THE URINARY SYSTEM: KIDNEYS 43S 



the outer fibrous investments and joins the main renal veins where they emerge from 

 the kidney (Figs. 31, 32), sending also independent small trunks to the postcava. The 

 external network and the central venous system of the kidney are in communication with 

 one another at intervals throughout the length of the organ by means of lateral connecting 

 branches. These correspond roughly to the positions of the interlobar septa, and are 

 roughly parallel with the primary branches of the urinary duct which also have an 

 interlobar position. They also receive branches from the groups of renculi bordering 

 upon them. 



The lobes which form the architectural basis of the kidney are thus surrounded by 

 venous arcades, into which smaller veins surrounding the constituent lobules drain 

 themselves. The lobes are penetrated by arteries from the central arterial system, 

 which ramify irregularly among the lobules. Each lobule, consisting of from four to six 

 renculi, receives its arteries from these (Fig. 31). 



Fig. 33 represents the appearance of the renculi in a horizontal section of an injected 

 kidney. The renculi, which lie in all directions to the plane of the section, are seen to 

 present very various and irregular outlines, but the grouping of them into lobules of four 

 to six renculi can be made out in the figure. On the right the main arteries and veins of 

 the central system can be seen, and portions of the cross-connecting venous trunks 

 running through the kidney substance. The lower sketch represents two lobes containing 

 two lobules each, while the upper one represents one larger lobe containing perhaps five 

 or six lobules. It can be seen that the arterioles penetrate into the centres of the lobules, 

 together with the secondary branches of the urinary duct, and enter the renculi at their 

 pelves. The venules, on the other hand, run in general round the peripheries of the 

 lobules, so that while the arterial supply to the renculi is intralobular the venous drainage 

 is interlobular. Further, since the kidney lobes are penetrated in all directions by arteries 

 from the central system, the arterial supply to them may be said to be intralobar ; but the 

 venous system, whose main channels lie outside the lobes, is interlobar. 



Within the renculi themselves (Fig. 34) the arterioles can be seen in injected sections 

 entering on either side at the bases of the pelves. They divide up within the renculus 

 to form the arterial arcade usually found in all mammalian kidneys — a network of 

 major capillaries between the cortex and the medulla. The arterioles of the arcade are 

 surrounded by fibrous tissue and can be seen in section at the apices of the pyramids 

 between the medullary rays (Fig. 34 a). Fig. 34 also shows part of this major network 

 within the renculus running in the plane of the section. The capillary supply to the 

 glomeruli is very irregularly distributed; but its general arrangement is based, as 

 Fig. 34 shows, on a series of major capillaries running outwards among the tubules and 

 capsules from the arcade towards the periphery of the renculus, where they anastomose 

 with one another. They give off minute capillaries to the neighbouring glomeruli, and 

 since, in the foetus, the glomeruli are orientated so that their necks are directed centri- 

 fugally, these minute capillaries seem to have in general a centripetal direction. The 

 glomerular supply has thus the appearance of a series of festoons of major capillaries 

 from which the minor ones depend (Fig. 34). 



