The Urixogexital 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, on 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- 



«»■»„ ™. «.-.«. « T « dition appears internallv in the division 

 FORM IN MAMMALS. l l - . . 



ot che 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. 50. 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. 



