THE URINARY ORGANS. 



329 



from 23 fj. to 28 // in diameter, possesses a columnar epithelium 

 similar to that of the proximal convoluted portion. Here, however, 

 the basal striation of the cells is not so distinct, the lumen is some- 

 what larger than that of the descending limb, and by treatment 

 with certain reagents the epithelium may often be separated as a 

 whole from the underlying basement membrane. 



The distal convoluted or intercalated portion (segment of 

 Schweigger-Seidel), from 39 fj. to 45 /j. in diameter, is only slightly 

 curved (2 to 4 convolutions). Its epithelium is relatively high, 

 though not so high as that lining the proximal convoluted portion 

 and not so distinctly striated, though containing numerous granules. 

 The cells are provided with large nuclei and their basal portions are 

 joined by interlacing projections. 



Fig. 266. Section of medulla of human kidney ; X about 300 : a, a, a, Ascending 

 limb of Henle's loop ; b, b, l>, blood vessels ; c, c, c, descending limb of Henle's loop. 



The next important segment is the short arched collecting portion, 

 which has nearly cubical epithelial cells and a lumen somewhat wider 

 than that of the intercalated tubule. The smaller straight collecting 

 tubules have a low columnar epithelium with cells of somewhat ir- 

 regular shape, the basal portions of which are provided with short, 

 irregular, intertwining processes, which serve to hold the cells in 

 place. The diameter of the collecting tubules measures from 45 fi 

 to 75 p. 



