346 THE UEINAKY SYSTEM 



The caliber also depends, to some extent, upon the secretory 

 activity of the epithelium, whose cells become shrunken, and the 

 lumen correspondingly dilated, during active secretion. The con- 

 voluted tubules are most actively engaged in the secretion of urine, 

 but the further changes accompanying their secretion have not yet 

 been satisfactorily demonstrated. 



4. Descending Limb of Henle's Tubule (the Thin or Narrow Tu- 

 bule of Henle). In this portion, which is, typically, located in the 

 boundary zone of the medulla, the uriniferous tubule becomes very 

 much narrowed, but the decreased diameter is the result of dimin- 

 ished height of the lining epithelium rather than of any change in 

 the caliber of the tubule. The length of this portion of the tubule 

 is very variable ; typically it corresponds very nearly with the 

 breadth of the medullary boundary zone. 



The lining epithelium of the descending limb is of a peculiar 

 flattened shape. Its cells possess an ovoid nucleus which, being 

 thicker than the surrounding portions of the cell, projects slightly 

 into the lumen of the tubule. The bulging nuclei of opposite 

 sides of the tubule are not in apposition but interlock with one 

 another, the nuclei of one side of the tubule being opposed to the 

 cell margins of the opposite side. The lumen of longitudinal sec- 

 tions through the axis of the tubule thus acquires a sort of zigzag 

 outline. The nuclei stain deeply but possess an evenly distributed 

 chromatin. The cytoplasm of the epithelium is very finely granu- 

 lar, and although its cells are intimately adherent at their lateral 

 margins they do not present the typical striations which are char- 

 acteristic of the preceding portion. 



5. The Loop of Henle. As the descending limb enters the loop 

 of Henle the tubule makes an abrupt turn and returns toward the 

 cortex. The location of the loop, being dependent upon the un- 

 certain length of its descending and ascending limbs, is very vari- 

 able. It may be found in any portion of the medulla except the 

 extreme tip of the Malpighian pyramids ; its most frequent site, 

 however, is near the junction of the boundary and papillary zones. 



The structure of the loop may be that of either the descending 

 or the ascending limb. It is also subject to great variations, since 

 the change in structure from the narrow to the broad type, though 

 it typically occurs just prior to the formation of the loop, is fre- 

 quently delayed until well into the ascending limb. As a rule the 

 change in type occurs earlier when the loop lies in the boundary 

 zone, and later when it occurs nearer the apex of the Malpighian 



