72 HUMAN PHYSIOLOGY. 



pighi, consist of a number of straight tubes, which commence at the apex 

 by from 10 to 20 openings; and as they pass toward the cortical portion, 

 they divide and subdivide at acute angles, until a large mass of tubes is 

 produced. These tubes are on the average about -5-^ of an inch in 

 diameter, and composed of a thin, but firm, elastic, structureless membrane, 

 lined by polygonal nucleated cells, which reduce the diameter of the lumen 

 of the tube about two-thirds ; these are the straight tubes of Bellini. 



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 cortical 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 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, 18 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 mucous. The orifice of the bladder is controlled by the 

 sphincter vesica, 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. 



