CHAP, in.] ELIMINATION OF WASTE PRODUCTS. :>i: 



In the living body these spontaneous movements, beat* they 

 might be called, are subordinated to the flow of urine into the 

 pelvis ; the more active the secretion of urine the more fre- 

 quent and vigorous are the beats of the pelvis and ureter ; but 

 the exact mechanism by which the secretion and the movements 

 are maintained in harmony has not yet been cleared up. 



Micturition. 



345. In the urinary bladder, the urine is collected, its 

 return into the ureters being prevented by the oblique entrance 

 into the bladder and valvular nature of the orifices of those 

 tubes ; and its discharge from thence in considerable quantity 

 is effected from time to time by a somewhat complex muscular 

 mechanism, of the nature and working of which the following 

 is a brief account. 



The involuntary muscular fibres forming the greater part of 

 the vesical walls are arranged partly in a more or less longitu- 

 dinal, and partly in a circular manner. The bladder after it 

 has been emptied is contracted and thrown into folds; as the 

 urine gradually collects, the bladder becomes more and more 

 distended. The escape of the fluid is in part prevented by tlie 

 resistance offered by the elastic fibres in the walls of the urethra 

 which help to keep the urethral channel closed. But this is 

 not all ; for observation shews that fluid is retained within the 

 bladder up to a pressure of 20 inches of water so long as the 

 bladder is governed by an intact spinal cord, but gives way to 

 a pressure of 6 inches only when the lumbar spinal cord is de- 

 stroyed or the vesical nerves are severed. This affords very 

 strong evidence that the obstruction at the neck of the bladder 

 to the exit of urine depends on some tonic muscular contraction 

 maintained by a reflex or automatic action of the lumbar spinal 

 cord. It has been maintained that the circularly disposed hbres 

 specially developed around the neck of the bladder are the sub- 

 jects of this tonic contraction and thus the chief instrument <>f 

 the retention ; hence the name sphincter vesicse. The continu- 

 ity of these fibres, however, with the rest of the circular fibres 

 of the bladder suggests that they probably do not act as a 

 sphincter, but that their use lies in their contracting after the 

 rest of the vesical fibres, and thus finishing the evacuation >f 

 the bladder. The resistance in question is supplied by a tonic 

 contraction not of these rirmiur lilnvs of the l>hul<ler itself but 

 of the muscular fibres, partly plain, partly striated, surrounding 

 the prostatic portion of the urethra, and constituting the tyhinctcr 

 vesicce externus or prost aticua or sphincter of Henle. It is stated 

 that artificially excited contractions of these fibres will resist a 

 pressure of fluid in the bladder. 



35 



