Cellulose, or the substance of the vegetable cell, resists the action of the digestive fluids, and reappears unchanged in the fteces.
Vinegar is probably only of use indirectly in softening animal textures which are taken as food.
3. The oleaginous group includes all the fats and oils, whether derived from the ani mal or the vegetable kingdom. The members of this extensive group are composed of carbon (ranging from CO to SO per cent) and hydrogen, with a little oxygen. Fat which has been taken with the food is mainly absorbed by the lactcals, although a portion of it passes directly into the capillaries of the as has been shown by microscopic examination, which has revealed the presence of fat-granules amongst the blood-cor puscles. The modifications which are impressed upon the fats, in order to prepare them for absorption, are explained in the article DIGESTION. Their uses in the system are various. In their oxidation in the organism, whether the process be gradual or rapid, a large amount of heat is liberated; and that they are oxidized, and for the most part ultimately resolved into carbonic acid and water, is evident, because they neither appear in any quantity in the excretions nor accumulate beyond a certain point in the organism. Moreover, in artificial, and doubtless iu natural digestion, the presence of a little fat accelerates the solution of nitrogenous matters taken as food. Lastly, the occurrence of fat in milk, in the egg, in all plastic exudations, and in all highly cellular organs, is a clear indication that this substance plays an important part in the process of cell-formation; hence we may probably explain the therapeutic use of such medi cines as cod-liver oil.
4. The albuminous group contains all those substances which are chemically known as the proteine-bodies (see PROTEINE), viz., albumen, fibriue, caseine, and the allied vegetable compounds, all of which are composed of very nearly the same proportions of carbon, hydrogen, nitrogen, and oxygen; while additionally they contain a little sul phur or phosphorus, or both: they all contain on an average abous 15 per cent of nitro gen, a substance which has not occurred in the preceding groups. All these proteine bodies, such as occur in the fluids of the egg, iu animal flesh, in the curdy matter of milk, etc., are dissolved by the gastric-juice and intestinal fluid, and converted into matters termed peptones, which, although similar in their ultimate composition to the substances from which they are derived, differ from them in their greater solubility and more their ready diffusibility through animal membranes. Like the fats, they are
chiefly absorbed by the lacteals, but to some extent by the capillaries. A reference to the chemical composition of the milk and of the fluids of the egg. shows that all the nitrogenous tissues in the body of the young animal must have been primarily derived from albumen or caseine; and it is established beyond all doubt that these substances are throughout life the essential producers of blood, and consequently of the various nitrogenous structures which are are built up from that fluid.
5. The gelatinous group (which formed a part of Prout's albuminous group) includes the different varieties of gluten, obtained by boiling, from many animal tissues; as, for example, bone-cartilage, tendons, skin, hoofs, etc. All soups and jellies which stiffen on cooling contain it, and such substances are popularly, but erroneously, regarded as highly nourishing. Unlike the preceding group, from which they only slightly differ in ultimate composition, they do not appear to form new blood, and their uses are still so questionable, that we will merely notice one function of them, which has been suggested by Liebig—viz., that these substances may go directly to the formation of such tissues as yield gluten on boiling, and which, if this food were not taken, would have to derive their nourishment from the members of the preceding group.
6. The saline group. With the exception of common salt (chloride of sodium), which we take instinctively in additional quantity with most kinds of food, the members of this group are unconsciously taken in the different articles of solid and fluid food on which we live. Want of space prevents us from noticing the different foods which yield the individual salts; but when we invariably find phosphate and carbonate of lime m the bones, in fixed and definite proportions; when we invariably find a nearly fixed proportion of chloride of sodium, alkaline phosphates, and other soluble salts, in the blood, flesh, milk, etc.; and when, further, we find that these substances are being constantly eliminated by the urine, it is obvious that they must be replaced by the food if we would keep the organism in its normal state. The evil consequences of a deficiency of any of these ingredients of food are well known; thus, when too little phosphate of lime (to which bone owes its firmness and hardness) is taken into the system, or when too much is again taken out (as occasionally, during pregnancy, when the fetal bones require it for their ossification), fractures do not readily unite.