424 



EXCRETION 



tubes, form larger ducts which finally open at the apex of the papilla. These 

 collecting tubes are lined with nucleated columnar or cubical cells. 



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 



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

 is Supposed 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, collecting tube; e, spiral tube; /, narrow section of ascending limb. 

 X 380. (Klein and Noble Smith.) 



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- 

 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 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 



