When a microscopic section of the kidney, made so as to include both cortical and pyramidal substance, is examined, the contrast between these two regions is sufficiently stliking in respect that in the cortical substance the uriniterous tubules have a markedly irregular and convoluted course. If attention be now confined to the cortex alone, as in Fig. 417, it will be seen that convoluted tubules are not the only kind present. There are also straight tubules prolonged up from the pyramids in the form of tapering bundles (a) between which lie convoluted tubules (b). These tapering bundles, the medullary rays or pyramids of Ferrein, do not reach the surface, the most superficial part of the cortex presenting a continuous layer of convoluted tubules. In this way the deeper parts of the cortex present a regular division into alternating areas of straight tubules or medullary rays, and convoluted tubules. Among the convoluted tubules lie the Malpighian bodies or glomeruli (e in figure). These occur somewhat frequently, and at tolerably regular intervals.
Fig. 417. - Section of normal kidney including cortex and base of pyramid with a very low magnifying power, a, medullary rays; 6, convoluted tubules; e, region of arterin recte in pyramids; d, larger vessels running between pyramid and cortex; e, Malphigiau tufts; f, capsule, x 12.
In addition to these arrangements of the tubules, the bloodvessels must receive attention. The larger arteries (d, Fig. 417) run between pyramids and cortex, and send up stems given off at right angles into the cortex. These pass at intervals into the region of convoluted tubules, and as they ascend they give off lateral branches to the glomeruli. It will thus appear that the areas of convoluted tubules are also the areas of the ascending arteries (which are also called interlobular arteries) and glomeruli. In the glomerulus the afferent vessel breaks up into a congeries of capillary vessels, called the tuft. These gather together to form the efferent vessel, and this again breaks up into capillaries which surround the tubules with a rich network.
The large arteries which run between the pyramids and the cortex also give off occasional arterial branches downwards to the pyramids. These arteries break up into bunches of straight arterioles (arteriae rectae, c in Fig. 417) which are increased by branches coming down from the afferent vessels in the deeper parts of the cortex. These bunches of arterioles taper as they pass down the pyramids, so that they form small pyramids with their bases towards the cortex. They correspond in position with the areas of convoluted tubules, which areas they, as it were, prolong down into the pyramids.
Fig. 418. - Diagram of course of uriniferous tubules from Malphigian body (I) to pyramid (IX). See text. (Quair).