MAMMARY GLAND. 337 



(/) Mammary Gland. 



The mammary gland is a cutaneous gland, which is present 

 in both sexes, and up to the beginning of puberty is not well 

 developed. Its epithelial beginning (milk line or ridge) is seen 

 in the first months of embryonic life. After the commence- 

 ment of puberty the gland continues to develop in the female, 

 but undergoes a retrogression in the male. The highest de- 

 velopment in the female is reached at the end of pregnancy. 

 Shortly after the birth of the child the milk secretion or 

 lactation begins. The function of the mammary gland is 

 thus dependent on the sexual life. 



Before puberty this whole organ, in both sexes, consists 

 of connective tissue in which branched tubules are imbedded. 

 These represent the ducts of the completely developed gland, 

 and end blindly in saccular dilatations. In the adult female 

 there occur branched tubular gland bodies, but it is only dur- 

 ing pregnancy that these develop in large quantities at the 

 sides of the branched ducts. The newly formed branches of 

 the gland bodies possess also side twigs. 



The well-developed mammary gland (at the end of preg- 

 nancy and during lactation) consists of fifteen to twenty 

 conical lobes, which are arranged radially. Each lobe con- 

 sists of numerous smaller lobules, which represent a, large group 

 of gland bodies lying close together. These have the form of 

 alveoli, and lead into small ducts, which join to form the duct 

 of the mammary gland. Before the latter opens to the outside 

 through the nipple, it is widened to form the sinus lactiferus. 

 Each individual lobe represents really a separate compound 

 alveolar gland, since it opens into the nipple by an orifice of its 

 own, the porus lactiferus. The individual lobes are separated 

 from one another by loose connective tissue, which often con- 

 tains a quantity of fat. 



The finer structure of the alveolus (Fig. 258) differs accord- 

 ing to whether it is at rest or secreting. When at rest, the 

 round or pyriform alveoli are small and lined with cubical 

 granular epithelial cells. During the transition to the active 

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