THE BLOOD-VESSEL ARRANGEMENTS OF THE KIDNEY.
It has been mentioned in passing that the expanded ends of the tubules contain bundles of fine blood-vessels. Each kidney receives an artery front the main arterial trunk--the aorta —as it passes through the belly. The artery, after entering the kidney, splits up into various branches, which penetrate into the substance of the kidney. They reach the junction between the cortex and medullary regions, from which twigs pass up between groups of the convo luted tubules towards the surface of the kid ney. On their way they give off branches.
These branches penetrate the expanded ends of the tubules and immediately break up into a ball or tuft of capillaries, for which the expan sion serves as a covering or cap sule. The ball of capillaries is called a glomer ulus, and' it, to gether with its capsule, forms a Malpighian body of the kid ney. The capil laries ultimately reunite to form a small vessel which passes out of the capsule, and speedily thereafter that outgoing vessel gives rise to a number of capillary blood -vessels which ramify over the convoluted tubules, affording, we may suppose, nourishment to their cells.
These capillaries are then gathered together to form a small vein which joins other veins till large veins are formed, whose junction forms one large vein which goes off from the kidney, carrying away the blood brought by the artery and pouring it into the vena cave (Fig. 157). Other arterial twigs proceed from
the junction of medulla and cortex into the region of the pyramids of straight tubes, over which they ramify, to end in veins as the others. The chief point to notice is that the vessel that enters the capsule—the afferent vessel (Latin, ad, to, and fero, I carry)—gives rise, directly or indirectly, to two sets of capil laries, those of the glomerulus and those that proceed from the vessel that issues from the capsule—the efferent vessel (ex, out of, and fero). The outgoing vessel is smaller than the ingoing ; and the significance of this will ap pear immediately. Fig. 161 represents a Mal pighian body with its entering and issuing vessels, surrounded by the capsule, the blind end of a tubule. Fig. 159 is a representation of the afferent vessel (1), arising from au ar terial branch, forming the glomerulus, which ends in the efferent vessel (2), whose capillaries ramify over the convoluted tubes and end in a venous twig. In the Malpighian body the tuft of vessels is covered with a layer of small cells, so also is the inner surface of the capsule.
These details of structure show, to a great extent, how the kidney discharges its duties in separating waste matters from the blood, and what is the nature of the apparatus by which this is effected.