The Urinary System 187 



a slender tube, the ureter, leads to the saclike urinary bladder. From 

 the bladder, the urethra carries the urine to the outside. 



The Kidneys. — The kidneys are situated on the dorsal wall of the 

 abdomen in the lumbar region, one on each side of the vertebral col- 

 umn. Each is covered ventrally by the peritoneal lining of the coelom ; 

 the kidneys, thus, are not actually a part of the coelomic viscera, rather 

 they are retroperitoneal (behind the peritoneum) organs. Each kidney 

 has a convex outer surface and a concave inner surface and is some- 

 what "bean" shaped. On this inner surface is a place called the hilus 



ADRCNAL GLANO 



BLAOOER 



URETHRA 



Fig. 55. — The urinary system of a mammal. 



where the blood vessels enter and leave the kidney and the ureter origi- 

 nates. One kidney is often more anterior than the other, although 

 there may be some variation in their relative positions. Each is im- 

 bedded in a mass of fat which helps to absorb the shock of movements. 

 Surrounding each kidney and its capsule of fat is a sheath of connective 

 tissue which helps to hold it in place. 



A longitudinal section through the kidney reveals that it is di- 

 vided into two principal regions, an outer cortex and an inner medulla. 



