The Urinogenital System. 



93 



THE KIDNEYS. 



urinogenital sinus. This development reaches its extreme in the 

 higher mammalia, where the urinogenital. sinus is completely 

 separated from the digestive tube, and where the urinary ducts are 

 also transferred 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 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 inter- 

 mediate cell mass from which they are formed. 

 That of the left side is displaced backward, out of the position 

 of symmetry, oa account of the posterior 

 development 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 expanded end of the ureter. 

 In the rabbit the kidney appears as an 

 almost continuous mass, in which, how- 

 ever, 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 in foetal life, and 

 though much more concentrated in the adult, the lobulated con- 

 dition appears internally in the division 

 of the ureter into several renal calyces, 

 each of them connected with a corresponding renal papilla. 



When horizontally divided (Fig. 50), the kidney is seen to be 

 made up of a more vascular and granular external layer, termed the 

 cortex, and of a somewhat radially striated, central mass, termed 

 the medulla. In the rabbit, there is a single renal papilla, and 

 the expended end or pelvis of the ureter is undivided. Notwith- 

 standing the solid appearance of cortex and medulla, the kidney is 

 made up of a system of tubules, the relation of which to the vascular 

 system and to the outside of the body is such that fluid materials 



Fig. so. The left kidney, divid 

 ed horizontally lengthwise, cut sur- 

 face of dorsal half, c, cortical sub- 

 stance; m, medullary substance; 

 p, renal papilla; u, ureter. 



FORM IN MAMMALS. 



