HISTOLOGY. The kidney is a highly differentiated compound tubular gland. It is composed of an external portion, or cortex. and an internal por tion, or medulla, the latter terminating internally in a number of apices known as papilla.. Each papilla contains the terminations of a system of tubules which, extending out into the vortex. eon itide a kidney pyramid or lobule. The division into lobules is readily seen in focal kidneys, and in the kidneys of certain of the lower animals.
In the adult human kidney, however, the lobula tion has disappeared, and in the cortex and me dulla there is no demarkation between the lobules. If a section be mane through the kidney it is easy with the naked eye to distinguish cortex and medulla. Strands of medullary substance can also be seen extending up into the cortex. These are known as medullary rays, and that portion of the cortex included between two medul lary rays is known as a cortical pyramid. Under the microscope both cortex and medulla are seen to he made up of innumerable sections of tubules which are entirely unintelligible in the ordinary section of the kidney. They all represent, how ever, sections of different portions of what are known as the uriniferous tubules. The structure of these tubules has been carefully worked out, and may be described as follows: Each urinifer ous tubule origi nates in the cortex in an expansion which surrounds a tuft of capillary blood-vessels called the glomerulus or Ma pigh ian body.
Leaving the glom embus by a narrow neck, it expands into the first or proximal convolut ed tubule. This is lined by rather large irregular granular epit he lium. The tubule now passes into the medulla• where its epithelium be e o nt e s flattened.
and it is known as the desvending arm of Ilenle's loop. In the deeper part of the medulla the tubule turns abruptly upon itself, forming the loop of Ilenle, and ascends in the medulla and medullary ray. Benle's loop and its ascended arm are lined by euboidal epithelium. In the cortex the tubule leaves the medullary ray, be comes lined with large irregular cells. and is known as the second or distal convoluted tubule. After a more or less tortuous course in the cor tical pyramid the tubule again enters a medullary ray and joins with other similar tubules to form what is known as a collecting tubule. This tu
bule is lined with euboidal epithelium and passes down through medulla and papilla to open into the pelvis of the kidney. The larger of these tubules situated in the papilla are called the ducts of Bellini. The different portions of the uriniferous tubules are thus seen to make up the structure of the kidney, being held together and supported by a very small amount of con nective tissue. The cortical pyramids are seen to lie formed by the glomernli and the first and seennd convoluted tubules, the medullary rays by the ascending arms of Ilenle's loops and the collecting tubules, the medulla by the as cending and descending arms of Henle's loop, by the loops themselves. and by the collect ing tubules, the papilla by the duets of Bel lini. These uriniferous tubules constitute the so called parenchyma or functionating part of the kidney, NI hile the connective tissue which sup ports them is known as the interstitial part of the kidney.
Btoon-VEsst:Ls OF THE Kinxi.x. Blood reaches the kidney through the renal artery, which enters the hilum and breaks up into several large branches. The:se branenes pass up between the ducts of i3alini to the junction of niedulla and cortex. Here they form a series of arches known as the renal arches. From these are given off two of branches. One set, the interlobular cortical arteries, pass upward to the cortex, forming a capillary network among the convoluted tubules and in the glomeruli; the other, known as the arterbe reetie, pass downward into the medulla, where they torm a capillary network among the tubules of the medulla. From the cortical capil lary network blood enters the interlobular vein, whence it is carrica to the veins of the renal arches. From the medullary capillaries blood enters the venue recta• and thence goes to the veins of the renal arches. The lymphatics of the kidney Consist of two sets of vessels, a superficial set which ramify in the deeper layers of the capsule, and a deeper set which accompany the blood-vessels into the interior of the organ, and communicate with the lymph-spaces of the intertubular eonneetive tissue. The nerves of the kidney belong to the sympathetic system. and are mainly of the non-nushillated variety. They pass in between the tubules, where they form a tine network.