Since the secretion of milk is known to proceed with the same regularity whether the nerves to the mammary glands are cut or intact, it seems probable that is is provoked by other than nervous stimuli. It cannot, indeed, be contested that the secretion is influenced through the nervous system, but this may be indirect, if the formation and outpouring of the secretion can be shown to be produced by chemical agents (hormones) circulating in the blood, such as have been found to excite secretion in the pancreas, which is stimulated to active secretion by a material obtained from the mucous membrane of the duodenum, and in the kidney, which is stimulated by a material yielded by the posterior or infundibular portion of the pituitary body. We have investigated the action of a large number of animal extracts upon the flow of milk from the mammary glands of lactating animals, chiefly cats, but including some dogs. The extracts, which were made with Ringer’s solution and were in most cases previously boiled, were injected slowly and in small amount (not more than 5 c. c at a time) into a superficial vein, and the flow of milk, if any, was recorded by one of two methods, or by both methods simultaneously. The simpler method consists in recording the rate of exudation of milk from a small and superficial cut into one of the mammary glands (exudation method). The other method consists in recording the flow of milk led from a canula tied into a cut nipple (nipple method); in either case the milk is allowed to drop upon an electric recorded, and the drops are marked by an electromagnetic signal upon the paper of a kymograph. On this paper are also recorded at the same time in some of our experiments the blood-pressure, the volume of the kidney, and the rate of excretion of urine. The animals were anæsthetised either with chloroform alone or with chloroform followed by chloral, the latter being administered either intravenously or subcutaneously; after the complete effect of the chloral is established, the chloroform administration is stopped.
