CHAP, in.] ELIMINATION OP WASTE PRODUCTS. 645 



and medulla branches are also given off to the medulla, that is to 

 say to the pyramids. These running in a straight or rather radial 

 direction down the pyramids, as arterice rectce, but soon breaking 

 up into bundles of smaller vessels also running radially, supply 

 all the medullary substance of the pyramids with blood, forming 

 capillary networks with meshes elongated radially. 



From the capillaries of the pyramids veins are gathered up, 

 and these running radially upwards fall into venous arches, which, 

 like to and even better developed than the arterial arches, are 

 placed at the boundary between the cortex and medulla. Following 

 reversely the course of the arteries these venous arches, forming 

 more numerous anastomoses than do the arteries, fall into veins 

 which running radially between the pyramids join together over 

 the pelvis of the kidney and form eventually the renal vein ; 

 this, running in company with the renal artery, falls into the 

 vena cava inferior. 



From the capillaries of the cortex, including the medullary 

 rays, the blood, some of which as we have seen has passed through 

 the glomeruli of the Malpighian capsules, but some of which has 

 not, is gathered up into radiate veins which running radially 

 inwards to the boundary zone fall into the venous arches spoken 

 of above. At the surface of the cortex the small veins are apt to 

 be arranged in a somewhat star-shaped fashion, and are spoken of 

 as vence stellatce. 



Relatively to the bulk of the kidney the renal artery has large 

 dimensions. Coming off directly from the aorta, where the blood- 

 pressure is very high, and being comparatively short, it affords 

 favourable conditions for an ample supply of blood to the organ, the 

 conditions being made still more favourable by the low pressure 

 existing in the vena cava inferior. And, as a matter of fact, the 

 blood-supply to the kidney is very large. That blood is carried, 

 as we have seen, in the first instance almost straight to the 

 boundary of cortex and medulla, and is distributed from that 

 region. Hence it results that the blood-supply of the pyramids 

 consisting chiefly of conducting tubules is to a very large extent 

 distinct from that of the cortex where the tubules are chiefly 

 secreting tubules. 



We may repeat that for its size the kidney is most abundantly 

 supplied with blood. In sections of hardened and prepared kidneys, 

 the arteries, capillaries, and to a large extent the veins are emptied 

 of their blood, and the capillaries collapsed. Hence, judging by 

 such specimens alone, the kidney appears to be made up almost of 

 tubules alone ; but it must be borne in mind that during life every 

 tubule is netted round with fairly close-set capillaries which always 

 are more or less filled with blood, and at times largely distended 

 with blood. As we shall see later on, the kidney by mere decrease 

 or increase of the blood flowing through it may vary very widely 

 in volume. 



