200 THE EXCRETORY SYSTEM 



into the capillary tuft. The capillary tuft is itself closely invested 

 with the visceral wall of Bowman's capsule, which fits in close around 

 the arterioles as they join the capillaries of the glomerulus. This 

 same inner, or visceral wall, of Bowman's capsule is reflected back 

 as the outer, or parietal, wall. The ])arietal wall continues down 

 around the inner wall and forms a narrow neck at the end of the 

 renal corpuscle opposite the entrance of the arterioles. The neck 

 continues into a portion of the nephridial tubule known as the 

 proximal convoluted tubule. The wall of the neck portion consists 

 first of low cuboidal and further on of cuboidal and columnar 

 epithelial cells. The proximal con\oluted portion is very much 

 contorted, extending somewhat toward the surface at first, then 

 turning toward the medulla and finally continuing into the medulla 

 as a very much narrower portion, known as the descending limb. 

 This portion extends radially in the medulla for some distance, then 

 forms a loop, known as Henle's loop, and turns back as the ascending 

 limb running alongside the descending limb. The ascending limb 

 is somewhat thicker than the descending limb. As it passes into 

 the cortex, the tubule l^ecomes the much thicker and contorted distal 

 convoluted tubule situated near the proximal convoluted portion. 

 Toward the cortical surface it joins a collecting tubule by a short 

 junctional tubule. These collecting tubules join the t)thers which in 

 turn join larger tubules, until the largest collecting ducts (papillary 

 ducts) are found in the papillse of the renal pyramids. These 

 I)apillary ducts then join the calyces. (Fig. 126.) 



Let us return for a moment to the cortex. A pars convoluta con- 

 sists chiefl>' of renal corpuscles, together with proximal and distal 

 convoluted portions of the nephridial tu})ules. A pars radiata is 

 formed mainly by collecting tubules. The medulla consists chiefly 

 of collecting tubules, together with the descending and ascending 

 limbs, and Henle's loop. Little has been said about the blood- 

 vessels involved in all these regions, for the \ascular supply will be 

 considered separately later. 



As indicated, a number of branchings, about seven, occur between 

 a given papillary duct and the smallest collecting tubules draining 

 into it. Each ])apillary duct is like a tree with many branches, the 

 collecting tubules of different grades originating from the single 

 trunk. All of these branches constitute parts of the collecting sys- 

 tem and ha\'e the same origin as the calyces, pelvis, and ureter in 

 the evagination of the mesone])hric duct. The ne])hridial tubule 

 pr()i)er, which develoj^s from ne])hrogenic tissue and takes ])art in 



