424 COMPARATIVE PHYSIOLOGY. 



THE EZPUIiSION OF URINB. 



We now present in concise form certain facts on which to 

 base opinions as to the nature of the processes by which the 

 bladder is emptied. 



It will be borne in mind that the secretion of urine is con- 

 stant, though of course very variable, that the urine is con- 

 veyed in minute quantities by rhythmically contractile tubes 

 (ureters) which open into the bladder obliquely ; and that the 

 bladder itself is highly muscular, the cells being arranged both 

 circularly and obliquely, with a special accumulation of the 

 circular fibers around the neck of the organ to form the sphinc- 

 ter vesicae. 



1. It is found that the pressure which the sphincter of the 

 bladder can withstand in the dead is much less (about one 

 third) than in the living subject. 2. We are conscious of being 

 able to empty the bladder, whether it contains much or little 

 fluid. 3. We are equally conscious of an urgency to evacuation 

 of the vesical contents, according to the fullness of the organ, 

 the quality of the urine, and a variety of other conitions. 



4. Emotions may either retard or render micturition urgent. 



5. In a dog in which the cord has been divided in the dorsal 

 region some months previously, micturition may be induced 

 reflexly, as by sponging the anus. 6. In the paralyzed there 

 may be retention or dribbling of urine. 7. In cases of urethral 

 obstruction from a calculus, stricture, etc., there may be excess- 

 ive activity of the muscular tissue of the bladder walls. 8. 

 Evacuation of the bladder may occur in the absence of con- 

 sciousness (sleep). 



The most obvious conclusions from these facts are that— 1. 

 The urine finds its way -to the bladder partly through muscular 

 (peristaltic) contractions of the ureters, partly through gravity, 

 in man at all eyents, and partly from the pressure within the 

 tubules of the kidneys themselves. 2. The evacuation of urine 

 may take place independently of the will (see 8), and is a reflex 

 (5) act. 3. Micturition may be initiated by the will, which is 

 usually the case, when by the action of the abdominal muscles 

 a little urine is squeezed into the urethra, upon which afferent 

 impulses set up contractions of the bladder by acting on the 

 detrusor center of the cord and at the same time inhibit the 

 center presiding over the sphincter (if such there be), permit- 

 ting of its relaxation. 4. Emotions seem to interfere with the 



