334 EXCRETION 



that when they contract they diminish its capacity chiefly by shorten- 

 ing its vertical diameter. The fibres of the external layer are of a 

 pinkish hue, being much more 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 superfi- 

 cial 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 bundles anas- 

 tomosing 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 is 

 sometimes called the plexif orm 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 millimeters) in breadth and one-eighth of an inch (3.2 milli- 

 meters) in thickness, embracing the neck of the bladder and the poste- 

 rior half of the prostatic portion of the urethra. The tonic contraction 

 of these fibres prevents the discharge of urine, and during the ejacula- 

 tion of semen, 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. 

 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 sur- 

 face of the trigone, the mucous membrane is closely adherent, and it is 

 not thrown into folds even when the bladder is entirely empty. 



The bloodvessels going to the bladder are finally distributed to its 

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

 where the mucous membrane is very vascular. Lymphatics are 

 found in the walls of the bladder. The nerves of the bladder are 

 derived from the hypogastric plexus. 



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



