366 REPRODUCTION 



Growth of the Mammary Glands 



The glands undergo a slight increase in size at puberty, 

 and a further temporary increase coincides with the 

 menstrual periods. 



The enlargement at pregnancy begins (in multiparse, 

 or those who have been previously pregnant) soon 

 after the second month, in virgins immediately after 

 conception ; the nipples at the same time become pig- 

 mented. During the latter stages of pregnancy a clear 

 fluid known as colostrum can be squeezed out. 



The growth of the gland at puberty is due to an internal 

 secretion elaborated by the ovary, for it does not occur 

 when the ovaries are removed. The congestion which occurs 

 with menstruation also appears to be of ovarian origin. 



The hypertrophy of pregnancy, similarly, is not due to 

 a nervous influence, for it occurs when all nervous con- 

 nection between the pelvic organs and breasts have been 

 severed by transection of the spinal cord. 



In the first half of pregnancy mammary growth is due 

 to a hormone poured into the blood by the corpus luteum. 

 The continued development of the glands in the second 

 half of pregnancy is also due to the corpus luteum, the 

 persistence of which probably depends upon the presence 

 of the foetus. 



Since no secretion occurs until after parturition, it is 

 held that the responsible hormone, at the same time as it 

 stimulates the growth of the gland, inhibits its activity. 

 On the removal of this inhibiting agent secretion occurs. 

 However that may be, the secretion, when once started, 

 depends for its continuance upon the act of suckhng. It 

 is also readily influenced by nervous agencies. The flow 

 of milk ceases at the onset of a new pregnancy. 



Composition of Milk 



Milk is amphoteric in reaction and has a specific gravity 

 lying between 1-028 and 1-034. From the following Table 



