Chap. XXI] THE ORGANS OF GENERATION 455 



Blood-vessels and nerves. — The mammary glands are well 

 supplied with blood brought to them by branches of the axillary, 

 internal mammary, and intercostal arteries. The nerves are 

 chiefly intercostal nerves. 



Development of the mammary glands. — The increase in the 

 size of the mammary glands at the time of puberty is due to an 

 increased development of the connective tissue and fat. The 

 glandular tissue remains undeveloped and does not function unless 

 conception takes place. When conception occurs the glandular 

 tissue undergoes a process of gradual development that produces 

 marked changes. The breasts become larger and harder, the 

 veins on the surface become more noticeable, the areola becomes 

 enlarged and darkened, the nipple becomes more prominent, and 

 toward the end of pregnancy a fluid called colostrum can be 

 squeezed from the orifice of the ducts. After delivery the amount 

 of colostrum increases for a day or two, and then its composition 

 changes to that of milk. 



The primary development and later functioning of the mam- 

 mary glands suggests an intimate connection between these glands 

 and the uterus and ovaries. The present theory is that the in- 

 crease in the size of the breasts at the time of puberty is influenced 

 by the internal secretion of the ovaries, for if the ovaries are re- 

 moved before puberty, the breasts do not develop, or if the ovaries 

 are removed after puberty, the breasts are apt to atrophy. The 

 development of the glandular tissue that follows conception is 

 thought to be due to some chemical substance that results from the 

 metabolism of the foetus. The chemical nature of this substance is 

 not known, but presumably it stimulates the development of the 

 gland, and also prevents secretion, as active secretion does not 

 commence until after delivery, and if conception occurs during the 

 months of lactation, the character of the milk is changed and its 

 secretion checked. The stimulus which causes the active secre- 

 tion of milk is thought to result from the emptying of the milk 

 ducts, because of the fact that when a woman does not nurse her 

 infant, the secretion of milk is checked, and the breasts return 

 to their usual size. The active secretion of milk is also influenced 

 by the nervous system, and this influence is probably exerted 

 through the vasomotor nerves which control the size of the blood- 

 vessels, and consequently the amount of blood sent to the gland. 



