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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
  The renal tubules, commencing in the renal corpuscles, present, during their course, many changes in shape and direction, and are contained partly in the medullary and partly in the cortical substance. At their junction with the glomerular capsule they exhibit a somewhat constricted portion, which is termed the neck. Beyond this the tubule becomes convoluted, and pursues a considerable course in the cortical substance constituting the proximal convoluted tube. After a time the convolutions disappear, and the tube approaches the medullary substance in a more or less spiral manner; this section of the tubule has been called the spiral tube. Throughout this portion of their course the renal tubules are contained entirely in the cortical substance, and present a fairly uniform caliber. They now enter the medullary substance, suddenly become much smaller, quite straight in direction, and dip down for a variable depth into the pyramids, constituting the descending limb of Henle’s loop. Bending on themselves, they form what is termed the loop of Henle, and reascending, they become suddenly enlarged, forming the ascending limb of Henle’s loop, and reënter the cortical substance. This portion of the tubule ascends for a short distance, when it again becomes dilated, irregular, and angular. This section is termed the zigzag tubule; it ends in a convoluted tube, which resembles the proximal convoluted tubule, and is called the distal convoluted tubule. This again terminates in a narrow junctional tube, which enters the straight or collecting tube.
  The straight or collecting tubes commence in the radiate part of the cortex, where they receive the curved ends of the distal convoluted tubules. They unite at short intervals with one another, the resulting tubes presenting a considerable increase in caliber, so that a series of comparatively large tubes passes from the bases of the rays into the renal pyramids. In the medulla the tubes of each pyramid converge to join a central tube (duct of Bellini) which finally opens on the summit of one of the papillæ; the contents of the tube are therefore discharged into one of the calyces.
  Structure of the Renal Tubules.—The renal tubules consist of a basement membrane lined with epithelium. The epithelium varies considerably in different sections of the tubule. In the neck the epithelium is continuous with that lining the glomerular capsule, and like it consists of flattened cells each containing an oval nucleus (Fig. 1132). The two convoluted tubules, the spiral and zigzag tubules and the ascending limb of Henle’s loop, are lined by a type of epithelium which is histologically the same in all. The cells are somewhat columnar in shape and dovetail into one another of their lateral aspect. Each has a striated border next the lumen of the tube, its inner part is granular and its outer portion vertically striated. The nucleus is spherical and situated about the center of the cell. In the descending limb of Henle’s loop the epithelium resembles that found in the glomerular capsule and the commencement of the tube, consisting of flat, clear epithelial plates, each with an oval nucleus (Fig. 1131). The nuclei alternate on opposite surfaces of the tubule so that the lumen remains fairly constant.
  In the straight tube the epithelium is clear and cubical: in its papillary portion the cells are distinctly columnar and transparent (Fig. 1132).

The Renal Bloodvessels.—The kidney is plentifully supplied with blood (Fig. 1133) by the renal artery, a large branch of the abdominal aorta. Before it enters the kidney, each artery divides into four or five branches which at the hilum lie mainly between the renal vein and ureter, the vein being in front, the ureter behind; one branch usually lies behind the ureter. Each vessel gives off some small branches to the suprarenal glands, to the ureter, land to the surrounding cellular tissue and muscles. Frequently a second renal artery, termed the inferior renal, is given off from the abdominal aorta at a lower level, and supplies the lower portion of the kidney, while occasionally an additional artery enters the upper part of the kidney. The branches of the renal artery, while in the sinus, give off a few twigs for the nutrition of the surrounding tissues, and end in the arteriæ propriæ renales, which enter the kidney proper in the renal columns. Two of these pass to each renal pyramid, and run along its sides for its entire length, giving off in their course the afferent vessels of the renal corpuscles in the renal columns. Having arrived at the bases of the pyramids, they form arterial arches or arcades which lie in the boundary zone between the bases of the pyramids and the cortical arches, and break up into two distinct sets of branches devoted to the supply of the remaining portions of the kidney.


FIG. 1131– Longitudinal section of descending limb of Henle’s loop. a. Membrana propria. b. Epithelium. (See enlarged image)

  The first set, the interlobular arteries (Fig. 1128), are given off at right angles from the side of the arterial arcade looking toward the cortical substance, and pass directly outward between the medullary rays to reach the fibrous tunic, where they end in the capillary net-work of this part. These vessels do not anastomose with each other, but form what are called end-arteries.

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