KIDNEY AND SKIN AS EXCRETORY ORGANS. 763 



layers is not so definite as in the case of the intestine; they seem, 

 in fact, to form a continuous layer, one passing gradually into the 

 other by a change in the direction of the fibers. At the cervix the 

 circular layer is strengthened, and has been supposed to act as a 

 sphincter with regard to the urethral orifice the so-called sphinc- 

 ter vesicae internus. Around the urethra just outside the blad- 

 der is a circular layer of striated muscle that is frequently desig- 

 nated as the external sphincter or sphincter urethrse. The urine 

 brought into the bladder accumulates within its cavity to a certain 

 limit. It is prevented from escaping through the urethra at first 

 by the mere elasticity of the parts at the urethral orifice, aided per- 

 haps by tonic contraction of the internal sphincter, although this 

 function of the circular layer is disputed by some observers. When 

 the accumulation becomes greater the external sphincter is brought 

 into action. If the desire to urinate is strong the external sphincter 

 seems undoubtedly to be controlled by voluntary effort, but whether 

 or not, in moderate filling of the bladder, it is brought into play 

 by an involuntary reflex is not definitely determined. Backflow 

 of urine from the bladder into the ureters is effectually prevented 

 by the oblique course of the ureters through the wall of the blad- 

 der. Owing to this circumstance, pressure within the bladder 

 serves to close the mouths of the ureters, and, indeed, the more 

 completely, the higher the pressure. At some point in the filling 

 of the bladder the pressure is sufficient to arouse a conscious sen- 

 sation of fullness and a desire to micturate. Under normal condi- 

 tions the act of micturition follows. It consists essentially in a 

 strong contraction of the bladder, with a simultaneous relaxation 

 of the external sphincter, if this muscle is in action, the effect of 

 which is to obliterate more or less completely the cavity of the blad- 

 der and drive the urine out through the urethra. 



The force of this contraction is considerable, as is evidenced by 

 the height to which the urine may spurt from the end of the urethra. 

 According to Mosso, the contraction may support, in the dog, a 

 column of liquid two meters high. The contractions of the blad- 

 der may be and usually are assisted by contractions of the walls 

 of the abdomen, especially toward the end of the act. As in defeca- 

 tion and vomiting, the contraction of the abdominal muscles, when 

 the glottis is closed so as to keep the diaphragm fixed, serves to in- 

 crease the pressure in the abdominal and pelvic cavities, and thus 

 assists in or completes the emptying of the bladder. It is, 

 however, not an essential part of the act of micturition. The last 

 portions of the urine escaping into the urethra are ejected, in the 

 male, in spurts produced by the rhythmical contractions of the 

 bulbocavernosus muscle. 



Considerable uncertainty and difference of opinion exists as to 



