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In ordinary flesh or butcher's meat, we have the albuminoid matter of the muscular fibre and the fat of the adipose tissue.

COMPOSITION OF ORDINARY BUTCHER'S MEAT.

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From what has been said above, it will easily be seen that the nutritious character of any substance, or its value as an article of food, does not depend simply upon its containing either one of the alimentary substances mentioned above in large quantity; but upon its containing them mingled together in such proportion as is requisite for the healthy nutrition of the body. What these proportions are cannot be determined from simple chemical analysis, nor from any other data than those derived from direct observation and experiment.

The total quantity of food required by man has been variously estimated. It will necessarily vary, indeed, not only with the constitution and habits of the individual, but also with the quality of the food employed; since some articles, such as corn and meat, contain very much more alimentary material in the same bulk than fresh fruits or vegetables. Any estimate, therefore, of the total quantity should state also the kind of food used; otherwise, it will be altogether without value. From experiments performed while living on an exclusive diet of bread, fresh meat, and butter, with coffee and water for drink, we have found that the entire quantity of food required during twenty-four hours by a man in full health, and taking free exercise in the open air, is as follows:

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That is to say, rather less than two and a half pounds of solid food, and rather over three pints of liquid food.

Another necessary consideration, in estimating the value of any substance as an article of food, is its digestibility. A vegetable or animal tissue may contain an abundance of albuminoid or starchy matter, but may be at the same time of such an unyielding consist ency as to be insoluble in the digestive fluids, and therefore useless as an article of food. Bones and cartilages, and the fibres of yel low elastic tissue, are indigestible, and therefore not nutritious. The same remark may be made with regard to the substances contained in woody fibre, and the hard coverings and kernels of various fruits. Everything, accordingly, which softens or disintegrates a hard alimentary substance renders it more digestible, and so far increases its value as an article of food.

The preparation of food by cooking has a twofold object: first, to soften or disintegrate it, and second, to give it an attractive flavor. Many vegetable substances are so hard as to be entirely indigestible in a raw state. Ripe peas and beans, the different kinds of grain, and many roots and fruits, require to be softened by boil ing, or some other culinary process, before they are ready for use. With them, the principal change produced by cooking is an alteration in consistency. With most kinds of animal food, however, the effect is somewhat different. In the case of muscular flesh, for example, the muscular fibres themselves are almost always more or less hardened by boiling or roasting; but, at the same time, the fibrous tissue by which they are held together is gelatinized and softened, so that the muscular fibres are more easily separated from each other, and more readily attacked by the digestive fluids. But beside this, the organic substances contained in meat, which are all of them very insipid in the raw state, acquire, by the action of heat in cooking, a peculiar and agreeable flavor. This flavor excites the appetite and stimulates the flow of the digestive fluids, and renders, in this way, the entire process of digestion more easy and expeditious.

The changes which the food undergoes in the interior of the body may be included under three different heads: first, digestion, or the preparation of the food in the alimentary canal; second, assimilation, by which the elements of the food are converted into the animal tissues; and third, excretion, by which it is again decomposed, and finally discharged from the body.

CHAPTER VI.

DIGESTION.

DIGESTION is that process by which the food is reduced to a form in which it can be absorbed from the intestinal canal, and taken up by the blood vessels. This process does not occur in vegetables. For vegetables are dependent for their nutrition, mostly, if not entirely, upon a supply of inorganic substances, as water, saline matters, carbonic acid, and ammonia. These materials constitute the food upon which plants subsist, and are converted in their inte. rior into other substances, by the nutritive process. These materials, furthermore, are constantly supplied to the vegetable under such a form as to be readily absorbed. Carbonic acid and ammonia exist in a gaseous form in the atmosphere, and are also to be found in solution, together with the requisite saline matters, in the water with which the soil is penetrated. All these substances, therefore, are at once ready for absorption, and do not require any preliminary modification. But with animals and man the case is different. They cannot subsist upon these inorganic substances alone, but require for their support materials which have already been organized, and which have previously constituted a part of animal or vegetable bodies. Their food is almost invariably solid or semi-solid at the time when it is taken, and insoluble in water. Meat, bread, fruits, vegetables, &c., are all taken into the stomach in a solid and insoluble condition; and even those substances which are naturally fluid, such as milk, albumen, white of egg, are almost always, in the human species, coagulated and solidified by the process of cooking, before being taken into the stomach.

In animals, accordingly, the food requires to undergo a process of digestion, or liquefaction, before it can be absorbed. In all cases, the general characters of this process are the same. It consists essentially in the food being received into a canal, running through the body from mouth to anus, called the "alimentary canal," in which it comes in contact with certain digestive fluids, which act

upon it in such a way as to liquefy and dissolve it. These fluids are secreted by the mucous membrane of the alimentary canal, and by certain glandular organs situated in its neighborhood. Since the food always consists, as we have already seen, of a mixture of various substances, having different physical and chemical properties, the several digestive fluids are also different from each other; each one of them exerting a peculiar action, which is more or less confined to particular species of food. As the food passes through the intestine from above downward, those parts of it which become liquefied are successively removed by absorption, and taken up by the vessels; while the remaining portions, consisting of the indigestible matter, together with the refuse of the intestinal secretions, gradually acquire a firmer consistency owing to the absorption of the fluids, and are finally discharged from the intestine under the form of feces.

In different species of animals, however, the difference in their habits, in the constitution of their tissues, and in the character of their food, is accompanied with a corresponding variation in the anatomy of the digestive apparatus, and the character of the secreted fluids. As a general rule, the digestive apparatus of herbivorous animals is more complex than that of the carnivora; since, in vegetable substances, the nutritious matters are often present in a very solid and unmanageable form, as, for example, in raw starch and the cereal grains, and are nearly always entangled among vegetable cells and fibres of an indigestible character. In those instances, where the food consists mostly of herbage, as grass, leaves, &c., the digestible matters bear only a small proportion to the entire quantity; and a large mass of food must therefore be taken, in order that the requisite amount of nutritious material may be extracted from it. In such cases, accordingly, the alimentary canal is large and long; and is divided into many compartments, in which different processes of disintegration, transformation, and solution are carried on.

In the common fowl, for instance (Fig. 16), the food, which consists mostly of grains, and frequently of insects with hard, coriaceous integument, first passes down the oesophagus (a) into a diverticulum or pouch (b) termed the crop. Here it remains for a time, mingled with a watery secretion in which the grains are macerated and softened. The food is then carried farther down until it reaches a second dilatation (c), the proventriculus, or secreting stomach. The mucous membrane here is thick and

glandular, and is provided with numerous secreting follicles or crypts. From them an acid fluid is poured out, by which the food is subjected to further changes. It next passes into the gizzard (d), or triturating stomach, a cavity inclosed by thick, muscular walls, and lined with a remarkably tough and horny epithelium. Here it is subjected to the crushing and grinding action of the muscular parietes, assisted by grains of sand and gravel, which the animal instinctively swallows with the food, by which it is so triturated and disintegrated, that it is reduced to a uniform pulp, upon which the digestive fluids can effectually operate. The mass then passes into the intestine (e), where it meets with the intestinal juices, which complete the process of solution; and from the intestinal cavity it is finally absorbed in a liquid form, by the vessels of the mucous membrane.

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ALIMENTARY CANAL OF FOWL.

-a. Esophagus. b. Crop. c. Proventriculus, or secret

ing stomach. d. Gizzard, or

triturating stomach. e. In

testine. f. Two long cæcal

In the ox, again, the sheep, the camel, the deer, and all ruminating animals, there are four distinct stomachs through which the food passes in succession; each lined with. mucous membrane of a different structure, and adapted to perform a different part in the digestive process (Fig. 17). When first swallowed, the food is received into the ru men, or paunch (b), a large sac, itself partially divided by incomplete partitions, and lined by a mucous membrane thickly set with long prominences or villi. Here it accumulates while the animal is feeding, and is retained and macerated in its own fluids. When the animal has finished browsing, and the process of rumination commences, the food is regurgitated into the mouth by an inverted action of the muscular walls of the paunch and oesophagus, and slowly masticated. It then descends again along the oesophagus; but instead of entering the first stomach, as before, it is turned off by a muscular valve into the second stomach, or reticulum (c), which is distinguished by the intersecting folds of its mucous membrane, which give it

tubes which open into the intestine a short distance above its termination.

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