402 



EXCRETION 



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. 

 These form venous arches corresponding to the arterial arches situated 

 between the medulla and cortex. 



Thus, in the kidney, the blood entering by the renal artery traverses 

 two sets of capillaries before emerging by the renal vein, an arrangement 

 which may be compared to the portal system. 



The tuft of vessels within the Malpighian capsule in the course of de- 

 velopment has been thrust into the dilated extremity of the urinary tubule, 

 which finally completely invests it. Thus within the Malpighian capsule 



r 



FIG. 289. Transverse Section of a Renal Papilla, a, Large tubes or papillary ducts; 

 b, c, and d, smaller tubes of Henle; e, f, blood capillaries, distinguished by their flatter 

 epithelium. (Cadiat.) 



there are two layers of squamous epithelium, a parietal layer lining the cap- 

 sule proper, and a visceral or reflected layer immediately covering the vas- 

 cular tuft, figure 290, and sometimes dipping down into its interstices. This 

 reflected layer of epithelium is readily seen in young subjects, but cannot 

 always be demonstrated in the adult, figures 290 and 291. 



The vessels which enter the medullary layer break up into smaller arte- 

 rioles, which form a fine arterial meshwork around the tubes of the papillary 

 layer and end in a similar plexus from which the venous radicles arise. The 

 vessels do not form a double set of capillaries. 



Besides the small afferent arteries of the Malpighian bodies there are, 

 of course, others which are distributed in the ordinary manner, for the nutri- 

 tion of the different parts of the organ; and there are numerous straight 



