THE URINIFEROUS TUBULES 347 



pyramid ; the thick ascending limbs do not occur in the papillary 

 zone of the medulla. 



6. The Ascending Limb of Henle's Loop (the Broad or Thick 

 Limb}. This portion of the tubule returns through the boundary 

 zone of the medulla and enters a medullary ray, its course being 

 parallel to that of the descending limb. It then travels toward the 



FIG. 285. A GROUP OF TUBULES FROM A TRANSECTION OF A MALPIGHIAN PYRAMID OF 



THE HUMAN KIDNEY; THE SECTION PASSES THROUGH THE BOUNDARY ZONE. 



a, collecting tubule ; i, ascending limb of Henle's loop ; c, descending limb ; </, loop of 

 Henle. Hematein and eosin. Photo, x. 275. 



surface of the kidney, but finally leaves the ray and enters the 

 labyrinth to reach that Malpighian body from which the urinif- 

 erous tubule took origin (Golgi) and in relation to which the tu- 

 bule again acquires a tortuous course (distal convoluted portion). 

 Within the boundary zone of the medulla this portion of the 

 tubule is much broader than the preceding division, but it becomes 

 somewhat reduced in size in its course through the medullary ray. 

 The epithelium of the ascending limb is of a short cuboidal 

 form. Its cytoplasm resembles that of the lining epithelium of 

 the convoluted portion, although the nuclei in the tubule of Henle 



