KIDNEY 



313 



A. 



epithelial lining of the ureter, pelvis, calyces and collecting tubules, includ- 

 ing all of their branches. 



The second part of the kidney, which consists of dense mesenchyma, 

 becomes subdivided into masses enveloping the enlarged tips of the branch- 

 ing collecting tubules. Some of its cells 

 become arranged so as to form vesicles 

 (Fig. 308), one of which is shown in the 

 reconstruction, Fig. 309, A. The vesi- 

 cles are at first entirely separate from 

 the collecting tubules. Each vesicle 

 becomes elongated, making an S-shaped 

 tubule (Fig. 309, B, C), and its outer 

 or upper end unites with the collecting 

 tubule (Fig. 309, D). A glomerulus 

 develops in the lower curve of the S, 

 and is gradually enveloped in the 

 terminal part of the tubule, which thus 

 forms its capsule. Between the cap- 

 sule and the collecting tubule, the renal 

 tubules become greatly convoluted. 

 One of the loops in the coils thus formed 

 elongates downward, lying close beside 

 and parallel with the collecting tubule; 

 this is the loop of Henle (Fig. 309, J). 



Three tubules of the adult kidney 

 are shown diagrammatically in Fig. 310. 

 Each capsule connects with a proximal 

 convoluted tubule, which, after extending, 

 outward toward the surface of the kid- 

 ney, turns downward as the descending 

 limb of Henle's loop. The descending 

 limb is a straight tubule, the lower por- 

 tion of which is of small diameter owing 

 to the flatness of the cells in its walls; 

 its lumen is not reduced. This "thin 

 segment," as shown in the diagram, 

 does not form the entire descending 

 limb, but only its lower part. Frequently it passes around the bend into 

 the ascending limb. The tubule, after turning the bend, forms the ascend- 

 ing limb of Henle's loop. It returns to the vicinity of the capsule from 

 which it arose, and makes a few coils, thus constituting the distal convo- 

 luted tubule (intercalated or intermediate tubule). By means of the 

 functional tubule it joins the arched collecting tubule and this passes into 



D. 



FIG. 310. DIAGRAM OF THREE URINIFEROUS 

 TUBULES IN RELATK>N WITH A COLLECT- 

 ING TUBULE. (Modified from Huber.) 



a. 1., Ascending limb of Henle's loop; c., cap- 

 sule; c. t., collecting tubule; d. c., distal 

 convoluted tubule; d. 1., descending limb; 

 j,, junctional tubule; p. c., proximal con- 

 voluted tubule; p. d.,papillary duct. 



A, cortex; B-D, medulla, subdivided into an 

 inner zone (D) and an outer zone (B-C) ; 

 the latter includes an inner band or stripe 

 (C), and an outer band (B). 



