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DISTINCTIVE CHARACTERS OF ANIMALS.

many which are fixed to one spot during nearly their whole lives, and which grow and extend themselves like plants, the movements of the body are but few in number, and trifling as to their variety; these movements are only destined to assist in the performance of the organic functions, as by bringing food to the mouth, and water to the respiratory organs; and the nervo-muscular apparatus by which they are effected, bears so small a proportion to the organs of nutrition, as to seem like a mere appendage to them, and is sometimes altogether undiscoverable. This is the case, for example, in the lowest kinds of shell-fish, such as the Oyster, and in the Coral-polypes.

12. Hence we perceive, as we descend the Animal scale, a nearer and nearer approach to the character of Plants; and this we shall find to be the case, not only in the general arrangement of the organs, but also in the nature of the elementary tissues of which these are composed. For in the higher animals, the whole organism is constructed in such a manner as to admit a free motion in its individual parts. The different portions of the skeleton or hard framework are connected with each other by flexible ligaments, which are adapted to resist a very powerful strain; the muscles are attached to these by fibrous cords or tendons, which, also, can support a vast weight; and the several muscles and other parts, which need to be mutually connected, but also require a certain power of moving independently of one another, are bound together by a very elastic loosely-arranged tissue, consisting of fibres crossing and interlacing in every direction, the interstices between which are filled with fluid. Now to these fibrous tissues, there is nothing analogous in plants, because no freedom of motion is required, or even permitted, among their parts; and we find them bearing a less and less proportion to the whole, as we descend the animal scale. On the other hand, we find the various forms of true cellular tissue, such as predominate in plants (VEGET. PHYS. Chap. III.), becoming more and more abundant, as we pass from the highest to the lowest animals, and having more and more important duties to fulfil. But even in the highest Animals, as will hereafter appear, they are the immediate instruments of the most important among the organic functions, just as they are in Plants.

CHEMICAL CONSTITUENTS:-ALBUMEN.

Chemical Constitution of the Animal Body.

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13. By far the larger proportion of the Animal fabric is formed at the expense of the substance termed Albumen; the composition and properties of which, therefore, claim our first attention. The fundamental importance of albumen in the animal economy, is shown by the fact that it constitutes, with fat, and a small proportion of certain mineral ingredients, the whole of that mass of nutrient material stored up in the eggs of oviparous animals, which, being appropriated by the germ to the building up of its fabric, is converted by it into the bones, muscles, nerves, tendons, ligaments, glands, membranes, &c. of the embryo. We find it also constituting a large proportion of the solid matter of the blood and other nutrient fluids of the adult animal; and it is the fundamental form to which the various azotized substances employed as food (§ 153)—such as animal flesh, or the gluten of bread— are first reduced by the act of digestion. It is composed of 49 carbon, 36 hydrogen, 14 oxygen, 6 nitrogen, with a minute proportion of sulphur; it is generally blended, also, with more or less of fatty matter, and with saline and earthy substances.

14. Albumen may exist in two states,-the soluble and insoluble. In the animal fluids it exists in its soluble form; and is united (as an acid to its base) with about 1 per cent. of soda, forming an albuminate of soda. It is not altered by being dried at a low temperature, but still retains its power of being completely dissolved in water. When a considerable quantity of it exists in a fluid (as in the white of the egg), it gives to it a glairy tenacious character; but it is nearly tasteless. When such a fluid is exposed to a temperature of about 150°, a coagulation or 'setting' takes place, as in the familiar process of boiling an egg. But if the albumen be present in smaller quantity, the fluid does not form a consistent mass, but only becomes turbid; and this only after being boiled. Albumen which has been dried at a low temperature, however, may be heated to the boiling point of water, without passing into the insoluble condition; a fact which is of peculiar interest in relation to the power which the Tardigrada (ZOOL. § 841) possess, of sustaining a very high temperature without the loss of their vitality, when

32

CHEMICAL CONSTITUENTS:-ALBUMEN, CASEIN.

their bodies have been completely dried up in the first instance. No trace of organization can be detected in coagulated albumen, which seems to be composed only of a mass of granules; and in this respect it differs in an important degree from fibrin-as we shall presently see. Albumen may also be made to coagulate readily by the action of acids, especially the nitric (aqua-fortis); so that a very small quantity of it may be detected in water, by the turbidity produced by adding to it a drop or two of nitric acid, and then heating it. Now, when thus coagulated, albumen cannot be dissolved again by any ordinary process; but its solution may be accomplished by rubbing it in a mortar with a caustic alkali, potass or soda. From this solution it may be precipitated again on the addition of an acid in sufficient quantity to neutralise the alkali. Albumen is distinguished, then, by its peculiar property of coagulating on the application of heat, or on being treated with certain acids.

15. Nearly allied to albumen is the substance termed Casein, which replaces it in milk; and this is specially worthy of notice here, because it is the sole form in which the young Mammal receives albuminous nourishment during the period of suckling, in which it draws its sustenance from its parent. Like albumen, this substance may exist in two forms, the soluble, and the insoluble or coagulated; and the presence of a small quantity of free alkali seems essential to its continuance in the soluble form. Casein differs from albumen, however, in this, that it does not coagulate by heat, and that it is precipitated from its solution by organic acids, such as the acetic and lactic, which have no coagulating action on albumen. It is further remarkable for the facility with which its coagulation is effected by the contact of certain animal membranes; as we see when a small piece of rennet (which is the dried stomach of the calf) is put into a large pan of milk in the process of cheese-making, the 'curd' which then separates being composed of casein entangling the oily particles of the milk. In the coagulated state, casein differs but very little from albumen, and is readily converted into it by the gastric fluid. It is remarkable for its power of dissolving the earthy phosphates, as much as 6 per cent. of phosphate of lime being usually obtainable from it; and it is in this combination, that the large quantity of bone-earth

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CHEMICAL CONSTITUENTS:-CASEIN, SYNTONIN, FIBRIN. required for the consolidation of the skeleton of the young animal, is introduced into its system. A substance resembling casein is obtainable from the serum of the blood, especially in pregnant females; and also from the serous fluid which occupies the interstices of the tissues. It is found, also, mingled with albumen, in the yolk of the egg, forming a compound which (before its true character was known) has been distinguished as vitellin. Now as all the liquids containing casein have it for their special function to supply formative materials to rapidly-growing tissues, we may with much probability regard it as still more closely related to them than is albumen itself. It differs from albumen but little, if at all, in the ultimate proportions of its elements (§ 13).

16. The substance of which muscles are composed, has been commonly considered to be Fibrin (§ 17); but it differs essentially from fibrin in its properties, and is now distinguished as Syntonin. Its chief peculiarity is its solubility in very dilute muriatic acid (1 part to 100 of water), and its precipitation in the form of a jelly when the acid is neutralised; this jelly treated with dilute alkalies forms a solution which coagulates by heat; and thus it seems to be reduced nearly to the condition of albumen. This is, in fact, very much what takes place in the act of digestion of flesh-meat; the muscle-substance being first dissolved by the muriatic or other acid of the gastric fluid, and the solution being then rendered alkaline by the mixture of bile and other secretions in the small intestine.

17. In the blood and other nutrient fluids of the animal body, there is found a substance which is so closely related to albumen in its ultimate chemical composition, as not to be distinguishable from it with any certainty; but which, though fluid whilst circulating in the living vessels, coagulates spontaneously after having been for a short time withdrawn from them, the coagulum or clot being distinguished from that of albumen or fibrin by the fibrillar arrangement of its particles, which indicates an incipient organization. This substance, termed Fibrin, may be obtained in a separate form, by stirring fresh-drawn blood with a stick, to which it adheres in threads. In this condition it possesses the softness and elasticity which characterise the flesh of animals, and contains about three-fourths of its weight of water. It may be

D

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CHEMICAL CONSTITUENTS:-FIBRIN.

deprived of this water by drying, and then becomes a hard and brittle substance; but, like dried flesh, it imbibes water again when moistened, and recovers its original softness and elasticity. From the recent experiments of Dr. Richardson, it appears that the coagulation of blood-fibrin depends upon the escape of ammonia, being accelerated by such conditions as favour the liberation of this gas, and retarded or prevented by such as cause its retention in the liquid; whilst, even after the clot has been formed, it may be dissolved by ammonia, forming again when that gas is set free. Fibrin differs from syntonin or muscle-substance in not being dissolved by very dilute muriatic acid, but being merely caused to swell up into a gelatinous mass, which contracts again when more acid is added. It combines with the earthy phosphates, of which as much as 24 per cent. is sometimes found in the ash left by its combustion.

18. There can be no doubt that fibrin is formed in the blood and in the other fluids in which it presents itself, at the expense of albumen. What is its precise destination, cannot as yet be clearly specified; but there are several circumstances which point to the conclusion that it is to be regarded as a transitional stage in the metamorphosis of albumen into the simple fibrous tissues (§ 23.) Thus, when the ordinary clot of blood is examined microscopically, it is found to consist, not, like an albuminous coagulum, of a homogeneous mass of granules, but of a network of imperfectly-formed fibres, enclosing the red corpuscles in its interstices. A much more distinct network of the same kind may be seen in the colourless coagulum formed by the liquid which may be skimmed off the surface of the blood drawn from persons suffering under any severe inflammation; such blood coagulates slowly, and its red corpuscles and the fluid in which they float have an unusual tendency to separate from each other; and the fibrin previously dissolved in the latter sets into definite fibres, which continue for some days to increase in firmness. It is a liquid of the same kind, charged with fibrin in a peculiarly "plastic" condition, that is poured forth for the formation of new tissue when the reparative processes are at work for the healing of a wound or the reunion of divided parts; and it is by a plug of coagulated fibrin, which gradually comes to present a more and more