650 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



almost totally emptied, the glottis closes, and the abdominal muscles, 

 contracting, serve to force down the abdominal contents into the pelvis, 

 and so by pressure from above aid the emptying of the bladder. Section 

 of the spinal cord above the level of this centre first causes retention of 

 urine by increasing the reflex activitjr of the urethral sphincter and by 

 interfering with the conduction of inhibitory impulses from the brain. 

 As soon as the bladder becomes distended the sphincter becomes mechan- 

 ically dilated, and the urine trickles away in drops, but less rapidly than 

 it enters from the kidney ; so the bladder becomes enormously dilated 

 and the retained urine is apt to undergo ammoniacal fermentation. 

 Goltz, however, has seen dogs micturate in a perfectly normal manner 

 after complete division of the spinal cord above the lumbar region. 



While urination is thus originally a purely reflex action, it is pos- 

 sible by education to bring it, to a very considerable extent, under the 

 control of the will. The contact of the first few drops of urine with the 

 mucous membrane of the urethrse, instead of inaugurating the act of 

 micturition, may by an exertion of the will lead to an increase of 

 the tonic contraction of the sphincter of the bladder instead of to its 

 inhibition. The act of micturition is thus voluntarily postponed 

 (Fig. 270). 



In addition to this voluntary control of the act of micturition, this 

 process is also largely governed by the emotions, and the starting point 

 of the act may originate not only in the distention of the bladder, but 

 in various forms of irritation of the genital apparatus. Such a cause 

 will frequently be found to explain the urinary incontinence of children. 



