CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 681 



The involuntary muscular fibres forming the greater part of the 

 vesical walls are arranged as we have said partly in a more or less 

 longitudinal direction, and partly in a circular manner. 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 in part prevented 

 by the 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 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 main- 

 tained 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 contraction and thus the chief cause of the 

 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 

 these circular fibres of the bladder itself but of the muscular 

 fibres, partly plain, partly striated, surrounding the prostatic portion 

 of the urethra, and constituting the sphincter vesicce externus 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 contraction of an obscurely 

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

 overcome the resistance offered by the urethra, and the urine issues 

 in a stream, the sphincter vesicse externus 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, at all events of the last portions, is forwarded by irregularly 

 rhythmic contractions of the bulbo-cavernosus or ejaculator urinse 

 muscle, the contractions of which compress the urethra ; 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 

 defalcation. 



In the case of the rectum we were able ( 276) to distinguish 



