THE KIDNEYS 



123 



mals, where the urinogenital sinus becomes completely separated 

 from the digestive tube, and where the urinary ducts are also trans- 

 ferred from a posterior or hypocystic position on the wall of the 

 urinogenital sinus to an anterior or epicystic position on the dorsal 

 wall of the bladder. 



/ 



The Kidneys 



The chief organs of the urinary system are the kidneys. They 

 are paired organs, lying against the dorsal abdominal wall, approxi- 

 mately in the position of the embryonic intermediate cell mass 

 (Fig. 22, n.) from which they are formed. During development, 

 one kidney is often displaced more than the other by the pressure 

 of adjacent organs so that the symmetrical disposition of the pair 

 is destroyed. Thus in the human adult 

 the right kidney is situated lower than 

 the left on account of the pressure of the 

 right lobe of the liver. In the rabbit, on 

 the other hand, the left kidney is dis- 

 placed further back than the right by 

 the posterior expansion of the greater 

 curvature of the stomach. 



The kidneys appear as solid organs, 

 brownish in colour and bean-like in 

 general shape, enclosed by a fibrous coat, 

 and connected medially with the ex- 

 panded end of the ureter. In the rabbit 

 the kidney appears as an almost con- 

 tinuous mass, in which, however, slight 

 traces of lobulation can be distinguished. In many mammals, 

 such as sheep and bear, the organ is composed of distinct and 

 separable lobules. This condition is clearly shown in the human 

 kidney during foetal life, and though the organ is much more 

 concentrated in the adult, the lobulated condition there appears 

 internally in the division of the ureter into several renal calyces, 

 each of them connected with a corresponding renal papilla. In the 

 rabbit, however, there is only a single renal papilla and the ex- 

 panded end of the ureter, the renal pelvis, is undivided. The pelvis 

 has a lobulated form not readily displayed in dissection but strlklng- 



FiG. 70. The left kidney, 

 divided horizontally lengthwise, 

 cut surface of dorsal half: c, 

 cortical substance; m, medullary 

 substance; p, renal papilla; u, 

 ureter. 



