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The Structure of the Kidneys

kidney, tubule, tubules, tubes, tube, called, pelvis and pyramids


The kidneys are two in number, and are situated in the cavity of the belly, one on each side of the back-bone, between the eleventh rib and the crest of the haunch bone. The liver is above the right kidney, the spleen (p. 201) above the left ; while both lie close against the back wall of the belly, so that the intestinal canal is in front of them. The human kidney is about 4 inches long, 2 inches broad, and 1 inch thick, and weighs usually about 5i ounces.

The shape of the human kidney is the same as that of a sheep, or rabbit, and is well known.

The connections of the kidney are shown in Fig. 157, which represents the outline of the belly, opened, the intestinal canal being re moved. A kidney appears on each side of the back-bone, blood-vessels being connected with each, and from each a tube—the ureter— passes downwards to the bladder, situated in the cavity of the pelvis.

Reference should be made to Plates XXII., XXIII., where the location of the kidneys is shown as if projected on to the front and back wall of the body; and where also is shown the direction a coloured chalk line would take which represented, on the skin, the course of the ureters to the bladder.

If a kidney be cut open in the direction of its length an appearance exhibited in Fig. 158 is seen. The ureter (u) where it joins the kid ney expands into a wide cavity (e), which is called the pelvis of the kidney. Into the pelvis conical processes of the fleshy substance of the kidney project.. The processes are called pyramids, or the pyramids of Malpighi, after the anatomist who described them, and in the human kidney there are about twelve of them. The point of each pyramid is invested by a part in continuation of the pelvis, which surrounds it like a cup or calyx.

The tubes of the kidney.

The distinction between the part of the kidney containing the twisted tubes and that contain ing the straight tubes is easily made out with the naked eye, the former part appearing granu lar, and being called the cortex or rind, while the central parts appear streaked, and are called the medulla, or marrow. The distinction is —The fleshy substance, of which the main mass of the kidney consists, and which towards the centre forms the conical projections referred to as pyramids, is made up of very fine tubes or tubules, the tubuli uriniferi, or urine-carrying tubules. Towards the surface of the kidney the tubules run a very irregular course, but towards the centre they run a straight course.

represented in Fig. 158, where it is evident that the medullary portion is formed of the pyramids whose bases rest on the cortex.

It is in the tubules that the urine is formed.

The tubules all ultimately open by a group of mouths on the surface of the pyramids, so that the urine formed in them finds its way into the pelvis of the kidney, and thence down the ureter into the bladder.

The tubuli uriniferi, of which the bulk of the substance of the tissue is made up, are very fine tubes, about the of an inch in dia meter. They run a very irregular course, under going various changes in different stages. They begin in the cortical or outer portion of the kidney in a blind extremity which is widened into a pouch or capsule. Into the capsule, as will be noted hereafter, a bundle of capillary blood-vessels projects. From this expanded extremity the tubule passes off by a narrow neck, and winds a very irregular course in the cortex, twisting and turning upon itself for some distance. This part of the tubule is called the convoluted tubule. Then, after a short portion in which the tube is more spiral than twisted, it suddenly contracts to a very narrow diameter and courses straight on towards the central or medullary part of the kidney, into which it enters. But shortly after entering the medullary region it turns upon itself and pro ceeds, still in a more or less straight direction, and as a very narrow tube, back into the cor tex. This narrower portion that doubles on itself is called the looped tubule of Heide, after the observer who first described it. Ul timately the tubule mingles again with the convoluted tubules, and again becomes itself wider and wavy or spiral. This second spiral part of the tube leads into a wider straight tube which passes down through the cortex to the medulla. As it proceeds through the medulla it joins, now and again, similar tubes at acute angles, or is joined by them, becoming thereby gradually wider until it opens on the surface of a pyramid. Fig. 159 gives a view of the passage of a tubule from its expanded extremity in the cortex to its mouth on the point of a pyramid.

In the different parts of their course different diameters of the tubule have been noticed, and there are other corresponding differences. The tubules are formed of a delicate membrane whose inner surface is lined with cells. In some parts of the tube the cells are large and cloudy, presenting the appearance of cells engaged in the active work of secretion. In other parts they are small and insignificant, evidently not for secreting purposes, but simply to act as a lining to the tube. Fig. 160 shows the appearance of a cross section of a part of a convoluted tubule and of a part of such a tubule opened up, the cells being large and of the actively secreting kind.