The renal arteries occasionally present some anomalies as to their origin, mode of division, or number. In some instances they arise below the usual situation, from the aorta, or even from the common iliac or hypogastric artery. The two last-mentioned origins are usually associated with an unusual po sition of the kidney, either in the iliac fossa or in the cavity of the pelvis. Meckel# has observed the two renal arteries arising by a common trunk from the anterior part of the aorta.
The artery sometimes divides into two or more branches immediately after its origin, in which case one branch usually leaves the others to enter one of the extremities of the kidney. This irregularity forms an approach to another, which consists in an increase in the number of renal arteries, each kidney re ceiving two, three, or four branches having a separate origin from the aorta.
The emulgent or renal vein commences in the substance of the kidney by numerous mi nute branches, which unite into four or five trunks, and these again unite to form a single trunk, either in the fissure of the kidney, or at a short distance from this point. The vein passes almost transversely inwards to the vena cava, the right vein being shorter than the left, on account of the position of the vena cava to the right of the spine; the junction of the right vein with the cava is also somewhat higher than that of the left. The vena cava presents a marked increase of size immediately after receiving the renal veins. Each renal vein is placed in front of the corresponding artery ; the vein on the left side crosses over the aorta. The renal veins receive some small branches from the supra-renal capsule, and from the reticular and adipose tissue sur rounding the kidney, and the left renal vein is usually joined by the spermatic of the same side.
The lynrphatics of the kidney are but little known ; they are said to consist of a super ficial and a deep set, the latter being the most abundant ; they pass from the fissure of the kidney to the lymphatic glands which surround the aorta and the vena cava.
The nerves of the kidney are very numerous, consisting of several small branches from the lower and outer parts of the semilunar ganglion and solar plexus, joined by the descending branches of the small splanchnic nerve. The renal plexus thus constituted accompanies the artery into the fissure of the kidney. This plexus sends numerous filaments to the sper matic plexus, and hence probably the sympa thetic connexion which exists between the testicle and the kidney.
Structure of the Kidney.— In order to exa mine the general structure of the kidney, it is necessary to make a longitudinal section from the convex towards the concave border. On examining the surface of such a section, it will be seen that the substance of the kidney is composed of two portions, differing from each other in their general appearance and arrangement ; an external cortical, and an in ternal nzedullary portion. It will also be seen that the entire organ is invested by a fibrous capsule, which at the hilum becomes conti nuous with the pelvis of the kidney and with the ureter. It appears desirable to examine the general appearance and structure of these several parts before proceeding to the consi deration of the minute anatomy of the kidney.
The cortical substance forms a layer about two lines in thickness, which occupies the surface of the kidney, and sends inwards pro longations, from one to three lines in thick ness, between the conical divisions of the tubular substance. The colour of the cortical substance is much influenced by the quantity of blood which it contains. It is usually of a lightish red colour, but in anwmic subjects it very frequently presents a yellowish-white appearance, this being the colour of the renal tissue when deprived of blood. The cortical substance is of a softish consistence, readily tearing beneath the pressure of the finger, and presenting an irregular granular surface. In an injected specimen there may be seen scat tered through the cortical substance, in every part, except near the surface of the organ, numerous minute red granules; these bodies have been named after the distinguished ana tomist who discovered them, the Malpighian bodies of the kidney. The great mass of the cortical substance is made up of secreting tubules, the existence of which was first clearly demonstrated by Ferrein; hence they have been named the tubes of Ferrein. The course of these tubes is, for the most part, very tortuous; but near the basis of what will presently be described as the pyramids of Malpighi there is an appearance, visible even to the naked eye, of straight lines radiating towards the surface of the kidney; these lines result from bundles of tubes passing upwards from the pyramids of Malpighi and retaining their straight course until they reach the sur face, where they become tortuous, and pass backwards deeply into the cortical substance (fig. 113).