196 



THE ESSENTIALS OF HISTOLOGY. 



as being ciliated. They certainly often exhibit a brush of cilium- 

 like processes projecting into the lumen, but it is doubtful if these 

 are vibratile. In the narrow descending limb of the looped tubule 

 (fig. 231, c), and in the loop itself, the cells are clear and flattened 

 and leave a considerable lumen ; in the ascending limb they again 

 acquire the striated structure and nearly fill the lumen. The 



FIG. 230. PART OF A CON- 

 VOLUTED TUBULE FROM THE DOG'S 

 KIDNEY. (Heidenhain). 



FIG. 231. PORTIONS OF TUBULES, ISOLATED. 

 (Cadiat.) 



a, large collecting tubule ; b, loop of Henle ; c, descend- 

 ing tubule of Henle. 



fibrillations of the cells are still more marked in the zigzag tubules 

 (fig. 228, b), and a similar structure is present also in the second 

 convoluted tubules, into which these pass. On the other hand, the 

 functional tubule has a large lumen and is lined by clear flattened 

 cells, and the collecting tabes have also a very distinct lumen and 

 are lined by a clear cubical or columnar epithelium (figs. 228, d : 

 231, a). 



