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 a little chloride of sodium, 1½} _Phosphates_, chiefly of _lime_, 2¼} to 10 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. [25] In warm weather the milk contains more butter, in cold weather more cheese and sugar. 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.