cows, dogs or man's. Some animals produce milk which contains ten times as much fat as is contained in cow's milk. Thus we find proper adaptation in nature of the only suitable food for young mammals. Again, the composition varies in individuals of the same species or race, and during the period of lactation. As the young grow older the concentration of the milk increases. When the calf begins to suck its mother's milk the milk is of thinner consistency than after the calf is several weeks old. It is evident from this fact that, when we substitute cow's milk for human milk as food for infants, the relative increase of milk components is not proportionate to the growth of the infant, but to the growth of the calf. It is, therefore, preferable to feed infants with mixed milk from a herd of cows rather than from an individual
cow. In a herd we have cows in various stages of lactation and the mixture of milk results in a uniform product, which can be modified if this is desired. Practical experience has proved that the composition of milk obtained from a herd runs nearly the same from day to day.
It is well known that there are differences in composition between cow's milk and human milk. In human milk there is more butter fat and more milksugar. The nitrogenous part, that is, the part which is necessary to replace the cells of the body and enable development to take a normal course, is about half the amount in human milk as compared with cow's milk. The quality of these components is also different in the two kinds of milk. The protein of cow's milk consists chiefly of two parts, one is casein, the other lactalbumin. The latter is more readily digested than the former, but is present in small proportion. Human milk contains less casein and more lactalbumin than cow's milk, and