Nutrition may be defined as the process by which the waste which is constantly going on in the animal's system is compensated by the deposit of fresh material derived from food, solid and liquid. To understand exactly what is required it is necessary to know something of the constitution of the body, and it may be stated, in the first instance, that four elementary substances, oxygen, hydrogen, carbon, and nitrogen, are the principal constituents of the important tissues of the animal body, muscle, nerve, and the solid particles of the blood.
Constitution of the Body. . Organic bodies include albuminoids or proteids, gelatine, horny matter, and fats (composed of carbon, oxygen, and hydrogen, nitrogen being absent). All these constituents are combustible. Besides the combustible constituents there is a considerable proportion of incombustible, and when burned with those which are combustible they are left as ash, of which bone ash forms the largest proportion. Bone ash consists chiefly of phosphate of lime and magnesia, with a small quantity of the carbonate. The ash obtained from the burning of muscle or flesh contains a considerable proportion of the phosphate of potassium. The whole of the constituents of the animal body are combined with a very large proportion of water, which, however, varies in different animals, and at different ages, from a little over 40 per cent to nearly 70. The following table will show the proportion of water, nitrogenous matter, fat, and ash in the bodies of cattle, sheep, and pigs:-
Half Fat Ox.