THE DISCHARGE OF URINE. 427 



kidney and ureter removed from the body, or even in an isolated excised 

 piece of the ureter. 



The rhythmically repeated contractions arise spontaneously in the mus- 

 cular coat of the ureter much in the same way as the similar cardiac con- 

 tractions arise in the muscular substance of the heart ; and it may here be 

 mentioned in support of what was urged in 141 with regard to the heart- 

 beats not being started by nerve-cells, that rhythmically repeated spontane- 

 ous peristaltic contractions have been observed in isolated pieces of ureter 

 taken from the middle of its course, in which no nerve-cells and indeed no 

 distinct nerve-fibres could be observed. 



In the living body these spontaneous movements, beats they might be 

 called, are subordinated to the flow of urine into the pelvis ; the more active 

 the secretion of urine, the more frequent 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. 



% 361. In the urinary bladder the urine is collected, its return into the 

 ureters being prevented by the oblique entrance into the bladder and valvu- 

 lar 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 longitudinal direction and 

 partly in a circular manner. After it has been emptied the bladder is con- 

 tracted and thrown into folds; as the urine gradually collects, the bladder 

 becomes mere and more distended. The escape of the fluid is in part pre- 

 vented by the resistance offered by the elastic fibres in the walls of the ure- 

 thra, which help to keep the urethral channel closed. But this is not all, 

 for observation shows that fluid is retained within the bladder up to a pres- 

 sure of twenty inches of water, so long as the bladder is governed by an 

 intact spinal cord, but gives way to a pressure of six inches only when the 

 lumbar spinal cord is destroyed 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. And it has been 

 maintained that it is the circularly disposed fibres specially developed 

 around the neck of the bladder which are the subjects of this tonic contrac- 

 tion and thus the chief cause of retention ; hence the name sphincter vesicse. 

 The continuity 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 of the bladder. The resistance in question is 

 supplied by a tonic contraction not of the circular fibres of the bladder 

 itself, but of the muscular fibres partly plain, partly striated surround- 

 ing the prostatic portion of the urethra, and constituting the sphincter vesicce 

 externm or prostaticus, or sphincter of Henle. It is stated that artificially 

 excited contractions of these fibres will resist a pressure of fluid in the 

 bladder. 



When the bladder has become full, we feel the need of making water, the 

 sensation being heightened, if not caused, by the trickling of a few drops of 

 urine from the full bladder into the urethra. We are then conscious of an 

 effort; during this effort the bladder is thrown into a long-continued con- 



