160 NORMAL HISTOLOGY. 



external diameters, the diameters of their lumina, and the character 

 of their epithelial linings. The appearance of the epithelial cells 

 differs, however, in accordance with their state of functional activity 

 (Figs. 135 and 136). 



The first convoluted tubule is relatively large, and is lined with 

 large epithelial cells, which project into the tubule about one-third 

 of its diameter. The cells have round nuclei situated near their 

 centres, and are granular, with an appearance of radiate striation 

 in their deeper halves when charged with secretion. 



The descending branch of Henle's tube has a smaller diameter, 

 but its lumen is wide in consequence of the thinness of the clear 

 epithelial cells lining it. In the ascending branch the lumen is 

 again smaller, although the diameter of the tube is larger, because 

 the lining cells are thicker, somewhat resembling those of the first 

 convoluted tubule. The transition from the character of the de- 

 scending to that of the ascending branch does not always take place 

 exactly at the loop. 



The second convoluted tubule is a little smaller than the first, and 

 is lined with cells that are not quite so granular and a little more 

 highly refracting. 



The collecting tubules are lined with columnar epithelium, the 

 cells of which become longer as the diameter of the tube increases 

 in its progress toward the apex of the pyramid. 



The epithelial lining throughout the course of the renal tubule 

 is said to rest upon a thin, homogeneous basement-membrane inter- 

 posed between it and the interstitial fibrous tissue. The latter is 

 present in small amount, and partakes of the character of an areolar 

 tissue, holding the tubules and bloodvessels in place. It is rather 

 abundantly supplied with lymphatics. 



For the study of the uriniferous tubules sections made trans- 

 verse to the course of the straight tubules will be found very use- 

 ful. In the cortex the medullary rays, with their descending and 

 ascending branches of Henle's tubes and their collecting tubules, 

 will appear surrounded by the labyrinth, made up of the con- 

 voluted tubules, Malpighian bodies, and larger vessels, the latter in 

 cross-section. Near the apex of the pyramid cross-sections of the 

 larger collecting tubes and of the vasa recta will be seen ; and near 

 its base the smaller collecting tubes and the tw 7 o limbs of Henle's 

 tube, with, possibly, here and there a " loop " in nearly longitudinal 

 section, will appear. Among all these sections of the tubules the 



