THE KIDNEY. l8l 



primitive renculi persist as separate structures, and 

 we thus have in the adult human kidney just as 

 many medullary pyramids and papillae as there were 

 original renculi. We find also, which is a striking 

 feature in the adult human kidney, that the cortical 

 substance is not confined to the cortex, but extends 

 into the pelvis between the papillae, often as far as, 

 and sometimes farther than the papillae themselves. 

 Not very infrequently the divisions between the 

 primitive renculi are not entirely obliterated in the 

 process of development, and the surface of the kid- 

 ney, even in the adult, is distinctly lobulated. 



Having thus acquainted ourselves with the general 

 structure of the kidney, we have now to study the 

 elements of which it is composed, and the way in 

 which they are grouped to form the different parts 

 above described. The kidney is a tubular gland ; 

 the innumerable tubes of which it is mainly com- 

 posed are lined with epithelial cells, and run a very 

 tortuous course from their origin in the cortex to 

 their termination in the tiny openings, above men- 

 tioned, at the apex of the papillae. They constitute, 

 with the cells lining the walls of the glomeruli, in 

 which they originate, the parenchyma of the kidney. 

 In addition to the parenchyma we have, then, to 

 study the connective tissue or interstitial tissue of 

 the organ and the blood-vessels. 



The tubes of the kidney, called in general urin- 

 iferous tubules, consist of an apparently homogeneous 



