1 ii ^A«^i* ■.*^ih^f^' kiMt i ^^'^ --■^or '*,.,*^' >,^,,<*/'s~/',> 1^§ mi WW"4 ■:;*•£. : :',;^! 1^1 m^!^^^'-^w^. mmf m mljjf ::^m 4A vo"-?? WSiL f.5?¥^fl >^'P^^.V -J«'*' THE BRIDGEWATER TREATISES ON 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 ROGET, M.D. SEC. R. S. ETC. IN TWO VOLUMES VOL II " AnU rill';IIL AUt UIVI.KMllt.S Ot 01•i■UATllJ^S, BUI IT IS TIIK SAVlt (ioO WHICH UORKKIH ILI, IN ALL." 1 CoK. XU. 0. ANIMAL AND VEGETABLE PHYSIOLOGY CONSIDERED WITH REFERENCE TO NATURAL THEOLOGY BY PETER MARK ROGET, M.D. SIXRF.TARY OF THE ROYAL SOCIETY, VICE PRESIDENT OF THE SOCIETY OF ARTS, MEMHER OF THE SENATE OF THE UNIVERSITY OF LONDON, AND EXAMINER IN PHYSIOLOGY AND COMPARATIVE ANATOMY IN THAT UNIVERSITY, FELLOW OF THE ROYAL COLI.ECE OF PHYSICIANS, CONSULTING PHYSICIAN TO QUEEN CHARLOTTE'S LYING-IN HOSPITAL, AND TO THE NORTHERN DISPENSARY, ETC. ETC. VOL II 9 TH]RD EDITION WITH NUMEROUS ADDITIONS AND EMENDATIONS LONDON WILLIAM PICKERING 1840 6L ^7 i^i . ^ C. WHITTINGHAM, TOOKS COURT, CHANCERY LANE. CONTENTS OF THE SECOND VOLUME. PART II.— THE VITAL FUNCTIONS. Page Chapter I. — Objects of Nutrition 1 Chapter II. — Nutrition in Vegetables 13 §1. Food of Plants 13 2. Absorption of Nutriment by Plants 16 3. Exhalation 23 4. Aeration of the Sap 25 5. Return of the Sap 31 6. Secretion in Vegetables 39 7. Excretion in Vegetables 46 Chapter III. — Animal nutrition in General 52 § 1 . Food of Animals 52 2. Series of Vital Functions 62 Chapter IV. — Nutrition in the lower orders of Ani- mals , 66 Chapter V. — Nutrition in the higher orders of Ani- mals 94 Chapter VI. — Preparation of Food 101 § 1 . Prehension of Liquid Food 101 2. Prehension of Solid Food 1 05 3. Mastication by means of Teeth 125 VI CONTENTS. Page 4. Formation and Developement of the Teeth 139 5. Trituration of Food in Internal Cavities 149 6. Deglutition 156 7. Receptacles for retaining- Food 1.59 Chapter VII. — Digestion 161 Cjiapter VIII. — Chyliiicatiox 180 Chapter IX. — Lacteal Ap.sorption 200 Chapter X. — Circulation 204 § 1 . General Plans of Circulation 204 2. Details of Structure in the different orders of Animals 214 3. Distribution of Blood Vessels 255 Chapter XI. — Respiratiox 263 § 1 . Respiration in general 263 2. Aquatic Respiration 266 3. Atmospheric Respiration 279 4. Chemical Changes effected by Respiration 299 Chapter XII. — Secretion ., 307 Chapter XIII. — Absorption 316 Chapter XIV. — Nervous Poaveu 318 PART III.— THE SENSORIAL FUNCTIONS. Chapter I. — Sensation 326 CiiAPTKR 11. — Touch " 338 Chapter 111. — Taste 352 Chapter IV. — Smelt 364 CONTENTS. Vll Page Chapter V. — Hearing 370 § 1. Acoustic Principles 370 2. Physiology of Hearing- in Man 376 3. Comparative Physiology of Hearing 389 Chapter VI.— Vision 398 § 1 . Object of the Sense of Vision 398 2. Modes of accomplishing the object of Vision .... 402 3. Structure of the Eye 411 4. Physiology of Perfect Vision 419 5. Comparative Physiology of Vision 426 Chapter VII. — Perception 453 Chapter Vlll. — Comparative PuYsioLociy up the Nervous System 477 § 1. General view uf the functions of the Nervous System 477 2. Nervous System of Invertebrate Animals 483 3. Nervous System of Vertebrated Animals 495 4. Functions of the Brain 504 5. Comparative Physiology of Perception 508 PART IV.— THE REPRODUCTIVE FUNCTIONS. Chapier I. — Reproduction 52 1 Chapter 11. — (Jkganic Developement 5;j7 Chapter III. — Decline of the System 55^ Chapter VI. — Unity oe Design 5G0 I ^ "ii^ i ■ 575 ANIMAL AND VEGETABLE PHYSIOLOGY. PART IL THE VITAL FUNCTIONS. Chapter L objects of nutrition. 1 HE mechanical structure and properties of the organized fabric, which have occupied our attention 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 admirable the skill and the foresight which have been exercised 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 dissolution. The animal machine, in common with 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 VOL. II. B THE VITAL FUNCTIONS. of the internal organs, we may trace the operation of many causes inevitably leading to their ultimate destruction. 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 con- traction, and which, if suffered to continue, would, in no long time, render them incapable of exer- cising their proper functions ; and the same obser- vation applies also to the nerves, and to all the other systems of organs. Provision must accord- ingly be made for remedying these constant causes of decay by the supply of those peculiar materials, which the organs require for recruiting their de- clining energies. It is obvious that the developement of the organs, and general growth of the body, 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 pur- pose, be incorporated with those which previously existed, and become identified with the living sub- stance. Thus we often find structures 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 materials is also wanted for the reparation of occasional injuries, to which, in the course of its long career, the body is unavoidably exposed. Like a ship OBJECTS OF NUTRITION. 3 fitted out for a long voyage, and fortified against the various dangers of tempests, of icebergs, 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 acci- dental 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 enumerated would of itself be sufficient to establish the neces- sity of supplies of nourishment for the maintenance of life. But there are other considerations, equally important in a physiological point of view, and derived from the essential nature of organization, which likewise create 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 vitality must therefore be selected and arranged with a view to their continual modification, corresponding to these ever varying changes of condition. 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 and other causes 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 4 THE VITAL FUNCTIONS. 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, displace- ments, 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, therefore, neither the elementary bodies nor even their simpler and more permanent combinations, but such of their compounds as are of a more plastic quality, and which allow of a variable proportion of ingredients, and of considerable diversity in the modes of their combination. So great is the complexity of these arrangements, that although chemistry is fully competent to the analysis of or- ganized 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 refined ope- rations of vitality, the only power which could produce such an 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 accuracy by modern chemists : yet to reproduce this sugar, by the artificial combination of its sinq)le elements. ORGANIC CHEMISTRY. O is a problem which has hitherto baffled all the efforts of philosophy. Chemistry, notwithstanding the proud rank it justly holds among the physical sciences, and the noble discoveries with which it has enriched the arts; notwithstanding it has un- veiled 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 notwith- standing it is armed with full powers to destroy, can- not, 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 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 pro- cesses which the most inventive chemist can devise have ever yet approached those delicate and refined operations which nature silently conducts 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 phospho- rus, together with a few of the alkaline, earthy, and metallic bases. These substances are variously united, so as to form certain specific compounds, which although they are susceptible, in different instances, of endless modifications, yet possess such a general character of uniformity, as to allow of their being arranged in certain classes ; the most characteristic 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 6 THE VITAL lUNCTlONS. kingdom, in like manner, furnishes Gelatin, Albu- 7nen, Fibrin, 3Iucus, Entomoline, Elearin, Stearin, and many others. The chemical constitution of these organic pro- ducts, formed, as they are, of but few primary elements, is strikingly contrasted with that of the bodies belonging to the mineral kingdom. The catalogue of elementary, or simple bodies, exist- ing in nature, is, indeed, more extensive than the list of those which enter into the composition of animal or vegetable substances. But in the mineral world they occur in simpler combinations, resolv- ible, for the most part, into a few definite ingre- dients, 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 diversity. Simple binary compounds are rarely ever met with ; but, in place of these, we find three, four, five, or even a greater number of constituent 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 wliich their several ingredients are held together, being very numerous, require to be much more nicely balanced, in order to retain them in combination. Slight causes are sufficient to dis- turb, or even overset, this equipoise of affinities, and often produce rapid changes of form, or even com- plete decomposition. The principles, thus retained in a kind of forced union, have a constant tendency ORGANIC CHEMISTRY. 7 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 disunion of the elements of an animal or vegetable body. Organic substances are, in like manner, unable to resist the slower, but equally destructive 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 M'hen 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 re- quiring for its counteraction, in the living system, that perpetual renovation of materials which is sup- plied 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 unfitting 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 which have been modified for that purpose, and which, when they afterwards lose these salutary properties, 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 8 THE VITAL FUNCTIONS. possess an extensive system of circulating and ab- sorbing vessels, the changes which are effected are so considerable and so rapid, that even in the densest textures, such as the bones, scarcely any portion of the substance which originally composed them is permanently 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 cer- tain time. The period assigned by the ancients for this entire change of the substance of the body was seven or eight years ; but modern inquiries, \vhich show us the rapid reparation 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 renovatioii of every portion of the living fabric* Imperfect as is our knowledge of organic che- mistry, we see enough to convince us that a series of the most refined and artificial operations is re- quired in order to bring about the complicated and elaborate arrangements of elements which consti- tute 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 ex- * See the article " Age" in the Cyclopsedia of Practical Medicine, where I have enlarged upon this subject. ORGANIC CHEMISTRY. 9 tended knowledge of that series of subtle processes, consisting of chemical combinations and decompo- sitions, 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 pre- science which are even now manifested 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, which have been termed vital, and the final object of which may be termed Nutrition, 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 application of properly prepared materials to their developement and growth. The functions subservient to nutrition may be distinguished according as they relate to seven principal periods in the natural order of their suc- cession. The first series of processes they comprise has for its objects the reception of the materials from without, and their gradual conversion into proper nutriment, that is, into matter having the same chemical properties with the substance of the organs v^^^ith 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 comprises those which are designed to convey the nutritive fluids thus elaborated to all the organs that are to 10 the: viial functions. be nourished by them. In the more developed systems of organization this purpose is accom- plished by means of tubes, called vessels, through which the nutritive fluids move in a kind of circuit : in this case the function is denominated the Circu- lation. It is not enough that the nutritive juices are assi- milated ; 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 wanted in dift'erent parts of the economy, are required to be formed by a peculiar set of organs, of which the intimate structure eludes observation ; although we may perceive that in many instances, among the higher orders of beings, a special appa- ratus of tubes, 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 ofiice performed by them, as well as by the simpler forms of structure above mentioned, is termed Secretion. Fifthly, similar processes o( secretion are also emi)loyed 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 PROCESSES OF NUTUITION. 11 measures are taken for tiie 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 accom- plished by a peculiar set of vessels; and the function they perform is termed Absorption. Lastly, the conversion of the fluid nutriment into the solids of the body, and its immediate applica- tion 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 harmonious system of opera- tions. 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 increase in complexit}^ corres- ponding, in some measure, to the greater refine- ment 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 surrounding them ; mate- rials which consist almost wholly of water, with a 12 THE VITAL I'UNCTIONS. small proportion of carbonic acid, and a few saline ingredients, of which that water is the vehicle. But these, after having been converted by the powers of vegetable assimilation, into the substance of the plant, acquire the characteristic properties of orga- nized 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 scat- tered over the soil, they are fitted to become more highly nutritive to other plants, which absorb them, and adapt them with more facility to the purposes of their own systems. Here they receive a still higher degree of elaboration ; and thus the same materials may pass through several successive series of modifications, till they become the food of animals, 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 vege- table 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 appertaining 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 carnivorous animals, which * Nitrogen, however, is generally present in vegetables; but in a much smaller proportion than in animal structures. It is an essen- tial ingredient in the food of plants, and is contained in their juices and secretions ; although it never enters into the chemical compo- sition of their membranes. The farinaceous seeds, more especially, contain a considerable quantity of this element. It abounds in the cambium, and generally in every part which is beginning to be deve- loped. Payen : Comptes Rendus, 1838, p. 132. VEGETABLE NUTRITION. 13 generally hold a higher rank in the scale of organi- zation, than those that subsist only on vegetables. Thus has each created being been formed with reference, not merely to its own welfare, but also to that of multitudes of others which are dependent on it for their support, their preservation, — nay, even for their existence. In contemplating this mutual relationship, this successive subordination of the different races to one another, and this con- tinual tendency to increased refinement, we cannot shut our eyes to the magnificent unfolding of the great scheme of nature for the progressive attainment of higher objects ; until, in the perfect system, and exalted endowments of Man, we behold the last result which 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 modern che- mistry, it was very generally believed that plants could subsist on water alone ; and Boyle and Van Helmont, in particular, endeavoured 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 14 THE VITAL FUNCTIONS. had previously ascertained 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 addi- tional weight of one hundred and seventy 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-chestnut 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, and evidently derived little or no sustenance from the water with which they were supplied. Experi- ments of a similar nature were made by Bonnet, and with the like result. When plants are con- tained in closed vessels, and regularly supplied with water, but denied all access to carbonic acid gas, they are developed only to a very limited ex- tent, determined by the store of nutritious matter which had been already collected in each plant when the experiment commenced, and which, by combining with the water, may have afforded a temporary supply of nourishment. FOOD OF PLANTS. 15 But the water which nature furnishes to the vege- table organs is never perfectly pure ; for, besides containing air, in which there is constantly a cer- tain 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 sub- stance had 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 quan- tity of carbon is also introduced into the fluids of the plant, derived from the decomposed animal and vegetable materials which the water generally con- tains. The peculiar fertility of each kind of soil depends principally on the quantity of these or- ganic 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 ascer- * Elements of Agricultural Chemistry, Lect. VI. p. 234. I(> THE VITAL FUNCTIONS. tained, by the researches of Berzelius, 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 surface, 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 construc- tion of the solid vegetable fabric, is, probably, one of the reasons why plants require so large a supply of water for their subsistence. § 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 AlgcB, 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 absorbing 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 VKGETABLE ABSORPTION. 17 the case with by far the greater nninber, the roots are the special organs to which this office of ab- sorbing nourishment is assigned : but it occasion- ally happens that, under certain circumstances, 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 the natural action of stomata ; and they assume it only in forced situations, when the plant can procure no water by means of the roots, either from its having been deprived of these organs, or from its being left totally dry. Thus a branch, separated from the trunk, may be preserved from withering for a long time, if its 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 situated, that any regular absorption takes place ; for the surface of the root, being covered in every other part by a layer of epidermis, is incapable of per- forming this office. It was long ago remarked by Duhamel, 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 other was made to dip into the water, and found that equal quantities were absorbed in both cases; while on VOL. II. c 18 THE VITAI, FUNCTIONS. 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 absorption 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 down- A\ ards, or at anj' other inclination. Thus they con- tinually arrive at new portions of soil, of which the nutritive matter has not yet been exhausted ; and as a constant relation is preserved 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 distance from the trunk, where, after it has soaked through the earth, it will be received by the extremities of the roots, and readily sucked in by the spongioles. We have here a striking in- stance of that beautiful correspondence, which has been established between processes belonging to different departments of nature, and which are made to concur in the production of such remote effects, as could never have been accomplished without these preconcerted nnd harmonious adjust- ments. 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 a In/gro- scopic power. But though these principles may VEGETABLE ABSORPTION. 19 sufficiently account for the simple entrance of the fluid, they are inadequate to explain its continued ascent through the substance of the root, or along the stem of the plant. The most probable expla- nation of this phenomenon is that the progressive movement of the tluid is produced by alternate contractions and dilatations of the cells themselves, which compose the texture of the plant ; these actions being themselves referable 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 consistence to pass readily through them are liable to obstruct or en- tirely block up these passages. Thus if the spon- gioles 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 solu- tion, with only a small proportion of the salts, being taken up ; and the remaining part of the fluid 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 operation, and that it furnishes no evidence of any discriminating faculty in the spongiole ; for it is found that, pro- 20 THK VITAL FUNCTIONS. vided 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 salu- brious. Solutions of sulphate 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 abed of manure, and is consequently loaded with carbonaceous particles, proves exceedingly injurious when admitted into the system of the plant, from the excess of nutri- ment it contains. But in the ordinary course of vegetation, no danger can arise from this general power of absorption, since the fluids supplied by nature are always such as are suitable to the organs which 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 necessary 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 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 prin- cipally the ligneous substance of the stem : in trees, its passage is chiefly through the alburnum, or more * The experiments of Dr. Daubeny, however, lead to the con- clusion that plants possess, to a certain extent at least, a power of selection, with respect, more particularly, to the earthy constituents which form the basis of their solid parts. (Trans. Linn. Soc. xvii. 253, 266.) ASCENT OF THE S\\\ 21 recently formed wood, and not through the bark, as was at one time believed. The course of the saji, 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 supplying. As the leaves are not yet completed, the sap is at first applied to purposes somewhat different from those it is des- tined to fulfil at a more advanced period, when it has to nourish the fully expanded organs : this fluid has, accordingly, received a distinct appellation, being termed the nurslino- 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 se- veral layers of wood, till it reaches the buds, which it is to supply with nourishment. During this cir- cuitous passage, it probably undergoes a certain degree of elaboration, fitting it for the office which it has to perform : it apparently combines with some nutriment, which had been previously de- posited 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 nursling 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 re(i aired for tlic rearin«//o/i of vegetables. Theefiect appears to be proportionate to the number of stomata which the plant contains. It is a process which takes place only in the 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 effiect 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 28 THIi Vri'AL FUNCTIONS. contain. This is also the case with the leaves them- selves, 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, absorb a portion of that element. The oxygen thus absorbed enters immediately into conjbination 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 pre- sented to the leaf, when daylight returns, and when a fresh decomposition is again effected. This reversal at night of what was done in the day may, at first sight, appear to be at variance M ith the nnity of plan, which we should expect 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 neces- sary, 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 contain carbon. This carbon is transmitted to the leaves, where, during the night, it is made to conibine W'ith the oxygen they have absorbed. It is thus converted into car- bonic acid, which, when daylight prevails, is de- composed ; 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 disintegration, to which it is reduced at the moment of its separation from carbonic acid by the AERATION (H Vt'.E SAl'. '19 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 water which conveys it into the interior of the plant. Hence the necessity of its having to undergo this double operation of first combining with oxy- gen, and then being precipitated from its combina- tion 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 reduce 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 vege- table organization ; in the latter, it is that of dis- charging 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 atmo- sphere 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 .'50 THE VITAL FUNCTIONS. receiving from the vegetable kingdom a large ac- cession of oxygen, and is, at the same time, freed from an equal portion of carbonic acid gas ; both of which effects tend to its purification, and to its continuing adapted to the respiration of animals. Nearly the whole of the carbon accumulated by vegetables is so much taken from the atmosphere, which is the primary source from which they de- rive that element. x4t the season of the year when vegetation is most active, the days are longer than the nights ; so that the diurnal process of purifica- tion 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 carbonic 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 atmosphere, not only from the tendency of all gases to xmiform diffusion, but also from the action of the winds, which are continu- ally agitating the whole mass, and promoting the thorough mingling of its different portions, so as to render it perfectly homogeneous 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 suspended. It is impossible to contemplate so special an adjustment of opposite effects without admiring this beautiful dispensa- RETURN OF THF SAP. 31 tion of Providence, 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 Sdp. 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 labora- tory, the double processes of exhalation and aera- tion, 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, perfectly assimilated to that particular kind of organization, with which it is to be afterwards incorporated. From the cir- cumstance of its being sent back from the leaves for distribution to the several organs where its pre- sence is required, it has received the name of the returning sap, that it might be distinguished from the crude fluid which arrives at the leaves, and which is termed the ascending sap. 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 dis- solved in it. It appears, however, that a large pro- 32 THE VIIAL I UNCIIONS. portion of the water, which was not exhaled by the leaves, has been actually decomposed,* and that its separated elements, the oxygen and the hydrogen, have been combined with certain pro- portions of carbon, hydrogen, nitrogen,! and various earths, metals, and salts, so as to form the proxi- mate 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 to be the principal basis of vegetable nutriment: and its simple and definite composition points it out as being the immediate result of the chemical changes which the sap experiences in the leaves. During the * BoussingauU was led to this result by a careful and elaborate series of experiments, by which he has proved that plants contain a hydrogenated substance, containing a proportionate quantity of hydrogen three times greater than that which is requisite to form water with the oxygen present. In ordinary cases, the quantity of hvdrogen which is thus assimilated varies from 0..5 to 2. per cent. — Annales des Sciences Naturelles, serie 2, xi. 31. t It also appears, from Boussingault's researches, that plants in full vegetation abstract nitrogen from the atmosphere, and assimi- late it into their system. A much larger quantity is thus obtained and appropriated by those plants, such as clover, which may be advantageously ploughed into the soil, than by the cerealia, the cultivation of which impoverishes the land, because they derive their azote almost exclusively from the organic substances it con- tains.— Ibid. X. 257. RETURN OF THE SAP. .33 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 alburnum, or outermost layer of the wood. With regard to the exact channels through which it passes, the same degree of uncertainty prevails as with regard to those which transmit the ascending sap. De Can- dolle maintains that, in either case, the fluids find their way through the intercellular spaces : other physiologists, however, are of opinion, that parti- cular vessels are appropriated to the office of trans- mitting the descending sap. The extreme mi- nuteness of the organs of vegetables has hitherto presented insuperable obstacles to the investigation of this important question ; and consequently our reasonings respecting it can be founded only on indirect evidence. The processes of the animal economy, where the channels of distribution, and the organs of propulsion are plainly observable, afford but imperfect analogies to guide us in this intricate inquiry ; for although it is true that in the higher classes of animals the circulation of the nutrient fluid, or blood, through distinct vessels, is sufficiently obvious, yet in the lower departments of the animal kingdom, and in the embryo condi- tion even of the more perfect species, the nutritious juices are distributed without being confined within VOL. II. D 34 THE VITAL FUNCTIONS. any visible vessels ; and they either permeate ex- tensive cavities in the interior of the body, or pene- trate through the interstices of a celkdar tissue. That this latter is the mode of transmission adopted in the vegetable system has been considered pro- bable, from the circumstance that the nutritious juices are diffused throughout those plants which contain no vessels whatsoever with the same faci- lity 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 dis- tribution to different parts, is involved in equal obscurity with the nature of the powers which con- tribute 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 man- ner, through those parts which have been latest formed ; that is, through the innermost layers of their structure. The returning sap, while traversing these several parts of the plant, deposits in each the particular 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 with- out impediment to the roots, to the extension of which, after it has nourished the inner layer of bark, it particularly contributes : that portion, on the other hand, which descends along the alburnum meets with the stream of ascending sap, which, during the day at least, is rising with considerable force. A certain mixture of these fluids probably RETURN OF THE SAP. 35 now takes place, and new modifications are in con- sequence produced, which, from the intricacy 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 results, namely, the gradual deposition of the materials of the future alburnum and liber. These materials are first depo- sited in the form of a layer of glutinous substance, termed the Cmnhium ; a substance which appears to consist of the solid portion of the sap, precipi- tated from it by the separation of the greater part of the water that held it in solution.* The cam- bium becomes in process of time more and more consolidated, and acquires the organization proper to the plant of which it now forms an integrant part : it constitutes two layers ; the one, belonging to the wood, being the alburnum ; the other, be- longing to the bark, being the liber. The alburnum and the liber, which have been thus constructed, perform an important part in inducing ulterior changes on the nutrient materials which the returning sap continues to supply. Their cells absorb the gummy substance from the sur- rounding fluid, and by their vital powers effect a still further elaboration in its composition ; con- verting it either into starch, or sugar, or lignin, according to the mode in which its constituent * A substance, containing a large proportion of nitrogen, and yielding ammonia on distillation, is deposited on the walls of the cellules of the roots, and especially of the spongioles. This sub- stance abounds in the cambium, prior to its becoming oxygenized ; and it exists in all the young parts of plants, during their develope- ment. See Payen, Comptes Rendus, 1838, p. 132. 36 THE VITAL FUNCTIONS. 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 ingredients ; and we may infer that the real chemical alterations, which are required in order to effect these conversions, are compara- tively slight, and may readily take place in the simple cellular tissue.* In the series of decompositions which are arti- 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 accordingly hoarded in ma- gazines, with a view to future employment, being to vegetables, what the fat is to animals, a resource for exigencies which may subsequently arise. * 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. This, according to the doctrine of chemical equiva- lents, corresponds to one molecule of water, and one molecule of carbon. Dried Starch or Fecula of 560 water, and 440 carbon. Pure crystallized Sugar . . 572 428 Lignin from Boxwood . . . 500 500 Phil. Trans, for 1827, 584. RETURN OF THE SAP, 37 With this intention, it is carefully stored in small cells, the coats of which protect it from the imme- diate dissolving action of the surrounding watery sap, but allow of the penetration of this fluid, and of its solution, when required by the demands of the system. The tuberous root of the potatoe, that in- valuable gift of Providence to the human race, is a remarkable 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 latter structures may be compared to the bones of animals ; composing by their union the solid framework, or skeleton of the organized system. The woody fibres do not seem to be capable of further alteration in the living vegetable ; and they 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 convertible kind. The sap holds in solution, besides carbonaceous matter, some saline compounds, and a few earthy and metallic bases ; bodies which, in however minute a quantity they may be present, have un- questionably a powerful influence in determining certain chemical changes among the elements of organic products, and in imparting to them peculiar properties; for it is now a well ascertained fact that a scarcely sensible portion of any one in- gredient is capable of producing important dif- ferences in the properties of the whole compound. 38 THE VITAL FUNCTIONS. 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 neighbourhood of melted gold, have the power of destroying its ductility. In the experiments made by Sir John Herschel on some remarkable motions excited in fluid conductors by the trans- mission of electric currents, it was found that minute portions of calcareous matter, in some instances less than the millionth part of the whole compound, are sufficient to communicate sensible mechanical mo- tions, 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 exhalation of the water which held it dissolved ; and it is found, accord- ingly, that the greater portion of this silica is actually deposited in the leaved, and the parts adjacent to them. When once deposited, it seems incapable of being again taken up, transferred to other parts, or ejected from the system ; and hence, in course of time, a considerable accumulation of siliceous particles takes place, and by clogging up the cells and vessels of the plant, tends more and more to obstruct the passage of nourishment into these organs. This change has been assigned as a principal cause of the decay and ultimate destruc- * Philosophical Transactions for 1824, p. 162. VEGETABLIi SECRETION. 39 tioii of the leaves : their foot-stalks, more especially suffering from this obstruction, perish, 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 con- verted into gummy, saccharine, amylaceous, or ligneous products ; there are other cellular 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 is unknown, and is therefore referred generally to the vital energies of vegetation ; 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 combination with oxygen as the repre- sentative of water : this is the case with all the oily secretions, whether they be fixed or volatile, and also with those secretions which are of a resinous quality. Some, on the contrary, are found to have an excess of oxygen ; and this is the condition of most of the acid secretions : while others, again, appear to have acquired an addition of nitrogen. 40 THE VITAL FUNCTIONS. All these substances have their respective uses, although it may frequently be difficult to assign them correctly. Some are intended to remain •permanently 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, w hich are pre- sently to be noticed, are specially designated as excretions. Many of these fluids find their way from one part of the plant to another, without appearing to be conducted along any definite channels; and others are conveyed by vessels, which appear to be specially appropriated to this office. When the plant is in full vegetation, a quantity of fecula is, as we have seen, contained in the form of minute granules, enclosed in the globules of chlorophyllite ; the amount of nutritive substance thus accumulated, when the extent of the leaves is taken into account, nuist be very considerable : and it appears to be deposited in reserve for future occasions, when extraordinary supplies of nourish- ment are wanted for the developement of the fruit. In those plants which lose their leaves in autumn, this fecula is absorbed and carried into the interior of the stem, where it remains deposited until the ensuing spring, when it furnishes materials for the evolution of the buds.* The following are examples of the uses to which the peculiar secretions of plants are applied. Many lichens, which fix themselves on calcareous rocks, * See Mohl, Ann. deb Sc. Nat. Bot. serie 2, ix, 166. VEGETABLE SECRETION. 41 such as the Patellaria immersa, are observed, 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 substance, analogous to that which many marine worms are known to possess. The agent appears in both instances to be an acid, which here is probably the oxalic, acting upon the carbonate of lime, and producing the gradual ex- cavation of the rock. This view is confirmed by the observation that the same species of lichen, when attached to rocks which are not calcareous, remains always at the surface, and does not pene- trate below it. In many stinging plants, as in the Dolichos pru- riens, or Cowhage, and the Urtica dioica, or Nettle, the fluid circulating in their tubular hairs which penetrate into the skin, is found to be highly acrid. The extremities of the hairs, which are often curi- ously barbed, or terminating in bulbs, or hooks, are extremely hard and brittle, so that after entering the skin they easily break off, and being retained there, produce violent mechanical irritation ; an eflect which, in the above-mentioned instances, is much exasperated by the caustic liquor that is at the same time instilled into the punctured part, and which, in the case of the nettle, has been ascertained by M. De CandoUe, junior, to be of an alkaline nature. The entrance of the sting is often facilitated by its surface having spiral grooves, giving it the eflect of a screw.* 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 * Ch. Morrcu, Bulletin de TAcad. de Bruxelles, vi, 239. 42 THE VITAL FUNCTIONS. the injurious effects of moisture; and 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 Ceroxylon, and the Iriartca 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 MesembrinDilhemum, or Fig-marigold, of the Atri- plex, or Orache, and of the Brassica, or Cabbage, maybe given as examples 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 plumage 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 Batraclwspermum, are, in like manner, protected by a viscid layer, which renders the leaves slippery to the touch, and which is im- permeable to water. Several tribes of plants contain opaque liquids, having a white milky appearance ; this is the case v.ith the Poppij, 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 Chelidoninm, or Celandine. All these juices are of a resinous nature, usually his2:hly acrid, and some- times even poisonous in their qualities ; and their opacity is occasioned by the presence of a great number of minute globules, visible with the micro- scope. The vessels in which these fluids are con- tained are of a peculiar kind, and exhibit ramifi- cations and junctions, resembling those of the blood CIRCULATION IN PLANTS. 43 vessels of animals. They are usually found con- nected with the cellular parts of plants ; and, possessing, as they evidently do, proper membranes, they ought not to be regarded as merely enlarged in- tercellular spaces. They are beautifully seen under the microscope in the young stipules of various species of Ficus, or Fig ; in the bractes of the Calistegia septum, or Bird-weed ; and in the petals of the Papaver somnifenim, or Poppy. They had been seen by Grew, who termed them proper vessels. Schultz, who observed them with greater care, denominated them the vilal, or laticiferous vessels, and their fluid contents, the latex. We may also discover, by the aid of the microscope, that this latex is moving in currents with considerable ra- pidity, as appears from the visible motions of its globules ; thus presenting a remarkable analogy with the circulation of the blood in some of the inferior tribes of animals. This curious pheno- menon was first observed in the Chelidonium by I Schultz,* in the year 1820 ; and he designated it by the term Cyclosis, in order to distinguish it from the true circulation of the nutrient fluids, which is effected in the higher animals ; and also from another kind of circulatory motion, of a more partial nature, of which I shall presently speak, and which is termed rotation. The circular movement, or cyclosis, which has been thus observed in the milky juices of plants, has lately attracted much attention among botan- ists : but considerable doubt still prevails whether * " Die Natiir der lebendigen Pflanze." See also Annales des Sciences Natiuelles, xxiii, 75. 44 THE VITAL FUNCTIONS. the appearances afford sufficient evidence of the existence of a general circulation of nutrient juices in the vegetable systems of those plants that exhibit them, which is the case with all those that contain milky juices. Are these moving fluids, it may be asked, to be regarded as the proper nutrient juices of the plant, or as the products of secretion ? And again, it may be made a still further question, wliether such circulation exists universally in vas- cular plants. The difficulties of accurate obser- vation are here so formidable, that a long time will probably elapse before these problems can be satisfactorily solved. We find that they cease the moment that the plant has received an injury, and that they are more active in proportion as the temperature of the atmosphere is higher. But we are perfectly in the dark with respect to the cause of these motions ; and we must be con- tent for the present to refer them, with Treviranus and De Candolle, generally to vital contractions of the vessels. Phenomena, in some respects analogous to these, and of a nature equally curious, are exhibited by cellular plants, in which no vessels can be traced, and of which the juices are nearly transparent, and contain only a few floating globules. Of this kind are the Clicua, or Stone- wort, and the Caulinia ftagilis: in each of the enlarged cells of which, the contained fluid may be seen by the aid of a power- ful microscope, moving in a continued current, ascending on one side and descending on the other, so as to perform a complete circulation within the space bounded by the transverse partitions which close the cell at each end. In the stem of the CIRCULATION IN PLANTS. 45 Chara, this space is the whole interval between two adjacent knots or joints. The appearance of this partial circulation, or rotation, as it has been more specifically termed, is beautifully seen in the cells of the Caulinia fra- gilis ; and is represented in Fig. 239, where the directions of the streams are indicated by arrows.* In each of the cells, of which the jointed hairs projecting from the cuticle of the calyx of the Trades- cantia virginica are com- posed, a similar appear- ance (as shown in Fig. 240) of partial circulations is presented. t Schultz, however, maintains that it is properly an example, not of rotation, but of cyclosis, and that it is merely a branch of the general circulation of the plant, which is performed by a system of vessels communicating extensively with one another.^ Some light has of late been thrown on the cause of these motions of rotation in the fluid contents of the cells of cellular plants, by the discovery of Donne. It had already been shown by Amici, that in the Chara fragilis, the currents follow the course of the spiral lines formed by rows of green I * This phenomenon was first observed by Corti, in 1774. t This was first observed by Mr. Robert Brown. Mr. Slack found that the movement took place in channels which were circumscribed by a fine pellucid membrane. X Ann. des. Sc. Nat. Bot. serie 2, x, 327. 40 THE VITAL FUNCTIONS. globules, which adhere to the inner coat of the cell in great numbers, and the presence of which seems to determine the existence, as well as the course of each current : and he inferred from this fact, that the fluid was actuated by a force emanating from these globules, and which he conceived might be electrical. Becquerel and Dutrochet have suffi- ciently proved that it is not due to electricity ; but still the fact of the dependence of the motion on some action of the green globules, remained estab- lished. Donne, however, observed that each of these globules, when detached from the membrane to which it had been adherent, spontaneously revolved rapidly round its axis; a motion which was quite independent of the general circulatory current, and was apparently derived from some inherent power of rotation. All the agents and cir- cumstances which retard, accelerate, or put a stop to the rotation of the current, produce the same effect on that of the individual granules. These observa- tions, which have been confirmed by those of A. Brongniart and Dutrochet, appear to warrant the conclusion that the same unknown power which causes the rotation of the granules when they are free to move, produces a current in the adjacent fluid when the granules are fixed, as they are in the natural condition of the pfant.^ § 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 VEGETABLE EXCRETION. 47 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 suit- able to their continuance in a vigorous condition, than the soil in the same spot was originally ; and also why plants of a different species are frequently found to flourish remarkably well in the same situation where this apparent deterioration of the soil has taken place. The truth of this sagacious conjecture has been established in a very satis- factory manner by the recent experiments of M. Macaire.* The roots of the Cliondrilla muralis 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 its smell and taste, the presence of a bitter narcotic substance, analogous to that of opium ; a result which was farther confirmed by the appli- cation of chemical tests, and by the reddish brown residuum obtained from the water by evaporation. M. Macaire ascertained that neither the roots nor the stems of the same plants, when completely detached, and immersed in water, could produce this effect, which he therefore concludes is the result of an exudation from the roots, continually 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 * 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. 48 THE VITAL FUNCTIONS. French bean (Phaseoliis vulgaris) during the night and the day, he found it to be much more con- siderable 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 confirmed 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 immediately much augmented : but, even when exposed to the light, there is always some exudation, 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 imbibed through the roots, was also proved by another set of experi- ments on the Merciirialis aimna, the Senecio vul- garis, and Brassica 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 separate vessel. After some days, during which th^ plants continued to vegetate tolerably well, the water in th.e 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 VEGETABLE EXCRETION. 4.9 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 with the like results. De Candolle has ascertained, that certain maritime plants which yield soda, and which flourish in situations very distant from the coast, provided they occasionally receive breezes from the sea, communicate a saline impregnation to the soil in their immediate vicinity, derived from the salt which they doubtless 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 supplying salutary nourishment to other kinds of plants : thus it has been observed that the Salicaria 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 advan- tage derived from a continued rotation of different crops in the same field, in increasing the produc- VOL. II. E 50 THE VITAL FUNCTIONS. tiveness of the soil.* It also gives a satisfactory explanation of the curious phenomenon oi fair if 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 /mw^«, or mushrooms, spread- ing from a central point-t The soil, which has once contributed to the support of these fungi, be- comes exhausted or deteriorated 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 enlarge- ment from the centre outwards. An appearance * There is also another reason why it is advantageous to observe a certain rotation in crops ; namely, that different kinds of plants differ considerably in their respective capacities to extract from the soil, and from the atmosphere, some of the principal elements of their nourishment ; and in particular carbon and nitrogen. This fact is well established t^ Boussiiigault, who, by a series of careful experiments, found that a crop of potatoes, raised from a given extent of ground, had, in the course of their growth, obtained from the atmosphere a quantity of carbon nearly live times greater than, and a quantity of nitrogen twice as great as that which they had derived from the soil and the manure which had been laid upon it. On the other hand, wheat, and the Cerealia generally, derive by far the largest proportion of their nourishment from the earth, and but a comparatively small quantity from the atmosphere. Hence the latter tend to impoverish the soil in a much greater degree than the former. Ann. Sc. Nat. serie 2. xi. 41. t Phil. Trans, for 1807, p. 133. VEGETABLE EXCRETION. 5l of luxuriance of the grass follows as a natural con- sequence ; 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 year's growth. It often happens, indeed, during the growth of these fungi, that they so completely absorb all nutriment from the soil beneath, that the herbage is for a time totally de- stroyed, giving rise to the appearance of a ring bare of grass, surrounding 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 progress, they not only do not cross each other, but both circles are invariably obliterated between the points of contact ; for the exhaustion occasioned 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 interference 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 de- trimental to the growth of the other, as if it had been occasioned by the previous 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 certain organic products calculated to supply sustenance 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 52 THE VITAL FUNCTIONS. together a certain number of elements derived from the mineral kingdom, in order to subject them to the operations of vital chemistry, a power too subtle for human science to detect, or for human art to 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 structures, 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 subsequent and more extended vegetation, or else, buried in the bosom of the earth, it forms part of that vast magazine of combustible matter, destined to benefit future communities of mankind, when the arts of civilization shall have developed the mighty ener- gies of human power. Chapter III. ANIMAL NUTRITION IN GENERAL. § I . Food of Animals. Nutrition constitutes no less important a part of the animal, than of the vegetable economy. En- dowed with more energetic powers, and enjoying a wider range of action, animals, compared with plants, require a considerably larger sujjply of nu- ANIMAL NUTRITION. 53 tritive materials for their sustenance, 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 combination the powers of organization 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 important as the former, take place to a great extent 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 pro- vided for their subsistence, apportioning to each the quantity and the kind most consonant to en- larged views of prospective beneficence, is calcu- lated to excite our highest wonder and admiration. While the waste is the smallest possible, we find * 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 ultimate analysis. They found, in particular, the same proportion of nitrogen in the chyle, whatever kind of food the animal habitually con- sumed ; and it was also the same in the blood, whether of carni- vorous or herbivorous animals ; although this last fluid contains more nitrogen than tlie chyle. {Mcmoires de lu Socictc dc Physi,jue ct d'Hlstoire Naturellc de Geucvcy v. 389.) 54 THE VITAL FUNCTIONS. 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, become the food of some species of insect, or contribute 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 designed as the ap- propriate nutriment of particular species, which would perish if these ceased to grow : thus the silk- worm subsists almost exclusively upon the leaves of the mulberry tree ; and many species of cater- pillars are respectively attached 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 Euphorhiiim, Henbane^ and Nightshade, afford a wholesome ahd delicious food to others. Innumerable tribes of animals sub- sist upon fruits and seeds ; w hile others feast upon the juices which they extract from flowers, or other parts of plants ; and 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, ANIMAL NUTRITION. OO birds, or fish; others feeding on reptiles or mol- lusca, and some satisfying their appetite with in- sects alone. The habits of the more diminutive tribes are not less predatory and voracious than those of the larger quadrupeds ; for the spiders on the land, and the Crustacea in the sea, are but re- presentatives of the lions and tigers of the forest, displaying an equally ferocious and insatiable ra- pacity. 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 multitudes 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 carnivo- rous quadrupeds, it never fails to be soon attacked by swarms of insects, which speedily consume its softer textures, leaving only the bones. These, again, are the favourite repast of the Hyaena, whose powerful jaws are peculiarly 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 in- habitants of the waters, where innumerable fishes, Crustacea, annelida, and moUusca 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, per- haps, but performing a no less important part in the economy of the creation. It may be observed 56 THE VITAL FUNCTIONS. 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 contem- plated, besides their own individual existence. They seem as if commissioned to act as the sca- vengers of organic matter, destined to clear away all those particles, of which the continued accumu- lation would have tainted the atmosphere or the waters with infection, 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 admir- ing 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 be- stowed upon the soil, reappearing in the substance of some plant, and being in this way made to con- tribute eventually to the same ultiiijate 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 organiza- tions, 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 circumstances, again partaking of ani- mation, and invested with various forms of infusory animalcules, which sport in countless myriads their ECOXOMY OF NUTRITIVE MATTER. 57 ephemeral existence within the ample regions of 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 exis- tence incalculable hosts of animal beings, which people every portion of the wide expanse of ocean, and which rise in regular gradation from the mi- croscopic monad, and scarcely visible medusa,* through endless tribes of mollusca and of fishes, up to the huge Leviathan of the deep. Even those portions of organic matter, which, in the course of decomposition, escape in the form of gases, and are widely diffused through the atmo- sphere, are not 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 pro- ducts, destined again to run through the same never-ending cycle of vicissitudes and transmuta- tions. The diffusion of animals over wide regions of the globe is a consequence of the necessity which prompts them to search for subsistence wherever * The immensity of the numbers of these animalcules, which people every region of the ocean, may be judged of from the phe- nomenon of the phosphorescent light which is so frequently exhi- bited by the sea, when agitated, and which, as I have already observed, is found to arise from the presence of an incalculable multitude of these minute animals. 58 THE VITAL FUNCTIONS. 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 move- ments among the predaceous tribes which subsist upon them. Thus are continual interchanges pro- duced, contributing to colonize the earth, and ex- tend 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 inter- mediate and necessary link between the mineral and the animal kingdoms. All those regions, which are incapable of supporting an extensive vegeta- tion, are, on that account, unfitted for the habita- tion 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 oc- cupy the summits of the highest mountain chains ; and such is the wide expanse of sai^d, which covers the largest portions both of Africa and of Asia : and often have we heard of the sunken spirits of the traveller through the weary desert, from the appalling silence that reigns over those regions of eternal desolation ; but no sooner is his eye re- freshed by the reappearance of vegetation, than he again traces the footsteps and haunts of ani- mals, and welcomes the cheering sound of sensi- tive beings. The kind of food which nature has assigned to each particular race of animals has an important influence, not merely on its internal organization, INFLUENCE OF THE DEMAND FOR FOOD. 51) but also on its active powers and disposition ; for the faculties of animals, as well as their structure, have a close relation to the circumstances con- nected with their subsistence, such as the abun- dance 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 deve- lopement : such, for instance, is the condition of herbivorous quadrupeds, 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 affords 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 muscles are more vigorous, their bones are stronger, their limbs more robust, their senses more delicate 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 animals are more accurate ; their sagacity embraces a greater variety of objects ; and in feats of strength and agility they far surpass the herbivorous tribes. A tiger will take a spring of lifteen 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 constantly employed in eating, carnivorous animals are able 60 THE VITAL FUNCTIONS. 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, and he then rushes to the attack, reckless of consequences, 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 ele- phant, subsisting upon the vegetable productions of the forest, superior in size ftnd 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 destruction of animal life by modes the most cruel and re- volting to all our feelings, we naturally recoil with horror from the sanguinary scene; and cannot re- frain from asking how all this is consistent with the wisdom and benevolence so conspicuously INFLUENCE OF THE DEMAND FOR FOOD. 61 manifested in all other parts of the creation. 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 con- sider the pain and suffering thus created, as one of the necessary consequences of those general laws which secure, on the whole, the greatest and most permanent good. There can be no doubt that the scheme, by which one animal is made directly con- ducive to the subsistence of another, leads to the extension of the benefits of existence to an infi- nitely greater number of beings than could other- wise have enjoyed them. This system, besides, is the spring of motion and activity in every part of nature. While the pursuit of its prey forms the occupation, and constitutes the pleasure of a consi- derable part of the animal creation, the employ- ment 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 propor- tion of instances, successful ; for wherever nature has inspired 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, re- treats within its shell, as within a fortress ; the * See, in particular, Paley's Natural Theology, chap. xxvi. 62 THE VITAL FUNCTIONS. 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 ; the cuttle- fish screens itself from pursuit by effusing an in- tensely dark coloured ink, which renders the surrounding waters so black and turbid as to con- ceal the animal, and favour its escape ; the torpedo defends itself from molestation by reiterated dis- charges from its electric battery ; the butterfly avoids capture by its irregular movements 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, Respira- tion, Secretion, Excretion, Absorption, and Nutri- tion; some of which admit of further subdivision. This is the case more particularly with the pro- cesses of assimilation, which are generally nume- rous, and require a very complicated apparatus for acting on the food in all the stages of its conversion RECEPTACLES OF FOOD. 63 into blood ; a fluid which, like the returning sap of plants, consists of nutriment in its completely assimilated state. It will be necessary, therefore, to enter into a more particular examination of the objects of these different processes. In the more perfect structures belonging to the higher orders of animals, contrivances must be adopted and organs provided for seizing the appro- priate food, and conveying it to the mouth. A mechanical apparatus must there be placed for effecting that minute subdivision, which is neces- sary to prepare it for the action of the chemical agents to which it is afterwards to be subjected. From the mouth, after it has been sufficiently mas- ticated, 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 con- verted into Chyle, which is a milky fluid, con- sisting 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 im- pulsive force, and of distributing it through appro- priate channels of circulation, not only to the res- piratory 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 04 THE VITAL FUNCTIONS. 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 more and more simplified, by the concentration of organs and the union of many oftices in a single organ, till we arrive in the very lowest orders, at little more than a simple digestive cavity, perform- ing 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, whether internal or external, the muscular and nervous systems, the glands, vessels, and organs of sense, have one after another disappeared, we still continue to find the diges- tive 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- 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, how- ever diminutive ; and even to the minutest of the microscopic animalcules.! The mode in which the food is received into the * See the first chapter of this volume, p. 9. t In some species of animals belonging to the tribe of Mediisge, as the Eudora, Berenice, Orythia, Favonia, Lyynnoria, and Geryonia, and also in the Liyula, among the Entozoa, no central INFLUENCE OF THE DEMAND FOR FOOD. 65 body is, in general, very different in the two orga- nized kingdoms of nature. Plants receive their nourishment by a slow, but nearly constant supply, and have no receptacle for collecting it at its im- mediate entry ; the sap, as we have seen, passing at once into the cellular tissue of the plant, where the process of its gradual elaboration is com- menced. Animals, on the other hand, are capable of receiving at once large supplies of food, in con- sequence of having an internal cavity, adapted for the immediate reception of a considerable quantity. A vegetable may be said to belong to the spot from which it imbibes its nourishment ; and the sur- rounding 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 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, corresponding in their office to the roots of vegetables, imbibe nourishment. Im- portant consequences flow from this plan of struc- ture ; 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, absolutely necessary to beings which have their subsistence to seek. It is this necessity, again, cavity corresponding to a stomach has been discovered : they appear, therefore, to constitute exceptions to the general rule, being pro- bably nourished solely by absorption from the surface of the body. See Peron, Annales de Museum, xiv, 227 and 326. VOL. II. F 66 THE VITAL FUNCTIONS. that calls for the continued exercise of their senses, intelligence, and more active energies ; and for the possession of those higher powers, which elevate them so far above the level of the vegetable crea- tion. Chapter IV. Nuttition 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 sim- plest instruments for catching food, — and nothing- more. A simple sac, or tube, adapted to receive and digest food, is the only visible organ of its body. It exhibits not a trace of either brain, nerves, or organs of sens*, nor any part corres- ponding to lungs, heart, or even vessels of any sort ; all these organs, so essential to the main- tenance of life in other animals, being here dispensed with. In the magnified view of the hydra, exhi- bited in Fig. 241, the cavity into which the food is received and di- gested is laid open by a longi- tudinal section, so as to show the comparative thickness of the walls of this cavity. The thinness and transparency of the walls of this cavity allow of our distinctly following the NUTRITION IN POLYPI. '67 digestion of the food by the aid of the microscope. Trembley watched this process with unwearied perseverance for days together, and has given the following account 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 filaments which the animal spreads out, in different direc- tions, like a net, in search of food. The struggles of the captive, which finds itself entangled in the folds of these tentacula, are generally ineffectual ; and the hydra, like the boa constrictor, contrives, by enormously expanding its mouth, slowly to draw into its cavity animals 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 the vermiform larva of a Tijmla^ 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 succeeded, though not without a severe contest, in swallowing a minnow, or other small fish, the form of M'hich is still visible through the transparent sides of the body, which are stretched to the utmost. It occa- sionally 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 68 THE VITAL FUNCTIONS. Stronger, having obtained the victory, swallows at a single gulp, not only the object of contention, but its antagonist also. This scene is represented in Fig. 244, m here the tail of the hydra, of which the body has been swallowed by the victor, is seen protruding from the mouth of the latter. It soon, however, extricates itself from this situation, appa- rently without having suffered the smallest injury. The voracity of the hydra is very great, especially after long fasting ; and it Vill then devour a great number of insects, one after another, at one meal, gorging itself till it can hold no more ; its body becoming dilated to an extraordinary size ; and yet the same animal can continue to live for more than four months without 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 resolved into a kind of jelly ; leaving unaltered only a few frag- ments of the harder and less digestible parts. These changes are accompanied by a kind of NUTRITION IN POLYPI. 69 undulation of the contents of the stomach, back- wards and forwards, throughout the whole tube ; apparently produced by the contraction and dilata- tion of its different portions. The undigested mate- rials being collected together and rejected by the mouth,* the remaining fluid is seen to contain opaque granules of various sizes, some of which are observed to penetrate through the sides of the stomach, and enter into the cellular 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. By watching attentively the motions of the granules, it will be perceived that they pass backwards 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 imparting its hue to the granules, and allowing of its progress being traced into the vesicles which compose the substance of the body. The vesicles that are nearest to the cavity of the stomach are those which are first tinged, and which therefore first im- bibe the nutritious juices : the others are coloured successively, in an order determined by their distance from the surface of the stomach. Trembley ascertained that a living hydra introduced into the * Corda found that although the larger masses of undigested materials are rejected by the mouth, yet that an aperture exists at the opposite end of the cavity, for the extrusion of the smaller and more divided particles. This orifice had long ago been r>oticed by Baker. 70 THi: VITAL FUNCTIONS. 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 tentacula, which had encircled it ; but these tentacula always ultimately come out of the stomach, sometimes after having remained there twenty-four hours, without the least detriment. The researches of Trembley have brought to light the extraordinary fact that not only the in- ternal surface of the stomach of the polypus is en- dowed with the power of digesting food, but that the same property belongs also to the external surface, or what we might call the skin of the animal. He found that, by a dexterous manipu- lation, the hydra may be turned inside out, like the linger of a glove ; and that the animal, after having undergone this violent evertion, soon recovers the power of performing its o^iinary 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 surface 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 astonishment, and many naturalists were incre- dulous 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 NUTRITION IN POLYPI. 71 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 wonder- ful power of repairing all sorts of injuries, and of restoring parts which have been removed. These animals are found to bear with impunity all kinds 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 asunder ; 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, another 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 replace 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 becoming inde- pendent 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 surfaces, and pressing them together ; for by this means they quickly unite, and become a compound animal. When 72 THE VITAL FUNCTIONS. 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 individual. A number of heads and bodies may thus be joined together artificially, so as to compose 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 pro- lific 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 ex- tended 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 stationary 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 vibra- tions, determine currents of water to flow towards the mouth, carrying with them the floating animal- cules on which the entire polypus subsistg. Each mouth leads into a separate stomach ; whence the food, after its digestion, passes into several channels, generally five in number, which 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 neighbouring stomachs : so that the food, which has been taken in by one of the mouths, contributes to the general NUTUITION IN POLYPI. 73 nourishment of the whole mass of aggregated polypi. Cuvier discovered this structure in the Veretilla, which belongs to this order of polypi : he also found it in the Pemiatula, and it is probably similar in most of the aggregated polypi. Fig. 246 represents three of the polypes of the Veretilla, with their communicating vessels seen below. The pre- vailing 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 whole society. But it is perhaps more consonant 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 con- sider the entire polypus, or mass composed of nu- merous polypes, as a single individual animal ; for there is no more inconsistency in supposing that an individual animal may possess any number of mouths, than that it may be provided with a mul- titude of distinct stomachs, as we shall presently find is actually exemplified in many of the lower animals.* Some of the Etitozoa, or parasitic worms, exhibit * Milne Edwards has traced a similar intercommunication of nutrient vessels amongst the Alcyonidce : he observes, however, that at a subsequent period of the developement of these animals, the passages of communication become gradually choked up by the accumulation of ova, and are ultimately obliterated by the adhesion of the sides of the vessels; so that the nutrient system of each polype then becomes isolated. He is of opinion that while the whole group is nourished in common by one communicating system of vessels, yet still each polype which composes that group may pos- sess individual sensibihty. Ann. Sc. Nat. serie 2, iv. 328, 330, 339. 74 THE VITAL FUNCTIONS. a general ditfusion, or circulation of nourishment through numerous channels of communication, 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 Taniia, 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 oT[)enings for im- bibing nourishment from the surrounding fluids. Although each segment is thus provided with a nutritive apparatus complete within itself, and so far, therefore, independent of the rest, the indi- viduality of the whole animal is sufficiently deter- mined by its having a distinct head at one extre- mity, 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 representation,) furnished with four suckers, and a tubular neck, NUTRITION IN THE ENTOZOA. 75 which terminates in a globular sac. When this sac, which may be regarded either as the stomach, or as a general reservoir of nutriment, is fully dis- tended 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 distinguished 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 Ccenurus, which is found in the brain of sheep, has a structure a little more complicated ; 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 Sponge has been already fully described ; and the course of the minute channels pointed out, in which a kind of circulation of sea water is carried on for the nourishment of the animal. The mode by which nutriment is ex- tracted from this circulating fluid, and made to contribute to the growth of these plant-like struc- tures, is entirely unknown. The apparatus for nutrition possessed by animals belonging to the tribe of Medusce is of a peculiar kind. I have already described the more ordinary form of these singular animals, which resemble a mushroom, from the hemispherical form of their bodies, and their central foot-stalk, or pedicle. In the greater number of species there exists, at the 7G THE VITAL FUNCTIONS. extremity of this pedicle, a single aperture, which is the beginning of a tube leading into a large central cavity in the interior of the body, and which may therefore 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 sufficient width to admit of the entrance of prey of consider- able size, as appears from the circumstance that fishes, of some inches in length, are occasionally found entire in the stomachs of those medusas 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 formed simply by a separation of the soft structure of the body. Its form varies in different species; having gene- rally, however, more or less of a star-like shape, composed of four curved rays, which might almost be considered as constituting four stomachs, joined at a common centre. Such, indeed, is the actual structure in the 3Icdnsa anrita, in which Gaede found the stomach to consist of four spherical sacs, completely 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 communi- cations, so as to compose a complicated network of vessels. These ramifications at length unite to form an annular vessel, which encircles the margin of the disk. It appears also, from the observations of Gaedc, that a further communication is established NUTRITION IN MEDUSA. 77 between this latter vessel and others, which per- meate 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 elongations 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 calcu- lated to distribute the nutritious juices to every part of the organization. There are other species of Medusse, composing 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. f 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 cen- tral orifice ; the only avenues to the stomach being these elongated 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 oeso- phagus, terminating in the general cavity of the stomach. The Medusa pulmo, of which a figure was given in vol. i., p. 175, belongs to this modern genus, * Journal de Physique, Ixxxix, 146, t It is from this circumstance that the genus has received the name it now bears, and which is derived from two Greek words, sig- nifying root-like mouths. 78 THE VITAL FUNCTIONS. and is now termed the Rhizosloma Cuvieri. It is evident that this conformation allows of the admit- tance of none but very minute particles of organized materials into the recipient orifices and almost capillary tubes of these singular animals. The course of these absorbent vessels is most conveniently traced after they have been filled with a dark coloured liquid. The appearances they pre- sent in the Rliizostoma Cuvieri, after being thus injected, are represented in the annexed figures ; the first of which (Fig. 249), shows the under sur- face 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 ''^"'«i\jJv( that its section presents the square surface, q, q. Fig. 25*2 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 NUTRITION IN MEDUSA. 79 base of the hemisphere presented in Fig. 249. On the other hand, the dotted Hne v, v, in Fig. 249, is that along which the vertical section of the same 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 ten- tacula, may be seen the canals, (marked by the dark lines, c, c, c), which arise from numerous ori- fices 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, or stomach (s), situated in the upper part of the cupola. The section of this ceso- 00 THE VITAL FUNCTIONS. phagus is visible at the centre of Fig. 249, where its cavity has the form of a cross. The stomach has a quadrangular shape, as in the ordinary me- dusae ; and from each of its four corners there proceed vessels, which are continuous with its cavity, and are distributed by endless ramifications over tlie substance of the cupola, extending even to the fringed margin all round its circumference. The mode of their distribution, and their numerous commimications by lateral vessels, forming a com- plete vascular network, is seen in Fig. 251, which represents, on a larger scale, a portion of the mar- ginal part of the disk. The two large figures (24.9 and 2o2) also show the four lateral cavities (r, k, Fig. 252), which are contiguous to the stomach, but separated from it by membranous partitions : these cavities have by some been supposed to per- form an office in the system of the Medusa, cor- responding to respiraticyi ; an opinion, however, which is founded rather on analogy than on any direct experimental evidence. The entrances into these cavities are seen open at e, in Fig. 249, and at E, E, in the section Fig. 252. A transverse sec- tion 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 tube, the cavities of some of the smaller branches (u, b), which are proceeding to join it, are also visible. The pendent tentacula of the Physalida formerly described,* are tubular, having at their extremities, absorbing orifices, which may * Vol. i. p. 179. NUTRITION IN MEDUSiE. 81 be considered as so many mouths : and all termi- nating above, in a common receptacle or intestine. Many gradations are observable in the com- plexity of the digestive cavities of this extensive tribe, for while some, as the Eudora, have, to all appearance, 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 destitute 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, apparently 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 a regular progression may be followed, through various species, in which the aperture pf this pouch is more and more com- pletely closed, and where the tube which enters it branches out into ramifications more or less nu- merous, as we have seen in the Rhizostoma.* In the Beroe, the alimentary tube is no longer a closed sac, but terminates in an external orifice. It is difficult to conceive in what mode nutrition is performed in the agastric tribes, or those destitute of any visible stomach ; unless we suppose that their nourishment is imbibed by direct absorption from the surface. Ever since the discovery of the animalcula of infusions, naturalists have been extremely desirous * See Peron, Annales du Museum, xiv. 330. VOL. II. G 82 THE VITAL FUNCTIONS. of ascertaining the nature of the organization of these curious beings ; but as no mode presented itself of dissecting objects of such extreme minute- ness, it was only from the external appearances they presented 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 ani- malcule 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 infer- ence was obvious that, in common with all other animals, they also must possess a stomach. But as no such structure had been rendered visible in the smallest species of infusoria, such as monads, it was too hastily concluded that these species were formed on a different and a simpler model. Lamarck characterized them as beino; throuo:hout 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 lijiht has been thrown on the subject by Professor Ehrenberg, whose researches have recently dis- » Vol. i. p. 53, Fig. 1. NUTRITION IN THE INFUSORIA. 83 closed fresh scenes of interest and of wonder in microscopic worlds, peopled with hosts of animated beings, almost infinite in number as in minuteness. In endeavouring to render the digestive organs of the infusoria more conspicuous, he hit upon the fortunate expedient of supplying them with coloured food, which might communicate its tinge to the cavities into which it passed, and exhibit their 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 at- tempts with regard to the infusoria ; but, in conse- quence of his having employed metallic or earthy colouring 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 substance highly deleterious to all animals ; but, at length, by em- ploying an indigo which was quite pure, he suc- ceeded perfectly.* The moment a minute particle * 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. 84 THE VITAL FUNCTIONS. of a highly attenuated solution of this substance is applied to a drop of water in which are some pedunculated Vorticellae, occupying the field of the microscope, the most 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 distinguished 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 trans- parent, becomes dotted with a number of distinctly circular spots, of a dark blue colour, evidently produced by particles of indigo accumulated in those situations. In some species, particularly those which have a contracted part, or neck, be- tween the head and the body, as the Rotifer vul- garis^ these particles may be traced in a continuous line in their progress from the mouth, through the neck, into the internal cavities. Following up this train of observations, Ehren- berg persuaded himself that he had discovered not only the existence of a system of digestive cavities in all the known genera of this tribe of animals ; but that he could also trace out their forms, situa- tions, and arrangement. They appeared to exhibit great variety of structure in the different tribes; but without any obvious relation to the form or magnitude of the animalcule. The Monas atomus, the minutest of the whole tribe, presented a number of sacs, opening by as many separate orifices, from a circumscribed part of the surface. In others, as NUTRITION IN THE INFUSORIA. 85 in the Leucoplua patula* the appearance of which under the microscope is shown in Fig. 255, he traced a long ahmentary canal, traversing the greater part of the body, taking several spiral turns, and furnished with a great number of cceca ; a term which denotes blind pouches, proceeding laterally from any internal canal, and having no other outlet. He stated that these cavities became filled with coloured particles immediately after their entrance into the alimentary canal ; and drew the inference that they are so many stomachs provided for the digestion of the food thus received : hence he gave them the general name of Poly gastric Jufusoria.^ But he found that they are not all filled at the same time ; for some continued long in a contracted state, so as not to be visible ; while, at another time, they readily admitted the coloured food. It was only by dint of patient watching that he made out the whole extent of the alimentary tube, and its appa- ratus of stomachs. Fig. 255, above referred to, * Trichoda patula. Muller. t From two Greek words, signifying many stomachs. 86 THE VITAL FUNCTIONS. exhibits the representation given by him of the Leucophra patula, with a few of its stomachs filled with the opaque particles ; and Fig. 256 shows the whole series of organs, as he conceives they would appear if they 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 globules may be counted, connected with the in- testinal tube. Many of the larger species, as the Hydatina senta,^ exhibit a greater concentration of organs, having only a single oval cavity of con- siderable size, situated in the fore part of the body. In the Rotifer vulgaris, the alimentary canal is stated to be a slender tube, considerably dilated near its termination. In some VorticeUce, the intestine, from which proceed numerous caeca, appears to make a complete circular turn, ending close to its commencement : Ehrenberg forms of these the tribe of Ci/cIocceIu, of which the Vorticella citriiia, and the Stentor polymorphus, are examples. The accuracy of these conclusions of Ehrenberg with regard to the existence of an intestinal tube, with accompanying csecal appendages, has been called in question by many able observers. Du- jardin, in particular, denies the permanence of those cells which Ehrenberg has regarded as stomachs ; and maintains that they are cavities arising spontaneously in the gelatinous substance of the animal, to which he gives the name of sarcode.^ Professor Rymer Jones was unable, not- withstanding the most patient and long-continued * Vorticella senta. Muller. f Ann. cles Sc. Nat. serie 2, iv. 367, et seq. and x. 258, 272, &c. NUTRITION IN THE INFUSORIA. 87 efforts, to detect the arrangement depicted in tiie figures given by Ehrenberg. He observed that the apparent vesicles, far from exhibiting a connexion with a central canal, are in fact, in a continual rotatory circulation, like the coloured granules which are visible in the gelatinous substance of the hydra, and which themselves receive a tinge from coloured food.* J. Meyen also declares his inabi- lity to see these pretended intestines and stomachs ; and observes that the opaque globules supposed to be lodged in them, move in the interior of the body, in many species with great rapidity, in the same manner as the granules which circulate in the joints of the chara: and what strengthens the analogy, the membrane enclosing the mucous sub- stance which constitutes the bulk of the animalcule, has an obviously spiral structure. In the large infusoria, a recipient cavity may be seen, the lower end of which dilates on the introduction of food, which it forms into a ball by the action of its cilia ; this ball then passes out of the stomach into the general cavity ; and this process being continually repeated, the balls are pushed forwards and distri- buted in various parts of the substance of the animalcule, giving rise to all the appearances described by Ehrenberg. Meyen remarks that similar cavities are formed in the mucus of true cellular plants, particularly in certain aquatic Cryptogamia.t The Hydatina senta, one of the largest of the Infusoria, was found by Ehrenberg to possess a * " General Outline of the Animal Kingdom," p. 57 and .58. t Annales des Sc. Nat. serie 2, xii, 122. 88 THE VITAL FUNCTIONS. highly developed structure with respect to many systems of organs, which we should never have expected to meet with in animals situated so low in the scale. As connected with the nutritive func- tions, it may here be mentioned that the head of this animalcule is provided with a regular appa- ratus 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 particles 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 ani- mal creation ; namely, digesting cavities, whence proceed various canals, which form a system for the transmission of the prepared nourishment to different parts; but all these cavities and canals being simply hollowed out of the solid substance of the body. As we ascend a step higher in the scale, we find that the stomach and intestinal tube, to- gether with their appendages, 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 elabo- ration in these organs. The Actinia, or Sea Ane mone, for example, resembles a polypus in its gene- ral 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 Po- lypus, the sides of the stomach constitute also those of the body, the whole being one simple sac ; NUTRITION IN THE ACTINIA. 89 in the Actinia, spaces intervene between the coats of the stomach, and the skin of the animal. As the sto- mach is not a closed sac, but is open below, these ca- vities are, in fact, continuous with that of the stomach : they are divided by nume- rous membranous partitions passing vertically between the skin, and the mem- brane of the stomach, and giving support to that organ. Fig. 257, representing a vertical section of the Actinia coriacea, displays this internal struc- ture. B is the base 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 the stomach (s), of which the lower orifice is open, and which is sus- pended in the general cavity by means of vertical partitions, of which the cut edges are seen below, uniting at a central point (c), and passing between the stomach and the integument. These muscular partitions are connected above with three rows of tentacula, of which the points are seen at t. These tentacula are here represented as they appear when retracted within the mouth ; ready, however* to be protruded by the injection of water into their cavities from the respective cells with which they communicate. The ovaries (o) are seen attached to the partition ; and the apertures in the lower part of the stomach, by which they communicate with its cavity, may also be perceived. All these 90 THE VITAL FUNCTIONS. internal surfaces are very generally beset with ex- tremely minute cilia, maintaining, by their vibra- tions, continual currents of fluids in the cavities, both of the body and of the tentacula.* If we considered the Medusa as having four sto- machs, we might in like manner regard the Aste- rias, 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, and w^iich 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 membranes, connecting them with the sides of the cavity in which they are sus- pended. Numerous cilia are found spread over all these internal surfaces. In the various species of Echini, we find that the alimentary tube has attained a greater develope- ment; for instead of constituting merely a blind pouch, it passes entirely through the body of the * Sharpcy, Cyclop, of Anat. and Physiol, art. Cilia, I. 614. NUTRITION IN THE ASTERIAS. 91 animal. We here find an cesophagus, or narrow tube, leading from the mouth to the stomach ; and the stomach is continued into a regular intestine, which takes two turns in the cavity of the body, before it terminates. In these animals, as in the former, ciliary motions take place over nearly the whole surface of the internal cavities. 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 pro- perly denominated the great intestine, by way of distinction from the middle portions of the tube, which are generally narrower, and are termed the small intestine. We often see blind pouches, or ccoca, projecting from different parts of the canal ; this is the case with the intestine of the Aphroclita aculeata, or sea-mouse. The intestine, being gene- rally 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 Lumbricus terrestris, or earth- worm. In the Nais, on the contrary, as is shown in Fig. 259, the alimentary tube pre- sents a series of dilatations, which, from the transparency of the skin, may be easily seen in the living animal. The food taken in by 02 THE VITAL FUNCTIONS. these worms is observed to be transferred from the one to the other of its numerous stomachs, back- wards and forwards many times, before its diges- tion is accomplished.* 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 the 262 C-/ great capacity, occupying 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. 260 M^IH^ Newport, who has examined its structure with great care, finds that each of the ten portions into which it is divided sends out, on the part most remote from the oesophagus (o), two lateral pouches, or caeca ; which, as they are traced along the canal, become both wider and longer, so that the tenth pair of caeca (a) extends to the hinder ex- tremity of the animal ; the intes- tine (i), which is very short, lying between them.f It has long been known, that if, after the leech, has fastened on the skin, a portion of the tail be cut off, the animal will continue to suck blood for * The internal surface of the intestine and caecal appendages of the Aphrodita, and other Annelida; and also of the entire ali- mentary canal of the MoUusca is furnished with cilia. + This figure was engraved from a drawing made, at my request, by Mr. Newport, from a specimen which he dissected. Fig. 261 represents the mouth, within which are seen the three teeth ; and Fig. '262, one of the teeth detached. COMPLEX APPARATUS FOR NUTRITION. 93 263 264 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 met with in the digestive organs of the Glossopora tuberculata, (Hirudo complanata, Linn.) of which Fig. 263 represents a mag- nified 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, prolonged 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.* * In both these figures, t is the tubular tongue, projected from the mouth. In Fig. 263, e are the six eyes, situated on the extre- mity which corresponds to the head; and a double longitudinal row of white tubercles is also visible, extending along the back of the animal. £, in Fig. 264, is the entrance into a cavity, or pouch, provided for the reception of the young. See Johnson, Phil. Trans, for 1817, p. 343. 94 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 per- form them are more numerous, and more compli- cated in their structure. The long series of pro- cesses requisite for the perfect elaboration of nutri- ment, 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, where 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, conducted on 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 further division of labour introduced. Of this we have already seen the commencement in the multi- plication of the digesting cavities of the Leech and other Annelida; but, in animals which occupy a still higher rank, we observe a more complete sepa- ration of offices, and a still greater complication of COMPLEX APPARATUS FOR NUTRITION. 95 organs ; the principle of the division of labour being carried to a much greater extent than in the inferior departments of the animal creation. Be- sides the stomach, or receptacle for the unassimi- lated food, another organ, the heart, is provided for the uniform distribution of the nutritious fluids elaborated by the organs of digestion. This sepa- ration 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, accord- ing 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 exhibits them in their natural order of connexion, and as they might be supposed to appear in a side view of the interior of a quadruped. To this dia- 9G THE VITAL FUNCTIONS. gram 1 shall make frequent reference in the fol- lowing description of this system. The food is, in the first place, prepared for di- gestion hy several mechanical operations, which loosen its texture and destroy its cohesion. 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 muscular tube, called the Oesophagus, (repre- sented in the diagram by the letter o,) into the sto- mach (s), of which the entrance (c) is called the car did . 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 far- ther subjected to the action of several fluid secre- tions, 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, which are exceed- ingly numerous, are spread over the 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 mesen- tery (m), or membrane connecting the intestines with the back ; forming larger and larger trunks, COMPLEX APPARATUS FOR NUTRITION. 97 till they terminate in an intermediate reservoir (u), 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 Thoracic 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 pouring 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 distributed by the heart through appropriate channels of circula- tion called arteries (of which the common 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 which these functions are conducted in the several 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 re- turning sap of plants, the blood is made to undergo further modifications in the minute vessels through which it circulates : new arrangements of its ele- ments take place during its passage through the subtle organization of the glands, which no micro- scope has yet unravelled : new products are here formed, and new properties acquired, adapted to VOL. II. H 98 THE VITAL FUNCTIONS. the respective purposes which they are to serve in the animal economy. The whole is one vast Labo- ratory, where Mechanism is subservient to Che- mistry, where Chemistry is the agent of the higher powers of Vitality, and where these powers them- selves minister to the more exalted faculties of Sensation and of Intellect. The digestive functions of animals, however com- plex 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 respective ends, than the mechanical functions. This arises from the circumstance that the processes 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 producing 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 intermediate stages before they can attain their final state of elaboration. Hence, whenever we can ascertain the degrees of difference existing between the chemical condi- tion of the substance taken into the body, and that of the product derived from it, we are furnished with a kind of scale whereby we may estimate the length of the process required, and the amount of power necessary for its conversion into that pro- duct. 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 COMPLEX APPARATUS FOR NUTRITION. .00 considerably greater than those required to convert animal food into the same fluid ; because the latter is itself derived, with only slight modification, im- mediately from the blood. We accordingly find it to be an established rule, that the digestive organs of animals which feed on vegetable materials are remarkable for their size, their length, and their complication, 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, 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 of it. 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 por- tion of vegetable and animal materials that happen to be intermixed with the soil ; and this slender nutriment is sufficient for the subsistence of that animal. Many marine worms, in like manner, feed apparently on sand alone ; but that sand is gene- rally intermixed with fragments of shells, which have been pulverized by the continual rolling of the tide and the surge ; and the animal matter con- * Corda observed, that in the Hydra fusca the digestion of animal substances is performed witli much greater rapidity than that of vegetable ; and relates that he saw a larva of an insect, with a thick skin, disappear almost entirely, by being digested in the sto- mach of the hydra, in four minutes ; whilst fragments of the plant called Vaucheria clavnta were rejected by it unchanged. 100 THE VITAL FUNCTIONS. tained in these fragments, affords them a supply of nutriment adequate to their wants. It is evident, 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 struc- ture 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, as well as the structure of the organs, will also be regu- lated by a great variety of other circumstances in the condition of the animal, independent of the mechanical or chemical nature of the food. The greater the energy with which the more peculiarly 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 exertions. 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 chameleon, 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, can yet endure long fasts with impunity. The rapidity of developement has also great influence on the quantity of food which an animal requires. Thus the caterpillar, which grows very COMPLEX APPARATUS FOR NUTRITION. 101 quickly, and must repeatedly throw off its integu- ments 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 considerable size. Chapter VI. PREPARATION OF FOOD. § 1. Prehension of Liquid Food. In studying the series of processes which constitute assimilation, our attention is first to be directed to the mode in which the food is introduced into the body, and to the mechanical changes it is made to undergo before it is subjected to the chemical action of the digestive organs. The nature of these preliminary processes will, of course, vary according to the texture and mechanical condition of the food. Where it is already in a fluid state, mastication is unnecessary, 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 * Some species of Fasciolce, or flukes, are furnished with two, three, six, or more sucking disks, by which they adhere to surfaces : to these animals the names Distoma, Tristoma, Hexastoma, and Polystoma have been given ; but these denominations, implying a plurahty of mouths, are evidently incorrect, since the sucking disks 102 THE VITAL FUNCTIONS. Hydatid, for instance, has four sucking apertures disposed around what may be called its head : the Tceuia has oral orifices in each of its jointed seg- ments: but the higher entozoa, as the ^.w«m, have a single mouth. In the Annelida, the margin of the mouth is often divided, so as to compose lips ; of these there are generally two, but in the Leech there are three. When the instrument for suction extends for some length from the mouth, it is generally termed a proboscis : such is the apparatus of the butterfly, the moth, the gnat, the house-fly, and other insects that subsist on fluid aliment. The proboscis 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 form- ing a tube on each side, but leaving also another tube, inter- mediate to the two lateral ones. This middle tube is formed by the junction of two grooves, which, by the aid of a curious apparatus of hooks, resembling those of the laminae of a feather already described, lock into each other, and can be either united into an air tight canal, or be instantly sepa- rated at the pleasure of the animal. Reaumur 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 Philos. Journal, xx, 101. The Sipnnculns, among the Echinodermata, has a long protractile tube for the [)urpose of suction. PREHENSION OF LIQUID FOOD. 103 conceives that the lateral tubes are intended for the reception of air, while the central canal conveys 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.* In the Hemiptera, the proboscis is a tube, either straight or jointed, guarded by a sheath, and acting- like a pump. The Diptera have a more compli- cated instrument for suction, consisting of a tube, of which the sides are strong and fleshy, and move- able in every direction, like the trunk of an elephant : it has, at its extremity, 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.'^ In the gnat they are five or six in number, finer than a hair, exceedingly sharp, and generally barbed on one side: in the Tuhanus, or horse-fly, they are flat, 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, lap up 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 this peculiar organ. It is pro- * Kirby and Spence's Entomology, vol, ii. p. 390. t Ibid. vol. iii. p. 467. 104 THE VITAL FUNCTIONS. tected by the mandibles, and is projected forwards by being carried on a pedicle, which can be folded back when not in use. The extremity of the pro- boscis is furnished, all round, with a nmltitude of short hairs for taking up the honey. In the state of repose, these hairs lie flat ; but whenever the organ is protruded into a flower, they are raised at right angles to the surface of the proboscis, by some peculiar mechanism, forming around it a thick brush, by which the honey is entangled, and taken into the mouth. The mouths of the Acephalous 3Iollusca are merely sucking apertures, with folds like lips, and without either jaws, tongue, or teeth ; but having often ten- tacula arising from their margins. Among fishes, we meet with the family of Cyclos- lomutd, so called from their having a circular 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 mecha- nism and mode of its attachment being similar to that of the leech. To this family belong the Mijxine and the Lamprey. So great is the force of adhesion exerted by this 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, and for a similar purpose, namely, sucking the juices of flowers. Among the mam- malia, the Vampire Bat affords another instance of suction by means of the tongue, which is PREHENSION OF LIQUID FOOD. 105 folded into a tubular shape for that purpose. But suction among the mammalia is generally per- formed 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 dipping 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 nmst be provided ; first, for their prehension and introduction into the mouth ; secondly, for their detention when so introduced ; and thirdly, for their mechanical division into smaller fragments. Of those instruments of prehension which are not portions of the mouth itself, and which form a series of variously constructed organs, extending 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 num- ber 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 princi- pally from the mechanical 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, 100 THE VITAL FUNCTIONS. likewise materially contribute; although their pri- mary 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 mouth, tend to compress and propel the contained food. We find, accordingly, 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 develope- inent are exhibited by the Echinodermata ; and in them we find a remarkable perfection in the organs of mastication. The oesophagus of the Echinus is surrounded by a framework of shell, consisting of five converging pieces, each armed with a long tooth ; and for the movement of these parts there are provided twenty separate muscles, of which the anatomy has been minutely described by Cuvier. In the shells of the echini which are cast upon the shore, this calcareous frame is usually found entire in the inside of the outer case; and x4ristotle having noticed its resemblance to a lantern, it has often gone by the whimsical name of the lantern of Aris- totle. 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 di- * Ehrenberg has found tliat a regular apparatus of jaws, armed with rows of teeth, exists in most of the rotatory infusoria, and even in some species of polygastrica. Ann. Sc. Nat. serie 2, iii. 281. JAWS OF THE ECHINUS. 107 versify 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 compre- hended in any general description. In most insects, also, their minuteness is an additional obstacle to the accurate observation of their anatomy, and of the mechanism of their action. The researches, however, of Savigny* and other modern entomolo- gists have gone far to prove, that amidst the infinite variations observable in the form and ar- rangement of the several parts of these organs, there is still preserved, in the general plan of their construction, a degree of uniformity quite as great as that which has been remarked in the fabric of the vertebrated classes. Not only may we re- cognise 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 different 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 uniformity of plan preserved in the con- struction 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 con- formation 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 * See his •' Theorie des Organes de la bouche des Animaiix invertebres et articules," which forms the first part of the " Meinoires sur les Animaux sans verlebres." Paris, 1816. 108 THE VITAL FUNCTIONS. fasliioned, 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. Pur- suing the examination of these organs in another series of articulated animals, we find them gradually assuming the characters, as well as the uses of instruments of prehension, of weapons for w^arfare, of pillars for support, of levers for motion, or of limbs for quick progression. Some of these re- markable metamorphoses 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 supernumerary organs of each description. In the Arachnida, in particular, we frequently meet with supernumerary jaws, together with various appen- dices, w hich 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 complete 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.f These parts in the Locusta viridissima, or common * Vol. i. p. 259. t All these parts, taken together, were termed by Fabricius, instrumenta cibaria ; and upon their varieties of structure he founded his celebrated system of entomological classification. Kirby and Spcnce have denominated them trophi. See their Intro- JAWS OF ARTICULATA. 109 grasshopper, are delineated in their relative situa- tions, but detached from one another, in Fig. 267. The upper jaws (m), which are termed the mandi- bles, are those principally employed for the masti- cation 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 de- signed to be applied. Thus the mandibles of some Melolonthce have a projection, rendered rough by numerous deep transverse furrows, converting it into a file for wearing down the dry leaves, which are their natural food.* In most cases, indeed, we ductlon to Entomology, vol. iii. p. 417. To the seven elements above enumerated Savigny adds, in the Hemiptera, an eighth, which he terms the Epiglossa. * Knoch, quoted by Kirby. 1 10 THE VITAL FUNCTIONS. are, in like manner, enabled, from a simple inspec- tion of the shape of the teeth, to form tolerably accurate ideas of the kind of food on which the insect naturally subsists.* Above, or rather in front of the mandibles, is situated the lahrum, 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 in- sects. The lower pair of jaws (j), or maxillcE, as thev 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 lahnim does above. In the grasshopper, each max- illa consists of an outer and an inner plate (o and i). The jaws of insects are confined, by their articula- tions with the head, to motions in a horizontal plane only, so that they open and close by lateral movements, and not upwards and downwards, as is the case with the jaws of vertebrated animals. The maxillae are, in most cases, employed principally for holding the substances on which the dividing or grinding apparatus of the mandibles is exerted. The parts denominated Palpi or Antennulcs (p, q), are probably organs of sense : they consist of jointed filaments, or processes, attached to ditferent parts of the mouth, and most usually to the maxillae and the labium ; the former (i») being termed the maxillary, and the latter (q) the labial palpi. In addition to these parts, another, which, from its supposed use, has been denominated Glossa, or tongue (g), is also generally found. * See a memoir by Marcel des Serres, in the Annales du Museum d'Hist. Nat. xiv. 56. JAWS OF INSECTS. Ill 268 269 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 ni- gricornis. This insect belongs to the order He- miptera, which has been usually characterised as being destitute of both mandibles and jaws; and as having, instead of these parts, an apparatus of very different construction, designed to pierce the skin of animals and suck their juices. But Savigny, on apply- ing the principles of his theory, has recognised, in the proboscis of the Cimex, the existence of all the constituent elements which are found in the mouth of insects formed for the masti- cation of solid food. This pro- boscis consists of four elongated filaments, contained in a kind of sheath : the filaments are re- presented in Fig. 268, separated to a little distance from each other, in order that their re- spective origins may be dis- tinctly seen ; the one set («), being prolongations of the man- dibles (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 112 THE VITAL FUNCTIONS. (g), which is tlie glossa, or tongue ; and the aperture seen at its root is the passage into the cesophagus. 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 de- scribed, 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 Aphro- dite^ 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. 261, (p. 92), in their relative positions; and Fig. 262 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. Among the Mollusca, the great majority of those inhabiting bivalve shells are either immoveably fixed to submarine rocks, or embedded in sand or other deposits near the shores of the ocean ; and they derive their nourishment only from the minute organic particles which are suspended in sea water, and are brought to them by currents, either natural or artificially created by the vibrations of cilia, and directed by them towards the entrance of the ali- mentary canal. The mouth, although furnished JAWS OF FISHES. 1 13 with folds, like lips, generally four in number, has on the whole more the character of an organ for suction, than of a cavity adapted to prehension and mastication. The Gasteropoda, on the other hand, being capable of locomotion, are enabled to seek and to select food of a more solid kind ; and they seize it with a mouth provided w4th moveable lips, retain it by means of hooks, and masticate and swallow it by the aid of a tongue and jaws ; but we frequently find them possessing a tubular organ. Most of the Mollusca which inhabit univalve shells are provided with a tubular organ, of a cylin- dric or conical shape, capable of elongation and contraction, and of eversion and inversion, by cir- cular 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 exist in most of the testaceous gastero- pods ; and they are of great size in the Baccinum, the Murex, and the Voluta ; as also in the Doris, and other Nudibranchiate Gasteropods. 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 carti- lage, having several tooth-like projections, which 270 assist in laying hold of the substances taken as food. That of the snail is repre- sented in Fig. 270. Most gasteropods have also a tongue, which in the Patella, and the Turbo pica, is of enormous length, and beset with numerous spines, to enable them to act as a file for rasping rocks and other hard bodies. All the Cephalopoda, or cuttle fish tribe, are fur- VOL. II. J 114 THE VITAL FUNCTIONS. nished, at the entrance of the mouth, with two horny jaws, having a remarkable resemblance to the bill of a parrot ; excepting that the lower piece is the larger of the two, and covers the upper one, which is the reverse of what takes place in the parrot. These, moved by strong muscles, and con- joined with a large fleshy tongue, constitute a powerful instrument for breaking the shells of the mollusca and Crustacea, which compose the usual prey of these animals. In all Ascidiw the proper mouth, by which the food is received for the pur- pose of digestion, is an orifice situated internally at the bottom of the branchial sac. Fishes almost always swallow their food entire ; so that their jaws and teeth are employed princi- pally 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 numerous than the corresponding bones in the higher classes of verte- brata ; and they appear, therefore, as if their developement had not proceeded sufficiently 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 * Attempts have been made to trace analogies between the dif- ferent segments of the jaws of fishes and corresponding parts of the mouths of Crustacea and of insects; but the justness of these ana- logies is yet far from being satisfactorily proved. JAWS OF FISHES. 1 15 various shapes, though usually sharp 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. Fishes which subsist on testaceous mol- lusca 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 implanted 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 formidable 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 whatever may be their uses, they are not sufficiently 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. 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 116 THE VITAL FUNCTIONS. like a valve in closing the air passage into the lungs. If, when this animal has approached within a cer- tain 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 Cliameleon also has a very long and lender 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 at a considerable distance, by a si- milar manoeuvre to that practised by the frog.* As Serpents swallow their prey entire, so the bones of their jaws and face are formed to admit of great expansion, and freedom of motion upon one another. Serpents and Lizards have generally curved or conical teeth, calculated rather for tearing and holding the food than for masticating it : like those of tishes, they are affixed partly to the jaws, and partly to the palate. The Chelonian reptiles * Mr. Houston lias given a description of the structure of this organ, and of the muscles by which it is moved, in a paper contained in tlie Transactions of the Royal Irish Academy, vol. xv. p. 177. He also describes a peculiar vascular structure by the rapid filling of whicli with blood he conceives the elongation of the tongue may be produced ; an effect, however, to which such a structure appears to be wholly inadequate. Mr. Crampton has lately endeavoured to explain the phenomenon of the quick [Hopulsion of the tongue of the Chameleon by the sudden blowing ir)to its tubular cavity of a small quantity of air furnished from a sac in the throat which com- municates with the trachea. JAWS OF BIRDS. 117 have no teeth ; their office being supplied 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 appropriated to each tribe : thus predaceous birds, as the eagle and the hawk tribe, have an exceedingly strong hooked beak, for tearing and dividing the flesh of the ani- mals on which they prey ; while those that feed upon insects, or upon 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 3Ierif-us, or Merganser, where the whole length of the margin of the bill is beset with small sharp pointed teeth, directed backwards : they are particularly conspicuous in the Mergus senator, or red-breasted Merganser. The beak of the Hcematopiis, or Oyster-catcher, has a wedge-shape, and acts like an oyster-knife for opening bivalve shells. In the Loxia 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 insinuating the bill between the scales. It can split clierry- stones with the utmost 118 THE VITAL FUNCTIONS. 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 swallowing. The Ri/nchops, or Black Skimmer, has a very singularly formed beak : it is very slender, and the lower mandible very much exceeds in length the upper one ; so that while skimming the waves in its flight, it cuts the water like a plough-share, catching the prey which is on the surface of the sea. The WoodpecTier is furnished with a singular ap- paratus 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 de- lineated in Fig. 271, which 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 (b) beset with barbs, of which the points are di- rected backwards : it is supported on a slender Os Hyoidcs, 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, nearly in * See a paper on the mechanism of the bill of this bird, by Mr. Yarrell, in the Zoological Journal, iv. 459. t These cartilages correspond in situation, at the part, at least, where they are joined to the os hyoides, to what are called the cornuciy or horns of that bone, in other animals. TONGUE OF THE WOODPECKER. 110 the whole extent of its course, at c, d, e, f, 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 obliquely 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 en- closed in a common sheath, are conducted together along a groove, which extends forwards, along the middle line of the cranium (e), till it arrives be- tween 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 repre- sented in the figure (at d), drawn out of the groove provided to receive and protect them.* A long and * S is the laige Scilivaiy gland on the right side. 120 THE VITAL FUNCTIONS. 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 car- tilages, so that their curvature is diminished, and the tongue protruded to a considerable distance, 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, however, 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 muscu- lar power.* This was the more necessary, because, while the bird is on the tree, it repeats these mo- tions almost incessantly, boring holes in the bark, and picking up the minutest insects, with the utmost celerity and precision. On meeting with an anthill, the woodpecker easily lays it open by the combined efforts of its feet and bill, and soon makes a plen- tiful meal of the ants and their eggs. Among the Mammalia which have no teeth, the Mynnecophaga, or Ant-eater, practises a remark- able manoeuvre for catching its prey. The tongue of this animal is very long and slender, and has a sreat resemblance to an earth-\\ orm : that of the two-toed ant-eater is very nearly one-third of the length of the whole body ; and at its base is scarcely * An account of this mechanism is given by Mr. Waller, in the Phil. Trans, for 1716, p. 509. TONGUE OF THE ANT-EATER. 121 tliicker than a crow-quill. It is furnished with a long and powerful muscle, which arises from the sternum, and is continued into its substance, afford- ing the means of a quick retraction, as well as lateral motion ; while its elongation and other move- ments are effected by circular fibres, which are ex- terior to the former. When laid on the ground 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 minute- ness, they require no mastication, they are swal- lowed undivided, and without there being any ne- cessity for teeth. The lips of quadrupeds are often elongated for the more ready prehension of food, as we see exem- plified in the Rhinoceros, whose upper lip is so ex- tensible as to be capable of performing the office of a small proboscis. The Sorex moschatus, or musk shrew, whose favourite food is leeches, has like- wise a very moveable snout, by which it gropes for, and seizes its prey from the bottom of the mud. More frequently, however, this office of prehension is performed by the tongue, which, for that pur- pose, is very flexible and much elongated ; as we see in the Camelopard, where it acts like a hand in grasping and bringing down the branches of a tree.* In the animals belonging to the genus Felis, the papillae in the fore part of the tongue are each 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 * Home, Lectures, &c. vi. Plate 32. 122 THE VITAL FUNCTIONS. 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 Ornithorhynchus has a form of construction intermediate between that of quadru- peds 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 horny bill, greatly resembling that of the duck, or the spoonbill. 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 perpendicularly 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 their general form and appear- ance, as they hang from the roof of the palate, are shown in Fig. 272, which represents only six of these plates.* They are connected with the bone by means of a white ligamentous substance, to which they are immediately attached, and from * 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. MOUTH OF THE WHALE. 1-2:3 which they appear to grow : at their inner mar- gins, the fibres, of which their texture is throughout composed, cease to adhere together ; but, being loose and detached, form a kind of fringe, calculated 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 within, which, though nar- row anteriorly, is wider as it extends backwards, and is ca- pable of holding a large quan- tity of water. Thus the whale is enabled to collect a whole shoal of mollusca, 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 M^halebone. Some contrivance of this kind was 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 with teeth to reduce the food into smaller parts. The principal food of the Balcsna Mi/sticetiis, or great whalebone whale of the Arctic Seas, is the small Clio JBorealis, which swarms in immense numbers in those regions 1-24 THE VITAL FUNCTIONS. of the ocean ; and which has been already deh- neated in Fig. ] 20.* These remarkable organs for filtration entirely supersede the use of ordinary teeth ; and accord- ingly no traces of teeth are to be discovered either in the upper or lower jaw. Yet a tendency to con- form to the type of the mammalia is manifested in the early conformation of the whale ; for rudiments of teeth exist in the interior of the lower jaw before birth, lodged in deep sockets, and forming a row on each side. The developement of these imper- fect teeth proceeds no farther ; they even disappear at a very early period, and the groove which con- tained them closes over, and after a short time can no longer be seen. For the discovery of this cu- rious fact we are indebted to GeofFroy St. Hilaire.f 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 external appearance exactly resembling the rudiments of teeth, but without being implanted into regular sockets in the maxil- lary bones : they are formed, however, by a process precisely similar to that of dentition ; that is, by deposition in the cells of avascular pulp, connected with the jaw. These tubercles are afterwards conso- lidated 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 elephant, and which I * Vol. i. p. 231. t Cuvier, Ossemens Fossiles, 3me edition, torn. v. p. 360. OFFICES OF THE TEETH. 125 shall presently describe. The original indentations are obliterated as the beak advances in growth ; but they are permanent in the bill of the duck, where the structure is very similar to that above described in the embryo of the parrot. § 3. 3Iastication 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 ani- mals ; and in none more so than in the Mammalia, the food of which generally requires considerable preparation previously 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, struc- ture, and position of the teeth ; and similar relations may also be traced in the shape of the jaw, in the mode of its articulation with the head, in the pro- portional 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 indicated by the structure and arrangement of the teeth, it is evident that these latter organs, in particular, will afford to the naturalist most important characters for establish- ing a systematic classification of animals, and more especially of quadrupeds, where the differences 126 THE VITAL FUNCTIONS. among the teeth are very considerable ; and these differences 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 princi- pally 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 grind- ing its harder portions. Occasionally some par- ticular teeth are much enlarged, in order to serve as weapons of attack or of defence ; for which pur- pose they extend beyond the mouth, and are then generally denominated tusks; this we see exempli- fied in the Elephant, the Narivhal, the Walrusy the Hippopotamus, the Boar, and the Sahiroussa. Four principal forms have been given to teeth, which accordingly may be distinguished into the conical, the sharp-edged, the flat, and the tubercu- lated teeth ; though we occasionally find a iew 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 especially adapted. The conical teeth, which are generally also sharp- pointed, are principally employed 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 TEETH OF CETACEA. 127 the Cetarea, where similar forms and arrange- ments of teeth prevail. All the Dolphin tribe, such as the Porpus, the Grampus, 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 constructed very much on the model of those of fish ; while those Cetacea, on the other hand, which are herbivorous, as the 3Ianatns and the Dugong, or Indian Walrus, have teeth very differently formed. The tusks of animals must necessarily, as respects their shape, be classed among the conical teeth. The sharp-edged teeth perform the office of cutting 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 conjunc- tion with those meeting them in the oppsite jaw, for grinding down the food by a lateral motion ; in a manner analogous to the operation of millstones 128 THE VITAL FUNCTIONS. in a mill. The tuberculated teeth, of which the surfaces present a number of rounded eminences, correspondinju; 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 in a mortar. The position of the teeth in the jaws is another ground of distinction. In those Mammalia M^hich exhibit the most complete set of teeth, the fore- most in the row have the sharp-edged or chisel shape, constituting the blades of a cutting instru- ment ; and they are accordingly denominated in- cisors. The incisors of the upper jaw are always implanted in a bone, intermediate between the two upper jaw bones, and called the intermaxillary bones. ^ The conical teeth immediately following the incisors, are called cuspidate, or canine teeth, from their being particularly conspicuous in dogs ; as they are, indeed, in all the purely carnivorous tribes. In the larger beasts of prey, as the Lion and the Tiger, they become most powerful wea- pons of destruction : in the Boar they are likewise of great size, and constitute the tusks of the ani- mal. All the teeth that are placed farther back in the jaw are designated by the general name of molar teeth, or grinders, but it is a class va hich in- cludes several different forms of teeth. Those teeth which are situated next to the canine teeth, partake of the conical form, having pointed eminences : * Those teeth of the lower jaw wliich correspond with the incisors of the upper jaw, are also considered as incisors. In Man, and in the species of Quadrumana that most nearly resemble him, the su- tures which divide the intermaxillary from the maxillary bones are obliterated before birth, and leave in the adult no trace of their former existence. TEETH AND JAWS OF MAMMALIA. 1 2D these are called the false molar teeth, and also, from their having generally two points, or cusps, the hicKspidale 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 frugivorous animals, their sur- face presents prominent tubercles, either pointed or rounded, for pounding the food ; while in quadru- peds 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 is likewise varied according to the particular move- ments required to act upon the food in the different tribes. The articulation of the lower jaw with the temporal bone of the skull approaches to a hinge joint; but considerable latitude is allowed to its motions by the interposition of a moveable carti- lage between the two surfaces of articulation, a contrivance admirably answering the intended purpose. Hence, in addition to the principal move- ments 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 and forwards to a certain extent. The muscles which effect the closing of the jaw are principally 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 internally, between the sides of the jaw and the basis of the skull. VOL. II. K 130 THE VITAL FUNCTIONS. In the conformation of the teeth and jaws, a re- markable contrast is presented between carnivorous and herbivorous animals. In the former, of which the Tiger, Fig. 274, may be taken as an example, the whole apparatus for mastication is calculated for the destruction of life, and for tearing and di- viding the fleshy fibres. The molar teeth are armed with pointed eminences, 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 enor- mous 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 diffi- culty 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 * 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 HEKBIVORA. lol large and 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 herbivorous animals, on the contrary, as may be seen in the skull of the Antelope, Fig. 27-'!), 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 comparatively small and feeble; the condyles of the jaw are broad and rounded, and more loosely connected with the skull by ligaments ; the muscles in the in- terior of the jaw, which move it from side to side, are very strong and thick ; and the bone itself is extended downwards, so to 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, by a provision which will be hereafter explained, kept rough, like those of mill- stones ; their office being in fact very similar to 132 THE VITAL FUNCTIONS. that performed by these implements for grinding. All these circumstances of difference are exempli- fied in the most marked manner, in comparing together the skulls of the larger beasts of prey, as the tiger, the wolf, or the bear, witli those of the antelope, the horse, or the ox. The Hodentia, 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 mate- rials, from which but little nutriment can be ex- tracted ; such as the bark, and roots, and even the woody fibres of trees, and the harder animal tex- tures, which would appear to be most difficult of digestion. They are all animals of diminutive size, whose teeth are express- ly formed for gnawing, nibbling, and wearing away by continued at- trition, the harder tex- tures of organized bodies. The Rat, whose skull is delineated in Fig. 276, belongs to this tribe. They are all furnished with two incisor teeth in each jaw, generally very long, and having the exact shape of a chisel ; and the molar teeth have surfaces, irre- gularly marked with raised zig-zag lines, rendering them very perfect instruments 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 particularly adapted, and by which the grinding operation is performed. The Seaver, TEETH OF QUADRAMANA. ^ ^^^ the Rat, the 3Iarmot, and the Porcupine, present examples of this structure, among the omnivorous rodentia : and the Hare, the Rabbit, and the Squirrel, among those which are principally herbivorous. The Quadrumana, or Monkey tribes, approach nearest to the human structure in the conformation j 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 grada- tions 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 Hycena are formed more especially for break- ing down bones, and in so doing exert prodigious force ; and those of the Sea Otter have rounded eminences, which peculiarly fit them for breaking sheels. The teeth, though composed of the same che- mical 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 peculiar office. The sub- stances of which they are composed 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 some peculiarities in the mode of its for- mation, the name of ivory has been generally given to it.* Its earthy ingredient consists almost en- * It has also been termed dentine. Owen's Odontography, p. 9. 134 THE VITAL FUNCTIONS. tirely of phosphate of lime ; the proportion of the carbonate of that earth entering into its composition being very small ; and the animal portion is albu- men, 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 sub- stances, and the apparent 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, con- taining no albumen, and scarcely a trace of gelatin : it is the hardest of all animal substances, and is capable of striking fire with steel. It exhibits a fibrous stnicture, approaching to a crystalline arrangement ; and the direction of its fibres, as shown by the form of its fragments when broken, is every where perpendicular 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 sub- stances upon which the teeth are made to act; and STRUCTURE OF TEETH. 135 the effect of that friction in wearing the enamel is thus rendered the least possible. By the skilful application of the microscope, it has recently been discovered that the ivory is com- posed of extremely minute tubular fibres, passing through a transparent and apparently homogeneous, but in reality cellular, substance. The fibres are closely compacted together, and disposed in radi- ating, and slightly undulating lines, having direc- tions perpendicular, in every part, to the surface of the tooth.* The deceptive appearance of con- centric layers, parallel to the surface, which is represented at o, in Fig. 277, is produced by the different refractions of light accompanying the undulations of the fibres. Each of the main tubes, in proceeding outwards, generally divides itself into two branches, each of which is itself again divided into two others, and so on successively. The interior of the tubes is filled with calcareous granules, and they have hence been termed by Mr. Owen calcigerous tubes. In the teeth of some quadrupeds, as of the Rhi- noceros, the Hippopotamus, and most of the Ro- dentia, the enamel is intermixed with the ivory; and the two so disposed as to form jointly the sur- face 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 the grinding * Leuwenhoeck liad long ago observed that these fibres were tubes. (Phil. Trans. 1678, p. 1002.) But the form and direction of the fibres were accurately determined by Purkinje : and their tubular nature by MuUer. The cells of the intertubular substance were detected by Retzius. 136 THE VITAL FUNCTIONS. surface; preserving it always in that rough con- dition, which best adapts it for the bruising and comminuting of hard substances. The incisors of tlie rodentia are guarded by a plate of enamel on their anterior convex surfaces only ; so that by the wearing down of the ivory behind this plate, a wedge-like form, of which the enamel constitutes the tine 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 con- tinue to grow only during a limited period. The same arrangement, attended with similar advan- tages, is adopted in the structure of the tusks of the Hippopota mns. In teeth of a more complex structure, a third substance is found, uniting the vertical plates of ivory and enamel, and performing the ofhce of an external cement. This substance has received various names, but it is most commonly known by that of tlie cement, or Crusta petrosa. It resembles ivory both in its composition, its extreme hardness, and in the circumstance of its intimate structure being composed of minute tubular fibres; but it is generally more opaque and yellow than that sub- stance ; and inasmuch as it contains cells disposed in radiating lines,* it approaches more nearly in its structure to true bone. In the pharyngeal teeth of the Scams, or parrot- * These radiated cells have been described by Purkinje as being characteristic of true bone. STRUCTURE OF TEETH. 137 fish, a fourth substance is added to the structure by the coarser ossification of the pulp, after its periphral portion has been converted into the dense ivory. The teeth, thus consisting of dentine, enamel, cement, and coarse bone, are the most complicated as regards their substance, that have yet been discovered.* Other herbivorous quadrupeds, as the horse, and animals belonging to the ruminant tribe, have also complex teeth composed of the three first of these 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 sur- face 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 pelrosa (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 in- wardly in the upper jaw ; thus providing for the crossing of the ridges of the two surfaces ; an arrangement similar to that which is practised in constructing 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 * Owen. Ibid. p. 9. 138 THE VITAL FUNCTIONS. their surfaces opposite to each other alternately on both sides. Fig. 279 shows the grinding surface of the tooth of a Sheep, 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 of the grinding tooth of the elephant, in which they are more completely and equally intermixed 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 structure 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. DENTITION. 139 The same tooth in its usual state, as worn by mas- tication, gives us a natural and 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), having the ivory on their interior, and the yellow crusta petrosa on their outer sides ; which latter substance also composes the whole circum- ference 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 constructed ; and being commenced at a very early period, they aiford a signal instance of Nature's provident anti- cipation of the future necessities of the animal. The teeth, being the hardest parts of the body, require a peculiar system of operations for giving them this extraordinary density, which no gradual consolidation could have imparted. The formation of the teeth is in some respects analogous to that of shell ; inasmuch as all their parts, when once depo- sited, remain as permanent structures, hardly ever admitting of removal or of renewal by the vital powers. Unlike the bones, which contain within their solid substance vessels of different kinds, by which they are nourished, modified, and occasion- ally removed, the closeness of the texture of the 140 THE VITAL FUNCTIONS. teeth is such as scarcely to admit of any passages for nutrient fluids.* 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 foundations of the teeth, in the young animal, are laid at a very early period of its evolution ; and considerable progress 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 in the interior of its cells the calcareous substance, which is to compose the ivory, at the most prominent points of that part of the vesicle, which corresponds in situation to the outer layer of the crown of the * Many anatomists have altogether denied the existence of any such channels; while others allow that they exist, but to a very limited extent. So gradual, indeed, as Mr. Owen observes, are the changes from one modification of dental structure to another, from bone to the densest ivory, that the physiologist who believes the teeth to be inorganic, would be at a loss to draw the line, and to determine where the vital forces cease to m.anifest themselves, and at which step in the series of tubular structures the tooth becomes an inert body. Ibid, p. 13, It will be seen from the account given in the text that I have adopted the views of Mr. Owen relative to the formation of the teeth. See Comptes Rendus, Dec. 1839, p. 784. DENTITION. 141 tooth. The thin scales of ivory thus formed in- crease by further depositions made in the cells at the surface of 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 imme- diate contact with this layer, secretes the substance that is to compose the enamel, and deposits it in the cells contiguous to the ivory. This double operation proceeds step by step ; fresh layers of ivory being formed, and thus building up the body of the tooth, into which the outer portion of the pulp is converted ; and only the remaining portion fills up the small cavity which remains in the centre of the tooth. The ivory has by this time received from the capsule a complete coating of enamel, which constitutes the wdiole outer surface of the crown ; after which no more calcareous matter is deposited ; and the function of the capsule having ceased, it shrivels and disappears. But the for- mation of ivory still continuing at the part most remote from the crown, the fangs are gradually 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 absorbed, 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 and formation is beautifully illustrated by feeding a young animal at difterent times with madder ; the teeth which are formed at that period exhibiting, in consequence, alternate layers of red and of white ivory.* * Cuvier. Dictionnaire des Sciences Medicales, t. viii. p. 320. 142 THE VITAL FUNCTIONS. The formation of the teeth of herbivorous qua- drupeds, which have three kinds of substance, is conducted in a still more artificial and complicated manner. Thus in the Elephant, the pulp which constructs 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 membrane in the intervals between the plates. Hence the ivory constructed by the pulp, and the enamel formed over it, are variously intermixed ; but besides this, the crusta petrosa is formed by a similar process on the out- side of the enamel. Cuvier asserts that it is con- structed 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 per- formed by a third membrane, w holly 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 the waved appearance, due to the undulating course of the fibres, which is 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 described, 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 the successive formation of layers, having a conical shape, from deposits of DENTITION. 143 calcareous phosphate in the cells of the pulp which occupies the axis of the growing tusk. 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 unossified portion of the pulp, becoming smaller in size, as the osseous part of the tusk increases in its dimensions, occupies only its lower and central part. 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 en- larges, and the teeth it contains admit not of any corresponding increase, it becomes necessary 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 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 gradu- ally acquire 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 * It is stated by Rousseau that the shedding- of the first molar tooth both of the Guinea-piy , and the Capibara, and its replace- ment by the permanent tooth, take place a few days before birth. (Anatomie Comparee du Systeme Dentaire, p. 164.) 144 THE VITAL FUNCTIONS. 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-teetli ; because they do not usually make their appearance above the gum till the person has attained the age of twenty. In the Negro, how- ever, where the jaw is of greater length, these teeth have sufficient room to come into their places, and are, in general, fully as large as the other molares. The teeth of carnivorous animals are, from 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 preserve 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 remark- able 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 succession of teeth, each of which is larger than that which has preceded it, during the whole period that the animal lives. A similar succession of teeth takes pbice in the Wild Boar, and also, though to DENTITION. 145 a less extent, in the Sus JEthiopicns* This mode of dentition appears to be peculiar to animals of great longevity, and which subsist on vegetable substances containing a large proportion 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 con- tained at the same time in the alveolar portion of the jaw. An expedient of a different kind has been re- sorted to in the Roclentia, for the purpose of pre- serving the long chisel-shaped incisors in a state fit for use. 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 consequence, 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 des- cribed; but in order to effect this object, the roots of these teeth are of great size and length, and are deeply imbedded in the jaw, in a large bony socket provided for that purpose ; and their cavity is always filled with the vascular pulp, within which a continued secretion and deposition of the materials both of ivory and enamel, take place. The tusks of the Elephant and of the Hippopotamus exhibit the same phenomenon of constant and uninter- rupted growth. In the Shark, and some other fishes, the same object is attained in a different manner. Several * Home, Phil. Trans, for 1799, p. 237 ; and 1801, p. 319, VOL. 11. L 146 THE VITAL FUNCTIONS. 284 rows of teeth are lodged in each jaw ; but only one of these rows projects and is in use at the same time ; the rest lying flat, but ready to rise in order to replace those which have been broken or worn down. In some fishes the teeth advance in propor- tion as the jaw lengthens, and as the fore teeth are worn away : in other cases they rise from the sub- stance of the jaw, which presents on its surface an assemblage of teeth in 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 ena- mel, 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 in- closed 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 conical form ; so that, when the latter is shed, it is already in its place, and fit for immediate use. This succession 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 with a recep- tacle at their base for a poisonous liquor, which is squeezed out by 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 FANGS OF SERPENTS. 147 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 w hen wanted : for which purpose there are com- monly provided two or three half-grown fangs, which are connected only by soft parts with tlie jaw, and are successively moved forwards into the socket to replace those that were lost.* The tube through w^hich the poison flows is formed by the folding in of the edges of a deep lon- gitudinal 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 struc- ture was discovered by Mr. T. Smith in the Coluber 7iaia^ or Cobra de Capcllo ;t and is shown in Fig. 28.3, 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 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, exhibiting 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 * Home, Lectures, &c. I. 333. t Philosopliical Transactions, IRIS, p. 471, 148 THE VITAL FUNCTIONS. representations of sections of the fangs of another species of serpent, resembling the rattle-snake. 288 290 292 291 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 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 per- ceptible 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 entirely 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 ve- nomous fangs. GASTRIC TEETH. 149 Among the many instances in which teeth are converted to uses widely different from mastication, may be noticed that of the Squalls pristis, or Saw- fish, where the teeth are set horizontally on the two lateral edges of the upper jaw, which is prolonged in the form of a snout (seen in a, Fig. 293), obviously - is the right or systemic auricle ; e the right or pulmonary ventricle ; f the pulmonary artery ; k the left or pulmonary auricle ; l the left or sys- temic ventricle ; and a the aorta. A similar separation of the ventricles though not to the same extent, is observed in the apex of the heart of the Manati, (Trichechus ma- natus.) WARM-BLOODED CIRCULATION. 253 and left cavities are kept perfectly distinct from one another, and are separated by thick partitions, allowing of no direct transmission of fluid from the one side to the other. These two hearts may there- fore be compared to two sets of chambers under the same roof; having each their respective entrances and exits, with a party- wall of separation between them. This junction of the two hearts is conducive to their mutual strength ; for the fibres of each in- termix and even co-operate in their actions, and both circulations are carried on at the same time ; that is, both ventricles contract or close at the same instant; and the same applies to the auricles. The blood which has just returned from the body, and that from the lungs, the former by the vense cavae, the latter by the pulmonary veins, fill their respec- tive auricles at the same instant ; and both auricles, contracting at the same moment, discharge their contents simultaneously into their respective ven- tricles. In like manner, at the moment when the left ventricle is propelling its aerated blood into the aorta, for the purposes of general nutrition, the right ventricle is likewise driving the vitiated blood into the pulmonary artery, in order that it may be purified by the influence of the air. Thus the same blood which, during the interval of one pulsation, was circulating through the lungs, is, in the next, circulating through the body ; and thus do the contractions of the veins, auricles, ventricles, and arteries all concur in the same general end, and establish the most beautiful and perfect harmony of action. Evidence is afforded of the human conformation being expressly adapted to the erect position of the body by the position of the heart, as compared 254 THE VITAL FUNCTIONS. with quadrupeds ; for in the latter the heart is placed directly in the middle of the chest, with the point towards the abdomen, and not occupying any portion of the diaphragm ; but in man, the heart lies obliquely on the diaphragm, with the apex turned towards the left side. The communications of the capillary arteries with the veins are beautifully seen, under the micro- scope, in the transparent membranes of frogs or fishes. The splendid spectacle, thus brought within the cognizance of our senses, of unceasing activity in the minutest filaments of the animal frame, and of the rapid transit of streams of fluid, bearing along with them minute particles, which appear to be pressing forwards, like the passengers in the streets of a crowded city, through multitudes of narrow and winding passages, can never fail, when first beheld, to fill the mind with astonishment ;* a feeling, which must be exalted to the highest ad- miration on reflecting that what we there behold is at all times going on within ais, during the whole period of our lives, in every, even the minutest por- tion of our frame. How inadequate, then, must be any ideas we are capable of forming of the incal- culable number of movements and of actions, which are conducted in the living system ; and how infi- nite must be the prescience and the wisdom, by * Leuwenhoeck, speaking of the delight he experienced on viewing the circulation of the blood in tadpoles, uses the following ex- pressions. " This pleasure has oftentimes been so recreating to me, that I do not believe that all the pleasure of fountains, or water- works, either natural or made by art, could have pleased my sight so well, as the view of these creatures has given me." — Phil. Trans. xxii. 453. DISTRIBUTION OF BLOOD-VESSELS. 255 which these multifarious and complicated opera- tions were so deeply planned, and so harmoniously adjusted ! <§ 4. Distribution of Blood-vessels. In the distribution of the arteries in the animal system, we meet with numberless proofs of wise and provident arrangement. The great trunks of both arteries and veins, which carry on the circu- lation in the limbs, are conducted always on the interior sides, and along the interior angles of the joints, and generally seek the protection of the adjacent bones. Grooves are formed in many of the bones, where arteries are lodged, with the evident intention of affording them a more secure passage. Thus the principal arteries which supply the mus- cles of the chest, proceed along the lower edges of the ribs, in deep furrows formed for their protection. Arteries are often still more effectually guarded against injury or obstruction by passing through complete tubes of solid bone. An instance occurs in the arteries supplying the teeth, which pass along a channel in the lower jaw, excavated through the whole length of the bone. The aorta in fishes, after having supplied arteries to the vis- cera of the abdomen, is continued to the tail, and passes through a channel, formed by bony pro- cesses from the vertebrae ; and the same kind of protection is afforded to the corresponding artery in the Cetacea. In the fore leg of the Lion, which is employed in actions of prodigious strength, the artery, without some especial provision, would have 256 THE VITAL FUNCTIONS. been in danger of being compressed by the violent contractions of the muscles : to guard against this inconvenience, it is made to pass through a per- foration in the bone itself, where it is completely secure from pressure.* In like manner the coffin bone of the Horse is perforated for the safe con- veyance of the arteries going to the foot. The energy of every function is regulated in a great measure by the quantity of blood which the or- gans exercising that function receive. The muscles employed in the most vigorous actions are always found to receive the largest share of blood. It is commonly observed that the right fore leg of qua- drupeds, as well as the right arm in man, is stronger than the left. Much of this superior strength is, no doubt, the result of education ; the right arm being habitually more used than the left. Bnt still the different mode in which the arteries are distri- buted to the two arms constitutes a natural source of inequality. The artery supplying the right arm with blood is conjoined at its origin with the artery conveying blood to that side of the head, and pro- ceeds in a more direct course from the heart than the artery of the left arm, which arises separately, and at a greater distance from the heart than even the artery going to the left side of the head. Hence it has been inferred that the right arm is originally better supplied with nourishment than the left. It may be alleged, in confirmation of this view, that * The brachial artery passes, in hke manner, through a foramen in the inner condyle of the humerus, in many species of Quadru- mana, Carnivora and Rodentia ; this protection appearing to have been given to those tribes more especially, which are endowed with great freedom of action in the fore limbs. DISTRIBUTION OF BLOOD-VESSELS. 257 in birds, where any inequality in the actions of the two wings would have disturbed the regularity of flight, the aorta, when it has arrived at the centre of the chest, divides with perfect equality into two branches, so that both wings receive precisely the same quantity of blood ; and the muscles, being thus equally nourished, preserve that equality of strength, which their function rigidly demands. Wlien a large quantity of blood is wanted in any particular organ, and yet the force with which it would arrive, if sent immediately by large arteries, might injure the texture of that organ, contrivances are adopted for diminishing its impetus, either by making the arteries pursue very winding and cir- cuitous paths, or by subdividing them, before they reach their destination, into a great number of smaller arteries. The delicate texture of the brain, for instance, would be greatly injured by the blood being impelled with any considerable force against the sides of the vessels which are distributed to it ; and yet a very large supply of blood is required by that organ for the due performance of its functions. Accordingly we find that all the arteries which go to the brain are very tortuous in their course ; every flexure tending considerably to diminish the force of the current of blood. In animals that graze, and keep their heads for a long time in a dependent position, the danger from an excessive impetus in the blood flowing towards the head is much greater than in other animals; and we find that an extraordinary pro- vision is made to obviate this danger. The arteries which supply the brain, on their entrance into the basis of the skull, suddenly divide into a great VOL. II. s 258 THE VITAL FUNCTIONS. number of minute branches, forming a compli- cated network of vessels ; an arrangement which, on the well known principles of hydraulics, must greatly check the velocity of the blood conducted through them. That such is the real purpose of this structure, which has been called the rete mirabile, is evident from the branches afterwards uniting into larger trunks when they have entered the brain, through the substance of which they are then dis- tributed exactly as in other animals, where no such previous subdivision takes place.* In the Sradypus tridactylus, or great American Sloth, an animal remarkable for the slowness of its movements, a plan somewhat analogous to the former is adopted in the structure of the arteries of the limbs. These arteries, at their entrance into both the upper and lower extremities, suddenly divide into a great number of cylindric vessels of equal size, communicating in various places by col- lateral branches. These curiously subdivided ar- teries are exclusively distributed to the muscles of the limbs ; for all the other arteries of the body branch off in the usual manner. This structure, which was discovered by Sir A. Carlisle,t is not confined to the Sloth, but is met with in other ani- mals, as the Lemur tardigradus, and the Lemur * The rete mirabile is particularly large in the ruminant tribe, and in the hog. It is much developed in the cat, but scarcely percep- tible in the dog. It does not exist in plantigrade and rodent animals, t Phil. Trans, for 1800, p. 98, and for 1804, p. 17. Professor Baer has discovered that in the Manati, (Trichechus manatus) and in the Porpus, (Delphinus phocsena) the axillary artery forms a similar plexus, which is continued down the limb. Mem. Imp. Acad. Sc. of St. Petersburg, XI. 204. DISTRIBUTION OF BLOOD-VESSELS. 259 loris, which resemble the sloth in the extreme slug- gishness of their movements. It is extremely pro- bable, therefore, that this peculiarity in the mus- cular power results from this remarkable structure in the arteries ; or is at least in some way connected with it. In the Lion, and some other beasts of prey, a similar construction is adopted in the arteries of the head ; probably with a view to confer a power of more permanent contraction in the muscles of the jaws for holding a strong animal, such as a buffalo, and carrying it to a distance. In the Cetacea a remarkable provision has been made for affording to the system a sufficient supply of arterial blood during the occasional suspension of respiration which occurs while they dive into the deep recesses of the ocean. The intercostal arteries, which are sent off from the aorta to the spaces be- tween the ribs on each side, are, in these animals, enormously dilated, and contorted into thousands of convolutions, united by a very elastic cellular tissue, and form an accumulation (or plexus) of vessels, of great bulk, which extends along both sides of the vertebral column, and back of the chest. These vessels are thus capable of retaining, as in a reservoir, a very large quantity of blood which has been arterialised, and which may be available, for a certain time, to the purposes of the circulation, during the temporary privation of atmospheric air.* * This curious structure was first noticed in the whale, by Hunter, (Phil. Trans, for 1787); and has since been minutely described in other cetacea by Breschet. (Histoire Anatomique et Physiologique d'uix organe de nature vasculaire decouvert dans les Cetaces, &c., Paris, 1838.) 260 THE VITAL FUNCTIONS. That we may form an adequate conception of the immense power of the ventricle, or prime mover in the circulation of the blood, we have but to reflect on the numerous obstacles impeding its passage through the arterial system. There is, first, the natural elasticity of the coats of the arteries, which must be overcome before any blood can enter them. Secondly, the arteries are, in most places, so con- nected with many heavy parts of the body, that their dilatation cannot be effected without, at the same time, communicating motion to them. Thus, when we sit cross-legged, the pulsation of the artery in the ham, which is pressed upon the knee of the other leg, is sufficiently strong to raise the whole leg and foot, at each beat of the pulse. If we con- sider the great weight of the leg, and reflect upon the length of the lever by which that weight acts, we shall be convinced of the prodigious force which is actually exerted by the current of blood in the artery in thus raising the whole limb. Thirdly, the winding course, which the blood is forced to take, in following all the oblique and serpentine flexures of the arteries, must greatly impede its motion. But notwithstanding these numerous and powerful impediments, the force of the heart is so irreat, that, in defiance of all obstacles or causes of retardation, it drives the blood with immense velo- city into the aorta. The ventricle of the human heart does not contain more than an ounce of blood, and it contracts at least seventy times in a minute; so that above three hundred pounds of blood are passing through this organ during every hour that we live. " Consider," says Paley, " what an affair FORCE OF THE HEART. '2(31 this is when we come to very large animals. The aorta of a whale is larger in the bore than the main pipe of the water-works at London Bridge ; and the water roaring in its passage through that pipe is in- ferior in impetus and velocity to the blood gushing through the whale's heart. An anatomist who understood the structure of the heart, might say beforehand that it would play ; but he would ex- pect, from the complexity of its mechanism, and the delicacy of many of its parts, that it should always be liable to derangement, or that it would soon work itself out. Yet shall this wonderful machine go on, night and day, for eighty years together, at the rate of a hundred thousand strokes every twenty-four hours, having at every stroke a great resistance to overcome, and shall continue this action, for this length of time, without disorder and without weari- ness. To those who venture their lives in a ship, it has often been said that there is only a plank be- tween them and destruction ; but in the body, and especially in the arterial system, there is in many parts only a membrane, a skin, a thread." Yet how well has every part been guarded from injury: how providentially have accidents been anticipated : how skilfully has danger been averted ! The impulse which the heart, by its powerful contraction, gives to the blood, is nearly expended by the time it has reached the veins : nature has accordingly furnished them with numerous valves, all opening in a direction towards the heart ; so that, as long as the blood proceeds in its natural course, it meets with no impediment ; while a retro- grade motion is effectually prevented. Hence ex- 262 THE VITAL lUNCTIONS. ternal pressure, occasionally applied to the veins, assists in promoting the flow of blood towards the 355 heart ; and hence the effects of exer- cise in accelerating the circulation. Valves are more especially provided in the veins which pass over the muscles of the extremities, or which run immediately beneath the skin ; while they are not found in the more internal veins belonging to the viscera, which are less exposed to unequal pressure. These valves are delineated in Fig. 365, which represents the interior of one of the large veins. The situation and structure of the valves be- longing to the hydraulic apparatus of the circulation furnish as unequivocal proofs of design as any that can be adduced. It vras the observation of these valves that first suggested to the mind of Harvey the train of reflexions which led him to the dis- covery of the real course of the blood in the veins, the arteries, and the heart. This great discovery was one of the earliest fruits of the active and rational spirit of inquiry, which at the era of Bacon's writings, was beginning to awaken the human mind from its long night of slumber, and to dissipate the darkness which had, for so many ages, overshadowed the regions of philosophy and science. We cannot but feel a pride, as English- men, in the recollection, that a discovery of such vast importance as that of the circulation of the blood, which has led to nearly all the modern im- provements in the medical art, was made by our own countryman, whose name will for ever live in RESPIRATION. 203 the annals of our race as one of its most distin- guished benefactors. The consideration, also, that it had its source in the study of comparative ana- tomy and physiology, affords us a convincing proof of the great advantages that may result from the cultivation of these sciences ; to which Nature, indeed, seems, in this instance, expressly to have invited us, by displaying to our view, in the organs of the circulation, an endless diversity of combina- tions, as if she had purposely designed to elucidate their relations with the vital powers, and to assist our investigations of the laws of organized beings. Chapter XI. RESPIRATION. § 1. Respiration in General. The action of atmospheric air is equally necessary for the maintenance of animal and vegetable life. As the ascending sap of plants cannot be perfected unless exposed to the chemical agency of air in the leaves; in like manner the blood of animals requires the perpetual renovation of its vital pro- perties by the purifying influence of respiration. The great importance of this function is evinced by the constant provision which has been made by Nature, in every class of animals, for bringing each portion of their nutritive juices, in its turn, into contact with air. Even the circulation of these juices is an object of inferior importance, compared 264 THE VITAL FUNCTIONS. with their aeration ; for we find that insects, which have but an imperfect and partial circulation of their blood, still require the free introduction of air into every part of their system. The necessity for air is more urgent than the demand for food ; many animals being capable of subsisting for a consider- able time without nourishment, but all speedily perishing when deprived of air. The influence of this element is requisite as well for the production and developement, as for the continuance of or- ganized beings in a living state. No vegetable seed will germinate, nor will any egg, even of the smallest insect, give birth to a larva, if kept in a perfect vacuum. Experiments on this subject have been varied and multiplied without end by Spal- lanzani, who found that insects under an air pump, confined in rarefied air, in general lived for shorter periods in proportion to the degree to which the exhaustion of air had been carried. Those species of infusoria, which are most tenacious of life, lived in very rarefied air for above a month : others perished in fourteen, eleven, or eight days; and some in two days only. In this imperfect vacuum, they were seen still to continue their accustomed evolutions, wheeling in circles, darting to the sur- face, or diving to the bottom of the fluid, and pro- ducing vortices by the rapid vibration of their cilia, to catch the floating particles which serve as their food : in course of time, however, they invariably gave indications of uneasiness ; their movements became languid, a general relaxation ensued, and they at length expired. But when the vacuum was rendered perfect, none of the infusions of animal or vegetable substances, which, under ordinary cir- RESPIRATION. 265 cn instances, soon swarm with millions of these microscopic beings, ever exhibited a single animal- cule ; although they soon made their appearance in great numbers, if the smallest quantity of air was admitted into the receiver. Animals which inhabit the waters, and remain constantly under its surface, such as fishes, and the greater number of mollusca, are necessarily pre- cluded from receiving the direct action of atmos- pheric air in its gaseous state. But as all water exposed to the air soon absorbs it in large quan- tities, it becomes the medium by which that agent is applied to the respiratory organs of aquatic ani- mals ; and the oxygen it contains may thus act upon the blood with considerable effect; though not, perhaps, to the same extent as when directly applied in a gaseous state. The air which is pre- sent in water is, accordingly, as necessary to these animals as the air of the atmosphere is to those which live on land : hence in our inquiries into the respiration of aquatic animals, it will be sufficient to trace the means by which the surrounding water is allowed to have access to the organs appropriated to this function ; and in speaking of the action of the water upon them, it will always be understood that I refer to the action of the atmospheric air which that water contains. Respiration, in its different modes, may be dis- tinguished, according to the nature of the medium which is breathed, into aquatic or atmospheric; and in the former case, it is either cutaneous, or branchial, according as the respiratory organs are external or internal. Atmospheric respiration, again, is either tracheal, or pulmonary, according <*>. 266 THE VITAL FUNCTIONS. as the air is received by a system of air tubes, denominated trachece, or into pulmonary cavities, composing the lungs. § 2. Aquatic Respiration. Zoophytes appear m general to be unprovided with any distinct channels for conveying aerated water into the mterior of their bodies, so that it may act in succession on the nutritive juices, and after performing this office, may be expelled, and ex- changed for a fresh supply. It has accordingly been conjectured, on the presumption that this function is equally necessary to them as it is to all other animals, that the vivifying influence of the surrounding element is exerted through the medium of the surface of the body. Thus it is very possible that in Polypi, while the interior surface of the sac digests the food, its external surface may perform the office of respiration ; and no other mode of accomplishing this function has been distinctly traced in the AcalephcB. Medusae, indeed, appear to have a further object than mere progression in the alternate expansions and contractions of the floating edges of their hemispherical bodies ; for these movements are performed with great regu- larity under all circumstances of rest or motion ; and they continue even when the animal is taken out of the water and laid on the ground, as long as it retains its vitality. The specific name of the Medusa pulmo* (the Puhnone Marino of the Ita- * See the delineation of this animal in Fii;. 81, vol. i. p. 175. AQUATIC RESPIRATION. 267 lians), is derived from the supposed resemblance of these movements to those of the lungs of breath- ing animals. The large cavities adjacent to the stomach, and which have been already pointed out in Fig. 249 and 252,* have been conjectured to be respiratory organs, chiefly, I believe, because they are not known to serve any other purpose. The Entozoa, in like manner, present no appear- ance of internal respiratory organs ; so that they probably receive the influence of oxygen only through the medium of the juices of the animals on which they subsist. Planaricu, which have a more independent existence, though endowed with a system of circulating vessels, have no internal respiratory organs ; and whatever respiration they perform must be wholly cutaneous. Such is also the condition of several of the simpler kinds of Aniielida; but in those which are more highly organized, an apparatus is provided for respiration, which is wholly external to the body, and appears as an appendage to it ; consisting generally of tufts of projecting fibres, branching like a plume of feathers, and floating in the surrounding fluid. The Lumbricus marinus, or lob-worm, ^ for example, has two rows of branchial organs of this description, one on each side of the body ; each row being com- posed of from fourteen to sixteen tufts. In the more stationary Annelida, which inhabit calcareous tubes, as the Serpula and the TereheUa, these arborescent tufts are protected by a sheath, which envelopes their roots ; and they are placed on the * Pages 78 and 79 of this volume. t Arenicola piscatorum (Lam.) See a delineation of this marine worm in Fig. 135, vol. i. p. 247. 208 THE VITAL FUNCTIONS. ^ head, as being the only part which comes in contact with the water. Most of the smaller Crustacea have branchiae in the form of feathery tufts, attached to the paddles near the tail, and kept in incessant vibratory mo- tion, which gives an appearance of great liveliness to the animal, and is more especially striking in the microscopic species. The variety of shapes which these organs assume in different tribes is too great to allow of any specific description of them in this place: but amidst these varieties it is sufficiently apparent that their construction has been in all cases designed to obtain a considerable extent of surface over which the minute subdivi- sions of the blood-vessels might be spread, in order to expose them fully to the action of aerated water. The Mollusca, also, present great diversity in the forms of their respiratory organs, although they are all, with but a few exceptions, adapted to aquatic respiration. In many of the tribes which have no shell, as the Thetis, the Doris, and the Tritonia, there are arborescent gills projecting from diffe- rent parts of the body, and floating in the water. In the Lepas, or barnacle, a curious family, con- stituting a connecting link between molluscous and articulated animals, these organs are attached to the bases of the cirrhi, or jointed tentacula, which are kept in constant motion, in order to obtain the full action of the water on the blood- vessels they contain. We are next to consider the extensive series of aquatic animals in which respiration is carried on by organs situated in the interior of the body. The AQUATIC RESPIRATION. 269 first example of internal aquatic respiration occurs in the Holothuria, where there is an organ com- posed of ramified tubes, the stem of which commu- nicates with the expanded end of the intestine. This organ is situated in a cavity having an ex- ternal opening for the admission of the aerated water, which is brought to act on a vascular net- work spread over the tubes, containing the nutri- tive juices of the animal, and apparently perform- ing a partial circulation of these juices. A still more complicated system of respiratory channels occurs, both in the Echiims and Asterias, where they open by separate, but very minute orifices, distinct from the larger apertures through which the feet protrude ; and the water admitted through these tubes is allowed to permeate the general cavity of the body, and is thus brought into con- tact with all the organs. The animals composing the family of Ascidi^ have a large respiratory cavity, receiving the water from without, and having its sides lined with a membrane, which is thrown into a great number of folds ; thus considerably extending the surface on which the water is designed to act. The en- trance into the oesophagus, or true mouth, is si- tuated at the bottom of this cavity ; that is, at the part most remote from the external orifice ; so that all the food has to pass through the respiratory cavity, before it can be swallowed, and received into the stomach. In several of the Annelida, also, we find internal organs of respiration. The Lnmhricus terrestris, or common earth-worm, has a single row of apertures, about 120 in number, placed along the back, and 270 THE VITAL FUNCTIONS. opening between the segments of the body : they each lead into a respiratory vesicle, situated be- tween the integument and the intestine.* The Leech has sixteen minute orifices of this kind on each side of the body, opening internally into the same number of oval cells, which are respiratory cavities ; the water passing both in and out by the same orifices. -f The Aphrodila aculeata has thirty-two orifices on each side, placed in rows, opening into the abdo- minal cavity, and admitting the water, which is afterwards received into numerous pouches, con- taining cfEcal processes of the intestine ; so that the nutriment is aerated almost as soon as it is pre- pared by the digestive organs-l In all the* higher classes of aquatic animals, where the circulation is carried on by means of a muscular heart, and where the whole of the blood is subjected, during its circuit, to the action of the aerated water, the immediate organs of respira- tion consist of long, narrow filaments, in the form of a fringe ; and the blood-vessels belonging to the respiratory system are extensively distributed over the whole surface of these filaments. Organs of this description are denominated BranchicB, or Gills; and the arteries which bring the blood to them are * A minute description of these organs is given by Morren, in pages 53 and 148 of his work aheady quoted. + The blood, after being aerated in these cells, is conveyed into the large lateral vessels, by means of canals, which pass transversely, forming loops, situated between the cseca of the stomach. These loops are studded with an immense number of small rounded bodies of a glandular appearance, resembhng those which are appended to the venge cavse of the cephalopoda. X Home, Philos. Trans, for 1815, p. 259. AQUATIC RESPIRATION. 271 called the branchial arteries; the veins, which con- vey it back, being, of course, the branchial veins. The larger Crustacea have their branchiae situated on the under side of the body, not only in order to obtain protection from the carapace, which is folded over them, but also for the sake of being attached to the haunches of the feet-jaws, and thoracic feet; and thus participating in the movements of those organs. They may be seen in the Lobster, or in the Crab, by raising the lower edge of the carapace. The form of each branchial lamina is shown at g, in Fig. 354 :* they consist of assemblages of many thousands of minute filaments, proceeding from their respective stems, like the fibres of a feather ; and each group having a triangular, or pyramidal figure. The number of these pyramidal bodies varies in the different genera ; thus the Lobster has twenty-two, disposed in rows on each side of the body ; but in the Crab, there are only seven on each side. To these organs are attached short and flat paddles, which are moved by appropriate muscles, and are kept in incessant motion, pro- ducing strong currents of water, evidently for the purpose of obtaining the full action of that element on every portion of the surface of the branchiae. In the greater number of Mollusca, these impor- tant organs, although external with respect to the viscera, are within the shell, and are generally situated near its outer margin. They are composed of parallel filaments, arranged like the teeth of a fine comb ; and an opening exists in the mouth for admitting the water which is to act upon them.t * Page 237 of this volume. t These filaments have, in most instances, tlie power of producing 272 THE VITAL rUNCTIONS. In the Gasteropoda, or inhabitants of univalve shells, this opening is usually wide. In the Ace- phala, or bivalve inollusca, the gills are spread out, in the form of laminaj, round the margin of the shell ; as exemplified in the Oyster, where it is commonly known by the name of beard. The aerated water is admitted through a fissure in the mouth ; and when it has performed its office in respiration, is usually expelled by a separate open- ing. The part of the mouth through which the M ater is admitted to the branchiae is sometimes pro- longed ; forming a tube, open at the extremity, and at all times allowing free ingress and egress to the water, even when the animal has withdrawn its body wholly within its shell. Sometimes one, and sometimes two tubes of this kind are met with; and they are often protected by a tubular portion of shell, as is seen in the 3Iiirex, Buccimim, and Strom- bus ; in other instances, the situation of the tube is only marked by a deep notch in the edge of the shell. In those mollusca which burrow in the sand, this tube can be extended to a considerable length, so as to reach the water, which is alternately sucked in and ejected by the muscular action of the mouth. In those Acephala which are unprovided with any tube of this kind, the mechanism of respiration currents of water in their vicinity by the action of minute cilia, similar to those belonging to the tentacula of many polypi, where the same phenomenon is observable. Thus if one of the branchial filaments of the fresh water muscle be cut across, the detached por- tion will be seen to advance in the fluid by a spontaneous motion, like the tentaculum of a polype, under the same circumstances. Similar currents of water, according to the recent observations of Mr. Lister, and apparently determined by the same mechanism of vibratory cilia, take place in the branchial sac of Ascidiae: but it is doubtful whether they exist in the Cephalopoda. RESPIRATION IN FISHES. 273 consists simply in the opening and shutting of the shell. By watching them attentively we may per- ceive that the surrounding water is moved in an eddy by these actions, and that the current is kept up without interruption. All the Sepiae have their gills enclosed in two lateral cavities, which com- municate with a funnel-shaped opening in the middle of the neck, and alternately receiving and expelling the water by the muscular action of its sides. The forms assumed by the respiratory organs in this class are almost infinitely diversified, while the general design of their arrangement is still the same. As we rise in the scale of animals, the respiratory function assumes a higher importance. In Fishes the gills form large organs, and the continuance of their action is more essential to life than it appears to be in any of the inferior classes : they are situated, as is well known, on each side of the throat in the immediate vicinity of the heart. Their usual form is shown at g g. Fig. 366, where they are repre- VOL. II. T 274 THE VITAL FUNCTIONS. sented on one side only, but in their relative situa- tions with respect to the auricle (d), and ventricle (e), of the heart ; the bulbus arteriosus (b), and the branchial artery (f). They have the same fringed structure as in the moUusca, the fibres being set close to each other, like the barbs of a feather, or the teeth of a fine comb, and being attached, on each side of the throat, in double rows, to the con- vex margins of four cartilaginous or osseous arches, which are themselves connected with the jaws by the bone called the 06- hyoides. The mode of their articulation is such as to allow^ each arch to have a small motion forwards, by which they are separated from one another ; and by moving backwards they are again brought together, or collapsed. Each filament contains a slender plate of cartilage, giving it mechanical support, and enabling it to preserve its shape while moved by the streams of water, which are perpetually rushing past. When their surfaces are still more minutely examined, they are found to be covered with innumerable minute processes, crowded together like the pile of velvet ; and on these are distributed myriads of blood- vessels, spread, like a delicate network, over every part of the surface. The whole extent of this sur- face exposed to the action of the aerated water, by these thickly set filaments, must be exceedingly great.* A large flap, termed the Operculum^ extends over the whole organ, defending it from injury, and leaving below a wide fissure for the escape of the water, which has performed its office in respiration. * Dr. Monro computed that in the Skate, the surface of the gills is, at the least, equal to the whole surface of the human body. RESPIRATION IN FISHES. 275 For this purpose the water is taken in by the mouth, and forced by the muscles of the throat through tlie apertures which lead to the branchial cavities: in this action the branchial arches are brought forwards, and separated to a certain dis- tance from each other ; and the rush of water through them unfolds and separates each of the thousand minute filaments of the branchiae, so that they all receive the full action of that fluid as it passes by them. Such appears to be the principal mechanical object of the act of respiration in this class of animals; and it is an object that requires the co-operation of a liquid, such as water, capable of acting by its impulsive momentum in expanding every part of the apparatus on which the blood vessels are distributed. When a fish is taken out of the water, this effect can no longer be produced ; in vain the animal reiterates its utmost efforts to raise the branchiae, and relieve the sense of suffocation it experiences in consequence of the general collapse of the filaments of those organs, which adhere together in a mass, and can no longer receive the vivifying influence of oxygen.* * It has been generally stated by physiologists, even of the highest authority, such as Cuvier, that the principal reason why fishes cannot maintain life, when surrounded by air instead of water, is that the branchiae become dry, and lose the power of acting when thus deprived of their natural moisture ; for it might otherwise natu- rally be expected that the oxygen of atmospheric air would exert a more powerful action on the blood which circulates in the branchiae, than that of merely aerated water. The rectification of this error is due to Flourens, who pointed out the true cause of suffocation, stated in the text, in a Memoir entitled " Experiences sur le Me- chanisme de la Respiration des Poissons." — Annales des Sciences Naturelles, xx, 5. 276 THE VITAL FUNCTIONS. Death is, in like manner, the consequence of a ligature passed round the fish, and preventing the expansion of the branchiae and the motion of the opercida. In all osseous fishes the opening under the oper- culum for the exit of the respired water, is a simple fissure ; but in most of the cartilaginous tribes, there is no operculum, and the water escapes through a series of apertures in the side of the throat. Sharks have five oblong orifices of this description, as may be seen in Fig. 367.* As the Lamprey employs its mouth more con- stantly than other fish in laying hold of its prey, and adhering to other bodies, the organs of respira- tion are so constructed as to be independent of the mouth in receiving the water. There are seven external openings on each side (Fig. 368), leading into the same number of separate oval pouches, situated horizontally, and the inner membrane of which has the same structure as gills: these pouches are seen on a larger scale than in the preceding figure, in Fig. 369. There is also an equal number of internal openings, seen in the lower part of this last figure, leading into a tube, the lower end of which is closed, and the upper terminates by a fringed edge in the oesophagus. The water which is received by the seven lateral openings, enters at one side, and after it has acted upon the gills, passes round the projecting membranes. The greater part makes its exit by the same orifices ; but a portion escapes into the middle tube, and thence passes, * They are also visible in Fig. 293, (page 149), which is that of of the Squalls pristis, a species belonging to this tribe. RESPIRATION IN FISHES. 277 either into the other cavities, or into the oeso- phagus.* In the Myxine, which feeds upon the internal parts of its prey, ^.nd buries its head and part of its body in the flesh, the openings of the respiratory organs are removed sufficiently far from the head to admit of respiration going on while the animal is so employed ; and there are only two external openings, and six lateral pouches on each side, with tubes similar to those in the lamprey. The Perca scandens (DaldorfF),t which is a fish inhabiting the seas of India, has a very remarkable structure, adapting it to the maintenance of respi- ration, and consequently to the support of life for a considerable time when out of the water ; and hence it is said occasionally to travel on land to some distance from the coast. J The pharyngeal bones of this fish have a foliated and cellular struc- ture, which gives them a capacity for retaining a suflicient quantity of water, not only to keep the gills moist, but also to enable them to perform their proper office ; while not a particle of water is suf- fered to escape from them, by the opercula being accurately closed. The same faculty, resulting from a similar struc- ture, is possessed by the Op/iicephalus, which is also met with in the lakes and rivers of India and China. * It was commonly supposed that the respired water is ejected through the nostril ; but this is certainly a mistake, for the nostril has no communication with the mouth, as was pointed out by Sir E. Home. Phil. Trans, for 1815, p. 259. These organs have also been described by Bloch and Gsertner. t Anthias testudineus (Bloch) : Ayiabas (Cuv.) X This peculiar faculty has been already alluded to in volume i. p. 386. 278 THE VITAL FUNCTIONS. Eels are enabled to carry on respiration when out of water, for a certain period, in consequence of the narrowness of the aperture for the exit of the water from the branchial cavity, which enables it to be closed, and the water to be retained in that cavity.* 1 have already stated that in all aquatic animals, the water which is breathed is merely the vehicle by which the air it contains is brought into contact with the organs of respiration. This air is con- stantly vitiated by the respiration of these animals, and requires to be renewed by the absorption of a fresh portion, which can only take place when the water freely communicates with the atmosphere ; and if this renewal be by any means prevented, the water is no longer capable of sustaining life. Fishes are killed in a very few hours, if confined in a li- mited portion of w ater, w hich has no access to fresh air. When many fishes are enclosed in a narrow vessel, they all struggle for the uppermost place, (where the atmospheric air is first absorbed), like the unfortunate men imprisoned in the black-hole at Calcutta. When a small fish-pond is frozen over, the fishes soon perish, unless holes be broken in the ice, in order to admit air : they may be seen flock- ing towards these holes, in order to receive the benefit of the fresh air as it is absorbed by the water ; and so great is their eagerness on these occa- sions, that they often allow^ themselves to be caught by the hand. Water, from which all air has been * Dr. Hancock states that the Doras costatus, {Silurus costatus, Linn.) or Hassar, in very dry seasons, is sometimes seen, in great numbers, making- long marches over land, in search of water. Edin. Phil. Journal, xx. 396. RESPIRATION IN FISHES. 279 extracted, either by the air-pump, or by boiling, is to fishes what a vacuum is to a breathing terrestrial animal. Humboldt and Provencal made a series of experiments on the quantities of air which fishes require for their respiration. They found that river- water generally contains about one 36th of its bulk of air ; of which quantity, one-third consists of oxygen, being about one per cent, of the whole volume. A tench is able to breathe when the quan- tity of oxygen is reduced to the 5000th part of the bulk of the water ; but soon becomes exceedingly feeble by the privation of this necessary element. The fact, however, shows the admirable perfection of the organs of this fish, which can extract so minute a quantity of air from water to which that air adheres with great tenacity.* § 3. Atmospheric Respiration. The next series of structures which are to come under our review, comprehends all those adapted to the respiration of atmospheric air in its gaseous * The swimming bladder of fishes is regarded by many of the German naturalists as having some relations to the respiratory func- tion, and as being the rudiment of the pulmonary cavity of land animals; the passage of communication with the oesophagus being conceived to represent the trachea. The air contained in the swim- ming bladder of fishes has been examined by many chemists, but although it is generally found to be a mixture of oxygen and nitro- gen, the proportion in which these gases exist is observed to vary considerably. Biot concluded from his experiments, that in the air- bladder of fishes inhabiting the greatest depths of the ocean, the quantity of oxygen is greater, while in those of fishes which come often to the surface, the nitrogen is more abundant; and De la Roche came to the same conclusion from his researches on the fishes 280 THE VITAL FUNCTIONS. form ; and their physiology is no less diversified than that of the organs by which water is respired. Air may be respired by trachece, or by pulmonary cavities ; the first mode is exemplified in insects ; the second is that adopted in the larger terrestrial animals. The greater part of the blood of insects being diffused by transudation through every internal organ of their bodies, and a small portion only being enclosed in vessels, and circulating in them, the salutary influence of the air could not have been generally extended to that fluid by any of the ordinary modes of respiration, where the function is carried on in an organ of limited extent. As the blood could not be brouglit to the air, it became necessary that the air should be brought to the blood. For this purpose there has been provided, in all insects, a system of continuous and ramified vessels, called trackecE^ distributing, and, as it were, circulating air through every part of the body. The external orifices, from which these air tubes commence, are called spiracles, or stigmata, and are usually situated in rows on each side of the body, of the Mediterranean. From the experiments of Humboldt and Proven9al, on the other liand, we may conclude, that the quality of the air contained in the air-bladder is but remotely connected with respiration, (Memoires de la Societe d'Arcueil, ii, 359.) According to Ehrmann, the Cobitis, or Loche, occasionally swal- lows air, which is decomposed in the alimentary canal, and effects a change in the blood-vessels, with which it is brought into contact, exactly similar to that which occurs in ordinary respiration. It is also believed that in all fishes a partial aeration of the blood is the result of a similar action, taking place at the surface of the body under the scales of the integuments. Cuvier, sur les Poissons, I, 383. RESPIRATION IN INSECTS. 281 as is shown in Fig. 370, which represents the lower or abdominal surface of the Dytiscus marginalis. They are seen very distinctly in the caterpillar, which has generally ten on each side, correspond- ing to the number of abdominal segments. In many insects we find them guarded by bristles, or tufts of hair, and sometimes by valves, placed at the orifice, to prevent the entrance of extraneous bodies. The spiracles are opened and closed by muscles provided for that purpose. Fig. 371 is a magnified view of spiracles of this description, from the Cerambyx heros. (Fab.) They are the begin- nings of short tubes, which open into large trunks (as shown in Fig. 372), extending longitudinally on each side, and sending off radiating branches from the parts which are opposite to the spiracles ; and these branches are farther subdivided, in the same manner as the arteries of the larger animals, so that their minute ramifications pervade every organ in the body. This ramified distribution has 282 THE VITAL FUNCTIONS. frequently occasioned their being mistaken for blood vessels. In the wings of insects the nervures, which have the appearance of veins, are only large air- tubes. Jurine asserts that it is by forcing air into these tubes that the insect is enabled suddenly to expand the wings in preparing them for flight, giving them by this means greater buoyancy, as well as tension. The tracheee are kept continually pervious by a curious mechanism ; they are formed of three coats, the external and internal of which are membranous; but the middle coat is constructed of an elastic thread coiled into a helix, or cylindrical spiral (as seen in Fig. .372) ; and the elasticity of this thread keeps the tube constantly in a state of expansion, and therefore full of air. When examined under water, the tracheae have a shining silvery appear- ance, from the air they contain. This structure has a remarkable analogy to that of the air vessels of plants, which also bear the name of tracheae ; and in both similar variations are observed in the contexture of the elastic membrane by which they are kept pervious.* The tracheae, in many parts of their course, present remarkable dilatations, which apparently serve as reservoirs of air ; they are very conspicuous in the Dytisciis marginalise which resides princi- pally in water; but they also exist in many insects, as the Melolontha and the Cerambyx, which live * According to the observation of Dr. Kidd these vessels are often annular in insects, as is also the case with those of plants. Recon- siders the longitudinal tracheae as connecting channels, by which the insect is enabled to direct the air to particular parts for occa- sional purposes. Phil. Trans, for 1825, p. 234. RESPIRATION IN INSECTS. '283 wholly ill the air.* Those of the Scolia hortorum (Fab.) are delineated in Fig. 373, considerably magnified. If an insect be immersed in water, air will be seen escaping in minute bubbles at each spiracle ; and in proportion as the water enters into the tubes, sensibility is destroyed. If all the spiracles be closed by oil, or any other unctuous substance, the insect immediately dies of suffocation ; but if some of them be left open, respiration is kept up to a considerable extent, from the numerous communi- cations which exist among the air vessels. Insects soon perish when placed in the receiver of an air- pump, and the air exhausted ; but they are gene- rally more tenacious of life under these circum- stances than the larger animals, and often, after being apparently dead, revive on the readmission of air. Aquatic insects have tracheae, and sometimes also branchiae ; and are frequently provided with tubes, which are of sufficient length to reach the surface of the water, where they absorb air for respiration. In a few tribes a complicated mode of respiration is practised ; aerated water is taken into the body, and introduced into cavities, where the air is ex- tracted from it, and transmitted by the ordinary tracheae to the different parts of the system. t Such, then, is the extensive apparatus for aera- tion in animals, which have either no circulation of * Leon Dufour, Annales des Sciences Naturelles; viii. 26. f Dutrochet conceives that the principle on which this operation is conducted is the same with that by which gases are reciprocally transmitted through moistened membranes ; as in the experiments of Humboldt and Gay Lussac, who, on enclosing mixtures of oxy- 284 THE VITAL FUNCTIONS. their nutritious juices, or a very imperfect one ; but no sooner do we arrive at the examination of animals possessing an enlarged system of blood vessels, than we find nature abandoning the system of tracheae, and employing more simple means of effecting the aeration of the blood. Advantage is taken of the facility afforded by the blood-vessels of transmitting the blood to particular organs, where it may conveniently receive the influence of the air. Thus Scorpions are provided on each side of the thorax, with four pulmonary cavities, seen at L, on the left side of Fig. 374, into each of which gen, nitrogen, and carbonic acid gases, in any proportion, in a membranous bladder, which was then immersed in aerated water, found that there is a reciprocal transit of the gases ; until at length ))ure atmospheric air remains in the cavity of the bladder. RESPIRATION IN INSECTS. 28-3 air is admitted by a separate external opening. A, B, is the dorsal vessel, which is connected with the pulmonary cavities by means of two sets of muscles, the one set (m, m) being longer than the other (m, m, m). The branchial arteries (v) are seem ramifying over the inner surface of the pul- monary cavities (r) on the right side, whence the blood is conveyed by a corresponding set of bran- chial veins to the dorsal vessel ; and other vessels, which are ordinary veins, are seen at o, proceeding from the abdominal cavity to join the dorsal vessel. The membrane which lines the pulmonary cavities is curiously plaited ; presenting the appearance of the teeth of a comb, and partaking of the structure of gills ; and on this account these organs are termed by Latreille pneumo-brcmchice . Organs of a similar description exist in Spiders ; some species have eight ; others four ; and some only two : but there is one entire order of Arachnida which resj^ire by means of tracheae, and in these the circulation is as imperfect as it is in insects. It may here be remarked that an essential dif- ference exists in the structure of the respiratory organs, according to the nature of the medium which is to act upon them ; for in aquatic respira- tion the air contained in water is made to act on the blood circulating in vessels which ramify on the external surface of the filaments of the gills ; while in atmospheric respiration the air in its gaseous state is always received into cavities, on the inter- nal surface of which the blood-vessels, intended to receive its influence, are distributed. It is not difficult to assign the final cause of this change of plan ; for in each case the structure is accommo- 286 THE VITAL FUNCTIONS. dated to the mechanical properties of the medium respired. Ahquid, being inelastic and ponderous, is adapted, by its momentum alone, to separate and surround the loose floating filaments composing the branchiae ; but a light gaseous fluid, like air, is, on the contrary, better fitted to expand dilatable cavities into which it may be introduced. Occasionally, however, it is found that organs constructed like branchiae, and usually performing aquatic respiration, can be adapted to respire air. This is the case with some species of Crustacea, of the order Decapoda, such as Crabs, which, by means of a peculiar apparatus, discovered by Audouin and Milne Edwards, retain a quantity of water in the branchial cavity so as to enable them to live a very long time out of the water. It is only in their mature state of developement, however, that they are qualified for this amphibious exist- ence, for at an early period of growth they can live only in water. • There is an entire order of Gasteropodous Mol- lusca which breathe atmospheric air by means of pulmonary cavities. This is the case with the Limax, or slug, and also with the Helix, or snail, the Testacella, the Clausilia, and many others, which, though partial to moist situations, are, from the conformation of their respiratory organs, essen- tially land animals. The air is received by a round aperture near the head, guarded by a sphincter muscle, which is seen to dilate or contract as occa- sion may require, but which is sometimes com- pletely concealed from view by the mantle folding over it. The cavity, to m hich this opening leads, is lined with a membrane delicately folded, and RESPIRATION BY LUNGS. 287 overspread with a beautiful network of pulmonary vessels. Other mollusca of the same order, which are more aquatic in their habits, have yet a similar structure, and are obliged at intervals to come to the surface of the water in order to breathe atmos- pheric air : this is the case with the Onchiditim, the Planorbis, the Lymnaa, &c. The structure of the pulmonary organs becomes gradually more refined and complicated as we ascend to the higher classes of animals. In all vertebrated terrestrial animals they are called lungs, and consist of an assemblage of vesicles, into which the air is admitted by a tube, called the trachea, or wind-pipe, extending downwards from the back of the mouth, parallel to the oesophagus. Great care is taken to guard the beginning of this passage from the intrusion of any solid or liquid that may be swallowed. A cartilaginous valve, termed the epiglottis, is generally provided for this purpose, which is made to descend by the action of the same muscles that perform deglutition, and which then closes accurately the entrance into the air-tube. It is an exceedingly beautiful contrivance, both as to the simplicity of the mechanism, and the accuracy with which it accomplishes the purpose of its for- mation. At the upper part of the chest the trachea divides into two branches, called the bronchia, passing to the lungs on either side. Both the wind- pipe and the bronchia are prevented from closing by the interposition of a series of firm cartilaginous ringlets, interposed between their inner and outer coats, and placed at small and equal distances from one another. The natural elasticity of these ring- lets tends to keep the sides of the tube stretched, 288 THE VITAL FUNCTIONS. and causes it to remain open : it is a structure very analogous to that of the trachea of insects, or of the vessels of the same name in plants. The lungs of Reptiles consist of large sacs, into the cavity of which the bronchia, proceeding from the bifurcation of the trachea, open at once, and without further subdivision. Cells are formed within the sides of this great cavity, by fine mem- branous partitions, as thin and delicate as soap bubbles. The lungs of serpents have scarcely any of these partitions, but consist of one simple pul- monary sac, situated on the right side, having the slender elongated form of all the other viscera, and extending nearly the whole length of the body- The lung on the left side is in general scarcely discernible, being very imperfectly developed. In the Chameleon the lungs have numerous processes which project from them like caeca. In the Sauria, the lungs are more confined to the thoracic region, and are more completely cellular. The mechanism, by which, in these animals, the air is forced into the lungs, is exceedingly peculiar, and was for a long time a subject of controversy. If we take a frog as an example, and watch its respiration, we cannot readily discover that it breathes at all, for it never opens its mouth to receive air, and there is no motion of the sides to indicate that it respires ; and yet, on any sudden alarm, we see the animal blowing itself up, as if by some internal power, though its mouth all the while continues to be closed. We may perceive, how- ever, that its throat is in frequent motion, as if the frog were economising its mouthful of air, and transferring it backwards and forwards between its RESPIRATION IN RKPTILLS. 289 mouth and lungs ; but if we direct our attention to the nostrils, we may observe in them a twirling motion, at each movement of the jaws; for it is, in fact, through the nostrils that the frog receives all the .air which it breathes. The jaws are never opened but for eating ; and the sides of the mouth form a sort of bellows, of which the nostrils are the inlets; and by their alternate contraction and re- laxation the air is swallowed, and forced into the trachea, so as to inflate the lungs. If the mouth of a frog be forcibly kept open, it can no longer breathe, because it is deprived of the power of swallowing the air required for that function ; and if its nostrils be closed, it is, in like manner, suffo- cated. The respiration of most of the Reptile tribes is performed in a similar manner; and they may be said rather to swallow the air they breathe, than to draw it in by any expansive action of the parts which surround the cavity of the lungs ; for even the ribs of serpents contribute but little, by their motion, to this effect, being chiefly useful as organs of progression. The Chelonia have lungs of great extent, passing backwards under the carapace, and reaching to the posterior part of the abdomen. Turtles, which are aquatic, derive great advantages from this structure, which enables them to give buoyancy to their body (encumbered as it is with a heavy shell), by introducing into it a large volume of air ; so that the lungs, in fact, serve the purposes of a large swimming bladder. That this use was con- templated in their structure is evident from the volume of air received into the lungs being much greater than is required for the sole purpose of VOL. II. V 290 THE VITAL FUNCTIONS. respiration. The section of the hmgs of the turtle (Fig. 375), shows their interior structure, composed of lara-e cells, into which the trachea (t) opens. Few subjects in animal physiology are more de- serving the attention of those whose object is to trace the operations of nature in the progressive develope- ment of the organs, than the changes which occur in the evolution of the tadpole, from the time it leaves the egg till it has attained the form of the perfect frog. We have already had occasion to notice several of these transformations in the organs of the mechanical functions, and also in those of digestion and circulation : but the most remark- able of all are the changes occurring in the re- spiratory apparatus, corresponding with the oppo- site nature of the elements which the same animal is destined to inhabit in the different stages of its existence. No less than three sets of organs are provided for respiration ; the first two being branchiae, adapted to the fish-like condition of the tadpole ; and the last being pulmonary cavities, for RESPIRATION IN REPTILES. 291 receiving air, to be employed when the animal exchanges its aquatic for its terrestrial life. It is exceedingly interesting to observe that this animal at first breathes by gills, which are furnished with cilia,* project in an arborescent form from the sides of the neck, and float in the water ; but these struc- tures are merely temporary, being provided only to meet the immediate exigency of the occasion, and being raised at a period when none of the internal organs are as yet perfected. As soon as another set of gills, situated internally, can be constructed, and are ready to admit the circulating blood, the external gills are superseded in their office ; they now shrivel, and are removed, and the tadpole per- forms its respiration by means of branchiae, formed on the model of those of fishes, and acting by a similar mechanism. By the time that the system has undergone the changes necessary for its conversion into the frog, a new and very ditFerent apparatus has been evolved for the respiration of air. These are the lungs, which now coming into play, direct the current of blood from the branchiae, and take upon themselves the whole office of respiration. The branchiae, in their turn, become useless, are soon obliterated, and leave no other trace of their former existence than the original division of the arterial trunks, which had supplied them with blood directly from the heart, but which, now- uniting in the back, form the descending aorta.| There is a small family, called the Perenni- branchia, belonging to this class, which, instead of * It has been found that in the young- animal nearly the whole surface of the body gives rise to ciliary motions : but these motions disappear as the animal advances in growth. t See Fig. .'^57, p. 248. 292 THE VITAL FUNCTIONS. undergoing all the changes I have been describing, present, during their whole lives, a great similitude to the first stage of the tadpole. This is the case with the Axolotl, the Proteus anguinns, the Siren lacertina, and the Menobranchus lateralis, which permanently retain their external gills, while at the same time they possess imperfectly developed lungs. It would therefore appear that these animals, in their adult state, are in a condition similar to that of the tadpole ; with this difterence, that in them the developement of the organs stops there ; whereas, in the latter, it proceeds to the metamor- phosis of the animal into a quadruped breathing only atmospheric air.* In all warm blooded animals respiration becomes a function of much greater importance, the con- tinuance of life being essentially dependent on its vigorous and unceasing exercise. The whole class of Mammalia have lungs of an exceedingly de- veloped structure, composed of an immense number of minute cells, crowded together as closely as pos- sible, and presenting a vast extent of internal sur- face. The thorax, or cavity in which the lungs, together with the heart and its great blood-vessels, are inclosed, has somewhat the shape of a cone ; and its sides are defended from compression by the * GeofFroy St. Hilaire thinks there is ground for believing that Crocodiles and Turtles possess, in addition to the ordinary pulmonary respiration, a partial aquatic abdominal respiration, effected by means of the two channels of communication which have been found to exist between the cavity of the abdomen and the external surface of the body : and also that some analogy may be traced between this aquatic respiration in reptiles, by these peritoneal canals, and the supposed function of the swimming bladder of Hshes, in subserviency to a species of aerial respiration. RESPIRATION IN MAMMALIA. 293 arches of the ribs, which extend from the spine to the sternum, or breast-bone, and produce mecha- nical support on the same principle that a cask is strengthened by being girt with hoops, which, though composed of comparatively weak materials, are yet capable, from their circular shape, of pre- senting great resistance to any compressing force. While Nature has thus guarded the chest with such peculiar solicitude, against the efforts of any external force tending to diminish its capacity, she has made ample provision for enlarging or contracting its diameter in the act of respiration. First, at the lower part, or that which corresponds to the base of the cone, the only side, indeed, which is not protected by bone, there is extended a thin expansion, partly muscular, and partly ten- dinous, forming a complete partition, and closing the cavity of the chest on the side next to the ab- domen. This nmscle is called the Diaphragni : it is perforated, close to its origin from the spine, by four tubes, namely, the oesophagus, the aorta, the vena cava, and the thoracic duct. Its surface is not flat, but convex above, or towards the chest ; and the direction of its fibres is such that when they contract they bring down the middle part, which is tendinous, and render it more flat than before, (the passage of the four tubes already mentioned, not interfering with this action,) and thus the cavity of the thorax may be considerably enlarged. It is obvious that if, upon the descent of the diaphragm, the lungs were to remain in their original situation, an empty space would be left between them and the diaphragm. But no vacuum can take place in the body ; the air cells of the lungs^ must always 294 THE VITAL FUNCTIONS. contain, even in their most compressed state, a cer- tain quantity of air ; and this air will tend, by its elasticity, to expand the cells : the lungs will con- sequently be dilated, and will continue to fill the chest ; and the external air will rush in through the trachea in order to restore the equilibrium. This action is termed inspiration. The air is again thrown out when the diaphragm is relaxed, and pushed upwards, by the action of the large muscles of the trunk ; the elasticity of the sides of the chest concurs in producing the same effect ; and thus expiration is accomplished. The muscles which move the ribs conspire also to produce dilatations and contractions of the ca- vity of the chest. Each rib is capable of a small degree of motion on that extremity by which it is attached to the spine ; and this motion, assuming the chest to be in the erect position, as in man, is chiefly upwards and downwards. But, since the inclination of the ribs is such that their lower edges form acute angles with the spine, they bend downwards as they proceed towards the breast; and the uppermost rib being a fixed point, the action of the intercostal muscles, which produces an approximation of the ribs, tends to raise them, and to bring them more at right angles with the spine ; the sternum also, to which the other extre- mities of the ribs are articulated, is elevated by this motion, and consequently removed to a greater distance from the spine. The general result of all these actions is to increase the capacity of the chest. Thus there are two ways in which the cavity of the thorax may be dilated ; namely, by the action of RESPIRATION IN BIRDS. 296 the diaphragm, and by the action of the intercostal muscles. It is only in peculiar exigencies that the whole power of" this apparatus is called into action ; for in ordinary respiration the diaphragm is the chief agent employed, and the principal effect of the action of the intercostal muscles is simply to fix the ribs, and thus give greater purchase to the dia- phragm. The muscles of the ribs are employed chiefly to assist the diaphragm, when, from any cause, a difficulty arises in dilating the chest. In Birds the mechanism of respiration proceeds upon a different plan, of which an idea may be derived from the view given of the lungs of the Ostrich, at l, l, Fiy^. 377. The construction of the lungs of birds is such as not to admit of any change in their dimensions ; for tlicy are very compact in 296 THE VITAL FUNCTIONS. their texture, and are so closely braced to the ribs, and npper parts of the chest, by firm membranes, as to preclude all possibility of motion. They in part, indeed, project behind the intervals between the ribs, so that their whole mass is not altogether contained within the thoracic cavity. There is no large muscular diaphragm by which any change in the capacity of the chest could be effected, but merely a few narrow slips of muscles, arising from the inner sides of the ribs, and inserted into the thin transparent membrane which covers the lower surface of the lungs. They have the effect of lessening the concavity of the lungs towards the abdomen, at the time of inspiration ; and they thereby assist in dilating the air cells.* The bron- chia, or divisions of the trachea (r), after opening as usual into the pulmonary air-cells, do not ter- minate there, but pass on to the surface of the lungs, where they open by numerous apertures. The air is admitted, through these apertures, into several large air-cells (c c c), which occupy a con- siderable portion of the body, and which enclose most of the large viscera contained in the abdo- men, such as the liver, the stomach, and the in- testines ;! and there are, besides, many lateral cells in immediate communication with the lungs, and extending all down the sides of the body. Numerous air-cells also exist between the muscles, and underneath the skin ; and the air penetrates even into the interior of the bones themselves ; fill- * Hunter on the Animal Economy, p. 78. t It was asserted by the Parisian Academicians, that the air gets admission into the cavity of the pericardium, in which the heart is lodged. That this was an error was fir^t pointed out by Dr. Ma- cartney. (See Rees's Cyclopeedia. — Art. Bird.) UESPIKATION IN BIRDS. 297 ing the spaces usually occupied by the marrow, and thus contributing materially to the lightness of the fabric* All these cells are very large and numerous in birds which perform the highest and most rapid flight, such as the Eagle. The bill of the Toucan, which is of a cellular structure, and also the cells between the plates of the skull in the Old, are, in like manner, filled with air, derived from the lungs. The barrels of the large quills of the tails and wings are also supplied with air from the same source. In birds, then, the air is not merely received into the lungs, but actually passes through them, being drawn forwards by the muscles of the ribs when they elevate the chest, and produce an ex- pansion of the subjacent air-cells. The chest is depressed, for the purpose of expiration, by another set of muscles, and the air driven back : this air, consequently, passes a second time through the lungs, and acts twice on the blood which circidates in those organs. It is evident that if the lungs of birds had been constructed on the plan of those of quadrupeds, they must have been twice as large to obtain the same amount of aeration in the blood ; and consequently must have been twice as heavy, which would have been a serious inconvenience in an animal formed for flying. | The diffusion of so * In birds, not formed for extensive flight, as the g^alhnaceous tribes, the humerus is the only bone into which air is introduced. — Hunter on the Animal Economy, p. 81. f I must mention, however, that the correctness of this view of the subject is contested by Dr. Macartney, who thinks it probable that the air, on its return from the large air-cells, pa<;ses directly by the large air-holes into the bronchia, and is not brought a second time into contact with the blood. 298 THE VITAL FUiNCTIONS. large a quantity of air throughout the body of animals of this class presents an analogy with a similar purpose apparent in the conformation of insects, where the same object is effected by means of tracheae.* Thus has the mechanism of respiration been varied in the different classes of animals, and adapted to the particular element, and mode of life designed for each. Combined with the peculiar mode of circulation, it atibrds a tolerably accurate criterion of the energy of the vital powers. In Birds, the muscular activity is raised to the highest degree, in consequence of the double effect of the air upon the whole circulating blood in the pulmo- nary organs. The Mammalia rank next below birds, in the scale of vital energy; but they still possess a double circulation, and breathe atmos- pheric air. The torpid and cold-blooded Reptiles are separated from Mammalia by a very wide in- terval ; because, although they respire air, that air influences only a part of the blood ; the pulmonary, being merely a branch of the general circulation. In Fishes, again, we have a similar result ; because, although the whole blood is brought by a double circulation to the respiratory organs, yet it is acted * The peculiarities of structure in the respiratory system of birds have probably a relation to the capability we see them possess, of bearing with impunity, very quick and violent changes of atmos- pheric pressure. Thus the Condor of the Andes is often seen to descend rapidly from a height of above 20,000 feet, to the edge of the sea, where the air is more than twice the density of that which the bird had been breathing. We are as yet unable to trace the connexion which probably exists between the structure of the luiigs, and this extraordinary power of accommodation to such great and sudden variations of atmosphciic pressure. CHEMICAL EI FECTS OF RESPIRATION. 291) upon only by that portion of air which is contained in the water respired, and which is less powerful in its action than the same element in its gaseous state. We may, in like manner, continue to trace the connexion between the extent of these func- tions and the degrees of vital energy throughout the successive classes of invertebrate animals. The vigour and activity of the functions of Insects, in particular, have an evident relation to the effec- tive manner in which the complete aeration of the blood is secured by an extensive distribution of tracheae through every part of their system. § 4. Chemical Changes effected hy Respiration. We have next to direct our attention to the chemi- cal offices which respiration performs in the animal economy. It is only of late years that we may be said to have obtained any accurate knowledge as to the real nature of this important function ; and there is perhaps no branch of physiology which exhibits in its history a more humiliating picture of the wide sea of error in which the human intellect is prone to lose itself, when the path of philosophi- cal induction is abandoned, than the multitude of wild and visionary hypotheses, devoid of all solid foundation, and perplexed by the most inconsistent reasonings, which formerly prevailed with regard to the objects and the processes of respiration. To give an account, or even a brief enumeration of these theories, now sufficiently exploded, would be incompatible with the purpose to which I must .*300 THE VITAL FUNCTIONS. confine mjself in this treatise.* I shall content myself, therefore, with a concise statement of such of the leading facts relating to this function, as liave now, by the labours of modern physiologists, been satisfactorily established, and which serve to elucidate the beneficent intentions of nature in the economy of the animal system. Atmospheric air acts without difficulty upon the blood, while it is circulating through the vessels which are ramified over the membranes lining the air cells of the lungs; for neither these membranes, nor the thin coats of the vessels themselves, present any obstacle to the transmission of chemical ele- ments from the one to the other. The blood beino; a highly compound fluid, it is exceedingly difficult to obtain an accurate analysis of it, and still more to ascertain with precision the different modifica- tions which occur in its chemical condition at dif- ferent times : on this account, it is scarcely possible to determine, by direct observation, what are the exact chemical changes which that fluid undergoes during its passage through the lungs; and we have only collateral evidence to guide us in the inquiry-t The most obvious effect resulting from the action of the air is a change of colour from the dark purple * For an account of the liistory of the various chemical theories wliich have prevailed on this interesting" department of Physiology, I must refer to the " Essay on Respiration," by Dr. Bostock, and also to the " Elementary System of Physiology," by the same author, which latter work comprises the most comprehensive and accurate compendium of the science that has yet appeared. t Some experiments very recently made by Messrs. Macaire and Marcet, on the ultimate analysis of arterial and venous blood, taken from a rabbit, and dried, have shown that the former contains a CHEMICAL EFFECTS OF RESPIRATION. 301 hue, which the blood has when it is brought to the lungs, to the bright vermillion colour, which it is found to assume in those organs, and which accom- panies its restoration to the qualities of arterial blood. In what the chemical difference between these two states consists may, in some measure, be collected from the changes which the air itself, by producing them, has experienced. The air of the atmosphere, which is taken into the lungs, is known to consist of about twenty per cent, of oxygen gas, seventy-nine of nitrogen gas, and one of carbonic acid gas. When it has acted upon the blood, and is returned from the lungs, it is found that a certain proportion of the oxygen, which it had contained, has disappeared, and that the place of this oxygen is almost wholly supplied by an addition of carbonic acid gas, together with a quantity of watery vapour. It appears also pro- bable that a small portion of the nitrogen gas is consumed during respiration. For our knowledge of the fact of the disappear- ance of oxygen, we are indebted to the labours of Dr. Priestley. It had, indeed, been long before suspected by Mayow, that some portion of the air inspired is absorbed by the blood ; but the merit of the discovery that it is the oxygenous part of the larger proportion of oxygen than the latter ; and that the latter contains a larger proportion of carbon than the former: the pro- portions of nitrogen and hydrogen being nearly the same in both. The following are the exact numbers expressive of these proportions : Carbo7i. Oxxigen. Nitrogen. Hydrogen. Arterial blood . . . 50.2 . . . 26.3 ... 16.3 ... 6.6 Venous blood . . . 55.7 . . . 21.7 . . . 16.2 ... 6.4 Memoires de la Societe de Physique et d'Hist. Naturelle de Geneve. T. v. p. 400. .002 THE VITAL FUNCTIONS. air which is thus consumed is unquestionably due to Dr. Priestley. The exact quantity of oxygen which is lost in natural respiration varies in dif- ferent animals, and even in different conditions of the same animal. Birds, for instance, consume larger quantities of oxygen by their respiration ; and hence require, for the maintenance of life, a purer air than other vertebrated animals. Vau- quelin, however, found that many species of insects and worms possess the power of abstracting oxygen from the atmosphere in a much greater degree than the larger animals. Even some of the terrestrial mollusca, such as snails, are capable of living for a long time in the vitiated air in which a bird had perished. Some insects, which conceal themselves in holes, or burrow under ground, have been known to deprive the air of every appreciable portion of its oxygen. It is observed by Spallanzani, that those animals, whose modes of life oblige them to remain for a great length of time in these confined situa- tions, possess this power in a greater degree than others, which enjoy more liberty of moving in the open air: so admirably have the faculties of ani- mals been, in every instance, accommodated to their respective wants. Since carbonic acid consists of oxygen and carbon, it is evident that the portion of that gas which is exhaled from the lungs is the result of the combination of either the whole, or a part, of the oxygen gas, which disappears during the act of respiration, with the carbon contained in the dark venous blood, which is brought to the lungs. The blood having thus parted with its superabundant carbon, which escapes in the form of carbonic acid CHEMICAL EFFECTS OF RESPIRATION. 303 gas, regains its natural vermillion colour, and is now qualified to be again transmitted to the dif- ferent parts of the body for their nourishment and growth. As the blood contains a greater propor- tion of carbon than the animal solids and fluids which are formed from it, this superabundant carbon gradually accumulates in proportion as its other principles, (namely, oxygen, hydrogen, and nitrogen) are abstracted from it by the processes of secretion and nutrition. By the time it has re- turned to the heart, therefore, it is loaded with carbon, a principle, which, when in excess, becomes noxious, and requires to be removed from the blood, by combining it with a fresh quantity of oxygen obtained from the atmosphere. It is not yet satis- factorily determined whether the whole of the oxygen, which disappears during respiration, is employed in the formation of carbonic acid gas : it appears probable, however, from the concurring testimony of many experimentalists, that a small quantity is permanently absorbed by the blood, and enters into it as one of its constituents. A similar question arises with respect to nitrogen, of which, as I have already mentioned, it is pro- bable that a small quantity disappears from the air when it is respired ; although the accounts of experimentalists are not uniform on this point. The absorption of nitrogen during respiration was one of the results which Dr. Priestley had deduced from his experiments : and this fact, though often doubted, appears, on the whole, to be tolerably well ascertained by the inquiries of Davy, Pfaff, and Henderson. With regard to the respiration of cold-blooded animals, it has been satisfactorily 30 4 THE VIIAL FUNCTIONS. established by the researches of Spallanzani, and still more by those of Humboldt and Provencal, on fishes, that nitrogen is actually absorbed. A con- firmation of this result has recently been obtained by Messrs. Macaire and Marcet, who have found that the blood contains a larger proportion of nitrogen than the chyle, from which it is formed. We can discover no other source from which chyle could acquire this additional quantity of nitrogen, during its conversion into blood, than the air of the atmosphere, to which it is exposed in its passage through the pulmonary vessels.* According to these views of the chemical objects of respiration, the process itself is analogous to those artificial operations which effect the com- bustion of charcoal. The food supplies the fuel, which is prepared for use by the digestive organs, and conveyed by the pulmonary arteries to the place where it is to undergo combustion : the dia- phragm is the' bellows, which feeds the furnace with air ; and the trachea is the chimney, through which the carbonic acid, which is the product of the combustion, escapes. t It becomes an interesting problem to determine whether this analogy may not be farther extended ; and whether the combustion of carbon, which takes place in respiration, be not the exclusive source of * See the note at page 300. f It is now generally believed that the combination of the absorbed oxygen with the carbon contained in the blood takes place, not so much in the vessels of the lungs, as in the course of the systemic circulation. It has been found that carbonic acid exists in larger proportion in venous than in arterial blood : a fact which is decidedly in accordance with this view of the process. CHEMICAL EFFECTS OF RESPIRATION. 305 the increased temperature, which all animals, but more especially those designated as luarm-blooded, usually maintain above the surrounding medium. The uniform and exact relation which may be observed to take place between the temperature of animals and the energy of the respiratory function, or rather the amount of the chemical changes in- duced by that function, affords very strong evidence in favour of this hypothesis. The coincidence, in- deed, is so strong, that notwithstanding the objec- tions that have been raised against the theory founded upon this hypothesis, from some apparent anomalies which occasionally present themselves, we must, I think, admit that it affords the best explanation of the phenomena of any theory yet proposed, and that, therefore, it is probably the true one. The maintenance of a very elevated temperature appears to require the concurrence of two con- ditions ; namely, first, that the whole of the blood should be subjected to the influence of the air, and, secondly, that that air should be presented to it in a gaseous state. These, then, are the circum- stances which establish the great distinction be- tween warm and cold-blooded animals ; a distinc- tion which at once stamps the character of their whole constitution. It is the condition of a high temperature in the blood which raises the Quadru- ped and the Bird to a rank, in the scale of vitality, so far above that of the Reptile : it is this which places an insuperable boundary between Mam- malia and Fishes. However the warm-blooded Cetacea, who spend their lives in the ocean, may be found to approximate in their outward form, VOL. II. X 306 THE VITAL FUNCTIONS. and in their external instruments of motion, to the other inhabitants of the deep, they are still, from the conformation of their respiratory organs, de- pendent on another element. If a Seal, a Porpoise, or a Dolphin were confined, but for a short time, under the surface of the M^ater, it would perish with the same certainty as any other of the mammalia, placed in the same situation. We observe them continually rising to the surface in order to breathe, under every circumstance of privation or of danger; and however eagerly they may pursue their prey, however closely they may be pressed by their enemies, a more urgent want compels them, from time to time, to respire air at the surface of the sea. Were it not for this imperious necessity, the Whale, whose enormous bulk is united with cor- responding strength and swiftness, would live in undisturbed possession of the widely extended do- mains of the, ocean, might view without dismay whole fleets sent out against him, and might defy all the efforts that man could practise for his cap- ture or destruction. But the constitution of his blood, obliging him to breathe at the surface of the water, brings him within the reach of the fatal harpoon. In vain, on feeling himself wounded, does he plunge for refuge into the recesses of the deep; the same necessity recurs, and compelling him again to present himself to his foes, exposes him to their renewed attacks, till he falls in the unequal struggle. His colossal form and gigantic strength are of little avail against the power of man, feeble though that power may seem, when phy- sically considered, but deriving resistless might from its association with an immeasurably superior intellect. 307 Chapter XII. SECRETION. The capability of effecting certain cliemical clianges in the crude materials introduced into the body, is one of the powers which more especially charac- terize life ; but although this power is exercised both by vegetable and by animal organizations, we perceive a marked difference in the results of its operation in these two orders of beings. The food of plants consists, for the most part, of the simpler combinations of elementary bodies, which are ela- borated in cellular or vascular textures, and con- verted into various products. The oak, for ex- ample, forms, by the powers of vegetation, out of these elements, not only the green pulpy matter of its leaves, and the light tissue of its pith, but also the densest of its woody fibres. It is from similar materials, again, that the olive prepares its oil, and the cocoa nut its milk ; and the very same ele- ments, in different states of combination, compose, in other instances, at one time the luscious sugar of the cane, at another the narcotic juice of the poppy, or the acrid principle of the euphorbium ; and the same plant which furnishes in one part the bland farina of the potatoe, will produce in another the poisonous extract of the nightshade. Yet all these, and thousands of other vegetable products, differing widely in their sensible quali- ties, agree very nearly in their ultimate chemical analysis, and owe their peculiar properties chiefly nOS THE VITAL FUNCTIONS. to the order in which their elements are arranged ; an order dependent on the processes to which they have been subjected in the system of each particular vegetable. In the animal kingdom we observe these pro- cesses multiplied to a still greater extent ; and the resulting substances are even farther removed from the original condition of unorganized matter. In the first place, the food of animals, instead of being simple, like that of plants, has always undergone previous preparation ; for it has either constituted a portion of some other organized being, or it has been a product of organization ; in each case, there- fore, partaking of that complexity of composition which characterises organized bodies. Still, what- ever may be its qualities when received into the stomach, it is soon converted by the powers of digestion into a milky, or transparent fluid, having nearly the same uniform properties. We have seen that there is scarcely any animal or vegetable substance, however dense its texture, or virulent its qualities, but is capable of affording nourishment to various species of animals. Let us take as an example the elytra of cantharides, which are such active stimulants when applied in powder to the skin in the ordinary mode of blistering : we find that, notwithstanding their highly acrid qualities, they constitute the natural food of several species of insects, which devour them with great avidity; and yet the fluids of these insects, though derived from this pungent food, are perfectly bland, and devoid of all acrimony. Cantharides are also, according to Pallas, the favourite food of the Hedge-hog ; although to other mammalia they are SECRETION. 309 highly poisonous. It has also been found that even those animal secretions, (such as the venom of the rattle-snake,) which, when infused into a wound, even in the minutest quantity, prove quickly fatal, may be taken into the stomach without producing any deleterious effects. These, and a multitude of other well-known facts, fully prove how completely substances received as aliment may be modified, and their properties changed, or even reversed, by the powers of animal digestion. No less remarkable are the transmutations which the blood itself, the result of these previous pro- cesses, is subsequently made to undergo in the course of circulation, and when subjected to the action of the nutrient vessels and secreting organs ; being ultimately converted into the various textures and substances which compose all the parts of the animal frame. All the modifications of cellular substance, in its various states of condensation ; the membranes, the ligaments, the cartilages, the bones, the marrow ; the muscles, with their tendons ; the lubricating fluid of the joints; the medullary pulp of the brain; the transparent jelly of the eye; in a word, all the diversified textures of the various organs, which are calculated for such different offices, are derived from the same nutrient fluid, and may be considered as being merely modified arrangements of the same ultimate chemical ele- ments. In what, then, we naturally ask, consists this subtle chemistry of life, by which nature effects these multifarious changes ; and in what secret recesses of the living frame has she constructed the refined laboratory in which she operates her 310 THE VITAL FUNCTIONS. marvellous transformations, far surpassing even those which the most visionary alchemist of former times had ever dreamed of achieving? Questions like these can be fairly met only by the confession of profound ignorance ; for, although the subject of secretion has long excited the most ardent curiosity of physiologists, and has been prosecuted with extraordinary zeal and perseverance, scarcely any positive information has resulted from their labours; and the real nature of the process remains involved in nearly the same degree of obscurity as at first.* It was natural to expect that in this inquiry material assistance would be derived from an accurate anatomical examination of the organs by wliich the more remarkable secretions are formed ; yet, notwithstanding the most minute and careful scrutiny of these organs, our knowledge of the mode in which they are instrumental in effect- ing the operations which are there conducted, has not in reality advanced a single step. To add to our perplexity we often see, on the one hand, parts, to all appearance very differently organized, giving rise to secretions of a similar nature ; and on the * It is not yet precisely determined to what extent the organs of secretion are immediately instrumental in producing the substance secreted ; and it has been even suggested that possibly their office is confined to the mere separation, or filtration from the blood, of certain animal products, which are always spontaneously forming in that fluid in the course of its circulation. This hypothesis, in which the glands, and other secreting apparatus are regarded as only very fine strainers, is supported by a few facts, which seem to indicate the presence of some of these products in the blood, independently of the secreting processes by which they are usually supposed to be formed ; but the evidence is as yet too scanty and equivocal to warrant the deduction of any general theory on the subject. SECRETION. 311 Other hand, substances of very difterent properties produced by organs, which, even in their minutest details, appear to be identical in their structure. Secretions are often found to be poured out from smooth and membranous surfaces, such as those which line the cavities of the abdomen, the chest, and the head, and which are also reflected inwards so as to invest the organs therein contained, as the heart, the lungs, the stomach, the intestines, the liver, and the brain.* In other instances, the secreting membrane is thickly set with minute processes, like the pile of velvet : these processes are called villi, and their more obvious use, as far as we can perceive, is to increase the surface from which the secretion is prepared. At other times we see an opposite kind of structure employed ; the secreting surface being the internal lining of sacs or cells, either opening at once into some larger cavity, or prolonged into a tube, or duct, for con- veying the secreted fluid to a more distant point. These cells, or follicles, as they are termed, are generally employed for the mucous secretions, and * Sometimes the secreting organ appears to be entirely com- posed of a mass of vessels covered with a smooth membrane ; in other cases, it appears to contain some additional material, or IKirenchyma, as it is termed. Vertebrated animals present us with numerous instances of glandular organs employed for special pur- poses of secretion : thus, in the eyes of fishes there exists a large vascular mass, which has been called the choroid tj land, and which is supposed to be placed there for the purpose of replenishing some of the humours of the eye, in proportion as they are wasted. Within the air-bladder of several species of fishes there is found a vascular organ, apparently serving to secrete the air v^ith which the bladder is filled; numerous ducts, filled with air, having been observed pro- ceeding from the organ, and opening on the inner surface of the air- bladder. 312 THL VITAL FUNCTIONS. are often scattered throughout the surfaces of mem- branes:* at other times the secreting cavities are collected in great numbers into groups; and they then frequently consist of a series of lengthened tubes, like caeca, examples of vrhich we have already seen in the hepatic and salivary glands of insects. A secretory organ, in its simplest form, consists of short, narrow and undivided tubes ; we next find tubes which are elongated, tortuous or convoluted, occasionally presenting dilated portions, or even having altogether the appearance of a collection of pouches, or sacs; while in other cases they are branched, and extend into minute ramifications. Sometimes they are detached, or isolated ; at other times they are collected into tufts, or variously grouped into masses, where still the separate tubes admit of beinpr un ravel led. -j" The secreting fila- ments of insects float in the general cavity, con- taining the mass of nutrient fluid, and thence im- bibe the materials they require for the performance of their functions. It is only when they receive a firm investment of cellular membrane, forming what is termed a capsule, and assuming the appear- ance of a compact body, that they properly consti- tute a gland; and this form of a secreting organ is met with only among the higher animals. | Great variety is observable both in the form and * Seep. 165 of this volume ; and in particular Fig. 305. Seba- ceous follicles are also noticed in vol. i. p. 102. t See " Muller on the intimate structure of secreting glands," edited by S. Solly, 1839. X Dr. Kidd, however, describes bodies apparently of a glandular character, disposed in rows on the inner surface of the intestinal canal of the Gryllotalpa, or mole-cricket. Phil. Tran. for 1825, p. 227. SECRETION. 313 Structure of different glands, and in the mode in which their blood-vessels are distributed. In ani- mals which are furnished with an extensive circu- lation, the vessels supplying the glands with blood are distributed in various modes ; and it is evident that each plan has been designedly selected with reference to the nature of the particular secretion to be performed, although we are here unable to follow the connexion between the means and the end. In some glands, for example, the minute arteries, on their arrival at the organ, suddenly divide into a great number of smaller branches, like the fibres of a camel-hair pencil : this is called the penicillated structure. Sometimes the minute branches, instead of proceeding parallel to each other after their division, separate like rays from a centre, presenting a stellated, or star-like arrange- ment. In the greater number of instances, the smaller arteries take a tortuous course, and are sometimes coiled into spirals, but generally the convolutions are too intricate to admit of being unravelled. It is only by the aid of the microscope that these minute and delicate structures can be rendered visible ; but the fallacy, to which all ob- servations requiring the application of high magni- fying powers are liable, is a serious obstacle to the advancement of our knowledge in this department of physiology. Almost the only result, therefore, which can be collected from these laborious re- searches in microscopic anatomy, is that nature has employed a great diversity of means for the accomplishment of secretion ; but we still remain in ignorance as to the kind of adaptation, which must assuredly exist, of each structure to its respec- 314 THE VITAL FUNCTIONS. tive object, and as to the nice adjustment of chemical affinities which has been provided in order to ac- complish the intended effects.* Electricity is, no doubt, an important agent in all these processes ; but in the absence of all certain knowledge as to the mode in which it is excited and brought into play in the living body, the chasm can for the present be supplied only by remote conjecture.! The process which constitutes the ultimate stage of nutrition, or the actual incorporation of the new material with the solid substance of the body, of which it is to form a part, is involved in equal obscurity with that of secretion. Some light, indeed, has recently been thrown on * The only instance in which we can perceive a correspondence between the ciiemical properties of the secretion, and the kind of blood from'which it is prepared, is in the liver, which, unlike all the other glands, has venous, instead of arterial blood, sent to it for that purpose. The veins, which return the blood that has circulated through the stomach, and other abdominal viscera, are collected into a large trunk, called the vena portce, which enters the liver, and is there again subdivided and ramified, as if it were an artery: its minuter branches here unite Avith those of the hepatic artery, and ramify through the minute lobules which compose the substance of the liver. After the bile is secreted, and carried off by hepatic ducts, the re- maining blood is conducted, by means of minute hepatic veins, which occupy the centres of each lobule, into larger and larger trunks, till they all unite in the vena cava, going directly to the heart. (See Kiernan's Paper on the Anatomy and Physiology of the Liver, Phil. Trans, for 1833, p. 711.) A similar system of venous ramifi- cations, though on a much smaller scale, has been discovered by Jacobson, in the kidneys of most fishes and reptiles, and even in some birds. t It appears, however, to be established by the researches of Donne that there exist, both in plants and in animals, electric currents, determined by the acid or alkaline quality of their fluids, and being probably at the same time the chief source of these pro- perties. Ann. Sc. Nat. serie 2, i. 125. SECRETION. 315 these obscure processes by the discoveries of Schlei- deii and Schwann, relative to the modes of organic developement :* for it is probable that the mode in which the losses of substance in the organs are repaired is similar to that in which those organs were originally formed, and their growth effected. This latter process is now known to consist in the successive evolution of vesicles, or cells, from certain primitive nuclei : and it is probable that the corpuscles with which the blood abounds con- stitute the nuclei from which the nascent organiza- tion takes its rise. Dr. Barry has found that these corpuscles, when effused from the blood-vessels, spontaneously undergo rapid and incessant changes of form, during a certain period; and that they ultimately assume the appearance of cells, which, in the progress of their further developement, ac- quire different characters according to the parti- cular nature of the elementary texture they are destined to compose, whether it be cellular, mus- cular, or nervous.! Valentin has traced the forma- * See Vol. I. p. 56, and 86, notes. t An account of these observations was given in a paper lately read at the Royal Society, (June 4th, 1840), and of which an account will shortly appear in its proceedings. In another paper read to the same Society, on the 18th of June, Mr. Bowman has shown that the primitive fibres, or fasciculi, as he terms them, of the muscles of voluntary motion, are composed of fibrils, (JibrillcB), marked by alternate dark and light points, sometimes resembling a string of beads, and presenting, by the apposition of the fibrils com- posing a fasciculus, the appearance of those transverse stri« which have been often noticed in the microscopical examination of the muscular structure. In these, as well as in other organic structures, he has traced the presence of numerous minute corpuscles, being apparently the nuclei of the cells from which the developement of the fibres had originated. 316 THE VITAL FUNCTIONS. tion of muscular fibres to the linear aggregation and coalescence of blood-globules ; the threads thus constituted having at first the appearance of a necklace of beads, but, subsequently losing the traces of divisions, are gradually converted into perfect cylinders. Schwann added the conjecture that the globules thus observed undergo an inter- mediate change by becoming cells, and that the cylindric muscular fibril is the result of the coales- cence of these cells. Chapter XIII. ABSORPTION. Absor'ption is another function, related to nutri- tion, which deserves special notice. The principal objects of this function are the removal of such materials as have been already deposited, and have become either useless or injurious, and their con- veyance into the general mass of circulating fluids; purposes which are accomplished by a peculiar set of vessels, called the Lymphatics. These vessels contain a fluid, which, being transparent and co- lourless like water, has been denominated the lymph. The lymphatics are perfectly similar in their stnic- ture, and probably also in their mode of action, to the lacteals, which absorb the chyle from the intes- tinal cavity : they are found in all the classes of vertebrated animals, and pervade extensively every part of the body. Exceedingly minute at their origin, they unite together as they proceed, forming larger and larger trunks, generally following the ABSORPTION. 317 course of the veins, till they finally discharge their contents either into the thoracic duct, or into some of the large veins in the vicinity of the heart. Throughout their whole course they are, like the lacteals, provided with numerous valves, 378 T'^IM which, when the vessel is distended with lymph, give it a resemblance to a string of beads, Fig. 378. In warm-blooded animals, the lymphatics are made to traverse, in some part of their course, certain bodies of a compact structure, resembling glands, and termed accordingly, the lymphatic glands. One of these is represented in Fig. 378. They correspond in structure, and probably also in their functions, to the mesenteric glands, through which, in the mammalia, the lacteals pass, before reaching the thoracic duct. It is chiefly in the mammalia, indeed, that these glands are met with, for they are rare among birds, and still more so among fishes and reptiles. In the lower animals it appears that the veins are occasionally endowed with a power of absorption, similar to that possessed by the lymphatics. None of the invertebrata, in- deed, possess lymphatics, and absorption must con- sequently be performed by the veins, when these latter vessels exist. The addition of the system of lymphatic vessels, as auxiliaries to the veins, may therefore be regarded as a refinement in organiza- tion, peculiar to the higher classes of animals.* Muller has lately discovered that the frog, and * Fohmann, who has made extensive researches on the absorbent vessels throughout all the classes of vertebrated animals, has found that they terminate extensively in the veins. See his work, entitled *' Anatomische Untersuchungen uber die Verbindung der Saugadern mit den Venen." 310 THE VITAL FUNCTIONS. several other amphibious animals, are provided with large receptacles for the lymph, situated im- mediately under the skin, and exhibiting distinct and regular pulsations, like the heart. The use of these lymphatic hearts, as they may be called, is evidently to propel the lymph in its proper course along the lymphatic vessels. In the frog four of these organs have been found ; the two posterior hearts being situated behind the joint of the hip, and the two anterior ones on each side of the trans- verse process of the third vertebra, and under the posterior extremity of the scapula. The pulsations of these lymphatic hearts do not correspond with those of the sanguiferous heart ; nor do those of the right and left sides take place at the same times, but they often alternate in an irregular manner.* Chapter XIV. NERVOUS POWER. The organs which are appropriated to the per- formance of the various functions conducive to nutrition, are generally designated the vital organs, in order to distinguish them from those which are subservient to sensation, voluntary motion, and the other functions of animal life. The slightest re- flection on the variety and complication of actions * Phil. Trans, for 1833, p. 89. Muller has discovered similar organs in the Toad, the Salamander, and the Green Lizard ; they have been found in the Crocodile, and other Saurian reptiles, and also in Serpents ; but not in any of the Chelonia. NERVOUS POWER. 319 comprised under the former class of functions in the higher animals, will convince us that they must be the result of the combined operation of several dif- ferent agents ; but the principal source of mechani- cal force required by the vital organs, is still, as in all other cases, the muscular power. The coats of the stomach and of the intestinal tube contain a large proportion of muscular fibres, the contractions of which effect the intermixture and propulsion of the contents of these cavities, in the manner best calculated to favour the chemical operations to which they are to be subjected, and to extract from them all the nourishment they may contain. In like manner, all the tubular vessels which transmit fluids, are endowed with muscular powers adapted to the performance of that office. The heart is a strong hollow muscle, with power adequate to propel the blood with immense force, through the arterial and venous systems. The blood-vessels, also, es- pecially the minute, or capillary arteries, besides being elastic, are likewise endowed with muscular power, which contributes its share in forwarding the motion of the blood, and completing its circu- lation. The quantity of blood circulating in each part, the velocity of its motion, and the heat which it evolves, are regulated in a great measure by the particular mode of action of the blood-vessels of that part. The quantity, and sometimes even the quality of the secretions, are dependent, in like manner, on the conditions of the circulation ; and the action of the ducts, which convey the secreted fluids to their respective destinations, is also re- solvible into the effects of a muscular power. The immediate cause which, in these organs, 320 THE VITAL FUNCTIONS. excites the muscular fibre to contraction, may fre- quently be traced to the forcible stretching of its parts. This is the case in all hollow and tubular muscles, such as the stomach, the heart, and the blood-vessels, when they are mechanically dis- tended, beyond a certain degree, by the presence of contained fluids, or other substances. At other times, the chemical quality of their contents ap- pears to be the immediate stimulus inciting them to contraction. But numerous instances occur, in the higher orders of animals, in which these causes alone are inadequate to explain the phenomena of the vital functions. No mechanical hypothesis will suffice to account for the infinite diversity in the modes of action of the organs which perform these functions, oc aftbrd any clue to the means by which they are made to co-operate, with such nicety of adjustment, in the production of the ultimate effects. Still less will any theory, comprising only the agency of the muscular power, and the ordinary chemical affini- ties, enable us to explain, how an irritating cause, applied at one part, shall produce its visible effects on a distant organ ; or in what way remote and apparently unconnected parts shall, as if by an invisible sympathy, be brought at. the same moment to act in concert, in the production of a common effect. Yet such co-operation must, in innumerable cases, be absolutely indispensable to the perfect ac- complishment of the vital functions of animals. Nature has not neglected objects so important to the success of her measures ; but has provided, for the accomplishment of these purposes, a controlling faculty, residing in the nervous system, and deno- minated the nervous power. Experiments have NERVOUS POWER. 321 shown that the due performance of the vital func- tions of digestion, of circulation, and of secretion, requires the presence of an agency, derived from different parts of the brain and spinal cord, and regulating the order and combinations of the actions of the organs which are to perform those functions. The same influence, for example, which increases the power of secretion in any particular gland, is found to increase, at the same time, the action of those blood-vessels which supply that gland with the materials for secretion ; and conversely, the increased action of the blood-vessels is accompa- nied by an increased activity of the secreting organ. Experience also shows that when the in- fluence of the brain and spinal cord is intercepted, although the afflux of blood may, for a time, con- tinue, yet the secretion ceases, and all the functions dependent upon secretion, such as digestion, cease likewise. Thus the nervous power combines toge- ther different operations, adjusts their respective degrees, and regulates their succession, so as to ensure that perfect harmony which is essential to the attainment of the objects of the vital functions ; and thus, not only the muscular power which re- sides in the vital organs, but also the organic affini- ties which produce secretion, and all those unknown causes which effect the nutrition, developement, and growth of each part, are placed under the con- trol of the nervous power.* * As the functions of plants are sufficiently simple to admit of being conducted without the aid of muscular power, still less do they require the assistance of the nervous energy ; both of which proper- ties are the peculiar attributes of animal vitality. We accordingly find no traces either of nervous or of muscular fibres in any of the vegetable structures. VOL. II. Y 322 THE VITAL FUNCTIONS. Although we are entirely ignorant of the nature of the nervous power, we know that, when em- ployed in the vital functions, it acts through the medium of a particular set of fibres, which form part of the nervous system, and are classed, there- fore, among the nerves. The principal filaments of this class of nerves compose what is called the sympathetic nerve, from its being regarded as the medium of extensive sympathies among the organs ; but the whole assemblage of these nerves is more commonly known by the name of the ganglionic system, from the circumstance of their being con- nected with small masses of nervous substance, termed ganglia, which are placed in different parts of their course. Fig. 379, represents a ganglion (g), at which numerous filaments of nerves are collected ; some entering into the ganglion, and others passing- out of it, and afterwards divided into smaller fila- ments F and B. Within the ganglion, there is always found a mass, more or less considerable, of a substance apparently similar to the soft exterior, or cortical portion of the brain : or, as it has also been termed o^ grey neurine. The nervous filaments Avhich are connected with this substance may be distinguished into the afferent nerves, or those which convey impressions arising at a distant, or peri- phral part, to the central neurine ; and the efferent nerves, or those which transfer impressions from these centres to remoter parts, whether those parts be muscular fibres, blood-vessels, or secreting- organs. The ganglia of the sympathetic nerve are con- nected by nervous filaments with every part of the brain and spinal cord, the great central organs of the nervous system ; and they also send out innu- NERVOUS POWER. 323 merable branches to be distributed all over the body. All the parts receiving blood-vessels, and more especially the organs of digestion, are abun- dantly supplied with ganglionic nerves ; so that, by their intervention, all these parts have extensive connexions with the brain and spinal cord, and also with one another. A great portion of the spinal cord itself may also be considered as performing the functions I have here assigned to the ganglia, and consequently as forming part of the ganglionic system : for although it differs much from the ganglia in form and appearance, yet it is found to correspond with them in all the essential charac- ters of its structure, being composed of a mass of grey neurine, at which white nervous filaments either arise or terminate. These filaments, like those of the ganglia, may be regarded, according to their functions, as either afferent or efferent ;* the former serving to convey impressions from the remote extremity of the filament to the central grey neurine, and the latter to transfer from thence a corresponding impression to other parts. All this may be done without participation of the brain, or of any other part of the nervous system ; and with- out the occurrence of sensation, volition or any other mental affection. The numerous communications and interchanges of filaments, which subsequently take place at various parts, forming what is called a plexus, are shown in Fig. 380 : where four trunks (t, t) divide into branches, which are again sepa- * Dr. Marshall Hall, who is the author of this distinction, has termed these two classes of fibres, respectively, the incident and the reflex nerves : and the whole system, comprehending the spinal cord and ganglia, performing the above mentioned functions, he terms the excito-motory system. .324 THE VITAL FUNCTIONS. rated, and variously reunited in their course, like a ravelled skein of thread, before they proceed to their respective destinations. Thus the ganglionic, or excito-motory system of Dr. M. Hall, furnish points of union between ner- vous fibres belonging to various organs, which are often situated very remotely from one another ; and these uniting parts may be regarded as so many separate centres of nervous power: the functions of this system being, first, to serve as the channels through which the affections of one organ may be enabled to influence a distant organ ; and secondly, to be the medium through which the powers of several parts may be combined and concentrated for effecting particular purposes, requiring such co-operation. Hence it is by means of this system that all the organs and all the functions are ren- dered efficient in the production of a common ob- ject, and are brought into one comprehensive and harmonious system of operation.* The nervous power, the efl'ects of which we are * We shall afterwards find that the greater part of the nervous systems of invertebrate animals execute functions of this description : not only the movements of the vital organs, but also many of those of the external instruments of motion being perfoi-med apparently without sensation or consciousness. NERVOUS POWER. 32o here considering, should be carefully distinguished from that power which is an attribute of another portion of the nervous system, and which, being connected with sensation, volition, and other intel- lectual operations, has been denominated sensorial power* The functions of digestion, circulation, absorption, secretion, and all those included under the class of nutrient or vital functions, are carried on in secret, are not necessarily, or even usually attended with sensation, and are wholly removed from the control of volition. Nature has not per- mitted processes, which are so important to the preservation of life, to be in any way interfered with by the will of the animal. We know that in ourselves they go on as well during sleep as when we are awake, and whether our attention be di- rected to them or not; and though occasionally influenced by strong emotions, and other affections of the mind, they are in general quite independent of every intellectual process. In the natural and healthy condition of the system, all its internal operations proceed quietly, steadily, and con- stantly, whether the mind be absorbed in thought or wholly vacant. The kind of existence resulting from these functions alone, and to which our atten- tion has hitherto been confined, must be regarded as the result of mere vegetative, rather than of ani- mal life. It is time that we turn our views to the higher objects, and more curious field of inquiry, belonging to the latter. * This distinction has been most clearly pointed out, and illus- trated by Dr. A. P. W. Philip. See his " Ex|)erimental Inquiry into the Laws of the Vital Functions." PART III. THE SENSORIAL FUNCTIONS. Chapter I. SENSATION. The system of mechanical and chemical functions which we have been occupied in reviewing, has been established only as a foundation for the en- dowment of those higher faculties which constitute the great objects of animal existence. It is in the study of these final purposes that the scheme of nature, in the formation of the animal world, opens and displays itself in all its grandeur. The whole of the phenomena we have hitherto considered concur in one essential object, the maintenance of a simply vital existence. Endowed with these pro- perties alone, the organized system would possess all that is absolutely necessary for the continuance and support of mere vegetative life. The machinery provided for this purpose is perfect and complete in all its parts. To raise it to this perfection, not only has the Divine Architect employed all the properties and powers of matter which science has yet revealed to man, but has also brought into play the higher and more mysterious energies of nature, and has made them to concur in the great SENSATION. 327 work that was to be performed. On tlie organized fabric there has been conferred a vital force ; witli the powers of mechanism have been conjoined those of chemistry ; and to these have been super- added the still more subtle and potent agencies of caloric and of electricity : every resource has been employed, every refinement practised, every com- bination exhausted that could ensure the stability, and prolong the duration of the system, amidst the multifarious causes which continually menace it with destruction. It has been supplied with ample means of repairing the accidents to which it is ordinarily exposed ; it has been protected from the injurious influence of the surrounding elements, and fitted to resist for a lengthened period the inroads of disease, and the progress of decay. But can this, which is mere physical existence, be the sole end of life ? Is there no further purpose to be answered by structures so ex(piisiteiy con- trived, and so bountifully provided with the means of maintaining an active existence, than the mere accumulation and cohesion of inert materials, dif- fering from the stones of the earth only in the more artificial arrangement of their particles, and the more varied configuration of their texture ? Is the growth of an animal to be ranked in the same class of phenomena as the concretion of a pebble, or the crystallization of a salt? Must we not ever asso- ciate the power of feeling with the idea of animal life? Can we divest ourselves of the persuasion that the movements of animals, directed, like our own, to obvious ends, proceed from voluntary acts, and imply the operation of an intellect, not wholly 328 THE SENSORIAL FUNCTIONS. dissimilar in its spiritual essence from oiir own? In vain may Descartes and his followers labour to sustain their paradox, that brutes are only auto- mata,— mere pieces of artificial mechanism, insen- sible either to pleasure or to pain, and incapable of internal aft'ections, analogous to those of which we are conscious in ourselves. Their sophistry will avail but little against the plain dictates of the understanding. To those who refuse to admit that enjoyment, which implies the powers of sensation and of voluntary motion, is the great end of animal existence, the object of its creation must for ever remain a dark and impenetrable mystery ; by such minds must all further inquiry into final causes be at once abandoned as utterly vain and hopeless. But it surely requires no laboured refutation to overturn a system that violates every analogy by which our reasonings on these subjects must neces- sarily be guided ; and no artificial logic or scholastic jargon will long prevail over the natural sentiment, which must ever guide our conduct, that animals possess powers of feeling and of spontaneous action, and faculties appertaining to those of intellect. The functions of sensation, perception, and volun- tary motion require the presence of an animal sub- stance, which we find to be organized in a })eculiar manner, and endowed with very remarkable pro- perties. It is called neurine, or the medullary substance; and it composes the greater part of the texture of the brain, spinal cord, and nerves; organs, of which the assemblage is known by the general name of the nervous system. Certain affec- tions of particular portions of this medullary sub- stance, generally occupying some central situation. NKRVOUS SYSTEM. 3*29 are, in a way that is totally inexplicable, connected with affections of the sentient and intelligent prin- ciple ; a principle which we cannot any otherwise conceive than as being distinct from matter ; although we know that it is capable of being affected by matter operating through the medium of this nervous substance, and that it is capable of reacting upon matter through the same medium. Of the truth of these propositions there exist abun- dant proofs ; but as the evidence which establishes them will more conveniently come under our notice at a subsequent period of our inquiry, I shall post- pone their consideration ; and, proceeding upon the assumption that this connexion exists, shall next inquire into the nature of the intervening steps in the process, of which sensation and perception are the results. Designating, then, by the name of hrain this primary and essential organ of sensation, or the organ of which the physical affections are immedi- ately attended by that change in the percipient being which we term sensation; let us first inquire what scheme has been devised for enabling the brain to receive impressions from such external objects, as it was intended that this sentient being- should be capable of perceiving. As these objects can, in the first instance, make impressions only on the organs situated at the surface of the body, it is evidently necessary that some medium of com- munication should be provided between the ex- ternal organ and the brain. Such a medium is found in the nerves, which are white cords, consist- ing of bundles of threads or filaments of medullary matter, enveloped in sheaths of membrane, and .330 THE SENSORIAL FUNCTIONS. extending continuously from the external organ to the brain, where they all terminate. It is also in- dispensably requisite that these notices of the pre- sence of objects should be transmitted instantly to the brain ; for the slightest delay would be attended with serious evil, and might even lead to fatal con- sequences. The nervous power, of which, in our review of the vital functions, we noticed some of the operations, is the agent employed by nature for this important office of a rapid communication of im- pressions. The velocity with which the nerves sub- servient to sensation transmit the impressions they receive at one extremity, along their whole course, to their termination in the brain, exceeds all mea- surement, and can be compared only to that of electricity passing along a conducting wire. These nerves may, in fact, be regarded as constituting a system of electric telegraphs, established by nature as the general medium of instantaneous transmis- sions of sensorial agencies between all, and even the most distant parts of the body. It is evident, therefore, that the brain requires to be furnished with a great number of these nerves, which perform the office of conductors of the subtle influence in question ; and that these nerves must extend from all those parts of the body which are to be rendered sensible, and must unite at their other extremities in that central organ. It is of especial importance that the surface of the body, in particular, should communicate all the impressions received from the contact of external bodies ; and that these impressions should produce the most distinct perceptions of touch. Hence we find that the skin, and all those parts of it more particularly NERVOUS SYSTEM. 331 intended to be the organs of a delicate touch, are most abundantly supplied with nerves ; each nerve, however, communicating a sensation distinguishable from that of every other, so as to enable the mind to discriminate between them, and refer them to their respective origins in different parts of the sur- face. It is also expedient that the internal organs of the body should have some sensibility ; but it is better that this should be very limited in degree, since the occasions are few in which its exercise would be useful, and many in which it would be positively injurious: hence the nerves of sensation are distributed more sparingly to these organs. It is not sufficient that the nerves of touch should communicate the perceptions of the simple pressure or resistance of the bodies in contact with the skin: they should also furnish indications of other qua- lities in those bodies, of which it is important that the mind be apprized; such, for example, as warmth, or coldness. Whether these different kinds of im- pressions are all conveyed by the same nervous fibres it is difficult, and perhaps impossible to determine. When these nerves are acted upon in a way which threatens to be injurious to the part im- pressed, or to the system at large, it is also their province to give warning of the impending evil, and to rouse the animal to such exertions as may avert it ; and this is effected by the sensation of pain, which the nerves are commissioned to excite on all these occasions. They act the part of sentinels, placed at the outposts, to give signals of alarm on the approach of danger. , Sensibility to pain must then enter as a neces- 332 THE SENSORIAL FUNCTIONS. sary constituent among the animal functions; for had this property been omitted, the animal system would have been but of short duration, exposed, as it must necessarily be, to perpetual casualties of every kind. Lest any imputation should be at- tempted to be thrown on the benevolent intentions of the great Author and Designer of this beautiful and wondrous fabric, so expressly formed for varied and prolonged enjoyment, it should always be borne in mind that the occasional suffering, to which an animal is subjected from this law of its organization, is far more than counterbalanced by the consequences arising from the capacities for pleasure, with which it has been beneficently or- dained that the healthy exercise of the functions should be accompanied. Enjoyment appears uni- versally to be the main end, tlie rule, the ordinary and natural condition ; while pain is but the casualty, the exception, the necessary remedy, which is ever tending to a remoter good, in subordi- nation to a higher law of creation. It is a wise and bountiful provision of nature that each of the internal parts of the body has been endowed with a particular sensibility to those im- pressions which, in the ordinary course, have a ten- dency to injure its structure ; while it has at the same time been rendered nearly, if not completely, insensible to those which are not injurious, or to which it is not likely to be exposed. Tendons and ligaments, for example, are insensible to many causes of mechanical irritation, such as cutting, pricking, and even burning ; but the moment they are violently stretched, (that being the mode in which they are most liable to be injured,) they SENSATION. 333 instantly communicate a feeling of acute pain. The bones, in like manner, scarcely ever communi- cate pain in the healthy state, except from the appli- cation of a mechanical force which tends to fracture them. anin all animals provided with a nervous system, those nerves which convey the impressions of touch, are universally present in all classes ; and among- the lowest orders, they appear to constitute the sole medium of communication with the external world. As we rise in the scale of animals we find the faculties of perception extending to a wider range ; and many qualities, depending on the chemical action of bodies, are rendered sensible, more espe- cially those which belong to the substances em- ployed as food. Hence arises the sense of taste, which may be regarded as a new and more refined species of touch. This difference in the nature of the impressions to be conveyed, renders it neces- sary that the structure of the nerves, or at least of those parts of the nerves which are to receive the impression, should be modified and adapted to this particular mode of action. As the sphere of perception is enlarged, it is made to comprehend, not merely those objects which are actually in contact with the body, but also those which are at a distance, and of the exist- ence and properties of which it is highly important that the animal, of whose sensitive faculties we are examining the successive endowment, should be apprized. It is more especially necessary that he should acquire an accurate knowledge of the dis- tances, situations, and motions of surrounding- objects. Nature has accordingly provided suitable 334 THE SENSORIAL FUNCTIONS. organizations for vision, for hearing, and for the perception of odours; all of which senses establish extensive relations betvAeen him and the external world, and give him the command of various objects which are necessary to supply his wants, or procure him gratification ; and which also apprize him of danger while it is yet remote, and may be avoided. Endowed with the power of combining all these perceptions, he commences his career of sensitive and intellectual existence ; and though he soon learns that he is dependent for most of his sensations on the changes which take place in the external world, he is also conscious of an internal power, which gives him some kind of control over many of those changed, and he knows that he moves his limbs by his own voluntary act ; move- ments which originally, and of themselves, appear, in most animals, to be productive of great en- joyment. To a person unused to reflection, the phenomena of sensation and perception may appear to require no elaborate investigation. That he may behold external objects, nothing more seems necessary than directing his eyes towards them. He feels as if the sight of those objects were a necessary consequence of the motion of his eye-balls, and he dreams not that there can be any thing marvellous in the func- tion of the eye, or that any other organ is concerned in this simple act of vision. If he wishes to ascer- tain the solidity of an object within his reach, he knows that he has but to stretch forth his hand, and to feel in what degree it resists the pressure he gives to it. No exertion even of this kind is re- quired for hearing the voices of his companions, or SENSATION. 335 being apprized, by the increasing loudness of the sound of falling waters, as he advances in a par- ticular direction, that he is coming nearer and nearer to the cataract. Yet how much is really implied in all these apparently simple phenomena! Science has taught us that these perceptions of external objects, far from being direct or intuitive, are only the tinal results of a long series of opera- tions, produced by agents of a most subtle nature, which act by curious and complicated laws, upon a refined organization, disposed in particular situa- tions in our bodies, and adjusted with admirable art to receive their impressions, to modify and combine them in a certain order, and to convey them in regular succession, and without confusion, to the immediate seat of sensation. Yet this process, complicated as it may appear, constitutes but the first stage of the entire function o^ perception : for before the mind can arrive at a distinct knowledge of the presence and peculiar qualities of the external object which gives rise to the sensation, a long series of mental changes must intervene, and many intellectual operations must be performed. All these take place in such rapid succession, that even when we include the move- ment of the limb, which is consequent upon the perception, and which we naturally consider as part of the same continuous action, the whole appears to occupy but a single instant. On a careful analysis of the phenomena, however, as I shall afterwards attempt to show, we find that no less than twelve distinguishable kinds of changes, or rather processes, some of which imply many changes, must always intervene, in regular succes- 330 THK SENSORIAL FUNCTIONS. sioii, between the action of the external object on the organ of sense, and the voluntary movement of the limb which it excites. The external agents, which are capable of affect- ing the different parts of the nervous system, so as to produce sensation, are of different kinds, and are governed by laws peculiar to themselves. The structure of the organs must, accordingly, be adapted, in each particular case, to receive the impressions made by these agents, and must be modified in exact conformity with the physical laws they obey. Thus the structure of that portion of the nervous system which receives visual im- pressions, and which' is termed the retina, must be adapted to the action of light ; and the eye, through which the rays are made to pass before reaching the retina, must be constructed with strict reference to the laws of optics. The ear must, in like manner, be formed to receive delicate impressions from those vibrations of the air which occasion sound. The extremities of the nerves, in these and other organs of the senses, are spread out into a delicate expansion of surface, having a softer and more uniform texture than the rest of the nerve ; whereby they acquire a susceptibility of being affected by their own appropriate agents, and by no other. The function of each nerve of sense is determinate, and can be executed by no other part of the nervous system. These functions are not interchangeable, as is the case with many others in the animal system. No nerve, but the optic nerve, and no part of that nerve, except the retina, is capable, however impressed, of giving rise to the sensation of light : no part of the nervous system, but the SENSATION. <7 3;37 auditory nerve can convey that of sound ; and so of the rest.* In almost every case the impression made upon the sentient extremity of the nerve which is appro- priated to sensation, is not the direct effect of the external body, but results from the agency of some intervening medium. There is always a portion of the organ of sense interposed between the object and the nerve on which the impression is to be made. The object is never allowed to come into direct contact with the nerves ; not even in the case of touch, where the organ is defended by the cuticle, through which the impression is made, and by which that impression is modified so as to produce the proper effect on the subjacent nerves. This observation applies with equal force to the organs of taste and of smell, the nerves of which are not only sheathed with cuticle, but defended from too violent an action by a secretion expressly provided for that purpose. In the senses of hear- ing and of vision, the changes which take place in the organs interposed between the external im- pressions and the nerves, are still more remarkable and important, and will be respectively the subjects of separate inquiries. The objects of these senses, as well as those of smell, being situated at a dis- tance, produce their first impressions by the aid * The credulity of the pubhc has sometimes been imposed upon by persons who pretended to see by means of their fingers : thus, at Liverpool, the celebrated Miss M'Avoy contrived for a long time to persuade a great number of persons that she really possessed this miraculous power. Equally unworthy of credit are all the stories of persons, under the influence of animal magnetism, hearing sounds addressed to the pit of the stomach, and reading the pages of a book applied to the skin over that organ. VOL. II. Z 338 THE SENSORIAL FUNCTIONS. of some medium, exterior to our bodies, through which their influence extends; thus, the air is the usual medium through which both lis-ht and sound are conveyed to our organs. Hence, in order to understand the wliole series of phenomena be- longing to sensation, regard must be had to the physical laws which regulate the transmission of these agents. We are now to consider these intermediate processes in the case of each of the senses. Chapter II. TOUCH. I HAVE already had occasion to point out the struc- ture of the integuments, considered in their mecha- nical office of protecting the general frame of the body ; but we are now to view them in their rela- tion to the sense of touch, of which they are the immediate organ. It will be recollected that the corium forms the principal portion of the skin ; that the cuticle composes the outermost layer ; and that between these there occurs a thin layer of a substance, termed the corpus mucosum. The co- rium is constructed of an intertexture of dense and tough fibres, through which a multitude of blood vessels and nerves are interspersed ; but its ex- ternal surface is more vascular than any other part, exhibiting a fine and delicate network of vessels, which is termed by anatomists the vas- cular plexus. This portion of the skin is most TOUCH. 330 acutely sensible in every point: hence, we may infer that it contains the terminations of all the nervous filaments distributed to this organ, which are here found to be divided to an extreme degree of minuteness. When examined with the microscope, this ex- ternal surface presents a great number of minute projecting filaments. Malpighi first discovered this structure in the foot of a pig ; and he gave these prominences the name of papillce. The ulti- mate ramifications of the nerves of touch have been distinctly traced by Breschet* into the sub- stance of the papillae, where they terminate by forming arches at the extremities of each of these processes. We may therefore consider these papillae, of which the assemblage has been termed the corpus papillare, as the principal and immediate organ of touch. This structure is particularly conspicuous on those parts of the skin which are more especially appropriated to this sense, such as the tips of the fingers, the tongue, and the lips ; in other parts of the surface, which are endowed with less sensibility, the papillae are scarcely visible, even with the aid of the microscope. The surface of the corium is exquisitely sensible to all irritations, whether proceeding from the con- tact of foreign bodies, or from the impression of atmospheric air. This extreme sensibility of the corium would be a source of constant torment, were it not defended by the cuticle, which is unprovided with either blood-vessels or nerves, and is, there- fore, wholly insensible. For the same reason, also, * Recherches sur la structure de la peau. Paris, 1835. 340 THE SENSORIAL FUNCTIONS. it is little liable to change, and is thns, in both respects, admirably calculated to afford protection to the finely organized corium. Although the cuticle exhibits no traces of vascu- larity, it is by no means to be regarded as a dead or inorganic substance, like the shells of the mol- lusca. That it is still part of the living system is proved by the changes it frequently undergoes, both in the natural and the diseased conditions of the body. It is perpetually, though slowly, under- going decay and renovation ; its external surface drying off in minute scales, and in some animals peeling off in large portions. When any part of the human skin is scraped with a knife, a grey dust is detached from it, which is found to consist of minute scales. By repeated friction, or pressure of any part of the skin, the cuticle soon acquires an increase of thickness and of hardness : this is observable in the soles of the feet, and palms of the hands, and in the fingers of those who make much use of them in laborious work. But this greater thickness in the parts designed by nature to suffer considerable pressure, is not entirely the effect of education ; for the cuticle, which exists before birth, is found, even then, to be much thicker on the soles of the feet, and palms of the hands, than on other parts. This example of provident care in originally adjusting the structures of parts to the circumstances in which they are to be placed at an after period, would of itself, were it a solitary instance, be well fitted to call forth our admiration. But as we study each department of the animal economy in detail, the proofs of design in the adaptation of TOUCH. 341 organs to their respective purposes multiply upon us in such profusion, that we are apt to overlook individual instances, unless they are especially brought to our notice. How often have we wit- nessed and profited by the rapid renewal of the cuticle, when by any accident it has been de- stroyed, without adverting to the nature of the pro- cess which it implies; or reflected that different sets of glands, with all their varied apparatus of ducts, blood-vessels and absorbents must, on all these occasions, supply the materials, out of which the new cuticle is to be formed, must effect their combination in the requisite proportions, and must deposit them in the precise situations in which they are wanted ! Different animals present remarkable differences in the thickness and texture of the cuticle, accord- ing to the element they are destined to inhabit, and the situations in which they are most frequently placed. Provision is in many cases made for pre- serving the cuticle from the injury it would receive from the long continued action of the air or water; for it is apt to become rigid, and to peel off, from exposure to a very dry atmosphere; and the con- stant action of water, on the contrary, renders it too soft and spongy. In order to guard against both these evils, the skin has been furnished, in various parts of its surface, with a secreting apparatus, which pours out unctuous or mucilaginous fluids ; the oily secretions being more particularly em- ployed as a defence against the action of the air, and the mucilaginous fluids as a protection against that of water. The conditions on which the perfection of the .342 THE SENSORIAL FUNCTIONS. sense of touch depends are, first, an abundant provi- sion of soft papillae supplied with numerous nerves ; secondly, a certain degree of fineness in the cuticle; thirdly, a soft cushion of cellular substance beneath the skin ; fourthly, a hard resisting basis, such as that which is provided in the nails of the human fingers ; and lastly, it is requisite that the organ be so constructed as to be capable of being readily applied, in a variety of directions, to the unequal surfaces of bodies ; for the closer the contact, the more accurate will be the perceptions conveyed. In forming an estimate of the degree of perfection in which the sense is exercised in any particular animal, we must, accordingly, take into account the mobility, the capability of flexion, and the figure of the parts employed as organs of touch. As touch is the most important of all the senses, inasmuch as it is the foundation of all our know- ledge of the material world, so its relative degrees of perfection establish marked differences in the intellectual sagacity of the several tribes, and have a considerable influence on the assignment of their proper station in the scale of animals. Although the power of receiving obscure impres- sions from the contact of external bodies, and of perceiving variations of temperature, is probably possessed by all animals, a small number only are provided with organs specially appropriated for conveying the more delicate sensations of touch. The greater part of the surface of the body in the testaceous Mollusca is protected by a hard and in- sensible covering of shell. The integuments of Insects, especially those of the Coleoptera, are in general too rigid to receive any fine impressions TOUCH. 343 from the bodies which may come in contact with them ; and the same observation applies, with even greater force, to the Crustacea. The scales of Fishes, and of Reptiles, the solid encasements of the Chelonia, the plumage of Birds, the dense coating of the Armadillo, the thick hides of the Rhinoceros, and other Pachydermata, are evidently incompatible with any delicacy of touch. This nicer faculty of discrimination can be enjoyed only by animals having a soft and flexible integument, such as all the naked Zoophytes, Worms and Mol- lusca, among the lower orders, and Serpents, among the higher. The flexibility of the body or limbs is another condition which is extremely necessary towards procuring extensive and correct notions of the relative positions of external objects. It is es- sential therefore that those instruments which are more particularly intended as organs of touch, should possess this property. It will not be necessary to enter into a minute description of these organs, because they have, for the most part, been already noticed as instruments of motion or prehension ; for the sense of touch is in general exercised more particularly by the same parts which perform this latter function. Thus the tentacula of the various tribes of Polypi, of Acti- niae, and of Annelida, are organs both of prehension and of touch. The tubular feet of the Asteriasand Echinus are subservient both to the sense of touch, and to the faculty of progressive motion. The feet of Insects and of Crustacea are well calculated, indeed, by their jointed structure, for being applied to the surfaces, and different sides of bodies ; but they are scarcely ever employed in this capacity ; 344 THE SENSORIAL FUNCTIONS. being superseded by the palpi, which are situated near the mouth. When insects are walking, the palpi are incessantly applied to the surface on which they advance, as if these organs were em- ployed to feel their way.* There can be little doubt, however, that, in most insects, the principal organs of touch are the Antennce, also denominated, from their supposed office, the Jeelers.\ Some idea of the great variety in the forms of the antennae of insects may be obtained from the specimens delineated in Fig. 381, which shows a few of the most remarkable.f The universality of these organs among every species of this extensive class of animals, their great flexibility, arising from their jointed struc- * The more special function of the palpi is to examine the sub- stances used for food, before they are taken into the mouth. t The German name for them, fiihlhoriier, or the feeling horns, is founded on the same notion. I In this fi^^ure, A represents the form of antennae, technically denominated Antenna capitulo uncinato, as exemplified in the Pausus. B. is the A. piioso-verticillata, as in the Psychoda ocellaris. C. .A. biclavata ( Claviger longicornis ) . D. .A. triangularis, (Lophosia). E. . A. clavata, ( Masaris). F. .A. capit lamellato, ( Melolontha tnas). G..A. capit. fissile, ( Aj)hodius fossor ) . H.,A. fusiformis, ( Zygcsna). I. .A. capitata, (Ascalaphus). K..A. furcata, ( Scliizocerus furcatus ) : L. .A. bipectinata, ( Ctenophora). M..A. irregularis, ( Agaon paradoxum). N. . A. cordata, ( Diaperis boleti). O. .A. bipectinata, ( Bombyx). P. .A. palmata, ( Nepa cineren). (J.. A. ensiformis, ( Truxalis). 11.. A. setacea, ( Cerambyx). TOUCH. 345 ture,* their incessant motion when the insect is walking, and their constant employment in exa- minins: the surfaces of all the bodies with which they come in contact, sufficiently point them out as instruments of a very delicate sense of touch. Organs of this kind were particularly necessary to insects, since the horny nature of the integuments of the greater number precludes them from impart- ing any accurate perceptions of touch. It has been conjectured that the antennae of insects are the organs of other senses besides that of touch. If an insect be deprived of its antennae, it either remains motionless, or if it attempt to walk or fly, appears bewildered, and moves without any * The number of segments into which these organs are divided is often very great. In the Gryllotalpa, or mole cricket, it amounts to above 100. (Kidd, Phil. Trans, for 1825, p. 211.) This insect has, besides the antennae on the head, two posterior or caudal an- tennae, which are not jointed, excepting at their very com.mencement. These are extremely sensible, and serve probably to give the animal notice of the approach of any annoyance from behind, lb. p. 216. 346 THE SENSORIAL FUNCTIONS. apparent object. Huber found that Bees are en- abled, by feeling with their antennae, to execute their various works in the interior of the hive, where, of course, they can have no assistance from light. They employ these organs perpetually while building the combs, pouring honey into the maga- zines, ascertaining the presence of the queen, and feeding and tending the larvae. The same naturalist observes, also, that it is principally by means of the antennae that these social insects communicate to one another their impressions and their wants. The different modes in which Ants, when they happen to meet during their excursions, mutually touch one another with their antennae, or other parts of the body, appears to constitute a kind of natural language understood by the whole tribe. This contact of the antennae evidently admits of a great variety of moditications, and seems capable of supplying all the kinds of information which these insects have occasion to impart. It would seem impossible, indeed, for all the individuals composing these extensive societies to co-operate effectually in the execution of many works, calcu- lated for the general benefit of the community, unless some such means of communication existed. There is no evidence that sound is the medium of this intercourse ; for none, audible to us at least, was ever known to be emitted by these insects. Their mode of communication appears to be simply by touching one another in different ways with the antennae. Ruber's observations on this subject are exceedingly curious.* He remarks that the signal * See his " Rechercbes sur les mceiirs dcs founnis indigenes." I TOUCH. 347 denoting the apprehension of danger, is made by the ant striking its head against the corselet of every ant which it chances to meet. Each ant, on receiving this intimation, immediately sets about repeating the same signal to the next ant which comes in its way ; and the alarm is thus dissemi- nated with astonishing rapidity throughout the whole society. Sentinels are at all times stationed on the outside of the nests, for the purpose of ap- prizing the inhabitants of any danger that may be at hand. On the attack of an enemy, these guar- dians quickly enter into the nest, and spread the intelligence on every side ; the whole swarm is soon in motion, and while the greater number of ants rush forwards with desperate fury to repel the attack, others who are entrusted with the office of guarding the eggs and the larvae, hasten to remove their charge to places of greater security. When the queen bee is forcibly taken away from the hive, the bees which are near her at the time, do not soon appear sensible of her absence, and the labours of the hive are carried on as usual. It is seldom before the lapse of an hour, that the working bees begin to manifest any symptoms of uneasiness: they are then observed to quit the larvae which they had been feeding, and to run about in great agita- tion, to and fro, near the cell which the queen had occupied before her abduction. They then move over a wider circle, and on meeting with such of their companions as are not aware of the disaster, communicate the intelligence by crossing their an- tennae and striking lightly with them. The bees which receive the news become in their turn agi- tated, and conveying this feeling wherever they go, 348 THE SENSORIAL FUNCTIONS. the alarm is soon spread among all the inhabitants of the hive. All rush forwards with tumultuous precipitation, eagerly seeking their lost queen ; but after continuing the search for some hours, and finding it to be fruitless, they appear resigned to their misfortune ; the noisy tumult subsides, and the bees quietly resume their labours. A bee, deprived of its antennae, immediately becomes dull and listless ; it desists ^from its usual labours, remains at the bottom of the hive, seems attracted only by the light, and takes the first opportunity of quitting the hive, never more to return. A queen bee, thus mutilated, ran about, without apparent object, as if in a state of delirium, and was incapable of directing her trunk with pre- cision to the food which was offered to her. Latreille relates that, having deprived some labouring ants of their antennae, he replaced them near the nest ; but they wandered in all directions, as if bewildered, and unconscious of what they were doing. Some of their companions were seen to notice their dis- tress, and approaching them with apparent com- passion, applied their tongues to the wounds of the sufferers, and anointed them with their saliva. This trait of sensibility was repeatedly witnessed by Latreille, while watching their movements with a magnifying glass. The Arachnida, from the mobility of their limbs, and the thinness of their cutaneous investment, have a very delicate sense of touch. Among the Mollusca, it is only the higher orders of Cephalo- poda that enjoy this sense in any considerable degree ; and they are enabled to exercise it by means of their long and flexible tentacula. Many TOUCH. 349 bivalve mollusca have, indeed, a set of tentaciila placed near the mouth, but they are short, and of little power. It is probable that the foot may also be employed by these animals as an organ of touch. Fishes are, in general, very ill -constructed for the exercise of this sense; and their fins are used for no other purposes than those of {progressive motion. That part of the surface which possesses the most acute feeling is the under-side, where the integuments are the thinnest. The chief seat of the sense of touch, however, is the lip, or end of the snout, which is largely supplied with nerves ; and perhaps the cirrhi, or little vermiform pro- cesses called barbels, which in some species are appended to the mouth, may be subservient to this sense. These kind of tentacula are remarkable for their length and mobility in the Lophius piscalorius, or Angler; and it is said that they are employed by the fish, while lurking in ambush, as a decoy to other fishes, which they entice by their resem- blance to worms. These processes in the Silurus gianis are moved by particular muscles. Serpents, from the great flexibility of their spine, are capable of grasping and twining round objects of almost any shape, and of taking, as it were, their exact measure. This conformation must be exceedingly favourable to the acquisition of correct perceptions of touch. As it is these perceptions, which, as we shall afterwards find, lay the founda- tion of the most perfect acquaintance with the tan- gible properties of surrounding bodies, we may presume that this power contributes much to the sagacity possessed by these animals. It has been .350 THE SENSORIAL FUNCTIONS. said of Serpents, that their whole body is a hand, conferring some of the advantages of that instru- ment. Helhnan has shown that the slender bifur- cated tongue of these animals is used for the pur- poses of touch. In those species of Lizards which are enabled by the structure of their feet to clasp the branches of trees, as the Gecko and the Chameleon, and whose tails also are prehensile, we must, for the same reason, presume that the sense of touch exists in a more considerable degree than in other Saurian Reptiles, which do not possess this advantage. The toes of Birds are also well calculated to per- form the office of organs of touch, from the number of their articulations and their divergent position, and from the papillae with which their skin abounds ; accompanied as they are with a large supply of nerves. Those birds, which, like the Parrot, employ the feet as organs of prehension, probably enjoy a greater developement of this sense. The skin which covers the bills of aquatic birds is sup- plied by very large nerves, and consequently pos- sesses great sensibility. This structure enables them to find their food, which is concealed in the mud, by the exercise of the sense of touch residing in that organ. A similar structure, probably serving a similar purpose, is found in the Orni- thorhynclius. Among Mammalia, we find the seat of this sense frequently transferred to the lips, and extremity of the nostrils ; and many have the nose prolonged and flexible, apparently with this view. This is the case with the Shrew and the Mole, which are burrowing animals, and still more remarkably with TOUCH. 351 the Pachyderniata, where this greater sensibility of the parts about the face seems to have been be- stowed as some compensation for the general obtuseness of feeling resulting from the thickness of the hide which covers the rest of the body. Thus the Rhinoceros has a soft, hook-shaped ex- tension of the upper lip, which is always kept moist, in order to preserve its sensibility as an organ of touch. The Hog has the end of the nose also constructed for feeling ; though it is not so well calculated for distinguishing the form of objects, as where the organ is prolonged in the form of a snout, which it is in the Tapir, and in a still higher degree in the admirably constructed proboscis of the Elephant, which as an organ, both of prehension and of touch, forms the nearest ap- proach to the perfect structure of the human hand. The Lion, Tiger, Cat, and other animals of the genus Felis, have whiskers, endowed at their roots with a particular sensibility, from being largely supplied with nerves. The same is the case with the whiskers of the Seal. The prehensile tails of the American Monkeys are doubtless fitted to convey accurate perceptions of touch, as well as the feet and hands ; as may be inferred from the great size of the nervous papillae, and the thinness of the cuticle of those parts. The sense of touch attains its greatest degree of excellence in the human hand, in which it is asso- ciated with the most perfect of all instruments of prehension. But as the structure and functions of this organ are the exclusive subjects of another of the Bridgewater Treatises, I shall refrain from any further remarks respecting it. 352 THE SENSORIAL FUNCTIONS. Chapter III. TASTE. The senses of Taste and Smell are intended to convey impressions resulting from the chemical qualities of bodies, the one in the fluid, the other in the gaseous state.* There is a considerable analogy between the sensations derived from these two senses. The organ of taste is the surface of the tongue, the skin of which is furnished with a large proportion of blood-vessels and nerves. The vascular plexus immediately covering the corium is here very visible, and forms a distinct layer, through which a great number of papillae pass, and project from the surface, covered with a thin cuticle, like the pile of velvet. In the fore part of the human tongue these papillEe are visible even to the naked eye ; and especially in certain morbid con- ditions of the organ. t They are of different kinds ; but it is only those which are of a conical shape that are the seat of taste. If these papillae be * Bellini contended that the different flavours of saline bodies are owing to the peculiar figures of their crystalHne particles. It is strange that Dumas should have thought it worth while seriously to combat this extravagant hypothesis, by a laboured refutation. t This is particularly the case in scarlatina, in the early stage of which disease they are elongated, and become of a bright red colour, from their minute blood-vessels being distended with blood. As the fever subsides the points of the papillae collapse, and acquire a brown hue ; giving rise to the appearance known by the name of the straw- berry tongue. TASTE. :V)3 touched with a fluid, which has a strong taste, such as vinegar, applied by means of a camel-hair pencil, they will be seen to become elongated by the action of the stimulus; an effect which probably always accompanies the perception of taste. The primary use of this sense, the organ of which is placed at the entrance of the alimentary canal, is evidently to guide animals in the choice of their food, and to warn them of the introduction of a noxious substance into the stomach. With respect to the human species, this use has been, in the present state of society, superseded by many acquired tastes, which have supplanted those ori- ginally given to us by nature; but in the inferior animals it still retains its primitive office, and is a sense of great importance to the safety and welfare of the individual, from its operation being coin- cident with those of natural instincts. If, as it is said, these instincts are still met with among men in a savage state, they are soon weakened or effaced by civilization. The tongue, in all the inferior classes of ver- tebrated animals, namely Fishes, Reptiles, and Birds, is scarcely ever constructed with a view to the reception of delicate impressions of taste; being generally covered with a thick, and often horny cuticle ; and being, besides, scarcely ever employed in mastication. This is the case, also, with those quadrupeds which swallow their food entire, and which cannot, therefore, be supposed to have the sense of taste much developed. Insects which are provided with a tongue or a proboscis may be conceived to exercise the sense of taste by means of these organs. But many VOL. II. A A 354 THE SENSORIAL FUNCTIONS. insects possess, besides these, a pair of short feelers, placed behind the maxillary palpi ; and it has been observed that, while the insect is taking food, these organs are in incessant motion, and are continually employed in touching and examining the food, before it is introduced into the mouth : hence, some entomologists have concluded that they are organs of taste. But it must be obvious that in this, as in every other instance in which our researches extend to beings of such minute dimensions, and which occupy a station, in the order of sensitive existence, so remote from ourselves, we are wandering into regions where the only light that is afforded us must be borrowed from vague and fanciful analo- gies, or created by the force of a vivid and deceptive imagination. Chapter IV. SMELL. Animal life being equally dependent on the salu- brious qualities of the air respired, as of the food received, a sense has been provided for discrimi- nating the nature of the former, as well as of the latter. As the organs of taste are placed at the entrance of the alimentary canal, so those of smell usually occupy the beginning of the passages for respiration, Mhere a distinct nerve, named the olfactory, appropriated to this office, is distributed. The sense of smell is generally of greater im- portance to the lower animals than that of taste ; SMELL. 355 and the sphere of its perceptions is in tlieni vastly more extended than in man. The agents, which give rise to the sensations of smell, are certain effluvia, or particles of extreme tenuity, which are disseminated very quickly through a great extent of atmospheric air. It is exceedingly ditiicult to conceive how matter so extremely rare and subtile as that which composes these odorous effluvia can retain the power of producing any sensible impres- sion on the animal organs ; for its tenuity is so extraordinary as to exceed all human compre- hension. The most copious exhalations from a variety of odoriferous substances, such as musk, valerian, or assafoetida, will be continually ema- nating for years, without any perceptible loss of weight in the body which supplies them. It is well known that if a small quantity of mnsk be enclosed for a few hours in a gold box, and then taken out, and the box cleaned as carefully as possible with soap and water, that box will retain the odour of musk for many years ; and yet the nicest balance will not show the smallest increase of its weight from this impregnation. No facts in natural philosophy ati'oid more striking illustra- tions of the astonishing, and indeed inconceivable divisibility of matter, than those relating to odorous effluvia. It would appear that most animal and vegetable bodies are continually emitting these subtle effluvia, of which our own organs are not sufficiently deli- cate to apprize us, unless when they are much concentrated, but which are readily perceived and distinguished by the lower animals ; as may be in- ferred from their actions. A do2c is known to follow 35<3 THE SENSORIAL FUNCTIONS. its master by the scent alone, through the avenues and turnings of a crowded city, accurately distin- guishing his track amidst thousands of others. The utility of the sense of smell is not confined to that of being a check upon the respiration of noxious gases; for it is also a powerful auxiliary to the sense of taste, which of itself, and without the aid of smell, would be very vague in its indications and limited in its range. What may have been its extent and delicacy in man while he existed in a savage state, we have scarcely any means of deter- mining ; but in the present artificial condition of the race, resulting from civilization and the habitual cultivation of other sources of knowledge, there is less necessity for attending to its perceptions, and our sensibility to odours has probably diminished in the same proportion. It is asserted both by Soemmerring and Bluinenbach that the organ of smell is smaller in Europeans and other civilized races of mankind, than in those nations of Africa or America, which are but little removed from a savage state : it is certainly much less developed in man than in most quadrupeds. To the carnivorous tribes, especially, it is highly useful in enabling them to discover their natural food at great distances. The cavity of the nostrils, in all terrestrial verte- brated animals, is divided into two by a vertical partition ; and the whole of its internal surface is lined by a soft membrane, called the Schneiderian membrane* which is constantly kept moist, is sup- plied with numerous blood-vessels, and upon which are spread the ultimate ramifications of the olfac- * It has been so named in honour of Schneider, the first anato- mist who gave an accurate description of this membrane. SMELL. 357 tory nerves. The relative magnitude of these nerves is much greater in carnivorous quadrupeds tlian in those which subsist on vegetable food. In quadru- peds, as well as in man, these nerves, in their course towards the brain, are not collected into a single trunk, but compose a great number of filaments, which pass separately through minute perforations in a plate of bone, (called the ethmoid bone), before they enter into the cavity of the skull, and join that part of the cerebral substance with which they are ultimately connected. In every class of vertebrated animals, it has been fonnd that the internal surface of this membrane is furnished with cilia, in great numbers, producing the usual currents in the fluid in contact with them. The surface of the membrane which receives the impressions from odorous effluvia, is considerably increased by several thin plates of bone, which pro- ject into tlje cavity of the nostrils, and are called the turbinated hones. These are delineated at t, t, in Fig. 382, as they appear in a vertical and lougi- 30,S TIIK SENSORIAL FUNCTIONS. tndinal section of the cavity of the human nostril, where they are seen covered by the Schneiderian membrane.* A transverse and vertical section of these parts is given in Fig. .383.'] The turbinated hones are curiously folded, and often convoluted in a spiral form, with the evident design of obtaining as great an extent of surface as possible within the confined space of the nasal cavity. This tur- binated or spiral shape, chiefly characterises these bones among herbivorous quadrupeds : in the * This figure shows the branches of the olfactory nerve (o), pass- ing through the thin cribriform plate of the ethmoid bone, and distributed over that membrane. Several of the cells, which open into the cavity, are also seen ; such as the large sphenoidal sinus (s), the frontal sinus (f), and one of the ethmoidal cells (c). n is the nusul bone ; p, the palate ; and e, the mouth of the Eustachian tube, which leads to the ear. t In this figure, s, is the septum, or partition of the nostrils, on each side of which are seen the sections of the tuibinated bones projecting into the cavity ; the ethmoid cells (c), situated between the orbits (o) ; and the Antrum maxillare (a), which is another large cavity coniniunicating with the nostrils. SMELL. 359 Horsey for example, the turbinated bones are of a large diameter, and extend the whole length of the prolonged nostrils. Their structure is exceedingly intricate ; for while they retain externally the ge- neral shape of an oblong spiral shell, they are pierced on all their internal sides with numerous perforations, through which the membrane, toge- ther with the fine branches of the nerves, passes freely from one side to the other. The cavities resulting from the convolutions are intersected by unperforated partitions of extraordinary tenuity ; serving both to support the arches of bone, and to furnish a still greater surface for the extension of the olfactory membrane. In the Sheep, the Goat, and the Deer, the structure is very similar to that just described; but the convolutions are double, with an intermediate partition, so as to resemble in its transverse section the capital of an Ionic column. They are shown at r, Fig. 384, which exhibits the transverse section of the nostrils of a sheep.* * In a species of Antelope described by IMr. Hodgson, cavities exist, situated immediately behind the ordinary nostrils, and com- municalinff with them. These accessory nostrils are conjectured to oGO THE SENSORIAL FUNCTIONS. Ill carnivorous quadrupeds the structure of these bones is still more intricate, and is calculated to afford a far more extensive surface for the distri- bution of the olfactory nerve. In the Seal this con- formation is most lully developed ; and the bony plates are here not turbinated, but ramified, as shown at t, in Fig. ."385. Eight or more principal branches arise from the main trunk ; and each of these is afterwards divided and subdivided to an extreme degree of minuteness, so as to form in all many hundred j)lates. Tlie olfactory membrane, with all its nerves, is closely applied to eveiy plate in this vast assemblage, as well as to the main trunk, and to the internal surface of the surround- ing cavity ; so that its extent cannot be less than 1*20 square inches in each nostril. An organ of such exquisite sensibility requires an extraordinary provision for securing it against injury, by the power of voluntarily excluding noxious vapours ; and nature has supplied a mechanism for this ex- press purpose, enabling the animal to close at plea- sure the orifice of the nostril. The Ho^\ which, in its natural state, subsists wholly on vegetable food, resembles herbivorous tribes in the external form and relative magnitude of the turbinated bones ; but they are more simple in their structure, being formed of single, and slightly convoluted plates, without partitions or perforations. In this respect they approach to the human structure, which is even less complicated, be useful to this exceedingly fleet animal by facilitating its breathing, while it is exerting its utmost speed ; for the expansion of the nostrils opens also these posterior cavities, the sides of which, being- elastic, remain dilated. (Journal of the Asiatic Society, Feb. 1832, p. 59.) SMELL. 3()l and indicates a greater affinity to vegetable than to animal feeders. Man, indeed, distinguishes more accurately vegetable odours than those pro- ceeding from animal substances; while the reverse is observed with regard to quadrupeds whose habits are decidedly carnivorous. A dog, for instance, is regardless of the odour of a rose or violet ; and probably, as he derives from them no pleasure, is unable to discriminate the one from the other. Predaceous animals, as Sir Busick Harwood ob- serves, require both larger olfactory nerves, and a more extensive surface for their distribution, than the vegetable eaters. The food of the latter is generally near at hand ; and as they have occa- sion only to select the wholesome from the noxious plants, their olfactory organs are constructed for the purpose of arresting the effluvia of odorous substances immediately as they arise. The former are often under the necessity of discovering the lurking places of their prey at a considerable dis- tance, and are therefore more sensible to the weak impressions of particles widely diffused through the surrounding medium, or slightly adhering to those bodies, with which the object of their pursuit may have come into contact. The olfactory bones of Birds are constructed very much on the model of the spiral bones of herbivorous quadrupeds, and vary but little in the different species. Fig. 38G exhibits their ap- pearance in the Turkey : but the size of the olfac- tory nerves of birds of prey greatly exceeds that of the same nerves in granivorous birds. In the latter, indeed, they are exceedingly small ; and as the natural food of that tribe has but little odour, we 302 THE SENSORIAL FUNCTIONS. find that they are easily deceived by anything which bears a resemblance to it. Sir Busick Har- vvood relates that some poultry, which were usually fed with a mixture of barley meal and water, were found to have swallowed, by mistake, nearly the whole contents of a pot of white paint. Two of the fowls died, and two others became paralytic. The crops of the latter were opened, and considerably more than a pound of the poisonous composition taken from each ; and the crops, either naturally, or from the sedative effects of the paint, appear to have so little sensibility, that, after the wounds were sewed up, both the fowls eventually recovered. The olfactory nerves are conspicuous in the L>ncJi, both from their size and mode of distri- bution. They are seen in Fig. 387, passing out 387 through the orbit of the eye (o) in two large branches, an upper one (u), and a lower one (l), SMELL. 363 the ramifications of wliich are spread over the mandibles, both within and without. For the pro- tection of the highly sensible extremity of the beak against the injurious impressions of hard bodies, a horny process (p), similar, both in form and office, to the human nail, is attached to it, and its edges guarded by a narrow border of the same horny material ; these receive a first, and fainter impression, and admonish the animal of approach- ing danger ; if none occur, the matter is then sub- mitted to the immediate scrutiny of the nerves themselves, and is swallowed or rejected according to their indication.* The cells in the bill of the Toucan communi- cate with the nostrils, and, being highly vascular, confer, no doubt, great delicacy of smell.t It has been generally asserted that Vultures, and other birds of prey, are gifted with a highly acute sense of smell ; and that they can discover by means of it the carcass of a dead animal at great distances : but it appears to be now sufficiently established bv the observations and experiments of Mr. Au- dubon, that these birds in reality possess the sense of smell in a degree very inferior to carnivorous quadrupeds; and that so far from guiding them to their prey from a distance, it affords thein no in- dication of its presence, even when close at hand. The following experiments appear to be perfectly conclusive on this subject. Having procured the skin of a deer, Mr. Audubon stuffed it full of hay ; and after the whole had become perfectly dry and * Such is the account given by Sir Busick. Harwood, in his ' System of Comparative Anatomy and Physiology," p. 26. t Tiaill, Trans. Linn. Soc. xi, '288. ^C)A THE SENSORIAL FUNCTIONS. hard, he placed it in the middle of an open field, laying it down on its back, in the attitude of a dead animal. In the course of a few minutes after- wards, he observed a vulture flying towards it, and alighting near it. Quite unsuspicious of the decep- tion, the bird immediately proceeded to attack it, as usual, in the most vulnerable points. Failing in his object, he next, with much exertion, tore open the seams of the skin, where it had been stitched together, and appeared earnestly intent on getting at the flesh, which he expected to find within, and of the absence of which, not one of his senses was able to inform him. Finding that his efforts, which were long reiterated, led to no other result than the pidling out large quantities of hay, he at length^, though with evident reluctance, gave up the at- tempt, and took flight in pursuit of other game to which he was led by the sight alone, and which he was not long in discovering and securing. Another experiment, the converse of the first, was next tried. A large dead hog was concealed in a narrow and winding ravine, about twenty feet deeper than the surface of the earth around it, and filled with briers and high cane. This was done in the month of July in a tropical climate, where pu- trefaction takes place with great rapidity. Yet, although many vultures were seen, from time to time, sailing in all directions over the spot where the putrid carcass was lying, covered only with twigs of cane, none ever discovered it ; but in the mean while, several dogs had found their way to it^ and had devoured large quantities of the flesh. In another set of experiments it was found that young vultures enclosed in a cage, never exhibited any SMELL. 365 token of their perceiving food, when it coiihl not be seen by them, however near to them it was brought.* It has been doubted whether fishes, and other aquatic animals possess the sense of smell ; in some of the Whale tribe, indeed, neither the organ of smell nor the olfactory nerves are found. f Some physiologists have gone the length of denying the capability of water to serve as the vehicle of odorous effluvia. But as water is known to contain a large quantity of air which acts upon the organs of respi- ration, it is easy to conceive that it may also convey to the nostrils the peculiar agents which are calcu- lated to excite perceptions of smell. Fishes are, in fact, observed to be attracted from great dis- tances by the effluvia of substances thrown into the water ; and they are well known to have a strong predilection for all highly odoriferous substances. Baits used by anglers are rendered more attractive by being impregnated with volatile oils, or other substances having a powerful scent, such as assa- foetida, camphor, and musk. Mr. T. Bell| has dis- covered in the Crocodile and Alligator, a gland, which secretes an unctuous matter, of a strong musky odour, situated beneath the lower jaw, on each side. The external orifice of this gland is a small slit, a little within the lower edge of the jaw ; and the sac, or cavity containing the odoriferous * Edinburgh New Journal of Science, ii. 172. The accuracy of these results, which had been contested by Mr. Waterton, is fully established by the recent observations and experiments of Mr. Bach- man, which are detailed in Loudon's Magazine of Natural History, vii. 167. t Home ; Lectures on Comparative Anatomy, i. 17. I Phil. Trans, for 18-27, p. 132. 366 THE SENSORIAL FUNCTIONS. substance, is surrounded by two delicate bands o*' muscular fibres, apparently provided for the pur- pose of first bringing the gland into a proper posi- tion, and then, by compressing it, discharging its contents. Mr. Bell conceives that the use of this secretion is to act as a bait for attracting fish towards the sides of the mouth, where they can be readily seized in the mode usual to the alligator, which is that of snapping sideways at the objects he aims at devouring. The organs of smell in Fishes are situated in cavities, placed one on each side, in front of the head : they are merely blind sacs, having no com- munication with the mouth or throat, and indeed no other outlet but the external openings, which are generally two to each sac. The principal entrance is furnished with a valve, formed by a moveable membrane, appearing like a partition dividing each nostril into two cavities, and serving the purpose of preventing the introduction of any foreign body. The organ itself is situated behind this valve, and consists either of a membrane, curiously plaited into numerous semicircrdar folds, or of tufted or arborescent filaments. Cilia have been discovered on the surface of these cavities by Purkinje and Valentin. Fig. .388 shows the nasal cavities (s, s,) with their plaited membrane in the Percb ; and Fig. 389, in the Skate ; the laminae in the former being radiated, and in the latter, foliated, or parallel to each other. On the surface of these organs, what- ever be their shape, the olfactory nerves (n), arising from the anterior lobes (o) of the brain are distri- buted ; and the great size of these nerves would lead us to infer considerable acuteness in the sense SMELL. 367 which they supply. When the fish is swimming, their situation in front of the snout exposes them to the forcible impulse of the water which strikes N'f44^^^/„„|,>^ against them. According to GeofFroy St. Hilaire, the water enters the cavity by the upper orifice, and escapes by the lower. Scarpa alleges that fishes exercise this sense by compressing the water against the membrane. On the other hand it is contended by Dumeril, that the perceptions communicated by this organ, being the result of the action of a liquid instead of a gas, should be classed under the head of taste rather than of smell. This seems, however, to be a mere verbal criticism, in making which it appears to have been forgotten that the impressions of odorous effluvia, even in animals breathing at- mospheric air, always act upon the nerve through the intermedium of the fluid which lubricates the membrane of the nostril. That the nasal cavities of fishes are rudimental forms of those of the mammalia, although they do not, as in the latter class, open into the res|3iratory organs, is shown by the curious transformation of the one into the other during the developement of the tadpole, both of the Frog and of the Salaman- der. We have already seen that during the first periods of their existence, these animals are per- fectly aquatic ; breathing water by means of gills, 368 THE SENSORIAL FUNCTIONS; and having all their organs formed on the model of the fish. Their nasal cavities are not employed for respiration at this early period ; nor even for some time after they have begun to take in air, which they do by the mouth, swallowing it in small por- tions at a time, and afterwards throwing it out in bubbles by the same orifice. But when they quit the water, and become land animals with pul- monary respiration, the nostrils are the channels through which the air is received and expelled ; and it is here also that the sense of smell continues to be exercised. We know very little respecting the seat of the sense of smell in any of the invertebrate animals, though it is very evident that insects, in particular, enjoy this faculty in a very high degree. Analogy would suggest the spiracles as the most probable seat of this sense, being the entrances to the respi- ratory passages. This office has, however, been assigned by many to the antennae; while other entomologists have supposed that the palpi are the real organs of smell.* Experiments on this subject are attended with great difficulty, and their results must generally be vague and inconclusive. Those which Mr. P. Huber made on bees seem, however, to establish, with tolerable certainty, that the spi- racles are insensible to strong odours, such as that of oil of turpentine which is exceedingly offensive to all insects. It was only when a fine camel-hair pencil containing this pungent fluid was presented near the cavity of the mouth, above the insertion * On the subject of this sense in insects, see Kirby and Spence's Introduction to Entomology, vol. iv. p. 249. SMELL. 369 of the proboscis, that any visible effect was pro- duced upon the insect, which then gave decisive indications of strong aversion. M. Lefebvre found that a bee was apparently unaffected by the ap- proach of ether to the abdominal region, or even by its application to the stigmata; but manifested great agitation when the effluvia reached the antennae. From his experiments on a wasp he concluded that the sensibility to odours resides exclusively in the last segment of the antennae.* Mr. Kirby has discovered in the anterior part of the nose of the Necrophorus vespillo, or burying-beetle, which is an insect remarkable for the acuteness of its smell, a pair of circular pulpy cushions, covered with a membrane, beautifully marked with fine transverse furrows. These he considers as the organs of smell ; and he has found similar structures in several other insects.t No distinct organs of smell have been discovered in any of the Mollusca ;:|; but as there is evidence that some of the animals belonging to that class possess tills sense, it has been conjectured that it resides either in the whole mucous surface of the mantle, or in the respiratory organs. Swammerdam observed, long ago, that snails are evidently affected by odours ; and the cuttle-fish is said to show a decided aversion to strongly scented plants. * Ann. Soc. Entomol. de France, 1838 : and Ann. Sc. Nat. ser. 2. xi. 191. t Ibid. vol. iii. 481 ; and iv. 254. X A group of laminse, closely resembling the olfactory organs of Fishes, has been lately observed by Mr. Owen in the Nautilus. VOL. II. B P. 370 THE SENSORIAL FUNCTIONS. Chapter V. HEARING. '§ 1 . Acoustic Principles. The knowledge acquired by animals of the pre- sence and movements of distant objects is derived almost wholly from the senses of hearing and of sight; and the apparatus, necessary for the exercise of these senses, being more elaborate and refined than any of the organs we have yet examined, exhibits still more irrefragable evidence of those profound designs, and that infinite intelligence^ vrhich have guided the construction of every part of the animal frame. Sound results from certain tremulous or vibratory motions of the particles of an elastic medium, such as air or water, excited by any sudden impulse or concussion given to those particles by the move- ments of the sounding body. These sonorous vi- brations are transmitted with great velocity through those fluids, till they strike upon the external ear; and then, after being concentrated in the internal passages of the organ, they are made to act on the filaments of a particular nerve called the acousticy or auditory nerve, of which the structure is adapted to receive these peculiar impressions, and to com- municate them to the brain, where they produce changes, which are immediately followed by the sensation of sound. Sound cannot traverse a void HEARING. 371 space, as light does ; but always requires a ponder- able material vehicle for its transmission ; and, accordingly, a bell suspended in the vacuum of an air-pump, gives, when struck, no audible sound, although its parts are visibly thrown into the usual vibratory motions. In proportion as air is admitted into the receiver, the sound becomes more and more distinct ; and if, on the other hand, the air be condensed, the sound is louder than when the bell is surrounded by air of the ordinary density.* The impulses given by the sounding body to the contiguous particles of the elastic medium, are pro- pagated in every direction, from particle to particle; each in its turn striking against the next, and com- municating to it the whole of its own motion, which is destroyed by the reaction of the particle against which it strikes. Hence, after moving a certain definite distance, (a distance, indeed, which is in- calculably small,) each particle returns back to its former situation, and is again ready to receive a second impulse. Each particle, being elastic within a certain range, f suffers a momentary compression, and immediately afterwards resumes its former shape : the next particle is, in the mean time, impelled, and undergoes the same succession of changes; and so on, throughout the whole series of particles. Thus the sonorous undulations have an analogy to waves, which spread in circles on the * These facts were first ascertained by Dr. Hauksbee. See Phi- losophical Transactions for 1705, vol. xxiv. p. 1902, 1904. t The particles of water are as elastic, within a limited distance, as those of the most solid body; although, in consequence of their imperfect cohesion, or rather their perfect mobility in all directions, this property cannot be so easily recognised in masses of fluids, as it is in solids. 372 THE SENSORIAL FUNCTIONS. surface of water around any body which by its motion ruffles that surface ; only that instead of merely extending in a horizontal plane, as waves do, the sonorous undulations spread out in all directions, forming, not circles in one plane, but spherical shells ; and, whatever be the intensity of the sounds, the velocity with which the undulations advance is uniform, as long as they continue in a medium of uniform elasticity. In air, this velocity is, on an average, about 1100 feet in a second, or twelve and a half miles in a minute : it is greater in dense, and smaller in rarefied air; being, in the same medium, exactly proportional to the elasticity of that medium. All other aeriform elastic fluids, such as steam, and the vapour of alcohol or ether, are capable of propagating sounds with the same facility as air.* Water is the medium of sound to aquatic animals, as the air is to terrestrial animals. Sounds are, indeed, conveyed more quickly, and to greater dis- tances, in water than in air, on account of the greater elasticity of the constituent particles of water, within the minute distance required for their action in propagating sound. Stones, struck to- gether under water, are heard at great distances by a person whose head is under water. Franklin found, by experiment, that sound, after travelling above a mile through water, loses but little of its intensity. Chladni estimated the velocity of sound, when conveyed by water, to be about 4900 feet in a second, or between four and five times greater than it is in air. M. Colladon has determined it, * Biot, Traite de Physique, ii, 4. HEARING. 373 with greater precision, to be equal to 4708 feet in a second, when the water is at a temperature of 46° of Fahrenheit. Solid bodies, especially such as are hard and elastic, and of uniform substance, are also excellent conductors of sound. Of this we may easily con- vince ourselves by applying the ear to the end of a log of wood or a long iron rod, in which situation we shall hear very distinctly the smallest scratch made with a pin at the other end; a sound, which, had it passed through the air only, would not have been heard at all. In like manner, a poker sus- pended by two strings, the ends of which are ap- plied to the two ears, communicates to the organ, when struck, vibrations which would never have been heard under ordinary circumstances. It is said that the hunters in North America, when de- sirous of hearing the sounds of distant footsteps, which would be quite inaudible in any other way, apply their ears close to the earth, and then readily distinguish them. Ice is known to convey sounds, even better than water ; for if cannon be lired from a distant fort, where a frozen river intervenes, each flash of light is followed by two distinct reports ; the first being conveyed by the ice, and the second by the air. In like manner, if the upper part of the wall of a high building be struck with a ham- mer, a person standing close to it on the ground, will hear two sounds after each blow, the first de- scending through the wall, and the second through the air. Cast iron, at the temperature of 51" Fahr. was found by Biot to convey sounds with a velocity of 11,090 feet in a second ; which is about ten times greater than that in air. 374 THE SENSORIAL FUNCTIONS. As sounds are weakened by diffusion over a larger sphere of particles, so they are capable of having their intensity increased by concentration into a smaller space ; an effect which may be pro- duced by their being reflected from the solid walls of cavities, shaped so as to bring the undulations to unite into a focus. It is on this principle that the ear-trumpet, for assisting persons dull of hearing, is constructed ; and that echoes occasionally reflect a sound of greater loudness than the original sound which was directed towards them. If the impulses given to the nerves of the ear be repeated at equal intervals of time, provided these intervals be not greater than the sixteenth part of a second, the impressions become so blended together as not to be distinguishable from one another : and the sensation of a uniform continued sound, or musical note, is excited in the mind. If the intervals between the vibrations be long, the note is grave ; if short, that is, if the number of vibrations in a given time be great, the note is, in the same pro- portion, acute.^ The former is called a lo2v, the latter a high note; designations which were, per- haps, originally derived from the visible motions of the throat of a person who is singing these differ- ent notes; for, independently of this circumstance, the terms of high and low are quite arbitrary ; and it is well known that they were applied by the ancients in a sense exactly the reverse of that in wiiich we now use them. The different degrees of tension given to the * In tones produced by the friction of the teeth of a revolving wheel against a hard body, Savart found that the highest audible note consisted of 24,000 impulses in a second. HEARING. 375 chord or wire of a stringed musical instrument, as well as its different lengths, determine the fre- quency of its vibrations ; a greater tension, or a shorter length, rendering them more frequent, and consequently producing a higher note ; and on the contrary, the note is rendered more grave by either lessening the tension, or lengthening the chord or wire. In a wind instrument, the pitch depends chiefly on the length of the tube producing the sound. There are, therefore, two principal qualities in sound recognisable by the ear, namely, its loudness, or intensity, and its pitch, or musical note ; the former depending on the force of the vibrations ; the latter, on their frequency. To these a third may be added, namely, the tone, or the quality called by the French " le timbre :" and which ap- pears to depend on the greater or less abruptness of the individual impulses. These acoustic principles are to be borne in mind in studying the comparative physiology of hearing; and since the functions of the different parts of the organ of this sense are, as yet, but imperfectly understood, I shall, in treating of this subject, de- viate from the plan I have hitherto followed, and premise an account of the structure of the ear in its most highly developed state, as it appears to be in Man. 376 THE SENSORIAL FUNCTIONS. § 2. Physioloiyy of Hearing in Man. That part of the organ of hearing, which, above all others, is essential to the performance of this func- tion, is the acoustic nerve, of which the fibres are expanded, and spread over the surface of a fine membrane, placed in a situation adapted to receive the full impression of the sonorous undulations which are conveyed to them. This membrane, then, with its nervous filaments, may be regarded as the immediate organ of the sense ; all the other parts constituting merely an accessory apparatus, de- signed to collect and to condense the vibrations of the surrounding medium, and to direct their con- centrated action on the auditory membrane. I have endeavoured, in Fig. 390, to exhibit, in one view, the principal parts of this complicated I organ, as they exist in man, in their relative situa- tions, and of their natural size ; thereby affording a scale by which the real dimensions of those HEARING. 377 portions, which I shall afterwards have occasion to explain by magnified representations, may be pro- perly appreciated.* The Concha, or external ear (c), is formed of an elastic plate of cartilage, covered by integument, and presenting various elevations and depressions, which form a series of parabolic curves; apparently for the purpose of collecting the sonorous undu- lations of the air, and of directing them into a funnel-shaped canal (m), termed the meatus audi- torius, which leads to the internal ear. This canal is composed partly of cartilage and partly of bone; and the integument lining it is furnished with numerous small glands, which supply a thick oily fluid, of an acrid quality, apparently designed to prevent the intrusion of insects : the passage is also guarded by hairs, which appear intended for a similar purpose. The meatus is closed at the bottom by a mem- brane (d), which is stretched across it like the skin of a drum, and has been termed, from this resem- blance, the membrane of the ttjmpanum, or the ear- drum.'f It performs, indeed, an office correspond- ing to its name ; for the sonorous undulations of the air, which have been collected, and directed inwards by the grooves of the concha, strike upon the ear-drum, and throw it into a similar state of vibration. The ear-drum is composed of an ex- ternal membrane, derived from the cuticle which * In this and all the following figures, the parts of the right ear are shown, and similar parts are always indicated by the same letters. t The inner surface of the ear-drum is shown in this figure ; the cavity of the tympanum, which is behind it, being laid open. 378 THE SENSORIAL FUNCTIONS. lines the meatus ; an internal layer, which is con- tinuous with that of the cavity beyond it ; and a middle layer, which consists of radiating muscular fibres, proceeding from the circumference towards the centre, where they are inserted into the ex- tremity of a minute bony process (h), presently to be described.* This muscular structure appears designed to vary the degree of tension of the ear- drum, and thus adapt the rate of its vibrations to those communicated to it by the air. There is also a slender muscle, situated internally, which by acting on this delicate process of bone, as on a lever, puts the whole membrane on the stretch, and ena- bles its radiating fibres to effect the nicer adjust- ments required for tuning, as it may be called, this part of the organ. t Immediately behind the membrane of the ear- drum, there is a hollow space (t), called the cavity of the tympanum, of an irregular shape, scooped out of the most solid part of the temporal bone, which is here of great density and hardness. This cavity is always filled with air; but it would obviously defeat the purpose of the organ if the air were confined in this space ; because unless it were allowed freely to expand or contract, it could not long remain in equilibrium with the pressure ex- erted by the atmosphere on the external surface of the ear-drum ; a pressure which, as is well known, is subject to great variations, indicated by the rise and fall of the barometer. These variations would * In many quadrupeds their insertion into this process is at some distance from the centre of the membrane. These muscular fibres are delineated in Fig. 45, vol. i. p. 125. t Home, Lectures, &c. iii. 268. HEARING. 379 expose the membrane of the ear-drum to great inequalities of pressure on its outer and inner surfaces, and endanger its being forced, according to the state of the weather, either outwards or inwards ; conditions which would affect its degree of tension, and alter the character of its vibrations. Nature has guarded against these evils by esta- blishing a passage of communication between the tympanum and the external air, by means of a tube (e), termed the Eiistachian tube, which begins by a small orifice from the inner side of the cavity of the tympanum, and opens by a wide mouth at the back of the nostrils.* This tube performs in the ear an office similar to that of the hole which it is found necessary to make in the side of a drum, for the purpose of opening a communication with the external air; a communication which is as necessary for the functions of the ear, as it is for the proper sounding of the drum. We find, accord- ingly, that a degree of deafness is induced when- ever the Eustachian tube is obstructed ; which may happen either from the swelling of the membrane lining it, during a cold, or from the accumulation of secretion in the passage. It is also occasionally useful as a channel through which sounds may gain admittance to the internal ear ; and it is perhaps for this reason that we instinctively open the mouth when we are intent on hearing a very faint or distant sound. On the side of the cavity of the tympanum, which is opposite to the opening of the Eustachian tube, is situated the beginning of another passage, * This opening is seen at e, in Fig. 382, p. 357, representing a vertici.ll and longitudinal section of the right nostril. 380 THE SENSORIAL FUNCTIONS. leading into numerous cells, contained in the mas- toid process of the temporal bone, and therefore termed the mastoid cells: these cells are likewise filled with air. The innermost side of the same cavity, that is the side opposite to the ear-drum, and which is shown in Fig. 391, is occupied by a rounded eminence (p), of a triangular shape, termed the promontory ; on each side of which 1 393 there is an opening in the bone, closed, however, by the membrane lining the whole internal surface of the cavity. The opening (o), which is situated at the upper edge of the promontory, is called the fenestra ovalis, or oval window ; and that near the under edge (r), is the fenestra rotunda^ or round window. Connected with the membrane of the ear-drum, at one end, and with the fenestra ovalis at the other, there extends a chain of very minute moveable bones, seen at b, in Fig. 390 ; but more distinctly at M, I, s, in Fig. 393, which is drawn on a some- what larger scale, and in which, as before, p is the promontory ; and r the fenestra rotunda. These bones, which may be called the tympanic ossicular are four in number, and are represented, enlarged to twice the natural size, in Fig. 392. The names they have received are more descriptive of their HEARING. 381 shape than of their office. The first is the malleus^ or hammer (m) ; and its long handle (h) is affixed to the centre of the ear-drum : the second is the incus, or anvil (i) ; the third, which is the smallest in the body, being about the size of a millet seed, is the orbicular bone (o) ;* and the last is the stapes, or stirrup (s), the base of which is applied to the membrane of the fenestra ovalis. These bones are regularly articulated together, with all the ordinary apparatus of joints, and are moved by small muscles provided for that purpose. Their office is apparently to transmit the vibrations of the ear- drum to the membrane of the fenestra ovalis, and probably, at the same time, to increase their force. The more internal parts of the ear compose what is designated, from the intricacy of its winding passages, the lahyriyith. It is seen at s v k in Fig. 390, in connexion with the tympanum ; but in Fig. 394, it is represented, on a very large scale, detached from every other part, and sepa- rated from the solid bone in which it lies embedded. It consists of a middle portion, termed the vestibule (v), from which, on its upper and pos- terior side, proceed the three tubes (x, Y, z), called, from their shape, the semi- * Blumenbach, and other anatomists, consider this as not being a separate bone, but only a process of the incus ; a view of the sub- ject which is supported by the observations of Mr. Shrapnell, de- tailed in the Medical Gazette, xii. 172. 382 THE SENSORIAL FUNCTIONS. circular canals; while to the lower anterior side of the vestibule there is attached a spiral canal, re- sembling in appearance the shell of a snail, and on that account denominated the Cochlea (k). All these bony cavities are lined with a very delicate membrane, or periosteum, and are filled with a transparent watery, or thin gelatinous fluid, which is termed by Breschet the perilymph* Within the cavity of the osseous labyrinth now described, are contained membranes having nearly the shape of the vestibule and semicircular canals, * Recherches Anatomiques et Physiologiques sur I'organe de I'ouie et de I'audition dans THomme et les Animaux vertebres. Paris, 1836. It has also been called the Aqua labyrinthi, and the fluid of Cotunnius, from the name of the Anatomist who first dis- tinctly described it. HEARING. 383 but not extending into the cochlea. These mem- branes, which compose what has been termed, for the sake of distinction, the membranous labyrinth^ form one continuous, but closed sac, containing a fluid,* perfectly similar in appearance to the peri- lymph, which surrounds it on the outer side, and intervenes between it and the sides of the osseous labyrinth, preventing any contact with those sides. In Fig. 395, which is on a still larger scale than the preceding figure, the osseous labyrinth is laid open, so as to show the parts it encloses, and more especially the membranous labyrinth, floating in the perilymph (p). The form of this latter part is still more distinctly seen in Fig. 396, where it is represented in a position exactly corresponding to the former figure, but wholly detached from the bony labyrinth, and connected only with the nervous filaments which are proceeding to be distributed to its different parts. A simple inspection of these figures, in both of which the corresponding parts are marked by the same letters, will show at once the form and the connexions of the three semicircular canals, (x, y, z), each of which present, at their origin from the vestibule, a considerable dilatation, termed an am- pulla (a, a, a), while, at their other extremities, where they terminate in the vestibule, there is no enlargement of their diameter ; and it will also be seen that two of these canals (x and v) unite into one before their termination. The same descrip- tion applies in all respects both to the osseous and * De Blainville has termed this fluid " la vitrine auditive," from its supposed analogy to the vitreous humour of the eye. 384 THE SENSORIAL FUNCTIONS. to the membranous canals contained within them ; the space (p) whicli intervenes between the two, being filled with the perilymph. But the form of the membranous vestibule demands more particular notice, as it is not so exact an imitation of that of the osseous cavity ; being composed of two distinct sacs, opening into each other: one of these (u) is termed the utricle;* and the other (s), the sac- culus. Each sac contains in its interior a small mass of white calcareous matter, (o, o), resembling powdered chalk, which seems to be suspended in the fluid contained in the sacs by the intermedium of a number of nervous filaments, proceeding from the acoustic nerves (g and n), as seen in Fig. 396. From the universal presence of these cretaceous substances in the labyrinth of all the mammalia, and from their much greater size and hardness in aquatic animals, there can be little doubt that they perform some office of great importance in the phy- siology of hearing. t Their size and appearance in the Dog are shown in Fig. 397 ; and in the Hare, in Fig. 398. The Cochlea, again, is an exceedingly curious structure, being formed of the spiral convolutions of a double tube ; or rather of one tube, separated into two compartments by a partition (l), called the lamina spiralis, which extends its whole length, except at the very apex of the cone, where it sud- denly terminates in a curved point, or hook (h), * Scarpa and Weber term it the sinus, or alveus utriculosus ; it is called by others the sacculus vestibuli. Breschet gives it the name of le sinus median. t These cretaceous bodies are termed by Breschet otolithes, and otoconies, according as they are of a hard or soft consistence. HEARING. 30o leaving an aperture by which the two portions of the tube communicate together. In Fig. 3i)5, a bristle (b, b) is passed through this aperture. The central pillar, round which these tubes take two and a half circular turns, is termed the modiolus. Its apex is seen at m. One of these passages is distinguished by the name of the vestibular tube* in consequence of its arising from the cavity of the vestibule ; and the other by that of the tympanic tube,'\ because it begins from the inner side of the membrane which closes the fenestra rotunda, and forms the only separation between the interior of that tube, and the cavity of the tympanum. The trunk of the auditory nerve occupies a hollow space immediately behind the ventricle ; and its branches pass through minute holes in the bony plate which forms the wall of that cavity ; being finally expanded on the different parts of the mem- branous labyrinth. I Great uncertainty prevails with regard to the real functions performed by the several parts of this very complex apparatus. It is most probable, however, that the sonorous vibrations of the air which reach the external ear, after being collected and reinforced by the grooves in that organ, are * Scala vestibuli. + Scala tymjjani. X In Fig. 396, the anterior trunk of the auditory nerve is seen (at g) distributing branches to the ampullae (a, a), the utricle (u), and the calcareous body it contains; while the posterior trunk (n) divides into a branch, which supplies the sacculus (s) and its cal- careous body (o), and a second branch (k) which is distributed over the cochlea, (d) is the nerve called the portio dura, which merely accompanies the auditory nerve, but has no relation to the sense of hearing. In Fig. 390, the auditory nerve (n) is seen en- tering at the back of the vestibule. VOL. II. C C 386 THE SENSORIAL FUNCTIONS. directed down the meatus, and striking against the ear-drum which closes the passage, throw that membrane into vibrations of equal frequency. The extent of these vibrations is regulated by the degree of tension given to it by the action of the muscles of the malleus, the handle of which is fixed to the centre of the membrane.* The vibrations of the ear-drum must excite corresponding motions in the air contained in the cavity of the tympanum ; which, again, communicates them to the mem- brane of the fenestra rotunda ; while, on the other hand, the membrane closing the fenestra ovalis re- ceives similar but stronger impressions from the stapes, conveyed through the chain of tympanic ossicula, which serve as solid conductors of the same vibrations.! Thus the perilymph, or fluid * Savart ascertained by a series of experiments that the dry membrane tympani was thrown into stronger vibrations by a given sound, when it was in a state of laxity than when tense : and drew the inference, that hearing becomes less acute when the tension of the ear-drum is increased. See Muller's Elements of Physiology, by Baly, p. 1256. f Savart has proved by decisive experiments, that vibrations thus freely transmitted by a chain of solid bodies, such as the ossicula, connected at both ends by membranes, and traversing a cavity filled with air, acquire an intensity considerably greater than the vibra- tions communicated by the air alone. Muller has also shown that the transmission of sonorous vibrations from air to water is much facilitated by the intervention of a tense membrane extended be- tween these two media : and still more so if a solid conductor is applied to that membrane, and affixed at its other end to the middle of a tense membrane which has atmospheric air on both of its sides. It is easy to perceive that these are exactly the conditions ob- served in the structure of the tympanic apparatus. The resonance of the air in the several cavities of the external meatus, tympanum and mastoid cells, and of the perilymph in those of the labyrinth will also contribute to modify the vibrations, and generally increase their intensity. HEARING. 387 contained in the labyrinth, is affected by each ex- ternal sound, both through the medium of the air in the tympanum, and by means of the ossicula : the undulations thus excited produce impressions on the extremities of the nervous filaments, which are spread over the membranous labyrinth ; and these impressions being conveyed to the brain, are imme- diately followed by the sensation of sound. With regard to the precise purposes which are answered by the winding passages of the semi- circular canals, and cochlea, hardly any plausible conjecture has been offered ; yet no doubt can be entertained that the uses of all these parts are of considerable importance, both as to delicacy and correctness of hearing. There is an obvious cor- respondence between the positions of the three semicircular canals, (two of which are vertical and one horizontal, and of which the planes are reci- procally perpendicular to one another,) and the three dimensions by which the geometrical relations of space are estimated; and it might hence be con- jectured that the object of this arrangement is to allow of the transmission of vibrations of every kind, in whatever direction they may arrive ; for as the vibrations thus transmitted will produce the strongest impression on different parts of the canals according to their direction, the corresponding differences in the sensations produced may indicate the position of the external source of the sound.* It is not an * Autenrieth and Kerner are of opinion that the semicircular canals contribute to the capacity of judging of the direction of sounds : and they remark that the corresponding canals on the right and left sides are not parallel to one another, and consequently do not receive similar or equal impulses from the same sound : an inequahty which may lead to the discrimination of its direction. They also state that 388 THE SENSORIAL FUNCTIONS. improbable supposition that the return into the ves- tibule, of undulations which have passed through these canals, has the effect of putting a stop to all further motion of the fluid ; thus preventing all interferences among successive undulations, and preserving the distinctness of each impression made on the nerves. The same use may be assigned to the double spiral convolutions of the tubes of the cochlea : for the undulations of the fluid in the tympanic tube, received from the membrane of the fenestra rotunda, will meet those proceeding along the vestibular tube, derived from the membrane of the fenestra ovalis, and like two opposing waves, will tend to destroy one another. Thus each external sound will produce but a single momentary impres- sion ; the continuance of the undulations of the fluid of the labyrinth being prevented by their mutual collision and consequent destruction.* Sounds may produce impressions on the auditory nerves by vibrations communicated directly to them through the solid bones of the skull ; and indepen- dently of the tympanic apparatus. This is shown animals in whom these canals are highly developed possess the greatest power of" distinguishing the direction of sounds. The same view of the subject is taken by Professor Wheatstone. Todd, Cyclopaedia of Anat. and Physiology, ii. 570. * The preliminary steps in the process above described are not absolutely essential to hearing, for many instances have occurred in which the power of hearing has been perfectly retained after the membrane of the ear-drum, and also the ossicula had been destroyed by disease. A small aperture in the membrane does not interfere v\ith its power of vibration ; but if the whole ear-drum he destroyed, and the ossioula lost, an almost total deafness generally ensues. After a time, however, the hearing may be in a great measure re- covered, with an undiminished power of distinguishing musical tones. See two papers by Sir Astley Cooper, in the Phil. Trans, for 1800, p. 151 ; and for 1801, p. 437. i HEARING. 389 by the sound of a tuning fork, made to vibrate by striking it against any solid body, being heard more loudly and distinctly when the handle is pressed against the head, and still more so when applied to the teeth, than when not in contact with any part of the head. It is chiefly in this way that a person hears his own voice ; those sounds being- best heard, as Professor Wheatstone remarks, which are principally articulated within the mouth.* Weber concludes from his experiments that the sonorous vibrations conveyed by the cranial bones are communicated more immediately to that por- tion of the auditory nerve which is distributed to the cochlea, while those which are conveyed by the tympanic apparatus are received principally by the nerves of the vestibule. § 3. Comparallce Physiology of Hearing. The structure of the organs of hearing in the lower animals presents a gradation from the simple vesti- bule, with its membranous sac, supplied with nervous filaments, which may be regarded as the only essential part of this organ, through the suc- * He observed that while the sounding tuning fork is placed on any part of the head, if both ears be closed by being covered with the hands, a considerable augmentation of the sound takes place. When both ears are open, the sound seems to be heard chiefly by the ear in the vicinity of which the stem of the fork is placed ; but when the opposite ear is closed, it appears as if the sound were trans- ferred from the open to the closed ear ; and if the ear be alternately opened and closed, the sound will appear to be transferred alter- nately from the one to the other. See Journal of the Royal Insti- tution for July, 1827. 390 THE SENSORIAL FUNCTIONS. cessive additions of semicircular canals, fenestra ovalis, tympanic cavity, ossicula, ear-drum, meatus auditorius, cochlea, and concha, till we arrive at the combination of all these parts in the higher orders of the Mammalia, The simpler forms are generally met with in aquatic animals ; probably because the sonorous undulations of water are communicated more readily, and with greater force, than those of air, and require no accessory appa- ratus for their concentration. The Lobster, for instance, has a vestibular cavity (seen at v, in Fig. 39y), containing a membranous sac, with a striated groove (g),* and receiving the filaments of the audi- tory nerve. This vestibule is protected by the shell on all sides, except at one part, where it is closed only by a membrane (e), which may there- fore be considered as corresponding to the fenestra ovalis. The outer side of this membrane in the 399 Astacus fluviatilis, or cray-fish, is seen at f in Fig. 401 ; while Fig. 402, shows an interior view of the same membrane (f), with the vestibule (v) laid open, and the auditory nerve (n) passing through the shell to be distributed on the sacculus. It appears from a variety of observations that Insects, both in their larva and their perfect state, possess the faculty of hearing ; but no certain knowledge has been obtained of the parts which * This groove is represented magnified in Fig. 400. HEARING. 391 exercise this sense. The prevailing opinion among entomologists is that it resides in some part of tlie antennae ; organs, which are supposed to have a peculiar sensibility to aerial undulations. This hypothesis is founded principally on the analogy of the Crustacea, whose antennae contain the vestibular cavity already described ; but on the other hand it is opposed by the fact that Spiders, which hear very acutely, have no antennae ; and it is also reported that insects, when deprived of their an- tennae, still retain the power of hearing.* None of the Mollusca appear to possess, even in the smallest degree, the sense of hearing, with the exception only of the highly organized Cephalo- poda ; and in them we find, at the lower part of the cartilaginous ring, (which has been supposed to exhibit the first rudiment of a cranium,) a tubercle, containing in its interior two membranous vesicles, contiguous to each other, and surrounded by a fluid. They evidently correspond to the vestibular sacs, and contain each a small calcareous body, suspended from the vesicles by slender nervous filaments, like the clapper of a bell, and probably performing an office analogous to that instrument ; for, being thrown into a tremulous motion by every undulation of the surrounding fluid, they must strike against the membrane, and communicate similar and still stronger impulses to the nerves by which they are suspended, thus increasing the * Comparetti has described structures in a great number of insects, which he imagined were organs of hearing ; but his observations have not been confirmed by subsequent inquirers, and their accu- racy is therefore doubtful. See De Blainville " De I'Organisation des Animaux," i, 565. .092 THE SENSORIAL FUNCTIONS. impression made on those nerves. The mechanical effect of an apparatus of this kind is shown by the simple experiment, mentioned by Camper, of en- closing a marble in a bladder full of water, and held in the hand ; when the slightest shaking of the bladder will be found instantly to communicate motion to the marble, the reaction of which on the bladder gives an unexpected concussion to the hand. The ear of Fishes contains, in addition to the vestibule, the three semicircular canals, which are in general greatly developed.* An enlarged view of the membranous labyrinth of the Lophius pisca- toriiis is given in Fig. 403, showing the form and complication of its parts, which are represented of twice the natural size, x, y, z, are the semicircular canals, with their respective ampullae (a, a, a), m is the Sinus medianus, or principal vestibular sac, with its anterior expansion, terrried the Utricle (u). The Sacculus (s) has, in like manner, a posterior * In the Lamprey, these canals exist only in a rudimental state, appearing as folds of the membrane of the vestibule ; and there are also no cretaceous bodies in the vestibular sac. HEARING. o9.3 appendage (c) termed the Cysticule. The hard calcareous bodies (o, o, o) are three in number;* and the branches of nerves (i, i, i), by which they are suspended in the fluid contained in the mem- branes, are seen passing into them ; while the ampullar are supplied by other branches (n, n, n). In all the osseous fishes the labyrinth is not en- closed in the bones of the cranium, but projects into its cavity ; but in the larger cartilaginous fishes, as the Ray and Shark tribes, it is surrounded by solid bone, and is not visible within the cranium. In these latter fishes, we first meet with a rudiment of the meatus, in a long passage extending from the inner side of the vestibule to the upper and back part of the skull ; where it is closed by a mem- brane, which is covered by the skin.^ Aquatic reptiles have ears constructed nearly on the same plan as those of fishes ; thus the Triton or Newt, has a vestibule containing only one cre- taceous body, and three semicircular canals, un- protected by any surrounding bone. In the Frog, however, we first perceive the addition of a distinct cavity, closed by a membrane, which is on a level with the integuments, on each side of the head. From this cavity, which corresponds to that of the tympanum, there proceeds an Eustachian tube, and within it, extending from the external mem- * The shape of these otolithes is so definite in osseous fishes, as to be capable of furnishing, according to Cuvier, characters available for the determination of species. They are considered by Weber as supplying the place of the cochlea in this class of animals, and of making direct impressions on the auditory nerves. See Todd's Cycl. ii, 567. t Scarpa and others do not consider these passages as performing the office of auditory canals. 394 THE SENSORIAL FUNCTIONS. brane, which must here be regarded as an ear-drum, to the membrane of the vestibule, or fenestra ovalis, is found a bone, shaped hke a trumpet, and termed the Columella. This bone is seen at c in Fig. 404, attached by its base (b) to the fenestra ovalis of the vestibule (v), which contains the cre- taceous body (o). There is also a small bone (i) attached in front to the columella. In the Chelonia, the structure of the ear is essentially the same as in the Frog, but the tympanum and columella are of greater length. In the saurian reptiles the cavity of the tympanum is still more capacious, and the ear-drum very distinctly marked ; and these ani- mals possess great delicacy of hearing. The laby- rinth of the Crocodile is enclosed in bone, and contains three calcareous bodies : it presents also an appendage which has been regarded as the earliest rudiment of a cochlea ; and there are two folds of the skin, resembling eye-lids, at the ex- ternal orifice of the organ, which appear like the first step towards the developement of an external ear. The structure of the ear in the Crocodile is but an approximation to that which we find prevailing in Birds, where the organ is of large size compared with that of the head. The rudimental cochlea, as seen at k in Fig. 405, which represents these organs HEARING. 395 in the Turkey, is of large size, and slightly curved. In the cavity of the tympanum (t) is seen the columella, which extends to the fenestra ovalis; and beyond it, the semicircular canals (s), the bony cells (b) which communicate with the tympanum, the OS quadratum (q), the zygomatic process (z), and the lower jaw (j). The ear-drum is now no longer met with at the surface, but lies concealed at the bottom of a short meatus, the orifice of which is surrounded with feathers, arranged so as to serve as a kind of imperfect concha, or external ear. In Owls these feathers are a prominent and charac- teristic feature; and in these birds there is, besides, a membranous flap, acting as a valve to guard the passage. The chief peculiarity observable in the internal ears of Mammalia is the great developement of the cochlea, the tubes of which are convoluted, turn- ing in a spiral, and assuming the figure of a turbi- nated shell. From an extensive comparison of the relative size of the cochlea in difierent tribes of quadrupeds, it has been inferred that it bears a tolerably constant proportion to the degree of acuteness of hearing, and that, consequently, it contributes essentially to the perfection of that faculty : Bats, for instance, which are known to possess exquisite delicacy of hearing, have a cochlea of extraordinary size, compared with the other parts of the ear. The tympanic ossicula are com- pletely developed only in the Mammalia.* It is * These tympanic ossicula are regarded by Spix and by Geoffroy St. Hilaire as corresponding to the opercular bones of fishes, where, according to his theory, they have attained their highest degree of developement. But Weber discovered the existence of the ossicula auditus in some fishes, quite apart from the opercula. 3y6 THE SENSORIAL FUNCTIONS. Iso in this class alone that we meet with a concha, or external ear, distinctly marked ; and the utility of this part, in catching and collecting the sonorous undulations of the air, may be inferred from the circumstance, that a large and very moveable concha is generally attended with great acuteness of hearing. This is more particularly the case with feeble and timid quadrupeds, as the Hare and Rabbit, which are ever on the watch to catch the most distant sounds of danger, and whose ears are turned backwards, or in the direction of their pur- suers ; while, on the contrary, the ears of preda- ceous animals are directed forwards, that is, towards the objects of their pursuit. This difference in di- rection is not confined to the external ear, but is observable also in the bony passage leading to the tympanum. The Cetacea, being strictly inhabitants of the water, have no external ear ; and the passage lead- ing to the tympanum is a narrow and winding tube, formed of cartilage instead of bone, and having a very small external aperture. In the Dolphin tribe the orifice will barely admit the entrance of a pin ; it is also exceedingly small in the Dugong ; these structures being evidently intended for pre- venting the entrance of any quantity of water.* It is apparently with the same design that in the Seal the passage makes a circular turn ; and that, in the OniitliorhyncliHS paradoxus, it winds round the temporal bone, and has its external orifice at a great distance from the vestibule. The internal parts of the organ of hearing in the Whale and * It is probable that in these animals the principal channel by which sounds reach the internal ortran is the Eustachian tube. HEARING. 397 Other Cetacea, are inclosed in a bone of extraordi- nary hardness, which, instead of forming a con- tinuous portion of the skull, is connected to it only by ligaments, and suspended in a kind of osseous cavity, formed by the adjacent bones. The cochlea is less developed than in quadrupeds ; for it only takes one turn and a half, instead of two and a half. The existence of the semicircular canals in the Cetacea was denied by Camper ; but they have since been discovered by Cuvier. Several quadrupeds, which are in the habit of burrowing, or of diving, as the Sorex fodiens, or water-shrew, are furnished with a valve, composed of a double membrane, capable of accurately closing the external opening of the meatus, and protecting it from the introduction of water, earth, or other extraneous bodies.* In like manner the external ear of the Hippopotamus, which feeds at the bottom of rivers, is guarded by an apparatus which has the effect of a valve. We find, indeed, the same provident care dis- played in this, as in every other department of the animal economy : every part, however minute, of the organ of this important sense, being expressly adapted, in every species, to the particular circum- stances of their situation, and to that degree of acuteness of perception, which is best suited to their respective wants and powers of gratification. | * GeofFroy St. Hilaire ; Menioires du Museum, i. 305. t The Comparative Physiology of the Voice, a function of which the object, in animals as well as in man, is to produce sounds, ad- dressed to the ear, and expressive of their ideas, feelings, desires and passions, forms a natural sequel to that of Hearing ; but Sir Charles Bell having announced his intention of introducing it in his Treatise on the Hand, 1 have abstained from entering into this extensive sub- ject. 398 THE SENSORIAL FUNCTIONS. Chapter VI. VISION. ^ 1 . Object of the Sense of Vision. To those who study nature with a view to the dis- covery of final causes, no subject can be more in- teresting or instructive than the physiology of Vision, the most refined and most admirable of all our senses. However well we may be acquainted with the construction of any particular part of the animal frame, it is evident that we can never form a correct estimate of the excellence of its mecha- nism, unless we have also a knowledge of the pur- poses to be answered by it, and of the means by which those purposes can be accomplished. Innu- merable are the works of creation, the art and con- trivance of which we are incompetent to under- stand, because we perceive only the ultimate effects, and remain ignorant of the operations by which those effects are produced. In attempting to investigate these obscure functions of the animal or vegetable economy, we might fancy ourselves engaged in the perusal of a volume, written in some unknown language, where we have penetrated the meaning of a few words and sentences, sufficient to show us that the whole is pregnant with the deepest thought, and conveys a tale of surpassing interest and wonder, but where we are left to gather the sense of connecting passages by the guidance VISION. 39.9 of remote analogies or vague conjecture. Wherever we fortunately succeed in decyphering any con- tinued portion of the discourse, we find it charac- terized by that perfection of style, and grandeur of conception, which at once reveal a masters hand, and which kindle in us the most ardent de- sire of supplying the wide chasms perpetually in- tervening in the mysterious and inspiring narrative. But in the subject which now claims our attention we have been permitted to trace, for a considerable extent, the continuity of the design, and the lengthened series of means employed for carrying that design into execution ; and the view which is thus unfolded of the magnificent scheme of crea- tion is calculated to give us the most sublime ideas of THE WISDOM, THE POWER, AND THE BENEVOLENCE OF God. On none of the works of the Creator, which we are permitted to behold, have the characters of intention been more deeply and legibly engraved than on the organ of vision, where the relation of every part to the effect intended to be produced is too evident to be mistaken, and the mode in which they operate is at once placed within the range of our comprehension. Of all the animal structures, this is, perhaps, the one which most admits of being brought into close comparison with the works of human art ; for the eye is, in truth, a refined optical instrument, the perfection of which can never be fully appreciated until we have instituted such a comparison ; and the most profound scientific investigations of the anatomy and physiology of the eye concur in showing that the whole of its structure is most accurately and skilfully adapted 400 THE SENSORIAL FUNCTIONS. to the physical laws of light, and that all its parts are finished with that mathematical exactness which the precision of the effect requires, and which no human effort can ever hope to approach, — far less to attain. V To the prosecution of this inquiry we are farther invited by the consciousness of the incalculable advantages we derive from the sense of sight, the choicest and most enchanting of our corporeal en- dowments. The value of this sense must, indeed, appear inestimable, when we consider of how large a portion of our sensitive and intellectual existence it is the intermediate source. Not only has it given us extensive command over the objects which sur- round us, and enabled us to traverse and explore the n)ost distant regions of the globe, but it has introduced us to the knowledge of the bodies which compose the solar system, and of the countless hosts of stars which are scattered through the firmament; thus expanding our views to the re- motest confines of creation. As the perceptions supplied by this sense are at once the quickest, the most extensive, and the most varied, so they be- come the fittest vehicles for the introduction of other ideas. Visual impressions are those which, in infancy, furnish the principal means of deve- loping the powers of the understanding: it is to this class of perceptions that the philosopher resorts for the most apt and perspicuous illustrations of his reasonings ; and it is also from the same inex- haustible fountain that the poet draws his most pleasing and graceful, as well as his sublimest imagery. VISION. 401 The sense of Vision is intended to convey to its possessor a knowledge of the presence, situation, and colour of external and distant objects, by means of the light which those objects are con- tinually sending off, either spontaneously, or by reflection from other bodies. We know of only one part of the nervous system so peculiarly organ- ized as to be capable of being affected by luminous rays, and conveying to the mind the sensation of light ; and this part is the Retina, so named from the thin and delicate membranous network, on which the pulpy extremities of the optic nerves, establishing an immediate communication between that part and the brain, are expanded. If the eye were so constructed as to allow the rays of light, which reach it from surrounding objects, simply to impinge on the retina as they are received, the only perception which they could excite in the mind, would be a general sensation of light, proportionate to the total quantity which is sent to the organ from the whole of the opposite hemisphere. This, however, does not properly constitute Vision ; for in order that the presence of a particular object in its real direction and position with respect to us, may be recognised, it is neces- sary that the light, which comes from it, and that light alone, should produce its impression exclu- sively on some particular part of the retina ; it being evident that if the light, coming from any other object, were allowed to act, together with the former, on the same part, the two actions would interfere with one another, and only a confused impression would result. The objects in a room, for example, are all throwing light on a sheet of VOL. II. D D 402 THE SENSORIAL FUNCTIONS. paper laid on the floor ; but this light, being spread equally over every part of the surface of the paper, furnishes no means of distinguishing the sources from which each portion of the light has proceeded; or, in other words, of recognising the respective figures, situations, and colours of the objects them- selves. We shall now proceed to consider the modifications to be introduced into the structure of the organ, in order to accomplish these purposes. § 2. Modes of accomplishing^ the Objects of Vision. Let us suppose that it were proposed to us as a problem to invent an apparatus, by which, availing ourselves of the known properties of light, we might procure the concentration of all the rays, proceed- ing from the respective points of the object to be viewed, on separate points of the retina, and obtain likewise the exclusion of all other rays ; and also to contrive that the points of the retina, so illumi- nated, should have the same relative situations among one another, which the corresponding points of the surrounding objects have in nature. In other words, let us suppose ourselves called upon to devise a method of forming on the retina a faithful delineation, in miniature, of the external scene. As it is a fundamental law in optics that the rays of light, while they are transmitted through the same medium, proceed in straight lines, the sim- plest mode of accomplishing the proposed end would be to admit into the eye, and convey to each particular point of the retina, only a single ray VISION. 403 proceeding directly from that part of the object which is to be depicted on it, anfl ^o exclude all other rays. For carrying this design into effect we have the choice of two methods, both of which we find resorted to by nature under different circumstances. The first method consists in providing for each of these single rays a separate tube, with darkened sides, allowing the ray which traverses it, and no other, to fall on its respective point of the retina, which is to be applied at the opposite end of the tube. The most convenient form to be given to the surface of the retina, which is to be spread out to receive the rays from all these tubes, appears to be that of a convex hemisphere ; and the most eligible distribution of the tubes is the placing them so as to constitute diverging radii, perpendicular, in every part, to the surface of the retina. This arrangement will be understood by reference to Fig. 406, which represents a section of the whole organ ; t, t, being the tubes disposed in radii every where per- pendicular to the convex hemis- pherical surface of the retina (r). Thus will an image be formed, composed of the direct rays from each respective point of the objects, to which the tubes are directed ; and these points of the image will have, among themselves, the same relative situation as the external objects, from which they originally proceeded, and which they will accord- ingly faithfully represent. The second method, which is nearly the inverse 404 THE SENSORIAL FUNCTIONS. of the first, consists in admitting the rays through a small aperture into a cavity, on the opposite and internal side of which the retina is expanded, forming a concave, instead of a convex hemisphe- rical surface. The mode in which this arrange- ment is calculated to answer the intended purpose will be easily understood by conceiving a chamber (as represented in Fig. 407), into which no light is 4or allowed to enter, except what is admitted through a small hole in a shutter, so as to fall on the opposite side of the room. It is evident that each ray will, in that case, illuminate a different part of the wall ; and that the whole external scene will be there faithfully represented ; for the several illuminated points, which constitute these images, preserve among themselves the same relative situation which the objects they represent do in nature ; although with reference to the actual objects they have an inverted position. This inversion of the image is a necessary consequence of the crossing of all the rays at the same point; namely, the small aper- ture in the shutter, through which they are ad- mitted. VISION. 405 One inconvenience attending the limiting of the illumination of each point of the wall to that of a single ray, in the mode last pointed out, is that the image produced must necessarily be very faint. If, with a view of remedying this defect, the aper- ture were enlarged, the image would, indeed, become brighter, but would at the same time be rendered more indistinct, from the intermixture and mutual interference of adjacent rays ; for all the lines would be spread out, the outlines shaded off, and the whole picture confused. The only mode by which distinctness of image can be obtained with increased illumination, is to collect into one point a great number of rays pro- ceeding from the corresponding point of the object to be represented. Such a collection of rays pro- ceeding from any point, is termed, in the language of optics, a pejicil of rays; and the point into which they are collected is called a focus. For the pur- pose of collecting a pencil of rays into a focus, it is evident that all of them, except the one which proceeds in a straight line from the object to that focus, must be deflected, or bent from their rectili- neal course. This effect may be produced by re- fraction, which takes place according to another optical law; a law of which the following is the exposition. It is only when the medium which the rays are traversing is of uniform density that their course is constantly rectilineal. If the density change, or if the rays pass obliquely from one medium into another of a different density, they are refracted ; each ray being deflected towards a line situated in the medium of greatest density, and drawn from 406 THE SENSORIAL FUNCTIONS. the point where the ray meets the new medium, perpendicular to the refracting surface. Thus the ray r, Fig. 408, striking obliquely on the surface of a denser medium, at the point s, instead of pur- suing its original course along the line s o, is re- fracted, or turned in the direction s t, Mhich is a line situated between s o, and s p ; this latter line being drawn perpendicularly to the surface of the medium, at the point s, and within that medium. When the ray arrives at t, and meets the posterior 408 surface of the dense medium, passing thence into one that is less dense, it is again refracted accord- ing to the same law ; that is, it inclines towards the perpendicular line t t, drawn from t, within the denser medium, and describes the new course t u instead of t v. The amount of the deflection corresponds to the degree of obliquity of the ray to the surface which refracts it ; and is mathematically expressed by the law, that the sines of the two angles formed with the perpendicular by the inci- dent and the refracted rays retain, amidst all the variations of those angles, the same constant pro- portion to one another. We may hence derive a simple rule for placing the plane of the refracting surface so as to produce the particular refraction we wish to obtain. When a ray is to be deflected VISION. 407 from its original course to a particular side, we have only to turn the surface of the medium in such a manner as that the perpendicular line to that surface, contained within the denser medium, shall lie still farther on the same side. Thus, in Fig. 408, if we wish to turn the ray r s, from s o to s T, we must place the dense medium so that the perpendicular s p, which is within it, shall be still farther from s o, than s t is ; that is, shall lie on the other side of s r. The same rule applies to the contrary refraction of the ray s t from t v to t u, when it passes out of a dense, into a rare medium ; for the perpendicular t i must still be placed on the same side of t v as t u is situated. Let us now apply these principles to the case before us ; that is, to the determination of the form to be given to a dense medium, in order to collect a pencil of rays, proceeding from a distant object. 409 accurately to a focus. We shall suppose the object in question to be very remote, so that the rays com- posing the pencil may be considered as being parallel to each other ; for at great distances their actual deviation from strict parallelism is wholly insensible ; and let a, b, c, d, e, (Fig. 409), repre- sent these rays. There must evidently be one of these rays (c), and only one, which, by continuing its rectilineal course, would arrive at the point (u) 408 THE SENSORIAL FUNCTIONS. intended to be the focus of the rays. This ray, then, may be suffered to pass on, without being subjected to any refraction ; the surface of the medium should, therefore, be presented to the ray (at i) perpendicularly to its course, so that it may pass through at right angles to that surface. Those rays (b and d) which are situated very near to this direct, or central ray (c), will require but a small degree of refraction in order to reach the focus (r) : this small refraction will be effected by a slight degree of obliquity in the medium at the points (h and k) where those rays meet it. In proportion as the rays (such as those at a and e) are more distant from the central ray, a greater amount of refraction, and consequently a greater obliquity of the surfaces (g and l) will be required, in order to bring them to the same focus. The convergence of these rays, after they have passed this first surface, which would have brought them to the point r, may be farther increased by interposing new surfaces of other media at the proper angles. If the new medium be still denser than the last, the inclination of its surface must be similar to that already described ; if rarer, they must be in an opposite direction. This last case, also, is illustrated in the figure, where m, n, o, p, q, show the inclinations of the surfaces of a rarer medium, calculated to increase the convergence of the rays ; that is, to bring them to a nearer focus (f). The result of the continued change of direction in the refracting surface, is a regular curvilineal sur- face, which, in the present case, approaches very nearly to that of a sphere. Hence by giving these refractive media spherical surfaces, we adapt them, VISION. 409 with tolerable exactness, to produce the con- vergence of parallel rays to a focus; and by making the denser medium convex on both sides (as shown in Fig. 410), both surfaces will conspire in pro- ducing the desired effects. Such an instrument is termed a double convex lens; and it has the pro- perty of collecting into a focus rays proceeding from distant points. Having obtained this instrument, we may now venture to enlarge the aperture through which the light was admitted into our dark chamber, and fit into the aperture a double convex lens. We have thus constructed the m ell-known optical instrument called the Camera Obscura, in which the images of external objects are formed upon a white surface of paper, or a semi-transparent plate of glass ; and these images must evidently be in an inverted posi- tion with respect to the actual objects which they represent. Such is precisely the construction of the eye, which is, to all intents, a camera obscura : for in both these instruments, the objects, the principles of construction, and the mode of operation are ex- actly the same ; and the only difference is, that the former is an infinitely more perfect instrument than the latter can ever be rendered by the utmost efforts of human art. 410 THE SENSORIAL FUNCTIONS. The refraction by spherical surfaces does not, strictly speaking, unite a pencil of parallel or divergent rays into a mathematical point, or focus ; for in reality the rays which are near the central line are made to converge to a point a little more distant than that to which the remoter rays con- verge : an effect which I have endeavoured to illus- trate by the diagram Fig. 411; where, in order to 411 render it obvious to the eye, the disparity is exagge- rated : for on ordinary occasions, where great nicety is not required, this difference in the degree of convergence between the central rays and those near the circumference of the lens, giving rise to what is termed the Aberration of Sphericity, is too small to attract notice. 414 With a view of simplifying the subject, I have assumed, that the rays which arrive at the eye are VISION. 411 parallel, which in mathematical strictness they never are. The focus of the rays refracted by a convex lens is more remote in proportion as the rays are more divergent ; or, in other words, pro- ceed from nearer objects. This is illustrated by Figures 412, 413, and 414 ; to which I shall again have occasion to refer in the sequel. ^ 3. Structure of the Eye. One of the many points of superiority which the eye possesses over the ordinary camera obscura is derived from its spherical shape, adapting the retina to receive every portion of the images produced by refraction, which are themselves curved ; whereas had they been received on a plane surface, as they usually are in a camera obscura, a considerable portion of the image would have been indistinct. This spherical form is preserved by means of the firm membranes which protect the eye, and which are termed its Coats; the transparent media which they enclose, and which effect the convergence of the rays, are termed the Humours of the Eye. There are in this organ three principal coats, and three humours ; composing altogether what is called the Globe of the Eye. Fig. 415, which gives an enlarged view of a horizontal section of the right eye, exhibits distinctly all these parts. The outermost coat (s), which is termed the Sclerotica, is exceedingly firm and dense, and gives to the globe of the eye the mechanical support it requires for the performance of its delicate func- tions. It is perforated behind by the optic nerve 412 THE SENSORIAL FUNCTIONS. (o), wliich passes onwards to be expanded into the retina (r). Tlie sclerotica does not extend farther than about four-fifths of the globe of the eye ; its place in front being supplied by a transparent convex membrane (c), called the Cornea, which is more prominent than the rest of the eye-ball. A line passing through the centre of the cornea and the centre of the globe of the eye is called the axis of the eye. The Sclerotica is lined internally by the Choroid coat (x), which is chiefly made up of a tissue of blood-vessels, for supplying nourishment to the eye. It has on its inner surface a layer of a dark coloured viscid secretion, known by the name of the Pigmentum nigrum^ or black pigment. Its use is to absorb all the light which may happen to be irregularly scattered through the eye, in conse- quence of reflection from different quarters ; and it serves, therefore, the same purpose as the black VISION. 413 paint, with which the inside of optical instruments, such as telescopes, microscopes, and cameras ob- scnrae, is darkened. Within the pigmentum nigrum, and almost in immediate contact with it,* the Retina (r) is expanded ; forming an exceedingly thin and delicate layer of nervous matter, supported by a fine membrane. More than three-fourths of the globe of the eye are filled with the vitreous humour (v), which has the appearance of a pellucid and elastic jelly, con- tained in an exceedingly delicate texture of cellular substance. The Crystalline humour (l), which has the shape of a double convex lens, is formed of a denser material than any of the other humours, and occupies the fore-part of the globe of the eye, im- mediately in front of the vitreous humour, which is there hollowed to receive it. The space which in- tervenes between the lens and the cornea is filled with a watery secretion (a), called the Aqueous hmnour. This space is divided into an anterior and a posterior chamber by a flat circular partition (i), termed the Iris. The iris has a central perforation (p), called the Pupil; and it is fixed to the edge of the choroid coat, by a white elastic ring (q), called the Ciliary Ligament. The posterior surface of the iris is called the Uvea, and is lined with a dark brown pigment. The structure of the iris is very peculiar, being composed of two layers of contractile fibres ; the one, forming concentric circles ; the other, dis- posed like radii between the outer and inner * Between the pigmentum and the retina there is found a very fine membrane, discovered by Dr. Jacobson : its use has not been ascertained. 414 THE SENSORIAL FUNCTIONS. margin.* When the former act, the pupil is con- tracted ; when the latter act, the breadth of the iris is diminished, and the pupil is, of course, dilated. By varying the size of the pupil, the quantity of light admitted into the interior of the eye is regulated, and accommodated to the sensibility of the retina. When the intensity of the light would be injurious to that highly delicate organ, the pupil is instantly contracted, so as to exclude the greater portion ; and, on the contrary, when the light is too feeble, it is dilated, in order to admit as large a quantity as possible. The iris also serves to intercept such rays as would have fallen on parts of the crystalline lens less fitted to produce their regular refraction, the object of which will be better understood when we have examined the functions of this latter part. But before engaging in this inquiry, it will be proper to complete the sketch of the anatomy of the eye by describing the principal parts of the apparatus belonging to that organ, which are exte- rior to the eye-ball, and may be considered as its appendages. The purposes answered by the parts exterior to the eye-ball are chiefly those of motion, of lubrica- tion, and of protection. As it is the central part of the retina which is endowed with the greatest share of sensibility, it is necessary that the images of the objects to be viewed should be made to fall on this part; and consequently that the eye should be capable of having its axis instantly directed to those objects, wherever they may be situated. Hence muscles are * See Fig. 47, vol. i, p. 125. VISION. 415 provided within the orbits, for effecting the motions of the eye-ball. A view of these muscles, with their at- tachments to the ball of the eye, but separated from the other parts, is given in Fig. 410. Four of these proceed in a straight course from the bottom of the orbit, arising from the margin of the aperture through which the optic nerve passes, and being inserted by a broad tendinous expansion into the fore-part of the sclerotic coat. Three of these are marked a, r., and c in the figure ; and the edge of the fourth is seen behind and above b. These straight muscles, as they are called, surround the optic nerve and the eye-ball, forming four longitu- dinal bands ; one (a) being situated above, for the purpose of turning the eye upwards ; a second (c), situated below, for turning it downwards ; and the two others, on either side, for performing its lateral motions to the right or left. The cavity of the orbit being considerably larger than the eye-ball, the intervening space, especially at the back part^ is filled up by fat, which serves as a soft cushion for its protection, and for enabling it to roll freely in all directions. Besides these straight muscles, there are also two others (s and i), termed the oblique muscles, which give the eye-ball a certain degree of rotation on its axis. When these act in conjunction, they draw the eye forwards, and serve as antagonists to the combined power of the straight muscles. The upper oblique muscle (s) is remarkable for the arti- ficial manner in which its tendon passes through a 410 THE SENSORIAL FUNCTIONS. cartilaginous pulley (p) in the margin of the orbit, and then turns back again to be inserted into the eye-ball ; so that the effect produced by the action of the muscle is a motion in a direction exactly the reverse of that in which its fibres contract. This mechanism, simple as it is, affords one of the most palpable instances that can be adduced of express contrivance ; for in no other situation could the muscle have been so conveniently lodged as within the eye-ball ; and in no other way could its tendon have been made to pull in a direction contrary to that of the muscle, than by the interposition of a pulley, turning the tendon completely round. The fore-part of the globe of the eye, which is of a white colour, is connected with the surrounding integuments by a membrane termed the Conjunc- tiva* This membrane, on arriving at the base of the eye-lids, is folded forwards so as to line their inner surfaces, and to be continuous with the skin which covers their outer sides. The surfaces of the conjunctiva and of the cornea are kept con- stantly moist by the tears, which are as constantly secreted by the Lacrymal glands. Each gland, (as shown at l. Fig. 417,) is situated above the eye, in a hollow of the orbit ; and the ducts (o) proceed- ing from it open upon the inner side of the upper eye-lid (e). This fluid, the uses of which are ob- viously to wash away dust, or other irritating sub- * An abundant supply of nerves has been bestowed on this mem- brane for the purpose of conferring upon it that exquisite degree of sensibiUty which was necessary to give immediate warning of the sHghtest danger to so important an organ as the eye from the in- trusion of foreign bodies. That this is the intention is apparent from the fact that the internal parts of the eye possess but little sensibi- hty compared with the external surface. VISION. 417 stances which may happen to get introduced, is distributed over the outer surface of the eye by means of the eye-lids. Each lid is supported by an elastic plate of cartilage, shaped like a crescent, and covered by integuments. An orbicular muscle, the fibres of which run in a circular direction, im- mediately underneath the skin, all round the eye,* is provided for closing them. The upper eye-lid is raised by a separate muscle, contained within the orbit, immediately above the upper straight muscle of the eye-ball. The eye-lashes are curved in op- posite directions, so as not to interfere with each other wdien the eye-lids are closed. Their utility in guarding the eye against the entrance of various substances, such as hairs, dust, or perspiration, and also in shading the eye from too strong impres- sions of light, is sufficiently apparent. The eye- lids, in closing, meet first at the outer corner of the eye ; and their junction proceeds along the line of their edges, towards the inner angle, till r" VOL. II. * See Fig:. 46, vol. i. p. l?^. E K 418 THE SENSORIAL FUNCTIONS. the contact is complete : by this means the tears are carried onwards in that direction, and accumu- lated at the inner corner of the eye ; an effect which is promoted by the bevelling of the margins of the eye-lids, which, when they meet, form a channel for the fluid to pass in that manner. When they arrive at the inner corner of the eye, the tears are conveyed away by two slender ducts, the ori- fices of which, called the puncta lacrymalia (p, p), are seen at the inner corner of each eye-lid, and are separated by a round projecting body (c), con- nected with a fold of the conjunctiva, and termed the lacrymal caruncle. The two ducts soon unite to form one passage, which opens into a sac (s), si- tuated at the upper part of the sides of the nose, and terminating below (at n) in the cavity of the nostrils, into which the tears are ultimately con- ducted. When the secretion of the tears is too abundant to be carried off by this channel, they overflow upon the cheeks ; but when the quantity is not excessive, the tendency to flow over the eye- lid is checked by an oily secretion proceeding from a row of minute glands, situated at the edge of the eye-lids, and termed the Meibomian glands. The eye-brows are a further protection to the eyes ; the direction of the hairs being such as to turn away from them any drops of rain or of per- spiration, which may chance to fall from above. Excepting in front, where the eyes are covered and protected by the eye-lids, these important organs are on all sides eflfectually guarded from injury by being contained in a hollow bony socket, termed the orbit, and composed of seven portions of bone. These seven elements may be recognised VISION. 419 in the skulls of all the mammalia, and perhaps also in those of all other vertebrated animals ; afFordina: an illustration of the unity of the plans of nature in the construction of the animal fabric. § 4. Physiology of perfect Vision. The rays of light, proceeding from a distant object, strike upon the convex surface of the cornea, which being of greater density than the air, refracts them, and makes them converge towards a distant focus. This effect, however, is in part counteracted on their emergence from the concave posterior surface of the cornea, when the rays enter into the aqueous humour. On the whole, however, they are consi- derably refracted, and made to converge to a degree equal to that which they would have undergone if they had at once impinged against the convex sur- face of the aqueous humour, supposing the cornea not to have intervened. A considerable portion of the light which has thus entered the aqueous humour is arrested in its course by the iris; so that it is only those rays which are admitted through the pupil that are sub- servient to vision. These next arrive at the crys- talline lens, where they undergo two refractions; the one at the anterior, the other at the posterior surface of that body. Both these surfaces being convex outwardly, and the lens being a denser sub- stance than either the aqueous or the vitreous humours, the effect of both these refractions is to increase the convergence of the rays, and to bring them to unite in a focus on the retina at the bottom 420 THE SENSORIAL FUNCTIONS. of the eye. The most effective of these refractions is the first; because the difference of density be- tween the air and the cornea, or rather the aqueous humour, is greater than that of any of the humours of the eye compared with one another. The accurate convergence of all the rays of light, which enter through the pupil, to their respective foci on the retina, is necessary for the perfection of the images there formed ; but for the complete attainment of this end various nice adjustments are still requisite. In the first place, the Aberration of Sphericity, already noticed,* which is a consequence of the geometrical law of refraction, introduces a degree of confusion in the image ; which is scarcely per- ceptible, indeed, on a small scale, but which be- comes sensible in instruments of much power; being one of the greatest difficulties which the optician has to overcome in the construction of the telescope and the microscope. Nature, in framing the human eye, has solved this difficulty by the simplest, yet most effectual means, and in a manner quite inimitable by human art. She has in the first place given to the surfaces of the crystalline lens, instead of the spherical form, curvatures more or less hyperbolical or elliptical ; and has, in the next place, constructed the lens of an infinite number of concentric layers, which increase in their density, as they succeed one another from the surface to the centre. The refracting power, being proportional to the density, is thus greatest at the centre, and diminishes as it recedes from that centre. This admirable adjustment exactly cor- * See Fig. 411, p. 410. VISION. 421 rects the deficiency of refraction, which always takes place in the central portions of a lens com- posed of a material of uniform density, as compared with the refraction of the parts more remote from the centre.* The second adjustment for perfect vision has reference to the variations in the distance of the focus which take place according as the rays arrive at the eye from objects at different distances, and which may be called the Aberrations of Parallax, When the distance of the object is very great, the rays proceeding from each point arrive at the eye with so little divergence, that each pencil may be considered as composed of rays which are parallel to each other ; the actual deviation from parallelism being quite insensible. But if the same object be brought nearer to the eye, the divergence of the rays becomes more perceptible; and the effect of the same degree of refraction is to collect them into a focus more remote than before. This is illus- trated by Fig. 412, 413, and 414 ;t the first of which shows the rapid convergence of rays proceeding from a very distant object, and which may be con- sidered as parallel. The second shows that di- vergent rays unite at a more distant focus; and the third, that the focus is more distant the greater the divergence. For every distance of the object there is a corresponding focal distance ; and when the eye is in a state adapted for distinct vision at * Sir David Brewster has ascertained that the variations of den- sity producing the doubly refracting structure, in the crystalline lens ,of fishes, are related, not to the centre of the lens, but to the diameter which forms the axis of vision ; an arrangement peculiarly adapted for correcting tiie spherical aberrations. Philos. Trans, for 18 1 6, p. 317. t Pages 409 and 410. 422 THE SENSORIAL FUNCTIONS. one distance, it will have confused images of objects at another distance ; because the exact foci of the rays will be situated either before or behind the retina. It is evident that if the retina be not placed exactly at the point where the focus is situated, it will either intercept the pencil of rays before they are united into a point, or receive them after they have crossed one another in passing through the focus ; in either of which cases, each pencil will throw upon the retina a small circle of light, brighter at the middle and fainter at the edges, which will mix itself with the adjacent pencils, and create confusion in the image. It is found, however, that the eye has a power of accommodating itself to the distinct vision of objects at a great variety of distances, according as the attention of the mind is directed to the par- ticular object to be viewed. That the refractive powers of the eye really change, is easily proved by the experiment of admitting only two separate pencils of rays proceeding from any small object, such as the head of a pin, to fall upon the eye ; as we can then at once ascertain whether or not they meet at the retina ; for when they do so, they form one image only, and the object appears single ; whereas if they do not unite, two images are formed on the retina, giving rise to the perception of a double object. This separation of the pencils may be obtained by viewing the object through two pin holes, made nearer to one another than the diameter of the pupil, in a blackened card held close to the eye. It will then be found that, with- out altering the situation either of the eye, or the object, or the interposed card, we possess the power, vibiOxN. 423 by a voluntary effort, of converting tlie double image into a single one, or vice versa ; a change which, being quite independent of the contraction or dilatation of the pupil, and of any change ex- ternal to the globe of the eye, can result only from a change effected in the refracting powers of the eye.* The mode in which this change in the state of the eye is effected has been the subject of much controversy. The increase of the refracting power of the eye necessary to adapt it to the vision of near objects is evidently the result of a muscular effort, of which we are distinctly conscious when we accurately attend to the accompanying sen- sations. The researches of Dr. Young have ren- dered it probable that some change takes place in the figure of the lens, whereby its convexity, and perhaps also its distance from the retina, are in- creased. He has shown by a very decisive experi- ment, that any change which may take place in the convexity of the cornea has but little share in the production of the effect; for the eye retains its power of adaptation when immersed in water, in which the form of the cornea can in no respect influence the refraction. But the rays of light are of different kinds; some exciting the sensation of red, others of yellow, and others again of blue ; and these different species of * This decisive experiment was first pointed out by Dr. Porter- field, in 1759. On the same principle Dr. Young contrived an instrument, which he termed the Optometer, (in which a line in a - flat graduated ruler is viewed through two narrow vertical slits made in a piece of brass), for measuring with exactness the distance to which the refractive powers of the eye are adjusted. 4'24 THE SENSORIAL FUNCTIONS. light are refracted, under similar circumstances, in dift'erent degrees. Hence the more refrangible rays, namely the violet and the blue, are brought to a nearer focus, than those which are less re- frangible, namely the orange and the red rays ; and this want of coincidence in the points of con- vergence of these different rays, (all of which enter into the composition of white light), necessarily impairs the distinctness of all the images produced by refraction ; shading off their outlines with va- rious colours, even when the object itself is colour- less. This defect, which is incident to the power of a simple lens, and which is termed the Chro- matic Aberration, is remedied almost perfectly in the eye, by the nice adjustment of the powers of the different refracting media, which the rays of light have to traverse before they arrive at the retina, producing what is called an achromatic combi- nation;* and it is found that the eye, though not an absolutely achromatic instrument, as was asserted by Euler,t is yet sufHciently so for all the ordinary practical purposes of life. The object, then, of the whole apparatus ap- pended to the optic nerve, is to form inverted images of external objects on the retina, which, as we have seen, is the expanded extremity of that nerve. That this effect is actually produced, may be easily shown by direct observation : for if the sclerotic and choroid coats be . carefully dissected * For the exposition of the principles on which these achromatic combinations of lenses correct this source of aberration,! must refer to works which treat professedly on Optics. t For the rectification of this error ue are indebted to Dr. Yoiin^j. VISION. 425 off from the posterior part of the eye of an ox, or any other large quadruped, leaving only the retina, and the eye so prepared be placed in a hole in a window-shutter, in a darkened room, with the cornea on the outside, all the illuminated objects of the external scene will be beautifully depicted, in an inverted position, on the retina. Few spectacles are more calculated to raise our admiration than this delicate picture, which nature has, with such exquisite art, and with the finest touches of her pencil, spread over the smooth canvass of this subtle nerve ; a picture, which, though scarcely occupying a space of half an inch in diameter, contains the delineation of a boundless scene of earth and sky, full of all kinds of objects, some at rest, and others in motion, yet all accu- rately represented as to their forms, colours and positions, and followed in all their changes, with- out the least interference, irregularity, or confusion. Every one of those countless and stupendous orbs of fire, whose light, after traversing immeasurable regions of space, at length reaches our eye, is collected on its narrow curtain into a luminous focus of inconceivable minuteness ; and yet this almost infinitesimal point shall be sufficient to convey to the mind, through the medium of the optic nerve and brain, a knowledge of the exist- ence and position of the far distant luminary, from which that light has emanated. How infinitely surpassing all the limits of our conception must be the intelligence, and the power of that Being, who planned and executed an instrument comprising, within such limited dimensions, such vast powers as th( eye, of which the perceptions comprehend 426 THE SENSORIAL FUNCTIONS. alike the nearest and most distant objects, and take cognizance at once of the most minute por- tions of matter, and of bodies of the largest mag- nitude ! § 5. Comparative Physiology of Vision. In the formation of every part of the animal ma- chinery we may generally discern the predomi- nance of the law of gradation ; but this law is more especially observed in those organs which exhibit, in their most perfect state, the greatest complication and refinement of structure ; for on following all their varieties in the ascending series, we always lind them advancing by slow gradations of im- provement, before they attain their highest degree of excellence. Thus the organ of vision presents, amidst an infinite variety of constructions, succes- sive degrees of refinement, accompanied by cor- responding extensions of power. So gradual is the progress of this developement, that it is not easy to determine the point where the faculty of vision, properly so called, begins to be exercised, or where the first rudiment of its organ begins to appear. Indications of a certain degree of sensibility to light are afibrded by many of the lower tribes of Zoophytes, w hile no visible organ appropriated to receive its impressions can be traced. This is the case with many microscopic animalcules ; and still more remarkably with the Hydra, and the Actinia, which show by their movements that they feel the influence of this agent ; for, when confined in a VISION. 427 vessel, they always place themselves, by prefer- ence, on the side where there is the strongest light.* The Veietillum ci/uomorium, on the other hand, seeks the darkest places, and contracts itself the moment it is exposed to light. f In a perfectly calm sea, the 3Iedasce which are rising towards the surface, are seen to change their course, and to descend again, as soon as they reach those parts of the water which receive the full influence of the sun's rays, and before any part of their bodies has come into contact with the atmosphere.^ But, ill all these instances a doubt may arise whether the observed actions may not be prompted by the mere sensation of warmth excited by the calorific rays which accompany those of light ; in vv hich case they would be evidence only of the operation of a finer kind of touch. The first appearance of visual organs is met with among the Infusoria. According to Ehrenberg, they are distinctly seen in some of the genera in the family of Monadina, and other polygastric infusoria of the smallest dimensions, such as the 3Iicroglena puuctifera,^ and monadina, Glenonicrum, P hacelomonas pulvisculus,\\ and JSodo,^ and in several of the Cryptomonadina, and Volvocina. In the Eiifrlena viridis** Nitzsch had before dis- covered three red points, which he regarded as eyes. In the Rotatoria these organs are almost * Such is the uniform report of Trembley, Baker, Bonnet, Goeze, Hanow, Roesel, and Scheeffer. t Rapp ; Nov. Act. Acad. Nat. Cur. of Bonn. xiv. 645. \ Grunt; Edin. Journal of Science: No. 20. § Enchelis ■punctifera,VL\\\\ NERVOUS SYSTEM OF VERTEBRATA. 497 section, Fig. 450), is composed of six parallel 4^0 ^. columns, two posterior (p), two middle (m), and two anterior (a) ; ^ which last are continuous with one 'p another, and are joined in front transversely by a broad commissure. On each side of the spinal cord, and between all the adjacent ver- tebrae, there proceed two sets of nervous filaments ; those which are continuous with the posterior columns, being appropriated to sensation ; and those arising from the anterior columns, being motor nerves. The former, soon after their exit from the spine, pass through a small ganglion, and then unite with the nerves from the anterior column ; com- posing, by the intermixture of their fibres, a single nervous trunk, which is afterwards divided and subdivided in the course of its further distri- bution, both to the muscular and the sentient organs of the body. Each of these spinal nerves also sends branches to the ganglia of the sympa- thetic nerve, which, as was formerly described, passes down on each side, parallel and near to the spine. Enlargements of the spinal cord are observed in those parts (w and l. Fig. 449), which supply the nerves of the extremities ; the increase of diameter being proportional to the size of the limbs requiring these nerves. In Serpents, which are wholly des- titute of limbs, the spinal cord is not enlarged in any part, but is a cylindrical column of uniform diameter. In Fishes, these enlargements appear to have a relation to the size of the organs of motion or sensation, and correspond to them in their situa- tion. Thus in the Tiigla lyra (the Red or Piper VOL. II. K K 498 THE SENSORIAL FUNCTIONS. Gurnard), and the Trigla gumardus (the Grey Gurnard), there are, at the commencement of the spinal cord, numerous enlargements, presenting a double row of tubercles (as seen in the space between m and s. Fig. 451). The nerves from these tubercles supply the detached rays, or feelers, anterior to the pectoral tin.* Fishes which possess electrical organs have a considerable dilatation of the spinal cord, answering to the large nerves which are distributed to those organs. Birds which fly but imperfectly, as the Gallinaceous tribe and the Scansores, have the posterior enlargement much greater than the anterior ; a disproportion which is particularly remarkable in the Ostrich. On the contrary, the anterior enlargement is much more considerable than the posterior in birds which have great power of flight. In the Dove, of which the brain and whole extent of the spinal cord are shown in Fig. 449, the enlargements (w and l) corre- sponding to the wings and legs respectively, are nearly of equal size. In Quadrupeds, we likewise And the relative size of these enlargements corre- sponding to that of fore and hind extremities. When the latter are absent, as in the Cetacea, the posterior dilatation does not exist. The brain (b) has been regarded as an expansion of the anterior or upper end of the spinal cord ; and its magnitude, as well as the relative size of its several parts, vary much in the diff"erent classes and families of vertebrated animals. This will appear from the inspection of the figures I have * The relation of these tubercles to the powers of sensation, rather than to those of motion, is apparent from their absence in the true Flying-fish, or Exocoetus voUtans. NERVOUS SYSTEM OF VERTEBRATA. 499 given of this organ in various species, selected as specimens from each class, view ed from above ; and in all of which I have indicated corresponding parts by the same letters of reference. The portion (m) of the brain, which appears as the immediate continuation of the spinal cord (s), is termed the medidla oblongata. The single tuber- cle (c), arising from the expansion of the posterior columns of the spinal cord, is termed the cere- bellum, or little brain. Next follow the pair (t), which are termed the optic tubercles, or lobes* and appear to be productions from the middle columns of the spinal cord. These are succeeded by ano- ther pair of tubercles (h), which are called the cerebral hemispheres, 2inA the origin of which maybe traced to the anterior columns of the spinal cord. There is also generally found, in front of the hemispheres, another pair of tubercles (o), which, being connected with the nerves of smelling, have been called the olfactory lobes, or tubercles. Several cavities, termed Ventricles, are found in the interior of the principal tubercles of the brain ; they appear to be formed by the apposition of the surfaces of the masses composing the brain, which are folded upon one another, leaving spaces lined by the pia mater, the surfaces being moistened by a fluid secretion. These are the principal parts of the cerebral mass to be here noticed ; for 1 purposely omit the mention of the minuter divisions, which, though they have been objects of much attention to anatomists, unfortunately furnish no assistance in * In the Mammalia, and in Man, they have been often designated by the very inappropriate name of Corpora quadriyemina. 500 THE SENSORIAL FUNCTIONS. understanding the physiology of this complicated and wonderful organ. On comparing the relative proportions of the brain and of the spinal cord in the four classes of vertebrated animals, a progressive increase in the size of the former will be observed, as we ascend from Fishes to Reptiles, Birds, and Mammalia. This increase in the magnitude of the brain arises chiefly from the enlargement of the cerebral hemis- pheres (h), which in the inferior orders of fishes, as in the Trigla lyra, or Piper Gurnard (Fig. 451), and in the MurcEna conger, or Conger Eel (Fig. 452), are scarcely discernible. They are very small in the Perca Jluviatilis, or common Perch (Fig. 453); but more developed in Reptiles, as in the Testudo mydas, or Green Turtle (Fig. 454), and in the Crocodile (Fig. 455) ; and still more so in Birds, as is seen in the brain of the Dove (Fig. 449) ; but most of all in Mammalia, as is exemplified in the brain of the Lion (Fig. 456). On the other hand, the optic tubercles (t) are largest, compared with the rest of the brain, in Fishes ; and their relative size diminishes as we ascend to Mammalia; and the same observation applies also to the olfac- tory lobes (o). The relative positions of the parts of the brain are much influenced by their proportional deve- lopement. This will be rendered manifest by the lateral views of the brains of the Perch, the Turtle, the Dove, and the Lion, presented in Figures 457, 458, 459, and 460, respectively, where the same letters are employed to designate the same parts as in the preceding figures. In Fishes, all the tuber- cles which compose this organ, are disposed nearly NERVOUS SYSTEM OF VERTEBRATA. 501 in a straight line, continuous with the spinal cord, of which, as they scarcely exceed it in diameter, they appear to be mere enlargements. As the skull expands more considerably than the brain, this organ does not fill its cavity, but leaves a large space, filled with fluid. Some degree of shortening, however, may be perceived in the brain of the Perch (Fig. 457) ; for the medulla oblongata (m) is doubled underneath the cerebellum (c), pushing it upwards, and rendering it more 23rominent than the other tubercles. This folding inwards, and shortening of the whole mass, proceeds to a greater extent as we trace the structure upwards, as may be seen in the brain of the Green Turtle, (Fig. 458). In that of Birds, of which Fig. 459 presents a vertical section, the optic tubercles have descended from their former place, and assumed a lateral position, near the lower surface of the brain, lying on each side of the medulla oblongata, at the part indicated by the letter t. In Mammalia, as in the Lion (Fig. 460), they are lodged quite in the interior of the organ, and concealed by the ex- panded hemispheres (h) ; their position only being marked by the same letter (t). These changes are consequences of the increasing developement of the brain, compared with that of the cavity in which it is contained, requiring every part to be more closely packed ; thus the layers of the hemispheres in Mammalia are obliged, from their great extent, to be plaited and folded on one another, presenting at the surface curious windings, or convolutions, as they are called (seen in Fig. 456), which do not take place in the hemispheres of the inferior classes. The foldings of the substance of the cerebellum i02 THE SENSORIAL FUNCTIONS. produce, likewise, even in birds, transverse furrows on the surface ; and from the interposition of a substance of a grey colour between the laminae of the white medullary matter, a section of the cere- bellum presents the curious appearance (seen in Fig. 459), denominated, from its fancied resem- blance to a tree, the Arbor Vitce. Thus far we have followed an obvious gradation in the developement and concentration of the dif- ferent parts of the brain ; but on arriving at Man, the continuity of the series is suddenly disturbed by the great expansion of the hemispheres, (Fig. 461), which, compared with those of quadrupeds. bear no sort of proportion to the rest of the nervous system. Both Aristotle and Pliny have asserted that the absolute, as well as the comparative size of the human brain is greater than in any other known animal ; exceptions, however, occur in the case of the Elephant, and also in that of the Whale, whose brains are certainly of greater absolute bulk NERVOUS SYSTEM OF VERTEBRATA. 503 than that of man. But all the large animals, with which we are familiarly acquainted, have brains considerably smaller ; as will readily appear from an examination of their skulls, which are narrow, and compressed at the part occupied by the brain; the greater part of the head being taken up by the developement of the face and jaws. In Man, on the other hand, the bones of the skull rise perpen- dicularly from the forehead, and are extended on each side, so as to form a capacious globular cavity for the reception and defence of this most impor- tant organ. It is chiefly from the expansion of the hemispheres, and the developement of its con- volutions, that the human brain derives this great augmentation of size.* Several expedients have been proposed for esti- mating the relative size of the brain in different tribes of animals, with a view of deducing conclu- sions as to the constancy of the relation which is presumed to exist between its greater magnitude and the possession of higher intellectual faculties. The most celebrated is that devised by Camper, and which he termed the facial angle, composed of two lines, one drawn in the direction of the basis of the skull, from the ear to the roots of the upper incisor teeth, and the other from the latter point, * This will be apparent from the vertical section of the human brain, Fig. 461; where, as before, s is the spinal cord; m, the medulla oblongata; c, the cerebellum, with the arbor vita ; t, the optic tubercles, or corpora quadrigemina, dwindled to a very small size, compared with their bulk, in fishes; p, the pineal gland, sup- posed by Des Cartes to be the seat of the soul ; v, one of the lateral ventricles; q, the corpus callusuni, or principal commissure; and H, n, 11, the hemispheres. 504 THE SENSORIAL FUNCTIONS. touching the most projecting part of the forehead. Camper conceived that the magnitude of this angle would correctly indicate the size of the brain, as compared with the organs of the principal senses which compose the face ; but the fallacy of this criterion of animal sagacity has been shown in a great many cases. § 1. Functions of the Brain. Physiologists have in all ages sought for an elu- cidation of the functions of the brain by the accu- rate examination of its structure, which evidently consists of a congeries of medullary fibres, ar- ranged in the most intricate manner. Great pains have been bestowed in unravelling the tissue of these fibres, in the hope of discovering some clue to the perplexing labyrinth of its organization ; but nearly all that has been learned from the labo- rious inquiry, is that the fibres of the brain are continuous with those which compose the columns of the spinal cord ; that they are variously inter- mixed with grey neurine, as they are in ganglia, in their course through masses of nervous matter, which appear to be analogous to ganglia ; that their remote extremities extend to the surface of the convolutions of the brain and cerebellum, which are entirely composed of grey neurine: and that these latter portions, again, are connected exten- sively by commissural fibres.* * By the application of high magnifying powers Ehrenberg has found that white neurine is composed of very minute fibres of dif- ferent sizes ; the larger being cylindrical tubes, containing a granular FUNCTIONS OF THE BRAIN. oOo It is a remarkable fact, that in vertebrated ani- mals all the organs which are subservient to the sensorial functions are double ; those on one side being exactly similar to those on the other. We see this in the eyes, the ears, the limbs, and all the other instruments of voluntary motion ; and in like manner, the parts of the nervous system which are connected with these functions are all double, and arranged symmetrically on the two sides of the body. The same law of symmetry extends to the brain : every part of that organ, which is found on one side, is repeated on the other ; so that, strictly speaking, we have two brains, as well as two optic nerves and two eyes. But in order that the two sets of fibres may co-operate, and consti- tute a single organ of sensation, corresponding with our consciousness of individuality, it was necessary that a free communication should be established between the parts on both sides : and it is for this purpose, apparently, that a set of medullary pulpy matter ; and the smaller, which he terms articulated Jibres, having, at regular intervals, dilated portions, connected by narrower filaments, so as to bear a resemblance to strings of pearls. These two kinds of fibres are united by cellular tissue, and a fine network of blood vessels. The nerves of the special senses, that is, the olfactory, optic, and auditory nerves, are composed entirely of arti- culated fibres, which also form the great bulk of the white substance of the brain. But the ordinary nerves contain only the straight cylindrical tubular fibres, which, however, are continuous with the former at their junction with the brain. The grey neurine also con- tains minute articulated fibres, but in less number, of smaller dia- meter, and more irregularly disposed : they are contained in a denser network of blood vessels, and are intermixed with layers of granules, and thin plates of grey pulp. The chemical analysis of neurine shows that it contains a large proportion, (about four fifths of its weight) of water. 506 THE SENSORIAL FUNCTIONS. fibres, passing directly across from one side of the brain to the other, has been provided ; these constitute what are called the Commissures of the Brain. The question, however, still recurs ; — What rela- tion does all this artificial intertexture and accu- mulation of fibres bear to the mental operations of which we are conscious, such as memory, ab- straction, thought, judgment, imagination, volition ? Are there localities set apart for our different ideas in the store-house of the cerebral hemispheres; and are they associated by the material channels of communicating fibres? Are the mental pheno- mena the effects, as was formerly supposed, of a subtle fluid, or animal spirits, circulating with great velocity along invisible canals in the nervous sub- stance ; or shall we, with Hartley, suppose them to be the results of vibrations and vibratiimcles, agi- tating in succession the finer threads of which this mystic web has been constructed? A little reflec- tion will suffice to convince us that these, and all other mechanical hypotheses, which the most fan- ciful imagination can devise, make not the smallest approach to a solution of the difficulty ; for they, in fact, do not touch the real subject to be explained, namely, how the affections of a material substance can influence and be influenced by an immaterial agent. All that we have been able to accomplish has been to trace the impressions from the organ of sense along the communicating nerve to the sensorium : beyond this the clue is lost, and we can follow the process no farther. The exact locality of the sensorium has been PERCEPTIONS OF ANIMALS. .507 eagerly sought for by physiologists in every age. It would appear, from the results of the most recent inquiries, that it certainly does not extend to the whole mass of the brain, but has its seat more especially in the lower part, or basis of that organ . But beyond this point we can derive no further aid from anatomy ; for the intellectual operations of which we are conscious bear no conceivable analogy to any of the configurations or actions of a material substance. Although the brain is con- structed with evident design, and composed of a number of curiously wrought parts, we are utterly unable to penetrate the intention with which they are formed, or to perceive the slightest correspon- dence which their configuration can have with the functions they respectively perform. The map of regions which modern Phrenologists have traced on the surface of the head, and which they suppose to have a relation to different faculties and pro- pensities, does not agree either with the natural divisions of the brain, or with the metaphysical classification of mental phenomena.* Experiments and pathological observations, however, seem to show that the hemispheres of the brain are the chief instruments by which the intellectual opera- tions are carried on ; that the central parts, such * For a summary of the doctrines of Drs. Gall and Spurzheim, I heg leave to refer the reader to an account which I drew up, many years ago, for the Encyclopaedia Britannica, and which composed the article " Cranioscopy" in the last supplement to that work, edited by Mr. Napier. It has since been republished, with additions, in the last edition, under the title of" Phrenology." 508 THE SENSORIAL FUNCTIONS. as the optic lobes and the medulla oblongata, are those principally concerned in sensation ; and that the cerebellum has considerable influence in regu- lating voluntary motion. § 5. Comparative Physiology of Perception. Of the perceptions of the lower animals, and of the laws which they obey, our knowledge must, of necessity, be extremely imperfect, since it must be derived from a comparison with the results of our own sensitive powers, which may differ very essentially from those of the subjects of our obser- vation. The same kind of organ which, in our- selves, conveys certain definite feelings, may, when modified in other animals, be the source of very diflerent kinds of sensations and perceptions, of which our minds have not the power to form any adequate conception. Many of the qualities of surrounding bodies, which escape our more obtuse senses, may be distinctly perceived, in all their gradations, by particular tribes of animals, fur- nished with more delicate organs. Many quadru- peds and birds possess powers of vision incompa- rably more extensive than our own ; in acuteness of hearing, we are excelled by a great number of animals ; and in delicacy of taste and smell, there are few quadrupeds that do not far surpass us. The organ of smell, in particular, is often spread over a vast extent of surface, in a cavity occupying the greatest part of the head ; so that the perceptions of this sense must be infinitely diversified. PERCEPTIONS OF ANIMALS. 509 Bats have been supposed to possess a peculiar, or sixth sense, enabling them to peirceive the situa- tions of external objects without the aid either of vision or of touch. The principal facts upon which this opinion has been founded were discovered by Spallanzani, who observed that these animals would fly about rapidly in the darkest chambers, although various obstacles were purposely placed in their way, without striking against or even touching them. They continued their flight with the same precision as before, threading their way through the most intricate passages, when their eyes were completely covered, or even destroyed. Mr. Jurine, who made many experiments on these animals, concludes that neither the sense of touch, of hear- ing, or of smell, was the medium through which bats obtain perceptions of the presence and situa- tion of surrounding bodies ; but he ascribes this extraordinary faculty to the great sensibility of the skin of the upper jaw, mouth, and external ear, which are furnished with very large nerves.* The wonderful acuteness and power of discri- mination which many animals exercise in the dis- covery and selection of their food, has often sug- gested the existence of new senses, different from those which we possess, and conveying peculiar and unknown powers of perception. An organ, which appears to perform some sensitive function of this kind, has been discovered in a great num- ber of quadrupeds by Jacobson.-j In the human * Sir Anthony Carlisle attributes this power to the cxtrcino deli- cacy of hearing in this animal. t See Annales tin Miisee; xviii. 412. f)\0 THE SENSORIAL FUNCTIONS. skeleton there exists a small perforation in the roof of the mouth, just behind the sockets of the incisor teeth, forming- a communication with the under and fore part of the nostrils. This canal is perceptible only in the dried bones ; for, in the living body, it is completely closed by the membrane lining the mouth, which sends a prolongation into it ; but in quadrupeds, this passage is pervious, even during life, and is sometimes of considerable width. Ja- cobson found, on examining this structure with attention, that the canal led to two glandular organs of an oblong shnpe, and enclosed in carti- laginous tubes : each gland has in its centre a cavity, which communicates above with the general cavity of the nostrils. These organs lie concealed in a hollow groove within the bone, where they are carefully protected from injury: and they receive a great number of nerves and blood-vessels, resembling in this respect the organs of the senses. Their structure is the same in all quadrupeds in which they have been examined ; but they are largest in the family of the Rodeutia, and next in that of the Rmninantia : in the Horse, they are still very large, but the duct is not pervious ; while in carnivorous quadrupeds, they are on a smaller scale. In Monkci/s, they may still be traced, although ex- tremely small ; appearing to form a link in the chain of gradation connecting this tribe with the human race, in whom every vestige of these organs has disappeared, excepting the aperture in the bones already noticed. Any use that can be at- tributed to these singularly constructed organs must evidently be quite conjectural. The ample supply of nerves which they receive would indicate their PERCEPTIONS OF ANIMALS. 511 performing some sensitive function ; and their situation would point them out as (ittiiig them for the appreciation of objects presented to the mouth to be used as food : hence it is probable that the perceptions they convey have a close affinity with those of smell and taste. The palate of the Carp is furnished with a very singular organ, of a reddish grey colour, soft and fleshy in its texture, abundantly supplied with nerves, and having the appearance of a gland. It is exceedingly irritable, immediately becoming swollen and turgid on the slightest puncture, or even touch of a foreign body. The sensations it conveys are probably analogous to those of taste : but the precise nature of its function can only be matter of conjecture.* The larger cartilaginous fishes, as Sharks and Rays, have been supposed by Treviranus to be endowed with a peculiar sense, from their having an organ of a tubidar structure on the top of the head, and immediately under the skin. Roux considers it as conveying sensations intermediate between those of touch and hearing ; while De Blainville and Jacobson regard it merely as the organ of a liner touch. The perceptive powers of Insects must embrace a very different, and in many respects, more ex- tended sphere than our own. These animals mani- fest by their actions that they perceive and anti- cipate atmospheric changes, of which our senses give us no information. It is evident, indeed, that the impressions made by external objects on their sentient organs must be of a nature widely different * Cuvicr, Hist. Nat. ties Poissons, i. 352. 512 THE SENSORIAL FUNCTIONS. from those which the same objects communicate to ourselves. While with regard to distance and mag- nitude our perceptions take a wider range, and ap})ear infinitely extended when compared with those of insects, yet they may, in other respects, be greatly inferior. The delicate discrimination of the more subtle affections of matter is perhaps com- patible only with a minute scale of organization. Thus the varying degrees of moisture or dryness of the atmosphere, the continual changes in its pressure, the fluctuations in its electrical state, and various other physical conditions, may be objects of distinct perception to these minute animals. Organs may exist in them, appropriated to receive impressions, of which we can have no idea ; and opening avenues to various kinds of knowledge, to which we must ever remain utter strangers. Art, it is true, has supplied us with instruments for dis- covering and measuring many of the properties of matter, which our unassisted senses are inadequate to observe. But neither our thermometers, nor our electroscopes, our hygrometers, nor our galvano- meters, however skilfully devised or elaborately constructed, can vie in delicacy and perfection with that refined apparatus of the senses, which nature has bestowed on even the minutest insect. There is reason to believe, as Dr. Wollaston has shown, that the hearing of insects comprehends a range of perceptions very different from that of the same sense in the larger animals ; and that a class of vibrations too rapid to excite our auditory nerves, is perfectly audible to them. Sir John Herschel has also very clearly proved that, if we admit the truth of the undulatory theory of light, it is easy to PERCEPTIONS OF ANIMALS. -513 conceive how the limits of visible colour may be established; for if there be no nervous fibres in unison with vibrations more or less frequent than certain limits, such vibrations, though they reach the retina, will produce no sensation. Thus it is perfectly possible that insects, and other animals, may be incapable of being affected by any of the colours which we perceive ; while they may be susceptible of receiving distinct luminous impres- sions from a class of vibrations which, applied to our visual organs, excite no sensation.* The func- tions of the antennae, which, though of various forms, are organs universally met with in this class of animals, must be of great importance, though obscurely known ; for insects when deprived of them appear to be quite lost and bewildered. The Torpedo, the Gymnotus, and several other fishes, are furnished with an electrical apparatus, resembling the Voltaic battery, which they have the power of charging and discharging at pleasure. An immense profusion of nerves is distributed upon this organ ; and we can hardly doubt that they communicate perceptions, with regard to electri- city, very different from any that we can feel. In general, indeed, it may be remarked, that the more an organ of sense differs in its structure from those which we ourselves possess, the more uncertain must be our knowledge of its functions. We may, without any great stretch of fancy, conceive our- selves placed in the situation of the beasts of the forest, and comprehend what are the feelings and motives which animate the quadruped and the * Encyclopsedia Metropolitana, Article " Light." VOL. H. L L 514 THE SENSORIAL FUNCTIONS. bird. But how can we transport ourselves, even in imagination, into the dark recesses of the ocean, which we know are tenanted by multitudinous tribes of fishes, zoophytes, and moUusca ? How can we figure to ourselves the sensitive existence of the worm or the insect, organized in so different a manner from ourselves, and occupying so remote a region in the expanse of creation ? How can we venture to speculate on the perceptions of the ani- malcule, whose world is a drop of fluid, and whose fleeting existence, chequered perhaps by various transformations, is destined to run its course in a few hours ? Confining our inquiries, then, to the more intel- ligible intellectual phenomena displayed by the higher animals, we readily trace a gradation which corresponds with the developement of the central nervous organ, or brain. That the comparison may be fairly made, however, it is necessary to distin- guish those actions which are the result of the exercise of the intellectual faculties, from those which are called instinctive, and are referable to other sources. The actions of animals appear on various occasions to be guided by a degree of sagacity not derivable from experience, and appa- rently implying a fore-knowledge of events, which neither experience nor reflection could have led them to anticipate. We cannot sufliciently admire the provident care displayed by nature in the pre- servation both of the individual and of the species, which she has entrusted not to the slow and uncer- tain calculations of prudence, but to innate faculties, prompting, by an unerring impulse, to the per- formance of the actions required for those ends. PERCEPTIONS OF ANIMALS. Ol5 We see animals providing against the approach of winter, the effects of which they have never expe- rienced, and employing various means of defence against enemies they have never seen. The parent consults the welfare of the offspring she is destined never to behold ; and the young discovers and pur- sues without a guide that species of food which is best adapted to its nature. All these unexplained, and perhaps inexplicable facts, we must content ourselves with classing under the head of instinct ; a name which is, in fact, but the expression of our ignorance of the nature of that agency, of which we cannot but admire the ultimate effects, while we search in vain for the efficient cause. In all the inferior orders of the animal creation where instincts are multiplied, while the indications of intellect are feeble, the organ which performs the office of the brain is comparatively small. The sensitive existence of these animals appears to be circumscribed within the perceptions of the mo- ment, and their voluntary actions have reference chiefly to objects which are present to the sense. In proportion as the intellectual faculties of animals are multiplied, and embrace a wider sphere, addi- tional magnitude and complication of structure are given to the nervous substance which is the organ of those faculties. The greater the power of com- bining ideas, and of retaining them in the memory, the greater do we find the developement of the cerebral hemispheres. These parts of the brain are comparatively small, as we have seen, in fishes, reptiles, and the greater number of birds ; but in the mammalia they are expanded in a degree nearly proportional to the extent of memory, saga- /)10 THE SENSORIAL FUNCTIONS. city, and docility. In man, in whom all the facul- ties of sense and intellect are so harmoniously combined, the brain is not only the largest in its size, but beyond all comparison the most compli- cated in its structure.* A large brain has been bestowed on man, evi- dently with the design that he should exercise superior powers of intellect; the great distinguishing features of which are the capacity for retaining an immense variety of impressions, and the strength, the extent, and vast range of the associating prin- ciple, which combines them into groups, and forms them into abstract ideas. Yet the lower animals also possess their share of memory, and of reason : they are capable of acquiring knowledge from ex- perience ; and, on some rare occasions, of devising expedients for accomplishing particular ends. But still this knowledge and these efforts of intellect are confined within very narrow limits ; for nature has assigned boundaries to the advancement of the lower animals, which they can never pass. If one favoured individual be selected for a special educa- tion, some additional share of intelligence, may, perhaps, with infinite pains, be infused ; but the improvement perishes with that individual, and is wholly lost to the race. By far the greater portion of that knowledge which it imports them to possess is the gift of Him, who has wisely implanted such instinctive impulses as are necessary for their pre- * All the parts met with in the brain of animals exist also in the brain of man ; while several of those found in man are either extremely small, or altogether absent in the brains of the lower animals. Soemmering has enumerated no less than fifteen mate- rial anatomical differences between the human brain and that of the ape. INTELLECTUAL FACULTIES OF MAN. 517 servation. Man also is born with instincts, but they are few in number compared with those of tlie lower animals, and unless cultivated and improved by reason and education, would, of themselves, produce but inconsiderable results. That of which the effects are most conspicuous, and which is the foundation of all that is noble and exalted in our nature, is the instinct of Sympathy, The affections of the lower animals, even between individuals of the same species, are observable only in a few in- stances ; for in general they are indifferent to each other's joys or sufferings, and regardless of the treatment experienced by their companions. The attachment, indeed, of the mother to her offspring, as long as its wants and feebleness require her aid and protection, is as powerful in the lower animals, as in the human species ; but its duration, in the former case, is confined, even in the most social tribes, to the period of helplessness ; and the ani- mal instinct is not succeeded, as in man, by the continued intercourse of affection and kind offices, and those endearing relations of kindred, which are the sources of the purest happiness of human life. While Nature lias apparently frowned on the birth of man, and brought him into the world weak, naked, and defenceless, unprovided with the means of subsistence, and exposed on every side to de- struction, she has in reality implanted in him the germ of future greatness. The helplessness of the infant calls forth the fostering care and tenderest affections of the mother, and lays the deep foun- dations of the social union. The latent energies of his mind and body are successively, though '"Sia THE SENSORIAL FUNCTIONS. slowly developed. While the vital organs are actively engaged in the execution of their different offices, while the digestive apparatus is exercising its powerful chemistry, while myriads of minute arteries, veins, and absorbents are indefatigably at work in building and modelling this complex frame, the sentient principle is no less assiduously and no less incessantly employed. From the earliest dawn of sensation it is ever busy in arranging, in com- bining, and in strengthening the impressions it receives. Wonderful as is the formation of the bodily fabric, and difficult as it is to collect its history, still more marvellous is the progressive construction of the human mind, and still more arduous the task of tracing the finer threads which connect the delicate web of its ideas, which fix its fleeting perceptions, and which establish the vast system of its associations ; and of following the long series of gradations, by which its affections are expanded, purified, and exalted, and the soul pre- pared for its higher destination in a future stage of existence. Here, indeed, we perceive a remarkable inter- ruption to that regular gradation, which we have traced in all other parts of the animal series; for between man and the most sagacious of the brutes there intervenes an immense chasm, of which we can hardly estimate the magnitude. The functions which are purely vital, and are necessary for even the lowest degree of sensitive existence, are pos- sessed equally by all animals : in the distribution of the faculties of mere sensation a greater in- equality may be perceived : the intellectual facul- ties, again, are of a more refined and nobler INTELLECTUAL FACULTIES OF MAN. 519 character, and being less essential to animal life, are dealt out by nature with a more sparing and partial hand. Between the two extremities of the scale we find an infinite number of intermediate degrees. The more exalted faculties are possessed exclusively by man, and constitute the source of the immense superiority he enjoys over the brute creation, which so frequently excels bim in the perfection of subordinate powers. In strength and swiftness he is surpassed by many quadrupeds. In vain may he wish for the power of flight possessed by the numerous inhabitants of air. He may envy that range of sight which enables the bird to discern, from a height at which it is itself invisible to our eyes, the minutest objects on the surface of the earth. He may regret the dullness of his own senses, when he adverts to the exquisite scent of the hound, or the acute hearing of the bat. While the delicate perceptions of the low er animals teach them to seek the food which is salutary, and avoid that which is injurious, man alone seems stinted in his powers of discrimination, and is compelled to gather instruction from a painful and hazardous experience. But if nature has created him thus apparently helpless, and denied him those instincts with which she has so liberally furnished the rest of her offspring, it was only to confer upon him gifts of infinitely higher value. While in acuteness of sense he is sur])assed by inferior animals, in the powers of intellect he stands unrivalled. In the fidelity and tenacity with which impressions are retained in his memory, in the facility and strength with which they are asso- ciated, in grasp of comprehension, in extent of .'320 THE SENSORIAL FUNCTIONS. reasoning, in capacity of progressive improvement, he leaves all other animals at an immeasurable distance behind. He alone enjoys in perfection the gift of utterance ; he alone is able to clothe his thoughts in words ; in him alone do we find im- planted the desire of examining every department of nature, and the power of extending his views beyond the confines of this globe On him alone have the high privileges been bestowed of recog- nising and of adoring the Power, the Wisdom, and the Goodness of the Author of the Universe, from whom his being has emanated, to whom he owes all the blessings which attend it, and by whom he has been taught to look forward to brighter skies and to purer and more exalted conditions of existence. Heir to this high destination, Man discards all alliance with the beasts that perish ; confiding in the assurance that the dissolution of his earthly frame destroys not the germ of immor- tality which has been implanted within him, and by the develupement of which the great scheme of Providence here commenced, will be carried on, in a future state of being, to its final and perfect consummation. PART IV. THE REPRODUCTIVE FUNCTIONS. Chapter I. REPRODUCTION. Limits have been assigned to the duration of all living beings. The same power to whom they owe their creation, their organization, and tlieir en- dowments, has also subjected them to the inexo- rable Laiv of Mortality ; and has ordained that the series of actions which characterize the state of life, shall continue for a definite period only, and shall then terminate. The very same causes which, at the earlier stages of their existence, promoted their developement and growth, and which, at a maturer, age, sustained the vigour and energies of the system, produce, by their continued and silent operation, gradual changes in the balance of the functions, and, at a later period, effect the slow demolition of the fabric they had raised, and the successive destruction of the faculties they had ori- ginally nurtured and upheld.* With the germs of life, in all organized structures, are conjoined the * See the article " Age," in the Cijclopcedia of Practical Medi- cine, where 1 have enltirged on tliis s^ubject. 522 THE REPRODUCTIVE FUNCTIONS. seeds of decay and of death ; and however great may be the powers of their vitality, we know that those powers are finite, and that a time must come when they will be expended, and wlien their re- newal, in that individual, is no longer possible. But although the individual perishes. Nature has taken special care that the race shall be constantly preserved, by providing for the production of new individuals, each springing from its predecessor in endless perpetuity. The process by which this formation, or rather this apparent creation, of a living being is effected, surpasses the utmost powers of the human comprehension. No conceivable combinations of mechanical or of chemical powers bear the slightest resemblance, or the most remote analogy, to organic reproduction, or can afford the least clue to the solution of this dark and hopeless enigma. We must be content to observe and gene- ralize the phenomena, in silent wonder at the mar- vellous manifestation of express contrivance and design exhibited in this department of the economy of created beings. Throughout the whole, both of the vegetable and animal world, Nature has shown the utmost soli- citude to secure not only the multiplication of the species, but also the dissemination of their numbers over every habitable and accessible region of the globe ; and has pursued various plans for the accomplishment of these important objects. The simplest of all the modes of multiplication consists in the spontaneous division of the body of the parent into two or more parts ; each part, when separated, becoming a distinct individual, and soon acquiring the size and shape of the parent. We REPRODUCTION 523 meet with frequent examples of this process of Jissiparous generation, as it is termed, among the infiisory animalcules. Many species of Monads^ for instance, which are naturally of a globular shape, exhibit at a certain period of their develope- ment a slight circular groove round the middle of their bodies, which by degrees becoming deeper, changes their form to that of an hour-glass ; and the middle part becoming still more contracted, they present the appearance of two balls, united by a mere point. The monads in this state are seen swimming irregularly in the fluid, as if animated by two different volitions; and, apparently for the purpose of tearing asunder the last connecting fibres, darting through the thickest of the crowd of surrounding animalcules ; and the moment this slender ligament is broken, each is seen moving away from the other, and beginning its independent existence. This mode of separation is illustrated by Fig. 462, representing the successive changes 462 Q (J ti O © Q 463 of form during its progress. In this animalcule the division is transverse, but in others, for example in the Vorticella (as shown in Fig. 403), and in most of the larger species, the line of separation is longi- tudinal. Some species of Monads, again, separate themselves at once, by a crucial line of division into four equal parts ; and the Gonium pectorals, by a double process of the same kind, divide them- 524 THE REPRODUCTIVE FUNCTIONS. selves always into sixteen parts. This subdivision of the Gonium begins in the interior of the parent animalcule, and is completed within the external, or teguinentary membrane, long before the con- tained young are liberated by the rupture of that membrane ; so that there is a certain period when the appearance presented is that of an aggregate of sixteen individual animalcules enclosed in a com- mon and perfectly transparent envelope. Each animalcule, thus formed by the subdivision of its predecessor, soon grows to the size which again determines a further spontaneous subdivision into two other animalcules ; these, in course of time, themselves undergo the same process, and so on, to an indefinite extent. The most singular circum- stance attending this mode of multiplication is that it is impossible to pronounce which of the new indi- viduals thus formed out of a single one should be regarded as the parent, and which as the offspring; for they are both of equal size. Unless, therefore, we consider the separation of the parts of the parent animal to constitute the close of its indi- vidual existence, we must recognise an unbroken continuity in the vitality of the animal, thus trans- mitted in perpetuity from the original stem, through- out all succeeding generations. This, however, is one of those metaphysical subtleties for which the subject of reproduction affords abundant scope, but which it would be foreign to the object of this work to discuss. A mode of multiplication referable to this head occurs in the polypi of the genus Campatmlaria* * Clytie of Lamouroux. REPRODUCTION. 525 of which, at a certain period of its existence, tlie ends, constituting tlie soft and contractile portion of the animal, each having its separate mouth and tentacula, detach themselves from the horny stem to which they had previously adhered, and begin their independent existence by swimming freely in the water: in this state they have a great resem- blance in form to small medusae. The stem which has been thus abandoned, continues to live and soon repairs its losses by the reproduction of new polypes.* It is in the animal kingdom only that we meet with instances of this spontaneous division of an organic being into parts, where each reproduces an individual of the same species. All plants, how- ever, are capable of being multiplied by artificial divisions of this kind : thus a tree may be divided longitudinally into a great number of portions, or slips, as they are called, any one of which, if planted separately and supplied with nourishment, may continue to grow, and may, in time, reproduce a tree similar in all respects to the one from which it originated. This inherent power of reproduction exists even in smaller fragments of a plant; for, when all circumstances are favourable, a stem will shoot from the upper end of the fragment, and roots will be sent forth from its lower end ; and ulti- mately a complete plant will be formed. | These * Nordman, Comptes Rendus, ix. 704. t Among the conditions necessary for these evolutions of organs are, fir&t, the previous accumulation of a store of nourishment in the detached fragment, adequate to supply the growth of the new parts; and secondly, the presence of a siifhcientquantity of circulating sap, as a vehicle for the transmission of that nourishment. It has been found that when ihese conditions are present, even the leaf of an 52f) THE REPRODUCTIVE FUNCHONS. facts, which are well known to horticultiiralists, exhibit only tlie capabilities of vegetative power under circumstances whicli do not occur in the natural course of things, but have been the effect of human interference. Reproductive powers of a similar kind are exhi- bited very extensively in the lower departments of the animal kingdom. The Hydra, or fresh water polype, is capable of indefinite multiplication by simple division : thus, if it be cut asunder trans- versely, the part containing the head soon supplies itself with a tail ; and the detached tail soon shoots forth a new head, with a new set of tentacula. If divided longitudinally, each half will, in the course of an hour, have formed itself into a separate tube by the folding in and adhesion of the cut edges, and will employ its tentacula in laying hold of objects. Even if divided into several strips, each fragment becomes a new tube, not, however, as before, by the folding in of its edges, but by the formation of a new cavity within its substance. If any of the tentacula, or any portion of one of them, be cut off, the mutilation is soon repaired ; and if the whole animal be divided into a great number of pieces, each fragment acquires, in a short time, all the parts which are wanting to render it a complete individual. The same phenomena are observed, and nearly to the same extent, in the Planaria. orange tree, when planted in a favourable soil, sends down roots, and is capable of giving origin to an entire tree. According to the ob- servations of Mirandola, the leaf of the Bryopliyllum, when simply laid on moist ground, strikes out roots, which quickly penetrate into the soil. (De Candolle, Physiologie Vegetale, ii. 677.) The leaves of the monocotyledonous plants often present the same phenomenon. RKPRODUCTION. 527 The Asterias, the Actinia, and some of the lower species of Annelida, as the Nais, are also capable of being multiplied by artificial divisions ; each segment having the power of supplying others, and containing within itself a kind of separate and individual vitality. A power of more partial regeneration of muti- lated parts by new growths, which is very analogous to that of complete reproduction, exists in the higher orders of animals, though it does not extend to the entire formation of two individuals out of one. The claws, the feet, and the antennae of the Crustacea^ and the limbs of the Aracluiida, are restored, when lost, by a fresh growth of these organs. If the head of a Snail be amputated, the whole of that part of the animal, including the telescopic eyes, and other organs of sense, will be reproduced. Even among the Vertebrata we find instances of these renovations of mutilated parts; as happens with respect to the tins of fishes : for Broussonet found that in whatever direction they are cut, the edges easily unite ; and the rays themselves are reproduced, provided the smallest part of their base has been left. The tails of Neivts, and of some species of Lizards, will grow again, if lost; and what is more remarkable, the eyes themselves, with all their complex apparatus of coats and humours, will, if removed, be replaced by the growth of new eyes as perfect as the former. We have seen that the teeth of Sharks, and other fishes, are renewed with the utmost facility, when by accident they have been lost. Among Mammalia, similar powers exist, although they are restricted within much narrower limits ; as is exemplified in the formation of new bones, replacing those which have perished. 528 THE REPRODUCTIVE FUNCTIONS. When we advert to the numberless instances of the reparation of injuries happening to various parts of our own frame, we have abundant reason to admire and be grateful forthe wise and bountiful provisions which Nature has made for meeting these con- tingencies. The multiplication of the species by buds, or Gemmiparous reproduction, is exemplified on the largest scale in the vegetable creation. Almost every point of the surface of a plant appears to be capable of giving rise to a new shoot, which, when fully developed, exactly resembles the parent stock, and may, therefore, be regarded as a separate organic being. The origin of buds is wholly be- yond the sphere of our observation; for they arise from portions of matter too minute to be cognizable to our organs, with every assistance which the most powerful microscopes can supply. These imper- ceptible atoms, from which organic beings take their rise, are called germs. Vegetable germs are of two kinds ; those which produce stems, and those which produce roots ; and although both may be evolved from every part of the plant, the former are usually developed at the axill(B of the leaves ; that is, at the angles of their junction with the stem ; and also at the extremities of the fibres of the stems ; their developement being determined by the accumulation of nourishment around them. They first produce buds, which ex- panding, and putting forth roots, assume the form of shoots ; and the successive accumulation of shoots, which remain attached to the parent plant,* * In some rare instances the shoots are removed to a distance from the parent plant, by a natural process : this occurs in some creeping plants, which propagate themselves by the horizontal extension of REPRODUCTION. 529 and to each other, is what constitutes a tree. Wliat are called knots in wood are the result of germs, which, in consequence of the accumulation of nourishment around them, are developed to a cer- tain extent, and then cease to grow. The Lemna, or common Duckweed, which consists of a small circular leaf, floating on the surface of stagnant pools, presents a singular instance of the develope- ment of germs from the edges of the leaves, and the subsequent separation of the new plant thus formed. In this respect the process is analogous to the natural mode of multiplication met with in the lower orders of Zoophytes, such as the Hi/dni. At the earliest period at which the young of this ani- mal is visible, it appears like a small tubercle, or bud, rising from the surface of the parent hydra: it grows in this situation, and remains attached for a considerable period ; at first deriving its nourish- ment, as well as its mechanical support, from the parent ; then occasionally stretching forth its tenta- cula, and learning the art of catching and of swal- lowing its natural prey. The tube, which constitutes its stomach, at first communicates by a distinct opening with that of its parent : but this opening afterwards closes ; and the filaments by which it is connected with the parent, becoming more and more slender, at length break, and the detached hydra immediately moves away, and commences its career their branches on the ground, where they dip, and strike out now roots, giving rise to stems independent of the original plant. This also sometimes happens in the case of tuberous roots, as the potatoe, which contain a number of germs, surrounded by nutritive matter, ready to be developed when circumstances are favourable. These portions are called eyes ; and each of them, when planted separately, are readily evolved, and give rise to an individual plant. VOL. II, M M 530 THE REPRODUCTIVE FUNCTIONS. of independent existence. This mode of multipli- cation, in its first period, corresponds exactly with the production of a vegetable by buds ; and may therefore be classed among the instances of gem- miparous reproduction ; although at a later stage, it differs from it in the complete detachment of the offspring from the parent. That the parent and its offspring have, while still united, distinct individual existences appears from the curious fact that both have been seen contending for the same worm, each having seized and beginning to devour the end nearest to it. Sometimes, also, new buds are found sprouting from the sides of the young, before it is itself detached from its parent ; thus exhibiting three generations united together in this living genealogical tree. Hydatids often, in like manner, multiply by the evolution of buds from the surface. Another plan of reproduction is that in which the germs are developed in the interior of the animal, assuming, at the earliest period when they become animated, the form of the parent. In this case they are termed gemmules instead of buds. This, which has been termed the gemmuliparous mode of reproduction, is exemplified in the Hydra^ which, besides giving rise to buds, as already described, produce in their interior little gelatinous globules, which, as the autumn advances, make their escape from the parent, and remain undeveloped at the bottom of the w ater till the approach of spring : but the animals thus produced do not grow to so large a size as those which have sprung from buds. Similar phenomena are presented by many of the Infusoria. Among the Entozoa, we find that in some species of Hydatids^ the young are developed REPRODUCTION. 531 within the parent; and this proceeds successively for an indefinite number of generations.* In most cases of the spontaneous evolution of gemmules within the parent, channels are provided for their exit ; but the gemmules of the Actinia force their way through the sides of the body, which readily open to give them passage; after which, the lace- rated part soon heals. * The mode in which infusory animalcules are produced and multi- plied is involved in much obscurity. Many distinguished naturalists, adopting the views of BufFon, have regarded them as the product of an inherent power belonging to a certain class of material particles, which, in circumstances favourable to its operation, tends to form these minute organizations ; and in this manner they explain how the same organic matter which had composed former living aggregates, on the dissolution of their union, reappears under new forms of life, and gives rise to the phenomenon of innumerable animalcules, starting into being, and commencing a new, but fleeting career of existence. Yet the analogy of every other department of the animal and vege- table kingdoms is directly opposed to the supposition that any living being can arise without its having been originally derived from an in- dividual of the same species as itself, and of which it once formed a part. The difficulty which the hypothesis of the spontaneous pro- duction of infusory animalcules professes to remove, consists in our inability to trace the pre-existence of the germs in the fluid, where these animalcules are found to arise; and to follow the operations of nature in these regions of infinite minuteness. The discoveries of Ehrenberg relative to the complexity of their organization, and the constancy with which each species preserves its form, go far towards placing these diminutive beings more on a level, both in structure and in functions, with the larger animals, of whose history and economy we have a more familiar and certain knowledge ; and in superseding the hypothesis above referred to, by showing that the bold assump- tion on which it rests, is not required for the explanation of the ob- served phenomena. In many of these animalcules, he has seen the ova excluded in the form of extremely minute globules, the 12,000th of an inch in diameter. When these had grown to the size of the 1700th of an inch, or seven times their original diameter, they were distinctly seen to excite currents, and to swallow food. The same 532 THE KEPKODU(TIVE FUNCTIONS. In the instances which have now passed under our review, the progeny is, at first, in direct com- munication with its parent, and does not receive the special protection of membranous envelopes, containing a store of nourishment for its subsequent growth. But in all the more perfect structures, diligent observer detected the young of the Rotifer vulgaris, per- fectly formed, moving in the interior of the parent animalcule, and excluded i-n a living state ; thus constituting them viviparous ani- mals, as the former were oviparous. Other species, again, imitate the hydra, in being what is termed gemmiparous, that is, producing gemmules (like the budding of a plant), which shoot forth from the side of the parent, and are soon provided with cilia, enabling them, when separated, to provide for their own subsistence ; although they are of a very diminutive size when thus cast otf. Some of the German naturalists go much farther than BufFon ; and profess their belief that organized beings may be produced out of an assemblage of perfectly inorganic materials ; such as the earths, when mixed with pure water, to which atmospheric air has access, under a certain temperature; or, in other words, by the con- current but exclusive agency of the four elements of the ancients. There is, however, little doubt that in all these cases, invisible germs were introduced through the medium of the air in which they are con- stantly floating: for when sufficient care is taken to exclude them, no organic beings make their appearance. This has been satisfac- torily shown by the negative result of an experiment recently made by F. Schultze, of Berhn, who placed various organic materials in a flask with water, the mixture being kept boiling for some time so as to destroy the vitaHty of all the germs that might have been present; and bent tubes being then adapted so as to allow no air to be ad- mitted, except such as had passed either through sulphuric acid on the one side, or a solution of potass on the other. The vessel was allowed to remain in this state for above two months, during the whole of which time, neither confervse, infusoria, nor any other organic being could be discovered in any part of the contents of the vessel : while in another vessel, containing the same materials, but to which the air had free access, they made their appearance in great numbers in the course of a few days. Edin. Phil. Journal, xxiii, 165. REPRODUCTION. 533 both of animals and vegetables, the germ is pro- vided with auxiliary coverings of this kind ; the whole together composing what is called a seed, or an ovum ; the former term being usually applied to vegetable, and the latter to animal productions ; and in both cases the organ which originally con- tained them is termed the ovary. The formation and evolution of vegetable seeds takes place, not indiscriminately at every point, as we have seen is the case with simple germs, but only in particular parts of the plant. The Filires, or fern tribes, may be taken as examples of this mode of reproduction ; the seeds being formed at the under surface of the leaves, apparently by a simple process of evolution ; and when detached and scattered on the ground being further developed into a plant similar to the parent. The Linnean class of Cryptogamia includes all the plants coming under this description. In Animals, likewise, it is only in the particular organs termed ovaries, that ova are formed ; and these organs are generally divided into compartments ; the whole being en- closed in a membranous covering, bearing a great resemblance to the seed-capsules of plants. The propagation of living beings by means of ova or seeds, is a process of a totally different class from their multiplication by mere slips or buds ; and the products of the former retain less of the peculiar characters of the individual from which they spring, than those of the latter. This is re- markably exemplified in the case of orchard trees, such as apples and pears ; for all the trees which derive their origin from shoots, or grafts, from the same individual, partake of the same properties, 534 THE REPRODUCTIVE FUNCTIONS. and produce a fruit of nearly the same flavour and qualities : whereas trees of the same species, which grow from seed, have the characters of distinct individuals, and losing all the peculiarities that may have distinguished the parent, revert to the original type of the species to which they belong. Thus from the seeds of the golden pippin, or non- pareil, arise trees bearing the common crab apple, which is the natural fruit of the species. By con- tinued graftings, after a long period, the vitality of the particular variety is gradually exhausted, and the grafts no longer bear the same fruit. This has already happened with regard to the two varieties of apples just mentioned. For these curious facts, and the theory which explains them, we are in- debted to the observation and sagacity of Mr. Andrew Knight.* The plans hitherto noticed are suited only to the simplest of vegetable or animal beings ; but for the continuance of the higher races in both kingdoms of nature there is required a more complex proce- dure. The latent germ, contained in the seed or ovum, is never developed beyond a certain point, unless it be vivified by the action of a peculiar fluid, which is the product of other organs. Thus there are established two distinct classes of struc- tures ; the office of the one being the formation of the seed or ovum, and that of the other the pro- duction of the vivifying fluid. The effect of this vivifying fluid upon the dormant germ is termed Fecundation; and the germ, when fecundated, re- ceives the name of Embryo . * See his various papers in the Philosophical Transactions, REPRODUCTION. 5:35 The modes in which the fecundation of the germ is accomplished are exceedingly various in differ- ent classes of. organized beings. In all P/muero- gamous plants, (so named in contradistinction to those which are Cryptogamous), the whole of the double apparatus required for reproduction is con- tained in the flower. One set of organs contains the rudiment of the seed, enclosed in various envelopes, of which the assemblage constitutes an ovary, and to which is appended a tube, {the pistil), terminated by a kind of spongiole, (the stigma). ,,The fecundating organs are the stamens, which are columns (or fllaments), placed generally near and parallel to the pistil, and terminated by a glandular organ, (the anther). This organ, when mature, contains, enclosed in a double envelope, a fine pow- der, {the pollen), consisting of very minute vesicles, filled with a viscous liquor, {the fovilla), in which are seen extremely small granules. Fecundation ^kes place by a portion of the pollen being re- ceived by the stigma, and conveyed through the tubular pistil to the seed, which it impregnates by imparting to it the fluid it contains. By far the greater number of plants composing the vegetable kingdom have these two sets of organs contained in the same flower ; or at least in flowers belonging to the same individual plant. In the animal kingdom this arrangement is also adopted ; but only in a comparatively small number of tribes. In these the ova, in their passage from the ovary, along a canal termed the oviduct, are fecundated by receiving a secretion from another set of organs in the same system, which is conveyed by a duct, opening into the oviduct in some part of 536 THE REPRODUCTIVE FUNCTIONS. its course. In a limited number of plants com- posing the class Dloecla, the individuals of the same species are distinguished by. their bearing flovt^ers which contain only one of the kinds of reproductive apparatus ; so that the stamens and the pistils are situated on separate plants ; and the impregnation of the ovaries in the latter, can be eft'ected only by the transference of the pollen from the former. A similar separation of offices is established among all the higher classes of the animal kingdom. In most Fishes, and in all Ba- trachian reptiles, the ova are impregnated after their expulsion from the body : in all other cases, their impregnation is internal ; and their subse- quent developement takes place in one or other of the four following ways. 1. The ovum, when defended by a firm enve- lope, which contains a store of nutriment, is termed an egg^ and is deposited in situations most favour- able for the developement of the embryo; and also for its future support when it emerges from the egg. Birds, as is well known, produce eggs which are encased in a calcareous shell, and hatch them by the warmth they communicate by sitting on them with unwearied constancy. All animals which thus lay eggs are termed oviparous. 2. There are a few tribes, such as the Viper and the Salamander, whose eggs are never laid, but are hatched in the interior of the parent ; so that they bring forth living offspring, although origi- nally contained in eggs. Such animals are said to be Ovo-viviparous. There are other tribes, again, which, according to circumstances, are either ovi- ORGANIC DEVELOPEMENT. 537 parous, or ovo-viviparoiis : this is the case with the Shark. 3. Viviparous animals are those in which no egg, properly so called, is completed ; but the ovum, after proceeding through the oviduct, sends out vessels, which form an attachment to the interior of a cavity in the body of the parent, whence it draws nourishment, and therefore has attained a consider- able size at the time of its birth. 4. Marsupial animals are those, which, like the Kangaroo, and the Opossum, are provided with abdominal pouches, into which the young, born at a very early stage of developement, are received and nourished with milk, secreted from glands con- tained within these pouches. As the young, both in this and in the last case, are nourished with milk prepared by similar glands, or Mammce, the whole class of viviparous and marsupial animals has re- ceived, from this characteristic circumstance, the name of 3Iammalia. Chapter II. ORGANIC developement. Although the study of organic structures in their finished state must tend to inspire the most sub- lime conceptions of the Great Creator of this vast series of beings, extending from the obscurest plant to the towering tenant of the forest, and from tlie lowest animalcule to the stately elephant and gigantic whale, there yet exists another depart- 538 THE REPRODUCTIVE FUNCTIONS. ment of the science of Nature, removed, indeed, from the gaze of ordinary observers, but present- ing to the philosophic inquirer subjects not less replete with interest, and not less calculated to exalt our ideas of the transcendent attributes of the Almighty. To a mind nurtured to reflection, these divine attributes, whether of power, of wis- dom, or of beneficence, are no where manifested with greater distinctness, or arrayed in greater glory, than in the formation of these various beings, and in the progressive architecture of their won- drous fabric. Our attention has already been directed, in a former part of these inquiries, to the successive changes which constitute the metamorphoses of M^inged insects,* and of Batrachian reptiles, phe- nomena which are too striking to have escaped the notice of the earliest naturalists : but the patient investigations of modern inquirers have led to dis- coveries still more curious, and have shown that all vertebrated animals, even those belonging to the higher classes, such as birds and mammalia, not excepting man himself, undergo, in the early stages of their developement, a series of changes fully as great and as remarkable as those which constitute the transformations of inferior animals. They have also rendered it extremely probable that the organs of the system, instead of existing simultaneously in the germ, arise in regulated succession, and are the results not of the mere ex- pansion of pre-existing rudiments, but of a real * The researches of Nordmann, on different species of Lerncea, have brought to light the most singular succession of forms during the progress of developement of the same individual animal. ORGANIC DEVELOPEMENT. 539 formation by the union of certain elements ; which elements are themselves successively formed by the gradual coalescence or juxta-position of their constituent materials. On contemplating the in- finitely lengthened chain of means and ends, and of causes and effects, which, during the construc- tion and assemblage of the numerous parts com- posing the animal machine, are in constant ope- ration, adapting them to their various purposes, and combining them into one efficient and har- monious system, it is impossible not to be deeply impressed with the extent and the profoundness of the views of Providence, which far exceed the re- motest boundaries of our vision, and surpass the utmost powers of human imagination.* The clearest evidence of enlarged and provident designs may be collected from observing the order in which the nascent organs are successively brought forwards, and added to the growing fabric; such order appearing, in all cases, to be that best calculated to secure the due performance of their appointed functions, and to promote the general objects of the system. The apparatus first per- fected is that which is immediately necessary for ii * " Si Ton applique," says Cuvier, when speaking of the anatomy of insects, " k chacune de ces esp^ces, par la pensee, ce qu'il seroit bien impossible qu'un homme entreprit de verifier en eftet pour toutes, une organisation a-peu-pr^s egale en complication i\ celle qui a ete decrite dans la chenille par Lyonet, et tout recemment dans le hanneton par M. Straus, et cependant plus ou moins diffe- rente dans chaque insecte, I'iraagination commencera k concevoir quelque chose de cette richesse effrayante, et de ces millions de millions de parties, et de parties de parties, toiijours correlatives, toujours en harmonie, qui constituent le grand ouvrage de la nature." (Histoire des Progres des Sciences Naturelles, iv. 145.) 540 THE REPRODUCTIVE FUNCTIONS. the exercise of the vital functions, and which is therefore required for the completion of all the other structures ; but provision is likewise made for the establishment of those parts which are to give mechanical support to each organic system in proportion as it is formed ; while the foundations are also preparing for endowments of a higher kind, by the early developement of the organs of the external senses, the functions of which so essentially minister to the future expansion of the intellectual faculties, embracing a wide range of perceptions and of active powers. Thus in the early, as well as in all the subsequent periods of life, the objects of nature vary as the respective necessities of the occasion change. At first, all the energies of vitality are directed to the raising of the fabric, and to the extension of those organs which are of greatest immediate utility ; but still having a prospective view to further and more important ends. For the accomplishment of this primary object unremitting exertions are made, commensurate with the magnitude of the design, and giving rise to a quick succession of varied forms, both with regard to the shape of each indi- vidual organ, and to the general aspect of the whole assemblage. In the phenomena of their early evolution, Plants and Animals present a striking contrast, corres- ponding to essential differences in the respective destinations of these two orders of beings. The primary object of vegetable structures appears to be the establishment of the functions of nutrition ; and we accordingly tind that whenever the seed begins to germinate, the first indication of deve- ORGANIC DEVELOPEMENT. 541 lopement is the appearance of the part called the piumula, which is a collection of feathery fibres, bursting from the enveloping capsule of the germ, and which, whatever may have been its original position, proceeds immediately to extend itself vertically ; while, at the same time, slender fila- ments, or radicles, shoot out below to form the roots. Thus early are means provided for the absorption and the aeration of the nutrient matter, which is to constitute the materials for the subsequent growth of the plant, and for the support and pro- tection of the organs by which these processes are to be carried on.* But animal vitality, being designed to minister to a higher order of endow- ments, is placed in subordination to a class of functions, of which there exists no trace in vege- tables, namely, those of the nervous system. By attentively watching the earliest dawn of organic formation, in the transparent gelatinous molecule, for example, which, with its three investing pelli- cles, constitutes the embryo of a bird, (for the eggs * Payen and Persoz have lately discovered that during the germi- nation of seeds, a vegetable principle hitherto unknown, and which they have termed Diastase, is formed. When dry, it has the appearance of a white tasteless powder, incapable of crystallization, insoluble in pure alcohol, but dissolving in water, and also in diluted alcohol : the solution gives no precipitate on the addition of subacetate of lead. It has no chemical action on gum, vegetable albumen, inuline, lignin, or, indeed, any of the vegetable proximate principles, with the sole exception of fecnla, on which it exerts a remarkable solvent power, and which by this combination is gradually converted into sugar. In effecting this chaiif^e, the operation of diastase is vastly more eneriretic than that of sidphuric acid, which, it has long been known, produces a similar conversion into sugar, not only of starch, but also of each of the four vegetable 542 THE REPRODUCTIVE FUNCTIONS. of that class of animals best admit of our following this interesting series of changes,) the first opaque object discoverable by the eye is a small dark line, called the primitive trace, formed on the surface of the outermost pellicle. Two ridges then arise, one on each side of this dark line ;* and by the union of their edges, they soon form a canal, containing a deposit of semi-fluid matter, which, on acquiring greater consistence and opacity, discloses two slen- der and delicate threads, placed side by side, and parallel to one another, but separated by a certain space. These are the rudiments of the spinal cord, or the central organ of nervous power, on the endowments of which the whole character of the being to be formed depends. We may next discern a number of parallel equidistant dots, arranged in two rows, one on the outer side of each of the filaments already noticed : these are the rudiments of the vertebrae, parts which will afterwards be wanted for giving protection to the spinal marrow, principles above mentioned, with regard to which diastase is totally inert. The quantity of saccharine product resulting from the action of diastase on starch, is sixty times greater than that from the action of sulphuric acid on the same substance in an equal time. Diastase does not exist in the seeds of oats, barley, wheat, &c. previously to the period of their germination ; it must, therefore, be formed during that process. This substance has also been met with surrounding the insertion of the buds of the potatoe, and of the Ailanthus glandulosa, and in the vicinity of masses of fecula, which are laid up as a store of nutriment to be used for the develope- ment of the bud, after being dissolved and modified by the action of the diastase. Ann. Sc. Nat. serie 2, Bot. x, 165. * The plicce primitivcB of Pander ; the laniince dorsales of Baer. See a paper on embryology by Dr. Allen Thomson, in the Edin. New Phil. Journal for 1830 and 1831. ORGANIC DEVELOPEMENT. 543 and which soon form, for this purpose, a series of rings embracing that organ.* The appearance of the elementary filaments of the spinal cord is soon followed by the develope- mentof its upper or anterior extremity, from which there arise three vesicles, each forming white tubercles; these are the foundations of the future brain. The tubercles are first arranged in pairs and in a longitudinal series, like those we have seen constituting the permanent form of the brain in the inferior fishes : but, in birds, they are soon folded together into a rounded mass ; while, in the mean time, the two filaments of the spinal cord have approached each other, and united into a single column, the form which they ever after retain. Even at this early period the rudiments of the organs of the higher senses, (first of the eye, and next of the labyrinth of the ear,) make their appearance ; but, on the other hand, those of the legs and wings do not show themselves until the brain has acquired greater solidity and developement. The nerves which are to connect these organs of sensation and of motion with the spinal cord and brain are formed afterwards, and are successively united to the nervous centres. Although the plan of the future edifice has thus been sketched, and its foundations laid in the homogeneous jelly, by the simpler efforts of the vital powers, the elevation of the vast superstructure demands the aid of other machinery, fitted to col- lect and distribute the requisite materials. Here, then, we might, perhaps, expect to meet with a * These rings have, by speculative physiologists, been supposed to be analogous to those which form the skeleton of the Annelida. 544 THE REPRODUCTIVE FUNCTIONS. repetition of those vegetative processes, liaving similar objects in view, and the adoption of ana- logous means for their accomplishment ; but so M'idely different in character is the whole organic economy of these two orders of beings, that we perceive no resemblance in the mechanism em- ployed for their formation. For the purposed of animal Ufe the nutrient juices must be brought into active circulation by means of vessels extensively pervading the system. Nature, then, hastens to prepare this important hydraulic apparatus, with- out which the work of construction could not proceed. What may be the movements of the transparent nutrient juices at the very earliest period must, of course, remain unknown to us, since we can only follow them by the eye after the nu- tritive substance they contain has become consoli- dated in the form of opaque globules. These glo- bules are at first seen to meander through the mass, unconfined by investing vessels ; presently, how- ever, a circular vessel is discovered, formed by the foldings of the membrane of the embryo, along which the fluids undulate backwards and forwards, without any constancy.* A delicate network of vessels is next formed in various parts of the area of the circle, which are seen successively to join by the formation of communicating branches, and ulti- mately to compose larger trunks, so as to establish a more general system of vascular organization. But increased power for carrying on this extended circulation will soon be wanted ; and for this pur- pose there must be provided a central organ of pro- * These phenomena are similar to those which were noticed as presented by the huvee of some insects and other inferior animals. ORGANIC DF.VELOPrMENT. 540 pulsion, or heart, the construction of which is now commenced, at a central point, by the folding- inwards of a lamina of the middle membrane, forming first a simple groove, but, after a time, converted, by the union of its outer edges, into a kind of sac, which is soon extended into a longi- tudinal tube,* The next object is to brintr this tube, or rudimental heart, into communication with the neighbouring vascular trunks ; and this is effected by their gradual elongation, till their cavi- ties meet, and are joined; one set of trunks (the future veins,) first uniting with the anterior end of the tube ; and then another set (the future arteries,) joining its other end. The addition of this central tube to the vessels previously formed completes the continuity of their course ; so that the uniform circulation of the blood is established in the direc- tion in which it is ever after to flow ; and we may now recognise this central organ as the heart, which, under the name of the punclnm salieihs, tes- tifies by its quick and regular pulsations that it has already begun to exercise its appropriate func- tion. It is long, however, before it acquires the form which it is permanently to retain ; for from being at first a mere lengthened tube, presenting three dilatations, which are the cavities of the future auricle, ventricle, and bulb of the aorta, it assumes in process of time a rounded shape, by the folding of its parts, the whole of which are coiled, as it were, into a knot; by which means the dif- ferent cavities acquire relative situations more * The discovery of this fact is due to Pander. See also the works of Rolando, Wolff', Prevost and Dumas, and Serres. VOL. II. N N »546 THE REPUODUCTIVE FUNCTIONS. nearly corresponding to their positions in the de- veloped and finished organ. The blood-vessels, in like manner, undergo a series of changes quite as considerable as those of the heart, and totally altering their arrangement and distribution. Serres maintains that the pri- mitive condition of all the organs, even those which are generally considered as single, is that of being double, or being formed in pairs ; one on the right, and another exactly similar to it on the left of the middle, or mesial plane, as if each were the re- flected image of the other.* Such is obviously the permanent condition of all the organs of sensation, and also of the apparatus for locomotion ; and it has just been shown that those portions of the nervous system which are situated in the mesial plane, such as the spinal cord and the brain, con- sisted originally of two separate sets of parts, which are brought together and conjoined into single organs. In like manner we have seen that the constituent laniinre of the heart are at first double, and afterwards form by their union a single cavity. The operation of the same law has been traced in the formation of those vascular trunks, situated in * A remarkable exemplification of this tendency to symmetric duplication of organs occurs in a very extraordinary parasitic animal, which usually attaches itself to the gills of the Cyprinus brama, and which has been lately examined by Nordmann, and named by him the Diplozoon paradoxum, from its having the semblance of two distinct animals of a lengthened shape', each bent at an obtuse angle, and joined together in the form of the letter X. The right and left halves of this cross are perfectly similar in their organiza- tion, having each a complete and independent system of vital organs; excepting that the two alimentary canals join at the centre of the cross to form a single cavity, or stomach. (Annales des Sciences Naturellos, xxx. 373.) ORGANIC DEVELOPEMENT. 547 the mesial plane, which are usually observed to be single, such as the aorta and the vena cava ; for each were originally formed by the coalescence of double vascular trunks, running parallel to each other, and at first separated by a considerable in- terval ; then approaching each other, adhering together, and quickly converted, by the oblitera- tion of the parts which are in contact, into single tubes, throughout a considerable portion of their length.* Nature, ever vigilant in her anticipations of the wants of the system, has accumulated round the embryo ample stores of nutritive matter, sufficient for maintaining the life of the chick, and for the building of its frame, while it continues in the egg, and is consequently unable to obtain supplies from without ; yet, with the same foresight of future cir- cumstances, she dela3^s not, longer than is necessary for the complete establishment of the circulation, to construct the apparatus for digestion, on which the animal is to rely for the means of support in after life. The alimentary canal, of which no trace exists at an earlier period, is constructed by the formation of two laminae, arising from folds of the innermost of the pellicles which invest the embryo ; that is, on the surface opposite to the one which has produced the spinal marrow. These laminae, which are originally separate, and apart from one another, are brought together, and by the junction * These facts were first observed by Serres (Annales des So. Nat. xxi. 8.), and their accuracy has been confirmed by the observations of Dr. Allen Thomson. In Reptiles this union of the two constituent trunks of the aorta is effected only at the posterior part, while the anterior portion remains permanently double. (See Fig. 357, vol. ii. p. 248.) 548 THE REPRODUCTIVE FUNCTIONS. or soldering of their opposite edges, formed into a tube,* whicii, from being at first uniform in dia- meter, afterwards expands into several dilated por- tions, corresponding with the cavities of the stomach, crop, gizzard, &c. into which they are to be con- verted, when the time shall come for their active employment. These new organs are, however, even in this their rudimental state, trained to the performance of their proper offices ; receiving into their cavities, through a tube temporarily provided for that purpose, the fluid of the yelk, and prepar- ing nourishment from it. In the mean time, early provision is made for the aeration of the fluids by an extensive but tem- porary system of vessels, spread over the membrane of the egg, and receiving the influence of atmos- pheric oxygen through the substance of the shell, which is sufficiently porous to transmit it ; and these vessels, being brought into communication with the circulatory system of the chick, convey to its blood this vivifying agent. As the lungs can- not come into use till after the bird is emancipated from its prison, and as it was sufficient that they should be in readiness at that epoch, these organs are among the last that are constructed ; and as the mechanism of respiration in this class of ani- mals does not require the play of the diaphragm, this muscular partition is only begTin, but not com- pleted, and there is no separation between the cavities of the thorax and the abdomen. The succession of organic metamorphoses is equally remarkable in the formation of the diver- * Wolff is the autlior of this discovery. ORGANIC DEVELUPEMLNT. o49 sified apparatus for aeration, which is required to be greatly modified, at different periods, in order to adapt it to different elements : of this we have already seen examples in those insects which, after being aquatic in their larva state, emerge from the water when they have acquired wings ; and also in the steps of transition from the tadpole to the frog. But similar, though less conspicuous changes occur in the higher vertebrated animals, during the early periods of their formation, corresponding to the differences in the modes of aeration employed at different stages of developement. In the primeval conditions this function is always analogous to that of aquatic animals, and requires for its performance only the simpler form of heart already described, consisting of a single set of cavities ; but the system being ultimately designed to exercise atmospheric respiration, requires to be gradually adapted to this altered condition ; and the heart of the Bird and the Quadruped must be separated into two compartments, corresponding to the double function it will have to perform. For this purpose a parti- tion wall is built in its cavity ; and this wall is begun around the interior circumference of the ventricle, and is gradually carried on towards the centre ; there being, for a time, an aperture of communication between the right and left cavities ; but this aperture is soon closed, and the ventricle is now effectually divided into two. Next the auricle, which at first was single, becomes double; not, however, by the growth of a partition, but by the folding in of its sides, along a middle line, as if it were encompassed by a cord, which was gradually tightened. In the mean while the partition, which 550 THE REPRODUCTIVE FUNCTIONS. had divided the ventricle, extends itself into the trunk of the main artery, which it divides into two channels ; and these afterwards become two sepa- rate vessels; that which issues from the left ven- tricle being the aorta; and the other, which pro- ceeds from the right ventricle, being the pulmo- nary artery ; and each of these vessels is now prepared to exercise its appropriate function in the double circulation which is soon to be established.* A mode of subdivision of blood vessels, very similar to that just described, takes place in those which are sent to the first set of organs provided for aeration, and which resemble branchige. These changes may be very distinctly followed in the Batrachia;-\ for we see, in those animals, the trunk of the aorta undergoing successive subdi- visions, by branches sent off from it and forming loops, which extend in length and are again sub- divided, in a manner not unlike the unravelling of the strands of a rope ; each subdivision, how- ever, being preceded by the formation of a double partition in the cavity of the tube; so that at length the whole forms an extensive ramified sys- tem of branchial arteries and veins. Still all these are merely temporary structures; for when the period of change approaches, and the branchi^ are to be superseded in their office, every vessel, one after another, becomes obliterated ; and there remain only the two original . aortae, which unite into a single trunk lower down, and from which * The principal authorities for the facts here stated are Baer and Rolando. See the paper of Dr. Tliomsou aheady quoted. t See the investigations of Rusconi, and of Baer, on this subject. ORGANIC DEVELOPEMENT. 55 J proceed the pulmonary arteries, conveying either the whole, or a portion of the blood, to the newly developed respiratory organs, the lungs. By a similar process of continued bifurcation, or the detachment of branches in the form of loops, new vessels are developed in other parts of the body; as has been particularly observed in the finny tail, and the external gills of the frog and the newt, parts which easily admit of microscopical examination * Progress is in the mean while making in the building of the skeleton ; the forms of the principal bones being modelled in a gelatinous substance, which is converted into cartilage; beginning at the surface, and gradually advancing towards the centre of each portion or element of the future bone ; and thus a temporary solid and elastic scaf- folding is raised, suited to the yielding texture of the nascent organs : lastly, the whole fabric is sur- rounded by an outer wall, the building of which is begun from the dorsal region, and conducted round the sides of the body, till the two portions come to meet in the middle abdominal line, where they are finally united into one general and con- tinuous integument. The eyes, which were hitherto unprotected, receive special means of defence, by the addition of eyelids, which are formed by a further extension and folding of these integuments ; and the greater part of the surface of the body gives rise to a growth of temporary down, which, as we have seen, is provided as a covering to the bird at the time it is ready to quit the shell. But * Such is the result of the concurring observations of SiJallanzaiii, Fontana, and Dollin^er. 552 THE REPRODUCTIVE FUNCTIONS. this hard shell, which had hitherto afforded it pro- tection, is now opposed to its emancipation; and the chick, in order to obtain its freedom, must, by main force, break through the walls of its prison ; its beak is, however, as yet too tender to apply the force requisite for that purpose. Here, again, we find Nature expressly interposing her assist- ance ; for she has caused a pointed horny pro- jection to grow at the end of the beak, for the special object of giving the chick the power of battering its shell, and making a practicable breach, through which it shall be able to creep out, and begin its new career of life. That this horn is provided only for this temporary use appears from the circumstance of its falling off spontaneously in the course of three or four days after it has been so employed.* But though the bird has now gained its liberty, it is still unable to provide for its own maintenance, and requires to be fed by its parent till it can use its wings, and has learned the art of obtaining food. The pigeon is furnished by nature with a secretion from the crop, with which it feeds its young. In the Mammalia the same object is pro- vided for still more expressly, by means of glands, whose office it is to prepare milk; a fluid which, from its chemical qualities, is admirably adapted to the powers of the digestive organs, when they first exercise their functions. The Cetacea have also mammary glands ; but as the structure of the mouth and throat of the young in that class does not appear adapted to the act of sucking, there has * Further details respecting this curious process are given by Mr. Yarrell, in the Zoological Journal, ii. 436. ORGANIC UEVELOPEMENT. 553 always been great difficulty in understanding how they obtain the nourishment so provided. A recent discovery of Geoffroy St. Hilaire appears to have resolved the mystery with respect to the Delphimis globiceps, for he found that the mammary glands of that animal contain each a large reservoir, in which milk is accumulated, and which the dolphin is capable, by the action of the surrounding mus- cles, of emptying at once into the mouth of its young, without requiring from the latter any effort of suction.* The rapid sketch which I have attempted to draw of the more remarkable steps of the early stages of organic developement in the higher ani- mals, taken in conjunction with the facts already adverted to in various parts of this Treatise, and particularly those relating to ossification, dentition, the formation of hair, of the quills of the porcupine, of the antlers of the stag, and of the feathers of birds, will suffice to show that they are regulated by laws which are definite, and preordained ac- cording to the most enlarged and profound views of the future circumstances and wants of the ani- mal. The double origin of all the parts of the frame, even those which appear as single organs, and the order of their formation, which, in each system, commences with the parts most remote from the centre, and proceeds inwards, or towards the mesial plane, are among the most singular and unexpected results of this train of inquiries.f We * The account of this discovery is contained in a memoir which was read at the " Institut." March 24, 1834. + The first of these two laws is termed by Serres, who has zea- lously prosecuted these investigations, " la lot de symmetrie ;" and 554 THE REPRODUCTIVE FUNCTIONS. cannot but be forcibly struck with the numerous forms of transition through which every organ has to pass before arriving at its ultimate and compa- ratively permanent condition : we cannot but won- der at the vast apparatus which is provided and put in action for effecting all these changes; nor can we overlook the instances of express contriv- ance in the formation of so many temporary struc- tures, which are set up, like the scaffold of an edifice, in order to afford the means of transporting the materials of the building in proportion as they are wanted ; nor refuse to recognise the evidence of provident design in the regular order in which the work proceeds, every organ growing at its appointed time, by the addition of fresh particles brought to it by the arteries, while others are car- ried away by the absorbents, and are gradually acquiring the form which is to qualify it for the performance of its proper office in this vast and complicated system of animal life. the second, " la loi de conjugaison." He maintains that they are strictly applicable to all the parts of the body having a tubnlar form, such as the trachea, the Eustachian tube, the canals and perfora- tions of bones, &c. See the preliminary discourse to his *' Anatomie comparee du cerveau," p. 25 ; and also his several memoirs in the " Annales des Sciences Naturelles," vols. xi. xii. xvi. and xxi. An excellent summary of the principal facts relating to the deve- lopement of the embryo is given by Mr. Herbert Mayo, in the third edition of his " Outlines of Human Physiology." 555 Chapter III. DECLINE OF THE SYSTEM. To follow minutely the various steps by which Nature conducts the individual to its state of ma- turity, would engage us in details incompatible with the limits of the present work. I shall only remark, in general, that during the period when the body is intended to increase in size, the powers of assimilation are exerted to prepare a greater abundance of nourishment, so that the average supply of materials rather exceeds the consump- tion ; but when the fabric has attained its pre- scribed dimensions, the total quantities furnished and exj^ended being nearly balanced, the vital powers are no longer exerted in extending the fabric, but are employed in consolidating and per- fecting it, and in qualifying the organs for the continued exercise of their respective functions, during a long succession of years. Yet, while every function is thus maintained in a state of healthy equilibrium, certain changes are in progress which, at the appointed season, will inevitably bring on the decline, and ultimate de- struction of the system.* The process of consoli- * It would appear from the researches of De Candolle, that the vegetable system is not, like the animal, subject to the destructive operation of internal causes ; for the agents which destroy vegetable life are always extraneous to its economy. Each individual tree is composed of an accumulation of the shoots of every successive year since the commencement of its growth; and although, from the continued deposition of lignin, and the consequent obliteration of 556 DECLINE OF THE SYSTEM. datioii, begun from the earliest period of develope- ment, is still advancing, and is producing in the fluids greater thickness, and a reduction of their total quantity ; and in the solids, a diminution in the proportion of gelatin, and the conversion of this element into albumen. Hence, all the textures acquire increasing solidity, the cellular substance becomes firmer and more condensed, and the solid structures more rigid and inelastic : hence the tendons and ligamentous fibres growing less flex- ible, the joints lose their suppleness, and the con- tractile power being also impaired, the muscles act more tardily as well as more feebly, and the limbs no longer retain the elastic spring of youth. The bones themselves grow harder and more brit- tle ; and the cartilages, the tendons, the serous membranes, and the coats of the blood-vessels, acquire incrustations of ossific matter, which inter- fere with their uses. Thus are all the progressive modifications of structure tending, slowly but in- evitably, to disqualify the organs for the due per- formance of their functions. Among the most important of the internal changes consequent on the progress of age are those which many of its cells and vessels, the vitality of the interior wood may be destroyed, and it then becomes liable to decay by the action of foreign agents, yet the exterior layers of the liber still vegetate with undiminished vigour; and, unless injured by causes extraneous to its own system, the life of the tree will continue to be sustained for an indefinite period. If, on the other hand, we were to regard each separate shoot as an individual organic body, and every layer as constituting a distinct generation of shoots, the older being covered and enclosed in succession by the younger, the great longevity of a tree would, on this hypothesis, indicate only the permanence of the species, not the indefinitely protracted duration of the individual plant. DECLINE OF THE SYSTEM. 5r>7 take place in the vascular system. A large pro- portion of the numerous arteries, which were in full activity during the building of the fabric, being now no longer wanted, are thrown, as it were, out of employment ; they, in consequence, contract, and becoming impervious, gradually disappear. The parts of the body, no longer yielding to the power applied to extend them, oppose a gradually increasing resistance to the propelling force of the heart ; while, at the same time, this force, in com- mon with all the others, is slowly diminishing. Thus do the vital powers become less equal to the demands made upon them ; the waste of the body exceeds the supply, and a diminution of energy becomes apparent in every function. Such are the insensible gradations by which, while gliding down the stream of time, we lapse into old age, which insidiously steals on us before we are aware of its approach. But the same provi- dent power which presided at our birth, which superintended the growth of all the organs, which infused animation into each as they arose, and which conducted the system unimpaired to its ma- turity, is still exerted in adjusting the conditions under which it is placed in its season of decline. New arrangements are made, new energies are called forth, and new resources are employed, to accommodate it to its altered circumstances, to prop the tottering fabric, and retard the progress of its decay. In proportion as the supply of nutri- tive materials has become less abundant, a more strict economy is practised with regard to their disposal ; the substance of the body is husbanded with greater care ; the absorbent vessels are em- 558 DECLINE OF THE SYSTEM. ployed to remove such parts as are no longer use- ful ; and when all these adjustments have been made, the functions still go on for a considerable length of time without material alteration. The period prescribed for its duration being at length completed, and the ends of its existence accomplished, the fabric can no longer be sus- tained, and preparation must be made for its inevi- table fall. In order to form a correct judgment of the real intentions of nature with regard to this last stage of life, its phenomena must be observed in cases where the system has been wholly entrusted to the operation of her laws. When death is the simple consequence of age, we find that the ex- tinction of the powers of life observes an order the reverse of that which was followed in their evolution. The sensorial functions, which were the last perfected, are the first which decay ; and their decline is found to commence with those mental faculties more immediately dependent on the physical conditions of the sensorium, and more especially with the memory, which is often much impaired while the judgment remains in full vigour. The next faculties which usually suffer from the effects of age are the external senses ; and the failure of sight and of hearing still far- ther contributes to the decline of the intellectual powers, by withdrawing many of the occasions for their exercise. The actual demolition of the fabric commences whenever there is a conside- rable failure in the functions of assimilation ; but the more immediate cause of the rapid extinction of life is usually the impediment which the loss of the sensorial power, necessary for maintaining the DECLINE OF THE SYSTEM. 559 movements of the chest, creates to respiration. The heart, whose pulsations gave the first indications of life in the embryo, generally retains its vitality longer than any other organ ; but its powers being dependent on the constant oxidation of the blood in the lungs, cannot survive the interruption of this function ; and on the heart ceasing to throb, death may then be considered as complete in every part of the system. It is an important consideration, with reference to final causes, that generally long before the com- mencement of this " Last scene of all. That ends this strange eventful history," the power of feeling has wholly ceased, and the physical struggle is carried on by the vital powers alone, in the absence of all consciousness of the sentient being, whose death may be said to pre- cede, for some time, that of the body. In this, as well as in the gradual decline of the sensorial faculties, and the consequent diminution both of mental and of physical sensibility in advanced age, we cannot fail to recognise the wise ordinances of a superintending and beneficent providence, kindly smoothing the path along which we descend the vale of life, spreading a narcotic mantle over the bed of death, and giving to the last moments of departing sensation the tranquillity of approaching sleep. 5G0 Chapter IV. UNITY OF DESIGN. The inquiries on Animal and Vegetable Physiology in which we have been engaged, lead to the gene- ral conclusion that unity of design and identity of operation pervade the whole of nature ; and they clearly point to one Great and only Cause of all things, arrayed in the attributes of infinite power, wisdom, and benevoleiice, whose mighty works extend throughout the boundless regions of space, and whose comprehensive plans embrace eternity. In examining the manifold structures and diver- sified phenomena of living beings, we cannot but perceive that they are extensively, and perhaps universally connected by certain laws of Analogy ; a principle, the recognition of which has given us enlarged views of a multitude of important facts, which would otherwise have remained isolated and unintelligible. Hence naturalists, in arranging the objects of their study, according to their similarities and analogies, into classes, orders and genera, have but followed the footsteps of Nature herself, who in all her operations combines the apparently opposite principles of general resemblance, and of specific variety ; so that the races which she has united in the same group, though possessed of features indi- vidually different, may easily be recognised by their family likeness, as the offspring of a common parent. " Facies non omnibus una ; Nee diversa tamen ; qualem decet esse sororum." DMTY OF DESIGN. 561 We have seen that in each of the two great divi- sions, or kingdoms of organic nature, the same general objects are aimed at, and the same general plans are devised for their accomplishment; and also that in the execution of those plans similar means and agencies are employed. In each divi- sion there prevails a remarkable uniformity in the composition and properties of their elementary tex- tures, in the nature of their vital powers, in the arrangement of their organs, and in the laws of their production and developement. The same principle of analogy may be traced, amidst endless moditications of detail, in all the subordinate groups into which each kingdom admits of being sub- divided, in respect both to the organization and functions of the objects comprehended in each assemblage; whether we examine the wonders of their mechanical fabric, or study the series of pro- cesses by which nutrition, sensation, voluntary motion, and reproduction are effected. To specify all the examples which might be adduced in con- firmation of this obvious truth is here unnecessary; for it would be only to repeat the numerous facts already noticed in every chapter of this treatise, relative to each natural group of living beings ; and it was, indeed, chiefly by the aid of such analogies, that we were enabled to connect and generalize those facts. We have seen that, in constructing each of the divisions so established, nature appears to have kept in view a certain definite type, or ideal standard, to which, amidst innumerable mo- difications, rendered necessary by the varying cir- cumstances and different destinations of each species, she always shows a decided tendency to VOL. II. o o 562 UNITY OF DESIGN. conform. It would almost seem as if, in laying the foundations of each organized fabric, she had com- menced by taking an exact copy of this primitive model; and, in building the superstructure, had allowed herself to depart from the original plan only for the purpose of accommodation to certain specific and ulterior objects, conformably with the destination of that particular race of created beings. Such, indeed, is the hypothetical principle, which, under the title of unitij of composition, has been adopted, and zealously pursued in all its conse- quences, by many naturalists of the highest emi- nence on the continent. As the facts on which this hypothesis is supported, and the views which it unfolds, are highly deserving of attention, I shall here briefly state them ; but in so doing I shall beg to premise the caution that these views should for the present be regarded as quite hypothetical, and far from possessing the certainty of philosophical generalizations. The hypothesis in question is countenanced, in the first place, by the supposed constancy with which, in all the animals belonging to the same natural group, we meet with the same constituent elements of structure, in each respective system of organs ; notwithstanding the utmost diversity which may exist in the forms of the organs, and in the uses to which they are applied. This principle has been most strikingly exemplified in the osteology of vertebrated animals : but its truth is also inferred from the examination of the mechanical fabric of Insects, Crustacea, and Arachnida ; and it appears to extend also to the structures subservient to other functions, and particularly those of the nervous UNITY OF DESIGN. 563 system. Thus nature has provided for the loco- iijotion of the serpent, not by the creation of new structures, foreign to the type of the vertebrata, but by employing the ribs in this new office ; and in giving wings to a lizard, she has extended these same bones to serve as supports to the superadded parts. In arming the elephant with tusks, she has merely caused two of the teeth in the upper jaw to be developed into these formidable weapons ; and in providing it with an instrument of prehension has only resorted to a greater elongation of the snout. The law of Gradation, in conformity to which all the living, together with the extinct races, of organic nature, arrange themselves, more or less, into cer- tain regular series, is one of the consequences which have been deduced from the hypothesis we are considering. Every fresh copy taken of the original type is supposed to receive some addi- tional extension of its faculties and endowments by the graduated developement of elements, which existed in a latent form in the primeval germ, and which are evolved, in succession, as nature ad- vances in her course. Thus we find that each new form which arises, in following the ascending scale of creation, retains a strong affinity to that which had preceded it, and also tends to impress its own features on those which immediately succeed ; and thus their specific differences result merely from the difll'erent extent and direction given to these organic developements ; those of inferior races pro- ceeding to a certain point only, and there stopping ; while in beings of a higher rank they advance farther, and lead to all the observed diversities of conformation and endowments. /ifU UNITY OF DESIGN. It is remarked, in further corroboration of these views, that the animals which occupy the highest stations in each series possess, at the commence- ment of their existence, forms exhibiting a marked resemblance to those presented in the permanent condition of the lowest animals in the same series ; and that, during the progress of their developement, they assume, in succession, the characters of each tribe, corresponding to their consecutive order in the ascending chain ; so that the peculiarities which distinguish the higher animal, on its attaining its ultimate and permanent form, are those which it had received in its last stage of embryonic evolu- tion. Another consequence of this hypothesis is that we may expect occasionally to meet, in inferior animals, with rudimental organs, which from their imperfect developement may be of little or no use to the individual, but which become available to some superior species, in which they are suffi- ciently perfected. The following are among the most remarkable facts in illustration of these pro- positions. In the series of Articulated Animals, of which the Annelida constitute the lowest, and winged Insects the highest terms, we find that the larvae of the latter are often scarcely distinguishable, either in outward form, or in internal organization, from Vermes of the lowest orders ; both being equally destitute of, or but imperfectly provided with ex- ternal instruments of locomotion ; both having a distinct vascular circulation, and multiple organs of digestion : and the central filaments of the nervous system in both being studded with numerous pairs of equidistant ganglia. In the worm all these fea- tures remain as permanent characters of the order: UNITY OF DKSIGN. .565 in the insect they are subsequently moditied and altered during its progressive metamorphoses. The embryo of a crab resembles in appearance the permanent forms of the 3I^riapoda, and of the lower animals of its own class, but acquires, in the progress of its growth, new parts; while those al- ready evolved become more and more concen- trated ; passing, in their progress, through all the forms of transition which characterize the inter- mediate tribes of Crustacea; till the animal attains its last state, and then exhibits the most developed condition of that particular type.* However different the conformations of the Fish, the Reptile, the Bird, and the Warm blooded Quadruped, may be at the period of their maturity, they are scarcely distinguishable from one another in their embryonic state ; and their early develope- ment proceeds for some time in the same manner. They all possess at first the characters of aquatic animals : and the Frog even retains this form for a considerable period after it has left the egg. The young tadpole is in truth a fish, whether we regard 'the form and actions of its instruments of pro- 'gressive motion, the arrangement of its organs of ^circulation and of respiration, or the condition of the central organs of its nervous system. We have seen by what gradual and curious transitions all these aquatic characters are changed for those of a * This GUI ions analogy is particularly observable in tlie successive forms assumed by the nervous system, which exhibits a gradual ^.passage from that of the Talitrus, to its ultimate greatest concen- tration in the Maia. (See Figures 439 and 441, p. 488 and 490.) Mln^ Edwards has lately traced a similar progression of develope- ment in the organs of locomotion of the Crustacea. (Annates des Sciences Naturellcs ; xxx, 354.) 5(J6 UNITY OF DLSIGN. terrestrial quadruped, furnished with limbs for moving on the ground, and with lungs for breathing- atmospheric air ; and how the plan of circulation is altered from branchial to pulmonary, in propor- tion as the gills wither and the lungs are developed. If, while this change is going on, and while both sets of organs are together executing the function of aeration, all further developement were pre- vented, we should have an amphibious animal, fitted for maintaining life both in air and in water. It is curious that this precise condition is the per- manent state of the Siren and the Proteus ; animals which thus exemplify one of the forms of transition in the metamorphoses of the Frog. In the rudimental form of the feet of serpents, which are so imperfectly developed as to be con- cealed underneath the skin, and to be useless as organs of progressive motion, we have an example of the first stage of that process, which, when car- ried farther in the higher animals, gives rise to the limbs of quadrupeds, and which it would almost seem as if nature had instituted with a prospective view to these more improved constructions. An- other, and a still more remarkable instance of the same kind occurs in the rudimental teeth of the young of the Whale, which are concealed within the lower jaw, and which are afterwards removed, to give place to the curious filtering apparatus, which occupies the roof of the mouth, and which nature has substituted for that of teeth ; as if new objects, superseding those at first pursued, had arisen in the progress of developement. Birds, though destined to a very different sphere of action from either fishes or reptiles, are yet ob- UMTV UF UKSIGN. ,3(J7 served to pass, in tlie embryonic stage of tlieir existence, through forms of transition, which suc- cessively resemble these inferior classes. The brain presents, in its earliest formation, a series of tubercles, placed longitudinally, like those of fishes, and only assuming its proper character at a later period. The respiratory organs are at first branchiae, placed, like those of fishes, in the neck, where there are also found branchial apertures similar to those of the lamprey and tlie shark ; and the heart and great vessels are constructed like those of the tadpole, with reference to a branchial circulation. In their conversion to the purposes of aerial respi- ration, they undergo a series of changes precisely analogous to those of the tadpole. Mammalia, during the early periods of their developement, are subjected to all the transfor- mations which have been now described ; com- mencing with an organization corresponding to that of the aquatic tribes; exhibiting not only branchiae, supported on branchial arches, but also branchial apertures in the neck; and thence pass- ing quickly to the conditions of structure adapted to a terrestrial existence. The developement of various parts of the system, more especially of the brain, the ear, the mouth, and the extremities, is carried still farther than in birds. IN or is the human embryo exempt from the same metamor- phoses; possessing at one period branchije and branchial apertures similar to those of the carti- lap:inous fishes,* a heart with a single set of cavities, ■ These facts are given on the authorities of Rathke, Baer, Huschke, Breschet, &c. Ann. des Sc. Naturelles, xv. 266. See also the paper of Dr. A. Thomson, already quoted. 568 UiNlTY OF DESIGN. and a brain consisting of a longitudinal series of tubercles; next loosing its branchiae, and acquiring lungs, while the circulation is yet single, and thus imitating the condition of the reptile ; then ac- quiring a double circulation, but an incomplete diaphragm, like birds ; afterwards, appearing like a quadruped, with a caudal prolongation of the sacrum, and an intermaxillary bone ; and lastly, changing its structure to one adapted to the erect position, accompanied by a great expansion of the cerebral hemispheres, which extend backwards so as completely to cover the cerebellum. Thus does the whole fabric arrive, by a gradual process of mutation, at an extent of elaboration and refinement, which has been justly regarded as constituting a climax of organic developement, unattainable by any other race of terrestrial beings.'^ It must, I think, be admitted that the analogies, on which the hypothesis in question is founded, are numerous and striking ; but great care should be taken not to carry it farther than the just * A popular opinion has long prevailed, even among the well informed, that mis-shapen or monstrous productions, or lusus naturce, as they were termed, exhibit but the freaks of nature, who was believed, on these occasions, capriciously to abandon her usual course, and to amuse herself in the production of grotesque beings, without any special object. But it is now found that all defective formations of this kind are occasioned by the imperfect develope- ment of some parts of the embryo, while the natural process is carried on in the rest of the system ; and thus it happens that a reseml)]ance may often be traced, in these malformations, with the type or the permanent condition of some inferior animal. Hence all these apparent anomalies are, in reality, in perfect harmony with the established laws of organic developement, and afford, indeed, striking confirmations of the truth of the theory here explained. UNITY OF DESIGN. 569 interpretation of the facts themselves may warrant. It should be borne in mind that these facts are few, compared with the entire history of animal deve- lopement ; and that the resemblances which have been so ingeniously traced, are partial only, and fall very short of that universality, which alone constitutes the solid basis of a strictly philosophical theory. Whatever may be the apparent similarity between one animal and another, during- different periods of their respective developements, there still exist specific differences, establishing between them an impassable barrier of separation, and effectually preventing any conversion of one species into another, however nearly the two may be mutually allied. The essential characters of each species, amidst occasional varieties, remain ever constant and immutable. Although gradations, to a greater or less extent, may be traced among the races both of plants and animals, yet in no case is the series strictly continuous ; each step, however short, being in reality an abrupt transition from one type of conformation to another. In many instances the interval is considerable ; as for ex- ample in the passage from tlie invertebrate to the vertebrated classes ; and indeed in every instance where great changes in tlie nature and arrangement of thg functions take place.* It is in vain to allege that the original continuity of the series is indicated by a few species presenting, in some respects, intermediate characters, sucli as the Omilhorliyn- chus, between birds and mammalia, and the Cctacea, * See a paper on this subject, by Cnvier, in the Ann. dcs Sciences Naturelles, xx. 241 : and also his remarks contained in tlie second edition of his Le9ons d'Anat. Comparce. i. bl, lOU. IbO, &c. 570 UNITY OF DESIGN. between fishes and warm-blooded quadrupeds ; for these are but detached links of a broken chain, tending, indeed, to prove the unity of the designs of Nature, but showing also the specific character of each of her creative efforts. The pursuit of remote and often fanciful analogies has, by many of the continental physiologists, been carried to an unwarrantable and extravagant length ; for the scope which is given to the imagination in these seductive speculations, by leading us far away from the path of philosophical induction, tends rather to obstruct tlian to advance the progress of real knowledge. By confining our inquiries to more legitimate objects, we shall avoid the delusion into which one of the disciples of this transcen- dental school appears to have fallen, when he announces, with exultation, that the simple laws he has discovered have now explained the uni- verse;* nor shall we be disposed to lend a patient ear to the more presumptuous reveries of another system-builder, who, by assuming that there exists in organized matter an inherent tendency to per- fectibility, fancies that he can supersede the opera- tions of Divine agency. t Very diff'erent was the humble spirit of the great Newton, who, struck with the immensity of nature, * '* L'univers est explique, et nous le voyons ; c'est un petit nombre de principes generaux et feconds qui nous en ont donne la clef." Serres, Ann. des Sc. Nat, xi. 50. t Allusion is here made to the celebrated theory of Lamarck, as exposed in his " Philosophic Zoologique." He conceives that there was originally no distinction of species, but that each race has, in the course of ages, been derived from some other, less perfect than itself, bv a spontaneous effort at improvement; and he supposes that infusorial animalcules, spontaneously formed out of organic molecules, TNITY OF DESICiN. 571 compared our knowledge of her operations, into which he had himself penetrated so deeply, to that of a child gathering pebbles on the sea shore. Compared, indeed, with the magnitude of the uni- verse, how narrow is the field of our perceptions, and how far distant from any approximation to a knowledge of the essence of matter, of the source of its powers, or even of the ultimate configurations of its parts ! How remote from all human cogni- zance are the intimate properties of those impon- derable agents, Light, Heat, and Electricity, which pervade space, and exercise so potent a control over all the bodies in nature ! Doubtless there exist around us, on every side, influences of a still more subtle kind, which "eye hath not seen, nor ear heard," neither can it enter into the heart or imagination of man to conceive. How scanty is our knowledge of the mind ; how incomprehensible is its connexion with the body ; how mysterious are its secret springs, and inmost workings ! What ineffable wonders would burst upon us, were we admitted to the perception of the spiritual world, now encompassed by clouds impervious to mortal vision ! The Great Author of our being, who, while he has been pleased to confer on us the gift of reason, has-prescribed certain limits to its powers, permits gave birth, by successive transformations, to all otlier animals now existing on the globe. He believes that tribes, originally aqnatic, acquired by their own efForts, prompted by their desire to walk, both feet and legs, fitting them for progression on the ground ; and that these members, by the long continued operation of the wish to fly, were transformed into wings, adapted to gratify that desire. If this be philosophy, it is such as might have emanated from the college of Laputa. 572 UNITY OF DESIGN. lis to acquire, by its exercise, a knowledge of some of the wondrous works of his creation, to interpret the characters of wisdom and of goodness with which they are impressed, and to join our voice to the general chorus which proclaims " His Might, Majesty, and Dominion." From the same gracious hand we also derive that unquenchable thirst for knowledge, which this fleeting life must ever leave unsatisfied ; those endowments of the moral sense, with which the present constitution of the world so ill accords ; and that innate desire of perfection which our present frail condition is so inadequate to fulfil. But it is not given to man to penetrate into the counsels, or fathom the designs of Omni- potence ; for in directing his views into futurity, the feeble light of his reason is scattered and lost in the vast abyss. Although we plainly discern intention in every part of the creation, the grand object of the whole is placed far above the scope of our comprehension. It is impossible, however, to conceive that this enormous expenditure of power, this vast accumulation of contrivances and of machinery, and this profusion of existence re- sulting from them, can thus, from age to age, be prodigally lavished, without some ulterior end. Is Man, the favoured creaiure of nature's bounty, " the paragon of animals," whose spirit holds communion with celestial powers, formed but to perish with the wreck of his bodily frame ? Are generations after generations of his race doomed to follow in endless succession, rolling darkly down the stream of time, and leaving no track in its pathless ocean ? Are the operations of Ahnighty power to end with the present scene? May we UNllY OF DESIGN. 573 not discern, in our spiritual constitution, tlie traces of higher powers, to which those we now possess are but preparatory; some embryo faculties which raise us above this earthly habitation ? Have we not in the imagination, a power but little in har- mony with the fetters of our bodily organs ; and bringing within our view purer conditions of being, exempt from the illusions of our senses and the infirmities of our nature, our elevation to which will eventually prove that all these unsated desires of knowledge, and all these ardent aspirations after moral good, were not implanted in us in vain ? Happily there has been vouchsafed to us, from a higher source, a pure and heavenly light to guide our faltering steps, and animate our fainting spirit, in this dark and dreary search ; revealing those truths which it imports us most of all to know ; giving to morality higher sanctions ; elevating our hopes and our affections to nobler objects than belong to earth, and inspiring more exalted themes of thanksgiving and of praise. INDEX. Abdomen ot" insects, i. '290 Abdomen of spiders, i. 254 Aberration, chromatic, ii. 4'24 Aberration, parallactic, ii. 421 Aberration, spherical, ii. 4 1 0, 420 Absorption, ii. 1 1 Absorption, animal, ii. 316 Absorption, lacteal, ii. 200 Absorption, vegetable, ii. 16 Acalepha, i. 175, 195, 266, 360 ; ii. 104, 240, 266, 272 Acetabulum, i. 252 Achatina, i. 216, 217 Achias, ii. 436 Achromatic combinations, ii. 424 Acid secretions, ii. 39 Acoustic principles, ii. 370 Acrida, ii. 187 Acridium, ii. 298 Acromion, i. 359 Actinia, i. 165, 179; ii. 88, 343, 426, 485, 531 Adanson, i. 226 Adductor muscle, i. 196 Adipose substance, i. 110 Aeration of sap, ii. 25, 29 Aeration, animal, ii. 29, 549 iEshna, i. 314 Afferent nerves, ii. 322, 478 Affinities, organic, i. 7 Affinity of structure, i. 323 Agaon, ii. 344 Agassiz, i. 385 Agastric medus-cS, ii. 64, 81 Age, progress of, ii. 558 Age of trees, i. 74 Agouti, i. 441 Agrion, i. 314; ii. 220 Ailanthus, ii. 542 Air-bladder, i. 382 ; ii. 279, 311 Air-cells of birds, ii. 296 Air, rarefaction of, in birds, i. 491 Air-tubes of plants, i. 64 Albumen, i. 91 ; ii. 6 Alburnum, i. 74; ii. 33 AlgsD, ii. 16 Alimentary canal, formation of, ii. 547 Alitrunk, i. 289, 309 Alligator, i. 409 ; ii. 365 Alveus utriculosus, ii. 384 Amceba, i. 169 Amble, i. 438 Ambulacra, i. 183 Amici, i. 67 ; ii. 45 Amphibia, i. 388,431 Amphinoma, ii. 231 Amphisbasna, i. 399 Amphitrite, i. 251 Ampulla, ii. 383 Anabas, ii. 277 Analogy, law of, i. 41 ; ii. 560 Analysis of bone, i. 329 Analysis, vegetable, ii. 36 Anarrhichas, ii. 115, 179 Anchylosis, i. 340 Ancillaria, i. 241 Anemone, sea. See Actinia Angler, i. 376; ii. 349, 392 Anguis, i. 398, 399 Animal functions, i. 32; ii. 318 Animal magnetism, ii. 337 Animal organization, i. 82 Annelida, i. 242; ii. 204, 210, 212,229,267,269, 343,428, 487 !f 570 INDEX. Annular vessels, i. 04 Annulose animals, i. 242 Anodon, i. 208 Anomata, ii. 432 Ant, i. 320; ii. 346,432,434 Ant-eater, i. 463; ii. 120 Antelope, i. 443, 455; ii. 131, 359 Antennge, i. 258 ; ii. 344 Antennulge, ii. 110 Anther, ii. 535 Anthias, ii. 277 Anthopliora, i. 314 Antipathes, i. 150 Antler of deer, i. 450 Antrum maxillare, ii. 358 Aorta, ii. 97, 209 Apatura, i. 319 Aphodius, ii. 344 Aphrodite, ii. 91, 112, 270 Aplysia, ii. 113, 151, 195, 242, 4*95 Apodes, i. 377 Aptei-ous insects, i. 265 Aqua labyrinthi, ii. 382 Aquatic animals, i. 134 Aquatic birds, i. 522 ; ii. 42 Aquatic insects, i. 299 Aquatic larvae, i. 277 Aquatic plants, ii. 42 Aquatic respiration, ii. 266 Aqueous humor, ii. 413 Arachnida, i. 252 ; ii. 211, 229, 285, 348, 433 Aranea. See Spider Arbor vitce, ii. 502 Architecture, organic, i. 504 Area, ii. 240 Arenicola, i. 248 ; ii. 234, 267 Argonaut, i. 237 Argvroneta, i. 256 Aristotle, i. 234; ii. 502 Aristotle's lantern, ii. 106 Arm, human, i. 480 Armadillo, ii. 157, 343 Arteries, i. 182 ; ii. 97 Articulata, i. 240 Ascalaphus, ii. 344 Ascaris, ii. 91, 102, 486 , Ascending sap, ii, 31 Ascidia, i. 137; ii. 114, 241, 269, 272 Ashtoji, i. 320 Ass, i. 456 Assimilation, i. 34; ii. 9. Astacus, ii. 390, 439 Astata, ii. 435 Asterias, i. 181 ; ii. 90, 184, 214, 216, 269, 343, 428 Ateles, i. 472; ii. 351, 355 Atriplex, ii. 42 Auditory nerve, ii. 370 Audouin, i. 260; ii. 287, 289, ii. 223, 237, 286,487 Audubon, ii. 363 Aulostomata, i. 385 Auricle, ii. 238 Auricula, i. 224 Autenrieth, ii. 387 Avicula, i. 210 Axillae of plants, i. 78 ; ii. 528 Axis of zoophyte, i. 135 Axolotl, ii. 292 Babirousa, ii. 126 Baccillaria, i. 169 Eacculites, i. 239 Bachman, ii 365 Bacon, ii. 252 Badger, i. 460 Baer, ii. 258,542,550, 567 Baker, ii. 427 Balsena. See Whale Balance of affinities, ii. 6 Balistes, i. 384 Banks, i. 403 Barbels of fish, ii. 349 Bark, i. 74 Barnacle, i. 230; ii.268 Barry, ii. 315 Barthez, i. 513 Bat, i. 94, 486, 490 ; ii. 395, 450 Batrachia,i. 388; ii. 115,550 Batrachospermum, ii. 42 Bauer, i. 53 Beard of oyster, ii. 272 Bear, i. 470; ii. 130 INDEX. 577 Beaver, i. 463 ; ii. 132, 169 Bee, i. 314; ii. 103, 3-16,435 Becquerel, ii. 46 Belchier, i. 342 Bell, (Sir C.) ii. 397, 475 Bell, (Thomas) i. 227, 255, 427 ; ii. 365 Bellini, ii. 352 Berheris, i. 1 19 Berenice, ii. 64 Berkeley, ii. 462, 463 Beroe, i. 177; ii. 81, 210, 485 Berzelius, ii. 16 Bibio, ii. 435 Bicuspid teeth, ii. 129 Bile, ii. 184, 314 Binney, i. 174 Biot, ii. 279, 373 Bipes canaliculatus, i. 407 Birds, i. 489; ii. 178, 197,295, 353,361, 394,449,498,501, 566 Bird-weed, ii. 43 Bivalve shells, i. 194 Blade bone, i. 359 Blennogenous glands, i. 99 Block, ii. 277 Blood, ii. 97, 202, 300 Blood-corpuscles, ii. 202, 315 Blood-vessels, distribution of, ii. 255 Blood-vessels, formation of, ii. 546 Blue-bottle fly, i. 297 Blumenbach, ii. 381 Boa, i. 399, 401 Boap, i. 47 ; ii. 126, 128 Bodo, i. 169 ; ii. 427 Bombyx,i. 272, 279,317, 318; ii. 344, 434 Bone, i. 96, 326 Bo7inet, ii. 14, 70, 427 Borelli, i. 513, 519 Bostock, ii. 300 Botany, i. 12 Bound of deer, i. 439 Boussignault, ii. 32 Bowerbank, ii. 220 Bowman, i. 174; ii. 316 VOL. II. Buyle, ii. 13 :;;mui / Brachial artery, ii. 256 Brachiopotla, ii. 240 Bractese, i. 82 Bradypus, i. 426, 427 ; ii. 258 Brain, i. 30 ; ii. 329, 479 Brain, formation of, ii. 543 Brain, functions of, ii. 460, 504 Branchiae, i. 391 ; ii. 270 Brancliial circulation, ii. 209 Branchio-cardiac veins, ii. 212 Brassica, ii. 42, 48 Braula, ii. 432 Breschet, i. 99 ; ii. 259, 339, 382, 567 Brewster, 1.50, 208; ii. 42 1,443 Bristles, i. 103 Brocken, spectre of, ii. 473 Brodcrip, ii. 197 Bronchia, ii. 287 Bronyniart, ii. 46 Broom-rape, ii. 49 Broussonet, ii. 527 Broivn, (R.) i. 57 ; ii. 45 Bruyuiere, i. 222 Bryophyllum, ii. 526 Buccinum, i. 205, 216 ; ii. 113, 272 Buckland, ii. 182 Buds, ii. 528 Buffon, i. 167; ii. 471, 531 Bulb of feather, i. 508 Bulb of hair, i. 106 Bulbulus glandulosus, ii. 165 Bulbus arteriosus, ii. 245 Buhmus, i. 223, 228 Bulla, ii. 150 Buprestidae, i. 285 Burmeister, i. 307 Burr of antler, i. 451 Burrowing of mole, i. 464 Burying beetle, ii. 369 Butterfly, i. 272 Cabbage, ii. 42 Cachalot, i. 328, 423, 427, 429 ; ii. 127 Caeca, ii. 91 Caecilia, ii. 444 P P o78 INDEX. Calaraary, i. 234 Calcigerous tubes, ii. 1 35 Calistegia, ii. 43 Callionymus, ii. 449 Calosoma, i. 286, 290 Cambium, ii. 35 Camel, i. 94; ii. 157, 176 Camelopard, i. 426, 441, 453; ii. 121 Camera obscura, ii. 404, 409 Camerated shells, i. 237 Campanulavia, ii. 216, 524 Cavijyer, ii. 397, 503 Canada rat, ii. 159 Canals, semicircular, ii. 381 Canine teeth, ii. 128 Cantharides as food, ii. 308 Capibara, ii. 143 Capillary circulation, ii. 254 Capillaries, ii. 208 Capsular ligaments, i. 92 Capsule, ii. 312 Capsules, seed, ii. 533 Caput Medusae, i. 192 Carapace, i. 260 Carbon, non-absorption of, by plants, ii. 15 Carbon, ii. 30 Carbonic acid, decomposition of, by plants, ii. 26 Cardia, ii. 96, 162 Cardiuni, i. 122, 199, 200, 210 Carduus, i. 1 19 Carinated sternum, i. 498 Carlisle, ii. 258, 509 Carnivora, i. 467 Carp, i. 366, 382; ii. 511 Carpus, i. 360 Carrion crow, i. 490 Cartilage, i. 94, 490 Caruncula lacrymalis, ii. 418 Carus, i. 235, 287, 327, 356; ii. 193, 220, 223, 451 Cassowary, ii. 198 Cat, ii. 258, 351,451 Caterpillar, i. 273, 281 ; ii. 433 Caudal vertebrae, i. 360 Caulinia, ii. 44 Causes, final, i. 19 Cavity of tympanum, ii. 378 Cavolini, ii. 216 Celandine, ii. 42 Cells, vegetable, i. 56 Cellular plants, i. 59, 62 Cellular texture, animal, i. 83 Centaurea, i. 119 Cephalic ganglion, ii. 487 Cephalo-thorax, i. 253 Cephalopoda, i. 231, 362; ii. 113, 195, 242, 348, 391, 441, 495 Cerambyx, i. 293; ii. 281, 282, 344 Cercaria, i. 169; ii. 427 Cerebellum, ii. 499 Cerebral ganglion, ii. 487 Cerebral hemispheres, ii. 499 Cerithium, i. 222 Ceroxylon, ii.42 Cervus, ii. 203 Cetacea, i. 112, 352, 353, 426, 427 ; ii. 255, 259, 396, 444, 498, 552, 569 Chabrier, i. 289, 309, 513 Chafer, i. 320 Chain of beino;, i. 44, 53 ; ii. 563 Chalcides, i. 399, 407 Chalcis, i. 304 Chameleon, i. 102, 41 1 ; ii. 1 16, 250, 288, 446 Chamois, i. 448 Chara, ii. 44, 45 Chelidonium, ii. 42 Chelonia, i. 411 ; ii. 289, 343, 394 Chemical changes by respiration, ii. 300 Chemistry, organic, i. 7 ; ii. 5 Chenodonta, i. 385 Cheselden, ii. 463 Clievveuil, i. 110 Chiaje, ii. 216 Children, i. 284 ; ii. 439 Chimera, ii. 246 Chitine, i. 284 Chladni, ii. 372 Chlorophyllite, i. 61 ; ii. 27 Chondrilla, ii. 47 INDEX. 579 Choroid coat, ii. 412 Choroid, common, ii. 438 Choroid gland, ii. 311, 442 Chromatic aberration, ii. 424 Chromatogenous glands, i. 99 Chromulite, i. 61 Chrysalis, i. 274 Chyle, ii. 53, 63, 96 Chylification, ii. 180 Chyme, ii. 162 Cicada, i. 304 Cicindela, ii. 187 Cilia, i. 114, 143; ii. 272 Ciliary ligament, ii. 413 Cimbex, i. 295, 297 Cimex, ii. Ill Circular theory, i, 35 Circulation, i. 35; ii. 10, 204 Circulation, discovery of, ii. 262 Circulation, vegetable, ii. 43 Cirrhi, i. 230 ; ii. 268, 349 Cirrhopoda, i. 230 Clausilia, ii. 286 Clausium, i. 226 Clavicle, i. 359 Clavicle of birds, i. 499 Claviger, ii. 344, 432 Claw in lion's tail, i. 469 Claws, i. 104 Cleridae, i. 296 Clio, i. 231; ii. 123 Cloquet, ii. 445 Clypeaster, i. 191 Clytie, ii. 524 Coats of the eye, ii. 411 Cobitis, ii. 280 Cobtade capello, i. 484; ii. 147 Coccygeal bone, i. 360 Cochlea, ii. 382 Cockle, i. 195 Cod, lens of, i. 382 ; ii. 443 Coenurus, ii. 75 Co-existence of forms, i. 42 Coffin-bone, i. 457 ; ii. 256 Coleoptera, i. 290, 291, 303; ii. 342, 369 Collar-bone, i. 215, 359 Colon, ii. 198 Colours af insects, i. 284 Colour, perception of, ii. 423 Colpoda, i. 1 69 Coluber, ;. 399, 401, 403; ii. 147 Columella, i. 217; ii. 394 Commissures, ii. 479, 506 Comparetti, ii. 223, 391 Complementary colours, ii. 472 Compound eyes, ii. 432 Concha of the ear, ii. 377 Condor, ii. 298 Conglomerate eyes, ii. 432 Conjugation, law of, ii. 554 Conjunctiva, ii. 416 Consumption of animal matter, ii. 54 Contractility, muscular, i. 113 Conus, i. 218, 223 Convergence of rays, ii. 407 Convolutions, cerebral, ii. 501 Convolvulus, ii. 42 Cooper, ii. 388 Coracoid bone, i. 359 Coral, jointed, i. 151 Coralhum, i. 150 Corda, i. 164 ; ii. 69 Cordylura, ii. 436 Corium, i. 97 ; ii. 338 Cornea, ii. 412 Corneule, ii. 436 Cornu Ammonis, i. 239 Coronet bone, i. 457 Corpora quadrigemina, ii. 499 Corpus callosum, ii. 503 Corpus mucosum, i. 97, 98; ii. 338 Corpus papillare, i. 98 ; ii. 339 Corpuscles of blood, ii. 202 Corselet, i. 289 Corti, i. 52; ii. 45 Cortical part of the brain, ii. 322 Cossus, i. 269, 279, 317 Cotunnius, ii. 382 Cowhage, ii. 41 Cowrie, i. 221 Crab, i. 260; ii. 236, 271,286, 441 Crab's eyes, i. 263 Cribriform plate, ii. 358 580 INDEX. Crampton, ii. 116 Cranial vertebrae, i. 356, 374 Granium, i. 355 Craw, ii. 151 Craw-fish, ii. 150, 439 Crinoidea, i. 181 Crocodile, i. 104, 352, 411 ; ii. 126, 146, 250, 292, 365, 500 Cross-bill, ii. 117 Crotalus, i. 400, 402 (.'row, carrion, i. 492 Crust, i. 96, 260 Crusta petrosa, ii. 136, 137 Cnistacea, i. 257 ; ii. 268, 271, 343, 391, 440, 487, 489 Cryptogamia, i. 62 ; ii. 533 Cryptomonadina, ii. 427 Crystalline lens, ii. 413 Ctenoides, i. 385 Ctenophora, ii. 344 Cupola, ii. 78 Curculio, i. 293 Cushions of insects, i. 298 Cuspidate teeth, ii. 128 Cuticle, i. 97; ii. 3L'8, 340 Cuticle, vegetable, i. 67 Cuttle-fish. See Sepia. Cuvier, (G.) i. 466, 521 ; ii. 77, 102, 142, 275, 280, 393, 397, 442, 539, 569 Cuvier, (F.) i. 110, 506; ii. 142 Cyclidium, i. 168 Cyclocsela, ii. 86 Cycloides, i. 385 Cyclosis, ii. 43 Cyclostomata, ii. 104 Cydippe, ii. 485 Cygnet, ii. 42 Cymbia, i. 215 Cymbulia, i. 231 Cymothoa, ii. 489 Cyprsea, i. 209, 221 Cyprinus, i. 366, 385 ; ii. 546 Cysticule, ii. 393 Cytoblast, i. 56 DaMorff, i. 386 ; ii. 277 Darwin, i. 77 Darwin, (R.) ii. 471 Dauheny, ii. 20 Davy, ii. 15, 303 Davy, (J.) ii. 247 Death, ii. 559 DeBlainville, i. 204, 222, 326 ; ii. 391, 383, 431, 445, 511 De Candolle, i. 60, 81 ; ii. 16, 22, 24, 26, 33, 44, 46, 49, 555 De Candolle, {A.) ii. 41 Decapoda, ii. 286 Decline of the system, ii. 558 Deer, ii. 359 Deflection of light, ii. 405 Defrance, i. 229 De Geer, i. 304, 319, 320 Deglutition, ii. 156 Delaroche, ii. 279, 444 Delphinus, ii. 258, 553 Dentine, ii. 133 Derm, i. 101 Dermo-skeleton, i. 327 De Saussure {Th.) ii. 25, 27 Descartes, ii. 328, 503 Deschanqys, i. 318 Design, evidence of, i. 24 Developement, vegetable, i. 5Q, 72 Developement, animal, i. 86 ; ii. 541 Developement of bone, i. 335 Diaperis, ii. 344 Diaphragm, ii. 293 Diastase, ii. 541 Dibrancliiate, ii. 244 Digiligrada, i. 470 Digestion, i. 34; ii. 63, 161 Diglena, i. 169 Diodon, i. 386 Dioecia, ii. 536 Dionsea, i 1 19 Diopsis, ii, 435 Diplozoon, ii. 546 Diptera, i. 288, 316; ii. 103 Diquemare, i. 198 Discoid shells, i. 216 Distoma, ii. 101 INDEX. 581 Distribution of blood-vessels, ii. 255 Dog, ii. 258, 384, 477 Dolichos, ii. 41 Dollinger, ii. 551 Dolphin, i. 423, 426; ii. 127, 396 Donne, ii. 45, 46, 314 Doras, ii. 278 D'Orbigny, i. 238 Doris, ii. 113, 268 Dormouse, ii. 169 Dorsal vessel, ii. 230 Dory, i. 375 Dove, ii. 498, 500 Down, i. 82 Draco volans, i. 47, 483, 484 Dragon-fly, i. 277 Dromedary, ii. 198 Drone-fly, i. 311 Duck, ii. 117, 362 Duckweed, ii. 529 Ductus pneumaticus, i. 383 Dufour, Lton, ii. 184, 283 Dugh, i. 392; ii. 223, 230, 235, 428, 436, 437, 440 Dugong, ii. 127, 252 Duhamel, i. 344; ii. 14, 17 Dujardin, i. 163, 238 Dujnas, ii. 352, 545 Dumcril, i. 356, 521 Dumortier, i. 327 Duodenum, ii. 184 Du Petit Thouars, i. 60 Dutrochet, i. 60, 66, 394 ; ii. 46, 283 Dyirastidse, i. 297 D'ytiscus, i. 25, 278, 297, 298, 300; ii. 281, 282 Eagle, ii. 297 Ear, ii. 376 Ear-drum, ii. 377 Earle, i. 494, 497 Echinoderniata, i. 181, 385 ; ii. 90, 106,216,269, 343,493 Edwards, i. 101, 114; ii. 73, 210, 230,237,242,286,487, 565 Eel, i. 377, 387; ii. 246, 278 Efferent nerves, ii, 322, 479> Effluvia, ii. 355 Egeriac, i. 319 Egg, ii. 536 Ehrenberg, i. 115, 117, 238; ii. 82, 106, 182, 216,427, 484, 485, 504 Ehrmann, ii. 280 Eichhorn, i. 52 Elaboration, successive, ii. 11 Elain, i. 110 Elastic ligaments, i. 93 Elater, i. 304 Elearin, ii. 6 Electric organs, i. 26; ii. 513 Electricity, ii. 314 Elements, organic, ii. 5 Elephants, i. 47, 94, 106, 125, 435, 458; ii. 126, 138, 145, 177, 198, 351, 450,502 Ellis, i. 137 Elytra, i. 289, 312 Elytra, analysis of, i. 284 Embryo, ii. 534 Embryo forms, ii. 565 Emu, i. 518 Emys, i. 420 Enamel of teeth, ii. 134 Enchelis, ii. 427 End of life, ii. 327 Endogenous plants, i. 72 Entomoline, i. 104; ii. 6, 284 Entomostraca, ii. 441 Entozoa, i. 252; ii. 73, 101, 217,267, 486, 530 Ephemera, i. 278; ii. 220,224, 227 Epidermis, vegetable, i. 75, 77 Epidermis, animal, i. 97, 100 Epidermis of shells, i. 207 Epiglossa, ii. 109 Epiglottis, ii. 287 Epiphragma, i. 226 Equivocal generation, ii. 531 Equorea, ii. 76 Erato, i. 221 Erect vision, ii. 461 Eraslalis, i. 31 1 582 INDEX. Erpobdella, i. 243; ii. 231 Eryx, i. 399 Esox, i. 380 Ethmoid bone, ii. 357 Euclamis, i. 164 Eudora, ii. 64, 81 Euglene, ii. 427 Eunice, ii. 233, 429 Euphorbium, ii. 54 Euryale, i. 192 Eustachian tube,ii. 358,379,554 Evil from animal warfare, i. 39 ; ii. 60 Excito-motory system, ii. 323, 477 Excretion, ii. 10 Excretion, vegetable, ii. 40, 46 Encretories, ii. 10 Exhalation by leaves, ii. 23 Exocetus, i. 483 ; ii. 498 Exogenous plants, i. 72 Expiration, ii. 294 Extremities, i. 359 Eye, i. 26, 27; ii. 401,411 Eye, formation of, ii, 543 Eye of Cod-fish, i. 51 Eyes, conglomerate, or com- pound, ii. 432 Eyes of potatoe, ii. 529 Eyelid, ii. 417 Eyelid, formation of, ii. 551 Fabricius, ii. 108 Facets, ii. 435 Facial angle, ii. 503 Fairy rings, ii. 50 Fallacies of perception, ii. 458 Fangs of serpents, ii. 146_ Faraday, ii. 466 Farre, i. 116 Fasciculi of muscle, ii. 315 Fasciola, ii. 101 Fasciolaria, i. 222 Fat, i. 110 Fata Morgana, ii. 473 Favonia, ii. 64 Feathers, i, 501 Fecula, i, 59; ii. 5 Fecundation, ii. 534 Feelers, i. 258 ; ii. 344 Feet-jaws, i. 259 Feet of birds, i. 516 Felis, ii. 121 Femur, i. 258, 360 Fenestrse of the ear, ii. 380 Ferns, i. 72 Fertility of soils, ii. 15 Fibre, primitive, i. 82 Fibrillse of muscles, ii. 315 Fibrin, ii. 5 Fibrous textures, i. 91 Fibrous capsules, i. 92 Fibula, i. 360 Ficus, ii. 43 Fig, ii. 43 Fig-marigold, ii. 42 Fig-tree, ii. 42 Filaments of feathers, i. 502 Filaments of flowers, ii. 535 Filaria oculi, i, 54 Filices, i. 72 ; ii. 533 Final causes, i. 19, et passim Fins of fishes, i. 375 Fins of cetacea, i. 431 Firn of glaciers, i. 12 Fishes, i. 10, 102, 363, ii. 114, 179, 244,273, 343, 349, 353, 365, 392, 442, 497, 500 Fissiparous reproduction, ii. 523 Flea, i. 266, 320 Flight of insects, i. 308 Flight of vertebrata, i. 481 Flourens, i. 344 ; ii. 275 Flower, ii. 535 Fluid of Cotunnius, ii. 382 Fluke, ii. 101 Flustra, i. 150, 153 Flying fish, i. 483 ; ii. 498 Flying lizard, i. 483 Flying squirrel, i. 485 Focus' of rays, ii. 405 Fohmann, ii. 317 Follicles, ii. 164 Follicles, sebaceous, i. 101 Follicles, secreting, ii. 311 Food of animals, ii. 52 Food of plants, ii. 13 Fontana, ii. 551 INDEX. 583 Foramen in humerus, ii, 256 Force of heart, ii. 260 Force, mechanic, i. 113 Forces, physical, i. 5 Fordyce, ii. 153 Fossil shells, i. 238 Fossil infusoria, i. 173 Fovilia, ii. 535 Franklin, ii. 372 French bean, ii. 48 Frigate-bird, i. 524 Frog, i. 489; ii. 115, 197, 247, 318, 393, 394, 5Q5 Fucus vesiculosus, i. 57, 62 FUllibrner, ii. 344 Functions, division of, i. 32 Functions of life, i. 28 Functions, animal, ii. 318 Functions, mechanical, i. 131 Functions, reproductive, ii. 521 Functions, sensorial, ii. 326 Functions, vital, ii. 9 Fungi, ii. 50 Fur, i. 103 Furcocerca, i. 169 Furcular bone, i. 499 Furcularia, i. 52 Fusiform fibres, i. 66 Fusiform roots, ii. 18 Future existence, ii. 572 Gadoides, i. 385 Gaede, ii. 76 GcErtner, ii. 277 Gaillon, i. 142 Gaimard, i. 84 Gcdileo, i. 71 Galiopithecus, i. 485 Galionella, i. 169 Gallinee, ii. 498 Gallop, i. 438 Galvanism, ii. 314 Ganglion, ii. 322 Ganglion, optic, ii. 438 Ganglionic system, ii. 322, 478 Ganoides, i. 385 Garner, ii. 429 Gasteropoda, i. 204; ii. 113, 240,272,429,431 Gastric glands, ii. 164 Gastric juice, ii. 163 Gastric teeth, ii. 150 Gastrobranchus, i. 362, 372 r ii. 445 Gay Lussac, ii. 283 Gazelle, i. 443 Gecko, i. 409 ; ii. 350 Gelatin, i. 91 ; ii. 6 Gemmiparous reproduction, ii. 528 Gemmules, i. 142 Gemrauliparous reproduction, ii. 530 Generation, equivocal, ii. 531 Geometer caterpillar, i. 315 Germs, vegetable, i. 75 ; ii. 528 Germs, animal, ii. 528 Gervais, ii. 434 Geryonia, ii. 64 Gibson, i. 344 Gillaroo- trout, ii. 179 Gills, ii. 209, 270 Gimbals, i. 294 Giorna, i. 320 Gizzard, ii. 151 Glands, ii. 10, 312 Glands, blennogenous, i. 99 Glands, clioroid, ii. 442 Glands, chromatogenous, i. 99 Glands, gastric, ii. 164 Glands, lacrymal, ii, 416 Glands, lymphatic, ii. 317 Glands, mesenteric, ii. 201 Glands, sudorific, i. 100 Glands, vegetable, i. 67 ; ii. 39 Gleichen, ii. 83 Glenoid cavity, i. 3.59 Glenomerum, ii. 427 Globe of the eye, ii. 411 Globules, i. 85 Globules in fluids, i. 54 Globules of the blood, ii. 202, 315 Glossa, ii. 110 Glossopera, ii. 93 Gnat, ii. 1 03 Goat, i. 455 ; ii. 359 Goethe, i. 356 584 INDEX. Go'eze, ii. 427 Gonium, i. 169; ii. 523 Goose, ii. 155 Gordius, i. 53, 247 Gorgon la, i. 150 Goring, ii. 220 Gradation of being, i. 44 Gradation, law of, ii.. 563 Grallge, i. 510 Grampus, ii. 127 Grant, i. 137, 178, 518; ii. 427, 485 Grtaj, i. 214, 215 Grew, ii. 43 Grinders, ii. 128 Growth, vegetable, i. 72; ii. 18 Grvithuisen, ii. 428 Gryllotalpa, i. 306; ii. 312, 345 Gryllus, ii. 186, 223 Guinea-pig, i. 441 ; ii. 143 Gulliver, ii. 203 Gulstonian lecture, ii. 473 Gum, ii. 32 Gurnard, ii. 50O Gymnodonta, i. 385 Gymnotus, i. 377, 385; ii. 513 Gyrinus, i. 307 ; ii. 435 Hsematopus, ii. 117 Haidinger, i. 186 Hair, vegetable, i. 82 Hair, animal, i. 103 Hair of insects, i. 285 Hair-worm, i. 53, 247 Hales, ii. 22 Haliotis, i. 208 Hall, ii. 246, 323, 324, 477 Halter, i. 85 Halteres, i. 316 Haltica, i. 320 Hamster, ii. 159 Hancock, ii. 278 Hanow, ii. 427 Hare, i. 440; ii. 169, 384, 396 Harpalidae, i. 297 Hartley, ii. 506 Harvey, ii. 262 Harwood, ii. 361, 362, 363 Hassar, ii. 278 Hauksbee, ii. 371 Haunch of insects, i. 258 Havers, i. 331 Hawk, ii. 117 Hawk-moth, ii. 191, 223 Head of insects, i. 286 Hearing, ii. 370 Heart, i. 127; ii. 211 Heart, force of, ii. 260 Heart in embryo, ii. 545 Hearts, lymphatic, ii. 318 Hedge-hos:, i. 463, 466, 470 ; ii. 308 Hedwig, i. 64 Hedysarum, i. 1 18 Helix, 1.216, 226; ii. 113,286, 430 Hellman, ii. 350 Hemi-elytra, i. 313 Hemiptera, i. 276, 313 ; ii. 432 Hemispheres, cerebral, ii. 499 Henbane, ii. 54 Henderson, ii. 303 Hepatic system, ii. 184 Hepatic blood-vessels, ii. 314 Herring, i. 375 Herschel, (Sir W.) ii. 470 Herschel, (Sir J.) i. 208; ii. 38, 512 Hesperia, i. 318 Hexastoma, ii. 101 Hippopotamus, i. 136; ii. 126, 135, 136, 145, 171, 397,430 Hirudo. See Leach Hodghin, i. 1 18 Hodgson, ii. 359 Hog, i. 357, 358, 460; ii. 171, 351, 360 Holothuria, i. 181 ; ii. 184,216, 269, 494 //ome, i. 403, 410; ii. 166, 173, 178, .197, 277, 365, 431. Homoptera, i. 320 Honey-comb stomach, ii. 173 Hooded snake, i. 484; ii. 147 Hoofs, i. 104 Hooke, ii. 435 Hooks on feet of insects, i. 295 Hooks on wings of insects, i. 319 INDEX. 585 Hop, i. 79 Horn, i. 103, 453 Horn on beak of chick, ii. 552 Horse, i. 456; ii. 14'2, 170,510 Horse-fly, i. 297 ; ii. 103, 435 Hostilities, animal, i. 39; ii. 60 Houston, ii. 116, 250 Hoivshij), i. 331 Huber, ii. 346 Huber, (P.) ii. 368 Human fabric, i. 473 ; ii. 501 Humboldt, i. 176; ii. 279, 283 Humerus, i. 360 Humming-bird, ii. 104 Humours of the eye, ii. 411, 413 Hunter, i. 94; ' ii. 152, 153, 167, 259, 296, 297 Huschke, ii. 567 Hyscna, i. 442 ; ii. 55, 133 Hybernation, ii. 477 Hydatid, ii. 74, 102, 530 Hydatina, ii. 86, 87, 216, 484 Hvdra, i. 147, 158 ; ii. 66, 68, 426, 526, 529, 530 Hydrogen in plants, ii. 32 Hydrophilus, i. 278 Hydrostatic acalephae, i. 179 Hygroscopic power, ii. 18 Hyla, i. 397 Hymenoptera, i. 288, 292, 314; ii. 103 Hyoides, os, ii. 274 Hyrax, ii. 170 Ichneumon, i. 320 Ichthyosaurus, i. 416 Ilium, i. 360 Illusions, visual, ii. 466 Imago, i. 275, 283 Imbibition, ii. 206 Impulse of the heart, ii. 260 Incident nerves, ii. 323 Incisions of insects, i. 291 Incisors, ii. 128 Incus, ii. 381 Indian walrus, ii. 127 Individuality of jtlants, i. 78 Individuality of polypes, ii. 73 Individuality of trees, ii. 555 Infusoria, i. 114, 166; ii. 57, 182,427, 531 Infusoria, fossil, i. 173 Insectivora, i. 464 Insects, i. 9, 265; ii. 108, 183, 211,217,312,342,343, 353, 432,491, 511 Inspiration, ii. 294 Instinct, ii. 515 Instrumenta cibaria, ii. 108 Integuments, i. 97 ; ii, 338 Intercellular spaces, i. 60; ii. 33 Intercostal plexus, ii. 259 Intermaxillary bones, ii. 128 . Interspinous bones, i. 353, 378 Intestines, ii. 91, 181 Iriartea, ii. 42 Iridescence, i. 208 Iris, i. 125; ii. 413 Ischium, i. 360 Isis, i. 151 Ivory, ii. 133 Ivy, i. 79 Jacobson, i. 247; ii. 413, 509, 511 Jaquemin, i. 490 Jaws of insects, ii. 108 Jerboa, i. 440, 475 Jones (Rymer), ii. 86, 485 Jurine, ii. 282, 509 Julus, i. 267, 268; ii. 434 Kaleidoscope, ii. 473 Kanguroo, i. 355, 440, 475 ; ii. 171,537 Kater, ii. 439 Kerner, ii. 387 Keule, ii. 451 Kidd, i. 307; ii. 282, 312, 345 Kiernan, ii. 314 Kieser, i. 57, 60 Kirby, i. 289, 292; ii. 108,368, 369, 434, 435 Knee-pan, i. 361 Knujht, ii. 534 Knock, ii. 109 Knots in wood, ii. 529 Koala, i. 465 586 INDEX. Koch, ii. 451 Kolpoda, i. 169 Labium of insects, ii. 110 Labrum, ii. 110 Labyrinth of ear, ii. 381 Lacepede, i. 386 Lacerta, i. 407 Lacrymal caruncle, ii. 418 Lacryinal gland, ii. 416 Lacteal absorption, ii. 200 Lacteals, ii. 96 Lamarck, ii. 82, 570 Lamina spiralis, ii. 384 Lamina dorsalis, ii. 542 Lamprey, i. 372; ii, 104, 105, 276, 392 Lancets of diptera, ii. 103 Language of insects, ii. 346 Lantern of Aristotle, ii. 106 Lark, i. 514 Larva, i. 274 Lassaigne, ii. 163 Latham, ii. 168 Latreille,\. 260; ii. 285, 348, 441 Latex, ii. 43 Laticiferous vessels, ii. 43 Law of analogy, i. 41 ; ii. 560 Law of co-existence of forms, i. 42 Law of gradation, ii. 563 Law of mortality, ii. 521 Law of symmetry, ii. 553 Laws of nature, i. 4 Leach, i. 197 Leaping, i. 302 Leaves, fibres of, i. 67 Leaves of vertebrae, i. 351 Leech, i. 127; ii. 92, 102, 112, 231,270 Lefehvre, ii. 369 Lemna, ii. 529 Lemur, ii. 258, 450 Lemur volans, i. 485 Lens, convex, ii. 409 Lens, crystalHne, ii. 413 Lenticellsc, i. 81 Lepas, i. 230 ; ii. 268 Lepidoptera, i. 316, 272; ii. 102, 191 Lepisma, i. 266, 267, 317, 320 Lernsca, i. 271 ; ii. 538 Leuioenhoeck, ii. 135, 254 Leuchs, ii. 431 Leucophra, ii. 85, 86 Leucopsis, i. 304 Leuret, ii. 163 Levers, i. 333 Libellula, i. 277, 311, 314; ii. 434, 435, 439 Liber, i. 75, 76 Lichen, ii. 16 Life, i. 35 Ligament, i. 92 Ligament, ciliary, ii, 413 Ligament, elastic, i. 93 Llgamentum nuchse, i. 93, 444 Light, properties of, ii. 405 Light on plants, ii. 25 Lignin, i. 62 ; ii. 5 Ligula, ii. 64 Lilium, i. 68 Limax, ii. 113, 286 Limpet, i. 194; ii. 194 Link, i. 60 Linnceus, i. 183 Lion, i. 94, 439, 467, 468 ; ii. 122, 255,259, 351, 500 Lister, ii. 216, 242, 272 Living nature, i. 8 Liver, ii. 96 Liver, vessels of, ii. 314 Lizards, i. 406; ii. 318, 350, 444, 445 Lob-worm, ii. 267 Lobster, i. 260; ii. 149, 271, 236, 489, 490 Lobularia, i. 146 Loche, ii. 280 Locomotion, powers of, i. 131 Locust, i. 320 Locusta, ii. 108 Logarithmic spiral, i. 218 Loiigo, i. 234, 362 ; ii. 243 Longevity of trees, ii. 555 Longicora, il. 435 Longicornua, i. 297 INDEX. 587 Lophius, i. 376 ; ii. 349, 392 Lophosia, ii, 344 Loris, ii. 259 Loxia, ii. 117 Lubbock, i. 369 Lucanus, i. 321 Lumbricus, i. 243, 248; ii. 91, 234, 269 Lumbricus marinus, ii. 267 Lungs, ii. 280, 287 Lusus natures, ii. 568 Lycopodium, i. 68 Lycoris, ii. 429 Lymnoria, ii. 64 Lymneea, ii. 287 Lymphatic, ii. 316 Lymphatic glands, ii. 317 Lymphatic hearts, ii. 318 Lyonet, i. 279, 317 Macaire, ii. 47, 49, 53, 300 Macartney, i. 521 ; ii. 296, 297 M'Avoy, ii. 337 Mackerel, i. 378 Macleay, i. 45 Macrobius Huflandii, i. 53 Madder, i. 342 Madrepora, i. 150 Mayendie, ii. 451, 475 Magilus, i. 223 Magnetism, animal, ii. 337 Maia, ii. 489 Malleus, ii. 381 Malpighi,\. 99; ii. 186 Mammaj, i. 423 ; ii. 537 Mammalia, i. 423 ; ii. 395, 537, .552 567 Man, i. 473 ; ii. 502, 503 Man of war, Portuguese, i. 179 ; ii. 80 Manatus, ii. 127, 252, 258 Mandibles, i. 259 Mandl, i. 105 Manitrunk, i. 289 Mantis, ii. 187 Mantle, i. 98, 193, 212 Manyplies stomach, ii. 174 Marcel des Serres, ii. 110, 185 Marcet, ii. 200 Marcet (F.), ii. 53, 300 Marginelh',, i. 221 Marmot, li. 133 Marrow, spinal, ii. 497 Marsiyli, i. 137 Marsupial bones, i. 400 Marsupialia, ii. 537 Marsupium, ii. 447 Masaris, ii. 344 Mastication, ii. 106, 125 Mastoid cells, ii. 380 Matrix of feathers, i. 507 Matter, ii. 454, 460 Blaunoir, ii. 469 Maxillae, ii. 110 Mayer, i. 398 Mayo, ii. 481, 554 Mayow, ii. 301 Meatus auditorius, ii. 377 Mechanical functions, i. 32, 131 Meckel, i. 235, 356 ; ii. 429 Medulla oblongata, ii. 499 Medullary rays, i. 75 Medullary substance, ii. 328, 504 Medusa, i. 84, 175; ii. 75, 214, 266, 427, 485 Meibomian glands, ii. 418 Melolontha, i. 293, 269, 285, 320; ii. 188, 218, 282, 344, 434, 435 Melophagus, ii. 432 Membrana nictitans, ii. 446, 447 Membrana tympani, ii. 377 Membranes, i. 88 Membranes, serous, i. 89 Membranous labyrinth, ii. 383 Membranous shells, i. 207 Meniscus, ii. 436 Menobranchus, ii. 292 Mercurialis, ii. 48 Mergaiisor, ii. 117 Mcrgus, ii. 117 Merrythought of a fowl, i. 499 Mesembryanthemum, ii. 42 Mesenteric glands, ii. 201 Mesial plane, i. 242; ii. 546 Mesmerism, ii. 337 Mesothorax, i. 288 508 INDEX. Metacarpus, i. 361 Metamorphosis, i. 271 Metatarsus, i. 361 Metathorax, i. 288 Meyen, ii. 87 Miescher, i. 332 Milk, ii. 552 Millepedes, ii. 434 Millepores, i. 151 Mimosa pudica, i. 119 Mind, ii. 454 Mint, ii. 15 Mirandola, ii. 526. Mirbel, i. 60, 78 Mitra, i. 222 Mizaud, i. 343 Modiolus, ii. 385 Mohl,\. 61, 81 ; ii. 40 Molar teeth, ii. 128 Moldcnhawer, i. 65 Mole, i. 463, 464, 470 ; ii. 350, 451 Mole-cricket, i. 306 Mollusca, i. 193; ii. 112,213, 240, 268,271,342,369,391, 429, 494 Monadina, ii. 427 Monas, i. 11, 166; ii. 84, 523 Monilitbrm vessels, i. 63 Monkey,!, 472; ii. 510 Monkey, American, ii. 351 Monoculus, ii. 441 Monotremata, ii. 261 Monro, i. Ill, 122,274 Monsters, ii. 568 Montegre, ii. 163 Mordella, ii. 434 Morpho, i 316 Morren,n. 41,235, 270 Mortality, law of, ii. 521 Moseleij, i. 218 Moth, i. 272 Mother of pearl, i. 208 Motion, voluntary, i. 31 ; ii. 475 Motor nerves, ii.475 Mouth of insects, ii. 108 Mucilage, ii. 5. Mucous glands, ii. 164 Mucus, ii. 6. Muller, i. 254; ii. 82, 135, 312,317, 318, 386,433,437 Mullet, ii. 179 Multilocular shells, i. 237 Multivalve shells, i. 194, 229 Murena, ii. 500 Murex, i.220,225; ii. 113,272, 431 Mus bursarius, ii. 159 Mus typhlus, ii. 451 Musca, i. 297 ; ii. 434 Muscle, i. 117 Muscle (shell-fish), i. 194 Muscular power, i. 112 Musical note, ii. 374 Mya, i. 201 Myriapoda, i. 266 Myrmecophaga, ii. 120 Mysis, i. 259 Mytilus, i. 194, 200 Myxine, i. 362, 372; ii. 104, 277 Nacreous structure, i. 208 Nail,i. 104 Nais, ii.91, 231, 428 Narwhal, i. 47, 423; ii. 126 Nature, i. 11 Nature, laws of, i. 5 Nautilites, i. 238 Nautilus, i. 216; ii. 244,369 Navier, i. .513 Navicula, i. 169 Necrophorus, ii. 369 Needles in biliary ducts, ii. 194 Negro, i. 99 ; ii. 144 Nemazoaria, i. 142 Nepa, ii. 344 Nereis,i. 243, 250; ii. 231, 428 Nerve, sympathetic, ii. 322 Nerves, i. 30; ii. 329,504 NerveSj afferent and efferent, ii, 322 Nerves, motor, ii. 475 Nervous power, ii. 318, 320 Nervous system, ii. 328 Nervures, i. 309 Nettle, ii. 41. Ncurilema, ii. 480 INDEX. 589 Neurine, i. 30 ; ii. 322, 328, 504 Neuroptera, i. 314; ii. 432 Neuro-skeleton, i. 327 Neivport, i. 314; ii. 92. 189, 192,223,22.5,491 Newt, ii. 393 Nightshade, ii. 54 Nitrogen, ii. 12 Nitrogen in plants, ii. 32 Nifzsch, ii. 427 Nordmann, ii. 525, 538, 546 Notonecta, i. 25, 301 Nuclei of cells, ii. 315 Nucleolus of cells, i. 56 Nudibranchiata, ii. 113 Nursling sap, ii. 21 Nutrition, ii. 1 , 9, 1 1 Nutrition in lower orders, ii. 66 Nutrition in higher orders, ii. 94 Nutritive functions, i. 32 Nycteribia, ii. 432 Oblique muscles, i. 122 Oblique muscles of the eye, ii. 415 Oblique processes, i. 352 Ocelli,!. 318 Octopus, i. 233; ii. 442 Oculisr spectra, ii. 467, 471 Ocythbe, i. 238 Oriier, i. 284 Odorous effluvia, ii. 355 (Esophagus, ii. 96 Oil, ii. 5 Oy^en,i. 312,356 Olfgfctory lobes, ii. 499 Olfactory nerve, ii. 354 Oliva, i. 215, 223 Onchidium, ii. 287 Oniscus, ii. 489 Onocrotalus, i. 490 Operculum, i. 225; ii. 274 Ophicephalus, ii. 277 Ophidia, i. 398 Ophiosurus, i. 404, 407 Ophiura, i. 192 Opossum, ii. 122, 537 Oplic axis, ii. 412,449 Optic ganglion, ii. 438 Oplic lobe:s, ii. 499 Optic nerves, ii. 401 Optical principles, ii. 402 Optometer, ii. 423 Opuntia, i. 1 19 Orache, ii. 42 Orbicular bone, ii. 381 Orbicular muscles, i. 125 Orchidiae, i. 60 Organization, i. 50 Organization, animal, i. 83 Ornithorhynchus, i. 352 ; ii. 122, 350, 396, 444, 569 Orobanche, ii. 49 Orthoceratites, i. 239 Orthoptera, i. 276, 313 ; ii. 151, 432 Orythia, ii. 64 Os coccygis, i. 360 Os hyoides, ii. 274 Osier, i. 187, 198, 201, 248, 251 Osseous fabric, i. 326 Ossicula, tympanic, ii. 380, 395 Ossification, i. 335 Ostracion, i. 384, 385 Ostrich, i. 497, 518, 521; ii. 165, 198, 295, 498 Otter, sea, ii. 133 Otoconies, ii. 384 Otolithes, ii. 384 Ovary, ii. 533 Oviduct, ii, 535 Oviparous animals, ii. 536 Ovula, i. 221 Ovum, ii. 533 Owen, ii. 133, 140, 157, 247, 369 Owl, ii. 297, 395, 449 Ox, i. 453; ii. 142 Oxygen, exhalation of, ii. 30 Oy.ster,i. 121, 194,195; ii. 429 Oyster-catcher, ii. 117 Paces of quadrupeds, i. 436 Pachydermala, i. 456, 458 ; ii. 343, 351 Package of organs, i. 89 Paget, i. 344 590 INDEX. Pain, ii, 331 Palemon, ii. 489 Paley, i. 89, 503 Palinurus, ii. 489 Pallas, i. 137; ii. 308 Palleal ganglion, ii. 494 Palm squirrel, ii. 1.59 Palm tribe, i. 72 Palmella nivalis, i. 12 Palmer, ii. 26 Palpi, i. 259; ii. 110 Pancreas, ii. 96, 195 Pander, ii. 542 Pangonia, ii. 435 Panniculus carnosus, i. 466 Panorpa, i. 291 Papaver, ii. 43 Paper nautilus, i. 237 Papilio, ii. 434 Papillse, ii. 339 Par vagum, ii. 493 Parallactic aberration, ii. 421 Parasitica, i. 266 Parenchyma, ii. 311 Parrot, i. 520; ii. 124, 160, 350, 477 Pascal, ii. 456 Pastern, i. 457 Patella, i. 194, 205, 216, 225; ii. 113, 194, 495 Patella, (knee-pan) i. 361 Patellaria, ii. 41 Paunch, ii. 173 Pausus, ii. 344 Payen, ii. 12, 35, 541 Pearl, i. 208 Peccari, ii. 171 Pecten, ii. 429 Pediculus, i. 266 Pelican, i. 490, 524 Pelvis, i. 360 Pencil of rays, ii. 405 Penguin, i. 522 Penicihated structure, ii, 313 Penitentiary, epidemic in, ii. 168 Pennatula, i. 157 Pcnniform muscle, i. 123 Pentacrinus, i. 190, 192 Perca, ii. 500, 501 Perca scandens, i. 386 ; ii. 277 Perception, i. 30 ; ii. 335, 453 Perch, ii. 442, 500, 501 Percoides, i. 385 Perennibranchia, ii. 291 Peridinium, i. 176 Perilymph, ii. 382 Periostracum, i. 212 Peristaltic motion, ii. 181 Peritoneal canals, ii. 292 Peron, i. 84; ii. 65, 81 Persoz, ii. 541 Pfaff, ii. 303 Phacelomonas, ii, 427 Phalanges, i. 361 Phalena, ii. 223, 434, 435 Phanerogamous plants, ii. 535 Phantasmagoria, ii. 473 Phantasmoscope, ii. 466 Phaseolus, ii. 48 Phenikisticope, ii. 466 Philip, ii. 169, 325 Philosophy, province of, i. 5 Phoca, i. 431 Pholas, i. 197, 229 Phosphorescence of the sea, i. 176; ii. 57 Phrenology, ii. 507 Phyllosoma, ii. 488 Physalida, i. 176; ii. 80 Physiology, object of, i, 18 Physsophora, i, 179 Phytozoa, i, 134 Picture on retina, ii, 462 Pierard, ii. 177 Pigeon, ii. 160 Pigmentum of skin, i. 99 Pigmentum of eye, ii. 412 Pike, i. 380 Piked whale, ii. 122 Pileopsis, i. 225 Pineal gland, ii. 503 Pinna, i. 202, 210 Piper gurnard, ii. 500 Pistil, ii. 535 Pitch of sounds, ii, 375 Pith of quill, i. 511 Placoides, i. 385 Placuna, i. 209 INDEX. 591 Planaria, ii. 230, 267, 428 Planorbis, i. 204 ; ii. 287 Plantigrada, i. 470 Plastron, i. 412 Pleurobranchus, ii. 194, 195 Pleuronectes, i. 383, 385 ; ii. 449 Plexus of nerves, ii. 323 Plexus of vessels, ii. 338 Plicae primitivse, ii. 542 Pliny, ii. 502 Plumula, ii. 541 Plumularia, ii. 216 Pneumobranchise, ii. 285 Pneumogastric nerve, ii. 493 Podura, i. 266, 267 Podura nivalis, i. 12 Poisers, i. 316 Poison of nettle, ii. 41 Poli, i. 211 Polirschriefer, i. 173 Pollen, ii. 535 Pollini, i. 60 Polyacanthus, i. 385 Polygastrica, ii. 85 Polypi, i. 114; ii. 66, 266,343, 483 Polypifera, i. 146 Polypus of Aristotle, i. 234 Polystoma, ii. 101 Polythalamous shells, i. 237 Pontia, i. 316 Pontobdella, i. 243 Poppy, ii. 42, 43 Porcellaneous shells, i. 207 Porcupine, i. 108, 466 ; ii. 133, Pores of sponge, i. 136 Porpita, i. 178 Porpus, i. 423, 427; ii. 127, 258 Porterfield, ii. 423 Portio dura, ii. 385 Potatoe, ii. 37 Predaceous birds, i. 514 Prehension of liquid food, ii. 101 Prehension of solid food, ii. 105 Prevost, ii, 545 Priestley, ii. 25, 301, 303 Primitive trace, ii. 542 Prislis, i. 47 ; ii. 149, 276 Pritchard, ii. 220 Privet hawk-moth, ii, 191, 223, 491 Proboscis of elephant, i. 460 Proboscis of insects, ii. 102 Proboscis of mollusca, ii. 113 Processus recurvatus femoris, i. 457 Progressive motion, i. 131 Prolegs, i. 280 Promontory of ear, ii. 380 Proper vessels of plants, ii. 43 Proteus, i. 169 Proteus anguinus, ii. 203, 292, 566 Prothorax, i. 289 Protococcus nivahs, i. 12 Prout, ii. 36 Prorocentrem, ii, 176 Provencal, ii. 279 Proximate principles, ii. 5 Psychoda, ii. 344 Pterocera, i. 220 Pteropoda, i. 230 Pteropus, ii. 122 Pubic bone, i. 360 Pubis, i. 360 Pulex, i. 266, 320 Pulmone marino, ii. 266 Pulmonary cavities, ii. 280 Pulmonary circulation, ii. 209 Pulvilli, i, 296 Puncta lacrymalia, ii. 418 Punctum saliens, ii. 545 Pupa, i. 274 Pupil of the eye, ii, 413 Pupipara, ii. 432 Purkinje, i. 115, 331; ii. 135, 136, 196 Pyloric appendices, ii. 196 Pylorus ii. 162 Pyramidalis muscle, ii. 448 Pyrosoma, ii. 242 Python, i, 399 Quadrumana, i. 470; ii. 128, 133, 199, 449 592 INDEX. Quagga, i. 456 Quail, i. 514 Quills, i. 103 Quills of porcupine, i. 108 Rabbit, i. 440; ii, 169, 396 Racoon, i. 98 Radiata, i. 148 Radicle, ii. 541 Radius, i. 360 Ranunculus aquaticus, i. 69 Rapji, ii. 427 Rat, ii. 132 Bathke, ii. 567 Rattle-snake, i. 400, 402 Mat/, i. 9 Ray, i. 95, 375, 377; ii. 196, 393,449, 511 Rays of fishes, i. 378 Reaction not a moving force, i. 513 Reaumur, i. 184, 212, 213, 262; ii. 102, 163, 152, 440 Recti muscles of the eye, ii. 415 Receptacles of food, ii. 63 Receplaculum chyli, ii. 97, 202 Red snow, i. 12 Reed of ruminants, ii. 175 Reflex functions, ii. 477 Reflex nerves, ii. 323 Refraction of light, ii. 405 Regeneration of parts, ii. 527 Remora, i. 410 Renal system, ii. 209 Rennet, ii. 175 Renovation of materials, ii. 8 Reparation, ii. 527 Repetition, law of, i. 48 Reproduction, i. 36; ii. 521 Reptiles, i. 387; ii. 288, 343, 353, 445, 547 Respiration, ii. 10, 263 Respiration, aquatic, ii. 266 Respiration, atmospheric, ii. 279 Respiration, vegetable, ii. 27 Respiration, chemical effects of, ii. 300 Respiratory circulation, ii. 209 Resinous secretions, ii. 41 Rete mirabile, ii. 258 Rete mucosum, i. 100 Reticule, ii. 173 Retina, ii. 336, 401, 413 Retina, general, ii. 438 Return of sap, ii. 31 Retzius, ii. 135 Reversed shells, i. 217 Reviviscence, i. 53 Reviviscence of snails, i. 228 Rhea, i. 518 Rhinoceros, i. 105,456; ii. 121, 135, 343, 351,450 Rhipiptera, i. 313 Rhizostoma, ii. 77 Rhynchops, ii. 118 Ribs, i. 357 Ricinius, i. 266 Right arm, ii. 256 Rings of annelida, i. 243 Rodentia, i. 462; ii. 132, 145, 156, 169, 256, 450, 510 Roesel, ii. 427 Rof/et, ii. 168, 466, 473, 507, .521 Rolando, ii. 545, 550 Roosting, i. 519 Roots of plants, i. 55 Ross, i. 12 Rostrum, ii. Ill Rotation in vegetables, ii. 43 Rotation of crops, ii. 50 Rotatoria, i. 172; ii. 427, 484 Rotifer, i. 52 ; ii. 82, 86, 532 Rotifera, ii. 216 Rousseau, ii. 143 Roux, ii. 511 Rudimental conditions, i. 46 Rudolphi, i. 66 Rumford, i. 66 Ruminants, i. 442 ; ii. 450, 510 Rumination, ii. 174 Rusconi, ii. 249 Sabella, i. 248 Sacculus of the ear, ii. 384, 392 Sacrum, i. 360 Sagra, i. 304 INDEX. 593 St. Hilaire,\. 327, 350,356,447, 461 ; ii. 124, 292,451, 553 Salamander, i. 397; ii. 115, 318, 445, 536 Salicaria, ii. 49 Saline substances in plants, ii. 37 Saliva, ii. 156 Salmon, i. 369, 386; ii. 192 Salmon, velocity of, i. 386 Salpa, ii. 242 Sand-hopper, ii. 487 Sap, nursling;, ii. 2 1 Sap, returning, ii. 31 Sarcode, i. 163 Sauria, i. 406; ii. 288 Savart, ii. 386 Savigny, i. 245, 260; ii. 107, 109, 111 Saw-fish, ii. 149 Scala tympani et vestibuli, ii. 385 Scales of fishes, i. 105 Scansores, i. 517 ; ii. 498 Scapula, i. 359 Scarabseus, ii. 434 Scarf-skin, i, 97 Scarlatina, ii. 352 Scarjm, i. 88 ; ii. 384, 393 Scarus, ii. 136 Schoeffer, ii. 427 Schizoccrus, ii. 344 Schleiden, i. 56, 64, 78, 86 ; ii. 315 Schneider, ii. 356 Schneiderian membrane, ii. 356 Sckultz, ii. 43, 45, 532 Schwann, i. 86; ii. 315 Scienoides, i. 385 Sciurus volans, i. 485 Sclerodermata, i. 385 Sclerotica, ii. 411 Scolia, ii. 283 Scolopendra, i. 267 ; ii. 434 Scomberoides, i. 385 Scoresby, i. 177 Scorpion, ii. 284, 433 Scorpionides, i. 385 Scuta, abdominal, i. 403 Scutella, i. 191 VOL. II. Scyllsea, i. 206 Sea, phosphoresence of, i. 176; ii. 57 Sea-anemone. See Actinia Sea-hare, ii. 1 13 Sea-mouse, ii. 112 Sea-otter, ii. 133 Sea-turtle, ii. 179 Sea-urchin, i. 185 Seal, i. 423, 431 ; ii. 351, 360, 396, 452 Sebaceous follicles, i. 102 Secretion, ii. 10, 307 Secretions, vegetable, i. 67 ; ii. 39 Secretion, theory of, ii. 310 Seed, ii. 533 Seed capsules, ii. 533 Segments of insects, i. 286 Semblis, ii. 221, 223 Semicircular canals, ii. 381 Senecio, ii. 48 Sennebier, ii. 17, 25, 26 Sensation, ii. 326, 329, 453 Sensibility, variations of, ii. 467 Sensitive plant, i, 119 Sensorial power, ii. 325 Sensorial system of nerves, ii. 478 Sensorium, ii. 453, 482 Sepia, i. 234; ii. 180,242,273, 441, 495 Seps, i. 407 Scries of organic beings, i. 44 Serous membrane, i. 89 Serpents, i. 398 ; ii. 343, 349, 445, 497 Serpula, i. 247 ; ii. 267 Serres, ii. 547, 553, 570 Sertularia, i. 149; ii. 215 Serum, i. 89 Sesamoid bones, i. 361 Sesise, i. 319 Setse, i. 245 Shark, i. 95, 377, 387; ii. 115, 145, 182, 196, 240, 276, 393,443,511, 537 Sharpey, i. 117; ii. 90 Sheep, i. 4.53; ii. 138, 172,359 Q Q 594 INDEX. Shell, i. 96, 194 Shell, formation of, i. '206 Shell of echinus, i. 189 Shell of lobster, i. 262 Sheltopusic, i. 407 Shrapnell, ii. 381 Shrew, ii. 350 Shuckard, i. 306 ; ii. 432, 435 Shuttle-bone, i. 457 Silica in plants, ii. 38 Silurus, i. 385; ii. 278, 349 Sinistral shells, i. 217 Sinus medianus, ii. 384, 392 Sinus utriculosus, ii. 384 Sinus venosus, ii. 245 Siphonaria, i. 225 Siphuncle, i. 239 Sipunculus, ii. 217 Siren, i. 407 ; ii. 248, 292, 56e> Skate, ii. 274, 443 Skeleton, i, 326 Skeleton of leaf, i. 83 Skimmer, ii. 118 Skin, i. 97 ; ii. 338 Skull, i. 339 Slack, i. 57 ; ii. 45 Sleep, ii. 476 Slips, propagation by, ii. 525 Sloth, i. 441,463; li. 258 Slow-moving quadrupeds, ii. 258 Slug, ii. 430 Smee, i. 332 Smell, ii. 354 Smith (T), ii. 147 Snail, i. 228 ; ii. 430 Snake-lizard, i. 399 Soemmerring, ii. 516 Soils, fertility of, ii. 15 Solar light, ii. 423 Solen, i. 199 Solipeda, i. 456; ii. 170 Solly, ii.zn Sonorous undulations, ii. 371 Sorex, ii. 121, 397,451 Sound, ii. 371 ^ Sound in fishes, i. 382 Soup diet, effects of, ii. 168 Spullanzaiii, i. 53 ; ii. 70, 152, 154, 163, 264,302,509,551 Sparoides, i. 385 Spatangus, i. 186 Spectra, ocular, ii. 467 Spectre of the Brocken, ii. 473 Speed of birds, i. 524 Speed of fishes, i. 386 Speed of quadrupeds, i. 440 Spermaceti, i. 429 Sphenoidal sinus, ii. 358 Spherical aberration, ii. 420 Sphincter muscles, i. 125 Sphinx, i. 311, 319; ii. 191, 223, 491 Spiciila in sponge, i. 140 Spider, i. 252; ii. 391, 433 Spider-crab, ii. 489 Spider-monkey, i. 355; ii. 351 Spinal cord, or marrow, ii. 497 Spine, i, 346 Spines, i. 103 Spino-cerebral axis, i. 345 Spinous process, i. 350 Spiracles, ii. 280 Spiral, logarithmic, i. 218 Spiral stems of plants, i. 78 Spiral threads in plants, i. 59 Spiral valve in fishes, ii. 182 Spiral vessels, i. 64 Spirits, animal, ii. 506 Spirula, i. 216 Spix, i. 356 ; ii. 395, 484 Spleen, ii. 198 Splint bones, i. 457 Spokes, curved spectra of, ii. 466 Spondylus, ii. 429 Sponge,!. 135; ii. 75, 483 Spongioles, i. 69; ii. 18 Spotted cells of plants, i. 60 Spring-tail, i. 266, 267 Spur of cock, i. 518 Spurious legs, i. 280 Squalus. See Shark Squalus pristis, ii. 149 Squirrel, i. 463 Squirrel, palm, ii. 159 Stability of trees, i.'71 Stability. of human frame, i. 477 Stag, i. 450 Stag-beetle, i. 321 INDEX. 595 Stamen, ii. 535 Stapes, ii. 381 Star-fish, ^ee Asterias Starch, i. 59 ; ii. 5 Staunton, ii. 472 Stearine, i. 110 ; ii. 6 Steifensand, ii. 431 Stellated vessels, ii. 313 Stems, vegetable, i. 70 Stemmata, ii. 431 Stentor, ii. 86 Sterno-costal appendices, i. 358 Sternum, i. 358 Stevens, ii. 163 Stigma, vegetable, ii. 280, 535 Stigmata of insects, ii. 280 Sting of bee, i. 315 Sting of nettle, ii. 41 Stipulse, i. 82 Stomach, i. 127 ; ii. 63 Stomata, i. 68 Stones, swallowing of, ii. 153 Stone-wort, ii. 44 Stork, i. 520 Stratiomys, i. 278 Stra.us, i. 269, 288, 289 ; ii. 438 Strawberry tongue, ii. 352 Strepsiptera, i. 313 Striee in shells, i. 219 Strombus, ii. 272 Sturgeon, i. 95, 373 ; ii. 245 Sturiones, i. 385 Styloid bone, i. 457 Stylops, ii. 435 Subbrachieni, i. 377 Suckers, i. 126, 233 Suckers of flies, i. 297 Sudorific glands, i. 100 Sugar, analysis of, ii. 4 Sugentia, i. 266 Sun, action of, on growth of plants, i. 79 Supra-oesophageal ganglion, ii. 487 Surveyor caterpillars, i. 281 Sus iEthiopicus, ii. 145 Sutures, i. 340 Swallow, flight of, i. 513 Swammerdam, ii. 186, 369,431 Swan, ii, 151, 155 Swimming bladder, i. 382 ; ii. 279 Symmetry, lateral, i. 48 ; ii. 505, 546 Symmetry, law of, ii. 553 Sympathetic nerve, ii. 322 Sympathy, ii. 517 Sympathy among ants, ii. 348 Synchaeta, i. 176 Synedra, i. 169 Synovia, i. 89 Syphon, i. 239 Syrphus, ii. 435 Systemic circulation, ii. 209 Tabanus, i. 297 ; ii. 103, 435 Tactual impressions, ii. 463 Tadpole, i. 389 Taenia, ii. 74 Tail, i. 360 Talitrus, ii. 487 Tapetum, ii. 451 Tape-worm, ii. 74 Tapir, i. 460; ii. 351 Tarsus, i. 258, 360 Taste, ii. 352 Teeth, ii. 139 Tegmina of orthoptera, i. 313 Telegraphic eyes, ii. 441 Tellina, i. 202 Temperature, animal, ii. 305 Tendons, i. 92, 124 Tendrils, i. 82 Tension of chords, ii. 374 Tentacula, i. 146, 232 Tenthredo, i. 295 Terebella, i. 248, 249 ; ii. 233, 267 Terebra, i. 222 Teredo, i. 210 Terrestrial larvae, i. 278 Testacella, ii. 286 Testudo, i. 417; ii. 500 Tetrabranchiata, ii. 244 Tetrodon, i. 375, 386 Tetrops, ii. 435 Textures, vegetable, i. 55 Textures, animal, i. 84 596 INDEX. Thetis, ii: 268 Thompson, (J. V.) i. 230 Thomson, (Allen) ii. 542, 547, 567 Thoracic duct, ii. 97, 202 Thorax, i. 357 Thorax of insects, i. 288 Thorns, i. 82 Thought, ii. 460 Threads, elastic, in plants, i. 59 Thysanura, i. 266 Tibia, i. 360 Tiedeniann, i. 184; ii. 216 Tiger, i. 439; ii. 128, 130,351 Tipula, ii. 67 Toad, ii. 318 Tone, musical, ii. 375 Tongue of insects, ii. 110 Tongue, strawberry, ii. 352 Torpedo, ii. 513 Tortoise, i. 41 1 ; ii. 250 Tortryx, i. 399 Toucan, ii. 197, 297, 363 Touch, ii. 338 Trachea, ii. 287 Trachese of insects, ii. 280 Tracheae of plants, i. 64 Tradescantia, ii. 45 Transverse processes, i. 350 Trapezius muscle, i. 125 Trees, ii. 555 Tree-frog, i. 397 Trembley, i. 159; ii. 67, 70, 83, 427 Treviramts, i. 60; ii. 44, 511 Trichechus, i. 431 ; ii. 252, 258 Trichloptera, i. 295 Trichoda, ii. 85 Trigla, ii. 498, 500 Triouyx, i. 421 Tripoh, i. 173 Tristoma, ii. 101 Triton, i. •125, 398 ; ii. 393 Tritonia, ii. 268 Trituration of food, ii. 149 Trochanter, i. 258 Trochilus, ii. 104 Tronc alifere, i. 289 Tropin, ii. 108 Trot, actions in, i. 438 Trout, ii. 179 Trunk of elephant, i. 459 Truxaiis, ii. 344 Tuberous roots, ii. "37 Tubicola?, i. 248 Tubipora, i. 149 Tubularia, ii. 215 Tunicata, ii. 241 Turbinated bones, ii. 357 Turbinated shells, i. 195, 216 Turbo, ii. 113 Tubular form, i. 334 Turkey, ii. 154, 361, 395 Turrited shells, i. 222 Turritella, i. 222 Turtle, i. 411 ; ii. 201, 250, 289, 292, 501 Tusks, ii. 126 Tympanic bones, ii. 380 Tympanic ossicula, ii. 380 Tympanic tube, ii. 385 Tympanum of ear, ii. 378 Type, definite, i. 40 ; ii. 561 Typhlopone, ii. 432 Typhlops, i. 398, 399 Ulna, i. 360 Undulations, sonorous, ii. 371 Ungual bone, i. 361 Unio batava, i. 195 Unity of design, ii. 560, 562 Univalve shells, i. 194 Uranoscopus, ii. 449 Urchin, sea. See Echinus Urtica, ii. 41 Utricle of labyrinth, ii. 384, 392 Uvea, ii. 413 Vacuum a non-conductor of sound, ii. 371 Valentin, i. 115; ii. 196 Valves of lacteals, ii. 201 Valves of shells, i. 194 Valves of veins, ii. 262 Valves of vessels, i. 90 Valve in stomach of horse, ii. 170 Vuinpiro bat, ii. 104 INDKX. 5^>7 Va?i Hasselt, ii. 242 Van Helmont, ii. 13 Vane of feather, i. 508 Variety of forms, i. 10 Variety, law of, i. 40 Vascular plexus, ii, 338 Vascularity of bone, i. 331 Vauquelin, ii. 202 Vegetable kingdom, i. 12, 33 Vegetable organization, i. 55 Vegetable nutrition, ii. 13 Velella, i. 178 Velocity of birds, i. 523 Velocity of salmon, i. 386 • Velocity of sound, ii. 372, 373 Velvet coat of antler, i. 451 Vena cava, ii. 209 Vena portse, ii. 314 Venous sinus, ii. 245 Ventricles of brain, ii, 499 Ventricles of heart, ii, 238 Veretillum, ii, 73, 427 Vertebra, i, 346 Vertebrata, i. 322, 347 Vertebrse offish, i. 370 Verticillated arrangement, i, 78 Vesicles of plants, i, 56 Vessels, i, 90; ii. 10 Vessels, annular, i, Q5 Vessels, beaded, i. 63 Vessels, moniliform, i. 63 Vessels, proper, ii. 43 Vessels, spiral, i. 64 Vestibule of the ear, ii. 381 Vestibular tube, ii. 385 Vibrations, nervous, ii. 506 Vibrio, i. 53 Villi, secreting, ii. 311 Viper, ii. 536 Vision, ii. 398, 419 Vision, erect, ii. 463 Visual perceptions, ii. 462 Vital functions, i. 32 ; ii. 6, 62 Vital organs, ii. 318 Vitality, i. 35 Vitreous humour, ii. 413 Vitreous shells, i. 212 Vitrine auditive, ii. 383 Viviparous animals, ii. 537 Voice, ii. 397 Voltaic battery of torpedo, ii. 513 Voluntary motion, i. 31 ; ii. 475 Voluta, i. 222; ii. 1 13, 431 Volvocina, ii. 427 Volvox, i. 169, 170 Voracity of hydra, ii. 68 Vorticella, i. 165, 169; ii. 84, 523 Vulture, ii. 363 Wading birds, i. 516 Walking, i. 436, 479 Waller, ii. 120 Walrus, i. 423; ii. 126 Warfare, animal, i. 39 ; ii. 60 Warm-blooded circulation, ii. 251 Wasserbar, i. 52 Water-rat, ii. 171 Waterton, ii, 365 Water not the food of plants, ii. 14, 15 Water-beetle. See Dytiscus Water-boatman. See Notonecta Weber, ii, 384, 393, 395, 429 Whale, i, 328, 423, 427, 428 ; ii, 122, 123, 201, 259, 396, 450, 502 Whalebone, ii. 123 Wheutstone, ii. 388, 389 Wheel animalcule, i. 52 Wheel spokes, spectrum of, ii. 466 Whelk. See Buccinum Whiskers, i, 108 ; ii. 351 Whorls of plants, i, 78 Whorls of shells, i. 217 Wild-boar, ii. 144 Willow, spongiole of, i, 69 Wings, i, 308 Winged insects, i. 268 Withers, i. 458 Wolf-fish, ii, 179 Wolff, ii, 545, 548 Wollaston, i, 80; ii. 50, 439, 512 598 liNUEX. Wombat, i. 465 Yarrell, ii. 118, 552 Wood, i. 74 Young, ii. 423, 424 Woodhouse, ii. 25 Woodpecker, i. 515 ; ii. 118 Zebra, i. 456 Woody fibres, i. 66 Zenini, ii. 451 Wool, i. 103 Zoanthus, i. f47, 164 Worms. See Annelida and En- Zoophytes, i. 132, 134; ii. 266 tozoa 426, 483, 529 Zostira, ii. 179 Xiphoid cartilage, i. 359 Zygaena, ii. 246, 344 Zygodactyli, i. 517 FINIS. C. 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