diameter, is only slightly curved (2 to 4 convolutions). Its epithelium is relatively high, though not so high as that lining the proximal convoluted portion and not so distinctly striated. The cells are provided with large nuclei and their basal portions are joined by interlacing projections.

The next important segment is the short arched collecting portion, which has nearly cubical epithelial cells and a lumen somewhat wider than that of the intercalated tubule. The smaller straight collecting tubules have a low columnar epithelium with cells of somewhat irregular shape, the basal portions of which are provided with short, irregular, intertwining processes, which serve to hold the cells in

Fig. 250.-Section of medulla of human kidney; X about 300: a, a, a, Ascending limb of Henle's loop; b, b, b, blood-vessels; c, c, c, descending limb of Henle's loop.

place. The diameter of the collecting tubules measures from 45 μ to 75 μl.

In the larger collecting tubules the epithelium is more regular and becomes higher as the tube widens. These tubules gradually unite within the Malpighian pyramid and the regions adjacent to the columns of Bertini to form about 20 papillary ducts from 200 μ to 300 μ in diameter. The latter have a high columnar epithelium, and empty into the pelvis of the kidney at the apex of the papilla, forming the foramina papillaria.

Besides the epithelium, the uriniferous tubules possess an ap

parently structureless membrana propria, that of the collecting tubules being very thin. According to Rühle (97), the membrana propria of the uriniferous tubules consists of fine circular and longitudinal fibers which are at no point connected with the cells, and which represent nothing more than a thickened and more regularly distributed layer of the interstitial reticular tissue. The basement membrane of the vascular loops in the glomeruli also appears to have a fibrous structure and presents numerous fine openings.

Between the Malpighian pyramids are found the columns of Bertini, presenting a structure similar to that of the cortex of the kidney, and extending to the hilum of the kidney.

Between the uriniferous tubules and surrounding the bloodvessels of the kidney there is found normally a small amount of connective tissue. Between the convoluted portions of the tubules this is present only in small quantity, a somewhat greater amount

being found in the neighborhood of the Malpighian corpuscles, in the boundary zone between the cortex and medulla and between the larger collecting tubules in the apices of the Malpighian pyramids.

From what has been said concerning the uriniferous tubule it must be evident that its course is a very tortuous one. Beginning with the Malpighian corpuscles, situated in the cortex between the medullary rays, the tubule winds from the cortex to the medulla. and back again into the cortex, where it ends in a collecting tubule, which passes to the medulla to terminate at the apex of a Malpighian pyramid. The different portions of the tubules have the following positions in the kidney: In the cortex between the medullary rays are found the Malpighian corpuscles, the neck, the proximal and distal convoluted portions of the uriniferous tubule, and the

arched collecting tubules. The medullary rays are formed by the cortical portions of the straight collecting tubules and a portion of the ascending limbs of Henle's loops. The medulla is made up mainly of straight collecting tubules of various sizes and of the descending limbs and loops of Henle's loops, the latter being often found in the boundary zone between the cortex and medulla. (See Fig. 250.)

The blood-vessels of the kidney have a characteristic distribution, and are in the closest relationship to the uriniferous tubules.

Fig. 252.-Section through junction of two lobules of kidney, showing their
coalescence; from new-born infant.

The renal artery divides in the neighborhood of the hilum into two branches, a dorsal and a ventral,-which again divide, the resulting trunks giving off lateral branches to the renal pelvis, supplying its mucous membrane and then breaking up into capillaries which extend as far as the "area cribrosa." The venous capillaries of this region empty into veins which accompany the arteries. Besides these, other arteries originate from the principal branches, or from their immediate offshoots, and pass backward to supply the walls of the renal pelvis, the renal capsule, and the ureter. The main

trunks themselves penetrate at the hilum, and divide in the columns of Bertini to form arterial arches (arteriæ arciformes) which extend between the cortical and medullary substances. Numerous vessels, the intralobular arteries, originate from the arteriæ arciformes and penetrate into the cortical pyramids between the medullary rays. Here they give off numerous twigs, each of which ends in the glomerulus of a Malpighian corpuscle. These short lateral twigs are the vasa afferentia. Each glomerulus is formed by the breaking down of its afferent vessel, which, on entering the Malpighian corpuscle, divides into a number of branches, each in turn subdividing into a capillary net. From each of these nets the blood passes into a somewhat larger vessel constituting one of the branches of the efferent vessel which carries the blood away from the glomerulus. Since the afferent and efferent vessels lie in close proximity, the capillary nets connecting them are necessarily bent in the form of loops. The groups of capillaries in a glomerulus are separated from each other by a larger amount of connective tissue than separates the capillaries themselves, so that the glomerulus may be divided into lobules. In shape the glomerulus is spheric, and is covered by a thin layer of connective tissue over which lies the inner membrane of the capsule, the glomerular epithelium. On its exit from the glomerulus the vas efferens separates into a new system of capillaries, which gradually becomes venous in character. Thus, the capillaries which form the glomerulus, together with the vas efferens, are arterial, and may be included in the category of the so-called arterial retia mirabilia. Those capillaries formed by the vas efferens after its exit from the Malpighian corpuscle lie both in the medullary rays and in the cortical pyramids. The meshes of the capillary networks distributed throughout the medullary rays are considerably longer than those of the networks supplying the cortical pyramids and labyrinth, the latter being quadrate in shape. The glomeruli nearest the renal papillæ give off longer vasa efferentia which extend into the papillary region of the Malpighian pyramids (arteriolæ rectæ spuriæ) and form there capillaries which ramify throughout the papillæ with oblong meshes.

Arterial retia mirabilia also occur in the course of the vasa afferentia between the intralobular arteries and the glomeruli, but nearer the latter. Each is formed by the breaking down of the small afferent vessels into from two to four smaller branches, which then reunite to pass on as a single vessel. In structure these retia differ greatly from the glomeruli in that here the resulting twigs are not capillaries and have nothing to do with the secretion of urine (Golubew).

From the vasa afferentia arterial twigs are occasionally given off, which break down into capillaries within the cortical substance. Other arteries originate from the lower portion of the intralobular arteries or from the arciform arteries themselves and enter the medullary substance, where they form capillaries. These

66

Their

vessels constitute the so-called arteriolæ rectæ veræ." capillary system is in direct communication with the capillaries of the vasa afferentia and "vasa recta spuria." The intralobular arteries are not entirely exhausted in supplying the vasa afferentia which pass to the glomeruli. A few extend to the surface of the kidney and penetrate into the renal capsule, where they terminate in capillaries which communicate with those of the recurrent, suprarenal, and phrenic arteries, etc. Smaller branches

Fig. 253.-Diagrammatic scheme of uriniferous tubules and blood-vessels of kidney. Drawn in part from the descriptions of Golubew.

from these latter vessels may penetrate the cortex and form glomeruli of their own in the renal parenchyma (arteriæ capsulares glomeruliferæ). These relations, first described by Golubew, are of importance not only in the establishment of a collateral circulation, but also as a partial functional substitute in case of injury to the renal arteries. The same author also confirms the statements of Hoyer (77) and Geberg, that between the arteries and veins of the kidney, in the cortical substance, in the columns of Bertini, and