-^•fltL-;«E UNIVERSITY OF CALIFORNIA SAN DIEGO rev. Co T- ^.c<7 <■ < • < c »^ cc c C^ \aV CI 39 (2/95) UCSD Lb. THE BRIDGEWATER TREATISES OJM THE POWER WISDOM AND GOODNESS OF GOD AS MANIFESTED IN THE CREATION TREATISE V ANIMAL AND VEGETABLE PHYSIOLOGY CONSIDERED WITH REFERENCE TO NATURAL THEOLOGY BY PETER MARK IIOGET. M. J). SEC. 11. S. ETC. IN TWO VOLUMES VOL 11 " Am) thkkk ark iiivgrsities ok opku*tions, bit it is tiik ssmh God UHllll WURKtTH ALL IN ALL." ' I CuR. 3Ui.fl. ANIMAL AND VEGETABLE PHYSIOLOGY CONSIDERED WITH REFERENCE TO NATURAL THEOLOGY BY PETER MARK ROGET, M. D. SECRETARY TO THE ROYAL SOCIETY, Fl'LLERIAN PROFESSOR OF PHYSIOl.OOY IN THE ROYAI. INSTITUTION OP GREAT BRITAIN, VICE PRESIDENT OP THE SOCIETY Ol' ARTS, FELLOW OF THE ROYAL COLLEGE OF PHYSICIANS, CONSULTING PHYSICIAN TO THE yULEN CH\R1.0TTK's LYINO-IN HOSPITAL, and TO THE NORTHERN DISPENSARY, ETC. ETC. VOL II LONDON WILLIAM PICKERING 1834 \!y- ^^■\ LIBRARY KCRlPPSXNS/flTUTIO^, OF oceaWgraphy UNIVERS.jy ofbALIFORNIA LA JOLLa/calIFORNIA / C. WlilTTINGRAM, TOOKS OOPRT, CHANCRnV I.ANR. CONTENTS OF TH^ SECOND VOLUME. PART II.— THE VITAL FUNCTIONS. Page Chapter I. — Objpxts of Nutrition 1 Chapter II. — Nutrition in Vegetables l-^ § 1. Food of Plants 15 2. Absorption of Nutriment by Plants 19 3. Exhalation 27 4. Aeration of the Sap 29 5. Return of the Sap 36 6. Secretion in Vegetables 45 7. Excretion in Vegetables 51 Chapter III. — Animal Nutrition in general 57 § 1 . Food of Animals 57 2. Series of Vital Functions 69 Chapter IV. — Nutrition in the lower orders of Animals 74 Chapter V. — Nutrition in the higher orders of Animals 104 Chapter VI. — Preparation of Food 113 § 1 . Prehension of Liquid Food 113 2. Prehension of Solid Food 117 3. Mastication by means of Teeth 1 40 VI CONTENTS. Page 4. Formation and Developement of the Teeth . . 155 5. Trituration of Food in Internal Cavities 167 6. Deglutition 174 7. Receptacles for retaining Food 178 Chapter VII. — Digestion 180 Chapter VIII. — Chylification 203 Chapter IX. — Lacteal Absorption 226 Chapter X. — Circulation 229 § 1 . Diffused Circulation 229 2. Vascular Circulation 235 3. Respiratory Circulation 265 4. Distribution of Blood Vessels 281 Chapter XI. — Respiration 290 ^ 1 . Respiration in general 290 2. Aquatic respiration 293 3. Atmospheric Respiration 310 4. Chemical Changes effected by Respiration . . 333 Chapter XII. — Secretion 342 Chapter XIII. — Absorption 351 Chapter XIV. — Nervous Power 354 PART III.— THE SENSORIAL FUNCTIONS. Chapter 1. — Sensation 362 Chapter II. — Touch 377 Chapter III. — Taste 393 CONTENTS. Vll Page Chapter IV. — Smell 396 Chapter V. — Hearing 414 § 1. Acoustic Principles 414 2. Physiology of Hearing in Man 420 3. Comparative Physiology of Hearing 434 Chapter VI. — Vision 444 § 1. Object of the Sense of Vision 444 2. Modes of accomplishing the objects of Vision 449 3. Structure of the Eye 460 4. Physiology of Perfect Vision 469 5. Comparative Physiology of Vision 477 Chapter VII. — Perception 508 Chapter VIII. — Comparative Physiology of the Nervous System 537 § 1. Nervous System of Invertebrated Animals. . . . 537 2. Nervous System of Vertebrated Animals .... 553 3. Functions of the Brain 561 4. Comparative Physiology of Perception 566 PART IV.— THE REPRODUCTIVE FUNCTIONS. Chapter I. — Reproduction 581 Chapter II. — Organic Developement 599 Chapter III. — Decline of the System 619 Chapter IV. — Unity of Design 625 Index 643 ANIMAL AND VEGETABLE PHYSIOLOGY. PART II. THE VITAL FUNCTIONS. Chapter I. OBJECTS OF NUTRITION. 1 HE mechanical structure and properties of the organized fabric, which have occupied our atten- tion in the preceding volume, are necessary for the maintenance of life, and the exercise of the vital powers. But however artificially that fabric may have been constructed, and however admi- rable the skill and the foresight that have been displayed in ensuring the safety of its elaborate mechanism, and in preserving the harmony of its complicated movements, it yet of necessity contains within itself the elements of its own dis- solution. The animal machine, in common with VOL. II. B 2 THE VITAL FUNCTIONS. every other mechanical contrivance, is subject to wear and deteriorate by constant use. Not only in the greater movements of the limbs, but also in the more delicate actions of the internal organs, we may trace the operation of many causes inevitably leading to their ultimate des- truction. Continued friction must necessarily occasion a loss of substance in the harder parts of the frame, and evaporation is constantly tending to exhaust the fluids. The repeated actions of the muscles induce certain changes in these organs, both in their mechanical properties and chemical composition, which impair their powers of contraction, and which, if suffered to continue, would, in no long time, render them incapable of exercising their proper functions ; and the same observation applies also to the nerves, and to all the other systems of organs. Provision must accordingly be made for remedying these constant causes of decay by the supply of those peculiar materials, which the organs require for recruiting their declining energies. It is obvious that the developement of the organs, and general growth of the body, must imply the continual addition of new particles from foreign sources. Organic increase consists not in the mere expansion of a texture previously condensed, and the filling up of its interstices by inorganic matter ; but the new materials that are added must, for this purpose, be incorporated with those which previously existed, and become OBJECTS OF NUTRITION. 3 identified with the living substance. Thus we often find stnictures forming in the bodies of animals of a nature totally different from that of the part from which they arise. In addition to these demands, a store of mate- rials is also wanted for the reparation of occa- sional injuries, to which, in the course of its long career, the body is unavoidably exposed. Like a ship fitted out for a long voyage, and fortified against the various dangers of tempests, of ice- bergs, and of shoals, the animal system, when launched into existence, should be provided with a store of such materials as may be wanted for the repair of accidental losses, and should also contain within itself the latent source of those energies, which may be called into action when demanded by the exigencies of the occasion. Any one of the circumstances above enume- rated would of itself be sufficient to establish the necessity of supplies of nourishment for the maintenance of life. But there are other consi- derations, equally important in a physiological point of view, and derived from the essential nature of organization, which also produce a continual demand for these supplies ; and these I shall now endeavour briefly to explain. Constant and progressive change appears to be one of the leading characteristics of life ; and the materials which are to be endowed with vi- tality must therefore be selected and arranged with a view to their continual modification, cor- 4 THE VITAL FUNCTIONS. responding to these ever varying changes of con- dition. The artificer, whose aim is to construct a machine for permanent use, and to secure it as much as possible from the deterioration arising from friction or other cases of injury," would, of course, make choice for that purpose of the most hard and durable materials, such as the metals, or the denser stones. In constructing a watch, for instance, he would form the wheels of brass, the spring and the barrel-chain of steel ; and for the pivot, where the motion is to be incessant, he would employ the hardest of all materials, — the diamond. Such a machine, once finished, being exempt from almost every natural cause of decay, might remain for an indefinite period in the same state. Far different are the objects which must be had in view in the formation of organized structures. In order that these may be qualified for exercising the functions of life, they must be capable of continual alterations, displacements, and adjustments, varying perpetually, both in kind and in degree, according to the progressive stages of their internal developement, and to the different circumstances which may arise in their external condition. The materials which nature has employed in their construction, are, there- fore, neither the elementary bodies, nor even their simpler and more permanent combinations; but such of their compounds as are of a more plastic nature, and which allow of a variable ORGANIC CHEMISTRY. O proportion of ingredients, and of great diversity in the modes of their combination. So great is the complexity of these arrangements, that althougli chemistry is fully competent to the analysis of organized substances into their ultimate elements, no human art is adequate to effect their reunion in the same state as that in which they had existed in those substances ; for it was by the relined operations of vitality, the only power that could produce this adjustment, that they have been brought into that condition. We may take as an example one of the simplest of organic products, namely Sugar ; a substance which has been analysed with the greatest accu- racy by modern chemists : yet to reproduce this sugar, by the artificial combination of its simple elements, is a problem that has hitherto baffled all the efforts of philosophy. Chemistry, not- withstanding the proud rank it justly holds among the physical sciences, and the noble discoveries with which it has enriched the arts ; notwith- standing it has unveiled to us many of the secret operations of nature, and placed in our hands some of her most powerful instruments for acting upon matter; and notwithstanding it is armed with full powers to destroy, cannot, in any one organic product, rejoin that which has been once dissevered. Through the medium of chemistry we are enabled, perhaps, to form some estimate of the value of what we find executed by other 6 THE VITAL FUNCTIONS. agencies ; but the imitation of the model, even in the smallest part, is far beyond our power. No means which the laboratory can supply, no process, which the most inventive chemist can devise, have ever yet approached those delicate and refined operations which nature silently con- ducts in the organized texture of living plants and animals. The elements of organic substances are not very numerous ; the principal of them being oxygen, carbon, hydrogen, nitrogen, sulphur, and phosphorus, together with a few of the alka- line, earthy, and metallic bases. These sub- stances are variously united, so as to form cer- tain specific compounds, which, although they are susceptible, in different instances, of endless modifications, yet possess such a general cha- racter of uniformity, as to allow of their being arranged in certain classes ; the most character- istic substance in each class constituting what is called a proximate organic principle. Thus in the vegetable kingdom we have Lignin, Tannin, Mucilage, Oil, Sugar, Fecula, &c. The animal kingdom, in like manner, furnishes Gelatin, Albumen, Fibrin, Mucus, Entomoline, Elearin, Stearin, and many others. The chemical constitution of these organic products, formed, as they are, of but few pri- mary elements, is strikingly contrasted with that of the bodies belonging to the mineral ORGANIC CHEMISTRY. 7 kingdom. The catalogue of elementary, or simple bodies, existing in nature, is, indeed, more extensive than the list of those which enter into the composition of animal or vege- table substances. But in the mineral world they occur in simpler combinations, resolvable, for the most part, into a few definite ingredients, which rarely comprise more than two or three elements. In organized products, on the other hand, although the total number of existing elements may be smaller, yet the mode of com- bination in each separate compound is infinitely more complex, and presents incalculable diver- sity. Simple binary compounds are rarely ever met with ; but, in place of these, we find three, four, five, or even a greater number of consti- tuent elements existing in very complicated states of union. This peculiar mode of combination gives rise to a remarkable condition, which attaches to the chemical properties of organic compounds. The attractive forces, by which their several ingredients are held together, being very nume- rous, require to be much more nicely balanced, in order to retain them in combination. Slight causes are sufficient to disturb, or even overset, this equipoise of affinities, and often produce rapid changes of form, or even complete decom- position. The principles, thus retained in a kind of forced union, have a constant tendency 8 THE VITAL FUNCTIONS. to react upon one another, and to produce, from slight variations of circumstances, a totally new order of combinations. Thus a degree of heat, which would occasion no change in most mineral substances, will at once effect the complete dis- union of the elements of an animal or vegetable body. Organic substances are, in like manner, unable to resist the slower, but equally destruc- tive agency of water and atmospheric air ; and they are also liable to various spontaneous changes, such as those constituting fermentation and putrefaction, which occur when their vitality is extinct, and when they are consequently abandoned to the uncontrolled operation of their natural chemical affinities. This tendency to decomposition may, indeed, be regarded as inherent in all organized substances, and as requiring for its counteraction, in the living system, that perpetual renovation of materials which is supplied by the powers of nutrition. It would appear that during the continuance of life, the progress of decay is arrested at its very commencement ; and that the particles, which first undergo changes imfitting them for the exercise of their functions, and which, if suffered to remain, would accelerate the destruction of the adjoining parts, are immediately removed, and their place supplied by particles that have been modified for that purpose, and which, when they afterwards lose these salutary pro- ORGANIC CHEMISTRY. 9 perties, are in their turn discarded and replaced by others. Hence the continued interchange and renewal of particles which take place in the more active organs of the system, especially in the higher classes of animals. In the fabric of those animals which possess an extensive system of circulating and absorbing vessels, the changes that are effected are so considerable and so rapid, that even in the densest textures, such as the bones, scarcely any portion of the sub- stance which originally composed them is per- manently retained in their structure. To so great an extent is this renovation of materials carried on in the human system, that doubts may very reasonably be entertained as to the identity of any portion of the body after the lapse of a certain time. The period assigned by the ancients for this entire change of the sub- stance of the body was seven or eight years : but modern inquiries, which show us the rapid re- paration that takes place in injured parts, and the quick renewal of the bones themselves, tend to prove that even a shorter time than this is adequate to the complete renovation of every portion of the living fabric* Imperfect as is our knowledge of organic chemistrj^ we see enough to convince us that a * See the article *'Age" in the Cyclopaedia of Practical Medicine, where I have enlarged upon this subject. 10 THE VITAL FUNCTIONS. series of the most refined and artificial opera- tions is required in order to bring about the com- plicated and elaborate arrangements of elements which constitute both animal and vegetable products. Thus in the very outset of this, as of every other inquiry in Physiology, we meet with evidences of profound intention and consummate art, infinitely surpassing not only the power and resources, but even the imagination of man. Much as the elaborate and harmonious me- chanism of an animal body is fitted to excite our admiration, there can be no doubt that a more extended knowledge of that series of subtle pro- cesses, consisting of chemical combinations and decompositions which are continually going on in the organic laboratory of living beings, would reveal still greater wonders, and would fill us with a more fervent admiration of the infinite art and prescience which are even now mani- fested to us in every department both of the vegetable and animal economy. The processes by which all these important purposes are fulfilled comprise a distinct class of functions, the final object of which may be termed Nulrition, that is, the reparation of the waste of the substance of the organs, their maintenance in the state fitting them for the exercise of their respective offices, and the appli- cation of properly prepared materials to their developement and growth. PROCESSES OF NUTRITION. 11 The functions subservient to nutrition may be distinguished, according as the processes they comprise relate to seven principal periods in the natural order of their succession. The first series of processes has for its objects the re- ception of the materials from without, and their preparation and gradual conversion into proper nutriment, that is, into matter having the same chemical properties with the substance of the organs with which it is to be incorporated ; and their purpose being to assimilate the food as much as possible to the nature of the organic body it is to nourish, all these functions have been included under the term Assimilation. The second series of vital functions com- prise those which are designed to convey the nutritive fluids thus elaborated, to all the organs that are to be nourished by them. In the more developed systems of organization this purpose is accomplished by means of canals, called vessels^ through which the nutritive fluids move in a kind of circuit : in this case the function is de- nominated the Circulation. It is not enough that the nutritive juices are assimilated ; another chemical process is still required to perfect their animalization, and to retain them in their proper chemical condition for the purposes of the system. This third object is accomplished by the function of Respiration. Fourthly, several chemical products which are 12 THE VITAL FUNCTIONS. wanted in different parts of the economy, are required to be formed by a peculiar set of organs, of which the intimate structure ekides observa- tion ; although we may perceive that in many instances, among the higher orders of beings, a special apparatus of vessels, sometimes spread over the surface of a membrane, at other times collected into distinct masses, is provided for that purpose. These specific organs are termed glands, and the office performed by them, as well as by the simpler forms of structure above mentioned, is termed Secretion. Fifthly, similar processes of secretion are also employed to carry off from the blood such animal products as may have been formed or introduced into it, and may possess or have acquired noxious properties. The elimination of these materials, which is the office of the excretories, constitutes the function of Excretion. Sixthly, changes may take place in various parts of the body, both solid and fluid, rendering them unfit to remain in their present situation, and measures must be taken for the removal of these useless or noxious materials, by transferring them to the general mass of circulating blood, so as either to be again usefully employed, or altogether discarded by excretion from the system. This object is accomplished by a peculiar set of vessels ; and the function they perform is termed Absorption. POWERS OF ASSIMILATION. 13 Lastly, the conversion of the fluid nutriment into the solids of the body, and its immediate application to the purposes of the developement of the organs, of their preservation in the state of health and activity, and of the repair of such injuries as they may chance to sustain, as far as the powers of the system are adequate to such reparation, are the objects of a seventh set of functions, more especially comprised under the title of Nutrition, which closes this long series of chemical changes, and this intricate but har- monious system of operations. Although the order in which the constituent elements of organized products are arranged, and the mode in which they are combined, are entirely unknown to us, we can nevertheless perceive that in following them successively from the simplest vegetables to the higher orders of the animal kingdom, they acquire continually increasing degrees of complexity, corresponding, in some measure, to the greater refinement and complication of the structures by which they have been elaborated, and of the bodies to which they are ultimately assimilated. Thus plants derive their nourishment from the crude and simple materials which they absorb from the earth, the waters, and the air that surround them ; mate- rials which consist almost wholly of water, with a small proportion of carbonic acid, and a few saline ingredients, of which that water is the 14 THE VITAL FUNCTIONS. vehicle. But these, after having been converted by the powers of vegetable assimilation, into the substance of the plant, acquire the charac- teristic properties of organized products, though they are still the simplest of that class. In this state, and when the fabric they had composed is destroyed, and they are scattered over the soil, they are fitted to become more highly nutritive to other plants, which absorb them, and with more facility adapt them to the purposes of their own systems. Here they receive a still higher degree of elaboration ; and thus the same mate- rials may pass through several successive series of modifications, till they become the food of ani- mals, and are then made to undergo still further changes. New elements, and in particular nitrogen, is added to the oxygen, hydrogen and carbon, which are the chief constituents of vegetable substances : * and new properties are acquired, from the varied combinations into which their elements are made to enter by the more energetic powers of assimilation apper- taining to the animal system. The products which result are still more removed from their original state of inorganic matter: and in this condition they serve as the appropriate food of * Nitrogen, however, frequently enters into the composition of vegetables : though in general, in a much smaller proportion than into the substance of animals, of which last it always ap- pears to be an essential constituent. VEGETABLE NUTRITION. 15 carnivorous animals, which generally hold a higher rank in the scale of organization, than those that subsist only on vegetables. Thus has each created being been formed in reference, not merely to its own welfare, but also to that of multitudes of others which are dependent on it for their support, their preser- vation,— nay, even for their existence. In con- templating this mutual relationship, this suc- cessive subordination of the different races to one another, and this continual tendency to increased refinement, we cannot shut our eyes to the mag- nificent unfolding of the great scheme of nature for the progressive attainment of higher objects ; nntil, in the perfect system, and exalted endow- ments of man, we behold the last result that has been manifested to us of creative power. Chapter II. nutrition in vegetables. § 1 . Food of Plants. The simplest kind of nutrition is that presented to us by the vegetable kingdom, where water may be considered as the general vehicle of the nutriment received. Before the discoveries of 16 THK VITAL FUNCTIONS. modern chemistry it was very generally believed that plants could subsist on vrater alone ; and Boyle and Van Helmont in particular endea- voured to establish by experiment the truth of this opinion. The latter of these physiologists planted a willow in a certain quantity of earth, the weight of which he had previously ascer- tained with great care ; and during five years, he kept it moistened with rain water alone, which he imagined was perfectly pure. At the end of this period he found that the earth had scarcely diminished in weight, while the willow had grown into a tree, and had acquired an ad- ditional weight of one hundred and fifty pounds : whence he concluded that the water had been the only source of its nourishment. But it does not seem to have been at that time known that rain water always contains atmospheric air, and frequently also other substances, and that it cannot, therefore, be regarded as absolutely pure water : nor does it appear that any precautions were taken to ascertain that the water actually employed was wholly free from foreign matter, which it is easy to conceive it might have held in solution. In an experiment of Duhamel, on the other hand, a horse-chesnut tree and an oak, exposed to the open air, and watered with distilled water alone, the former for three, and the latter for eight years, were kept alive, indeed, but they were exceedingly stinted in their growth, FOOD OF PLANTS. 17 and evidently derived little or no sustenance from the water with which they were supplied. Experiments of a similar nature were made by Bonnet, and with the like result. When plants are contained in closed vessels, and regularly supplied with water, but denied all access to carbonic acid gas, they are developed only to a very limited extent, determined by the store of nutritious matter which had been already col- lected in each plant when the experiment com- menced, and which, by combining with the water, may have afforded a temporary supply of nourishment. But the water which nature furnishes to the vegetable organs is never perfectly pure ; for, be- sides containing air, in which there is constantly a certain proportion of carbonic acid gas, it has always acquired by percolation through the soil various earthy and saline particles, together with materials derived from decayed vegetable or animal remains. Most of these substances are soluble, in however minute a quantity, in water : and others, finely pulverized, may be suspended in that fluid, and carried along with it into the vegetable system. It does not appear, however, that pure carbon is ever admitted, for Sir H. Davy, on mixing charcoal, ground to an impalpable powder, with the water into which the roots of mint were immersed, could not discover that the smallest quantity of that substance had VOL. II. c 18 THE VITAL FUNCTIONS. been, in any case, absorbed.* But in the form of carbonic acid, this element is received in great abundance, through the medium of water, which readily absorbs it : and a considerable quantity of carbon is also introduced into the fluids of the plant, derived from the decom- posed animal and vegetable materials, which the water generally contains. The peculiar fertility of each kind of soil depends principally on the quantity of these organic products it contains in a state capable of being absorbed by the plant, and of contributing to its nourishment. The soil is also the source whence plants derive their saline, earthy, and metallic ingredients. The silica they often contain is, in like manner, conveyed to them by the water, which it is now well ascertained, by the researches of Berzilius, is capable of dissolving a very minute quantity of this dense and hard substance. It is evident that, however small this quantity may be, if it continue to accumulate in the plant, it may in time constitute the whole amount of that which is found to be so copiously deposited on the sur- face, or collected in the interior of many plants, such as the bamboo, and various species of grasses. The small degree of solubility of many substances thus required for the construction of the solid vegetable fabric, is, probably, one of the reasons why plants require so large a supply of water for their subsistence. * Elements of Agricultural Chemistry, Lect. VI. p. 234. VEGETABLE ABSORPTION. 19 § 2. Absorption of Nutriment by Plants. The greater number of cellular plants absorb water with nearly equal facility from every part of their surface : this is the case with the Algce, for instance,which are aquatic plants. In Lichens, on the other hand, absorption takes place more partially ; but the particular parts of the surface where it occurs are not constantly the same, and appear to be determined more by mechanical causes than by any peculiarity of structure : some, however, are found to be provided in certain parts of the surface with stomata, which De Candolle supposes may act as sucking orifices. Many mushrooms appear to be capable of ab- sorbing fluids from all parts of their surface indiscriminately ; and some species, again, are furnished at their base with a kind of radical fibrils for that purpose. In plants having a vascular structure, which is the case in by far the greater number, the roots are the special organs to which this office of absorbing nourishment is assigned : but it occasionally happens that, under certain circum- stances, the leaves, or the stems of plants are found to absorb moisture, which they have been supposed to do by the stomata interspersed on their surface. This, however, is not their natural action ; and they assume it only in forced situa- 20 THE VITAL FUNCTIONS. tions, when they procure no water by means of the roots, either from having been deprived of these organs, or from their being left totally dry. Thus a branch, separated from the trunk, may be preserved from withering for a long time, if the leaves be immersed in water : and when the soil has been parched by a long drought, the drooping plants will be very quickly revived by a shower of rain, or by artificial watering, even before any moisture can be supposed to have penetrated to the roots. It is by the extremities of the roots alone, or rather by the spongioles which are there situ- ated, that absorption takes place : for the surface of the root, being covered in every other part by a layer of epidermis, is incapable of performing this office. It was long ago remarked by Du- hamel, that trees exhaust the soil only in those parts which surround the extremities of the roots : but the fact that absorption is effected only at those points has been placed beyond a doubt by the direct experiments of Sennebier, who, taking two carrots of equal size, immersed in water the whole root of the one, while only the extremity of the otlier was made to dip into the water, and found that equal quantities were absorbed in both cases ; while on immersing the whole surface of another carrot in the fluid, with the exception of the extremity of the root, which was raised so as to be above the surface, no ab- VEGETABLE ABSORPTION. 21 sorption whatever took place. Plants having a fusiform, or spindle-shaped root, such as the carrot and the radish, are the best for these ex- periments. In the natural progress of growth, the roots are constantly shooting forwards in the direction they have first taken, whether horizontally, or vertically, or at any other inclination. Thus they continually arrive at new portions of soil, of which the nutritive matter has not yet been exhausted ; and as a constant relation is pre- served between their lateral extension and the horizontal spreading of the branches, the greater part of the rain which falls upon the tree, is made to drop from the leaves at the exact dis- tance from the trunk, where, after it has soaked tlu-ough the earth, it will be received by the ex- tremities of the roots, and readily sucked in by the spongioles. We have here a striking instance of that beautiful correspondence, which has been established between processes belonging to diffe- rent departments of nature, and which are made to concur in the production of remote effects, that could never have been accomplished without these preconcerted and harmonious adjustments. The spongioles, or absorbing extremities of the roots, are constructed of ordinary cellular or spongy tissue : and they imbibe the fluids, which are in contact with them, partly by capillary action, and partly, also, by what has been termed •22 * THE VITAL FUNCTIONS. a hygroscopic power. But though these principles may sufficiently account for the simple entrance of the fluids, they are inadequate to explain its continued ascent through the substance of the root, or along the stem of the plant. The most probable explanation of this phenomenon is that the progressive movement of the fluid is produced by alternate contractions and dilatations of the cells themselves, which compose the texture of the plant ; these actions being themselves refer- able to the vitality of the organs. The absorbent power of the spongioles is limited by the diameter of their pores, so that fluids which are of too viscid or glutinous a con- sistence to pass readily through them are liable to obstruct or entirely block up these passages. Thus if the spongioles be surrounded by a thick solution of gum, or even of sugar, its pores will be clogged up, scarcely any portion of the fluid will be absorbed, and the plant will wither and perish : but if the same liquids be more largely diluted, the watery portion will find its way through the spongioles, and become available for the sustenance of the plant, while the greater part of the thicker material will be left behind. The same apparent power of selection is exhibited when saline solutions of a certain strength are presented to the roots : the water of the solution, with only a small proportion of the salts, being- taken up, and the remaining part of the fluid VEGETABLE ABSORPTION. 23 being found to be more strongly impregnated with the salts than before this absorption had taken place. It would appear, however, that all this is merely the result of a mechanical opera- tion, and that it furnishes no evidence of any discriminating faculty in the spongiole : for it is found that, provided the material presented be in a state of perfect solution and limpidity, it is sucked in with equal avidity, whether its qualities be deleterious or salubrious. Solutions of sul- phate of copper, which is a deadly poison, are absorbed in large quantities by the roots of plants, which are immersed in them : and water which drains from a bed of manure, and is consequently loaded with carbonaceous particles, proves ex- ceedinglyinjurious when admitted into the system of the plant, from the excess of nutriment it con- tains. But in the ordinary course of vegetation, no danger can arise from this general power of absorption, since the fluids which nature supplies are always such as are suitable to the organs that are to receive them. The fluid, which is taken up by the roots, and which, as we have seen, consists chiefly of water, holding in solution atmospheric air, together with various saline and earthy ingredients neces- sary for the nourishment of the plant, is in a perfectly crude state. It rises in the stem of the plant, undergoing scarcely any perceptible change in its ascent; and is in this state conducted 24 THE VITAL FUNCTIONS. to the leaves, where it is to experience various important modifications. By causing the roots to imbibe coloured liquids, the general course of the sap has been traced with tolerable accuracy, and it is found to traverse principally the ligneous substance of the stem : in trees, its passage is chiefly through the alburnum, or more recently formed wood, and not through the bark, as was at one time believed. The course of the sap, however, varies under different circumstances, and at different epochs of vegetation. At the period when the young buds are preparing for their developement, which usually takes place when the genial warmth of spring has penetrated beyond the surface, and expanded the fibres and vessels of the plant, there arises an urgent demand for nourishment, which the roots are actively employed in sup- plying. As the leaves are not yet completed, the sap is at first applied to purposes somewhat different from those it is destined to fulfil at a more advanced period, when it has to nourish the fully expanded organs : this fluid has, ac- cordingly, received a distinct appellation, being termed the nursling sap. Instead of rising through the alburnum, the nursling sap ascends through the innermost circle of wood, or that which is immediately contiguous to the pith, and is thence transmitted, by unknown channels, through the several layers of wood, till it reaches ASCENT OF THE SAP. 25 the buds, which it is to supply with nourishment. During this circuitous passage, it probably un- dergoes a certain degree of elaboration, fitting it for the office which it has to perform : it appa- rently combines with some nutriment, which had been previously deposited in the plant, and which it again dissolves ; and thus becoming assimilated,, is in a state proper to be incorporated with the new organization that is developing. This nurs- ling sap, provided for the nourishment of the young buds, has been compared to the milk of animals, which is prepared for a similar purpose at those times only when nutriment is required for the rearing of their young. Several opinions have been entertained with regard to the channels through which the sap is conveyed in its ascent along the stem, and in its passage to its ultimate destination. Many observations tend to show, that, in ordinary cir- cumstances, it is not transmitted through any of the distinguishable vessels of the plant : for most of these, in their natural state, are found to con- tain only air. The sap must, therefore, either traverse the cells themselves, or pass along the intercellular spaces. That the latter is the course it takes is the opinion of De Candolle, who adduces a variety of arguments in its sup- port. The sap, he observes, is found to rise equally well in plants whose structure is wholly cellular ; a fact which proves the vessels are not 26 THE VITAL FUNCTIONS. in all cases necessary for its conveyance. In many instances the sap is known to deviate from its usual rectilinear path, and to pursue a cir- cuitous course, very different from that of any of the known vessels of the plant. The diffusion of the sap in different directions, and its sub- sidence in the lowest parts, on certain occasions, are facts irreconcileable with the supposition that it is confined in these vessels. Numerous experiments have been made to discover the velocity with which the sap rises in plants, and the force it exerts in its ascent. Those of Hales are well known : by lopping off the top of a young vine, and applying to the truncated extremity a glass tube, which closed round it, he found that the fluid in the tube rose to a height, which, taking into account the specific gravity of the fluid, was equivalent to a perpendi- cular column of water of more than forty-three feet; and consequently exerted a force of propul- sion considerably greater than the pressure of an additional atmosphere. The velocity, as well as the force of ascent, must, however, be liable to great variation ; being much influenced by eva- poration, and other changes, which the sap undergoes in the leaves. Various opinions have been entertained as to the agency by which the motion of the sap is effected ; but although it seems likely to be resolved into the vital move- ments of the cellular structure already mentioned, VEGETABLE EXHALATION. 27 the question is still enveloped in considerable obscurity. There is certainly no evidence to prove that it has any analogy to a muscular power; and the simplest supposition we can make is that these actions take place by means of a contractile property belonging to the vege- table tissue, and exerted, under certain circum- stances, and in conformity to certain laws, which we have not yet succeeded in determining. '§ .3. Exhalation. The nutrient sap, which, as we have seen, rises in the stem, and is transmitted to the leaves without any change in its qualities or compo- sition, is immediately, by the medium of the stomata, or orifices which abound in the surface of those organs, subjected to the process of exhalation. The proportion of water which the sap loses by exhalation in the leaves is generally about two-thirds of the whole quantity received ; so that it is only the remaining third that returns to nourish the organs of the plant. It has been ascertained that the water thus evaporated is perfectly pure ; or at least does not contain more than a 10,000,000th part of the foreign matter with which it was impregnated when first ab- sorbed by the roots. The water thus exhaled, 28 THE VITAL FUNCTIONS. being dissolved by the air the moment it escapes, passes off in the form of invisible vapour. Hales made an experiment with a sun -flower, three feet high, enclosed in a vessel, which he kept for fifteen days : and inferred from it that the daily loss of the plant by exhalation was twenty ounces ; and this he computes is a quantity seventeen times greater than that lost by insensible perspi- ration from an equal portion of the surface of the human body. The comparative quantities of fluid exhaled by the same plant at different times are regu- lated, not so much by temperature, as by the intensity of the light to which the leaves are exposed. It is only during the day, therefore, that this function is in activity. De Candolle has found that the artificial light of lamps pro- duces on the leaves an efl'ect similar to that of the solar rays, and in a degree proportionate to its intensity.* As it is only through the stomata that exhalation proceeds, the number of these pores in a given surface must considerably in- fluence the quantity of fluid exhaled. By the loss of so large a portion of the water which, in the rising sap, had held in solution various foreign materials, these substances are rendered more disposed to separate from the fluid, and to become consolidated on the sides * Physiologie Vegetale, i. 112. AERATION OF TflE SAP. 29 of the cells or vessels, to which they are con- ducted from the leaves. This, then, is the first modification in the qualities of the sap which it undergoes in those organs. § 4. Aeration of the Sap. A CHEMICAL change much more considerable and important than the preceding is next effected on the sap by the leaves, when they are subjected to the action of light. It consists in tJie decom- position of the carbonic acid gas, which is either brought to them by the sap itself, or obtained directly from the surrounding atmosphere. In either case its oxygen is separated, and is dis- engaged in the form of gas ; while its carbon is retained, and composes an essential ingredient of the altered sap, which, as it now possesses one of the principal elements of vegetable structures, may be considered as having made a near ap- proach to its complete assimilation, using this term in the physiological sense already pointed out. The remarkable discovery that oxygen gas is exhaled from the leaves of plants during the day time, was made by the great founder of pneumatic chemistry. Dr. Priestley : to Senne- bier we are indebted for the first observation 30 THE VITAL FUNCTIONS. that the presence of carbonic acid is required for the disengagement of oxygen in this process, and that the oxygen is derived from the decom- position of the carbonic acid ; and these latter facts have since been fully established by the researches of Mr. Woodhouse, of Pensylvania, M. Theodore de Saussure, and Mr. Palmer. They are proved in a very satisfactory manner by the following experiment of De CandoUe. Two glass jars were inverted over the same water-bath ; the one filled with carbonic acid gas, the other filled with water, containing a sprig of mint ; the jars communicating below by means of the water-bath, on the surface of which some oil was poured, so as to intercept all com- munication between the water and the atmosphere. The sprig of mint was exposed to the light of the sun for twelve days consecutively : at the end of each day the carbonic acid was seen to dimi- nish in quantity, the water rising in the jar to supply the place of what was lost, and at the same time the plant exhaled a quantity of oxygen exactly equal to that of the carbonic acid which had disappeared. A similar sprig of mint, placed in ajar of the same size, full of dis- tilled water, but without having access to carbonic acid, gave out no oxygen gas, and soon perished. When, in another experiment, conducted by means of the same apparatus as was used in the first, oxygen gas was substituted in the first jar AERATION OF THE SAP. 31 instead of carbonic acid gas, no gas was disen- gaged in the other jar, which contained a sprig of mint. It is evident, therefore, that the oxygen gas obtained from the mint in the first experi- ment was derived from the decomposition, by the leaves of the mint, of the carbonic acid, which the plant had absorbed from the water. Solar light is an essential agent in effecting this chemical change ; for it is never found to take place at night, nor while the plant is kept in the dark. The experiments of Sennebier would tend to show that the violet, or most re- frangible of the solar rays have the greatest power in determining this decomposition of car- bonic acid : but the experiments are of so deli- cate a nature, that this result requires to be con- firmed by a more rigid investigation, before it can be admitted as satisfactorily established. That the carbon resulting from this decompo- sition of carbonic acid is retained by the plant, has been amply proved by the experiments of M. Theodore de Saussure, who found that this process is attended with a sensible increase in the quantity of carbon which the plant had j^re- viously contained. It is in the green substance of the leaves alone that this process is conducted : a j^rocess, which, from the strong analogy that it bears to a similar function in animals, may be considered as the respiration of vegetables. The effect appears to •i'Z THE VITAL FUNCTIONS. be proportionate to the number of stomata which the plant contains. It is a process which takes place only in a living plant ; for if a leaf be bruised so as to destroy its organization, and consequently its vitality, its substance is no longer capable either of decomposing carbonic acid gas under the influence of solar light, or of absorbing oxygen in the dark. Neither the roots, nor the flowers, nor any other parts of the plant, which have not this green substance at their surface, are capable of decomposing carbonic acid gas : they produce, indeed, an effect which is in some respects the opposite of this ; for they have a tendency to absorb oxygen, and to convert it into carbonic acid, by uniting it with the carbon they themselves contain. This is also the case with the leaves themselves, whenever they are not under the influence of light : thus, during the whole of the night, the same leaves, which had been exhaling oxygen during the day, ab- sorb a portion of that element. The oxygen thus absorbed enters immediately into combina- tion with the carbonaceous matter in the plant, forming with it carbonic acid : this carbonic acid is in part exhaled ; but the greater portion either remains attached to the substance of the leaf, or combines with the fluids which constitute the sap : in the latter case it is ready to be again presented to the leaf, when daylight returns, and when a fresh decomposition is again effected. AERATION OF THE SAP. .33 This reversal at night of what was done in the day may, at first sight, appear to be at variance with the unity of plan, which we should ex- pect to find preserved in the vegetable economy : but a more attentive examination of the process will show that the whole is in perfect harmony, and that these contrary processes are both of them necessary, in order to produce the result intended. The water which is absorbed by the roots generally carries with it a certain quantity of soluble animal or vegetable materials, which con- tain carbon. This carbon is transmitted to the leaves, where, during the night, it is made to combine with the oxygen they have absorbed. It is thus converted into carbonic acid, which, when daylight prevails, is decomposed ; the oxygen being dissipated, and the carbon retained. It is evident that the object of the whole process is to obtain carbon in that precise state of disin- tegration, to which it is reduced at the moment of its separation from carbonic acid by the action of solar light on the green substance of the leaves ; for it is in this state alone that it is avail- able in promoting the nourishment of the plant, and not in the crude condition in which it exists when it is pumped up from the earth, along with the v/ater which conveys it into the interior of the plant. Hence the necessity of its having to undergo this double operation of first combining VOL. II. D 34 THE VITAL FUNCTIONS. with oxygen, and then being precipitated from its combination in the manner above described. It is not the whole of the carbon introduced into the vegetable system, in the form of carbonic acid, which has to undergo the first of these changes, a part of that carbon being already in the condition to which that operation would re- duce it, and consequently in a state fit to receive the decomposing action of the leaves. The whole of these chemical changes may be included under the general term Aeration. Thus the great object to be answered by this vegetable aeration is exactly the converse of that which we shall afterwards see is effected by the respiration of animals : in the former it is that of adding carbon, in an assimilated state, to the vegetable organization ; in the latter, it is that of discharging the superfluous quantity of carbon from the animal system. The absorption of oxygen, and the partial disengagement of carbonic acid, which constitute the nocturnal changes effected by plants, must have a tendency to deteriorate the atmosphere with respect to its capability of supporting animal life ; but this effect is much more than compensated by the greater quantity of oxygen given out by the same plants during the day. On the whole, therefore, the atmosphere is continually receiving from the vegetable kingdom a large accession of oxygen, and is, at the same time, freed from an equal portion of carbonic acid gas, both of which effects AERATION OF THE SAP. 35 tend to its purification and to its remaining adapted to the respiration of animals. Nearly the whole of the carbon accumulated by vege- tables is so much taken from the atmosphere, which is the primary source from which they derive that element. At the season of the year when vegetation is most active, the days are longer than the nights ; so that the diurnal pro- cess of purification goes on for a greater number of hours than the nocturnal process by which the air is vitiated. The oxygen given out by plants, and the car- bonic acid resulting from animal respiration, and from the various processes of combustion, which are going on in every part of the world, are quickly spread through the atmosj^here, not only from the tendency of all gases to uniform diffusion, but also from the action of the winds, which are continually agitating the whole mass, and promoting the thorough mingling of its dif- ferent portions, so as to render it perfectly ho- mogeneous in every region of the globe, and at every elevation above the surface. Thus are the two great organized kingdoms of the creation made to co-operate in the execution of the same design : each ministering to the other, and preserving that due balance in the constitution of the atmosphere, which adapts it to the welfare and activity of every order of beings, and which would soon be destroyed, were the operations of any one of them to be sus- 36 ^ THE VITAL FUNCTIONS. pended. It is impossible to contemplate so special an adjustment of opposite effects without admiring this beautiful dispensation of Provi- dence, extending over so vast a scale of being, and demonstrating the unity of plan on which the whole system of organized creation has been devised. § 5. Return of the Sap. The sap, which, during its ascent from the roots, contains but a small proportion of nutritious par- ticles, diluted with a large quantity of water, after undergoing in the leaves, as in a chemical laboratory, the double processes of exhalation and aeration, has become much more highly charged with nutriment ; and that nutriment has been reduced to those particular forms and states of composition which render it applicable to the growth of the organs, and the other purposes of vegetable life. This fluid, therefore, corresponds to the blood of animals, which, like the elaborated sap, may be regarded as fluid nutriment, per- fectly assimilated to that particular kind of or- ganization, with which it is to be afterwards in- corporated. From the circumstance of its being sent back from the leaves for distribution to the several organs where its presence is required, it has received the name of the returning sap, that it miglit be distinguished from the crude fluid RETURN OF THE SAP. 37 which arrives at the leaves, and which is termed the asceiiclirig saj). The returning sap still contains a considerable quantity of water, in its simple liquid form; which was necessary in order that it might still be the vehicle of various nutritive materials that are dissolved in it. It appears, however, that a large proportion of the water, which was not ex- haled by the leaves, has been actually decom- posed, and that its separated elements, the oxygen and the hydrogen, have been combined with certain proportions of carbon, hydrogen, nitrogen, and various earths, metals, and salts, so as to form the proximate vegetable products, which are found in the returning sap. The simplest, and generally the most abundant of these products, is that which is called Gum* From the universal presence of this substance in the vegetable juices, and more especially in the returning sap, of all known plants, from its bland and unirritating qualities, from its great solubility in water, and from the facility with which other vegetable products are convertible into this product. Gum may be fairly assumed * According to the investigations of Dr. Prout, 1000 grains of gum are composed of 586 grains of the elements of water, that is, of oxygen and hydrogen, in the exact proportions in which they would have united to form 586 grains of water ; together with 414 of carbon, or the base of carbonic acid. This, accord- ing to the doctrine of chemical equivalents, corresponds to one molecule of water, and one molecule of carbon. Phil. Trans. 1827, 584. 38 THE VITAL FUNCTIONS. to be the principal basis of vegetable nutriment ; and its simple and definite composition points it out as being the immediate result of the che- mical changes which the sap experiences in the leaves. During the descent of the sap, however, this fluid undergoes, in various parts of the plant, a further elaboration, which gives rise to other products. We are now, therefore, to follow it in its progress through the rest of the vegetable system. The returning sap descends from the leaves through two different structures : in exogenous plants the greater portion finds a ready passage through the liber, or innermost layer of bark, and another portion descends through the albur- num, or outermost layer of the wood. With re- gard to the exact channels through which it passes, the same degree of uncertainty prevails as with regard to those which transmit the as- cending sap. De Candolle maintains that, in either case, the fluids find their way through the intercellular spaces : other physiologists, how- ever, are of opinion, that particular vessels are appropriated to the office of transmitting the des- cending sap. The extreme minuteness of the organs of vegetables has hitherto presented insuperable obstacles to the investigation of this important question : and consequently our rea- sonings respecting it can be founded only on indirect evidence. The processes of the animal RETURN OF THE SAP. 39 economy, where the channels of distribution, and the organs of propulsion are plainly obser- vable, afford but imperfect analogies to guide us in this intricate inquiry : for although it is true that in the higher classes of animals the circula- tion of the nutrient fluid, or blood, through dis- tinct vessels, is sufficiently obvious, yet in the lower departments of the animal kingdom, and in the embryo condition even of the more perfect species, the nutritious juices are distributed with- out being confined within any visible vessels ; and they either permeate extensive cavities in the interior of the body, or penetrate through the interstices of a cellular tissue. That this latter is the mode of transmission adopted in the vege- table system has been considered probable, from the circumstance that the nutritious juices are diffused throughout those plants w hich contain no vessels whatsoever with the same facility as throughout those which possess vessels ; from which it has been concluded that vessels are not absolutely necessary for the performance of this function. The nature of the forces which actuate the sap in its descent from the leaves, and its distribu- tion to different parts, is involved in equal ob- scurity with the nature of the powers which contribute to its motion upwards along the stem, from the roots to the leaves. In endogenous plants the passage of the sap in its descent, is, in like manner, through those parts which have 40 THE VITAL FUNCTIONS. been latest formed ; that is, through the inner- most layers of their structm^e. The returning sap, while traversing these se- veral parts of the plant, deposits in each the par- ticular materials which are requisite for their growth, and for their maintenance in a healthy condition. That portion which flows along the liber, not meeting with any ascending stream of fluid, descends without impediment to the roots, to the extension of which, after it has nourished the inner layer of bark, it particularly contri- butes : that portion, on the other hand, which descends along the alburnum, meets with the stream of ascending sap, w hich, during the day at least, is rising with considerable force. A certain mixture of these fluids probably now takes place, and new modifications are in conse- quence produced, which, from the intricacies of the chemical processes thus conducted in the inner recesses of vegetable organization, we are utterly baffled in our attempts to follow. All that we are permitted to see are the general re- sults, namely the gradual deposition of the mate- rials of the future alburnum and liber. These materials are first deposited in the form of a layer of glutinous substance, termed the Cam- bium; a substance which appears to consist of the solid portion of the sap, precipitated from it by the separation of the greater part of the w ater that held it in solution. The cambium becomes in process of time more and more consolidated, RETURN OF THE SAP. 41 and acquires the organization proper to the plant of which it now forms an integrant part : it con- stitutes two layers, the one, belonging to the wood, being the alburnum ; the other, belonging to the bark, being the liber. The alburnum and the liber, which have been thus constructed, perform an important part in in- ducing ulterior changes on the nutrient materials which the returning sap continues to supply. Their cells absorb the gummy substance from the surrounding fluid, and by their vital powers effect a still further elaboration in its compo- sition ; converting it either into starch, or sugar, or lignin, according to the mode in which its constituent elements are arranged. Although these several principles possess very different sensible properties, yet they are found to differ but very slightly in the proportions of their in- gredients ; and we may infer that the real che- mical alterations, which are required in order to effect these conversions, are comparatively slight, and may readily take place in the simple cellular tissue.* In the series of decompositions which are arti- * According to the analyses of Dr. Prout, the following is the composition of these substances : 1000 parts of Pure Gum Arabic consist of 586 of oxygen and hydrogen, united in the proportions in which they exist in water, and 414 of carbon. Dried Starch or Fecula of 560 water, and 440 carbon. Pure crystallized Sugar . . 572 428 Lignin from Boxwood . . . 500 500 42 THE VITAL FUNCTIONS. ficially effected in the laboratory of the chemist, it has been found that gum and sugar are inter- mediate products, or states of transition between various others ; and they appear to be peculiarly calculated, from their great solubility, for being easily conveyed from one organ to another. Starch, and lignin, on the other hand, are compounds of a more permanent character, and especially adapted for being retained in the organs. Starch which, though solid, still possesses considerable solubility, is peculiarly fitted for being applied to the purposes of nourishment : it is accord- ingly hoarded in magazines, with a view to future employment, being to vegetables, what the fat is to annuals, a resource for the exigencies that may subsequently arise. With this inten- tion, it is carefully stored in small cells, the coats of which protect it from the immediate dissolving action of the surrounding watery sap, but allow of the penetration of this fluid, and of its solution, when the demands of the system require it. The tuberous root of the potatoe, that invaluable gift of Providence to the human race, is a remark- able example of a magazine of nutritive matter of this kind. The lignin, on the contrary, is deposited with the intention of forming a permanent part of the vegetable structure, constituting the basis of the woody fibre, and giving mechanical support and strength to the whole fabric of the plant. These RETURN OF THE SAP. 43 latter structures may be compared to the bones of animals, composing by their union the solid frame work, or skeleton of the organized system. The woody fibres do not seem to be capable of farther alteration in the living vegetable, and are never, under any circumstances, taken up and removed to other parts of the system, as is the case with nutritive matter of a more conver- tible kind. The sap holds in solution, besides carbona- ceous matter, some saline compounds and a few earthy and metallic bases : bodies which, in how- ever minute a quantity they may be present, have unquestionably a powerful influence in determining certain chemical changes among the elements of organic products, and in im- parting to them peculiar properties ; for it is now a well ascertained fact that a scarcely sensible portion of any one ingredient is capable of pro- ducing important differences in the properties of the whole compound. An example occurs in the case of gold, the ductility of which is totally destroyed by the presence of a quantity of either antimony or lead, so minute as barely to amount to the two thousandth part of the mass ; and even the fumes of antimony, when in the neighbour- hood of melted gold, have the power of destroy- ing its ductility.* In the experiments made by * Hatchett. 44 THE VITAL FUNCTIONS. Sir John Herschel on some remarkable motions excited in fluid conductors by the transmission of electric currents, it was found that minute portions of calcareous matter, in some instances less than the millionth part of the whole com- pound, are sufficient to communicate sensible mechanical motions, and definite properties to the bodies with which they are mixed.* As Silica is among the densest and least soluble of the earths, we might naturally expect that any quantity of it taken into the vegetable system in a state of solution, would very early be precipitated from the sap, after the exhala- tion of the water which held it dissolved ; and it is found, accordingly, that the greater portion of this silica is actually deposited in the leaves, and the parts adjacent to them. When once deposited, it seems incapable of being again taken up, and transferred to other parts, or ejected from the system : and hence, in course of time, a considerable accumulation of silicious particles takes place, and by clogging up their cells and vessels, tends more and more to ob- struct the passage of nourishment into these organs. This change has been assigned as a principal cause of the decay and uhimate des- truction of the leaves : their foot-stalks, more especially suffering from this obstruction, perish, * Philosophical Transactions for 1824, p. 162. VEGETABLE SECRETION. 45 and occasion the detachment of the leaves, which thus fall off at the end of each season, making way for those that are to succeed them in the next. § 6. Secretion in Vegetables. While the powers of the simpler kinds of cells are adequate to produce in the returning sap the modifications above described, by which it is converted into gummy, saccharine, amylaceous, or ligneous products ; there are other celhdar organs, endowed with more extensive powers of chemical action, which effect still greater changes. The nature of the agents by which these changes are produced are unknown, and are therefore referred generally to the vital energies of vege- tation ; but the process itself has been termed Secretion, and the organs in which it is conducted, and which are frequently very distinguishable as separate and peculiar structures, are called Glands. When the products of secretion are chemically analysed, the greater number are found to contain a large quantity of hydrogen, in addition to that which is retained in combi- nation with oxygen as the representative of water : this is the case with all the oily secre- tions, whether they be fixed or volatile, and also with those secretions which are of a resinous 46 THE VITAL FUNCTIONS. quality. Some, on the contrary, are found to have an excess of oxygen ; and this is the condi- tion of most of the acid secretions ; while others, again, appear to have acquired an addition of nitrogen. All these substances have their respective uses, although it may frequently be difficult to assign them correctly. Some are intended to remain per- manently inclosed in the vesicles where they were produced ; others are retained for the purpose of being employed at some other time ; while those belonging to a third class are destined to be thrown off from the system as being superfluous or noxious : these latter substances, which are presently to be noticed, are specially designated as excretions. Many of these fluids find their way from one part of the plant to another, with- out appearing to be conducted along any definite channels, and others are conveyed by vessels, which appear to be specially appropriated to this office. The following are examples of the uses to which the peculiar secretions of plants are applied. Many lichens, which fix themselves on calcareous rocks, such as the Patellaria immersa, are ob- served, in process of time, to sink deeper and deeper beneath the surface of the rock, as if they had some mode of penetrating into its sub- stance, analogous to that which many marine worms are known to possess. The agent appears VEGETABLE SECRETIONS. 47 in both instances to be an acid, which here is probably the oxalic, acting upon the carbonate of lime, and producing the gradual excavation of the rock. This view is confirmed by the ob- servation that the same species of lichen, when attached to rocks which are not calcareous, re- mains always at the surface, and does not pene- trate below it. A caustic liquor is sometimes collected in vesicles, situated at the base of slender hairs, having a canal which conducts the fluid to the point. This is the case with the Nettle. The slightest pressure made by the hand on the hairs growing on the leaves of this plant, causes the fluid in their vesicles to pass out from their points, so as to be instilled into the skin, and occasion the well known irritation which ensues. M. De Candolle junior has ascertained by chemical tests that the stinging fluid of the nettle is of an alkaline nature. In some species of this genus of plants, the hairs are so large that the whole mechanism above described is visible to the naked eye. This apparatus bears a striking re- semblance to that which exists in the poisonous teeth of serpents, and which is hereafter to be described. As the resinous secretions resist the action of water, we find them often employed by nature as a means of effectually defending the young buds from the injurious effects of moisture ; and 48 THE VITAL FUNCTIONS. for a similar purpose we find the surface of many plants covered with a varnish of wax, which is another secretion belonging to the same class : thus the CeroxyloHy and the Iriartea have a thick coating of wax, covering the whole of their stems. Sometimes the plant is strewed over with a bluish powder, possessing the same property of repelling water : the leaves of the Mesembryanthemum, or Fig-marigold, of the At?iplex, or Orache, and of the Brassica, or Cabbage, may be given as ex- amples of this curious provision. Such plants, if completely immersed in water, may be taken out without being wetted in the slightest degree ; thus presenting us with an analogy to the plu- mage of the cygnet, and other aquatic birds, which are rendered completely water-proof by an oily secretion spread over their surface. Many aquatic plants, as the Satrachosjjennuin^ are, in like manner, protected by a viscid layer, which renders the leaves slippery to the feel, and which is impermeable to water. Several tribes of plants contain liquids that are opaque, and of a white milky appearance ; this is the case with the Poppy ^ the Fig-tree, the Convolvulus, and a multitude of other genera ; and a similar kind of juice, but of a yellow colour, is met with in the Chelidonium, or Celan- dine. All these juices are of a resinous nature, and usually highly acrid, and even poisonous in their qualities; and their opacity is occasioned CIRCULATION IN PLANTS. 49 by the presence of a great number of minute globules, visible with the microscope. The vessels in which these fluids are contained are of a pe- culiar kind, and exhibit ramifications and junc- tions, resembling those of the blood vessels of animals. We may also discover, by the aid of the microscope, that the fluids contained in these vessels are moving in currents with considerable rapidity, as appears from the visible motions of their globules ; and they present therefore a re- markable analogy with the circulation of the blood in some of the inferior tribes of animals. This curious phenomenon was first observed in the Chelidonium by Schultz, in the year 1820 ; and he designated it by the term Cyclosis, in order to distinguish it from a real circulation, if, on farther inquiry, it should be found not to be entitled to the latter appellation.* The circular movements, which have been thus observed in the milky juices of plants, have lately attracted much attention among botanists: but considerable doubt still prevails whether these appearances afford sufficient evidence of the existence of a general circulation of nutrient juices in the vegetable systems of those plants which exhibit them ; for it would appear that in reality the observed motions of the fluid, are, in every case, partial, and the extent of the circuit * " Die Natur der lebendigen Pflanze." See also Annales des Sciences Naturelies, xxiii, 75. VOL. II. E so THE VITAL FUNCTIONS. very limited. The cause of these motions is not yet known ; but probably they are ultimately referable to a vital contraction of the vessels ; for they cease the moment that the plant has re- ceived an injury, and are more active in pro- portion as the temperature of the atmosphere is higher. These phenomena are universally met with in all plants that contain milky j uices ; but they have also been observed in many plants, of which the juices are nearly transparent, and contain only a few floating globules, such as the Chara, or stone- wort, the Caulinia fragilis, &c.,* where the double currents are beautifully seen under the microscope, performing a complete circulation within the spaces of the stemthatliebeween two adj acent knots or joints ; and where, by the pro- per adjustment of the object, it is easy to see at one view both the ascending and descend- ing streams passing on opposite sides of the stem. Fig. 239 shows this circulation in the cells of the Caulinia fragilis very highly magnified, * Aniici, Annales des Sciences Naturelles, li. p. 41. CIRCULATION IN PLANTS. 51 the direction of the streams being indicated by the arrows. Fig. 240 represents the circulation in one of the jointed hairs, projecting from the cuticle of the calyx of the Tradescantia vir- ginica* in each cell of which the same circu- latory motion of the fluids is perceptible. § 7. Excretion in Vegetables. It had long been conjectured by De Candolle, that the superfluous or noxious particles contained in the returning sap are excreted or thrown out by the roots. It is evident that if such a process takes place, it will readily explain why plants render the soil where they have long been cultivated less suitable to their continuance in a vigorous condition, than the soil in the same spot w as origi- nally ; and also why plants of a different species are frequently found to flourish remarkably well in the same situation where this apparent dete- rioration of the soil has taken place. The truth of this sagacious conjecture has been established in a very satisfactory manner by the recent ex- periments of M. Macaire.t The roots of the * Fig. 239 is taken from Amici, and Fig. 240 from that given by Mr. Slack, Trans. Soc. Arts, vol. xlix. t An account of these experiments was first published in the fifth volume of the " Memoires de la Societe de Physique et d'Histoire Naturelle de Geneve," and repeated in the " Annales des Sciences Naturelles," xxviii, 402. 52 THE VITAL FUNCTIONS. Cliondrilla ynuralis were carefully cleaned, and immersed in filtered rain water : the water was changed every two days, and the plant continued to flourish, and put forth its blossoms : at the end of eight days, the water had acquired a yellow tinge, and indicated, both by the smell and taste, the presence of a bitter narcotic sub- stance, analogous to that of opium ; a result which was farther confirmed by the application of chemical tests, and by the reddish brown re- siduum obtained from the water by evaporation. M. Macaire ascertained that neither the roots nor the stems of the same plants, when com- pletely detached, and immersed in water, could produce this effect, which he therefore concludes is the result of an exudation from the roots, con- tinually going on while the plant is in a state of healthy vegetation. By comparative experiments on the quantity of matter thus excreted by the roots of the French bean (Phaseolus vulg-aris) during the night and the day, he found it to be much more considerable at night ; an effect which it is natural to ascribe to the interruption in the action of the leaves when they are deprived of light, and when the corresponding absorption by the roots is also suspended. This was con- firmed by the result of some experiments he made on the same plants by placing them, during day time, in the dark, under which circumstances the excretion from the roots was found to be VEGETABLE EXCRETIONS. 53 immediately much augmented : but, even when exposed to the light, there is always some exu- dation, though in small quantity, going on from the roots. That plants are able to free themselves, by means of this excretory process, from noxious materials, which they may happen to have im- bibed through the roots, was also proved by ano- ther set of experiments on the 3Iercurialis cuiniia, the Senecio vulgaris, and Srassica campestris, or common cabbage. The roots of each specimen, after being thoroughly washed and cleaned, were separated into two bunches, one of which was put into a diluted solution of acetate of lead, and the other into pure water, contained in a sepa- rate vessel. After some days, during which the plants continued to vegetate tolerably well, the water in the latter vessel being examined, was found to contain a very perceptible quantity of the acetate of lead. The experiment was varied by first allowing the plant to remain with its roots immersed in a similar solution, and then removing it, after careful washing, in order to free the roots from any portion of the salt that might have adhered to their surface, into a vessel with rain water ; after two days, distinct traces of the acetate of lead were afforded by the water. Similar experiments were made with lime-water, and with a solution of common salt, instead of the acetate of lead, and were attended 54 THE VITAL FUNCTIONS. with the like resuUs. De Candolle has ascer- tained, that certain maritime plants which yield soda, and which flourish in situations very distant from the coast, provided they occasionally re- ceive breezes from the sea, communicate a saline impregnation to the soil in their immediate vi- cinity, derived from the salt which they doubt- less had imbibed by the leaves. Although the materials which are thus excreted by the roots are noxious to the plant which rejects them, and would consequently be injurious to other individuals of the same species, it does not therefore follow that they are incapable of sup- plying salutary nourishment to other kinds of plants : thus it has been observed that the Sali- caria flourishes particularly in the vicinity of the willow, and the Orobanche, or broom-rape, in that of hemp. This fact has also been established experimentally by M. Macaire, who found that the water in which certain plants had been kept was noxious to other specimens of the same species, while, on the other hand, it produced a more luxuriant vegetation in plants of a different kind. This fact is of great importance in the theory of agriculture, since it perfectly explains the advantage derived from a continued rotation of different crops in the same field, in increasing the productiveness of the soil. It also gives a satisfactory explanation of the curious pheno- VEGETABLE EXCRETIONS. 55 menon oi fairy rings, as they are called, that is of circles of dark green grass, occurring in old pastures : these Dr. Wollaston has traced to the growth of successive generations of certain yk?*^?, or mushrooms, spreading from a central point.* The soil, which has once contributed to the sup- port of these fungi, becomes exhausted or dete- riorated with respect to the future crops of the same species, and the plants, therefore, cease to be produced on those spots ; the second year's crop consequently appears in the space of a small ring, surrounding the original centre of vegetation ; and in every succeeding year, the deficiency of nutriment on one side necessarily causes the new roots to extend themselves solely in the opposite direction, and occasions the circle of fungi continually to proceed by annual en- largement from the centre outwards. An appear- ance of kixuriance of the grass follows as a na- tural consequence ; for the soil of an interior circle will always be enriched and fertilized with respect to the culture of grass, by the decayed roots of fungi of the preceding years' growth. It often happens, indeed, during the growth of these fungi, that they so completely absorb all nutriment from the soil beneath, that the her- bage is for a time totally destroyed, giving rise to the appearance of a ring bare of grass, sur- * Phil. Trans, for 1807, p. 133. 56 THE VITAL FUNCTIONS. rounding the dark ring ; but after the fungi have ceased to appear, the soil where they had grown becomes darker, and the grass soon vegetates again with peculiar vigour. When two adjacent circles meet, and interfere with each other's pro- gress, they not only do not cross each other, but both circles are invariably obliterated between the points of contact : for the exhaustion occa- sioned by each obstructs the progress of the other, and both are starved. It would appear that different species of fungi often require the same kind of nutriment ; for, in cases of the in- terference of a circle of mushrooms with another of puff-balls, still the circles do not intersect one another, the exhaustion produced by the one being equally detrimental to the growth of the other, as if it had been occasioned by the pre- vious vegetation of its own species. The only final cause we can assign for the series of phenomena constituting the nutritive functions of vegetables is the formation of cer- tain organic products calculated to supply suste- nance to a higher order of beings. The animal kingdom is altogether dependent for its support, and even existence, on the vegetable world. Plants appear formed to bring together a certain number of elements derived from the mineral kingdom, in order to subject them to the opera- tions of vital chemistry, a power too subtle for human science to detect, or for human art to VEGETABLE EXCRETIONS. 57 imitate, and by which these materials are com- bined into a variety of nutritive substances. Of these substances, so prepared, one portion is con- sumed by the plants themselves in maintaining their own stmctures, and in developing the em- bryos of those which are to replace them ; another portion serves directly as food to various races of animals ; and the remainder is either employed in fertilizing the soil, and preparing it for subse- quent and more extended vegetation, or else, buried in the bosom of the earth, it forms part of that vast magazine of combustible matter, des- tined to benefit future communities of mankind, when the arts of civilization shall have developed the mighty energies of human power. Chapter III. ANIMAL NUTRITION IN GENERAL. ^ 1 . Food of Animals. Nutrition constitutes no less important a part of the animal, than of the vegetable economy. Endowed with more energetic powers, and en- joying a wider range of action, animals, com- pared with plants, require a considerably larger supply of nutritive materials for their sustenance, 58 THE VITAL FUNCTIONS. and for the exercise of their various and higher faculties. The materials of animal nutrition must, in all cases, have previously been combined in a peculiar mode ; which the powers of organ- ization alone can effect. In the conversion of vegetable into animal matter, the principal changes in chemical composition which the former undergoes, are, first, the abstraction of a certain proportion of carbon ; and secondly, the addition of nitrogen.* Other changes, however, less easily appreciable, though perhaps as im- portant as the former, take place in greater quantity, with regard to the proportions of saline earthy, and metallic ingredients ; all of which, and more especially iron, exist in greater quantity in animal than in vegetable bodies. The former also contain a larger proportion of sulphur and phosphorus than the latter. The equitable mode in which nature dispenses to her innumerable offspring the food she has provided for their subsistence, apportioning to * The recent researches of Messrs. Macaire and Marcet tend to establish the important fact that both the chyle and the blood of herbivorous and of carnivorous quadrupeds are identical in their chemical composition, in as far, at least, as concerns their ulti- mate analysis. They found, in particular, the same proportion of nitrogen in the chyle, whatever kind of food the animal habi- tually consumed : and it was also the same in the blood, whether of carnivorous or herbivorous animals ; although this last fluid contains more nitrogen than the chyle. {Memoires de la Socicte de Physique et d'Histoire Naturelle de Geneve, v. 389.) ANIMAL NUTRITION. 59 each the quantity and the kind most consonant to enlarged views of prospective beneficence, is calculated to excite our highest wonder and admiration. While the waste is the smallest possible, we find that nothing which can afford nutriment is wholly lost. There is no part of the organized structure of an animal or vegetable, however dense its texture, or acrid its qualities, that may not, under certain circumstances, be- come the food of some species of insect, or con- tribute in some mode to the support of animal life. The more succulent parts of plants, such as the leaves, or softer stems, are the principal sources of nourishment to the greater number of larger quadrupeds, to multitudes of insects, as well as to numerous tribes of other animals. Some plants are more particularly assigned as the appropriate nutriment of particular species, which would perish if these ceased to grow : thus the silkworm subsists almost exclusively upon the leaves of the mulberry tree ; and many species of caterpillars are attached each to a particular plant which they prefer to all others. There are at least fifty different species of insects that feed upon the common nettle ; and plants, of which the juices are most acrid and poisonous to the gene- rality of animals, such as Euphorbimn, Henbane, and Nightshade, afford a wholesome and deli- cious food to others. Innumerable tribes of ani- mals subsist upon fruits and seeds ; while others 60 THE VITAL FUNCTIONS. feast upon the juices which they extract from flowers, or other parts of plants; others, again, derive their principal nourishment from the hard fibres of the bark or wood. Still more general is the consumption of animal matter by various animals. Every class has its carnivorous tribes, which consume living prey of every denomination ; some being formed to devour the flesh of the larger species, whether quadrupeds, birds, or fish ; others feeding on reptiles or moUusca, and some satisfying their appetite with insects alone. The habits of the more diminutive tribes are not less predatory and voracious than those of the larger quad- rupeds ; for the spiders on the land, and the Crustacea in the sea, are but representatives of the lions and tigers of the forest, displaying an equally ferocious and insatiable rapacity. Other families, again, generally of still smaller size, are designed for a parasitic existence, their organs being fitted only for imbibing the blood or juices of other animals. No sooner is the signal given, on the death of any large animal, than mvdtitudes of every class hasten to the spot, eager to partake of the repast which nature has prepared. If the carcass be not rapidly devoured by rapacious birds, or car- nivorous quadrupeds, it never fails to be soon attacked by swarms of insects, which speedily consume its softer textures, leaving only the ECONOMY OF NUTRITIVE MATTER. 6*1 bones.* These, again, are the favourite repast of the Hyeena, whose powerful jaws are pecu- liarly formed for grinding them into powder, and whose stomach can extract from them an abundant portion of nutriment. No less speedy is the work of demolition among the inha- bitants of the waters, where innumerable fishes, Crustacea, annelida, and mollusca are on the watch to devour all dead animal matter which may come within their reach. The consumption of decayed vegetables is not quite so speedily accomplished ; yet these also afford an ample store of nourishment to hosts of minuter beings, less conspicuous, perhaps, but performing a no less important part in the economy of the creation. It may be observed that most of the insects which feed on decomposing materials, whether animal or vegetable, consume a much larger quantity than they appear to require for the purposes of nutrition. We may hence infer that in their formation other ends were contemplated, besides * So strongly was Linnaeus impressed with the immensity of the scale on which these works of demolition by insects are carried on in nature, that he used to maintain that the carcass of a dead horse would not be devoured with the same celerity by a lion, as it would be by three flesh flies (Musca vomitoria) and their immediate progeny : for it is known that one female fly will give birth to at least 20,000 young larvae, each of which will, in the course of a day, devour so much food, and grow so rapidly, as to acquire an increase of two hundred times its weight : and a few days are sufficient to the production of a third generation. 62 THE VITAL FUNCTIONS. their own individual existence. They seem as if commissioned to act as the scavengers of organic matter, destined to clear away all those particles, of which the continued accumulation would have tainted the atmosphere or the waters with infec- tion, and spread a wide extent of desolation and of death. In taking these general surveys of the plans adopted by nature for the universal subsistence of the objects of her bounty, we cannot help ad- miring how carefully she has provided the means for turning to the best account every particle of each product of organic life, whether the material be consumed as food by animals, or whether it be bestowed upon the soil, reappearing in the substance of some plant, and being in this way made to contribute eventually to the same ulti- mate object, namely, the support of animal life. But we may carry these views still farther, and following the ulterior destination of the minuter and unheeded fragments of decomposed organizations, which we might conceive had been cast away, and lost to all useful purposes, we may trace them as they are swept down by the rains, and deposited in pools and lakes, amidst waters collected from the soil on every side. Here we find them, under favourable circum- stances, again partaking of animation, and in- vested with various forms of infusory animalcules, ECONOMY OF NUTRITIVE MATTER. G3 which sport in countless myriads their ephemeral existence within the ample regionsof every drop. Yet even these are still qualified to fulfil other objects in a more distant and far wider sphere ; for, borne along, in the course of time, by the rivers into which they pass, they are at length conveyed into the sea, the great receptacle of all the particles that are detached from the objects on land. Here also they float not uselessly in the vast abyss, but contribute to maintain in ex- istence incalculable hosts of animal beings, which people every portion of the wide expanse of ocean, and which rise in regular gradation from the microscopic monad, and scarcely visible medusa,* through endless tribes of mollusca, and of fishes, up to the huge Leviathan of the deep. Not even are these portions of organic matter, which, in the course of decomposition, escape in the form of gases, and are widely diffused through the atmosphere, wholly lost for the uses of living nature : for, in course of time, they, also, as we have seen, re-enter into the vegetable system, resuming the solid form, and reappearing as organic products, destined again to run through * The immensity of the numbers of these microscopic medusae, which people every region of the ocean, may be judged of from the phenomenon of the phosphorescent light which is so fre- quently exhibited by the sea, when agitated, and which, as I have already observed, is found to arise from the presence of an incal- culable multitude of these minute animals. 64 THE VITAL FUNCTIONS. the same never ending cycle of vicissitudes and transmutations. The diffusion of animals over wide regions of the globe is a consequence of the necessity which prompts them to search for subsistence wherever food is to be met with. Thus while the vegetation of each different climate is regulated by the sea- sons, herbivorous animals are in the winter forced to migrate from the colder to the milder regions, where they may find the pasturage they require; and these migrations occasion corresponding movements among the predaceous tribes which subsist upon them. Thus are continual inter- changes produced, contributing to colonise the earth, and extend its animal population over every habitable district. But in all these changes we may discern the ultimate relation they ever bear to the condition of the vegetable world, which is placed as an intermediate and necessary link between the mineral and the animal king- doms. All those regions which are incapable of supporting an extensive vegetation, are, on that account, unfitted for the habitation of animals. Such are the vast continents of ice, which spread around the poles ; such are the immense tracts of snow and of glaciers, which occupy the sum- mits of the highest mountain chains ; and such is the wide expanse of sand, which covers the largest portions both of Africa and of Asia : and often have we heard of the sunken spirits of the i| INFLUENCE OF THE DEMAND FOR FOOD. 65 traveller through the weary desert, from the appalling silence that reigns over those regions of eternal desolation ; but no sooner is his eye refreshed by the reappearance of vegetation, than he again traces the footsteps and haunts of animals, and welcomes the cheering sound of sensitive beings. The kind of food M'hich nature has assigned to each particular race of animals has an impor- tant influence, not merely on its internal organ- ization, but also on its active powers and dispo- sition ; for the faculties of animals, as well as their structure, have a close relation to the cir- cumstances connected with their subsistence, such as the abundance of its supply, the facility of procuring it, the dangers incurred in its search, and the opposition to be overcome before it can be obtained. In those animals whose food lies generally within their reach, the active powers acquire but little developement : such, for in- stance, is the condition of herbivorous quad- rupeds, whose repast is spread every where in rich profusion beneath their feet ; and it is the chief business of their lives to crop the flowery mead, and repose on the same spot which aftbrds them the means of support. Predaceous animals, on the contrary, being prompted by the calls of appetite to wage war with living beings, are formed for a more active and martial career ; their mus- cles are more vigorous, their bones are stronger, VOL. II. 1 66 THE VITAL FUNCTIONS. their limbs more robust, their senses more deli- cate and acute. What sight can compare with that of the eagle and the lynx ; what scent can be more exquisite than that of the wolf and the jackall ? All the perceptions of carnivorous ani- mals are more accurate, their sagacity embraces a greater variety of objects, and in feats of strength and agility they far surpass the herbi- vorous tribes. A tiger will take a spring of fif- teen or twenty feet, and seizing upon a buffalo, will carry it with ease on its back through a dense and tangled thicket : with a single blow of its paw it will break the back of a bull, or tear open the flanks of an elephant. While herbivorous animals are almost con- stantly employed in eating, carnivorous animals are able to endure abstinence for a great length of time, without any apparent diminution of their strength : a horse or an ox would sink under the exhaustion consequent upon fasting for two or three days, whereas the wolf and the martin have been known to live fifteen days without food, and a single meal will suffice them for a whole week. The calls of hunger produce on each of these classes of animals the most opposite effects. Herbivorous animals are rendered weak and faint by the want of food, but the tiger is roused to the full energy of his powers by the cravings of appetite ; his strength and courage are never so great as when he is nearly famished. INFLUENCE OF THE DEMAND FOR FOOD. (J7 and he rushes to the attack, reckless of conse- quences, and undismayed by the number or force of his opponents. From the time he has tasted blood, no education can soften the native ferocity of his disposition : he is neither to be reclaimed by kindness, nor subdued by the fear of punishment. On the other hand, the elephant, subsisting upon the vegetable productions of the forest, superior in size and even in strength to the tiger, and armed with as powerful weapons of offence, which it wants not the courage to em- ploy when necessary, is capable of being tamed with the greatest ease, is readily brought to submit to the authority of man, and requites with affection the benefits he receives. On first contemplating this extensive destruc- tion of animal life by modes the most cruel and revolting to all our feelings, we naturally recoil with horror from the sanguinary scene ; and cannot refrain from asking how all this is consis- tent with the wisdom and benevolence so conspi- cuously manifested in all other parts of the crea- tion. The best theologians have been obliged to confess that a difficulty does here exist,* and that the only plausible solution which it admits of, is to consider the pain and suffering thus created, as one of the necessary consequences of those general laws which secure, on the whole, * See, in particular, Paley's Natural Theology, chap. xxvi. (58 THE VITAL FUNCTIONS. the greatest and most permanent good. There can be no doubt that the scheme, by which one animal is made directly conducive to the subsis- tence of another, leads to the extension of the benefits of existence to an infinitely greater number of beings than could otherwise have en- joyed them. This system, besides, is the spring of motion and activity in every part of nature. While the pursuit of its prey forms the occupa- tion, and constitutes the pleasure of a considerable part of the animal creation, the employment of the means they possess of defence, of flight, and of precaution is also the business of a still larger part. These means are, in a great proportion of instances, successful ; for wherever nature has in- spired sagacity in the perception of danger, she has generally bestowed a proportionate degree of ingenuity in devising the means of safety. Some are taught to deceive the enemy, and to employ stratagem where force or swiftness would have been unavailing : many insects, when in danger, counterfeit death to avoid destruction ; others, among the myriapoda, fold themselves into the smallest possible compass, so as to escape detec- tion. The tortoise, as we have already seen, retreats within its shell, as within a fortress ; the hedge-hog rolls itself into a ball, presenting bristles on every side ; the diodon inflates its globular body for the same purpose, and floats on the sea, armed at all the points of its surface ; SERIES OF VITAL FUNCTIONS. 69 the cuttle-fish screens itself from pursuit by effu- sing an intensely dark coloured ink, which renders the surrounding waters so black and turbid as to conceal the animal, and favour its escape ; the torpedo defends itself from molestation by reite- rated discharges from its electric battery ; the butterfly avoids capture by its irregular move- ments in the air, and the hare puts the hounds at fault by her mazy doublings. Thus does the animated creation present a busy scene of activity and employment : thus are a variety of powers called forth, and an infinite diversity of pleasures derived from their exercise ; and existence is on the whole rendered the source of incomparably higher degrees, as well as of a larger amount of enjoyment, than appears to have been compatible with any other imaginable system. § 2. Series of Vital Functions. In the animal economy, as in the vegetable, the vital, or nutritive functions are divisible into seven kinds, namely. Assimilation, Circulation, Respi- ration, Secretion, Excretion, Absorption, and Nutrition ; some of which even admit of farther subdivision. This is the case more particularly with the processes of assimilation, which are generally numerous, and require a very compli- cated apparatus for acting on the food in all the 70 THE VITAL FUNCTIONS. stages of its conversion into blood, a fluid which, like the returning sap of plants, consists of nutri- ment in its completely assimilated state. It will be necessary, therefore, to enter into a more par- ticular examination of the objects of these diffe- rent processes. In the more perfect structures belonging to the higher orders of animals, contrivances must be adopted, and organs provided for seizing the appropriate food, and conveying it to the mouth. A mechanical apparatus must there be placed for effecting that minute subdivision, which is necessary to prepare it for the action of the che- mical agents to which it is afterwards to be sub- jected. From the mouth, after it has been sufficiently masticated, and softened by fluid secretions prepared by neighbouring glands, the food must be conveyed into an interior cavity, called the Stomach, where, as in a chemical laboratory, it is made to undergo the particular change which results from the operation termed Digestion. The digested food must thence be conducted into other chambers, composing the intestinal tube, where it is converted into Chyle, which is a milky fluid, consisting wholly of nutritious matter. Vessels are then provided, which, like the roots of plants, drink up this prepared fluid, and convey it to other cavities, capable of imparting to it a powerful impulsive force, and of distributing it through appropriate RECEPTACLES OF FOOD. 71 channels of circulation, not only to the respi- ratory organs, where its elaboration is completed by the influence of atmospheric air, but also to all other parts of the system, where such a supply is required for their maintenance in the living state. The objects of these subsequent functions, many of which are peculiar to animal life, have already been detailed.* This subdivision of the assimilatory processes occurs only in the higher classes of animals, for in proportion as we descend in the scale, we find them juore and more simplified, by the con- centration of organs, and the union of many ofiices in a single organ, till we arrive, in the very lowest orders, at little more than a simple digestive cavity, performing at once the functions of the stomach and of the heart ; without any distinct circulation of nutrient juices, without vessels, — nay without any apparent blood. Long after all the other organs, such as the skeleton, whe- ther internal or external, the muscular and ner- vous systems, the glands, vessels, and organs of sense, have one after another disappeared, we still continue to find the digestive cavity retained, as if it constituted the most important, and only indispensable organ of the whole system. The possession of a stomach, then, is the pecu- liar characteristic of the animal system as con- * See the first chapter of this volume, p. 11. 72 THE VITAL FUNCTIONS. trasted with that of vegetables. It is a distinctive criterion that applies even to the lowest orders of zoophytes, which, in other respects, are so nearly allied to plants. It extends to all insects, however diminutive ; and even to the minutest of the microscopic animalcules.* The mode in which the food is received into the body is, in general, very different in the two organized kingdoms of nature. Plants receive their nourishment by a slow, but nearly constant supply, and have no receptacle for collecting it at its immediate entry ; the sap, as we have seen, passing at once into the cellular tissue of the plant, where the process of its gradual elabo- ration is commenced. Animals, on the other hand, are capable of receiving at once large supplies of food, in consequence of having an in- ternal cavity, adapted for the immediate recep- tion of a considerable quantity. A vegetable may be said to belong to the spot from which it imbibes its nourishment, and the surrounding soil, into which its absorbing roots are spread on every side, may almost be considered as a part of its system. But an animal has all its * In some species of animals belonging to the tribe of medusse, as the Eudora, Berenice, Orythia, Favonia, Lymnoria, and Geryonia, no central cavity corresponding to a stomach has been discovered : they appear, therefore, to constitute an exception to the general rule. See Peron, Annales de Museum, xiv, 227 and 326. INFLUENCE OF THE DEMAND FOR FOOD. 73 organs of assimilation within itself, and having receptacles in which it can lay in a store of provisions, it may be said to be nourished from within ; for it is from these interior receptacles that the lacteals, or absorbing vessels, corres- ponding in their office to the roots of vege- tables, imbibe nourishment. Important conse- quences flow from this plan of structure ; for since animals are thus enabled to subsist for a certain interval without needing any fresh supply, they are independent of local situation, and may enjoy the privilege of moving from place to place. Such a power of locomotion was, indeed, abso- lutely necessary to beings which have their sub- sistence to seek. It is this necessity, again, which calls for the continued exercise of their senses, intelligence, and more active energies ; and that lead, in a word, to the possession of all those higher powers, which raise them so far above the level of the vegetable creation. 74 Chapter IV. Nutrition in the lower Orders of Animals. The animals which belong to the order of polypi present us with the simplest of all possible forms of nutritive organs. The hydra, for instance, which may be taken as the type of this formation, consists of a mere stomach, provided with the simplest instruments for catching food, — and no- thing more. A simple sac, or tube, adapted to receive and digest food, is the only visible organ of the body. It exhibits not a trace of either brain, nerves, or organs of sense, nor any part corresponding to lungs, heart, or even vessels of any sort ; all these organs, so essential to the maintenance of life in other animals, being here dispensed with. In the magnified view of the hydra, exhibited in Fig. 241, the cavity into which the food is 241 iP'^^f received and digested is laid open by a longitudinal section, so as to show the comparative thick- ness of the walls of this cavity. The structure of these walls must be adapted not only to prepare and pour out the fluids by which the food is di- gested, but also to allow of the transudation NUTRITION IN POLYPI. 75 through its substance, probably by means of in- visible pores, of the nutritious particles thus ex- tracted from the food, for the purpose of its being incorporated and identified with the gelatinous pulp, of which the body appears wholly to consist. The thinness and transparency of the walls of this cavity allow of our distinctly following these changes by the aid of the microscope. Trembley watched them with unwearied perseverance for days together, and has given the following ac- count of his observations. The hydra, though it does not pursue the animals on which it feeds, yet devours with avidity all kinds of living prey that come within the reach of its tentacula, and which it can overcome and introduce into its mouth. The larvae of insects, naides, and other aquatic worms, minute Crustacea, and even small fishes, are indiscriminately laid hold of, if they happen but to touch any part of the long fila- ments which the animal spreads out, in different directions, like a net, in search of food. The struggles of the captive, which finds itself en- tangled in the folds of these tentacula, are gene- rally ineffectual, and the hydra, like the boa constrictor, contrives, by enormously expanding its mouth, slowly to draw into its cavity ani- mals much larger than its own body. Worms longer than itself are easily swallowed by being previously doubled together by the tentacula. Fig. 242 shows a hydra in the act of devouring 76 THE VITAL FUNCTIONS. the vermiform larva of a Tipula, which it has encircled with its tentacula, to which it has applied its expanded mouth, and of which it is absorbing the juice, before swallowing it. Fig. 243 shows the same animal after it has suc- ceeded, though not without a severe contest, in swallowing a minnow, or other small fish, the form of which is still visible through the trans- parent sides of the body, which are stretched to the utmost. It occasionally happens, when two of these animals have both seized the same object by its different ends, that a struggle between them ensues, and that the strongest, having ob- tained the victory, swallows at a single gulp, not only the object of contention, but its antagonist also. This scene is represented in Fig. 244, where the tail of the hydra, of which the body has been swallowed by the victor, is seen pro- truding from the mouth of the latter. It soon, however, extricates itself from this situation, NUTRITION IN POLYPI. 77 apparently without having suffered the smallest injury. The voracity of the hydra is very great, especially after long fasting ; and it will then devour a great number of insects, one after ano- ther, at one meal, gorging itself till it can hold no more, and its body becoming dilated to an extraordinary size : and yet the same animal can continue to live for more than four months with- out any visible supply of food. On attentively observing the changes induced upon the food by the action of the stomach of these animals, they appear to consist of a gradual melting down of the softer parts, which are re- solved into a kind of jelly, leaving unaltered only a few fragments of the harder and less digestible parts. These changes are accompanied by a kind of undulation of the contents of the stomach, backwards and forwards, throughout the whole tube, apparently produced by the contraction and dilatation of its different portions. The un- digested materials being collected together and rejected by the mouth, the remaining fluid is seen to contain opaque globules of various sizes, some of which are observed to penetrate through the sides of the stomach, and enter into the gra- nular structure which composes the flesh of the animal. Some portion of this opaque fluid is distributed to the tentacula, into the tubular cavities of which it may be seen entering by passages of communication with the stomach. 78 THE VITAL FUNCTIONS. By watching attentively the motions of the glo- bules, it will be perceived that they pass back- wards and forwards through these passages, like ebbing and flowing tides. All these phenomena may be observed with greater distinctness when the food of the animal contains colouring matter, capable of giving a tinge to the nutritious fluid, and allowing of its progress being traced into the granules which are dispersed throughout the substance of the body. Trembley is of opinion that these granules are vesicular, and that they assume the colour they are observed to have, from their becoming flUed with the coloured particles contained in the nou- rishment. The granules which are nearest to the cavity of the stomach are those which are first tinged, and which therefore first imbibe the nutritious juices : the others are coloured succes- sively, in an order determined by their distance from the surface of the stomach. Trembley ascertained that a living hydra introduced into the stomach of another hydra, was not in any degree acted upon by the fluid secretions of that organ, but came out uninjured. It often happens that a hydra in its eagerness to transfer its victim into its stomach, swallows several of its own ten- tacula, which had encircled it : but these tenta- cula always ultimately came out of the stomach, sometimes after having remained there twenty- four hours, without the least detriment. NUTRITION IN POLYPI. 79 The researches of Trembley have brought to light the extraordinary fact that not only the internal surface of the stomach of the polypus is endowed with the power of digesting food, but that the same property belongs also to the ex- ternal surface, or what we might call the skin of the animal. He found that by a dexterous mani- pulation, the hydra may be completely turned inside out, like the finger of a glove, and that the animal, after having undergone this singular operation, will very soon resume all its ordinary functions, just as if nothing had happened. It accommodates itself in the course of a day or two to the transformation, and resumes all its natural habits, eagerly seizing animalcules with its tentacula, and introducing them into its newly formed stomach, which has for its interior sur- face what before was the exterior skin, and which digests them with perfect ease. When the discovery of this curious phenomenon was first made known to the world, it excited great asto- nishment, and many naturalists were incredulous as to the correctness of the observations. But the researches of Bonnet and of Spallanzani,who repeated the experiments of Trembley, have borne ample testimony to their accuracy, which those of every subsequent observer have farther contributed to confirm. The experiments of Trembley have also proved that every portion of the hydra possesses a won- 80 thf: vital functions. derful power of repairing all sorts of injuries, and of restoring parts which have been removed. These animals are found to bear with impunity all sorts of mutilations. If the tentacula be cut off, they grow again in a very short time : the whole of the fore part of the body is, in like manner, reproduced, if the animal be cut asun- der ; and from the head which has been removed there soon sprouts forth a new tail. If the head of the hydra be divided by a longitudinal section, extending only half way down the body, the cut portions will unite at their edges, so as to form two heads, each having its separate mouth, and set of tentacula. If it be split into six or seven parts, it will become a monster with six or seven heads ; if each of these be again divided, ano- ther will be formed with double that number. If any of the parts of this compound polypus be cut off, as many new ones will spring up to re- place them ; the mutilated heads at the same time acquiring fresh bodies, and becoming as many entire polypi. Fig. 245 represents a hydra with seven heads, the result of several operations of this kind. The hydra will sometimes of its own accord split into two ; each division be- coming independent of the other, and growing to the same size as the original hydra. Trembley found that different portions of one polype might be engrafted on another, by cutting their sur- faces, and pressing them together ; for by this NUTRITION IN POLYPI. 81 means they quickly unite, and become a com- pound animal. When the body of one hydra is introduced into the mouth of another, so that their heads are kept in contact for a sufficient length of time, they unite and form but one in- dividual. A number of heads and bodies may thus be joined together artificially, so as to com- pose living monsters more complicated than the wildest fancy has conceived. Still more complicated are the forms and eco- nomy of those many-headed monsters, which prolific nature has spread in countless multitudes over the rocky shores of the ocean in every part of the globe. These aggregated polypi grow in imitation of plants, from a common stem, with widely extended flowering branches. Myriads of mouths open upon the surface of the animated mass ; each mouth being surrounded with one or more circular rows of tentacula, which are extended to catch their prey : but as the station- ary condition of these polypes prevents them from moving in search of food, their tentacula are generally furnished with a multitude of cilia, which, by their incessant vibrations, determine currents of water to flow towards the mouth, carrying with them the floating animalcules on which the entire polypus subsists. Each mouth leads into a separate stomach ; whence the food, after its digestion, passes into several channels, generally five in number, which VOL. II. G 82 THE VITAL FUNCTIONS. proceed in different directions from the cavity of each stomach, dividing into many branches, and being distributed over all the surrounding portions of the flesh. These branches communicate with similar channels proceeding from the neigh- bouring stomachs : so that the food which has been taken in by one of the mouths, contributes to the general nourishment of the whole mass of aggregated polypi. Cuvier discovered this struc- ture in the Veret ilia, which belongs to this order of polypi : he also found it in the Pennatula, and it is probably similar in all the others. Fig. 246 represents three of the polypes of the Veretilla, with their communicating vessels seen below. The prevailing opinion among naturalists is, that each polypus is an individual animal, associated with the rest in a sort of republic, where the labours of all are exerted for the common benefit of the w hole society. But it is perhaps more con- sonant with our ideas of the nature of vitality to consider the extent of the distribution of nutritive fluid in any organic system as the criterion of the individuality of that system, a view which would lead us to consider the entire polypus, or mass composed of numerous polypes, as a single individual animal ; for there is no more incon- sistency in supposing that an individual animal may possess any number of mouths, than that it may be provided with a multitude of distinct stomachs, as we shall presently find is actually exemplified in many of the lower animals. NUTRITION IN THE ENTOZOA. 83 Some of the Entozoa, or parasitic worms, ex- hibit a general diffusion, or circulation of nou- rishment through numerous channels of commu- nication, into which certain absorbing vessels convey it from a great number of external orifices, or mouths, as they may be called. This is the case with the Tcenia, or tape worm, which is composed of a series of flat jointed portions, of which two contiguous segments are seen, highly magnified, in Fig. 247, exhibiting round the margin of each portion, a circle of vessels (v), which communicate with those of the adjoining segments ; each circle being provided with a tube (o), having external openings for imbibing nourishment from the surrounding fluids. Al- though each segment is thus provided with a nutritive apparatus complete within itself, and so far, therefore, independent of the rest, the individuality of the whole animal is sufficiently determined by its having a distinct head at one 84 THE VITAL FUNCTIONS. extremity, provided with instruments for its attachment to the surfaces it inhabits. The Hydatid (Fig. 248) is another parasitic worm of the simplest possible construction. It has a head (o), of which h is a magnified repre- sentation, furnished with four suckers, and a tubular neck, which terminates in a globular sac. When this sac, which is the stomach, is fully distended with fluid, its sides are stretched, so as to be reduced to a very thin transparent membrane, having a perfectly spherical shape ; after this globe has become swollen to a very large size, the neck yields to the distension, and disappears ; and the head can then be distin- guished only as a small point on the surface of the globular sac. It is impossible to conceive a more simple organic structure than this, which may, in fact, be considered as an isolated living stomach. The Ccemirus, which is found in the brain of sheep, has a structure a little more com- plicated ; for instead of a single head, there are a great number spread over the surface, opening into the same general cavity, and when the sac is distended, appearing only as opaque spots on its surface. The structure of the Spoflge has been already fully described ; and the course of the minute channels pointed out, in which a kind of circu- lation of sea water is carried on for the nourish- ment of the animal. The mode by which nutri- NUTRITION IN MEDUSiE. 85 ment is extracted from this circulating fluid, and made to contribute to the growth of these plant- like structures, is entirely unknown. The apparatus for nutrition possessed by animals belonging to the tribe of MeduscB is of a peculiar kind. I have already described the more ordinary form of these singular animals, which resemble a mushroom, from the hemis- pherical form of their bodies, and their central foot-stalk, or pedicle. In the greater number of species there exists at the extremity of this pedicle, a single aperture, wdiich is the begin- ning of a tube leading into a large central cavity in the interior of the body, and which may there- fore be regarded as the mouth of the animal : but in those species which have no pedicle, as the Equorea, the mouth is situated at the centre of the under surface. The aperture is of suffi- cient w idth to admit of the entrance of prey of considerable size, as appears from the circum- stance that fishes of some inches in length are occasionally found entire in the stomachs of those medusae which have a single mouth. The central cavity, which is the stomach of the animal, does not appear to possess any proper coats, but to be simply scooped out of the soft structure of the body. Its form varies in diflerent species ; having generally, however, more or less of a star-like shape, composed of four curved rays, which might almost be considered as consti- 86 THE VITAL FUNCTIONS. tilting four stomachs, joined at a common centre. Such, indeed, is the actual structure in the Medusa aurita, in which Gaede found the stomach to consist of four spherical sacs, com- pletely separated by partitions. These arched cavities, or sacs, taper as they radiate towards the circumference, and are continued into a canal, from which a great number of other canals proceed, generally at first by successive bifurcations of the larger trunks, but afterwards branching off more irregularly, and again uniting by lateral communications so as to compose a complicated net- work of vessels. These rami- fications at length unite to form an annular vessel, which encircles the margin of the disk. It appears also, from the observations of Gaede, that a farther communication is established between this latter vessel, and others which permeate the slender filaments, or tentacula, that hang like a fringe all round the edge of the disk, and which, in the living animal, are in perpetual motion. It is supposed that the elon- gations and contractions of these filaments are effected by the injection or recession of the fluids contained in those vessels.* Here, then, we see not only a more complex stomach, but also the commencement of a vascular system, taking its rise from that cavity, and calculated to * Journal de Physique, Ixxxix, 146. NUTRITION IN MEDUSA. 87 distribute the nutritious juices to every part of the organization. There are other species of Medusae, com- posing the genus Rhizostoma of Cuvier, which, instead of having only one mouth, are provided with a great number of tubes which serve that office, and which bear a great analogy to the roots of a plant.* The pedicle terminates below in a great number of fringed processes, which, on examination, are found to contain ramified tubes, with orifices opening at the extremity of each process. In this singular tribe of animals there is properly no mouth or central orifice, the only avenues to the stomach being these elon- gated canals, which collect food from every quarter where they extend, and which, uniting into larger and larger trunks as they proceed towards the body, form one central tube, or oesophagus, which terminates in the general cavity of the stomach. The Medusa piilmo, of which a figure was given in Vol. i., page 192, belongs to this modern genus, and is now termed the Rhizostoma Cuvieri. The course of these absorbent vessels is most conveniently traced after they have been filled with a dark coloured liquid. The appearances they present in the Rhizostoma Cuvieri, after * It is from this circumstance that the genus has received the name it now bears, and which is derived from two Greek words, signifying root-like mouths. m THE VITAL FUNCTIONS. being thus injected, are represented in the annexed figures ; the first of which (Fig. 249), shows the under surface of that animal, after the pedicle has been removed by a horizontal section, at its origin from the hemispherical body, or cupola, as it may be termed, where it has a square prismatic form, so that its section presents the square surface, q, q. Fig. 252 is a vertical section of the same specimen ; both figures being reduced to about one-half of the natural size. The dotted line, h, h, in the latter figure, shows the plane where the section of the pedicle was made in order to give the view of the base of the hemisphere presented in Fig. 249. On the other hand, the dotted line v, v, in Fig. 249, is that along which the vertical section of the same NUTRITION IN MEUUSiE. 89 animal, represented in Fig. 252, was made, four of the arms (a, a, a, a), descending from the pedicle, being left attached to it. In these arms, or tentacula, may be seen the canals, marked by the dark lines (c, c, c), which arise from numerous orifices in the extremities and fringed surface of the tentacula, and which gradually uniting, like the roots of a plant, converge towards the centre of the pedicle, and terminate by a common tube, which may be considered as the oesophagus (o), in one large central cavity. 90 THE VITAL FUNCTIONS. or stomach (s), situated in the upper part of the cupola. The section of this oesophagus is visible at the centre of Fig. 249, where its cavity has the form of a cross. The stomach has a quad- rangular shape, as in the ordinary medusae, and from each of its four corners there proceed vessels, which are continuous with its cavity, and are distributed by endless ramifications over the substance of the cupola, extending even to the fringed margin all round its circumference. The mode of their distribution, and their nume- rous communications by lateral vessels, forming a complete vascular net-work, is seen in Fig. 251 , which represents, on a larger scale, a portion of the marginal part of the disk. The two large figures (249 and 252) also show the four lateral cavities (r, r, Fig. 252), which are contiguous to the stomach, but separated from it by mem- branous partitions : these cavities have by some been supposed to perform an office in the system of the Medusa corresponding to respiration ; an opinion, however, which is founded rather on analogy than on any direct experimental evi- dence. The entrances into these cavities are seen open at e, in Fig. 249, and at e, e, in the section Fig. 252. A transverse section of one of the arms is given in Fig. 253, showing the form of the absorbent tube in the centre : and a similar section of the extremity of one of the tentacula is seen in Fig. 254, in which, besides the central NUTRITION IN MEDUSA. 91 tube, the cavities of some of the smaller branches (b, b), which are proceeding to join it, are also visible. The regular gradation which nature has ob- served in the complexity of the digestive cavities and other organs, of the various species of this extensive tribe, is exceedingly remarkable : for while some, as the Euclora, have, to all appear- ance, no internal cavity corresponding to a stomach, and are totally unprovided with either pedicle, arms, or tentacula ; others, furnished with these latter appendages, are equally desti- tute of such a cavity ; and those belonging to a third family possess a kind of pouch, or false stomach, at the upper part of the pedicle, appa- rently formed by the mere folding in of the integument. This is the case with the Geronia, depicted in Fig. 250, whose structure, in this respect, approaches that of the Hydra, already described, where the stomach consists of an open sac apparently formed by the integuments alone. Thence may a regular progression be followed, through various species, in which the aperture of this pouch is more and more com- pletely closed, and where the tube which enters it branches out into ramifications more or less numerous, as we have seen in the Rhizostoma.* It is difficult to conceive in what mode nutrition ' See Peron, Annales du Museum, xiv. 330. 92 THE VITAL FUNCTIONS. is performed in the agastric tribes, or those destitute of any visible stomach ; unless we sup- pose that their nourishment is imbibed by direct absorption from tlie surface. Ever since the discovery of the animalcula of infusions, naturalists have been extremely de- sirous of ascertaining the nature of the organi- zation of these curious beings : but as no mode presented itself of dissecting objects of such extreme minuteness, it was only from the ex- ternal appearances they present under the microscope that any inferences could be drawn with regard to the existence and form of their internal organs. In most of the larger species, the opaque globules, seen in various parts of the interior, were generally supposed to be either the ova, or the future young, lodged within the body of the parent. In the Rotifer, or wheel animalcule of Spallanzani,* a large central organ is plainly perceptible, which was by some imagined to be the heart ; but which has been clearly ascertained by Bonnet to be a receptacle for food. Muller, and several other observers, have witnessed the larger animalcules devouring the smaller ; and the inference was obvious that, in common with all other animals, they also must possess a stomach. But as no such struc- ture had been rendered visible in the smallest species of infusoria, such as monads, it was * V\)l. i. ]). fi'2, Fis:. 1 . NUTRITION IN THE INFUSORIA. 93 too hastily concluded that these species were formed upon a different and a simpler model. Lamark characterized them as being throughout of a homogeneous substance, destitute of mouth and digestive cavity, and nourished simply by means of the absorption of particles through the external surface of their bodies. The nature and functions of these singular beings long remained involved in an obscurity which appeared to be impenetrable ; but at length a new light has been thrown upon the subject by Professor Ehrenberg, whose re- searches have recently disclosed fresh scenes of interest and of wonder in microscopic worlds, peopled with hosts of animated beings, almost infinite in number as in minuteness.* In en- deavouring to render the digestive organs of the infusoria more conspicuous, he hit upon the for- tunate expedient of supplying them with coloured food, which might communicate its tinge to the cavities into which it passed, and exhibit their * The results of Ehrenberg's labours were first communicated to the BerHn Academy ; they have since been published in two ■works in German : the first of which appeared at Berlin in 1830, under the title of " Organisation, Systejnatik und Geo- graphisches Verhultniss der Infusionsthierchen." The second work appeared in 1832, and is entitled " Zur Erkenntniss der Organisation in der Richtung des kleinsten Raumes." Both are in folio, with plates. An able analysis of the contents of the former of these works, by Dr. Gairdner, is given in The Edin- burgh New Philosophical Journal for 1831, p. 201, of which I have availed myself largely in the account which follows. 94 THE VITAL FUNCTIONS. situation and course. Obvious as this method may appear, it was not till after a labour of ten years that Ehrenberg succeeded in discovering the fittest substances, and in applying them in the manner best suited to exhibit the phenomena satisfactorily. We have already seen that Trembley had adopted the same plan for the elucidation of the structure of the hydra. Gleichen also had made similar attempts with regard to the infusoria; but, in consequence of his having employed metallic or earthy colour- ing materials, which acted as poisons, instead of those which might serve as food, he failed in his endeavours. Equally unsuccessful were the trials made by Ehrenberg with the indigo and gum-lac of commerce, which are always contaminated with a certain quantity of white lead, a sub- stance highly deleterious to all animals ; but, at length, by employing an indigo which was quite pure, he succeeded perfectly.* The moment a minute particle of a highly attenuated solution of this substance is applied to a drop of water in which are some pedunculated vorticellae, oc- cupying the field of the microscope, the most * The colouring matters proper for these experiments are such as do not chemically combine with water, but yet are capable of being diffused in a state of very minute division. Indigo, sap green, and carmine, answer these conditions, and being also easily recognised under the microscope, are well adapted for these observations. Great care should be taken, however, that the substance employed is free from all admixture of lead, or other metallic impurity. NUTRITION IN THE INFUSORIA. 95 beautiful phenomena present themselves to the eye. Currents are excited in all directions by the vibrations of the cilia, situated round the mouths of these animalcules, and are readily dis- tinguished by the motions of the minute particles of indigo which are carried along with them ; the currents generally all converging towards the orifice of the mouth. Presently the body of the vorticella, which had been hitherto quite transparent, becomes dotted with a number of distinctly circular spots, of a dark blue colour, evidently produced by particles of indigo accu- mulated in those situations. In some species, particularly those which have a contracted part, or neck, between the head and the body, as the Rotifer vulgaris, these particles can be traced in a continuous line in their progress from the mouth to these internal cavities. In this way, by the employment of colouring- matters, Ehrenberg succeeded in ascertaining the existence of a system of digestive cavities in all the known genera of this tribe of animals. There is now, therefore, no reason for admitting that cuticular absorption of nutritive matter ever takes place among this order of beings. Whole generations of these transparent gelatinous ani- malcules may remain immersed for weeks in an indigo solution, without presenting any coloured points in their tissue, except the circumscribed cavities above described. Great variety is found to exist in the forms, 96 THE VITAL FUNCTIONS. situations, and arrangement of the organs of digestion in the Infusoria. They differ also in their degree of compUcation, but without any obvious relation to the magnitude of the ani- malcule. The Monas atomiis, the minutest of the whole tribe, exhibits a nmnber of sacs, opening by as many separate orifices, from a circumscribed part of the surface. In others, as in the Leucophra jxitula, of which Fig. 255 represents the appearance under the micro- scope, there is a long alimentary canal, tra- versing the greater part of the body, taking several spiral turns, and furnished with a great number of blind pouches, or cceca, as sacs of this description, proceeding laterally from any internal canal, and having no other outlet, are technically termed. These cavities become filled with coloured particles immediately after their entrance into the alimentary canal ; and must therefore be considered as so many stomachs NUTRITION IN THE INFUSORIA. .97 provided for the digestion of the food which they receive.* But they are not all filled at the same time, for some continue long in a con- tracted state, so as not to be visible ; while, at another time, they readily admit the coloured food. It is, therefore, only by dint of patient watching that the whole extent of the alimentary tube, and its apparatus of stomachs, can be fully made out. Fig. 255, above referred to, exhibits the Leucophra patula of Ehrenberg,t with a few of its stomachs filled with the opaque particles: but Fig. 256 shows the whole series of organs as it would appear if it could be taken out of the body, and placed in the same relative situation with the eye of the observer as they are in the first figure. In some species, from one to two hundred of these sacs may be counted, connected with the intestinal tube. Many of the larger species, as the Hydatina serittty exhibit a greater concentration of organs, having only a single oval cavity of considerable size, situated in the fore part of the body. In the Rotifer vulgaris, the alimentary canal is a slender tube, considerably dilated near its termi- nation. In some VorticellcBy the intestine, from which proceed numerous cseca, makes a complete circular turn, ending close to its commencement: * Ehrenberg terms these Polygastric infusoria, from the Greek, signifying with many stomachs. t Tiichoria patiilo. MuUer. VOL. II. H 98 THE VITAL FUNCTIONS. Ehrenberg forms of these the tribe of Cycloccela, of which the Vorticella citrina, and the Stentor polymoiyhus, are examples. Thus do we dis- cover the same diversity in the structure of the digestive organs of the several races of these diminutive beings, as is found in the other classes of animals. The Hydatina senta, one of the largest of the infusoria, was found by Ehrenberg to possess a highly developed structure with respect to many systems of organs, which we should never have expected to meet with so low in the scale of ani- mals. As connected with the nutritive functions, it may here be mentioned that the head of this animalcule is provided with a regular apparatus for mastication, consisting of serrated jaws, each having from two to six teeth. These jaws are seen actively opening and shutting when the animal is taking its food, which consists of par- ticles brought within reach of the mouth by means of currents excited by the motions of the cilia. Such are the simple forms assumed by the organs of assimilation among the lowest orders of the animal creation ; namely, digesting cavities, whence proceed various canals, which form a system for the transmission of the prepared nou- rishment to different parts ; but all these cavities and canals being simply hollowed out of the solid substance of the body. As we ascend a NUTRITION IN THE ACTINIA. 99 step higher in the scale, we find that the stomach and intestinal tube, together with their appen- dages, are distinct organs, formed by membranes and coats proper to each, and that they are themselves contained in an outer cavity, which surrounds them, and which receives and collects the nutritious juices after their elaboration in these organs. The Actinia, or Sea Anemone, for example, resembles a polypus in its general form, having a mouth, which is surrounded with tentacula, and which leads into a capacious stomach, or sac, open below, and occupying the greater part of the bulk of the animal ; but while, in the polypus, the sides of the stomach constitute also those of the body, the whole being one simple sac ; in the actinia, spaces inter- vene between the coats of the stomach, and the skin of the animal. As the stomach is not a closed sac, but is open below, these cavities are, in fact, continuous with that of the stomach : they are divided by numerous membranous partitions passing vertically between the skin, and the membrane of the stomach, and giving support to that organ. Fig. 257, repre- senting a vertical section of the Actinia coriacea, displays this internal structure, b is the base 100 THE VITAL FUNCTIONS. or disk, by which the animal adheres to rocks : I is the section of the coriaceous integument, showing its thickness : m is the central aperture of the upper surface, which performs the office of a mouth, leading to s, the stomach, of which the lower orifice is open, and which is suspended in the general cavity, by means of vertical par- titions, of which the cut edges are seen below, uniting at a central point, c, and passing between the stomach and the integument. These mus- cular partitions are coimected above with three rows of tentacula, of which the points are seen at T. The ovaries (o) are seen attached to the partition ; and also the apertures in the lower part of the stomach, by which they communicate with its cavity. If we considered the medusa as having four sto- machs, we might in like manner regard the Aste- riaSy or star-fish, as having ten, or even a greater number. The mouth of this radiated animal is at the centre of the under surface ; it leads into a capacious bag, situated immediately above it, NUTRITION IN THE ASTERIAS. 101 and which is properly the stomach. From this central sac there proceed ten prolongations, or canals, which occupy in pairs the centre of each ray, or division of the body, and subdivide into numerous minute ramifications. These canals, with their branches, are exhibited at c,c, Fig. 258, which represents one of the rays of the Asterias, laid open from the upper side. The canals are supported in their positions by mem- branes, connecting them with the sides of the cavity in which they are suspended. In the various species of Echini, we find that the alimentary tube has attained a more perfect developement ; for instead of constituting merely a blind pouch, it passes entirely through the body of the animal. We here find an (esophagus, or narrow tube, leading from the mouth to the sto- mach ; and the stomach continued into a regular intestine, which takes two turns in the cavity of the body, before it terminates. The alimentary tube in the lower animals fre- quently exhibits dilatations in different parts; these, if situated in the beginning of the canal, may be considered as a succession of stomachs ; while those that occur in the advanced portions are more properly denominated the great intes- tine, by way of distinction from the middle por- tions of the tube, which are generally narrower, and ■ are termed the small intestine. We often see blind pouches, or cteca, projecting from difie- 102 THE VITAL FUNCTIONS. rent parts of the canal ; this is the case with the intestine of the Aphrodita aculeata, or sea-mouse. The intestine, being generally longer than the body, is obliged to be folded many times within the cavity it occupies, and to take a winding course. In some cases, on the other hand, the alimentary tube passes in nearly a straight line through the body, with scarcely any variation in its diameter ; this is the case with the Ascaris, which is a long cylindric worm ; and nearly so with the Lumhricus terrestris, or earth-worm. In the Nais, on the contrary, as shown in Fig. 259, the alimentary tube presents a series of dilatations, which, from the transparency of the skin, may be easily seen in the living animal. The food taken in by these worms is observed to be transferred from the one to the other of its numerous stomachs, backwards and forwards many times, before its digestion^ is ac- complished. The stomach of the Leech is very peculiar in its structure : its form, when dissected off, and removed from the body, is shown in Fig. 260. It is of great capacity, occupying the larger part of the interior of the body ; and its cavity is expanded by folds of its internal membrane into several pouches (c, c, c). Mr. Newport, who has lately examined its structure with great care, NUTRITION IN THE ANNELIDA. 103 finds that each of the ten portions into which it is divided sends out, on the part most remote from the oesophagus (o), two hiteral pouches, or caeca ; which, as they are traced along the canal, become both wider and longer, so that the tenth pair of cffica (a) extends to the hinder extremity of the animal; the intestine (i), which is very short, lying between them.* It has long been known, that if, after the leech has fastened on the skin, a portion of the tail be cut off, the ani- mal will continue to suck blood for an indefinite time : this arises from the circum- stance that the caecal portions of the stomach are laid open, so that the blood received into that cavity flows out as fast as it is swallowed. A structure very similar to that of the leech is * This figure was engraved from a drawing made, at my re- quest, by Mr. Newport, from a specimen which he dissected, and which he was so obliging as to show me. Fig. 261 repre- sents the mouth, within which are seen the three teeth ; and Fig. 262, one of the teeth detached. A paper, descriptive of the structure of the stomach of the leech, by Mr. Newport, was ' lately read at a meeting of the Royal Society. See the abstracts of the proceedings of the Society, for June, 1833. 104 THE VITAL FUNCTIONS. met with in the digestive organs of the Glosso- pora tuherculata, (Hirudo complanata, Linn.) of which Fig. 263 represents a magnified view from the upper side. When seen from the under side, as is shown in Fig. 264, the cavity of the stomach is distinctly seen, pro- longed into several cells, divided by partitions, and directed towards the tail. The two last of these cells (c c) are much longer than the rest, and terminate in two blind sacs, between which is situated a tortuous intestinal tube.* Chapter V. Nutrition in the higher orders of Animals, In proportion as we rise in the animal scale, we find that the operations of Nutrition become still farther multiplied, and that the organs which perform them are more numerous and more com- * In both these figures, t is the tubular tongue, projected from the mouth. In Fig. 263, e are the six eyes, situated on the extremity which corresponds to the head ; and a double lon- gitudinal row of white tubercles is also visible, extending along the back of the animal, e, in Fig. 264, is the entrance into a cavity, or pouch, provided for the reception of the young. See Johnson, Phil. Trans, for J817, p. 343. COMPLEX APPARATIJS FOR NUTRITION. 105 plicated in their structure. The long series of processes requisite for the perfect elaboration of nutriment, is divided into different stages ; each process is the work of a separate apparatus, and requires the influence of different agents. We no longer find that extreme simplicity which we noticed as so remarkable in the hydra and the medusa, Avhere the same cavity performs at once the functions of the stomach and of the heart. The manufacture of nutriment, if we may so express it, is, in these lower zoophytes, con- ducted upon a small scale, by less refined methods, and with the strictest economy of means ; the apparatus is the simplest, the agents the fewest possible, and many different operations are carried on in one and the same place. As we follow the extension of the plan in more elevated stages of organic developement, we find a farther division of labour introduced. Of this we have already seen the commencement in the multiplication of the digesting cavities of the Leech and other Annelida : but, in animals which occupy a still higher rank, we observe a more complete separation of offices, and a still greater complication of organs. The principle of the division of labour is carried tp a much greater extent than in the inferior departments of the animal creation. Besides the stomach, or receptacle for the unassimilated food, another organ, the heart, is provided for the uniform dis- H)6 THE VITAL FUNCTIONS. tribution of the nutritious fluids elaborated by the organs of digestion. This separation of functions, again, leads to the introduction of another system of canals or vessels, for trans- mitting the fluids from the organs which prepare them to the heart, as into a general reservoir. In the higher orders of the animal kingdom, all these processes are again subdivided and varied, according to the species of food, the habits, and mode of life, assigned by nature to each individual species. For the purpose of conveying clearer notions of the arrangement of this extensive system of vital organs, I have drawn the annexed plan (Fig. 265), which ex- hibits them in their natural order of connexion, and as they might be supposed to apj^ear in a side view of the interior of a quadruped. To COMPLEX APPARATUS FOR NUTRITION. 107 this diagram I shall make frequent reference in the following description of this system. The food is, in the first place, prepared for digestion by several mechanical operations, which loosen its texture and destroy its cohe- sion. It is torn asunder and broken down by the action of the jaws and of the teeth; and it is, at the same time, softened by an admixture with the fluid secretions of the mouth. It is then collected into a mass, by the action of the muscles of the cheek and tongue, and swallowed by the regulated contractions of the different parts of the throat. It now passes along a mus- cular tube, called the (Esophagus, (represented in the diagram by the letter o,) into the stomach (s), of which the entrance (c;) is called the cardia. In the stomach the food is made to undergo various chemical changes ; after which it is con- ducted through the aperture termed the pylorus (p), into the canal of the intestine (i i), where it is further subjected to the action of several fluid secretions derived from large glandular organs situated in the neighbourhood, as the liver (l) and the pancreas ; and elaborated into the fluid which is termed Chyle. The Chyle is taken up by a particular set of vessels, called the Lacteals, which transmit it to the heart (h). These vessels are exceedingly numerous, and arise by open orifices from the 108 THE VITAL FUNCTIONS. inner surface of the intestines, whence they absorb, or drink up the chyle. They may be compared to internal roots, which unite as they ascend along the mesentery (m), or membrane connecting the intestines with the back ; forming- larger and larger trunks, till they terminate in an intermediate reservoir (r), which has been named the Receptacle of the Chyle. From this receptacle there proceeds a tube, which, from its passing through the thorax, is called the Tho- racic duct (t) ; it ascends along the side of the spine, which protects it from compression, and opens at v, into the large veins which are pour- ing their contents into the auricle, or first cavity of the heart (u), whence it immediately passes into the ventricle, or second cavity of that organ (h). Such, in the more perfect animals, is the circuitous and guarded route, which every particle of nourishment must take before it can be added to the general mass of circulating fluid. By its admixture with the blood already con- tained in these vessels, and its purification by the action of the air in the respiratory organs (b), the chyle becomes assimilated, and is distri- buted by the heart through appropriate channels of circulation called arteries (of which the com- mon trunk, or Aorta, is seen at a), to every part of the system ; thence returning by the veins (v, V, V,) to the heart. The various modes in COMPLEX APPARATUS FOR NUTRITION. 109 which these functions are conducted in the seve- ral tribes of animals will be described hereafter. It will be sufficient for our present purpose to state, by way of completing the outline of this class of functions, that, like the returning sap of plants, the blood is made to undergo farther modifications in the minute vessels through which it circulates ; new arrangements of its elements take place during its passage through the subtle organization of the glands, which no microscope has yet unravelled : new products are here formed, and new properties acquired, adapted to the respective purposes which they are to serve in the animal economy. The whole is one vast Laboratory, where mechanism is sub- servient to Chemistry, where Chemistry is the agent of the higher powers of Vitality, and where these powers themselves minister to the more exalted faculties of Sensation and of Intellect. The digestive functions of animals, however complex and varied, and however exquisitely contrived to answer their particular objects, yet afford less favourable opportunities of tracing . distinctly the adaptation of means to the re- spective ends, than the mechanical functions. This arises from the circumstance that the pro- cesses they effect imply a refined chemistry, of which we have as yet but a very imperfect knowledge ; and that we are also ignorant of the nature of the vital agents concerned in pro- 110 THE VITAL FUNCTIONS. ducing each of the chemical changes which the food must necessarily undergo during its assimi- lation. We only know that all these changes are slowly and gradually effected ; the materials having to pass through a great number of inter- mediate stages before they can attain their final state of elaboration. Hence we are furnished with a kind of scale whereby, whenever we can ascertain the degrees of difference existing between the chemical con- dition of the substance taken into the body, and that of the product derived from it, we may estimate the length of the process required, and the amount of power necessary for its conversion into that product. It is obvious, for example, that the chemical changes which vegetable food must be made to undergo, in order to assimilate it to blood, must be considerably greater than those required to convert animal food into the same fluid, because the latter is itself derived, with only slight modification, immediately from the blood. We accordingly find it to be an esta- blished rule, that the digestive organs of animals which feed on vegetable materials are remark- able for their size, their length, and their com- plication, when compared with those of car- nivorous animals of the same class. This rule applies, indeed, universally to Mammalia, Birds, Reptiles, Fishes, and also to Insects : and below these we can scarcely draw the comparison. COMPLEX APPARATUS FOR NUTRITION. 1 1 1 because nearly all the inferior tribes subsist wholly upon animal substances. Many of these latter animals have organs capable of extracting nourishment from substances which we should hardly imagine contained any sensible portion. Thus, on examining the stomach of the earth- worm, we find it always filled with moist earth, 'which is devoured in large quantities, for the sake of the minute portion of vegetable and animal materials that happen to be intermixed with the soil ; and this slender nutriment is suf- ficient for the subsistence of that animal. Many marine worms, in like manner, feed apparently upon sand alone ; but that sand is generally intermixed with fragments of shells, which have been pulverized by the continual rolling of the tide and the surge ; and the animal matter con- tained in these fragments, afibrds them a supplj^ of nutriment adequate to their wants. It is evi- dent, that when, as in the preceding instances, large quantities of indigestible materials are taken in along with such as are nutritious, the stomach and other digestive cavities must be rendered more than usually capacious. It is obvious also that the structure of the digestive organs must bear a relation to the mechanical texture, as well as the chemical qualities of the food ; and this we find to be the case in a variety of instances, which will hereafter be specified. The activity of the digestive functions and the 112 THE VITAL FUNCTIONS. Structure of the organs, will also be regulated by a great variety of other circumstances in the condition of the animal, independently of the mechanical or chemical nature of the food. The greater the energy with which the more pecu- liarly animal functions of sensation and muscular action are exercised, the greater must be the demand for nourishment, in order to supply the expenditure of vital force created by these exer- tions. Compared with the torpid and sluggish reptile, the active and vivacious bird or quadruped requires and consumes a much larger quantity of nutriment. The tortoise, the turtle, the toad, the frog, and the chamelion, will, indeed, live for months without taking any food. Fishes, which, like reptiles, are cold-blooded animals, although at all times exceedingly voracious when supplied with food, yet can endure long fasts with impunity. The rapidity of developement has also great influence on the quantity of food which an ani- mal requires. Thus the caterpillar, which grows very quickly, and must repeatedly throw off* its integuments, during its continuance in the larva state, consumes a vast quantity of food compared with the size of its body ; and hence we find it provided with a digestive apparatus of consi- derable size. 113 Chapter VI. PREPARATION OF FOOD. § 1 . Prehension of Liquid Food, In studying the series of processes which con- stitute assimilation, our attention is first to be directed to the mode in which the food is in- troduced into the body, and to the mechanical changes it is made to undergo before it is sub- jected to the chemical action of the digestive organs. The nature of these preliminary pro- cesses will, of course, vary according to the tex- ture and mechanical condition of the food. Where it is already in a fluid state, mastication is unne- cessary, and the receiving organs consist simply of an apparatus for suction. This is the case very generally with the Entozoa, which subsist upon the juices of other animals, and which are all provided with one or more sucking orifices, often extended in the form of a tube or proboscis.* The Hydatid, for instance, has four sucking apertures disposed round the head of the animal : * Some species of Fasciolce, or flukes, are furnished with two, three, six, or more sucking disks, by which they adhere to sur- faces : to these animals the names Distoma, Tristoma, Hexas- fowrt, and Polystoma have been given; but these denominations, implying a plurality of mouths, are evidently incorrect, since the VOL. II. I 114 THE VITAL FUNCTIONS. the TcBiiia has orifices of this kind in each of its jointed segments : the Ascaris and the Earth- worm have each a simple mouth. The margin of the mouth is often divided, so as to compose lips ; of these there are generally two, and in the leech there are three. In some rare cases, as in the Planaria, there is, besides the ordinary mouth, a tube also provided for suction, in a dif- ferent part of the body, and leading into the same stomach.* When the instrument for suction extends for some length from the mouth, it is generally termed ^proboscis: such is the apparatus of the butterfly, the moth, the gnat, the house fly, and other insects that subsist on fluid aliment. The pro- boscis of the Lepidoptera, (Fig. 266), is a double tube, constructed by the two edges being rolled longitudi- nally till they meet in the middle of the lower surface, thus forming a tube on each side, but leaving also another tube, intermediate to the two lateral ones. This middle tube is formed by the junction sucking' disks are not perforated, and do not perform the office of mouths ; and the true mouth for the reception of food is single. Cuvier discovered an animal of this class furnished with above a hundred of these cup-shaped sucking organs. See Edinburgh Pliilos. Journal, XX. 101. » Phil Trans, for 1822, 442. PREHENSION OF LIQUID FOOD. 1 1 O of two grooves, which, by the aid of a curious apparatus of hooks, resembUng those of the la- minae of a feather already described,* lock into each other, and can be either united into an air tight canal, or be instantly separated at the pleasure of the animal. Reaumur conceives that the lateral tubes are intended for the reception of air, while the central canal is that which con- veys the honey, which the insect sucks from flowers, by suddenly unrolling the spiral coil, into which the proboscis is usually folded, and darting it into the nectary. t In the Hemiptera, the proboscis is a tube, either straight or jointed, guarded by a sheath, and acting like a pump. The Dipt era have a more complicated instrument for suction, con- sisting of a tube, of which the sides are strong and fleshy, and moveable in every direction, like the trunk of an elephant : it has, at its ex- tremity, a double fold, resembling lips, which are well adapted for suction. The gnat, and other insects which pierce the skin of animals, have, for this purpose, instruments termed, from their shape and office, lancets. I In the ouat they are five or six in number, finer than a hair, ex- ceedingly sharp, and generally barbed on one side. In the Tabanus, or horse-fly, they are flat * Volume i. page .570. + Kirby and Spence's Entomology, vol. ii. p. 390. \ Ibid, vol. iii. p. 467. 116 THE VITAL FUNCTIONS. like the blade of a knife. These instruments are sometimes constructed so as to form, by their union, a tube adapted for suction. In the flesh- fly, the proboscis is folded like the letter Z, the upper angle pointing to the breast, and the lower one to the mouth. In other flies there is a single fold only. Those insects of the order Hymenoptera, which, like the bee, suck the honey of flowers, have, together with regular jaws, a proboscis formed by the prolongation of the lower lip, which is folded so as to constitute a tube : this tube is protected by the mandibles, and is pro- jected forwards by being carried on a pedicle, which can be folded back when the tube is not in use. The mouths of the Acephalous 3Iollnsca are merely sucking apertures, with folds like lips, and without either jaws, tongue, or teeth, but having often tentacula arising from their margins. Among fishes, we meet with the family of Cyclostomata, so called from their having a cir- cular mouth, formed for suction. The margin of this mouth is supported by a ring of cartilage, and is furnished with appropriate muscles for producing adhesion to the surfaces to which it is applied ; the mechanism and mode of its attach- ment being similar to that of the leech. To this family belong the Myxine and the Lamprey. So great is the force of adhesion exerted by this PREHENSION OF LIQUID FOOD. 117 sucking apparatus, that a lamprey has been raised out of the water with a stone, weighing ten or twelve pounds, adhering to its mouth. Humming birds have a long and slender tongue, which can assume the tubular form, like that of the butterfly or the bee, and for a similar pur- pose, namely, sucking the juices of flowers. Among the mammalia, the Vampire Bat affords another instance of suction by means of the tongue, which is said to be folded into a tubular shape for that purpose. But suction among the mam- malia is almost always performed by the muscles of the lips and cheeks, aided by the movements of the tongue, which, when withdrawn to the back of the cavity, acts like the piston of a pump. In the lamprey this hydraulic action of the tongue is particularly remarkable. Many quadrupeds, however, drink by repeatedly dip- ping their tongue into the fluid, and quickly drawing it into the mouth. § 2. Prehension of Solid Food. When the food consists of solid substances, organs must be provided ; first, for their pre- hension and introduction into the mouth ; se- condly, for their detention when so introduced ; and thirdly, for their mechanical division into smaller fragments. 118 THE VITAL FUNCTIONS. Of those instruments of prehension which are not portions of the mouth itself, and which form a series of variously constructed organs, extend- ing from the tentacula of the polypus to the proboscis of the elephant, and to the human arm and hand, some account has already been given in the history of the mechanical functions: but, in a great number of instances, prehension is performed by the mouth, or the parts which are extended from it, and may be considered as its appendices. The prehensile power of the mouth is derived principally from the mecha- nical form and action of the jaws, which open to receive, and close to detain the bodies intended as food ; and to this latter purpose, the teeth, when the mouth is furnished with them, likewise materially contribute, although their primary and more usual office is the mechanical division of the food, by means of mastication, an action in which the jaws, in their turn, co-operate. Another principal purpose effected by the jaws is that of giving mechanical power to the muscles, which, by acting upon the sides of the cavity of the moiith, tend to compress and propel the contained food. We find, accord- ingly, that all animals of a highly developed structure are provided with jaws. Among the animals which are ranked in the class of Zoophytes, the highest degrees of deve- lopement are exhibited by the Echinodermata, JAWS OF THE ECHINUS. 119 and in them we find a remarkable perfection in the organs of mastication. The mouth of the Echinus is surrounded by a frame-work of shell, consisting of five converging pieces, each armed with a long tooth ; and for the movement of each part there are provided separate muscles, of which the anatomy has been minutely de- scribed by Cuvier. In the shells of the echini that are cast on the shore, this calcareous frame is usually found entire in the inside of the outer case ; and Aristotle having noticed its resem- blance to a lantern, it has often gone by the whimsical name of the lantern of Aristotle. In all articulated animals which subsist on solid aliment, the apparatus for the prehension and mastication of the food, situated in the mouth, is exceedingly complicated, and admits of great diversity in the different tribes ; and, indeed, the number and variety of the parts of which it consists is so great, as hardly to admit of being comprehended in any general descrip- tion. In most insects, also, their minuteness is an additional obstacle to the accurate obser- vation of their anatomy, and of the mechanism of their action. The researches, however, of Savigny* and other modern entomologists have gone far to prove, that amidst the infinite vari- * See his " Theorie des Ore;anes de la bouche des Animaux invert^bres et articules," which forms the first part of the '^Vle- moires siir les Animaux sans vertebres." Paris, 1816. 120 THE VITAL FUNCTIONS. ations observable in the form and arrangement of the several parts of these organs, there is still preserved, in the general plan of their con- struction, a degree of uniformity quite as great as that which has been remarked in the fabric of the vertebrated classes. Not only may we recognise in every instance the same elements of structure, but we may also trace regular chains of gradation connecting forms apparently most remote, and organs destined for widely dif- ferent uses : so that even when there has been a complete change of purpose, we still perceive the same design followed, the same model copied, and the same unifonnity of plan pre- served in the construction of the organs of every kind of mastication ; and there prevails in them the same unity of system as is displayed in so marked a manner in the conformation of the organs of progressive motion. The jaws, which in one tribe of insects are formed for breaking- down and grinding the harder kinds of food, are, in another, fitted for tearing asunder the more tough and fibrous textures ; they are fashioned, in a third, into instruments for taking up the semi-fluid honey prepared by flowers ; while, again, in a fourth, they are prolonged and folded into a tubular proboscis, capable of suction, and adapted to the drinking of fluid aliment. Pursuing the examination of these organs in another series of articulated animals. JAWS OP ARTICULATA. 121 we find them gradually assuming the characters, as well as the uses of instruments of prehension, of weapons for warfare, of pillars for support, of levers for motion, or of limbs for quick pro- gression. Some of these remarkable metamor- phoses of organs have already attracted our attention in a former part of this treatise.* Jaws pass into feet, and feet into jaws, through every intermediate form ; and the same individual often exhibits several steps of these transitions ; and is sometimes provided also with super- numerary organs of each description. In the Arachnida, in particular, we frequently meet with supernumerary jaws, together with various ap- pendices, which present remarkable analogies of form with the antennae, and the legs and feet of the Crustacea. The principal elementary parts which enter into the composition of the mouth of an insect, when in its most perfect state of developement, are the seven following: a pair of upper jaws, a pair of lower jaws, an upper and a lower lip, and a tongue. t These parts in the Locusta * Vol. i. p. 289. t All these parts, taken together, were termed by Fabricius instruinenta cibaria ; and upon their varieties of structure he founded his celebrated system of entomological classification. Kirby and Spence have denominated them trophi. See their Introduction to Entomology, vol. iii. p. 417. To the seven elements above enumerated Savigny adds, in the Hemiptera, an eighth, which he terms the Epiylossa. 122 THE VITAL FUNCTIOMS. viriclisslma, or common grasshopper, are deli- neated in their relative situations, but detached from one another, in Fig. 267. The upper jaws (m), which are termed the mandibles^ are those principally employed for the mastication of hard substances ; they are accordingly of greater strength than the lower jaws, and their edges are generally deeply serrated, so as to act like teeth in dividing and bruising the food. Some of these teeth are pointed, others wedge-shaped, and others broad, like grinders ; their form being in each particular case adapted to the mechanical texture of the substances to which they are designed to be applied. Thus the mandibles of some MelolonthcB have a projection, rendered rough by numerous deep transverse furrows, converting it into a file for wearing down the I JAWS OF INSECTS. 123 dry leaves which are their natural food.* In most cases, indeed, we are, in like manner, enabled, from a simple inspection of the shape of the teeth, to form tolerably accurate ideas of the kind of food on which the insect naturally subsists, t Above, or rather in front of the mandibles, is situated the Jabnim, or upper lip (u). It is usually of a hard or horny texture, and admits of some degree of motion : but its form and direction are exceedingly various in different tribes of insects. The lower pair of jaws (j), or maxill(By as they have been termed, are behind the mandibles, and between them is situated the labium, or lower lip (l), which closes the mouth below, as the labnim does above. In the grass- hopper, each maxilla consists of an outer and an inner plate (o and i). The jaws of insects are confined, by their articulations with the head, to motions in a horizontal plane only, so that they open and close by a lateral movement, and not vertically upwards and downwards, as is the case with the jaws of vertebrated animals. The maxillae are, in most cases, employed prin- cipally for holding the substances on which the dividing or grinding apparatus of the mandibles * Kiioch, quoted by Kirby. t See a memoir by Marcel des Series, in the Annales du Museum d'Hist. Nat. xiv. 56. 124 THE VITAL FUNCTIONS. is exerted. A similar use may be assigned also to the organs denominated Palpi, or AntennulcB (p, q), which are jointed filaments, or processes, attached to different parts of the mouth, and most usually to the maxillae and the labium ; the former (p) being termed the maxillary, and the latter (q) the labial jxilpi. In addition to these parts, another, which, from its supposed use, has been denominated Glossa, or tongue (g), is also generally found. For an account of the various modifications which these parts receive in different tribes and species, I must refer to works which treat pro- fessedly of this branch of comparative anatomy. I shall content myself with giving a single example of the conversion of structure here alluded to, in that of the rostrum, or proboscis of the Cimex nigricornis. This insect belongs to the order Hemiptera, which has been usually characterised as being destitute of both man- dibles and jaws, and as having, instead of these parts, an apparatus of very diflferent construc- tion, designed to pierce the skin of animals and suck their juices. But Savigny, on applying the principles of his theory, has recognised, in the proboscis of the Cimex, the existence of all the constituent elements that are found in the mouth of insects formed for the mastication of solid food. This proboscis consists of four elon- gated filaments, contained in a kind of sheath : JAWS OF INSECTS. 125 268 269 these filaments are represented in Fig. 208, separated to a little distance from each other, in order that their respective origins may be distinctly seen ; the one set (q) being prolongations of the mandibles (j), and the other set (p) being, in like manner, prolongations of the maxillae (m). Between these filaments, and near their com- mencement, is seen a pointed cartilaginous body (g), which is the glossa, or tongue ; and the aperture seen at its root is the passage into the oesopha- gus. The sheath is merely the elongated labium, of which the base is seen at l, in Fig. 268 ; but is represented in its whole length in Fig. 269, where the groove for containing the filaments above described, is apparent. In the mouths of the Annelida we often meet with hard bodies, which serve the purposes of jaws and of teeth. The retractile proboscis of the Aphrodite, or sea-mouse, is furnished with four teeth of this description. The Leech has, immediately within its lips, three semi-circular teeth, with round and sharp cutting edges : they are delineated in Fig. 262, in their relative 126 THE VITAL FUNCTIONS. positions ; and Fig. 263 represents one of the teeth detached from the rest. It is with these teeth that the leech pierces the skin of the animals whose blood it sucks ; and as soon as the wound is inflicted, the teeth, being moveable at their base, fall back, leaving the opening of the mouth free for sucking. The wound thus made is of a peculiar form, being composed of three lines, radiating from a centre, where the three teeth had penetrated. Most of the Mollusca which inhabit univalve shells are provided with a tubular organ, of a cylindric or conical shape, capable of elongation and contraction, by circular and longitudinal muscular fibres, and serving the purpose of a proboscis, or organ of prehension for seizing and conveying food into the mouth. These tubes are of great size in the Buccinum, the Murex, and the Valuta, as also in the Doris, which, though it has no shell, is likewise a gasteropode. In those mollusca of this order which have not a proboscis, as the Limax, or slug, the Helix, or snail, and the Aplysia, or sea-hare, the mouth is furnished with broad lips, and is supported by an internal cartilage, having several tooth-like 270 projections, which assist in laying hold of the substances taken as food. That of the snail is represented in Fig. 270. All the Sepice, or cuttle fish tribe, are fur- nished, at the entrance of the mouth, with two JAWS OF FISHES. 127 horny jaws, having a remarkable resemblance to the bill of a parrot ; excepting that the lower piece is the largest of the two, and covers the upper one, which is the reverse of what takes place in the parrot. These constitute a powerful instrument for breaking the shells of the mol- lusca and Crustacea which compose the usual prey of these animals. Fishes almost always swallow their food entire, so that their jaws and teeth are employed prin- cipally as organs of prehension and detention ; and the upper jaw, as well as the lower one, being moveable upon the cranium, they are capable of opening to a great width. The bony pieces which compose the jaws are more nume- rous than the corresponding bones in the higher classes of vertebrata, and they appear, therefore, as if their developement had not proceeded suf- ficiently far to effect their consolidation into more compact structures.* Fishes which live upon other animals of the same class having a soft texture, are furnished with teeth constructed merely for seizing their prey, and perhaps also for slightly dividing it, so as to adapt it to being swallowed. These teeth are of various shapes, though usua^'y sharp * Attempts have been made to trace analogies between the different segments of the jaws of fishes and corresponding parts of the mouths of Crustacea and of insects : but the justness of these analogies is yet far from being satisfactorily proved. 128 THE \ITAL FUNCTIONS. at the points, and either conical or hooked at the extremity, with the points always directed backwards, in order to prevent the escape of the animal which has been seized. Those fishes which subsist on testaceous moUusca have teeth with grinding surfaces, and their jaws are also adapted for mastication. Every part of the mouth, tongue, and even throat, may afford lodgement for teeth in this class of animals. Almost the whole cavity of the mouth of the Anarrhichas lupus, or wolf-fish, may be said to be paved with teeth, a triple row being im- planted on each side ; so that this fish exerts great power in breaking shells. The Shark has numerous rows of sharp teeth, with serrated margins : these at first sight appear to be for- midable instruments ; but as the teeth in the opposite jaws do not meet, it is evident that they are not intended for cutting, like the incisors of mammalia. Among Reptiles, we find the Batrachia almost wholly destitute of teeth. Frogs, indeed, exhibit two rows of very fine points ; the one in the upper jaw, and the other passing transversely across the palate : they may be considered as teeth existing in a rudimental state ; for they are not sufiiciently developed to be useful in mastication. There are about forty of these minute teeth on each side in the frog. In the Salamander, there are sixty above and below ; and also thirty on each side of the palate. TONGUES OF REPTILES. 129 The tongue of the frog is of great length ; its root is attached close to the fore part of the lower jaw, while its point, which is cloven, is turned backwards, extending into the throat and acting like a valve in closing the air passage into the lungs. If, when this animal has ap- proached within a certain distance of the insect it is about to seize, we watch it with attention, we are surprised to observe the insect suddenly disappear, without our being able to perceive what has become of it. This arises from the frog having darted out its tongue upon its victim with such extreme quickness, and withdrawn it, with the insect adhering to it, so rapidly, that it is scarcely possible for the eye to follow it in its motion. The Chameleon also has a very long and slender tongue, the extremity of which is dilated into a kind of club, or spoon, and covered with a glutinous matter : with this instrument the animal catches insects from a considerable distance, by a similar manoeuvre to that prac- tised by the frog.* Serpents and Lizards have generally curved or conical teeth, calculated rather for tearing and holding the food, than for masticating it : like those of fishes, they are affixed partly to the * Mr. Houston has given a description of the structure of this organ, and of the muscles by which it is moved, in a paper con tained in the Transactions of the Royal Irish Academy, vol. xv. p. 177. VOL. IL K 1.30 THE VITAL FUNCTIONS. jaws, and partly to the palate. The Cheloiiian reptiles have no teeth ; their office being sup- plied by the sharp cutting edges of the horny portion of the jaws. Birds, as well as serpents, have a moveable upper jaw ; but they are also provided with beaks of various forms, in which we may trace an exact adaptation to the kind of food appro- priated to each tribe : thus predaceous birds, as the eagle and the hawk tribe, have an exceed- ingly strong hooked beak, for tearing and di- viding the flesh of the animals on which they prey ; while those that feed on insects, or on grain, have pointed bills, adapted to picking up minute objects. Aquatic birds have generally flattened bills, by which they can best select their food among the sand, the mud, or the weeds at the bottom of the water ; and their edges are frequently serrated, to allow the fluid to filter through, while the solid portions are retained in the mouth. The duck affords an instance of this structure ; which is, however, still more strongly marked in the genus 31ergus, or Mergansor, where the whole length of the margin of the l)ill is beset with small sharp pointed teeth, directed backwards : they are par- ticularly conspicuous in the Mergus serrator, or red-breasted Mergansor. The object of the barbs and fringed processes which are appended to the tongue in many birds, such as that of the I \ JAAVS OF BIRDS. 131 Toucan and the Parrakeet, appears, in like manner, to be the detention of substances intro- duced into the mouth. The beak of the Hcematopus, or Oyster-catcher, has a wedge shape, and acts like an oyster- knife for opening bivalve shells. In the LiOxia curvirostra, or Cross-bill, the upper and lower mandibles cross each other when the mouth is closed, a structure which enables this bird to tear open the cones of the pine and fir, and pick out the seeds, by insi- nuating the bill between the scales. It can split cherry stones with the utmost ease, and in a very short time, by means of this peculiarly shaped bill.* Birds which dive for the purpose of catching fish have often a bill of considerable length, which enables them to secure their prey, and change its position till it is adapted for swal- lowing. The Rliyiichops, or black Skimmer, has a very singularly formed beak ; it is very slender, but the lower mandible very much exceeds in length the upper one, so that while skimming the waves in its flight, it cuts the Avater like a plough-share, catching the prey which is on the surface of the sea. The Woodpecker is furnished with a singular "" See a paper on the mechanism of the bill of this bird, by Mr. Yarrell, in the Zooloo^ical Journal, iv. 459. ^:V2 THE VITAL FUNCTIONS. apparatus for enabling it to dart out with great velocity its long and pointed tongue, and transfix the insects on which it principally feeds ; and these motions are performed so quickly that the eye can scarcely follow them. This remarkable mechanism is delineated in Fig. 271, vrhicli represents the head of the woodpecker, with the skin removed, and the parts dissected. The tongue itself (t) is a slender sharp-pointed horny cylinder, having its extremity (u) beset with barbs, of which the points are directed backwards: it is supported on a slender Os Hyoides, or lingual bone, to the posterior end of which the extremities of two very long and narrow cartilaginous processes are articulated.* The one on the right side is shown in the figure. { * These cartilages conespond in situation, at the part, at least, where they are joined to the os liyoides, to wliat are called the corniia, or horns of that bone, in other animals. TONGUE OF THE MOODPECKEK. 1 .'J."> nearly in the whole extent of its course, at c, n, E, E, and a small portion of the left cartilage is seen at l. The two cartilages form, at their junction with the tongue, a very acute angle, slightly diverging as they proceed backwards ; until, bending downwards (at c), they pass ob- liquely round the sides of the neck, connected by a membrane (m) ; then, being again inflected upwards, they converge towards the back of the head, where they meet, and, being enclosed in a common sheath, are conducted together along a groove, which extends forwards, along the middle line of the cranium (e), till it arrives between the eyes. From this point, the groove and the two cartilages it contains, which are now more closely conjoined, are deflected towards the right side, and terminate at the edge of the aperture of the right nostril (f), into which the united cartilages are finally inserted. In order that their course may be seen more distinctly, these cartilages are represented in the figure (at d), drawn out of the groove provided to receive and protect them.* A long and slender muscle is attached to the inner margin of each of these cartilages, and their actions conspire to raise the lower and most bent parts of the cartilages, so that their curvature is diminished, and the tongue protruded to a considerable dis- S is tlio laiLre salixarv ^^.-laiitl uii tlio riiiht sk\v. 134 THE VITAL FUNCTIONS. tance, for the purpose of catching insects. As soon as this has been accomplished, these muscles being suddenly relaxed, another set of fibres, passing in front of the anterior portion of the cartilages nearly parallel to them, are thrown into action, and as suddenly retract the tongue into the mouth, with the insect adhering to its barbed extremity. This muscular effort is, how- ever, very materially assisted by the long and tortuous course of these arched cartilages, which are nearly as elastic as steel springs, and effect a considerable saving of muscular poM^er.* This was the more necessary, because, while the bird is on the tree, it repeats these motions almost incessantly, boring holes in the bark, and pick- ing up the minutest insects, with the utmost celerity and precision. On meeting with an ant- hill, the woodpecker easily lays it open by the combined efforts of its feet and bill, and soon makes a plentiful meal of the ants and their eggs. Among the Mammalia which have no teeth, the Myrmecophaga, or Ant-eater, practises a re- markable manoeuvre for catching its prey. The tongue of this animal is very long and slender, and has a great resemblance to an earth-worm : that of the two-toed ant-eater is very nearly one-third of the length of the whole body ; and * An account ot" this uiechanism is given by Mr. Waller, in the Phil. Trans, lor 171(S, p, 509. TONGUE OF THE ANT-EATElt. 13o at its base is scarcely thicker than a crow-quill. It is furnished with a long and powerful muscle, which arises from the sternum, and is continued into its substance, affording the means pf a quick retraction, as well as lateral motion ; while its elongation and other movements are effected by circular ffbres, which are exterior to the former. When laid on the groinid in the usual track of ants, it is soon covered with these insects, and being suddenly retracted, transfers them into the mouth ; and as, from their minuteness, they require no mastication, they are swallowed un- divided, and without there being any necessity for teeth. The lips of quadrupeds are often elongated for the more ready prehension of food, as we see exemplified in the R/ii/ioceroSf whose upper lip is so extensible as to be capable of performing the office of a small proboscis. The Sorex moschatus^ or musk shrew, whose favourite food is leeches, has likewise a very moveable snout, by which it gropes for, and seizes its prey from the bottom of the mud. More frequently, how- ever, this office of prehension is performed by the tongue, which for that purpose is very flexible and much elongated, as we see in the Cameleopardj where it acts like a hand in grasp- ing and bringing down the branches of a tree.* * Home, Lectures, &c. vi. Phitc 32. 136 THE VITAL FUNCTIONS', In the animals belonging to the genus FeliSf each of the papillae of the tongue is armed with a horny sheath terminating in a sharp point, which is directed backwards, so as to detain the food and prevent its escape. These prickles are of great size and strength in the larger beasts of prey, as the Lion and the Tiger ; they are met with also in the Opossum, and in many species of bats, more especially those belonging to the genus Pteropus : all these horny productions have been regarded as analogous to the lingual teeth of fishes, already noticed. The mouth of the Ornithorhyncus has a fonn of construction intermediate between that of quadnipeds and birds ; being furnished, like the former, with grinding teeth at the posterior part of both the upper and lower jaws, but they are of a horny substance ; and the mouth is terminated in front by a homy bill, greatly resembling that of the duck, or the spoon- bill. The Whale is furnished with a singular appa- ratus designed for filtration on a large scale. The palate has the form of a concave dome, and from its sides there descends vertically into the mouth, a multitude of thin plates set parallel to each other, with one of their edges directed towards the circumference, and the other towards the middle of the palate. These plates are known by the name of whalebone^ and iheir general form MOUTH OF THE WHALE. 137 and appearance, as they hang from the roof of the palate, are shown in Fig. '272, which repre- sents only six of these plates.* They are con- nected to the bone by means of a white liga- mentous substance, to which they are imme- diately attached, and from which they appear to grow : at their inner margins, the fibres, of which their tex- ture is throughout composed, cease to adhere together ; but, being loose and detached, form a kind of fringe, calcu- lated to intercept, as in a sieve, all solid or even gelatinous substances that may have been admitted into the cavity of the mouth, which is exceedingly capacious; for as the plates of whalebone grow only from the margins of the upper jaw, they leave a large space with- in, which though narrow^ an- teriorly is wider as it extends backwards, and is capable of holding a large quantity of water. Thus the whale is enabled to collect a whole shoal of mol- mm. im W^\i * In the Piked Whale the plates of whalebone are placed very near together, not being a quarter of an inch asunder; and there are above three hundred plates in the outer rows on each side of the mouth. 138 THE VITAL FUNCTIONS. lusca, and other small prey, by taking into its" mouth the sea water which contains these ani- mals, and allowing it to drain off through the sides, after passing through the interstices of the net work formed by the filaments of the whale- bone. Some contrivance of this kind was even necessary to this animal, because the entrance into its oesophagus is too narrow to admit of the passage of any prey of considerable size ; and it is not furnished M'ith teeth to reduce the food into smaller parts. The principal food of the Balcena 3Iijsticetus, or great whalebone whale of the Arctic Seas, is the small Clio Borealis, which swarms in immense numbers in those regions of the ocean ; and which has been al- ready delineated in Fig. 120.* These remarkable organs for filtration entirely supersede the use of ordinary teeth ; and ac- cordingly no traces of teeth are to be discovered either in the upper or lower jaw. Yet a ten- dency to conform to the type of the mammalia is manifested in the early conformation of the whale ; for rudiments of teeth exist in the in- terior of the lower jaw before birth, lodged in deep sockets, and forming a row on each side. The developement of these imperfect teeth pro- ceeds no farther ; they even disappear at a very early period, and the groove which contained * Vol. i. p. 258. :mouth of the whale. 1.3J) them closes over, and after a short time can no longer be seen. For the discovery of this curious fact we are indebted to Geoftroy St. Hilaire.* In connexion with this subject, an analogous fact which has been noticed in the parrot may here be mentioned. The young of the parrot, while still in the egg, presents a row of tubercles along the edge of the jaw, in ex- ternal appearance exactly resembling the rudi- ments of teeth, but without being implanted into regular sockets in the maxillary bones : they are formed, however, by a process precisely similar to that of dentition ; that is, by depo- sition from a vascular pulp, connected with the jaw. These tubercles are afterwards consoli- dated into one piece in each jaw, forming by their union the beak of the parrot, in a manner perfectly, analogous to that which leads to the construction of the compound tooth of the ele- phant, and which I shall presently describe. The original indentations are obliterated as the beak advances in growth ; but they are per- manent in the bill of the duck, where the structure is very similar to that above described in the embryo of the parrot. * Cuvier, Ossemens Fossilcs, 3iue edition, torn. v. p. 3(i0. 140 THE VITAL FUNCTIONS. § 3. Masticatkm by means of Teeth. The teeth, being essential instruments for seizing and holding the food, and effecting that degree of mechanical division necessary to prepare it for the chemical action of the stomach, perform, of course, a very important part in the economy of most animals ; and in none more so than in the Mammalia, the food of which generally re- quires considerable preparation previous to its digestion. There exist, accordingly, the most intimate relations between the kind of food upon which each animal of this class is intended by nature to subsist, and the form, structure, and position of the teeth ; and these relations may, indeed, be also traced in the shape of the jaw, in the mode of its articulation with the head, in the proportional size and distribution of the muscles which move the jaw, in the form of the head itself, in the length of the neck, and its position on the trunk, and indeed in the whole conformation of the skeleton. But since the nature of the appropriate food is at once indi- cated by the structure and arrangement of the teeth, it is evident that these latter organs, in particular, will afford to the naturalist most im- portant characters for establishing a systematic classification of animals, and more especially of quadrupeds, where the differences among the OFFICES OF THE TEETH. 141 teeth are very considerable ; and these diifer- ences have, accordingly, been the object of much careful study. To the physiologist they present views of still higher interest, by exhibiting most striking evidences of the provident care with which every part of the organization of animals has been constructed in exact reference to their respective wants and destinations. The purposes answered by the teeth are prin- cipally those of seizing and detaining whatever is introduced into the mouth, of cutting it asunder, and dividing it into smaller pieces, of loosening its fibrous structure, and of breaking- down and grinding its harder portions. Occa- sionally some particular teeth are much enlarged, in order to serve as weapons of attack or of defence ; for which purpose they extend beyond the mouth, and are then generally denominated tuski>; this we see exemplified in the Elephant ^ the Narwhal, the Walrus, the Hippopotamus, the Hoar, and the JBabiroussa. Four principal forms have been given to teeth, which accordingly may be distinguished into the conical, the sharp-edged, the flat, and the tuberculated teeth ; though we occasionally find a few intermediate modifications of these forms. It is easy to infer the particular functions of each class of teeth, from the obvious mechanical actions to which, by their form, they are espe- cially adapted. The conical teeth, Avhich are 142 THE VITAL FUNCTIONS. generally also sharp-pointed, are principally em- ployed in seizing, piercing, and holding objects : such are the offices which they perform in the Crocodile, and other Saurian reptiles, where all the teeth are of this structure ; and such are also their uses in most of the Cetacea, where similar forms and arrangements of teeth prevail. All the Dolphin tribe, such as the Porpiis, the {rfampus, and the Dolphin, are furnished with a uniform row of conical teeth, set round both jaws, in number amounting frequently to two hundred. Fig. 273, which represents the jaws of the Porpus, shows the form of these simply prehensile teeth. The Cachalot has a similar row of teeth, which are, however, confined to the lower jaw. All these animals subsist upon fish, and their teeth are therefore constnicted very much on the model of those of fish ; while those Cetacea, on the other hand, which are her- bivorous, as the Manatus and the Diigoiig, or Indian Walrus, have teeth very differently formed. The tusks of animals must necessarily, as respects their shape, be classed among the <^onical teeth. TEETH OF CETACEA. 143 The sharp-edged teeth perform the office of cuttmg and dividing the yielding textures pre- sented to them ; they act individually as wedges or chisels ; but when co-operating with similar teeth in the opposite jaw, they have the power of cutting like shears or scissors. The flat teeth, of which the surfaces are generally rough, are used in conjunction with those meeting them in the opjjosite jaw for grinding down the food by a lateral motion, in a manner analogous to the operation of mill-stones in a mill. The tuber- culated teeth, of which the surfaces present a number of rounded eminences, corresponding to depressions in the teeth opposed to them in the other jaw, act more by their direct pressure in breaking down hard substances, and pounding them, as they would be in a mortar. The position of the teeth in the jaws has been another ground of distinction. In those Mam- malia which exhibit the most complete set of teeth, the foremost in the row have the sharp- edged or chisel shape, constituting the blades of a cutting instrument; and they are accordingly denominated incisors. The incisors of the u})per jaw are always implanted in a bone, intermediate between the two upper jaw bones, and called the intermaxillary bones.* The conical teeth, * Those teeth of the lower jaw which correspond with the incisors of the upper jaw, are also considered as incisors. In Man, and in the species of quadrumana tliat most nearly re- 144 THE VITAL FUNCTIONS. immediately following the incisors, are called cuspidate, or canine teeth, from their being par- ticularly conspicuous in dogs ; as they are, in- deed, in all the purely carnivorous tribes. In the larger beasts of prey, as the lion and the tiger, they become most powerful weapons of destruc- tion : in the boar they are likewise of great size, and constitute the tusks of the animal. All the teeth that are placed farther back in the faw are designated by the general name of molar, teeth, or grinders, but it is a class which includes several different forms of teeth. Those teeth which are situated next to the canine teeth, partake of the conical form, having pointed emi- nences ; these are called the false molar teeth, and also, from their having generally two points, or cusps, the hicuspidate teeth. The posterior molar teeth are differently shaped in carnivorous animals, for they are raised into sharp and often serrated ridges, having many of the properties of cutting teeth. In insectivorous and fru- givorous animals their surface presents pro- minent tubercles, either pointed or rounded, for pounding the food ; while in quadrupeds that feed on grass or grain they are flat and rough, for the purpose simply of grinding. The apparatus for giving motion to the jaws semble him, the sutures which divide the intermaxillary from the maxillary bones are obliterated before birth, and leave in the iidult no trace of their former existence. MOVEMENTS OF TrlK JAWS. 145 is likewise varied according to the particular movements required to act upon the food in the different tribes. The articulation of the lower jaw with the temporal bone of the skull ap- proaches to a hinge joint ; but considerable lati- tude is allowed to its motions by thfe interposi- tion of a moveable cartilage between the two surfaces of articulation, a contrivance admirably answering the intended purpose. Hence, in ad- dition to the principal movements of opening and shutting, which are made in a vertical direction, the lower jaw has also some degree of mobility in a horizontal or lateral direction, and is likewise capable of being moved backwards or forwards to a certain extent. The muscles which effect the closing of the jaw are princi- pally the temporal and the masseter muscles ; the former occupying the hollow of the temples, the latter connecting the lower angle of the jaw with the zygomatic arch. The lateral motions of the jaw are effected by muscles placed inter- nally between the sides of the jaw and the basis of the skull. In the conformation of the teeth and jaws, a remarkable contrast is presented between car- nivorous and herbivorous animals. In the for- mer, of which the Tiger, Fig. -274, may be taken as an example, the whole apparatus for masti- cation is calculated for the destruction of life, and for tearing and dividing the fleshy fibres. VOL. II. L 146 THE VITAL FUNCTIONS. The molar teeth are armed witli pointed emi- nences, which correspond in the opposite jaws so as exactly to lock into one another, like wheelwork, when the mouth is closed. All the muscles which close the jaw are of enormous size and strength, and they imprint the bones of the skull with deep hollows, in which we trace marks of the most powerful action. The temporal muscles occupy the whole of the sides of the skull (t, t) ; and by the continuance of their vigorous exertions, during the growth of the animal, alter so considerably the form of the bones, that the skulls of the young and the old animals are often with difficulty recognised as belonging to the same species.* The process of the lower jaw (seen between t and t), to which this temporal muscle is attached, is large and * This is remarkably the case with the Bear, the skull of which exhibits in old animals a large vertical crest, not met with at an early period of life. JAWS AND TEETH OF HERP.IVORA. 147 prominent; and the arch of bone (z), from which the masseter arises, takes a wide span outwards, so as to give great strength to the muscle. The condyle, or articulating surface of the jaw (c), is received into a deep cavity, constituting a strictly hinge joint, and admitting simply the motions of opening and shutting. In herbivoro^lS animals, on the contrary, as may be seen in the skull of the Antelope, Fig. 275, the greatest force is bestowed, not so much on the motions of opening and shutting, as on those which are necessary for grinding, and which act in a lateral direction. The temporal muscles, occupying the space t, are compara- tively small and feeble ; the condyles of the jaw are broad and rounded, and more loosely con- nected with the skull by ligaments ; the muscles in the interior of the jaw, which move it from side to side, are very strong and thick ; and the bone itself is extended downwards, so as to 1:48 THE VITAL FUNCTIONS. afford them a broad basis of attachment. The surfaces of the molar teeth are flattened and of great extent, and they are at the same time kept rough, hke those of mill-stones, their office being in fact very similar to that performed by these implements for grinding. All these cir- cumstances of difference are exemplified in the most marked manner, in comparing together the skulls of the larger beasts of prey, as the tiger, the wolf, or the bear, with riiose of the antelope, the horse, or the ox. The Rodent ia, or gnawing quadrupeds, which I have already had occasion to notice, compose a well-marked family of Mammalia. These animals are formed for subsisting on dry and tough materials, from which but little nutriment can be extracted ; such as the bark, and roots, and even the woody fibres of trees, and the harder animal textures, which would appear to be most difficult of digestion. They are all animals of diminutive stature, whose teeth are expressly formed for gnawing, nibbling, and wearing away by continued attrition, the harder textures of organized bodies. The i2«^, whose skull is delineated in Fig. 276, belongs to this tribe. They are all furnished with two incisor teeth in TEETH OF QUADRUMANA. 140 each jaw, generally very long, and having the exact shape of a chisel ; and the molar teeth have surfaces irregularly marked with raised zig-zag lines, rendering them very perfect in- struments of trituration. The zygomatic arch is exceedingly slender and feeble ; and the condyle is lengthened longitudinally to allow of the jaw being freely moved forwards and backwards, which is the motion for which the muscles are adapted, and by which the grinding operation is performed. The Beaver, the Rat, the Mai-mot, and the Porcupine, present examples of this structure, among the omnivorous rodentia : and the Hare, the Rabbit, the Shrew, among those that are principally herbivorous. The Quadncmana, or Monkey tribes, approach nearest to the human structure in the confor- mation of their teeth, which appear formed for a mixed kind of food, but are especially adapted to the consumption of the more esculent fruits. The other orders of Mammalia exhibit intermediate gradations in the structure of their teeth to those above described, corresponding to greater varieties in the nature of their food. Thus the teeth and jaws of the Hi/cena are formed more especially for breaking down bones, and in so doing exert prodigious force ; and those of the Sea Otter have rounded eminences, which peculiarly fit them for breaking shells. The teeth, though composed of the same 150 THE VITAL FUNCTIONS. jfl chemical ingredients as the ordinary bones, differ from them by having a greater density and compactness of texture, whence they derive that extraordinary degree of hardness which they require for the performance of their pecuUar office. The substances of which they are com- posed are of three different kinds : the first, which is the basis of the rest, constituting the solid nucleus of the tooth, has been considered as the part most analogous in its nature to bone, but from its much greater density, and from its differing from bone in the mode of its formation, the name of ivory has been generally given to it. Its earthy ingredient consists almost entirely of phosphate of lime, the proportion of the car- bonate of that earth entering into its composition being very small ; and the animal portion is | albumen, with a small quantity of gelatin. A layer of a still harder substance, termed the enamel, usually covers the ivory, and, in teeth of the simplest structure, forms the whole of their outer surface : this is the case with the teeth of man and of carnivorous quadrupeds. These two substances, and the direction of their layers, are seen in Fig. 277, which is the section of a simple tooth. E is the outer case of enamel, o the osseous portion, and p the cavity where the vascular pulp which formed it was lodged. The enamel is composed almost wholly of phosphate of lime, containing no albumen, and scarcely a STRUCTURE OF TEETH. 151 trace of gelatin ; it is the hardest of all animal substances, and is capable of striking fire with steel. It exhibits a fibrous structure, approach- ing to a crystalline arrangement, and the direc- tion of its fibres, as shown by the form of its fragments when broken, is every where perpen- dicular to the surface of the ivory to which it is applied. The ends of the fibres are thus alone exposed to the friction of the substances on which the teeth are made to act; and the effect of that friction in wearing the enamel is thus rendered the least possible. In the teeth of some quadrupeds, as of the Rhinoceros, the Hippopotmmis, and most of the Rodentia, the enamel is intermixed with the ivory, and the two so disposed as to form jointly tlie surface for mastication. In the progress of life, the layers of enamel, being the hardest, are less worn down by friction than those of the ivory, and therefore form prominent ridges on 152 THE VITAL FUNCTIONS. the grinding surface, preserving it always in that rough condition, which best adapts it for the bruising and comminuting of hard substances. The incisors of the rodentia are guarded by a plate of enamel only on their anterior convex surfaces, so that by the wearing down of the ivory behind this plate, a wedge-like form, of which the enamel constitutes the fine cutting edge, is soon given to the tooth, and is constantly retained as long as the tooth lasts (Fig. -280). This mode of growth is admirably calculated to preserve these chisel teeth fit for use during the whole life-time of the animal, an object of greater consequence in this description of teeth than in others, which continue to grow only during a limited period. The same arrangement, attended with similar advantages, is adopted in the struc- ture of the tusks of the Hippopotamus. In teeth of a more complex structure, a third substance is found, uniting the vertical plates of ivory and enamel, and performing the office of an external cement. This substance has re- ceived various names, but it is most commonly known by that of the Criista petrosa: it resem-- bles ivory both in its composition and its extreme hardness; but is generally more opaque and yellow than that substance. Other herbivorous quadrupeds, as the horse, and animals belonging to the ruminant tribe, have also complex teeth composed of these three STRUCTURE OF TEETH. 153 substances : and their grinding surfaces present ridges of enamel intermixed in a more irregular manner with the ivory and crusta petrosa ; but still giving the advantage of a very rough surface for trituration. Fig. 278 represents the grinding surface of the tooth of a horse, worn down by long mastication, e is the enamel, marked by transverse lines, showing the direction of its fibres, and enclosing the osseous portion (o), which is shaded by interrupted lines. An outer coating of enamel (e) is also visible, and between that and the inner coat, the substance called crusta petrosa (c), marked by waving lines, is seen. On the outside of all there is a plate of bone, which has been left white. In ruminants, the plates of enamel form crescents, which are convex outwardly in the lower, and inwardly in the upper jaw; thus providing for the crossing of the ridges of the two surfaces, an arrange- ment similar to that which is practised in con- structing those of mill-stones. The teeth of the lower jaw fall within those of the upper jaw, so that a lateral motion is required in order to bring their surfaces opposite to each other alternately on both sides. Fig. 279 shows the grinding sur- face of the tooth of a SJteep, where the layers of bone are not apparent, there being only two layers of enamel (e), and one of crusta petrosa (c). These three component parts are seen to most advantage in a vertical and longitudinal section 154 THE VITAL FUNCTIONS. of the grinding tooth of the elephant, in which they are more completely and equally inter- mixed than in that of any other animal. Fig. 281 presents a vertical section of the grinding tooth of the Asiatic Elephant, in the early stage of its growth, and highly polished, so as to exhibit more perfectly its three component structures. The enamel, marked e, is formed of transverse fibres; the osseous, or innermost struc- ture is composed of longitudinal plates. The general covering of crusta petrosa, c, is less regularly deposited, p is the cavity which had been occupied by the -pulp. In this tooth, which is still in a growing state, the fangs are not yet added, but they are, at one part, beginning to be formed. The same tooth in its usual state, as worn by mastication, gives us a natural and DENTITION. 155 horizontal section of its interior structure, in which the plates of white enamel are seen forming waved ridges. These constitute, in the Asiatic Elephant, a series of narrow transverse bands (Fig. 283), and in the African Elephant, a series of lozenge-shaped lines (Fig. 282), hav- ing the ivory on their interior, and the yellow crusta petiosa on their outer sides; which latter substance also composes the whole circumference of the section. § 4. Formation and Developement of the Teeth. Few processes in animal developement are more remarkable than those which are employed to form the teeth ; for they are by no means the same as those by which ordinary bone is con- structed ; and being commenced at a very early period, they afford a signal instance of Nature's provident anticipation of the future necessities of the animal. The teeth, being the hardest parts of the body, require a peculiar system of opera- tions for giving them this extraordinary^ density, which no gradual consolidation could have im- parted. The formation of the teeth is in some respects analogous to that of shell; inasmuch as all their parts, when once deposited, remain as permanent structures, hardly ever admitting of removal or of renewal by the vital powers. 156 THK VITAL FUNCTIONS. Unlike the bones, which contain within their solid substance vessels of different kinds, by which they are nourished, modified, and occa- sionally removed, the closeness of the texture of the teeth is such as to exclude all vessels what- soever. This circumstance renders it necessary that they should originally be formed of the exact size and shape which they are ever after to possess : accordingly the foundation of the teeth, in the young animal, are laid at a very early period of its evolution, and considerable pro- gress has been made in their growth even prior to birth, and long before they can come into use. A tooth of the simplest construction is formed from blood-vessels, which ramify through small masses of a gelatinous appearance ; and each of these pulpy masses is itself enclosed in a delicate transparent vesicle, within which it grows till it has acquired the exact size and shape of the future tooth. Each vascular pulp is farther protected by an investing membrane of greater strength, termed its capsule, which is lodged in a small cavity between the two bony plates of the jaw. The vessels of the pulp begin at an early period to deposit the calcareous substance, which is to compose the ivory, at the most prominent points of that part of the vesicle, which corres- ponds in situation to the outer layer of the crown of the tooth. The thin scales of ivory thus formed increase by farther depositions made on DENTITION. 157 their surfaces next to the pulp, till the whole has formed the first, or outer layer of ivory : in the mean time, the inner surface of the capsule, which is in immediate contact with this layer, secretes the substance that is to compose the enamel, and deposits it in layers on the surface of the ivory. This double operation proceeds step by step, fresh layers of ivory being depo- sited, and building up the body of the tooth, and in the same proportion encroaching upon the cavity occupied by the pulp, which retires before it, until it is shrunk into a small compass, and fills only the small cavity which remains in the centre of the tooth. The ivory has by this time received from the capsule a complete coat- ing of enamel, which constitutes the whole outer surface of the crown ; after which no more is deposited, and the function of the capsule having ceased, it shrivels and disappears. But the formation of ivory still continuing at the part most remote from the crown, the fangs are gra- dually formed by a similar process from the pulp ; and a pressure being thereby directed against the bone of the socket at the part where it is the thinnest, that portion of the jaw is ab- sorbed, and the progress of the tooth is only resisted by the gum ; and the gum, in its turn, soon yielding to the increasing pressure, the tooth cuts its way to the surface. This process of successive deposition is beautifully illustrated 158 THE VITAL FUNCTIOMS, by feeding a young animal at different times with madder; the teeth which are formed at that period exhibiting, in consequence, alternate layers of red and of white ivory.* The formation of the teeth of herbivorous quadrupeds, which have three kinds of substance, is conducted in a still more artificial and com- plicated manner. Thus in the elephant, the pulp which deposits the ivory is extended in the form of a number of parallel plates ; while the capsule which invests it, accompanies it in all its parts, sending down duplicatures of mem- brane in the intervals between the plates. Hence the ivory constructed by the pulp, and the enamel deposited over it, are variously inter- mixed ; but besides this, the crusta petrosa is deposited on the outside of the enamel. Cuvier asserts that this deposition is made by the same capsule which has formed the enamel, and which, previously to this change of function, has become more spongy and vascular than before. But his brother, M. Frederic Cuvier represents the deposit of crusta petrosa, as performed by a third membrane, wholly distinct from the two others, and exterior to them all, although it follows them in all their folds. In the horse and the ox, the projecting processes of the pulp, have more of a conical form, with undulating sides ; and hence * Cuvier. Dictionnaire des Sciences Medicales, t. viii. p. 320. DtlNTITION. lo9 the waved appearance presented by the enamel on making sections of the teeth of these animals. The tusks of the elephant are composed of ivory, and are formed precisely in the same manner as the simple conical teeth already des- cribed, excepting that there is no outer capsule, and therefore no outer crust of enamel. The whole of the substance of the tusk is constructed by successive deposits of layers, having a conical shape, from the pulp which occupies the axis of the growing tusk ; just as happens in the forma- tion of a univalve shell which is not turbinated, as, for instance, the patella. Hence any foreign substance, a bullet, for example, which may happen to get within the cavity occupied by the pulp, becomes, in process of time, encrusted with ivory, and remains embedded in the solid substance of the tusk. The pulp, as the growth of the tusk advances, retires in proportion as its place is occupied by the fresh deposits of ivory. The young animal requires teeth long before it has attained its full stature ; and these teeth must be formed of dimensions adapted to that of the jaw, while it is yet of small size. But as the jaw enlarges, and the teeth it contains admit not of any corresponding increase, it becomes neces- sary that they should be shed to make room for others of larger dimensions, formed in a more capacious mould. Provision is made for this necessary change at a very early period of the 160 THE VITAL FUNCTIONS. growth of the embryo. The rudiments of the human teeth begin to form four or five months before birth : they are contained in the same sockets with the temporary teeth, the capsules of both being connected together. As the jaw enlarges, the second set of teeth gradually ac- quire their full dimensions, and then, by their outward pressure, occasion the absorption of the fangs of the temporary teeth, and, pushing them out, occupy their places.* As the jaw bone, during its growth, extends principally backwards, the posterior portion, being later in forming, is comparatively of a larger size than either the fore or the lateral parts ; and it admits, therefore, of teeth of the full size, which consequently are permanent. The molar teeth, which are last formed, are, for want of space, rather smaller than the others, and are called the ivisdom-teeth, because they do not usually make their appearance above the gum till the jierson has attained the age of twenty. In the negro, however, where the jaw is of greater length, these teeth have sufficient room to come into their places, and are, in gene- ral, fully as large as the other molares. The teeth of carnivorous animals are, from * It is stated by Rousseau that the shedding of the first molar tooth both of the Guinea-pig , and the Capibara, and its re- placement by the permanent tooth, take place a few days before birth. Anatomic Comparee du systems dentaire, p. 164. DENTITION. 161 the nature of their food, less liable to be worn, than those of animals living on grain, or on the harder kinds of vegetable substances ; so that the simple plating of enamel is sufficient to pre- serve them, even during a long life. But in many herbivorous quadrupeds we find that in proportion as the front teeth are worn away in mastication, other teeth are formed, and advance from the back of the jaw to replace them. This happens in a most remarkable manner in the elephant, and is the cause of the curved form which the roots assume ; for in proportion as the front teeth are worn away, those immediately behind them are pushed forwards by the growth of a new tooth at the back of the jaw ; and this process goes on continually, giving rise to a suc- cession of teeth, each of which is larger than that which has preceded it, during the w^hole period that the animal lives. A similar suc- cession of teeth takes place in the icilcl boar, and also, though to a less extent, in the Sits (Ethio- picjis* This mode of dentition appears to be peculiar to animals of great longevity, and which subsist on vegetable substances con- taining a large proj^ortion of tough fibres, or other materials of great hardness ; and requiring for their mastication teeth so large as not to admit of both the old and new tooth being: o * Home, Phil. Trans, for 1799, p. 237 ; and 1801, p. 319. VOL. II. M 1()2 THE VITAL FUNCTIONS. contained at the same time in the alveolar por- tion of the jaw. An expedient of a different kind has been resorted to in the Rodentia, for the purpose of preserving the long chisel-shaped incisors in a state fit for nse. By the constant and severe attrition to which they are exposed, they wear away very rapidly, and would soon be entirely lost, and the animal would perish in conse- quence, were it not that nature has provided for their continued growth, by elongation from their roots, during the whole of life. This growth proceeds in the same manner, and is conducted on the same principles, as the original formation of the simple teeth already described : but, in order to effect this object, the roots of these t^eth are of great size and length, and are deeply imbedded in the jaw, in a large bony canal provided for that purpose; and their cavity is always filled with the vascular pulp, from whicli the continued secretion and deposition of fresh layers, both of ivory and enamel, take place. The tusks of the Elephant and of the Hippo- potamus exhibit the same phenomenon of con- stant and uninterrupted growth. In the Shark, and some other fishes, the same object is attained in a different manner. Several rows of teeth are lodged in each jaw, but one only of these rows projects and is in use at the same time ; the rest lying flat, but ready to rise DENTITION. 163 284 ill order to replace those that have been broken or worn down. In some fishes the teeth ad- vance in proportion as the jaw lengthens, and as the fore teeth are worn away : in other cases they rise from the substance of the jaw, which presents on its surface an assemblage of teeth m different stages of growth : so that in this class of animals the greatest variety occurs in the mode of the succession of the teeth. The teeth of the Crocodile, which are sharp- pointed hollow cones, composed of ivory and enamel, are renewed by the new tooth (as is shown at a, in Fig. 284), being formed in the cavity of the one (b) which it is to replace, and not being inclosed in any separate cavity of the jaw bone (c). As this new tooth increases in size, it presses against the base of the old one, and entering its cavity, acquires the same co- nical form ; so that when the latter is shed, it is already in its place, and fit for immediate use. This suc- cession of teeth takes place several times during the life of the animal, so that they are sharp and perfect at all ages. The fangs of serpents are furnished, like the stings of nettles, with a receptacle at their base for a poisonous liquor, which is squeezed out by 164 THE VITAL FUNCTIONS. the pressure of the tooth, at the moment it inflicts the wound, and conducted along a canal, opening near the extremity of the tooth. Each fang is lodged in a strong bony socket, and is, by the intervention of a connecting bone, pressed forwards whenever the jaw is opened sufficiently wide ; and the fang is thus made to assume an erect position. As these sharp teeth are very liable to accidents, others are ready to supply their places when wanted : for which purpose there are commonly provided two or three half- grown fangs, which are connected only by soft parts with the jaw, and are successively moved forwards into the socket to replace those that were lost.* The tube through which the poison flows is formed by the folding in of the edges of a deep longitudinal groove, extending along the greater part of the tooth ; an interval being left between these edges, both at the base and extremity of the fang, by which means there remain apertures at both ends for the passage of the fluid poison. This structure was discovered by Mr. T. Smith in the Coluber naia, or Cobra de Capello;-\ and is shown in Fig. 285, which represents the full grown tooth, where the slight furrow, indicating the junction of the two sides of the original groove, may be plainly seen; as also the two * Home, Lectures, &c. I. 333. t Philosophical Transactions, 1818, p. 471. FANGS OF SERPENTS. 165 apertures (a and b) above mentioned. This mode of formation of the tube is farther illus- trated by Fig. 286, which shows a transverse section of the same tooth, exjiibiting the cavity (p) which contains the pulp of the tooth, and which surrounds that of the central tube in the form of a crescent. Figures 287 and 288 are delineations of the same tooth in different stages of growth, the bases of which, respectively, are shown in Figures 289 and 290. Figures 291 and 292 are magnified representations of sections of the fangs of another species of serpent, resem- bling the rattle-snake. Fig. 291 is a section of the young fang taken about the middle : in this stage of growth, the cavity which contains the pulp, almost entirely surrounds the poison tube, and the edges of the depression, which form the suture, are seen to be angular, and present so large a surface to each other, that the suture is completely filled up, even in this early stage of 166 THE VITAL FUNCTIONS. growth. Fig. 292 is a section of a full-grown fang of the same species of serpent, at the same part as the preceding; and here the cavity of the pulp is seen much contracted from the more advanced stage of growth. It is a remarkable circumstance, noticed by Mr. Smith, that a similar longitudinal furrow is perceptible on every one of the teeth of the same serpent ; and that this appearance is most marked on those which are nearest to the poisonous fangs : these furrows, however, in the teeth that are not venomous, are confined en- tirely to the surface, and do not influence the form of the internal cavity. No trace of these furrows is discernible in the teeth of those serpents which are not armed with venomous fangs. Among the many instances in which teeth are converted to uses widely different from masti- cation, may be noticed that of the Squalus pristis^ I ^iwrr^^^"^^^^ or Saw-fish, where the teeth are set horizontally GASTRIC TEETH. 1(57 on the two lateral edges of the upper jaw, which is prolonged in the form of a snout (seen in a, Fig. 293), constituting a most formidable weapon of oftence. b is a more enlarged view of a portion of this instrument, seen from the under side. *§ 5. Tritnraliou of Food In Tntenial Cavities. The mechanical apparatus, provided for tritu- rating the harder kinds of food, does not belong exclusively to the mouth, or entrance into the alimentary canal, for in many animals we find this office performed by interior organs. Among the inferior classes, we find examples of this conformation in the Crustacea, the Mollusca, and above all in Insects. Thus there is found in the stomach of the Lobster, a cartilaginous frame-work, in which are implanted hard cal- careous bodies, having the form, and performing the functions of teeth. They are delineated in Fig. 294, which presents a view of the interior of the sto- mach of that animal. The tooth A is situated in the middle of this frame, has a rounded conical shape, and is smaller than the others (b, c). 294 168 THE VITAL FUNCTIONS. which are placed one on each side, and which resemble in their form broad molar teeth. When these three teeth are brought together by the action of the surrounding muscles, they fit exactly into each other, and are capable of grinding and completely pulverising the shells of the mollusca introduced into the stomach. These teeth are the result of a secretion of cal- careous matter from the inner coat of that organ, just as the outer shell of the animal is a pro- duction of the integument : and at each casting of the shell, these teeth, together with the whole cuticular lining of the stomach to which they adhere, are thrown off, and afterwards renewed by a fresh growth of the same material. In the Craw-fish, the gastric teeth are of a different shape, and are more adapted to divide than to grind the food. Among the gasteropodous Mollusca, several species of Bullce have stomachs armed with calcareous plates, which act as cutting or grind- 235 ing teeth. The Sulla aperta has three instruments of this descrip- tion, as may be seen in Fig. 295, which shows the interior of the stomach of that species. Similar organs are found in the Sulla lig7iaria. The Aplysia has a con- siderable number of these gastric teeth. An apparatus of a still more complicated kind is GIZZARDS OF BIRDS. 169 provided in most of the insects belonging to the order of Orthoptera ; but I shall not enter at present in their description, as it will be more convenient to include them in the general ac- count of the alimentary canal of insects, which will be the subject of future consideration. The internal machinery for grinding is exem- plified on the largest scale in granivorous birds ; where it forms part of the stomach itself, and is termed a Gizzard. It is shown in Fig. 298, repre- senting the interior of the stomach of a Stvan. Both the structure and the mode of operation of this organ bear a striking analogy to a mill for grinding corn, for it consists of two powerful muscles (g), of a hemispherical shape, with their flat sides applied to each other, and their edges united by a strong tendon, which leaves a vacant space of an oval or quadrangular form between their two surfaces. These surfaces are covered by a thick and dense horny substance, which, when the gizzard is in action, performs an office similar to that of mill-stones. In most birds, there is likewise a sac, or receptacle, termed the Crmv, (represented laid open at c) in which the food is collected for the purpose of its 170 THE VITAL FUNCTIONS. being dropped, in small quantities at a time, into the gizzard, in proportion as the latter gra- dually becomes emptied.* Thus the analogy between this natural process and the artificial operation of a corn-mill is preserved even in the minuter details ; for while the two flat surfaces of the gizzard act as mill-stones, the craw sup- plies the place of the hopper, the office of which is to allow the grain to pass out in small quan- tities into the aperture of the upper mill- stone, which brings it within the sphere of their action. Innumerable are the experiments which have been made, particularly by Reaumur and Spal- lanzani, with a view to ascertain the force of compression exerted by the gizzard on its con- tents. Balls of glass, which the bird was made to swallow with its food, were soon ground to powder : tin tubes, introduced into the stomach, were flattened, and then bent into a variety of shapes ; and it was even found that the points of needles and of lancets fixed in a ball of lead, were blunted and broken ofl" by the power of the gizzard, while its internal coat did not appear to be in the slightest degree injured. These results were long the subject of admiration to physio- logists ; and being echoed from mouth to mouth, were received with a sort of passive astonishment, * The gastric glands, which are spread over the greater part of the internal surface of the craw, and which prepare a secretion for macerating the grain, are also seen in this part of the figure. ACTION OF THE GIZZARD. 171 till Hunter directed the powers of his mind to the inquiry, and gave the first rational explanation of the mechanism by which they are produced. He found that the motion of the sides of the gizzard, when actuated by its muscles, is lateral, and at the same time circular ; so that the pres- sure it exerts, though extremely great, is directed nearly in the plane of the grinding surfaces, and never perpendicularly to them ; and thus the edges and points of sharp instruments are either bent or broken off by the lateral pressure, without their having an opportunity of acting directly upon those surfaces. Still, however, it is evident that the effects we observe produced upon sharp metallic points and edges, could not be accom- plished by the gizzard without some assistance from other sources ; and this assistance is pro- cured in a very singular, and, at the same time, very effectual manner. On opening the gizzard of a bird, it is con- stantly found to contain a certain quantity of small pebbles, which must have been swallowed by the animal. The most natural reason that can be assigned for the presence of these stones, is, that they aid the gizzard in triturating the contained food, and that they, in fact, supply the office of teeth in that operation. Spallanzani, however, has called in question the soundness of this explanation, and has contended that the pebbles found in the gizzard are swallowed 172 THE VITAL FUNCTIONS. merely by accident, or in consequence of the stupidity of the bird, which mistakes them for grain. But this opinion has been fully and satisfactorily refuted both by Fordyce and by Hunter, whose observations concur in establishing the truth of the common opinion, that in all birds possessing gizzards, the presence of these stones is essential to perfect digestion. A greater or less number of them is contained in every gizzard, when the bird has been able to meet with the requisite supply, and they are never swallowed but along with the food. Several hundred were found in the gizzard of a turkey ; and two thousand in that of a goose: so great an accumulation could never have been the result of mere accident. If the alleged mistake could ever occur, we should expect it to take place to the greatest extent in those birds which are starving for want of food ; but this is far from being the case. It is found that even chickens, which have been hatched by artificial heat, and which could never have been instructed by the parent, are yet guided by a natural in- stinct in the choice of the proper materials for food, and for assisting its digestion : and if a mixture of a large quantity of stones with a small proportion of grain be set before them, they will at once pick out the grain, and swallow along with it only the proper proportion of stones. The best proof of the utility of these substances fl I GIZZARDS OF BIRDS. 173 may be derived from the experiments of Spal- lan^ani himself, who ascertained that grain is not digested in the stomachs of birds, when it is protected from the effects of trituration. Thus the gizzard may, as Hunter remarks, be regarded as a pair of jaws, whose teeth are taken in occasionally to assist in this internal mastica- tion. The lower part of the gizzard consists of a thin muscular bag, of which the office is to digest the food which has been thus triturated. Considerable differences are met with in the structure of the gizzards of various kinds of birds, corresponding to differences in the texture of their natural food. In the Turkey, the two muscles which compose the gizzard are of un- equal strength, that on the left side being consi- derably larger than that on the right ; so that while the principal effort is made by the former, a smaller force is used by the latter to restore the parts to their situation. These muscles pro- duce, by their alternate action, two effects ; the one a constant trituration, by a rotatory motion ; the other a continued, but oblique, pressure of the contents of the cavity. As this cavity is of an oval form, and the muscle swells inwards, the opposite sides never come into contact, and the interposed materials are triturated by their being intermixed with hard bodies. In the Goose and Swan, on the contrary, the cavity is flattened, and its lateral edges are very thin. The surfaces 174 THE VITAL FUNCTIONS. aj^plied to each other are mutually adapted m their curvatures, a concave surface being ev^ry where applied to one which is convex : on the left side, the concavity is above ; but on the right side, it is below. The horny covering is much stronger, and more rough than in the turkey, so that the food is ground by a sliding, instead of a rotatory motion of the parts opposed, and they do not require the aid of any inter- vening hard substances of a large size. This motion bears a great resemblance to that of the grinding teeth of ruminating animals, in which the teeth of the under jaw slide upwards, within those of the upper, pressing the food be- tween them, and fitting it, by this peculiar kind of trituration, for being digested.* § 6. Deglutition. The great object of the apparatus which is to prepare the food for digestion, is to reduce it into a soft pulpy state, so as to facilitate the chemical action of the stomach upon it : for this purpose, solid food must not only be sub- jected to mechanical trituration, but it must also be mixed with a certain proportion of fluid. Hence all animals that masticate their food are * Home, Phil. Trans, for 1810, p. 188. SyVLIVAUV APPARATUS. 175 provided with organs which secrete a fluid, called the Saliva, and which pour this fluid into the mouth as near as possible to the grinding sur- faces of the teeth. These organs are glands, placed in such a situation as to be compressed by the action of the muscles which move the jaw, and to pour out the fluid they secrete in greatest quantity, just at the time when the food is undergoing mastication. Saliva contains a large quantity of water, together with some salts and a little animal matter. Its use is not only to soften the food, but also to lubricate the pas- sage through which it is to be conveyed into the stomach ; and the quantity secreted has always a relation to the nature of the food, the degree of mastication it requires, and the mode in which . it is swallowed. In animals which subsist on vegetable materials, requiring more complete maceration than those which feed on flesh, the salivary glands are of large size : they are parti- cularly large in the Rodentia, which feed on the hardest materials, requiring the most complete trituration ; and in these animals we find that the largest quantity of saliva is poured out opposite to the incisor teeth, which are those principally employed in this kind of mastication. In Birds and Reptiles, which can hardly be said to mas- ticate their food, the salivary glands are compa- ratively of small size ; the exceptions to this rule occurring chiefly in those tribes which feed on 176 THE VITAL FUNCTIONS. vegetables, for in these the glands are more consi- derable.* In Fishes there is no structure of this kind provided, there being no mastication per- formed : and the same observation applies to the Cetacea. In the cephalopodous and gastero- podous 3Iolhisca, we find a salivary apparatus of considerable size : Insects^ and the Annelida,^' also, generally present us with organs which appear to perform a similar office. The passage of the food along the throat is facilitated by the mucous secretions, which arc poured out from a multitude of glands inter- spersed over the whole surface of the membrane lining that passage. The Camel, which is formed for traversing dry and sandy deserts, where the •atmosphere as well as the soil is parched, is spe- cially provided with a glandular cavity placed behind the palate, and which furnishes a fluid for the express purpose of moistening and lubri- cating the throat. In the structure of the (Esophagus, which is the name of the tube along which the food passes from the mouth to the stomach, we may trace a similar adaptation to the particular kind of food taken in by the animal. When it is swallowed entire, or but little changed, the * The large salivary gland in the woodpecker, is seen at s, Fig. 271, page 132. t The bunch of filaments, seen at s, Fig. 260 (p. 103) are the salivary organs of the leech. DEGLUTITION. 177 oesophagus is a very wide canal, admitting of great dilatation. This is the case with many carnivorous birds, especially those that feed on fishes, where its great capacity enables it to hold, for a considerable time, the large fish which are swallowed entire, and which could not con- veniently be admitted into the stomach. Blu- menbach relates that a sea-gull, which he kept alive for many years, could swallow bones of three or four inches in length, so that only their lower ends reached the stomach, and were digested, w hile their upper ends projected into the oesophagus, and descended gradually in proportion as the former were dissolved. Ser- pents, w hich swallow animals larger than them- selves, have, of course, the oesophagus, as well as the throat, capable of great dilatation ; and the food occupies a long time in passing through it, before it reaches the digesting cavity. The turtle has also a capacious oesophagus, the inner coat of which is beset with numerous firm and sharp processes, having their points directed towards the stomach ; these are evidently in- tended to prevent the return of the food into the mouth. Grazing quadrupeds, who, while they eat, carry their heads close to the ground, have a long oesophagus, with thick muscular coats, capable of exerting considerable power in pro- pelling the food in the direction of the stomach, which is contrary to that of gi'avity. VOL. II. N 178 THE VITAL FUNCTIONS. § 7. Receptacles for retaining Food. Provision is often made for the retention of the undigested food in reservoirs, situated in different parts of the mouth, or the oesophagus, instead of its being immediately introduced into the sto- mach. These resei*voirs are generally employed for laying in stores of provisions for future consumption. Many quadrupeds have cheek pouches for this purpose : this is the case with several species of Monkeys and Baboons; and also with the 3Ius cricetns, or Hamster. The 3Ius bursarius, or Canada rat, has enormous cheek pouches, which, when distended with food, even exceed the bulk of the head. Small cheek pouches exist in that singular animal, the Orni- thorhyncus. The Sciurus palmarnm, or palm squirrel, is also provided with a pouch for laying in a store of provisions. A remarkable dilatation in the lower part of the mouth and throat, answering a similar purpose, takes place in the Pelican; a bird which displays great dexterity in tossing about the fish with which it has loaded this bag, till it is brought into the proper position for being swallowed. The Whale has also a receptacle of enormous size, extending from the mouth to a considerable distance under the trunk of the body. RECEPTACLES FOR RETAINING FOOD. 179 Analogous in design to these pouches are the dilatations of the oesophagus of birds, deno- minated crops. In most birds which feed on grain, the crop is a capacious globular sac, placed in front of the throat and resting on the furcular bone. The crop of the Parrot is repre- sented at c, Fig. 29D; where, also, s indicates the cardiac portion of the stomach, and g the giz- zard, of that bird. The inner coat of the crop is furnished with numerous glands, which pour out considerable quantities of fluid for macerating and softening the dry and hard texture of the grain, which, for that purpose, remains there for a considerable time. Many birds feed their young from the contents of the crop ; and, at those seasons, its glands are much enlarged, and very active in preparing their peculiar secretions : this is remarkably the case in the Pigeon (Fig. 300), which, instead of a single sac, is provided with two (seen at c, c. Fig. 300), one on each side of the oesophagus (o). The pouting pigeon has the faculty of filling these cavities with air, which produces that dis- tended appearance of the throat from which it derives its name. Birds of prey have, in general, 180 THE VITAL FUNCTIONS. very small crops, their food not requiring any previous softening ; but the Vulture^ which gorges large quantities of flesh at a single meal, has a crop of considerable size, forming, when filled, a visible projection in front of the chest. Birds which feed on fish have no separate dila- tation for this purpose, probably because the great width of the oesophagus, and its having the power of retaining a large mass of food, render the further dilatation of any particular part of the tube unnecessary. The lower portion of the oesophagus appears often, indeed, in this class of animals, to answer the purpose of a crop, and to effect changes in the food which may properly be considered as a preliminary stage of the digestive process. Chapter VII. Digestion. All the substances received as food into the stomach, whatever be their nature, must neces- sarily undergo many changes of chemical com- position before they can gain admission into the general mass of circulating fluids; but the extent of the change required for that purpose will, of course, be in proportion to the difference be- DIGESTION. 1551 tweeii the qualities of the nutritive materials in their original, and in their assimilated state. The conversion of vegetable into animal matter necessarily implies a considerable modification of properties ; but even animal substances, how- ever similar may be their composition to the body which they are to nourish, must still pass through certain processes of decomposition, and subsequent recombination, before they can be brought into the exact chemical state in which they are adapted to the purposes of the living system. The preparatory changes we have lately been occupied in considering, consist chiefly in the reduction of the food to a soft consistence, which is accomplished by destroying the cohesion of its parts, and mixing them uniformly with the fluid secretions of the mouth ; effects which may be considered as wholly of a mechanical nature. The first real changes in its chemical state are produced in the stomach, where it is converted into a substance termed Chyme ; and the process by which this lirst step in the assimilation of the food is produced, constitutes what is properly termed Digestion. Nothing has been discovered in the anato- mical structure of the stomach tending to throw any light on the means by which this remark- able chemical change is induced on the materials it contains. The stomach is in most animals 182 THE VITAL FUNCTIONS. ! a simple sac, composed of several membranes, enclosing thin layers of muscular fibres, abun- dantly supplied with blood-vessels and with nerves, and occasionally containing structures which appear to be glandular. The human sto- mach, which is delineated in Fig. 301, exhibits one of the simplest forms of this organ ; c being the cardiac portion, or part where the oesophagus opens into it ; and p the pyloric portion, or that which is near its termination in the intestine. At the pylorus itself, the diameter of the pas- sage is much constricted, by a fold of the inner membrane, which is surrounded by a circular band of muscular fibres, performing the office of a sphincter, and completely closing the lower orifice of the stomach, during the digestion of its contents. The principal agent in digestion, as far as the ordinary chemical means are concerned in that operation, is a fluid secreted by the coats of the i DIGESTION. 183 stomach, and termed the Gastric juice. This fluid has, in each animal, the remarkable pro- perty of dissolving, or at least reducing to a pulp, all the substances which constitute the na- tural food of that particular species of animal ; while it has comparatively but little solvent power over other kinds of food. Such is the conclusion which has been deduced from the extensive researches on this subject made by that indefatigable experimentalist, S})allanzani, who found in numberless trials that the gastric juice taken from the stomach, and put into glass vessels, produced, if kept at the usual tempera- ture of the animal, changes to all appearance exactly similar to those which take place in natural digestion.* In animals which feed on flesh, the gastric juice was found to dissolve only animal substances, and to exert no action on vegetable matter ; while, on the contrary, that taken from herbivorous animals, acted on grass and other vegetable substances, without pro- * The accuracy of this conclusion has been lately contested by M. De Montegre, whose report of the effects of the gastric juice of animals out of the body, does not accord with that of Spallanzani ; but the difference of circumstances in which his experiments were made, is quite sufficient to account for the discrepancy in the results ; and those of M. De Montegre, therefore, by no means invalidate the general facts stated in the text, which have been established by the experiments, not only of Spallanzani, but also of Reaumur, Stevens, Leuret, and Lassaigne. See Alison's Outlines of Physiology and Pathology, p 170. 184 THE VITAL FUNCTIONS. f ducing any effect on flesh ; but in those animals, which, like man, are omnivorous, that is, par- take indiscriminately of both species of aliment, it appeared to be fitted equally for the solution of both. So accurate an adaptation of the che- mical powers of a solvent to the variety of sub- stances em2iloyecl as food by different animals, displays, in the most striking manner, the vast resources of nature, and the refined chemistry she has put in action for the accomplishment of her different purposes. A In the stomachs of many animals, as also in the human, it is impossible to distinguish with any accuracy the organization by which the secretion of the gastric juice is effected ; but where the structure is more complex, there may be observed a number of glandular bodies inter- spersed in various parts of the internal coats of the stomach. These, which are termed the Gastric glands, are distributed in various ways in different instances : they are generally found in greatest number, and often in clusters, about the cardiac orifice of the stomach ; and they are frequently intermixed with glands of another kind, which prepare a mucilaginous fluid, serving to protect the highly sensible coats of the sto- mach from injurious impressions. These latter are termed the mucous glands, and they are often constructed so as to pour their contents into intermediate cavities, or small sacs, which are DIGESTION. 185 denominated yoZ//c/^*, where the fluid is collected before it is discharged into the cavity of the sto- mach. The gastric glands of birds are larger and more conspicuous than those of quadrupeds : but, independently of those which are situated in the stomach, there is likewise found, in almost all birds, at the lower termination of the cesophagus, a large glandular organ, which has been termed the bulhulus glaudidosus. In the Ostrich, this organ is of so great a size as to give it the appearance of a separate stomach. A view of the internal surface of the stomach of the African ostrich is given in Fig. 302 ; where 305 303 304 c is the cardiac cavity, the coats of which are studded with numerous glands; g, g, are the two sides of the gizzard. Fig. 30.3 shows one of the gastric glands of the African ostrich ; Fig. 304, a gland from the stomach of the American 186 THE VITAL FUNCTIONS. ostrich; and Fig. 305, a section of a gastric gland in the beaver, showing the branching of the ducts, which form three internal openings. In birds that live on vegetable food, the structure of the gastric glands is evidently different from that of the corresponding glands in predaceous birds ; but as these anatomical details have not as yet tended to elucidate in any degree the pur- poses to w hich they are subservient in the pro- cess of digestion, I pass them over as being foreign to the object of our present inquiry.* It is essential to the perfect performance of digestion, that every part of the food received into the stomach should be acted upon by the gastric juice ; for which purpose provision is made that each portion shall, in its turn, be placed in contact with the inner surface of that organ. This is the more necessary, as many facts ren- der it probable, as will be noticed more parti- cularly hereafter, that, besides the chemical action of the gastric juice, an influence, derived from the nerves, essentially contributes to the accomplishment of the chemical changes which the food undergoes in the stomach. For this reason it is that the coats of the stomach are provided with muscular fibres, passing, some * These structures have been examined with great care and minuteness by Sir Everard Home, who has given the results of his inquiries in a series of papers, read from time to time to the Royal Society, and published in their Transactions. DIGESTION. 187 in a longitudinal, others in a transverse, or circular direction ; while a third set have an oblique, or even spiral course.* When the greater number of these muscles act together, they exert a considerable pressure upon the contents of the stomach ; a pressure which, no doubt, tends to assist the solvent action of the gastric juice. When different portions act in succession, they propel the food from one part to another, and thus promote the mixture of every portion with the gastric juice. We often find that the middle transverse bands contract more strongly than the rest, and continue con- tracted for a considerable time. The object of this contraction, which divides the stomach into two cavities, appears to be to separate its contents into two portions, so that each may be subjected to different processes ; and, indeed, the differences in structure, which are often observable between these two portions of the stomach, would lead to the belief that their func- tions are in some respects different. During digestion the exit of the food from the stomach into the intestine is prevented by the pylorus being closed by the action of its sphinc- ter muscle. It is clear that the food is required to remain for some time in the stomach in order to be perfectly digested, and this closing of the * See Fig. 51, vol. i. p. 137, and its description, p. 138. 180 THE VITAL FUNCTIONS. pylorus appears to be one means employed for attaining this end ; and another is derived from the property which the gastric juice possesses of coagulating, or rendering solid, every animal or vegetable fluid susceptible of undergoing that change. This is the case with fluid albumen ; the white of an egg, for instance, which is nearly pure albumen, is very speedily coagu- lated when taken into the stomach ; the same change occurs in milk, which is immediately curdled by the juices that are there secreted, and these effects take place quite independently of any acid that may be present. The object of this change from fluid to solid appears to be to detain the food for some time in the stomach, and thus to allow of its being thoroughly acted upon by the digestive powers of that organ. Those fluids which pass quickly through the stomach, and thereby escape its chemical action, however much they may be in themselves nutritious, are very imperfectly digested, and consequently aflbrd very little nourishment. This is the case with oils, with jelly, and with all food that is much diluted.* Hunter ascer- I * A diet consisting of too large a proportion of liquids, although it may contain much nutritive matter, yet if it be incapable of being coagulated by the stomach, will not be sufficiently acted upon by that organ to be properly digested, and will not only aftord comparatively little nourishment, but be very liable to produce disorder of the alimentary canal. Thus DIGESTION. 189 tained that this coagulating power belongs to the stomach of every animal, wliich he exa- mined for that purpose, from the most perfect down to reptiles*; and Sir E. Home has pro- secuted the enquiry with the same result, and ascertained that this property is possessed by the secretion from the gastric glands, Avhich commu- nicates it to the adjacent membranes. t The gastric juice has also the remarkable property of correcting putrefaction. This is par- ticularly exemplified in animals that feed on carrion, to whom this property is of great im- portance, as it enables them to derive wholesome soups will not prove so nutritive when taken alone, as when they are united with a certain proportion of solid food, capable of being detained in the stomach, during a time sufficiently long to allow of the whole undergoing the process of digestion. I was led to this conclusion, not only from theory, but from actual observation of what took place among the prisoners in the Mil- bank Penitentiary, in 1823, when on the occasion of the extensive prevalence of scorbutic dysentery in that prison. Dr. P. M. Latham and myself were appointed to attend the sick, and enquire into the origin of the disease. Among the causes which concurred to produce this formidable malady, one of the most prominent appeared to be an impoverished diet, consisting of a large proportion of soups, on which the prisoners had subsisted for the preceding eight months. A very full and perspicuous account of that disease has been drawn up, with great ability, by my friend Dr. P. M. Latham, and published under the title of " An Account of the disease lately prevalent in the General Peniten- tiary." London, 1825. * Observations on the Animal Economy, p. 172. t Phil. Trans, for 1813, p. 96. 190 THE VITAL FUNCTIONS. nourishment from materi-als which would other- wise taint the whole system with their poison, and soon prove destructive to life. It would appear that the first changes which constitute digestion take place principally at the cardiac end of the stomach, and that the mass of food is gradually transferred towards the pylorus, the process of digestion still con- tinuing as it advances. In the Rabbit it has been ascertained that food newly taken into the stomach is always kept distinct from that which was before contained in it, and which has begun to undergo a change : for this pur- pose the new food is introduced into the centre of the mass already in the stomach ; so that it may come in due time to be applied to the coats of that organ, and be in its turn digested, after the same change has been completed in the latter.* As the flesh of animals has to undergo a less considerable change than vegetable materials, so we find the stomachs of all the purely carni- vorous tribes consisting only of a membranous bag, which is the simplest form assumed by this organ. But in other cases, as we have already seen, the stomach exhibits a division into two compartments, by means of a slight * See Dr. Philip's Experimental Enquiry into the Laws of the Vital Functions, 3d. edition, p. 122. STOMACHS OF MAMMALIA. 191 contraction ; a condition which, as Sir E. Home has remarked, is sometimes found as a tem- porary state of the human stomach;* while, in other animals, it is the natural and per- manent conformation. The Rodentia furnish many examples of this division of the cavity into two distinct portions, which exhibit even differences in their structure : this is seen in the Dormouse, (Fig. 300) the Beaver, the Hare, the Rahhit, and the Cape Hi/rax, (Fig. .307). The first, or cardiac portion, is often lined with cuticle, while the lower portion is not so lined ; as is seen very conspicuously in the stomachs of the Solipeda. The stomach of the Horse, in particular, is furnished at the cardia, with a * The figure given of the human stomach, p. 182, shows it in the state of partial contraction here described. 192 THE VITAL FUNCTIONS. spiral fold of the inner, or cuticular membrane, which forms a complete valve, offering no impe- diment to the entrance of food from the oeso- phagus, but obstruct- ing the return of any part of the contents of the stomach into that passage.* This valve is shown in Fig. 311, which represents an inner view of the car- diac portion of the sto- mach of the horse ; o being the termination of the oesophagus. The stomach of the Water Rat is composed of two distinct cavities, having a narrow passage of communication : the first cavity is lined ^ith cuticle, and is evidently intended for the mace- ration of the food before it is submitted to the agents which are to effect its digestion ; a process which is completed in the second cavity, pro- vided, for that purpose, with a glandular surface. In proportion as nature allows of greater lati- tude in diet, we find her providing greater com- plication in the digestive apparatus, and subdi- viding the stomach into a greater number of * The total inability of a horse to vomit is probably a conse- quence of the impediment presented by this valve. See Mem. du Museum d'Hist. Nat. viii. 111. STOMACHS OF MAMMALIA. 193 cavities, each having probably a separate office assigned to it, though concurring in one general effect. A gradation in this respect may be traced through a long line of quadrupeds, such as the Hog, the Peccari, the Porcupine, (Fig.308), and the Hippopotamus, where we find the number of separate pouches for digestion amounting to four or five. Next to these we may rank the very irregular stomach of the Kanguroo, (Fig. 309) composed of a multitude of cells, in which the food probably goes through several prepa- ratory processes : and still greater complication is exhibited by the stomachs of the Cetacea, as, for example, in that of the Porpus (Fig. 310). As the fishes upon which this animal feeds are swallowed whole, and have large sharp bones, which would injure any surface not defended by cuticle, receptacles are provided, in which they may be softened and dissolved, and even con- verted into nourishment, by themselves, and without interfering with the digestion of the soft parts. The narrow communications between these several stomachs of the cetacea are pro- bably intended to ensure the thorough solution of their contents, by preventing the exit of all such portions as have not perfectly undergone that process- Supernumerary cavities of this kind, be- longing to the stomach, are more especially provided in those animals which swallow food VOL. n. o 194 THE VITAL FUNCTIONS. either in larger quantity than is immediately wanted, or of a nature which requires much pre- paration previous to digestion. The latter is more particularly the case with the horned ruminant tribes that feed on the leaves or stalks of vege- tables, a kind of food, which, in proportion to its bulk, affords but little nutriment, and requires, therefore, a long chemical process and a compli- cated digestive apparatus, in order to extract from it the scanty nutritious matter it contains, and prepare it for being applied to the uses of the system. This apparatus is usually considered as consisting of four stomachs ; and in order to convey a distinct idea of this kind of structure I have selected for representation, in Fig. 312, that of the Sheep, of which the four stomachs are marked by the numbers 1, 2, 3, 4, respectively, in the order in which they occur when traced from the oesophagus (c) to the intestine (p). STOMACHS OF RUMINANTS. 195 The grass which is devoured in large quan- tities by these animals, and which undergoes but little mastication in the mouth, is hastily swallowed, and is received into a capacious reservoir, marked 1 in the figure, called the paunch. This cavity is lined internally with a thick membrane, beset with numerous flattened papillae, and is often divided into pouches by transverse contractions. While the food remains in this bag, it continues in rather a dry state ; but the moisture with which it is surrounded contributes to soften it, and to prepare it for a second mastication ; which is effected in the following manner. Connected with the paunch is another, but much smaller sac (2), which is considered as the second stomach ; and, from its internal membrane being thrown into numerous irregular folds, fonning the sides of polygonal cells, it has been called the Jioneycomh stomachy or reticule. Fig. 313 exhibits this reticulated appearance of the inner surface of this cavity. A singular connexion exists between this sto- mach and the preceding ; for while the oesophagus appears to open naturally into the paunch, there is on each side of its termination, a muscular ridge which projects from the orifice of the latter, so that the two together form a channel leading into the second stomach ; and thus the food can readily pass from the oesophagus into either of these cavities, according as the orifice of the one or the other is open to receive it. 196 THE VITAL FUNCTIONS. It would appear, from the observations of Sir E. Home, that liquids drank by the animal pass at once into the second stomach, the entrance into the first being closed. The food contained in the paunch is transferred, by small portions at a time, into this second, or honey-comb stomach, in which there is always a supply of water for moistening the portion of food intro- duced into it. It is in this latter stomach, then, that the food is rolled into a ball, and thrown up, through the oesophagus, into the mouth, where it is again masticated at leisure, and while the ani- mal is reposing ; a process which is well known by the name oi chewino- the cud, or rumination. When the mass, after being thoroughly ground down by the teeth, is again swallowed, it passes along the oesophagus into the third stomach (3), the orifice of which is brought forwards by the muscular bands, forming the two ridges already noticed, which are continued from the second stomach, and which, when they con- tract, effectually prevent any portion of the food from dropping into either of the preceding cavities. In the ox, this third stomach is de- scribed by Sir E. Home as having the form of a crescent, and as containing twenty-four septa, or broad folds of its inner membrane. These folds are placed parallel to one another, like the leaves of a book, exce23ting that they are of unequal breadths, and that a narrower fold is placed between each of the broader ones. STOMACHS OF RUMINANTS. 197 Fig. 314 represents this plicated structure in the interior of the third stomach of a bullock. Whatever food is introduced into this cavity, which is named, from its foliated structure, the many -plies stomach, must pass between these folds, and describe three-fourths of a circle, before it can arrive at the orifice leading to the fourth stomach, which is so near that of the third, that the distance between them does not exceed three inches. There is, however, a more direct channel of communication between the oeso- phagus and the fourth stomach (4), along which milk taken by the calf, and which does not require to be either macerated or ruminated, is conveyed directly from the oesophagus to this fourth stomach ; for at that period the folds of the many-plies stomach are not yet separated, and adhere closely together ; and in these ani- mals rumination does not take place, till they begin to eat solid food. It is in this fourth stomach, which is called the reed, that the proper digestion of the food is performed, and it is here that the coagulation of the milk takes place ; on which account the coats of this stomach are employed in dairies, under the name of rennet^ to obtain curd from milk. A regular gradation in the structure of rumi- nating stomachs may be traced in the different genera of this family of quadrupeds. In rumi- nants with horns, as the bullock and the sheep, there are two preparatory stomachs for retaining 1.98 THE VITAL FUNCTIONS. the food previous to rumination, a third for receiving it after it has undergone this process, and a fourth for effecting its digestion. Rumi- nants without horns, as the Camel, Dromedary, and Lama, have only one preparatory stomach before rumination, answering the purpose of the two first stomachs of the bullock ; a second, which I shall presently notice, and which takes no share in digestion, being employed merely as a reservoir of water ; a third, exceedingly small, and of which the office has not been ascertained ; and a fourth, which both receives and digests the food after rumination. Those herbivorous animals which do not ruminate, as the horse and ass, have only one stomach ; but the upper portion of it is lined with cuticle, and appears to perform some preparatory office, which renders the food more easily digestible by the lower por- tion of the same cavity.* The remarkable provision above alluded to in the Camel, an animal which nature has evidently intended as the inhabitant of the sterile and arid regions of the East, is that of reservoirs of water, which, when once filled, retain their contents for a very long time, and may minister not only to the wants of the animal that possesses it, but also to those of man. The second stomach of the Camel has a separate * Home, Phil. Trans. 8vo. 1806, p. 370. DIGESTION. 199 compartment, to which is attached a series of cellular appendages ; (exhibited on a small scale, in Fig. 315) : in these the water is retained by strong muscular bands, which close the orifices of the cells, while the other portions of the stomach are performing their usual functions. By the relaxation of these muscles, the water is gradually allowed to mix with the contents of the stomach, and thus the Camel is enabled to support long marches across the desert without receiving any fresh supply. The Arabs, who traverse those extensive plains, accompanied by these useful animals, are, it is said, sometimes obliged, when faint, and in danger of perishing from thirst, to kill one of their camels, for the sake of the water contained in these reservoirs, which they always find to be pure and wholesome. It is stated by those who have travelled in Egypt, that camels, when accustomed to go journeys, during which they are for a long time deprived of water, acquire the power of dilating the cells, so as to make them contain a more than ordinary quantity, as a supply for their journey.* When the Elephant, while travelling in very hot weather, is tormented by insects, it has been observed to throw out from its proboscis, directly upon the part on which the flies fix themselves, a quantity of water, with such force as to dislodge * Home, Lectures on Comparative Anatomy, vol. i. p. 171. 200 THE VITAL FUNCTIONS. them. The quantity of water thrown out, is in proportion to the distance of the part attacked, and is commonly half a pint at a time : and this Mr. Pierard, who resided many years in India, has known the elephant repeat eight or ten times within the hour. The quantity of water at the animal's command for this purpose, observes Sir E. Home, cannot therefore be less than six quarts. This Avater is not only ejected immedi- ately after drinking, but six or eight hours after- wards. Upon receiving this information, Sir E. Home examined the structure of the stomach of that animal, and found, in it a cavity, like that of the camel, perfectly well adapted to afford this occasional supply of water, which may, at other times, be employed in moistening dry food for the purposes of digestion.* In every series of animals belonging to other classes, a correspondence may be traced, as has been done in the Mammalia, between the nature of their food and the conformation of their diges- tive organs. The stomachs of birds, reptiles and fishes, are, with certain modifications, formed very much upon the models of those already described, according as the food con- sists of animal or of vegetable materials, or presents more or less resistance from the co- hesion of its texture. As it would be impos- * Supplement to Sir E. Home's Lectures on Comparative Anatomy, vol. vi. p. 9. DIGESTION IN BIRDS. 201 sible in this place to enter into all the details necessary for fully illustrating this proposition, I must content myself with indicating a few of the most general results of the inquiry.* As the food of birds varies, in different spe- cies, from the softest animal matter to the hardest grain, so we observe every gradation in their stomachs, from the membranous sac of the carnivorous tribes, which is one extreme, to the true gizzard of granivorous birds, which occu- pies the other extremity of the series. This gradation is established by the muscular fibres, which surround the former, acquiring, in dif- ferent tribes, greater extent, and forming stronger muscles, adapted to the corresponding variations in the food, more especially as it partakes of the animal or vegetable character. In all the cold-blooded vertebrata, where di- gestion is not assisted by any internal heat, that operation proceeds more slowly, though in the end not less effectually, than in animals where the contents of the stomach are constantly main- tained at a high temperature. They almost all * The comparative anatomy of the stomach has been investi- gated with great diligence by the late Sir E. Home, and the results recorded in the papers he communicated from time to time to the Royal Society, and which have been republished in his splendid work, entitled " Lectures on Comparative Anatomy," to which it will be seen that I have been largely indebted for the facts and observations relating to this subject, detailed in the text. 202 THE VITAL FUNCTIONS. rank as carnivorous animals, and have accord- ingly stomachs, which, however they may vary in their form, are alike simply membranous in their structure, and act by means of the solvent power of their secretions. Among reptiles, only a few exceptions occur to this rule. The common sea-turtle that is brought to our tables, is one of these ; for it is found to feed exclu- sively on vegetable diet, and chiefly on the sea- weed called zostira maritima, and the structure of its stomach corresponds exactly to the gizzard of birds. Some tortoises, also, which eat grass, make an approach to the same structure. In fishes, indeed, although the membranous structure of the stomach invariably accompanies the habit of preying upon other fish, yet there is one species of animal food, namely, shell-fish, which requires to be broken down by powerful means before it can be digested. In many fish, which consume food of this kind, its trituration is effected by the mouth, which is, for this pur- pose, as I have already noticed in the wolf-fish, armed with strong grinding teeth. But in others, an apparatus similar to that of birds is employed ; the office of mastication being trans- ferred to the stomach. Thus the Mullet has a stomach endowed with a degree of muscular power, adapting it, like the gizzard of birds, to the double office of mastication and digestion ; and the stomach of the Gilluroo trout, a fish DIGESTION IN FISHES. 203 peculiar to Ireland, exhibits, though in a less degree, the same structure. The common trout, also, occasionally lives upon shell-fish, and swallows stones to assist in breaking the shells. Among the invertebrated classes we occa- sionally meet with instances of structures ex- ceedingly analogous to a gizzard, and probably performing the same functions. Such is the organ found in the Sepia ; the earth-worm has both a crop and a .gizzard ; and insects offer numerous instances, presently to be noticed, of great complexity in the structure of the stomach, which is often provided, not only with a me- chanism analogous to a gizzard, but also with rows of gastric teeth. Chapter VIII. Chylification. The formation of Chyle, or the fluid w^hich is the immediate and exclusive source of nutriment to the system, takes place in the intestinal tube, into which the chyme prepared by the stomach is received, and where farther chemical changes are effected in its composition. The mode in which the conversion of chyme into chyle is accomplished, and indeed the exact nature of the changes themselves, being, as yet, very imper- 204 THE VITAL FUNCTIONS. fectly known, it is consequently impossible to trace distinctly the correspondence which, in all cases, undoubtedly exists between the objects, to be answered, and the means employed for their attainment. No doubt can be entertained of the importance of the functions that are per- formed by structures so larg^e and so complicated as arc those composing the alimentary canal, and its various appendages. We plainly per- ceive that provision is made in the interior of that canal, for sidijecting its contents to the action, first, of an extensive vascular and nervous surface ; and secondly, of various fluid secretions, derived from different sources, and exercising powerful chemical agencies on the digested aliment; that a muscular power is supplied, by means of the layers of circular and longitudinal fibres, contained between the outer and inner coats of the intestine,* for exerting a certain pressure on their contents, and for propelling them forwards by a succession of contractions, which constitutes what is termed their peristaltic motion ; and lastly, that contrivances are at the same time resorted to for retarding the progress of the aliment in its passage along the canal, so that it may receive the full action of these several agents, and yield the utmost quantity of nutri- ment it is capable of affording. * See vol. i. p. 137. CHYLIFICATION. 205 The total length of the intestinal tube differs much in different animals, being in general, as already stated, smaller in the carnivorous tribes, than in those which feed on substances of diffi- cult digestion, or affording but little nourishment. In these latter animals, the intestine is always of great length, exceeding that of the body many times ; hence it is obliged to be folded into a spiral or serpentine course, forming many con- volutions in the abdominal cavity. Sometimes, probably for greater convenience of package, instead of these numerous convolutions, a similar effect of increasing the surface of the inner membrane is obtained by raising it into a great number of folds, which project into the cavity. These folds are often of considerable breadth, contributing not only to the extension of the surface for secretion and absorption, but also to the detention of the materials, with a view to their more complete elaboration. Remarkable examples of this kind of struc- ture occur in most of the carti- laginous fishes, when the inner coat of the large intestine is ex- panded into a broad fold, which, as is seen in Fig. 316, repre- senting this stRicture in the in- terior of the intestine of the shark, takes a spiral course ; and this is continued nearly the whole 206 THE VITAL FUNCTIONS. length of the canal, so that the internal surface is much augmented without any increase in the length of the intestine.* When the nature of the assimilatory process is such as to require the complete detention of the food, for a certain time, in particular situa- tions, we find this object provided for by means of cceca, or separate pouches, opening laterally from the cavity of the intestine, and having no other outlet. Structures of this description have already been noticed in the infusoria t, and they are met with, indeed, in animals of every class, occurring in various parts of the alimentary tube, sometimes even as high as the pyloric portion of the stomach, and frequently at the commence- ment of the small intestine. Their most usual situation, however, is lower down, and especially at the part where the tube, after having remained narrow in the first half of its course, is dilated into a wider cavity, which is distinguished from the former by the appellation of the great intes- tine, and which is frequently more capacious than the stomach itself. It is exceedingly pro- bable that these two portions of the canal per- form different functions in reference to the * Structures of this discription have a particular claim to attention from the light they throw on the nature of several fossil remains, lately investigated with singular success by Dr. Buckland. t Page 96, of this volume. CHYLIFICATION. 207 assimilation of the food : but hitherto no clue has been discovered to guide us through the intricacies of this difficult part of physiology ; and we can discern little more than the ex- istence, already mentioned, of a constant relation between the nature of the aliment and the structure of the intestines, which are longer, more tortuous, and more complicated, and are furnished with more extensive folds of the inner membrane, and with larger and more numerous caeca, in animals that feed on vegetable sub- stances, than in carnivorous animals of the same class. The class of insects supplies numberless exemplifications of the accurate adaptation of the structure of the organs of assimilation to the nature of the food which is to be converted into nutriment, and of the general principle that vegetable aliment requires longer processes, and a more complicated apparatus, for this purpose, than that which has been already animalized. In the herbivorous tribes, we find the oesophagus either extremely dilatable, so as to serve as a crop, or receptacle for containing the food pre- vious to its digestion, or having a distinct pouch appended to it for the same object : to this there generally succeeds a gizzard, or apparatus for trituration, furnished, not merely with a hard cuticle, as in birds, but also with numerous rows of teeth, of various forms, answering most effec- 208 THE VITAL FUNCTIONS. taally the purpose of dividing, or grinding into the minutest fragments, all the harder parts of the food, and thus supplying any deficiency of power in the jaws for accomplishing the same object. Thence the aliment, properly prepared, passes into the cavity appropriated for its digestion, which constitutes the true sto- mach.* In the lower part of this organ a pecu- liar fluid secretion is often intermixed with it, which has been supposed to be analogous to the bile of the higher animals. It is prepared by the coats- of slender tubes, termed hepatic vessels, which are often of great length, and sometimes branched or tufted, or beset, like the fibres of a feather, with lateral rows of filaments, and which float loosely in the general cavity of the body, attached only at their termination, where they open into the alimentary canal.t * It is often difficult to distinguish the portions of the canal, which correspond in their functions to the stomach, and to the first division of the intestines, or duodenum ; so that different naturalists, according to the views they take of the peculiar office of these. parts, have applied to the same cavity the term of chy- liferous stomach, or of duodenum. See the memoir of Leon Dufour, in the Annales des Sciences Naturelles, ii. 473. t The first trace of a secreting structure, corresponding to hepatic vessels, is met with in the Asterias, where the double row of minute lobes attached to the caecal stomachs of those animals, and discharging their fluid into these cavities, are considered by Carus, as performing a similar office. The flocculent tissue which surrounds the intestine of the Holothuria, is probably also an hepatic apparatus. DIGESTIVE ORGANS OF INSECTS. 209 In some insects these tubes are of larger dia- meter than in others : and in many of the or- thoptera, as we shall presently see, they open into large receptacles, sometimes more capacious than the stomach itself, which have been sup- posed to serve the purpose of reservoirs of the biliary secretion, pouring it into the stomach on those occasions only when it is particularly wanted for the completion of the digestive process.* The distinction into small and great intestine is more or less marked, in different insects, in proportion to the quantities of food consumed, and to its vegetable nature ; and in herbivorous tribes, more especially, the dilatations in the lower part of the canal are most conspicuous, as well as the duplicatures of the inner mem- brane, which constitute imperfect valves for retarding the progress of the aliment. It is generally at the point where this dilatation, of the canal commences, that a second set of hepatic vessels is inserted, having a structure essentially the same as those of the first set, but generally more slender, and uniting into a small number of ducts before they terminate. The number and complication of both these sets of hepatic vessels, appear to have some relation to * A doubt is suggested, by Leon Dufour, whether the liquid found in those pouches is real bile, or merely aliment in the pro- gress of assimilation. Ann. Sc Nat. ii. 473. VOL. II. P 210 THE VITAL FUNCTIONS. the existence and developement of the gizzard, and consequently also to the nature and bulk of the food. Vessels of this description are, indeed, constantly found in insects ; but it is only where a gizzard exists, that two sets of these secreting organs are provided ; and in some larvae, remark- able for their excessive voracity, even three orders of hepatic vessels are met with.* A muscular power has also been provided, not only for the strong actions exerted by the gizzard, but also for the necessary propulsion, in dif- ferent directions, of the contents both of the stomach and intestinal tubes. The muscular fibres of the latter are distinctly seen to consist of two sets, the one passing in a transverse or circular, and the other in a longitudinal direc- tion. Glandular structures, analogous to the mucous follicles of the higher animals, are also plainly distinguishable in the internal coat of the canal, more especially of herbivorous insects. | The whole tract of the alimentary canal is at- tached to the sides of the containing cavity by a fine membrane, or peritoneum, containing numer- ous air-vessels, or trachecB.\ * See the Memoirs of Marcel des Serres, in the Annales du Museum, xx. 48. f Lyonet. X It has been stated by Malpighi and by Swammerdam, and the statement has been repeated by every succeeding ana- tomist, that almost all the insects belonging to the tribe of i DIGESTIVE ORGANS OF INSECTS. 211 To engage in a minute description of the end- less variations in the structure of the digestive organs, presented in the innumerable tribes which compose this class of animals, would be incompatible with the limits of this treatise. I shall content myself, therefore, with giving a few illustrations of their prin- cipal varieties, selected from those in which the leading characters of structure are most strongly marked. I shall, with this view, exhibit first one of the simplest forms of the alimentary organs as they oc- cur in the 3Ia?itis religiosa, (Linn.) which is a purely car- nivorous insect, belonging to the order of Orthoptera. Fig. 317 represents those of this insect, freed from their attach- ments, and separated from the body. The whole canal, as is seen, is perfectly straight : it commences by an oesophagus (o), of great length, which is succeeded by a Grylli, possessed the faculty of ruminating their food; but this error has been refuted by Marcel des Serres, who has offered satis- factory evidence that in no insect is the food subjected to a true rumination, or second mastication, by the organs of the mouth. See Annales du Museum, xx. 51 and 364. 212 THE VITAL FUNCTIONS. gizzard (g) ; at the lower extremity of this organ the upper hepatic vessels (b,b), eight in number, and of considerable diameter, are inserted : then follows a portion of the canal (d), which may be regarded either as a digesting stomach, or a chyliferous duodenum : farther downwards, the second set of hepatic vessels, (h h), which are very numerous, but as slender as hairs, are received : and after a small contraction (n) there is again a slight dilatation of the tube (c) before it terminates. The alimentary canal of the Cicindela ca7npes- tris, (Lin.) which preys on other insects, is re- presented in Fig. ;n8 ; where we see that the lower part of the oesophagus (o), is dilated into a crop (p), succeeded by a small gizzard (g), which is provided for the purpose of bruising the elytra, and other hard parts of their victims : but, their mechanical division being once effected, we again find the true digesting stomach (s) simply membranous, and the intestine (i) very short, but dilated, before its termination, into a large colon (c). The hepatic vessels (h), of which, in this insect, there is only one set, ter- minate in the cavity of the intestine by four ducts, at the point where that canal commences. A more complicated structure is exhibited in the alimentary tube of the Melolontha vulgaris, or common cockchaffer, which is a vegetable DIGESTIVE ORGANS OF INSECTS. 213 feeder, devouring great quantities of leaves of plants, and consequently requiring a long and capacious canal for their assimilation ; as is shown in Fig. 319, which represents them pre- pared in a similar manner to the former. In this herbivorous insect, the oesophagus (o) is, as might be expected, very short, and is soon dilated into a crop (p) ; this is followed by a very long, wide, and muscular stomach (s), ringed like an 214 THE VITAL FUNCTIONS. 320 earth-worm, and continued into a long and tor- tuous intestine (i, i), which presents in its course several dilatations (c, c), and receives very elongated, convo- luted, and ramified hepatic vessels (h,h). Fig. 320 is a highly magnified view of a small portion of one of these vessels, showing its branched form. In the alimentary canal (Fig. 321*) of the Acrida aptera (Stephens), which is a species of grass- hopper, feeding chiefly on the dewberry, we observe a long oesophagus (o), which is very dilatable, enlarging occasion- ally into a crop (i), and suc- ceeded by a rounded or heart- shaped gizzard (g), of very complicated structure, and connected with two remark- ably large biliary pouches (u and b), which receive, at their anterior extremity, the upper set of hepatic vessels (v v). A deep furrow in the pouch (b), which, in the horizontal posi- * The figures relating to this insect were engraved from the drawings of Mr. Newport, who was also kind enough to supply me with the description of the parts they represent. Fig. 321 is twice the natural size. DIGESTIVE ORGANS OF INSECTS. 215 tion of the body, lies underneath the gizzard, divides it apparently into two sacs. The intes- tinal canal is pretty uniform in its diameter, re- ceives in its course a great number of hepatic vessels (h h), by separate openings, and after making one convolution, is slightly constricted at N, and is dilated into a colon (c), on the coats of which the longitudinal muscular bands are very distinctly seen. Fig. 322 is a magnified view of the gizzard laid open, to show its internal structure. It is furnished with six longitudinal rows of large teeth, and six intermediate double rows of smaller teeth ; the total number of teeth being 270. One of the rows of large teeth is seen, detached, and still more magnified, in Fig. 323 ; it contains at the upper part, five small hooked teeth (f), succeeded below by four broad teeth (d), consisting of quadrangular plates, and twelve tricuspid teeth (t) ; that is, teeth having three cusps, or points at their edges. Fig. 324 shows the profile of one of these teeth ; a, being the sharp point by which the anterior acute angle of the base terminates. Fig. 325 exhibits the base of the same tooth seen from below, e, e, e, being the three cusps, and m, the triangular hollow space for the insertion of the muscles which move them, and which compose part of the muscular apparatus of the gizzard. The smaller teeth, which are set in double lines between each of the larger rows, consist of twelve 216 THE VITAL FUNCTIONS. small triangular teeth in each row. All the teeth contained in this organ are of a brown colour and horny texture, resembling tortoise- shell. The same insect, as we have seen, often exhibits, at different periods of its existence, the greatest contrast, not only in external form, but also in its habits, instincts, and modes of subsistence. The larva is generally remarkable for its voracity, requiring large supplies of food to furnish the materials for its rapid growth, and frequently consuming enormous quantities of fibrous vegetable aliment : the perfect insect, on the other hand, having attained its full dimen- sions, is sufficiently supported by small quantities of a more nutritious food, consisting either of animal juices, or of the fluids prepared by flowers, which are generally of a saccharine quality, and contain nourishment in a concen- trated form. It is evident that the same appa- ratus, which is necessary for the digestion of the bulky food taken in during the former period, would not be suited to the assimilation of that which is received during the latter ; and that in order to accommodate it to this altered condition of its function, considerable changes must be made in its structure. Hence, it will be interest- ing to trace the gradual transitions in the confor- mation of the alimentary canal, during the pro- gressive developement of the insect, and more DIGESTIVE ORGANS OF INSECTS. 217 especially while it is undergoing its different metamorphoses. These changes are most conspicuous in the Lepidoptera, where we may observe the suc- cessive contractions which take place in the im- mensely voluminous stomach of the caterpillar, while passing into the state of chrysalis, and thence into that of the perfect insect, in which its form is so changed that it can hardly be recognised as the same organ. I have oiven re- 328 presentations of these three different states of the entire alimentary canal of the Sphinx ligustri. 218 THE VITAL FUNCTIONS. or Privet Hawk-moth, in Figures 326, 327, and 328* ; the first of which is that of the caterpillar ; the second, that of the chrysalis ; and the third, that of the moth. The whole canal and its ap- pendages, have been separated from their at- tachments, and spread out, so as to display all their parts ; and they are delineated of the natural size, in each case, so as to show their comparative dimensions in these three states. In all the figures, a is the oesophagus ; b, the stomach ; c, the small intestine ; d, the caecal portion of the canal ; and e, the colon, or large intestine. The hepatic vessels are shown at f ; and the gizzard, which is developed only in the moth, at g. Fig. 328. It will be seen that in the caterpillar, (Fig. 326), the stomach forms by far the most considerable portion of the alimentary tube, and that it bears some resemblance in its structure and capacity to the stomachs of the Annelida, already de- scribed.! This is followed by a large, but short, and perfectly straight intestine. These organs in the pupa (Fig. 327) have undergone con- siderable modifications, the whole canal, but more especially the stomach, being contracted * These figures also have been engraved from the drawings of Mr. Newport, which he was so obliging as to make for me, from M preparations of his own, the result of very careful dissections. f See the figures and description of those of the Nais and the Leech, p. 10'2 and 103. DIGESTIVE ORGANS OF MOLLUSCA. 219 both in length and width* : the shortening of the intestine not being in proportion to that of the whole body, obliges it to be folded upon itself for a certain extent. In the moth, (Fig. 328), the contraction of the stomach has pro- ceeded much farther ; and an additional cavity, which may be considered as a species of crop or gizzard (g), is developed : the small intestine takes a great many turns during its course, and a large pouch, or c^cmn, has been formed at the part where it joins the large intestine. The hepatic vessels are exceedingly nume- rous in the Crustacea, occupying a very large space in the general cavity ; and they compose by their union an organ of considerable size, which may be regarded as analogous in its functions to the Liver of the higher classes of animals. This organ acquires still greater size and importance in the Mollusca, where it frequently envelopes the stomach, pouring the bile into its cavity by numerous ducts. f As the structure and course of the intestinal canal varies greatly in different tribes of Mollusca, they do not admit of being comprised in any * Carus states that he found the stomach of a pupa, twelve days after it had assumed that state, scarcely half as long', and only one-sixth as wide as it had been in the caterpillar. t Transparent crystalline needles, the nature and uses of which are quite unknown, are frequently found in the biliary ducts of this class of animals. 220 THE VITAL FUNCTIONS. 329 general description. The only examples I think it necessary to give, in this class, are those of the Patella, or Limpet, and of the Pleurohranchus. The in- testinal tube of the Patella is delineated in Fig. 329 ; where M is the mouth ; t, the tongue folded back ; o, the oesophagus ; and s, the stomach, from which the tortuous intestinal tube is seen to be continued. All the convolutions of this tube, as well as the stomach itself, are enclosed, or rather imbedded in the substance of the liver, which is the largest organ of the body. The P leurobvanchus Peronii {C\xw .) is remark- able for the number and compli- cation of its organs of digestion. They are seen laid open in Fig. 330 ; where c is the crop ; g, the gizzard ; p, a plicated stomach, re- sembling the third stomach of ru- minant quadrupeds ; and d, a fourth cavity, being that in which diges- tion is completed. A canal of com- munication is seen at t, leading from the crop to this last cavity : b is the point where the biliary duct enters. In the Cephalopoda, the structure of these DIGESTIVE ORGANS OF FISHES. 221 organs is very complicated ; for they are pro- vided with a crop, a muscular gizzard, and a caecum, which has a spiral form. In these ani- mals we also discover tlie rudiment of another auxiliary organ, namely, the Pancreas, which secretes a fluid contributing to the assimilation of the food. This organ becomes more and more developed as we ascend in the scale of animals, assuming a glandular character, and secreting a watery fluid, which resembles the saliva, both in its sensible and chemical properties. It has been conjectured that many of the vessels, which are attached to the upper portion of the alimentary canal of insects, and have been termed hepatic, may, in fact, prepare a fluid having more of the qualities of the pancreatic than of the biliary secretion. The alimentary canal of fishes is in general characterised by being short ; and the con- tinuity of the stomach with the intestines is often such as to offer no well marked line of distinc- tion between them. The cseca are generally large and numerous ; and a number of tubular organs, connected more especially with the pylorus, and called therefore the pyloric appen- dices, are frequently met with, resembling a cluster of worms, and having some analogy, in situation at least, to the hepatic or pancreatic vessels of insects. Their appearance in the •222 THE VITAL FUNCTIONS. Salmon is represented at p, in Fig. 331 . The pan- creas itself is only met with, in this class of animals, in the order of cartilaginous fishes, and more especially in the Ray and the Shark tribes. A distinct gall- bladder, or reservoir, is also met with in some kinds of fish, but is by no means general in that class. In the classes both of Fishes and of Reptiles, which are cold-blooded animals, the processes of digestion are conducted more slowly than in the more energetic systems of Birds and of Mammalia ; and the comparative length of the canal is, on the whole, greater in the former than in the latter : but the chief differences in this respect depend on the kind of food which is consumed, the canal being always shortest in those tribes that are most carnivorous.* As the Frog, in the different stages of its growth, lives upon totally different kinds of food, so we find that the structure of its alimentary canal, like that of the moth, undergoes a material change during these metamorphoses. The intestinal canal of the tadpole is of great length, and is collected into a large rounded mass, composed of a great number of coils, which may easily be distinguished, by the aid of a magnifying glass, through the transparent skin. During its gra- * See Home, Lectures, &c. I. 401. DIGESTIVE ORGANS OF MAMMALIA. 223 dual transformation into a frog, this canal be- comes much reduced in its length ; so that when the animal has attained its perfect form, it makes but a single convolution in the abdominal cavity. A similar correspondence exists between the length of the canal, and the nature of the food in the class of Birds. At the termination of the small intestine there are usually found two caeca, which in the gallinaceous and the aquatic fowls, are of great length : those of the ostrich contain in their interior a spiral valve. Sir E. Home is of opinion that in these animals the functions of the pyloric portion of the stomach are per- formed by the upper part of the intestine. In the intestines of the Mammalia contrivances are employed with the apparent intention of preventing their contents from passing along too hastily : these contrivances are most effectual in animals whose food is vegetable, and contains little nourishment, so that the whole of what the food is capable of yielding is extracted from them. Sir E. Home observes that the colon, or large intestine of animals which live upon the same species of food, is of greater length in proportion to the scantiness of the supply. Thus the length of the colon of the Elephant, which inhabits the fertile woods of Asia, is only 262 feet ; while in the Dromedary, which dwells in the arid deserts of Arabia, it is 42. This con- 224 THE VITAL FUNCTIONS. trast is still more strongly marked in birds. The Cassowary of Java, which lives amidst a most luxuriant supply of food, has a colon of one foot in length, and two caeca, each of which is six inches long, and one quarter of an inch in diameter. The African ostrich, on the other hand, which inhabits a country where the supply of food is very scanty, has the colon forty-five feet long ; each of the caeca is two feet nine inches in length, and, at the widest part, three inches in diameter; in addition to which there are broad valves in the interior of both these cavities.* On comparing the structure of the digestive organs of Man with those of other animals belonging to the class Mammalia, we find them holding a place in the series intermediate be- tween those of the purely carnivorous, and ex- clusively herbivorous tribes ; and in some mea- sure uniting the characters of both. The powers of the human stomach do not, indeed, extend to the digestion either of the tough woody fibres of vegetables on the one hand, or the compact texture of bones on the other ; but still they are competent to extract nourishment from a wider * Lectures, Blind-worm, i. 454, 457. Blood, ii. 334. Blood-vessels, ii. 281. Blumenbach, ii. 426. Boa, i. 447, 448. Boar, i. 56; ii. 141, 161. Bombyx, i. 300, 304, 312; ii. 486. Bone, i. Ill, 365, 375. Bonnet, i. 53; ii. 17, 79, 92, 252, 478. Borelli, i. 588. Bosc, i. 149. Bostock, ii. 333. Bound of deer, i. 495. Bowerbank, ii. 241. Boyle, ii. 16. Bracteae, i. 94. Bradypus, i. 481 ; ii. 284. Brain, i. 35 ; ii. 366, 555, 575. Brain, formation of, ii. 605. Branchiae, ii. 267, 293, 299. Brassica, ii. 48, 53. Braula, ii. 483. Breschet, ii. 427. Brewster, i. 232 ; ii. 472, 495. Brocken, spectre of, ii. 533. Broussonnet, ii. 587. Bruguiere, i. 149, 248. Bryophyllum, ii. 586. Buccinum, i. 215, 229, 242; ii. 126, 301. Buckland, ii. 206. Buds, i. 86; ii. 588. Buffon, i. 185; ii. 530, 591. Bulb of hair, i. 117. Bulb of feather, i. 577. Bulbus arteriosus, ii. 273. Bulbulus glandulosus, ii. 185. Bulimus, i. 249. Bulla, ii. 168. Burrowing of the mole, i. 525. Cabbage, ii. 48, 53. Cachalot,!. 484; ii. 142. Cffica, ii. 101,206. Ccecilia, ii. 497. Calamary, i. 261 . Callionymus, ii. 503. Calosoma, i. 320. Cambium, ii. 40. Camel, i. 108; ii. 176, 198. Cameleopard, i. 481, 498; ii. 135. Camera obscura, ii. 458. Camerated shells, i. 265. Campanularia, ii. 234. Camper, ii. 437, 443, 561. Canada rat, ii. 178. Cancelli, i. 374. Cannon bone, i. 505. Capibara, ii, 160. Capillaries, ii. 263. Capsular ligaments, i. 106. Caput Medusae, i. 212. Carapace, i. 290, 463. Carbon, non-absorption of, ii. 17. Carbonic acid, ii. 30, 337. Cardia, ii. 182. Cardium, i. 131, 221, 222, 224. Carduus, i. 127. Carinated sternum, i. 566. Carlisle, i. 426,434; ii. 285, 567. Carnivora, i. 528 ; ii. 66, 145. Carp, i. 411,429. Carpus, i. 405. Cartilage,!. 109. Caruncle, lacrymal, ii. 468. Cams, i. 366; ii. 208, 219, 240, 252, 505. Cassowary, i. 586; ii. 224. Cat, ii. 392, 505. Caterpillar, i. 305, 315; ii. 484. Caudal vertebrae, i. 404. Cavolini, i. 159. Celandine, ii. 48. Cells of plants, i. 66, 69. Cellular texture, animal, i. 99. Centaurea, i. 127. Cephalic ganglion, ii. 541. 646 INDEX. Cephalo-thorax, i. 282. Cephalopoda, i. 258 ; ii. 220, 551. Cerambyx, i. 328; ii. 311, 313. Cercaria, i. 186; ii. 479. Cerebellum, ii. 555. Cerebral ganglion, ii. 541. Cerebral hemispheres, ii. 556. Cerithium, i. 249. Ceroxylon, ii. 48. Cetacea, i. 481,482; ii. 142, 176, 193, 442, 555, 616. Chabrier, i. 108, 346. Chain of being, i. 53 ; ii. 629. Chalcides, i. 448, 457. Chameleon, i. 462; ii. 129, 390, 499. Chara, ii. 50, 254. Chelidonium, ii. 48. Chelonia, i. 463; ii. 130,276, 321,439. Chemistry, organic, ii. 5, 333. Cheselden, ii. 520. Chevreuil, i. 123. Children, i. 318; ii. 491. Chitine, i. 318. Chladni, ii. 417, Chondrilla, ii. 52. Choroid coat, ii. 462. Choroid gland, ii. 495. Chromatic aberration, ii. 474. Chromule, i. 70. Chrysalis, i. 307. Chyle, ii. 107, 203. Chyme, ii. 181. Cicada, i. 340. Cicindela, ii. 212. Cilia, i. 126, 154, 157, 173, 195,203,215. Ciliary ligament, ii. 463. Cimbex, i. 333. Cimex, ii. 124. Cineritious, ii. 561. Circulation, ii. 11, 229. Cirrhi, ii. 296, 389. Cirrhopoda, i. 257. Classification, i. 51 ; ii. 625.' Clausilia, ii. 317. Clausium, i. 253. Clavicle, i. 404, 523, 566. Claviger, ii. 483. Claw in lion's tail, i. 531. Clio, i. 258; ii. 138. Cloquet, ii. 498. Clypeaster, i. 211. Cobitis, ii. 309. Cobra de capello, i. 549 ; ii, 164. Coccygeal bone, i. 404. Cochlea, ii. 427. CockchafFer. See Melolontha. Cockle, i. 221. &eCardium. Cod, lens of, i. 59 ; ii. 496. Coenurus, ii. 84. Co-existence of forms, i. 50. Coffin-bone, i. 517. Coleoptera, i. 348 ; ii. 382. Collar-bone, i. 404. Colours of insects, i. 318. Colours, perceptions of, ii.531. Coluber, i. 448,450; ii. 164. Columella, i. 243 ; ii. 439. Commissures of brain, ii. 562. Comparetti, ii. 244, 436. Complementary colours, ii. 531. Compound eyes, ii. 483. Concha of the ear, ii. 421. Condor, ii. 331. Conger eel, ii. 556. Conglomerate eyes, ii. 483. Conjunctiva, ii. 466. Consumption of animal mat- ter, ii. 60. Contractility, muscular, i. 125. Conus, i, 250. Convolutions of the brain, ii. 558. Convolvulus, ii. 48. Cooper, ii. 434. Coracoid bone, i. 404, 566. Coral, i. 166. Coral islands, i. 15. INDEX. d47 Corium, i. 112. Cornea, ii. 461. Corneule, ii. 487. Cornu Ammonis, i. 267. Coronet bone, i. 517. Corpora quadrigemina, ii. 555. Corpus callosum, ii. 562. Corpus papillare, ii. 378. Cortical substance, ii. 561. Cossus, i. 300, 312,355. Cotuyinius, ii. 427. Cowrie, i. 247. Crab, i. 290; ii. 258, 299,317, 493. Cranium, i. 399, 400, 443, 470. Cranium of insects, i. 322. Craw, ii. 169. Cray-fish, ii. 435,491. Cribriform plate, ii. 400. Crinoidea, i. 212. Crocodile, i. 458, 460, 462; ii. 142, 163, 276, 409, 440, 557. Crop, ii. 179. Cross-bill, ii. 131. Crotalus, i. 450. Crust, i. Ill, 292. Crusta petrosa, ii. 152. Crustacea, i. 286 ; ii. 269, 295, 299, 542, 587. Cryptogamia, i. 71 ; ii. 593. Crystalline lens, i. 59 ; ii. 462. Crystalline needles in biliary ducts, ii. 219. Curculio, i. 328. Cushions of insects, i. 331. Cuticle, vegetable, i. 77. Cuticle, animal, i. 112; ii. 377. Cuttle-fish. See Sepia. Cuvier, passim. Cuvier (F.), i. 120, 574. Cyclidium, i. 186. Cyclocaela, ii. 98. Cyclosis, ii. 49, 233. Cyclostomata, ii. 116. Cymbia, i. 241. Cymothoa, ii. 544. Cyprsea, i. 247. Cyprinus, i. 1 16, 411. Cysticule, ii. 438. Daldo7-ff, i. 433 ; ii. 306. Darwin, i. 89. Darwin (Dr. R.), ii. 530. Davy, ii. 17,338. Davy (Dr.), ii. 274. Death, ii. 624. De Blainville, i. 63, 248, 366; ii. 252, 428, 482, 497, 570. De Candolle, i. 93 ; ii. 19,25, 28, 30, 38,51, 620. De Candolle (junior), ii. 47. Decapoda, ii. 258. Decline of the system, ii. 619. Decollated shells, i. 249. Deer, i. 507; ii. 402. Defrance, i. 256. De Geer, i. 341. Deglutition, ii. 174. Delarocke, ii. 309, 497. De Montegre, ii. 183. Dermo-skeleton, i. 366. De Saussure (Tli.), ii. 30. Des Cartes, ii. 364, 560. De Serres, ii. 211, 239, 485. Design, evidence of, i. 28. Design, unity of, ii. 625. Developement, vegetable, i. 82. Developement, animal, ii. 599. Diaphragm, ii. 326, 611. Diffusion of animals, ii. 64. Digestion, i. 41 ; ii. 180. Digitigrada, i. 533. Diodon, i. 433. Dioecia, ii. 596. Dionsea, i. 128. Diplozoon, ii. 608. Diptera, i. 323, 353; ii. 115. Diquemare, i. 220. Distoma, ii. 1 13. Divisibility of matter, ii. 397. 648 INDEX. Dollinger , ii. 614. Dolphin, ii. 142, 442,507,616. Doras costatus, ii. 307. D'Orbigmj, i. 265. Doris, ii. 126, 296. Dormouse, ii. 191. Dorsal vessel, ii. 236. Dory, i. 421. Dove, ii. 554, 557. Down of plants, i. 94. Down of birds, i. 572. Draco volans, i. 5Q, 547. Dragon-fly, i. 310, 351 ; ii. 487. Dreaming, ii. 536. Dromedary, ii. 223. Duckweed, ii. 589. Dufour (Leon), ii. 209, 313. Duges, ii. 244, 250, 479, 487, 491. Dugong, ii. 142, 279, 442. Duhamel, ii. 16, 20. Dumas, ii. 393. Dumeril, ii. 411. Dumortier, i. 366. Duodenum, ii. 208. Dutrochet, i. 75, 190 ; ii. 314. Dytiscus, i. 29,311,333,336; ii. 311, 313. Eagle, ii. 130. Ear, ii. 421. Ear-drum, ii. 422. Earle, i. 560. Earths in plants, ii. 43. Earth-worm, (see Lumbricus). Echinodermata, i. 199. Echinus, i. 203, 210; ii. 101, 119, 297, 383. Edwards, ii. 317, 542, 631. Eel, i. 424 ; ii. 307. Egg, ii. 597. Ehrenherg, i. 13, 186, 189; ii. 93, 478, 592. Ehrmann, ii. 309. Elaboration, successive, ii. 13. Elastic ligaments, i. 107. Elater, i. 341. Elearine, i. 123. Electric organs, i. 31. Electricity, ii. 350. Elements, organic, ii. 6. Elephant, i. 5Q, 108, 491, 51 8 ; ii. 141, 154, 162, 199, 223, 392, 504, 559. Ellis, i. 150. Elytra, analysis of, i. 318, 349. Embryo, ii. 595. Emu, i. 586. Emys, i. 474. Enamel of teeth, ii. 150. Endogenous plants, i. 83. Entomoline, i. 115, 318. Entomostraca, ii. 493. Entozoa, i. 282; ii. 83, 113, 235,294,540, 591. Ephemera, i. 311 ; ii. 241. Epidermis, vegetable, i. 88. Epidermis, animal, i. 1 1 2, 1 1 3, 231. Epiphragma, i. 253. Equivocal generation, ii. 591. Equorea, ii. 85. Erato, i. 247. Erect vision, ii. 521. Erpobdella, i. 272 ; ii. 252. Eryx, i. 447. Esox, i. 427. Ethmoid bone, ii. 400. Eudora, ii. 91. Euler, ii. 475. Eunice, ii. 480. Euphorbium, ii. 59. Euryale, i. 212. Eustachian tube, ii. 424. Evil from animal warfare, i. 46 ; ii. 67. Excretion, ii. 12. Excretion, vegetable, ii. 46, 51 . Exhalation by leaves, ii. 27. Exocetus, i. 547. Exogenous plants, i. 83. Eye, i. 31 ; ii. 460, 587, 589. Eye, formation of, ii. 605. INDEX. (>49 Eye-lids, formation of, ii. 615. Fabricius, i. 195. Facial angle, ii. 561. Fairy rings, ii. 55. Fallacies of perception, ii. 514. Fangs of serpents, ii. 163. Faraday, ii. 524. Fasciola, ii. 113. Fasciolaria, i. 249. Fat, i. 123. Fata Morgana, ii. 533. Feathers, i. 568, 591. Fecula, i. 70. Fecundation, ii. 595. Feelers, i. 288; ii. 383. Feet-jaws, i. 289. Feet of birds, i. 584. Femur, i. 287, 328, 405. Fenestrse of ear, ii. 425. Ferns, i. 83 ; ii. 593. Fibre, animal, i. 98, 105. Fibula, i. 405. Fig-tree, ii. 48. Fig Marygold, ii. 48. Filaments of feathers, i. 569. Filaria, i. 63. Filices, i. 83 ; ii. 593. Final causes, i. 1 ,22, et passim. Fins of fishes, i. 421. Fins of cetacea, i. 486. Fishes, i. 109, 408; ii. 127, 272, 389, 410, 494, et pas- sim. Fissiparous reproduction, ii. 583. Flea, i. 297. Flight, i. 344, 545. Flourens, ii. 305. Flower, ii. 595. Fluidity, organic, i. 61. Flustra, i. 165, 169, 172. Flying fish, i. 547. Flying lizard, i. 547. Flying squirrel, i. 550. Focus, ii. 453. Fohmann, ii. 353. Follicles, i. 114; ii. 185. Fontana, ii. 614. Food of plants, ii. 15. Food of animals, ii. 57. Foot of mollusca, i. 22 1 . Forces, physical, i. 6. Fordyce, ii. 172. Fovilla, ii. 596. French bean, ii. 52. Frog, i. 437;ii. 128,222,274, 439. Fucus vesiculosus, i. faQ. Functions, i. 34, 38 ; ii. 69. Fungi, ii. 55. Furcular bone, i. 566. Furcularia, i. 62. Fusiform roots, ii. 21. Future existence, ii. 580, 640. Gaede, ii. 86. Gaimard, i. 97. Galeopithecus, i. 550. Galileo, i. 81. Gallinae, ii. 554. Gallop, i. 495. Galvanism, ii. 514. Ganglion, ii. 358. Gasteropoda, i. 227; ii. 176, 300, 480. Gastric juice, ii. 183. Gastric teeth, ii. 167,214. Gastric glands, ii. 184. Gastrobranchus, i. 407, 416 ; ii. 116, 497. Gay Lussac, ii. 314. Gecko, i. 460 ; ii. 390. Gelatin, i. 105. Gemmiparous reproduction, ii. 588. Gemmule, i. 156 ; ii. 591. Geometer caterpillars, i. 315. Germs, vegetable, i. 86 ; ii. 588. Geronia, ii. 91. Gillaroo trout, ii. 202. Gills, i. 439 ; ii. 267, 299. Gimbals, i. 330. ()50 INDEX. Gizzard, ii. 169,214. Glands, vegetable, i. 77 ; ii. 45. Glands, animal, ii. 348. Glands in crocodile, ii. 409. Glands, gastric, ii. 1 84. Gleichen, ii. 94. Globules, i. 64, 98. Glossa, ii. 124. Glossopora, ii. 104. Gmelin, i. 149. Gnat, ii. 115. Goat, ii. 402. Goeze, ii. 478. Gonium, i. 187. Goose, ii. 173, 500. Gordius, i. 63, 276. Gorgonia, i. 166. Gradation of being, i. 53 ; ii. 629. Grampus, ii. 142. Grallse, i. 585, 592. Grant, i. 147, 151, 169, 172, 175, 195,203,215,587; ii. 478. Gray, i. 219, 239, 254. Growth, vegetable, i. 84; ii. 21, 599. Gruit/misen, ii. 479. Gryllotalpa, i. 342 ; ii. 385. Gryllus, ii. 244. Guinea-pig, i. 498. Gulstonian lectures, ii. 532. Gum, ii. 37. Gurnard, ii. 554. Gymnotus, i. 424 ; ii. 572. Hsematopus, ii. 131. Haidinger, i. 205. Hair, vegetable, i. 94. Hair, animal, i. 117, 319. Hair-worm, i. 276. Hales, ii. 26. Haliotis, i. 231. Haller, i. 98. Halteres, i. 353. Hamster, ii. 178. Hancock, ii. 307. Hand, i. 544 ; ii. 392. Hanow, ii. 478. Hare, i. 497; ii. 149,191. Hartley, ii. 563. Harwood, ii. 404, 405. Hatchett, ii. 43. Haaksbee, ii. 415. Haunch in insects, i. 287, 328. Hawk, ii. 130. Head of insects, i. 322. Hearing, ii. 414, 571. Heart, i. 41,138; ii. 258,607. Hedge-hog, i. 524, 527. Hedysarum gyrans, i. 127. Hedwig, i. 74. Helix, i. 242, 253; ii. 126, 317,481. Hellman, ii. 390. Hemiptera, i. 309, 350; ii. 115. Hemispheres, cerebral, iii. 556. Henbane, ii. 59. Henderson, ii. 338. Hepatic vessels, ii. 208, 214. Herring, i. 421. Herschel (Sir W.), ii. 529. Herschel (Sir John), i. 232 ; ii.44, 571. Hervey, ii. 288. Hesperia, i. 356. Hexastoma, ii. 113. Hippopotamus, ii. 141, 151, 152, 162, 193, 443, 504. Hirudo, i. 138, 281 ; ii. 102, 125,252,298, 480. Hodgkin, i. 99, 127. Hodgson, ii. 403. Hog, i. 402,521 ; ii. 193, 392. Holothuria, ii. 208, 235, 296, 550. Home (Sir Everard), passim. Honey-comb stomach, ii. 195. Hooded snake, i. 549. Hooks on feet of insects, i. 331 . Hop, i. 91. Horn, i. 115,514. 1 INDEX. 631 Horn on beak of chick, ii. 615. Horse, i. 516; ii. 191, 401, 569. Horse-fly, ii. 115. Hostilities of animals, i. 46 ; ii. 67. Houston, ii. 129. Huber, ii. 386,413. Human fabric, i. 536 ; ii. 559. Humboldt, ii. 308, 314, 338. Humerus, i. 405. Humours of the eye, ii. 460. Hunter, i. 108; ii. 171, 188, 330. Hysena, i. 499; ii. 61, 149. Hybernation, ii. 536. Hydatid, ii. 84, 113, 591. Hydatina, ii. 97, 98, 479, 539. Hydra, i. 162, 176; ii. 74, 477, 538, 586, 590. Hydrogen, ii. 45. Hydrophilus, i. 311. Hydrostatic acalepha, i. 196. Hyla, i. 445. Hymenoptera, i. 323, 351 ; ii. 116, 244. Hyoid bone, ii. 132, 303. Hyrax, ii. 191. Ichthyosaurus, i. 469. Ilium, i. 405. Imago, i. 307, 317. Incisions of insects, i. 327. Incisor teeth, ii. 143. Incus, ii. 426. Indian walrus, ii. 142. Individuality of polypes, i. 173. Infusoria, i. 183 ; ii. 539, 583. Injuries, reparation of, ii. 3, 587. Inorganic world, i. 7. Insects, i. 11, 108, 296; ii. 207, 236, 395, 436, 546, 570. Insectivora, i. 525. Instinct, ii. 574. Integuments, i. Ill ; ii. 377. Intercellular spaces, i. 70. Intermaxillary bone, ii. 143, 634. Interspinous bones, i. 396. Intestine, ii. 101. Iriartea, ii. 48. Iridescence, i. 232. Iris, i. 136; ii. 463. Ischium, i. 405. Isis, i. 168. Ivy, i. 92. Jacobson, ii. 568, 570. Jerboa, i. 497, 538. Johnson, ii. 104. Julus, i. 298 ; ii. 485. Jurine, ii. 567. Kaleidoscope, ii. 533. Kanguroo, i. 399, 497, 538; ii. 193, 598. Kater, ii. 491. Kerona, i. 186. Kidd, i. 342; ii. 313, 348, 385. Kiernan, ii. 350. Kieser, i, 66, 74. Kirby, i. 327; ii 413,485. Knight, ii. 595. Knots in wood, ii. 589. Koala, i. 527. Kolpoda, i. 187. Labium of insects, ii. 123. Labrum of insects, ii. 123. Labyrinth, ii. 427. Lacerta, i. 457, 458. Lacrymal organs, ii. 466. Lacteals, ii. 107, 226. Lamarck, i. 149 ; ii. 93, 637. Lamina spiralis, ii. 430. Lamouroux, i. 149. Lamprey, i. 416; ii. 116,305, 437. Lancets of diptera, ii. 115. Language of insects, ii. 386. Lark, i. 582. (J52 INDEX. Larva, i. 304, 306. Lassaigne, i. 318 ; ii. 183. Latham, ii. 189. Latreille, i. 290 ; ii. 316, 389, 493. Laws of nature, i. 6. Law of mortality, i. 42. Law of co-existence of forms, i. 50. Law of gradation, ii. 629. Law of analogy, i. 49 ; ii. 625. Leach, i. 219. Leaves, ii. 29, 44. Leech, (see Hirudo). Lemur, i. 533, 550; ii. 285, 505. Lens, crystalline, i. 59 ; ii. 462, 496. Lenticellso, i. 93. Lepas, i. 257 ; ii. 296. Lepidoptera, i. 304, 354 ; ii. 114, 217. Lepisma, i, 297, 298, 356. Lernsea, i. 302 ; ii. 600, 608. Leuchs, ii. 482. Leucophra, ii. 96. Leuret, ii. 183. Lewenhoeck, i. 356 ; ii. 264. Libellula, i. 310, 351 ; ii.486, 487. Liber, i. 88; ii. 41. Lichen, ii. 19. Life, i. 34, 42. Ligaments, i. 106. Ligamentum nuchse, i, 108, 501. Light on plants, i. 91 ; ii. 28. Lignin, i. 70 ; ii. 41. Lilium, i. 78. Limax, ii. 126,317. Limpet, (see Patella). Link, i. 75. Lion, i. 108,496, 529; ii. 136, 392, 557. Lister, ii. 233, 300. Liver, ii. 219, 350. Lizard, ii. 129, 390, 497, 587. Lobster, i. 292; ii. 167, 258, 299, 435, 544. Lobularia, i. 161. Loche, ii. 309. Locomotion, i. 143. Locusta, ii. 122. Loligo, i. 261, 407; ii. 271. Longevity of trees, ii. 620. Lophius, i. 422; ii. 390, 437. Loxia, ii. 131. Lucanus, i. 359. Lumbricus marinus, i. 277,295. Lumbricus terrestris, ii. 102, 114, 254, 297. Lungs, ii. 267, 611. Lycopodium, i. 78. Lycoris, ii. 480. Lymphatics, ii. 352. Lymphatic hearts, ii. 353. Lyonet, i. 300, 312, 355. Macaire, ii. 51, 54, 58, 334. Macartney, i. 590 ; ii. 329, 331, 562. Macavoy, ii. 375. Mackerel, i. 425. Macleay, i. 54. Madder, i. 384. Madrepore, i. 166. Magendie, ii. 505, 535. Magilus, i. 249. Maia, ii. 269, 545. Malleus, ii. 426. Malpighi, ii. 378. Mammee, ii. 598, 616. Mammalia, i. 477 ; ii. 325, 441,598. Man, i. 536 ; ii. 559. Man of war, Portugese, i. 196. Manatus, ii. 142. Mandible, i. 289. Mantis, ii. 211. Mantle, i. 113, 237. Many-plies stomach, ii. 197. Marcet, ii. 58, 226, 334, 338. Marginella, i. 247. Marmot, ii. 149. INDEX. 653 Marsigli, i. 150. Marsupialia, ii. 277, 598. Maisupium, ii. 500. Mastication, ii. 140. Mastoid cells, ii. 425. Matrix of feather, i. 576. Matter, ii. 516. Maunoir, ii. 527. Maxillae, ii. 123. Mayer, i. 447. Matjo, ii. 618. Meatus auditorius, ii. 422. Mechanical functions, i. 38. Meckel, i. 482 ; ii. 480. Medulla oblongata, ii. 555. Medullary substance, ii. 365. Medullary rays, i. 86. Medusa, i. 96, 192; ii. 63, 72, 85, 294, 478. Meibomian glands, ii. 469. Melolontha, i. 300 ; ii. 122, 212, 236, 313,486, 490. Melophagus, ii. 483. Membrana nictitans, ii. 499, 501. Membrane, i. 101. Menobranchus, ii. 324. Mercurialis, ii. 53. Mergus, ii. 130. Merrythought of fowl, i. 566. Mesembryanthemum, ii. 48. Mesenteric glands, ii. 227. Mesentery, ii. 108. Mesothorax, i. 323. Metacarpus, i. 405. Metals in plants, ii. 43. Metamorphoses, i. 302, 437 ; ii. 632, 634. Metatarsus, i. 405. Metathorax, i. 323. Milk, ii. 616. Millepedes, ii. 485. Millepora, i. 167. Mimosa, i. 127. Mint, ii. 17, 30. Mirandola, ii. 586. Mirbel, i. 69, 72. Mite, i. 297. Mitra, i. 248. Modiolus, ii. 431. Molar teeth, ii. 144. Moldenhawer, i. 74. Mole,i. 524,525; ii. 391,505. Mole cricket, i. 342. MoUusca, i. 213; ii. 269, 389, 550. Monas, i. 13, 184; ii. 96, 583. Monkey, i. 533 ; ii. 149, 392, 569. Monoculus, ii. 493. Monothalamous shell, i. 265. Monotremata, ii. 277. Monro, \. 123, 132; ii. 303. Mordella, ii. 486. Morpho, i. 354. Morren, ii. 252, 255. Mortality, i. 42; ii. 581. Mother of pearl, i. 232. Motion, voluntary, i. 37 ; ii. 534. Motion, vegetable, i. 127. Motor nerves, ii. 535. Mucous membrane,!. 112. Mucous glands, ii. 184. Mulberry, ii. 59. Muller,' I. 183; ii. 92, 353, 480. Mullet, ii. 202. Multilocular shells, i. 265. Multivalves, i. 257. Mursena, ii. 497, 556. Murex, i. 245, 252; ii. 126, 301,482. Mus, ii. 178, 506. Musca, i. 332. Muscle (shell fish), i. 222, 224. Muscle, i. 124, 127, 300. Muscles of eye, ii. 464. Muscular power in plants, ii. 358. Muscular power in birds, i. 593. Mushroom, ii. 19. Musk shrew, ii. 135. 654 INDEX. Musical tone, ii. 419. Mya, i. 223. Myriapoda, i. 297, ii. 248. Myrmecophaga, ii. 134. Mysis Fabricii, i. 289. Mytilus, i. 222. Myxine, i. 407, 416; ii. 116, 497. Nacreous structure, i. 231. Nais, ii. 102, 251, 479, 586. Narwhal, i. 56; ii. 141. Nature, i. 6, 13. Nautilus, i. 242, 266 ; ii. 270. Necrophorus, ii. 413. Needles in biliary ducts, ii. 219. Nereis, i. 271, 274, 280; ii. 251. Nerve, i. 36 ; ii. 366. Nervous system, ii. 365, 537, 553. Nervous power, ii. 354. Nettle, ii. 47. Neuro-skeleton, i. 366. Neuroptera, i. 351. Newport, i. 352; ii. 102, 214, 218, 244, 547, 548. Newt, ii. 439, 587. Nightshade, ii. 59. Nitrogen, ii. 14, 338. Nordmann, ii. 600, 608. Notonecta, i. 29, 337. Nursling sap, ii. 24. Nutrition, ii. 1, 10, 13, 57. Nutrition in lower orders, ii. 74. Nutrition in higher orders, ii. 104. Nutritive functions, i. 38. Nycteribia, ii. 483. Octopus, i. 261 ; ii. 494. Ocular spectra, ii. 530. Odier,\. 318. (Esophagus, ii. 101, 107, 176. Oken, i. 349, 400. Olfactory nerve, ii. 396. Olfactory lobes, ii. 556. OHvse, i. 241, 250. Oniscus, ii. 544. Onocrotalus, i. 556. Operculum of Mollusca, i. 252. Operculum of fishes, ii. 303. Ophicephalus, ii. 307, Ophidia, i. 447. Ophiosaurus, i. 454, 457. Ophiura, i. 212. Opossum, ii. 136, 598. Optic axis, ii. 503. Optic ganglion, ii. 489. Optic lobes, ii. 555. Opuntia, i. 127. Orache, ii. 48. Orbicular bone, ii. 426. Orbicular muscle, i. 136. Orchidese, i. 69. Organic Mechanism, i. 59, 96. Organic developement, ii. 599. Ornithorhyncus, i. 395; ii. 136, 178, 391, 442, 497. Orobanche, ii. .54. Orthoceratite, i. 267. Orthoptera, i. 309, 349. Os hyoides, ii. 132, 303. Osier, i. 206, 220, 223, 277, 280. Osseous fabric, i. 365. Ossicula, tympanic, ii. 426. Ossification, i. 375, 556. Ostracion, i. 432. Ostrich, i. 563, 587, 590; ii. 185, 224,328, 554. Otter, sea, ii. 149. Ovary, ii. 593, 594. Oviduct, ii. 596. Oviparous animals, ii. 597. Ovo-viviparous animals, ii. 597. Ovula, i. 247. Ovum, ii. 593. Owen, i. 563. Owl, ii. 330,441,503. Ox, horn of, i. 515. Oxygen, ii. 29. Oyster, i. 131, 220, 221. Oyster-catcher, ii. 131. INDEX. 655 Paces of quadrupeds, i. 492. Pachydermata, i. 518; ii. 382, 391. Package of organs, i. 102. Pain, ii. 368. Palemon, ii. 544. Paley, i. 102,571 ; ii. 286. Palinurus, ii. 544. Pallas, i. 150; ii. 344. Palms, i. 83. Palm squirrel, ii. 178. Palmer, ii. 30. Palpi, i. 289; ii. 124. Pancreas ii. 221. Pander, ii. 607. Panniculus carnosus, i. 527. Panorpa, i. 326. Paper nautilus, i. 265. Papilio, i. 357 ; ii. 486. PapiUse, ii. 378, 394. Par vagum, ii. 549. Parakeet, ii. 131. Parallax, aberration of, ii. 472. Parrot, ii. 179, 391. Pastern, i. 517. Patella, i. 228; ii. 220,551. Patella of knee, i. 406. Patellaria, ii. 46. Paunch, ii. 195. Pearl, i. 232. Peccari, ii. 193. Pediculus, i. 297. Pelican, i. 556; ii. 178. Pelvis, i. 404. Pencil of rays, ii. 453. Penguin, i. 592. Penitentiary, ii. 189. Pennatula, i. 174; ii. 82. Penniform muscle, i. 133. Pentacrinus, i. 212. Perca, i. 116, 433; ii. 306, 410, 495, 557. Perception, i. 36; ii. 372, 508. Perch (See Perca). Perennibranchia, ii. 324. Perilymph, ii. 427. Periostracum, i. 237. Peristaltic motion, ii. 204. Peron, i. 97 ; ii. 72. Pfaff, ii. 338. Phalanges i. 405. Phalena, ii. 244, 486. Phanerogamous plants, ii. 595. Phantasmagoria, ii. 533. Phantasmascope, ii. 524. Phaseolus, ii. 52, Phenakisticope, ii. 524. Philip, ii. 190, 360. Phoca, i. 487. Pholas, i. 220, 256. Phosphorescence of the sea, i. 194; ii. 63. Phrenology, ii. 565. Phyllosoma, ii. 544. Physalia, i. 196. Physiology, i. 21. Physsophora, i. 197. Phytozoa, i. 146. Pierard, ii. 200. Pigeon, ii. 179, 616. Pigmentum of skin, i. 112. Pigmentum of the eye, ii. 462. Pike, i. 427. Pileopsis, i. 252. Pineal gland, ii. 560. Pinna, i. 224, 235. Pistil, ii. 596. Pith of plants, i. 85. Pith of quill, i. 580. Placuna, i. 233. Planaria, ii. 114, 236, 250, 294, 479, 586. Planorbis, i. 227, 242. Plantigrada, i. 533. Plastron, i. 463. Pleurobranchus, ii. 220. Pleuronectes, i. 431 ; ii. 503. Plexus, nervous, ii. 359. Pliny, ii. 559. Plumula, ii. 603. Plumularia, ii. 234. Pneumo-branchifp, ii. 316. Pneumo-gastric nerve, ii. 549. Podura, i. 297. 650 INDEX. Poisers, i. 353. Poison of nettle, ii. 47. Poll, i. 227, 235. Pollen, ii. 596. Polygastrica, ii. 97. Polypi, i. 161; ii. 74,81, 293, 383. Polystoma, ii. 113. Polythalamous shell, i. 265. Pontia brassica, i. 354. Pontobdella, i. 271. Poppy, ii. 48. Porcupine quills, i. 120. Porcupine, i. 527; ii. 149, 193. Porifera, i. 147. Porpita, i. 195. Porpus, ii. 142,193. Porterfield, i. 374. Potatoe, ii. 589. Prehension of food, ii. 113, 117. Priestley, ii. 29, 336, 338. Pristis, i. 56; ii. 166. Pritchard, ii. 241. Privet Hawk moth, ii. 218. Proboscis of insects, ii. 114, Proboscis of mollusca, ii. 126. Proboscis of Elephant, i. 520. Progressive motion, i. 144. Prolegs, i. 313. Promontory of ear, ii. 425. Proteus, i. 187; ii. 324,632. Prothorax, i. 323. Prout, ii. 37, 41. Provencal, ii. 308. Proximate principles, ii. 6. Pterocera, i. 246. Pteropoda, i. 257. Pteropus, ii. 136. Pubic bone, i. 405. Pulmonary organs, ii. 267. Puncta lacrymalia, ii. 468. Punctum saliens, ii. 607. Pupa, i. 304, 307. Pupil, ii. 463. Pupipara, ii. 483. Pyloric ajjpendices, ii. 221. Pylori-s, ii. 107, 182. Pyramidalis muscle, ii. 501. Python, i. 447. Quadratus muscle, ii. 500. Quadrumana, i. 533 ; ii. 149. Quadrupeds, i. 487. Quagga, i. 516. Quail, i. 582. Quills of porcupine, i. 120. Quills of feathers, i. 568. Quay, i. 97. Rabbit, i. 497; ii. 149, 190. Racoon, i. 112. Radiata, i. 164. Radicles, ii. 603. Radius, i. 405. Ranunculus, i. 79. Rapp, ii. 478. Rat, ii. 148, 149, 192. Rathke, ii. 634. Rattle-snake, i. 450. Ray, i. 11. Ray, i. 420,422,423; ii. 503, 569. Rays of fins, i. 424. Razor-shell-fish, i. 222. Reaumur, i. 199, 202, 227, 237,292; ii. 115, 170, 183. Receptacles of food, ii. 178. Receptaculum chyli, ii. 108, 228. Reed of ruminants, ii. 197. Refraction, law of, ii. 453. Regeneration of claw, i. 295. Rennet, ii. 197. Reparation, ii. 3, 9, 587. Repetition of organs, i. 57. Reproduction, i. 43; ii. 581. Reptiles, i. 435 ; ii. 273. Resinous secretions, ii. 47. Respiration, i. 41 ; ii. 11, 265. 290. Rete muscosum, i. 112. Reticulated cells, i. 69. Reticule of Ruminants, ii. 195. Retina, ii. 374, 448, 462. I INDEX. 657 Returning sap, ii. 36. Revelation, ii. 641. Reviviscence, i. 62 ; ii. 255. Rhea, i. 586. Rhinoceros, i. 515; ii. 135, 151, 382, 392,504. Rhipiptera, i. 350. Rhizostoma, ii. 87. Rhyncops, ii. 132. Ribs, i. 401 ; ii. 327. Ricinus, i. 297. Rings of annelida, i. 272. Rodentia, i. 523; ii. 148, 151, 162, 175, 191, 504. Roesel, ii. 478. Roget, ii. 9, 524, 532, 582. Rolando, ii. 613. Roosting, i. 588. Roots, i. 93 ; ii. 20. Ross, i. 16. Rostrum, ii. 124. Rotifer, i. 62, 189; ii.92,479, 539, 591. Roux, ii. 569. Rudimental organs, i. 55; ii. 632. Rudolphi, i. 75. Rumford, i. 76. Ruminantia, i. 499; ii. 196, 504. Rusconi, ii. 613. Sabella, i. 277. Sacculus of ear, ii. 430. Sacrum, 404. St. Ange, ii. 277. St. Hilaire, passim. Salamander, i. 446 ; ii. 128, 498, 597. Salicaria, ii. 54. Saline substances in plants, ii. 43. Saliva, ii. 175. Salmon, ii. 222. Sand-hopper, ii, 542. Sap, ii. 24. Sauria, i. 457 ; ii. 276. VOL. II. Savigny, i. 274, 290; ii. 119, 124. Saw-fish, ii. 166. Scala tympani et vestibuli, ii. 431. Scales of lepidoptera, i, 354. Scales of fishes, i. 116. Scansores, i. 586 ; ii. 554. Scapula, i. 404. Scarabseus, ii. 486. Scarf skin, i. 112. Scarpa, i. 101 ; ii. 411, 430. Schoeffer, ii. 478. Schneiderian membrane, ii. 399. Schultz, ii. 49. Sciurus, i. 550; ii. 178. Sclerotica, ii. 460. Scolopendra, i. 298; ii. 248, 485. Scoresby, i. 194. Scorpion, ii. 315, 485. Scuta, abdominal, i. 453. Scutella, i. 211. Scyllsea, i. 229. Sea, phosphorescence of, i. 194 ; ii. 63. Sea-hare, ii. 126, 168, 551. Sea-mouse, ii. 102, 125, 298. Sea-otter, ii. 149. Seal, i. 487; ii. 403, 442, 506. Sebaceous follicles, i. 114. Secretion, ii. 12, 45, 342. Seed, ii. 593. Segments of insects, i. 320. Semblis, ii. 242. Semicircular canals, ii. 427. Senecio, ii. 53. Sennebier, ii. 20, 29. Sensation, ii. 362. Sensibility, variations of, ii. 526. Sensitive plant, i. 127. Sensorial power, ii. 360. Sensorium, ii. 508. Sepia, i. 261 ; ii. 126, 203, 413,493. Seps, i. 458. u u G5ii INDKX. Series of organic beings, i. 53. Serous membranes, i. 102. Serpents, i. 447; li. 129, 163, 390. Serpula, i. 277 ; ii. 295. Serres, ii. 609, 617. Sertularia, i. 165; ii. 234. Serum, i. 102. Sesamoid bones, i. 406. Setae, i. 274. Shark, ii. 162, 205, 262, 495, 569, 587, 598. Sheep, ii. 153, 194, 402. Shell, i. 111,230. Sheltopusic, i. 457. Shrapnell, ii. 426. Shrew, ii. 149, 391. Shuttle bone, i. 517. Silica, ii, 18, 44. Silk worm, i. 305; ii. 59. Silurus, ii. 307, 390. Sinistral shells, i. 243. Siphonaria, i. 252. Siren, i. 457; ii. 324. Skate, ii. 303, 410, 49.5. Skeleton, i. 365, 386. Skeleton, vegetable, i. 95; ii. 42. Skimmer, ii. 132. Skin, ii. 377. Skull (see Cranium). Slack, i. 66. Sleep, ii. 536. Slips, propagation by, ii. 585. Sloth, i. 481, 498, 524; ii. 284. Slug, ii. 126, 317. Smell, ii. 396. Smith, ii. 166. Snail, ii. 317,413,587. Snake-lizard, i. 448. Snout, i. 521. Snow, red, i. 16. Soemmerring, ii. 575. Soils, fertility of, ii. 18. Solar light, ii. 31. Solen, i. 222. Solipeda, i. 516. Solly, ii, 354. Sorex, ii. 135,443, 505. Sound in fishes, i. 429. Sound, ii. 414. Spallanzani,\. 62; ii. 79, 170, 183, 338, 567, 614. Spatangus, i. 205, 211. Spectra, ocular, ii. 525, 530 Spectre of the Brocken, ii. 533. Speed of quadrupeds, i. 496. Spermaceti, i. 484. Spherical aberration, ii. 471. Sphincter muscle, i. i36. Sphinx, ii. 217, 244, 547. Spicula, in sponge, i. 154. Spider, i. 282, 284 ; ii. 248. Spider-crab, ii. 545. Spider-monkev, i. 399, 534. Spine, i. 387,*392. Spinal cord, or Spinal marrow, ii. 553, 604. Spiracles, ii. 311. Spiral threads in plants, i. 68. Spiral vessels, i. 73. Spiral growth of plants, i. 90. Spiral valve in fishes, ii. 205. Spirits, animal, ii. 563. Spirula, i. 242. Spix, ii. 252. Spleen, ii. 224. Splint bone, i. 517. Spokes, curved spectra of, ii. 524. Sponge, i. 147 ; ii. 84. Spongiole, i. 79; ii. 20, 21. Spotted cells of plants, i. 69. Spring-tail, i. 297. Spur of cock, i. 586. Squalus (see Shark). Squalus pristis, ii. 166. Squirrel, i. 524, 550; ii. 178. Stability of trees, i. 81. Stability of human frame, i. 54 1 . Stag, skeleton of, i. 507. Stamen, ii. 596. Stapes, ii. 426. Star-fish, i. 200 (sec Asterias). Starch, i. 70; ii. 41. Staunton, ii. 531. INDEX. (U)ii Steariue, i. 123. Steifeiisand, ii. 482. Stems, vegetable, i. 81. Stemmata, ii. 483. Stentor, ii. 98. Sternum, i. 402. Stevens, ii. 183. Stigma, vegetable, ii. 596. Stigmata of insects, ii. 31 1 . Sting of bee, i. 352. Stipulse, i. 94. Stomach, ii. 72, &c. Stomata, i. 77; ii. 19. Stones, swallowing of, ii. 171. Stone-wort, ii. 50. Stork, i. 590. Stratiomys, i. 310, 348. Straus Durckheim, i. 300, 323 ; ii. 490. Strepsiptera, i. 350. Striated structures, i. 232. Strombus, i. 246 ; ii. 301. Styloid bone, i. 517. Subbrachieni, i. 423. Suckers, i. 136,260,332. Sugar, ii. 5. Sun, action of, on plants, i. 91. Surveyor caterpillars, i. 315. Sus -^thiopicus, ii. 161. Suture, i. 381. Swammerdam, i. 352 ; ii. 413, 482. Swan, i. 559, 593; ii. 169. Swimming of fishes, i. 412. ' Swimming bladder, i. 429. Symmetry, lateral, i. 57 ; ii. 609. Sympathy, ii. 576. Sympathy of ants, ii. 389. Sympathetic nerve, ii. 358. Synovia, i. 102. Syphon of shells, i. 267. Systemic circulation, ii. 266. Tabanus, i. 333; ii. 115. Tadpole, i. 437 ; ii. 222, 322, 632. Tvenia, ii. 83, 114, 236. Tail, i. 398, 524, 531, 583; li. 392, 634. Talitrus, ii. 542. Tapetum, ii. 505. Tapeworm, ii. 83, 114, 236. Tapir, i. 521 ; ii. 392. Tarsus, i. 288, 328, 330, 405. Taste, ii. 393. Teeth, ii. 140. Tegmina of orthoptera, i. 349. Telegraphic eyes, ii. 493. Tellina, i. 224. Temperature, animal, ii. 340. Tendons, i. 106,134. Tendrils, i. 94. Tentacuia, i. 161, 171 ; ii. 383. Terebella,i. 277,278. Terebra, i. 249. Teredo, i. 235 ; ii. 295. Testacella, ii. 317. Testudo, i. 470 ; ii. 557 Tetrodon, i. 420, 433. Textures, vegetable, i. 66. Textures, animal, ii. 97. Thetis, ii. 296. Thoracic duct, ii. 108, 228. Thorax, i. 323; ii. 325. Thorns, i. 94. Thought, ii. 517. Threads, elastic, in plants, i. 68. Tibia, i. 328, 330, 405. Tick, i. 297. Tiedemann, ii. 235. Tiger, i. 496; ii. 136, 145, 146, 392. Tipula, i. 331. Tone, musical, ii. 419. Tongue of insects, ii. 124. Tongue, strawberry, ii. 394. Torpedo, i. 31 ; ii. 572. Tortoise, i. 463 ; ii. 499. Tortryx, i. 447, 448. Toucan, ii. 131, 330. Touch, ii. 377, 534. Tracheae of animals, ii. 293, 310. 660 INDEX. Tracheae of plants, i. 73. Tradescantia, ii. 51. Trapezius muscle, i. 135. Trembley,i. Ill ; ii. 79, 478. Treviranus, i. 73, 75 ; ii. 569. Trichechus, i. 487. Trichoda, ii. 97. Trigla, ii. 554. Trionyx, i. 475. Tristoma, ii. 113. Triton, i. 252, 446. Tritonia, ii. 296. Trituration of food, internal, ii. 167. Trochanter, i. 328. Trochilus, ii. 117. Trophi, ii. 121. Trot, actions in, i. 494. Trunk-fish, i. 432. Trunk of elephant, i. 520. Tuberose roots, ii. 589. Tubicolae, i. 277. Tubipora, i. 165. Tubularia, ii. 233. Turbinated shells, i. 216. Turbinated bones, ii. 400. Turkey, ii. 173,440. Turritella, i. 249. Turtle, i. 463; ii. 202, 557. Tusks, ii. 141. Tympanum, ii. 422. Type, i. 48 ; ii. 627. Typhlops, i. 447. Ulna, i. 405. Ungual bone, i. 405. Unio batava, i. 217. Unity of design, ii. 625. Uranoscopus, ii. 503. Urceolaria, i, 187. Urchin, sea. See Echinus, Utricle of labyrinth, ii. 430. Uvea, ii. 463. Valves, i.3 1,1 04: ii. 260, 288. Vanijjire bat, ii. 117. Van Hclmont, ii. 16. Vane of feather, i, 568. Variety, law of, i. 11, 48; ii. 626. Varley, ii. 254. Vascular circulation, ii. 235. Vascular plexus, ii. 377. Vauquelin, ii. 229, 336. Vegetable kingdom, i. 14, 40. Vegetable organization, i. 65. Vegetable nutrition, ii. 15. Veins, i. 41 ; ii. 108. Velella, i. 195. Velocity of fishes, i. 434. Velvet coat of antler, i. 510. Vena cava, ii. 263. Ventricle of heart, ii. 108, 259. Ventricles of brain, ii. 556. Veretillum, ii. 82, 478. Vertebra, i. 387 ; ii. 604. Vertebrata, i. 361. Verticillated arrangement, i. 90. Vesicles of plants, i. 66. Vespertilio, i. 551 ; ii. 136, 567. Vessels of plants, i. 71. Vessels of animals, i. 103; ii. 606, 613,621. Vestibule of ear, ii. 427. Vibrations, ii. 563. Vibrio, i. 63, 186. Vicq d'Azyr, i. 190. Villi, ii. 347. Viper, i. 447; ii. 597. Vision, ii. 444. Vision, erect, ii. 521. Visual perceptions, ii. 520. Vital functions, i. 38 ; ii. 1, 69. Vital organs, ii. 354. Vitality, i. 20. Vitreous humour, ii. 462. Vitreous shells, i. 236. Viviparous reproduction, ii. 598. Voice, ii. 444. Voltaic battery of torpedo, ii. 572. INDEX. 001 Voluntary motion, i. 37; ii. 534. Volute, i. 248 ; ii. 126, 482. Volvox, i. 186, 188; ii. 591. Voracity of hydra, ii. 77. Vorticella, i. 62, 182; ii. 97, 584. Vulture, ii. 180, 406. Wading birds, i. 585, 592. Walking, i. 492, 542. Waller, ii. 134. Walrus, i. 487; ii. 141. Warfare, animal, i. 46 ; ii. 67. Warm-blooded circulation, ii. 278. Water not the food of plants, ii. 16. Water-beetle (see Dytiscus). Water-boatman, i. 29, 337. Wax, vegetable, ii. 48. Web-footed birds, i. 592. Weber, ii. 430, 480. Whale, i. 55; ii. 178, 443, 504, 559. Whalebone, ii. 136. Wheel animalcule, i. 189. Wheel spokes, spectre of, ii. 524. Whelk (see Buccinum). Whiskers, ii. 392. Whorls of plants, i. 90. Whorls of shells, i. 243. Willow, i. 79. Wings, i. 344, 567. Winged insects, i. 299. Withers, i. 518. Wolf-fish, ii. 128. Wollasto7i, i. 92; ii. 55, 491, 571. Wom.bat, i. 527. Woodhouse, ii. 30. Woodpecker, ii. 132. Woody fibres, i. 71, 75. Worms (see Annelida and En- tozoa). Yarrell, ii. 131. Young, ii. 474, 475. Zebra, i. 516. Zemni, ii. 506. Zoanthus, i. 162, 182. Zoocarpia, i. 156. Zoophytes, i. 146 ; ii. 477, 537. Zostira, ii. 202. Zygodactyli, i. 586. FINIS. C. WHI'lTlKGHAHiTOOKS COURT, CHANCEBY LANK. AA 000 884 326 o CENTRAL UNIVERSITY LIBRARY University of California, San Diego DATE DUE JUN 30 1974 CARRFi nmpr.r JUN idril CI 39 I UCSD Libr. ) > > > > y y -> m> '^'^j"^^*^?^" ^^3* ^^:^^\ :>'- >:> ^:>.> W ;. ^H^^-.^ >:im : ^-s^, .,>^>'->^>^-' ^^:* >> :M ^Jsi^' ^^:)>_^ ^^^ Xl^ .>^^»-^-- ^#1