Retardation of Mammary Involution in the Mouse by Irritation of the Nipples *

Early studies of the mammary gland and its innervation led to the belief, at least in some quarters, that mammary activity is under the control of the nervous system.6 With the development of endocrinology, however, growth and function of the mammary apparatus were found to be determined by secretions of the ovaries and hypophysis. In a number of species mammary development identical with that occurring during pregnancy has been induced by the proper administration of ovarian hormones, and in some species full lactation has followed the administration of lactogenic substance from the hypophysis to animals whose mammary glands have grown under the influence of extrinsic ovarian hormones."4 In the normal sequence of events in the intact animal, however, full lactation does not occur until the parturient animal has been suckled. Moreover, cessation of suckling is rather promptly followed in the fully lactating animal by termination of lactation and by mammary involution.14 In explanation of these commonly observed facts it has been suggested0 11, 12, 13 that irritation of the nipples by suckling sets up centripetal nerve impulses which, by stimulation of the hypophysis, lead to maintenance of the mammary glands and of lactation. If this explanation is correct, adequate irritation of the nipples by artificial means in the complete absence of young should exert the same effect as suckling, without the complicating factor of a possible psychic effect of the mere presence of the young. The experiments reported here were designed to test this assumption, in the belief that retardation of mammary involution by chemical irritation of the nipples would establish nipple irritation as at least one of the means by which suckling maintains the lactating mammary gland. Mice were used as the test animals and spirits of turpentine was employed as the irritant. At autopsy the mammary glands were removed with the skin and fixed in Bouin's picro-formol. After freeing the glands from the skin they were stained with Mayer's