THE UEINAEY TRACT. 781 



the pyramidal substance (Henle). Outside the circular layer, 

 again, a varying number of longitudinal bundles are found, all 

 being freely connected with each other by oblique bundles, and 

 having marked septa of a fibrous perimysium. 



(3) The urinary bladder is, in all essential parts, identical with 

 the ureters ; but its muscle-layers are very broad, and arranged 

 like a complex hurdle-work. The inner and outer layers take a pre- 

 vailing longitudinal course ; the middle layers run mostly in a 

 circular direction. The longitudinal bundles are more marked 

 in the main walls of the bladder, while the circular fibers prevail 

 at the place of transition into the ureters and the urethra, pro- 

 ducing the internal sphincter muscle. These fibers are also well 

 developed in the mucosa between the two orifices of the ureters 

 the trigonum. The diameter of the walls of the bladder varies 

 from two to fifteen mm., according to its conditions of contrac- 

 tion and extension. The arteries enter the posterior wall of the 

 bladder, and are traceable through the muscle-coat into the 

 mucous layer, where they break up into a rich capillary plexus. 

 The nerves are most numerous in the lowest portion of the blad- 

 der the so-called neck. 



(4) The urethra is a prolongation of the bladder, and presents 

 a structure similar to that of the bladder in the female ; while 

 the male urethra, being a canal common to the urinary and the 

 genital tract, is of a more complicated character. 



The female urethra is lined with stratified epithelium; its 

 mucosa holds a few racemose mucous glands ; its muscle-layers 

 consist of an inner longitudinal and an outer circular the smooth 

 fibers of the latter blending, at the periphery of the urethra, with 

 the striated muscle-fibers. 



The male urethra has a stratified epithelium. It is maintained 

 by some authors that, in the whole cavernous and in a large part 

 of the membranous portion, there is but one layer of columnar 

 epithelia. As to the different portions of the urethra, the nature 

 of the covering epithelium requires more exact studies than have 

 yet been made up to the present time, and it is quite possible 

 that there are individual as well as local differences. The epithe- 

 lial prolongations, in the form of racemose mucous glands (so- 

 called Littre's glands), are quite numerous along the mucosa of 

 the urethra, more particularly along the posterior wall. These 

 often exhibit very long and winding ducts, and but few acini ; 

 the ducts sometimes open into larger, funnel-shaped pits at the 

 inner surface of the mucosa. The muscle-layer is double along 



