318 HISTOLOGY 



layer is a flat syncytium blending with the perivascular tissue of the 

 glomerulus, and following its lobulations. The outer layer of the capsule 

 is smooth, and is composed of flat polygonal cells. Terminal bars, which 

 have been found in all other divisions of the renal tubules, have not 

 been demonstrated in the capsule. The flat epithelium of the outer layer 

 changes at the "neck" of the capsule to the low columnar epithelium 

 of the proximal convoluted tubule. The neck may occur in various 

 positions, generally being opposite the aperture through which the vessels 

 enter and leave. The space between the layers of the capsule is continuous 

 with the lumen of the convoluted tubule. 



The proximal convoluted tubules are large (40-60 /* in diameter) , with 

 irregular lumens and indistinct cell walls. In some animals the walls 

 are folded so as to be vertically plaited. The cells show signs of secretory 

 activity and are believed to excrete urea and pigments; the fluid part of 

 the urine comes chiefly from the glomeruli. The nuclei are toward the base 



Collecting Thin Thick 



tubules. segment. segments. 



FIG. 314. CROSS SECTION OF A CON- 

 VOLUTED TUBULE FROM A RABBIT. 

 (Szymonowicz.) 



FIG. 315. TUBULES OF THE PARS RADIATA. FROM A RADIAL 

 SECTION OF A HUMAN KIDNEY. X 240. 



of the cells, and the protoplasm contains granules arranged in vertical 

 rows which form basal rods (Fig. 314). Toward the lumen there is a 

 "brush border" suggestive of short non-motile cilia. It is uncertain 

 whether this is normal or due to post-mortem 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; and the reverse 

 is true when the urine is scanty. 



The upper segment of the descending limb of Henle's loop is similar 

 in structure to the proximal convoluted tubules. It is a straight tubule, 

 however, and is found in the radiate part of the cortex (Fig. 313). 



The upper segments of the ascending limbs are also found in the pars 

 radiata. Their protoplasm is less granular than that of the descending 

 limbs, but closely resembles that of the distal convoluted tubules. The 

 latter are typically shown in Fig. 313 (there being one on either side of the 

 label line to the " capsule of the glomerulus"). Huber (loc. tit.) describes 

 these tubules as showing "an outer dark zone which is finely striated, 



