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exclusively organic origin, but they resemble the latter in their crystallizability and their definite chemical composition; in consequence of which their chemical investigation may be pursued in nearly the same manner, and their chemical changes expressed in nearly the same terms. But the proximate principles of the third class are in every respect peculiar. They have an exclusively organic origin; not being found except as ingredients of living or recently dead animals or vegetables. They have not a definite chemical composition, and are consequently not crystallizable; and the forms which they present, and the chemical changes which they undergo in the body, are such as cannot be expressed by ordinary chemical phraseology. This class includes such substances as albumen, fibrin, casein, &c.

CHAPTER II.

PROXIMATE PRINCIPLES OF THE FIRST CLASS.

THE proximate principles of the first class, or those of an inorganic nature, are very numerous. Their most prominent characters have already been stated. They are all crystallizable, and have a definite chemical composition. They are met with extensively in the inorganic world, and form a large part of the crust of the earth They occur abundantly in the different kinds of food and drink; and are necessary ingredients of the food, since they are necessary ingredients of the animal frame. Some of them are found universally in all parts of the body, others are met with only in particular regions; but there are hardly any which are not present at the same time in more than one animal solid or fluid. The following are the most prominent of them, arranged in the order of their respective importance.

1. WATER. Water is universally present in all the tissues and fluids of the body. It is abundant in the blood and secretions, where its presence is indispensable in order to give them the fluidity which is necessary to the performance of their functions; for it is by the blood and secretions that new substances are introduced into the body, and old ingredients discharged. And it is a necessary condition both of the introduction and discharge of substances naturally solid, that they assume, for the time being, a fluid form; water is therefore an essential ingredient of the fluids, for it holds their solid materials in solution, and enables them to pass and repass through the animal frame.

But water is an ingredient also of the solids. For if we take a muscle or a cartilage, and expose it to a gentle heat in dry air, it loses water by evaporation, diminishes in size and weight, and becomes dense and stiff. Even the bones and teeth lose water by evaporation in this way, though in smaller quantity. In all these solid and semi-solid tissues, the water which they contain is useful

by giving them the special consistency which is characteristic of them, and which would be lost without it. Thus a tendon, in its natural condition, is white, glistening, and opaque; and though very strong, perfectly flexible. If its water be expelled by evaporation it becomes yellowish in color, shrivelled, semi-transparent, inflexible, and totally unfit for performing its mechanical functions. The same thing is true of the skin, muscles, cartilages, &c.

The following is a list, compiled by Robin and Verdeil from various observers, showing the proportion of water per thousand parts, in different solids and fluids:

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According to the best calculations, water constitutes, in the human subject, between two-thirds and three-quarters of the entire weight of the body.

The water which thus forms a part of the animal frame is derived from without. It is taken in the different kinds of drink, and also forms an abundant ingredient in the various articles of food. For no articles of food are taken in an absolutely dry state, but all contain a larger or smaller quantity of water, which may readily be expelled by evaporation. The quantity of water, therefore, which is daily taken into the system, cannot be ascertained in any case by simply measuring the quantity of drink, but its proportion in the solid food, taken at the same time, must also be determined by experiment, and this ascertained quantity added to that which is taken in with the fluids. The entire quantity of water so introduced during twenty-four hours varies according to the researches of M. Barral' from 3 to 4 pounds.

After forming a part of the animal solids and fluids, and taking part in the various physical and chemical processes of the body, the water is again discharged; for its presence in the body, like that of all the other proximate principles, is not permanent, but only

In Robin and Verdeil, vol. ii. p. 139.

temporary. After being taken in with the food and drink, it is associated with other principles in the fluids and solids, passing from the intestine to the blood and from the blood to the tissues and secretions. It afterwards makes its exit from the body, from which it is discharged by four different passages, viz., in a liquid form with the urine and the feces, and in a gaseous form with the breath and the perspiration. Of all the water which is expelled in this way, about 48 per cent. is discharged with the urine and feces,' and about 52 per cent. by the lungs and skin. This estimate, however, is an average, calculated from the observations of different authors upon different individuals. The absolute and relative amount of water discharged, both in a liquid and gaseous form, varies according to circumstances. There is particularly a compensating action in this respect between the kidneys and the skin, so that when the cutaneous perspiration is very abundant the urine is less so, and vice versa. The quantity of water exhaled from the lungs varies also with the state of the pulmonary circulation, and with the temperature and dryness of the atmosphere. The water is not discharged at any time in a state of purity, but is mingled in the urine and feces with saline substances which it holds in solution, and in the cutaneous and pulmonary exhalations with animal vapors and odoriferous substances of various kinds. In the perspiration it is also mingled with saline substances, which it leaves behind on evaporation.

2. CHLORIDE OF SODIUM.-This substance is found, like water, throughout the different tissues and fluids of the body. The only exception to this is perhaps the enamel of the teeth, where it has not yet been discovered. Its presence is important in the body, as regulating the phenomena of endosmosis and exosmosis in different parts of the frame. For we know that a solution of common salt passes through animal membranes much less readily than pure water; and tissues which have been desiccated will absorb pure water more abundantly than a saline solution. It must not be supposed, however, that the presence or absence of chloride of sodium, or its varying quantity in the animal fluids, is the only condition which regulates their transudation through the animal membranes. The manner in which endosmosis and exosmosis take place in the animal frame depends upon the relative quantity of all the ingre

1 Op. cit., vol. ii. pp. 143 and 145.

dients of the fluids, as well as on the constitution of the solids themselves; and the chloride of sodium, as one ingredient among many, influences these phenomena to a great extent, though it does not regulate them exclusively.

solution of various Thus, in the blood perhaps also of the

It exerts also an important influence on the other ingredients, with which it is associated. it increases the solubility of the albumen, and earthy phosphates. The blood-globules, again, which become disintegrated and dissolved in a solution of pure albumen, are maintained in a state of integrity by the presence of a small quantity of chloride of sodium.

It exists in the following proportions in several of the solids and fluids:

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In the blood it is rather more abundant than all the other saline

ingredients taken together.

Since chloride of sodium is so universally present in all parts of the body, it is an important ingredient also of the food. It occurs, of course, in all animal food, in the quantities in which it naturally exists in the corresponding tissues; and in vegetable food also, though in smaller amount. Its proportion in muscular flesh, however, is much less than in the blood and other fluids. Consequently, it is not supplied in sufficient quantity as an ingredient of animal and vegetable food, but is taken also by itself as a condiment. There is no other substance so universally used by all races and conditions of men, as an addition to the food, as chloride of sodium. This custom does not simply depend on a fancy for gratifying the palate, but is based upon an instinctive desire for a substance which is necessary to the proper constitution of the tissues and fluids. Even the herbivorous animals are greedy of it, and if freely supplied with it, are kept in a much better condition than when deprived of its use.

The importance of chloride of sodium in this respect has been well demonstrated by Boussingault, in his experiments on the fattening of animals. These observations were made upon six

1 Robin and Verdeil.

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