KIDNEY. One of a pair of glands found in the body of every vertebrate (and represented by a similar organ in many invertebrates), whose function is to elaborate and eliminate urine, and thereby excrete waste organic products of destroyed tissue and nutritive changes. In the human being the kidneys are placed in the back part of the abdominal cavity, behind the perito neum, imbedded in fat which, together with their blood-vessels, keeps them in position. One is sit uated on each side of the spinal column, extend ing from about the eleventh rib to the neighbor hood of the crest of the ilium. Above each one is the pyramidal suprarenal capsule (q.v.). The kidney is of the shape of a plump Lima bean, with a concave notch at one side, the hilmn. It is of a dark-red color, firm, dense, but somewhat brittle. It is a little over four inches long, about two inches wide, and one inch thick, and varies in weight from four to six ounces, the female kid ney being slightly smaller than the male. The organ is covered by a thin but tough fibrous en velope (tuni•n propria). The hilum leads down int-o a cavity, the sinus, in which lie the renal vessels. nerves, and duct. The duet is continu ous with the ureter (which is the canal which conveys the urine into the bladder), and enlarges to become the pelvis of the kidney. The pelvis is tunnel-shaped, with the edges of the large end at tached to the margin of the sinus, thence turning inward and investing the sheaths of the vessels. The pelvis divides, and then subdivides, the pri mary segmeats of the duct being called ealices.
On longitudinal section, the kidney is seen to be composed of two principal portions: the ex ternal portion, or cortex, and the more central portion, or inedllla. The medulla consists of Iron) eight to eighteen conical segments called the pyramids of Alalpighi, the apices of which (the papilla.) project into the bottom of the sinus and are encircled by the ealices, and the bases of which are directed outward toward the surface and are contiguous to the cortex. Each pyramid is about th•ee-fourths of an inch high. and about two-thirds of an inch across the base, smooth in section, and marked with sirke running from base to apex which mark the course of the urinif crone tubules. The cortex consists of a periph eral layer, and several processes (columns of Bertini) which pass down between the pyramids of the medulla and reach the sinus. The cortex
is somewhat granular, owing to the presence of a itself and returns, after running down into the subjacent Alalpighian pyramid. The narrow loop is called the tube of lienIe. It thus runs into the cortex, where it becomes again wide and con voluted, and finally opens into a straight tube which forms the axis of a pyramid of Ferrein. The straight collecting tubes run into the Mal pighian pyramids, uniting and forming large trunks which terminate in openings in the papil la: of the pyramids, thus emptying their contents into the canoes.
The kidney is very well supplied with blood vessels. The arteries pass from the point at which they enter the organ at the bottom of the sinus, running up between the Ahdpighian pyra mids and subdividing at their bases in cortico number of very small pyramidal groups of tu bules, the pyramids of Ferrein. The tubules that carry the urine begin by a number of spherical capsules in the cortex. From each capsule a narrow tubule passes which becomes wide and convoluted, narrows again as it doubles upon medullary arches. These arches, lying between cortex and medulla, send out arterioles in all direct' s. tte cortical branches supplying dif ferent twigs to the glomeruli, which are inclosed in the capsules of Bowman. and the medullary branches passing inward and forming plexuses around the straight and looped tubes of the Mal. pighian pyramids. The efferent vessels of the glomeruli form a capillary plexus around the uriniferous tubules. and terminate in the renal veins.
The leading valieties of the kidneys are as fol lows: In shape. resulting in the long. globular. or triangular kidney. In size, i•suhing in one very small and one very large kidney. In number, some individuals having only one kid ir rarely three kidneys. the third being in the median line or • accompanying one of the others, in the hunbar region. In position, one or both of the kidneys being dislocated. By fu sion. the two kidneys being joined, and a 'horse shoe kidney' resulting. In mobility, a laxity of the subperitoneal tissue allowing of the 'floating' of a kidney.