330 



THE GENITOURINARY ORGANS. 



In the larger collecting tubules the epithelium is more regular 

 and becomes higher as the tube widens. These tubules gradually 

 unite within the Malpighian pyramid and the regions adjacent to 

 the columns of Bertini to form 1 5 to 20 papillary ducts from 200 fj. 

 to 300 // in diameter. The latter have a high columnar epithelium, 

 and empty into the pelvis of the kidney at the apex of the papilla, 

 forming the foramina papillaria in an area known as the area 

 cribrosa. 



Besides the epithelium, the uriniferous tubules possess an ap- 

 parently structureless membrana propria, that of the collecting 

 tubules being very thin. This membrane may be isolated, as has 

 been shown by F. P. Mall, by macerating frozen sections in a cold 

 saturated solution of bichromate of soda for several days. This 

 membrane is digested in pancreatin. 



Papillary duct. 



Blood-vessel. 



Fig. 267. From longitudinal section through papilla of injected kidney ; X 4 : a > Epi- 

 thelium of collecting tubule under greater magnification. 



Between the Malpighian pyramids are found the columns of 

 Bertini, presenting a structure similar to that of the cortex of the 

 kidney, and extending to the hilum of the kidney. 



Between the uriniferous tubules and surrounding the blood- 

 vessels of the kidney there is found normally a small amount of 

 stroma tissue, consisting of white fibrous and reticular fibers, elastic 

 fibers being found in connection with the blood-vessels (F. P. Mall, 

 Riihle). Between the convoluted portions of the tubules this is 

 present only in small quantity, the fibrils being felted to form 

 sheaths for the tubules ; a somewhat greater amount being found 

 in the neighborhood of the Malpighian corpuscles, in the boundary 

 zone between the cortex and medulla and between the larger col- 

 lecting tubules in the apices of the Malpighian pyramids. 



From wrmt has been said concerning the uriniferous tubule it 

 must be evident that its course is a very tortuous one. Beginning 



