KIDNEYS, THE, are two glands having for their office the secretion of the urine. That this office or function is of extreme importance is sufficiently shown by the facts that if, in consequence of disease, it is altogether suspended in the human subject, even for a day or two, death not unfrequently occurs, and that urinary glands corresponding in function to our kidneys are found, not only in all vertebrate animals, but in almost all mollusks, in the arachnidans, in insects, and in myriapods.
' The human kidneys are situated in the region of the loins, on each side of the spine, and are imbedded in a layer of fatty tissue. Their form is too well known to require any description. The average length of each kidney is more than four inches, and its usual weight is from four to six ounces. The substance of the kidneys is dense, extremely fragile, and of a deep red color. On making a vertical section of the kidney, it is seen to consist of two different substances, which are named, from their position, the external or cortical, and the internal or medullary substance.
The cortical substance forms by far the greater part of the gland, and sends numerous prolongations inwards between the pyramids of the medullary substance. It is soft, granular, and contains numerous minute red globular bodies diffused throughout it, which are called, from their discoverer, the Malpighian bodies, and which will be pres ently noticed more fully. Its substance is made up of the urinifel Wes tubes (which are 'described in the notice of the medullary portion), capillaries, lymphatics, and nerves, held together by an intermediate parenchymatous substance.
The medullary substance consists of pale-reddish conical masses, called the "pyra mids of They are usually about twelve in number, but vary from eight to eighteen, and their apices (the papilke) point towards the hollow space (termed the sinus or pelvis) which occupies the interior of the gland. The medullary structure is firmer than the cortical, and instead of being granular, presents a striated appearance, from its being composed of minute diverging tubes (the uriniferous tubes, or tubes of Bellini), which run in straight lines through this portion of the kidneys, after having run in a highly convoluted course through the cortical portion.
The cavity occupying the interior of the kidneys (the sinus or pelvis) is lined by mucous membrane, which, through the medium of the ureter, is continuous with that of the bladder, and which extends into the tissue of the kidneys, to line the uriniferous tubes. The mucous membrane forms a cup-like cavity around the termination of each pyramid, and the cavity, termed the calyx, receives the urine from the open terminations, of the tubes and conveys it towards the pelvis, from whence it passes down the ureter into the bladder.
Each kidney is supplied with blood by a renal artery, a large trunk which comes off at right angles to the aorta. The blood, after the separation of the various matters which constitute the urine (q.v.), is returned into the venous system by the renal or emulgent vein, which opens into the inferior vena cava.
The nerves are derived from the renal plexus, which is formed by filaments of the • solar plexus and the lesser splanchnic nerve. They belong entirely to the ganglionic or sympathetic system.
The Malpighian bodies are found in all vertebrate animals. In mammals, which are the only animals in which there is a division into a cortical and a medullary portion, these bodies are only found in the former. In an injected specimen, they appear to the naked eye as mere colored spots. They are for the most part of a spherical, oval, or flask-like form. Their diameter in man may range from to of an inch, the mean being I th. A small artery, termed the afferent vessel, may be traced into each Malpighian body, while a minute venous radicle, the efferent 'vessel, emerges from it close to the point at which the artery had entered. The Malpighian body itself consists of a rounded bunch or tuft of capillaries, derived from the afferent, and terminating in the efferent vessel, and inclosed in a clear and transparent capsule, lined at its lower part. with epithelium, continuous with that of the uriniferous tube which springs from each capsule.