568 



THE URINARY APPARATUS 



[CH. XXXVI TI. 



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

 such as frogs, where the neck is longer, the epithelium is ciliated. 



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

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

 protoplasm is granular. The cells interlock laterally and are difficult 

 to isolate. 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 functional tubule 

 has a large lumen, and is lined by clear flattened cells; the col- 

 lecting tubules and ducts' of Bellini are lined by clear cubical or 

 columnar cells (see fig. 373). 



a 



nn ?h7 T? a Ve mL cal sec j; lon through the kidney of a dog-the capsule of which is supposed to be 

 on we right, a, The capillaries of the Malpighian corpuscle, which are arranged in lobules ; n, neck 

 of capsule ; c, convoluted tubes cut in various directions; 6, zigzag tubule; d, e, and/, are straight 

 ?m ^SdlawJ'^) 80 * 1 ^ tUbe; e ' s P^ tube; /.narrow section of ascending limb. 



The extent of the zone of clear cells in the loop of Henle varies 

 a good deal in different animals ; a diminution of this part of the 

 tubule lessens the length of the total loop ; in most animals there 

 is an admixture of long and short loops, though the proportion of the 

 two varies greatly in different parts of the animal kingdom. 



