KIDNEYS. 151 



The nerves of the kidney follow the course of the blood-vessels and 

 are derived from the renal plexus. 



The ureter is a membranous tube, situated behind the peritoneum 

 about the diameter of a goose-quill, eighteen inches in length, and extends 

 from the pelvis of the kidney to the base of the bladder, which it perforates 

 in an oblique direction. It is composed of three coats: fibrous, muscle 

 and mucous. 



The bladder is a reservoir for the temporary reception of the urine prior 

 to its expulsion from the body; when fully distended it is ovoid in shape, 

 and holds about Vne pint. It is composed of four coats', serous, muscle 

 (the fibers of which are arranged longitudinally and circularly), areolar, and 

 mucous. The orifice of the bladder is controlled by the sphincter vesica, a 

 muscular band about % of an inch in width. 



As soon as the urine is formed it passes through the tubuli uriniferi 

 into the pelvis and thence through the ureters into the bladder, which it 

 enters at an irregular rate. Shortly after a meal, after the ingestion of 

 large quantities of fluid, and after exercise, the urine flows into the bladder 

 quite rapidly, while it is reduced to a few drops during the intervals of 

 digestion. It is prevented from regurgitating into the ureters by the oblique 

 direction they take between the mucous and muscular coats. 



Nerve Mechanism of Urination. When the urine has passed into 

 the bladder, it is there retained by the sphincter vesicae muscle, kept in a 

 * state of chronic contraction by the action of a nerve center in the lumbar 

 region of the spinal cord. This center can be inhibited and the sphincter 

 relaxed, either reflexly, by impressions coming through sensory nerves 

 from the mucous membrane of the bladder, or directly, by a voluntary 

 impulse descending the spinal cord. When the desire to urinate is expe- 

 rienced, impressions made upon the vesical sensory nerves are carried to 

 the centers governing the sphincter and detrusor urina muscles and to the 

 brain. If now the act of urination is to take place, a voluntary impulse 

 originating in the brain passes down the spinal cord and still further inhibits 

 the sphincter vesicae center, with the effect of relaxing the muscle and of 

 stimulating the center governing the detrusor muscle, with the effect of 

 contracting the muscle and expelling the urine. If the act is to be suppressed, 

 voluntary impulses inhibit the detrusor center and possibly stimulate the 

 sphincter center. 



' The genitospinal center controlling these movements is situated in that 

 portion of the spinal cord corresponding to the origin of the third, fourth, 

 and fifth sacral nerves. 



