EXCRETION. 



467 



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 in miniature. 



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 there are two layers of squamous epithelium, a parietal layer 

 lining the capsule proper, and a visceral or reflected layer immediately 

 covering the vascular tuft (fig. 295), and sometimes dipping down into 

 its interstices. This reflected layer of 

 epithelium is readily seen in young- 

 subjects, but cannot always be demon- 

 strated in the adult. (See figs, 295 

 and 296.) 



Fig. 295. 



Fig. 296. 



Fig. 295. Transverse section of a developing Malpighian capsule and tuft (human). X 300. 

 From a fcetus at about the fourth month; a, flattened cells growing to form the capsule; b, more 

 rounded cells, continuous with the above, reflected round c, and finally enveloping it; c, mass of 

 embryonic cells which will later become developed into blood-vessels. (W. Pye.) 



Fig. 296. Epithelial elements of a Malpighian capsule and tuft, with the commencement of a 

 urinary tubule showing the afferent and efferent vessel ; a, layer of flat epithelium forming the 

 capsule; b, similar, but rather larger epithelial cells, placed in the walls of the tube; c, cells, covering 

 the vessels of the capillary tuft; d, commencement of the tubule, somewhat narrower that the rest 

 of it. (W. Pye.) 



The vessels which enter the medullary layer break up into smaller 

 arterioles, which pass through the boundary layer, and proceed in a 

 straight course between the tubules of the papillary layer, giving off on 

 their way branches, which form a fine arterial meshwork around the 

 tubes, and ending in a similar plexus from which the venous radicles 

 arise. 



Besides the small afferent arteries of the Malpighian bodies, there 

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

 the nutrition of the different parts of the organ; and in the- pyramids, 

 between the tubes, there are numerous straight vessels, the vasa recta, 

 some of which are branches of vasa efferentia from Malpighian bodies, 

 and therefore comparable to the venous plexus around the tubules in 



