172 



THE ESSENTIALS OF HISTOLOGY 



The urinary bladder has a muscular wall lined by a strong mucous 

 membrane and covered in part by a serous coat. 



The muscular coat consists of three layers, but the innermost is 

 incomplete. The principal fibres run longitudinally and circularly, 

 and the circular fibres are collected into a layer of some thickness 

 which immediately surrounds the commencement of the urethra, form- 

 ing the sphincter vesicce. The mucous membrane is lined by a transi- 

 tional stratified epithelium like that of the ureter. The shape and 

 structure of the cells have already been studied. 



The penis is mainly composed of cavernous tissue which is collected 

 into two principal tracts the corpora cavernosa, one on each side, and 

 the corpus spongiosum in the middle line inferiorly. All these are 

 bounded by a strong capsule of fibrous and muscular tissue, containing 

 also many elastic fibres and sending in strong septa or trabeculas, 



FIG. 208. SECTION OF ERECTILE TISSUE. (Cadiat.) 



a, trabeculse of connective tissue, with elastic fibres, and bundles of plain muscular 

 tissue (c) ; b, venous spaces. 



which form the boundaries of the cavernous spaces of the erectile tissue 

 (fig. 208). The arteries of the tissue run in these trabeculae, and their 

 capillaries open into the cavernous spaces. On the other hand, the 

 spaces are connected with efferent veins. The arteries of the cavernous 

 tissue may often in injected specimens be observed to form looped or 

 twisted projections into the cavernous spaces (helicine arteries of 

 Miiller). 



Urethra. The cross-section of the urethra appears in the middle 

 of the corpus spongiosum in the form of a transverse slit. It is lined 

 by columnar epithelium, except near its orifice, where the epithelium is 

 stratified. The epithelium rests upon a vascular mucous membrane, 



