436 



THE URINARY SYSTEM 



mentioned. In the outer zone Peter describes an outer and an inner stripe, 

 the line of demarcation being the very definite level where the proximal 

 convoluted tubule passes into the thin, clear, narrow limb of Henle's loop 

 (Fig. 400). The point of transition from outer to inner zone is marked by a 

 change in the character of the epithelium lining the distal limb of the long 

 Henle's loops; the epithelium becomes cloudy (granular) and thicker. In 

 view of the great variability in the character of the epithelium at the vari- 



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v * ^ * '' ' 



X%f'^ 



FIG. 406. A GROUP OF TUBULES FROM A TRAN SECTION OF A RENAL PYRAMID OF 

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



a, collecting tubule; b, ascending limb of Henle's loop; c, descending limb; d, loop 

 of Henle. Hematein and eosin. Photo. X 275. 



ous levels in loops of different lengths, and the recognition by Peter of a 

 thicker cloudy segment, it seems better to speak of thin clear, thick clear, 

 and thick cloudy segments of the loop. In kidneys of the dog and cat 

 there are no really short loops, none lying within the outer zone. In the 

 kidney of the pig many loops lie in the cortex; such loops lack entirely 

 the thin, clear narrow limb. 



7. The Distal Convoluted Portion (Intercalary or Intermediate 

 Portion}. This portion of the uriniferous tubule begins close to the 

 vascular pole of the renal corpuscle, and, after several irregular contor- 



