The provision is very beautiful by which the young animal,—the muscle and bones of which are rapidly growing,—is supplied with a larger portion of nitrogenous food and of bone-earth, than are necessary to maintain the healthy condition of the full grown animal. The milk of the mother is the natural food from which its supplies are drawn. The sugar of the milk supplies the comparatively small quantity of carbon necessary for the respiration of the young animal; as it gets older, the calf or young lamb crops green food for itself to supply an additional portion. The curd of the milk (casein) yields the materials of the growing muscles, and of the animal part of the bones,—while dissolved along with the curd in the liquid milk is the phosphate of lime, of which the earthy part of the bones is to be built up. A glance at the constitution of milk will shew us how copious the supply of all these substances is,—how beautifully the constitution of the mother’s milk is adapted to the wants of her infant offspring. Cow’s milk consists in 1000 parts by weight of—
| Butter, | 27 | to | 35 |
| Cheesy matter (casein), | 45 | to | 90 |
| Milk sugar, | 36 | to | 50 |
| Chloride of potassium, and | 1½ |  | to 10 |
| a little chloride of sodium, | |||
| Phosphates, chiefly of lime, | 2¼ | ||
| Other salts, | 6 | ||
| Water, | 882¼ | to | 815 |
| 1000 | 1000 |
The quality of the milk, and, consequently, the proportions of the several constituents above mentioned, vary with the breed of the cow,—with the food on which it is supported,—with the time that has elapsed since the period of calving,—with its age, its state of health, and with the warmth of the weather;[25] but in all cases this fluid contains the same substances, though in different quantities.
Milk of the quality above analyzed contains, in every ten gallons, 4½ lbs. of casein, equal to the formation of 18 lbs. of ordinary muscle, and 3½ ounces of phosphate of lime (bone-earth), equal to the production of 7 ounces of dry bone. But from the casein have to be formed the skin, the hair, the horn, the hoof, &c. as well as the muscle, and in all these is contained also a minute portion of the bone-earth. A portion of all the ingredients of the milk likewise passes off in the ordinary excretions, and yet every one knows how rapidly young animals thrive, when allowed to consume the whole of the milk which nature has provided as their most suitable nourishment.
And whence does the mother derive all this gluten and bone-earth, by which she can not only repair the natural waste of her own full grown body, but from which she can spare enough also to yield so large a supply of nourishing milk? She must extract them from the vegetables on which she lives, and they again from the soil.
The quantity of solid matter thus yielded by the cow in her milk is really very large, if we look at the produce of an entire year. If the average yield of milk be 3000 quarts, or 750 gallons in a year—every 10 gallons of which contain bone-earth enough to form about 7 ounces of dry bone—then the milking of the cow alone exhausts her of the earthy ingredients of 33 lbs. of dry bone. And this she draws necessarily from the soil!
If this milk be consumed on the spot, then all returns again to the soil in the annual manuring of the land. Let it be carried for sale to a distance, or let it be converted into cheese and butter, and in this form exported, there will then be a yearly drain upon the land of the materials of bones, from this cause alone, equal to 30 lbs. of bone-dust. After the lapse of centuries, it is conceivable that old pasture lands in cheese and dairy countries should become poor in the materials of bones—and that in such districts, as now in Cheshire, the application of bone-dust should entirely alter the character of the grasses, and renovate the old pastures.
Thus, as was stated at the commencement of the present section the study of the nature, and functions of the food of animals throws additional light upon the nature also and final uses of the food of plants. It even teaches us what to look for in the soil—what a fertile soil must contain that it may grow nourishing food—what we must add to the soil when chemical analysis fails to detect its actual presence, or when the food it produces is unable to supply all that the animal requires.
The principles above explained, therefore, shew that the value of any vegetable production, considered as the sole food of an animal, is not to be judged of—cannot, in short, be accurately determined—by the amount it may contain of any one of those substances, all of which together are necessary to build up the growing body of the young animal, and to repair the natural waste of such as have attained to their fullest size.
Hence the failure of the attempts that have been made to support the lives of animals by feeding them upon pure starch or sugar alone. These substances would supply carbon for respiration, but all the natural waste of nitrogen, of saline matter, and of earthy phosphates, must have been drawn from the existing solids and fluids of their living bodies. The animals in consequence pined away, and sooner or later died.