NORMAL HISTOLOGY. 



5. The conspicuous diminution in diameter which marks the pas- 

 sage of the spiral tubule into the descending limb of Henle's loop 

 is accompanied by a change in the character of the lining epithelium. 

 The low columnar cells are replaced by flattened, transparent plates, 

 whose nuclei, thicker than the bodies of the cells, encroach upon the 

 lumen of the tubule as minute spindle-shaped projections ; since the 

 latter are situated often on opposite sides of the tube, its lumen in 

 section appears as a wavy channel. 



6. Shortly before reaching the loop, at a point within the de- 

 scending limb corresponding with the increased diameter of the 

 tubule, the epithelium becomes polyhedral, possessing flattened 

 nuclei and faint striations ; the lumen is distinct in this region, 



although narrow. This 



FIG. 241. character is retained by the 



epithelium throughout the 

 loop and the ascending 

 limb as far as the succeed- 

 ing portion of the tube. 



7. The irregular tubule 

 is distinguished by its small 

 and uncertain lumen and 

 its distinctly striated epi- 

 thelium ; the thickness of 

 the latter and, conse- 

 quently, the size of the 

 canal vary with the con- 

 ditions of secretion. 



8. The lining of the dis- 

 tal convoluted tubule 

 resembles that of the proxi- 

 mal, the epithelium being 

 granular, indistinctly sepa- 

 rated into individual cells, 

 and presenting a striated 

 outer zone ; the lumen of 

 the canal depends largely 



upon the thickness of the lining cells, which changes with the func- 

 tional activity of the secretory elements. 



9. The succeeding segment, the arched collecting tubule, con- 

 tains low cuboidal, transparent cells, which, with slight alteration, 

 become the epithelium of the straight collecting tubule. 



10. Passing into the medulla, the cells of the collecting tubules 

 become markedly columnar, which form they retain with increasing 

 distinctness throughout the remainder of their course. The large 



Section of medulla of human kidney : -w, large collect- 

 ing tubules ; x andy, descending and ascending limbs of 

 Henle's loops ; z, loops of Henle ; v, blood-vessels. 



