ANATOMY OF THE URINARY PASSAGES. 181 



and lateral bundles anastomosing with each other as they- 

 descend toward the neck of the bladder, by oblique bands 

 of communication, and the posterior bundles interlacing in 

 every direction, forming an irregular plexus. Here they 

 are not to be distinguished from the fibres of the middle 

 layer. This arrangement has given to these fibres the name 

 of the plexiform layer, and it gives to the interior of the 

 bladder its reticulated appearance. This layer is continuous 

 with the muscular fibres of the urachus, the ureters, and the 

 urethra. 



The sphincter vesicas is composed of a band of smooth 

 fibres, about half an inch in breadth and one eighth of an 

 inch in thickness, embracing the neck of the bladder and 

 the posterior half of the prostatic portion of the urethra. 

 The tonic contraction of these fibres prevents the flow of 

 urine, and during the ejaculation of the seminal fluid, it 

 ofiers an obstruction to its discharge into the bladder. 



It is seen, from this arrangement of the muscular fibres 

 of the bladder, that they are capable by their contraction of 

 expelling the greatest part of the urine when the sphincter 

 is relaxed. 



The mucous membrane of the bladder is smooth, rather 

 pale, thick, and loosely adherent to the submucous tissue, 

 except over the corpus trigonum. The epithelium exists in 

 several layers, and presents the same diversity in form that 

 is observed in the pelvis of the kidney and the ureters ; viz., 

 the deeper cells are elongated and resemble the columnar 

 epithelium, while the cells on the surface are flattened. In 

 the neck and fundus of the bladder are a few mucous 

 glands ; some in the form of simple follicles, and others 

 collected so as to form glands of the simple racemose variety. 



The corpus trigonum is a triangular body, lying just 

 beneath the mucous membrane at the base of the bladder, 

 and extending from the urethra in front to the openings of 

 the ureters. It is composed of white fibrous tissue, with a 

 few elastic and muscular fibres. At the opening of the 



