In considering the various stages of the nu tritive process in animals, we shall do well to bear constantly in mind the leading facts in re gard to the same process in the simplest cellu lar plant : for we shall find that the elementary parts of the most complex animal organism go through a series of changes essentially the same; so that the type of the function is everywhere uniform, notwithstanding the vast apparent dif ferences in the mode in which it is performed. The cell of the red snow or yeast plant, for instance, is developed from an almost imper ceptible germ, by its own power of attracting to itself certain nutritive materials in its neigh bourhood, which it combines into the new forms required both for its own grow th and in crease, for the elaboration of certain peculiar matters contained in its cavity, and for the pro duction of the germs of new cells; and these. being liberated in time by the death of their parent, go through, in their turn, the same series of changes. We shall now trace these changes in the highest and most complex form in which they are presented to us ;—that is, as they oc cur in man, or any vertebrated animal.
Elaboration ty. organizable materials.—The alimentary substances taken in by the absorbent vessels require to undergo very important changes within the body, before they can be applied to the nutrition of its structure. The chief constituents of the chyle, as at first ab sorbed, are albumen and tia ; the former is destined to be converted into the material of the solid tissues ; the latter is chiefly designer' for the maintenance of the animal tempemture, by the combination it is made to undergo with the oxygen introduced through the lungs. It is questionable, as already explained, whether fatty matter, or any other non-izotized com pound, can ever be applied to the nutrition of the animal body. Even if it should be ever proved to be subservient to the re-construction of the azotized tissues, there can be no doubt that it must have been first converted into an albuminous compound—that is, into some mo dification of protein ; and as the evidence that such a transformation ever takes place is far from being satisfactory, we have as yet ncr data for examining the mode in which it is effected. We shall, therefore, consider albumen as the starting-point of the animal tissues, and shall endeavour to trace, so far as t'ne present state of our knowledge admits, the processes by which this is converted into the organized fabric.
In this assumption we seetn justified by two very obvious considerations. First, in the egg of a bird, (or any other oviparous animal,) we find that, putting aside the fatty. matter of t yolk, albumen is the sole organic compound, the expense of which all its tissues are to formed ; so that, by the wonderful processes chemical and vital transformation, which ta place during the period of incubation, the bumen which it conMined at first is me morphosed into bone, cartilage, nerve, mr • tendon, ligament, membrane, areolar gelatinous matter horny substance, f &c., &c. Secondy, a similar metamo appears to be continually taking place body of the adult animal ; for every compound employed as food appears to duced to the form of albumen in the digesti process,; so that this becornes the essential co stituent of whatever fluid is absorbed for t nutrition of the tissues. It is true that gelati taken in as food, may be absorbed and carri into the current of the circulation ; but there no doubt that it is altogether incapable of bei applied to the re-constniction of any but t gelatinous tissues; and, as already stated, seems questionable whether, even iu these, exists in a condition that can rightly he termed organised. Moreover, as it is clear that the gelatinous tissues may be formed at the ex pense of albumen, we are justified in regard ing this substance as the common pabulum for all.
In order to form a definite conception of the nature of the transformations, which this prin ciple is destined subsequently to undergo, it is important to bear in mind, in limine, that al bumen cannot be regarded as possessed of any properties that characterize it as a vital com pound—or, in other words, that essentially distinguish it from compounds of an ordinary chemical nature. In its coagulability by heat or by acids—in its combination with alkalies as an acid, or with acids as a base--and in the absence of any power of spontaneously passing into forms more decidedly organic than the granules which are seen when it is made to coagulate slowly—it is closely analogous to many substances which belong to the domain of inorganic chemistry. It appears then, to be
totally unpoisessed of the property of plasticity; by which we mean the power of being at once converted into organised tissue : so that any deposit, whether fluid or solid, which mainly consists of albuminous matter, must be regard ed as aplastic. This is a principle of great importance, as we shall see further on.
Before albumen is ready to Ile appropriated by the tissues as the material for their nutri tion, it must undergo a very important change— not so much, however, in its chemical compo sition, as in the re-arrangement of its particles in a new mode, by which its properties are es sentially changed. There seems reason to believe that, in ahe proportions of its ultimate elements, it is identical with the substance termed fibrin, into which it is changed during its passage through the chyliferous and sangui ferous vessels. [See ALBUMEN and FIBRIN.] But there are such decided and well-marked differences between these two compounds, as indicate that they fulfil entirely different pur poses in the animal economy ; and that whilst, chemically speaking, they are isomeric, the fibrin is endowed with properties of a distinctly vital character—that is, altogether different from any with which mere chernistry brings us acquainted. One of the most obvious mani festations of this difference is the property which is universally regarded as distinctive of fibrin—its tendency to coagulate spontaneously when withdrawn from the living vessels, and to pass into the form of a tissue more or less definitely organised.' As will presently be shown, the completeness of this transformation depends upon two circumstances in particu lar ;—the perfect elaboration. of the fibrin it self, and the vitality of the surface upon which the concretion takes place. When the fibrin is highly elaborated, it will coag-ulate in the form of a definite network of minute even upon a dead surface, as a slip of glass ; this is the case, for instance, with the fibrin of the buffy coat of the blood, or with that of the liquor sanguinis (coagulated lymph,) poured out for the reparation of an injured part. But in the ordinary fibrin of the blood, the fibrilla tion is less distinct, when the concretion takes place upon a dead surface. When ,it occurs in contact with a living surface, however, the co agulation takes place more graduAlly ; and it seems as if the particles, having miim time to arrange themselves, become aggregated into more definite forms, so that a more regular tissue is produced—just as crystals are most perfectly formed, when the crystalline action takes place slowly. It was formerly imagined, that the muscular tissue is the only one pro duced at the expense of the fibrin of the blood; the other tissues being formed from its albumen. This, however, is unquestionably erroneous. There is no proof whatever, that albumen, as long as it remains in that condition, ever be comes organised; whilst, on the other hand, there is abundant evidence that the plasticity of any fluid deposit—that is, its capability of being metamorphosed into organised tissue—is in direct relation with the quantity of fibrin which it contains. Thus the liquor sanguinis or coagulated lymph, thrown out for the repa ration of injuries, contains a large amount of fibrin ; and this substance is converted, not at first into muscular fibre, but (whatever may be the tissue to be ultimately produced in its place) into a fibrous network, which filk up the breach, and holds together the surrounding structure. This may be regarded as a simple form of areolar tissue, which gradually be comes more perfectly organised by the exten sion of vessels and nerves into its substance, and in which other forms of tissue rnay subse quently make their appearance. This process will be more particularly described hereafter; it is at present noticed here, as an illustration of the general fact, thatfibrin is to be regarded as the plastic element of the nutritive fluids.