CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 675 



again bear long cilia, constituting a second conducting portion. 

 This second conducting ciliated portion is in turn succeeded by a 

 division of much larger diameter in which the cells are most 

 distinctly striated and otherwise resemble the cells of the~ con- 

 voluted tubules of the mammalian kidney, thus constituting the 

 second secreting portion. The succeeding portions of the tubule 

 have the character of conducting tubules, and join their fellows 

 to fall ultimately into the ureter. Obviously the second secreting 

 portion is represented in the mammal by the second convoluted 

 tubule, the zigzag tubule and the ascending limb of Henle's 

 loop, while the descending limb of Henle's loop corresponds to 

 the second conducting ciliated portion of the amphibian tubule; 

 the cilia however have entirely disappeared, and the likeness is 

 confined to the narrowness of the whole tubule and the absence 

 of secreting characters in the cells. Why however the kidney 

 of the lower animal possesses this reduplication of secreting and 

 conducting portions, and why remains and remains only of the 

 reduplication should thus be preserved in the mammalian kidney, 

 has not yet been satisfactorily explained. 



397. The vascular arrangements of the kidney deserve 

 special attention. The renal artery approaching the kidney at 

 the hilus divides into branches which slipping round the pelvis 

 enter into the substance of the kidney at the angles formed by 

 the branching of the pelvis into calyces, and therefore between 

 the pyramids. Running radially between the pyramids the 

 branches, reaching the boundary between cortex and medulla, 

 divide and, spreading laterally, form at the bases of the pyramids 

 arches more or less concentric with the hilus. From these arches, 

 which anastomose to a certain extent with each other, vessels proceed 

 on the one hand to the cortex and on the other to the medulla. 



To the cortex are given off relatively large arteries which run 

 in a radial direction towards the surface in the masses of cortex 

 between the medullary rays. From each of these interlobular or 

 radiate arteries as they are called, short relatively thick branches 

 are given off at intervals on all sides ; these taking a course 

 somewhat curved, with the convexity directed towards the surface 

 of the kidney, end, without branching, in Malpighian capsules; 

 they are the afferent vessels spoken of previously. Other branches 

 of the same radiate arteries break up into capillaries surrounding 

 the tubules, this being especially the case near the surface of 

 the kidney. The efferent vessels from the Malpighian capsules also 

 break up into a capillary network which, embracing the tubules, 

 becomes continuous with the other network, the meshes being 

 rounded or polygonal in the cortical substance, but more elongated 

 radially in the medullary rays. The blood-supply here repeats on 

 a small scale the portal system of the liver, since a vessel formed 

 by the union of capillaries breaks up in capillaries once more. 



From the same arterial arches at the boundary of the cortex 



