348 HISTOLOGY 



muscles, consist of simple low epithelium. They may connect directly 

 with the end pieces, or a secretory duct may intervene. 



The muscularis of the prostatic part of the urethra consist of an 

 inner longitudinal and an outer circular layer of smooth muscles. Both 

 layers continue throughout the membranous part; the circular layer ends 

 in the beginning of the cavernous urethra leaving only oblique and lon- 

 gitudinal bundles in its distal part. 



Corpus cavernosum urethra. In the submucosa of the cavernous 

 urethra there are many veins (Fig. 349) which become larger and more 

 numerous in and beyond the muscularis. This vascular tissue which 

 surrounds the urethra is limited by a dense elastic connective tissue layer, 

 the tunica albuginea, and the structure which is thus bounded is the corpus 

 cavernosum urethra. Toward the perineum it ends 

 in a round enlargement, the bulbus urethra, and 

 distally it terminates in the glans penis. The urethra 

 enters the upper surface of this corpus cavernosum 

 near the bulbus. Branches of the internal pudendal 

 (pudic) artery, namely, the arteriae bulbi and the 

 urethral arteries, penetrate the albuginea, and the 

 former pass the length of the cavernous body and 



PIG. 35o.-CRosi SECTION end in the g lans - These arteries have particularly 

 thick walls of circular muscle, and in cross sections 

 the initma may be seen to form coarse rounded pro- 

 Ela8ti (AfterEberth S .) ain ' jections into the lumen. These projections contain 

 longitudinal muscles and subdivisions of the inner 

 circular elastic membrane (Fig. 350). The arteries in the corpus caver- 

 nosum produce capillaries found chiefly toward the albuginea. The 

 capillaries empty into thin-walled venous spaces which appear as endothe- 

 lium-lined clefts in a connective tissue containing many smooth muscle 

 fibers. The cavernous body is permeated with these spaces which, at 

 times of sexual excitement, become distended with blood, reducing the 

 tissue between them to thin trabeculae. Such distensible vascular 

 tissue is known as erectile tissue. Some arteries connect directly with the 

 venous spaces, and such as appear coiled or C-shaped in a collapsed 

 condition are called arteries helicina. The vena cavernosa have such very 

 thick walls that they resemble arteries. They contain an abundance 

 of inner longitudinal muscle fibers, and since these are not evenly dis- 

 tributed but occur in columns, the lumen of the veins is usually crescentic 

 or stellate in cross section. Emissary veins pass out through the albu- 

 ginea and empty into the median dorsal vein of the penis. 



The corpora cavernosa penis are a pair of structures similar to the 

 cavernous body of the urethra, and are found side by side above it (Fig. 

 351). The septum between them is perforated distally so that they 



