392 MANUAL OF PHYSIOLOGY. 



the greater portion of its diameter. This convoluted tubule (/) 

 leads into a tube (e) of much less external diameter, but about 

 equal lumen, owing to the thinness of its lining epithelium, the 

 cells of which are more flattened and much thinner than those 

 in the tortuous tubes. This thin tube forms a loop extending 

 down into the medullary pyramid and returning to the cortex, 

 where it can be seen to become again convoluted (d) and then 

 to open into a straight collecting tube. The collecting tubes 

 (c, 6) receive many similar tributary tubes on their way toward 

 the apex of the Malpighian pyramid, where they pour their 

 contents into the pelvis of the kidney. The epithelial lining of 

 these collecting tubes is of the ordinary cylindrical type. 



We thus find four kinds of epithelial cells in the various parts 

 of the urinary tubules, viz. : scaly cells in the capsule ; peculiar 



FIG. 173. 



Portion of Convoluted Tubule, showing peculiar fibrillated epithelial cells. 

 (Heidenhain.) 



rod-beset glandular cells in the convoluted tubes; flattened cells 

 in a great part of the loop; and ordinary cylindrical cells in the 

 large straight tubes. (Figs. 172 and 173.) 



BLOOD VESSELS. 



The renal artery, on its way from the hilus to the boundary 

 between the cortical and medullary portions of the kidney, breaks 

 up suddenly into numerous small branches ; these vessels then 

 form arches which run along the base of the pyramids. From 

 the latter, straight branches, called interlobular arteries, pass 

 toward the surface, and give off" lateral branchlets which form the 

 afferent vessels to the neighboring Malpighian capsules. Within 

 the capsules the afferent arteries at once break up into a series 



