TEE URINARY APPARATUS. 575 



prevents the "flowing back of the urine into the nreter during its expulsion, the 

 intermembranous portion of that canal being strongly compressed by the external 

 pressure then exerted by the muscular coat, and by the internal resistance which 

 the accumulation of urine in the bladder opposes to this pressure. So well are 

 Nature's intentions fuhilled in this respect, that we may inflate the bladder by 

 the ureter, after tying the canal of the urethra, and press vigorously on the dis- 

 tended organ, without being able to make a single bubble of air pass through the 

 perfectly pervious canal. 



Steuctuee. — The excretory canal of the kidney is composed of three tunics : 



1. An internal mucous tunic, continuous, in front, with that lining the pelvis 

 of the kidney, and behind, with that of the bladder. It is very tliin, pale, 

 plicated longitudinally, and has a stratified tesselated epithelium. (It has some 

 mucous follicles, but no villi.) 



2. A middle inuscular layer arranged in two orders — a superficial, the fibres 

 of which are circular, and a deep set, passing in a longitudinal direction. (Leyh 

 and other authorities describe the arrangement of the muscular planes — which are 

 composed of smooth fibres — to be the reverse of this, the longitudinal being super- 

 ficial, and the deep circular.) 



3. An external tunic, composed of connective tissue and elastic fibres. 



The muscular tissue of the ureter, by contracting, accelerates the flow of the 

 urine. 



3. The Bladdee (Fig. 338). 



Position. — This is a membranous reservoir, lodged in the pelvic cavity, where 

 it occupies more or less space, according to the quantity of urine it contains ; it 

 may extend beyond the pubis, into the abdominal cavity. 



Form. — Considered in a moderate state of plentitude, the bladder is ovoid in 

 figure ; its large extremity, being turned forward, forms a rounded cul-de-sac 

 {fundus), at the bottom of which is remarked a kind of cicatrice, caused by the 

 obliteration of the urachus. The other extremity terminates, posteriorly, by a 

 well-marked constriction — the neck (or cervix) of the bladder — which gives rise 

 to the urethral canal. 



Weight. — The average weight of the empty bladder is about sixteen ounces. 



Relations and mode of attachment. — The bladder is related : above, to the 

 vesiculae seminales, to the pelvic dilatations of the vasa deferentia, as well as to 

 the rectum ; below, to the inferior wall of the pelvis, on which it rests (by its 

 base) ; on the sides, to the lateral walls of that cavity. In the female, the upper 

 face of the bladder is in relation with the uterus and vagina, which entirely 

 separate it from the rectum. The posterior extremity or neck (cervix), flanked on 

 each side by the lobes of the prostate, is fixed below to the ischio-pubic symphysis, 

 by means of a particular ligament or fasciculus of elastic and contractile fibres, 

 which are detached from the muscular layer, and expanded over the lower face of 

 Wilson's muscle, to be carried backwards and downwards, and terminate on the 

 surface of the internal obturator muscle. The anterior extremity — or fundus — 

 is usually related to the pelvic flexure of the large colon. 



This extremity is covered by a serous cap, which is prolonged backwards on 

 its body, further above than below. This covering is continuous with the 

 parietal layer of peritoneum, and adheres closely to the muscular tunic -of the 

 bladder, so that it constitutes its chief attachment ; its arrangement is precisely 

 similar, in principle, to that of the other serous visceral membranes. 



