Kidney, a special organ in vertebrated animals, whose office is to separate from the blood certain effete substances, to be thrown out of the system in the urine; it has no direct connection with any of the nutritive operations concerned in digestion. Taking these organs in man as typical, the kidneys are situated in the lumbar region, one on each side of the spine, on a level with the last two dorsal and the first two lumbar vertebrae; they are of a brownish red color, bean-shaped, flattened from before backward, and grooved on the interior border for the great vessels; the anterior surface is in relation on the right with the duodenum and the ascending colon, and on the left with the descending colon, the posterior surface is imbedded in fat, resting against the muscles; the upper extremity is embraced by the supra-renal capsules; the lower extremity, which is somewhat smaller than the upper, projects a little further downward upon the right side than the left. The kidneys are well supplied with blood, in accordance with the importance of their function; the renal arteries come directly from the aorta, and the large veins terminate in the vena cava; the nerves a. Cortical substance, b. Medullary substance, c. Conical bundles of the medullary substance, d.

Membranous ducts, into which the conical bundles are received, e. Pelvis of the kidney. f.. Ureter, g. Renal artery, h, Renal vein. come from the renal plexus of the sympathetic system. They are covered by a thin, firm, transparent cellular envelope; internally they are composed of two substances, an exterior or cortical and an interior or medullary. From the researches of Bowman, Gerlach, Kolliker, and others, it is ascertained that the cortical substance, the seat of the greater part of the secretory process, is made up of a great number of uriniferous tubes, much convoluted and inosculating with each other, and lined with epithelial cells of a spheroidal and projecting form; scattered through the plexus formed by these tubes and the blood vessels are dark points which have been called corpora Mal-pighiana from their discoverer; these last are convoluted masses of minute blood vessels included in flask-like dilatations of the uriniferous tubes, forming a close relation between the circulating and the secreting systems.

Human Kidney, in Vertical Section.

The medullary substance is composed principally of tubes passing nearly straight inward to the central receptacle of the secretion. Both these substances are imbedded in interlacing a. Small arteries of the cortical portion, b. Corpora Mal-pighiana. d. Capillary blood vessels of the cortical portion, e, n, p. External surface of the kidney, g, h, i. Blood vessels of the medullary portion. I. Straight uriniferous tubes of the medullary portion, becoming convoluted in the cortical portion. fibres, most abundant in the medullary. In mammals the kidneys are supplied with blood directly from the arterial system, but the renal artery divides very soon after entering the organs into minute twigs which pierce the capsule of the Malpighian tufts; from the convolutions of these tufts arise the efferent vessels which surround the uriniferous tubes, and from which the renal veins are formed; thus the urinary secretion is produced from blood which has passed through the Malpighian capillaries, the efferent trunks from which have been compared to a portal system within the kidney.

The uriniferous tubes end in from 12 to 18 conical bundles, pointing toward the interior, and there embraced by 6 or 12 membranous ducts received into the central reservoir or pelvis of the kidney, from which arises the ureter, the membranous tube which conducts the renal secretion to the bladder. Without entering upon physiological questions which will be more properly treated under Urine, it will be sufficient to state that the kidneys serve to regulate the quantity of water in the system, a large amount of which may be got rid of through their agency. As the skin and lungs, the other channels through which superfluous water is removed from the blood, are liable to be greatly affected by external circumstances, the kidneys perform a very important office in relation to that fluid. Hence the quantity of the renal secretion will depend on the amount of fluid passed off by the skin, being greatest when the cutaneous secretion is least, and vice versa; the amount of solid ingredients being dependent on the amount of waste and the excess of nitrogen in the system.

The kidneys serve to free the blood from highly nitrogenized compounds formed from the decomposition of the albuminous and gelatinous tissues and from some portions of the food; they also remove certain excrementitious compounds, of which carbon is a principal ingredient, abnormally increased when the liver and the lungs do not act freely; by them the superfluous water and various saline matters in excess, and foreign substances introduced into the blood as medicines or otherwise, which would be injurious if retained, are carried off. The kidneys are subject to many painful and dangerous diseases, which can only be alluded to here; among these are vascular congestion, inflammation, fatty and waxy degeneration, and diseased states produced by retention of urine, by calculi, external violence, and extension from other organs. Bright's disease is one of their most common and fatal affections, the so-called granular degeneration, consisting in the distention of the tubules, the surrounding tissue, and the Malpighian capsules, with exudation matter, and the subsequent atrophy of portions or even the whole of the cortical substance. Invertebrates have special organs for the secretion of urine, opening into the intestines or into the branchial cavity.

In fishes

Vertical Section through a portion of the Medullary and Cortical Portions of the Rabbit's Kidney.

Malpighian Tuft from near the base of one of the Medullary Cones. a. Arterial branch, of. Afferent vessel, m. Malpighian tuft. ef. Efferent vessel; b, its branches, entering the medullary cone. (Magnified 70 diameters.) the kidneys are very long, extending the whole length of the spine, even to the head, formed of a mass of simple globules, the ureter opening into the cloaca or a urinary bladder; in reptiles they are generally situated within the pelvis, but in serpents they come further forward and are made up of numerous lobes of a compressed reniform shape. In birds they are elongated, commencing immediately below the lungs, extending on each side of the spine to the rectum, and variously divided into lobes. In mammals they resemble those of man, except that in cetaceans and some other lower families they are more or less subdivided into lobes, as in the human foetus; in mammals only is there the marked distinction into cortical and tubular substance. In the foetus at an early period, while the kidneys are very small and imperfect, their office is performed by the " Wolffian bodies," two organs analogous to them in structure, which afterward become atrophied and disappear.

The two kidneys, which first make their appearance just behind the Wolffian bodies, grow rapidly as the latter diminish in size, and in the human subject have fully taken their place by the end of the second month of foetal life. In fishes, on the other hand, the Wolffian bodies remain as permanent organs, no true kidneys being developed.