THE SECRETION OF URINE 1267 



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 epithe- 

 lium similar to that of the convoluted tubules, but with less marked 

 striation. The junctional and collecting tubules are lined with 

 cubical or columnar cells with a clear protoplasm. The marked 

 differences between the structure of these various parts point to a 

 differentiation of function and division of labour among them in the 

 preparation of the fully formed urine. This conclusion is borne out 

 by a study of the blood-supply of this kidney. The large renal artery 

 divides in the pelvis into four or five branches, which pass up to 

 the boundary zone and there give off arteries in different directions ; 

 those which run towards the surface are the inter lobular arteries. 

 Each of these, which is an end- artery presenting no anastomoses with 

 its fellows, gives off on all sides short wide branches, which pass 

 to the glomeruli and constitute the vasa afferentia of these bodies. 

 Each vas afferens has a thick muscular wall. The glomerulus itself 

 consists of a number of anastomosing wide capillaries invested by an 

 extremely thin wall, which is sometimes said to consist simply of a 

 protoplasmic film devoid of nuclei. The glomerular capillaries are col- 

 lected together to form an efferent vessel, the vas efferens, which is 

 narrower than the vas afferens, but, like the latter, presents a well- 

 marked muscular coat. The vas efferens breaks up again into a second 

 set of capillaries, which ramify around the tubules of the cortex and 

 communicate with a similar network round the tubules of the medulla. 

 The medullary pyramids are also provided with blood by a plexus of 

 capillaries taking their origin from little bunches of vessels, the vasa 

 recta (v. Fig. 527), which leave the concave side of the arterial arches of 

 the boundary zone to run towards the papilla, and receive also a few 

 vessels which spring from the vasa afferentia of the cortical vessels. 

 From the capillaries of the tubules the blood is collected again into 

 veins, which leave the kidney partly by the cortex and capsular vessels, 

 partly by large venous trunks which join to form the renal vein at the 

 hilum of the kidney. The kidney is richly supplied with nerves, which 1 

 are chiefly distributed to the muscular walls of its blood-vessels. Some ] 

 authors have described a fine nerve-plexus surrounding the tubules and 

 sending branches between and into the cells of the convoluted tubules 

 themselves. 



The main points in the above description of the structure of 

 the kidney were made out by Bowman in 1840, and suggested the 



