THE EXCRETION OF URINE. 233 



in the region called the cortex, as a body called the Malpighian 

 corpuscle. This corpuscle consists of the dilated end of a tubule 

 which is invaginated to form a cup-shaped vessel, within the cup 

 of which lies a tuft of capillaries. The capillaries compose the 

 structure known as the glomerulus, and the tubular part, the 

 capsule of Bowman. 



From Bowman 's capsule a short neck leads into what is known 

 as the convoluted tubule, which is a very tortuous vessel lined 

 with very large epithelial cells. This structure lies in the cortex 

 of the kidney and is nourished by the blood which has already 

 been through the glomerular capillaries. A loop of the tubule 

 leads down into the center or medullary portion of the kidney 

 and back again to the cortex, where the cortex again becomes 

 very tortuous. This finally empties, in company with many other 

 similar vessels, into a common collecting tubule, which leads to 

 the pelvis of the kidney. 



THE BLOOD SUPPLY OF THE KIDNEY is very large compared 

 with that of the other organs of the same size. The renal artories 

 come from the aorta and distribute their blood directly to the 

 glomeruli and the inner medullary portions of the kidney. The 

 vessels of the glomerulus are collected into an afferent vein, which 

 again breaks up into capillaries to supply the remaining struc- 

 tures of the cortical portions of the kidney (Fig. 37). 



THE NERVES OF THE KIDNEY. The kidney is very richly sup- 

 plied with vasomotor nerve fibers, which are carried to it in the 

 splanchnic nerves. Whether there are nerve fibers in either the 

 vagus or splanchnic nerves which have a secretory influence on 

 the kidney cells, is at present an unsettled question. 



The Nature of Urine Excretion. In spite of the many at- 

 tempts to explain the nature of urine excretion, there remain 

 many steps in the process which are not fully understood. The 

 constituents of the urine are formed by other organs than the 

 kidney, and are present in the blood plasma. Thq function of 

 the kidney is to remove these substances from the blood. Many 

 bodies are present in the blood plasma which are not found in the 

 urine, and again some of the urinary constituents are found in 

 far greater concentration in the urine than in the blood plasma. 



