THE URINARY SYSTEM 329 



part of the cell which contains the nucleus bulges into the lumen, and 

 as the nuclei of opposite sides of the tubule usually alternate, the 

 lumen is apt to present a wavy appearance in longitudinal sections. 

 5. Ilcnlcs Loop. — The epithelium here changes from the flat of 

 the descending arm to the cuboidal of the ascending arm. The exact 

 point where the transition occurs varies. It may take place during 

 the turn of the loop, or in either the ascending or descending arm. 



I S. 



■1 



7- "^^ 



.1 



■Si, 







m< m 







Fig. 227. — Tubules of Human Kidney. X560. From longitudinal section. 

 (Technic 2, p. 338.) i. Descending arm of Henle's loop; 2, ascending arm of Henle's 

 loop; 3, collecting tubule; 4, duct of Bellini. Beneath the longitudinal sections are seen 

 cross sections of the same tubules. 



6. The ascending arm of Henle's loop (Fig. 227, 2) is broader 

 than the descending, measuring from 20 to 30,". in diameter. Its 

 epithelium is cuboidal with granular striated protoplasm. The cells 

 thus resemble those of the convoluted tubule, but are smaller, more 

 regular, and less granular. 



7. The second convoluted tubule has a diameter of 40 to 5o/-«. It is 

 much less tortuous than the first convoluted tubule. Its epithelium 

 is similar to that lining the first convoluted tubule except that it is 

 shghtly lower and less distinctly striated. 



8. The arched tubule has a somewhat narrower lumen (about 25") 

 than the second convoluted. It is lined with a low cuboidal epithe- 

 lium with only slightly granular cytoplasm. 



