MAMMARY GLANDS IN ENDOCRIN RELATIONSHIPS 641 



the stimulus for its specific preparation for the taking on of the secretory 

 function. In many individuals there is to be noted between these two 

 phases the regular occurrence of a swelling of the mamma at the menstrual 

 periods, which enlargment has been considered as due to the reception and 

 response to the third or menstrual growth impulse. With the onset of 

 pregnancy, there takes place the fourth type of growth and development, 

 due to the fourth growth impulse, the changes occurring at this time being 

 the final steps in the preparation of the gland for its function of milk 

 secretion. Repetition of the changes induced by the third and fourth 

 growth impulses occurs as circumstances direct, until the regressive stages 

 accompanying and following the climacterium set in, and the glands, 

 having accomplished their function, become inactive and sterile in common 

 with the other generative organs. 



It is thus possible to classify the growth changes of the mamma into 

 four distinct groups, each of which is dependent upon the reception of and 

 response to & definite stimulus. The development of the mammary glands, 

 however, is 'merely the preparation of this tissue for taking on its par- 

 ticular function, the elaboration and secretion of milk, and this act of 

 secretion is also apparently dependent upon the reception of and response 

 to a definite stimulus. 



Control of Development. Attention was first directed to nerve im- 

 pulses as the source of the stimuli bringing about these very evident 

 changes. Goltz observed that even after the lumbar portion of the spinal 

 cord had been removed from a bitch, the animal, on becoming pregnant, 

 obtained a normal hypertrophy of the mammary glands and was able to 

 suckle the young after parturition. Probably the most conclusive results 

 were obtained by Ribbert. This investigator removed a mamma of a 

 guinea pig from its customary location and transplanted it to the region 

 of the ear. The animal was then allowed to become pregnant. During 

 pregnancy the transplanted gland became enlarged, and parturition was 

 followed by lactation. This was confirmed by Pfister. These and similar 

 observations by Mironow, Kehrer, and others yield positive evidence that 

 the growth of the mammary glands, at least during pregnancy and the 

 attainment of their secretory function after delivery, is independent of the 

 direct effect of nerve stimulation, and it will be later shown that the 

 growth impulses at other times is likewise largely dependent upon factors 

 other than nerve impulses. 



The question whether or not mechanical stimulation of the breast, 

 by sucking, with accompanying mechanical acceleration of the local cir- 

 culation, is sufficient of itself to induce secretion, was investigated by 

 Cliquet and Lacassagne (quoted by Halban (c), 1905), and by Hilde- 

 brandt. The results were negative. Freund was inclined to believe that 

 the initiation of actual secretion was correlated with the time of greatest 

 involution of the uterus, and that at this contraction there was squeezed 



