The glandular substance of the kidney is divided into two parts, an external or convex portion, called the cortical substance, or cortex, and an internal or concave portion, the medullary substance, or medulla.
This division can be readily seen by cutting a kidney into two equal parts in the line of its long diameter. An intermediate zone, which separates the cortical from the medullary substance, is called the boundary layer of the kidney. The whole organ is enveloped in a fibrous membrane, the capsule.
The medullary substance contains the pyramids of the kidney, and is therefore also called the pyramidal portion. The apex of each pyramid, the papilla, projects into a special arm of the renal pelvis, viz., a calyx; the base or expanded portion is directed toward the cortical substance, and sends prolongations into the latter.
An examination of the cortical substance shows it to be composed of two distinct varieties of tissue, running parallel to one another toward the free surface. One has a fibrous appearance, and is composed of cylindrical cords. It is a continuation of the pyramids. These pyramidal prolongations (Henle) are also called medullary rays. The other portion, situated between the prolongations, is a granular-look ing material, called the cortical substance proper, or labyrinth of Ludwig.
The latter contains numerous small bodies, which are of a distinctly red color when there is a large amount of blood in the kidney ; they are the Malpighian bodies, or glomeruli.
The boundary layer is characterized by numerous blood vessels, some of which unite to form an arcade, which is parallel to the convex surface of the kidney, and from which branches are given off to the cortical substance proper.
The renal artery, before it enters the hilum of the kidney, divides into branches, which pierce the medulla between the pyramids and ascend toward the cortical substance until they reach the boundary layer. Here they divide obliquely or at right angles to give off smaller branches, which have the direction and arched appearance above referred to these arched vessels then send off the branches already mentioned, which traversing the centres of the cortical substance proper, at right angles to the parent stem extend almost to the capsule of the kidney. On their way they in turn give off smaller twigs, each of which bears a glomerulus upon its extremity. In this way there is an alternate arrangement of pyramidal prolongation and cortical substance proper. Though he pyramidal prolongations almost reach the capsule of the kidney, they never quite touch it, being separated by the interposition of some cortical substance proper.
Specimens for study should be made from a fresh kidney, in which the renal artery has been injected with carmine-gelatine, the whole organ having been subsequently immersed in alcohol of 50 per cent, strength. When in that fluid it is to be divided into four or more parts, allowed to remain therein for twenty-four hours, afterward transferred
The substance of the kidney is composed of secreting and collecting tubules, vessels, and a stroma, which fills the interspaces between the tubules, and is more abundant in the medullary than in the cortical substance. In human adults this connective material is found in small quantity and is a sort of colloid substance. In the lower animals it is more abundant, and assumes the character of real connective tissue. In young infants there is said to be a greater proportionate amount of this tissue than in subsequent life.
The Renal Tubules
The tubules are found both in the cortical and medullary substances ; they are of different diameters and pursue either a straight or tortuous course. Some have a basement membrane (membrana or tunica propria), on which the epithelium rests ; others appear to have none.
The tubules are clothed with epithelium of different varieties. Specimens should be made from a kidney that has lain for twenty four hours in a 5 per cent, solution of chromic acid.
Each tubule takes its origin in an expansion that surrounds the glomerulus, and is called Bowman's or Muller's capsule. Miiller, in 1830, described the capsules, but regarded them as vesicles which had no connection whatever with the uriniferoua tubules.
See Bowman, in Philosoph. Trans act., 1842.
It is round or elliptical in shape, and has a diameter of about 0. 2 mm. where the capsule empties its contents into the tubule, there is a slight constriction known as the neck ; it is very distinct in some of the lower animals. The canal then en larges and begins to pursue a tortuous course in the cortical substance ; it is now called a convoluted tube. It next undergoes sudden diminution in size and passes straight through the medulla until, at a variable point, it bends upon itself, forming a loop ; then, ascending, it increases in calibre, and in the cortical substance becomes convoluted for the second time.
Those canals that are nearest the glomeruli are called convoluted tubules of the first order, the others convoluted tubules of the second order. Between these two are the looped tubules of Henle, just described, each being divided into a descending and ascending branch. The convoluted tubules of the second order terminate by emptying into tubules of greater diameter, called collecting tubules, which descend through the cortical and medullary substances, and, receiving other collecting tubules on the way, finally empty into the pelvis of the kidney.
At the base of each pyramid there are a vast number of collecting tubules, but as they successively empty into larger collecting tubes, the area they occupy is thereby diminished ; at the apex of the papillae, where they ultimately discharge the urine into the pelvis of the kidneys, there are only about twenty in number. This gradual coalescence of the tubes gives to the pyramids a conical shape, but the breadth of the base is also partly due to the presence of the looped tubules which pass down into the pyramids for a varying depth.
The larger collecting tubules may be readily injected with Beale's blue fluid ' or carmine-gelatine, either directly or from the ureters ; it will be found, however, that the injection will seldom extend beyond the looped tubules, owing to the small diameter of the descending branches.
Bowman's capsule is composed of a structureless basement membrane surrounding each glomerulus. Upon the inner sur face of these capsules is a .continuous layer of flat, epithelioid cells, 1 which are continued over the glomerulus itself. 3 Occa sionally an epithelioid cell may be seen between the vessels of the coil composing the glomerulus.
Each capsule is pierced by two vessels, called, respectively, afferent and efferent. The former enters the capsule and forms the glomerulus, while the latter makes its exit close to the en trance of the former. The layer of epithelium above described passes over from the inner surface of the capsule on to the glomerulus about the points of entrance and exit just men tioned. On the opposite side, the capsule becomes continuous with a convoluted tubule. To obtain specimens, the renal artery of a fresh kidney should be injected with blue gelatine and then placed in alcohol. Vertical and transverse sections of the cor tical substance may then be made. They should be stained in carmine and examined in glycerine, or the artery may be injected with absolute alcohol and the sections stained as above.
The epithelium of the tubules
The basement-membranes of the convoluted tubules of the first order are in direct continuation with the basement-membranes of the capsules. Their diameter averages 0.04 mm. The epithelium of these canals is peculiar, and was first correctly described by Heidenhain.
According to this writer, the greater part of the cell-protoplasm assumes the form of small, cylindrical bodies, the so-called rods of Heiderihain, giving the epithelium a striated appearance.
