MAMMARY GLANDS 31 



Mammary Glands. 



Structure. The mammary glands are two in number in 

 the human being, and are loosely attached to the great pectoral 

 muscles. They are rudimentary in both sexes until puberty, 

 and in men throughout life. At puberty the gland in the female 

 enlarges markedly, but is never fully developed before preg- 

 nancy. At this time the gland vesicles make their appearance, 

 and the rudimentary ducts come to be more and more ramified. 

 These ramifications do not reach their full development, how- 

 ever, until lactation begins. The skin covering the areola of 

 the nipple is dark, especially during pregnancy, and much thinner 

 than over other parts. The dark color is due to a deposit of 

 pigment. 



The mammary gland belongs to the compound tubulo-race- 

 mose type, and consists of fifteen or twenty lobes bound together 

 by areolar connective tissue. Each lobe is made up of a num- 

 ber of lobules, containing the alveoli or secreting portions. 

 The secretion from all the alveoli and lobules of a lobe converges 

 to a single duct, which discharges its contents upon the surface of 

 the nipple without anastomosis with any duct. There are, 

 therefore, some fifteen or twenty ducts thus opening upon the 

 surface. Each of them has a dilatation beneath the nipple, and 

 it is in these sinuses largely that the milk accumulates during 

 lactation. When lactation has ceased the ducts retract, the sin- 

 uses disappear, the alveoli undergo retrograde changes, and the 

 whole gland is inclined to become flabby and pendulous. It 

 does not regain after pregnancy the firmness which character- 

 ized it before. 



Secretion of Milk. After parturition the first discharge 

 from the gland is colostrum, a liquid resembling milk in some 

 respects. In two or three days the true milk appears. Besides 

 water and salts, all the constituents of milk are formed by the 

 cells of the mammary gland. During the period of gestation the 



