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HANDBOOK OF PHYSIOLOGY. 



layer the tube again narrows, forming the ascending spiral of Henle's 

 loop, but is still lined with polyhedral cells. At the point where the tube 

 enters the cortex (9) the ascending limb narrows, but the diameter 

 varies considerably; here and there the cells are more flattened, but 

 both in this as in (8), the cells are in many places very angular, branched, 

 and imbricated. It then joins (10) the "irregular tubule," which has a 

 very irregular and angular outline, and is lined with angular and imbri- 

 cated cells. The tube next becomes convoluted (11), forming the distal 

 convoluted tube or intercalated section of Schweigger-Seidel, which is 

 identical in all respects with the proximal convoluted tube (12 and 13). 

 The curved and straight collecting tubes, the former entering the latter 



r 



ft 



Fig. 291. Transverse section of a renal papilla; a, large tubes or papillary ducts; 6, c, andtf, smaller 

 tubes of Henle; e, /, blood capillaries, distinguished by their flatter epithelium. (Cadiat.) 



at right angles, and the latter passing vertically downward, are lined 

 with polyhedral, or spindle-shaped, or flattened, or angular cells. The 

 straight collecting tube now enters the boundary layer (14) and passes 

 on to the papillary layer, and, joining with other collecting tubes, forms 

 larger tubes, which finally open at the apex of the papilla. These col- 

 lecting tubes are lined with transparent nucleated columnar or cubical 

 cells (14, 15). 



The cells of the tubules with the exception of Henle's loop and all 

 parts of the collecting tubules, are, as a rule, possessed of the intra- 

 nuclear as well as of the intra-cellular network of fibres, of which the 

 vertical rods are most conspicuous. 



In some places, it is stated that a distinct membrane of flattened 

 cells can be made out lining the lumen of the tubes (centrotubular mem- 

 brane). 



