KIDNEY 



319 



and an inner zone which is lighter, the nuclei being placed at the junction 

 of the two zones." It is probable, from their position, that the distal 

 convoluted tubules in Fig. 313 are parts of the tubule which connects 

 with the glomerulus shown in the figure. 



The arched collecting tubules, into which the distal convoluted tubules 

 empty, pass into the collecting tubules of the rays, which are readily 

 identified. They have round and clear-cut lumens; cell walls are distinct 

 (in all but the smallest), and the nuclei are regularly arranged. Thus 

 the collecting tubule resembles an excretory duct. 



The structures seen in the radiate part of the cortex are therefore the 

 ascending and descending limbs of Henle's loops, and the collecting tubules; 



Large collecting tubule. 



Thick segments 



of Henle's loop 



(ascending). 



Thin segments 



of Henle's loop 



(descending). 



PIG. 316. TRANSVERSE SECTION THROUGH THE MEDULLA OF A HUMAN KIDNEY. X 320. (Schaper. ) 



they are shown in longitudinal section in Fig. 315. The convoluted part 

 of the cortex contains proximal and distal convoluted tubules and glomeru- 

 lar capsules. 



The medulla (Fig. 316) contains the same elements as the rays. The 

 collecting tubules are larger, and their walls are more distinct. Among 

 their columnar cells a few are decidedly darker than the others. The thick 

 segments of Henle's loops are easily distinguished from the thin segments. 

 The latter are slender (9-16 A* in diameter) but have large lumens. Cell 

 walls are absent, and the cells are so flat that their nuclei cause elevations. 

 The thin segments are generally descending, but they may also ascend, as 

 seen in the inner zone of the medulla ; Fig. 3 1 5 is from the outer zone, in which 

 most, if not all, of the thin segments are descending. (In comparing 

 Fig. 316 with Fig. 313, it should be noted that the former is more highly 

 magnified, and the thick ascending limbs appear more granular than those 

 tubules of the cortex with which they are continuous.) 



