KIDNEYS. 79 



enter the Malpighian body and form a mass of convoluted vessels, the 

 glomerulus. After circulating through the Malpighian tuft, the blood is 

 gathered together by two or three small veins, which again subdivide and 

 form a fine capillary plexus, which envelops the convoluted tubules; from 

 this plexus the veins converge to form the emulgent vein, which empties 

 into the vena cava. 



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, 1 8 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 3 coats, fibrous, muscular 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 one pint. It is composed of four coats, serous, 

 muscular, the fibres of which are arranged longitudinally and circularly, 

 areolar and mucotts. The orifice of the bladder is controlled by the 

 sphincter vesicce, a muscular band about half an inch in width. 



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

 into the pelvis, and from 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 blad- 

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

 digestion. It is prevented from regurgitating into the ureters on account of 

 the oblique direction they take between the mucous and muscular coats. 



Nervous 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 tonic contraction by the action of a nerve centre in the lumbar 

 region of the spinal cord. This centre 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 experienced, 

 impressions made upon the vesical sensory nerves are carried to the centres 

 governing the sphincter and detrusor urines 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 vesicee centre, with the effect of relaxing the muscle, and of 

 stimulating the centre governing the detrusor muscle, with the effect of con- 



