328 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



exhibit periodic peristaltic contractions. It does not seem possible at present 

 to decide between these two views as to the cause of the contractions. The 

 nature of the contractions, their mode of progression, and the way in which 

 they force the urine through the ureter seem, however, to be clearly established. 

 Efforts to show a regulatory action upon these movements through the central 

 nervous system have so far given only negative results. 



Movements of the Bladder. The bladder contains a muscular coat of plain 

 muscle-tissue, which, according to the usual description, is arranged so as to 

 make an external longitudinal coat and an internal circular or oblique coat. 

 A thin longitudinal layer of muscle-tissue lying to the interior of the circular 

 coat is also described. The separation between the longitudinal and circular 

 layers is not so definite as in the case of the intestine ; they seem, in ^act, to form 

 a continuous layer, one passing gradually into the other by a change in the 

 direction of the fibres. 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 sphincter vesicse internus. Round the urethra just outside the blad- 

 der is a circular layer of striated muscle which is frequently designated 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 escape 

 through the urethra at first by the mere elasticity of the parts at the urethral 

 orifice, aided perhaps by tonic contraction of the internal sphincter, although 

 this function of the circular layer at this point 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 undoubt- 

 edly 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. Back-flow of urine from the bladder into the ureters is effectually 

 prevented by the oblique course of the ureters through the wall of the 

 bladder. Owing to this circumstance pressure within the bladder serves to close 

 the mouths of the ureters, and indeed the more completely the higher the pres- 

 sure. At some point in the filling of the bladder the pressure is sufficient to 

 arouse a conscious sensation of fulness and a desire to micturate. Under nor- 

 mal conditions 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 bladder 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 spirt 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 bladder may be and usually are assisted by 

 contractions of the walls of the abdomen, especially toward the end of the act. 

 As in defecation and vomiting, the contraction of the abdominal muscles, when 

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

 pressure in the abdominal and pelvic cavities, and is thus used to assist in or 



