BLOODVESSELS OF THE KIDNEY. 97 



Cobitis fossilis. The capsule is here relatively long, the neck very 

 long, and the tortuous tube extends from the neck to the collecting 

 tube, which is of large diameter at its commencement and termination, 

 but in the middle of its course, and for a short distance, is considerably 

 attenuated. The height of the epithelium varies with the diameter 

 of the canal ; it is everywhere transparent. 



The simplest form of kidney with which we are at present acquainted 

 is that of Bdellostoma Fosteri. The urinary tubules in this instance 

 are composed of a capsule, the neck of which becomes continuous 

 with a wide tube that after a short course opens into the collecting 

 tube. Nothing is known in regard to the epithelium of this primitive 

 form of kidney. 



BLOODVESSELS. 



The kidneys receive their chief supply of blood through the 

 renal arteries, but occasionally they are partially supplied by 

 the phrenic, lumbar, and suprarenals, very delicate branches of 

 which anastomose with those of the renal artery on the fibrous 

 capsule. Although the medulla, cortex, and capsule are thus 

 supplied by a single trunk, the capillaries and ultimate arterial 

 divisions are separable into three parts, corresponding to these 

 several portions of the kidney. 



BLOODVESSELS OF THE CORTEX. The renal arteries send 

 by far the greatest part of their blood through the cortex, and 

 the trunks, without the formation of plexuses, undergo division 

 so rapidly that very shortly after their entrance into the 

 cortex they have broken up into very fine arteries, the 

 arteriolse interlobulares. 



In longitudinal sections of the kidney these vessels are seen 

 occupying the interspace between two adjoining medullary 

 rays, and also where several primitive cones are in contact 

 with each other. By far the greatest part of these branches, 

 as soon as the medullary rays cease, becomes indistinguishable 

 to the naked eye, though a small portion penetrates to the 

 most superficial layer of the cortex, and reaches the fibrous 

 capsule. Every arteria interlobularis, in its course between the 

 tortuous tubules, gives off branches in quick succession, and 

 so that small trunklets extend to the dilated extremity of a 



VOL. II. H 



