KIDNEY 



321 



with that of the fibrous capsule. The latter contains elastic fibers, which 

 increase in abundance with age, and also smooth muscle fibers. 



Lobes and lobules. In embryonic life the kidney is divided into lobes, 

 bounded by the renal columns, and indicated by grooves 

 upon the outer surface (Fig. 318). The grooves become 

 obliterated during the first year. In the ox similar 

 grooves are permanent; in many mammals as in the cat 

 and rabbit, they never exist, since the kidney has but 

 one lobe, papilla and pyramid. The lobules or structural 

 units of the kidney are the areas centering around each 

 radiate division of the cortex, by which they are drained 



(Fig. 317)- 

 septa. 



Blood vessels. The kidney has a capillary circulation. The renal 

 artery passes from the aorta to the hilus, or notch on the medial border of 

 the kidney. It divides into several branches, most of which pass over the 



FIG. 318. KIDNEY OF 



They are not bounded by connective tissue * CHILD AT BIRTH. 



J J ( After Hertwig.) 



Partly injected glomerulL 



Jwterlobular artery. 

 ""Interlobular vein. 



FIG. 319. FROM A SECTION OF THE INJECTED CORTEX OF AN ADULT HUMAN KIDNEY. X 30. 



ventral surface of the pelvis into the fat around the calyces (Fig. 311). 

 Thence, as interlobar arteries, they extend to the boundary layer between 

 the cortex and medulla where they are known as arciform arteries (Fig. 

 317). These send interlobular arteries through the convoluted part of the 

 cortex and their terminal branches enter the fibrous capsule. It will be 

 noted that the kidney is exceptional in having its arteries at the periphery 

 of its lobules. From the interlobular arteries small stems pass to the glo- 

 meruli, each of which receives a single twig (Fig. 319). This is resolved into 

 a knot of capillary loops, the endothelium of which seems to blend with the 

 surrounding syncytium and indirectly with the inner layer of the capsule. 



