If a kidney be split longitudinally, it will be noticed that its outer part, the cortex, is darker in colour than its inner part, the medulla (Fig. 9). The glomeruli already referred to occur in the cortex. The medulla is occupied by radiating tubules, collected into groups. Those of each group converge towards a common duct. From twelve to eighteen ducts open into the expanded end of the ureter, each at the apex of a pyramid. If the section of the kidney be examined with a lens, it will be seen that narrow rays from the medulla extend into the cortex. The cortex is therefore made up of interdigitating pyramids of dark substance, consisting of glomeruli and the contorted tubules, about to be described, and of lighter substance, consisting of straight tubules continuous with those of the medulla. [Illustration: FIG. 9.—THE UPPER END OF THE LEFT KIDNEY, VERTICALLY DIVIDED, AND MAGNIFIED. It is invested by a capsule with which, at the hilus, the dilated end of the ureter blends. A portion of a papilla (the end of a pyramid) is shown projecting into one of the calices into which the ureter dilates. The peripheral portion of the kidney containing glomeruli and contorted tubes is termed its cortex, the central portion medulla. At A is shown a single urinary tubule. Commencing at the third glomerulus, it winds in the cortex, descends into the medulla, turns in a loop of Henle, again winds in the cortex, and ends in a collecting tube, which joins a duct. The arrangement of the bloodvessels is shown at B. A straight artery and a straight vein lie side by side. The artery gives branches to the glomeruli. The venules from the glomeruli again divide into capillaries, which supply the contorted tubes and loops of Henle. The ducts are supplied by long arterial capillaries. C shows the structure (magnified) of a glomerular tuft of capillary vessels, invested by a capsule which closes into a contorted tube, _ct_; _dH_, a descending limb; _aH_, an ascending limb of a loop of Henle; _d_, a duct.]