In considering the principles of feeding, it is necessary to point out, in the first instance, that the plant and animal are composed of the same chemical elements, hence the food supplied to the latter invariably contains all the substances it requires for the maintenance of its functions. And not only is this the case, but these elements are to a great extent combined together in a similar manner,—the fibrine, caseine, albumen, and fatty matters contained in animals corresponding in all respects with the compounds extracted from plants under the same name; and though the starchy and saccharine substances do not form any part of the animal body, they are represented in the milk, the food which nature has provided for the young animal. It has been frequently assumed that the nitrogenous and fatty matters are simply absorbed into the animal system, and deposited unchanged in its tissues; but it is probable that the course of events is not quite so simple, although, doubtless, the decomposition which occurs is comparatively trifling. The starchy matters, on the other hand, are completely changed, and devoted to purposes which will be immediately explained.
It is a matter of familiar experience, that if the food be properly proportioned to the requirements of the animal, its weight remains unchanged; and the inference to be drawn from this fact obviously is, that the food does not remain permanently in the system, but must be again got rid of. It escapes partly through the lungs, and partly by the excretions, which do not consist merely of the part which has not been digested, but also of that portion which has been absorbed, and after performing its allotted functions within the system, has become effete and useless. When the weights of the excretions, the carbon contained in the carbonic acid expired by the lungs and the small quantity of matter which escapes in the form of perspiration, are added together, they are found in such a case to be exactly equal to the food. If the animal be deprived of nutriment, it immediately begins to lose weight, because its functions must continue—carbon must still be converted into carbonic acid to maintain respiration—and the excretions be eliminated, although diminished in quantity, because they no longer contain the undigested portion of the daily food, and the substances already stored up in the body are consumed to maintain the functions of life. Universal experience has shewn that, under such circumstances, the fat which has accumulated in various parts of the body disappears, and the animal becomes lean; but it is less generally recognised that the muscular flesh, that is the lean part of the body, also diminishes, although it is sufficiently indicated by the fact that nitrogen still continues to be found in the urine, and that the animal becomes feeble and incapable of muscular exertion. Respiration and secretion, in fact, proceed quite irrespective of the food, which is only required to repair the loss they occasion. When the course of events within the animal body is traced, it is found to be somewhat as follows: The food consumed is digested and absorbed into the blood, where it undergoes a series of complicated changes, as a consequence of which part of it is converted into carbonic acid, and eliminated by the lungs, and part is deposited in the tissues as fat and flesh. After the lapse of a certain period, longer or shorter according to circumstances, a new set of actions comes into play, by which the complex constituents of the tissues are resolved into simpler substances, and excreted chiefly by the lungs and kidneys. The changes thus produced are, to a great extent, identical with those which would take place if the fat and flesh were consumed in a fire; and the animal frame may, in a certain sense, be compared to a furnace, in which, by the daily consumption of a certain quantity of fuel and air inhaled in the process of respiration, its temperature is maintained above that of the surrounding atmosphere. If the daily supply of fuel, that is of food, be properly adjusted to the loss by combustion, the weight of the animal remains constant; if it be reduced below this quantity, it diminishes; but if it be increased, the stomach either refuses to digest and assimilate the excess, or it is absorbed and stored up in the body, increasing both the fat and flesh.
When an animal is fed in such a manner that its weight remains constant, a balance is produced between the supply of nutriment contained in the food and the waste of the tissues, the gain from the former exactly counterpoising the loss occasioned by the latter. If in this state of matters an additional supply of food be given, this balance is deranged, and the nutriment being in excess of the loss, the animal gains weight, and it continues to do this for some time, until it reaches a point at which a new balance is established, and its weight again becomes constant; and this is due to the fact that the animal becomes subject to an additional waste, consequent on the increased weight of matter accumulated in its tissues. If, after the animal has attained its new constant weight, the food be a second time increased, a further gain is obtained, and so on, with every addition to the supply of nutriment, until at length a certain point is reached, beyond which its weight cannot be forced. In fact, each successive increase of weight is obtained at a greater expenditure of food. If, for example, a lean animal is taken, and its food increased by a given quantity, it will rapidly attain a certain additional weight, but if another extra supply of food be given, the increase due to it will be much more slowly attained, and so on until at length an additional increase can only be secured by the long-continued consumption of a very large quantity of food. The great object of the feeder is to obtain the greatest possible increase with the smallest expenditure of food, and to know the point beyond which it is no longer economical to attempt to force the process of fattening. To do this it is necessary first to consider the composition of the animal itself, then that of its food, and lastly, the mode in which it may be most economically used.
It has been already observed that the animal tissues are composed of albuminous or nitrogenous compounds, fat, mineral matters, and water; but the proportions of these substances have, until lately, been very imperfectly known. Water is well known to be by far the largest constituent, and amounts in general to about two-thirds of the entire weight, and it has been generally supposed that the nitrogenous matters stood next in point of abundance, but a most important and elaborate series of experiments by Messrs. Lawes and Gilbert have shewn that they are greatly exceeded by the fatty matters. The following table contains a summary of the composition of ten different animals in different stages of fattening. The first division gives the composition of the carcass, that is, the portion of the animal usually consumed as human food; the second that of the offal, consisting of the parts not usually employed as food; and the third that of the entire animals, including the contents of the stomach and intestines:—
[Transcriber's note: Column titles are printed vertical, which is not possible to do here. Therefore they are replaced with a 2-3 character code, explained here]
Column titles:
MM = Mineral Matter
NC = Nitrogenous Compounds
TDS = Total Dry Substance
CSI = Contents of Stomachs and Intestine in moist state.
Wat = Water
| Per cent in Carcass | Per cent in Offal, excluding contents of Stomachs and Intestines. | ||||||||||
| MM | NC | Fat | TDS | WAT | MM | NC | Fat | TDS | WAT | ||
| Fat Calf | 4·48 | 16·6 | 16·6 | 37·7 | 62·3 | 3·41 | 17·1 | 14·6 | 35·1 | 64·9 | |
| Half-fat Ox | 5·56 | 17·8 | 22·6 | 46·0 | 54·0 | 4·05 | 20·6 | 15·7 | 40·4 | 59·6 | |
| Fat Ox | 4·56 | 15·0 | 34·8 | 54·4 | 45·6 | 3·40 | 17·5 | 26·3 | 47·2 | 52·8 | |
| Fat Lamb | 3·63 | 10·9 | 36·9 | 51·4 | 48·6 | 2·45 | 18·9 | 20·1 | 41·5 | 58·5 | |
| Store Sheep | 4·36 | 14·5 | 23·8 | 42·7 | 57·3 | 2·19 | 18·0 | 16·1 | 36·3 | 63·7 | |
| Half-fat old Sheep | 4·13 | 14·9 | 31·3 | 50·3 | 49·7 | 2·72 | 17·7 | 18·5 | 38·9 | 61·1 | |
| Fat Sheep | 3·45 | 11·5 | 45·4 | 60·3 | 39·7 | 2·32 | 16·1 | 26·4 | 44·8 | 55·2 | |
| Extra fat Sheep | 2·77 | 9·1 | 55·1 | 67·0 | 33·0 | 3·64 | 16·8 | 34·5 | 54·9 | 45·1 | |
| Store Pig | 2·57 | 14·0 | 28·1 | 44·7 | 55·3 | 3·07 | 14·0 | 15·0 | 32·1 | 67·9 | |
| Fat Pig | 1·40 | 10·5 | 49·5 | 61·4 | 38·6 | 2·97 | 14·8 | 22·8 | 40·6 | 59·4 | |
| Mean of all | 3·69 | 13·5 | 34·4 | 51·6 | 48·4 | 3·02 | 17·2 | 21·0 | 41·2 | 58·8 | |
| Mean of 8, viz., the half-fat, fat, and very fat animals | 3·75 | 13·3 | 36·5 | 53·6 | 46·4 | 3·12 | 17·4 | 22·4 | 42·9 | 57·1 | |
| Mean of 6, viz., of the fat and very fat animals | 3·38 | 12·3 | 39·7 | 55·4 | 44·6 | 3·03 | 16·9 | 24·1 | 44·0 | 56·0 | |
| Per cent in Entire Animal. | ||||||
| MM | NC | Fat | TDS | CSI | WAT | |
| Fat Calf | 3·80 | 15·2 | 14·8 | 33·8 | 3·17 | 63·0 |
| Half-fat Ox | 4·66 | 16·6 | 19·1 | 40·3 | 8·19 | 51·5 |
| Fat Ox | 3·92 | 14·5 | 30·1 | 48·5 | 5·98 | 45·5 |
| Fat Lamb | 2·94 | 12·3 | 28·5 | 43·7 | 8·54 | 47·8 |
| Store Sheep | 3·16 | 14·8 | 18·7 | 36·7 | 6.00 | 57·3 |
| Half-fat old Sheep | 3·17 | 14·0 | 23·5 | 40·7 | 9·05 | 50·2 |
| Fat Sheep | 2·81 | 12·2 | 35·6 | 50·6 | 6·02 | 43·4 |
| Extra fat Sheep | 2·90 | 10·9 | 45·8 | 59·6 | 5·18 | 35·2 |
| Store Pig | 2·67 | 13·7 | 23·3 | 39·7 | 5·22 | 55·1 |
| Fat Pig | 1·65 | 10·9 | 42·2 | 54·7 | 3·97 | 41·3 |
| Mean of all | 3·17 | 13·9 | 28·2 | 44·9 | 6·13 | 49·0 |
| Mean of 8, viz., the half-fat, fat, and very fat animals | 3·23 | 13·3 | 29·9 | 46·4 | 6·26 | 47·3 |
| Mean of 6, viz., of the fat and very fat animals | 3·00 | 12·7 | 32·8 | 48·5 | 5·48 | 46·0 |
From this table it appears that, in the carcass, the proportion of fat is, in general, even in lean animals, much greater than that of nitrogenous compounds. In one case only, that of the fat calf, are they equal. But in the lean sheep there is more than one and a half times as much fat as nitrogenous matters, in the half fat sheep twice, in the fat sheep four times, and in the very fat sheep about six times as much. As a general result of the analyses it may be stated, that in the carcass of an ox in good condition, the quantity of fat will be from two to nearly three times as great as that of the so called albuminous compounds; in a sheep three or four times, and in the pig four or five times as great. In the offal, including the hide, intestines, and other parts not usually consumed as food, the proportion is very different,—the quantity of fat being much smaller, and that of nitrogenous compounds considerably larger.