URINARY SYSTEM. The urinary system in the fully developed human being consists of (I) the kidneys, (2) the ureters, (3) the urinary bladder, and (4) the urethra.
The kidneys are two firm, reddish brown organs about 41in. long, placed obliquely behind the other abdom areolar tissue. The degree to which the kidney is fixed in position varies somewhat. In so-called moveable or floating kidney the mo bility may give rise to symptoms. The kidney in the foetus is lobu lated, but becomes smooth in later years of childhood. Each organ has a firm, fibrous capsule, easily stripped off when the kidney is healthy. The inner and ventral margin of each kidney is concave (hi/um) and contains the renal artery and vein and the ureter which is always behind and below the blood vessels. Longitudinally divided from hilum to outer edge, the cut surface is seen to consist of two parts—an outer layer, the cortex, and an inner part, the medulla (fig. 1). The latter consists of a series of eight to sixteen pyramids, whose bases and sides are invested with cortical matter, and whose apices or papillae project into the pelvis of the kidney, where they are severally surrounded by membranous tubes (calices), which by their union make up the ureter. In minute structure the kidney is the most complex gland in the body.
The ureter or duct of the kidney begins at the hilum and descends on the back wall of the abdominal cavity to open into the bladder. It is usually about I zin. in length and as thick as a goose quill. At its termination it passes obliquely through the coats of the bladder, so that when the bladder is distended the lumen of its end is closed. The urinary bladder is a membranous bag lying in the pelvic cavity directly behind and above the dorsal surface of the pubes. In the foetus and infant, however, the bladder lies in the abdomen, not in the pelvis. During life it is seldom distended so as to hold more than about I ooz. When distended it rises and is applied closely against the back of the ventral abdominal wall. The bladder has a strong
wall of unstriped muscle in several layers, which are innervated by branches from the sacral nerves. It has a peculiar epithelial lining of several strata, the superficial cells of which are cubical when the sac is collapsed, but flattened and scale-like when it is distended. At the lower part of the bladder is a triangular space known as the trigone, the angles of which are formed by the openings of the two ureters and the urethra. In this space the mucous membrane is smooth and firmly bound to the subjacent muscle; elsewhere it is thrown into numerous folds when the bladder is empty. The female urethra is I2in. in length and is comparable only with that part of the male urethra which extends from the bladder to the openings of the seminal ducts (fig. 3). The male urethra is described under REPRODUCTIVE SYSTEM.
The excretory organs are developed as a series of small tubes situated in the intermediate cell mass, the ventral part of which projects to form the Wolffian ridge. Three sets of these tubes appear in succession and occupy the whole length of the body from the cervical to the lumbar region. The most anterior—pronephros or head kidney—is represented in man by only two or three small tubules on each side which appear as ingrowths from the neighbouring coelom (fig. 4). It is prob able that these are only mere vestiges. Although the pronephros is rudimentary, the duct which in lower types carries away its excre don is well developed. This is the Wolffian duct, which appears in man before the pronephric tubes are formed, and runs longi tudinally back in each intermediate cell mass to open into the cloaca (fig. 4). In certain parts of its course it is, at an early date, close to the skin on the dorsal side of the intermediate cell mass, and many embryologists hold that it is originally ecto dermal and has sunk into the mesoderm secondarily. Morpholog ically, this view seems likely.