KIDNEYS. 215 



The part these cells take in the secretion of urine, and the 

 changes that take place in them during this process, have not 

 definitely been made out. In secretion the cells become lower 

 and the lumen of the tubule wider than during rest. Secretory 

 capillaries have not been demonstrated. 



The descending arm of Henle's loop is a thin-walled tube 

 made up of flat epithelial cells, whose nuclei bulge out into the 

 lumen. The cells are so arranged in the tubule that in a lon- 

 gitudinal section they alternate on the two sides that is, two 

 cells are never opposite one another. In cross-section the canal 

 is not unlike a blood capillary. The membrana propria usually 



FIG. 167. 



Cross-section of a convoluted tubule from the kidney of a rabbit. The boundaries of 

 the epithelial cells cannot be seen. Only three nuclei are shown. The rod-like structure 

 is plainly visible. X 1100. 



is seen distinctly, and the whole diameter of the canal is from 

 9 to 15 ^. 



The ascending arm of the loop of Henle is considerably 

 thicker, being 25 ^ in diameter. The epithelial cells are cub- 

 ical, and the size of the lumen narrow in relation to the thick- 

 ness of the walls. The transition from the flat cells to the 

 cubical takes place usually in the lower part of the descending 

 arm of the loop. The cubical cells show the striation spoken 

 of in the convoluted tubules. 



The convoluted tubules of the second order are much 

 shorter and have a wider lumen than those of the first order. 

 The canals are 39-46^ in diameter. The epithelial cells are 

 low, and show a finely granular and striated appearance. 



