Taking a general average of the whole, the following may be assumed as representing approximately the general composition of a lean and a fat animal:—
| Lean. | Fat. | |
| Mineral matters | 5 | 3 |
| Nitrogenous compounds | 15 | 12·5 |
| Fat | 24 | 33 |
| Water | 56 | 48·5 |
| — | —— | |
| 100 | 100·0 |
The data given in the preceding table, coupled with a knowledge of the relative weights of the lean and fat animals, enable us to ascertain the composition of the increase during the fattening process. It is obvious, from the material diminution of the per centage of water, that the matters deposited in the tissues must contain a much larger proportion of dry matters than the whole body; and the reduced per centage of nitrogenous matters shews that the fat must also greatly preponderate. This is still more distinctly illustrated by the following table, giving the per centage composition of the increase in fattening oxen, sheep, and pigs:—
| Mineral Matters. | Nitrogenous Compounds. | Fat. | Water. | |
| Oxen | 1·47 | 7·69 | 66·2 | 24·6 |
| Sheep | 2·34 | 7·13 | 70·4 | 20·1 |
| Pigs | 0·06 | 6·44 | 71·5 | 22·0 |
Hence it may be stated in round numbers, that for every pound of nitrogenous matters added to the weight of a fattening animal, it will gain ten pounds of fat, and three of water. These are the proportions over the whole period of fattening, but it is probable that during the last few weeks of the process the ratio of fat to nitrogenous matters is still higher.
In considering the composition of the food of animals, it will be readily admitted that the milk, the nutriment supplied by nature for the maintenance of the young animal, must afford special instruction as to its requirements during the early stages of existence, and indicate, at least, some of the points to be attended to under the altered conditions of mature life. The following table gives the average composition of the milk of the most important farm animals:—
| Cow. | Ewe. | Goat. | |
| Caseine | 3·4 | 4·50 | 4·02 |
| Butter | 3·6 | 4·20 | 3·32 |
| Sugar of milk | 6·0 | 5·00 | 5·28 |
| Ash | 0·2 | 0·68 | 0·58 |
| Water | 86·8 | 85·62 | 86·80 |
| ——— | ——— | ——— | |
| 100·00 | 100·00 | 100·00 |
In examining these, and all other analyses of food, it is necessary to draw a distinction between the flesh-forming and the respiratory elements; the former including the nitrogenous compounds which are used in the production of flesh, the latter, the non-nitrogenous substances which produce fat and support the process of respiration. The former, however much they may differ in name, are nearly or altogether identical in chemical composition, the latter embracing two great classes—the fats which exist in the body and the saccharine compounds, including the different kinds of sugar and starch which are not found in the animal tissues. It was at one time supposed that these substances were entirely consumed in the respiratory process, and eliminated by the lungs in the form of carbonic acid and water, but it has been clearly shewn that they may be and often are converted into fat, and accumulated in the system. Careful experiments on bees have demonstrated that when fed on sugar they continue to produce wax, which is a species of fat, and animals retain their health and become fat, even when their food contains scarcely any oil. There is, however, an important difference between these two classes of substances as regards their fat-producing effect. A pound of fat contained in the food is capable of producing the same quantity within the animal; but the case is different with starch and sugar, the most trustworthy experiments shewing that two and a half pounds of these substances are necessary for that purpose. Hence we talk of the fat equivalent of sugar, by which is meant the amount of fat it is capable of producing, and which is obtained by dividing its quantity by 2·5. Applying this principle to the analyses of the milk, it appears that the relative proportions of the two great classes of nutritive substances stand thus:—
| Flesh forming | Respiratory, expressed in their fat equivalent | |
| Cow | 3·4 | 6·0 |
| Ewe | 4·5 | 6·2 |
| Goat | 4·0 | 5·4 |