ko.A dm. /M ': Boston Medical Library Association, 19 BOYLSTON PLACE, POPULAR CYCLOPAEDIA NATURAL SCIENCE ANIMAL PHYSIOLOGY. WILLIAM B. CARPENTER, M.D., AUTHOR OF " PRINCIPLES OF GKNERAL AND COMPARATIVE PHYSIOLOGY," AND " PRINCIPLES OF HUMAN PHYSIOLOGY." LONDON : Wm. S. ORR and Co., PATERNOSTER ROW. W. AND R. CHAMBERS, EDINBURGH ; AND W. CURRY AND Co., DUBLIN. MDCCCXLIII. LONDON : LAUBURY AND EVANS, PRINTERS, WHITBFR1ARS. TO SIR JAMES CLARK, BART., M.D., F.R.S. physician in ordinary to the queen and to prince albert, etc., etc. My dear Sir James, I cannot more appropriately inscribe this Treatise, having for its object the general diffusion of sound Physiological knowledge, than to one, whose Professional eminence is founded on his enlightened application of it to the prevention 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. I remain, my dear Sir James, Your obliged Friend and Servant, WILLIAM B. CARPENTER, Bristol, Aug. 27, 1843. PREFACE. The want of a good Elementary Treatise on Animal Physi- ology, has been very greatly felt by those, who have desired to gain a general acquaintance with the Science, or who are entering upon the professional study of it. Indeed, the Author is inclined to believe, that one cause of the almost complete exclusion of the subject from the Educational system of this country,* may be traced to that deficiency. In France it has long been otherwise. A competent knowledge of Animal Physiology and Zoology is there required from every Candidate for University honours ; and men of the highest scientific reputation do not think it beneath them, to write elementary books, for the instruction of the beginner. The general plan of this volume, is the same with that of the Treatise on Physiology, contributed by M. Milne-Edwards, one of the most eminent Naturalists in France, to the " Elementary Course of Natural History," adopted by the French Government as the Text-Book of instruction, in the Colleges connected with the University of Paris. It has the advantage of possessing the same admirable and beautifully-executed series of Illustrations, as those which have been prepared for that work, together * The University of London have introduced it into the course of study required for the degree of Bachelor of Arts. PREFACE. with many additions ; and a continuation of the same will appear in the Treatise on Zoology, which will be next in order of publication. In the execution of the details, however, much difference will be found. Whilst careful to omit nothing of any real import- ance in the work of M. Milne-Edwards, the Author of the present Treatise has found it preferable to give to it a character altogether distinct. The increased size of the volume (which is twice the bulk of the " Anatomie et Physiologic") admitted of the intro- duction of a large amount of matter, which he believes will be found of great interest and importance ; and to have interwoven this with a formal translation of the French Treatise, would have been a much greater labour than the composition of an original work ; whilst he ventures to think that its result would have been less satisfactory. Moreover, there is a considerable difference in the character and objects of the two Treatises. That of M. Milne-Edwards is almost entirely composed of details ; whilst the Author of the following Volume has been desirous to make it conformable, as far as possible, to the plan of the Series of which it forms part, by combining with these as many general principles, as the present state of the Science might warrant his introducing. It has been his constant endeavour to make his Treatise interesting to the intelligent reader, by stating, not only what is, but why it is so. And if there should seem a needless repetition of certain principles or facts, which are often referred to in the course of the volume, he would observe, that he has found the advantage, in his labours as a Teacher, of frequently bringing these promi- nently before his Pupils ; and that he has therefore considered, that it would be advantageous for his Readers also. Although this Volume cannot be regarded as an abridgment PREFACE. Vll of either of the Author's larger works (Principles of General and Comparative Physiology, and Principles of Human Physiology,) yet it necessarily contains much in common with them, and may be advantageously used as an introduction to them. He has been desirous of putting forward in this Treatise, such general views only, as are entitled to take rank among the established principles of the science ; and he has consequently avoided all reference to many interesting questions, which will be found touched upon in his larger works ; and has excluded everything of a controversial character. Acting upon this plan, he has thought it right to include the substance of an Essay of his own, on the action of Cells in the Animal body, which has been characterised by an able Critic as " commanding the highest respect of any one who knows the extreme rarity of the power of systematic and comprehensive generalisation, and can value every approach to the explanation of various and complicated phenomena, by reducing them to a few comparatively simple principles";* whilst he has admitted, to a very limited extent only, the recently promulgated chemical doctrines of Liebig, many of which, although they present a specious probability, will be found to have a very limited application, and to be, in consequence, un-entitled to take rank as established principles. 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 Preserving 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, * Westminster Review, August, 1843, p. 247. Vlll PREFACE. of the Great First Cause; which are more obvious, although, perhaps, not really more clear and valid, in this Science, than in any other. Believing, as he does, that these inferences are more satisfactorily founded upon the principles, than upon the facts, of the Science, — or in other words, upon the general manifesta- tions of Laid 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 a Special Treatise on Natural Theology, to build up the applications. CONTENTS. PAGE INTRODUCTION 1 CHAPTER I. ON THE VITAL OPERATIONS OF ANIMALS, AND THE INSTRUMENTS BY WHICH THEY ARE PERFORMED 15 STRUCTURE OF THE PRIMARY TISSUES .... 29 CHAPTER IT. GENERAL VIEW OF THE ANIMAL KINGDOM 52 DIVISION OF THE ANIMAL KINGDOM INTO CLASSES . . 67 CHAPTER III. NATURE AND SOURCES OF ANIMAL FOOD 117 CHAPTER IV. DIGESTION AND ABSORPTION 139 PREHENSION OF FOOD ........ 140 MASTICATION ......... 143 INSALIVATION ......... 154 DEGLUTITION . . . . . . . . .156 CHYMIF1CATION . . . . . . . . . 159 X CONTENTS. CHAPTER IV.— continued. NATURE OF THE DIGESTIVE PROCESS CHYLIFICATION DEFECATION . ABSORPTION OF CHYLE SANGUIFICATION PAGE 166 171 173 174 178 CHAPTER V. OF THE BLOOD, AND ITS CIRCULATION ...... 181 PROPERTIES OF THE BLOOD ..... 182 CIRCULATION OF THE BLOOD . . . . . . 192 CIRCULATING APPARATUS OF THE HIGHER ANIMALS . .198 MECHANISM OF THE CIRCULATION . . . . 208 COURSE OF THE BLOOD IN THE DIFFERENT CLASSES OF ANIMALS . . . . . . . .21/ CHAPTER VI. OF RESPIRATION .......... 235 NATURE OF THE CHANGES ESSENTIALLY CONSTITUTING RESPIR- ATION ......... 236 STRUCTURE AND ACTIONS OF THE RESPIRATORY APPARATUS ] . 242 CHAPTER VII. OF SECRETION .......... 269 GENERAL PURPOSES OF THE SECRETING PROCESS . ih. NATURE OF THE SECRETING PROCESS. STRUCTURE OF THE SECRETING ORGANS ....... 275 CHARACTERS OF PARTICULAR SECRETIONS . . ... "282 CHAPTER VIII. general review of the nutritive operations. — formation of tissues .......... 292 GENERAL REVIEW OF THE NUTRITIVE OPERATIONS . il). FORMATION OF THE TISSUES ...... 296 CONTENTS. XI CHAPTER IX. PAGE ON THE EVOLUTION OF LIGHT, HEAT, AND ELECTRICITY BY ANIMALS 303 ANIMAL LUMINOUSNESS ANIMAL HEAT ANIMAL ELECTRICITY ib. 309 318 CHAPTER X. FUNCTIONS OF THE NERVOUS SYSTEM 325 GENERAL STRUCTURE AND ACTIONS OF THE NERVOUS SYSTEM . .32/ STRUCTURE AND ACTIONS OF THE NERVOUS SYSTEM IN THE PRINCIPAL CLASSES OF ANrMALS ...... 331 FUNCTIONS OF THE SPINAL CORD. REFLEX ACTION . . . 354 FUNCTIONS OF THE GANGLIA OF SPECIAL SENSE. INSTINCTIVE ACTIONS ......... 362 FUNCTION OF THE CEREBELLUM. COMBINATION OF MUSCULAR ACTIONS .......... 365 FUNCTIONS OF THE CEREBRUM. — INTELLIGENCE AND WILL . 366 CHAPTER XL ON SENSATION, AND THE ORGANS OF THE SENSES SENSE OF TOUCH .... SENSE OF TASTE .... SENSE OF SMELL ..... SENSE OF HEARING .... SENSE OF SIGHT .... 369 371 378 381 385 397 CHAPTER XII. ANIMAL MOTION ......... 430 STRUCTURE AND ACTIONS OF MUSCULAR FIBRE . . . 431 OF THE APPARATUS OF MOVEMENT IN GENERAL . . . 443 DESCRIPTION OF THE MOTOR APPARATUS OF MAN . . 454 OF THE ATTITUDES OF THE BODY, AND THE VARIOUS KINDS OF LOCOMOTION ... ..... 478 Xll CONTENTS. CHAPTER XIII. PAGE OF THE VOICE ' . . 502 CHAPTER XIV. OF INSTINCT AND INTELLIGENCE 516 MANIFESTATIONS OF INTELLIGENCE ... ... 538 CHAPTER XV. OF REPRODUCTION ......... 544 INTRODUCTION. The importance of the study of Animal Physiology, as a branch of General Education, can scarcely be 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 on which the due performance of these actions, — in other words, his health, — depends, would have been an object of universal pursuit. That it has not hitherto been so, may be 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 curiosity of the vulgar, whilst the movements of the sun, moon, and planets are regarded by them 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 ex- plain, 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 be born, to grow, to be subject to occasional disease, to decline, to die, is his lot in common with other animals; and what knowledge can avail (it may be asked) to avert the doom imposed on him by his Creator ? '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 pain and weakness, that might have been prevented by a simple attention to those principles, which it is the province of Physiology to unfold. From the moment of his birth, the infant is almost entirely dependent upon the condition in which he is placed, for the future development of his frame ; and 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 become weakly, fretful, and self-willed, a source of constant discomfort to himself and to others ; or form one of that vast proportion, whose lot it is to be removed from this world, before infancy has expanded into childhood. The due supply of warmth, food, and air, are the principal points then to be attended to ; and on every one of them 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 sustaining its own temperature, and is con- sequently 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 quantity 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 careless- ness. And, lastly, one of the most fertile sources of infantile disease, 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 INTRODUCTION. 3 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 diseases of in- fancy and childhood ; and those whose native vigour of consti- tution 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, care- fully 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 con- ditions for the support of life, — the warmth, food, and air, on which the body depends for sustenance, no less than for its early development, — 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 on upon the most extensive scale. "Wherever we find the conditions, which the Physiologist asserts to be most favourable to the preservation of the health of the body, most completely fulfilled, there do sick- ness 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 1 in every 4J before the end of the b2 4 INTRODUCTION. 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 23 deaths in every 24 infants, under the age of one year. For a long time, this frightful devastation was allowed to go on as 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 2600 to 450 in a year. Here, then, was a total of 2150 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 their own negligence; but against the ignorance, indifference, or cruelty of man. And what a lesson of vigilance 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 birth /"* A recent visitor to the island of St. Kilda, the most northern of the Hebrides, states that 8 out of every 10 children die between the eighth and twelfth day of their existence ; in consequence of which terrible mortality, the population of the island is diminish- ing rather than increasing. This is due, not to anything injuri- ous in the position or atmosphere of the island ; for its " air is good, and the water excellent :" but to the li 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 ; . * Dr. A. Combe on the Physiological and Moral Management of Infancy. INTRODUCTION. 5 whereas, according to the average mortality around him, at least three out of the four would have been dead within the first fort- night. One of the most terrible instances ever recorded, of infant mor- tality resulting from mismanagement, is that which occurred at the end of last century, in the Dublin Foundling Hospital. During the space of 21 years, ending in 1796, out of 10,272 sick children sent to the infirmary, only 45 recovered. In this case, not only deficient ventilation and improper food, but the most criminal treatment, was concerned in the fearful result. The children were not provided with nurses, but were fed by hand ; "when they cried and became troublesome, they were dosed with laudanum to keep them still; and the laudanum did succeed in keeping them still, for many of them never awoke." In another institution in Dublin, a most important improvement has been effected by simple attention to cleanliness and ventila- tion. At the conclusion of 1782, out of 17,650 infants born alive, 2944, or nearly every sixth child, died within the first fortnight. By the employment of additional means of ventilat- ing the wards, the number of deaths was speedily reduced to only 419 out of 8033, or about one in J 9 J, instead of one in every 6 ; and it has recently been still further diminished. A vast number of facts of a similar kind might be brought together, all proving the same thing. It may be sufficient to add the following statement of the comparative number of deaths of children under five years of age, in London, during successive periods of 20 years; as proving the benefit derived from increased attention to the physiological conditions requisite for health. In the 20 years subsequently to 1730, out of every 100 children born, 74^, or nearly three out of four, died before they were five years old. In the succeeding 20 years, the proportion of deaths was reduced to 63 in 100, or less than two-thirds. Between 1770 and 1790, it was only 51.1 in 100, or little more than one- lialf. In the 20 years succeeding 1790, it was farther reduced 6 INTRODUCTION. to 41-i- in 100, or little'more than two-fifths. And between 1810 and 1830, it was no more than 32 in 100, or less than one- third. Now although the introduction of Vaccination has un- questionably had a share in reducing the mortality of infants, by mitigating that terrible scourge, the small-pox, yet it will be perceived that the principal diminution took place previously to the time when this came into general use, which was not until the commencement of the present century. It is not only, however, in diminishing mortality, but in pro- moting health of body and mind, that attention to the laws of life, as ascertained and applied by the physiologist, proves efficacious. A remarkable and interesting " case in point," is that of the Orphan Asylum in Albany (New York), wdiich was opened in the end of 1829 with about 70 children, the number being sub- sequently increased to 80. " During the first three years, when an imperfect mode of management was in operation, from 4 to 6 children were constantly on the sick list, and sometimes more ; one or two assistant nurses were necessary ; the physician was in regular attendance twice or thrice a week ; and the deaths amounted in all to between 30 and 40, or about one in every month. At the end of this time, an improved system of diet and general management was adopted ; and notwithstanding the disadvantages inseparable from the orphan state of the children, the results were in the highest degree satisfactory. The nursery was soon entirely vacated, and the services of the nurse and phy- sician no longer needed ; and for more than two years, no case of sickness or death took place. It is also stated that, since the new regimen has been fully adopted, there has been a remark- able increase of health, strength, activity, vivacity, cheerfulness, and contentment, among the children. The change of temper is also very great ; they have become less turbulent, irritable, peevish, and discontented; and far more manageable, gentle, peaceable, and kind to each other." * * Combe on Infancy. INTRODUCTION. 7 But this improvement has taken place, not only in regard to infant health, but also in the duration of life among adults. Notwithstanding the common impression, that the men of the present race have sadly degenerated from their ancestors, both as to bodily and mental vigour, a closer examination shows that this is a fallacy ; and that we are misled by the pre-eminence attained among their fellows, by men of athletic frames and animal courage, at the time wThen brute force commanded the chief respect and obedience. In regard to the average duration of life, which may be regarded as affording a tolerably accurate test of bodily vigour, it is unquestionable that a vast improve- ment has taken place during the last 1800 years. It has been ascertained from historical records, that the average duration of life among the ancient Romans, when compared with that of the English of the present day, was as two to three ; that is, out of thirty Romans, as many would have died in a given time, as out of forty-five Englishmen. The term of human life has undergone a considerable increase, even within the last hundred years. Not only has the average mortality of the whole civilised world decreased, in consequence of the greater care and judgment exercised in the treatment of infants and children, but the value of life — that is, the probable number of years which any one may expect to live — has considerably increased. This is proved by the fact, that the tables which were computed seventy or eighty years ago, of the average number of deaths for each year of life, and which served as the basis for the calculations of Insurance companies, are now found to have under-rated the duration of life very considerably ; the average number of deaths that would take place in a year, out of one thousand adults of any given age — say thirty-five — being much less at present than it was when those tables were constructed ; and a larger proportion, therefore, living to an advanced period of life. But when we examine the abodes of squalid poverty, and witness the filth, destitution, and wretchedness, which prevail 8 INTRODUCTION, tli ere, we cannot but feel that a yet greater improvement Is destined still to result from any measures, that shall convert these into the dwellings of a cheerful, clean, well-fed, thriving population. It appears from the examination of the tables of mortality in France, that the number of deaths per annum, among the poor, is more than twice as great, in proportion to the whole number, as it is among those in easy circumstances ; and it can scarcely be doubted that the same proportion holds good in this country. If the average duration of life, and freedom from sickness, among the poor, could be raised to the standard which prevails among the higher classes, the whole average mor- tality of this country would doubtless be reduced, by an amount %t least as great as it is already less than that of the most unhealthy countries of Europe. Whilst in England and Scotland, no more than one in fifty-eight now die every year, out of the whole population, — one in forty-five annually die in Germany, one in thirty-nine in France, one in thirty in Turkey and in Italy in general, and one in twenty-eight in the Homan and Venetian states ; so that it would almost seem that, the more favourable the climate, the greater carelessness is there respecting the other means that conduce to the preservation of life and health. It is a principle now universally admitted, that the life or vital actions of no one species of animal can be correctly under- stood, 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 Astronomer to restrict himself to the observation of a single planet, or for the Chemist to endea- vour to determine the properties of a metal by the study of those of one alone. 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 INTRODUCTION. 9 great light upon the great object of all our inquiries, the Phy- siology of Man. For 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 per- form. So that we often have to look to the 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. Hence, even if we restrict our aims to the investigation of that wonderful series of actions, of which the sum makes up the life of the Human being, we are obliged to refer to a number of other tribes, for the assistance which we gain from the comparison of their structure and opera- tions with his. But if we go further, and aim to build up Physiology as a science, and to place it on the same footing with mechanics, chemistry, or any of those sciences which are founded upon the phenomena of inorganic matter, we must consider Man but as one out of many hundred thousand tribes of living beings, all whose actions have to be studied, the whole history of their lives unveiled, and their minutest structure determined. Until this has been done, Physiology will be deficient in the exactness which it may hope ultimately to possess ; but there is much in its recent progress and present state, which encourages the hope, that the time is not far off when its claims to attention will be universally recognised. There is certainly no science which more constantly and forcibly brings before the mind the power, wis- dom, and goodness of the Creator. For 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 realise, the physiologist finds them in the meanest worm that we tread beneath our feet, or in the humblest zoophyte dashed by the waves upon our shores, no 10 INTRODUCTION. 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. The diffusion of animal life is only one degree less extensive 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 kind 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. No extremes of temperature in our atmosphere seem inconsistent with animal life. In the little pools formed by the temporary influence of the sun upon the surface of the arctic snows, animalcules have been found in a state of activity ; and the tracts of red snow, which frequently cover the surface of arctic and alpine regions for miles in extent, are formed, not merely by the little cryptogamic plant elsewhere described (Veget. Phys. §. 48), but by incalculable multitudes of certain species of animalcules, and by the eggs of other kinds. 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 the 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 have existed there. The humble and toilsome ants make these their food, and become INTRODUCTION. 1 1 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 boiling water with the preservation of animal life. Thus in a hot spring at Manilla, which raises the thermo- meter to 187°, and in another in Barbary, whose usual tempera- ture is 172°, fishes have been seen to flourish. Fishes have been thrown up in very hot water from the crater of a volcano, which, from their lively condition, was apparently their natural resi- dence. 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. In- testinal 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 organi- sation and wonderful powers. Vast flights of butterflies, the emblems of summer and sunshine, may sometimes be seen above the highest peaks of the Alps, almost 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 Chimbo- razo, where the barometer 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 Artesian well lately sunk in Paris to a depth of about 1800 feet, the water has recently 1 2 INTRODUCTION. brought up a number of small black fishes, which, being without eyes, seem to have been produced in the depths from which it springs. The object of the preceding remarks has been, in the first place, to show the importance of the study of Physiology, as leading to the knowledge of those laws, by attention to which the health of the body and mind of man may be most effectually preserved ; and, in the second, to show the extent of the field which lies open for cultivation. Their application will be made more apparent in subsequent pages. But as the Author's object is not merely to communicate the results of his own inquiries in the science, but also to stimulate his readers to observe for themselves, and thus to add to its stores, he would add a few remarks on the pleasure and advantage which the intelligent mind may derive, from even a moderate degree of attention, as opportunity serves, to the same pursuit. Every one can do something towards 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 beginner, should have the effect of stimulating his exertions. Of by far the larger part of the organised creation, 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 imperfect. Of the meanest and commonest tribes, 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 metamorphoses, more wonder- ful than those of the Insect, has been discovered in the Jelly-fish INTRODUCTION. 13 of our seas, the Barnacles that attach themselves to floating pieces of timber, and 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 nature 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 expansion 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, comparable at least to that which the man of generous benevolence 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 is not this pleasure increased, when we are made aware, — as in the study of Nature we soon become, — that the sources of them are never-ending, and that our enjoyment of them becomes more intense in proportion to the comprehensive- ness of our knowledge ? And does not the feeling that we are not looking upon the arts or inventions of a skilful human artificer, but studying the wonders of a Creative design infinitely more skilful, immeasurably heighten all these sources of grati- fication ? If it is not every one who can feel all these motives, cannot every one feel the force of some ? But it is not only in affording us such interesting objects of regular study, that the bounty of Nature is exhibited. Perhaps 14 INTRODUCTION. it is even more keenly felt by the mind which, harassed by the cares of the world, or vexed by its disappointments, 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 hreathing sweets, The melody of woods, and winds, and waters, — Till he relent, and can no more endure To be a jarring and a 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. CHAPTER I. ON THE VITAL OPERATIONS OP ANIMALS, AND THE INSTRU- MENTS BY WHICH THEY ARE PERFORMED. 1. The general characters of living beings, and their chief distinction from inert matter, have been elsewhere explained (See. Vegetable Physiology, Chap, i.) ; and it will not be requisite, therefore, to do more than recapitulate them here. — Living beings, whether Plants or Animals, are distinguished from the inert matter which is commonly said to form the Mineral kingdom, by their peculiarities of structure and of action. In a living being, no matter how simple its conforma- tion, we find two or more distinct parts or organs ', adapted for different purposes ; thus, in the simple cell which constitutes the entire plant of Red Snow, or the Yeast Fungus ( Veg. Phys. §§. 48 and 55), we have a containing membrane which absorbs liquids and gases from the surrounding elements, — a contained fluid of peculiar characters, formed out of these materials, — and a number of minute granules which are to become the germs of new cells. On the other hand, in mineral matter, the same structure and the same properties may prevail through a mass of any size. Hence the structure of living beings is said to be organised; whilst that of inert mineral matter is said to be un- organised or inorganic. 2. Again, living beings are distinguished by their actions. Continual change seems an essential part of their character ; and the alterations they undergo are not the result of accidental cir- cumstances, but the consequence of their own peculiar proper- ties, and take place with great regularity. Thus if the life of one of the simple plants just mentioned, be attentively watched, 16 FUNCTIONS OF LIVING BEINGS. a set of actions will be observed, which may be enumerated briefly as follows. The germ consists of a minute granule, in which no distinction of parts can be observed ; but this, by im- bibing water and other materials, soon enlarges ; and a distinc- tion between the containing and contained parts, the cell-wall and the cavity of the cell, is speedily observed. The enlarge- ment continues, until the full size of the individual is arrived at ; and the fluid the cell contains is then observed to have a number of minute granules diffused through it, which resemble the ori- ginal germ. These granules are at last set free by the bursting of the parent cell, which now ceases to exist, or dies; and its progeny commence life for themselves, and go through the same series of actions as those performed by the parent. These actions are termed functions ; and their number and variety cor- respond with, the number of different organs existing in tho structure. Thus in the simple beings just adverted to, we can only distinguish two sets of operations, — those by which the growth of the parent cell was effected, and those by which the germs of a new generation were produced and set free. The- former are termed functions of nutrition ; and the latter, func- tions of reproduction. •3. But it has been shown that, in the higher Plants, a large number of distinct parts or organs may be observed, — such as the root, stem, leaves, &c. ; and that these parts have distinct uses in the economy of the plant. Thus the roots, besides fixing the plant in its position, absorb or suck up liquid from the soil around ; and this liquid usually contains, dissolved in it, some of the solid particles which the plant requires as the materials of its growth. The stem has for its office to convey this liquid upwards to the leaves and flowers, where it may be exposed to the air and light. One important function of the leaves is to get rid of a large quantity of this superfluous fluid, by the pro- cess termed exhalation ; whilst these organs have also the power of absorbing additional fluid, if needed by the plant. Another function of the leaves, is that of taking in an additional most important element, carbon, from the air, by decomposing the carbonic acid it contains ; and this being combined in the interior FUNCTIONS, ORGANIC AND ANIMAL. 17 of their cells with part of the water taken in by the roots, forms the materials by which the tissues of the plant are nourished, and their growth provided for, and whence their peculiar products are supplied. Yet even these most important functions are per- formed, in the highest plant, as in the lowest, by simple cells ; for the leaf is but an assemblage of such cells, with a framework or skeleton of harder tissue ; and the action of each cell is the same as that performed by the rest. Moreover, when certain products, — such as. oil, resin, starch, &c. — are separated from the juices that have been elaborated by the cells of the leaves, and are stored up in particular receptacles, these receptacles are themselves cells, the walls of which have the peculiar property of selecting from the juices the materials they are destined to contain. 4. Now all these actions in the Plant are classed under the head of functions of organic life, being entirely concerned with the nutrition of the individual. But we have in Animals another series of actions, by which that individual is connected in a peculiar manner with the world around. All animals possess, in however slight a degree, a consciousness of what is going on around them ; — that is, they are sensible to the impressions of external objects. And they all possess, though often to an almost imperceptible amount, the power of acting on objects around them, by spontaneous motion. These two functions, — sensibility and the power, of spontaneous motion, — being peculiar to animals, are called the functions of animal life ; and they are sometimes called functions of relation, from the peculiar connection they establish between the individual and the world around. 5. The difference between an Animal and a Plant essentially consists in the presence or absence of these powers. Every being which is conscious, in however slight a degree, of its own condition, and of the circumstances affecting it, must be regarded as an animal ; and there is reason to believe, that no being pos- sesses such a property, which does not also possess some power of adapting itself to these circumstances by a movement of its body, so as to render its condition more desirable. But it is often difficult to distinguish between a spontaneous movement IS DISTINCTION BETWEEN ANIMALS AND PLANTS. of this kind, and those motions which are performed by many plants, without any consciousness or design ; and consequently it is not easy to draw the line precisely between the Animal and Vegetable kingdoms. There can be no doubt, however, that such a boundary really exists ; although we may not every- where be able to trace it. We shall find that many animals, though obviously possessing the faculty of giving motion to the individual parts of their bodies, are yet incapable of moving from place to place, being fixed to one spot during all but the earliest period of their lives ; and descending still lower, we come to beings which are not only thus rooted, like plants, to the same situation, but which have so little power of moving any part of their mass, and seem to be so destitute of sensibility, that it is difficult to imagine them to possess any distinct con- sciousness. This is the case, for example, with the Sponge. And yet there is such a resemblance in the structure of the sponge, to that of other beings whose animal character is un- doubted, that it is equally difficult to remove it from the animal kingdom; and ifc is probable that this creature, low as it evidently is in the scale of being, may be endowed with just so much sensibility as may be sufficient to give it a pleasing consciousness of existence, whilst it may be incapable of receiving painful sensations. Its selection of food appears to be no more go- verned by its will, than the same process in vegetables ; but it may possess, when satisfied, the same general sense of comfort, as that which we enjoy after a sufficient meal of wholesome food. 6. But, it may be asked, is there nothing in the structure of animals, which distinguishes them from plants ? And to this it may be replied, that in the higher animals, not only the prin- cipal organs, but the greater part of their elementary parts or tissues, are formed upon a plan so entirely different from that which prevails in plants, that there would be no danger of mis- taking the one for the other. All the arrangements of the organism or corporeal edifice of the higher animals, are made, as we shall presently find, for the purpose of enabling them to perform, in the most advantageous manner possible, those peculiar functions with which they have been endowed, — to receive DISTINCTIVE PECULIARITIES OF ANIMALS. 19 sensations, — to feel, think, and will, — and to move in accordance with the directions of the instinct or the judgment. For these purposes we find a peculiar apparatus, termed the nervous sys- tem, 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 extre- mities, and of conveying them to the central masses of nervous matter, (known in the higher animals as the brain and spinal marrow,) where they are communicated to the mind. 7. From these centres, other cords proceed to the various muscles, by which the body is moved. These muscles, com- monly known as the 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 movements of plants are pro-luced (Veget. Phys. §. 420) ; but it differs from them in being thrown into action, not only by stimuli that are applied directly to itself, but by the action conveyed through the nervous system. Thus, in an animal recently dead, we may excite any muscles to contraction, by sending a current of electricity through 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 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 will; and the will causes a certain stimulus, 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 pro- duces 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 strong desire (owing to the unfitness of the time and place) to restrain it. The emotion also produces a change in the nervous c2 20 DISTINCTIVE PECULIARITIES OF ANIMALS. 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 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. 8. In the higher Animals, then, the presence of this nervous and muscular apparatus is an essential and obvious distinction 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 to which they give motion ; and it is only in the interior of his trunk, that we find organs correspond- ing; with those which form the whole structure of the Plant. These organs of nutrition have for their main purpose, 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 continually nourished and repaired, by the materials provided by what may be termed the vegetative organs (§. 53). But in the lower tribes of animals, we do not find the animal functions to possess this predomin- ance. In fact, among the 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 func- tions, as by bringing food to the mouth, and water to the respiratory organs ; and the nervous and muscular 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 PECULIARITIES OF ANIMAL STRUCTURE. 21 sometimes altogether undiscoverable. This is the case, for example, in the lowest kinds of shell-fish, such as the oyster ; and in the coral-animals. 9. 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 they are composed. For 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 frame wofk are attached to 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 connected, but also require a certain power of gliding over one another, are bound together by a very elastic loosely-arranged tissue, consisting of fibres cross- ing and interlacing in every direction, the interstices between which are filled with fluid. Now 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 wTe descend the animal scale. On the other hand, we find the various forms of true cellular tissue, such as predominate in plants (Yeget. Phys. Chap, m.), becoming more and more abundant, as we pass from the highest to the lowest, and having more and more important duties to fulfil. But even in the highest Animals, as will hereafter appear, they are the essential instruments of the most important among the organic functions,, just as they are in Plants. 10. It is evident, from what has been already remarked as to the peculiar life to which animals are destined, that the mode in which they obtain their nourishment must be very different from that which prevails in the vegetable kingdom. Plants extend their roots through the soil in search of fluid, and spread out their leaves to the air for the purpose of obtaining its carbon ; but the animal could not so exist, and be at the same time 22 MODE OF RECEPTION OF FOOD PECULIAR TO ANIMALS. endowed with the power of moving from place to place. Hence he requires some other plan of obtaining his food ; and this is provided for, in the peculiar character of the food itself, and in the apparatus by which it is taken into his system. We have seen (Veget. Phys. Chap, vi.) that Plants will live and thrive, when supplied with the requisite amount of the four elements, oxygen, hydrogen, carbon, and nitrogen, united in the simple forms of water, carbonic acid, and ammonia, — together with certain mineral ingredients ; and that hence they can grow at the expense of the elements around, where no living being ever existed previously. But this is not the case with animals. They can only exist upon substances, which have been already combined, either by plants or by other animals, into certain peculiar compounds; and thus the whole animal kingdom is directly or indirectly dependent upon the vegetable kingdom for its support; and no animal can exist where a plant has not preceded it. 11. Now, these substances usually exist in a solid form; and since the animal, as well as the plant, is incapable of absorbing nutriment into its body in any but the fluid form, there is need of a preparatory process by which this conversion may be effected. For this purpose, animals are provided with an internal cavity, or stomachy in which this conversion is performed. This stomach is nothing else than a bag formed by the prolongation of the external covering of the body into its interior. The cavity serves to hold the food which is 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 dissolve it ; and through its walls are absorbed those portions of the food, which are capable of being applied to the nutrition of the system, while the remainder is cast forth again by the aperture of the bag. Hence the lining membrane of this simple stomach performs the same function as the general surface of the sea weeds, every part of which is endowed with the power of absorbing fluid from the surrounding elements (Veget. Phys. §. 102). And as in the higher plants, we find that the function of absorption is specially committed to a set of root-fibres prolonged from the general surface into the soil, so do PURPOSES OF THE STOMACH OF ANIMALS. 23 we find that in the higher animals it is performed by a set of vessels spread out upon the sides of the digestive cavity or stomach, which take up the nutritious fluid, and carry it to distant parts of the body. 12. Thus it is seen that the possession of a stomach is neces- sarily connected with the peculiar mode in which Animals are destined to live, and the food on which they are to be sup- ported. By it they are enabled to carry about with them supplies of food, which cannot in general be obtained constantly, but are only to be secured occasionally ; and this food they can reduce in it to the fluid form, and thus prepare it for being absorbed. It is obvious that, until it is so absorbed, it is no more within the body of an animal — though contained in its stomach — than is the fluid in the soil which surrounds the roots of plants. 13. Now, this digestive cavity, or stomach, with the organs by which the food is introduced into it, constitute the whole structure of the simplest animals. There seems reason to believe, that many of the lower class of animalcules (§. 136), are little else than single cells, differing from those of the red-snow or yeast plant chiefly in this, — that there is an aperture or mouth by which nourishment is introduced into their cavity, so that they do not absorb by their external surface only, but also by their internal, — and that they are furnished with cilia, or little hair-like filaments, by the vibration of which, they are moved actively from place to place, and their food conveyed into their mouths. And it is quite certain that animals much more highly organised, are composed of little else than an assemblage of such cells, so arranged as to form a membranous bag, with a mouth, and a circle of arms or tentacula around it, serving to lay hold of and draw in its food. 14. This is the case, for example, with the Hydra, or fresh- water polype, a little animal which is very abundant in many of our ponds, and the study of which has been a most fertile source of interest to the microscopic observer. It commonly attaches itself to a stick, straw, or other floating substance, by means of a kind of sucker at its lower extremity ; and stretches out 24 STRUCTURE OF THE HYDRA. Fig. 1.— Hydra, or Fresh-water Polype. its arms 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 arms, and are drawn into the mouth, a, which 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. A kind of circulation, or flow of nutritious fluid through channels, appears to take place even in this simple animal, for the conveyance of nourishment to the arms. The portions of the food which are not capable of being digested, are cast out through the mouth ; but in the higher po- lypes, as in all the more com- plex animals, a second aperture is provided for this purpose. — Now, that the lining mem- brane of the stomach is nothing else than an inward prolon- gation of that which covers its surface, is evident from a very curious experiment which has been many times performed upon this little animal, with the same result. It may actually be turned inside-out, as we should turn a glove or a stocking ; so that the lining of the stomach shall become the external cover- ing, and the external membrane the lining of the stomach ; and yet the functions of the animal seem to go on as if they had not been in the least disturbed. 15. All the chief functions which have been described as taking place in plants, are performed by animals also, besides those which are peculiar to the latter. Thus, the nutriment which has been absorbed in the state which maybe called the raw mate- rial, has to be converted into a substance fit for the nourishment of the tissues, and capable of being appropriated by them ; this process, resembling the " elaboration of the sap" in plants, is termed assimilation (or rendering-like). This assimilated fluid GENERAL ACCOUNT OF THE FUNCTIONS. 25 has to be conveyed into every part of the body, so as to afford it the supply which it needs for its maintenance and growth ; and the process by which it is so conveyed is termed the circu- lation. By the same means, the waste or dead matter which is being continually set free in the action of the several organs, particularly of the nerves and muscles, is conveyed away. In order to get rid of this waste, part of it is united with oxygen^ and is thrown off by the process of respiration or breathing, in the form of carbonic acid and water ; whilst part is got rid of in other ways, by the various processes of excretion, which separate the injurious substances from the Iblood, and pour them, in a fluid form, into channels by which they are conveyed out of the body. The process of respiration serves also to introduce a sup- ply of oxygen, which is needed for various purposes in the economy ; and it is connected with the maintenance of the animal temperature. Moreover, the blood is conveyed by the circulation to various organs, by which certain fluids are separated, that have particular uses in the economy, — such as the tears, the saliva, the poison of serpents and insects, the odours of many mammalia, &e. ; these secretions, as they are termed, do not seem to carry off from the blood anything injurious to it, but are~destined for other purposes. Lastly, the animal, like the plant, is endowed with the powerof reproduction; and we shall hereafter find that this process is conducted, in all animals, on a plan closely resem- bling, in all its essential particulars, that which prevails in the phanerogamic plants. __! 1 6. The tissues of animals are composed of a substance, which,. in reference to its chemical properties, is nearly the same in all. It has been mentioned, that although most plants require the element nitrogen or azote as one of the materials of their growth, this element does not enter so much into the composition of their own tissue, as it does into that of certain products which they form for the use of animals (Yeget. Phys. §. 195). Indeed, the organised tissues of plants, if completely separated from the substances they contain, are found to consist of oxygen, hydro- gen, and carbon, only ; these being united in the same proportions as the elements of starch. But all the tissues of animals, if sepa- 26 PROPERTIES OF ALBUMEN. rated in like manner from the substances deposited in them, are found to be composed of the four elements, oxygen, hydrogen, carbon, and nitrogen ; and these are for the most part united in the same proportions, as those which form the substance called albumen. This we find in blood, in the white of egg, and in most of the fluids of the animal body ; it is the form into which all the substances which contribute to the nourishment of the tissues, — such as animal flesh, the gluten of bread, &c, — are converted before they are absorbed ; and it appears to perform, in the animal economy, a part almost precisely corresponding to that of gum in the vegetable (Yeget. Phys. §, 329). It is desirable, therefore, to consider its properties before we go further. 17. Albumen may exist in two states, — the soluble and insoluble. In the animal fluids it exists in its soluble form ; and 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 egg), it gives to it a glairy tenacious character ; but it is nearly tasteless. "When such a fluid is exposed to a temperature of 158°, a coagulation or setting takes place, as in the familiar process of boiling an egg. But 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. No trace of organisation 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 important degree, from fibrin — as 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 turbidity produced by adding to it a drop or two of nitric acid, and then heating it. This is seen, too, in the ordinary process of the formation of curd in milk, which takes place when it is mixed with vinegar, acid fruits, or the acid contained in the rennet (§. 199) which is commonly used in the making of cheese. Now, when thus coagulated, albumen cannot be dissolved again by any ordinary process; but its solution may be accomplished by PROPERTIES OF FIBRIN. 27 rubbing it in a mortar with a caustic alkali, potass or soda. — We may always distinguish albumen, then, by its peculiar pro- perty of coagulating on the application of heat, or on being treated with certain acids. 18. Now it does not appear that albumen can be made use of as the material for the production of tissue, until it has under- gone a further process, by which its properties are greatly altered, though the proportions of its elements remain the same or nearly so. This alteration is manifested in its tendency to spontaneous coagulation, when withdrawn from the living body ; and in the traces of organisation which are presented in the coagulum or clot. We do not meet with this converted albumen, we fibrin, in the first products of digestion ; but it is gradually formed at the expense of the albumen which these supply ; and as it is con- tinually withdrawn by the action of nutrition, it is re-formed with equal constancy by the act of assimilation. We meet with it in the greatest abundance in the blood; and it is that portion of it which causes the coagulation or clotting of the fluid, soon after it is withdrawn from the vessels. In the circulating fluid, the fibrin is completely dissolved, — as completely as the albumen in the white of egg ; but no sooner is the fluid at rest, and with- drawn from the contact of a living surface, than it passes into its insoluble form. The coagulum thus produced, however, is far from being as simple as that which is formed of albumen ; for it has an evidently fibrous texture ; and, when examined under favourable circumstances, a distinct network of fibres, crossing each other in every direction, can be seen in it. Hence we may consider this substance as intermediate between albumen and solid tissue; and the name "chair coulante," or liquid flesh, which has been proposed for it by some French physiologists, very happily expresses its peculiar properties. The act of nutrition seems to consist in the conversion of this material into new cells, fibres, &c, in connection with those previously formed ; and the mode in which these new parts originate will be presently con- sidered. 19. Fibrin may be obtained in a separate form, by stirring fresh- drawn blood with a stick, to which it adheres in threads. 2S PROPERTIES OF FIBRIN AND GELATIN. In this condition it possesses the softness and elasticity which characterise the flesh of animals, and contains about three-fourths of its weight of water. It may be deprived of this water by drying, and then becomes a hard and brittle substance ; but, like dried flesh, it imbibes water again when moistened, and recovers its original softness and elasticity. Fibrin may be converted back, into a substance resembling albumen, in various ways. This conversion takes place in the ordinary process of digestion ; all the animal flesh, or organised fibrin, being reduced to the form of albumen, before it can be dissolved and absorbed. And a similar change may be effected by the chemist, who can re- dissolve fibrin with the assistance of nitre, and make a solution closely resembling that of albumen. When it has undergone this change, fibrin of course loses the properties which peculiarly distinguish it ; and these properties, being entirely different from those of any ordinary chemical compound, and being observed in it only after being subjected to the action of the living tissues through which it flows, are called vital. 20. The greater part of the animal tissues seems composed of solidified fibrin ; that is, they agree with it in the proportion of the elements of which they are composed, and in the manner in which they are acted on by acids, alkalies, &c. But a variety of different substances may be deposited in these tissues, and may give them very peculiar characters. Thus mfat we have a col- lection of cells filled up with oily matter ; in bones and teeth, a large quantity of mineral matter is deposited ; and in the horny tissues, there is a similar deposit of a horny substance. Neither of these deposits present any organisation in themselves. There is one other substance, however, which exists largely in the ani- mal body, and the condition of which is not yet exactly known. This is gelatin, the substance commonly known as glue. It is characterised by being always soluble in water, especially with the aid of heat ; and by the thickening of its solution on cooling; so that, if a sufficiently large quantity be dissolved, it sets firmly. It is also distinguished by the mode in which it is acted upon by tannin, for a peculiar compound is formed by the two, which falls down in copious flocks when their solutions are mixed ; and MATERIALS OF ANIMAL ORGANISM. 29 the formation of this compound, which takes place when a piece of skin or other suhstance largely containing gelatin is immersed in water containing oak-bark, nut-galls, &c. (Veget. Phys. §. 364) is the change on which the conversion of skin into leather de- pends. Now gelatin may be obtained in large quantity, by boiling the skin, bones and cartilages of animals ; but it is Un- certain whether it always exists in them as such, or whether some chemical change is not occasioned by long boiling, which produces gelatin as its result. 21. The number of combining equivalents, or proportionals, of the four elements, which enter into albumen and gelatin respectively, are as follows : — - OXYGEN. HYDROGEN. CARBON. NITROGEN. Albumen 14 36 48 6 Gelatin. 18 41 48 7h The actual quantity of each element by weight in 1000 parts of the whole, is, OXYGEN. HYDROGEN. CARBON. NITROGEN, Albumen 213 68 557$ 16H Gelatin 246 70 502 182 Whence it appears that gelatin contains more oxygen and nitrogen (or azote), and less carbon, than albumen. Small quantities of sulphur and phosphorus are found in both fibrin and albumen ; but they cannot be traced in gelatin. Structure of the Primary Tissues. 22. The primary tissues of which the various organs of ani- mals are composed, seem divisible into two classes ; — 1st, Those which are formed simply by the consolidation of fibrin ; — and 2nd, those which, either in their perfect state, or at some period of their formation, are in the condition of cells, resembling those of plants. 23. It has been already stated (§. 18) that the clot which is formed by the coagulation of fibrin, even when this takes place out of the living body, has more or less of the fibrous character ; and this is more distinct, the more slowly and firmly the coagu- lation takes place. Now, when fibrin is poured out upon a 30 PRODUCTION OP FIBRES. AREOLAR TISSUE. living surface, as it is in the production of false membranes as a consequence of inflammation, or in the repair of injuries (Chap. viii.) its coagulation takes place more slowly and more firmly; and its tendency to assume this regular arrangement is therefore still greater. There are several tissues which seem to have been produced in this manner. The one which most satisfactorily shows a structure, that we may believe to have been produced by the simple coagulation of fibrin, is the membrane which encloses the white of the egg, and which forms the basis of the shell. If this membrane be soaked in water for some little time, it. may be separated into numerous layers, every one of which presents a beautiful matted appearance, being composed of fibres that cross one another in every direction. The egg-shell itself, after its chalky matter has been removed by acid, exhibits precisely the same structure ; and the most easy way of obtaining a sufficiently thin layer, is to take off the first delicate film that is seen on the inside of the shell (the lining membrane that surrounds the white having been previously peeled off) after it has been immersed for a few minutes in vinegar or any other weak acid. Now this fibrous membrane may (in the opinion of the author) be taken as a type or example of the fibrous tissues in general ; but there is this difference between them, — that this has to serve a pur- pose merely temporary, and consequently no provision is required for its growth and renovation ; — whilst they have to form a per- manent portion of the living body, and must be nourished like the rest, in order that the continual wear may be counterba- lanced, and that accidental injuries may be repaired ; so that we find them traversed by blood-vessels, of which wTe do not meet with a trace in it. But as they are but little liable to change, the amount of vessels distributed through them is usually small. 24. Of the fibrous tissues produced in this manner, the one which is most abundant, in the bodies of all the higher animals at least, is the one termed areolar * This is composed of a net- work of minute fibres, intermingled with thin plates of membrane that seem made up of fibres adhering together, side by side ; and * From the Latin areola, a small open space. This is the tissue commonly but erroneously termed cellular. AREOLAR TISSUE. 31 these are interwoven in such a manner, as to leave very nume- rous interstices and cavities amongst them, having a tolerably- free communication with each other. These cavities are filled during life with a serous fluid;* and it is a necessary result of the communication between them, that if an accumulation of this fluid takes place to an undue extent, as in dropsy, it descends by gravity to the lowest situation. Hence, the legs swell more frequently than any other part. In its natural state, this tissue possesses considerable elasticity ; 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 im- paired 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 composition 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 permitting 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 structure of almost 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. 26. 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 taken place accidentally, and has sometimes been purposely effected, does not produce any disorder in the general functions of the body. It has sometimes hap- pened, that in blowing the nose violently, a rupture or bursting has taken place in some part of the membrane lining its cavity, which has allowed air to pass into the areolar tissue of the face, * A fluid resembling tbe serum of the blood, diluted with water (§. 236). 32 AREOLAR TISSUE. SEROUS MEMBRANES. and especially into that contained in the eyelids, which is parti- cularly loose ; an enormous 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 occa- sioned 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 inflation of the areolar tissue of the head, in the living state, has been frequently practised by impostors, in order to excite commiseration. 27. The areolar tissue seems liable, from various circum- stances, to rapid decay ; and there is, consequently, a provision for its equally rapid renovation. It is more copiously supplied with vessels than are any other of the fibrous tissues ; and when a portion of it has been destroyed, it is very quickly replaced. 28. Now these fibres and shreds may be so interwoven as to form a continuous sheet of membrane, having a smooth and glis- tening surface, and this appears to be the mode in which the serous membranes are produced, 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 {§. 256). 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, a serous fluid is exhaled, which adds to its smoothness. It is by an accumulation 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 FIBROUS AND ELASTIC TISSUES. 33 tissues are produced, which are employed wherever a greater 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 sheaths that envelop and protect many of the most important organs. These do not in general possess much elasti- city ; but there is one kind of fibrous tissue, distinguished by its yellow colour (the rest being white, or gray), by which this property is manifested in a remarkable degree. One of the best examples of this is seen in the ligament of the neck of many quadrupeds, commonly known as the paccy-waxy. This 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 ; and carnivorous quadrupeds, such as the lion and tiger, are endowed with it, to give them additional power of carrying away heavy burdens in their mouths. In man we scarcely find a trace of it. This yellow fibrous tissue is found, however, in the walls of the arteries (§.248), to which it gives its peculiar elasticity ; and it also forms the vocal cords of the larynx (Chap, xiii.), as well as other parts. It is of the same kind of tissue, that the ligament which holds together the shells of the bivalve mollusca (§. 124) is composed of. 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 re- quire to be considered as vital. We may consider them, there- fore, as among the lowest forms of animal tissue; and accord- ingly we find that, when the higher forms degenerate, or waste away, these appear in their place. Such a degeneration 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 con- tractility (§. 7) ; 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 front of the 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 dis- 34 FORMATION OF SIMPLE MEMBRANE. — CELLS. use, will gradually degenerate into fibrous tissue. Moreover, this change may take place as a part of the regular actions of life ; 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 glan- dular structures remaining only as areolar tissue. 31. But it appears that fibrin may be consolidated, not merely into fibres, but also into very thin layers of membrane, in which no trace of structure can be perceived. Such a mem- brane is found covering every free surface of the body, both ex- ternal and internal. It forms the outer layer of the true skin, lying between it and the epidermis or scarf-skin (§. 35) ; in the same manner, it forms the lining of all the cavities that are pro- longed from it, such as the mouth, stomach, and intestinal tube, with all the canals opening into these ; it also forms the inner layer of the serous membranes ; and it lines the blood-vessels and other tubes. It may be obtained, too, from any shell, by dissolving away the mineral portion with the aid of an acid : such a combination of membrane and mineral matter forms the whole thickness of many shells, and the inner layer of all. This membrane is termed the basement or primary membrane ; and, like the elementary membrane of plants (see Veget. Phys. §. 69), it is remarkable for the readiness with which it is permeated by fluid, though no visible pores can be seen in it. 32. We now come to the second group of animal tissues ; those which are either composed of cells, or have had their origin in them. Between the animal and vegetable cell, there seems to be no other essential difference, than what relates to the chemical composition of the membrane which forms its wall. A cell is a minute bag or vesicle, formed of a colourless membrane, in which no structure can be detected ; and having its interior filled with fluid of some kind. The original form of the cell is globular or oval ; but when there are a number in contact with each other, and 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. Of such tissue the CELLS IN ANIMAL FLUIDS. 35 whole plant, in the lower tribes of the Vegetable kingdom, and all the softer portion in the higher, is composed. It does not form so large a part of the structure of Animals ; but we shall find that their vital functions are as much dependent upon the agency of cells as are those of plants. 33. In the blood or nutritious fluid, which circulates through the bodies of all but the very lowest animals, there may be seen a number of colourless cells floating in the liquid, and carried along in its current. These cells are also to be seen in the nutri- tious fluid which is taken up in the absorbent vessels (or lacteals) of higher animals, and which is gradually being converted into blood. They contain a number of minute granules, which appear to be the germs of new cells ; and it is probable that the life of each parent cell has its appointed period, and that, when that period has elapsed, it bursts (exactly in the manner of the red- snow or yeast plant, Veget. Phys. §§. 48 and 55), and sets free these germs, whose development into new cells has already commenced. Now as to the function of these floating cells, a considerable amount of evidence has been brought together to show,* that they are the real agents of the conversion of albu- men into fibrin, — a change of the utmost importance in the animal economy. 34. In the blood of all the higher animals, we also find a vast number of minute discs, sometimes round, sometimes oval ; these are not colourless, like the former, but contain a red fluid ; and to this the colour of the blood is entirely due. These discs, too, are flattened cells ; and they also seem to have a fixed term of life, and to be capable of reproducing one another, somewhat in the manner of the last. Their function appears to be that of serving as the carriers of oxygen from the lungs, into the tissues which need the supply ; and of bringing back the carbonic acid, which is set free in the latter, to be disengaged in the lungs (§.234). 35. The basement membrane just described, is covered, in almost every part, by one or more layers of flattened cells. On that of the skin, these layers are very numerous, and form a con- * See a Paper by the author, on the Functions of Cells, in the British and Foreign Medical Review, for January 1843. 36 EPIDERMIC CELLS, COVERING THE SURFACE. tinuous membrane, the epidermis, or scarf-skin ; this is separated from the true skin, when raised by a blister ; and it may also be separated after death, by soaking a piece of skin for some time in water. The cells of the outer layers dry up by exposure to the air, and become flat scales ; but those of the inner layers, which are kept moist by fluid that is supplied to them from the vessels of the skin, present their usual rounded figure. Now, the outer layers are being continually worn away ; and the inner layers are as continually pushed outwards, by the formation of new layers on the surface of the basement membrane. These seem to originate in germs which that membrane contains. The epidermis does not seem to perform any other function, than that of protection to the delicate and sensitive surface of the true skin. It undergoes various modifications for this purpose in different animals ; whose defences are for the most part formed as appendages to it or alterations of it. Thus in many shells, the exterior layers are formed of cells arranged side by side, and filled with mineral matter ; this is the ca§e with the brownish yellow substance which forms nearly the whole shell of Pinna, the outer layers of the Pearl-oyster, and the edges of the outer layers of the common Oyster. The scales of fishes and reptiles, the feathers of birds, — the hair, hoofs, claws, nails, and horns of mammalia, — all possess a structure very similar to that of the epidermis in its nature ; and may be regarded as belonging to it. 36, But the layers of cells which cover the internal pro- longations of the skin, have very different and more important functions to perform ; and in order that these may be understood, it is desirable to explain more fully the nature and offices of the membrane which forms these prolongations. It has been already stated that, in the lowest animals, the membrane which lines the stomach, and that which covers the body, so strongly resemble each other, that one may be made to perform the functions of the other (§. 14) ; and this is to a certain extent the case even in the highest animals, where, however, each has its own special offices to perform. The skin is the part by which chiefly the impressions of external objects are conveyed to us, through the nervous system (§.6). It is copiously supplied with nerves SKIN AND MUCOUS MEMBRANE. 37 over every part ; so that there is no portion of the surface, except that which is protected by the nails, on which we cannot feel. For the action of these nerves, a large supply of blood is required. Hence the true skin is principally made up of nerves and blood- vessels, bound together by various kinds of fibrous tissue. 37. On the other hand, it is by means of the membrane lining the digestive cavity, that the functions of digestion and absorption are performed ; and it consequently needs a very dif- ferent structure. It is not supplied with many nerves ; and it possesses in health so little sensibility, 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. But it is endowed with the power of secreting the fluids necessary for the solution of the food ; and with the power of selecting and absorbing the nutritious products which have been separated from it ; and for these purposes it is most copi- ously supplied with blood-vessels. The same general characters are presented by the membrane that lines the windpipe and air- passages, as well as by several others ; and these are all com- monly spoken of as mucous membranes, on account of the peculiar tenacious secretion, termed mucus, by which they are covered, and protected from the irritation that would be otherwise pro- duced by the contact of solid or liquid substances, or even of air, with their free surfaces. When this secretion is checked, which sometimes happens from injuries of the nervous system, the membrane becomes inflamed, and may even be completely de- stroyed by the diseased action thus occasioned. 38. Mucous membrane may either exist in the condition of a simple expanded surface, covered with one or more layers of flat epithelium cells, which form a kind of pavement to it ; or it may have a much more complex arrangement, by which its sur- face is greatly increased. The simple mucous membrane, such as that which lines the nose and air-passages, is found, for the most part, where no absorption has to be performed, and where only a moderate amount of secretion is necessary. But 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 38 STRUCTURE OF MUCOUS MEMBRANE. projections, and also a number of minute pits. These projec- tions sometimes have the form of long folds ; in other instances they are narrow filaments, crowded together so as almost to re- semble 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. 39. The whole surface of every mucous membrane, whether simple or involuted, is usually covered with cells, resembling those of the epidermis, but never dried up into scales ; and the layer formed by these cells, which sometimes adhere into a continuous membrane, whilst in other instances they readily separate from each other, is called the epithelium. This epithelium is frequently being cast off, like the epidermis ; especially from the parts that are most concerned in secretion : and it is as continually being replaced, by the development of new cells from germs contained in the basement membrane, at the expense of fluid that transudes it from the blood-vessels copiously distributed beneath. Not only are the flat ex- panded surfaces of the mucous mem- brane covered with epithelium cells, but the villi also are sheathed by them ; and the secreting follicles are lined by the same. It would appear, how- ever, that the epi- thelium cells of the have the pro- Fig. 2. Diagram representing the Mucous Membrane of the Intestinal Canal ; A, in the intervals of digestion ; B, tectlOfl of their deli- during digestion; a, a, absorbent vessels; b, b, basement membrane ; c, c, epithelium cells ; d, d, absorbent cells of villus; e, e, secreting cells of follicle. cate surface for their chief purpose ; and FUNCTIONS OF EPITHELIUM CELLS. 39 that they fall off when these minute but most important organs are performing their function of selecting and absorbing the nutritious elements of the food, — being replaced again in the time that elapses before the next digestion. On the other hand, it seems probable that the epithelium cells of the follicles are the real agents in the secreting process ; — that they draw from the blood, as materials for their own growth, certain elements con- tained in it ; — and that, when mature, they fall off, and 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. 40. 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 pur- poses in the system (§. 15). In all of them, the structure ultimately consists of such follicles, sometimes however 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. Hence we see that the act of secretion is, in animals as in plants, really performed by cells. It is necessary 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 ; and it is in this manner that the serous fluid of areolar tissue and serous membrane is poured out. 41. Recent discoveries enable us to go further, and to say that the selective absorption of nutritious matter seems to be performed by the same agency. It is necessary to distinguish the absorption by the villi of mucous membrane, which are the roots, as it were, of the vessels that take up the nutriment, from the mere imbibi- tion (or drinking in) of fluid, which may take place through any thin and soft animal tissue. We shall find hereafter, that water 40 OFFICES OF CELLS IN ABSORPTION AND EXCRETION. and any substances completely dissolved in it, which do not render it viscid, may be thus absorbed by the blood-vessels ; but that the villi seem to have the peculiar property of selecting nutritive substances only, and of conveying these into the vessels adapted to carry them into the circulation (§. 218). This they appear to accomplish by means of a set of cells developed at their extre- mities for this purpose, every time that the process takes place. In the intervals of the digestive action, only a few granules, which appear to be cell-germs, can be seen at the end of the villus (Fig. 2. A, d) ; but when absorption is taking place, the protective epithelium falls off, and a number of large round cells, rilled with a milky fluid, are seen to have been developed at the extremity of each villus (Fig. 2. B, d). These cells lie very close to the loops that form the commencement of the absorbent vessels; and when they have become filled with fluid, they either burst or dissolve away, and yield their contents to those. This process is repeated every time that the absorption of this peculiar fluid, termed chyle, takes place. 42. Thus it will be seen, that these apparently simple and insig- nificant bodies, cells, are the real agents in the two most important processes in the nutritive operations, — the absorption of nutritive matter, and the excretion of substances that are unfit to be retained in the system. We shall hereafter see that the latter is even the more constantly important of the two ; for that an animal may live for some time without food, whilst it is rapidly destroyed if its blood be not purified from the matter that is destined to be excreted. There is something extremely mysterious in the performance of these different operations, by instruments that appear so simple, and which so strongly resemble each other. There would not seem any obvious reason, why one set of cells should thus minister to absorption, and another to excretion. Perhaps, however, it may be partly explained from their respective situations. The absorbent cells are situated in the substance of the villus, beneath its covering of basement membrane ; hence, when they burst or dissolve, their contents are not set free, but they are delivered to the vessels in their neighbourhood, by which they are imbibed. On the other hand, the secreting cells cover the free surface of SECRETION BY CELLS. 41 the basement membrane ; and when they liberate their contents, these are cast into the tube or canal whose walls they help to form, and are thus conveyed away. In the fatty tissue, we have an example of cells, endowed with a similar power of secretion, but, through their difference of situation, remaining to form part of the regular structure of the body (§. 44). But of the reason why one secreting cell should separate from the blood a certain product, and another should draw off a product entirely different, we can give no account whatever. 43. We have cases exactly parallel in the vegetable, however; for in the same plant we may find one set of cells secreting (or drawing from the circulating fluid into their own cavities) a fixed oil — another set, a volatile oil — another set, starch — another set, colouring matter — and so on. In the tissue of a parti-coloured flower, we observe cells lying side by side, and apparently under precisely the same circumstances, but which secrete or produce different kinds of colouring matter, so that one (for example) shall be blue, and the other yellow. Or if we go to the opposite extremity of the scale, we find one species of plants, composed of separate cells only, adapted to live upon any cold damp surface, and to obtain its nutriment from the air and moisture around ; whilst another plant, also consisting of separate cells, can only live and vegetate, when supplied with animal or vegetable matter in a state of change or decay.* Of the reason of the respective pecu- liarities of these cells, no account whatever can be given ; and we are consequently as much in the dark respecting their cause, as we are in regard to the reason of the peculiar operation of cells in the animal economy. But it is an important and interesting step in the science of Physiology, to be able to show that, although the structure and mode of life of Animals appear so different from those of Plants, the manner in which those functions that are common to both are performed, is so precisely the same. 44. In the bodies of the higher animals, there are few organs of whose tissue cells form a permanent part ; except the epidermis and its appendages, and the epithelium. In almost every part * Sec the account of the Red Snow and Yeast Plant, in Veget. Physiol, §§. 48 and 55. 42 STRUCTURE AND USES OF FAT. of the body, however, we find oily matter or fat deposited in the areolar tissue. It does not lie freely there, in the meshes of that tissue ; for if it did, it would not be confined to particu- lar spots, but would find its way from one part of the body to another (§. 24). The fatty tissue is distinct, as well as the fatty matter itself; for it is composed of minute cells or vesicles, 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 ; 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.* 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 deposites 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 ; thus the eye rests on a sort of cushion of fat, on which it can freely turn, and through which the muscles pass that keep it in play. It also affords, by its power of resisting the passage of heat, 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 abun- dant, it serves as a store 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 (Chap, in.) 45. 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 gelatinous substance, through which are scattered a number of cells, at a greater or less distance from one another. In the gelatinous substance itself, no trace of structure can be seen, in the simple cellular cartilages, such as those which cover * Thus oil will not pass into blotting-paper, if this have been previously moistened with water. STRUCTURE AND USES OF CARTILAGE. 43 the ends of the bones, where they glide over one another so as to form movable joints. But in cartilages which have to resist not only pressure but also extension or strain, we find the space between the cells partly occupied by fibres, which resemble those of ligaments ; and such are termed fibro-cartilages. They are found in man between the vertebrae of which the spinal column is made up (§. 63) ; and also uniting the bones of the pelvis (§. 85.) Sometimes, where elasticity is required, the fibres are those of the yellow fibrous tissue (§. 29) ; 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 quantity of fluid is spread out over the cartilage, 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 transmitted 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 con- sists of cells imbedded in a gelatinous substance, and therefore bears a close resemblance to animal cartilage) obtains its nourish- ment from the surrounding fluid. ( Veget. Phys. §. 102). 46. 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 developed in its substance, by a process which will be described hereafter (Chap, vii.), when it becomes inflamed. This may be often seen to take place. The front of the eye is formed by a transparent cartilage, bulging like a watch-glass ; which is termed the cornea (Chap, xi.) This substance is properly nourished only 44 STRUCTURE OF BONE. 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 trans- parent as before. 47. Many parts exist in the state of cartilage in the young animal, which are afterwards to become bone ; and in fact we may say that all bones have their origin in a cartilaginous struc- ture. The transformation is a very curious one, and is not yet properly understood ; but the nature of the bony structure is easily explained. When we cut through a fully formed bone, such as that of the thigh, we find that the shaft or lengthened 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 structure 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 of areolar tissue, by their bony partitions, which cross each other in every direction. These chambers are filled with marrow, like the central cavity, with which they communicate. 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 this areolar 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, all of which proceed from the central cavity, and are filled, like it, with marrow. These canals usually run, in the shafts of long bones, in the direction of their length ; and are connected, every here and there, by cross branches. They are termed the Haversian canals, after the name of their discoverer, Havers. The lining membrane of the large central cavity is STRUCTURE OF BONE. 45 copiously supplied with blood-vessels ; and this sends off pro- longations into the areolae at the extremities of the bone, and into the Haversian canals. Thus blood is conveyed into the interior 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 carti- lage. These spaces are provided with nutriment by the follow- ing very remarkable arrangement. 48. When we cut across the shaft of a long bone, and ex- amine a thin section with a microscope, we of course see the open extremities of the Haversian canals (Fig. 3, a) ; just as we see the cut ends of the ducts and vessels of wood, when we make a transverse section of a stem (Veget. Phys. §. 129). Around each of these apertures, the bony matter is arranged in concentric rings, which are marked out and divided by circles of little dark spots ; and when these spots areex- amined with a higher power, it is seen that they are small flattened cavities, from which proceed a number of extremely minute tu- bules (A). These tubules pass out from the two flat sides of each cavity ; one set passes inwards, towards the centre of the ring, and the other outwards towards 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 Fig. 3. 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. 46 STRUCTURE OP BONE. those proceeding from different rings are so connected 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 supplied by the blood-vessels, and to transmit it to the outer circles, or those farthest 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 experi- ments, that the substance of bone is undergoing continual change ; and it is owing to the comparative activity of its nutritive pro- cesses, that bone is so readily and perfectly repaired, when it has been broken by violence, or injured by disease. 49. 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 deposition of a large quantity of mineral matter in the cartilaginous portion of its tissue. Such a deposition sometimes takes place in ordinary cartilages, especially in old persons ; thus the cartilages which unite the ribs to the breast-bone are often found to possess an almost bony hardness in persons who are advanced in life, although the organic structure is not changed. 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 ingre- dient 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 (spirit of salt) much diluted with water, the compounds of lime are entirely removed from it, and the cartilaginous substance remains ; the latter is now quite flexible, and almost transparent, so that the distribution of its vessels (if they have been previously injected with colouring matter) may be distinctly seen. On the other hand, if we sub- ject a bone to strong heat, the animal portion will be burnt out, and the earthy matter will remain. The form of the bone will CONVERSION OF CARTILAGE INTO BONE. 47 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 pressure, is given by the earthy matter ; whilst its tenacity, or power of holding together, depends upon the animal portion. The amount of mineral matter in the bones increases with the age ; thus in the child it forms about half the weight of the bone, in the adult four-fifths, and in the old person seven-eighths. 50. Of the conversion of cartilage into bone, it may be suffi- cient to say, that the Haversian canals, and the areolse of the spongy texture, seem to be formed by the union of a considerable number of cartilage-cells, which are much increased in number, and become arranged in rows or in clusters. By their union, a tube is formed (in the same manner as the ducts of plants origi- nate in cells which break down into one another, Veget. Phys. §. 82,) in the one case ; and a little chamber in the other. These tubes and chambers communicate with each other, and with the central cavity ; and from the latter, vessels are prolonged upon their lining membrane, from which the bony matter is deposited. The peculiar cavities and tubuli of bone seem to be spaces left, in which this deposit is not laid down. 51. The conversion of cartilage into bone is going on for some time after birth, as well as previously. In the long bones, it commences at several distinct points, being those near which the vessels enter, and from which they spread out or radiate. And the different portions first ossified or converted into bone, are not firmly united to each other until sometime afterwards. It is curious to observe that, in Reptiles and Fishes, many parts which are thus united in the higher animals, remain permanently separate. And there is a large group of Fishes, in which the skeleton retains the cartilaginous character throughout life ; a cer- tain quantity of mineral matter being deposited in the cartilage, but its conversion into bony structure never taking place. In a few, not even a firm cartilage is produced ; and all the trace of a skeleton is a tube surrounding the brain and spinal marrow, (§. 64), formed of a mass of hexagonal cells, resembling those of the pith of plants. Such a tube precedes the formation of the system of bones, which takes its place in the higher animals. 4S FORMATION AND DECAY OF TISSUES. 52. The various tubes and vessels through which blood and other fluids are conveyed, have their origin in cells, precisely as they have in plants. In the lowest animals no such tubes exist ; and the same is the case even with the highest, in their early condition. For these, as will be shown hereafter (Chap, v.), consist but of a mass of cells, of which some afterwards become cartilage, some are converted into bone, some into muscle, and so on ; whilst some break down into each other, so as to form the tubes required for the conveyance of fluids from one part of the system to another, just as they are in plants. Of these tubes some are straight and single, like the ducts in which the sap ascends ; whilst others form a network, like that in which the elaborated sap circulates in plants (Veget. Phys. §. 87). 53. "YVe have now completed our general survey of the prin- cipal tissues of animals ; excluding, however, the nervous and muscular, which depart more widely from the characters we have been considering, and which it will be preferable to describe when treating particularly of their functions (Chaps, x. andxii.). The mode in which these are severally formed will be explained hereafter ; and at present it will be enough to say that, as every organised being has its allotted term of life, which may be pro- longed or shortened to a certain degree, according to circum- stances, so every portion of its fabric appears destined to last only for a certain time, and to cease to exist as soon as it has performed its peculiar office. Thus the cells which float sepa- rately in the blood seem to be continually undergoing a change, — dying, and giving birth to others. "We have seen that the cells of the epithelium and the epidermis are also 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. But that of the muscular and nervous tissues seems to depend almost entirely on the use that is made of them. When an animal is quite inactive, these require but very little nutrition ; but every one of its movements, in which a certain expenditure of muscular and nervous power takes place, seems to involve the death and re-formation of a certain portion of these tissues ; for we find that any animal in a state of activity requires a far greater amount of food, and sets DECAY CONSTANT DURING LIFE. 49 free by excretion a far larger quantity of dead matter, than whilfe it is continuing in a state of rest. Thus it is that Animals expend so large an amount of nutritive matter, not in the extension of their own structure as Vegetables do, but in the performance of those movements which are peculiar to themselves. 54. From the foregoing statements we may justly say, — however 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. Many animals are capable of being reduced to a state of complete torpidity, with- out the loss of their vitality ; so that all their actions are renewed, when they are again placed in the requisite conditions. Some are reduced to this state by cold ; others by dryness. Amongst the most remarkable examples of this are the Wheel- Animalcules (§. 117) ; some species of which may be completely dried up, and may be even exposed to a temperature much exceeding that of boiling water, without losing the power of recovery when again moistened. An animal in this state strongly resembles a seed, which is prevented from germinating, by being kept at a moderate temperature, and excluded from the influence of air and moisture. (Veget. Phys. §. 446.) Instances have been recorded, in which seeds have been thus preserved for a known period of more than 2000 years ; and there are others in which the period was probably much longer. There are no positive facts which enable us to say, how long an animal may remain in a similar condition; but it is well known that revival has often taken place, after the body has been frozen or dried up for several years ; and there seems no reason why it should not occur after many times that period. It is to be remembered that this is a condition, in which the decay of even dead animal matter is resisted. Frozen animals are brought from the remotest parts of the Russian empire, to be sold as food in the markets of Petersburgh ; and their flesh, when thawed, is as good as ever. Again, meat, dried and pounded, forms the staple food of the North American hunters, by whom it is kept for weeks or months without change. Even in the moist state, animal and vegetable substances may be perfectly preserved for years, if 50 PECULIAR CONDITION OF THE LIVING BODY. they are completely excluded from the air. This is the plan now adopted in victualling ships for long voyages. The sub- stances, partially cooked, are put into tin vessels; these are completely filled with gravy, &c; and they are soldered down whilst the contents are hot. Thus we see that the decomposi- tion of the animal body is not a necessary consequence of its death ; whilst it is in fact continually going on during its life. 55. In what then, it may be asked, does the peculiar condi- tion of a living body, as distinguished from a dead one, consist ? To answer this question, it is necessary to consider the body as a whole, and as made up of several parts. Each of these parts has its own independent vitality ; that is, it has its own peculiar properties, which depend upon the mode in which its elements are combined and arranged (Veget. Phys. §. 8) ; and these it will continue to exercise, as long as it is supplied with the neces- sary conditions. Thus the secreting cells of the epithelium require a supply of blood, both for their own growth, and for the formation of their peculiar prod acts. The muscular tissue cannot act without a suwply of oxygen, and this is conveyed to it by the blood. All the tissues possessing peculiar vital pro- perties, are in like manner dependent upon the blood or nutritive fluid for the continued exercise of these. But after the circula- tion of the blood has ceased, so that the body is commonly said to be 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 necessary conditions are withheld by the stoppage of the cir- culation,— a function which thus binds, as it were, into one whole, the scattered elements, and causes the different operations to minister to one another. 56. The death of the body, therefore, is said to take place when its circulation permanently ceases ; since on this circulation all its functions depend. But, as we have just seen, the separate parts may retain their vitality for some time longer, — some of them, indeed, until they are actually beginning to become putrid. PECULIAR CONDITION OF THE LIVING BODY. Ol And, on the other hand, the circulation may cease for a time, as in fainting or temporary suffocation among the higher animals, or in torpidity among the lower. The time is short, however, during which it can be thus suspended among warm-blooded animals ; because their tissues very soon begin to undergo a destructive change. And the reason why it may be almost in- definitely prolonged, in those animals which can submit to being- frozen or dried up, appears to be simply this, — that this treat- ment prevents that destructive change from taking place. 57. Hence, life is not (as has sometimes been asserted), the condition in which decay is resisted ; but the condition in which, by the wonderful adaptation and mutual harmony of the opera- tions of the different parts, — a harmony which could only proceed from the mind of an All-powerful and All-wise Being, — they are all made to unite to one common end, the maintenance of the structure in a condition fit for the performance of its proper actions. So long as all these actions go on with regularity and completeness, so long the whole body lives ; but if any one of the more important among them be interrupted, the stoppage of the whole must be the result. If we could imagine a steam- engine capable not only of constantly supplying itself with water and fuel, but also of repairing its own wear and tear of mate- rials, we should have some notion, by analogy, of what may be termed the physical functions of the animal body. But it must be borne in mind, that the body which these functions are destined to maintain, is but the instrument, in Animals, of a higher set of operations, those of the mind; to which no actions performed by any piece of mechanism constructed by Man, or even by those most refined and beautiful fabrics of which the Vegetable kingdom is composed, bear the slightest resemblance. Of the mode in which the mind is connected with the body, and uses it as its instrument, we have not, and probably never shall possess in our present state of being, the most distant idea. K ?. CHAPTER II. GENERAL VIEW OF THE ANIMAL KINGDOM. 59. When we examine the Animal Kingdom as a whole, it is easy to distinguish in it four general plans or types of structure, by which, with almost infinite variations in detail, the formation of the several beings that compose it has been guided. As speci- mens of these four plans or types, we may name four animals which are familiar to almost every one, — the Dog, the Lobster, the Slug, and the Star-fish. 60. The differences by which these types are distinguished, are manifested 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 pecu- liarly distinguishes the Animal 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. For 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 blend- ing, in a particular animal, of the general characters of two of them, as to lead us to hesitate in assigning its place in the ani- mal kingdom. 61. But it is not only in the form and arrangement of the parts of the nervous system, that we find such important dif- GENERAL VIEW OF THE ANIMAL KINGDOM. VERTEBRATA. 53 ferences ; for there are other organs connected with the powers of sensation and motion, the arrangement of which also exhibits corresponding variations. Thus the power of motion is de- pendent upon the muscles, which are called into action by the nervous system ; and these cannot act with any force, unless they are attached to hard parts, connected together by joints, so as to form a skeleton. Now this skeleton sometimes consists of bones, which are clothed by the muscles, as in the dog. Some- times it consists of a jointed shell, enclosing the muscles, as in the lobster. And sometimes it is altogether absent ; and the muscles, having no firm points of attachment, act slowly and with little power, as in the slug. In regard to the arrangement of the organs of special sense, also, — those of sight, hearing, smell, and taste, — there is a corresponding variation. In the dog and the lobster, they are set upon a prominent part of the body, in the neighbourhood of the mouth ; and this is termed the head. In the slug, too, this is also the case ; but in the oyster, an animal of lower organisation, belonging to the same division, there is no head, and the few organs of sense which it possesses are almost buried (as it were) in the mass of the body. Lastly, in the star-fish, there is a similar absence of anything resembling a head ; the mouth is in the centre of the body ; and the eyes are placed at the greatest possible distance from it, one at the extremity of each ray. 62. 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, — which will be presently described. This is the group to which Man belongs ; and all the animals it contains bear a greater or less resemblance to him in structure. We notice in regard to their external form, that they are all alike on the two sides of their body ; every part having its fellow on the other side. This 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 extend to the 54 GENERAL STRUCTURE OF VERTEBRATA. organs of nutrition, which are unequally disposed on the two sides : thus, the heart and stomach are on the left side, the liver on the right, and the lungs much larger on the right side than on the left. 63. In all Vertebrated animals, the skeleton is internal; and consists of bones, which are capable of growing, and of being reproduced after injury, like any other part of the living tissue ; beino- 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 Ser- pents ; 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 Yertebrated animals ; though in a few among them (the lowest Fishes) it is very imperfect. It consists of seve- ral pieces jointed together, so as to fig. 4,-Skeleton of the Ostrich. possess great flex- ibility ; 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 ; but in many Serpents there are several hundred. Each of them STRUCTURE OP THE VERTEBRAL COLUMN. 55 is termed a vertebra; and the whole structure, composed of the united vertebra, is termed the vertebral column (Fig. 5.) 64. The essential 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 important parts of the nervous system, — the spinal cord (commonly but errone- ously termed the spinal marrow *) — is contained. The solid portion of the vertebra (Fig. 6, «), is termed its body ; and the projections, b andc, are termed its processes, the former spinous, the latter transverse. The row of spinous processes forms the ridge which we feel passing down the back ; it is seen on the right hand side of Fig. 5. To the transverse pro- cesses the ribs are attached. The vertebral column Bral column. is expanded (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 b of sense also. At the opposite extremity, we see it contracted into the tail ; which is composed of a c series of vertebras 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 Fig 6— single animals in which the skull is very large, the tail is Vertebra, 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 kangaroo (whose tail is like a third hind-leg) the head is comparatively small. But this rule does not hold good universally. 65. The nervous system of Yertebrated animals consists of a brain and spinal cord, which are lodged within the skull and * The marroiv of bones in general, is an oily matter, which seems to contri- bute to their nourishment. The spinal marrow is a part of the nervous system. 56 NERVOUS SYSTEM, AND GENERAL STRUCTURE, OF VERTEBRATA. vertebral column ; and of nervous trunks proceeding from these, which are distributed to all parts of the body. The brain and spinal cord are termed the nervous centres ; since it is in them that the power of this system resides; the trunks or cords being only conductors of its influ- ence. The distinguishing feature of this 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 ani- mals : and that it is enclosed in the bony casing, which has been described as pecu- liarly destined for its protection, 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 pecu- liar protection is quite necessary, in conse- quence of the much higher development which it attains, and the much greater importance which it possesses, in this divi- sion of the animal kingdom, than in any other. 66. The general arrangement of the other organs in Vertebrated animals, is shown in the succeeding page (Fig. 8). At m is seen the mouth, forming the entrance to the digestive cavity, of which the termination is at the oppo- site extremity of the body ; i, i, is the intestinal canal, and 7, 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, which conducts air into the lungs, p ; these organs, with the heart, h, are contained in the portion of the trunk called the thorax, or chest. At b is seen the position of the brain ; and at 5 that of the spinal cord. Fig. 7. — Brain and Spinal Cord of Man. GENERAL CHARACTERS OF - VERTEBRATED ANIMALS. 57 67. The foregoing characters apply, with greater or less modi- fication as to details, to the classes of Mammalia (commonly Fig. 8 — Diagram, showing the position of the principal Organs in Vertebrata. termed Quadrupeds), Birds, Reptiles, and Fishes ; and these fur- ther 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 Mammalia, we find all four present in every tribe save that of Whales, which want the hinder pair ; though the upper or ante- rior 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 Serpents ; and there are Lizards which have only one pair. And in Fislies, we usually find two pair, constituting the pectoral and ventral fins ; but one or both pairs are sometimes absent, as in the Eel, Lamprey, &c. 68. We have further to remark, in regard to the general characters of Vertebrated animals, that they have all red blood (§. 226) ; and that they possess a complex apparatus for circu- lating this through the body. Lastly, in all but the very lowest, all five kinds of sensation exist ; — namely, sight, hearing, smell, 58 INTELLIGENCE AND INSTINCT. ARTICULATA. 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 instinct to perform actions, of whose objects they are not aware. And we find, by observing and comparing the structure and actions of the different groups, that the intelli- gence 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 con- clude, that the brain is the organ of intelligence, or of the reasoning faculties. 69. All the animals which are destitute of a vertebral column, are called Invertebrata ; and this division into the vertebrated and in vertebrated 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 in vertebrated division are comprehended three groups, of which the members differ as much, from one another, as they do from vertebrated ani- mals ; 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 (§. 59) as characteristic specimens of these divisions ; and it will become more apparent as we proceed. 70. In the second division, that of Articulata, or Articulated (jointed) animals, we find a confor- mation very different from that which has been just described. The exterior of the body is still per- fectly symmetrical, as in the vertebrata : 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 fig. 9. body. But the skeleton, instead of being internal, Centipede. . . . is external ; and is composed ot a series ot pieces jointed together, which form a casing that includes the whole STRUCTURE OP ARTICULATED ANIMALS. 59 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. 9). The limbs are usually very numerous, where they exist at all ; and they have a jointed covering, 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 Vertebrata in their general organisation, 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 cover- ing is thrown off and renewed at intervals. 7 1 . The nervous system consists of a series of small masses or ganglia^ which are arranged in a cord or chain along the central line of the body. There is usually a large ganglion in the head, bearing a resemblance (in its peculiar connection with the eyes) to a certain part of the brain of Vertebrata ; and there is com- monly 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 Vertebrata. The cord which connects these ganglia is double ; and the ganglia themselves are composed of two halves which have little connection with each other. The chain thus formed, passes along the under side of the trunk of the animal (as seen at g, Fig. 11), not on what seems its back ; and it is by the presence of this double chain of ganglia that an Articulated animal maybe distinguished, even when, in its general structure, it should seem to belong to the group of Mollusca (see §. 113). 72. The general arrangement of the organs in the Articulata Fig. 10.— Nervous System of an Insect. 60 GENERAL STRUCTURE OF ARTICULATA. is shown in the accompanying figure of a cray-fish. The mouth, s f h i situated on a projecting head, opens into s, the stomach, from which passes backwards ( the intestinal tube, i, i, to terminate at the opposite ex- tremity of the body. The upper part of the tube is Fig. 11. -Diagram showing the position of the Surrounded by the liver, J\ principal Organs in the Articulata. ^^^ ig here yery ^^^ Jn the head are seen the ganglia, c ; and along the under side of the body is seen the chain of ganglia, g. The organs which answer to the lungs of Yertebrata are not connected with the mouth ; and are not usually restricted to one part of the body, but are diffused either on its outside, or through its substance. The organs of sense, in this group, are less numerous than in Yerte- brata, and are inferior in perfection. The blood is nearly colour- less ; and the heart, k, by which it is impelled through the body, is much less energetic. 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 remark- able 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 different individuals of the same species have not that difference of capacity and of dis- position, which we see in animals whose endowments are higher. 73. The general character of the animals composing the group or division Mollusca, is, in many respects, the very op- posite of that which belongs to the articulated animals. The body is soft (whence the name of the group is derived), neither possessing an internal skeleton, nor any proper external skeleton. In some of the most characteristic specimens of the group, such as the slug, there is no hard frame-work or skeleton ; and thus the body is alike destitute of support and protection. In many GENERAL STRUCTURE OP MOLLUSCA. 61 Fig. 12: — Lvmn^us Stagnalts. molluscs, 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 (Fig. 12). Wemustnot regard the shell as an essential *' ■= • part of the Mol- luscous animal ; because there are many tribes en- tirely destitute of it ; and also because some of the Articulata have the power of forming a shell, which bears a close resem- blance to that produced by the animals of this group. Not un- frequently we see that, of two animals whose general structure is almost exactly the same — as that of the snail and slug, one possesses a shell, into which it can withdraw its whole body for the sake of pro- tection, whilst the other has none ; and that several inter- mediate 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. 13). 74. The external form of the body of the Mollusca 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. For these are Fig. 13. — Testacella. 62 GENERAL STRUCTURE OF MOLLUSCA. almost always symmetrical, being arranged with equality on the two sides of a middle line ; whilst the rest of the body, con- taining the organs of nutrition, is often unequal on the two sides. This is the case, for instance, with the Lymnceus (Fig. 12), which is represented in the act of crawling on its large fleshy disc or foot, with its head, bearing its eyes and feelers, projected for- wards.— But in the lower Mollusca, which have little or no power of moving from place to place, this regularity of arrange- ment is altogether lost. 75. Few of the Mollusca have any powers of active move- ment; in fact, the term sluggishness, derived from a characteristic member of the group, very well expresses their general habit. They usually crawl upon a fleshy disc, by the successive con- tractions 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 disc already mentioned. 76. The accompanying figure of the interior of a Snail will show the very large size of the digestive apparatus, and of the other organs of nutrition. The muscular disc or foot is seen at// and at the extremity of this are seen the tentacula or feelers, t (com- monly termed horns), half contracted. The mouth, situated in the neighbourhood of these, opens into a tube that leads to the stomach, of which a portion is seen at s ; and from this cavity pro- ceeds the intestine, i, which passes forwards again as seen at r, to terminate at a. This termination of the intestine near the mouth is very frequent in Mollusca; and is obviously necessary, when the body is enclosed in a shell that has only one outlet. The liver, /, GENERAL STRUCTURE OF MOLLUSCA. 63 Fig. 14. s d f -Anatomy of the Snail. is a very large mass, surrounding the stomach and intestine. The heart is seen h v ap p r at h ; and from it , is seen to proceed a large vessel, a p, l that ramifies up- on the walls of a cavity, p, which answers to the ar lungs of higher animals ; this ca- 0 vity is separated from the other organs by a kind of diaphragm or partition, d, which is here turned to one side. At ar is shown the artery which proceeds from the heart, to convey blood to the general system. At o is seen the ovarium, in which the eggs are formed ; this ocupies the highest part of the shell ; but it has a canal which terminates near that of the intestine. And lastly, at v is pointed out a gland that secretes the viscous or slimy fluid, with which the body of the animal is covered ; and this is carried out by the canal cv. 77. Thus it is seen that, — whilst the body of an Articulated animal may be compared to that of a man, in whom the appa- ratus of nutrition (contained in the chest and abdomen) is of the smallest possible size, but whose limbs are strong, and his move- ments agile, — the body of a Mollusc resembles that of a man ' whose god is his belly,' his digestive apparatus becoming enor- mously developed, whilst his limbs are feeble, and his movements heavy. Such varieties, in a greater or less degree, are continu- ally presenting themselves to our notice. 78. 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 64 GENERAL STRUCTURE OF MOLLUSCA. parts of the body, and are connected with the preceding by nervous cords. The former correspond to those contained 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. 15 is represented one of the simpler forms of this nervous system, — that of the Pecten or scal- lop-shell ; A A are the ganglia near the mouth, from which the organs of sense are supplied ; B is the ganglia connected with the gills; and C is that from which power is given to the foot. The two first lie wide apart, but are connected by an arched band that passes over the gullet, e. The organs of sense among the Mollusca are but little more developed than those of mo- tion. They serve to direct the animal to its food, and to warn it of danger ; but there seems an absence, in all but the highest species, of that ready and acute sensibility which is so remark- able 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 discovered ; the organ of hearing is always imperfect, and frequently absent altogether ; and the eyes are very often wanting ; so that touch and taste (which is but a refined kind of touch) are the only senses left. — The blood of the Mollusca is white, as it is in the Articulata ; but the apparatus by which it is circulated through the body is much more powerful and complete. 79. The fourth and last 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-fish, which has five such rays, all having a pre- Fig. 1.5.— Nervous System of Pecten. GENERAL STRUCTURE OP RADIATA. 65 cisely similar structure, and thus repeating each other in e very- respect. The mouth of this animal is in the centre : and it Fig. 16.— Star Fish. opens into a stomach, which occupies the central disc, 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. 17 ; where a shows the position of the mouth, which is surrounded by a ring or nervous cord, having five ganglia, cor- responding 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 be- lieved to be an imperfectly developed eye. No other organs of special sense can be detected in any of these animals ; and it is only in a few that even these imperfect eyes can be discovered. All parts of their structure appear to be reduced to their greatest Fig. 17. — Nervous System of Star-Fish. 66 ANALOGY OF ZOOPHYTES AND PLANTS. simplicity of form ; and at last we come down to that which has been already noticed (§. 13, 14) as the simplest type of a decidedly animal structure, — a stomach or digesting sac, with an orifice that serves as the mouth, and a set of arms arranged in a circular manner, around this orifice, as we find in the Hydra. 80. It is only among the highest of the Radiata, that there exists a circulation of nutritive fluid in distinct vessels : in most of the animals which this group includes, the fluid and solid parts are in contact with each other through the whole body ; and their tissues bear a close resemblance to those of plants. The circular arrangement of their organs is a more obvious point of resemblance to the Vegetable kingdom ; and this has frequently caused mistakes to be made in regard to the sea-anemonies, 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 composite tribe of plants, as to have received the name of animal flowers. But there is yet a stronger analogy between the lowest of the Radiated group and the Yegetable kingdom ; for among the former, as in the latter, we find a union of many indi- viduals, which are capable of existing separately, into one com- pound 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 (See Veget. Phys. §. 305). In consequence of this remarkable union, it is often very difficult to say, whether a particular mass is to be regarded as a single animal, or as a collection of many. To the tree-like fabrics thus produced, the name of 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 per- form, for the purpose of obtaining and securing their food, is very little greater than that which has been described in the Sensitive plant, and Venus's fly-trap (See Veget. Phys. §. 421). DIVISION OF THE ANIMAL KINGDOM INTO CLASSES. 67 81. In regard to the situation of the skeleton of the Radiata, there is very great variety ; as there is in respect to the arrange- ment of the organs. In the star-fish and sea-urchin, the body is enclosed in a hard casing, furnished with prickles or spines. In the jelly-fish and sea- anemone, it is altogether destitute of any hard support or protection. And in the coral-forming tribe, a massive internal skeleton is generally produced ; though in some few instances the stony matter forms a tube, which envelops the soft parts. Division of the Animal Kingdom into Classes. 82. The various animals which are united into each of the four preceding groups, or primary divisions of the Animal King- dom, whilst agreeing among themselves in the general characters that have been enumerated, differ from each other in important points ; and it hence becomes necessary to subdivide these again, according to the manner in which their functions are respectively performed. Thus, among Yertebrated animals, there are some which produce their young alive, and which nourish them after- wards by suckling ; while the greater part rear them from an egg which contains a store of nutritive matter, and do not afford them any further nourishment from their own bodies. Again, some breathe air ; whilst others live constantly 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 differ- ence 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. These differences are so well marked, that they afford a ready means of subdividing the Yertebrated group into four classes, Mamma- lia, Birds, Reptiles, and Fishes. The chief peculiarities of these will now be explained. 83. The Mammalia are distinguished by the first of the F 2 68 GENERAL CHARACTERS OF MAMMALIA. characters mentioned in the last paragraph ; being the only animals that produce their young alive, and which nourish them after- wards by suckling. They are for the most part quadruped (that is, four-footed), and 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 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), having the hinder part of the body prolonged and spread out into a broad flattened tail, whilst the anterior limbs are converted into short fin-like paddles, and the posterior are altogether wanting. Notwithstanding these marked differences in external form, there is a great correspond- ence 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 hav- ing an active circulation. Their bodies 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 animals which inhabit warm climates, and which do not require this pro- vision. 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 exercise of his own ingenuity. 85. The general conformation of the skeleton of Mammalia, is shown in the subjoined figure, which represents that of a Camel, — the black ground showing the outline of its form, when clothed with flesh. The head is supported upon a neck, which, whether long or short, always contains seven vertebras (v c, the cervical vertebrae) ; the number of the vertebra? of the back («? d, the dorsal vertebrae), and that of the vertebras of the loins (v I, the lumbar vertebras), varies considerably in different animals. In most quadrupeds the spinous processes (§. 64) of the vertebrae of the back form a high ridge, for the purpose of giving attachment SKELETON OF MAMMALIA. 69 to the muscles and ligaments by which the heavy head and neck are supported. Several of the vertebrae at the hinder end of the Fig. 18.— Skeleton of the Camei. spinal column are united into a single bone (v sy the sacrum), by which they are connected with the framework that supports the legs. Behind this, they are again separate in the tail (v q, the caudal vertebrae). To the transverse processes of the vertebrsB are attached the ribs, c, by which the upper part of the cavity of the trunk is protected ; the ribs are prolonged by cartilages that meet under the body in the breast-bone, which in most of the mammalia is flat. Upon the ribs lies on each side the blade- bone, or scapula, o ; and this forms part of the shoulder-joint, and gives attachment to some of the muscles by which the fore- leg is moved. Each anterior limb consists, first, of the humerus, or arm-bone, h, which lies between the shoulder and the elbow ; then of two bones giving support to the fore-arm, which are commonly united, however, in herbivorous quadrupeds, into a single one, cu ; next, of the bones of the wrist, ca, which are also 70 GENERAL STRUCTURE OF MAMMALIA. few in number, in animals that do not possess separate fingers ; and lastly of the bones of the hand, roc, and those of the toes,jt?^. The hind-leg, united to the vertebral column by a framework called the pelvis^ contains a corresponding series of bones ; the femur or thigh-bone, fe ; the knee-pan, ro, a little bone which lies upon the knee-joint; the two bones of the leg, here united into one, ti ; the bones of the ancle, ta ; those of the foot and toes, mt. 86. The general arrangement of the internal organs will be seen from the accompanying figure of the body of a Monkey, laid Sub-maxillary Gland Windpipe ■•«. Parotid Gland Pharynx (Esophagus Liver Gall-Bladder Colon Caecum Small Intestines Fig. 19.— Interior of a Monkey. open in such a manner as to exhibit the chief of them. The cavity of the trunk is completely divided, by the muscular par- tition termed the diaphragm, into two portions, — the thorax, containing the heart and lungs, — and the abdomen containing the digestive apparatus. It is by the alternate contraction and re- laxation of this muscle, that the act of breathing is performed in Mammalia, as will be explained hereafter (§.331). GENERAL STRUCTURE OF BIRDS. 71 87. In Birds, there is a much closer conformity to one gene- ral plan than we find in Mammalia, The covering of feathers, by which we ordinarily distinguish the members of this class, prevails universally ; and there is no wide departure from the form, which we are accustomed to regard as characteristic of this interesting group. This class belongs to the oviparous division of the Vertebrata ; since the young are reared from eggs. But it is distinguished from Reptiles, which are also oviparous and air- breathing, by being warm-blooded ; and by having a very ener- getic, instead of a very slow circulation. The covering of feathers is given, not only to keep in the heat of the body, which is even greater than that of Mammalia, but also to afford the required surface for the wings, on which the bird is supported and pro- pelled 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 entirely absent, in the Ostrich and a few other birds, which pre- sent 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 the bird, which can neither walk nor fly with rapidity, can swim very quickly through the water. 88. The general form of the skeleton of Birds is shown in Fig. 20, which represents that of the Vulture. The head is supported upon a very flexible neck, of which the vertebrae are often very numerous (vc). The vertebrae of the back and loins, however, are usually few in number, and are connected together very firmly, so as to form a nearly inflexible column ; and this again is closely united to the sacrum, v s. The vertebrae of the tail, v q, are also small in number, and possess little motion. The ribs are very strongly connected to each other and to the vertebrae, and are united to the breast-bone, s t, by bony instead of cartilaginous prolongations. 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 greatest power which these 72 SKELETON OF BIRDS. organs require, is in the downward direction ; in order that, by their stroke, they may raise the bird in the air, or keep it from falling. Accordingly we find that the great mass of flesh or Fig. 20, — Skeleton of the Vulture. muscle that puts them in action, lies on the breast-bone, the centre of which, st, is raised into a high keel or ridge for its attachment. On the oth*er hand, the muscles that raise the limb, and draw it backwards, which are attached to the blade-bone of Mammalia, and are usually strong in them, are comparatively weak in birds, whose scapula (forming part of the side-bone) is very narrow. Moreover, in order to keep the wings properly apart, there is a very strong clavicle or collar-bone, c I, in birds ; and this, though it exists in Man, and in Mammalia that use their fore-arm for other purposes than support and motion, is deficient in the greater part of those that employ it only as a leg. 89. The bones with which the wings are connected are better seen in Fig. 21 ; where o represents the narrow blade- bone;/, the clavicle or collar-bone; £, a bone that seems like a second collar-bone, being an extension of what is a mere SKELETON OP BIRDS. 73 Fig. 21.— Bones <^f the Shoulder and Breast of Birds. process or projection on the scapula of Mammalia ; s, the sternum or breast-bone ; e, its ° lower border ; b, its keel or ridge projecting forwards; and s, the attachment of the ribs. The amount of the projection of the keel of the sternum cor- responds with the size of the muscles that are to be attached to it ; and upon this depends the power of flight which the bird possesses. In the Ostrich, and other birds whose wings are not developed, the sternum is flat as in Mammalia. — Return- ing to Fig. 20, we notice further the humerus or arm-bone, k, con- stituting the first bone of the pinion; the two bones of the fore- arm, 0 ; the bones of the wrist c a, which are here scarcely developed ; and the bones of the fingers, ph, 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 sepa- rated into distinct fingers ; since they are never applied to any other purpose than that of supporting the wing-feathers. In the hinder extremity of the bird we find a thigh-bone, /, (prin- cipally concealed in the figure by the bones of the wing), the two bones of the leg, t, which are commonly in part united ; the shank 'or ancle bones, ta, and the four separate toes, by the spread of which the body is firmly supported, though it rests only on two feet. 90. The arrangement of the organs contained in the cavity of the trunk of Birds differs from that which has been described in the Mammalia, chiefly in this, — that there is usually no dia- phragm to separate the chest from the abdomen, and that although the lungs themselves are confined to the upper part of this cavity, they are connected with a series of air-sacs, which are distributed through the whole of it. In the accompanying figure, which represents the internal organs of the Ostrich, the heart is seen at a, the stomach at &, and the intestinal tube at c. The windpipe, dy 74 GENERAL STRUCTURE OF BIRDS. REPTILES. opens into the lungs, e, which are themselves small, and are attached to the ribs, instead of lying freely in the cavity of the 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. The use of these air-cells in the respiration of Birds, will be explained hereafter (§. 356). 91. In the class of Reptiles we find a variety of form so re- markable, that, if we were influ- enced by this alone, we should ^ti\^z™::™^T-^™™y «*»* *> «**»]« it d the trachea, e the lungs, /// air-ceiis, contains as belonging to the same in which are also seen the tubes by which , ,, , , „ .. these air-ceiis communicate with the group ; yet the structure of the lungs- internal organs, on which classifi- cation is founded, is essentially the same in all. Four obviously different tribes, turtles, lizards, serpents, and frogs, are brought together by the following characters. They are all oviparous (Chap, xv.), 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 Birds ; and they breathe air by means of lungs, instead of breath- ing water by gills, in wThich 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 (§. 97). The first three of the tribes just mentioned, undergo 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 GENERAL STRUCTURE OF REPTILES. TURTLE TRIBE. 75 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. These differences are considered by some naturalists as sufficient to require the separa- tion of the Frog tribe into a distinct class, to which the name of Amphibia (which properly means animals that can live either in air or water) is given. 92. The Turtle tribe is peculiarly distinguished by the inclo- sure of the body in a bony covering ; of which the upper arched portion (termed the carapace) is formed by an expansion of the p f Fig. 23. — Skeleton of the Turtle. ribs, which grow together, as it were, at their edges, so as to form a continuous plate ; whilst the lower flat plate (termed the 76 GENERAL STRUCTURE OF REPTILES. — TORTOISES, LIZARDS. plastron) which is often incomplete, is formed by a similar ex- pansion of the sternum or breast-bone, which is here spread out sideways, instead of being raised into a projecting keel, as in Birds. The accompanying figure will show the general con- struction of the skeleton of this tribe, the sternum being re- moved. As in the preceding figures, vc are the cervical vertebra, and vd the dorsal vertebrae ; c are the ribs extended in width, so as to unite at their edges ; cs are the bony pieces which con- nect these ribs with the sternum ; 0, is the scapula or blade- bone, here very narrow, as in birds ; c/, the clavicle ; co, the additional clavicle, as in birds; &, the bones of the pelvis which support the lower limbs ; /, the thigh-bones ; p and t, the bones of the leg. Although the bones of the toes are separate, they are enclosed in a single horny casing ; this is flattened in the aquatic turtles, and forms a paddle ; whilst in the land tortoises it forms a stumpy foot. The carapace and plastron are covered with large horny plates, variously arranged in the different species, and constituting what is com- monly called tortoise-shell. These plates are often very beautifully disposed, form- ing a kind of tesselated pavement ; as in the common Tortoise, which is often pre- served alive in our gardens. 93. 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 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, the lizards approach the Mammalia, being all four- footed, and living for the most part on land ; but they differ from them in the very important particulars already mentioned, as well as in several others of less consequence. In general, their bodies are covered with scales, which lap over one another like the tiles of a roof ; but in the Crocodile tribe, many parts of the surface are covered with large knotted horny plates, that Ftg. 24.— Tortoise. GENERAL STRUCTURE OP REPTILES. LIZARDS, SERPENTS. 77 meet at their edges, like the scales of tortoise-shell, and afford Fig. 2.r> — Crocodile. an almost impenetrable covering. Although some of the Lizard tribe spend a large part of their time in water, yet they all Fig. 26. — Chalcis. breathe air; but, as their respiration is very inactive, they can spend a considerable time beneath the surface, without being obliged to come up to breathe. There are some lizards in which the feet are extremely small, and the body much prolonged, as shown in the Chalets (Fig. 26) ; and by these we pass to the next group. 94. The tribe of Serpents may be regarded as lizards without feet; their spinal column is immensely pro- longed; and their ribs are also very numerous ; and they are able to crawl upon the points of these, using them almost as the centipedes do their legs (§. 112). But in general, the movement of their bodies is accomplished by their being drawn up into folds, and then straightened so as to Naia Aspic. 78 GENERAL STRUCTURE OP REPTILES. SERPENTS. project the head. The prolonged form of the body in Serpents, occasions a curious variation in the arrangement of the prin- cipal organs, which is shown in the accompanying figure. The oesophagus or gullet, oe, which leads from the mouth to the stomach, is a long and very wide canal, being even larger than Fig. 28. — Anatomy of a Coluber. the stomach at its commencement ; a portion of it is removed at oe\ in order to show the heart, &c, which would otherwise be GENERAL STRUCTURE OF REPTILES. SERPENTS, FROGS. 79 concealed 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, cl9 which opens externally by the orifice a n. The liver, /, is also much lengthened. From the mouth also proceeds the long windpipe, 1 1, 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, oV, are produced ; and this also is very much lengthened, ex- tending 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 hereafter (§. 284). At vt is seen the single ventricle of the heart ; c is the right auricle, and cf the left auricle; ad and ag the two arches of the aorta or great artery, from which proceed ae, ac, the carotid arteries to supply the head, and which unite to form a' the aorta of the trunk ; v, the vena cava or great vein that returns the blood from the head and front of the body ; and vc, the vein that brings it from the trunk. 95. The 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 common 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 particulars, that of a fish, and undergo a gradual series of changes, by which their form and structure become analogous 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 the accompanying figures ; 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 adult frog. 80 METAMORPHOSIS OF TADPOLES INTO FROGS. Soon after the young tadpole has come forth from the egg, it presents the form which is shown in Fig. 29 : 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 members. The gills are long fringes, hanging loosely in the water on either side of the head; and by these the tadpole breathes, as do the aquatic Mollusca. 96. 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 fk Fro. 30. Fig. 31. Fig. 29. Fig. 33. Fig. 34. is represented in Fig. 30. In a short time afterwards, 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. 31. At a still later period, the fore-legs begin to be INCOMPLETE METAMORPHOSIS IN THE FROG TRIBE. 81 developed, as seen in Fig. 32 ; and from that time they are nourished at the expense of the tail, which gradually disappears (Fig. 33.) During this period, other important changes are taking place in the interior of the body ; the chief of which is the development of the lungs, and the gradual disuse of the gills, so that the animal be- comes 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. in Fig. 34. Fig. 35.— Water-Newt. The perfect form of the Frog is shown Fig. 36.— Axolotl. 97. It has been said that the Frog itself under- goes a more complete meta- morphosis than others of the group. Thus in the Water- Newt (Fig. 35), the tail is retained during the whole of life, and the animal con- tinues 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 sufficiently developed to enable the animals to breathe air, so that they can live either on land or in water, and are thus truly amphibious. In Fig. 36 is represented an animal of this kind, the Axolotl, which inhabits some of the lakes of Mexico. And in Fig. 37 is shown Fig. 37- — Lepidosiren. 82 GENERAL CHARACTERS OF FISHES. the form of a still more remarkable animal, the Lepidosiren, recently brought from the 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 a fish or a reptile, so complete is the mixture of characters which it presents. 98. The class of Fishes is distinguished from all other Ver- tebrata, by the adaptation of the animals composing it, to breathe by means of water in their adult state, so as to be capable of living in that element only. Like Reptiles, they are oviparous and cold-blooded ; and in these characters they differ completely from the whales and other Mammalia, 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 the latter it is flattened verti- cally, that its strokes from side to side may simply propel the fish through the water. This flattening or compression of the body is seen more or less in almost all fishes ; and it is inti- Fig. 38. — Skeleton of the Perch. mately connected with the nature of their motion through the element they inhabit, as it serves the double purpose of dimi- GENERAL STRUCTURE OF FISHES. 83 nishing the resistance which it offers to their progress, and of increasing the extent of the oar-like surface, by the stroke of which the body is propelled forwards (Chap. xn). 99. The preceding figure of the skeleton of the Perch shows the bony apparatus, by which this extended surface is supported. The spinous processes of the vertebrae, which pro- ject upwards from the vertebral column, are long, and are con- nected with another set of bones, which continue them, as it were, into the finny expansion that rises from the back, of which they constitute the framework. There is a corresponding series of bones below ; but they leave a part of the trunk free, to contain the viscera. They form another fin behind, however ; and they spread out at the tail to support its large expanded surface. It is, therefore, by bending its spinal column, that the side stroke of the tail and of the hinder part of the body, is made, for the propulsion of the fish through the water; and thus, in this lowest group of the vertebrated series, the act of motion is per- formed by the vertebral column itself, instead of being committed to the limbs as in Birds and Mammalia. But these limbs or members are not altogether wanting in fishes ; for there are usually one or two pairs of fins (those already mentioned are single, and are placed on the central line of the body) which evidently represent the arms and legs of man. The hinder pair of these is not unfrequently situated nearly as far forwards as the other ; this is the case in the perch, as well as in the mullet, of which a sketch is given in fig. 36. The single fins arranged on the central line, are the first and second dorsal, d 1 and d 2, the caudal or tail-fin, c, and the anal fin, a; at v is seen one of the ventral fins, which correspond to the legs; and above this is shown one of the pecto- _ ' „ _ 1 Fig. 39 — Bearded Mullet. ral fins, which are ana- logous to the arms. These fins are of little use in propelling the G 2 84 GENERAL STRUCTURE OF PISHES. — ARTICULATA. fish through the water; but they give great assistance in raising or lowering it, and in changing its direction. Sometimes one or both pairs of them are absent. 100. 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 manner as the air is in the higher Vertebrata. 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. 101. The bodies of Fishes are usually covered with scales or plates, which have sometimes a bony hardness, and which, in some species of fishes that do not now exist alive, appear to have been even of the density of enamel. Thus we have a sort of transition to the external skeletons of the Invertebrated animals. And in this class also we not unfrequently find the internal ske- leton so deficient in the stony matter from which bone derives its hardness, that it seems like cartilage or gristle ; and in a few of the lowest species, we do not even find a distinct vertebral column. So that the change of character from the Vertebrated to the Inver- tebrated series is a gradual, and not an abrupt one ; and would probably be found to be still more gradual, if we were acquainted not only with all the forms of animal life which now exist, but also with those which have existed in ages long gone by, and are now extinct. 102. In the Articulated subdivision of the animal kingdom, we meet with differences no less remarkable than those we have already seen. In some we find the body furnished with articu- lated members or legs, which constitute its instruments of motion, as in the Sandhopper (Fig. 40) ; and it is in these that the organs of sense are best developed, and that those ganglia of the nervous system which the head contains, exert the greatest influence over the rest. Sometimes, on the contrary, as in the leech, there are no GENERAL STRUCTURE OP INSECTS. 85 Fig. 40. — Sandhopper. jointed members, the nervous ganglia are but little developed even in the head, and they all have nearly the same functions. Hence we might subdivide this group into two, — those which possess articulated members, — and those which are destitute of them. Each of these is again subdivided into classes. 103. 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 pre- sence of wings, but to this there are excep- tions. It includes those of the higher Articulata, which breathe air by means of air -tubes distri- buted 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. 41). 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 ulti- mately to possess ; and it undergoes a series of changes, after the last of which only it presents the form of the perfect insect. In some tribes, the general form is the same from the first, and the wings Fig. 41 Humble Bee. 86 METAMORPHOSIS OF INSECTS. are the only parts deficient ; these gradually make their appear- ance, and the insect is then perfect. Such is the case with the grasshopper and cricket kind ; and a change of this kind is termed an incomplete metamorphosis. But in general, the change of form is much greater ; for that which the egg produces has a form quite unlike that of the perfect insect, and much more resembling the lower classes of Articulata. 104. The caterpillar or maggot of many Insects is com- pletely destitute of legs ; and where it does possess feet by which it may crawl, these are merely fleshy projections, and are not jointed members. The segments or divisions of the body are all equal, or nearly so, as in the worm tribe; and in its habits and mode of life, the larva (as it is termed) is en- tirely unlike the perfect insect. It is extremely vora- cious, and increases rapidly in size; and during this in- Fig. 42- silk- worm. crease, its skin is several times thrown off", and a new one formed, better adapted to its advancing growth. When it has attained its full deve- lopment as a caterpillar, it undergoes a very re- markable change, previously to which it usually forms a protection for itself ; either by weaving a silken thread of its own spinning, into a case or cocoon (as is done by the silk-worm) ; or by gluing together bits of stick, straw, or dead leaves, as is done by many other insects ; or by burying itself in the ground, as do most of the beetle tribe. Beneath this protection it under- goes its first metamorphosis into the state of chrysalis or pupa ; in which it remains completely or partially motionless, and Fig. 43.— Chrysalis of the Silk-worm. METAMORPHOSIS OF INSECTS. 87 takes no food, but lives upon the store which had been deposited in its tissues whilst yet in the larva condition. 105. The period during which it remains in this state is not, however, one of real inactivity ; for changes of a very im- portant nature are taking place within the body. The wings and other parts characteristic of the perfect insect are being deve- loped; and preparation is thus being made for its coming forth into the world, as if after its re-entrance into the egg, in a complete state. In this condition only it can re- produce its kind ; and the fer- tilization and deposition of the eggs, together with the pre- paration of a residence for the young, constitute the great business of the perfect Insect ; FlG- 44.— Silk-worm Moth. which, in many instances, takes no nourishment from the time that it undergoes its last metamorphosis. Those insects, in whose development these three stages are distinctly marked, are said to undergo a complete metamorphosis ; but there are some among these, in which the chrysalis does not completely lose its power of motion ; and whose history, therefore, has a certain resemblance to that of the insects whose metamorphosis is incom- plete. These general characters of the larva, chrysalis, and perfect states of the Insect, will hereafter be not unfrequently referred to. 106. The animals of the class Arachnida, which includes the spiders, scorpions, and mites, are, like insects, articulated, breath- ing 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 representa- tion of the arrangement of the parts contained in these cavi- ties. At c t is seen the cephalothorax 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 of the palpi, which are ap- pendages to the mouth ; pa is the foremost leg ; t, the large 88 GENERAL STRUCTURE OF ARACHNIDA. nervous mass, from which the legs are supplied ; a, the collec- tion of ganglia supplying the abdomen; a b, the abdomen ; p a, Fig. 45— House-spider. the respiratory chambers ; s, the stigmata or openings into these ; ct poab pa s Fig. 46.— Anatomy of Spider. /, the leaf-like folds within them (§.323.) ; m a. the muscles of GENERAL STRUCTURE OF CRUSTACEA. 89 the abdomen ; a n, the termination of the intestine ; /, the spinnerets ; o, the ovaries ; and o r, the opening of the oviduct. 107. The class of Crustacea, of which the crab, lobster, and crayfish 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 being tenants of the air. Moreover the body is enclosed 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 encloses the members, which are never less than Fig. 47 Thelphusa. fl b' Fig. 48. — Anatomy of a Crab. ten in number, and are frequently more numerous. There is great 90 GENERAL STRUCTURE OF CRUSTACEA. variety of form among the animals of this group, which is alto- gether 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 contains is exhibited in the preceding 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 the membrane which lines the carapace and covers in the viscera. On the central line, at c, is seen the heart, which in the Crus- tacea is large and powerful in its action ; from it there passes forwards the artery a 0, which supplies the eyes 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 b' are seen those of the right side, turned back to show their under surface, and to disclose the lower por- tion of the shell, fl. At e 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 convoluted intestine 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, fo. 108. 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- hopper (Fig. 40), 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. 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-Jish,(Fig.51)ih\s variation of form is much less remarkable ; and these members may be used either as legs or claws. And in the Sand-hopper, they closely resemble the other legs. But this is a very small part of the diversity just alluded to. Most of the Crustacea, like insects, come forth from the egg in a state very different from their adult form ; and afterwards undergo a series of changes, which are in some instances so remarkable as METAMORPHOSIS OF CRUSTACEA. 91 Fig. 49.- -Early Form of a Crab. to approach the complete metamorphoses 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. 49. 109. Now the immature Crustacea of different tribes bear much more resemblance to each other, than do the forms into which they are ultimately to be developed; and the differences they afterwards present are chiefly due to a variety in the amount of growth, which the different parts undergo. Thus in the crab, the thorax is developed at the expense of the abdomen or tail ; in the lobster and cray-fish, the hinder part of the body is developed at the ex- pense of the front. This variation in the development of the same parts is most remark- ably shown in regard to the members connected with the segments forming the head and the front of the thorax. In the Limulus or king-crab, of which the under surface is shown in Fig. 50, these members, p, are fully de- veloped around the mouth b ; and they are so formed, as to serve either as legs or claws ; whilst the joints nearest the mouth act the part of jaws. Beneath the hinder portion of the body are appendages, a b, w7hich support the gills. But, as may be readily supposed, these feet-jaws do not perform the functions of movement or of Fig. 50. — Liiuulus. 92 METAMORPHOSIS OF CRUSTACEA. mastication so completely as they would do, if they had either to perform, and were particularly adapted for it ; and accordingly we find, in the higher tribes of this class, that this separation of function does take place ; and that, whilst the members nearest the mouth are converted into jaws, those belonging to the trunk serve as legs and claws. 110. This is the case in the Cray-fish^ the several parts of which are shown in the accompanying figures. In Fig. 51 is ■ mm Fig. 51.— Cray-Fish. Fig. 52. — Masticatory Apparatus. seen the under side of the animal, exhibiting the general arrange- ment of its members. The letters a and b point to the two pairs of antennce or feelers, which many of the Crustacea have, in common with insects ; at c, are the eyes, and at d the imper- fect organ of hearing. At e are seen the external feet-jaws, which correspond to the legs of the limulus ; whilst between / and g are situated the five pairs of true legs developed from the CRUSTACEA. MYRIAPODA. 93 thorax. At h are shown the false legs beneath the abdomen ; at j the termination of the intestinal canal ; and at i, the broad fin- like expansion of the tail, by which chiefly the animal propels itself through the water. In the adjoined series of figures, are represented the various forms of the feet-jaws ; by which it is seen that there is a gradual change, from before backwards, into the character of the other limbs. The first of the six pairs, «, constitute the chief biting jaws or mandibles; the second and third pairs, b and c, are also termed jaws; whilst the three remaining pairs, d, converted < other animal > thrown off< disengaged by the respi- tised compounds J into j_ 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 supply 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 teachings of experience re- specting the fattening of cattle ; for it is well known that this may be accomplished much sooner, if the animals are shut up in a wrarm 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 propensities, 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 development of either his bodily or his mental powers ; but this cannot be said in regard to a diet of which bread is the chief ingredient, since the gluten it contains appears to be as well adapted for the nutrition of the animal tissues, as does 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 134 NUTRITION OF MAN. the climate he inhabits), by the necessity 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-azotised 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 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 possesses over the wandering tribes of hunters which still people a large part both of the old and new continents. 1 64. The mixture of the azotised and non-azotised compounds (gluten and starch), that exists in wheat-flour, seems to be just that which is most useful to Man ; and hence we see the ex- planation of the fact, that, from very early ages, bread has been regarded as the " staff of life." In regard to the nutritious properties of different articles of vegetable food, these may be generally measured by the proportion of azote they contain, which is in almost every instance less than that which exists in good wheat-flour. But it must not be forgotten that, owing to the varieties of constitution which have been pointed out among dif- ferent animals, the power of particular substances to nourish Man and Cattle is not the same, — the latter requiring a larger proportion of the saccharine and oleaginous compounds, than is beneficial to him, — especially when it is an object to cause a large quantity of fatty matter to be deposited in their tissues, or to be excreted in milk. Thus potatoes are found to increase the proportion of butter in the milk of a cow that feeds upon them ; their starch being probably converted into fatty matter. It has been also shown by recent experiments, that the proportion of butter in the milk of a cow allowed to feed during the day in a pasture, and shut up at night in a warm stall, was much greater jn the morning milk than in the evening, — the former containing 5-6 parts of butter in 100, and the latter only 3*7 parts. This was evidently due to the diminished demand for the materials NUTRITION OF MAN. 135 for respiration during the night, when the body was at rest and the skin kept warm. The experiment was then tried, of keep- ing the cow in a shed during the day, and feeding her with the same grass ; and the proportion of butter in her evening milk then rose to 5*1 parts in 100. But this plan diminished the proportion of casein or cheesy matter in the milk, which was increased again by allowing the cow to pasture in the open field. Hence it appears that stall-feeding is most favourable to the production of butter, and pasturing to that of cheese. 165. These principles should be kept in view in regulating the diet of individuals, especially in certain disordered states of the constitution, which require to be treated by strict attention to diet. Thus there are some persons who have a remarkable tendency to the deposition of fat ; and others in whom there is a morbid (or diseased) production of sugar in the body, which is carried off by the urine. In these cases, the diet should be so regulated as to contain the least possible quantity of the saccha- rine or oleaginous principles ; the food being made to consist entirely of animal flesh, with a very small quantity of bread, — or still better, with bread from which the greater part of the starch has been removed. On the other hand, there is a state of the system, known as that in which gout and gravel are liable to occur, in which there seems to be an excess of azotised mat- ter : and the diet of such persons should be so regulated, that very little or no animal flesh should be employed as food, the aliment being made to consist almost exclusively of farinaceous (starchy) substances, such as rice, potatoes, &c. 166. Besides these substances, there are certain mineral in- gredients, 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 (Yeget. Phys., Chap. vi.). Of this kind are common salt, and also phosphorus, sulphur, and lime, either in combination or separate. The uses of Salt are very numerous and important. It consists of two substances of opposite qualities, muriatic acid and soda ; and the former is the essential ingredient in the gastric juice (§. 204) ; whilst the latter performs a very important part 136 MINERAL MATTERS REQUIRED BY ANIMALS. in the production of bile. 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 bone is consolidated. Sulphur exists in small quantities in several animal tissues ; but its part is by no means so important as that performed by phosphorus. Lime is required for the consolidation of the bones, and for the pro- duction 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 crust of our globe is composed, a very large propor- tion is made up of the remains of animals that formerly existed in the ocean. Thus some are almost entirely made up of masses of coral, others of beds of shells, and others of the coverings of animalcules of extreme minuteness. To these ingredients we may also add iron, which is a very important element in the red blood of Vertebrated animals. 167. These substances are contained, more or less abundantly, in most articles generally used as food ; and where they are defi- cient, the animal suffers in consequence, if they are not supplied in any other way. Thus common salt exists, in no inconsiderable quantity, in the flesh and fluids of animals, in the milk, and in the egg: it is not so abundant, however, in plants; and the deficiency is usually supplied to herbivorous 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. Phosphorus 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 ; and 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. So abundant is this in oats, that the horse is subject to an earthy concretion in its intestinal canal ; in which phosphorus is a principal ingredient. 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 MINERAL MATTERS REQUIRED BY ANIMALS. 137 articles of food, more phosphorus is taken into the body than it requires ; and the excess has to be carried out in the excretions. Sulphur is derived alike from vegetable and animal substances. It exists in flesh, eggs, and milk ; also in the azotised 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 ; for there are very few animal or vegetable sub- stances in which it does not exist. It is most commonly taken in, among the higher animals, combined with phosphoric acid, so as to form bone-earth ; and in this state it exists largely in the seeds of most grasses, especially in wheat-flour. If it were not for their deficiency in phosphate of lime, beans and peas would be more nutritious than wheaten-flour, the proportion of azotised matter they contain being much larger. A considerable quan- tity of lime exists, in the state of carbonate and sulphate, in all hard w^ater. When an unusual demand exists for lime, how- ever, 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 carbonate of lime, which is required for the consolidation of the shell ; and if this is 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 the aquatic Mollusca depends greatly upon the quantity of lime in the sur- rounding 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 quantity 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. 169. It is a very curious question, from what source the coral-forming Polypes have obtained the enormous quantities of 138 SOURCES OF LIME IN ANIMAL STRUCTURES. lime that have served to build up their gigantic structures. It seems probable that, in the parts of the globe where they now most abound, which are the seats of frequent volcanic action, there are springs whose waters contain a large quantity of car- bonate of lime (as hot springs in the neighbourhood of volcanoes, such as the Geysers in Iceland usually do) ; and that these, emptying themselves beneath the sea, furnish the required sup- ply. Thus the materials required by the coral animals are being continually supplied from the deep-seated rocks beneath. And there is further reason to believe, that a similar process formerly took place through a much wider portion of the surface of the globe, and that the waters of the sea were much more loaded with carbonate of lime ; for in this way only can we account for the vast limestone beds, formed of successive growths of coral, which constitute a great part of our solid land, their thickness being many hundred feet, and in some places two or three thou- sand feet. We may reasonably suppose that, by the operations of these creatures, continued through many ages, the proportion of lime dissolved in the waters of the sea was gradually reduced, and at last brought down to its present amount; just in the same manner as the atmosphere seems to have been gradually purified from a large quantity of carbonic acid gas (which would have been fatal to warm-blooded animals), by the continued growth of those luxurious forests, which, having been buried beneath other deposits, have come down to us as coal. (See Yeget, Phys., §. 298.) 170. The mode in which the Crustacea, whose calcareous shell is periodically thrown off (§. 107), 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 thrown off, this matter is taken up by the blood, and is thrown out from the surface, mingled with the animal matter of which the shell is composed. This hardens in a day or two, and the new covering is complete. The concre- tions in the stomach are then found to have disappeared; but they are gradually replaced, before the supply of lime they con- tain is again required. CHAPTER 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 pro- cesses are performed, in some animals, in a part of the digestive tube intermediate between the mouth and stomach, and even in the latter itself. The stomach is usually situated at some dis- tance from the mouth, and is connected with a tube called the oesophagus or gullet ; and the passage of the food into this, con- stituting 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 its alimentary portion is completely dis- solved, 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 the nutritious por- tion, or chyle, is separated from the matters that are to be thrown off: this process, which is the second stage of true digestion, is termed chylijication. The rejected portions of the food, with 140 PREHENSION OF FOOD. secretions poured into the alimentary canal, find their way out through the intestinal tube ; and are voided at its second orifice by the act of defecation. And lastly, the nutritious chyle is taken up by vessels that are distributed upon the walls of the digestive cavity, and undergoes a gradual change by which it is converted into blood. These two processes are called absorption and sanguification, (or manufacture of blood). Each of the fore- going stages will now be separately considered. Prehension of Food. 172. The introduction of aliment within the entrance to the OUSTITI. digestive cavity is accomplished in various methods in different 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 (Fig. 80) the division of labour is car- ried further; the food being laid hold of by the ante- rior members, or hands, and by them carried to the mouth. Where tho hand has the power of grasping, and especially Fro. 81. — Sqwrhb PREHENSION OF FOOD. 141 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 grasping. 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 to glue it to the surface, which is covered with a viscid saliva. The Giraffe uses its long tongue to lay hold of the young shoots on which it browzes ; 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 of In- sects, of which a pair is attached to each jaw (Fig. 84) ; the tentacula of fig. 82—Head of ele^Int. the Mollusca, which are sometimes extremely prolonged, as in Fig. 84.— Jaws of the same Insect. Fig. 85 — Lougopsis. 142 RECEPTION OF FLUIDS. the cuttle-fish tribe (Fig. 85) ; and the similar organs of the polypes (Fig. 1). 173. The reception of fluids is accomplished in two ways. Sometimes the liquid is made to fall into the mouth, simply by its own weight ; in other instances it is drawn or pumped up by 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 sup- ported by liquids which they find in plants, or which they draw from the bodies of other animals on which they live as parasites. This is the case with many insects ; and their mouth, instead of pre- & « sen ting the ordinary structure, is formed into a sort of tube or trunk, very much extended, through which the juices are drawn up accord- ing to the wants of the animal. Such a conforma- tion exists in the butterfly and moth tribe, whose trunk, when not in use, is coiled up in a spiral beneath the head ; as is shown in Fig. 87, re- presenting the head of a 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 di- pterous (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. Chimpanzee drinking. Fig. 87-— Trunk ok a Butterfly. STRUCTURE AND DEVELOPMENT OF TEETH. 143 Mastication. 174. The act of Mastication, or the mechanical division of the alimentary matter, is effected in most of the higher animals, Fig. 88. — Nemestrina Longirostris. by teeth, which are implanted in the jaws. These are composed of a substance that bears a strong resemblance to bone in its texture and hardness ; and are so fixed as to act against one another, with a cutting, crushing, or grinding power, accord- ing 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 de- veloped in the interior of a d little membranous sac, which is lodged in the thickness of the b ° Fig. 89. — Development of Teeth. c jaw-bone; as seen in the ac- a, the gum ;&, the lower jaw ; ,3 angle of companying figure, which repre- the Jaw > d> dental capsules, sents 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 ivory of the tooth, which 144 DEVELOPMENT OF TEETH. in man constitutes nearly its whole structure ; this conversion takes place first at its highest points, 4> d. The crown or upper d d portion of the tooth receives a covering of enamel, a much harder substance, which is g&tflfa formed by the lining of the capsule. Gradually jfc^jl the process of conversion extends more and ^^^^Sf.. more to the interior of the pulp ; and at last the whole is changed into ivory, with the fig 90 —Dental exception of a small portion that still remains, capsule. occupying what is termed the cavity of the tooth, which is frequently laid open by decay of its external wall. As the 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 tendency 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, con- vulsive actions of various kinds, on the principles hereafter to be explained (Chap, x.) ; and as the removal of the source of irritation, in such cases, is of the greatest 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 interfere with the course of nature. 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 not to possess the power of repairing injuries occa- sioned by disease or accident. Hence a tooth which is broken or decayed, is not restored as a bone would be. Still, however, STRUCTURE AND DEVELOPMENT OF TEETH. 145 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 reme- dies 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 with pulp. Such have no proper root; for additional mat- ter is being continually formed at their base, and thus the Whole tooth is Fl<5- 91.-Jaw and Teeth of Rabbit. 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 Rabbits, Squirrels, Rats, and other gnawing animals (Fig. 91). The upper edges of these teeth are being constantly worn away by use ; but they are kept up to their proper level by the growth of the tooth from below. But it sometimes happens that one of these teeth is broken off; and the one opposite to it in the other jaw is then thrown into disuse. 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 incon- venience to the animal. 178. The teeth are generally made up of two or more distinct substances, differing in structure and properties. That which usually forms their principal part, is termed the ivory ; but the summit or crown of the tooth is generally clothed with a much harder substance, which is termed enamel; and its fang is covered with a substance closely resembling bone, and termed the cortical substance. Although the ivory, enamel, and cortical substance occupy these different positions in most teeth, they are all mixed together (as it were) in the grinding teeth of many herbivorous animals, to answer a particular purpose (§. 182). 146 STRUCTURE AND COMPOSITION OF TEETH. 179. All these substances contain a large proportion of mineral matter. The amount is the greatest in the enamel, which is generally hard enough to strike fire with steel ; this does not contain above two parts in one hundred of animal tissue. In the ivory, the proportion of animal matter is larger, varying from 20 to 29 per cent. And in the cortical substance, it is about 42 per cent.; which is nearly the same proportion as that which exists in bone. Of the mineral matter by far the largest part consists of phosphate of lime ; most of the remainder being the carbonate of lime (the same as limestone or chalk). 1 80. When examined with the microscope, the Ivory is seen to contain a number of very minute wavy tubes, which commence in the central cavity and pass towards the surface of the teeth ; these appear analogous to the minute tubes which have been described as contained in Bone (§. 48). The Enamel, when examined in a similar manner, is seen to consist of a multitude of six-sided columns or prisms, packed closely against each other, and directed perpendicularly to the surface on which they lie, so that one of the extremities of each column rests upon the ivory, whilst the other helps to form the surface of the tooth. When the mineral matter is dissolved away by an acid, the remaining very delicate animal tissue still presents the same form ; so that we may regard it as composed of a number of six-sided prismatic cells, precisely resembling those of Plants (See Veget. Phys. §. 71), in which the mineral matter is deposited. The structure of the Cortical substance is of the same character with that of Bone, but is less regular. 181. In Man, and 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. 92). Others have more of a conical form, and in many animals (especially those of carnivorous habits) project far beyond the former ; they are not adapted 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 a large irregular flattened surface, and are TEETH OF MAN AND OTHER MAMMALIA. 147 adapted to bruise and grind tlie food ; these are called molar (or mill-like) teeth. The manner in which these different teeth are Molars. Bicuspid. Canine. Incisors. Fig. 92 Human Teeth. implanted in the jaw, varies with the form of their crown, and is in accordance with their several uses. The incisors, whose action tends rather to bury them in their sockets, than 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, pene- trate the jaw more deeply than the incisors ; especially when they are large and long, as in the cat tribe (Fig. 93). 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 cutting edges, which work against each other like the blades of a pair of scis- sors (Fig. 93) ; whilst in animals that live on insects, these teeth are raised into conical points, which lock into correspond- ing depressions in the teeth of the opposite jaw (Fig. 94). When the nourishment of the animal consists principally of soft l 2 148 DIFFERENT FORMS OF TEETH. fruits, these teeth are simply raised into rounded elevations (Fig. 96) ; and when they are destined to grind harder vegetable Fig. 93.— Tekth of Carnivorous Animal. Fig. 94— Teeth of Insectivorous Animal. substances, they are terminated by a large flat and roughened surface (Fig. 95). The roughness of this surface is maintained, by the peculiar arrangement of the three substances, of which the Fig. 95. — Teeth of Herbivorous Animal. Fig. 96. — Teeth of Frugivorous Animal. 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 cortical substance. 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 sur- face, which is admirably adapted to the grinding action the tooth is destined to perform. In the great gnawing teeth of the squirrel, &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. 1 83. 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 ABSENCE OP TEETH IN THE WHALE. WHALEBONE. 149 Fig. 97 — Skull of Boar. 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 are chiefly employed among carnivorous animals, for the purpose of seiz- ing their living prey, and are never deficient in them; but they are less required in herbi- vorous animals; and either or both kinds are not unfrequently deficient. Sometimes, however, they are not only present, but are largely developed, serving as weapons of attack and defence ; as in the Boar (Fig. 97). 184. 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 there hang down into the mouth a number of plates of a fibrous substance, to which we give the name of whalebone, though it 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 water is drawn, in enormous quantities, whenever the Whale is in want of food ; and in this manner it strains out, as it were, the minute gelatinous ani- mals upon which it lives, — such as the lit- tle Pteropods (§. 122) and MedusEe (§. 130), which abound in the Fig. 90. F,g. 98,-Skull ok Whale. Seas it inhabits. The Whalebone. water thus taken in is 150 ABSENCE OF TEETH IN WHALE, ANT-EATER, &C. expelled from the nostrils or blow-holes, which are situated at the top 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 Ruminant quadrupeds (oxen, sheep, &c), though they never show them- selves above the gum. 185. 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. — Lastly, may be mentioned as a curious exception Fig. 100. — Skull o* the Ant-eater. Fig. ]<>] .— Ornithorhyncus TEETH OF REPTILES AND FISHES. 151 to the general rules respecting the teeth of Mammalia, the remark- able Ornithorhyncus, or Duck-billed Platypus of New Holland (Fig. 101). This animal feeds, like the duck, upon the water insects, shell-fish, and aquatic plants, that it obtains from the mud, into which it is continually 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. 186. Among Birds, there is an entire absence of teeth ; and the mechanical division and the reduction of food is performed in the stomach, in the manner hereafter to be mentioned (§. 200). The mouths of almost all Reptiles, excepting the Turtle tribe, are furnished with numerous teeth ; 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 ; and their shape is consequently nearly uniform, being for the most part simply conical. There are some Lizards, however, which are herbivorous; and these have large rough teeth, somewhat resembling the molars of Mammalia. Animals of this tribe attained a gigantic length, not less than from 90 to 120 feet, in past ages of the world. — In Fishes, the teeth are commonly very numerous ; but they have for their object only to separate and retain their food ; and there is little variety in their form. Frequently they have no bony attachment, being only held by the gum as in the shark ; and they are conse- quently often torn away, but they are 102.— Head of Gavial (Crocodile of the Ganges). Fig. 103.— Head of Shark. as readily replaced. a continuation of tl Sometimes, however, the tooth seems like le bone of the jaw, not being in any way 152 CHANGE OF FIRST SET OF TEETH. separated from it, and the tubular structure of the latter being con- tinued into it without any interruption. The teeth of Fishes are often set, not only upon the proper jaw-bones, but upon the sur- face of the palate, and even in the pharynx or swallow. — In the Invertebrata, there are generally no proper teeth ; but in both the Articulated and Molluscous series, we very commonly meet with firm horny jaws, which are often furnished with projections that answer the same purpose. It is very remarkable, however, that in an animal so low in the scale as the TLchinus or Sea-Urchin (§. J 28), a very complex 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, which seems to be chiefly of a vegetable nature. 187. 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 20 in number, — namely, four incisors in the front of each jaw, and two canines and four molars on each side. They 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 respectively ; 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. 92). 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 MOTION GIVEN TO THE TEETH. 153 known as wise or wisdom teeth. There are then 32 teeth in all, or 16 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 obliterated. 188. 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 independently 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. 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 instru- ment. 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 masti- cating its food, that the lower jaw has considerable power of motion from side to side. On the other hand, in the gnawing animals, furnished with two large front teeth, the lower jaw has no powder of moving from side to side, but is rapidly drawn backwards and forwards ; and, as the ridges of the molar teeth are arranged in the opposite direction, they become very powerful filing instruments, by which the toughest vegetable substances are quickly reduced. 1 89. 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, 154 MOTIONS OF JAW. INSALIVATION. 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 pro- , 7y, ^ cess or projection of the lower jaw (a, Fig. 104), about an inch in front of the joint. As the distance from the fulcrum, of the point at which the power is applied, is thus much less than that of the front of the jaw, where chiefly the re- sistance is encountered, the power of the muscle is applied at a me- chanical disadvantage; and, to overcome a given resistance, the Fig. 104. 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 (Mechanical Philosophy, §. 292). Insalivation. 190. The act of mastication is connected with another; which is also of great importance in preparing for the subsequent 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 neighbourhood of the mouth; of these there are three pair in man, two beneath the tongue (Fig. 105), 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 SECRETION OF SALIVA, AND ITS USES. 155 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 which 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 the mouth, and moving it from side to side. 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 the impressions are conveyed, over the act of secretion. 191. There are certain kinds of food, in which the admixture of saliva appears to occasion the commencement of those chemical changes in which digestion consists ; but in general, the benefit derived from this process 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 which he is about to dis- solve in a larger amount. If the preliminary operations of mas- tication 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 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 mas- tication 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 156 DEGLUTITION OR SWALLOWING. by artificial means, the next best remedy is to cut the food into very small portions, 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 movable curtain, which is known as the veil of the palate (Fig. 105) ; and this hangs down during mastica- tion, in such a manner as to prevent any of the food from pass- ing backwards. This partition, which does not exist 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 Veil of the palate Nose Pharynx I §§|flf|f|f|i'] ;' . - -., - afi§| Tongue ¥__ "*" Salivary glands Os hyoides 1 l/IV ' — ■ Larynx Thyroid gland Oesophagus Trachea Fig. 105.— Perpendicular Section of the Mouth and Throat. of the tongue into a kind of ball ; and this, being carried back- wards by the action of its muscles, presses against the partition just mentioned, and causes it to open. The food thus passes DEGLUTITION OR SWALLOWING. 157 into a sort of funnel, formed by the expansion of the top of the oesophagus or gullet ; this cavity, termed the pharynx, commu- nicates above with the nostrils, and in front with the larynx, which is at the top of the windpipe. The wsophagus is a long and narrow tube, which descends from the pharynx to the sto- mach, lying just in front of the vertebral column, and behind the heart and lungs. It is surrounded by muscular fibres, dis- posed 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 windpipe. In order 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 body, 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 of 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 sub- stance may slip backwards, by its own weight, into the glottis, when there was no intention of swallowing, and when the larynx was consequently not drawn forwards beneath the tongue. The presence of such a substance in the windpipe excites a violent, and frequently 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. But if this does not occur, it is necessary to remove the offending body in other ways ; sometimes it may be removed by an aperture made in the windpipe ; but if it cannot be laid hold of and drawn through this, the plan recently adopted in the case of Mr. Brunei (the celebrated engineer) into whose windpipe a half-sovereign had unfortunately found its way, may be advan- tageously employed. He was fixed upon a board that was made 158 MOVEMENTS OF DEGLUTITION. to revolve upon a pivot, in such a manner that his body was brought into a very inclined position, with the head downwards, a position which could not, of course, be retained for a long time at once. The coin then dislodged itself by its own weight, from its place in one of the bronchial tubes (§.328), and wTas felt to drop towards the glottis. But it there produced so violent an irritation, as to bring on a cough which threatened suffocation ; and the attempt was abandoned. It was renewed on another occasion, however, after an opening had been made into the windpipe (for the purpose of extracting the coin through it, which was attempted unsuccessfully) ; and as the admission of air into the lungs through this opening prevented any chance of suffocation, the inclined position of the body was continued, until the coin dropped through the glottis into the mouth. 195. The act of swallowing is itself involuntary, and may be even made to take place against the 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 ; but when it has arrived there, it is laid hold of, as it were, by the muscles of the pharynx, and is then carried down involun- tarily. It has several times happened, that a feather, with which the back of the mouth was being tickled to excite vomit- ing, 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 back- wards 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. This action is one of the kind now denominated reflex. It is produced through the nervous system ; for if the nerves supply- ing 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 make on the lining of the pharynx; this impression, conveyed through one set of nerves, excites in the MOVEMENTS OF DEGLUTITION. CHYMIFICATION. 15.9 spinal cord a motor power ; which, being transmitted 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 func- tions : 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 paralysed 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 performed. In the severest cases of apoplexy, however, the power of swallowing is lost ; and this is a symptom of great danger, since it showTs 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 (Chap, vi.) will probably soon cease, so that death must ensue. Chymification. 197. The food, thus propelled downwards by the action of the muscles of the pharynx and of the oesophagus (gullet) arrives in Man and the Mammalia, at the stomach; which is a large membranous bag, placed across the upper part of the abdomen (Fig. 106). 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 digestive tube ; this is the case in the Cat tribe, for example. In Herbivorous 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 digestive tube, but a bag or sac that bulges out, as it were, on the left side of that canal. By the degree of 160 FORM OF THE STOMACH. this bulging, we can judge of the nature of the food on which the animal is destined to live. Thus in Man (Fig. 106), the large end of the stomach, situated on the left side (the right side Gall-Bladder __ Large Intestines Spleen Colon Small Intestine Colon Small Intestine Rectum Fig. 106.— Digestive Apparatus of Man. 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 sometimes forms a distinct pouch. STOMACH OF RUMINATING ANIMALS. 161 198. The most complex form of the stomach among Mam- malia, 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 process of digestion. The external appearance of the stomach of the Sheep is seen in Fig. 107 ; and its interior is displayed in Fig. 108. The food of the Ruminant animals is not chewed by Oesophagus Intestine Pylorus 4th Stoni. 2d Stom. 1st Stom. Fig. 107 — Stomachs of the Sheep. them, before it is first swallowed. In their wild state, they are peculiarly exposed to the attacks of their carnivorous 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 exposed to long-conti- nued 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, and masticate it at their leisure, when they have retreated to a place of safety. The crude un- masticated 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 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 communication with the oesophagus, 162 STOMACH OF RUMINATING ANIMALS. and appears destined especially to receive the fluid that is swal- lowed ; for this passes immediately into it, without going into the first stomach at all. The folds of its lining membrane seem des- tined to present a large surface, through which fluid may be absorbed into the system. It is 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 are nothing else than the little pits which are seen in the honey- comb stomach of the Sheep ; they are much deeper, however, and each may be closed at the top by the drawing-together of its orifice. This is accomplished by a set of muscular fibres which form a net work, passing in every direction round the orifices ; and a similar arrangement of these fibres is seen even in Man, in whose stomach the cells do not exist. 199. In the second stomach, the food transmitted from the first is rolled up, as it were, into balls, which are returned at intervals to the mouth. There they are completely reduced by mastication and insalivation ; and are then ready for the real process of digestion. It is this mastication which is commonly Intestine Honeycomb Paunch Fig. 108. — Section of the Stomachs of the Shekf. 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 manyplies, from the peculiar manner in which its lining membrane is arranged. This presents STOMACH OF RUMINANTIA ; — OP BIRDS. 163 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 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 at that time, 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 consider- able modification of form, resulting from the fact that, as these animals do not masticate their food, they require some 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 oesophagus has a pouch-like dilatation, termed the crop or craw; in this it is retained, and exposed to the action 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 granivorous (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 M 2 164 DIGESTIVE APPARATUS OF BIRDS. solution ; the walls of their gizzard are consequently thin, pos- sessing but few tendinous fibres ; and the three cavities of the stomach are almost united into one. (Esophagus Ventriculus Succenturiatus Gizzard Pancreas Duodenum Cceca ~ Large Intestine Ureter - Oviduct .... Cloaca Anus Fig. 119. — Digestive Apparatus of Fowl. 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 DIGESTIVE APPARATUS OF INSECTS. 165 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 mas- tication in their mouths, the grinding down of their food (where it is of such a nature as to require it) must be per- formed in their stomach ; and accordingly we find many tribes, belonging to different divisions of the animal king- dom, in which a gizzard, or something analogous to it, exists. It is possessed by almost all the Cephalopoda, and by many of the Gaster- opoda. In the walls of the stomach of one 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 di- gestive stomach (Fig. 110, c). The accompanying figure ex- hibits the alimentary canal of a Beetle, from its commence- Fig. 1 10. — Digestive Apparatus of Beetle, 166 VARIOUS FORMS OF DIGESTIVE APPARATUS. ment to its termination. At a is seen the head, bearing the jaws, &c. ; from this the gullet passes straight backwards, and is dilated into a crop at £>, 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, , to the system at large ; and from this it is collected, in the state of ve- nous blood, by the veins Fig. 133.— Circulating Apparatus of Doris. , . . which terminate in the large CIRCULATION IN MOLLUSCA. 229 trunk//. By this trunk it is distributed to the gills e; and thence returns to the heart, after it has undergone aeration. Now if a second heart had been placed on the trunk//, 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 Mammalia. There is great variety in the position of the gills in Molluscous 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 form, round the termination 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 Reptiles ; 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 Fig. 134 a cs vv Apparatus of Cuttle-fish. of the heart in the Cephalopoda, or Cuttle-fish tribe ; which seems to form a connecting link between the plan of the circu- 230 CIRCULATION IN MOLLUSCA. lation that prevails among the Mollusca in general, and that 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. 134 ; where c represents the ventricle or systemic 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 v c, covered with a curious spongy mass c s, the use of which is not known ; this also receives the blood from the intestinal veins v v ; and it divides into two trunks which convey the blood to the gills or branchiae (br and br'), where it undergoes aeration. On each of these trunks is an enlargement, c b, which has the power of contracting and dilating, and thus of assisting the transmission of the blood through the arteries of the gills, ab. The blood is returned to the ventricle by the branchial veins, v b0 on each of which there is another dilatation, bu, which might be regarded as analogous to the auricle of the other Mol- lusca, but is not muscular. Thus in the Cuttle-fish, the blood receives an impulse from the systemic heart, when it is trans- mitted into the main artery; and when it returns by the sys- temic 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 Vertebrata. 292. In the Crab and Lobster, and other animals of the class Fig 135. — Circulating Apparatus of Lobster. CIRCULATION IN CRUSTACEA. 231 Crustacea, the blood for the most part follows the same course as in the Mollusca ; excepting that the heart contains but a single cavity ; and that there are no distinct vessels for veins, these being replaced by irregular channels which are excavated in the tissues, and which present occasional enlargements, termed venous sinuses. The arrangement of the circulating apparatus of a lobster is seen in Fig. 135, in which a is the heart ; b and c the arteries to the eyes and antennae ; d, the hepatic artery ; and e and/, the arteries which supply the thorax and abdomen. The blood that has been propelled through these by the action of the heart, finds its way into the great venous sinus gg, 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 h w c f vb Fig. 136, which repre- sents a transverse sec- tion of it in the region of the heart, with one va pair of gills. The heart is seen at c; and from its under side proceeds one of the arterial trunks, which conveys the blood to the system. Returning 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. 126. 293. In the class of Insects, we seem to lose all trace of distinct vessels for the conveyance of the blood. Neither arteries nor veins can be detected : but the nutritive fluid is diffused st ce 232 CIRCULATION IN INSECTS. through the channels or interstices that exist among the different organs. Nevertheless, it has a tolerably regular circulation ; and Pig. 137- —Circulation in Insects the organ by which this movement is chiefly effected is a long tube, termed the dorsal vessel^ which seems to propel it forwards, whilst other trunks on the two sides 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. By the successive contraction of these different portions, the blood, which entered at the poste- rior extremity of the dorsal vessel, is gradually propelled for- wards ; 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. It is to be remarked that in Insects no special arrange- ment of vessels for the aeration of the blood is required ; since this aeration is accomplished by the conveyance of air, by means of minute air-tubes, into every part of the body, however small (§. 320) ; a mode of respiration different from any that we notice CIRCULATION IN ANNELIDA AND ECHINODERMATA. 233 elsewhere. A very similar arrangement 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. 138 ; where a re- presents 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 respir- atory organs. 294. In the animals of the Worm tribe, belonging to the class Annelida, there is a general similarity in the course Of the blood to that which prevails in Fjg. 1 38.— Dorsal Vessel of Insects ; but as the respiration is ac- ~ SpiDER- complished by means of special organs, which are sometimes diffused along the entire body, and sometimes restricted to one part of it (§. 115), there is considerable variety in the provisions for submitting the blood to the influence of the air; and as these are rather curious than interesting, they will not be here described.* It seems probable that, in some at least of this class, the blood does not always flow through the vessels in the same direction, but that its course is occasionally changed. Such a change occurs in the lowest group of Mollusca, the Tunicata; and also in Zoophytes. 295. In the animals of the class Echinodermata, the Star- fish, Sea-urchin, &c, 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 enlargement 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, as in plants (§. 280). 296. This is still more evidently the case in the animals that * For a full account of them, see the Author's Principles of General and Com- parative Physiology, §. 345 — 350. 234 CIRCULATION IN ZOOPHYTES. unite into those remarkable compound structures, which we term Zoophytes. Even in the thin walls of the body of the Hydra, and of the polypes resembling it in structure, and also in the arms, a movement of fluid takes place, through channels which seem excavated in the soft tissue. But the most curious circu- lation is that which is seen in the stems and branches of the Ser- tularia, and some other compound polypes (§. 131). When these are examined with a high magnifying power, a current of fluid containing granular particles is seen running along the axis ; and this, after continuing one or two minutes in the same direction, changes and sets in the opposite one, in which it continues about as long, and then resumes the first ; — thus alternately flowing down the stem to the extremities of the branches, and back again. The rapidity and constancy of these currents seem to depend on the activity of the growth of the parts towards which they are directed. 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 com- pletely, a great disturbance soon takes place in its various func- tions ; shortly afterwards, the various actions of life cease to take place ; and a state of apparent death comes on, which speedily becomes real, if air be not readmitted. The influence of air is not less necessary to Plants than to Animals ; for they also die when excluded from it (See Veget. Phys., §. 286. ) : and thus it may be stated to be a general condition, necessary for the con- tinuance of the life of all organised beings. 298. At first sight it might be thought that Animals which always live beneath the surface of the water, such as Fishes, and many Mollusca, are removed from the influence of the air ; and that they hence constitute an exception to this general law. But this is not the case ; for the liquid which they inhabit has the power of absorbing, and of holding dissolved in it, a certain quantity of air, which they can easily separate, 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 they are prevented from inhaling air in the ordinary manner. 236 NATURE OF THE CHANGES CONSTITUTING RESPIRATION. 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 im- portant part of their vital actions ; and the changes themselves, 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 Bespiration . 300. Atmospheric air, it has been stated, is necessary to the continued life of all animals; but this fluid is not composed 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 contains 21 parts in 100 of oxygen, and 79 parts of nitrogen or azote ; with traces of carbonic acid gas. The question immediately presents itself, therefore, whe- ther these gases have the same action on animals ; or, if their actions are 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 living animal in a jar filled with air, and cut off all com- munication with the atmosphere, it perishes by suffocation 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 ajar filled with nitrogen gas, it perishes 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 disappears is replaced by a new gas, carbonic acid ; which, instead of being favourable, like the preceding, to the maintenance of life, causes CARBONIC ACID SET FREE, AND OXYGEN ABSORBED. 237 the death of animals which inhale it, even in small quantities. The production 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 ; and it appears that the principal use of this gas is to dilute the oxygen, which, in a state of purity, would act too powerfully on the animal system, and produce in it a state somewhat resembling that of fever. It ap- pears, however, that there is a continual absorption of nitrogen by the blood ; and as continual an exhalation of it : and this may be shown to be a necessary result of the laws regulating the dif- fusion of gases, by which the interchange of elements between the air and the blood is regulated. (See Mechanical Philo- sophy, §. 4J.) When the quantity exhaled, and the amount absorbed, are equal or nearly so (which is usually the case), 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. 303. The differences in the character of the blood which are produced by its exposure to the air have already been no- ticed (§. 227) ; and we now see that they are essentially due to the absorption of oxygen, and the setting-free of carbonic acid. The same change 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 drawn from a vein into a basin or cup, the dark hue of the sur- face is usually seen to undergo a rapid change, so as to present the arterial tint ; but this change is confined to the upper sur- face, 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 two be cut off by a mem- branous partition, as it is in the living animal : for if the blood 238 CARBONIC ACID SET FREE, AND OXYGEN ABSORBED. 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 mani- fested, not only by the alteration in the character of the blood, but by the disappearance of a certain quantity of oxygen from the air, 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 con- tinually change its surface, we render the change more complete. But 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 in- fluence 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 ? 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 considerable disengage- ment of heat. But 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 res- piration, and those of the combustion of carbon ; and this agreement in the results naturally leads to the belief that the causes of both are the same. 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 CARBONIC ACID SET FREE, AND OXYGEN ABSORBED. 239 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. 306. This has been proved in various ways. In the first place, it may 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 may also be shown that arterial blood contains a considerable quantity of oxygen. Hence the inference seems unavoidable, that the blood, in passing through the lungs, gives off carbonic acid, and takes in oxygen. This view receives full confirmation from the fol- lowing experiment. If Frogs, Snails, or other cold-blooded animals, be confined for some time in an atmosphere of nitrogen or hydrogen (two gases which do not themselves exert 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 sup- plied ; and showing that the carbonic acid must have been brought ready-formed. But this process 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 taken in, and of carbonic acid thus disengaged, bears a very regular proportion to the amount of muscular exertion which is made during the same time. Hence it is much greater in tribes whose habits are active than in those who are inert ; and it is also greater in the same individual, during a day spent in active exercise, than it is during the same period passed in repose. It is a well-known physiological fact, that, for the continued energy of a muscle, a continued supply of arterial blood is required ; and the use of this appears to be two- fold. There is much reason to believe that every muscular "240 CARBONIC ACID SET FREE, AND OXYGEN ABSORBED. contraction, or exercise of vital force, is necessarily accompanied by the death of a certain amount of muscular tissue ; 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 (§. 160), 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 muscular force ; and the elements of the blood itself are required to re-form the tissue which has been thus destroyed. 308. Hence we see why the quantity of carbonic acid thrown off by respiration should be increased by muscular exer- tion. It is among Birds and Insects that we find the greatest quantity produced, in proportion to the size of the animals ; and in both these classes, we find extraordinary provisions for the energetic performance of this function (§§• 320 and 326). The greater energy of the respiration of Birds than that of Mamma- lia, when compared with the greater number of the red cor- puscles 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 (J. 234). 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. 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 1 20 respiratory movements in a minute ; after the lapse of an hour, they had sunk to 58 ; and they subse- quently fell to 46. In the first hour of its confinement, it pro- duced 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. HYBERNATION . 24 1 In the larva state, which is usually one of comparative inactivity, the respiration is not above that of the neighbouring tribes, such as the Myriapoda ; and in the Chrysalis state of those which become completely inactive, the amount of respiration is still lower. 309. This Chrysalis state bears a strong resemblance to the torpid condition, in which many animals pass the winter. Rep- tiles 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 exhibit- ing all their usual actions, when the temperature rises again. In this state, their circulation and respiration appear to cease entirely; or, if the functions are performed at all, they take place with extreme feebleness ; and the animals may be prevented from reviving for a long time, without their vitality being per- manently 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. Snails may be reduced to a similar state by the want of water ; and may be kept in it for almost any length of time. Among Mammalia 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 ordi- nary deep sleep. Thus some of the animals which hyhernate 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 ordi- nary 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. It appears from Dr. M. HalFs experiments and observations, that the Bat, when in a state of complete torpidity, consumes no oxygen, and forms no carbonic acid, though its circulation continues languidly. But a very slight irritation is sufficient to produce respiratory move- ments. 310. Animals will in general bear deprivation of air well or 242 VARIOUS DEGREES OF ACTIVITY OF RESPIRATION. badly, according as the respiration is more or less active. Thus a warm-blooded animal usually dies, if sunk 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 continue for a much longer time without the loss of life. Thus the common Water- Newts naturally pass a quarter of an hour or more beneath the surface, without coming up to breathe ; and they may be kept down for many times that period, without serious injury to them. And, as we might expect from their peculiar condition, warm-blooded, hybernating animals may be kept beneath water for an hour or more, without apparent suffering ; although the same animals, in their active condition, would not survive above three minutes. There is reason to believe that a similar con- dition 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 organisation 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 Animalcules, 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 Frog 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 its structure modified for the advantageous performance of this function. This modifica- tion consists in the peculiar vascularity of this membrane, that is, EXTENT OF SURFACE GAINED BY SUBDIVISION. 243 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 we shall find that this surface may be immensely extended, without any increase in the size of the organ. Thus the small lungs of a Rabbit 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 sub- division 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. Now 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 cham- ber previously exposed. And thus, by the subdivision of the first chamber into four smaller ones, the extent of surface has been doubled. Now 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 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 prolonged outwardly, so as R ?. 244 ORGANS FOR AQUATIC RESPIRATION. to form gills, which hang from the exterior of the body (as is the case in most aquatic animals), its surface is very much ex- tended by disposing it in folds, and by dividing these folds into fringes, so as to expose a large surface to the water. 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 respiratory organs differ according as they are destined to come in contact with air in the state of gas, or to act upon water in which a certain amount of it is dissolved. In the former case, we usually find the respiratory membrane (which is but a prolongation 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 in- terior of the body, exists in all air-breathing animals ; and in the Vertebrata it receives the name of lung. On the other hand, in animals that breathe by means of water, the respiratory sur- face is prolonged externally, so as to be evidently but an exten- sion of the general surface, — just in the same manner as the roots of plants are prolonged into the soil around them. These pro- longations, termed 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 Fro. 139.— Gill-tuft of other instances, they are composed of a Eunice. number of branching tufts, which are more copiously supplied with vessels. In the class Annelida, we RESPIRATION IN AQUATIC ARTICULATA. 245 observe a great variety in the mode in which these tufts are dis- posed ; and this is connected with the general habits of the animal. Thus in the Serpula (Fig. 58), whose body is enclosed in a tube, the tufts are disposed around the head alone, and spread out widely into the semblance of a flower. In the Nereis (Fig. 57) 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. 139, 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 letters t and ci point to other pro- jections, which also seem connected with the move- ments of the animal. In the Arenkola (the lob- worm of fishermen) we find the respiratory tufts disposed on cer- tain segments only, and possessing more of an arborescent (tree-like) form (Fig. 140). 315. A somewhat similar arrange- ment is seen in the larvae of many aquatic Insects, which breathe by means of gills ; although all perfect Insects breathe air in the manner to be presently described. In Fig. 141 is represented the larva of the Ephe- mera (May-fly), which breathes by means of a fig. ho.— areni- 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 enclosed within a cavity, through which a stream of water is made constantly to flow, by mechanism adapted for the purpose. Their form and position in the Crab are shown at b, b\ Fig. 48. Although m Fig. 141.— Larva of Ephemera. 246 RESPIRATION IN MOLLUSCA. these animals usually reside in the water, or only quit it occa- sionally, there are some species, known under the name of land- crabs, which have the power of living for some time at a dis- tance 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 respi- ratory membrane is merely the lining of the large chamber formed by the mantle, through which a stream of water is con- tinually made to flow (§. 319) ; and this surface is sometimes extended, by the folding or plaiting of the membrane. In most Fig. 142. — Anatomy of the Oyster. v, one of the valves of the shell ; v', its hinge ; m, one of the lobes of the mantle ; m', a portion of the other lobe folded back ; e, muscles of the shell ; br, gills ; b, mouth ; t, tentacula, or prolonged lips ; /, liver ; i, intestine ; a, anus ; co, heart. of the Conchifera, however, we find four lamella? or folds of membrane (br, Fig. 142), 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 RESPIRATION IN MOLLUSCA. 247 these are united, so as to shut in the gills, there are two orifices, often prolonged into tubes (as in the Tellina, Fig. 143), 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 Glaucus and Tritonia, sometimes arranged Fig. 143 Tellina. Fig. 144.— Tritonia. Fig. 145.— Glaucus. in a circle around the end of the intestine, as in the Doris (see also Fig. 133) ; and are sometimes covered in, more or less completely, by a fold of the mantle. Tn most of the species that possess shells, the gills form comb-like fringes, which are lodged in a cavity enclosed in the last turn of the spiral shell ; and to this cavity the water is admitted, sometimes by a large open- ing, sometimes by a prolonged tube. In the Cephalopoda, we find the gills composed of a collection of little leaf-like folds, placed on a stalk (5r, 6r, Fig. 134) ; they are enclosed in a cavity which is covered in by the mantle ; and the walls of this cavity have the power of alternately dilating Fig. 146.— Doris. 248 RESPIRATION IN MOLLUSCA AND FISHES. and contracting, so as to draw in and expel water. It communi- cates with the exterior by two orifices, one of which, o (Fig. 147), a wide slit, is for the entrance 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 cartilaginous 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 subjoined Fig. 147 Gills of Poulp. Fig. 148. — Lamprey. 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 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 RESPIRATION IN FISHES. 249 orifices just described. Fishes, in common with other animals that breathe by gills, can only respire properly, when these are kept moist, and are so spread out as to expose their surface to the sur- rounding element. The act of respiration can take place when they are exposed to air, provided these conditions are fulfilled ; but 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 circumstances which are necessary to the life of an air- breathing animal. 318. There are certain Fishes, however, which are provided with an apparatus, somewhat resembling that which has been already described in the Land-crab, for keeping the gills moist. The bones of the pharynx are extended and twisted in such a remarkable manner, 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 supplied with a sufficient amount, to keep them moist during some time. The gill-filaments themselves are so arranged that they do not clog to- gether ; and by this com- bination 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 FlG- 149— Respiratory Apparatus of Anabas. journey for a considerable distance on land. Such a provision is Fig. 150.— Anabas. 250 RESPIRATION IN AQUATIC ANIMALS. especially desirable in tropical climates, where shallow lakes are often dried up by a 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 Andbas or climbing-perch of Tran- quebar ; 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 Amphibious 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 (§. 117) which cover the surfaces of the gills. Even in those which do not pos- sess 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 the Polypes, 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 pro- pels 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 ener- getically, on 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 continual current of water is kept up through the respiratory chamber of the lower Mollusca ; and even where this current is maintained in other ways, as in the Cephalopods and Fishes, we still find the gills covered with cilia, by the action of which an interchange in the water, that is in contact with each individual filament, is secured. This action may be well observed in the Tadpole of the common Water- Newt, whose gills hang from the neck on either side ; the cilia are themselves so minute that they ATMOSPHERIC RESPIRATION. 251 cannot be distinguished ; but the motion of the water may be readily perceived in a tolerable microscope, especially when small light particles, such as powdered charcoal, are diffused through it. 320. In animals whose blood is made to act directly upon Head Tracheae Stigmata Air-sacs Fig. 151.— Respiratory Apparatus of Insect (Nepa). the air, we usually find a provision of some kind for introducing the air into the interior of the body. The simplest arrangement is that which we meet with in the Snail and Slug ; and it con- sists merely of a cavity, resembling that in which the gills are 252 RESPIRATION IN INSECTS. disposed in the aquatic Mollusca, but having a free communica- tion with the external air, and having the blood minutely distri- buted by vessels upon its walls (/?, Fig. 14). In the air-breathing Annelida, such as the Earth-worm, 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 stigmata. The same is the case in the Myriapoda ; but the air-sacs now begin to send off branching air-tubes or trachea, which spread through the body ; these, however, have not much communication with each other. In Insects, this plan of structure is carried out in the most remarkable manner. The stigmata do not open into distinct air-sacs, but into canals, which lead to two large tracheee that run along the sides of the body, and are con- nected by several tubes that pass across it — one usually for each segment. From these tracheas others branch off, which again subdivide into more minute tubes : and by the ramifications of these, Fig. 152— Air- . ' J ' tube of insect, even the minutest parts of the body are pene- trated (Fig. 151). These tubes are formed upon a similar plan with the air-vessels of Plants, having a spiral fibre winding inside their outer membranous coat ; by the elasticity of which fibre, the tube is kept from being closed by pressure. 321. In this manner the air is brought into contact with almost every portion of the tissue, and is enabled to act most energetically upon it; and thus the feeble circulation of these animals (§. 293) is in a great degree counterbalanced by the extraordi- nary activity of their respiration. There are no animals which consume so much oxygen, in pro- portion to their size, as Insects do when they are in motion ; but when they are at rest, their respi- ration falls to the low standard, of the tribes to which they bear the greatest general resemblance. In the larva state, the respiration is sometimes ac- Fig. 153— larva complished by means of gills, and sometimes by tracheae, according to the habits and residence of the animal ; RESPIRATION IN INSECTS AND ARACHNIDA. 253 but it is never very active. Some aquatic larvae breathe air by- means of tracheae ; and they are consequently obliged, like the Whales and other aquatic Mammalia, 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 stigmata 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 (2, Fig. 153) being at the surface. 322. In most perfect Insects, we find the tracheae dilated at certain parts into large air-sacs (Fig. 151); 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 much larger portion of the trunk, than they do in the insect whose system of air- tubes has been just repre- sented,— 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 FlG- 154.— Nepa. 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 Ckeese-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 to the air, which is admitted through the spiracles. This arrangement is shown in Fig. 46. 324. Hitherto it has been seen that the respiratory apparatus 254 RESPIRATION IN FISHES AND REPTILES. is not connected with the mouth ; which, in the Invertebrated 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 Yertebrata, which are formed by an extension of the mucous membrane of the digestive cavity. The simplest condition of the true lung is that which constitutes the air-bladder 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 admitted to it from without. But in other cases, we find it connected with the intestinal tube by means of a short wide duct ; and, as many Fishes 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 Respira- tion is evidently more important. The canal by which it com- municates 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 (§. 97), 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. 37), 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 conse- quence of the small surface over which the two are brought together (§. 312). In Serpents we find but a single lung, that of one side not being developed ; and this lung extends along 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 of Frogs is of great im- RESPIRATION IN FROGS AND BIRDS. 255 portance in their respiration — in fact, of almost as much conse- quence as their lungs. The necessity for more energetic respira- tion 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 spring returns, and brings with it a warmer tem- perature, which arouses them from their inaction, they need a larger amount of respiration, and come occasionally 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 the ponds, in order 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. More- over, if the lungs be removed, and the animal be made to breathe by its skin alone, it may live for some time, if the tem- perature 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 Trachea Pulmonary vessels Lung . Orifice of bronchial tube " . Bronchial tube opened Bronchial tube "opened Ftg. 155. — Air-tubes and Lungs of Birds. any other Yertebrata ; that is, they consume more oxygen, and 256 RESPIRATION IN BIRDS. form more carbonic acid, in proportion to their size. Their lungs, however, are not so minutely subdivided as are those of Mam- malia ; 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 sub- divisions of the windpipe, pass from the lungs to these air-cells, is shown in Fig. 155. Now, by this arrangement, a much larger quantity of air is taken in at once, and a much more extensive surface 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 Reptiles, are destitute of the peculiar appa- ratus by which Mammalia are enabled to fill their lungs with air ; but it is introduced without any effort on their parts. For the cavity of the trunk is almost surrounded, as we have seen (§. 88), 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 increas- ing its size without adding to its weight, and thus rendering it more buoyant. 328. In Man and the other Mammalia, the lungs are con- fined to the upper portion of the cavity of the trunk, termed the thorax; which is separated from the abdomen by the diaphragm, RESPIRATORY APPARATUS OF MAMMALIA. 257 a muscular partition of which the use in respiration is very- important. An imperfect diaphragm is found in some Birds, which approach most nearly to the Mammalia in their general structure. The lungs are suspended, as it were, in this cavity, by their summit or apex ; and are covered by a serous mem- brane termed the pleura^ which also lines the thorax, being reflected from one surface to the other precisely in the manner of the pericardium. 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 therefore so smooth, as to glide over one another with the least possible friction. The lungs themselves are very minutely subdivided; a and thus expose a vast extent 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 ave- rage above the l-100th of an inch in diameter. In the accompany- ing figure is represented, on one side, the lung, d, presenting its natural appearance; and on the other, the ramifications of the air-passages or bronchial tubes, c, e, by which air is conveyed into every part of the lungs. The trachea or windpipe, b, opens into FlG- 156— Air-tubes and lung of Man. 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, xiii.) ; but it may be here mentioned that the entrance from the pharynx into the larynx consists of a narrow slit, which is capable of being en- larged or closed by the contraction of muscles. These muscles are made to act by a process corresponding with that which is 258 RESPIRATORY APPARATUS OP MAMMALIA. concerned in deglutition (§. 195) ; and the purpose of this is, to prevent the entrance of anything injurious into the windpipe. Tims if we attempt to breathe carbonic acid gas, which would produce an immediately fatal result if introduced into the lungs, the glottis, which forms the walls of this chink, immediately closes, and so prevents its entrance. The contact of liquids or of solid substances, too, usually causes the closure of the glottis, 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, bronchial tubes, and air-cells of the lungs, in all air-breathing Vertebrata, are lined by a mucous membrane, which is continued from the back of the mouth ; 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 (§. 193) ; 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 suffoca- tion. The vibration of the cilia is observed to be always in one direction, — towards the outlet ; and it is in this direction, there- fore, that the fluid is gradually but regularly conveyed. The ciliary movement may be seen to take place on the surface of the mucous membrane of the nose ; but not on that of the pha- rynx, 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 the Mammalia, by a peculiar mechanism, which accomplishes 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 RESPIRATORY APPARATUS OF MAMMALIA. 259 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. Now the size of this cavity may be made to vary considerably; — in the first place, by the movement of the diaphragm, — and in the second, by that of the ribs. 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 g, Fig. 157 ; 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 degree, rise and fall with it. It is obvious that, when the diaphragm descends, these organs, with the abdomi- nal viscera in general, must be pushed down- wards ; 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 e abdomen, force up the liver and stomach against the under side of the dia- phragm, and cause it to rise to its former height. 332. II. The play of the ribs is rather more complicated. Thesebones, c c, and c' c' (to the num- ber of twelve on each side in Man) are attached at one end by Fig. 157— Thorax of Man. 260 MECHANISM OF RESPIRATION IN MAMMALIA. a movable joint to the spinal column, a ; whilst at the other they are connected with the sternum (breast-bone) by an elastic cartilage. Now 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, i, 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. Now by the raising of the ribs, they are brought more nearly into a horizontal line ; as are also their cartilages ; and as 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 cartilages, and partly by the contraction of the abdominal muscles which are attached to their lower border. — For the full understanding of this descrip- tion, it is desirable that the reader should examine the move- ments 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. Now the cavity of the thorax is itself perfectly closed ; so that, if it were not for the expansion of the lungs, a void or vacuum would be left, when the diaphragm is drawn down, and the ribs are elevated. The air around presses to fill that vacuum (see Treatise on Pneumatics) ; but this it can only do by entering the lungs through the windpipe, and inflating them (or blowing them out), so as to increase their size in proportion to the increase of the space they have to fill. In this manner the lungs are QUANTITY OF AIR RESPIRED BY MAN. 261 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 movements of inspiration are performed, and the lung of that side* would not be dilated. The same thing would happen, if there were an aperture in the lung itself, allowing free communication between one of the larger bronchial tubes and the cavity of the chest ; for the air, although still drawn in by the windpipe, would pass directly into the cavity of the chest, rather than dilate the lung, which thus becomes entirely useless. Such an aperture is sometimes formed as the result of disease ; and if the action of both Jungs were thus prevented, death must immediately take place from suffocation. 334. The extent of the respiratory movements varies consi- derably ; but in general it is only such, as to change about the seventh part of the air contained in the lungs. It may be gene- rally noticed, that every fifth or sixth inspiration in Man is longer and fuller than the rest. Their number 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 general, from 14 to 18 inspira- tions take place every minute in an adult ; but the number be- comes greater, if the attention of the person, whose respirations are being counted, is directed to it. The average quantity of air taken in at each inspiration seems to be about 20 cubic inches ; so that, reckoning 16 inspirations to take place per minute, nearly 20,000 cubic inches pass through the lungs in an hour, and 460,224 cubic inches, or 266|- cubic feet, in the twenty-four hours. The air which has passed through the lungs contains about l-26th part of carbonic acid ; and thus about 17,856 cubic inches (or rather more than 10 cubic feet) of that gas, containing 2616 grains or about 5^ 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 * Each lung has its own cavity ; the two heing completely separated from each other by the pericardium (§ 255). 262 NECESSITY FOR FREE VENTILATION. animal life ; for it prevents the due excretion by the lungs, of that which has been formed within the body ; and the latter con- sequently accumulates in the blood, and exercises a very de- pressing influence on the action of the various organs of the body, but particularly on that of the nervous system. The usual pro- portion is not above 1 part in 1000 ; 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. Now it is evident, from the statements in the last paragraph, that, as a man pro- duces in twenty-four hours about 10 cubic feet of carbonic acid, if he were enclosed in a space containing 1000 cubic feet of air (such as would exist in a room 10 feet square 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, pro- vided that no interchange takes place between the air within and the air outside the chamber. The amount would be further increased by the carbonic acid thrown off by the skin, the quan- tity 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 insuf- ficiently provided with the means of ventilation. It is evident that if 12 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. Now we will suppose 1200 per- sons to remain in a church or assembly-room for two hours; they will jointly produce 1000 cubic feet of carbonic acid in that time. Let the dimensions of such a building be taken at 80 feet long, 50 broad, and 25 high ; then its cubical content will be (80x50x25) 100,000 cubic feet. And thus an amount of carbonic acid, equal to l-100th part of the whole, will be com- municated to the air of such a building, in the short space of two hours, by the presence of 1 200 people, if no provision is made for ventilating it. And the quantity will be greatly increased, and the injurious effects will be proportionably greater, if there NECESSITY FOR FREE VENTILATION. 263 is an additional consumption of oxygen, produced by the burning of gas-lights, lamps, or candles. 337. Hence we see the great importance of providing for free ventilation, wherever large assemblages of persons are col- lected 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, factories, and other places where a large number of persons remain during a consi-' derable 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 ventila- tion ; which particularly favours the spread of contagious dis- eases, by keeping in the poison, and thus concentrating it upon those who expose themselves to its influence. 338. When the quantity of carbonic acid in the air accumu- lates 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). Now, 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 the blood which is sent to it under such cir- cumstances becomes more and more venous in its quality, and is at the same time diminished in quantity, its actions first become 264 SUFFOCATION. irregular, producing violent convulsive movements, and at last cease altogether, the animal becoming completely insensible. In this condition, 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 con- tinue, 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 recovers its power of acting, and all goes on as before. 339. Now all these changes occur in exactly the same manner, when a warm-blooded animal is made to breathe nitro- gen 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. 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. This may be accomplished in various ways, such as blowing into them with the mouth or with a pair of bellows ; but the most effectual mode, where it can be employed, is the application of galvanism, by which the diaphragm is made to CAUSE OP THE RESPIRATORY MOVEMENTS. 265 contract, and thus to perform a natural inspiratory movement, which is much more efficient, as well as less injurious, than the forcible inflation of the lungs. 340. The ordinary movements of respiration belong, like those of swallowing, to the class of reflex actions (§. 195). 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 ; 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 breatli 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 application of cold to the skin. It is this which is the cause of the first inspiration in the new- born infant; which is not unfrequently prevented, by the seclu- sion of its face (the part most capable of receiving this impres- pression) 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 respiratory 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 T 266 VARIOUS MOVEMENTS CONNECTED WITH RESPIRATION. 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 respira- tory 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 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 con- traction of the muscles of the jaw, and also by a very great elevation of the ribs, in which the shoulders and arms partake. The purely involuntary character of this movement is some- times seen in a remarkable manner in cases of palsy; in which the patient cannot raise his shoulder by an effort of the will, but does so in the act of yawning. Nevertheless 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 com- pany 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 move- ment occurs ; and the glottis closes suddenly in the midst of it ; and the sound is occasioned by the impulse of the column of 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 sometimes goes on until the diaphragm is more arched, and the chest more com- pletely emptied of air, than it could be by an ordinary move- ment of expiration. The act of Crying, though occasioned by a VARIOUS MOVEMENTS CONNECTED WITH RESPIRATION. 267 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 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 combine them in the act of coughing. This act consists, 1st, in a long inspiration, which fills the lungs ; 2nd, in the closure of the glottis at the moment when expiration commences; and, 3rd, in the bursting open (as it were) of the glottis, by the violence of the expiratory move- ment, so that a sudden blast of air is forced up the air-passages, carrying before it anything that may offer an obstruction. The difference between coughing and Sneezing consists in this, — that in the latter, 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, and in such a way as to carry off any source of irrita- tion that may be present there. 343. Besides serving for the aeration of the blood, the respi- ratory organs of Animals, like those of Plants, have the power of throwing off, or exhaling, a considerable amount of watery vapour; and also (under certain conditions) of absorbing it. Every one is aware that the air he breathes forth contains a large quantity of vapour ; this is not perceptible in a warm atmo- 268 EXHALATION AND ABSORPTION BY THE LUNGS. sphere, because the watery particles remain dissolved in it, and do not affect its transparency ; but in a cold atmosphere, they are no longer held in suspension, and consequently 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 un- doubtedly proceeds from the moist lining of the mouth, throat, &c. ; but the greater part is thrown off by the lungs themselves. 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 frequently acquires a disagreeable taint, from the recep- tion of putrescent matters into the blood. 344. It has been ascertained by experiment, that, when a very warm atmosphere loaded with dampness is breathed, there is rather an absorption than an exhalation of aqueous vapour ; and the same may probably take place in a less degree, in an ordinary atmosphere, when the body has been drained of its fluid. In this manner, perhaps, we are partly to account for the extra- ordinary increase in weight which the body undergoes, by ab- sorption from the atmosphere, under peculiar circumstances (§. 220). That absorption may take place through the lungs, is evident, also, from the effects upon the system of certain gases, which 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 l-1500th part of sulphuretted hydrogen ; and a Dog will not live long in an atmosphere con- taining 1 -800 th 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. CHAPTER VII. OF SECRETION. General Purposes of the Secreting Process. 345. We have seen that the blood, in the course of its cir- culation, 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 con- tinually 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 for the purposes to which it is destined, it is requisite that these products should be drawn off from the current of the circulation, as constantly as they are re- ceived into it ; and this is accomplished by the various processes of Secretion, 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 the uninter- rupted 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 secretions be checked. We have a striking instance of this in the case of the Respiration, which may be regarded as a true function of Secretion, 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 a 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. 270 NATURE AND OBJECTS OF ANIMAL EXCRETIONS. 346. A distinction may be not improperly made between the true secretions, which are substances elaborated or separated from the blood, not so much to purify it, as to answer some pur- pose in the animal economy ; and the excretions, whose sole or principal object is to carry off substances that cannot be retained in the blood, consistently with the maintenance of its purity. The former vary considerably in the different classes of Animals ; the latter are the same, as to their essential characters at least, through the whole Animal kingdom, 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, fibrin, gelatin, and fatty matter — contain these ele- ments united in constant proportions, from whatever source we obtain them. Hence we should expect to find the products of their decomposition 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 Respi- ration, that excretion must be checked ; and it is required, in like manner, for uniting with hydrogen to form water, and with compounds of nitrogen to form urea. Hence there is no need of an organ to carry off the superfluous oxygen ; but an organ to introduce it is rather required ; and this purpose, as we have seen, is answered by the Respiratory apparatus. But we find organs of excretion specially destined to carry off the carbon, hy- drogen, 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. On the other hand, the Liver has for its office to separate the same elements from the blood in a different form, throwing them off in the condition of a peculiar fatty NATURE AND OBJECTS OF ANIMAL EXCRETIONS. 271 matter, which consists almost entirely of carbon and hydrogen. The function of the liver is the most active, when that of the respiratory organs is least so, and vice versa. Of this we shall presently meet with some remarkable examples (§. 365). Lastly, the Kidneys have for their chief object to throw off the azotised 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 com- pounds, urea, and uric or litkic acid, the latter of which is usually thrown off in combination with ammonia. 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 taken in. If these last have been no more in amount than was absolutely necessary for the maintenance of the body during that period, all the azotised portions of the food was first appropriated to the formation of the azotised tissues ; whilst the non-azotised 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 separate, or strain off, as it were, the products of the decomposition 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 muscu- lar tissue is not formed in proportion to the quantity of aliment supplied, but in proportion to the demand created by the exer- cise of it (§. 589) ; consequently, if more food is 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. 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. 272 EXCRETION OF SUPERFLUOUS AZOTISED FOOD. 348. Hence, if more azotised food be taken in, than is required to supply the waste of the muscular and other azotised tissues, the surplus must be carried off by the organs of excre- tion-— 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, accumu- late in it, and give rise to various diseases of a more or less serious character, which might have been almost certainly pre- vented 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 traced to the same cause, — the accumula- tion in the blood of lithic acid, which results from the decompo- sition of the superfluous azotised 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 azotised 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 decom- position of these. 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 azotised nutri- ment— whether contained in animal flesh, bread, or other arti- cles of vegetable diet, — but such as is required to supply the wants of the system. 349. The case of the late Dr. James Gregory, a celebrated Professor and Physician of Edinburgh, is an apposite illustration of the importance of this precept. He was descended from a decidedly gouty family ; and between the ages of 23 and 30, he himself suffered from several attacks. By using active exercise, however, avoiding all excesses, and using moderation in diet (although he did not abstain from animal food) during a period of twenty years, he so completely overcame the disposi- tion to the disease, that all symptoms of it disappeared in the latter part of his life. If such abstinence be carried too far, EXCRETION OF SUPERFLUOUS NON-AZOTISED FOOD. 273 however, it will produce injurious instead of beneficial results, weakening the system, 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-azotised food is taken into the system than can be got rid of by Respiration, it must either be deposited as fat, or it must be separated from the blood, and carried off by the excretion of the Liver. If too much work be thrown upon this organ, too, 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 mer- curial character) which shall excite the liver to increased secre- tion. The constant use of these medicines has a very pernicious effect upon the constitution, and careful attention to the regu- lation of the diet, and especially the avoidance of a superfluity of oily or farinaceous matter, will answer the same end in a much better manner. 351 . That the materials of the biliary and urinary excretions exist (like the carbonic acid thrown off by respiration) ready formed in the blood, being taken up by it in the course of its circulation, — and that the function of the Liver and Kidneys is (like that of the Lungs in regard to carbonic acid) to separate them, and not to form them, — 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 intro- duced into it. But if the excretions are 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, especially those of the nervous system. This sometimes happens in consequence of 274 NATURE AND PURPOSES OF ANIMAL SECRETIONS. disease, and it may be imitated by experiment ; for when the trunk of the blood-vessel, conveying 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 resulted from 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 they increase in intensity, as the amount of these substances is augmented, until death takes place. 352. It is unnecessary to say more then, to prove the great importance of the functions of Excretion in the Animal Economy ; and the necessity of carefully attending to them. The various Secretions which are formed in different parts of the body, have not so much for their object the purification of the blood, as certain purposes which are to be answered by them in the animal economy. 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) ; the Lachrymal secretion poured out upon the surface of the eye, for the purpose of washing it from impurities (§. 540) ; the secretion of Milk in Mammalia, for the nourishment of the young ; and various poisonous secretions in Serpents and Insects, for the destruction of their prey or for means of defence. Any one of these may be checked, with- out rendering the blood impure by the accumulation of any substances that should be drawn off from it ; but its cessation may produce effects fully as injurious, by disordering the func- tion 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 strongly resembles the ordinary constituents of the blood, espe- cially albumen. Hence we may believe that these secretions are formed by the several organs which elaborate them ; and that they do not pre-exist as such in the blood. 353. The various acts of Secretion and Excretion which are INFLUENCE OF EMOTIONS UPON THE SECRETIONS. 275 continually taking place in the living body, are, like those of Nutrition, completely removed from the influence of the will ; 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 secretion (§. 540). 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 the 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 nourishment of her off- spring. 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 communicate to it an abso- lutely poisonous character, which has occasioned the immediate death of the child.* The influence of the emotions 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. Notwithstanding the different characters of the products of Secretion and Excretion, and the variety of the purposes 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 (§. 44), of a mass of cells, bound together by areolar tissue, which allows the blood-vessels to gain access * See the Author's Principles of Human Physiology, Chap. vn. ; and Dr. A. Comhe on the Management of Infancy, Chap. x. 276 ACT OF SECRETION PERFORMED BY CELLS. 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. Phys. §. 361) — not being poured forth, as it is in most other cases, by the subsequent bursting of the cell. 355. 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 (§. 38) are covered with epithelium-cells, which have the power of separating the peculiar glairy viscid substance termed mucus, from the blood which is distributed to the mem- brane beneath them. They are continually being cast off, and are replaced as constantly by a fresh crop of cells, developed from germs contained in that membrane ; and the cells of each crop, as they are cast off, pour forth their contents, which thus cover the whole surface of the membrane with a layer of viscid fluid, that 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. 2). These epithelium-cells, in the progress of their development, draw into themselves certain products from the blood ; and when they have attained their full growth, they are cast off, and pour forth their contents, either by dissolving away, or by bursting. Such follicles are very abundant along the whole alimentary canal of Man ; and there is good reason to believe that a new crop of cells is developed in them, every time that the digestive process takes place (§. 39). 356. Now, although the most important Secretions and Excretions are elaborated, in Man and the higher animals, by organs of a much more complex structure, yet in the lower we find them produced by follicles, which are formed exactly upon this plan. Thus in the little Bowerbankia, (§. 134) the bile is secreted by minute follicles which are lodged in the walls of the STRUCTURE OF GLANDS. 277 stomach (Fig. 77, <0, an(^ Pour tneir secretion separately into its cavity, having no communication with one another. The glan- dular apparatus which surrounds the alimentary canal in the Wheel- Animalcule (Fig. 59, /£, k) seems to be made up of similar follicles, having separate openings into the cavity. In more complex forms of the same organ, however, several follicles open together into a tube, which either pours its contents directly into the alimentary canal, or unites with other tubes to do so. The condition of such a glandular organ very much resembles that of a bunch of grapes; as is seen in Fig. 158, which re- presents the structure of the Parotid gland (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 instruments 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 Crustacea, by referring to Fig. 48 (fo) ; and in the Mollusca it is nearly the same (Figs. 14, /, and 142,/). 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 Fig. 158. — Intimate Structure of a Composite Gland (the Parotid). 278 STRUCTURE OF GLANDS. this we have a remarkable example in the Kidney of the higher animals, which is entirely composed of such tubes, and of the blood-vessels distributed amongst them. If we make a vertical section of the kid- ney of Man or any of the higher Mam- malia, (Fig. 159, A) we find that it seems composed of two different substances, one surrounding the other ; to the outer, «, the name of cortical (bark-like) substance has been given ; whilst the inner, &, is termed medullary (or pith-like). In the cortical substance, no definite arrangement can be detected by the naked eye; it chiefly consists of a very intricate network of blood-vessels, surrounding the extremi- ties of the tubes. But in the medullary substance we can trace a regular passage of minute tubes, from the circumference towards the centre. They commence in the midst of the network of blood-vessels (B, «), and then pass down in clusters, nearly in a straight direction, and slightly converging 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. — These tubes, like the follicles, are lined with epithelium-cells, wThich are the real instruments in the production of their secretion. 358. That there is nothing in the form of the secreting appa- ratus, however, which determines the peculiar nature of its secre- tion, is evident from this fact, — that, in glancing through the Animal series, we find the same secretion elaborated by glandular structures 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 Fig. 159. — Structure of the Kidney of Man. A, vertical section of the kidney , a, cortical sub- stance ; b, tubular sub- stance ; c, calyx and pel- vis ; d, the ureter. B, portion of the gland enlarged ; a, extremity of the uriniferous tubes ; b, straight portion ; c, their termination in the calyx. STRUCTURE OF GLANDS. 279 as are those by which the mucous secretions are formed in the highest. But in Insects, the bile is secreted by a small number of long tubes, which open into the in- testinal canal just below the stomach (Fig. 110) ; and these tubes apparently differ in no respect from those that form the urinary secretion in the same animals, which open nearer the outlet of the intes- tinal 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. 160, which re- presents the digestive apparatus of the Cock-chafer, 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, they are beset with a series of short tubes opening from them, by which the extent of secreting surface is much increased. — On the other hand, although the urinary secretion is gener- ally 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 (§. 533) that line its ink-bag; and the corres- ponding secretion in other Mollusca is rendered purple by the same cause. The bile seems to be universally tinged with a yellow colouring matter, which may be regarded, therefore, as an essential part of the secretion ; in some herbivorous quadru- peds it has a green hue, and the colouring matter by which this is given, appears to be identical with that contained in the vegetable tissues on which they feed. Fig. 160.-— Digestive Appara- tus of Cock- chafer. 280 NATURE OF THE SECRETING PROCESS. 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. Although, therefore, there is a limit to our knowledge, beyond which it does not at present seem probable that we shall ever pass, it is an important step to have attained the general principle, that, in Animals as in Plants, the process of secretion is performed by cells ; the difference being that, in the latter, the secreted pro- ducts remain, for the most part, stored up within the cells, which form part of the general fabric ; whilst in the former, the secret- ing cells are generally subject to continual death and renewal, so as to cast the products they have elaborated, into channels, by which they may be carried out of the body. But the fat of Animals, as already mentioned (§. 44), is an instance of a mass of secreting cells remaining unchanged in the midst of the body. 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 surface, will perform its functions more or less perfectly ; this happens most frequently in regard to the important Excretions, as if Nature had especially provided for their continued separation 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 TRANSFERENCE OF SECRETING POWER. 281 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 re- garded as fabulous ; but they rest upon good authority, and they are quite consistent with physiological principles. For a little consideration will show that, as the membrane which lines the stomach and intestinal tube is but a prolongation of that which forms the external surface of the body (§. 14), so the membrane that lines the tubes and follicles of which the glands are com- posed, is but a prolongation or extension of the former. Now we have seen that, in the lowest animals, the lining membrane of the stomach, and that which covers the surface, may be made to take the places of each other, without detriment to their respective operations (§. 14) ; and it is not incredible, therefore, that the various divisions of the secreting surface, which ordina- rily have separate duties to perform, should be able, under particular circumstances, to take on, to a certain degree, each other's functions. 362. Some of the main ducts or channels, through which the glands pour forth their secretions, are provided with enlarge- ments 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 l duct which conveys into the intestinal tube the bile secreted by the liver, is also con- i nected 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 (§. 212) ; < when it is pressed out into the intestinal canal. It is a curious fact, that in most Fig. i6i.— urinary ap- persons who die of starvation, the gall- fl)lddneySA*t™ieters; c bladder is found distended with bile ; show- bladder ; a, its canal, the ing that the secretion has continued, but that it has not been poured into the intestine for want of the 282 CHARACTERS OF PARTICULAR SECRETIONS. stimulus occasioned by the presence of food in the latter. In many quadrupeds, especially those of the Ruminant tribe, the milk-ducts are in like manner dilated into a large receptacle, the udder, which retains the secretion as it is formed, until the period when it is needed. In all Mammalia, and in some Reptiles, Mollusca, and Insects, but not in Birds or Fishes, we find the ureters, which convey away the urinary excretion, dilated at their lower extremity into a bladder (Fig. 161), which serves to retain all the fluid that is poured forth by the gland during a considerable length of time, and thus to prevent the necessity for its being continually passed out of the body. Characters of Particular Secretions. 363. In regard to the particular characters of the various secretions, and the structure of the organs by which they are respectively formed, it would be unsuitable to the character of the present work to enter into any detail ; but it will be de- sirable to notice some of the more important circumstances con- nected with the chief of them. 364. The Bile, which is secreted by the Liver, is charac- terised, as already stated (§. 346), by the presence of a large quantity of fatty and other matters, which are chiefly composed of carbon and hydrogen. It is difficult to determine the exact characters of these ; but it seems pretty certain that one of them, at least, is a crystallisable fatty substance, somewhat resembling spermaceti ; this, to which the name of Cholesterine has been given, contains 37 equivalents of carbon, and 32 of hydrogen, with only one of oxygen, and no azote. Now the proportional amount and importance of this excretion seem to vary considerably in different classes of animals. In all, it appears to be concerned in the operation of digestion (§. 212) ; for we always find it poured into the alimentary canal, near the stomach, instead of being carried directly out of the body, like the urinary excretion. But whilst in some instances this seems to be almost its only function, in others it appears to have for its purpose to carry out of the system a large quantity of carbon and hydrogen, of which the respiratory apparatus cannot get rid. INVERSE RATIO OF RESPIRATION AND BILIARY SECRETION. 283 365. It is a general rule, to which there are few if any ex- ceptions, that the development and activity of the Respiratory organs and of those concerned in the secretion of Bile, stand in an inverse proportion to one another, through the whole animal series. 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 respira- tion 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), and its secreting power is evidently very great. 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 respiration and temperature are low, the liver is comparatively larger than in Birds and Mammalia, in which classes the respi- ration is more energetic, and the blood warm. And finally, in the embryo of the latter, whose respiration is very imperfect, the liver is much larger, in proportion, than it is in the adult ;* its weight in the new-born child being about l-18th that of the body, whilst in the adult it is not more than l-36th. Hence the conclusion seems almost indisputable, that the superfluous carbon and hydrogen of the body are chiefly carried off by the Respi- ratory organs, when it is required that the temperature of the body should be high, and that oxygen should be introduced into the system in large amount ; but that the office of excreting them devolves upon the Liver, when there is no necessity for keeping up the heat of the body, or for making the excretion the means of introducing oxygen (§. 307). 366. It is a remarkable point in reference to the secretion of Bile, that, in all the Yertebrata, it is formed from venous blood. The veins of the intestines and glands connected with them, as also in Fishes the veins from the tail and hinder part of the body, * These facts appear to the Author to furnish an important objection to the doc- trine of Liebig, as to the connexion between the secretion of bile and the respira- tory process. 284 BILE SECRETED FROM VENOUS BLOOD. UREA, URIC ACID. unite into a trunk, the vena portce (§. 266), which might almost be considered an artery, for it conveys the blood to the liver, and then subdivides into a set of minutely-divided capillaries, by which the fluid is carried into every part of the substance of the gland. From this venous blood, the secretion of bile is formed ; and it is not elaborated from the arterial blood that is sent to the liver for its nourishment, until this has passed through the capillaries — in which it is rendered venous. Hence it may be inferred that the materials from which the bile is formed, pecu- liarly exist in venous blood, which has become charged, whilst passing through the capillaries, with the products of their decom- position, and needs to be purified from these. 367. The Urinary 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 con- tains 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 (§. 21) of the proportions between the elements of albumen and gelatin, that the amount of azote in proportion 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 8 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 is combined with ammonia ; and in this state it ordinarily exists. When formed in too large quantity, however, it may be depo- sited 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 SECRETION OF URINE. 285 lithic acid ; thus a gravelly deposit may be found in the urine, when no undue amount of uric acid has been secreted, simply because, by disordered digestion and nutrition, there is an im- proper formation of lactic acid in the stomach, or in the blood, which precipitates the lithic acid in the urine. 368. 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 thus got rid of; and the means by which this is accomplished. The kidneys seem to form a kind of regulating valve, by which the quantity of water in the system is kept to its proper amount. The exhalation from the skin, which is the other principal means by which this is effected, 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 adjust- ing the quantity of fluid in the blood-vessels, it would be liable to continual and very injurious variation. This important function is performed by the kidneys, which allow such a quantity of water to pass into the urinary tubes, as may keep the pressure within the vessels very nearly at a uniform standard. 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 quantity 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 (§. 160) ; and also upon the quantity of surplus aliment, which is discharged through this channel, there being no other vent for it (§. 348). 369. The solid matter of the urine is elaborated, like that of other secretions, by the cells lining the tubes. But in animals which pass off a large quantity of water through this organ, there is a distinct and very curious provision for its separation. The extremity of the uriniferous tube is made to include a little knot or bunch of capillary vessels, which have extremely thin 286 SECRETION OF URINE. EXHALATION FROM THE SKIN. walls ; and a vast number of such knots are scattered through the cortical portion (§.357) of the kidney. To these the blood which is brought to the organ by the renal artery, is first con- veyed ; and the membranes that separate the interior of the capillary vessels from the cavity of the uriniferous tube, being 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 net-work upon the part of the tube that is lined by the secreting cells ; and it is there subser- vient to the elaboration of the solid part of the secretion, which is afterwards set free in the cavity of the tube, by the bursting of the cells, and is then dissolved in the fluid which has already found its way into the tube. 370. Next to the excretions formed by the liver and the kid- neys, 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 ; and this amount is greatly increased under particular circumstances. It will be remembered that, in Plants, a distinction was drawn between the simple evaporation of fluid, which takes place from the necessary action of the dry air upon the soft moist surface, and which would continue to take place from the similar surface of a dead plant, — and the exhalation, which was described as a special function, performed by the living plant only, and de- pendent as to its activity on the amount of light it is exposed to. (Veget. Phys. §§. 254 — 259). Precisely the same difference exists in Animals. A continual evaporation is taking 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 SENSIBLE AND INSENSIBLE PERSPIRATION. 287 charged with moisture. That all these influences affect the evaporation from the bodies of Animals, there is ample evidence derived from experiment. 371. But besides this continual evaporation, there is another mode in which loss of fluid takes place from the surface of the body. A vast number of minute glands are imbedded in the fatty layer just beneath the skin, and are copiously supplied with blood by its vessels ; their ducts pass through it, twisting in a spiral manner, towards its surface, where they open ; their orifice being covered by a little valve or flap of the epidermis (§. 35) which the fluid lifts up, when it is poured forth by the canal. These perspiratory glands seem to be continually exha- ling fluid, which is dissolved by the atmosphere 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 con- sequently 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, and is thus able 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 immediately be- comes 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 perspiration, is not the same, therefore, with the difference between simple evaporation and exhalation from the skin ; for a part of the latter is com- monly 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 macin- tosh cloak or coat, on a damp day. The water-proof garment keeps in the perspiration, almost as effectually as it keeps out the rain ; and consequently the air within it becomes loaded with x 2 288 COOLING EFFECT OF CUTANEOUS EXHALATION. fluid, and the skin remains in a most uncomfortable as well as prejudicial state of dampness. 372. Although no evaporation from the skin can take place when the surrounding atmosphere is loaded with vapour, the secretion of the perspiratory glands continues ; and does so even when the skin is immersed in fluid, provided the fluid be of high temperature. Hence we see that the conditions under which it is poured forth are peculiar to the living body alone, and entirely differ from those under which simple evaporation takes place. 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 consider- able quantity of heat is withdrawn from it, becoming latent in the change from fluid to vapour ;* of this we make use in ap- plying cooling lotions to inflamed parts. The more rapid the evaporation, the greater is the amount of heat withdrawn in a given time j hence, if we pour, on separate parts of the back of the hand, a small quantity 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 tem- perature ; and individuals 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 moulds were dried, while the floor was red-hot, * See Treatise on Heat. IMPORTANCE OF CUTANEOUS EXHALATION. 289 and a thermometer in the air stood at 350° ; and Chabert, the " Fire-king," was in the habit of entering an oven, whose tem- perature 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 been just stated ; since, if the perspiration that is poured forth upon the skin is not carried off with sufficient rapi- dity, on account of the previous humidity of the air, the tempe- rature 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 atmosphere, 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 If 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 substances which would be injurious if allowed to remain in the blood. That which is called the Hydropathic system proceeds upon the plan of in- creasing 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, Rheumatism, &c, are drawn off from it more speedily and certainly in this way, than in any other. 375. 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 con- siderable number of sebaceous follicles, which secrete a fatty 290 SEBACEOUS FOLLICLES. SECRETION OF MILK. substance that keeps the skin soft and smooth.* These are abun- dant on the most exposed parts of the face ; and their secretion prevents the skin from drying up and cracking, which it would be liable to do under the influence of the sun and air. They are more numerous in the skins of Negroes, producing in them that oily sleekness for which they are generally remarkable, and which prevents their skins from suffering by exposure to a tro- pical sun, as would those of Europeans. Besides the sebaceous follicles, the skin contains others in particular parts, for secret- ing peculiar substances ; as, for instance, those which form the cerumen, or bitter waxy substance that is poured into the canal leading to the internal ear, for the purpose (it would seem) of preventing the entrance of insects. 376. The secretion of Milk is important, not so much to the parent who forms it, as to the offspring for whose nourishment 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 con- tains ; nevertheless it is found that, when this secretion is once fully established, it cannot be suddenly checked, without produ- cing considerable disturbance of the general system. The struc- ture of the mammary gland closely resembles that of the parotid, already described (§. 356). 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 rami- fications of these ducts terminate in a multitude of little follicles, like those shown in Fig. 158, and 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 secretion of milk is made evident by the processes to which we commonly subject it. When allowed to stand for some time, the oleaginous part, forming the cream, rises to the top. This is still combined, however, with a certain * It has lately been discovered that, even in persons of cleanly habits, each of these follicles is the residence of a minute insect closely resembling the cheese-mite. SECRETION OF MILK. 291 quantity of albuminous matter, which forms a kind of envelope round each of the oil-globules. 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 decom- position, 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 coagu- lates it. The buttermilk and whey left behind after the sepa- ration of the other ingredients, contain a considerable quantity of sugar, and some saline matter. The proportion of these ingre- dients varies in different animals ; and also in the same animal, according to the substances upon which it is fed, and the quan- tity of exercise it takes (§. 164). The amount of casein seems to be greatest in the milk of the Cow, Goat, and Sheep ; that of oleaginous matter in the 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 be obtained. The important influence of mental emotion on this secretion has already been noticed (§. 353) ; and many more instances might be related, were not the ordinary facts in regard to it generally known. CHAPTER VIII. GENERAL REVIEW OF THE NUTRITIVE OPERATIONS.— FORMATION OF TISSUES. General Revieic of the Nutritive Operations. 378. In the preceding Chapters (in. 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 vn.) considered those functions, by which this fluid is kept free from the impurities it acquires during its circu- lation 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 (§. 54), re- quires 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 circu- lation, the products of the waste or decomposition of the various tissues, which it conveys to the various organs, — the Lungs, Liver, 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 COMPOSITION OF THE BLOOD. 293 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. 379. We have now to enter, in more detail, into the mode in which the circulating fluid is applied to the Nutrition and For- mation of the Tissues ; but, before proceeding to this, it will be advantageous to recapitulate briefly the properties of this fluid, and to compare them with those of the various kinds of struc- ture into which its elements are to be converted. 380. The circulating blood of Vertebrata consists of a clear fluid, the liquor sanguinis, in which float a vast number of red corpuscles (§. 229). As these last, however, are not present in Invertebrated animals, it is evident that they cannot be essential to the nutritive operations to which the Blood is subservient ; and strong reasons have been given for the belief, that they minister to the respiratory operations, by which oxygen is carried through the capillary vessels into every part of the system, and carbonic acid is, in like manner, conveyed away from them, to be set free in the lungs (§. 234). In considering the nutrition and formation of the tissues, therefore, we may probably leave the red corpuscles out of view. The liquor sanguinis contains the two animal substances, fibrin and albumen, in a state of solution; together with fatty matter and saline substances ; and, from some recent inquiries, it would seem to contain gelatin also, or, at least, a substance which may be easily converted into gelatin by long boiling. 381. The Albumen of the blood is derived at once from the food ; for it is in this form that all the albuminous portion of the aliment is received into the system, having been reduced to this condition by the digestive process, whatever may have been its previous character (§. 16) ; and it serves as the raw material, from which the other products are elaborated. Most of the animal Secretions contain a greater or less amount of albuminous matter (§. 352) ; but there is no sufficient evidence that albumen, 294 COMPOSITION OP THE ANIMAL TISSUES. as such, ever enters into the composition of the Animal Tissues* In fact it appears to be incapable of undergoing organisation, until its condition has been changed into that of Fibrin, The formation of this last substance appears to be continually taking place, during the motion of the circulating fluid through the living vessels. It is evidenly derived from the albumen derived from the food ; and it is probably elaborated by the colourless floating cells, which are found both in the chyle and in the blood of Yertebrata, and which exist in the blood of the Invertebrated animals (§. 235, 241). That this Fibrin is the material, at the expense of which the organised fabric is chiefly if not entirely formed, seems highly probable, from a variety of considerations formerly touched upon. It passes spontaneously, in the act of coagulation (§. 18), into a regular fibrous tissue of simple structure ; and all that seems necessary for the complete organisation of this, is that it should be permeated by vessels, which may furnish it with the materials of its growth and reno- vation. We shall presently find that the formation of blood- vessels takes place, in all instances, subsequently to the first produc- tion of a tissue, and is consequently not essential to it (§. 392). 382. It has been shown (Chap. I.) that the tissues which originate in this manner, are those which have functions simply mechanical, — such as affording support, resisting strains, or imparting elasticity. But the tissues which possess endow- ments peculiarly vital, — that is to say, entirely different from the physical properties of inorganic matter, and manifested only by a living organised tissue, — are formed in a different manner ; being either permanently composed of cells, or having their origin in them, and undergoing a subsequent transform- ation. We have seen, in preceding chapters (iv. and vi.), that the selection of alimentary materials from the chyme, and their introduction into the blood, is accomplished by the agency of cells, which rapidly grow, and disappear again as rapidly ; and that the selection from the blood of the materials of the secretions is accomplished in the same manner. These * Physiologists have been accustomed to speak of the albuminous tissues ; but the Author believes that he is justified in asserting, that no chemical difference exists, by which albumen and fibrin can be certainly distinguished. CONSTRUCTION OF THE ANIMAL TISSUES. 295 processes are of a nature peculiarly vital ; as we cannot imagine them to be in any way dependent upon the physical properties of matter ; the same may be said of the agency of the floating cells of the chyle and blood, in converting albumen into fibrin ; and of the red corpuscles in elaborating, from the fluid which surrounds them, the peculiar substance they contain. All these processes have reference only to the nutritive actions, or organic life (§. 4) of the being ; and their essential nature is the same, as we have already seen, in the Plant and in the Animal. There is this interesting circumstance to be observed in regard to them, — that the action of every cell is independent of that of its fellows, yet that by the wonderful adaptation of their several properties (of which we can give no account whatever, except that it is the will of the Creator), they all work together for one general end, the maintenance of the bodily fabric. 383. On the other hand, the Muscular and Nervous tissues, which are subservient to the actions of animal life, although originally formed from cells, have a very different structure when complete. It is easy to see that, so long as cells remain isolated from each other, they exist as so many distinct indivi- duals,— performing, it may be, the very same operations, — but doing this independently of one another. Now the very nature of the animal functions requires, that the actions of the several parts of the tissues which perform them, should be most inti- mately connected ; thus, when an impression is made upon any part of the surface of the body, it has to be instantaneously com- municated to the brain ; or an effort of the will, acting through the brain, has to call into immediate operation a large amount of muscular tissue. We could not conceive these functions to be performed by a number of isolated cells ; and we find, in fact, that the muscular and nervous tissues are composed of tubes, containing substances that are peculiar to each respectively (§. 428 and 578). These tubes originate, like the ducts of plants (Veget. Phys. §. 82), in cells laid together end to end, the partitions between which have broken down ; and the depo- sits that are found within them, on which their peculiar proper- ties seem to depend, are formed at a subsequent time. 296 ORIGIN OP TISSUES IN CELLS, Formation of the Tissues. 384. We see, then, that all the Animal tissues may be con- sidered as taking their origin in fibrin, — either directly in the fibrous network formed by its coagulation, or indirectly in the cells which are developed at its expense. The question next arises, — what is the origin of these cells ? 385. There is sufficient reason to believe that every living being is developed from a germ ; no organised structure being able to take its origin (as some have supposed) in a chance com- bination of inorganic elements. All the facts relating to the production of Fungi and Animalcules, which have been imagined to favour this doctrine, may be satisfactorily explained in other ways (Veget. Phys. §. 50, 51). Now the first structure deve- loped 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 structure, 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 becomes in its turn the parent of others ; and thus, by a continuance 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, containing 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. Of the cause of these transformations, and of the regu- larity with which they take place in the different parts, according to the type or plan upon which the animal is constructed, we are entirely in the dark ; and we may probably never know much more than we do at present. 386. "When once the several forms of tissue are developed, their nutrition is kept up by the supply of their respective materials, which they derive from the blood. Each tissue draws NUTRITION OF DIFFERENT TISSUES. 297 from the blood that which it requires ; and as some portions of it undergo decomposition, others are newly formed. The germs of these newly-formed parts may be supplied in some instances by the blood, in others by the tissues themselves; — on this subject nothing certain can be at present stated. Where the new struc- ture merely replaces that which has been removed by death and decay, it is probable that its germs are furnished by the part itself, drawing the materials of their development from the blood; just as the cell of the Red Snow or Yeast Plant, whilst itself dying, sets free the contained granules, which become the germs of new cells, obtaining their nourishment from the air and moisture around (Veget. Phys. §. 424). But where an entirely new structure is being formed, as in the process to be presently described (§. 393), it is probable that the blood both furnishes the germs and the materials for their development. 387. Though all the tissues derive the materials of their development from the blood which circulates in the vessels, yet there is considerable variety in the mode in which the supply is afforded; some tissues being supplied with blood much more copi- ously and directly than others, in consequence of the greater minuteness with which the capillaries are distributed 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 sanguinis that is brought into their neighbourhood. This is the case, for instance, with the epithelium and epidermis (§. 35, 39) ; the cells of which are developed at the expense of the fluid which they absorb, through the basement membrane 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 absorp- tion from the vessels which surround it (§. 45). 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 298 NUTRITION OF DIFFERENT TISSUES. one cell to another (as in the cellular tissue of Plants) is suf- ficient 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 areolar structure (§. 47), yet none of these pass into the actual substance of the bone. The Fibrous tissues are, for the most part, but sparingly supplied with blood-vessels, as they are but little liable to decay or injury ; but the delicate areolar tissue, which is continually undergoing change, receives a larger quantity of blood, being traversed by capillary vessels in every direction. 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 capillary vessels, not entered by them. The same important purpose 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 vessels, so that their contents must be nourished by fluid absorbed through their walls. 388. In no instance that we are acquainted with, in the 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 san- guinis seems, therefore, to furnish all that is wanting for this purpose ; and it readily permeates the walls of the capillaries, 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 nutrition of the body cannot take place with its proper energy. The same result happens, if its fibrin is not properly elaborated. The tissues are imperfectly nourished; and the strength of the body, and the vigour of the mind, are consequently alike impaired. NATURE OF TUBERCITLAR DISEASE. 299 389. This imperfect elaboration seems to be the essential condition of one of the most destructive diseases to which the human frame is liable, — that commonly known as Consumption. This is, however, but one out of several diseases, which may result from the same state of constitution. If the fibrin of the blood be imperfectly elaborated, it is less fit to undergo organis- ation ; and consequently, instead of being converted into living tissue, part of it is deposited as an unorganisable 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 pro- duce irritation and inflammation, in the same manner as other substances imbedded in the tissues would do ; so that the issue, although often postponed for a time, is almost invariably fatal, when once tubercular matter has been deposited in the lungs. Microscopic examination of tubercular matter shows that it consists of half- formed cells, fibres, &c, together with a granular 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 consti- tutional state from which it results. This is sometimes heredi- tary; and is sometimes induced by insufficient nutrition, habitual exposure to cold and damp, long-continued mental depression, &c. The treatment must be directed to the invigoration of the system by good food, active exercise, pure air, warm clothing, and cheer- ful occupations ; 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 to tubercular dis- ease. For the earnestness with which he has directed general attention to this important topic, the British public are much indebted to the writings of Sir James Clark. 390. There is another remarkable class of diseases resulting from a disordered condition of the nutritive processes, — those, namely, of a cancerous nature. The peculiarity of the structure of the various forms of Cancer consists in this, — that they are composed of cells, sometimes of a globular form, sometimes elongated or spindle-shaped, having a power of rapid multiplica- 300 CANCER. RELATION BETWEEN THE BLOOD AND THE TISSUES. tion, and not capable of changing into any other kind of tissue. It is this rapidity of increase, combined with the tendency which the diseased growth has, to appear in one part of the body when removed from another, which gives to these diseases their pecu- liar character of malignancy. "When a truly cancerous growth has once established itself in any part of the body, it may increase to any extent, obtaining its nourishment from the blood-vessels in its neighbourhood, and destroying the surrounding parts by its pressure, and by drawing off their supply of nourishment. When it has developed itself to a considerable degree, the Sur- geon is disinclined to remove it ; knowing that the disease will probably make its appearance in some other part of the body. It is probable that this extension of it is due to the conveyance of some of the germs of the Cancer-cells, by the blood, to distant parts of the body ; in the same manner as the germs of the peculiar Mould, which constitutes the Muscardine of Silk -worms, are conveyed through their bodies (Yeget. Phys. §. 54). Can- cerous diseases may be propagated, like Muscardine, by inocula- tion from one animal to another ; by which operation, some of the cell-germs are transplanted, as it were, into a new soil. 391. From the foregoing facts it is evident, that the opera- tions of Nutrition are due, on the one hand, to the independent 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 network : and the various elaborate tissues of the body could not of themselves select and assimilate their nourishment, and are consequently dependent upon the blood for their supply. We may illustrate the relation between the three states,- -that of aliment, blood, and organised 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. USE OF THE BLOOD-VESSELS. REPAIR OF INJURIES. 301 392. It is also evident, that the blood-vessels have no other purpose in the act of Nutrition, than to convey the circulating fluid into the neighbourhood of the part where it is to be employed : and the blood, or at least its organisable portion, the liquor sanguinis, must quit the vessels, before it can be employed in the development of new tissue. We might illustrate 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 distribu- tion 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 depen- dent 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 ; and they bear a close resemblance, at an early period, to the vessels through which their nutritious sap or latex circulates through their tissues (Veget. Phys. §. 87). 393. 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 resem- bling the first development of the entire fabric. The neighbour- ing 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, when it coagulates, it forms a connecting medium between the separated parts. It would appear that, when coagulating upon a living surface, the fibrin contained in this fluid assumes a more perfect arrangement, than that which it usually presents when it coagulates out of the body ; for the 302 REPAIR OF INJURIES. lymph which is thus poured out soon begins to show a regular organisation ; fibres and cells first appear in it ; some of the cells speedily break down into vessels, which form connections with those in the nearest living part ; the blood begins to circulate through the newly-forming tissue ; and in time, such a change takes place in it, as converts its several portions into fabrics resembling those with which they are connected, — whether bone, nerve, fibrous membrane, mucous membrane, or skin ; until the separation is complete and effectual. This is the mode of repair known to the Surgeon as healing by the first intention. — It often happens, however, that the destruction of tissue has been too great for its renewal in this manner ; and a gradual process of growth from the surrounding solid parts, is then necessary. This may take place in two ways, according to the mode in which it is regulated. Under the most favourable circumstances, when the wound is completely excluded from the contact of air and from other sources of irritation, and when the constitution is not in an inflammatory state, there is a gradual and complete repair of the parts that have been lost, by the growth of the surround- ing structures. This, which is termed by Dr. Macartney (who first described it) the 'modelling process, is nothing else than the simple natural process of development, analogous to that which takes place in the first production of the fabric, and in the regeneration of entire members that are lest among the lower animals (Chap. xv). But if inflammation be permitted to arise, the repair takes place by a process termed granulation, which consists in the sprouting forth of a rapidly- growing tissue (commonly known as proud-flesh) that fills up the cavity ; but this contracts after the skin has closed over it, and gives rise to an unsightly scar, which is completely avoided in the former method. CHAPTER IX. ON THE EVOLUTION OF 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, inter- spersed here and there by stars or ribands of more intense brilliancy. The uniform diffused 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 brilliantly displayed. Both belong, for the most part, to the class Acaleph^, all the species of which appear to be more or less phosphorescent, those of tropical seas being the most so. This interesting phenomenon, when it occurs on our own coasts, is chiefly produced by incalculable multitudes of a small species, having a nearly globular form, and of a size about equal to that of the head of an ordinary 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 Y 2 304 ANTMAL LUMINOUSNESS. the little animals, to which these beautiful appearances are due, might cause them to be overlooked, if they are not attentively sought; they much resemble grains of boiled sago in their aspect, but are much softer. 395. The light emitted by these animals appears to be due to the peculiar chemical nature of the mucus secreted from their bodies ; for this, when removed from them, retains its properties for some time, and may communicate them to water or milk, rendering them luminous for some hours, particularly when they are warmed and agitated. It is probably from this source, that the diffused luminosity of the sea is partly derived. The secre- tion appears to be increased in amount, by anything that irri- tates 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. 396. But the Acalephas are by no means the only luminous animals which tenant the deep. Many of the Polypifera appear to have this property in an inferior degree, and also some of the Echinodermata. Of the lowest class of Mollusca, 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 shell-bearing Molluscs, the phenomenon has also been observed ; and also in the 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 pro- bable that the luminosity attributed to many of the latter, is due to the disturbance they make in the surrounding water, which excites its phosphorescence in the manner just explained. In all these, the general phenomena are analogous, — the luminous ANIMAL LTJMIN0USNESS. 305 matter appearing 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 respiration. The light continues for some days after death ; but ceases at the commencement of putrefaction. 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 Elaterida?, 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 New World. Their light proceeds from two minute but brilliant points, which are situated one on each side the front of the thorax ; and from another beneath the hinder part of the thorax, which is only seen during flight. The light pro- ceeding from these points is sufficiently intense to allow small print to be read in the profoundest darkness, if the insect be held in the fingers and be moved along the lines; and the natives of the countries 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 Lampyridce contains about 200 species known to be luminous ; the greater part of these are natives of * This insect has been happily introduced by the poet Southey in his " Madoc" as furnishing the lamp by which the British hero was rescued from the hands of the Mexican priests : — " She beckoned and descended, and drew out From underneath her vest a cage, or net It rather might be called, so fine the twigs Which knit it, where, confined, two Fire-flies gave Their lustre. By that light did Madoc first Behold the features of his lovely guide." 306 GLOW-WORMS. America, whilst others are widely diffused through the Old World. In most of these, the light is most strongly displayed by the female, which is usually destitute of wings, so that it might be mistaken for a larva. The species of our own country is known as the Glotc-vcorm. " Who that has ever enjoyed the luxury of a summer evening's walk in the country, in the southern parts of our island, but has viewed with admiration these stars of the earth and diamonds of the night? And if, living like me in a district where it is rarely to be met with, the first time you saw this insect chanced to be, as it was in my case, one of those delightful evenings which an English summer rarely yields, when not a breeze disturbs the balmy air, and i every sense is joy,' and hundreds of these radiant worms, studding their mossy couch with mild effulgence, were presented to your wondering eye in the course of a quarter of a mile, — you could not help associating with the name of glow-worm the most pleasing recollections. No wonder that an insect which chiefly exhibits itself on occasions so interesting, and whose economy is so remarkable, should have afforded exquisite images and illus- trations to those poets who have cultivated Natural History." * 399. The light of the glow-worm issues from the under sur- face of the three last 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 num- ber 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 (§. 320) ; which ramify through every part of the granular substance ; and it appears that the access of air through these is a Fig HJ2.-MALE axd Female Glow- neceSsary condition of the pllOSpllO- WOP.M. J * ± rescence. 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 in- * Kirby and Spence's Entomology, vol. n. p. 331. FIRE-FLIES. 307 juring the trachea, the light is not interrupted. All the lumi- nous insects appear to have the power of extinguishing their lights ; and this they probably do when alarmed by approach- ing danger. This circumstance is beautifully alluded to, in the following elegant description, by the poet already quoted, of the first effect of the brilliant nocturnal spectacle, presented by these insects in the countries where they abound, upon the British visitors of the New World. " Sorrowing we beheld The night come on ; but soon did night display More wonders than it veiled ; innunierous tribes From the wood-cover swarm'd, and darkness made Their beauties visible : one while they stream'd A bright blue radiance upon flowers that closed Their gorgeous colours from the eye of day ; Now motionless and dark, eluded search, Self-shrouded ; and anon, starring the sky, Rose like a shower of fire." The sudden extinction of the light is probably due to the ani- mal's power of voluntarily or instinctively closing the aperture of the trachea. 400. There are a few other Insects not included in these fami- lies, which are reputed to possess luminous powers ; and of these the most remarkable are the Fulgorce^ or Lantern-flies, of which one species inhabits Guiana, whilst another is a native of China. These are insects of very remarkable form, having an extraordinary projec- tion upon the head; and this is the part said to be luminous. The autho- rity 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, Fig. 163.— Fulgora lanternarja. 308 USES OF ANIMAL LUMINOUSNESS. which is found in dark damp places, beneath stones, &c, is slightly luminous; and the common Earth-worm 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 conjectured. 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 per- petuation of the race, — is probably 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 Fire-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 appointed for the lovers' rendezvous," would not seem so incorrect as the ideas of poets on subjects of Natural History too frequently are. It may be objected, on the other hand, that there are many Moths and Beetles, which have a similar tendency to fly towards the light, but which themselves possess no shining lamps. Some of these, however, are faintly luminous ; and it would not seem improbable that the Insects which are attracted by flame, and thus show that they are seeking for objects which emit light, may be able to perceive feebler degrees of it — undistinguishable by our eyes — that may be possessed by the beings of which they are in search. 402. 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. It may serve for the illumination (however faintly) of those dark USES OF ANIMAL LUMINOUSNESS. 309 and rayless depths of the ocean, which are known to be tenanted by Fishes and other animals, although they receive no appre- ciable portion of solar light ; and it may be sufficient to direct these to their prey. It has also been suggested, that the pro- perty may be conferred upon them (like the stinging powers possessed by some) as a means of self-defence ; their bodies not being protected by a dense external covering, nor possessed of the means of escape from danger by rapid motion. Many of them have the power of throwing out momentary vivid flashes of light, which would not seem producible by the secretion already described ; and these may have the effect of scaring away the animals that would otherwise make them a prey. 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, Mol- lusca, 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 recently 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 (§. 167) ; and it is by no means impossible that some peculiar compound of this substance may be formed, during the early stages of decomposition, and even before death ; and 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 phosphorescent ; 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 vital action, is a certain amount of warmth ; and we have seen * See Sir H. Marsh, on the Evolution of Light from the Living Human Subject. 310 TEMPERATURE OP COLD-BLOODED ANIMALS. that the animals which alone are capable of retaining their ac- tivity 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 ren- dered completely inactive, even by a comparatively 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 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 the sources of its production, is one, therefore, of great prac- tical importance. 404. Our knowledge of the heat evolved by the lower Inver- tebrated 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, when it is 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 Earth-worms, Leeches, Snails, and Slugs, ascertained by intro- ducing 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, that would otherwise immediately freeze their bodies. 405. In the cold-blooded Yertebrata, also, the heat of the body is almost entirely dependent upon that of the surrounding element. Thus most Fishes 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 temperature 10 or 12 TEMPERATURE OP COLD-BLOODED ANIMALS. 311 degrees higher than that of the sea ; and these are peculiar for the activity of their circulation, and 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 Medi- terranean, where there are exten- sive 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 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 otherwise have suffered from cold, into deep waters ; and those species which are confined to shallow lakes and ponds, and which are thus liable to be frozen during the winter, are frequently en- dowed 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 temperature is somewhat greater ; being especially shown when the external temperature is reduced very low. Thus when the air is between 60 and 70 degrees, the body of a Reptile will be nearly of the same heat ; but when the air is between 40 and 50 degrees, it may be several degrees higher. Frogs and other aquatic Reptiles have a remarkable power of sustaining a tem- perature 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 degrees, the body of an edible frog has been found to be 37^ degrees; and even in ice of 9 degrees, the animal has maintained a temperature of 33 degrees. 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 312 HEAT OF BIRDS AND MAMMALIA. last it is congealed, and the body is also 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 the 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 tempe- rature considerably higher than that of the atmosphere ; thus in some of the more agile of the Lizard tribes, the high temperature of 86 degrees has been noticed, when the external air was but 71 degrees. 407. The only classes of animals in which a constantly-ele- vated temperature is kept up, are Birds and Mammalia. 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 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 the Mammalia seems to range from 96° to 104° ; but more accurate and extensive observations, especially on the temperature of the same species under different circumstances, are much wanting. From the observations of Dr. J. Davy upon the temperature of Man, it appears that the mean or average heat of his body is about 100° ; he has observed it as low as 96^°, when the tem- perature of the air was 60° ; and as high as 102°, when the air was at 82°. Thus we see that a variation of 5^ degrees was witnessed when the range of the temperature of the air was only from 60° to 82° ; and 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 HEAT OF YOUNG ANIMALS. 313 standard ; and no observations yet made, indicate that the tem- perature of the body ever falls below 95° in health. In Asthma and Asiatic Cholera, however, it has been found 20° below its usual standard ; and in Scarlet-fever and Tetanus (locked-jaw) it has been known to rise to 106° and 110f°. 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 (Chap.xv.) 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 Mammalia, that the young animals have the power of maintaining an independent 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 66^°, the temperature of the atmo- sphere 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, however, 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 to be the case among Mammalia. 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 Mammalia 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 314 TEMPERATURE OF INFANTS : — OF INSECTS. destitute of sight, at its entrance into the world, like the young of the Cat, Dog, or Rabbit, 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 born 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, consequent 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 extraor- dinary and interesting phenomena. In the larva and pupa states, the temperature of the body is never more than from J° to 4° above that of the surrounding medium ; but in many tribes, the temperature of the perfect Insect rises so high, when it is in a state of activity, that it might be 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 com- municated 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 TEMPERATURE OF BEES. 315 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 most active of the Beetle tribe. Those of the latter which seldom leave the ground, have little power of producing heat. 41 1 . 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. But the production of heat is increased to a most extraordinary degree when the pupce are about to come forth from their cells as perfect Bees, and require a higher tempera- ture for their complete development. This is furnished 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 ; at the same time being 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 temperature ; for a thermometer intro- duced into a hive during May has been seen to rise to 96° 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 supposed that Bees do not become torpid during the winter; but this is now known to be a mistake. Bees, like other Insects, pass the winter in a state of hyberna- tion ; 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 atmo- sphere; being kept at or above the freezing-point, when the air is far below it. Under such circumstances, their power of 316 PRODUCTION OF ANIMAL HEAT. generating heat is most remarkable. In one instance, the tem- perature 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 temperature of the air was 34^°. 412. The cause of the evolution of heat in the Animal body, may now be referred with tolerable certainty, to the union, by a process resembling ordinary combustion, of the carbon and hydrogen of the system, with the oxygen taken in from the air, in the process of Respiration. 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,, §. 411) ; and in all those Ani- mals 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). Or, again, it may be derived from the store laid up in the sys- tem in the form of fat; which seems destined to afford the requisite supply, when other sources fail. Thus in diseases which prevent the reception of food, the fat in the body rapidly diminishes ; being burnt off, as it were, to keep up the tempe- rature 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 being observed to come forth from their winter quarters, with the return of spring, in a very lean condition. — In animals which do not require the main- tenance of a high temperature, but which nevertheless eat with great voracity, such as the Mollusca and Crustacea, we find the superfluous portion of the carbon and hydrogen of the body car- ried off by means of the liver ; in the secretion from which, these elements form a very large part (§. 364). ANIMAL HEAT DEPENDENT ON CONSUMPTION OF OXYGEN. 317 413. On the other hand, we find in all Animals, that are endowed with the power of developing much heat, a provision for introducing a large quantity of oxygen into the body, to unite with these elements, and to carry them off in the form of carbonic acid gas and watery vapour. We have seen that, in Insects, the air is itself conveyed, by means of air-tubes, into every part of the body (§. 320) ; and that, in the warm-blooded Vertebrata, its oxygen is equally distributed through the system, by means of the blood, and chiefly by its red corpuscles (§. 234). We find the number of these corpuscles to correspond exactly with the amount of heat maintained by each class of animals : thus in Birds, the blood is redder than in the Mammalia ; in the Mammalia it is far redder than in Reptiles and Fishes ; and in those of the latter class which can maintain a higher temperature than the rest, it is redder than in the white-fleshed species of less active habits (§. 405). The consumption of oxygen, more- over, and the production of carbonic acid, are found to take place in every animal, exactly in proportion 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 proportionally low; and the diseases which cause a lowering of the temperature, are precisely those in which there is a dimi- nished consumption of oxygen. On the other hand, when- ever 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 ac- celerating 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 if the animal be exposed for too long a period to a very low temperature, it will not be able to resist its influence, and will perish. 318 ANIMAL HEAT MAINTAINED BY RESPIRATION. 414. The influence of respiration in maintaining the heat of the body, is well shown by the fact, that, if the brain be removed, but the top of the spinal cord (which is the centre of the actions of respiration, §, 450) be left, so that the movements of breathing continue, the temperature of the body is kept up with little diminution ; and even if this be removed, and the respiration be artificially kept up, by blowing air into the lungs and pressing it out again, the body will cool much more slowly than it would otherwise have done. That this substitute will not full?/ answer the purpose, is easily understood, when we contrast the natural with the artificial respiration. In the natural, the air-cells are first dilated, and the air rushes into them from the air-tubes ; in the artificial, the air is sent first into the air-tubes, and cannot properly distend the air-cells so as to act upon the blood, unless an injurious amount of force be employed. 415. It may be concluded, from this and other experiments, that the Nervous System has no direct influence in maintaining the temperature of the body, as some physiologists have sup- posed. It is true that the temperature of a paralytic limb is usually a degree or two lower than that of the sound one, and that it is more affected by changes of external temperature ; but this is readily accounted for by the fact, that its inactivity pre- vents or retards those changes in its substance, which contribute to the maintenance of its heat. And the fact that heat is deve- loped in Plants, which have no nervous system, to as high a degree as in Animals which possess it, when other circumstances are the same, should be enough to show that we are to look for its source in the various changes which the elements of the body are undergoing. Of these changes, the union of its carbon and hydrogen with oxygen taken in from without, are unquestion- ably the most constant and important ; and we know that this union would produce the same effect out of the living body as in it ; but there may be others, which are also concerned in the production of heat, though in a less degree. Animal Electricity. 416. Almost all chemical changes are attended with some alteration in the electric state of the bodies concerned : and when ELECTRICITY OF HUMAN BEING. 319 we consider the number and variety of these changes in the living animal body, it is not surprising that disturbances of its electric equilibrium (see Treatise on Electricity) should be con- tinually occurring. But these, when slight, can only be detected by very refined means of observation ; and it is only when they become considerable that they attract notice. Some individuals exhibit electric phenomena much more frequently and power- fully than others. There are persons, for instance, who scarcely ever pull off articles of dress which have been worn next the skin, without sparks and a crackling noise .being produced, espe- cially in dry weather. This is partly due, however, to the friction of these materials on the surface and with each other ; for it is greatly influenced by their nature. Thus, if a black and white silk stocking be worn, one over the other, on the same leg, the manifestation 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 connection with the body. 417. The most remarkable case of the production of elec- tricity in the Human being, at present on record, is one lately related on excellent authority in America. The subject of it, a lady, was for many months in an electric state so different from that of surrounding bodies, that, whenever she was but slightly insulated by a carpet or other feebly-conducting medium, sparks passed between her person and any object which she approached. From the pain which accompanied the passage of the sparks, her condition was a source of much discomfort to her ; when most favourably circumstanced, four sparks per minute would pass from her finger to the brass ball of a stove, at a distance of 1^ inch. The circumstances which appeared most favourable to the production of electricity were an atmosphere of about 80°, tran- quillity of mind, and social enjoyment ; while a low temperature and depressing emotions diminished it in a corresponding degree. The phenomenon was first noticed during the occurrence of a vivid Aurora Borealis; and though its first appearance was sudden, its departure was gradual. Various experiments were made, with the view of ascertaining if the electricity was pro- z 2 320 ELECTRICITY OF ANIMALS. duced by the friction of articles of dress ; but no change in these seemed to modify its intensity. 418. In most animals with a soft fur, sparks may be produced 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 Ley den 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 pro- ducing and accumulating electricity in large quantities, by means of organs specially adapted for the purpose ; and of discharging it at will, with considerable violence. It is remarkable that all these belong to the class of Fishes ; * and that they should differ alike in their general conformation, and in their geographical distribution. Thus, the two species of Torpedo, belonging to the Ray tribe, are found on most of the coasts of the Atlantic and Mediterranean ; and sometimes so abundantly, as to be a staple article of food. The Gymnotus, or Electric Eel, is con- fined to the rivers of South America. The Silurus (more correctly, the Malapterurus), which approaches more nearly to the Salmon tribe, occurs in the Niger, the Senegal, and the Nile ; and there are two other less known Fishes, said to possess electric properties, which inhabit the Indian seas. 420. Of all these, the Gymnotus 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 appear- ance ; its usual length is from six to eight feet ; but it is said occa- sionally to attain the length of twenty feet. This fish will attack and paralyse horses, as well as kill small animals ; and the dis- * Certain Insects and Mollusca have been said to possess electrical properties ; but no special electric organ has been discovered in them. ELECTRIC FISHES. 321 Fis. 165.— Gymnotus. charges of the larger individuals sometimes prove sufficient to deprive even men, of sense and mo- tion. 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 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 fee- ble ; but as the animal becomes more irritated, their power increases. After a considerable num- ber of powerful discharges, the energy is exhausted, and is not recovered for some time ; and this circumstance is taken advan- tage of in South America, both to obtain the fishes (which afford excellent food), and to make the rivers they infest passable to travellers. A number of wild horses are collected in the neigh- bourhood and 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 ex- hausted in the same degree, and they then become an easy prey to their captors. 421. The shock of the Torpedo is less power- ful ; 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 discharge is procured from the Torpedo, by touching its back and belly simultaneously ; the electricity of the back being positive, and Fig. 1 Torpedo. Common 322 ELECTRIC FISHES. 9 o 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 discharge is effected. If the fish be placed between two plates of metal, the edges of which are in contact, no shock is perceived by the hands placed upon them, since the metal is a better con- ductor 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 ob- tained from the Tor- pedo than from the Gymnotus. 422. The electric organs of the Torpedo are of flattened shape, and occupy the front and sides of the body, Fig. 167.— Electric Apparatus of Torpedo. c, brain ; me, spinal cord ; o, eye and optic nerve; e, electric organs ; np, pneumogastric nerve, supplying electric organs ; nl, lateral nerve ; n, spinal nerves. ELECTRIC FISHES. 323 forming two large masses, which extend backwards and out- wards from each side of the head. They are composed of two layers of membrane, the space between which is divided by vertical partitions, into hexagonal cells like those of a honey- comb (0, Fig. 167), the ends of which are directed towards the two surfaces of the body. These cells, — which are filled with a whitish soft pulp, somewhat resembling the substance of the brain, but containing more water, — are again subdivided horizontally by little membranous partitions; and all these partitions are profusely supplied with vessels and nerves. The electrical organs of the Gymnotus are essentially the same in structure, but differ in shape in accordance with the conformation of the animal ; they occupy one-third of its whole bulk, and run nearly along its entire length, being arranged in two distinct pairs, one much larger than the other. In the Malapterurus, there is not Fig. 168-— Electric Malapterurus. any electrical organ so definite as those just described ; but the thick layer of dense areolar tissue, which completely surrounds the body, appears to be subservient to this function, — being composed of tendinous fibres interwoven together, and contain- ing a gelatinous substance in its interstices, so as to bear a close analogy with the special organs of the Torpedo and Gymnotus. 423. In all these instances, the electrical organs are supplied with nerves of very great size, larger than any others in the same animals, and larger than any nerves in other animals of like bulk. These nerves arise from the top of the spinal cord, and seem analogous to the pneumogastric nerve (§. 458) of other animals. The influence of these nerves is essential to the action of the electric organs. If all the trunks on one side be cut, the power of the corresponding organ will be destroyed, but that of 324 ELECTRIC FISHES. the other may remain uninjured. If the nerves be partially destroyed on either or both sides, the power is retained by the portions of the organs which are still connected with the brain by the trunks that remain. Even slices of the organ entirely separated from the body, except by a nervous fibre, may exhibit electrical properties. Discharges may be produced, by irri- tating the part of the nervous centres from which the trunks proceed (so long, at least, as they are entire), or by irritating the trunks themselves. In all these respects, there is a strong ana- logy between the action of the nerves on the electric organs, and on the muscles (Chap, xn.) ; and there is the same objection to the idea that the influence of the nerves is itself electrical ; for if the nerves be tied, their power of exciting the electric organs is destroyed as completely as if they were cut. As to the mode in which the nerves cause these organs to generate electricity, we know nothing whatever; but we know nothing more of the manner in which they excite muscular contraction; and we must be satisfied for the present to remain ignorant of it. 424. As to the uses of the electrical organs to the animals 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 cap- tivity do not seem disposed to exercise their powers on small fishes placed in the water with them. The chief use of the electrical power appears to be, to serve as a means of defence, to the Fishes which possess it, against their enemies. CHAPTER X. FUNCTIONS OF THE NERVOUS SYSTEM. 425. The preceding Chapters have been devoted to the con- sideration of the Functions of Organic Life, — those changes, namely, in the Animal body, which are concerned in the mainte- nance of its own fabric ; and which, although performed in a different mode, and having different objects to fulfil, are essentially the same in character with those which take place in Plants. The first and most striking difference of mode results, as we have seen, from the nature of the food of Animals ; which requires that they should possess a cavity for its reception, and a chemical and mechanical apparatus for its digestion (or reduction to the fluid form), in order that it may be prepared for absorption into the vessels. In regard to the absorption of the aliment, and its cir- culation through the system, there is but little essential difference between Plants and the lower Animals ; but in the higher tribes of the latter, we find that a muscular organ, having the action of a forcing-pump, is appended to the system of tubes in which the fluid circulates, in order to drive it through them with the requisite certainty and energy. The respiration of Animals, again, is essentially the same with that of Plants ; the chief dif- ference being that, in order to secure the active performance of this important function, the higher Animals are provided with a complex apparatus of nerves and muscles, by which the air or water in contact with the aerating surface is continually re- newed. And in regard to the functions of secretion and excre- tion, we have seen that, though there is a wide difference in the form of the organs by which they are executed, they are the same in essential structure ; and that the difference in their mode of operation consists chiefly in this, that their products in the 326 PURPOSES OF THE ORGANIC FUNCTIONS IN ANIMALS. Animal are destined to be carried out of the body, instead of being retained within it, as in Plants. 426. In regard to the immediate object of these functions, also, there is but little essential difference ; for in both instances it is the conversion of alimentary materials into living organised tissue. But the ultimate purpose of this tissue is far from being the same in the two kingdoms. Nearly all the nourishment taken in by Plants is applied to the extension of their own fabric ; and hence there is scarcely any limit to the size they may attain. There is very little waste or decay of structure in them, the parts once formed (with the exception of the leaves and flowers) continuing to exist for an indefinite time ; this is a consequence of the simply physical nature of the functions of the woody structure, which has for its chief object to give support to the softer parts, and to serve as the channel for the movement of the fluid that passes towards and from them. — The case is very different in regard to Animals. With the exception of those inert tribes which may be compared with Plants in their mode of life, we find that the whole structure is formed for motion ; and that every act of motion involves a waste or decay of the fabric which executes it. An energetic performance of the nutritive actions is required, therefore, in the more active Ani- mals, simply to make good the loss which thus takes place ; we find, too, that their size is restrained within certain limits; so that, instead of the nourishment taken into the body being applied, as in Plants, to the formation of new parts, it is em- ployed for the most part in the simple repair of the old. Thus we may say that, whilst the object of Vegetable Life is to build up a vast fabric of organised structure, the purpose of the Organic Life of Animals is to construct, and to maintain in a state fit for use, the mechanism which is to serve as the instru- ment of their Animal Functions, — enabling them to receive sen- sations, and to execute spontaneous movements, in accordance with their emotions, instincts, or will. 427. This mechanism consists of two kinds of structure, — the Nervous and Muscular ; which have entirely different offices to perform. The Nervous system is that which is the actual GENERAL STRUCTURE AND ACTIONS OF NERVOUS SYSTEM. 327 instrument of the mind. Through its means, the individual becomes conscious of what is passing around him ; its operations are connected, in a manner we are totally unable to explain, with all his thoughts, feelings, desires, reasonings, and determi- nations; and it communicates the influence of these to his muscles, exciting them to the operations which he desires or determines to execute. But of itself it cannot produce any movement, or give rise to any action ; any more than the expansive force of Steam could set a mill in motion, without the machinery of the Steam-Engine for it to act upon. The Muscular System is the apparatus by which the movements of the body are immediately accomplished ; and these it effects by the peculiar power it pos- sesses, of contracting upon the application of certain stimuli, of which Nervous Agency is the most powerful. General Structure and Actions of the Nervous System. 428. The Nervous tissue consists of two distinct structures ; of one of which the trunks of the nerves a are entirely made up ; whilst the other enters largely into the composition of the ganglia or centres of action (§.65). The former, termed the white or fibrous tissue, consists of straight fibres, lying side by side, and bound together by areolar tissue into bundles ; 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 distributed through the body, especially to the skin and muscles. These fibres, however, differ entirely in their character from any which have been hitherto described. They are found, when examined with a good Microscope, to be tubes, containing a sort of pulpy substance, which may be squeezed from their ends when they are cut across. Each of these tubes runs its course completely separated from the rest, no union or Fig. 168.* — Structure of Nervs- Tubes. A, cylindrical tubes from trunk of nerve ; B, beaded tubes from brain. 328 FUNCTIONS OF THE NERVOUS TRUNKS. inosculation ever occurring among them : and there is good reason to believe that every tube extends, without break or interruption, from the central organ with which the trunk is connected, to the point at which it enters the muscular substance or skin, to which it is transmitted ; and that it has a function entirely distinct from those of the tubes by which it is surrounded. 429. There can be no doubt that the office of this kind of Nervous tissue 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 elec- tric influence, from the instrument which excites it, to some distant point, where it is to be applied to some use. There can be no doubt that the effects of such changes in the state of the Nervous System are propagated in two opposite directions ; — the impres- sions 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 sensations; — and the in- fluence of the emotions or volitions to which these sensations give rise (§. 7), being propagated from the central organs to the mus- cles which they excite to contraction. And by the discoveries of Sir C. Bell, hereafter to 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 origi- nate in the circumference; — and the other conveying to the circumference those which originate in the centre (§.451). The transmission of these changes is completely interrupted by divi- sion of the nervous trunk, or by pressure upon it ; and it some- times happens that one set of conducting fibres is thus affected, whilst the functions of the other are not impaired ; so that a limb may possess sensibility, and be totally destitute of the power of motion, or may be completely obedient to the will, though totally destitute of sensibility. In Vertebrated animals, we find some nerves in which there is only one set of fibres ; so that the trunk is only sensory or only motor (§. 458) : but in general, the two sets are bound up together in the same sheath. 430. The structure and functions of the gray substance, how- ever, are very different. This kind of nervous tissue is found in the interior of the ganglia of Invertebrated animals, and in the FUNCTIONS OF THE GRAY MATTER. 329 centre of the Spinal Cord of Vertebrata ; but it is disposed on the outside of the Brain, and forms a thin layer enclosing the fibrous substance, of which the greater part of the mass is com- posed. From this peculiarity of position in the Brain, it is not unfrequently termed the cortical (bark-like) substance. It is principally composed of a minute network of blood-vessels, which surrounds the extremities of the fibres of the white substance ; and in the midst of it, there are a large number of cells, that lie loosely in the interstices of the network. What is the precise manner in which this structure acts, we are unable to define ; but this much is tolerably certain, — that, in the gray matter of the nervous centres, all those changes originate, which are propa- gated by the motor fibres to the muscles ; and that these changes depend upon the continual supply of arterial blood. If this supply be cut off, by failure of the heart's action (as in ordinary fainting), or by pressure on the vessels that convey blood to the head, immediate insensibility, with loss of all power of motion, is the result. 431. The nervous fibres which are distributed to the muscles, spread forth from their respective trunks in loops, which fre- quently anastomose with each other, and thus form a network through the muscle. But the fibres which commence in the skin and organs of the senses, and convey the impressions made upon them to the sensorium, do not resemble these in their distribution. They originate in minute elevations of the surface, termed papilla? ; in every one of which there is a network of vessels, in- cluding cells in its meshes, around the extremity of the fibre. The action of the blood conveyed by these vessels is just as neces- sary for the production of the sensory impression upon the nervous fibre, which is to be conducted by it to the brain, and there to become a sensation, — as it is for the gray matter of the brain to excite a change in the motor fibres that calls a muscle into operation. If the circulation in the skin be checked by cold, or by pressure on the artery of a limb, we perceive a numbness, resulting from the want of power in its nervous fibres to receive impressions. Thus we see that a sort of gray matter exists at the commencement of every sensory fibre, as well as at the com- 330 GENERAL FUNCTIONS OF THE NERVOUS SYSTEM. mencement of every motor fibre ; but that which belongs to the former is diffused over the surface of the body, and the changes to which it gives rise are conducted towards the central organs, there to produce sensations ; whilst that which belongs to the latter is collected into a mass, in these same central organs, and its influence is propagated along the fibres which diverge from them to all parts of the body. 432. Hence we find that, before the mind of the individual can become conscious of what is passing around him, a change or impression must be effected by external objects upon the origins of the sensory nerves in the papillae ; this impression must be conducted by the nervous trunk to the sensorium, and there it becomes a sensation. On the other hand, before the mind can direct the body to perform any movement, an emotion, or an act of the will, must produce a change at the origin of the motor nerves in the brain ; and this change is conducted along the motor trunks to the muscles, where it excites a contraction adapted to the required purpose. 433. It is by actions of this kind, that the Nervous System ministers to those Functions, which are peculiarly distinguished as Animal (§. 4). But it is also concerned in producing certain movements of the body, which have for their object to maintain the Organic functions; — those, for instance, of Deglutition (§. 195), and Respiration (§. 340). Such movements require the same double set of nervous fibres, and the same kind of ner- vous centre containing gray matter ; but they do not require that sensation should intervene, or (in other words) that the individual should become conscious of the impression in which they originate, in order that the muscles may be excited to contraction. Move- ments of this class are termed reflex ; from the peculiar action of the ganglion in throwing back, or reflecting, along the motor nerves that pass from it, the impressions which it receives from the fibres that pass towards it. There can never be more than a single centre of sensation in any animal ; for if there were two or more, there must be two sets of feelings, and consequently two distinct individual minds. But there may be many centres of reflex action, having different purposes, because connected COMPARATIVE STRUCTURE OF THE NERVOUS SYSTEM. 331 with different functions. In the lower classes of Animals, these centres are often very numerous ; and the actions to which they minister, constitute a great part of the movements performed by them. Hence some of our best examples of reflex action are drawn from these classes. But as we ascend the scale, we find that these centres of reflex action are less important in compa- rison with the organ by which the mind operates ; and that the body is more influenced by the latter than by the former. In order to comprehend the mutual relations of the different parts of the nervous system, it will be better to commence with its simplest forms, and to pass gradually on to the more complicated. Structure and Actions of the Nervous System in the principal Classes of Animals. 434. In most of the Radiated classes, it is difficult to dis- cover any distinct traces of a Nervous System ; the general softness of their tissues being such, that it cannot be clearly made out amongst them. But in the highest group, the Echinoder- mata, it may be detected without difficulty ; and it presents an extremely simple form, which partakes of the general arrange- ment of parts in these animals. In the Star-fish, for instance, it forms a ring, which surrounds the opening into the stomach ; this ring \\X'~ -" ''// consists of nervous cords, which form \ ^^pr*0~*~a*y£' / communications between five ganglia, j / \ . one of which is placed at the base of L ( ** ) 1 '^... each ray. These ganglia appear to « cornea ; s, sclerotic ; s', portion of 1 • v • , j ii 7 -7/7\ tne sclerotic turned back to show the Which IS termed the Choroid (Ch); subjacent parts ; eh, choroid ; r, retina ; this is much more delicate in its »>.°Pticnerve; ca, anterior chamber ; i, iris, p, pupil ; cr, crystalline lens; pc, Structure, and Consists almost ciliary processes ; v, vitreous humour; ,. , » ,, , , , bb, conjunctiva. entirely of blood-vessels and nerves. It has a deep black colour, owing to its being covered with a layer of black pigment, which consists of cells that have 402 STRUCTURE OF THE EYE OP MAN. the power of secreting a black granular matter in their interior.* This coat also changes its character in the front of the eye ; being there continuous with the iris, or coloured portion (i), which forms a sort of curtain that hangs down behind the cornea. The surface of the iris is flat, or nearly so ; and there is consequently a space between it and the cornea, like that which intervenes be- tween the dial-plate and glass of a watch ; this space is termed the anterior chamber of the eye. The iris is perforated in its middle by an aperture, termed the pupil (p). This aperture is always round in Man ; but in animals whose range of vision is required to extend widely in a horizontal direction, (such as the Ruminantia, and others which feed upon herbage,) it is an ellipse with the long diameter horizontal ; whilst in animals which rather seek their food above or below them, (such as the Cat and other Carnivora, which naturally live among trees and high places,) the pupil is an ellipse, whose long diameter is vertical. 534. By the contraction and relaxation of certain fibres in the iris, the size of the pupil is changed, according to the degree of light to which the eye is exposed ; the pupil being made to contract in a strong light, in such a manner as to exclude the rays that would be superfluous, and to prevent too many from falling upon the expansion of the optic nerve ; whilst it dilates in a faint light, so as to admit as many rays as possible. If we notice the pupil of a Man who is looking towards the mid-day sun, we shall see that it is contracted to a small round speck ; but the pupil of a Sheep would be contracted, in similar circum- stances, into a horizontal slit ; and the pupil of a Cat into a ver- tical one. The alteration in the size of the pupil, in accordance with the degree of light, may be easily observed, by stationing one's self at a window provided with shutters, and holding a looking-glass in the hand. If the light be at first strong, the pupil will be seen in a contracted state ; but if the shutters be gradually closed, so as to diminish the amount of light that falls upon the eye, the pupil will be seen to enlarge ; and it will * Similar pigment-colls, having great variety in their form, are to be found com- posing the black spots on the skin of the Frog, Water Newt, &c. STRUCTURE OF THE EYE OF MAN. 403 diminish again when the shutters are re-opened. The blackness which the pupil always presents, in the healthy state of the eye, is due to our seeing the black lining of the back of the eye through it. In many quadrupeds, the black pigment is replaced, in one portion of the eye, by a layer of a bluish colour, having an almost metallic lustre ; and from this we see the light bril- liantly reflected, when we look at their eyes in certain directions. The back of the iris is always covered with black pigment, the use of which is, to absorb the rays that would otherwise be reflected from one side of the interior of the eye to another, ren- dering the image indistinct. 535. On turning back the choroid coat, we come to the third layer (r), of which the wall of the eye is composed ; this is an extremely delicate film, chiefly consisting of nervous fibres which spread out from the optic nerve (rt) ; and it is called the retina (or net). It advances nearly as far as the iris; but it is deficient in the front of the eye. 536. The cavity of the globe is occupied by three humours of different consistence, — the aqueous, vitreous, and crystalline. The aqueous humour is nearly pure water, being nothing else than the serum of the blood very much diluted ; it occupies the an- terior chamber (ca), and a small space behind the iris, in front of the crystalline lens. The vitreous humour (y) resembles thin jelly in consistence, and occupies the greater part of the globe of the eye behind the iris. The crystalline (cr) is of much firmer consistence, resembling very thick jelly, or soft gristle ; it has the form of a double convex lens, the posterior surface of which is more convex than its anterior ; and hence it is com- monly known as the crystalline lens. It is suspended in its place by a set of little bands proceeding from the choroid coat, and known as the ciliary processes (jt?c). 537. The cornea is covered externally by a membrane (bb) termed the conjunctiva. This membrane is perfectly transparent where it covers the cornea, and seems like the outer layer of it ; the front of the sclerotic also is covered by it, and it is there semi-opaque, having a whitish colour. The membrane does not pass back over the globe of the eye, however, but bends forward 404 CONJUNCTIVAL MEMBRANE. MUSCLES OF THE EYE. again, as seen at bb, so as to form the lining of the eyelids, at the edge of which it becomes continuous with the skin. Thus the smooth surfaces of the eye and of the under side of the lids, are both formed by this membrane ; the mucous secretion from which, serves to diminish the friction of one upon the other. But the smallest particle of any hard substance, which may become interposed between these surfaces, produces great irri- tation. It cannot pass far backwards, however, on account of the bend of the membrane at bb ; and thus it may be always removed (if loose) with little difficulty. The lower lid can be easily drawn down, so as to expose the membrane as far as this bend ; and any loose particle that is lying upon its surface may thus be detected and removed. But the upper lid, being longer, cannot be drawn out sufficiently for this purpose; and it is necessary to evert it, or turn it inside-out. As the knowledge of the mode of performing this very simple operation will often save a good deal of suffering, it will be here described. Nothing more is necessary than to close the upper lid — not forcibly, how- ever ; next to make pressure upon its upper part with a pencil, bodkin, knitting-needle, or other hard body of small diameter ; and then, taking hold of the eye -lashes, to draw the lower edge of the lid forwards and upwards. By a dexterous movement of this kind, the lid may be everted without any pain, — a little temporary discomfort being all that the displacement occasions ; its lining membrane is then exposed, and any offending particle may be readily removed. 538. The globe of the eye is moved by six muscles, which are lodged within the bony cavity or orbit, hollowed out in the skull. All these muscles, except one, originate at the back of the orbit, and are inserted into the sclerotic coat, near its front, by broad thin tendons. Four of them are termed recti or straight muscles. One of these, the superior rectus (e, Fig. 198), is inserted at the upper part of the eye, and consequently by its contraction rolls the globe upwards ; another, the inferior rectus (d) produces a corresponding movement downwards. A third, the internal rectus (which could not be shown in this figure) rolls the globe inwards or towards the nose ; whilst a MUSCLES AND MOVEMENTS OF THE EYE. 405 fourth, the external rectus, (the cut extremity of which is seen at /), turns it outward. Be- h e sides these, there is a remark- able muscle, the superior oblique (A), which originates at the back of the orbit, comes for- wards to the front, where its tendon passes through a pulley, and then turns backwards to be inserted into the sclerotic coat, at a point considerably d behind the pulley. The action Fig. 198.— Vertical Section of the Ormt, o ,, . , .11 i , ii showing the globe of the eye and its append- of this muscle will be to roll ages. a> cornea. b> scleiotic.C} optic nerve; the globe downwards and Some- d> inferior rectus muscle ; e, superior rectus ; & ft cut extremity of the external rectus ; g, what Outwards. The Sixth milS- end of the inferior oblique; h, superior ob- cle, termed the inferior ollique X^ST' * "" "PperM; *' '*" (^), does not arise like the rest from the back of the orbit, but from its lower border ; and its action is to roll the eye upwards and inwards. Of these muscles, the superior, inferior, and internal recti, together with the inferior oblique, and also the elevator muscle (i) of the upper eyelid, are supplied with motor nerves by the third pair (§. 458) ; whilst the superior oblique has a nerve to itself, the fourth ; and the external rectus has another nerve to itself, the sixth. 539. The relative actions of these muscles and nerves are not yet fully understood. There is this very peculiar circum- stance attending the movements of the two eyes, — that although they are harmonious, they are seldom symmetrical. Thus, when we direct our eyes towards an object on one side of us, they move harmoniously, that is, with a common purpose ; but their movement is not symmetrical, for one globe is rolled inwards by the internal rectus, whilst the other is rolled out- wards by the external rectus. These two different actions seem to be instinctively connected, and to be guided by the sensations which are received through the two eyes respectively.* It is only when they both move directly upwards, or directly downwards, * See the Author's Principles of Human Physiology, §. 251, 2b?.. 406 EYELIDS AND LACHRYMAL APPARATUS. that their movements are at the same time harmonious and sym- metrical. It may be shown that two of the recti muscles, act- ing in conjunction with one of the oblique, may produce (accord- ing to the laws of the composition of forces, Mechan. Philos. §. 162) any of the motions of the globe ; and it has been sug- gested that the symmetrical movements of the eyes are produced (in all cases in which they are not harmonious) by the voluntary action of one set, acting upon the eye which is directed towards the object ; whilst the other eye follows in the same direction by the instinctive movement of the other set. 540. The eyebrows, eyelids, and eyelashes, serve in various ways for the protection of the eyes. In Birds and Reptiles there is a third eyelid, which is drawn across the eye by a muscle that passes through a loop in it. This nictitating membrane, as it is termed, is semi-transparent ; and it serves to protect the eye from the too-powerful rays of light, without destroying the power of vision. The upper and lower eyelids of the Mammalia, and the nictitating membrane of Birds and Reptiles, are very frequently drawn over the front of the globe, during the waking state, for the purpose of sweeping from it the dust and other accidental impurities, which would otherwise be injurious. Beneath the upper eyelid, in the upper and outer portion of the orbit, is situated the lachrymal gland (&, Fig. 198) ; this is con- tinually pouring out a watery secretion over the globe of the eye, which serves to wash from it these impurities, and to keep it moist. It is only, however, when the quantity of this secretion is increased by mental emotion or by irritation in the eye itself, producing a flow of tears, that we become conscious of its existence. It is ordinarily drawn off as fast as it is formed, by a curious apparatus situated at the inner border of the eye. If the edges of the lids be carefully examined, there will be seen upon each of them, close to the inner corner of the eye, a minute spot, which is the entrance to a small canal, termed the lachrymal duct. The two ducts, one commencing at the corner of the upper lid, and the other at the lower, soon unite into one canal, which swells out into a sort of reservoir, the lachrymal sac, that lies upon the side of the upper part of the nose ; and from this SENSE OF VISION. 407 reservoir a canal passes down, through the bones of the nose, into its cavity. By this apparatus, the fluid which is poured by the lachrymal gland over the exterior of the eye, is drawn off at the interior, after washing its surface : whence it is carried into the nose, and is got rid of by the current of air that passes through its passages in breathing. The edges of the lids meet in such a manner, when they are closed, as to form a sort of gutter or channel, along which the lachrymal secretion flows from their outer to their inner corner during sleep. 541. Having thus described the structure of the Eye, and the general actions of the parts by which it is adapted to the per- formance of its remarkable function, we shall examine into the details of this function ; in other words, into the nature of vision. 542. It is through the medium of the light they emit, that we are enabled to take cognizance by the Eye of the forms, sizes, colours, and positions of surrounding objects. Some of these are self-luminous ; that is, they give off light from themselves, when that from all other sources is excluded. This is the case with the Sun, and with bodies in a state of combustion ; as well as with those that are phosphorescent, which may probably be regarded as in a state of slow combustion. But other bodies transmit to us only that light which their surfaces reflect from self-luminous bodies ; and hence, when they are excluded from the influence of the latter, they are not seen. Thus in day-light, the light of the sun is reflected to us from the clouds, from the surface of the earth, and from all terrestrial objects, — the more powerfully, in proportion as their respective surfaces are more highly polished or of brighter colours : but if we place ourselves in a room, from which the light of the sun is entirely excluded, we can see no objects, because no luminous rays are reflected by them to our eyes. 543. The rays of light which diverge from the several points of any object, and fall upon the front of the cornea, are refracted by its convex surface, whilst passing through it into the eye, and are made to converge slightly. They are brought more closely together by the crystalline lens, which they reach after passing through the pupil ; and its refracting influence, together with Fie. 199. 408 FORMATION OF A PICTURE ON THE RETINA. that produced by the vitreous humour, is such as to cause the rays that issued from each point to meet in a focus on the retina. As every point is thus represented in its proper position rela- tively to others (except that those which were above are now below, and vice versa), a complete inverted image or picture of the object is formed upon the retina. This is shown in Fig. 199; where, for the sake of convenience, two rays only are represented as issuing from each of the two extremities of an object, a c. These rays cross each other in the middle of the eye, those from a being brought to a focus at b, and those from c at d ; and as all the other rays are refracted in the same manner, a complete inverted picture of the object is formed at the back of the eye. 544. That this is really the case, may not only be inferred, but proved. If the eye of a Rabbit be removed from its socket, and cleansed of the muscles, fat, &c. adherent to its back part, and a candle be then brought in front of it, the transparency of the sclerotic coat will allow the image of the candle, that is formed upon the retina, to be distinctly seen. Or, if we take the eye of a Sheep or an Ox, and after cleansing it in the same manner, we cut out from the back of it a portion of the sclerotic and choroid coats, covering the part of the retina thus left bare, with a piece of tissue-paper (for the purpose of keeping in the vitreous humour, without interrupting our view of the image), a distinct but inverted miniature picture of all the objects in front of the eye will be seen at its back. It is necessary in these experiments that the eyes should be taken from animals recently killed ; as the cornea and humours soon lose their transparency, and the distinctness of the picture is consequently impaired. 545. The black pigment, which is situated immediately behind the retina, — that is, in contact with its external surface, — is destined to absorb the rays of light, immediately that they have passed through the retina ; so as to prevent them from ALBINOS. SPHERICAL ABERRATION. 409 being reflected from one part of the interior of the globe of the eye to another, which would cause a great confusion and indis- tinctness in the picture. Hence it is that, in those individuals (both among Man and the lower animals) in whose eyes this pig- ment is deficient, vision is extremely imperfect. The eyes of those individuals (termed Albinos), derive, from the absence of pigment, a very peculiar appearance. The iris does not possess its ordinary colour ; but, owing to the large quantity of minute blood-vessels which it contains, it presents a bright red hue. The choroid coat, seen through the pupil, has exactly the same aspect ; so that the pupil is not readily distinguished. During the day, the vision of these Albinos is very indistinct, and the glare of light is painful to them ; and it is only when twilight comes on, that they can see clearly and without discomfort. 546. The eye is a much more perfect optical instrument than we might be led to suppose, from the cursory view we have hitherto taken of its functions ; for by the peculiarities of its construction, certain faults and defects are avoided, to which all ordinary optical instruments are liable. One of these, termed spherical aberration, results from the fact, that rays falling upon the central and outer parts of an ordinary convex lens, whose surfaces form part of a sphere, are not brought to meet in one point, — the focus of the central portion being rather more distant than that of the outer part. This is shown in Fig. 200, where Fig. 200. l l is the lens, r l, r l, are rays falling upon its circumfer- ence, and r' l', r' l', are rays falling near its centre. The former set of rays meet in/; whilst the latter pass on to f, before they meet in a focus. This may be shown by covering the cen- tral and outer portions of the lens, alternately, with some opaque substance, which shall stop all the rays of light proceeding 410 SPHERICAL ABERRATION, CORRECTED IN THE EYE. through it. When the central portion is covered, a distinct image will be formed at /, by the rays falling upon the outer portion ; and when the outer portion is covered, a distinct image will be formed at f, by the rays that have passed through the inner portion. But when the whole lens is employed, no dis- tinct image is formed anywhere ; for if a screen be held at /, it will receive, not only the rays which are brought to a focus at that point, but also the rays which are going on to meet at p ; and, on the other hand, if the screen be held at f, it will receive, not only the rays which are brought to a focus there, but also those which, having met at /, have crossed and passed on to G and h. 547. Now this indistinctness is ordinarily got over in prac- tice, by employing only the central portion of the lens ; so that only those rays which correspond to r' l', r' i/, shall pass through it. This we observe in ordinary Microscopes and Tele- scopes ; — a stop or perforated partition being interposed behind the lenses, so as to allow the light to pass through only a small aperture in their centre. By this plan a great deal of light is cut off; and the image is rendered dark. The spherical aber- ration may be completely corrected, however, by a certain adaptation of two or more lenses, whose surfaces have different curvatures ; the effect of which is, to bring all the rays that have passed through every part of this compound lens to the same focus. Now this very adjustment is made in the eye, by the arrangement of the curvatures of the cornea and of the two surfaces of the crystalline lens ; and in the well-formed eye it is so perfect as to produce complete distinctness in the image or picture formed upon the retina. The only case in which this would not occur is, when an object is brought near the eye ; for then the rays diverge from each other at a greater angle than when the object is at a moderate distance; and those which fall upon different parts of the lens would not be all brought to the same focus. This error is corrected by the contraction of the pupil, which takes place involuntarily when we bring an object near the eye, and thus cuts off the rays that would otherwise render the picture indistinct. CHROMATIC ABERRATION. 411 548. But there is another imperfection to which ordinary optical instruments are liable, that is completely corrected in the eye. If we look through a common Microscope, especially when a high power is employed, by the light of a lamp or candle, we see that the edges of the image are bordered by coloured fringes, which very much impair its distinctness, and prevent it from being seen in its true aspect. This is the result of what is termed chromatic aberration ; and it results from the fact, that the rays of different colours, which are all blended in ordinary colourless light, are refracted by the same lens in different degrees, so as to be brought to a focus at different points. Thus we will suppose that the lens l l (Fig. 200) has been corrected for spherical aberration ; and that r l, r l, are violet rays falling upon it, whilst r' i/, r' i/, are red rays. The former are capable of being refracted in a much higher degree than the lat- ter ; so that they are brought to a focus at /, whilst the others do not meet until p. Hence if a screen be placed to receive the image at/, the picture will be formed by the violet rays only ; and it will be surrounded by red fringes, occasioned by the red rays which are passing on to their focus at f. On the other hand, if the screen be placed at F, the picture will be chiefly formed by the red rays ; and will be surrounded by violet fringes, produced by the violet rays, which, having met in/, have crossed and passed on to g and h. Now as from each point of almost every object, proceed rays in which the differently- coloured rays are blended, the refraction of an ordinary lens produces a separa- tion of these, and a consequent indistinctness and false colouring in the picture. This is particularly the case with regard to the rays that pass through the outer portion of the lens ; for, as these are subject to greater change in their direction than are those which pass through its centre, the separation of the differently- coloured rays of which they are composed is more considerable. 549. In practice, this error is got over, like the preceding, by very much contracting the aperture of the lens ; so that only the central rays, in which the colours are but little separated, are allowed to pass. But it may be perfectly corrected by combining lenses formed out of different materials, which possess 412 ADAPTATION OP THE EYE TO DISTANCE. a different refracting power ; the errors of these being made to counterbalance one another. Such lenses, which are termed achromatic, are now employed in all superior Telescopes and Microscopes ; and a most perfect combination exists in the Eye, the different density of whose humours is adapted in such a manner as completely to answer this purpose. The contraction of the pupil, which takes place when we look at a very near object, prevents the only imperfection which could occur; and thus the picture on the retina, in a healthy eye, is always rendered free from false colours. It is said that the first idea of uniting glasses of different kinds, so as to produce an achromatic lens, was taken from the Eye ; and this is not at all improbable. In this, as in many other instances, Nature has served as a guide to Art ; or, in other words, the Divine Artificer has thus con- descended to teach the human workman. 550. There is another wonderful arrangement in the healthy Eye, which the optician can only imitate in his instruments, in a very bungling manner. It is that by which the eye adapts itself to view objects at different distances from it, with equal distinct- ness. If we look at a near object with a Telescope, adjusting the instrument so as to see it distinctly, and then turn it towards a remote object, we shall not see the latter with equal clearness, until the instrument has been again adjusted. If we then turn it back to the nearer object, we shall find that the change in the adjustment occasions the representation of it to be now indis- tinct; and in order to bring back the image to its former clear- ness, it is requisite to re-adjust the instrument to its first condi- tion. This is a necessary consequence of the optical law, that the distance of the image from the lens which forms it, varies with that of the object, — being increased as the object is brought nearer, and diminished as it recedes. If the Eye were con- structed in the same manner, we should not be able to see dis- tinctly, without the aid of artificial assistance, at any other distance than that for which it is adjusted. Hence if a perfect picture of an object situated at 12 inohes distance from the eye, were formed upon the retina, we should not be able to see it clearly when brought to the distance of 6 inches, nor when ADAPTATION OP THE EYE TO DISTANCE. 413 removed to the distance of six feet ; because, in the first of these cases, the rays would not be brought to a focus upon the retina, but at a point behind it (if they were allowed to pass on un- checked) ; whilst in the second, they would be brought to a focus at a point nearer than the retina, and would consequently begin to separate again before they reach it. 551. But the healthy eye possesses a power of perfect adjust- ment to the viewing of objects situated at different distances ; and this without any effort or intention on our parts, but, as it were, by an instinctive operation. Of the mode in which this adjustment is accomplished, nothing certain is yet known ; but it is probable that either the position or the form of the crystal- line lens undergoes a change ; by which either its distance from the retina, or its refracting power, is increased when the object is brought near, and diminished when it is carried to a greater distance : so that, under all circumstances, the picture is formed distinctly upon the retina, and vision is consequently clear. That such a change really takes place, we may readily convince our- selves, by looking at a near and a distant object placed in the same line, — a pencil-case, for instance, held up at a few inches from the eye, and a chimney half a mile off. We shall find that no effort of attention will enable us to see them both distinctly at the same time; but that, on whichever of the two objects we fix our eyes, we shall see it clearly, whilst the other becomes indistinct. 552. In advanced life, however, the convexity of the cornea, and the refracting power of the humours diminish ; and the eye can no longer accommodate itself to near objects, their rays not being brought to a focus by the time that they reach the retina. But as it is still able to see distant objects clearly, it is said to be long-sighted. By the use of a convex glass, however, adapted to supply that additional amount of refraction which is required, near objects may be distinctly seen. — A contrary state of the eye not unfrequently exists, in which the cornea is too convex, and the refracting power of the humours is too high ; from which it happens that the rays proceeding from distant objects are brought to a focus too soon, so as to cross each other before they 414 CHOICE OP SPECTACLES. reach the retina. But as such an eye can form a very distinct picture of a near object, it is said to be near-sighted. This imperfection is remedied by interposing a concave lens between the object and the eye ; by which its refracting power is dimi- nished to the necessary degree. 553. In the choice of spectacles, or eye-glasses for these pur- poses, particular care should be taken, that they are not too powerful ; since great mischief is frequently done to the eye, by the employment of lenses of too great a curvature. A person who in youth and middle age has enjoyed good sight, very com- monly finds it necessary to employ spectacles for assistance in reading and writing, as his age advances towards 50 years ; and he should be very cautious, when first availing him- self of their assistance, to employ those of the longest focus. As his age advances, it will be necessary to substitute more power- ful lenses for these ; but this should be done very gradually ; and in no instance should a glass be employed that produces any apparent enlargement in the object, its proper use being only to render the object distinct. The evil influence of using spectacles of too high a power, soon manifests itself in the strained feeling which the eyes experience for some time ; but this feeling at last subsides, in consequence of the eye having adapted itself to the glasses, and thus undergone a change which it might other- wise have taken years to produce. In this manner the eyes of a person at 60 may be brought to the state, which, under more careful management, might have been deferred ten or fifteen years longer. — Similar remarks apply to the use of concave lenses by short-sighted persons. They should never be employed of a higher power, than is requisite to see objects with distinctness, when at a moderate distance ; and on no account should any glasses be used, that diminish their apparent size. As age ad- vances, the eyes of short-sighted persons usually become more flattened ; and they are then able to adapt themselves to objects at a variety of distances. Persons who have been short-sighted when young, are not unfrequently able to see distinctly at an advanced age, without the assistance of convex glasses. 554. Some interesting observations on the limits of vision, DISCERNMENT OF MINUTE OBJECTS. 415 in regard to the minuteness of the size of the particles which can be distinguished with the naked eye, have been made by the Prussian naturalist, Ehrenberg, to whose microscopical inqui- ries we are indebted for a large part of our present knowledge of the structure and habits of Animalcules. The smallest par- ticle of a white substance distinguishable by the naked eye upon a black ground, or of a black substance upon a white ground, is about the l-400th of an inch square. It is possible, by the closest attention, and by the most favourable direction of the light, to recognise particles that are only 1- 540th of an inch square ; but without sharpness or certainty. But particles which strongly reflect light may be distinctly seen, when not half the size of the least of the foregoing ; thus, gold-dust of the fineness of 1-1 125th of an inch may be discerned with the naked eye in common daylight. When particles that cannot be dis- tinguished by themselves with the naked eye, are placed in a row, they become visible ; and hence the delicacy of vision is greater for lines than for single particles. Thus, opaque threads of no more than 1 -4900th of an inch across, — or about half the diameter of the silkworm's fibre, maybe discerned with the naked eye, when they are held towards the light. 555. The degree in which the attention is directed to them, has a great influence on the readiness with which very minute or distant objects can be perceived ; and there is a much greater variation in this respect amongst different individuals, than there is in regard to the absolute limits of vision. Many persons can distinctly see such objects, when their situation is exactly pointed out to them, who cannot otherwise distinguish them. There must be few who have not experienced this, in regard to a bal- loon or a faint star, in a clear sky, or a ship in the horizon ; we easily see them after they have been pointed out to us ; but we withdraw our eyes for a few minutes, and then search in vain for them, until they are again pointed out to us by some one who has been watching in the interval. The faculty of rapidly descrying such objects, depends upon the habit of using the eyes in search of them ; thus a seaman will distinguish land, when to the landsman there is no appearance more distinct than ff2 416 INTERPRETATION OF NOTIONS DERIVED FROM VISION. that of a faint cloud on the horizon, with (to him) no definite outline ; or he will recognise the course and rig of a distant ship, which, to the landsman appears but as a white speck on the ocean : yet the landsman, placed before a piece of delicate machinery, might be able to astonish the seaman, by distinguish- ing the forms and movements of minute parts, which to the latter appear only as a confused mass. 556. The picture formed upon the retina, closely resembles that which we see in a camera obscura. It represents the out- lines, colours, lights and shades, and relative positions, of the objects before us ; but these do not necessarily convey to the mind the knowledge of their real forms, characters, or distances. It would appear, from the actions of the lower animals, that many of them have the power of intuitively or instinctively determining the latter from the former, from the moment when they come into the world ; thus a Fly-catcher just come out of its egg, has been seen to make a successful stroke with its bill, at an insect which was near it. If it were not so, those animals, which are thrown from the first upon their own resources, must perish almost inevitably ; being starved by want of food, during the time required for them to learn how to obtain it. But this is well known not to be the case in regard to Man. The infant is educating his senses, long before any indications of mind pre- sent themselves. By the combination, especially, of the sensa- tions of sight and touch, he is learning to judge of the nature of the surfaces of objects, as they feel, by the appearance they pre- sent,— to form an idea of their distance by the mode in which his eyes are directed towards them (§. 563), — and to estimate their size, by combining the notions obtained through the pic- ture on the retina, with those he acquires by the movement of his hands over their different parts. A simple illustration will show, how closely the ideas excited by the two sets of sensations are blended in our minds. The idea of smoothness is one which has reference to the touch ; and yet it constantly occurs to us, on looking at a surface which reflects light in a particular manner. On the other hand, the idea of polish is essentially visual, having reference to the reflection of light from the surface INTERPRETATION OF NOTIONS DERIVED FROM VISION. 417 of the object ; and yet it would occur to us from the sensation conveyed through the touch, even in the dark, 557. That this combination is not formed intuitively in Man, but is the result of experience, is particularly evident from cases, in which the sense of sight has been wanting during the first years of life, and has then been obtained by an operation. Several such cases are now on record. The earliest, which still remains the most interesting, is one recorded by Cheselden, a celebrated surgeon at the beginning of the last century. The youth (about 12 years of age), for some time after tolerably dis- tinct vision had been obtained, saw everything Jlat, as in a picture; the impression made upon his retina being simply transferred to his mind : and it was some time before he acquired the power of judging, by his sight, of the real forms, characters, and distances of objects around him. Thus, among other inter- esting circumstances, it is mentioned, that he was well acquainted with a Dog and a Cat by feeling, but could not remember their respective characters when he saw them ; one day, when thus puzzled, he took up the Cat in his arms and felt her attentively, at the same time looking steadfastly at her, so as to associate the two sets of ideas ; and then, setting her down, said, '* So, puss, I shall know you another time." A similar instance has come under the Author's own knowledge; but the subject of it was scarcely old enough to present facts of so striking a character. One curious circumstance, however, may be mentioned, as fully bearing out the view here given. The lad had been accustomed to find his way readily about his father's house, by the use of his hands ; and for some time after his sight was tolerably clear, he continued to do the same, being evidently puzzled, rather than assisted, by the impressions conveyed through his new sense ; but, when learning a new locality, he employed his sight, and evidently perceived the increase of facility which he derived from it. Hence we can have little hesitation in deciding upon the question, which has perplexed many able reasoners, whether a person born blind, who was able by the sense of touch to distinguish a cube from a sphere, would, on suddenly obtaining his sight, be able to dis- tinguish them by the latter sense. This question was answered 418 ERECT VISION. SINGLE VISION. in the negative, by the celebrated mental philosopher, Locke, and with perfect justice. 558. We shall now inquire into the mode in which we form our notions of the nature, sizes, distances, &c, of external objects, from their pictures impressed upon our retina. The first question is one on which a vast amount of discussion has taken place, with very little satisfactory result. It is, — why are the objects which we see, represented to our minds in their true erect posi- tion,— their images upon the retina being inverted ? Various replies to this question have been proposed at different times ; and amongst others, it has been actually assumed, that the Infant really does see objects inverted, and that his idea is only corrected by experience. The cases alluded to in the last paragraph, how- ever, satisfactorily prove this assumption to be incorrect ; since, although the individuals saw everything upon the same plane, as in a picture, the representation was erect from the first. The various other solutions that have been proposed, although at first sight appearing to possess more or less truth, are all open to objection ; and we are obliged, therefore, to have recourse to the doctrine of instinct or intuition (§. 476), as the real explanation. When it is remembered how much knowledge the lower animals evidently derive, without any experience, from the visual repre- sentation which their eyes produce, it is not difficult to imagine that a small part, — that, too, which is most necessary for obtaining all the rest, — is intuitive in Man. 559. The same may be said of the cause of single vision (that is, of our seeing but one object) although the picture is double, being formed upon both retinas. In animals which, like Man, look straight forwards, the field of vision of the two eyes is nearly the same ; so that most of the objects seen with one eye will be seen with the other also. The objects at the right and left sides of the field of vision, however, are seen with the right and left eyes singly; yet we perceive no difference in the aspect of these, from that of the objects towards which both our eyes are directed. It is evident, then, that the pictures formed on the two retinae are blended, as it were, by the mind, into a single perception. This seems to be, in part at least, the effect COMBINATION OF THE TWO PICTURES. 419 of habit. When the images do not fall upon parts of the two retinae which are accustomed to act together, double vision is the result. Thus if, when looking steadily at an object, we press one of the eyeballs sideways with the finger, the object is seen double. In the same manner, if an affection of the nerves or muscles of one eye (which may come on suddenly), prevents it from being directed to the same point with its fellow, double vision of all objects is the result. This is a not unfrequent symptom of disorder within the brain. If it continue, however, the individual becomes accustomed to the double images, or rather ceases to perceive that they are double ; probably because the mind becomes habituated to receive the impressions from the two parts of the retinas, which now act together. For if, after the double vision has passed away, the conformity of the two eyes is restored, as by the operation for the cure of squint- ing, there is double vision for some little time, although the two parts of the retina, which originally acted together, are now brought to do so again. 560. That the combination of the two images must be effected by an operation of the mind, is evident from another circumstance. It is easy to show, that no near object is seen by the two eyes in exactly the same manner. Thus, let the reader hold up a thin book, in such a manner that its back shall be exactly in front of his nose, and at a moderate distance from it ; he will observe, by closing first one eye and then the other, that his view of it is very different, according to the eye with which he sees it. With the right eye he will see its back and right side — the latter very much foreshortened, but none of the left side ; whilst with the left eye he will see its back and left side — the latter also foreshortened, but none of the right side. Hence, if he were to draw a perspective view of the object, as seen by each eye, the two delineations would be very different. But, on looking at the object with the two eyes conjointly, there is no confusion between these pictures ; nor does the mind dwell upon either of them singly ; but the union of the two intuitively gives us the idea of a solid projecting body — such an idea as we could have only acquired otherwise, by the exercise of the sense of touch. 420 COMBINATION OF THE TWO PICTURES. 561. That this is really the case, has been proved by expe- riments with a very ingenious instrument (invented by Professor Wheatstone), termed the Stereoscope. It consists of two plane mirrors, inclined with their backs to one another at an angle of 90°, the point of meeting being opposite to the middle of the forehead. Now if two drawings of any solid object, representing the different perspective views of it seen by the two eyes, be placed before these mirrors, in such a manner that their images shall be reflected to the two eyes respectively, and shall fall on corresponding parts of the two retinas, in the same manner as the two images formed by the solid object itself would have done, so that their apparent places are the same, — the mind will per- ceive, not one or other of the single representations of the object, nor a confused union of the two, but a body projecting in relief, the exact counterpart of that from which the drawings were made. But if two dissimilar objects be thus presented to the two eyes, the mind perceives only one of them, the other being completely excluded for a time ; but it commonly happens that, after one has been thus seen, the other begins to attract atten- tion, and takes its place, without creating any confusion or intermingling of images, except at the moment of change. The power of the will may to a certain extent determine which object shall be seen ; but not entirely ; for if one picture be more illumi- nated than the other, it will be seen during a larger proportion of the time. — Some interesting experiments of this kind may be performed without the aid of mirrors ; by simply placing the two pictures before the two eyes, and looking at each separately, by holding a piece of pasteboard or a thin book, with one edge upon the nose and forehead, and the other directed forwards, so as completely to separate the fields of view of the two eyes. By a slight effort, the apparent places of the two pictures may be made to coincide, so that one shall be made to cover the other, as when the Stereoscope is used ; and the mental percep- tion which results from the combination of the two images will be of the same kind as in the former case. There is a diffi- culty, however, in keeping the images in correspondence with one another, which renders the use of the Stereoscope much preferable. IDEA OP PROJECTION. IDEA OF DISTANCE. 421 562. It is, then, by the combination which is effected through a mental process of an instinctivekind, that the different pictures formed on the two retinae are made to blend into one representa- tion, which gives the idea of projection. We do not form this idea, except from reasoning and experience, when we look at a distant object; because the two pictures are then almost precisely the same; and hence it is impossible (without moving the head) to distinguish with certainty between a well-painted picture, in which the proportions, lights and shades, &c, are well preserved, and the objects it is intended to represent, if we are prevented from knowing that it is a picture. Some admirable illusions of this kind have been effected in the Diorama. But a slight movement of the head suffices to dispel the illusion ; since by this movement a great change would be effected in the perspective view of an object, — a little of the side of a projecting buttress or column being seen, for instance, where only the front was perceived before, — whilst the image formed by a picture is but slightly affected. 563. Our idea of the distance of objects is evidently acquired by experience. An infant, when a bright object is held before its eyes, attempts to grasp it with its little hands ; but obviously with no certain idea of its situation. The same is observed in persons, who have but recently acquired sight. Here, then, the impressions made upon the eyes have to be corrected by those received through the touch, before the power of judging of dis- tances is acquired. But when it is once acquired, we can accu- rately estimate the relative distances of near objects, without using our hands. This we do chiefly, by the interpretation which we have learned to make, of the sensations which are occasioned in the muscles of the eyes, by their various actions. When we fix both our eyes upon a distant object, their axes (lines drawn through the centre of each cornea and pupil, backwards through the centre of each sphere,) are nearly parallel to each other. But when we direct them to a near object, the axes of the eyes meet in the point at which we are looking. This is very easily seen, by watching the eyes of another person, when fixed upon an object, first held at arm's length, and then brought nearer and 422 ESTIMATION OF DISTANCE. nearer to the middle point between the eyes ; the two corneee are at first directed nearly straightfor wards ; but they gradually turn inwards, as the object is brought nearer, and at last a very decided inward squint is produced, which disappears as soon as the object is removed. Thus, for objects within a moderate dis- tance, the degree of convergence of the axes of the eyes, and the muscular sensations thereby produced, afford us sufficient means of judgment. 564. "We perceive this in another, as well as in ourselves; for by observing his eyes, we can judge, not only of the direc- tion, but of the distance, of the object he is looking at. Thus when a person, A., sees a friend, B., looking towards him, he can at once tell, by the appearance of B.'s eyes, whether he is looking at him,, or at an object nearer or more remote; or whether, being in a contemplative mood, his eyes are fixed upon no defi- nite object, but are looking into space. In the latter case, as in the case of blind persons in whose eyes there is no other indica- tion of loss of sight, the peculiar vacant expression is due to the want of any convergence between the axes of the eyes, such as would indicate that they are fixed upon an object. The assist- ance which the joint use of both eyes affords in the estimation of distance, is evident from the fact, that, if we close one eye, we are unable to execute with certainty many actions, which require a precise appreciation of the distance of near objects, — such as threading a needle, or snuffing a candle. Instances are not un- frequent, in which persons have experienced this difficulty, before they were aware that they had lost the sight of one of their eyes. 565. In regard to distant objects, our judgment is chiefly founded upon their apparent size, if their actual size be known to us ; and also upon the extent of ground, which we see to inter- vene between ourselves and the object. But if we do not know their actual size, and are so situated that we cannot estimate the intervening space, we principally form our judgment from the greater or less distinctness of their colour and outline. Hence, this estimate is liable to be very much affected by varying states of the atmosphere ; a distant ridge of hills, for example, some- ESTIMATION OF SIZE. 423 times appearing to be more remote, at other times to be compa- ratively near, according as the air is hazy or peculiarly clear. 566. Our notion of the size of an object is closely connected with that of its distance. It is founded on the dimensions of the picture, which is formed by the object upon the retina ; but it is corrected by the known or supposed distance of the object itself. Thus, I hold up a book at a certain distance from my eye, and it covers the whole of the opposite window ; the apparent size of the picture of both is just the same, therefore ; but knowing that the book is much nearer than the window, I infer that it is much smaller. Where we know their respective distances, the estimation of their real sizes is very easy ; but this is not the case, when we only guess at their distances. Hence our esti- mate of the size of objects, even moderately distant, is much influenced by states of the atmosphere. Thus, if we walk across a common in a fog, a child approaching us appears to have the size of a man, and a man seems like a giant ; for the indistinct- ness of the outline excites in the mind the idea of distance ; and the same picture, supposed to be that of a more remote object, will give rise to the idea of greater size. The want of innate power in Man to form a true conception of either size or distance, is well shown by the effect produced on the mind unprepared for such illusions, by a skilfully-painted picture, the view of which is so contrived, that its distance from the eye cannot be estimated in the ordinary manner ; the objects it represents are invested by the mind with their real sizes and respective distances, as if their real images were formed upon the retina. This illusion has been extremely complete, in some of those who have seen the panoramic view of London in the Colosseum. A lively and interesting account of it is given in the Journal of the Parsee Shipbuilders (from Madras) who visited England a few years since. 567. When a number of luminous impressions are made upon the retina at short intervals, they become blended into one, — the intervals being unclistinguishable. Thus, when we rapidly move the end of a lighted stick in a straight line or circle, the impression produced is that of a band or ring of light ; for the 424 DURATION OF LUMINOUS IMPRESSIONS. COLOUR. impression made by the light, as it passes each point, remains for some time subsequently. If the stick be whirled round with sufficient rapidity, for it to reach any point a second time, before the impression made by its previous passage has departed, an unbroken circle of light is produced. By experiments made in this manner, we may determine the longest interval during which impressions remain on the sensorium ; for if we find that a hot coal, whirling round at the rate of six times in a second, produces a continued circle of light, — but that the circle is broken, when it turns round only five times in a second, — we know that the length of the impression is l-6th of a second. By experiments of this kind, it has been found, that the duration varies in different individuals, and in the same individual at dif- ferent times, from l-4th to l-10fch of a second. On this prin- ciple, several very ingenious toys have been constructed, in which two or more images are combined, by the rapid revolution of a wheel on which they are painted. 568. The impressions of variety of colour are produced by the different rays, which objects reflect to the eye ; according to the principles that will be described in the Treatise on Optics. Some persons, whose sight is perfectly good for forms, distances, &c, are unable to discriminate colours. This is particularly noticed in regard to the complementary* colours, especially red and green ; so that such persons are not able to distinguish ripe cherries amongst the leaves of the tree, except by their form. 569. When the retina has been exposed for some time to a strong impression of some particular kind, it seems less suscep- tible of feebler impressions of the same kind ; just as the ear, when it has been exposed to a series of very loud sounds (as the discharge of cannon in a naval engagement), is for some time deaf to fainter ones. Hence several curious visual phenomena result. If we look at any brightly luminous object, and then * White or colourless light is spoken of as composed of three primary colours, red, blue, and yellow. By the complementary colour is meant that which would he required to make white light, when mixed with the original. Thus, red is the complement of green (which is composed of yellow and blue) ; blue is the comple- ment of orange (red and yellow) ; yellow of purple (red and blue) ; and vice versd in all instances. COMPLEMENTARY COLOURS. — COLOURED SHADOWS. 425 turn our eyes on a sheet of white paper, we shall perceive a dark spot upon it ; the portion of the retina which had been affected by the bright image, not being affected by the fainter rays reflected by that part of the paper. If the eye has received a strong impression from a coloured object, the spot afterwards seen exhibits the complementary colour ; thus, if the eye be fixed for any length of time upon a bright red spot on a white ground, and then be suddenly turned so as to rest upon the white surface, we see a green image of the spot. The same explanation applies to the curious phenomenon of coloured shadows. It may be not unfrequently observed at sunset, that, when the light of the sun acquires a bright orange colour, from the hue of the clouds through which it passes, the shadows cast by it have a blue tint. Again, in a room with red curtains, the light which passes through these produces green shadows. In both instances, a strong impression of one colour is made upon the general surface of the retina ; and at any particular spots, from which the coloured light is excluded, that particular hue is not perceived in the faint light that remains, and its complement only is visible. The correctness of this explanation is proved by the fact, that, if the shadow be viewed through a tube, in such a manner that the coloured ground is excluded, it seems like an ordinary shadow. 570. Upon these properties of the eye are founded the laws of harmonious colouring; a full knowledge of which should be possessed by artists of every kind who are concerned with con- trasts of colour, whether in pictures, architectural decorations, or even in dress. All complementary colours have an agreeable effect when judiciously disposed in combination ; and all bright colours, which are not complementary, have a disagreeable effect, if they are predominant : this is especially the case in regard to the simple colours (red, blue, and yellow) ; strong combinations of any two of which, without any colour that is complementary to either of them, are extremely offensive. Painters who are ignorant of these laws, introduce a large quantity of dull grey into their pictures, in order to diminish the glaring effects which they would otherwise produce ; but this benefit is obtained by a sacrifice of the vividness and force, which may be secured in 426 SENSIBILITY OF THE EYE. DIRECTION OF MOVEMENTS. combination with the richest harmony, by proper attention to physiological principles. 571. The Eye is endowed with common sensibility (§. 487) by the fifth pair of nerves ; and when this is paralysed, all parts of it are completely insensible to the touch, although the power of vision may remain unimpaired. It seldom preserves its healthy condition in this state, however ; for the lachrymal and mucous secretions which protect its surface, are no longer formed as they should be ; and inflammation, often terminating in the destruction of the eye, is the result. 572. The visual sensations obtained through the Eye have numerous and varied purposes among the lower animals. That they chiefly serve to direct their movements, is evident from observation of these movements ; and from the fact that, when the eyes are covered or destroyed, most animals make little attempt at determinate motions, though they frequently exhibit a great deal of restlessness. There are a few Yertebrata, how- ever, which do not possess perfectly-formed eyes, and which are consequently guided in their movements by other senses. This is the case with the Mole, which spends its whole life in bur- rowing beneath the ground ■ and also with the Proteus, aud some others of the lower Amphibia, which inhabit the dark recesses of subterranean lakes and channels. 573. In the Articulated series of animals, we meet with eyes of a kind entirely different from those which have been previously describ- ed. In most Insects we notice a large black or dark- brown hemispheri- cal body, situated on either side of the head; and in Crabs, Lobsters, &c, we see spherical bodies, of similar appearance, mounted on short 201 Hkad and Eyes of the Bee, showing the Division into Facets. a, antennae ; A, facets enlarged ; B, the same with hairs growing between them. . COMPOUND EYES OF ARTICULATA. 427 footstalks, which are capable of some degree of motion. When these are examined with the microscope, their surface is seen to be divided into a vast multitude of hexagonal (six-sided) facets. In a species of Beetle (Mordella) upwards of 25,000 of these have been counted ; in a Butterfly, above ] 7,000 ; in a Dragon fly, more than 12,500; and in the common House-fly, 4000. Every one of these facets may be regarded as the front of a distinct eye ; which, however, instead of being globular, is conical in its form,— the front being the base of the cone, and the apex or point being directed towards the optic nerve, which swells out into a bulbous expansion, that fills a large part of the interior of the hemisphere. Each individual eye consists, there- fore, of its facet, which (being convex on both surfaces) acts as a lens ; — of the transparent cone behind this, which may be com- pared to the vitreous humour ; — and of the fibre which passes from the bulbous expansion of the optic nerve, to the point of this cone. The several fibres are separated from one another by a considerable quantity of black pigment, which also fills up the spaces between the cones ; and it is to this, that the black appear- ance of the whole compound eye is due. The various lenses or facets adhere together strongly at their edges, and may be readily separated from the remaining parts of the hemisphere, by soaking it in water for some time ; a transparent horny membrane is thus obtained, in which the division into facets may be very beautifully shown, by transmitting light through it. 574. We must thus regard each of the cones, which, united together, constitute the hemispherical or globular mass, — in the light of a distinct eye, of which the horny facet constitutes the cornea, wThilst the short transparent cone behind it is probably to be considered as the vitreous humour. Between the two there is, in some instances, a partition or diaphragm perforated by an aperture, which evidently represents the iris and pupil ; and a lens-shaped body is occasionally found behind this, in the true position of the crystalline lens : in these instances, the number of cones is smaller than usual, and each individual eye is larger ; so as to approach more nearly, both in its structure and mode of action, to the eyes of the kind previously considered. Where there 428 COMPOUND EYES OF ARTICULATA. is no separate crystalline lens, the two surfaces of the cornea are both convex, so that each facet is a distinct lens ; and it has been ascertained that the focus of this lens is exactly equivalent to the length of the transparent cone behind it ; so that the image it produces, will fall upon the extremity of the filament of the optic nerve, which passes to its apex. The rays which have passed through the several prisms or separate cornese, are prevented from mixing with each other, by means of the layer of black pigment which surrounds each cone ; and thus no rays, except those which correspond in direction with the axis of the cone, can reach the fibres of the optic nerve. Hence it is evident that each separate eye must have an extremely limited range of vision, being adapted to receive but a very small collection of rays, proceeding from a single point in any object ; and as these eyes are usually immove- able, the animals which possess them singly would be very insuffi- ciently informed of the position of external things. But by the vast multiplication in the number of the eyes, their defects are compensated ; a separate eye being provided, as it were, for every point to be viewed. And it is quite certain, from observation of the movements of Insects, that their vision must be very perfect and acute*. 575. Although these Compound Eyes exist in all Insects and most Crustacea, Arachnida, and Myriapoda, they are in general not the only organs of vision which these animals possess. Most of them are also furnished with several simple eyes, ana- logous in their structure to those of higher animals, but less complex and perfect in their organisation ; these are for the most part disposed on the back of the head ; they are largest in Spiders. The larvae of some Insects possess the simple eyes * It is commonly believed that each of these compound eyes produces its own image of the same external object, as do our two eyes ; but from the description here given of their separate directions when united, it is evident that, in no two of tbem, can an image of the same object be formed at the same time. The mem- brane formed of all the lens-like cornene united together, when separated from the other parts of the eye, and flattened out, has the properties of a multiplying-glass, each lens forming a distinct image of the same object ; but this is not the case when they are arranged in their natural position, because no two of them then have the same direction. EYES OF INVERTEBRATA. 429 without the compound ; the latter being only developed at the time of the last metamorphosis. The simple eyes of Insects do not appear to be nearly so efficient, as instruments of vision, as are their compound ones ; for when the latter are covered, the animals seem almost as perplexed, as if they were perfectly blinded. Simple eyes, closely resembling those of Insects in structure, are found in most of the Mollusca which possess a head, — namely in the Gasteropods, Pteropods, and Cephalopods ; those of the last class present an evident approach to the eyes of Fishes, in the greater completeness of their structure, and in their adaptation for distinct vision. In many of the lower Mollusca, as in the Rotifera and several Annelida, and also at the end of the arms of the Star-fish, red spots may be seen, which appear to be rudiments of eyes ; but no distinct organs of vision can be seen in the Zoophytes and lowest Mollusca ; although many of them appear very sensible to the action of light. CHAPTER XII. ANIMAL MOTION. 576. The different modifications of the faculty of Sensation, which have been described in the preceding chapter, enable Man and other Animals to become acquainted with what is going on around them. But their connexion with the external world is not confined to this faculty ; for if they possessed it alone, they would be nearly as passive as are Plants, — experiencing, it is true, pain and pleasure from their sensations, but not having the power of avoiding the one or procuring the other. They are endowed, however, with another faculty, that of spontaneous movement ; which serves the double purpose, of enabling them to act upon the inanimate world around them, and of commu- nicating to each other their feelings and ideas. Thus, if we find ourselves scorched by a flame, we either withdraw our bodies from it, under the direction of the instinct which leads us to avoid suffering ; or we set about to extinguish the fire, by an act of the will, founded upon our knowledge of its injurious tendency. The Plant, even if it had sensation (which some naturalists have supposed), could do neither of these things, j^gain, it is entirely by the movements concerned in speech, by those giving expression to the countenance, and by the gestures of the body, that we convey to beings like ourselves a know- ledge of what is passing in our own minds ; of this power, we know that Plants are entirely destitute ; and it is possessed in a very limited degree by the lower Animals. 577. The movements of Animals are effected by the action of a peculiar property, with which different beings seem to be endowed in proportion to their sensibility ; this property is MUSCULAR TISSUE. 431 termed contractility ; and it consists in the power which certain parts of the body possess, of contracting suddenly when excited to do so, and of afterwards lengthening again by relaxation or slackening. In some animals of extremely simple structure, such as the Hydra (Fig. 1), all parts of the body appear equally susceptible of thus contracting ; but when we rise a little in the animal scale, we find that this property is nearly restricted to a peculiar tissue, of which it forms the peculiar endowment ; this tissue is commonly known as muscular fibre. In Yertebrated animals, the muscles, which are the active instruments of all their movements, form the greater part of the mass of the body, and constitute what is commonly known as the flesh or meat of animals. In the Mollusca, these muscles are for the most part few in number. Among the lower classes of that group, this tissue exists only in the mantle, in the muscles which draw the valves of the shell together, in the foot, and in the parts about the mouth. But in the Articulated tribes, the muscles are very numerous, and form (as in Yertebrata) a large part of the sub- stance of the body. Many of them are, however, but repetitions of one another ; and there is by no means that variety of move- ment among them, which we meet with in Yertebrata. Structure and Actions of Muscular Fibre. 578. Every muscle is formed by the union of a number of bundles, which are united together by means of 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 consists of a delicate mem- branous tube, enclosing a great number of fibrillar, or , . . _. ,, Fig. 202.— Muscular Firre separating extremely minute fibrils, into fibrill.^. which are not capable of 432 STRUCTURE AND ACTIONS OF MUSCULAR FIBRE. further division. Perfectly-formed muscular fibre exhibits a remarkable and very characteristic set of transverse bands or strice; and, when it is separated into its fibrillae, it is seen that this appearance is 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 it is really cylindrical in form. 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 ~%re presents. The form and diameter of the fibres vary con- siderably, both in different tribes, and in different parts of the same animal. In the higher classes, its 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 fibrillae are arranged in flat bands, so that the fibre often consists but of a single layer of them. The diameter of the fibrils is nearly the same in all classes, seldom departing much from l-10,000th of an inch; and the average distance of the dark striae from each other is nearly the same. 579. Under the influence of certain exciting causes, or stimuli, muscular fibres suddenly and forcibly 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, espe- cially towards its middle. Of this any one may convince him- self, 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 enlargement in diameter is exactly equivalent to the shortening of the distance between its extremities. 580. It appears, however, that when an ordinary muscle is thrown into contraction, all its fibres do not contract together, ALTERNATE CONTRACTION AND RELAXATION. 433 but only a small part of them. It seems to be a peculiar property of muscular fibre, however, that its contraction should be speedily followed by relaxation ; and it appears that, whilst a continued contraction is taking place in the whole muscle, so that a con- stant force is exerted by it, there is a continual interchange in the action of the fibres by which this takes place, — those which have been shortened becoming slack, and being replaced (as it were) 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 zig-zag form. It was supposed until lately, that this is the state of con- traction ; but it is now known to be otherwise. This peculiar arrangement gives place to the straight form, either when the fibre passes into the state of contraction, or when, by the relaxa- tion of the whole muscle, its ends are separated again to their full extent. 581. 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 together, and all to relax together, — those of the auricles contracting whilst the others are relaxing, and vice versa; — and in this way the alternate contrac- tions 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 propagated on (as it were) to the suc- ceeding part, which in its turn contracts and then relaxes, pro- ducing 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 intes- tinal tube from one extremity of it to the other ; just as the peculiar action of the heart is adapted to receive and propel the blood alternately ; and as the mode of contraction of the ordinary muscles, enables them to keep up a continued strain for a great length of time. 434 CONTRACTION OP MUSCLES. 582. The extremities of the muscles are usually attached to bones, which their contraction causes to move one upon the other. Of these attachments, one is usually called the origin, and the other the insertion, — the origin being in the part that is most fixed, and the insertion in that which moves upon it. Thus the muscle already spoken of as concerned in bending the elbow, has its origin at the shoulder, and its insertion in the bones of the fore-arm, — its general action being to move the latter, while the former is fixed or nearly so. But its attach- ment to the fore-arm may really become its origin, and its other attachment its insertion ; for, when a person is hanging by his hands from a beam or cord, and raises his body by bending his elbows, the fore-arm is the fixed point, and the shoulder is moved upon it. In like manner, the muscular mass at the bot- tom of the back, having one attachment to the bones of the pelvis and another to the thigh-bone, serves to draw the latter backwards, when the former is made the fixed point, as when we rise up from the sitting posture ; but it is also continually serving to keep the body upright upon the thighs, the latter being the fixed point, and brings it into this position when it has been bent forwards, as in stooping. Muscles are seldom directly at- tached to bones ; but are united with them by means of the peculiar substance which is called tendon. Sometimes the tendon is long and round ; this is the case with several of those that move the hand and fingers, which arise from the muscles forming the fleshy part of the fore-arm, and may be felt at the wrists as hard round cords. In other instances, however, the tendon is a broad flat band ; of such we find several within the shell of the body and limbs of the Crab or Lobster, when we have removed the muscle or flesh. 583. The peculiar contractility of muscular fibre may be called into action by various causes. As in certain vegetable tissues (Veget. Phys. §. 420), contraction may be excited by a stimulus directly applied to the muscle itself.' Thus, if the heart of an animal recently killed be touched with a pointed instru- ment, it will contract, and then dilate, as if performing its ordi- nary action ; and this may be repeated several times. In the if the walls of the intestinal canal be pricked or CONTRACTION OF HEART AND ALIMENTARY CANAL. 435 pinched, they will recommence and continue for a short time their peristaltic movement. And if any part of an ordinary muscle be irritated in the same manner, that particular bundle will contract ; but the rest will not be affected. Now these actions are analogous to those performed by the Sensitive Plant, the Yenus's Fly-trap, and many other plants, some part of whose tissues contracts in like manner when an irritation is applied to it, causing — it may be — extensive and important motions. It appears to be in this manner, that the contractions of the heart, and of the alimentary tube from the stomach to the rectum, are ordinarily excited in the living body. The contact of blood with the lining membrane of the heart stimulates its muscular walls to contraction, as long as they retain their perfect structure and properties, even though the connection of that organ with the nervous system has been completely cut off. But there must be some other cause for the continuance of its regular move- ments ; for the heart of many cold-blooded animals will continue to contract and dilate, for many hours after it has been removed from the body, when it neither receives nor propels blood, and can have no influence from the nervous system. There is an instance on record, in which the heart of a sturgeon, which had been removed from the body and inflated with air, continued to beat, until the auricle had become so dry as to rustle during its movements. In the same manner, the peristaltic motions of the intestine continue to propel its contents for some time after the ge- neral death (§. 55) of the body; and may even take place when the whole tube is removed from it. It is interesting to remark, that in these two instances, in which the contractility of mus- cular fibre is called into action in the same manner as that of the contractile tissues of Plants, it is applied in both cases to the maintenance of the organic or vegetative functions : and it serves the purpose of placing them altogether beyond the control of the will, and of keeping up the necessary movements, even with- out our consciousness. 584. But the muscles of the trunk, limbs, &c, are not called into action in this manner ; for, as just now stated, a stimulus applied to any one part of these does not excite contraction in 436 CONTRACTION OF VOLUNTARY MUSCLES. GALVANISM. the whole muscle (as it does in the case of the heart), but only in the individual bundle of fibres irritated. These muscles are all of them supplied with nerves from the Cerebro-spinal system, or the nervous centres that correspond to them in Invertebrated animals ; — and it is only by a stimulus transmitted to them along these trunks, that all the bundles of which the muscle is com- posed, can be called into action at once. The nervous trunks which enter the muscles divide into branches, which again sub- divide into minute fibres ; and these spread themselves through- out the muscle, forming loops, which return again to the trunks. The nerve-tubes pass among, and into, the bundles of muscular fibres ; but in no instance do they enter the tubular sheaths of the latter. 585. When the trunk of a nerve supplying a muscle is irri- tated by pricking, pinching, &c, in the body of a living animal, or in one recently dead, the whole muscle is thrown into con- traction ; and this contraction is peculiarly strong, when a current of electricity is transmitted along the nerve. In cold-blooded animals, whose muscular fibre retains its vital properties for a much longer time after death than that of warm-blooded, this contraction may be excited by a very feeble current of electricity, in a limb which has been separated from the body for 24 hours or more ; and it was owing, in fact, to this circumstance that the peculiar form of electricity which is now termed Galvanic or Voltaic, was discovered. The wife of Galvani, who was Pro- fessor of Medicine at Bologna, being about to prepare some soup from frogs, and having taken ofT their skins, laid them on a table in his study, near the conductor of an electrical machine, which had been recently charged ; and she was much surprised, upon touching them with the scalpel (which must have received a spark from the machine) to observe the muscles of the frog strongly convulsed. Her husband, on being informed of the circumstance, repeated the experiment ; and found that the muscles were called into action, by electricity communicated through the metallic substance, with which the limb was touched. 586. The experiment was repeated in various ways by Volta, who was Professor of Natural Philosophy at Pa via ; and INFLUENCE OF ELECTRICITY ON MUSCLES. 437 he found that the effects were much stronger, when the connect- ing medium through which the electricity was sent, consisted of two metals instead of one ; and from this circumstance he was led to the discovery, that electricity is produced by the contact of two different metals, — a discovery which has since been so fruit- ful in most important results. (See Treatise on Electricity.) The following simple experiment puts this in a striking point of view. If the skin of the legs of a Frog recently killed be removed, and the body be cut across, above the origin of the great (sciatic) nerve going to the legs, — if the spine and nerve9 be then enveloped in tin-foil, and the legs be laid upon a plate of silver or copper, — convulsive movements in the muscles will be excited, every time that the metals are made to touch each other, so as to complete the electric circuit. 587. Similar experiments have been tried with the Yoltaic battery, upon the dead bodies of criminals recently executed. If one wire be placed upon the muscles which it is desired to call into action, and the other upon the part of the spine from which the nerves proceed, movements of every kind may be produced ; and the application of electricity in this manner, so as to renew the motions of the diaphragm, is probably the best means of restoring vital activity, after it has been suspended by drowning, or suffocation (§. 338). No agent more effectually imitates the natural action of the nerves, in exciting the contractility of muscles, than Electricity thus transmitted along their trunks ; and it has been hence supposed, by some philosophers, that elec- tricity is the real agent by which the nerves act upon the mus- cles,— more especially since it is certain that, in those animals which generate large quantities of electricity, the nerves have a great share in this peculiar operation (§. 423). But there are many objections to such a view; and this very important one among the rest, — that electricity may be transmitted along a nervous trunk which has been compressed by a string tied tightly round it, whilst the passage of ordinary nervous power is as completely checked by this process, as if the nerve had been divided. We have already seen, too, that electricity, transmitted along the sensory nerves, excites the peculiar changes in the 438 INFLUENCE OF NERVES ON MUSCLES. brain, by which sensations are produced (§. 488) ; and thus it appears that, in their effects, as well as in their mode of action, there is more analogy between electricity and nervous agency, than there is between any other two powers of animated and inanimated nature. 588. The power, whatever be its nature, by which the Nerves act upon the Muscles in the living body, originates in the central organs, or ganglionic masses, of the nervous system, and is propagated from these, through the nervous fibres to the mus- cles,— in a mode precisely analogous to that in which the electric power, called forth by the action of an electrical machine or gal- vanic battery, is transmitted to any distance through conducting wires. If the conductor be divided, no action at the centre, however powerful, can produce any change at its extremities ; and in this manner, by division of the nervous trunk, the muscle supplied by it is palsied. The muscle itself does not thereby lose its contractility ; for it may still be made to contract, by a sti- mulus transmitted through the part of the trunk that remains attached to it, — as, for instance, by pricking or pinching the cut extremity, or by passing an electric current along it ; but it is completely withdrawn from the dominion of the nervous cen- tres, under which it previously was ; and can neither be called into action by the will, by an emotion, or by a reflex impulse. The part of the trunk in connection with it soon loses its power of conveying irritations ; and the muscle itself, being thrown into disuse, in time loses its contractility. 589. From this last fact it has been supposed, that the con- tractility of muscular fibre depends upon its connection with the nervous system, and is not an endowment peculiar to itself. But this idea is disproved by a number of circumstances. Thus the contractility of the heart and intestinal tube is exhibited, long after these parts have been separated from their nerves. The contractility of other muscles may be exhausted, by repeated ex- citement, so that even the stimulus of galvanism will not produce movement in them ; and yet it may be recovered after the ner- vous trunks have been divided. And it has been ascertained that if the muscles be frequently exercised, as by the application CONTRACTILITY AN INDEPENDENT PROPERTY OP MUSCLE. 439 of galvanism once or twice a day, they will retain their con- tractility for any length of time. This exercise is further found to have the effect, of preventing the wasting-away of the muscles which otherwise takes place ; and thus we see that the preserva- tion of this peculiar property is dependent upon the due nutri- tion of the muscle, whilst the loss of the property results from its want of nutrition, — as we find to be the case in regard to other tissues. Further, the activity of the nutrition of muscles depends in great part upon the use that is made of them ; and thus we find that any set of muscles in continual employment undergoes a great increase in size and vigour ; whilst those that are disused, even though their nervous connections remain entire, lose their firmness and diminish in bulk, until, if the inaction be continued long enough, almost all trace of proper muscular sub- stance disappears, and the contractility of the part is lost. 590. But a muscle may be palsied by some change taking place in the central organs, which shall prevent the nervous influence from being excited there. Thus by an effusion of blood in a certain part of the brain, the arm, leg, or the whole of one side may be paralysed to the influence of the will. But the muscles which are thus withdrawn from the power of the will, may yet be moved by an emotional or instinctive impulse, ovhy reflex actions; provided its connection with the parts of the nervous centres, in which these actions originate, be unim- paired (Chap, x.) Thus, a completely paralytic arm has been seen to be violently shaken, when the emotions of the patient were strongly excited by the approach of a friend. The muscles of the shoulder, in a case of complete paralysis of one side, were called into contraction in the reflex movement of j^awning (§. 341). And the muscles of the legs, when their communica- tion with the brain, — and consequently the control of the will over them, — has been completely cut off, have been made to act energetically when the feet were tickled, although the patient was not conscious either of the irritation or of the motion. When the muscles are thus aroused to occasional activity, their nutri- tion is not so much impaired, and their contractility does not depart nearly as completely, as when they are thrown into entire disuse, by dividing their nerves. 440 VOLUNTARY AND INVOLUNTARY ACTIONS OF MUSCLES. 591. Muscles are commonly divided into voluntary and in- voluntary, according as they act in obedience to the will, or are not under its dominion. But this is not a correct division; since, whilst nearly all the muscles of the body are more or less under the control of the will, they may all at times have an involuntary action. The heart and the muscular coat of the alimentary canal, with the muscles concerned in swallowing and in one or two other actions of a similar character, are the only muscles which the will cannot either set in action, or control when in action. There are several muscles whose usual movements are of a reflex and, therefore, involuntary character ; which are yet capable of being, to a certain extent, controlled and governed by the will. Such are the movements of respiration ; which will continue to take place after the brain has been removed, and which go on regularly during the profoundest sleep, and the most complete withdrawal of the attention from them. In the Invertebrated animals, these motions are probably not influenced by the will ; but in the air-breathing Vertebrata, they are placed in a certain degree under the dominion of the will, in order that they may be made to contribute to the production of the vocal actions of speaking, singing, &c, which are restricted to these classes. We can hold the breath for a certain time by a voluntary effort, or we can expel or draw it in more quickly than usual ; but no voluntary effort can cause the breath to be held for more than a few moments ; for the uneasiness which is then felt, and which is continually increasing, causes an involuntary action of the muscles, by which it is relieved. But again, there are other muscles, whose ordinary actions are voluntary ; but which are occasionally made to act independently of the will, or even against its direction. Such are those which are excited by the emotions, as in laughing, crying, sobbing, &c. We may have the strongest desire to check these actions, owing to the unfit- ness of the time and place for their manifestation ; and yet we may be unable to do so. And lastly, muscles, whose action is usually voluntary, may be occasionally called into powerful con- traction, which the will cannot in the least degree control or prevent ; this is the case in cramps, convulsions, Sec, of various kinds. CONDITIONS OF MUSCULAR ACTION. 441 592. All these facts are readily accounted for by the know- ledge we now possess, of the different sources of muscular move- ment. It has been already shown (§. 464) that every motor trunk proceeding to a muscle, contains filaments that are derived from at least two different sources, — one of these the brain, and the other the spinal cord. Now those muscles which are chiefly supplied by the latter, and usually receive from it their stimulus to contraction (which is the case with those of respiration) will be more involuntary in their character than those, which are chiefly supplied from the brain, and which are less connected with the spinal cord,— as is the case with the muscles that move the limbs. And those muscles which are called into action by the emotions, probably receive a third set of fibres, from a part of the nervous centres different from either of the preceding ; and the influence transmitted by these fibres may be so strong, as not to be capable of being resisted by others. 593. The vigorous action of the muscular structure is dependent upon several causes. In the first place, it requires (as we have already seen) an active nutrition of the muscles themselves. Firm, plump, and high-coloured muscles act with greater force than those which are pale and flabby, even though the size of the latter may be greater. Again, in all those animals whose activity is greatest, a constant supply of oxygen is requisite for muscular vigour. This is conveyed, in Birds and Mammalia, by the blood (§. 234) ; in Insects, on the other hand, it actually enters the muscular tissue, in the state of atmospheric air (§. 320). In Reptiles, again, the blood goes to the tissues very imperfectly oxygenated ; and their move- ments are comparatively slow and feeble. But it is a remark- able circumstance, that in the dead bodies of the latter, or in parts separated from the living body, the property of contracti- lity does not depart nearly so soon, as it does in similar parts of warm-blooded animals. By experiments on Mammalia it has been found, that the muscles of the trunk cannot be caused to contract by galvanism, for more than two or three hours after death ; though the auricles of the heart retain their contractility for some hours later. The muscles of Birds (whose respiration 44*2 CONDITIONS OF MUSCULAR ACTION. — -FATIGUE. is more active, and whose temperature is higher) lose their con- tractility yet sooner ; but those of Reptiles sometimes retain the power of contracting for several days. When venous or imper- fectly-aerated blood is made to circulate through the vessels of warm-blooded animals, it acts like a poison upon them, dimi- nishing or even destroying their contractility.* 594. Further, the energy of muscular contraction depends in great degree upon the power of the stimulus which is trans- mitted to it through the nervous system. We often have the opportunity of observing this, in the case of persons who are under the excitement of violent passion or of insanity ; a delicate female is frequently a match for three or four strong men, and can even break cords and bands that would hold the most powerful man in his ordinary state. The strength in such cir- cumstances seems almost preternatural ; but it is not greater than that which we see manifested in convulsive actions, where the movements depend only upon the spinal cord. Thus a slender girl affected with spasmodic affection of the muscles of the spine, which threw the back into an arch, of which the head and heels were the two resting-points, has been known to raise a weight of 9001bs. laid on the abdomen, with the absurd intention of straightening the body. 595. The sense of fatigue, which comes on after prolonged muscular exertion, is really dependent upon a change in the brain, though usually referred by us to the muscles that have been exercised. For it is felt after voluntary motions only; and the very same muscles may be kept in reflex action for a much longer time, without any fatigue being experienced. Thus, we never feel tired of breathing ; and yet a forced voluntary action of the muscles of respiration soon causes fatigue. The volun- tary use of the muscles of our limbs, in walking or running, soon causes weariness ; but similar muscles are used in Birds and Insects, for very prolonged flights, without apparent fatigue ; * Other substances do this with even greater rapidity; thus a strong solution of nitrate of potass (nitre) injected into the blood-vessels, and conveyed by them to the heart, causes the immediate cessation of its action — the poison finding its way, through its own vessels, into the capillaries of the muscular structure. ENERGY OF MUSCULAR ACTION. 443 and as we find that the actions of flight may he performed, after the brain, or the ganglia that correspond to it in Insects^ have been removed (§§. 444, 465), we may regard them as of a reflex character ; and the absence of fatigue is thus accounted for. 596. The energy of muscular contraction appears to be greater in Insects, in proportion to their size, than it is in any other animals. Thus a Flea has been "known to leap sixty times its own length, and to move as many times its own weight. The short-limbed Beetles that inhabit the ground have an enormous power, which is manifested both in their movement of heavy weights, and in the resistance they overcome with their jaws. Thus the Dung or Shard-borne Beetle can support uninjured, and even elevate, a weight equal to at least 500 times that of its body. And the Stag-Beetle has been known to gnaw a hole of an inch diameter, in the side of an iron canister in which it had been confined. The rapidity of their movements is also most extra- ordinarily great, and is especially seen in the vibrations of their wings. It would be impossible to form an estimate of the time occupied by these, were it not for the musical tone they produce ; and it may be calculated from these (upon principles which will be explained in the Treatise on Sound) that the wings of many Insects strike the air several hundred times, — and those of some of the smaller Insects many thousand times, — in a second of time. Of the Apparatus of Movement in general. 597. Muscular contraction performs an important part in nearly every one of the functions, of which we have already treated. Thus the reception of the food, and its propulsion along the alimentary canal, forming part of the function of Digestion, are accomplished through its means. The Circulation of the blood, again, depends mainly on the agency of a con- tractile organ, the heart. The Respiration cannot be kept up, in the higher animals at least, without the aid of certain move- ments, which are accomplished by the muscles. With the pro- cesses of Nutrition and Secretion, it is not so closely connected ; but the latter is dependent upon it so far as this, that its products are carried out of the body by the aid of muscular 444 LOCOMOTION OF ANIMALS. contraction. And even in Sensation, the peculiar endowment of muscular tissue comes into use ; by giving to the organs of sense those movements, which enable them to take a wider range, and to apply themselves most perfectly to the objects before them. But we have now to study its applications in those general and partial movements of the body, on which depend the locomotion (or change of place) of animals, their attitudes, and a number of other important actions, entirely of a mechanical nature. The organs by which these are effected, may be con- veniently divided into the active and passive. The active are those which have peculiar vital powers within themselves, and which exert these by giving motion to other parts. To this class, therefore, we refer the muscles, whose peculiar endowments have been just considered. The passive organs, on the other hand, are those which perform no action of themselves, which have no power but that of yielding a simply mechanical support, and which consequently perform no movement but such as they are made to do by the muscles. Of this kind are the hard parts which form the skeleton or solid framework of the body, whether this be internal or external. 598. In the lower tribes of animals, the muscles are all inserted in the soft and flexible membrane which covers the body; and it is by acting upon it, that they can change the form of the body, in such a manner as to cause it to move, either altogether or in part. This is the case, for example, in the Leech, Earth- worm, and other Annelida ; which are furnished with two sets of muscular fibres, one running along the body, and the other passing round it in rings. By the contraction of the former, the two ends are drawn together, so that the body is shortened ; whilst by that of the latter, its diameter is lessened, so that it is necessarily lengthened. By these two movements, which take place alternately, the progression of the animal is accomplished ; and by varying the contractions of one part or another, almost any form and direction can be given to the soft and flexible body. 599. But in the higher animals, we find the apparatus of movement to consist, not only of muscles, but also of a framework of solid pieces ; which serves to augment the precision, the force, GENERAL STRUCTURE OF THE SKELETON. 445 and the extent of the movements ; whilst, at the same time, it determines the general form of the body, and protects the viscera against external forces. This solid framework, or skeleton, to which the muscles are attached, may be, as we have seen, either internal or external. In the Vertebrated classes, the hard skeleton is internal ; in the Articulated series it is external ; in the Mollusca it can scarcely be said to exist, since no muscles are attached to the shell, save those that serve to attach the animal to it, or to draw together its valves ; and in the Radiata, its position is variable, being sometimes external, as in the Echinodermata, and sometimes internal, as in the stony Corals. 600. The peculiar structure of some of these parts has been already described. It has been stated that the bones of Verte- brata differ from all other substances of a similar hardness, in being penetrated by blood-vessels ; but it is not the only substance in which tubes exist ; for a system of tubes resembling those of teeth, or those which radiate from the minute cavities in bone (§. 48), is to be found in the shells of some Mollusca and Crus- tacea, and even in the stony Corals, as also in the bony scales of certain Fishes. All these parts are formed by the consolida- tion of living animal tissue, by deposits of carbonate of lime or other mineral matter; and the difference between them consists mainly, in the nature of the tissue thus consolidated. Thus in some shells, it is previously cellular, constituting a sort of epithelium ; in others it is simply membranous. In the Polypifera it seems to be a very delicate cellular tissue, resembling that of which the soft bodies of the animals are composed. And in the Echinodermata, the animal substance which remains after the lime has been dissolved away by acid, has rather the cha- racters of areolar tissue, — consisting, not of cells, but of plates and shreds of membrane, interwoven into a structure that con- tains numerous cavities communicating with each other. 601. The different portions of the skeleton are articulated, or united by joints to one another, in such a manner, that they can move with greater or less freedom. This we see both in the Vertebrated and Articulated classes. In the latter, the joints are for the most part very simple in their construction. The 446 ADHESION OF SEPARATE PIECES OF THE SKELETON. different rings or pieces are held together by a flexible membrane passing from one to the other ; this seems to be little else than a portion of the integument that originally covered the body, which has remained unconsolidated, whilst the rest has been hardened. And sometimes they are made to adhere to each other by a kind of soldering ; so as to be altogether immovable. But in the internal skeletons of the Yertebrata, we find a more complex mode of union, fitted to afford scope for the greater variety of motions which their parts perform. Here, too, we find some parts immovably united to each other, where support and pro- tection alone are required. These immovable articulations, of which there are several kinds, will be first considered. 602. All the bones of the head and face (with the exception of the lower jaw), in Man and the higher Yertebrata, have their edges in immediate contact with each other ; so that they hold together in the dry skull, as well as during life. Those bones of the skull, which inclose and protect the brain, are very firmly united by what are termed sutures ;* which are mostly formed by the interlocking of the jagged edges of one bone, into corre- sponding notches of the adjoining one ; though in some, this kind of union is incomplete : and in others it is replaced by a bevelling of the edges that are in contact, or by the reception of a ridge of one bone into a groove in the other. So firmly are the bones united in this manner, that it is difficult to separate them without breaking away some of their projecting parts ; and in the skulls of old persons, the sutures are almost obliterated by the complete union between the adjacent bones. In the infant, on the other hand, the bones of the skull are only united to each other by a membranous substance ; and there is a point at the top of the head, which is not even covered by a bony layer for some time after birth. It is only as the age advances, and the ossification becomes more complete (§. 51), that the firm bony union is effected. 603. In several other articulations, the bones do not come into direct contact with each other, but are united by an inter- vening layer of cartilage, and also by ligaments and other fibrous * From the Latin, sutura, a seam. ARTICULATIONS ; — HINGE-JOINT J BALL-AND-SOCKET-JOINT. 447 membranes encircling the articulations. The adjacent surfaces of the bones are flat, and have a slight gliding movement over one another ; but the extent of motion permitted is very small. This kind of articulation exists between the bodies of the ver- tebrae of Man and the higher Yertebrata, between the bones of the pelvis, and some other parts. 604. The proper movable articulations, by which the limbs are connected with the trunk, and their different divisions to each other, are those to which we commonly give the name of joints. In these, the surfaces of the adjacent bones are not united in any other way, than by the ligaments and muscles which surround them ; and they have a free gliding movement over each other. They are covered, it is true, by cartilage ; this, however, does not pass from one bone to the other, as in the previous case ; but forms a thin layer over the end of each, and presents a very smooth surface, which greatly facilitates the motion of the joint. These surfaces are kept moist by a fluid termed synovia; this is secreted by a serous membrane which covers them, and which forms a closed capsule or bag, much resembling that of the pericardium (§. 256;. The beautiful smoothness of the surfaces of the joints, and the manner in which the bones are held together by the muscles and ligaments, is well seen by examining the knuckle-joint at the lower end of a leg of mutton (before being cooked), and the other joint which connects it with the bones at the top. These two joints are ex- amples of the two principal varieties of freely-movable articula- tions,— the hinge-)omt) and the ball-and socket joint. In the first of these, the surfaces of the bones are so formed, that the movement, though free as regards its extent, is very limited in its direction, being in fact restricted to a backward and forward action in the same line, just like that given by a common hinge. In the second, the end of one bone is formed into a rounded head or ball; and this is received into a corresponding socket or cup in the other, the edge of which is usually deepened by cartilage ; in this manner, the bone which carries the ball is enabled to move upon the other in any direction, unless checked in other ways. Of the hinge-joint we have examples in the elbow, the HH 2 448 MOVEMENTS, AND DISLOCATIONS, OF JOINTS. knee, and the joints of the fingers and toes. Of the perfect ball-and-socket joint, we have in Man only two examples, — the shoulder, and the hip. In the former, the socket is much shal- lower than in the latter; and the motions of the arm are conse- quently more extensive than those of the thigh. Both, how- ever, are unchecked in regard to their direction, except when the limb is brought against the body or against its fellow. The wrist and the ankle-joint are of an intermediate character ; the former more resembling the ball-and-socket, and the latter the hinge-joint. 605. All these joints are more or less subject to dislocation, by violence of different kinds. This takes place by the slipping away from each other of the two surfaces, which ought to be in contact. Thus the head of the humerus (or arm-bone) may slip over the edge of its socket, so as to lie entirely on the outside of it ; and this, in consequence of the shallowness of the cup, hap- pens not unfrequently. The head of the thigh-bone, also, may slip out of its socket ; but this accident is more rare, on account of the deepness of its cup. The elbow and knee-joints, as also those of the wrists, ankle, fingers, and toes, may be dislocated by the slipping of one surface on the other, either forwards, backwards, to one side or to the other. One of the most com- mon dislocations is that of the thumb, the lowest articulation of which has rather the character of the ball-and-socket (with a very shallow cup), than of the hinge-joint. But in proportion to the liability of any joint to dislocation, is usually the ease with which it may be brought into place again. 606. The action of any muscle in producing a change in the position of a movable bone on which it acts, is determined in the first place, by the nature of the movement of which the bone is capable ; and in the second, by the direction in which the power of the muscle is applied to it. Having now considered the former of these conditions, we proceed to the latter. The contraction of a straight muscle which is attached to a fixed point at one end, and to a movable point at the other, will obvi- ously tend to draw the latter towards the former. Thus, the muscles which bend the fingers lie in the palm of the hand, and APPLICATION OF MUSCLES TO MOVEMENT OF BONES. 449 on the corresponding side of the fore-arm ; whilst those that straighten the fingers are situated on the opposite side. But we often find that the direction of a muscle's action is changed, by the passing of its tendon through a pulley-like groove or loop ; so that it draws the movable bone in a direction different from that of its fixed attachment. This is the case, for example, with some of the muscles that bend the toes ; these being situated in the calf of the leg, would draw the toes upwards, were it not that their tendons are carried beneath the bones of the heel, working in smooth pulley-like channels hollowed out in them ; hence, when the muscle contracts, the tendons draw the ends of the toes towards the heel, and thus bend them. 607. We generally find that even movements of a simple character are performed by the combined action of several muscles ; of which some may be considered as the principal, and others as assistants. Those which are principals in one move- ment may become assistants in another ; and vice versa. Thus, if we wish to bend the wrist directly downwards upon the fore- arm, we put in action, not only certain muscles whose tendency would be to produce this movement, but others which, acting by themselves, would produce a different motion. One of these would draw the wrist towards the thumb-side of the fore-arm ; and the other towards the little-finger-side ; and they become the principal muscles in these movements respectively : but when they act together, their several tendencies to draw the wrist to the opposite sides counterbalance one another, and they simply assist the principal muscles in bending the wrist downwards upon the fore-arm. 608. Almost every muscle has its antagonist, which performs an action precisely opposite to its own. Thus by one set of muscles, termed flexors, the joints are bent; by a contrary set, the extensors, they are straightened. One set of muscles draws the arm or leg away from the central line of the body ; another draws the limbs inwards. One set, again, closes the jaws ; and another opens it. In short, we shall find that probably every muscle in the human body has its antagonist in another muscle, or in some part of it. But we find an economy of muscular 450 ACTION OF MUSCLES ON BONES. substance in some of the lower animals, where parts are to he usually kept in a particular position, and this is only to be changed occasionally and for a short time. Thus the valves of the Conchiferous Mollusca (§. 124) are kept apart, not by a muscle, but by an elastic ligament ; but they are closed, when the animal is alarmed and wishes to protect itself, by muscular action. In the same manner, the sharp claws of the Cat tribe are usually drawn in by an elastic ligament, that their points may not be worn away by rubbing against the ground ; whilst they are forced outwards by the action of a muscle provided for the purpose, when the animal desires to fasten them into its prey. 609. We commonly find that, in order to preserve the necessary form of the animal body, muscles are applied at a great mechanical disadvantage, as regards the exercise of their power ; that is, a much larger force is employed, than would suffice, if differently applied, to overcome the resistance. But we generally find that, in this as in other forms of lever action, what is lost in power is gained in time ; and thus a very slight change in the length of a muscle is sufficient to produce a con- siderable movement. 610. The first source of disadvantage results from the di- rection in which the muscle is attached to the bone. This is rarely at right angles to it ; and consequently a considerable part of the power is lost (see Mechan. Philos., §. 299). Thus n a if the muscle m (Fig. 203), whose JJI . force we shall suppose equal to 10, m &^. w^S^ *s nxec* a* right angles to the bone ^H|^ ■••••"Ta/tF r ^ wnose extremity a is movable ••■'''^^. •',//: upon the point of support r; its < " ..-•-'' v ! force of contraction will be most d b i advantageously applied to overcome Fro. 203. the resistance, and will draw the bone from the position a b into the direction a c, making it traverse a space which we shall also represent by 10. But if this muscle act obliquely on the bone, in the direction of the line n b for example, it will be quite otherwise; for it will then tend to draw the bone in the direction bn, and consequently to make it ACTION OF MUSCLES ON BONES. 451 approach the articular surface r. But as this bears upon an immovable socket, and as the bone can move in no other way than by turning upon the point r, as upon a pivot, the contraction of the muscle to the same amount as before will carry the bone no further than into the direction ad; three quarters of the force employed will thus be lost, and the resulting effect will be no more than one-fourth of that, which the same power, applied perpendicularly to the bone, would have produced. 611. Now in the animal body, we usually find that the mus- cles are inserted so obliquely, that their power is applied at a great disadvantage ; but this disadvantage is rendered much less than it would have otherwise been, by a very simple con- trivance,— the very enlargement of the bones at the joints, which is necessary to give them the required extent of surface for working over each other. Thus, let r and o (Fig. 204) be two bones connected by a joint; and' let the muscle m, which moves the lower bone upon the upper, be attached to the former at i. Now as this muscle acts almost precisely in the line of the bones themselves, almost all its power will be expended FlG- 204- in drawing the lower bone against the upper. But by the enlarge- ment of the ends of the bones, as seen in Fig. 205, the direction of the tendon of the muscle m is so changed, near its insertion i, that the contraction of the muscle will cause the lower bone to turn upon the upper one with comparatively little loss of power. In the Knee we find a still greater change of direction effected, by the fig. 205. mterp0sition of a movable bone, the patella or knee- pan, in the substance of the tendon. 612. But the advantage or disadvantage with which the muscles act upon the bones, depends in great degree upon the distance of their point of attachment, from that of the point of support on which the bone moves, and from the point at which the resistance is applied. Every bone acted on by muscles may be regarded as a lever, having its fulcrum or point of support in the joint, its power where the muscle is attached to it, and its weight where the resistance is to be overcome ; and the distances 452 ACTION OF MUSCLES ON BONES. of the fulcrum from the power and the weight respectively, are termed the two arms of the lever. Now on the mechanical prin- ciples, fully explained elsewhere, (Mechan. Philos., §. 287,) the relative length of these two arms governs the force which is necessary to overcome a given resistance. Thus in the Steel- yard (Fig. 206), the beam is divided into two arms of unequal a length at the point of support or fulcrum a; at the end of the short arm, r, hangs the >•■■■ — )p body whose downward pressure we wish to determine ; and on the other, p, there O slides a weight, which will balance a fig. 206. greater or less amount of pressure at the opposite extremity, r, according as it is made to hang from a point which is more distant from the fulcrum or nearer to it, — that is, according as the length of the power-arm of the lever is increased or diminished, that of the weight-arm remaining the same. 613. Now in order that there may be an equilibrium, or balancing between the power and the weight, it is necessary that they should be inversely proportional to the lengths of their respective arms ; that is, the power multiplied by the length of its arm, should be always equal to the weight multiplied by the length of its arm. Thus, ~^ to balance a certain resist- ance, r, equal to 10, and applied at the end of a ^ lever, a b, whose length we shall call 20, it is neces- sary that a force, p, ap- no. 207. 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, at 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 ACTION OF MUSCLES ON BONES. 453 am 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 dis- tance at which it is applied may be equal to 200. Hence when a muscle is applied near the fulcrum, while the resistance is at a distance from it, its force must be proportionably greater. 614. But this arrangement greatly increases the rapidity of the motion, which is the consequence of the muscular action. For let us suppose that the muscle p (Fig. 208), acts upon the lever, a r, in such a manner that its point of insertion, c, tra- 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 dis- tance from the fulcrum, «, 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 Fig. 208. Fig. 209. the muscular actions in the animal economy. Thus the fore-arm 454 ACTION OF MUSCLES OP ARM. — BONES OF SKULL. is bent upon the arm by a muscle, d, which arises from the top of the latter, and which is inserted at — such as the Whale tribe among Mam- malia, 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 474 PELVIS. — FEMUR. MUSCLES OF THE THIGH. former more solidity at the expense of freedom of motion, and to make them organs of locomotion instead of organs of prehen- sion. 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 lower extremities, this framework is much more firmly attached to that of the trunk, than is the case with that which supports the arms. It consists on each side of a bone, which, in the adult state, is single ; though at an early age it is composed of three distinct pieces. This bone is closely connected with the sacrum behind, and in front it meets with its fellow ; in such a manner as to form a sort of bason, termed the pelvis. The spreading sides of this, formed by the iliac bones (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, which is so deep as to be almost a hemisphere, when completed by its cartilaginous border. The movements of the thigh-bone are consequently more limited than those of the arm ; but it is much less liable to displacement. 646. The thigh, like the arm, is supported by 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 nar- row portion, which is termed its neck. At the point where this neck joins the shaft of the bone, there are two large projections, — one on the outside, the other on the interior side, — which can be felt beneath the skin ; and these serve to give attachment to the muscles, by which the thigh is moved. Of these muscles, one descends from the lumbar vertebrae ; and this, with another that rises from the upper expanded surface of the pelvis, passes down over the front border of the pelvis, and is 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 be performed with any great force. The muscles which draw back the thigh, on the TIBIA, FIBULA, AND PATELLA. MUSCLES OF THE LEG. 475 other hand, arise from the under surface and back of the pelvis, where they form a very thick fleshy mass ; and they pass to the larger and external projection, and to a ridge which runs down the thigh-bone. Other muscles, which arise from the lower bor- der 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 large bone of the leg moves backwards and forwards. The knee is a good example of a pure hinge-joint ; all its move- ments being restricted to one plane. 647. The leg, although containing two bones like the fore- arm, does not possess the peculiar movement which characterises it. One of these bones, called the tibia, is much larger than the other, which is called the fibula; and it is the former alone which rests upon the thigh-bone, and which also gives the chief support to the foot, so that no movement of rotation is permitted in the leg. In fact, the fibula, which is a long slender bone, running nearly parallel with the tibia (Fig. 213), 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 prin- ciple formerly stated (§.611). In the elbow-joint, this change was not required; since the ulna projects sufficiently far backwards, to afford advantageous 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 476 BONES AND MUSCLES OP THE FOOT. 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. 648. The foot is composed, like the hand, of three distinct portions, which are called the tarsus, metatarsus, and toes. There are seven bones in the tarsus, all of which are larger than those of the carpus, and some of them of considerable size. The arti- culation with the leg is formed by one of these only, the astra- galus, which projects above the rest, and is imbedded between the projecting extremity of the tibia (which forms the inner boundary of the ankle-joint) and that of the fibula. The astra- galus rests on the os calcis, or bone of the heel, which projects considerably backwards, and is connected in front with the other bones of the tarsus. In front of the tarsus, we find the meta- tarsus, 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 converted, however, into a per- fect 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 of affording a firm basis of support. Even the Chimpanzee and the Orang, when they attempt to walk erect, rest upon the side of the foot ; and the absence of a projecting heel causes them to be 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 ELASTICITY OF THE FOOT. 477 foot itself, serve to bend the toes, — an action which gives great assistance in walking, running, leaping, &c. And they are straightened by an extensor muscle, which lies on the front of the leg, and of which the tendon runs under an annular ligament (§. 641) 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. When this arrangement is not perfect (as sometimes happens), so that the foot is too flat, the nerves are compressed by the weight of the body, and the act of walking cannot be continued for a long time without pain. 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 man- ner in which this is constructed (§. 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 arrangement of the bones of the foot, well adapted to diminish the shock produced by the sudden descent of the body upon the ground. 478 ATTITUDES OP ANIMALS. Of the Attitudes of the Body, and the various kinds of Locomotion. 650. A small number of Vertebrated 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 any firm horizontal basis. In main- taining 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 a set of muscles, depends in great degree upon the length of time during which they have been in 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 stea- diness in the standing posture ; for in order that the body may rest firmly upon the members, it must be in equilibrium. It has been shown (Mechan. Philos. Chap, iv.) that equilibrium ex- ists,— or in other words, that a body remains at rest in its position, — not only when it bears upon the whole of a broad surface, but also when it is so placed, that the tendencies of its different parts to descend or gravitate towards the earth, counterbalance each other. This is the case when its centre of gravity is supported, — that is, when a line drawn perpendicularly from it 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 which its feet cover and enclose between EQUILIBRIUM OF ANIMALS. 479 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. 217, the table a must be upset; because the line of direction, e, from the centre of gravity, c, falls outside the base of support, d: whilst the table b, although equally inclined, will not be upset, but will return to its proper place ; because the line of direction, two bronchial tubes, I b\ b each of which has its own glottis and vocal cords; the c ;nnAW i:« ^f «.„.* ~f *1 • Fig. 249.— Larynx of a Rook. inner lip or one of these is seen at a (Fig. 250) ; and at me is shown a drum-like membrane, forming the inner wall of the bronchial tube, which probably increases the resonance of the voice. These parts are acted on by several muscles, the number of which varies according to the compass and flexibility of the voice in the different species; being very considerable in the most esteemed of the singing- birds, and being reduced to a small amount in those which have no vocal powers. In some, indeed, they are altogether absent ; and the state of the glottis can be influenced only by those muscles which raise and lower the whole trachea. 686. The vocal sounds produced by the action of the larynx Fig. 250. — Vertical section of the 6AM E. 512 DIFFERENT KINDS OF VOICE. are of very different characters ; and may be distinguished into the cry, the song, and the ordinary or acquired voice. — The cry is generally a sharp sound, having little modulation, or accuracy of pitch, and being usually disagreeable in its timbre or quality. It is that by which animals express their unpleasing emotions, especially pain or terror ; and the Human infant, like many of the lower animals, can utter no other sound. — In song, by the regulation of the vocal cords, definite and sustained musical tones are produced, which can be changed or modulated at the will of the individual. Different species of Birds have their respective songs; which are partly instinctive, depending upon the construc- tion of their larynx ; and are partly governed by their education. In Man, the power of song is entirely acquired ; but, when once acquired, it is far more susceptible of variety and expression, than that of any other animal. In fact the larynx of Man may be said to be the most perfect musical instrument ever constructed. — The voice is a sound more resembling the cry, in this, — that it does not consist of sustained musical tones ; but it differs from the cry, both in the quality of its tone, and in the modulation of which it is capable by the will. In ordinary conversation, the voice passes through a great variety of musical tones, in the course of a single sentence, or even a single word,- — sliding im- perceptibly from one to another ; and it is when we attempt to fix it definitely to a certain pitch, that we change it from the speaking to the singing tone. 687. It is to the wonderful power that Man possesses, of producing articulate sounds, which form a medium by which he can communicate ideas of any kind to his fellows, — that much of his superiority to other animals is due. Nevertheless, it is not to this alone that we must attribute it ; for many animals, espe- cially Birds, can produce, by imitation, sounds as articulate as those of Man ; but the mind which originates them, and which uses them as expressions of its ideas and desires, is deficient. 688. All spoken language is made up of a certain number of elementary sounds, which are combined into syllables, words, and sentences. It may be easily shown, upon arithmetical principles, that from 20 or more of these elementary sounds, an ELEMENTARY ARTICULATE SOUNDS. 513 almost infinite variety of combinations may be produced ; and from such an inexhaustible store, there is no difficulty in deriving new combinations, to represent any new ideas that we may desire to express. These simple or elementary sounds ought to be represented by an equal number of single letters ; this is the case, however, in but few languages. Our own is particularly faulty in this respect ; for there are many simple sounds, that can be only represented by a combination of letters ; whilst others may be represented by more than one single letter ; and in some instances, a single letter represents a composite sound. Thus the sounds of au and tli are really simple ones, and ought to be represented by single letters. Again, the sound of k is repre- sented also by the hard c, as in the first syllable of concert ; and the sound of s by the soft c, as in the second syllable of the same word, where the c is sounded exactly as the s in consent. And the letter i (as usually pronounced in English) does not repre- sent a simple sound, but a combination of two, as will be pre- sently shown. Most of the Continental languages are superior to the English in this respect. 689. Yocal sounds are divided into Vowels and -Consonants ; the true distinction between which appears to be, that the Vowel sounds are continuous tojies, modified by the form of the aperture through which they pass out ; whilst in giving utter- ance to Consonants, there is a partial or complete interruption to the breath, in its passage through the organs in front of the larynx. Hence all true Vowels may be prolonged for any length of time, that the breath is supplied from the lungs ; whilst the sound of many Consonants is momentary only. It is easy for any one to convince himself that the Vowel sounds are governed simply by the form of the cavity of the mouth, and by that of the aperture of the lips, by passing, in one continued tone, from one of the following vowel sounds to another. English a . . as in ah . Continental a English a . as in all . Diphthong aw English a . . as in name . . Continental e English c . . as in theme . . Continental i English o as in cold Continental o English oo . . ' as in cool . Continental u 514 PRODUCTION OF ARTICULATE SOUNDS. The short vowel sounds, as a in fat, e in met, o in pot, &c, are not capable of being prolonged ; as they are formed in the act of preparation for sounding the succeeding consonant. The sound of the English i is a compound one, being formed in the act of transition from that of a as in ah, to that of e as in theme ; hence it cannot be prolonged ; and it is the very worst vowel sound upon which to sing a long note, since it is impossible to prevent its being heard as one of the sounds of which it is com- posed. Much discussion has taken place, with reference to the true characters of the letters w and y, when employed to commence words, as wall, yawl, wet, yet. A little attention to the state of the mouth in pronouncing them will show, how- ever, that they are then really vowel sounds, rapidly trans- formed into the succeeding ones ; for the sound of w in this situation is oo ; and that of y is the long e ; so that wall might be spelt ooall, and yaul eaul. 690. Consonants are naturally divided into those, which require a total stoppage of the breath, at the moment previous to their being pronounced, and which cannot therefore be prolonged ; and those, in pronouncing which the interruption is partial, and which can be prolonged like the vowels. The former are termed explosive consonants ; the latter continuous. The explosive con- sonants are b and p, d and t, the hard g and k. All the others are continuous ; but the sound is modified by the position of the tongue, palate, lips, and teeth ; and also by the degree in which the air is permitted to pass through the nose. 691 . The study of the mode in which the different consonants are produced, is of particular importance to those who labour under defective speech, especially that difficulty which is known as stammering. This very annoying impediment is occasioned by a want of proper control over the muscles concerned in arti- culation ; which are sometimes affected with a kind of spasmodic action. It is in the pronunciation of the consonants of the ex- plosive class, that the stammerer usually experiences the greatest difficulty ; for the total interruption to the breath, which they occasion, is frequently continued involuntarily, so that either the expiration is entirely checked, or the sound comes out in jerks. CURE OF STAMMERING. 515 Sometimes, on the other hand, in pronouncing vowels and con- tinuous consonants, the stammerer prolongs his expiration, with- out being able to check it. The best method of curing this defect (where there is no malformation of the organs of speech, but merely a want of power to use them aright), is to study the particular defect under which the individual suffers ; and then to make him practise systematically the various movements con- cerned in the production of the sounds in question, at first sepa- rately, and afterwards in combination, until he feels that his voluntary control over them is complete. * * For a fuller consideration of this subject, see the Author's Human Physiology, Chap. vi. CHAPTER XIV. OF INSTINCT AND INTELLIGENCE. 692. It will be remembered that, when the Nervous System was described (Chap, xi.), it was shown to be the instrument of three classes of operations, each of which seems to be per- formed by a distinct portion of the system. — I. The first of these is the class of Reflex actions, which are executed only in respon- dence or answer to the impression made, by certain agents ope- rating upon the nerves proceeding to a ganglionic centre ; as when a Dytiscus, whose head has been cut off, executes swim- ming movements, immediately that its feet come in contact with water. These actions are evidently performed without any choice or direction on the part of the animal, which, in execut- ing them, seems like a mere machine, adapted to perform certain actions when certain springs are touched ; and it has been shown that they may take place, even without its consciousness. Of these reflex movements, the Spinal Cord of Yertebrata, and in Invertebrata, the ganglia corresponding to it (in regard to their connections with the organs of locomotion, respiration, &c.) are the instruments. — II. The second class comprehends the Instinc- tive and Emotional actions, which differ from the preceding, in being dependent on the Sensations received by the animal, and in being, therefore, never performed without its consciousness. Nevertheless, the animal in executing them is not guided by any perception of the object to be attained, or by any choice of the means by which it is to be accomplished ; but acts blindly and involuntarily, in accordance with an irresistible impulse, im- planted in it by its Creator for the purpose of doing that, without or even against its Will, which it would not have chosen or INSTINCTIVE AND VOLUNTARY ACTIONS. 517 devised by its very imperfect intelligence. The Actions of this class are most wonderful in the Invertebrata, which possess the least Intelligence ; and, on the contrary, they are fewest and least remarkable in Man, whose Intelligence is highest. From the constant proportion which they bear to the size of the ganglia of sensation, which form nearly the whole nervous mass in the head of Insects, &c, and a large part of that of the lower Yertebrata, but which are comparatively small in the Mam- malia, and especially in Man, there seems good reason to regard these organs as their chief instruments. — III. The third and highest class of actions, is that in which Intelligence is the guide, and the Will the immediate agent. The animal receives sensa- tions, forms a notion of their cause, reasons upon the ideas thus excited, perceives the end to be attained, chooses or devises the means of accomplishing it, and voluntarily puts those means into execution. These actions are seen, in their highest and most complete form, in Man ; but they are not confined to him ; for, as will be shown hereafter, true reasoning processes are performed by many of the lower animals. There can be no doubt that the Cerebral hemispheres, which form the Brain properly so called, constitute the instrument by which these actions are executed ; for we find their size and development bearing a very regular proportion to the degree of Intelligence which the animal possesses. 693. It follows, then, that the lower we descend in the scale of Animal life, the larger is the proportion of the movements of any particular species, which we are to attribute to the Reflex and the Instinctive classes ; whilst the proportion, which is due to Intelligence and "Will, diminishes in a like degree. Thus we have seen that the ordinary movements of locomotion, which are for the most part voluntary in Man, are reflex in Insects (§. 445) ; and perhaps it would not be wrong to suppose, that the move- ments of the tentacula of the Hydra, by which it entraps its prey and draws it to the entrance of its stomach (§. 14), are of a reflex, rather than a voluntary or instinctive character, since they are obviously analogous to the movements of the pharyngeal muscles, by which the food is grasped, and carried into the alimentary 518 REFLEX MOVEMENTS OF HYDRA. — INSTINCTIVE ACTIONS. tube, in the highest animals (§. 195). There is one curious fact, which would seem to indicate a difference between them, but which is really a strong argument in favour of their analogy. It is continually observed that, when the stomach of the Polype is full, its arms do not make any attempt to seize objects that touch them ; so that small worms, insects, &c, which would at other times be entrapped, may now come near them with impunity. It has been supposed that this results from an act of choice on the part of the animal, and that its choice is influenced by its con- sciousness that its stomach is supplied with food. It must seem improbable that an Animal, which so nearly resembles Plants in its general habits, and in which the nervous system is so obscure that it has not yet been discovered, should possess mental endowments of so high a character ; and we may find, in studying our own functions, a circumstance exactly parallel to that just mentioned. For when we commence eating, with a good appetite, we may notice that the muscles of Deglutition act very readily ; but when we are completely satisfied, it is often difficult to excite these muscles to contraction, so as to swallow another morsel, even though for the gratification of our palate we may desire to do so. Thus we see how much better a guide we find in Nature, for the amount of food we require, than in our own pampered tastes. 694. The first class, that of Reflex Movements, has been already considered in sufficient detail ; but it is intended, in the present Chapter, to ofier some examples of those of the second and third classes, — those actions, namely, which are guided by Instinct and Intelligence respectively. These actions may be usually distinguished by the two following tests : — 1. Although in most cases, experience is required to give the Will command over the muscles concerned in its operations, no experience or education is required, in order that the different actions, which result from an Instinctive impulse, may follow one another with unerring precision. 2. Instinctive actions are performed by the different individuals of the same species, nearly, if not exactly, in the same manner; presenting no such variation of the means applied to the objects in view, and admitting of no such improve- CHARACTERS OF INSTINCTIVE AND INTELLIGENT ACTIONS. 519 merits in the progress of life, or in the succession of ages, as we observe in the habits of individual Men, or in the manners and customs of nations, which are for the most part adapted to the attainment of particular ends, by voluntary efforts guided and directed by reason. — Where, as in the examples hereafter to be mentioned (§. 7^7), we find individual animals "learning- wisdom by experience," and acquiring the power of performing actions which do not correspond with their natural instincts, we cannot do otherwise than regard them as possessed of a certain degree of intelligence, by which they are rendered susceptible of education. 695. The amount of intelligence displayed in these acquire- ments, can only be judged of, however, by carefully examining the circumstances under which they are made. If the new habits are gained by imitation simply, — as is the talking of a Parrot, — no great degree of intelligence is manifested ; but if it sponta- neously result from a reasoning process on the part of the animal, our idea of its sagacity is raised. There may be a combination of both these conditions ; as in the following curious circum- stance, related to the Author by a friend who has repeatedly witnessed it. Some horses kept in a paddock, were supplied with water by a trough, which was occasionally filled from a pump, — not, however, as often as the horses seem to have wished ; for one of them learned, of his own accord, to supply himself and his companions, by taking the pump-handle between his teeth, and working it with his head. The others, however, appear to have been less clever or more lazy ; and finding that this one had the power of supplying their wants, they would tease him, by biting, kicking, &c, until he had pumped for them, and would not allow him to drink until they were satis- fied. That this was not a mere act of imitation, appears from the circumstance, that the horse did not attempt to imitate the movement of the man, but performed the same action in a dif- ferent manner, — evidently because it had associated in its mind, the motion of the pump-handle with the supply of water. 696. The Instincts of Animals may be shown to have imme- diate reference, probably in every instance, to the supply of the 520 INSTINCT OF THE ANT-LION. wants of the individual, or to the continuance of the race. Thus we have Instincts which guide in the selection and acquirement of food ; others which govern the construction of habitations for the individual, and of receptacles for the eggs, — and these may influence a number at once, in such a manner as to unite them into a society ; others which direct their emigrations, whether in search of food, for the deposit of their eggs, or for other pur- poses. Of these, some examples will now be given. 697. Among the instincts which direct animals in the acquirement of their food, few are more remarkable than those possessed by the larva of the Ant-lion, a small' insect allied to the Dragon-fly. This animal is destined to feed upon ants and other small insects, whose juices it sucks; but it moves slowly and with difficulty; so that it could scarcely have obtained the requisite supply of food, if Nature had Fig. 251—Ant-lion, in perfect state. not g^ed it in the construction of a remarkable snare, which entraps the prey it could not acquire by pursuit. It digs in fine sand a little funnel-shaped pit (Fig. 253), and conceals itself at the bottom of this, until an insect falls over its edge ; and if its victim seeks to Fig. 252. — Larva ok the Ant-lion. Fh;. 253. — PlTKALL OK THE ANT-LION. escape, or stops in its fall to the bottom, it throws over it, by PITFALL OF THE ANT-LION. WEB OF THE SPIDER. 521 means of its head and mandibles, a quantity of sand, by which the insect is caused to roll down the steep, within reach of its captor. The manner in which the Ant-lion digs this pit is extremely curious. After having examined the spot where it purposes to establish itself, it traces a circle of the dimensions of the mouth of its pit ; then, placing itself within this line, and making use of one of its legs as a spade, it digs out a quantity of sand, which it heaps upon its head, and then, by a sudden jerk, throws this some inches beyond its circle. In this manner it digs a trench, which serves as the border of its intended excavation, moving backwards along the circle, until it comes to the same point again ; it then changes sides, and moves in the contrary direction, and so continues until its work is completed. If, in the course of its labours, it meets with a little stone, the presence of which would injure the perfection of its snare, it neglects it at first, but returns to it after finishing the rest of its work, and uses all its efforts to get it upon its back, and carry it out of its excavation ; but if it cannot succeed in this, it abandons its work and commences anew elsewhere. "When the pit is completed, it is usually about 30 inches in diameter, by 20 in depth ; and when the inclination of its walls has been altered by any slip, as almost always happens when an insect has fallen in, the Ant-lion hastens to repair the damage. 698. Snares of a still more singular character are constructed by many Spiders, which spin webs of the finest silk, for the purpose of entrapping their prey. The arrangement of these toils varies according to the species, and sometimes does not present any regu- larity ; but in several instances it is of ex- treme elegance ; and no one can watch the labours of a common garden spider (as, for instance, the Epeira Fig. 254.— Epkira Diadema. 522 BURROWS OF THE HAMSTER. MYGALE. diadema, Fig. 254), without being struck with the marvellous sagacity which it displays in the execution of its work, and the perfection with which its web is constructed. 699. An equally curious instinct is often displayed in the construction of the habitations which the animal designs for its abode. Thus the Hamster, a small rodent animal allied to the Rat, which is met with in most cultivated districts on the Con- tinent from Alsace to Siberia, and which is very injurious to agriculture, constructs a burrow in the soil, which has always two openings, — one in an oblique direction, which serves the animal for casting out the earth it has dug away, — the other perpendicular, which is the passage by which it enters and makes its exit. These galleries lead to a regular series of circular excavations, which communicate with each other by horizontal passages ; one of these cavities, furnished with a bed of dried herbage, is the abode of the Hamster ; and the others serve as magazines for the provisions, which it collects in large quantities. 700. There are certain Spiders known to Zoologists under the name of My gales, which per- form operations analogous to those of the Hamster, but still more com- plicated ; for not only do they ex- cavate in the ground a large and commodious habitation, but they line it with a silken tapestry, and furnish it with a door regularly hung upon a hinge. For this pur- pose, the Mygale digs, in a clayey soil, a sort of cylindrical pit, about 3 or 4 inches in length ; and plasters its walls with a sort of very consistent mortar. It then Fig. 255.— Hamster. Fig. 256. — Nest of Mygale. HABITATIONS OF SPIDERS AND INSECTS. 523 constructs, of alternate layers of earth, and of threads woven into a web, a trap-door exactly adapted to the orifice of its hole, and only capable of opening outwards ; and it attaches this by a hinge of the same thread, to the tapestried lining of its chamber. The outside of this trap-door is covered with mate- rials resembling the soil around ; and so little does it differ from this, that it is with difficulty distinguished, even by a person seeking to discover the Spider's habitation. If an attempt is made to lift it, when the animal is within its excavation, the effort is resisted by the whole force of the Spider, which holds down the door, by fixing its claws into small holes on its under surface, at the point most distant from the hinge, where its force may be most advantageously applied. 701. Among Insects, we find a great number of very curious processes, instinctively performed in the construction of their habitations. Many Caterpillars form themselves a protection, by rolling together portions of leaves, and attaching them by threads. In almost every garden, we may observe (at the proper season) nests of this kind, on the leaves of the Lilac or Goose- berry ; and a similar one, represented in Fi«\ 257, *s constructed in the leaves of the Oak, by the caterpillar of a small nocturnal Butterfly ; the Toririx viridis- sima. The larva of the little Clothes- Moth, again, forms a sort of tubular sheath, composed of the filaments it detaches from the stuff, through which it ex- cavates its galleries ; this sheath fig. 257— Nest of tORTrtx. it is continually prolonging at one extremity ; and when, in consequence of the growth of the larva, its tube becomes too small for its comfortable residence, it slits it down, and lets in a piece. The aquatic larva* of the Caddice-flies (Fig. 258, c), which are commonly known as Caddice- worms, house themselves in straws, pieces of hollow stick, rushes, &c. ; and those of some species glue together a number of minute stones, pieces of stick, small shells, &c, so 524 HABITATION OF THE CADDICE-WORM. Fig. 258.— c; a, its tube the tube. Phryganea or Caddice-fly. b, network at the entrance of as to make a tube (a), in which the animal creeps along the bottom and sides of the brook it inhabits, and sometimes rows itself on the surface of the water. "When full- grown, the larvae attach their cases to some large stone by threads; and then close the mouth of the case with an open net-work of threads (b), sufficiently close to pre- vent the entrance of insects, but with meshes permitting the water to pass through. In this state they undergo their metamorphosis into the Pupa state ; and a short time before their last metamorphosis, they cut the threads of the network by means of two hooks, with which their heads are furnished, and creep out of the water ; soon after which they change into the perfect insect. 702. It is scarcely possible to point to any actions better fitted to give an idea of the nature of instinct, than those which are performed by various Insects, when they deposit their eggs. These animals will never behold their progeny ; and cannot ac- quire any notion from experience, therefore, of that which their eggs will produce ; nevertheless they have the remarkable habit of placing, in the neighbourhood of each of these bodies, a supply of aliment fitted for the nourishment of the larva that is to pro- ceed from it ; and this they do, even when they are themselves living on food of an entirely different nature, such as would not be adapted for the larva. They cannot be guided in such actions by anything like reason ; for the data on which alone they could reason correctly, are wanting to them ; so that they would be led to conclusions altogether erroneous if they were not prompted, by an unerring instinct, to adopt the means best adapted for the attainment of the required end. 703. Of this kind of instinct, the Necrophorus, a kind of Beetle not uncommon in our fields, offers a good example. PREPARATION OF FOOD FOR LARV.E. 525 When the female is about to lay her eggs, she seeks for the dead body of a Mole, Shrew, or such other small quadruped; and having found one, she excavates beneath it a hole of sufficient dimensions to contain the body, which she gradually drags into it ; she then deposits her eggs in the carcase, so that the larvae, when they come forth, find themselves in the midst of a supply of carrion, on which they feed, like their parents. This instinct is still more remarkable, when an Insect, whose diet is exclusively vegetable, prepares for its larva a supply of animal food. Such is the case with the Pompilus, an Insect allied to the Wasp. In its perfect state, it lives entirely on the juices of flowers ; but the larvae are carnivorous ; and the mother provides for them the requisite supply of the food they require, by placing in the nest, by the side of the eggs, the body of a Spider or Caterpillar, which she had previously killed Fig. 259.— Necrophorus. -Xylocopa Fig. 261.— Nest of Xylocopa. by means of her sting. The Xylocopa or Carpenter-Bee, has very analogous habits ; the female makes long burrows in wood, palings, &c, in which she excavates a series of cells ; and in every one of these she deposits an egg, with a supply of pollen- paste. 704. The instinct of support and protection to the young 526 NESTS OF BIRDS. and helpless offspring, is seen in all animals in which it is needed ; and it is particularly observable in Birds. The nests which they construct are destined much more for the reception of their eggs, and for the protection of the young, than for their own residence ; for there are few Birds which pass much time in their nests, except at night, and during the period of incuba- tion. It is impossible to watch the process of their construction, without admiring the perseverance with which these interesting animals bring together the materials that are destined for their erection, and the art with which they are arranged. The form and structure of these habitations are always nearly the same, among the individuals of the same species ; but there is neces- Fig. 262. — Nbst of Goldfjnch. sarily a certain latitude in regard to the materials of which they are composed, since the same could not be everywhere procured. The nests of different species vary greatly, however, both as to form, structure, and materials ; and these are admirably adapted to the particular circumstances, in which the young families are respectively destined to live. Sometimes these habitations are constructed of earth, the particles of which are united by the NESTS OF BIRDS. 527 viscid saliva of the Bird, are then commonly built against the sides of a rock or wall. But, in general, they are com- posed of sticks, straws, and other vegetable sub- stances, and are placed either on the ground, or among the branches of trees. The greater num- ber of them have a some- what hemispherical form, resembling a little round basket ; and their interior is lined with moss and down (Fig. 262). into a tenacious mortar ; and they Fig. 263.— Nest gf the Baya. Fig. 264. — Nest of thf. Tailor-bird. 705. But sometimes the arrangement is much more complicated, in order that some particular danger may be avoided, or some special purpose answered. Thus the nest of the Baya, a little Indian bird allied to our Bulfinch, has the form of a bottle ; and it is suspended from a twig of such slenderness and flexibility, that neither Monkeys, Serpents, or Squirrels can reach it (Fig. 263). That it may be still more secure against the attacks of its numerous enemies, the Bird forms the en- trance of the nest on its under side, so that it can itself only reach it by the aid of its wings. This curious habitation is constructed of long grass ; and several chambers are found in its interior, of which one serves for the female to sit on her eggs, whilst another is occupied by the male, who solaces his companion with his song, whilst she is occupied in maternal N N 2 528 TAILOR-BIRD. BEAVER. cares. Another curious nest is that of the Sylvia suloria, or Tailor-bird, a little Eastern bird allied to our linnet ; which, by the aid of filaments of cotton drawn from the Cotton-plant, sews leaves together with its beak and feet, in such a manner as to conceal the nest which they enclose from the observation of its enemies (Fig. 264). 706. The association of a number of individuals of certain species, for the performance of labours in which they all unite to one common end, is another most remarkable example of the operation of instinct. Several Mammalia exhibit this tendency in a greater or less degree; but the most interesting of all, Fig. 265.— Beaver. in this point of view, is the Beaver, which is now chiefly found in Canada, though it formerly abounded on the Continent of Europe. During the summer, it lives solitarily in burrows, which it excavates for itself on the borders of lakes and streams; but as the cold season approaches, it quits its retreat, and unites itself with its fellows, to construct, in common with them, a winter residence. It is only in the most solitary places, that their architectural instinct fully develops itself. Having asso- ciated in troops of from two to three hundred each, they choose a lake or river, which is deep enough to prevent its being frozen to the bottom ; and they generally prefer running streams, for the sake of the convenience which these afford, in the transporta- OPERATIONS OF BEAVERS. 529 tion of the materials of their erection. In order that the water may be kept up to a uniform height, they begin by constructing a sloping dam ; which they form of branches interlaced one with another, the intervals between them being filled up with stones and mud, with which materials they give a coat of rough-cast to the exterior also. When the dam passes across a running stream, they make it convex towards the current ; by which it is caused to possess much greater strength, than if it were straight. This dam is usually eleven or twelve feet across at its base, and it is enlarged every year; and it frequently becomes covered with vegetation, so as to form a kind of hedge. 707. When the dam is completed, the community separates into a certain number of families ; and the Beavers then employ themselves in constructing huts, or in repairing those of a pre- ceding year. These cabins are built on the margin of the water; they have usually an oval form, and an internal diameter of six or seven feet. Their walls are constructed, like the dam, of branches of trees ; and they are covered, on two of their sides, with a coating of mud. Each has two chambers, one above the other, separated by a floor ; the upper one serves as the habita- tion of the Beavers ; and the lower one as the magazine for the store of bark, which they lay up for their provision. These chambers have no other opening, than one by which they pass out into the water. It has been said that the flat oval tail of the Beavers serves them as a trowel, and is used by them in laying on the mud of which their erections are partly composed; but it does not appear that they use any other implements, than their incisor teeth and fore-feet. With their strong incisors they cut down the branches, and even the trunks, of trees which may be suitable ; and by the aid of their mouths and fore-feet, they drag these from one place to another. When they establish themselves on the banks of a running stream, they cut down trees above the point where they intend to construct their dwell- ings, set them afloat, and, profiting by the current, direct them to the required spot. It is also with their feet that they dig up the earth they require for mortar, from the banks or from the bottom of the water. These operations are executed with extra- 530 BUILDING INSTINCT OF BEAVER. ordinary rapidity, although they are only carried on during the night. When the neighbourhood of Man prevents the Beavers from multiplying to the degree necessary to form such associa- tions, and from possessing the tranquillity which they require for the construction of the works now described, they no longer build huts, but live in excavations in the banks of the water. 708. The building instinct shows itself, even when the Beaver is in captivity, and in circumstances in which it could be of no use. A half-domesticated individual, in the possession of Mr. Broderip, began to build, as soon as it was let out of its cage, and materials were placed in its way. Even when it was only half-grown, it would drag along a large sweeping-brush or warming-pan, grasping the handle with its teeth, so that the load came over its shoulder ; and would endeavour to lay this with other materials, in the mode employed by the Beaver when in a state of nature. " The long and large materials were always taken first ; and two of the longest were generally laid cross- wise, with one of the ends of each touching the wall, and the other ends projecting out into the room. The area formed by the cross-brushes and the wall, he would fill up with hand- brushes, rush-baskets, books, boots, sticks, cloths, dried turf, or anything portable. As the work grew high, he supported him- self upon his tail, which propped him up admirably ; and he would often, after laying on one of his building materials, sit up over against it, appearing to consider his work, or, as the coun- try people say, 'judge it.' This pause was sometimes followed by changing the position of the material judged ; and sometimes it was left in its place. After he had piled up his materials in one part of the room (for he generally chose the same place), he proceeded to wall up the space between the feet of a chest of drawers which stood at a little distance from it, high enough on its legs to make the bottom a roof for him ; using for this pur- pose dried turf and sticks, which he laid very even, and filling up the interstices with bits of coal, hay, cloth, or anything he could pick up. This last place he seemed to appropriate for his dwelling; the former work seemed to be intended for a dam. When he had walled up the space between the feet of the chest IRRATIONAL CHARACTER OF INSTINCT. 531 of drawers, he proceeded to carry in sticks, cloths, hay, cotton, &c, and to make a nest ; and when he had done, he would sit up under the drawers, and comb himself with the nails of his hind feet." 709. We see, in this account, a very interesting example of the irrational character of Instinct. If the animal were guided in its ordinary building operations, by such an amount of intel- ligence, as would lead it to choose and execute its various move- ments, with a design to accomplish certain ends, the same intel- ligence would direct it to leave these actions unperformed, when the purpose no longer required it ; instead of which, we see that the animal is impelled by an internal impulse, to construct erections, as nearly resembling those which it would build up on the banks of its native streams, as the materials and circum- stances will permit. Other animals are, in like manner, occa- sionally conducted by their natural instincts, to the performance Fig. 266. — Nest of Republican Grosbeak. of actions equally irrational, and quite incapable of answering the purpose which the particular instinct is destined to serve. 532 SOCIAL BIRDS AND INSECTS. Thus the Hen will sit upon an egg-shaped piece of chalk, as readily as upon her own egg ; being deceived without difficulty by the general similarity of its appearance : and the Flesh-fly lays its eggs in the petals of the Carrion-flower, whose odour so much resembles that of tainted meat, as evidently to deceive the Insect into the belief, that it affords the proper receptacle for the eggs. 710. Societies like those of the Beaver are rare among Birds, whose associations are usually less perfect. There is a small species, however, termed the Republican Grosbeak (Loxia socio), which lives in numerous societies in the neighbourhood of the Cape of Good Hope, and constructs its nest under a sort of roof which is common to the whole colony (Fig. 266). Fig. 267.— Nest of Wasp. 711. It is among Insects, that we find the most remarkable SOCIAL WASPS. HIVE BEES. 533 examples of this kind of social instinct ; and the structures which are produced by the united labours of a large number, working together in harmony, are extremely interesting. The nests of Wasps are constructed in this manner. In order to form the materials for building them, these Insects detach with their mandibles the fibres of old wood, which they convert by masti- cation into a sort of pulp, that hardens into the consistence of pasteboard ; of this substance they construct several ranges of hexagonal cells; and the combs thus formed are arranged parallel to each other at a regular distance, and are united at intervals by little columns which serve to suspend them (Fig. 267). The whole is either hung in the air, lodged in the hollow of a tree, or buried in the ground ; and it is sometimes enclosed in a general enve- lope, sometimes left uncovered, according to the species. 712. The same community of labour is observed in the ordi- nary Hive-Bees, which present to the intelligent observer a source of interesting occupation that scarcely ever fails. The number and variety of instincts, each of them most perfectly adapted to the end in view, which these Insects exhibit, is most wonderful ; and many volumes have been written upon them, without by any means exhausting the subject. Nothing more than a very general sketch of these can be attempted in the present Treatise, for the reason already mentioned ; but the illustrations they afford of the general remarks heretofore made upon the nature of Instinct, are too valuable to be passed by. Each Hive contains but a single Queen ; and she is the only individual capable of laying eggs. There are usually from 6 to 800 males or Drones ; and from 10,000 to 30,000 Neuters or Working-bees. In their Fig. 268.— working-Bee. natural condition, they live in the hollows of trees ; but they appear equally ready to avail themselves of the habitations pre- pared for them by Man. The cells, of which their combs are composed, are built up of the material that we term wax. Of this, a part may be obtained direct from Plants, since it is secreted in greater or less abundance by several species ; but there seems 534 CONSTRUCTION OF BEE S CELL. Fig. 269.— Larvje of Bee, na tural size, and magnified. to be no doubt, that Bees can elaborate it for themselves from the saccharine materials of their aliment (§. 155). The wax is sepa- rated in little scales, from between the segments of the abdomen; these scales are kneaded together by the mandibles of the Insect, and are then applied to the construction of the cells. It is easy to understand, that the hexagonal form is that which enables the cells to be best adapted to the purposes for which they are built, whilst the least amount of material is expended. As they are *~ intended, not only to contain a store of honey, but also to serve as the resi- dence for the Larvas and Pupae, it is evident that their form must approach near to that of the cylinder, in order that there may be the greatest eco- nomy of space ; but it is also evident that, if their walls were circular, a large quantity of material would be required, to fill up the in- terspaces left between them ; whilst, by giving the cells the hexagonal form, their walls everywhere have the same thickness, and their cavity is sufficiently well adapted to the forms of the larva and grub. 713. Every comb contains two sets of cells, one opening on each of its faces. The cells of one side are not exactly opposite, however, to those of the other ; for the middle of each cell abuts against the point, where the walls of three cells meet on the oppo- site side ; and thus the partition that separates the cells of the opposite sides, is greatly strengthened. This partition is not flat, however, but consists of three planes, which meet each other at a particular angle, so as to make the centre of the cell its deepest part. Of the three planes which form the bottom of each fig. 271—Hkxagoxal CeIls, cell> one forms Part of tne bottom of each shotting the manner of union 0f the three cells, against which it abuts at the Base. . ., . on the opposite side, as shown in the accompanying figure. Now it can be proved, by the aid of Fig. 270 — Pupa of Bee. CONSTRUCTION OP BEE S CELL. 535 mathematical calculation of a very high order, that, in order to combine the greatest strength with the least expenditure of mate- rial, the angles formed by the edges of these planes should have a certain regular amount ; which was ascertained by the measure- ment of Maraldi to be, for one, 109° 28', and for the other 70° 32'. By the very intricate mathematical calculations of Koenig, it was determined that the angles should be 109° 26', and 70° 34', — a coincidence between the theory of the Mathematician and the practice of the Bee (untaught, save by its Creator), which has been ever regarded as truly marvellous, and as affording one of the most remarkable examples of the operation of instinct. The very small discrepancy, amounting to only 2 minutes of a degree (or l-10,800th part of the whole circle), has been usually sup- posed to result from a slight error, in the observation of the angle employed by the Bees ; but Lord Brougham, not satisfied with this explanation, has recently applied himself to a fresh ma- thematical investigation of the question ; and he has shown that, owing to the neglect of certain small quantities, the result formerly obtained was erroneous, to the exact amount of 2 Fig. 272.— Apiary. minutes ; so that the Bees proved to be right, and the Mathe- matician wrong.* * See his Supplement to New Edition of Paley's Natural Theology. 536 CONSTRUCTION OF CELLS. FOOD OF BEES. Fig. 273.— Royal Cell. 714. The ordinary cells of the comb are of two sizes ; one for the larvae of the working-bees, and the other for those of the drones. Both of these may be used for laying up a store of food, either for themselves or the progeny ; but it is observed that, in the breeding season, the central portion of each comb, only, is tenanted by the young Bees ; this being the part of the hive where they will most constantly obtain the warmth requisite for their development (§. 411). The deposition of the eggs in these cells only, therefore, is another remarkable instinct on the part of the Queen ; and this is further manifested in the fact, that she never deposits eggs in the comb, with which the glasses are filled that are sometimes placed on the top of a hive, as in Figure 272. The temperature of these glasses is ne- cessarily lower than that of the interior of the hive. — The royal cells, as they are termed, in which the larvae of the young queens are reared, are different in form from the rest; sometimes they lie in the midst of them, as shown in the accompanying figure ; but most commonly they project from the sides or edges of the comb. 715. The food which the Bees collect, is of two kinds, — the honey of flowers for themselves, and their pollen for the larvae. The honey, which they suck up by means of their proboscis-like tongue, seems to undergo some change in their digestive cavity; and the part not required for their own nourishment, is af- terwards returned from the stomach, and deposited in one of the cells, which, when filled, is sealed with a cover- ing of wax. The pollen is obtained by rubbing the body against the anthers, or the parts of the flower over which the pollen may have been scat- tered by their bursting ; and when the surface has been sufii- Fig. 274.— Bee's Mouth. FOOD OF LARV^. — DEVELOPMENT OF QUEENS. 537 ciently dusted in this manner, the fine particles are collected from it by little brushes^ with which the feet of the Bee are furnished, and are worked up into small pellets, which the Insect carries home in basket-shaped hollow's, of which there is one on each hind thigh. The pollen or farina thus collected is worked up with honey in a mass, to which the name of bee-bread has been given ; and with this the larvae are nourished, until the time when they are about to pass into the FlG- 275-Hind leg of ^ r . Worker. pupa state. The mouth of the cell is then sealed by a waxen cover ; and the larva spins a delicate silken cocoon, within which it undergoes its metamorphosis. In the chrysalis state it remains quite inactive for some days ; and during the latter part of this period, when it is rapidly approach- ing the condition of the perfect Insect, its development is aided by the heat supplied by the nurse-bees, which seem prompted by a remarkable instinct to generate it, in the manner formerly described (§. 411). 716. One of the most curious features in the whole economy of Bees, is the manner in which they manufacture new Queens, when, from any cause (as by the intentional removal of her from the hive), their sovereign has been lost. In order to un- derstand the process, it is necessary to be aware, that the ordi- nary working-bees may be regarded as females, with the repro- ductive organs undeveloped ; and it appears to depend on the manner in which they are treated in the larva state, whether the egg shall be made ultimately to produce a queen or a working- bee. For if, when the queen has been removed, the royal cells (which are usually among the last constructed) be not suffi- ciently forward, and contain no eggs, the bees select one or more worker-eggs or larvae, remove the egg or larva on either side of it, and throw the three cells into one. The larva thus pro- moted is liberally fed with royal jelly ^ a pungent food prepared by the working bees for the exclusive nourishment of the queen- larvae ; and in due time she comes forth a perfect queen. This 538 MANIFESTATIONS OF INTELLIGENCE. change is doubtless owing to the peculiar effect of the food ; and it is remarkable that it should operate, not only in developing the reproductive organs, but also in altering the shape of her tongue, jaws, and sting, in depriving her of the power of pro- ducing wax, and in obliterating the hollows just referred to, which would otherwise have been formed upon her thighs. Manifestations of Intelligence. 717. The amount of reasoning power possessed by some among the lower animals, may be considered as very much upon a par with that exhibited by an intelligent child, about the time when it is learning to speak. One of its first exercises is in the connection or association of ideas ; such as is shown in the following instance, related to the Author by an eye-witness. A Wren built its nest in the slate-quarries at Penrhyn, in such a situation as to be liable to great disturbance from the occasional explosions. It soon, however, learned to quit its nest, and fly to a little distance, on the ringing of the bell which warned the workmen. This was noticed, and was frequently shown to visitors, by ringing the bell when there was not to be an ex- plosion; so that the poor bird suffered many needless alarms. It at last learned, however, that the first notion it had formed, by the association of the ringing of the bell with the explosion, was liable to exceptions ; and it formed another more correct. For it was observed after a time, that the wren did not leave its nest, unless the ringing of the bell was followed by the moving away of the workmen. A similar process of association, carried rather further, but still quite simple enough to be readily believed, is shown in two Dogs, which have been taught by their master to play at Dominoes, and which go through the game with another person (under circumstances which render the idea of collusion with their master impossible) with the utmost regu- larity and correctness ; not only playing rightly themselves, but watching to see that their adversary does so too. This, also, is a feat which a very young child might be taught to perform. A third instance has reference to the patient endurance of bodily pain, in opposition to the instinctive tendency to struggle MANIFESTATIONS OF INTELLIGENCE. SIZE OF THE BRAIN. 539 against the infliction of it, and evidently occasioned by a volun- tary effort on the part of the animal, made by it in obedience to the dictates of its reason. Dr. Davy mentions having seen an Elephant, in India, that was suffering under a deep abscess in its back, which it was necessary to lay open, in order to effect a cure. " He was kneeling down, for the convenience of the operator, not tied; his keeper was at his head. He did not flinch, but rather inclined towards the surgeon, uttering a low suppressed groan. He seemed conscious that what was doing was intended for his good ; no human being could have behaved better ; and so confident were the natives that he would behave as he did, that they never thought of tying him." It were much to be wished, that all human beings would imitate this docile Elephant's self-control. It is sometimes manifested, however, even in Infancy ; the painful operation of lancing the gums being often sustained without a cry, from the consciousness of the benefit derived from it. 718. It has been stated that the relative amount of intelli- gence in different animals, bears a pretty constant proportion to the size and development of the Cerebral hemispheres (§. 452). That size alone does not produce the difference, is evident from a number of facts. As we advance from the lower to the higher Vertebrata, we observe a marked advance in the complexity of the structure of the brain. Its surface becomes marked by con- volutions (§. 456), which greatly increase the surface by which the blood-vessels enter it from the enclosing membranes ; and in proportion to the increase in the number and depth of these, do we find an increase in the thickness of the layer of gray matter, which seems to be the real centre of all the operations of the organ. The arrangement of the white or fibrous tissue, which forms the interior of the mass, also increases in complexity ; and as we ascend from the lower Mammalia up to Man, we trace a great difference in the number of the fibres, which establish com- munications between different parts of the surface. Still there can be no doubt that the size of the cerebrum, compared with that of the spinal cord and the ganglia at its top, usually affords a tole- rably correct measure of the intelligence of the animal; and that, 540 SIZE OF THE BRAIN. FACIAL ANGLE. even in comparing together different Men, we shall find the same rule to hold good, when due allowance has been made for the comparative activity of their general functions, such as is ex- pressed by the word temperament. Thus, two men having brains of the same size and general conformation, may differ greatly in mental vigour, because the general system of one performs its functions much more actively and energetically than that of the other. For the same reason, a man of small brain, but whose general habit is active, may have a more powerful mind than another whose brain is much larger, but whose system is inert, his perceptions dull, and his movements languid. But of two men alike in this respect, and having the same general configura- tion of head, it cannot be doubted that the one with the larger brain will surpass the other. It is a striking fact, that almost all those persons who have been eminent for the amount of their acquirements, or for the influence they have obtained by their talents for command, over their fellow-men, have had large brains ; this was the case, for example, with Newton, Cuvier, and Napoleon. 719. The size of the brain, and especially of its anterior lobes (which seem particularly connected with the higher reasoning powers), as compared with that of the face, may be estimated pretty correctly by the measurement of the facial angle; as proposed by Camper, an eminent Dutch naturalist. This is done by drawing a horizontal line (c d, Figs. 276 and 277), between a the entrance to the ear, and the floor of the cavity of the nose, so as to pass in the direction of the base of the skull ; this is met by another line {a 6), which passes from the most prominent part of the forehead to the front of the upper jaw. It is evi- dent that this last will be more in- Fig. 276—Skull of European. clme(j t() the former, SO as to make a more acute angle with it, in proportion as the face is more developed, and the forehead more retreating; whilst it will approach more nearly to a right angle (as in Fig. 276), if the SIZE OF THE BRAIN. — FACIAL ANGLE. 541 Skull of Negro forehead be prominent, and the muzzle project but little. Hence this facial angle will indicate, with tolerable correctness, the proportion which the brain bears to the face, — the instrument of intelligence, to the receptacle of the organs of sense. 720. Of all animals, there are none in which the facial angle is so open as in Man ; and there exist, in this respect, great variations, even among the different a human races. Thus, in European heads, the angle is usually about 80° (Fig. 276). The ancient Greeks, in those statues of Deities and Heroes to which they wished to give the appearance of the greatest intellectual power, made it 90°, or even more, by the projection they gave to the forehead. On the other hand, in the Negro races, it is commonly about 70° (Fig. 277) ; in the different species of the Monkey tribe, it varies from about 65° to 30° (Fig. 278) ; and as we descend still lower, we find it becoming still more acute. In the Horse and Boar, for example, it becomes impossible to draw a straight line from the forehead to the upper jaw ; in con- sequence of the retreating character of the former, and the projection of the nose ; this will be evident from an ex- amination of Fig. 279. In Birds, Reptiles, and Fishes, the facial angle, when it can be measured, is found to be fig. 27S—Skull of Boar. still further diminished. Fig. 278. — Skull of Macacos. 721. It appears, then, that the mind of Man differs from that of the lower animals, rather as to the degree in which the 542 DISTINCTIVE CHARACTER OF THE HUMAN SOUL. reasoning faculties are developed in him, than by anything pecu- liar in their kind. Among the more sagacious Quadrupeds, it is easy to discover instances of reasoning as close and prolonged, as that which usually takes place in early childhood ; and it is only with the advance of age, and the maturity of the powers, that the superiority of Man becomes evident. There is a ten- dency, however, by which he seems to be distinguished from all other animals ; — this is, the disposition to believe in the exist- ence of an unseen but powerful Being, which seems never to be wanting (under some form or other) in any race or nation, although (like other natural tendencies) it may be defective in individuals. Attempts have been made by some travellers to prove that particular nations are destitute of it ; but such assertions have been based upon a limited acquaintance with their habits of thought, and with their outward observances : for there are probably none, that do not possess the idea of some invisible Power external to themselves, whose favour they seek, and whose anger they deprecate, by sacrifice and other religious observances. It requires a higher mental cultivation than is commonly to be met with among savage races, to conceive of this Power as having a spiritual existence ; but it appears, from the reports of Missionaries who have laboured to spread Christianity amongst the Heathen, that an aptitude or readiness to receive this idea is rarely wanting ; so that the faculty is obviously present, though it has not been called into operation. 722. Closely connected with this tendency to belief in a Great unseen Power, is the desire to share in His spiritual existence, which seems to have been implanted by the Creator in the mind of Man, and which is one of the chief natural argu- ments for the immortality of the soul, — since it could scarcely be supposed, that such a desire should have been implanted by our beneficent Maker, if it were not in some way to be gratified. By the Immortal Soul, the existence of which is thus guessed by Man, but of whose presence within him, he derives the strongest evidence from Revelation, Man is connected with beings of a higher order, amongst whom Intelligence exists, unrestrained in its exercise by the imperfections of the bodily frame, with which IMMORTALITY OF THE SOUL. 543 it is here connected, and by which it here operates. — Such views tend to show us the true nobility of Man's rational and moral nature ; and the mode in which he may most effectually fulfil the ends for which his Creator designed him. "We learn from them the evil of yielding to those merely animal tenden- cies,— those " fleshly lusts which war against the soul," — that are characteristic of beings so far below him in the scale of existence, and tend to degrade him to their level ; and the dig- nity of those pursuits, which by exercising his intellect, and by expanding and strengthening his loftier moral feelings, raise him towards beings of a higher and purer order. But even the lof- tiest powers and highest aspirations, of which he is at present capable, may be regarded as but the germs or rudiments of those more exalted faculties, which the human mind shall possess, when, purified from the dross of earthly passions, and expanded into the comprehension of the whole scheme of Creation, the soul of Man shall reflect, without shade or diminution, the full effulgence of the Love and Power of its Maker. CHAPTER XV OF REPRODUCTION. 723. There is no one of the functions of living beings, that distinguishes them in a more striking and evident manner from the inert bodies which surround them, than the process of Re- production. By this function, each race of Plants and Animals is perpetuated ; whilst the individuals composing it successively disappear from the face of the earth, by that death and decay which is the common lot of all. There are some tribes, in which the death of the parent is necessary for the liberation of the germs, from which a new race is to spring up ; and there are many in which the life of the parent is not prolonged, after it has laid the foundation (as it were) of a new generation ; so that this function can be only exercised once by each individual. But, in general, the mode in which it is performed renders it less injurious to the parent ; and indeed it may be regarded as a function equally consistent with its well-being, with any of those we have been considering. A very unnecessary degree of mys- tery has been spread around this process. It has been regarded as one altogether inscrutable, — whose real nature could not be unveiled, even by the scientific inquirer, and whose secrets the uninitiated should never seek to comprehend. But so much light has been thrown upon it by recent investigations, that we now know at least as much of this, as of almost any other func- tion ; and the Author's experience has led him to believe, that such knowledge may be communicated to the general reader, without the least infringement of the purest delicacy of feeling. Indeed his own judgment would lead him to the belief, that the possession of such knowledge is the best possible check to that TWO MODES OF REPRODUCTION. 545 curiosity, which almost every one feels upon the subject, and which frequently leads to improper modes of gratifying it. 724. It has been elsewhere shown (Veget. Phys. Chaps, ix., xii.), that, in the Vegetable Kingdom, there are two distinct modes, by which the propagation of most kinds of Plants may take place ; — the extension of the parent structure into new por- tions, which are independent of each other, and which can maintain their lives when separated from it ; — and the formation of cer- tain bodies, the development of which does not commence, until they have been cast off from the parent, — these being nothing else, from the first, than the germs of new individuals. Now the bodies of the first class are known as leaf-buds in the Flow- ering Plants, and as gemmae among the Cryptogamia ; many of which last, as the Marchantia (Veget. Phys. §. 33) are fur- nished with a peculiar means of producing them. These buds may be developed in connexion with the parent structure, and may continue to form a part of it ; or they may be removed from it (as in the processes of budding, grafting, &c), and may be developed into new individuals. On the other hand, the bodies of the second class are known as seeds among the Flower- ing Plants, and as spores among the Cryptogamia. From the very first, these are destined to produce new individuals ; and their development does not take place, until they are cast off from the parent, which frequently (as in annual plants) dies as soon as it has matured them and set them free. Both these modes of Reproduction exist in the Animal Kingdom ; but the former is confined to its lowest tribes ; and among these, we not only find a tendency to multiplication by buds, but an extraordinary power of regenerating lost parts, and even of reproducing the whole structure from a small portion of it. 725. This is the case, to a greater or less degree, in most of the Radiated classes ; and in none more than in the Hydra^ already so frequently referred to. Not only will the body re- produce any of the arms that have been cut off, but, if it be cut up into a number of small portions, each of these will deve- lop itself into a complete polype. Although this interesting little animal appears sometimes to reproduce itself by ova or 546 REPRODUCTION BY BUDS. Fig. 280. Hydra, attached to duck-weed. yet its usual mode of propagation is by buds. From the side of the parent, a little knot or protuberance is seen to project ; this increases in size, and assumes more of the form of a young ani- mal ; tentacula sprout from around its extremity, and a mouth or opening into its interior cavity is formed there; this cavity, which at first communicates with the stomach of the parent, is gradually shut off from it ; the young animal begins to seize and digest its own food, whilst still attached by its base to the body of the parent ; and at last the connexion is broken, and it becomes completely independent. Several of these buds, in dif- ferent stages of development, may sometimes be seen on the body of a well-fed Hydra (as shown on the left-hand side of Fig. 280); and occasionally the young ones themselves begin to produce a third generation, before they are separated from the body of the parent ; so that as many as 18 individuals, in various stages of development, have been seen sprouting from a single one. The Sea-Anemone has a power of reproducing lost parts almost equal to that of the Hydra ; but it does not propagate itself in the same manner, its reproduction being always effected by ova. But these eggs are not unfrequently hatched, and the embryos partially developed, within the body of the parent ; so that the half-formed young ones are ejected from its mouth, along with the undigested remnants of its food (§. 132). 726. Among the compound Polypifera, we find the process of reproduction by budding carried to a great extent. The buds do not originate, however, from the individual polypes, but from the tree-like structure which connects them (§. 133). This structure has powers of growth in itself, independently of the polypes, which may be regarded as the mouths by which it obtains its food ; and when it extends itself, by commencing a REPRODUCTION BY BUDS. 547 new branch or twig, the polype-cell is first formed, and the polype itself does not appear until this is complete. A small portion of the gelatinous flesh peeled off from the stem of one of the stony corals (§ J 33), is able to reproduce the entire structure; for, absorbing nourishment from the surrounding fluid, it begins to deposit stony matter on the surface on which it may be lying, so as to lay the foundation of a cell ; within this, a polype is speedily developed ; and the stem and branches, with multitudes of new polypes, are in time produced, by the continuance of a similar process. 727. In the Star-fish, a considerable power of regenerating lost parts has been observed ; but this appears to be confined to the reproduction of the arms from the body. As it does not seem that the body can be regenerated from the arms, or from a half of itself, no multiplication of individuals can take place in this manner ; and in this class there is no propagation by buds, as in the group just mentioned. In Animalcules, however, we find this process, or a modification of it, to be almost the only means of re- production, which the beings composing that wonderful group pos- sess ; for the greater number of species never deposit eggs, but multiply themselves by the development of buds, or by the division of their own bodies. The former process may be continually wit- nessed by the microscopic observer, in the common Vorticella, a Fig. 2H1 — Various Forms of Animalcules. bell-shaped animalcule, attached by a stalk, and abundant in almost every pool in which aquatic vegetables grow, especially clus- tering around the stems of Duckweed (Fig. 281, a, a). Its various 548 REPRODUCTION BY BUDS. VOLVOX. stages closely resemble those, which have been already described in the Hydra. But in many other species, the body of the parent divides into two equal parts, in each of which we see a mouth and other parts resembling those of the original. This division is gradual. A narrowing of the body along or across its middle (for the fission or cleaving sometimes takes place lengthways, as at b, sometimes transversely, as at c), is first seen ; the indent- ation at the edge becomes gradually deeper, and at last the two parts hold together by but a narrow band, which finally breaks, and they become free. 728. But there are some species in which this process is stopped short, as it were, before the final separation; and in this case, a compound animal is gradually formed, by the union of many in one line or surface, — just as several cells remaining ad- herent together, but having independent powers of life, form the simple stalks of the bead-mould, or yeast-fungus ( Veget. Phys. §. 56). Some very remarkable compound structures are thus pro- duced ; and among them is one well known to microscopic obser- vers, under the name of the Volvox, or Globe-animalcule. This is large enough to be discernible to the naked eye, and is a most interesting and beautiful object under the microscope. It ap- pears in the form of a hollow globe with a transparent wall, inclosing other smaller but similar globes. The wall of each sphere is studded all over with minute green points, disposed at regular distances from each other; and when these are examined with a sufficiently high magnifying power, they are seen to be distinct animalcules, closely resembling in structure those which in other species are separate ; each having its own cilia, by the vibrations of which, in connection with the rest, the whole mass receives a rolling motion. They seem to be in some degree con- nected with each other, by means of vessels, which pass along the transparent glassy wall that supports them. The interior globes are young structures of a similar kind, which have had their origin in one of the animalcules of the parent mass. This, by dividing and subdividing, forms a new group of animalcules, which is at first attached to the interior of the wall of the parent structure ; but after a time it is separated, and floats in REPRODUCTION BY BUDS. 549 its cavity ; and, when it is mature, it is liberated by the bursting of the walls which inclose it, — the destruction of the parent being thus necessary for the propagation of the race. Not un- frequently, a third generation may be seen within the second, previously to the setting-free of the latter ; and, under the bril- liant light and high magnifying power of the solar microscope, even a fourth generation has been seen within the third. 729. Among many of the lower Articulata, the segments of the body appear to be capable of producing new individuals ; and there are some among the Annelida, whose ordinary pro- pagation is accomplished in this manner. In some species of the Earth-worm^ the reproductive power appears to be nearly as great as in the Hydra; for a single individual having been divided into twenty-six parts, almost all of them reproduced the head and tail, and became so many new and perfect individuals ; and in another experiment, the head of one of these animals was cut off eight times, and reproduced as often. In the Nais, one of the marine-worms, the last joint of the body gradually extends, and increases to the size of the rest of the animal ; and a separa- tion is made by a narrowing of the preceding joint, which at last divides. Previously to its separation, however, the young one often shoots out a young one from its own last joint, in a similar Fig. 282 Nereis Prolifera. manner; and three generations have thus been seen united. In some species of Nereis, the separation takes place nearer the middle of the body. 730. In the Planaria, — an aquatic animal, which has the same general structure with the Entozoa (particularly resembling the Fluke, common in sheep's livers), but which does not inhabit the bodies of other animals, — the power of reproducing the several 550 REPRODUCTION BY BUDS. parts of the body is nearly as great as in the Hydra ; although it is an animal of much more complex structure, possessing eyes, a regular digestive apparatus with a second orifice, and a system of vessels. A partial division of the body will produce an animal with two heads or two tails, according to the end that is left ; portions of different animals, and even of different species, may be grafted together ; and in fact any conceivable monstrosity may be produced in this manner. Among the higher Articulata, the power of this kind of reproduction appears to be much less ; but it is really not inferior, because the parts regenerated are much more complex in their structure. Both Crustacea and Spiders have the power of re-forming legs and claws which have been broken off; and in the former class it is very common to meet with individuals, in which the size of the two organs of the same pair is so different, as to show that one of them has been lost and is in process of renewal. Among perfect Insects, whose nutritive functions are for the most part comparatively inactive (§. 105), the reproduction of parts scarcely ever takes place ; but many of the Larvae possess this power in a considerable degree. 731. In the lowest Mollusca, we have some instances of the power of increase by budding, which seems so remarkably to connect the Animal and Vegetable kingdoms ; but these are restricted to those species, in which, as in Plants, there is an association of a number of individuals into one compound struc- ture (§.126). In regard to the amount of reproductive power possessed by the higher tribes of this group, few observations have been made ; but the head of the Snail has been known to be replaced after being cut off, provided the cephalic ganglion is not injured. 732. Even Vertebrated animals exhibit similar phenomena in no inconsiderable degree. In Fishes, this reproductive power is confined to the fins, which are sometimes regenerated after being lost by accident or disease. In many Reptiles, however, especially those of the Frog tribe, it is much more energetic. In the Salamander, for instance, new legs, with perfect bones, nerves, muscles, &c, are reproduced, after the loss or severe injury of the original ones ; and in the Water Newt, a perfect eye REPRODUCTION OF LOST PARTS. 551 has been formed, to replace one which had been removed. In the true Lizards, the tail when lost appears to be restored ; the new part contains no perfect vertebrae, however, but merely a cartilaginous column like that of the lowest fishes. — In Mam- malia in general, as in Man, the power of reproducing entire organs appears to be much less considerable ; but each tissue is capable of regenerating that of its own kind ; and as this process of renovation is constantly taking place in the living body, the act of Nutrition has been not unjustly spoken of, as a perpetual generation. This power is nowhere, perhaps, more remarkably manifested, than in the re-formation of a whole 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 attach- ments 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 regenerating power is retained to a much greater degree than it is by the class at large; thus, there is a well-authenticated instance, in which a super- numerary thumb on a boy's hand was twice reproduced, after having been removed from the joint. And in many cases in which the crystalline lens of the eye has been removed, in the operation for cataract, it has been afterwards regenerated. 733. We have now to consider the special means, by which the proper function of Reproduction is performed, in the various tribes of Animals; all the preceding phenomena being referrible to the operations of Nutrition, taking place under peculiar cir- cumstances. In order to understand the real nature of this function, it is simply necessary to recal the statements which have been elsewhere made, respecting its performance in the Vegetable kingdom (Veget. Phys., Chap. xn.). In nearly all Plants, we find certain cells appropriated to the development of germs ; which, when mature, are set free by the bursting of the parent-cell, and then commence their development into new 552 NATURE OF REPRODUCTION IN PLANTS. cells. These reproductive cells (which are themselves usually- cast off from the parent structure, before they set free their con- tained cell-germs,) are termed spores in the Cryptogamia, and pollen-grams in the Flowering-Plants. The cell-germs con- tained in the spore are developed into cells, without any further assistance, than that which they derive from the air, moisture, &c, that surround them ; from these first-formed cells, others are gradually produced ; and from them, the various organs of the new plant are gradually developed. But the cell-germs contained in the pollen-grains are not thus thrown upon their own resources in the first instance ; for they are received into another organ, which supplies them with nourishment already prepared for them by the parent, at the expense of which, their early development takes place. Into this organ, the ovule, the cell-germs are conveyed, by a very curious process, — the pas- sage of long tubes put out from the pollen-grain, down the style, and into the ovarium. The germs thus deposited in the ovule, there undergo their early development ; and the mature seed contains the embryo or young plant, developed up to a certain point, along with a further supply of nourishment, des- tined to afford the materials of its growth, until its roots, leaves, &c, are sufficiently advanced, to enable them to perform their proper functions in obtaining and preparing its food. 734. Thus in the higher Plants, we have two sets of organs concerned in the reproductive process ; — the germ- preparing and the germ-nourishing. In the lower, the former alone exist. The aid of the latter is evidently necessary, to enable the young- plant ultimately to attain a higher degree of development ; for it is a rule which we may trace in universal operation through- out Nature, that, the more highly-organised the being is ultim- ately to become, the longer does it require assistance in its early development. The two sets of organs are usually possessed by the same individual ; so that it requires no aid or influence from another, in the discharge of the Reproductive function. But they are sometimes separated, as in diwcious Plants (Veget. Phys. §. 435) ; and the aid of insects or of the wind, and some- times even the assistance of Man, are needed to convey the germs NATURE OF REPRODUCTION IN ANIMALS. 553 contained in the pollen- grains, to the ovules in which their deve- lopment is to commence. 735. Now in Animals we find, almost without any excep- tion, that the concurrence of these two sets of organs is neces- sary ; so that, even in the lowest tribes, a germ-preparing and germ-nourishing organ exist ; and a body is produced which is analogous to the seed, and not to the spore, of Plants. This body is the ovum, or egg ; and it contains, as we shall presently see, with the embryo, a store of nourishment laid up for its sup- port daring a longer or shorter time. Such an ovum is produced even by the lowest Radiata, whose mode of reproduction was formerly thought analogous to that of the Cryptogamia. But there is a remarkable difference in the degree of development which the embryo of different tribes attains, at the time of quitting the egg, and coming forth into the world ; and to this difference are due, the extraordinary metamorphoses, which are exhibited by some, especially among the lower classes, — It is not difficult to understand, why the embryo of every Animal should receive such assistance ; for it will be remembered as a general principle, that the animal tissues can only be nourished by matter that has previously formed part of a living being (§. 10) ; and that, as such matter does not exist diffused through water, in a state in which it can be appropriated by the germ (so long as this consists but of a mass of cells), it must be prepared and communicated to the embryo by the parent, or it could not maintain its existence. 736. As the history of the early development of Animals is in many respects parallel with that of Plants, it will be useful to recapitulate the chief phenomena of the latter. The first cells produced from the cell-germs of the Cryptogamia, usually form a row or string, from the sides of which other cells are developed ; and in this manner a leaf-like expansion is produced, which has the power of performing the functions of absorption, respiration, digestion, &c, and which, in fact, constitutes the permanent form of the lowest tribes of Plants. But though, in the highest Cryptogamia, the character of the Plant is ultimately to become very different from this, its formation commences in precisely 554 STAGES OF DEVELOPMENT OF PLANTS. the same manner ; so that the young Fern, which is afterwards to send a woody stem and beautifully-formed leaves into the air, and to transmit its solid roots deep into the ground, might be readily mistaken for an humble Liverwort, whose frond is not destined to raise itself from the ground, but creeps along its sur- face, and obtains its nourishment by the slight fibres which insinuate themselves into the soil. It is from the centre of this leafy expansion, termed the primary frond, that the true stem and roots of the Fern are subsequently put forth ; and all the remaining portion decays away, as soon as the first true leaf has unfolded itself. Even in the Flowering-Plants, the early development of the embryo takes place upon the same plan ; for the mass of cells of which it is composed does not at first take the form which the young plant is afterwards to pre- sent, but spreads itself out into the single or double cotyledon, which is a leaf-like expansion, closely resembling the primary frond of the Fern. This expansion, absorbs the nourishment provided in the ovule, and prepares it for the development of the young plant, which, even when the seed is mature, forms but a very small proportion of it. The development of the permanent structure takes place rapidly, however, during the process of germination ■ in which all the nourishment that the seed con- tains, is prepared for the embryo by the cotyledons, which serve the purpose of leaves, until the stem and roots have been deve- loped, and the true leaves unfolded. When the store has been exhausted, and the development of the embryo has advanced far enough, for it to be able to support itself, the cotyledons decay away. 737. Thus we see that even the highest Plants have to pass through the conditions, which are permanently shown in the lower ; and that the parts which are first formed, are destined for a temporary purpose only. We shall find, in tracing the history of the development of Animals, that exactly the same general fact may be observed in even a higher degree, — the number of different stages being greater, and an even larger proportion of the parts first formed in the higher tribes, having only a temporary purpose. GERM-PREPARING, AND GERM-MATURING ORGANS OF ANIMALS. 555 738. The germ-preparing organs of Animals differ consider- ably in their structure, arrangement, and position, in the dif- ferent classes ; and yet as regards their essential character, they are the same in the highest as in the lowest. In the part of the body of the Hydra near its mouth, there are certain cells, within which are produced numerous minute thread-like bodies, which are occasionally set free from it by an opening in its walls, very much in the same manner as the cell-germs of the Confervw are allowed to pass forth (Veget. Phys., §. 44) ; and, as in that tribe, these bodies have a spontaneous motion through the water, which they continue to exhibit for some time. Indeed they have been mistaken for distinct animalcules ; but to this cha- racter they have no more claim, than have many other parts of the body (for instance, epithelium-cells fringed with cilia), which exhibit similar motions, when detached from the entire structure. Now, although a separate organ, of much complexity of struc- ture, is appropriated in the higher animals to the preparation and expulsion of these germs, its essential portion still consists of cells; which are the parts that really form them within them- selves, setting them free when mature, by the rupture of their own wall. 739. The germ-maturing organs of Animals also differ in their form and arrangement ; but in the lowest, they present all the essential parts which they possess in the highest ; and we shall therefore draw our description from the former. In certain other cells of the Hydra, near its stem or foot, are developed a set of little bodies, which are analogous in structure to the ovules of Plants, being adapted to receive the germs, and to assist them in their early development, by means of the store of nutri- ment they contain. These ovules also are set free by an aper- ture which forms, when they are matured, in their containing cell ; when they have been fertilised by the introduction of the germ, they become true ara, or eggs ; and the embryo remains in them, until it is prepared to support its own life, — the period at which it can do this varying considerably in the different tribes. — Now although the structure of the organs in which the ovule is prepared, becomes much more complex in the higher 556 STRUCTURE OF THE OVULE. tribes, its essential character (as in the previous case) remains exactly the same. The ovarium of Birds consists of a mass of dense areolar tissue, in the interstices of which are a vast number of cells, whose function is precisely the same as that of the cells, in which the ovules of the Hydra are developed ; for within them are the rudiments of ovules, which are progres- sively matured and set free by the bursting of the cells. There is this difference, however, in the mode in which they are liberated ; that whereas in the Hydra, the ovules, set free by the bursting of the cells, at once pass into the water around, those of the Bird would fall into the cavity of the abdomen, were they not received into the funnel-shaped extremity of a tube, called the oviduct, by which they are conveyed to the ex- terior of the body, to be deposited by the animal in the place selected for it. 740. The essential structure of the ovule or unfertilised egg, as it quits the ovarium, is the same in all animals; and is shown in Fig. 284, A. It consists of a sac, or bag, d, containing a fluid, c, termed the yolk; which is composed of albumen and oil- globules, and in which cells are developed, as the egg becomes ready for fertilisation. The yolk answers precisely the same purpose, in the Animal, as the starchy and oily matter, laid up in the seed, does in the economy of the Plant; being destined to afford support and nourishment to the embryo, until it can obtain these for itself. The yolk contains all the substances which are necessary for the development of the organised frame- work ; and the germ has the wonderful power of appropriating these, in such a manner as to develop itself, from the condition of a simple cell-germ, to that of a highly-organised and most complex fabric. Floating in this fluid is a cell of peculiar aspect, b, which is termed the germinal vesicle; and upon its walls is a very distinct spot or nucleus, a, termed the germinal spot. These last parts, have, as we shall presently see, most important functions to perform, in the reception of the germ, and in aiding its early development. 741. Tn the eggs of Birds, and many others among the higher oviparous animals, we find the yolk-bag surrounded by STRUCTURE OF THE BIRD'S EGG. 5.") 7 a store of pure albumen, commonly termed the white of the egg (Fig. 283, g), which is gradually absorbed into the volk-basr Fig. 283. — Section of a Bjrd's Egg.