408 EXCRETION. 



The middle, or muscular coat, consists of fibres of the non-striated or involuntary 

 variety, arranged in three tolerably distinct layers. 



The external muscular layer is composed of longitudinal fibres, which arise from parts 

 adjacent to the neck, and pass anteriorly, posteriorly, and laterally over the organ, so 

 that when they are contracted they diminish its capacity chiefly by shortening its verti- 

 cal diameter. The anterior fibres of this layer arise from the body of the pubis and the 

 symphysis, by tendinous bands, known to most anatomists as the anterior ligaments. 

 These tendinous fibres spread out upon the prostate and are attached to its anterior sur- 

 face. As the fibres on the anterior surface pass over the summit of the bladder, they in- 

 terlace, and some of them are continuous with the fibres coming from the posterior sur- 

 face. The posterior fibres arise from the base of the prostate, and, after forming a dis- 

 tinct band an inch or an inch and a quarter in breadth, spread out upon the posterior sur- 

 face of the bladder. The lateral fibres arise from the sides of the prostate and spread 

 out upon the lateral surfaces of the bladder. In the female, the posterior fibres arise 

 from the dense fibrous membrane between the neck of the bladder and the vagina, and 

 the lateral fibres, from the perineal aponeurosis, the anterior fibres arising from the pubis, 

 as in the male. The fibres of the external layer are of a pinkish hue, being much more 

 highly colored than the other layers. 



The middle muscular layer is formed of circular fibres, arranged, on the anterior sur- 

 face 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 muscular layer is composed of excessively pale fibres arranged in longi- 

 tudinal fasciculi, the anterior 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 vesica3 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 offers 

 an obstruction to its passage into the bladder. 



It is seen, from the 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 sphinc- 

 ter is relaxed. 



The mucous membrane of the bladder is smooth, rather pale, thick, and loosely ad- 

 herent to the submucous tissue, except over the corpus trigonum. The epithelium exists 

 in several layers 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 the co- 

 lumnar 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 white 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 vesicas. 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 mem- 

 brane. They are not very numerous, except at the fundus, where the mucous mem- 



