URINARY SYSTEM. 475 



canal, urethra, has first a position vertically downwards, then 

 it turns and curves forwards ; thus, this canal is liable to be 

 compressed by the enormous distention of the bladder. 



Again, the prostate gland (P/>, Fig. 129) is a hard unyield- 

 ing organ, being composed of glands, fibrous tissue, and 

 muscular elements; this urethral opening penetrates and is 

 encircled by this prostate gland in such way as to have its 

 walls closed by contact. This forms the principal cause in 

 man of the retention of urine during the inaction of the 

 bladder. Should the prostate gland become hypertrophied, 

 a still greater obstacle (too much so in old men) is made to 

 the passage of urine, and becomes the cause of a pathological 

 retention. 



Finally, the flattening of the urethral canal and its closure 

 by contact are influenced by the arrangement of the perineal 

 fasciae, the fibres of which press upon the sides of the ure- 

 thral canal in their course from the ischium to the pubis ; and 

 a certain muscular and expulsive effort is required to over- 

 come this constraint, and dilate the orifice. 



It is not surprising in view of this explanation that the 

 urine is allowed to accumulate in this muscular, dilatable, and 

 elastic reservoir, and that no physiological act or contraction 

 is required to prevent the exit of the urine ; these conditions 

 are simply mechanical and continue after death, since urine 

 is often found in the bladder of the dead body. 



When the walls of the bladder become too much distended 

 by the presence of urine, we have said that a compression of 

 the contents is produced by contraction of the smooth mus- 

 cular fibres ; this overcomes the elasticity of the neck of the 

 bladder and of the prostate, and the urine passes into the 

 bulbous portion of the urethra : here it comes in contact with 

 a very sensitive mucous surface, the prostatic mucous mem- 

 brane, which presides over a large number of reflex phenom- 

 ena. It is owing to this contact of the urine with the mucous 

 surface that we experience that peculiar sensation of a neces- 

 sity or desire for micturition, and which we refer, in common 

 with almost all other sensations of this region, to the fossa 

 navicularis. If we pay no attention to this desire, a reflex 

 irritation is produced, which is followed by the contraction 

 of the constrictor urethra?, or urethral sphincter ; the urine 

 can then go no farther, and is even obliged to retrograde, on 

 account of the contraction of the muscles on the anterior por- 

 tion of the prostate, and so re-enters the bladder whose con- 

 tractions have ceased. 



