430 THE URINARY APPARATUS. 



first group causes feeble, and of the second strong, contraction of 

 the bladder. 



Function of the Bladder. The bladder acts as a reservoir for 

 the urine until such time as it is passed in the act of urination or 

 micturition. The urine is retained within the bladder by the tonic 

 contraction of the sphincter vesicse in the same manner as feces 

 are retained in the rectum by the sphincter ani. The pressure of 

 the urine when the bladder is full is equal to only 1 cm. of mer- 

 cury, while it takes a pressure of at least 3 cm. to overcome the 

 elasticity of the sphincter. When the bladder is about to be 

 emptied, as the result of an inhibitory impulse, the sphincter vesicse 

 becomes relaxed. At the same time the muscular coat of the 

 bladder and the abdominal muscles contract, and the urine begins 

 to flow. The pressure which is thus exerted equals 10 cm. of 

 mercury. Although the starting of the act is voluntary, when 

 once it has begun it continues tinder the influence of the vesico- 

 spinal center situated in the lumbar part of the cord until the 

 bladder is empty. 



Up to a certain point the brain is able to inhibit the center and 

 postpone the evacuation of the bladder, but after a time, if too 

 long delayed, the resistance of the sphincter is overcome and urine 

 will flow. It is more difficult to stop the act after it has once 

 begun than to delay its beginning, for the urine, flowing over the 

 mucous membrane of the urethra, stimulates the vesicospinal 

 center, and the efferent impulses to the contracting* muscles are 

 increased. 



If the mucous membrane of the bladder is inflamed, as in 

 cystitis, the stimulation of the center may be so great as to pre- 

 vent the brain from inhibiting the evacuation, and this may occur 

 when only a small quantity of urine is accumulated. Or it may 

 happen that the spinal cord is injured or diseased in the upper 

 or middle portion, and thus all sensation caused by a full bladder 

 may be abolished. Under these circumstances the bladder, when 

 full, will be emptied by the reflex action of the vesicospinal center. 

 Or, again, if the lesion of the cord is such as to disorganize this 

 center, then there will be no reflex action of the cord, and the 

 elasticity of the tissues about the neck of the bladder will keep the 

 urine in that viscus until the elasticity is overcome by the disten- 

 tion, when the urine will flow in drops as fast as it comes from 

 the kidneys ; but the bladder will not empty itself. Inexperi- 

 enced persons are often deceived by this dribbling of the urine, 

 thinking that its discharge is evidence that the bladder is perform- 

 ing its duty, while the fact is that it is evidence of paralysis and 

 retention. 



Urethra (Fig. 244). This canal extends from the neck of the 

 bladder to the meatus urmarius, and in the male is about 20.4 cm. 

 and in the female about 3.7 cm. in length. It is lined with 



