202 



THE EXCRETORY SYSTEM 



nephron, and extends alongside the collecting tuliules of the cortex. 

 This capillary network connects with the interlobnlar veins running 

 alongside the interlobular arteries, and then these join the arcuate 

 veins. The capillary system of the cortex joins with the capillary 

 supply of the medulla. Here the capillary network surrounds the 

 systems of collecting tu})ules and those portions of the nephridial 

 tubules (nephrons) which are located in the medulla. The capillary 

 network of the medulla connects with radially directed venulse 

 rectse, which are small veins connecting with the arciform veins. 

 The latter branches join to form interlobar veins, which occur 



alongside interlobar arteries 

 and eventually join with the 

 renal vein. Not only is the 

 kidney well supplied with 

 arteries, veins, and capillaries, 

 but there is also a rich lym- 

 ])hatic system. The nerves 

 su])plying the kidney are 

 branches of the sympathetic 

 ])lexus. 



THE URETERS. 



Urine is conveyed from 

 kidneys by tubes called ure- 

 ters. The ureter of a pro- 

 nephros is a pronephric duct 

 and posteriorly this duct joins 

 with the cloaca. The pos- 

 terior portion of this same duct also serves as the ureter for the 

 mesonephros, in which case it is known as the mesonephric, or 

 Wolffian, duct. As an example of the structural character of this 

 duct in the case of the Amphibia, the frog's ureter will serve. 



Mesonephric Duct. — Tn the frog, the ureter runs dorsally along the 

 anterior two-thirds of the kidney, then turns to accompany the 

 portal vein along the outer border. During its course along the 

 kidney, the ureter is inclosed in the fibrous connective tissue of 

 the kidney. In the remainder of its course, it lies free in the body 

 cavity associated with the Miillerian duct. The mucous membrane 

 of the ureter is longitudinally folded and varies from 1 to 3 cell 

 layers in thickness. (Fig. 127.) In the anterior portion along the 

 kidney, there may be a single layer of cuboidal or low cohnnnar 



Fig. 127. — Photograph of a cross-section 

 of the ureter of Necturus in the region of 

 the kidney. 



