MAMMARY GLANDS 31 



glands are only modified forms of sebum, and the structures 

 producing these secretions belong to the class of sebaceous 

 glands. 



Mammary Glands. 



Structure. The mammary glands are two in number in 

 the human being, and are loosely attached to the great pec- 

 toral 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 pregnancy. 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, however, until lactation begins. The skin cov- 

 ering the areola of the nipple is dark, especially during preg- 

 nancy, 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 number 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 dilata- 

 tion beneath the nipple, and it is in these sinuses largely that 

 the milk accumulates during lactation. When lactation has 

 ceased the ducts retract, the sinuses 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 characterized 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. Be- 

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



