350 



THE URINARY SYSTEM 



cross nearly the whole breadth of the renal cortex and entering 

 the medulla extend from base to apex of the Malpighian pyramid. 

 Throughout their whole course they progressively increase in 

 size and caliber. A corresponding progressive increase in the 

 height of their epithelial cells likewise occurs, so that the lumen 

 of the straight tubules of the medulla is not only actually greater 

 than that of those of the medullary rays, but the walls of the 

 former tubules are also considerably thicker. The extreme of 

 this progression is found in the broad lurnen and tall epithelium 

 of the ducts of Bellini. 



The epithelium of the straight tubules, like that of the arched, 

 possesses a clear cytoplasm, distinct and deeply staining chromatic 

 nuclei, and well defined cell outlines. Beginning in the medul- 

 lary rays with a low columnar type, it gradually increases in height 

 in the course of the tubule until, in the papillary zone, the epi- 

 thelium acquires a 

 tall columnar form. 

 The clear cyto- 

 plasm and distinct 

 nuclear membranes 

 of the epithelium 

 of the collecting 

 tubules stand out 

 in sharp contrast to 

 the granular cyto- 

 plasm and the even- 

 ly distributed chro- 

 matin in the nuclei 

 of the lining cells 

 in the secreting por- 

 tions of the urinif- 

 erous tubules. 



10. The Ducts of 

 Bellini (Papillary 

 Ducts). These are 

 the wide mouths 

 of the uriniferous 

 tubules which are 

 formed by the dicot- 



omous union of the collecting tubules and which empty their 

 secretion into the renal calyces at the apex of the Malpighian pyra- 



FIG. 287. DUCTS OF BELLINI IN OBLIQUE AND LONGITU- 

 DINAL SECTION, NEAR THE APEX OF A MALPIGHIAN 

 PYRAMID IN THE HUMAN KIDNEY ; FROM THE PAPILLARY 

 ZONE OF THE MEDULLA. 



Hematein and eosin. Photo, x 450. 



