238 NORMAL HISTOLOGY. 



short, very dilatable, and of large size. Its walls consist of a mucosa, 

 composed of fibrous tissue intermingled with many elastic fibres and 

 containing large numbers of leucocytes, and an epithelium of the 

 stratified squamous variety, continuous with the transitional epi- 

 thelium of the bladder on the one hand and with the epithelium of 

 the vestibule on the other. The mucous membrane of the urethra 

 presents longitudinal folds, especially on the posterior wall, and con- 

 tains many tubular mucous glands ; several of these, near the 

 urethral orifice, are of large size. Near the vestibular end of the 

 canal on either side are found the rudimentary paraurethral 

 ducts or tubes of Skene. Outside the submucosa follows in- 

 voluntary muscle, disposed as an inner thin longitudinal and an 

 outer thick circular layer. 



THE MAMMARY GLANDS. 



The mammary glands are usually described in connection with the 

 female reproductive organs, although these structures are only modi- 

 fied and specialized sebaceous integumentary glands ; strictly re- 

 garded, the mammae, therefore, belong to the consideration of the skin. 

 Each mamma consists of from fifteen to twenty distinct tubo- 

 racemose glands or lobes, which are held together by connective 

 tissue and united into a single hemispherical mass by adipose tissue, 

 which fills all irregularities and interspaces between the divisions of 

 the organ. Each lobe, supplied by its own excretory duct, is sub- 

 divided by penetrating fibrous septa into lobules, which, in turn, are 

 composed of groups of individual acini. The histological details of 

 p the secreting portions of 



the organ vary with the 

 stages of its functional ac- 

 tivity: the following de- 

 scription applies to the 

 active glands as seen 

 during lactation. 



The acini, usually tu- 

 bular or saccular in form, 

 are grouped together to 

 constitute the lobules ; 

 limited by a distinct 

 membrana propria, 

 they are lined by a single 

 layer of short columnar 

 or polyhedral epithe- 

 lial cells, whose protoplasm differs in appearance with the condition 

 of secretion. At rest, these cells are uniformly granular; as secre- 



Section of human mammary gland during lactation : a, a, 

 sections of the large tubular acini which constitute almost the 

 entire lobule; b, interlobular connective-tissue septa. 



