434 THE URINARY SYSTEM 



perfectly preserved often present a delicately striated, cuticular border 

 ('brush border'). The remaining portions of the cytoplasm are finely 

 granular. 



The nuclei of the epithelial cells of the convoluted tubules are spheri- 

 cal in shape, and do not stain very deeply with nuclear dyes as compared 

 with the more distinct and deeply staining nuclei of the collecting 

 tubules. Thus they appear as if partially clouded by the granular cyto- 

 plasm, an appearance which is greatly exaggerated with the onset of acute 

 inflammatory processes, which, on attacking the kidney, are prone to 

 involve the convoluted tubules. The chromatin is quite evenly distributed 

 throughout the nucleus and the nuclear membrane is not easily demon- 

 strated. 



The lumen of the convoluted portion of the uriniferous tubule is of 

 variable caliber; it presents frequent slight dilatations. The caliber also 

 depends, to some extent, upon the secretory activity of the epithelium, 

 whose cells become shrunken, and the lurnen correspondingly dilated, 

 during active secretion. The diameter varies from 40 to 60 microns. 

 The convoluted tubules are most actively engaged in the secretion of 

 urine, but the further changes accompanying their secretion have not 

 yet been satisfactorily demonstrated. It is generally believed that the 

 water and salts of the urine are secreted in the glomerular capsule, the 

 urea in the convoluted tubules. 



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 (8-15 p.), 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 sections through the axis of the tubule thus ac- 

 quires a sort of zigzag outline. The nuclei stain deeply but possess an 

 evenly distributed chromatin. The cytoplasm of the epithelium is very 

 finely granular, and although its cells are intimately adherent at their 



