at the bases of the Malpighian pyramids, etc., direct anastomoses exist by means of precapillary twigs.

From the capillaries the venous blood is gathered into small veins which pass out from the region of the medullary rays and cortical pyramids and unite to form the "intralobular veins." These have an arrangement similar to that of the corresponding arteries. The venous blood of the labyrinthian capillaries also flows into the intralobular veins, and as a result a peculiar arrangement of these vessels is seen at the surface of the kidney where the capillaries pass radially toward the terminal branches of the intralobular veins and form the stellate figures known as the vena stellata. This system is also connected with those venous capillaries of the capsule which do not empty into the veins accompanying the arteries of the capsule. The capillary system of the Malpighian pyramids unites to form veins, the "venulæ rectæ," which empty into the venous arches (venæ arciformes) which lie parallel with and adjacent to the corresponding arteries. The larger veins are found side by side with the arteries and pass out at the hilum of the organ.

A

B

Fig. 254.-A, Direct anastomosis between an artery and vein in a column of Bertin of child; B, bipolar rete mirabile inserted in the course of an arterial twig. Dog's kidney (after Golubew).

The lymph-vessels of the kidneys. need to be investigated further. Lymph clefts have been observed in the cortex between the convoluted tubules; these have been traced into larger vessels found in the capsule.

The kidneys receive their innervation through nonmedullated and medullated nerve-fibers. The former accompany the arteries and may be traced along these to the Malpighian corpuscles. From the plexuses surrounding the vessels small branches are given off, which end on the muscle-cells of the media. According to Berkley, small nerve-fibrils may be traced to the uriniferous tubules, which pierce the membrana propria and end on the epithelial cells. Dogiel has shown that medullated (sensory) nerve-fibers terminate in the adventitia of the arteries of the capsule.

The most important investigations into the secretory processes of the uriniferous tubules are those of R. Heidenhain (83) who used indigo-carmin in his researches. If a saturated aqueous solution of indigo-carmin be injected into the blood-vessels of a rabbit, the elimination of the substance will be found to take place through the kidneys as well as by means of the other excretions. Microscopic examination of such a kidney reveals the fact that the proximal convoluted tubules and ascending limbs of the loops of

Henle are alone concerned in the elimination of the substance, while apparently water alone is filtered through the remaining segments of the uriniferous tubules. Among others, Disse has recently taken up the subject of cellular secretion in the uriniferous tubules. According to him, we may distinguish in the convoluted tubules (1) those with a wide lumen, having low cells apparently with no cell limits and no distinct basilar zone, but with peculiar structures which may be likened to cuticulæ, so called, or a striated border (Tornier) (Fig. 247); (2) tubules with a narrow lumen and wedge-shaped epithelial cells, with indistinct cell limits and diffusely granular protoplasm; (3) tubules with an extremely narrow lumen and high epithelial cells with differentiated protoplasm, the basal portion of which is dark and striated, the upper clear and containing the nucleus. These results are not, however, confirmed by the painstaking researches of Sauer. This author finds that the secretory portions of the uriniferous tubules (convoluted portions of the tubules and part of the loops of Henle) always have the same unchanged epithelium, but that, during secretion, the lumina of the tubules are subject to great variation; in tubules with scarcely recognizable lumina the epithelial elements are high and narrow; in those with wide lumina, low and broad. In the former the striation of Heidenhain is naturally fine; in the latter, somewhat coarser. The peculiar terminations of Tornier are found by Sauer during all phases of secretion. According to this view, then, neither the striation of Heidenhain nor the terminations of Tornier are temporary appearances due to a particular phase of secretion, but represent permanent structural peculiarities of the cells in certain. definite portions of the uriniferous tubules. The volumetric changes in the uriniferous tubules also probably influence the form and number of the indentations in the epithelial cells described on page 291.

The permanent kidney is developed as early as the fifth week of embryonic life. The renal anlagen, from which the epithelium of the ureter, renal pelvis, and uriniferous tubules is formed, originate from the median portion of the posterior wall of the Wolffian duct. These buds grow with their blind ends extending anteriorly, and are soon surrounded by cellular areas, the blastema of the kidneys. After the renal bud has become differentiated into a narrow tube (the ureter) and a wider central cavity (the renal pelvis) hollow epithelial buds are developed from the latter. These extend radially toward the surface of the renal anlagen, where they undergo a T-shaped division. These latter are the first traces of the papillary ducts and collecting tubules. The cup-shaped capsules are formed by the invagination of the ends of the tubules by the glomeruli which originate separately and in this way become connected with the uriniferous tubules. The remaining portions of the adult uriniferous tubules are gradually formed from the tubes connecting the glomeruli with the collecting tubules.

300

2. THE PELVIS OF THE KIDNEY, URETER, AND BLADDER.

The renal pelvis, ureter, and urinary bladder are lined by stratified transitional epithelium. Its basal cells are nearly cubical; these support from two to five rows of cells of varying shape. They may be spindle-shaped, irregularly polygonal, conical, or sharply angular, and provided with processes. probably due to mutual pressure.

Their variation in form is The superficial cells are large

Superficial epithelial cells.

Mucosa.

Epithelium.

Mucosa.

Inner longitudinal muscular layer.

Middle circular

muscular

layer.

Outer muscular
layer.

Fig. 255.-Section of lower part of human ureter; X 140.

and cylindric, a condition characteristic of the ureter and bladder. Their free ends and lateral surfaces are smooth, but their bases present indentations and projections due to the irregular outlines of the underlying cells. The superficial cells often possess two or more nuclei.

The mucosa often contains diffuse lymphoid tissue, which is more highly developed in the region of the renal pelvis. A few

mucous glands are also met with in the pelvis and in the upper portion of the ureter. The ureter possesses two layers of nonstriated muscle-fibers the inner longitudinal, the outer circular. From the middle of the ureter downward a third external muscular layer is found with nearly longitudinal fibers.

The urinary bladder has no glands, and its musculature apparently consists of a feltwork of nonstriated muscle bundles, a condition particularly well seen in sections of the dilated organ. But even here three indistinct muscle layers may be distinguished, the outer and inner layers being longitudinal and the middle circular. remarkable peculiarity of these structures is the extreme elasticity of their epithelium, the cells flattening or retaining their natural shape according to the amount of fluid in the cavities which they line (compare London, Kann).

The nerve supply of the bladder has been studied by Retzius, Huber, and Grünstein in the frog and a number of the smaller mammalia. Numerous sympathetic ganglia are observed, situated outside of the muscular coat, at the base and sides of the bladder. The neuraxes of the sympathetic neurones of these ganglia are grouped into smaller or larger bundles which interlace and form plexuses surrounding the bundles of nonstriated muscle-cells. From these plexuses nerve-fibers are given off, which penetrate the muscle bundles and end on the muscle-cells. The cell-bodies of the sympathetic neurones are surrounded by the telodendria of small medullated fibers, which terminate in the ganglia. Passing through the ganglia large medullated fibers (sensory nerves) may be observed which pass through the muscular coat, branch repeatedly in the mucosa, and lose their medullary sheaths on approaching the epithelium in which they end in numerous telodendria, the small branches of which terminate between the epithelial cells.

The ureters are surrounded by a nerve plexus containing nonmedullated and medullated nerve-fibers. The former end on cells of the muscular layers; the latter pass through the muscular layer, and on reaching the mucosa branch a number of times before losing their medullary sheaths. The nonmedullated terminal branches form telodendria, the terminal fibers of which have been traced between the cells of the lining epithelium (Huber).

B. THE SUPRARENAL GLANDS.

The suprarenal gland is surrounded by a fibrous-tissue capsule containing nonstriated muscle-cells, blood- and lymph-vessels, nerves, and sympathetic ganglia. The glandular structure is divided into a cortical and a medullary portion. In the former are distinguished three layers, according to the arrangement, shape, and structure of its cells-an outer glomerular zone, a middle broad fascicular zone, and an inner reticular zone. According to Flint, who

worked in Mall's laboratory, and whose account will here be followed, the framework of the gland is made up of reticulum. In the glomerular zone this reticulum is arranged in the form of septa, derived from the capsule, which divide this zone into more or less regular spaces of oval or oblong shape. In the fascicular zone the reticulum is arranged in processes and fibrils running at right angles to the capsule. In the reticular zone the fibrils form a dense network, while in the medulla the reticular fibrils are arranged in processes and septa which outline numerous spaces.

The gland-cells of the glomerular zone are arranged in coiled columns of cells found in the compartments formed by the septa of reticulum above mentioned. The cells composing these columns are irregularly columnar, with granular protoplasm and deeply staining nuclei. In the fascicular zone the cells are arranged in regular columns, consisting usually of two rows of cells, and situated between the reticular processes, which run at right angles to the cap