The tendency of the lactating female to persist in secreting milk, even though deficient in one or more respects as a food for the suckling because of faults in her diet, is truly remarkable. Babcock (12) described experiments in which he deprived cows of common salt during lactation. The animals were not entirely deprived of salt since they were fed on ordinary farm products, all of which contain salt, but they were not given any supplementary supply, as is the custom among farmers. In Wisconsin an ordinary grain and roughage ration does not furnish even approximately enough sodium chlorid. Deer, living on grass and the leaves of shrubs, develop such an appetite for salt, that, although they are by nature very shy, they will brave any danger to visit their accustomed salt licks. In Babcock's experiments the period of deprivation of salt varied from two to fifteen months. Some of the animals died from lack of salt and others were saved from death only by its administration.

In no instance was there any noticeable decrease in the yield of milk until a short time before the cows began to fail rapidly. The fat content of the milk from cows suffering from partial salt starvation was somewhat higher than that of the milk of the control group. Long before the cows showed any sign of injury from lack of salt the milk became practically chlorin-free. This illustrates the fact that common foods do not necessarily furnish enough of certain elements to meet the needs of a lactating mother to enable her to secrete a milk of normal composition. This is true of calcium and probably of phosphorus.