THE REPRODUCTIVE ORGANS. 219 



fied epithelium of the epidermis extends for a short distance into 

 its lumen. A little below the base of the nipple the duct presents 

 a fusiform dilatation, called the " ampulla," which serves as a reser- 

 voir for the comparatively small amount of milk secreted in the 

 intervals between nursings. 



The main duct branches in its course from the nipple into the 

 deeper portions of the gland, and these branches give off' twigs, 

 which terminate in the alveoli of the gland. The columnar epithe- 

 lium lining the main duct gradually passes into a cubical variety 

 in the branches, and this becomes continuous with the epithelial 

 lining of the alveoli. The terminal branches of the ducts are short, 

 so that the alveoli opening into them lie close together and are col- 

 lectively known as a "lobule" of the gland. These lobules are, in 

 turn, grouped into lobes, each of which corresponds to one of the 

 main ducts of the breast. 



The individual alveoli and the lobules are surrounded by fibrous 

 tissue, which may be subdivided into an intralobular and an inter- 

 lobular portion, the latter more abundant than the former. This 

 fibrous tissue supports the vessels and nerves supplied to the gland. 



The character of the epithelium lining the alveoli varies with the 

 functional activity of the gland. 



Before puberty the secreting acini are only slightly, if at all, 

 developed, the mamma consisting of a little fibrous tissue and the 

 ducts of the gland, which possess slightly enlarged extremities. 



When the gland has become fully developed, at or about puberty, 

 the epithelial cells lining the acini are small and granular and nearly 

 fill the diminutive lumina. The fibrous stroma is, at this period, 

 abundant and makes up the chief bulk of the breast. 



When the gland assumes functional activity the cells enlarge 

 and multiply (Fig. 194), and the lumina of the acini become dis- 

 tinct and filled with a serous fluid. Into this fluid a few fat-globules 

 are discharged from the epithelial lining, forming an imperfect milk, 

 very poor in cream and differing in the proportions of the dissolved 

 constituents from the milk that is produced after the function of 

 the gland is fully established. This secretion is called " colostrum." 

 Besides the scant supply of fat-globules which it contains, it is fur- 

 ther characterized by the presence of so-called colostrum -corpuscles. 

 These are leucocytes which have wandered into the acini of the 

 gland from the bloodvessels in the interstitial tissue, and have taken 

 some of the fat-globules of the secretion into their cytoplasm. This 



