922

CHEMICAL NATURE OF THE FOOD OF ANIMALS.

has amply demonstrated that no one of these principles can be wholly withheld from any animal of the higher orders for a lengthened period without producing fatal results. It has been shown, for example, when animals are fed exclusively upon sugar, upon gelatin, or upon albumin, that they die of starvation; although, when these different materials are given in mixture, the animal lives and thrives. It has also been observed that those aliments which are admitted to be the most nutritious, and which are most largely used as articles of food, are those which contain all these principles in due proportion. A good example of this kind is afforded in bread made from wheaten flour: its starch and sugar represent the saccharine principle, its gluten furnishes the azotised or albuminous material; water it of course contains; and it also furnishes the phosphates and other salts of the alkalies and earths, including sodic chloride. It likewise contains small quantities of fatty or oily matter, though the amount of this constituent is hardly adequate to the wants of the system: this deficiency is, however, supplied by the common habit of eating it with butter, milk, fat bacon, or some other oleaginous substance.

Animals appear to possess to some extent the power of deoxidizing saccharine matter and converting it into fat. The removal of a certain number of atoms of carbonic anhydride, of water, and of oxygen, from glucose would furnish the elements of the ordinary forms of fat; for example:

20 € H12 = 27H1106 + 6€Ð1⁄2 + 10 H ̧Ð + 43 →1⁄2.

12

An interesting illustration of the artificial conversion of sugar into a body belonging to the series of the fats, is afforded in the production of butyric acid by the fermentation of a solution of sugar under the influence of casein (1285); and a striking proof of the convertibility of sugar in the animal economy into a fatty substance, is given by the experiments of Gundelach and others upon the feeding of bees. In these experiments it was found that the insects, when supplied with pure sugar only, secreted wax in abundance; about 20lb. of sugar being consumed by the bees whilst they were producing 1lb. of wax.

The exact relative value of the saccharine and oleaginous principles of the food is not clearly understood, but one important difference between their functions is made evident, in the mode in which fat admits of being stored up in the economy in a form which allows of its being subsequently absorbed when needed:

LIEBIG'S CLASSIFICATION OF THE COMPONENTS OF FOOD. 923

fat thus appears to serve as a sort of fly-wheel in the nutritive

economy.

(1719) Liebig's Classification of the Components of Food.The great objects of the supply of food were regarded by Liebig as twofold, viz., 1. the renewal of the tissues after they have become effete; and 2. the maintenance of animal heat. The constituents of the food were chiefly considered by him in their relation to these two great functions, and were subdivided into plastic nutritive materials, which consist exclusively of the albuminoid principles, and into respiratory food, including the saccharine and oleaginous principles of Prout. Liebig estimates that some of the more important materials used as food contain these two principal modifications in the following proportions; what he termed the respiratory materials being all calculated, for the convenience of comparison, in the quantities of starch to which they would be equivalent :

There is, however, a third great purpose to be attained by the assimilation of food, viz., the maintenance of the supply of energy, which by the actions of the living body becomes transformed into nervous or muscular power. The share which each of the principal constituents of the food of animals has in contributing to muscular force has within the last few years excited the close scrutiny of the physiologist and the chemist, and has not hitherto been decided (1726).

Liebig, and those who adopt his views, regard fat, sugar, starch, and alcohol, as articles which supply respiratory materials only; and he remarks that when from external circumstances, such as exposure to cold, there is a greater demand upon the respiratory function, there is an instinctive craving for additional quantities

924

RESPIRATORY MATERIALS OF FOOD.

of food which furnish those substances, by which the temperature of the body is maintained above that of the surrounding air: the Laplander and the Greenlander, for instance, drink train-oil, and are more prone to excess in spirits than the dweller in a more temperate climate. Alcohol, it must be observed, is not simply food it acts as a powerful stimulant upon the nervous system, and hence its effects are complicated. The non-azotised ingredients of the food which find their way into the blood do not leave the body until they have, by conversion into water and carbonic anhydride, experienced complete oxidation, so as to yield up in the body all their potential energy as actual energy.

The azotised ingredients of the food likewise evolve heat in the act of oxidation; but since these materials constitute the more highly elaborated, and less abundant, yet indispensable components of the food of animals, their consumption merely in the supply of heat must be considered as extremely wasteful; whilst at the same time these bodies are less perfectly adapted than the non-azotised constituents of the food for the maintenance of animal heat. Moreover, the azotised matters are chiefly excreted in the form of urea, a substance which is still combustible, and carries off a heat-producing power, amounting, according to Frankland's experiments, to about one-third of that of an equal weight of dry muscle.

The

From the table above quoted it appears that in milk and in the different varieties of corn (which are the most perfect forms of nutritive matter), the proportion which the plastic bear to the respiratory materials are, for I of the former, to from 3 to 6 of the latter. The proportions of these components, however, vary considerably in the food of different classes of animals, and their organs of digestion are modified to meet these differences. carnivora, for example, live on the flesh and blood of other animals, which in chemical and physiological properties are identical with their own, and which after digestion in the stomach are reduced to a soluble condition; in this form they are absorbed, and enter into the composition of the animal. In the herbivora the organs of digestion are more complicated, since their food contains only small quantities of nitrogen. It is found, on the whole, that animals of equal weight require more or less food, in proportion as the compounds which contain nitrogen are less or more abundant in it. The nitrogen occurs in the food principally in the form of compounds which have the same composition as the albumin and fibrin of animals. Vegetable albumin and fibrin are the principles which furnish plastic nutriment to the herbivora,

PROCESS OF DIGESTION.

925

and if they be withheld for any protracted period, starvation ensues. Hence it appears that one essential condition to the maintenance of animal life is the assimilation of certain materials, which are identical in constitution with the principal constituents of the blood. Animals do not possess the power of forming these principles, yet they produce a multitude of others from them by their decomposition; for example, plants cannot produce the gelatinous principles, neither do they furnish the constituents of the brain, both of which are elaborated iu the animal frame from the components of the food. The azotised compounds, when taken alone, are insufficient to support life; saccharine and oleaginous matters are absolutely necessary, and in the young, even of carnivora, they form, in the shape of milk, a most important part of their nutriment. Even the flesh diet of the carnivora contains a large proportion of fat, which supplies the necessary material of this kind.

In our estimate of the importance of the different materials which enter into the composition of food, we must not overlook the various salts; particularly common salt, calcic phosphate,' and the sulphates and phosphates of the alkali-metals. Common salt is essential for the supply of the hydrochloric acid of the gastric juice, and the sodium of the bile. Calcic phosphate is necessary to the building up of the bones; and not a cell or a fibre of the body can be formed without the presence of the soluble phosphates.

Liebig has pointed out the singular manner in which these different salts are associated in the different parts of the body; sodic chloride, for instance, abounds in liquids which, like the blood and the serous exudations, have an alkaline reaction, and is commonly accompanied by trisodic phosphate. Potassic phosphate, on the other hand, occurs in liquids which, like the juice of the flesh, have an acid reaction.

(1720) Process of Digestion.-The preliminary cooking to which much of our food is submitted is not essential to the process of digestion, but is generally resorted to in order to please the palate; but where a mixture of many kinds of food is to be assimilated by the stomach, the process of cooking may be made to assist the operations of digestion. The principal steps by which the food, after its reception into the mouth, is converted into a part of the living organism, are the following:-The food is first masticated and thoroughly mixed with the saliva. Animals which are carnivorous chew their food less completely, and are supplied less abundantly with saliva, than those that are herbivorous. The reason is obvious, since in the carnivora, the fleshy nature of their

926

PROCESS OF DIGESTION.

aliments enables the latter to be more readily assimilated than is the case in the herbivora, many of which, by ruminating, subject their food a second time to a thorough trituration and intermixture with the saliva. Birds that are granivorous are furnished with a strong hollow muscle, the gizzard, which is especially designed for the purpose of triturating the seed, to aid which, the birds swallow pebbles and gravel. The food, having been masticated and reduced to a pulp by intermixture with the saliva, is next transferred to the stomach, where it becomes mingled with the acid secretion of the gastric juice (1689). After remaining for a few hours in this organ, the principal proportion of the albuminous and fibrinous constituents of the food is brought into the liquid form; but the fatty matters remain unaffected, and the starchy components are but imperfectly converted into the soluble form of dextrin, or of sugar. From the stomach, the food, now reduced to a pultaceous mass termed chyme, passes to the duodenum, or commencement of the intestinal canal, where it is mixed, first with the secretion from the liver, and then with that from the pancreas.

Physiologists are not agreed upon the share which the bile takes in the function of digestion, but both the bile and the pancreatic secretion are alkaline, and the chyme, immediately after admixture with these liquids, assumes the appearance of an emulsion; in its further passage along the intestinal canal, it is mixed with a secretion from the inner surface of the jejunum and of the ileum, which still further modifies the food, completing the conversion of starch into a soluble compound, and rendering the fatty matters capable of assimilation. In the small intestines a separation of the aliment into two portions is effected; the larger of these portions is absorbed into the system, the smaller portion, consisting of vegetable fibre and insoluble and undigested matters, becomes excrementitious, acquiring a peculiar odour, and is ultimately rejected. The absorbed portion is taken up partially as a thin, milky liquid (the chyle), which is absorbed from the inner surface of the intestines by the lacteals, and these vessels empty themselves through the thoracic duct, into the subclavian vein: a still larger portion is absorbed by the mesenteric veins, which transmit their contents to the liver, to be there further elaborated before they are transferred by the hepatic veins to the right auricle of the heart.

One principal change wrought in the food, during its conversion into chyme, consists in its being rendered soluble, and combined with water; but it is evident that the mere fact of the conversion