374 



EXCRETION 



Renal Blood Supply. The renal artery divides into several branches 

 which pass in at the hilus of the kidney and are covered by a fine sheath 

 of areolar tissue derived from the capsule. They enter the substance of the 

 organ chiefly in the intervals between the papillae and at the junction between 

 the cortex and the boundary layer. The main branches then pass almost 

 horizontally, forming more or less complete arches and giving off branches 

 upward to the cortex and downward to the medulla. The former are for 

 the most part straight; they pass almost vertically to the surface of the kidney, 

 giving off laterally in all directions longer and shorter branches, which ulti- 



FIG. 288. From a Vertical Section through the Kidney of a Dog, the Capsule of which is Sup- 

 posed to be on the Right, a, The capillaries of the Malpighian capsule, the glomerulus, are arranged 

 in lobules; n, neck of capsule; c, convoluted tubes cut in various directions; b, irregular tubule: 

 d, e, and / are straight tubes running toward capsules forming a so-called medullary ray; d, collect- 

 ing tube; e, spiral tube; f, narrow section of ascending limb. X 380. (Klein and Noble Smith.)] 



mately supply the glomerulus. The small afferent artery, figures 287, a, 

 290, d, which enters the Malpighian capsule, breaks up in the interior into 

 a dense convoluted and looped capillary plexus, which is ultimately gathered 

 up again into several small efferent vessels, comparable to minute veins, 

 which leave the capsule at one or more places near the point at which the 

 afferent artery enters it. On leaving, they do not immediately join other 

 small veins as might have been expected, but again break up into a second 

 set of capillary vessels which form an interlacing network around the urinif- 

 erous tubules. This second capillary plexus terminates in small veins 

 which, by union with others, help to form the radicles of the renal vein. 



