RENAL CIRCULATION. 389 



parts essentially differing from one another both in their structure 

 and in their relation to the bloodvessels. 



The tubes begin by a small rounded dilatation (Malpighian 

 capsule) which is lined by thin flattened epithelium. Opening 

 from this capsule, Fig. 171 (#), is found a tortuous tubule (/), 

 lined by peculiar large rod-beset epithelial cells, which occupy 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 tor- 

 tuous 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 



FIG. 173. 



Portion of Convoluted Tubule, showing peculiar fibrillated epithelial cells. 

 (Heidenhain.) 



similar tributary tubes on their way towards the apex of the Mal- 

 pighian 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 

 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.) 



BLOODVESSELS. 



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

 between the cortical and medullary portions of the kidney, breaks 



