The kidney is covered by a fibrous capsule, which is slightly attached at its inner surface to the proper substance of the organ by means of very fine bundles of areolar tissue and minute blood vessels.

From the healthy kidney, therefore, it may be easily torn off without much injury to the subjacent cortical portion of the organ. At the hilus of the kidney, it becomes continuous with the external coat of the upper and dilated part of the ureter.

 On dividing the kidney into two equal parts by a section carried through its long convex border it is seen to be composed of two portions called respectively cortical and medullary; the latter is composed of about a dozen conical bundles of urinary tubules, each bundle forming what is called a pyramid. The upper part of the ureter or duct of the organ is dilated into the pelvis; and this, again, after separating into two or three principal divisions, is finally subdivided into still smaller portions, varying in number from about 8 to 12, called calyces. Each of these little calyces or cups receives the pointed extremity or papilla of a pyramid. The number of pyramids varies in different animals; in some there is only one.

The kidney is a compound tubular gland, and both its cortical and medullary portions are composed of tubes, the tubuli uriniferi, which, by one extremity, in the cortical portion, commence around tufts of capillary blood-vessels, called Malpighian bodies, and, by the other, open through the papillae into the pelvis of the kidney, and thus discharge the urine which flows through them. They are bound together by connective tissue.

In the pyramids the tubes are straight uniting to form larger tubes as they descend through these from the cortical portion; while in the latter region they spread out more irregularly, and become much convoluted.

But in the boundary zone between cortex and medulla, small collections of straight tubes called medullary rays project into the cortical region.

Tubuli Uriniferi

The tubuli uriniferi are composed of a basement membrane, lined internally by epithelium. They vary considerably in size in different parts of their course, but are, on an average, about of an inch (^T mm.) in diameter, and are found to be made up of several distinct portions which differ from one another very markedly, both in situation and structure.

Each begins in the cortex as a dilatation called the Capsule of .Bowman; this encloses a tuft or glomerulus of capillaries called a Malpighian corpuscle. The tubule leaves the capsule by a neck, and then becomes convoluted (first convoluted tubule), but soon after becomes nearly straight or slightly spiral (spiral tubule); then rapidly narrowing it passes down into the medulla as the descending tubule of Henle; this turns round, forming a loop (loop of Henle), and passes up to the cortex again as the ascending tubule of Henle. It then becomes larger and irregularly zigzag (zigzag tubule) and again convoluted (second convoluted tubule). Eventually it narrows into a junctional tubule, which joins a straight or collecting tubule. This passes straight through the medulla, where it joins with others to form one of the ducts of Bellini that open at the apex of the pyramid.

The character of the epithelium that lines these several parts of the tubules is as follows :

In the capsule, the epithelium is flattened and reflected over the gjlomerulus.

In the neck the epithelium is still flattened, but in some animals, such as frogs, where the neck is longer, the epithelium is ciliated.

In the first convoluted and spiral tubules, it is thick, and the cells show a fibrillated structure, except around the nucleus, where the protoplasm is granular. The cells interlock laterally and are difficult to isolate. In some animals they are described as ciliated. In the narrow descending tubule of Henle and in the loop itself, the cells are clear and flattened and leave a considerable lumen; in the ascending limb they again become striated and nearly fill the tubule. In the zigzag and second convoluted tubules the fibrillations become even more marked. The junctional tubule has a large lumen, and is lined by clear flattened cells; the collecting tubules and ducts of Bellini are lined by clear cubical or columnar cells.

Blood-vessels of Kidney

The renal artery enters the kidney at the hilus, and divides into branches that pass towards the cortex, then turn over and form incomplete arches in the region between cortex and medulla. From these arches vessels pass to the surface which are called the interlobular arteries; they give off vessels at right angles, which are the afferent vessels of the glomeruli; a glomerulus is made up of capillaries as previously stated. From each, a smaller vessel (the efferent vessel of the glomerulus) passes out, and like a portal vessel on a small scale, breaks up once more into capillaries which ramify between the convoluted tubules. These unite to form veins (interlobular veins) which accompany the interlobular arteries; they pass to venous arches, parallel to, but more complete than, the corresponding arterial arches; they ultimately unite to form the renal vein that leaves the hilus. These veins receive also others which have a stellate arrangement near the capsule (venae stellulae).

The medulla is supplied by pencils of fine straight arterioles which arise from the arterial arches. They are called arteriae rectae. The efferent vessels of the glomeruli nearest the medulla may also break up into similar vessels which are called false arteriae rectae. The veins (venae rectae) take a similar course and empty themselves into the venous arches. In the boundary zone groups of vasa recta alternate with groups of tubules, and give a striated appearance to this portion of the medulla.

The ureters

The duct of each kidney, or ureter, is a tube about the size of a goose quill, and from ten to sixteen inches in length, which, continuous above with the pelvis, ends below by perforating obliquely the walls of the bladder, and opening on its internal surface.

It is constructed of three coats: an outer fibrous coat; a middle muscular coat, of which the fibres are unstriped, and arranged in three layers the fibres of the central layer being circular, and those of the other two longitudinal in direction; the outermost longitudinal layer is, however, present only in the lower part of the ureter; and a mucous membrane continuous with that of the pelvis above, and of the urinary bladder below. It is composed of areolar tissue lined by transitional epithelium.

The Urinary Bladder, which forms a receptacle for the temporary lodgment of the urine, is pyriform ; its widest part, which is situate above and behind, is termed the fundus; and the narrow constricted portion in front and below, by which it becomes continuous with the urethra, is called its cervix or neck.

It is constructed of four coats, serous, muscular, areolar or submucous, and mucous.

The serous coat, which covers only the posterior and upper part of the bladder, has the same structure as the peritoneum, with which it is continuous. The fibres of the muscular coat, which are unstriped, are arranged in three layers, of which the external and internal have a general longitudinal, and the middle layer a circular direction.

The latter are especially developed around the cervix of the organ and form the sphincter vesicae. The areolar or submucous coat is constructed of connective-tissue with a large admixture of elastic fibres, The mucous membrane is like that of the ureters. It is provided with mucous glands, which are most numerous near the neck of the bladder.

The bladder is well provided with blood- and lymph-vessels, and with nerves. The latter consist of branches from the sacral and hypogastric plexuses. Ganglion cells are found, here and there, on the course of the nerve-fibres.

The Urethra

This occupies the center of the corpus spongiosum in the male. As it passes through the prostate it is lined by transitional, but elsewhere by columnar epithelium, except near the orifice, where the epithelium is stratified like the epidermis with which it becomes continuous. The female urethra has stratified epithelium throughout. The epithelium rests on a vascular corium, and this is covered by submucous tissue containing an inner longitudinal and an outer circular muscular layer. Outside this a plexus of veins passes insensibly into the surrounding erectile tissue.

Into the urethra open a number of oblique recesses or lacunae, a number of small mucous glands (glands of Littre), two compound racemose glands (Cowper's glands), the glands of the prostate, and the vas deferens. The prostate, which surrounds the commencement of the male urethra, is a muscular and glandular mass. Its glands are tubular and lined by columnar epithelium.

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