CHAPTER IX.

THE URINARY TRACT.

THE kidney is covered by a capsule of fibrous tissue, and upon longitudinal section is seen to be composed of two distinct portions.

The outer or cortical portion is granular in appearance and comprises about one-third of the organ. The inner or medullary portion constitutes the remaining two-thirds of the organ, and is seen to be composed of a number (from ten to fifteen) of pyramidal masses (the pyramids of Malpighi), with their bases at the inner edge of the cortex, and their apices pointing into the sinus, constituting the papilla.

They are finely striated radially, owing to the alternate arrangement of the uriniferous tubules and bloodvessels which compose them. Usually small extensions of cortical substance, called the cortical columns, or columns of Bertini, project down between the pyramids of Malpighi, separating them one from another.

At intervals along the junction of the cortex with the Malpighian pyramids projections from the latter extend into the cortical substance. These are called the medullary rays, or pyramids of Ferrein, and consist of continuations of the striations composing the Malpighian pyramids. That portion of the cortex situated between the pyramids of Ferrein has received the name of the labyrinth, on account of the tortuous course which its component tubules take (being composed

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chiefly of the first and second convoluted tubules, a description of which follows).

At the side of the kidney opposite the cortex there is a marked depression, called the hilum, at which point it flares out into a funnel-shaped tube (the pelvis) and expands into a central cavity, called the sinus.

Fig. 35.-Kidney, longitudinal section, exhibiting general relations of microscopic details (after Piersol): 4, Renal artery; U, ureter; C, one of the calyces, into which a papilla projects; 1, cortex containing labyrinth (7) and medullary rays (m); 2, medulla; M, Malpighian pyramids, some obliquely cut at 3, 3; b, boundary layer; B, columns of Bertini; 4, masses of adipose tissue; 5, 5, branches of renal artery (after Henle).

The sinus has several branches or divisions, into which project the papillæ (the infundibulæ, or calyces).

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sions (the glomeruli or Malpighian bodies) in the labyrinth of the cortex. At the end of the glomerulus a slight constriction occurs, called the neck. Next the tubule increases in size slightly and pursues a wavy course, entering the area of the medullary ray (the first or pyramidal convoluted tubule). Here it turns downward in a spiral course (the spiral tubule), until it reaches the lower border of the medullary ray, and enters one of the Malpighian pyramids. At this point the diameter becomes much smaller and the tubule descends in nearly a straight line as the descending limb of Henle's loop. When well down in the Malpighian pyramid, the tube expands somewhat, and then turns abruptly upward (the loop of Henle) and ascends in a straight line, nearly parallel to the descending tubule, until it is well up into the medullary ray (the ascending limb of the loop of Henle), when it expands still more, reenters the labyrinth, and follows a very irregular course for a short distance (the irregular or zigzag tubule).

The convolutions of the zigzag tubules become more regularly curved (the second or distal convoluted tube), and the tubule again enters the medullary ray. Here it forms a slight arch (the arched collecting or junctional tubule) and joins a large collecting tubule or the tubule of Bellini, which runs straight down toward the pelvis of the kidney, joining other similar collecting tubules to form principal tubules, several of which unite to form a papillary duct, which terminates at the end of one of the papillæ. A single papilla will contain about two hundred papillary ducts.

A Malpighian body is composed of a small afferent arteriole, which immediately branches into a spheric tuft of capillaries, which reunite to form one or, rarely, more efferent arterioles, which emerge beside the afferent vessel.

Fig. 36.-Diagram of kidney, showing the course of uriniferous tubules and of the blood-vessels (after Piersol); for convenience, the medulla is represented as greatly shortened. The various divisions of the tubule-Bowman's capsule, neck, proximal convoluted, spiral, descending and ascending limb of Henle's loop, irregular, distal convoluted, arched collecting, straight collecting, and excretory duct-are indicated by their initial letters: a, e, c, respectively the afferent, efferent, and capillary blood-vessels; s, stellate vein; vr, vasæ rectæ.

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Closely surrounding this tuft and dipping down between the capillaries is a thin layer of fibrous tissue called the capsule of Bowman, which serves as a basement membrane for a layer of flat cells, which are continuous with the cells lining the uriniferous tubules. At the upper part of the Malpighian body, where the arteriole enters, the capsule of Bowman, with its layer

Fig. 37.-Kidney in section: a, Glomerulus; b, cross-section of convoluted tubule; c, cross-section of Henle's tubule; d, capsule of Bowman e, fibrous tissue between tubules.

of cells, bends sharply around upon itself, and descends, inclosing a very narrow space, which is the beginning of the lumen of the tubule. The tuft, therefore, lies entirely on the outside of the tubule, as though it had been thrust into a hollow sphere, causing one side to be invaginated into the other.