538 



THE EEPRODUCTIVE SYSTEM 



canalized fibrin. Wherever the main stems are inserted into the decidua 

 the epithelium which formerly covered their tips appears to have also 

 degenerated into a peculiar hyalin border zone. Towards the end of preg- 

 nancy the cytotrophoblast becomes converted into plasmoditrophoblast. 



Within its syncytium the substance of the villus consists of the super- 

 ficial cells of Langhans with their large ovoid nuclei, and a core of con- 

 nective tissue of a delicate embryonic type, in which are the fetal blood- 

 vessels. Even the smallest villi contain capillary loops of broad caliber, 

 which are supplied by fetal arteries, derived from the umbilical arteries, 

 which distribute their branches throughout the chorionic connective tissue. 

 The fetal veins accompany the arteries. 



The Vagina 



The vagina is a fibromuscular sheath whose wall is divisible into 

 three coats mucous, muscular, and fibrous. 



The mucous membrane is clothed by a layer of stratified squamous 

 epithelium, and is thrown into numerous folds or rugae. The epithelium 



FIG. 472. VAGINAL MUCOSA. X 90. 

 ep., epithelium; p., papilla; c.t., connective tissue. (After Williams.) 



rests upon a fibrous basement membrane. The tunica propria is 

 formed by a close-meshed areolar tissue which, in its deeper and looser 

 portion, is permeated by vascular channels of considerable size. This 

 deep vascular layer is frequently described as a submucosa; it rests 

 directly upon the muscular wall. The surface of the mucosa presents 

 numerous conical papillae which project well into the epithelial layer. 

 The musculature of the vagina contains smooth or involuntary 

 fibers, and is divisible into an inner circular and an outer longitudinal 

 layer. The muscle fibers are long and slender. Considerable connective 



