It would be an error, however, to infer that all nitrogenous food, when once digested and absorbed into the blood, must necessarily leave the system in the urine. On the contrary, in the young and growing animal, all increase of the fibrous structures of the body is gained through the building up of those flesh-forming constituents into their substance; in the pregnant animal the growth of the offspring and its envelopes has a similar origin, and in the dairy cow the casein or curd of the milk is a means of constant elimination of these nitrogen-containing agents. Thus, in the breeding cow and, above all, in the milking cow, the womb or udder carries on a work in one sense equivalent to that otherwise performed by the kidneys. Not only are these organs alike channels for the excretion of albuminous products, but they are also related to each other structurally and by nervous sympathy, so that suffering in the one is liable to induce some measure of disorder in the other. As in the case of other mammals, this nitrogenous waste matter is mainly present in the urine of cattle in the form of urea, but also, to some extent, as hippuric acid, a derivative of vegetable food which, in the herbivora, replaces the uric acid found in the urine of man and carnivora. Uric acid is, however, found in the urine of sucking calves which have practically an animal diet, and it may also appear in the adult in case of absolute, prolonged starvation, and in diseases attended with complete loss of appetite and rapid wasting of the body. In such cases the animal lives on its own substance, and the product is that of the wasting flesh. The other products containing nitrogen are present in only small quantities and need not be specially referred to. The urine of cattle contains much less of carbonates than that of the horse, and effervesces less on the addition of an acid. As the carbonates form a large proportion of the solid deposits (gravel, stone) from the horse's urine, the ox may thus be held less liable, yet even in the ox the carbonates become abundant or scanty, according to the nature of the feed, and therefore gravel, formed by carbonate of lime, is not infrequent in cattle. When fed on beets, clover hay, or bean straw carbonates are present in large quantities, these aliments being rich in organic acids and alkaline carbonates; whereas upon oat straw, barley straw, and, above all, wheat straw, they are in small amount. In calves fed on milk alone no carbonates are found in the urine. Phosphates, usually in combination with lime, are, as a rule, present only in traces in the urine of cattle; however, on a dietary of wheat, bran, or other aliment rich in phosphates, these may be present in large amount, so that they render the liquid cloudy or are deposited in solid crystals. The liquid is rendered transparent by nitric acid. The cow's urine, on a diet of hay and potatoes, contained: