426 



THE UKINAEY SYSTEM 



of the organ and the tips of the cortical rays, and in which the renal 

 corpuscles, though present, are relatively few in number. 



The Renal Lobule. In fetal and infantile life the kidney is dis- 

 tinctly lobed. This condition is permanent in some animals e.g., rep- 

 tiles, birds, porpoise, ox, bear each lobe consisting of a renal pyramid 

 with its related portion of cortical substance. In 

 man, after the first year, the renal lobes com- 

 pletely fuse and eventually leave scarcely a trace 

 of the early lobed condition. 



Internally this fetal lobed condition is in part 

 indicated by the definitive renal pyramids, from 

 eight to eighteen in number, each representing the 

 product of fusion of from two to nine primitive 

 lobes. In certain mammals e.g., mouse, rabbit, 

 cat, guinea pig neither a primitive nor definitive 

 internal or external lobar arrangement appears. 

 Such kidneys are designated unipyramidal or uni- 

 papillary, in contradistinction to the multipapil- 

 lary type. In certain of the perissodactyls (ele- 

 phant, horse) no distinct pyramids are present. 



The term renal lobule or renculus, as applied 

 to the adult human kidney, refers to a still smaller 

 subdivision of the organ, one which includes a 

 single cortical ray together with that portion of the cortical labyrinth by 

 which it is immediately invested. Its peripheral boundary is marked by 

 the interlobular blood-vessels. This lobule is the anatomical unit of the 

 kidney and is thus comparable to the portal and to the pulmonary lobule, 

 except that its arterial supply enters at the periphery. The tortuous 

 secreting portions of its uriniferous tubules are contained in the laby- 

 rinth at the periphery of the lobule, while its straight conducting por- 

 tions lie in the cortical ray in the axis of the lobule. The larger inter- 

 lobular arteries and veins lie at the periphery, where they supply 

 branches to several adjacent lobules. 



FIG. 399. RECON- 

 STRUCTION OF A URI- 

 NIFEROUS TUBULE 

 OF AN INFANT. (After 

 Stoerk.) 



THE KENAL CONNECTIVE TISSUES 



The kidney is enveloped by a fibrous capsule, the tunica fibrosa, 

 which is loosely attached to the substance of the organ and contains the 

 usual proportion of elastic fibers together with a little smooth muscle. 

 A fatty layer, the tunica adiposa, invests the capsule. At the hilum of 



