196 THE REPRODUCTIVE SYSTEM. 



tophorous ducts, which are lined with a layer of low columnar 

 epithelium and gradually increase in size as they unite. The 

 ducts in each lobe unite to form a large outlet-duct, which 

 pursues a separate course to the nipple, distinct from the other 

 lobar ducts. These large lobar outlet-ducts have well-marked 

 walls of fibrous tissue and some unstriped muscle ; toward 

 the nipple they are dilated so as to form ampullce or sinuses, 

 which serve as temporary receptacles for the milk, and open 

 at the nipple by constricted orifices; they are lined with 

 simple columnar epithelium nearly to the nipple, where the 

 lining changes to stratified squamous. 



The nipple is covered with a sort of mucous membrane, 

 rosy or pigmented, lined with stratified squamous epithelium, 

 and containing numerous special sensory nerve-terminals. 

 Within the nipple is involuntary muscle, to which its erec- 

 tility is due. Scattered about the base of the nipple and the 

 surrounding areola are the small racemose glands of Mont- 

 gomery, especially developed during lactation, which corre- 

 spond to small projections on the surface. 



When not in a condition of active secretion, the alveoli are 

 smaller, less numerous, rudimentary or atrophic ; instead of 

 having a cavity their lumen is filled with cells, so that they 

 are represented by small solid nodules of epithelium. 



The system of ducts is present and well formed. 



Areolar and adipose tissue is more abundant, taking the 

 place of the undeveloped or degenerated alveoli, which are 

 imbedded in this increased amount of connective tissue. 



Such is the rudimentary and undeveloped structure of the 

 mamma in females before the first pregnancy, and in males. 



During pregnancy the gland undergoes active development, 

 especially the secretory alveoli, which increase in number and 

 size, while the intervening connective tissue is reduced, and 

 the cells in the centre of the solid alveolar epithelial masses 

 undergo fatty degeneration and at the beginning of lactation 

 are perhaps cast oif in the first milk as colostrum-corpuscles. 



After lactation is over the gland undergoes retrogressive 

 changes or involution to its former condition, the alveoli 

 atrophying and being replaced by adipose and areolar tissue. 



Milk, the secretion of the mammary gland, consists micro- 



