538 



THE URINARY APPARATUS 



fCH. XXXVI. 



The character of the epithelium that lines these several parts of 

 the tubules is as follows : 



In the capsule, the epithelium is flattened and reflected over the 

 glomerulus. 



The way in which this takes place in process of development is 

 shown in figs. 415 and 416. 



In the neck the epithelium is still flattened, but in some animals, 

 like frogs, where the neck is longer, the epithelium is ciliated. 



In the first convoluted and spiral tubules, it is thick, and the cells 



FIG. 415. Transverse section of a deve- 

 loping Malpighian capsule and tuft 

 (human), x 300. From a foetus at 

 about the fourth month ; a, flattened 

 cells growing to form the capsule; 

 b, more rounded cells; continuous 

 with the above, reflected round c, and 

 finally enveloping it ; c, mass of em- 

 bryonic cells which will later become 

 developed into blood-vessels. (VV. 

 Pye). 



FIG. 416. Epithelial elements of a MalpN 

 ghian capsule and tuft, with the com- 

 mencement of a urinary tubule show- 

 ing the afferent and efferent vessel ; a, 

 layer of flat epithelium forming the 

 capsule ; 6, similar, but rather larger 

 epithelial cells, placed in the walls of 

 the tube ; c, cells, covering the vessels 

 of the capillary tuft ; d, commence- 

 ment of the tubule, somewhat nar- 

 rower than the rest of it. (W. Pye). 



show a fibrillated structure, except around the nucleus, where the 

 protoplasm is granular. The cells interlock laterally and are difficult 

 to isolate. In some animals they are described as ciliated. In the 

 narrow descending tubule of Henle and in the loop itself, the cells are 

 clear and flattened and leave a considerable lumen ; in the ascending 

 limb they again become striated and nearly fill the tubule. In the 

 zigzag and second convoluted tubules the fibrillations become even more 

 marked. The junctional tubule has a large lumen, and is lined by 

 clear flattened cells; the collecting tubules and ducts of Bellini are 

 lined by clear cubical or columnar cells. 



Blood- Vessels of Kidney. The renal artery enters the kidney 



