858 PHYSIOLOGY OF DIGESTION AND SECRETION. 



strengthened to form a ring around the orifice and along the begin- 

 ning of the urethra which is supposed to function as a sphincter, the 

 internal sphincter or sphincter vesicce internus. Around the urethra, 

 in the prostate and membranous portions, is a circular layer of 

 striated muscle that 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 escaping through the urethra by a tonic contrac- 

 tion of the internal sphincter. When the accumulation becomes 

 greater the external sphincter may be brought into action. Back- 

 flow of urine from the bladder into the ureters is effectually pre- 

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

 the bladder. Owing to this circumstance, pressure within the blad- 

 der 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 internal sphincter, and of the external sphincter also if this 

 latter is in contraction. 



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. 



The act of micturition as it takes place in man is pictured by 

 Rehfisch* as follows: As the urine accumulates in the bladder the 

 pressure-stimulation of the sensory fibers leads to a reflex stimu- 

 lation of the internal sphincter. Further accumulation by a pres- 

 sure effect on the sensory fibers causes reflex contractions of the 

 muscle of the bladder, and the additional sensory stimuli produced 

 by these contractions spreading upward from the lower center occa- 

 sion the conscious desire to urinate. In the adult at least the 

 urination takes place by a voluntary act, which consists in an inhi- 

 * Virchow's "Archiv f. path. Anat.," etc., 150, 111, 1897. 



