THE SECRETION OF URINE 



1133 



tubules, but with less marked striation. The .functional 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 con- 

 clusion is borne out by a study of the blood-supply of the kidney. The large renal 



FIG. 530. A portion of convoluted tubule with ' rodded ' epithelium. 

 (HEIDENHAIN.) 



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 interlobular 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 



B 



FIG. 531. Cross-sections of convoluted tubules from kidney of rat. (SAUER.) 

 A, during slight secretion ; B, during maximal secretion. 



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 collected 

 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 



