KIDNEYS. 167 



Blood-vessels of the Kidney. The renal artery is of large size 

 and enters the organ at the hilum ; it divides into several large 

 branches, which penetrate the substance of the kidney between the 

 pyramids, at the base of which they form an anastomosing plexus, 

 which completely surrounds them. From this plexus vessels follow 

 the straight tubes toward the apex, while others, entering the corti- 

 cal portion, divide into small twigs, which enter the Malpighian 

 body and form a mass of convoluted vessels, the glomerulus. After 

 circulating through the Malpighian tuft, the blood is gathered to- 

 gether 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, 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, 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 fibers of which are arranged longi- 

 tudinally and circularly), areolar, and mucous. The orifice of the 

 bladder is controlled by the sphincter vesicce, a muscular band about 

 Y2 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 



