1906.] Growth and Activity of the Mammary Glands. 509 



an impulse is exerted by the developing ovum on the mammary glands which 

 acts as a stimulus to growth, and at the same time protects the cells of the 

 gland from those autolytic disintegrative processes which occur to a large 

 extent in the secreting gland. That the act of secretion can be ascribed to 

 autolytic processes of the gland seems highly improbable. We have no 

 evidence that the autolysis of the gland cells would give rise to the specific 

 constituents which characterise milk, but Hildebrandt's idea of an inhibitory 

 substance which excites hypertrophy of the gland, and whose removal leads 

 to secretion, is a valuable one. 



Our whole idea of inhibition involves the stopping of the normal dissimi- 

 lative processes in a tissue and the augmentation of the assimilative, so that 

 the continued reception of inhibitory impulses must lead to an actual increase 

 in the substance of the cells, with resulting division and growth of the gland. 

 But, as Hering has pointed out, the more this process of assimilation is 

 carried on, the greater is the tendency to autonomous dissimilation, and the 

 removal at the end of pregnancy of inhibitory impulses, or the attainment 

 towards the end of pregnancy of a maximum degree of hypertrophy, must 

 result in an autonomous dissimilation, i.e., activity of the tissue. The normal 

 activity of the mammary gland is secretion, and it is entirely in accordance 

 with the accepted principles of physiology that the stimulus, which during 

 pregnancy gave rise to growth, should by its removal at the end of pregnancy 

 be responsible for the act of lactation. 



We may conclude then that lactation is due not to excitation of the gland 

 by special substances produced in the other generative organs, but to the 

 removal of the stimulus which during pregnancy was responsible for the 

 growth of the gland. 



The Origin of the Substance or Substances by which the Hypertrophy of the 

 Mammary Glands is determined. 



We have seen reason for regarding the development of the mammary 

 glands, which occurs in the female at puberty, as being due to the production 

 of some substance in the ovaries which must reach these glands by way of the 

 blood. It is probable that the slight increase in size of these glands at each 

 oestral period is also determined by the greater activity of the ovaries at these 

 periods, an activity which has been shown by Marshall* to be probably 

 responsible for the changes which occur in the uterus. These changes are, 

 however, insignificant as compared with the enormous hyperplasia which is 

 associated with pregnancy. The question arises whether the greater growth 

 during pregnancy is to be ascribed to an increased production of the specific 



* ' Phi). Trans.,' B, vol. 198, 1905. 



