254 



HISTOLOGY. 



be identified (Fig. 268) ; they may be studied to better advantage, however, 

 in tangential sections of the kidney, one through the cortex and one through 

 the medulla. In these the tubules appear in cross section. The radiate 

 parts of the cortex are seen as islands of circular sections surrounded by 

 the irregular convoluted tubules and renal corpuscles. The greater part 

 of such an island is shown in Fig. 287. 



Finer structure of the renal tubules. The renal tubules are lined 

 throughout with simple epithelium. In the inner layer of the capsule of 

 the glomerulus, it is a flat syncytial layer blending with the small amount 

 of perivascular connective tissue beneath. The outer layer of the capsule 

 is also flat and is composed of polygonal cells. Terminal bars which occur 



in all other divisions of the renal 

 tubules have not been demonstrated 

 in the capsule. The flat epithelium 

 of the outer layer of the capsule 

 changes at the 'neck' of the capsule 

 to the low columnar epithelium of 

 the proximal convoluted tubule. 

 Here cell boundaries are indistinct. 

 The nuclei are toward the base of 

 the cells which rest on a structure- 

 less basement membrane continuous 

 with that of the capsule. The pro- 

 toplasm contains granules arranged 

 in vertical rows which toward the 

 base of the cell appear as rods (Fig. 

 289). In certain animals plaitings 

 in the cell wall have been found to 

 cause a rodded appearance in these 



cells. Toward the irregular lumen there is a 'brush border' (Fig. 289) 

 suggestive of short non-motile cilia. It is uncertain whether this is nor- 

 mal or due to disintegration. Clear spaces are sometimes seen in the 

 outer part of the cells. The lumen is wide and the cells are low after 

 copious urine production; reverse conditions occur when the urine is 

 scanty. It is in the two convoluted portions of the tubules that urea and 

 pigments are believed to be excreted; the fluid part of the urine comes 

 chiefly from the glomeruli. 



The descending limb both in the radiate cortex and in the medulia 

 (Figs. 287 and 288) is a thin walled conducting tube from 9 to 16 p. In 

 diameter. (The proximal convoluted tubule measures from 40 to 60 /*). 

 Cell boundaries are absent. Often in sections the flat nucleus causes 



Collecting tubule 



Descending limb. 



Ascending limb. 



Surface view of 

 an ascending 

 limb. 



FIG. 286. TUBULES OF THE PARS RADIATA. 

 From a radial section of a human kidney. X 240. 



