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elaboration. Owing to the differences between the food of animals and that of plants, these processes are by no means identical in their character, but they are analogous in their general relations to the economy. For their respective products, the blood and the descending sap, although chemically different, agree in being the nutritious fluids of their respective organisms, and in containing all that is essential to the maintenance, reparation, and extension of each, and to the formation of their peculiar secretions. Now, the elaboration of the crude materials of the absorbed fluid appears to be effected in animals, partly by the cells (blood-discs and chyle-corpuscles) which float in the stream of fluid, and partly by the cells of which the peculiar glandular bodies, connected with the vascular system, are chiefly made up. Of these glandular bodies, some are placed in the course of the absorbent vessels, so that every particle of fluid is intercepted as it passes along them. Others (as for instance the spleen) are connected with the blood-vessels, and appear to be continually drawing from them the least perfect portion of their fluid, which they pour back into the circulating current, after having subjected it to an elaborating action by the instrumentality of their component cells. The nutritive fluid, thus completely prepared, is appropriated by the several tissues of the body, each selecting that class of materials which it requires for its own growth or renovation; and excepting in the simple fibrous tissues, which appear to be formed by the mere fibrillation (or coagulation with a fibrous arrangement) of the plastic element, every act of development and reparation essentially consists in the production of cells, and in the functions which they perform.
So far, then, there is an essential conformity between the constructive operations of the animal and the plant. And there is this further point of correspondence,—that, in the animal, as in the plant, the more durable portions of the fabric are formed by the means of cells, whose term of existence is comparatively brief. This production of cells, which, as in the case of the leaves of plants, are destined to serve a temporary purpose only, is peculiarly remarkable in animals, from the rapidity of their growth, and the equal rapidity of their decay. A new set of absorbent cells would appear to be developed in the interval between every meal;
and the generation and growth of the elaborating cells can be scarcely less rapid, considering that they all die as soon as they have discharged their products.
We find, however, that there is a limit in animals (save in the Zoophytic tribes) to the extension of the individual structure. As soon as the adult form and dimensions are once acquired, the nutritive operations are for the future concerned solely in their perfect maintenance. This difference is the consequence of the fact, that whilst in living plants there is little or no waste by decay (except in the death of the leaves), a constant decay of tissue is going on in the animal body, with a rapidity proportionate to the energy with which the animal functions are exerted. This fact, though not unrecognised by preceding physiologists, was first brought prominently into notice by Prof. Liebig. An important difference between the animal and vegetable fabrics depends upon it. If the products of these changes were allowed to accumulate in the blood, they would speedily act as deadly poisons : it is requisite, therefore, that they should be regularly eliminated or removed from the body. For this purpose a complex respiratory apparatus is provided. It acts by setting free the carbonic acid, -one of the products of decomposition,-at the same time that it introduces the oxygen which is required for the energetic action of the nervous and muscular tissues. But this interchange of ingredients between the blood and the air is a physical or chemical, not a vital, operation ;-being simply accomplished by the exposure of the blood, in a very finely divided state, to the atmosphere, through the medium of a very thin membrane. It does not involve, therefore, the necessity for cell-agency. But another class of products is separated from the blood in a liquid state, by the agency of the cells, which make up the essential part of all glandular structures. Thus the substance of the liver (with the exception of its blood vessels, absorbents, nerves) and excreting ducts, and some connecting areolar tissue, consists entirely of such cells; and it is their important office, in their growth and development, to separate from the blood the biliary matter which constitutes their appropriate food; and afterwards, on their dissolution, to deliver up their product to the duct, by which it is conveyed into the alimentary canal. All the other glandular structures of animals are formed upon the same plan ;-whether their purpose be to separate from the blood a material which is to answer some special purpose in the economy, such as the gastric fluid for the solution of the food; or whether, like the kidney, to separate those products of decomposition, which are to be at once carried forth from the body. In every case, the selection of the particular ingredients of the circulating fluid is an act of cell-growth, analogous in its own nature to that by which nutritive matter is introduced into the body, though differing in the purpose it is to serve in the general economy. The only secreting function in the animal at all resembling the separating process by which nutritive material is stored up in the cells of the vegetable fabric is the growth of fat. This substance consists of an assemblage of cells resembling those of plants in all essential particulars, but held together by areolar tissue, and surrounded on their exterior by blood vessels. They separate from the circulating fluid its superfluous fatty matters, selecting these from it, just as the cells of the liver select bile, and those of the kidney urea. But instead of discharging these selected products, they keep them stored up in their cavities until there is a demand for them in the system ; when they are again taken back into the current of blood, to be used accordingly.
In this manner, the whole series of the nutritive actions of animals is seen to be but a repetition, under various modifications, of the phenomena of that cell-growth which is also shown to be the life of the simplest plant. It remains to consider, how far the functions peculiar to animals,—those of the nervous and muscular systems,may be included in the same general expression.
The nervous system consists of two different forms of elementary tissue; namely, the tubular fibres of which the nerve-trunks are composed, and the cells which are found in the central masses or ganglia. The tubular fibres are now well understood to be merely concerned in the transmission of nervous agency, their function being analogous to that of the conducting wires of a voltaic battery; they consequently bear some analogy to the vessels by which blood and other fluids are conveyed through the system. An uninterrupted continuity is indispensable to the performance of their function; and the function itself seems
at least as much allied to physical, as it is to vital agency. On the other hand, the nervous power,
be its nature, is generated by the agency of the cells, which are the essential components of the ganglionic centres, and which are also found at the origin of certain nerves that proceed from the organs of sense towards these centres. Here, then, in that class of actions which seems the most removed of all the animal functions from the simple operations of vegetation, we find that the life and growth, the death and succession, of cells, still constitutes the essential part of the phenomena.
And, lastly, the most recent inquiries have shown that the muscular system forms no exception to the general rule ; for this, too, in its ultimate analysis, is reduced to the same organic elements. There is no difficulty in showing that any muscle may be unravelled, as it were, into a vast number of bundles of fibres; and further, that, in the most perfect form of muscular structure, these fibres are tubular, and contain fibrillæ of extreme minuteness, which are, in fact, the real elements of the tissue. The peculiar markings upon these fibrillæ have been a source of much perplexity to microscopists, and many discrepant opinions have been entertained regarding them. Not only on this account, but also as being the last tissue which remained to be resolved, a peculiar interest attached to the question of its intimate nature. This interest was yet further increased by the consideration that, if it should prove to be cellular, the generalization, which had
previously included only the organic or vegetative functions, might now comprehend the animal functions also. The problem was resolved simultaneously and independently by Professor Sharpey and Dr. Carpenter. Each fibrilla of muscle was discovered by them to be in reality a series of cells arranged in a linear series, like those of a Conferva ; and the contraction of the muscle was shown to be simply due to an instantaneous change in the form of the component cells of the fibrillæ. The motions of the leaf-stalks
. of the sensitive-plant and Dionæa, those of the filament of the berberry, and others of like nature, all result from a change in the form of the component cells of the tissues of these parts, the change being due to the direct action of the stimulus by which they are called forth. The contraction of a muscular fibre may, in like manner, be excited by a mechanical or electrical stimulus immediately applied to it. The peculiarity which distinguishes it is its connection with the nervous system, and, through the nervous system, with the mind.
Thus, then, in all those material changes through whose instrumentality the mind is rendered conscious of the impressions of external objects; in those, too, which seem essential in our present state of being to the workings of the mind itself; and in those by which it reacts upon the world around it, by means of the movements which it excites in its corporeal framework; we have still the same agency at work.
With regard to the reproductive functions of animals, we must content ourselves with the simple statement that, whether in the extension of the parent structure by gemmation, in the detachment of buds capable of maintaining an independent existence, or in the origination of an entirely new generation, the foundation is invariably laid by the growth of cells, which may subsequently undergo more or less of transformation, according to the nature of the fabric to be evolved. Of the last-named of these processes—the origination of an entirely new generation-it would appear that here too, as in plants, the essential part consists in the mixture of the contents of two distinct cells, out of which combination the embryonic structure is built up. The embryo, when first distinguishable within the ovum, consists of a pair of cells. This soon multiplies itself, precisely after the fashion of the simplest cellular plant, so as to form a mass resembling a mulberry in shape and appearance, when highly magnified. And the component cells of this mass, which are at first precisely similar to each other in size and aspect, after a time present indications of the commencement of those changes by which they are gradually to be converted into the diverse organs and tissues of the complete creature. During the whole history of embryonic development, however, as in the nutrition and maintenance of the adult fabric, we recognise the same agency in continued operation ;-every stage of the process being referable, like all the other manifestations of life, to the act of cell-growth.
Thus, then, the same simple and comprehensive expres