113 2 PHYSIOLOGY 



into the underlying portion of the medulla, and consists of straight descending and 

 ascending limbs and the loop of Henle. The ascending limb passes into a distal convo- 

 luted tubule, and this by a ' junctional tubule ' joins with a number of others to form a 

 straight ' collecting tubule.' Several of these unite to form the papillary ducts, which 

 open on the surface of the papilla in the expanded part of the renal duct or ureter 

 (Fig. 529). The whole tubule consists of epithelium lying on a basement membrane ; 

 the epithelium varies in structure in different parts of the tubule. The bunch of 

 glomerular capillaries is covered with a very thin layer of endothelial cells, and a similar 

 layer forms the lining of Bowman's capsule. The convoluted tubules contain cells 

 which are roughly cubical or cylindrical in cross-section, but do not present very 

 definite cell outlines. These cells, which are similar in the two sets of convoluted 



Cortex 



Boundary zone 



Medulla 



FIG. 529. Diagram showing course of urinary tubules, and the distribution 

 of blood-vessels. (From YEO.) 



tubules, have long been distinguished as * rodded epithelium ' (Fig. 530) on account of 

 the ease with which a radial disposition of rods or granules is demonstrated in their 

 protoplasm. As ordinarily prepared, the free margin of these cells, where they abut 

 on the lumen, is irregular. This appearance is due to the readiness with which the 

 cells undergo alteration under the influence of different fixing reagents, especially of 

 such as contain water. When properly fixed it is seen that the rodded structure as 

 described by Heidenhain is due to rows of granules arranged vertically to the basement 

 membrane. Moreover the free margin of the cells, instead of being irregular, consist of 

 a well-marked striated border, formed of a number of very fine hairs closely set together 

 and springing from a row of granules in the peripheral part of the cell (Fig. 531). The 

 hairs, which make up the striated border (sometimes called the ' brush border '), have 

 not been observed to present ciliary movement, and are probably comparable with the 

 similar structures found clothing the free border of the epithelium of the intestinal 

 villus. Such cells are characteristic features of the epithelium lining the urinary organs 

 in all types of animals, and are well marked in the nephridia of worms. Besides these 

 rows of granules other granules are found, especially towards the free margin of the cell 

 and round about the nucleus. Some of the granules appear to be of a fatty, others of 

 a protein character. 



The descending limb of Henle's loop is narrow, and possesses flattened epithelial 

 cells, while the ascending limb presents an epithelium similar to that of the convoluted 



