here no long stiff feathers, by the projection of which from the limb itself the surface may be increased to almost any extent ; but the wing is formed by an expansion PTERODACTYLS!. — WINGS OF INSECTS. 509 This is accomplished by the enormous extension of the hones of the hand, especially the metacarpal me, which are here separate ; and the membrane is further sustained by the legs and tail. The thumb po is not included in the wing, but serves as a hook by which the animal can suspend itself. — The only true flying Eeptile is (or rather vms) the Pterodac - tyle, a kind of winged lizard, which does not now exist, but of whose character the skeletons that are found imbedded in the earth afford most convincing proof. The structure of its wing differed from that of either Birds or Bats ; for it appears, from the conformation of its anterior member (fig. 252), that the animal could have used it for resting or walking, the framework of the wing being formed by the enormous elonga¬ tion of one finger only. Fig. 253. — Dragon Fly. 670. The wings of Insects (fig. 253) have no correspon¬ dence whatever with those of Vertebrata, except in serving for the like use, and in being composed of an expanded sur¬ face of membrane, stretched upon a firm framework. This framework is not composed of solid pieces jointed together, but is merely an extension of the air- tubes and vessels within the body, which are strengthened by a continuation of its hard envelope. Their only action is a hinge-like movement at the point where they are united to the body ; and this is accomplished by powerful muscles contained within the thorax. 671. In all instances, the action of the wings must be such, that the air is struck with less force during the up-stroke than 510 POWER OP FLIGHT POSSESSED BY BIRDS. during the down-stroke ; otherwise the effect of the former would neutralise that of the latter. This is partly accom¬ plished by the great velocity of the down-stroke compared with the up- stroke, which causes the resistance of the air to be much greater against the former than against the latter.1 But it is by the alteration in the surface of the wing, as it acts upon the air, that the chief difference is made in Birds ; the arrange¬ ment of their great feathers being such, that they strike the air with their flat sides, but present only their edges in rising. What is called “ feathering the oar ” in rowing, is a similar operation, performed with the same intention, and deriving its name from this resemblance. 67 2. The degree in which the wings act in raising the body or in propelling it through the air, varies considerably in different species, according to the way in which they are set. Thus in Birds of Prey, which require a rapid horizontal motion, the surface of the wings is very oblique, so that they strike backwards as well as downwards, and thus impel the body forwards whilst sustaining it in the air. Such birds find a difficulty in rising perpendicularly ; and can in fact only do so by flying against the wind, which then acts upon the inclined surface of the wings just as it does upon that of a kite. On the other hand, the Lark, Quail, and such other birds as rise to great heights in a direction nearly vertical, have the wings so disposed as to strike almost directly down¬ wards. It has been estimated that a Swallow, when simply sustaining itself in the air, is obliged to use as much force to prevent its fall, as would raise its own weight to a height of about twenty-six feet in a second. Hence, we may form some idea of the enormous expenditure of force which must take place, when the body is not only supported, but raised and propelled through the air. The Eider-duck is said to fly 90 miles in an hour, and the Hawk 150. The Swallow and Swift pass nearly the whole of the long summer days upon the wing, in search of food for themselves and their 1 This resistance varies as the square of the velocity of the stroke. Hence, if the down-stroke be made three times as fast as the up-stroke, the resistance it experiences will be nine times as great. But as this only operates during one-third of the time, it is in effect equal to three times that which operates against the up-stroke, and which would tend to lower the Bird in the air. IMPOSSIBILITY OF HUMAN FLIGHT. 511 helpless offspring ; and the rapidity of their flight is such, that they can scarcely traverse less than seven or eight hun¬ dred miles in that time, although they go hut a short distance from home. The flight of Insects is even more remarkable for its velocity in proportion to their size ; thus a Swallow, which is one of the swiftest-flying of Birds, has been seen to chase a Dragon-fly for some time without success ; the Insect always keeping about six feet in advance of the Bird, and turning to one side and the other so instantaneously, that the Swallow, with all its powers of flight, and its tact in chasing Insects, was unable to capture it. 673. If the preceding estimate of the power expended by a Bird in sustaining itself in the air be correct, it may be easily proved that it would be impossible for a Man to sustain him¬ self in the air by means of his muscular strength alone, in any manner that he is capable of applying it. It is calculated that a man of ordinary strength can raise 13| lbs. to a height of 3^ feet per second ; and can continue this exertion for eight hours in the day. He will then exert a force capable of raising (13| X 60 X 60 X 8) 381,600 lbs. to a height of 3| feet ; or one-eighth that amount, namely 47,700 lbs., to the height of twenty-six feet, which, as we have seen, is that to which the Bird would raise itself in one second, by the force it is obliged to exert in order to sustain itself in the air. JSTow if we suppose it possible that a Man could by any means concen¬ trate the whole muscular power required for such a day’s labour, into as short a period as the accomplishment of this object requires, we might find the time during which it would support him in the air, by simply dividing this amount by his weight, which we may take to be 150 lbs. The quotient is 318, which is the number of seconds , during which the ex¬ penditure of a force that would raise 47,700 lbs. to a height of twenty-six feet, will keep his body supported in the air ; and this is but little more than five minutes. There is no possible means, however, by which a Man could thus concen¬ trate the force of eight hours’ labour, into the short interval in which he would have to expend it while supporting himself in the air. And we have elsewhere seen (Mechanics, § 285), that by no combination of mechanical powers can force be created ; as these only enable force to be more advantageously applied. Hence, the problem of human flight will never be 512 USE OF PREHENSILE ORGANS IN LOCOMOTION. solved, until some source of power shall be discovered, far surpassing that which his muscular strength affords, and so portable in its nature as not materially to add to his weight. 674. The only other organs of locomotion which we have to consider, are those of prehension. Of these, the principal have been elsewhere noticed, with reference to their use in laying hold of food and conveying it to the mouth (§ 172), and with regard to the differences between the hand of Man and the claspers of the Qaadrumana (§ 643). The hand of Man is seldom employed to assist in his locomotion, except in swimming (where it serves the purpose of a fin), and in climbing ; neither of which kinds of movement can be said to be natural to him. But the claspers of the Quadrumana (fig. 254) are most efficient instru¬ ments of locomotion ; enabling them not only to grasp the branches of the trees which they climb to despoil them of their fruit, but also to catch hold of them at the end of a long leap. This they do with the most wonderful agility ; as all who have seen Monkeys in circumstances at all like those of their natural habitations, must have observed. The Gibbons , or long-armed Apes of the East Indies, are probably the most remarkable in this respect. The Author has seen the U nghaputi leap round and round a room of about fifteen feet square, catching at each side by some small support attached to the wall ; and taking its next leap (if such it could be called) by merely swinging itself from this, without touching anything solid with its feet. There are many of the Monkey tribe, however, espe¬ cially in the Hew World, whose hands are less efficient as instruments of prehension ; and these are furnished with a prehensile tail; that is, a tail which can be coiled round the branch of a tree, and by which the animal can suspend itself PRODUCTION OF SOUNDS BY ANIMALS. 513 (fig. 255). A similar tail is possessed by some of the Opos¬ sum tribe ; and by the Chameleon among Eeptiles. CHAPTEE XIII. OF THE PRODUCTION OF SOUNDS : VOICE AND SPEECH. 675. It is not by their movements alone, that Animals are enabled to influence one another. Were it so, their commu¬ nication would be restricted to the small amount which can be effected by signs and gestures. This, however, is necessa¬ rily the case amongst aquatic animals in general ; since they are prevented by the nature of the medium they breathe from producing sounds through its means. Some of them appear to have the power of communicating with each other by the vibrations which they can excite in the water ; of this we have already noticed an example among the Whale tribe (§ 491) ; and there is reason to believe that certain Mollusks possess a similar means of communication. L L 514 PRODUCTION' OF SOUNDS BY INSECTS. 676. Many Insects have the power of producing a conti¬ nuous sound, which probably serves the purpose of intimating to each other the neighbourhood of their own kind ; and even, in some instances, of expressing them feelings : some of these sounds are produced only during flight. Of this kind is the sharp hum of the Gnat, Mosquito, Gad-fly, &c., which, though often a source of extreme annoyance to man and beast, serves to give warning of the proximity of these blood-thirsty Insects, and is therefore of real service to the animals they attack. From recent experiments, however, it appears that in Bees and Flies, at least, the sound is not produced so much by the vibrations of the wings (to which it is commonly attributed), as by those of a little mem¬ branous plate, situated in one of the spiracles or stig¬ mata (§ 321) of the thorax; for if the apertures of these be stopped, no sound is heard, though the wings remain in movement. But in Cock¬ chafers, and other noisy Beetles, Butterflies, &c., no such apparatus can be dis¬ covered. Other sounds are produced while the insect is feeding ; that occasioned by the armies of Locusts, when incalculable millions of powerful jaws are in action at the same time, has been compared to the crack¬ ling of a flame of fire driven by the wind. Certain two- winged Flies, distinguished by a long proboscis (fig. 256), make a humming sound whilst sucking honey from flowers ; and the same is the case with some of the Hawk-moths. 677. Some Insects are remarkable for a peculiar mode of calling , commanding , or giving an alarm . The neuters or soldiers among the White Ants make a vibrating sound, rather shriller and quicker than the ticking of a watch, by striking against hard substances with their mandibles ; this seems intended to keep the labourers, who answer it by a hiss, upon the alert and at their work. The well-known sound termed the “ death-watch” is produced by a small beetle termed Anobium (fig. 257), that burrows in old timber ; and PRODUCTION OF SOUNDS BY INSECTS. 515 it is occasioned by the striking of its mandibles upon the wood. The sound is evidently intended by the animal as a means of communication with its fellows ; for if it be an¬ swered it is continually repeated, whilst if no answer be returned the animal repeats the signal in another place. The noise exactly resembles that pro¬ duced by tapping moderately with the nail upon the table ; and, when familiarised, the insect will very readily answer this imi¬ tation. — The most remarkable example of the production of sounds for the purpose of autho¬ rity, is that of the Queen-Bee; which has the power of influencing the whole hive, especially about the time of swarming, by the peculiar notes she produces. 678. Many Insects have the power of expressing their passions, also, — as fear, anger, sorrow, joy, or love, — by the sounds they can generate. The most curious of those given out under the influence of alarm is that produced by the Sphinx Atropos or Death’s-head Hawk-moth (fig. 258); which Fig. 258. — Sphikx Atropos. when confined, or taken into the hand, sends forth a strong and sharp cry, resembling, some say, that of a mouse, but more plaintive and even lamentable. The means by which this cry is produced, have not yet been certainly ascertained. The influence of anger , sorrow , and joy, in modifying the tone of the hum of Bees, is well known to those who have studied their habits ; the first is particularly evident in the sharp angry tone which is heard when the hive has been disturbed, especially if some of the Bees have been killed ; the second l l 2 Fig. 257. — Anobium. Natural size and magnified. 516 SOUNDS PRODUCED BY INSECTS. is manifested in a low plaintive tone which, is given-out when the queen has been taken away ; and the cheerful humming which is immediately heard when the sovereign is restored, is an evident indication of the last. Of all the Insects inha¬ biting this country, the most noisy are the Crickets; whose Fig. 259. — House-Cricket. sound, which seems to be their expression of love , is produced by the rubbing of the elytra or wing-covers one against the other. In several species it may be distinctly seen that a very strong nervure on one of these has a jagged surface like that of a file ; and that this works against a collection of smaller nervures, which resemble so many strings. 679. The Cicada (fig. 260) was a very favourite insect among the ancient Greeks ; and was frequently mentioned by their poets with the most endearing epithets. Its song was considered particularly musical ; and it was regarded as the happiest as well as the most innocent of animals. The Cicadse of other countries produce an extremely shrill and disagreeable sound, which can be heard at a great distance. In the warmer parts of the United States, there is a species which, in the hotter months of summer, is a very troublesome and impertinent neighbour. The Cicadse of Brazil are said to be audible at the distance of a mile : this is as if a man of ordinary stature, supposing his powers of voice increased in the ratio of his size, could be heard all over the world. The organs by which the sound is produced are placed on the under side of the body, between the base of the hind legs and the abdomen, and consist externally of a pair of large flattened plates of a horny texture, varying in form in the different species. "When these are raised, they are found to conceal a large cavity partially covered with a membrane of a SOUNDS OF INSECTS. - VOICE OF VERTEBRATA. 517 much, more delicate nature than the external covering, with a horny plate in the middle, which lies along the bottom. Still more internally are two bun¬ dles of muscles, which are the real : agents in producing the sound ; for, when they are pulled and sud¬ denly let go, even in a dead speci¬ men, the sound is produced as well as though the insect were alive. They draw-in and force-out, by their alternate and rapid con¬ traction, a horny drum or mem¬ brane, stretched in such a manner as to vibrate readily ; the sound occasioned by the movements of which passes out through an aper¬ ture resembling the sound-holes of a violin. The Fulgorce , also, have considerable sound-producing powers, but exert them in the night, whilst the Cicadse perform in the day. The Great Lantern- fly of Guiana (§ 400, fig. 175) begins regularly at sunset; and its noise, resembling that of a razor-grinder at work, is so loud, that the insect is called u scare-sleep” by the Dutch colonists. 680. In all air-breathing Vertebrata, the production of sound depends upon the passage of air through a certain portion of the respiratory tube, which is so constructed as to set the air in vibration. In Eeptiles and Mammals, it is at the point where the windpipe opens into the front of the pharynx, that this vibrating apparatus is situated. Few of the animals of the former class, however, can produce any other sound than a hiss , occasioned by the passage of air through the narrow chink by which the trachea communicates with the pharynx ; but this sound, owing to the great capa¬ city of their lungs (§ 325), is often very much prolonged. Among Mammals, on the other hand, there are few, if any, which have not some vocal sound ; but the variety and expressiveness which can be given to it differ considerably in the several tribes of this class, being by far the. greatest in 518 STRUCTURE OF THE LARYNX. Man. This sound is produced by the apparatus termed the larynx , which is situated beneath the base of the tongue, and in front of the pharynx (§ 192, fig. 107). It is suspended, as it were, from the hyoid hone ( h , fig. 261), — a hone of a horse-shoe form, detached from the rest of the skeleton ; from two projections (l) on the upper side of which, several of the muscles of the tongue originate. The sides of the larynx are formed by two large cartilages (t, fig. 261), which i tr Fig. 261. Human Larynx, viewed SIDEWAYS. h, hyoid hone ; l, point of origin of muscles of the tongue ; t, thyroid car¬ tilage ; a, projection in front, commonly called Adam’s apple ; c, cricoid cartilage ; tr, trachea ; o, posterior side of the larnyx, in contact with the oesophagus. tr Fig. 262. Vertical Section oe the Larynx. ar, arytenoid cartilages ; v, ventricle of the glot¬ tis ; e, epiglottis; — the other references as be¬ fore. a a Fig. 263. Front view of the Larynx. The interior wall is mark¬ ed by the lines a, a, b,b; — It, inferior ligaments of the glottis, or vocal cords ; Is, superior liga¬ ments ; — the other re¬ ferences as before. are termed the thyroid cartilages ; where these meet on the middle line a projection is formed, which is particularly prominent in Man, and has received the name of Pomum Adami, or Adam’s apple (a). The thyroid cartilages rest upon another, termed the cricoid (c); this has the form of a ring, much deeper behind than in front, and surmounts the trachea, with the upper ring of which its lower edge is con¬ nected by a membrane. Upon the upper surface of the back of the cricoid cartilage, where there is an open space left between the two thyroid cartilages, are mounted two small cartilaginous bodies, the arytenoid (ar, fig. 262). These are movable to a certain extent ; and their position may be changed in various directions by several muscles which act upon them. STRUCTURE OF THE LARYNX. 519 681. To these arytenoid cartilages are attached two ligaments of elastic fibrous substance (§ 23), which pass forwards to he 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 termed the vocal cords or ligaments (fig. 263, li). By the meeting of these ligaments in front and their separation behind, the usual aperture has the form of a V ; but it may be narrowed by the drawing-together of the arytenoid cartilages, until the two vocal ligaments 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 regu¬ lated ; 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 ( l s , fig. 263), 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 epi¬ glottis ( c , 325, Z, l) on either side, which are the representatives of the arches of as many vertebrae. The furrow widens-out in the situation of the head, so as to form the receptacle (d) for the series of large ganglionic masses that is to constitute the brain (fig. 323, d, e,f) ; and though its sides do not there close-in for some time longer, it receives a special hood-like covering from a peculiar fold of the germinal membrane, the edge of which is seen at e , fig. 317. The cells, of which the parts of these laminae that bound the bottom &nd sides of this furrow are composed, appear to furnish the rudiments of the nervous centres that are afterwards to occupy the canal ; but beneath its deepest part there lies a continuous rod of peculiar nucleated cells (/), the chorda dorsalis , which marks-out the situation afterwards to be taken by the bodies of the vertebrae. This remains the only representative of the vertebral column in the Lamprey and other Fishes of a low grade, the develop¬ ment of whose bony skeleton is checked so early that it never advances beyond this simple embryonic type (§ 53). Fig. 316. — Yolk-Bag of Fowl’s Egg, after twelve hours’ incubation : t, yolk; b, primitive trace surrounded by pellucid area ; c, more opaque ring, the commencement of the vascular area. DEVELOPMENT OP CIRCULATING APPARATUS. 583 7 58. During the progress of this change, another very im¬ portant one is taking place, which is destined for the nourish¬ ment of the embryo during its further development. This is the formation of vessels in the substance of the germinal membrane ; which vessels serve to take up the nourishment supplied by the yolk, and to convey it through the tissues of the embryo. The space over which these vessels spread tebral arches ; d, dilatation for brain ; e e, cephalic hood ; /, chorda dorsalis. Fig. 318.— Yolk-bag of Fowl’s Egg, at the beginning of the third day of incu¬ bation : a, yolk ; 6, embryo ; c c, arteries of vascu¬ lar area; d d , veins ; e s, terminal sinus. themselves, is called the Vascular Area ; it makes its ap¬ pearance during the second day of incubation in the Fowl's egg (fig. 316, c), and soon spreads itself over the surface of the yolk (figs. 318, 319). Islets or points of a dark colour first appear in it ; these unite in rows ; and at last continuous vessels are formed. The heart makes its appearance at the twenty-seventh hour of incubation, as a simple dilatation of the trunk into which the blood-vessels unite (fig. 320, h). Its wall is at 584 DEVELOPMENT OF VESSELS AND DIGESTIVE CAVITY. first formed by a layer of cells ; and no muscular structure is , seen in it, until after its regular pulsations bave commenced. It is in these vessels that the first blood is formed ; and the same process appears to be continued through the whole period of incubation, the yolk being progressively converted into blood, and this blood being conveyed by the great trunks which collect it into the body of the embryo. Looking at the yolk-bag in the light of a temporary stomach, its vessels may be likened to those which take so large a share in the act of absorption from the digestive cavity of the adult (§ 218). 759. During the same early period of incubation, the layers of the germinal membrane begin to exhibit various folds, which after¬ wards serve for the forma¬ tion of the several cavities Fig. 319.— Embryo of Bird, with the of the body. The points of days’ incubation. it which lie beyond the extremities, and which spread-out from the sides of the embryo, are doubled-in so as to make a depression upon the yolk ; and their folded edges gradually approach one an- / ± T other under the abdomen, which lies next the in¬ terior of the egg. In this manner is formed the per¬ manent digestive cavity ; which is first a simple pouch communicating with the yolk-bag, by a wide opening, as seen at s, fig. 320 ; but which is gradually separated from it by the narrowing of this orifice (fig. 322), the connecting portion being elongated into a duct (fig. 321, b). Thus we may Fig. 320. — Diagram of the Formation of the Digestive Cavity: e, embryo ; /, g, layers of germinal mem¬ brane ; h, heart; s, stomach. DEVELOPMENT OF DIGESTIVE CAVITY AND ALLANTOIS. 585 say that the digestive cavity in Yertebrata is formed by the pinching-off (as it were) of a small portion of the general sac of the yolk. In the Mammalia, the remainder of the yolk-bag is completely separated from this by the closure of its narrow orifice, and is afterwards thrown off ; so that only a very small portion of the germinal membrane is received into the permanent structure. But in Birds and other oviparous animals, the whole of the yolk- bag is ultimately drawn into the abdomen of the embryo ; the former gradually shrinking as its contents are exhausted ; and the latter enlarging, so as to receive it as a little pouch or appendage. In Fishes, the hatching of the egg very com¬ monly takes place before this process has been completed ; so that the little Fish swims about with the yolk-bag hang¬ ing from its body. 760. The embryo, like the adult, has need of Bespiration ; partly that its own heat may be kept up; and partly that the carbonic acid liberated in the various processes of nutrition, may be set free. Owing to the peculiar structure of the membrane covering the albumen and forming the basis of the Fig. 321. — A, MORE ADVANCED EM¬ BRYO of Fowl, connected only by the vitelline duct b , with the yolk- bag a a, over which are distributed the blood-vessels c c; b, early form of the anterior extremity a, and of the posterior extremity b. Fig 322. — Diagram of the Formation of the Allantois, i. (The other references as in fig. 320.) shell (§ 755), the outer air is enabled to gain access to the interior of the egg; and at first its action upon the blood, 586 RESPIRATION OF THE EMBRYO : - ALLANTOIS. whilst circulating in the vascular area, is sufficient. In Fishes, no further provision is made for this process ; since, by the time it would be required, the egg is hatched; the young animal comes forth into the medium it is permanently to inhabit, its own gills come into play, and the air contained in the water can act directly upon the blood circulating in the vascular area. But in the higher oviparous animals, whose development proceeds further before they leave the egg, a special provision is made for this purpose. On the third day of incubation, in the Fowl, a bag termed the Allantois (fig. 322, i) begins to sprout (as it were) from the lower end of the body ; and gradually enlarges (fig. 323), passing round the embryo, and beneath its en¬ veloping membranes, so as ig. jirf.-i/MBjuu uj;’ rowii, wiui me , ■ , , . , Allantois, a, over which ramify the almost Completely to inclose embryo ; and as one side of it lies in close proximity with the membrane of the shell, it is very advantageously situated for receiving the in¬ fluence of the air. It thus serves as the temporary respi¬ ratory apparatus of the Chick, up to the time when it is pre¬ paring to quit the egg.1 There is reason to believe that the bird then receives air into its lungs, from the air-space formerly mentioned (§ 755), which increases in size as the contents of the egg diminish in bulk by the evaporation of their watery part. By the increased vigour which it thus acquires, it is enabled to perform the movements requisite for extricating 1 If the respiration of the embryo be prevented by rendering the shell impermeable to air, its development is completely checked. No means of accomplishing this is so effectual, as smearing the shell with oil or grease of any kind. Hence the effect of the well-known practice of buttering the surface of the egg, in preventing the chick from being reared ; and the same operation, if performed when the egg is quite fresh, will preserve it for some time fit for eating, its decomposition bei^g prevented by the complete exclusion of the air. DEVELOPMENT AND NUTRITION OF MAMMALIAN EMBRYO. 587 itself from its shell ; which it does entirely by its own exer¬ tions. When it thus becomes independent of the allantois, the circulation through the latter diminishes ; and almost the whole sac is separated from the body by the contraction of the connecting foot-stalk, which at last gives way. 761. The formation of the yolk-bag and the allantois takes place in Mammals (fig. 324) almost exactly on the same plan Fig. 324. — Embryo op Mole : a, entire ; b, with the abdomen laid open : — a, chorion ; &, footstalk of allantois and umbilical vessels; c, yolk-bag; d, vitelline duct; e, upper portion of intestinal canal ; /, lower portion ; g , eye ; h , indication of branchial arches ; i, auditory vesicle; Jc, anterior, and l, posterior extremity; m, n, Wolffian bodies or rudi¬ mentary kidneys ; o, rudimentary lung ; p, trachea ; q, ventricle of heart ; r , atrium of heart. as in Birds ; but on account of the absence of food-yolk, these sacs are comparatively small ; and the function of both is su¬ perseded, at an early period of the development of the embryo, by a new and remarkable contrivance. The ovum, in passing through the oviduct, has been already stated to receive a new envelope, analogous to that which forms the membrane of the shell in Birds ; this is termed the Chorion . It is then received into the cavity of the Uterus, a receptacle within which it is delayed for a considerable period, and continually supplied with nourishment drawn from the blood of the parent. From 588 FORMATION AND USES OF PLACENTA. the whole surface of the chorion, a number of little tufts shoot out (fig. 325), which come into contact with the lining mem¬ brane of the uterus ; and this is furnished with a multitude o£ glandular follicles, which secrete a nutritious fluid that is absorbed by the tufts of the chorion, and by them communi¬ cated to the embryo. When the allantois is formed, it serves to carry the blood-vessels of the embryo to the inner surface of one part of the chorion ; and they shoot through this, so as Fig. 325. — Interior oe Human Uterus at the seventh week of Pregnancy : b, outlet of the uterus, of which the walls c, c, c, c, laid open by incision, are turned hack to display its contents ; d d , its lining membrane ; g , tufted surface of the chorion ; g2, its internal aspect ; h , h , amnion ; i , yolk-bag ; k, umbilical cord ; l, embryo. to dip-down, as it were, into large expanded vessels that extend outwards from the walls of the uterus. In this manner is formed, in all the higher Mammalia, the important organ termed the Placenta; which essentially consists of the ramifi¬ cations of the foetal blood-vessels contained in the Umbilical Cord or “ navel-string,” ensheathed by prolongations of the large vessels of the maternal uterus. In the Marsupials and Monotremes (Zoology, §§ 309— -320), no placenta is ever formed, the embryo coming into the world in a stage scarcely more advanced than that represented in fig. 325. In either case, the vessels of the . embryo are enabled to absorb from the blood contained in those of the parent, through the thin DEVELOPMENT OF CIRCULATING APPARATUS. 589 walls of both, the materials requisite for its growth ; but there is no direct communication between the two. The same means serve for the aeration of the blood of the embryo ; for this, being brought from its body in the venous condition, is exposed to the influence of the arterial blood of the parent, through the thin walls of its vessels, — just as the venous blood of aquatic animals is aerated in their gill-tufts, — and passes back to the embryo in the arterial condition, having imparted its carbonic acid to the blood of the parent, and received from it oxygen. — Thus all but the very early stages of development are performed in Mammals, by means of which we scarcely find a trace in Oviparous animals ; yet the ova of both are originally formed on the same plan, and the first changes which they undergo are exactly analogous. 762. It would not be consistent either with the design or with the limits of this work, to enter in much detail into the considera¬ tion of the processes of develop¬ ment, although they present many points of the highest interest. The general history of the evolu¬ tion of the Circulating apparatus and of the Nervous centres may, however, be noticed, as character¬ istic examples of the mode in which the evolution of the several organs of the body takes place. — The Heart, in Man and other Mammals, as in the Bird (§ 758), is at first a simple tube, resembling the pulsatile trunk that remains as the sole organ of impulsion in the lowest forms of circulating apparatus. After a time this tube is doubled upon itself, and two cavities are formed, an auricle and a ventricle ; in this con¬ dition, it strongly resembles the heart of the Fish (§ 286). The circulation too is, at an early period, that of the Fish ; for the Fig. 326. — Embryo of the Fowl, from the Ovum shown in fig. 318, greatly enlarged : a, b, folds of germinal membrane* enveloping head and tail ; c, la¬ teral folds ; d, e, rudiments of optic ganglia and cerebrum ; /, heart ; g, dilated termination of venous trunk, forming atrium of heart; h, aorta; 1, 2, 3, 4, bran¬ chial arches ; i, i, vessels of vascular area; k, Jc, dorsal lami¬ nae; l, l, rudiments of vertebral arches. 590 DEVELOPMENT OF CIRCULATING APPARATUS. arterial trunk that springs from the ventricle, divides into a set of arches on each side (fig. 326), which closely resemble the branchial arches of Fishes and Tadpoles. Although no gills are present, yet there is a series of clefts on each side of the neck, passing through to the pharynx, which are analogous to the branchial apertures of the cartilaginous Fishes (§ 317). After a time, however, the auricle and ventricle of the heart are each divided by a vertical partition, so that four cavities are formed, out of the two which previously existed ; and at the same period, the arrangement of the vessels undergoes a change, by the division of some trunks, and the obliteration of others, so that they gradually assume the distribution which is charac¬ teristic of warm-blooded animals (§ 281). But even up to the time of the birth of the Mammalia, there is a communica¬ tion between the two sides of the heart, and between the pulmonary and systemic vessels, which is closely analogous to that which permanently exists in the Crocodile (§ 283). 763. Again, the space within the head of the embryo, into which the vertebral canal widens-out (§ 757), is occupied in the first instance by a succession of vesicles or bags, arranged in a linear series (fig. 323, d , e,f) ; each of which is the rudi¬ ment of one of those principal ganglionic masses, that col¬ lectively make-up the brain of the Fish (§ 453), in which they present a very similar aspect (fig. 192). As in many Fishes, too, the Cerebrum is inferior in size to the Optic ganglia, and only comes to surpass and finally (as it were) to overpower them (§§ 455, 456) in the later periods of embry¬ onic development. 764. The true representation of these and similar facts is not, as was maintained when they were first brought into view, that the several organs of the higher animals go through a series of forms which remain permanent in the lower; but that the development of all animals formed upon the same general plan commences in the same manner, their special differences manifesting themselves as development proceeds. Thus, as we have seen, the foundation of the Vertebral column is laid in all Vertebrata in precisely the same method (§ 757) ; in some of the lowest Fishes, the evolution of this structure is checked at so early a period that it never advances beyond the embryonic type (§ 53) ; but the fully-formed spine has a von baer’s law of development. 591 characteristically- different structure in each, of the classes of Yertebrata, which is not presented at any period in the history of the others. So, the evolution of the Circulating apparatus commences in all Yertebrata upon the same original plan ; and from this plan there is but little departure in the Fish ; but the circulating apparatus of the early Human embryo, how¬ ever like that of the adult Fish, differs from it in this essential particular, — the absence of gill-tufts receiving capillary vessels from the branchial arches (§ 286). The like is true in regard to the Nervous centres ; for although the earliest condition of the Human brain very closely resembles that of the brain of the foetal Fish, it never bears any exact analogy to that of the adult Fish. 7 65. Hence the principal facts of Organic Development admit of being stated in this general formula , which we owe to the sagacity of Yon Baer, — that the more special forms of structure arise progressively out of the more general , — a prin¬ ciple than which there is none more comprehensive or more important in the whole range of Physiological Science. 766. The Unity of Plan which is visible through the whole Animal Kingdom, is nowhere more remarkable than in the function of which an outline has now been given. We have seen that, however apparently different, the essential character of the Reproductive process is the same in the highest Animal as in the lowest. It has been shown that the development of the highly-organized body of Man, — though it is to serve as the instrument of those exalted faculties, by the right employment of which he is made “ but a little lower than the Angels,” — commences from the same starting-point with that of the meanest creature living : for even Man, in all the pride of his philosophy, and all the splendour of his luxury, was once but a single cell, undistinguishable, by all human means of observa¬ tion, from that which constitutes the entire fabric of the simplest Protozoon. And when the Physiologist is inclined to dwell unduly upon his capacity for penetrating the secrets of Nature, it may be salutary for him to reflect that, — even when he has attained the furthest limits of his Science, by advancing to those general principles which tend to place it 592 CONCLUSION. on the elevation which others have already reached, — he yet knows nothing of those wondrons operations, which are the essential parts of every one of those complicated functions by which the life of the body is sustained. Why one cell should absorb, — why another, that seems exactly to resemble it, should assimilate, — why a third should secrete, — why a fourth should prepare the reproductive germs, — and why, of the two germs that seem exactly similar, one should be developed into the simplest Zoophyte, and another into the complex fabric of Man, — are questions that Physiology is not likely ever to answer. All our science is but the investigation of the mode or plan on which the Creator acts ;.the Power which operates is Infinite, and therefore inscrutable to our limited compre¬ hension. . But when Man shall have passed through this embryo state, and shall have undergone that metamorphosis by which everything whose purpose was temporary shall be thrown aside, and his permanent or immortal essence shall alone remain, then, we are encouraged to believe, his finite mind shall be raised more nearly to the character of the Infi¬ nite, all his highest aspirations shall be gratified, and never- ending sources of delightful contemplation shall be continually opening to his view. The Philosopher who has attained the highest summit of mortal wisdom, is he who, if he use his mind aright, has the clearest perception of the limits of human knowledge, and the most earnest desires for the lifting of the veil that separates him from the Unseen. He, then, has the strongest motives for that humility of spirit and purity of heart, without which, we are assured, none shall see God. INDEX, N .B .—The numbers refer to the paragraphs. A. Aberration, chromatic, 548, 549; spherical, 546, 547; avoided in the eye, 549. Absorption, nutritive, 171 ; in Verte- hrata, through laeteals, 217 ; through blood-vessels, 218; in Invertebrata, 225; in non-vascular animals, 225; interstitial, performed by lymphatics, 219; from general surface, by veins, 220. Abstinence, power of, 140, 141, 152. Acalephve, 120 ; luminousness of, 395; (see Medusce ). Acephalous Mollusca, 111, 113. Actinophrys, 130. Adipose tissue, 46. Aeration. of blood, 253, 303. Aerating surface, 311, 312. Agamic Reproduction, of Insects, 746, 747 ; of Entomostraca, 748. Air, atmospheric, composition of, 300; need of in animals, 207; change of, by respiration, 300 — 302; effects of de¬ privation of, 297, 298, 310, 338. - bladder of Fishes, 324. - cells of Birds, 80, 326, 327. - tubes and sacs of Insects, 320 — 322. Albinos, 545. Albumen, chemical composition of, 13, 14; use of in blood, 240. Albuminous principles of food, 153; des¬ tination of, 158 — 164. Aliment, definition of, 132 (see Food). Allantois, 760, 761. Ammonite, 111. Amphibia, 85, 87 ; circulation in, 287— 289 ; (see Batrachia and Frog). Amphioxus, 53. Anabas, respiratory organs of, 318. Anastomosis, of arteries, 262 — 265; of nerves, 428. Anemone, Sea, 126, 127; reproduction of, 726, 739. Aneurism, 263. Animal Life, functions of, 6, 426, 427 ; nervous system of, 461. Animals, distinctive characters of, 6 — 12; dependent on Plants for support, 144—147. Animalcules, structure of, 13, 136, 137 ; heat produced by, 404 ; movements of, 577 ; reproduction of, 135, 725. Annelida, general characters of, 104; circulation in, 294 ; respiration of, 314; luminosity of, 396 ; nervous sys¬ tem of, 440; muscular motion of, 597 ; reproduction of, 727, 744. Annular ligament, of wrist, 641 ; of ancle, 648. Anobium, sound of, 677. Ant-eaters, 186. Antennae of Insects, uses of, 498. Anthozoa, 126; development of, 736. Ant-lion, instinct of, 697. Aorta, 258; valves of, 273. Aphides, agamic reproduction of, 727, 735, 746. Aplysia, nervous system of, 438. Aquatic animals, respiration in, 298. Aqueous humor, 536. Arachnida, general characters of, 106; circulation in, 293 ; respiration in, 323; development of, 751. Arachnoid membrane, 43. Area pellucida, 757. • — — vasculosa, 758. Areas, comparative, of arteries, 247. Arenicola, 314. Areolar tissue, structure and properties of, 24—27. Argonaut, reproduction of, 753. Arm, bones and muscles of, 638 — 640. Arteries, 246 ; comparative areas of, 247 ; walls of, 248; pressure of blood in, 249 ; protection given to, 250 ; division of into capillaries, 251 ; general distri¬ bution of, in Man, 258, 259 ; in Bird, 260, 261 ; peculiarities of distribution ©f, 262, 264, 265 ; flow of blood through, 274— 276 ; wounds of, 277. Articulata, general structure of, 93 — 96 ; skeleton of, 597—6 00 ; nervous system of, 440 — 447; eyes of, 573 — 575. Articulate sounds, 687 — 691. Q Q 594 INDEX. Articulation of bones, different modes of, 600—604. Arytenoid cartilages, 680, 681. Ascidia, 114; nervous system of, 435. Asphyxia, 280, 338 ; treatment of, 339. Assimilating glands, 223, 224. Astrsea, fission of, 726. Atlas-vertebra, 632. Attention, effect of, on sense of touch, 494; on hearing, 525; on sight, 555; on mental power, 721. Auditory nerve, 512. Auricles of heart, 257 ; action of, 270. Automatic movements, 479. Axis vertebra, 632. Axolotl, 87. Azotized principles of food, 153, 154; destination of, 158 — 165; effects of ex¬ cess of, 348. Azotized compounds, excretion of, 346 —348. B. Balancing of body, 480, 481. Balanus, 102; development of, 749. Ball-and-socket joint, 603. Barnacle, 102; development of, 749. Basement-membrane, 31. Bat, wings of, 669; ear of, 515; hyber¬ nation of, 407 ; peculiar sensibility of, 494. Batrachia , 86, 87 ; reparative power of, 390; development of egg of, 756. Baya, nest of, 705. Beat of heart, 269, 271. Beaumont, Dr., his experiments upon gastric fluid, 207, 208. Beaver, operations of, 706 — 708. Bees, formation of wax by, 156; heat produced by, 410, 411; use of antennse in, 498; sounds of, 678; instincts of, 712 — 716; reproduction of, 747. Beetle, digestive apparatus of, 202. Bell, Sir C., his discoveries, 429, 451. Bicuspid teeth, 187. Bile , chemical characters of, 364 ; use of, in digestion, 213 ; formed from venous blood, 266, 366 ; purposes of excretion of, 346 — 351, 365; effect of suspension of, 351. Bilious complaints, 350. Binary subdivision of cells, 33. Birds, general characters of, 78 — 80; digestive apparatus of, 200; digestive powers of, 201 ; blood-discs in, 230 ; arterial system in, 260 ; circulation in, 281, 282; respiration in, 326, 327; heat of, 407, 413; nervous system of, 455 ; intelligence of, 484, 717 ; wings of, 669; flight of, 672; larynx of, 685; nests of, 704, 705; social instinct in, 710; ovarium of, 754; structure of eggs of, 755 ; development of embryo of, 757—760. Bladder, gall, 362 ; urinary, 362. Blastema, 33; production of cells from, 34. Blood of Yertebrata, general characters of, 226; arterial and venous, 227; ge¬ neral purposes of, 228, 239 ; composed of liquor sanguinis and corpuscles, 229; coagulation of, 236 — 239; serum of, 238 ; huffy coat of, 236 ; flow of, means of checking, 277, 278; changes of, in disease, 233 ; assimilating power of, 242. Blood of Invertebrata, character of, 235. Blood Corpuscles, colourless, 35, 234. - red, 35, 229 — 234 ; ac¬ tion of endosmose on, 231 ; composi¬ tion of, 232 ; variety of proportion of, in different animals, 233; functions of, 235, 241 ; change of, in respiration, 310 ; connexion of, with heat libe¬ rated, 413. Blood-vessels, 244; formation of in new tissues, 393; in embryo, 758. Bombylius, sound of, 676. Bone , structure of, 49, 50 ; chemical composition of, 51 ; formation of, from cartilage, 52, 53 ; reparation of, 390. Bones of head, 617 — 693; of ear, 516; of spine, 626 — 632 ; of trunk, 623 ; of upper extremity, 635 — 644; of lower extremity, 645 — 648. Bowerbankia, 115, 356. Brain of Vertebrata, 72; not the only source of action, 465, 466 ; comparative size of, 718, 719; development of, 763; (see Cerebrum and Cerebellum'). Branchial arches, 2S6— 289 ; of embryo, 762. Bronchial tubes, 326, 328. Bryozoa , 115; (see Poly zoa). Buds, reproduction by (see Gemmation). Buffy coat of blood, 236. Bulk required in food, 205. Butter, 377. C. Caddice-worms, 701. Calamary, 111. Camel, stomach of, 198 ; skeleton of, 644. Campanularia, 124. Cancelli of bone, 49 ; formation of, 52. Canine teeth, 181, 183. Cantering, 660. Capillary vessels, 251; movement of blood in, 275, 280. Carapace of Turtles, 83. Carbon, modes of excretion of, 345 — 351, 365 ; combustion of, 157, 305, 306, 412, 413. Carbonic acid, set free in respiration, 301, 303; mode of its production, 305, 306 ; quantity of, proportional to ac¬ tivity of animal, 307 — 309 ; amount of, disengaged by Man, 334 ; deterioration of atmosphere by, 335—338. INDEX. 595 Carnivorous tribes of animals, 148 — 151 ; nutrition of, 161; teeth of, 179, 182. Carpenter-Bee, 703. Cartilage , structure of, 47 ; nutrition of, 48 ; transformation of, into bone, 52. Casein, 15. Cat, electricity of, 418. Cauda equina, 460. Cells , nature of, 32 ; multiplication of, 33; new production of, 34, 393 ; inde¬ pendent condition of, 35 ; origin of all organisms from, 379 ; differentiation of functions of, 380. Cells of Bee, 712, 713; royal, 714, 716. Cementum of Teeth, 54. Centipede, 93, 103 ; reflex actions of, 443. Cephalopoda, general characters of, 121 ; circulation in, 291 ; respiration in, 316; nervous system in, 448; . re¬ production of, 753. Cercaria-larva of distoma, 743. Cerebellum , 449; development of, in different classes, 452 — 456 ; in Man, 458; functions of, 480, 481. Cerebro-spinal nerves, 458—461. Cerebrum , 449 ; development of, in different classes of Vertebrata, 452 — 456 ; in Man, 458 ; development of, correspondent with intelligence, 452, 718; functions of, 483—485; effects of removal of, 465. Cerumen of ear, 375. Cheetodon rostratus, 476, Chalazae of egg, 755. Cheese, 377. Chemical constitution of organized bodies, 4; of albumen, 13, 14 ; of casein, 15; of syntonin, 16; of fibrin, 17, 18; of gelatin, 19; of chondrin, 20; of bone, 51 ; of teeth, 54. Cheselden’s case, 507. Childers, speed of, 660. Chimpanzee, 173, 674. Cholesterin, 364. Cholic acid, 364. Chondrin, 20. Chorda dorsalis, 53, 757. Chorion, 761. Choroid coat, 533, Chromatic aberration, 548, 549. Chrysalis, 97, 309, 745. Chyle , 171, 213, 222; change of, in lac- teals, 222, 223 ; delivery of into blood- Chylificaticm,1 171, 212—215. Chyme, 171, 211. Cicada, sound of, 679. Cicatricula, 7 55, 756. Cilia, 45, 143, 319, 329. Ciliary processes, 536. Circulation, 253 ; purposes of, 228, 253, 254: complete double, 282; greater, 253; lesser, 253, 268; pecu¬ liarity of, in liver, 267; mechanism of, 269, 270. Circulation, course of, in warm-blooded animals, 282; in foetus, 283,762; in Reptiles, 284, 285 ; in Fishes, 286 ; in Am¬ phibia, 287, 288; in Invertebrata, 289 ; in Mollusca, 290 ; in Cephalopoda, 291 ; in Crustacea, 292 ; in Insects, 293 ; in Spiders, 293; in Worms, 291; in Tuni- cata and Echinodermata, 295 ; in Zoo¬ phytes and Sponges, 295. Cirrhipeda, general structure of, 102; de¬ velopment of, 749. Claspers of monkeys, 643, 674. Clavicle, 634, 636. Climbing perch, 318. Coagulation of albumen, 14; of fibrin, 17 ; of blood, 236—238 ; of chyle, 222. Coagulable lymph, 391. Cochlea of ear, 518—520. Cockchafer, digestive apparatus of, 358. Cod, brain of, 453. Cod-liver oil, use of, 386. Coecum, 214. Cold sustained by animals, Introd., 405. Cold-blooded animals, temperature of, 403—406. Coloured shadows, 569. Colouring matter, formed by cells, 359, 360, 533. Colourless corpuscles of blood, 234. Colours, want of power to distinguish, 468; complementary, 568 — 570. Comatula, development of, 741. Combustion in animal body, 157, 305, 306,412,413. Commissures of nervous system, 434, 458. Complementary colours, 568 — 570. Compound eyes of Articulata, 572 — 575. - - Polypes, 124, 127. - Tunicata, 114. Conchifera, 113; respiration in, 316; luminosity of; 396; nervous system of, 437. Conjunctival membrane, 537. Consonants, 689, 690. Consumption, nature and treatment of, 386. Contractility of muscular fibre, 579, 590 ; dependent on its nutrition, 591. Contractions of heart, -269, 271, 581, 587. - - - of muscles, energy of, 592 — 594; use of, in organic functions, 595; in locomotion, 595. Convex surfaces, influence of, on light, 529—531. Convolutions of brain, 456, 458. Convulsive movements, 473, 474; energy of, 592. Cooling effects of evaporation, 372, 373. Coral-forming animals, 131 — 134. Cornea, 533 ; structure of, 46. Corpora striata, 458. Corpus callosum, 458. Q Q 2 596 INDEX. Corpuscles of blood, 35 (see Blood-cor¬ puscles). - chyle, 222. Cortical substance, of brain, 430. - kidney, 357. Coughing, act of, 342. Crab, anatomy of, 99 ; metamorphosis of, 101 ; nervous system of, 447. Cranium, bones of, 617 — 623. Crassamentum of blood, 236. Cray-fish, 95. Cricket, leaping powers of, 662; sound produced by, 678. Cricoid cartilage, 680. Crinoidea, 118. Crocodile, 93. Crusta petrosa of teeth, 54. Crustacea, general characters of, 99 — 101; formation of shell of, 170; teeth of, in stomach, 202 ; circulation in, 292 ; respiration in, 315 ; liver of, 356 ; luminousness of, 396; reproduction of claws in, 389 ; generation of, 748 ; development of, 101, 748. Crystalline lens, 536; reproduction of, 390; change of form of, 551. Cutis, structure of, 37 ; sensibility of, 490, 491. Cuttle-fish, 111; ink of, 359. Cynips, agamic reproduction of, 746. Cysticercus, development of, 742. D. Daphnia, reproduction of, 748. Death, of the body, 67 ; apparent, 66. - of parts, continually taking place, 65, 68. Death-watch, 677. Death’s-head Moth, 678. Decay of dead animal matter, 54, 160. Decay continually taking place in living body, 65, 68. Deer, foot of, 652. Defecation, 171, 216. Degeneration of tissues, from want of use, 30. Deglutition, 171, 192—196, 470. Dentine, 54. Development, first stages of in ovum, 736, 737; of Polypes, 738, 739 ; of Me¬ dusae, 740 ; of.Echinodermata, 741 ; of Entozoa, 742, 743 ; of Annelida, 744 ; of Insects, 745—747 ; of Crustacea, 748; of Cirrhipeda, 749 ; of Rotifera, 750 ; of Arachnida, 751; of Mollusca, 752; of Vertebrata, 756— 762. Diaphragm, 328; deficiency of, in Birds and Reptiles, 327 ; peculiar to Mam¬ mals, 330 ; action of, in respiration, 331—333. Diet, natural, of Man, 163 — 165. Differentiation of structure and function, 380. Digestion, several stages of, 171; gas¬ tric, 204 — 211; intestinal, 212 — 215. Digestive cavity, characteristic of Ani¬ mals, 8; different forms of, 197 — 203; formation of, in embryo, 759. Direction of action of muscles, 606 — 611. - visual objects, 558. Dislocations, 604. Distanbe, adaptation of the eye to, 550, 551 ; mode of estimating, 563—565. Distoma, development of, 743. Division of labour in living organisms, 2. Dogs, intelligence of, 717. Doris, circulation in, 290; respiration in, 316. Dorsal vessel of Articulata, 293. Double vision, 559. Draco volans, 668. Drowning, 338; treatment of, 339. Drum of the ear, 516. Duration of luminous impressions, 567. Dytiscus, 444. E. Ear, simplest forms of, 512, 514; of Man, external, 515; middle, 516, 517; in¬ ternal, 518 — 521 ; bones of, 516. Earthworm, 104, 142, 389, 400, 404, 597. Echinodermata, 118,119; circulation in, 295; luminousness of, 396; deve¬ lopment of, 741. Echinus, 118 ; teeth of, 189. Egg of Birds, structure of, 755 ; shell of, its permeability to gases, 760. Elastic fibrous tissue, 23, 29. Elasticity of arteries, 274, 275; of foot, 649; of vertebral column, 631. Elateridce, luminousness of, 397 ; leap¬ ing power of, 662. Electricity, animal, sources of, 416, 417 ; in Cat, 418; in Fishes, 419 — 423; of muscle and nerve, 424; analogy of to nervous agency, 488, 585. Electric organs, structure of, 421, 422; copiously supplied with nerves, 423. Elephant, trunk of, 172, 493; molar teeth of, 102; tusks of, 177; intelli¬ gence of, 717. Embryo, origin of, 737, 757; develop¬ ment of, 757 — 762 ; circulation in, 283, 758 , 762; respiration in, 760. Emotions, 477, 478 ; influence of on muscles, 590; on organic functions, 461. Enamel, structure and composition of, 54; arrangement of, 182. Encrinites, 118. Endosmose, action of, on blood cor¬ puscles, 232. Entomostraca , agamic reproduction of, 727, 748. Entozoa, 105; development of, 742, 743. Ephemera, 315. Ephippial eggs of Daphnia, 748. Epidermis, structure of, 38; use of, 492. Epidermic appendages, 38. Epiglottis, 193, 681. INDEX, 597 Epileptic fits, 473. Epithelium, structure of, 40 ; action of, in secretion, 41, 42, 355. Eunice, 314. Eustachian tube, 516, 517. Evaporation from surface, 370 — 373; cooling effects of, 372, 373. Excretion, objects of, 345 — 351 ; mate¬ rials of, formed in the blood, 351; effects of retention of, 351. Exhalation of moisture from the lungs, 343 ; from the skin, 371 — 374;*amount of, 374. Eye, an optical instrument, 532, 543 — 553 ; structure of, in Man, 533 — 536 ; muscles of, 538; motions of, 538, 539; aberration corrected in, 547, 549 ; adaptation of to distance, 550, 551; limits of vision by, 554 ; common sen¬ sibility of, 571 ; long and near-sighted, 552, 553; peculiar structure of, in Articulata, 573 — 575 ; deficient in some Vertebrata, 572; rudiments of, in the lower animals, 575. Eyelids, uses of, 537. F. Face, bones of, 690 — 623 ; muscles of, 624. Facial angle, 719, 720. Fat, structure and uses of, 46,412; de¬ position of, 157, 162; of blood, 232, 241. Fatigue, sense of, 595. Fertilization of ovum, 732, 734, 736. Fibre, muscular (see Muscular Fibre). Fibres, of nerves (see Nerves ). Fibrin, composition and properties of, 17,18; uses of in blood, 236 — 240. Fibrous membranes, 29. ■ - - tissues, general uses of, 12 ; for¬ mation of, 22 ; general characters of, 22 — 30 ; nutrition of, 384, 390. Fibro-cartilages, 47. Fins of fishes, 666. Fire-flies, 397. Fishes, general structure of, 88 — 91 ; teeth of, 188; circulation in, 286 ; re¬ spiration in, 317, 318; air-bladder of, 324; luminousness of, 396; heat of, 405 ; electricity of, 419 — 424; nervous system of, 453 ; organs of smell of, 509 ; vertebral column of, 629, 630; movements of, 666, 667 ; reproduction of, 754, 755, 759. Fission, multiplication of cells by, 33; of Infusoria by, 135, 725. Flea, leaping powers of, 594, 662. Flesh-fly, voracity of larva of, 141. Flight, action of, 667 — 672 ; impossible in Man, 673. Flying Fish, 667. - Lemur, 668. Flying Phalanger, 668. - Squirrel, 668. Follicles, of mucous membrane, 41. - of glands, 42, 355, 356. Food of Animals, 7, 8 ; derived from Plants, 144 — 147; from Animals, 148 — 150; chemical nature of, 153, 154, 164; mineral ingredients of, 166, 167. - demand for, 140, 141 ; economy of, 165. Food-yolk, 736, 752, 754. Foot, structure of, 648, 649. Foraminifera , 128, 131. Freezing of animal bodies, 67, 405. Frog tribe, 86, 87 ; blood-discs of, 230 ; metamorphosis of, 86, 87 ; change of circulating system in, 287—289 ; respi¬ ration of, 325 ; experiments on nervous system of, 466, 468 ; eggs of, 755 ; de¬ velopment of, 756. Fulgoridce, luminousness of, 400 ; sound emitted by, 679. Functions of living beings, 2; nutritive, 6; animal, 6; relation of organic and animal, 425 — 427. G. Gall-bladder, 362. Galloping, 660. Galvanic electricity, discovery of, 583. Ganglia, 61 (see Nervous System ). - of special sense, functions of, 475— 479. - olfactive and optic, 453 — 456. Gases, poisonous, 335, 344. Gasteropoda, 107, 108,112; palate of, 189 ; circulation in, 290; respiration in, 316; nervous system of, 438; develop¬ ment of, 752. Gastric follicles, 204. - juice, 204; properties of, 207 — 210 ; artificial, 210. Gavial, teeth of, 186. Gelatin, chemical compostion of, 19 ; use of as food, 159 ; present in blood, 380. Gelatinous nerve-fibres, 60. - principles of food, 153; des¬ tination of, 159. Gemmation , multiplication by, of Plants, 724; of Infusoria, 725; of Zoophytes, 726; of Medusae, 726, 741 ; of Echiro- dermata, 726, 741 ; of Articulata, 7i7 , 744; of Mollusca, 728; of Vertebrata, 729 ; antagonism of, to Generation, 735; (see Agamic Reproduction.) Gemmules of polypes, 738. Generation, sexual, essential nature of, 730 — 733; anragonism of, to gem¬ mation, 735 ; simplest form of, 734. Germ-cells of plants, 724; of animals, 732. Germ -yolk, 736, 752, 754. INDEX. 598 Germinal membrane, 737, 756. — — spot, 732. - vesicle, 732. Gizzard, of Birds, 200, 201 ; of Insects, 202 ; of Bryozoa, 202; of Rotifera, 202. Glands, secreting, structure of, 355—358. — mesenteric, 218; lymphatic, 219. Glaucus, 316. Globules of Blood (see Blood). Globulin, 232. Glosso-pharyngeal nerve, 459, 470. Glottis, 193, 681. Glow-worms, 398 — 401. Gluten of bread, composition of, 153. Glycine, 364. Glycocholic acid, 364. Gnat, larva of, 321. Goat -moth, larva of, 141. Goldfinch, nest of, 704. Gout, nature and cure of, 348, 349. Granulation, repair of wounds by, 392. Gravel, 348, 367. Gregory, Dr., case of, 349. Grey substance of nerves, 430, 431. Guiding Sensations, importance of, 478. Gymnotus, electricity of, 419 — 424. H. Habitual actions, 479. Hsematin, 232. Hairs, structure of, 38. Hall, Dr. Marshall, his treatment of asphyxia, 339. Hamster, instinct of, 699. Hands, use of, in prehension, 172, 173 ; in locomotion, 674; structure of, 641 — 644. Hare, leaps of the, 661. Hartz forest, devastated by Insects, 147. Haversian canals of bone, 49, 50 ; forma¬ tion of, 52. Head, definition of, 111 ; bones of, 617 — 623 ; muscles of, 624. Healing of wounds, 391, 392. Hearing, sense of, 510 — 524; improved by cultivation, 525. Heart, 245 ; structure of, in Man, &c., 256, 257 ; respiratory and systemic, 281; action of, 269, 270, 581, 583; valves of, 272, 273 ; number of pulsa¬ tions of, 271; development of, 758, 762. • - structure of, in Reptiles, 284 ; in Fishes, 286 ; in Mollusca, 290 ; in .Cephalopoda, 291; in Crustacea, 292; in Insects, 293. Heat, sustainable by Animals, Introd., 372, 373. - generated by Animals, 403 — 415; of Invertebrata, 404; of Fishes, 405; of Reptiles, 406; of Birds, 407; of Mammals, 407 ; of Man, 407 ; of young animals, 408, 409 ; of Insects, 410, 411. Heat, animal, dependent on combustion of carbon and hydrogen, 412, 413; on supply of oxygen, 413 ; maintained by respiration, 414; influence of nervous system on, 415. Hemispheres of Brain, 449. Herbivorous animals, 144 — 147 ; nutri¬ tion of, 162 ; teeth of, 179, 182. Hiccup, 341. Hinge-joint, 603. Hippuric acid, 367. Holothuria, 118, 119; reparative power of, 389; development of, 741. Horse, foot of, 652 ; intelligence of, 695. Howling Monkeys, 684. Humble-bee, heat produced by, 410. Hunger, sense of, 140, 205. Hyalaea, 112. Hybernation, 309. Hydatina, multiplication of, 750. Hydra, 121; referred to, 131, 296, 577; propagation of, by buds, 122, 726; by artificial division, 122; by eggs, 123, 734 ; development of ovum of, 738. Hydrogen, combustion of, in animal body, 343. Hydropathic system, 374. Hydrophobia, 474. Hydrozoa, 124, 125; development of, 726, 738. Hyoid bone, 625, 680. Hysteric disorder, 474. I. Iliac bones, 645. Images, formation of, by lenses, 531 ; on retina, 543, 544. Imago or perfect insect, 745. Immortality of the soul of man, 721, 722. Impressions on nervous system, 432,486. Incisor teeth, 181, 183. Infants, necessity of warmth to, 408, 409. Infusoria , 133 — 135 ; multiplication of, 725. Ink, of cuttle-fish, 359. Insalivation, 179, 190, 191. Insects, general characters of, 97 ; diges¬ tive apparatus of, 202 ; circulation in, 293 ; respiration in, 308, 321, 322 ; repa¬ rative powers of, 389 ; secreting appa¬ ratus in, 358; luminousness of, 397 — 401 ; heat of, 410, 411 ; nervous system of, 440 — 446 ; instincts of, 483, 484, 667 — 716; antennae of, 498,499; eyes of, 573 — 575 ; muscular power of, 594, 662 ; wings of, 670 ; production of sounds by, 676, 679 ; reproduction of, 745, 746. Instinctive actions, 692 ; predominance of, in Articulata, 96 ; characters of, 694; examples of, 696 — 716; corre¬ spondence of, with ganglia of special sense, 475 — 479; irrationality of, 709. INDEX. 599 Intelligence of Vertebrata, 73, 483— 485* 694; examples of, 695, 717; corre¬ spondence of, with development of the Cerebrum, 452, 692, 718—720. Intervertebral substance, 631. Intestinal juice, 213. - tube, motion of aliment through, 215 ; digestion continued in, 213, 214; relative length of, 213. - Worms, 105 ; development of, 742, 743. Invertebrata , 92 ; absorption in, 225 ; nature of circulating fluid in, 225, 235 ; heat of, 404 ; skeletons of, 598, 599. Involuntary movements, 589, 590. Iris, 533, 534. Iron, a constituent of animal bodies, 166, 167; of red corpuscles of blood, 232. Ivory, structure and composition of, 54. lulus, 103. J. Jaw, motion of, in Quadrupeds, 138; in Man, 189; articulation of, 623. Jelly-fish, 120. Joints, 603; dislocation of, 604. K. Kangaroo, leaping powders of, 661 ; skele¬ ton of, 661. Kidneys, structure of, 357, 358, 368, 369 ; purposes of their excretion, 346—348, 367. L. Labyrinth of ear, 519. Lachrymal apparatus, 540. - gland, 540. - sac, 540. Lacteals, 217, 218. Lactic acid, 349, 367. Lacunae of bone, 50. Lamprey, 317; chorda dorsalis of, 53, 757. Lampyridae, 397 — 399. Land-crabs, 315. Lantern-flies, 400. Larva, of Cirrhipeds, 749; of Crab, 101; of Fchinodermata, 741 ; of Entozoa, 742 ; of Insects, 97, 141, 745 ; of Medusae, 740. Larynx, 192; structure of, 680, 681; action of, 682—684 ; in Birds, 685. Laughing, act of, 341. Leaping, 661, 662. Leech, 104, 105. Leg, bones and muscles of, 647. Lepidosiren, 81; blood-discs of, 230; respiration of, 324. Leverage of bones, 612 — 615. Life, maintained by continual change, 68. Ligaments, structure of, 29. - vocal, 681 — 684. Light, emitted by living animals, 394 — 402 ; by dead bodies, 402. — — propagation of, 526 ; refraction of 527—532. Lime, amount of, in bones, 49; in teeth, 54; in egg-shell, 169; in shells of Mollusks, 169; in shells of Crustacea, 170. - sources of, in animal bodies, 166 — 170. Limulus, 100. Lingual nerve, 500. Liquids, reception of, 173. Liquor sanguinis, 229, 232, 236 — 241, 385, 387, 391. Lithic acid, 346, 348, 367. Liver, structure of, 356, 358, 363; circu¬ lation in, 267, 363 ; assimilating action of, 224; secreting action of, 364 — 366; formation of sugar by, 366. — objects of its excretion, 346, 350. Living beings, distinctive characters of, 1—5. Lizard tribe, 84; reparative powers of, 390. Lobster, 100; circulation in, 292. Lock-jaw, 473. Locomotion, reflex movements of, 471 ; organs of, 596. Locusts, voracity of, 148 ; multiplication of prevented, 149. Long-sighted eyes, 552, 553. Luminousness, animal, 394-400 ; uses of, 401 ; from decomposition, 402. Lungs, rudimentary in Fishes, 324 ; in Reptiles, 325 ; in Birds, 326, 327 ; in Mammals, 326 — 333* Lymnseus, parasites of, 743. Lymph, coagulable, 391. Lymphatics, 219, 220. M. Mactra, 113. Madrepore, 127. Malapterurus, electric, 419, 422. Malpighian bodies of kidney, 369. Mammals, general structure of, 77 ; di¬ gestive apparatus in, 197 — 199 ; blood- discs in, 229; circulation in, 281, 282; respiration in, 328 — 333 ; heat of, 407 ; nervous system of, 456 ; reproduction in, 756, 761. Mammary glands, structure of, 376. Man, food of, 163; stomach of, 197; heart of, 256, 257 ; arterial system of, 258; quantity of air required by, 334 —337 ; reproduction of lost parts in, 390; repair of injuries in, 391—393; heat of, 407 ; nervous system of, 456 — 462 ; peculiar characters of soul of, 721, 722. Mantis, actions of, 444. Mantle of Mollusca, 107. Marmot, hybernation of, 309. Marrow of bones, 49. - Spinal (see Spinal Cord). 600 INDEX. Mastication, 171, 174 — 189. Mastodon, teeth of, 182. Measles of pork, 742. Medulla oblongata, 450, 460. Medusce, 120; development of, 125, 740 ; circulation in, 296 ; rhythmical move¬ ments of, 578; gemmation of, 726. Membrana Tympani, 516. Membrane, basement or primary, 31. Membranes, fibrous, 29 ; serous, 28, 43 ; mucous, 39 — 41. Mesentery, 217. Mesenteric glands, 217. Metamorphosis of Frog tribe, 86, 87. - - Insects, 97, 745. - Crustacea, 101. Milk, different classes of aliment con¬ tained in, 158; chemical composition of, 377 ; influence of mind on, 353. Milk-teeth, 184. Mineral ingredients required by Ani¬ mals, 166— 170. Mitral valve, 272. Molar teeth, 181 — 183. Mollusca, general characters of, 106 — 110; circulation in, 290; respiration in, 316, 320 ; structure of liver in, 356 ; of kidneys, 358 ; luminousness of, 396 ; nervous system of, 435—439 ; develop¬ ment of, 752. Monkey, interior of, 77. Monstrosities by excess, 729. Mortality under different circumstances, Introd. Mucous membranes, general structure of, 39—41. Mulberry mass of ovum, 736, 7 37. Muscles, general purposes of, 10 ; general mode of action of, 605 — 615 ; of eye, 538; of face, 624; of trunk, 637; of arm, 638, 640 ; of hand, 641 ; of leg, 646, 647; of foot, 648. Muscular Contraction , 58, 59, 579 ; con¬ ditions of, 591 ; stimuli to, 580—586; influence of electricity on, 583 — 585 ; relation of, to nervous power, 586 — 588, 592, 593 ; voluntary and involun¬ tary, 589, 590; energy of, 592 — 594; use of, in organic functions, 595 ; in locomotion, 605 — 615. Muscular Fibre, structure of, 55 — 57 ; contraction of, 58 ; alternates with re¬ laxation, 58, 59. Musk, odour of, 504. Mygale, nest of, 700. Myriapod a, general structure of, 103; nervous system of, 440. N. Nails, structure of, 38. Nais, spontaneous fission of, 727. Near-sighted eyes, 552, 553. Necrophorus, instinct of, 703. Negro, skin of, 375. Nemestrina, trunk of, 173. Nepa, tracheal system of, 322. Nereis, 104, 314, 727. Nerita, palate of, 189. Nervous System, general structure of, 483; general objects of, 9, 10, 429 — 432 ; form of, in Vertebrata, 72; in Articulata, 94; in Mollusca, 110; in Radiata, 116. - - particular structure and actions of, in Radiata, 434; in Mollusca, 435 — 439 ; in Articulata, 440 — 446 ; in highest Invertebrata, 447, 448; inVertebrata, 449 — 452; in Fishes, 453; in Reptiles, 454; in Birds, 455; in Mammalia, 456 ; in Man, 457 — 462. - Sympathetic, 461,462. - - - the instrument of the mind, 427 ; influence of, on secretion, 190, 353 ; on muscular contraction, 584, 585 ; on animal heat, 415. Nervous Tissue, white or fibrous sub¬ stance of, 62, 63; distribution of, 63; grey or vesicular substance of, 61. Nests of Insects and Birds, 700 — 705, 710—714. Newt, 87. Nictitating membrane, 540. Nitrogen, absorption and exhalation of, 302. Non-azotized constituents of food, 154; destination of, 155 — 157, 162 — 165 ; effects of excess of, 350. Nose, structure of, 506, 507 ; common sensibility of, 508. Nurse-bees, 411. Nurses of Cercariae, 743. Nutrition of tissues, increased by use, 242, 589; dependent on liquor san¬ guinis, 240, 241, 385; mode of, in dif¬ ferent tissues, 384, 385 ; share of blood in, 387 ; share of blood-vessels in, 388; share of tissues in, 387 ; imperfect forms of, 386. O. Oak, caterpillars supported on, 145. Octopus, 121. Odours, 504, 505. (Esophagus, 192. Oleaginous principles, 153; destination of, 154—157, 162. Olfactive ganglia, 453 — 456, 458. - nerve, 507. Optic ganglia, 453 — 456, 458. — nerve, 459. Organic Life, 6, 425, 426; nervous system of, 461. - Functions, relation of, to ani¬ mal, 425—427; influenced by emo¬ tions, 461. Organized bodies, distinctive form of, 1; structure of, 2; consistence of, 3; chemical constitution of, 4; actions of, 5. INDEX. Organs of Sense (see Sensation , Organs of). Ornithorhyncus, 186, 664. Otolithes, 513. Ovarium, 732 ; of Bird, 754. Ovum, structure of, 732, 733. Oxygen, carried by blood-corpuscles, 235 ; by liquor sanguinis, 241 ; ab¬ sorbed in respiration, 300 — 306, 343, 346 ; consumption of, dependent on muscular action, 307 — 309. Oyster, 113, 316, 437. P. Palates of Gasteropods, 189. Palpi of Insects, 172, 503. Palsy of muscles, 586 — 588. Paludina, 112. Pancreatic fluid, use of in digestion, 213. Papillae of skin, 37, 490; of tongue, 500. Parotid gland, 356. Paxy-waxy, 29. Pecten, nervous system of, 110, 437. Pectinibranchiata, development of, 752. Pedal ganglia, of Mollusks, 437, 438; of Articulata, 446. Pelvis, 645. Penguin, 667. Pentacrinoid-larva of Comatula, 741. Perch, skeleton of, 666. Pericardium, 43. Peristaltic movement of intestines, 215, 579. Peritoneum, 43. Perspiration, 371 — 374. Pharyngeal ganglia, of Mollusks, 438; of Articulata, 446. Pharnyx, 192. Phosphorescence of the sea, 394, 395. Phosphorus in animal bodies, 166; sources of, 167; light produced by, 402. Pia mater, 458. Pigment, black, of eye, 533 ; use of, 545 Pigment-cells, 533. Pitch of sound, dependent on number of vibrations, 523, 682. Placenta of Mammals, 761. Planaria, reparative power of, 389. Plants, general comparison of with Animals, 6 — 12 ; afford food to Ani¬ mals, M4 — 147 ; resemblance of their life to organic life in Animals, 425, 426. Plastron, of Turtles, 83. Plethoric state of body, 233. Pleura, 328. Pneumogastric nerve, 459, 470. Podura, leaping power of, 662. Poisonous gases, 344. Polycystina, 132. Polype, fresh-water (see Hydra). Polypifera, 121 — 125 (see Zoophytes). Polyzoa, 115; gizzard of, 202; circula- 601 tion of, 295 ; gemmation of, 728 ; de¬ velopment of, 752. Pompilus, nest of, 703. Porifera, 136, 137; circulation in, 296. Portal system of blood-vessels, 267, 366. Poulp, 316, 448. Primitive trace, 757. Prehension, act of, 171 — 173, 643, 674. Projection, idea of, to what due, 560 — 562. Proteus, blood-discs of, 230, 231. Protozoa, 128; movements of, 5 77. Pseudopodia, 130, 131. Pterodactylus, 669. Pteropoda, 122. Pulmonary circulation, 268. Pulse, 276; influence of posture on, 655. Pupa, of insect, 97. Pupil, 553 ; dilatation and contraction of, 534. Pus, 393. Q. Quadrumana, extremities of, 643, 648, 674. Queen-bee, 712, 714, 716, 747. R. Rabbit, teeth of, 177 ; movements of, 661. Radiata, general characters of, 116, 117; stomach of, 203; nervous system of, 434. Radius, 63g. Ray, peculiar swimming of, 666. — electric, 419. Red Corpuscles of blood (see Blood- Corpuscles). Reflex actions, 195, 340, 430, 692; in Mollusca, 436, 439 ; in Articulata, 442 — 445, 693; in Vertebrata, 451; the spinal cord their instrument, 464— 474; dependent on stimuli, 466; not dependent on sensation, 467 — 469. Refraction of light, 527 — 532. Relief, perception of, 560 — 562. Rennet, action of, 15; nature of, 199. Repair of injuries, 389 — 393. Reproduction (see Development, Gemma¬ tion, and Generation). Reptiles, general characters of, 81 — 87 ; teeth of, 187; blood-discs in, 430; circulation in, 284, 285 ; respiration in, 325 ; importance'of skin, as respiratory organ in, 325 ; heat of, 406 ; nervous system of, 454; vertebral column of, 629 ; reproduction of, 75’6 — 760. Republican Grosbeak, 710. Resistance, sense of, 496. Respiration, 299; use of blood-cor¬ puscles in, 235 ; necessity for, 297 ; in aquatic animals, 298 ; changes pro¬ duced by, in air, 300 — 302, 334 — 336; in blood, 303 — 306; related to nervo- muscular activity, 307 ; energy of, in 602 INDEX. Birds, Mammalia, and Insects, 308 ; small amount of, in cold-blooded ani¬ mals, 309, 310; no special provision for in lowest, 311 ; uses of cilia in, 319, 329; an excreting process, 345, 346 ; subservient to maintenance of heat, 412 — 414; in embryo, 760, 761. Respiratory apparatus of Annelida, 314 ; of aquatic Insects, 315; of Crustacea, 315 ; of Moliusca, 316, 320; of Fishes, 317, 318, 324; of Myriapoda, 320; of Insects, 321 — 322; of Arachnida, 323; of Reptiles, 325 ; of Birds, 326, 327 ; of Mammals, 328 — 333; of embryo, 760, 761. - movements, 331 — 334, 340 — 342. - surface, extension of, 312 ; pro¬ longation of, externally, into gills, 313; internally into lungs, 313. - system of nerves, in Articu- lata, 446; in Moliusca, 437, 438; in Vertebrata, 450. Rete mucosum, 38. Retina, 535; yellow spot of, 554; in¬ sensible spot of, 554. Rhizopods, 129 ; substance of, 64. Rhythmical movements, 578, 581. Ribs, 633. Rodentia, teeth of, 177. Rooks, benefit of, 148. Rotifera , 105; reproduction of, 750; drying up of, 66. Ruminating Animals, stomach of, 198 ; foot of, 652. Running, act of, 660. S. Saccharine aliments, 153 ; destination of, 155 — 157, 162; conversion of, into olea¬ ginous, 156. Sacrum, 624, 645. Salamander, 732. Saliva, secretion of, 190; union of with food, 191. Salpa, reproduction of, 728. Salt, use of, 166, 167. Sandhopper, 100. Sanguification, 222 — 224. Sarcode, 128. Saunderson, case of, 496. Scapula, 634, 635, 637. Sclerotic coat, 533. Scurvy, 165. Seal, 664, 665. Sebaceous follicles, 38, 375. Secretion, general nature of, 245 ; act of, performed by cells, 42, 354 ; distin¬ guished from excretion, 352 ; influence of mind upon, 353 ; transference of, 361. Secreting follicles, 355, 356. - membranes, 355. - tubes, 357, 358. Segmentation of yolk, 736, 756. Semicircular canals, 518, 520. Semilunar valves, 273. Sensation , 432, 486 ; organs of, 9 ; general, 487 ; special, 488, 489 ; dependent on supply of blood, 63, 487 ; modes of exciting, 487, 488. Sensori-motor actions, 430. Sensorium, 429, 486. Sensory ganglia, 452; functions of, 475 — 479. Serous membranes, structure of, 28 ; arrangement of, 43. Serpents, 85, 203 ; lung of, 325 ; verte¬ bral column of, 629. Serpula, 314. Serum of blood, 238. Shark, teeth of, 188; brain of, 453. Shell of Moliusca, 106 ; of Crustacea, 99, 170; of Bird’s egg, 755. Siamese twins, 729. Sighing, 341. Sight, sense of, 526 — 57 5 (see Vision). Silk-worm, voracity of larva of, 141. Single Vision, 559. Sitting posture, 654, 655. Size, visual estimate of, 566. Skeleton , position of, in different animals, 598; internal, of 'Vertebrata, 71, 599; external, of Articulata, 93, 598; of Moliusca, 106, 598; of Radiata, 118', 124, 127, 131, 132, 598; of Man, 616; of Camel, 644; of Bird, 669; of Perch, 666 ; of Kangaroo, 661 ; of Seal, 664; of Dugong, 664 ; of Bat, 669 ; of Ptero- dactylus, 669. - articulation of pieces of, 601 — 605. Skin, structure of, 36 — 38 ; exhalation from, 370 — 374; secretions from, 375; sensory papillae of, 37, 490 ; sensibility of, 491—495. Skull, bones of, 617 — 619. Sloth, peculiar arterial distribution in, 264. Slug, 106, 107. Smell, sense of, 504 — 509 ; concerned in taste, 501. Snail, 106, 107; torpidity of, 67; respi¬ ration of, 320 ; reparative power of, 389. Sneezing, 342, 508. Sobbing, 341, Societies of animals, 706 — 711. Song of animals, 686. Soul of Man, 721, 722. Sounds, propagated by vibrations, 510 — 512; produced by insects, 676 — 679; by larynx, 682; pitch of, dependent on number of vibrations, 523, 682. Spatangus, 142. Spectacles, choice of, 553. Speech, articulate, 686—691. Spermatozoids, 731. Sperm-cells, of plants, 724 ; of animals, 730, 731. Spherical aberration, 546, 547. Sphinx atropos, sound produced by, 678. - - ligustri, nervous system of, 441. INDEX* 603 Spiders, 98; circulation in, 293; respi¬ ration in, 323 ; nervous system of, 447; instincts of, 698, 700. Spinal column, 71, 626 — 632. Spinal cord, 72, 451, 460; independent powers of, 464 — 474; nerves, 451, 457, 460. Spiracles of Insects, 320 — 322. Spleen, uses of, 224. Sponge, 136, 137 ; circulation in, 296. Stammering, 691. Standing posture, 650 — 654. Star-fish, 116 — 119; reparative power of, 389 ; nervous system of, 434 ; develop¬ ment of, 741. Sternum, 633. Stereoscope, 561. Stomach, need of in animals, 8 ; form of, 197; in Ruminants, 198, 199; move¬ ments of, 206. Stomato-gastric system of nerves, 447— 450. Stork, 653, Strychnia, action of, 474. Sturgeon, continued action of heart of, 583. Sucking, act of, 172, 472. Suffocation, 338, 339. Sugar, formation of by liver, 366. Sulphur in animal bodies, 166; sources of, 167. Supra-renal capsules, 224. Sutures, 602. Swallowing, act of, 192 — 196. Swimming, act of, 663 — 666. Symmetry of disease, 380. Sympathetic system of nerves, 60, 61, 461, 462. Syncope, 271. Synovial membranes, 44. Syntonin, 16. T. Tadpole, 95 — 97; circulation in, 287— 288. Tailor-bird, nest of, 705. Tape-worm, 105 ; development of, 742. Tardigrada, drying-up of, 66. Taste, sense of, 499, 503. Taurine, 364. Tauro-cholic acid, 368. Teeth , structure of, 54 ; development of, 174; cutting of, 175; cessation of growth of, 176; continued growth of, 177; structure of, ‘178 — 180; different kinds of, 181 — 183; first set of, 184; motion of, in mastication, 178 — 180. Teething, convulsions of, 174, 473, 474. Tellina, 316. Temperament, 718 Temperature, sense of, 497. Tendons, structure of, 29; attachment of muscles by, 605. Testacella, 106. Testis, 731. Tetanus, 380. Thalami optici, 458. Thigh, bone and muscles of, 646. Thoracic duct, 221. Thorax, 328; movements of, 332. Thumb, uses of, 643; reproduction of, 390. Thunny, temperature of, 405. Thymus gland, 224. Thyroid cartilage, 680. - gland, 224. Timbre of Sounds, 524. Tissues , of Animals, distinctive pecu¬ liarities of, 10 — 12; chemical composi¬ tion of, 13 — 21; fibrous, 22 — 30; mem¬ branous, 37 — 45; osseous, 49 — 54; cel¬ lular, 32 — 36, 46 — 48 ; muscular, 55 — • 59; nervous, 60 — 63; degeneration of, from want of use, 58 ; continual decay ■ and renewal of, 67, 68; self-formative power of, 382, 387 ; reproduction of, 390. Tongue, nerves of, 501 ; mechanical uses of, 503. Torpedo, electricity of, 419, 421. Torpidity of animals, 66, 309. Tortoise-shell, 92. Tortrix, nest of, 701. Touch, sense of, 490 — 499. Tracheae of Insects, 321, 322. Tranference of secretion, 361. Transfusion of blood, 239. Trematode Entozoa, 743. Tricuspid valve, 272. Tridacne, 119. Tritonia, 316. Trunk of Elephant, 172. - Insects, 173. Tubercle, nature of, 386. Tunicata, 114; circulation in, 295; re¬ spiration in, 3 16 ; luminousness of, 396 ; nervous system of, 435, 436; gemmi- parous reproduction of, 728 ; develop¬ ment of, 752. Turbo, anatomy of, 108. Turnip-fly, voracity of, 147. Turtle tribe, 83. Tympanum, 516, 517. U. Ulna, 639. Ungkaputi, 674. Unity of Design, 261, 763. Urea, 346, 367. Ureters, 362. Uric acid, 346, 348, 367. Urinary apparatus, 368, 369. • - - bladder, 362. - - excretion, purposes of, 346 — 348; effects of suspension of, 351; composition of, 367 ; water discharged by, 369. Uterus, 761. 604 INDEX. V. w, Valves of heart, 272, 273; of veins, 279. Vascular area, 758. Vegetative repetition of parts, 2. Veins, 246 ; structure of, 248 ; pressure of blood in, 249; arrangement of, 250, 266; flow of blood through, 277, 278; valves in, 279. Vena cava, 266. Vena portae, 267, 366. Ventilation, importance of, 336, 337. Ventricles of brain, 458. - of heart, 257 ; action of, 270. Ventriloquism, 525. Vertebrae, structure of, 71, 628; classifi¬ cation of, 626 ; number of, 627 ; con¬ nexion of, in Reptiles and Fishes, 629; in Man, 630; in Birds, 630. Vertebral column. 70, 71, 626 — 632 ; first development of, 757. Vertebrata, general characters of, 70 —76; nervous system of, 449 — 452; skeleton of, 599 ; gemmation in, 729 ; embryonic development of, 757 — 762. Vessels, origin of, 393. Vestibule of ear, 518, 521. Vibrations, sonorous, 510 — 512; pitch determined by number of, 523, 682. Villi of mucous membrane, 41 ; absorp¬ tion performed through, 41, 217. Vision, dependent on light, 526, 542; adaptation of eye to distinct, 543 — 553 ; influence of attention on, 555 ; infe¬ rences drawn from, 556 — 566 ; duration of impressions, 567 ; distinction of colours by, 568 — 570; erect, though picture inverted, 558 ; single, with two -eyes; 559; double, 559. Vitality, independent, of parts of organ¬ ism, 65. Vitreous humour, 536. Vocal cords, 681. Voice , confined to Vertebrata, 680 ; how produced in larynx, 682 ; differences in pitch and quality of, 683, 684. Voltaic electricity, discovery of, 583, 584. Voluntary movements, 589, 590. Vorticella, reproduction of, 725. Vowel sounds, 689, 690. Vulture, skeleton of, 668. Walking, act of, 657. Warm-blooded animals, 407 — 409. Wasps, nest of, 711. Waste of the system, 160, 307, 345. Water, passed off* by kidneys, 369 ; ex¬ haled from lungs, 343, 344 ; from skin, 370—374. Water-Newt, 87. Wax formed from sugar only, 155. Webs of Spiders, 698. Whale, mouth of, 185; peculiar arterial distribution in, 265; sensibility of surface in, 491; blow-holes of, 509; propulsion of, in water, 665, 666. Whalebone, 185. Wheel-animalcules, (see Rotifer a). ) White fibrous tissue, 23 — 29. White of egg, 14, 7 55. Wings, of Birds, 78, 668 ; of Bat, 669 ; of Pterodactylus, 669; of Insects, 670; action of, 667 — 672. Winter eggs of Hydra and Rotifera, 735. Wounds, healing of, 391 — 393; of ar¬ teries, treatment of, 277. Wren, intelligence of, 717. X. Xylocopa, nest of, 703. Y. Yawning, 341. Yellow Fibrous tissue, 23 — 29. Yellow spot of retina, 554. Yolk of egg, 733, 736, 754. Yolk-bag, 733, 754. Young animals, heat of, 408, 409. Z. 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