422 PHYSIOLOGY CHAP. 



The convoluted tubules, too, consist of a structureless basement 

 membrane covered with cubical epithelium, the turbid protoplasm 

 of which is thickly set with granules in such regular lines that 

 (more particularly in the outer basal half) they present an effect of 

 striation. At the inner edge facing the lumen, there is, according 

 to the observations of Fornier (confirmed by many workers, 

 particularly by Sauer, 1895, in Germany, and by R and A. Monti, 

 1900, in Italy), a sort of finely striated border, which looks 

 like the bristles of a brush, and is therefore known as the orlo a 

 spazzola ("brush border," Fig. 113). Later on we shall see what 



changes this characteristic epi- 

 thelium undergoes, according to 

 V the functional state of the kidney. 



'\ The epithelium of Heule's 



loops differs in the narrow de- 

 scending and the wider ascending 

 limb. In the former the basement 



/: * HHB& membrane is covered by a flattened 



/ , pi epithelium, the nuclei being pro- 



/ ; : |:| minent towards the lumen. As 



H9l shown in Fig. 114, three cells, 



vL ; '&/.] more often two, still more often 



one flattened cell, are enough to 

 line the entire lumen of the canal. 

 The alternating position of these 

 cells gives a spiral character to 

 the tubule. The lumen does not 

 exceed 9-15 /z, i.e. it is approxi- 

 FIG. us. section of convoluted tubuie of mately equal to that of the blood 



waking marmot, during functional activity ^ QT ,ill QT ,i OQ 

 of renal epithelia. (R. and A. Monti.) Capillaries. 



In the ascending limb of 



Henle's loops the epithelium is cubical, and the cells project so as 

 to restrict the lumen, and make it narrower than in the descending 

 limb, although as a whole the diameter of the tubules exceeds 

 25 p. 



In the second convoluted tubules, which unite Henle's loops 

 with the ducts of Bellini, the same characteristic cells recur as in 

 the first convoluted tubules. The cells, however, are smaller, 

 while the diameter of the tubule is about the same (36-46 /*). 



The epithelium of the straight collecting tubes has clear cells 

 with no appearance of striation. They are cubical at first, and 

 gradually become columnar as the size of the lumen increases. 

 The nucleus is always round and sharply defined. 



In the large collecting tubes or papillary ducts the basement 

 membrane is strengthened by connective tissue. The diameter at 

 the orifices upon the papillae may be 1 mm. 



The arrangement of Uood-vessels in the kidneys is no less 



