CHAP, iv.] RENAL SECRETION. 411 



vesicse. After it has been emptied the bladder is contracted and 

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

 becomes more and more distended. The escape of the fluid is 

 however prevented by the resistance offered by the elastic fibres of 

 the urethra which keep the urethra channel closed. Some 

 maintain that a tonic contraction of the sphincter vesicae aids in or 

 indeed is the chief cause of this retention. The continuity of the 

 sphincter vesicse with the rest of the circular fibres of the bladder 

 suggests that it probably is not a sphincter, but that its use lies in 

 its contracting after the rest of the vesical fibres, and thus finish- 

 ing the evacuation of the bladder. On the other hand, the fact 

 that the neck of the bladder can withstand a pressure of 20 inches 

 of water so long as the bladder is governed by an intact spinal 

 cord, but a pressure of 6 inches only when the lumbar spinal cord 

 is destroyed or the vesical nerves are severed, affords very strong 

 evidence in favour of the view 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. 



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 contraction of an obscurely 

 peristaltic nature, the force of which is more than sufficient to 

 overcome the elastic resistance of the urethra, and the urine issues 

 in a stream, the sphincter vesicse, if it act as a sphincter, being at 

 the same time either relaxed after the fashion of the sphincter ani, 

 or at least overcome. In its passage along the urethra, the exit 

 of the urine is forwarded by irregularly rhythmic contractions of 

 the bulbo-cavernosus or ejaculator urinse muscle, and the whole 

 act is further assisted by pressure on the bladder exerted by 

 means of the abdominal muscles, very much the same as in defse- 

 cation. 



We said just now, "when the bladder has become full," but this 

 must not be understood to mean, "when the bladder has received a 

 certain quantity of fluid." On the contrary, it is a matter of 

 common experience that we feel the desire to make water some- 

 times when a large quantity and sometimes when a small quantity 

 of urine has accumulated in the bladder. We have evidence that 

 the bladder possesses to a very high degree that obscure con- 

 tinuous contraction which we speak of as 'tone' ; and further that 

 the amount of its tone is exceedingly variable, the organ, quite 

 independently of distinct efforts at micturition, being at one time con- 

 tracted and at another flaccid and distended. When it is in a 

 contracted state, a small quantity of fluid may exert the same 

 pressure on the vesical walls as a larger quantity when the bladder 

 is flaccid. Hence the determining cause of the desire to make 



