MECHANISM OF THE DISCHARGE OF URINE. 371 



highly colored than the other layers. The middle layer is formed of circular 

 fibres, arranged, on the anterior surface of the bladder, in distinct bands at 

 right angles to the superficial fibres. They are thinner and less strongly 

 marked on the posterior and lateral surfaces. The internal layer is composed 

 of pale fibres arranged in longitudinal fasciculi, the anterior and lateral bun- 

 dles anastomosing with each other, as they descend toward the neck of the 

 bladder, by oblique bands of communication, and the posterior bundles inter- 

 lacing in every direction, forming an irregular plexus. Here they are not to 

 be distinguished from the fibres of the middle layer. This is sometimes called 

 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 vesicae is a band of non-striated fibres, about half an inch 

 (12'7 mm.) in breadth and one-eighth of an inch (3 - 2 mm.) 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 offers an 

 obstruction to its passage into the bladder. 



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 is stratified and presents the same diversity in form as that 

 observed in the pelvis of the kidney and the ureters ; viz., the deeper cells are 

 elongated and resemble 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 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 ordinary fibrous tissue, 

 with a few elastic and muscular fibres. At the opening of the urethra, it 

 presents a small, projecting fold of mucous membrane, which is sometimes 

 called the uvula vesicae. Over the whole of the surface of the trigone, the 

 mucous membrane is very closely adherent, and it is never thrown into folds, 

 even when the bladder is entirely empty. 



The blood-vessels going to the bladder are ultimately distributed to its 

 mucous membrane. They are not very abundant except at the fundus, where 

 the mucous membrane is quite vascular. Lymphatics have been described as 

 existing in the walls of the bladder, but Sappey has failed to demonstrate 

 them in this situation. The nerves of the bladder are derived from the hypo- 

 gastric plexus. 



The urethra is provided with muscular fibres, and it is lined by a mucous 

 membrane, the anatomy of which will be more fully considered in connection 

 with the physiology of generation. In the female the epithelium of the ure- 

 thra is like that of the bladder. In the male the epithelial cells are small, 

 pale and of the columnar variety. 



Mechanism of the Discharge of Urine. In the human subject the urine 



