THE URINARY ORGANS. 



199 



excretory ducts, or tubes of Bellini, in the papillary region 

 present a beautiful example of simple columnar epithelium in the 

 tall, transparent, and clearly-defined cells with which they are lined. 

 These cells, the largest epi- 

 thelial elements within the FIG. 242. 

 kidney, are defined from 

 one another with great dis- 

 tinctness, and possess oval 

 nuclei situated somewhat 

 nearer their outer bound- 



aries. 



The blood-vessels with- 

 in the kidney are very plen- 

 tiful. The renal artery, 

 entering at the hilum, passes 

 through the sinus within 

 the submucous tissue which 

 occupies the space between 

 the wall of the pelvis and 

 the neighboring paren- 



chyma; during itS COUrse 

 through the sinUS Several 

 Small twigS are given Off for 



the nutrition of the struc- 

 tures in the immediate vicinity. Before entering the glandular tissue 

 the renal artery breaks up into a number of large branches, which 

 traverse the parenchyma through oblique channels within the inter- 

 pyramidal tracts, or columns of Bertini, to gain a position at the 

 juncture of the cortex and medulla corresponding to the bases 

 of the Malpighian pyramids. At this point they bend sharply to 

 form a series of horizontal arches, from which two sets of vessels 

 spring the ascending interlobular cortical arteries and the 

 arteriae rectse of the medulla. 



The straight cortical branches, passing towards the free surface 

 of the organ, give off short, curved lateral twigs to supply the affer- 

 ent vessels of the glomeruli. These branches divide into groups 

 or lobules of convoluted capillaries ; the latter, in turn, join to form 

 the slightly smaller efferent vessels, which carry off the still arterial 

 blood from the Malpighian bodies. The efferent vessels soon break 

 up into capillary net-works which surround the tubules of the 

 labyrinth and the medullary ray. These net-works are taken up by 

 the interlobular veins which accompany the arteries, and pass to 

 the pelvis, where they aid in forming the large renal veins. The 

 vessels collecting the blood from the peripheral zone of the cortex 



Transverse section of papillary region of medulla of hu- 

 ^" ^ney, more h^hly magnified : C, large collecting 

 tubules ; x and y, descending and ascending limbs of 

 Henle's loops ; K, blood-vessels. 



