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Originally appearing in Volume V27, Page 799 of the 1911 Encyclopedia Britannica.
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URINARY SYSTEM. The urinary system in the fully developed human being consists of (1) the kidneys, (2) the ureters, (3) the urinary bladder, and (4) the urethra. As the greater part of the male urethra is a generative as well as a urinary canal, its description will be found in the article on the REPRODUCTIVE SYSTEM. The kidneys are two bean-shaped granular masses, firm in consistence and reddish brown in colour, about 41 in. long, and placed obliquely behind the other abdominal viscera—one on each Kidneys. side of the last thoracic and three upper lumbar vertebrae. Each is imperfectly covered on its ventral surface by peritoneum and is moulded to some extent by the viscera which press on it. Around them there is usually a considerable amount of fat and areolar tissue, by which, as well as by the peritoneum and by the presence of the surrounding viscera, the kidneys are retained in their place. In rare cases the kidney may slip from its usual place in the loins to a lower position (movable kidney), and may even be movable 2 The lots may have been small pebbles, or small tablets of wood or bone. in the abdominal cavity (floating kidney)—a condition often productive of serious consequences. The kidney in the foetus is lobulated, but the intervals between the lobes become smoothed out in later years of childhood. Each gland is invested Labyrinth Medullary ray From A. F. Dixon, Cunningham's Text-Book of Anatomy. forming a Kidney Lobe. In the middle part of the figure the course of one of the kidney tubules is indicated, and in the lateral parts the disposition of the larger arteries. A, cortex; B, intermediate zone; C, papillary portion. The diagram at the right-hand side of the lower part of the figure illustrates the connexions of the structures composing a Malpighian corpuscle. by a firm, closely adherent, fibrous capsule, under which is an imperfect lamina of unstriped muscle. The inner and ventral margin of each kidney is concave, and into this hilum or concavity the renal artery from the aorta passes. Here also the renal vein escapes and joins the vena cava inferior. The ureter or metanephric duct, always behind and below the blood vessels, emerges here and passes downward to the bladder. When the kidney is 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. I). 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 hilum, where they are severally surrounded by membranous tubes (calices), which by their union make up the ureter. The part of the ureter situated in the hilum is dilated, and is named the pelvis of the kidney. In minute structure the kidney is the most complex gland in the body. Each of the papillae consists of a large number of straight tubes—collecting tubules—which open by pores on its surface. When these are traced into the pyramid, they are seen to divide several times, their fine end-branches projecting in little tufts into the cortical matter at the base of each pyramid. Here the branches coming from the tube change in structure and become convoluted in the cortex—the convoluted tubules. Next, each suddenly dips back again as a long straight loop—the loop of Henle—into the pyramid, reaching nearly to the papillary region; then turning sharply on itself, passes back straight to the cortex, where it again becomes convoluted, ultimately ending by dilating into a flask-like bulb called a Malpighian corpuscle. The renal artery, after breaking up into branches between the pyramids, ends in minute end-arteries in the cortex. Each of these pierces into one of the flasks just described, and there becomes branched, the branches being collected into a little ball or glomerulus which nearly fills the flask. From this an efferent vessel escapes, which, joining with its neighbouring vessels of the same kind, makes a close network around the convoluted tubes, ultimately ending in the renal vein. It is sup-posed that the different constituents of the urine are eliminated in different 'parts of these tubes—some, especially the watery parts, in the flask, and some, especially the more solid constituents, in the convoluted tubular apparatus. A peculiar form of glandular epithelium lines the two convoluted areas of the tubes and the limb of the loop nearer the straight or collecting tubes. 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 12 in. in length and as thick as a goose Bladder quill. At its termination it passes obliquely through the and coats of the bladder, so that when the bladder is distended urethra. 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, how-ever, the bladder lies in the abdomen, not in the pelvis. During life it is seldom distended so as to hold more than about 10 oz., but when the abdomen is opened it can be dilated to more than double that size. When distended it rises and is applied closely against the back of the ventral abdominal wall. The bladder has a strong muscular investment of unstriped muscle in several layers, which Labyrinth Interlobular artery Glomerulus 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 become flattened and scale-like when it is distended. At the lower part of the bladder there 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. A muscular band called the torus uretericus Bladder apex Posterior surface of prostate Seminal vesicle From A. F. Dixon, Cunningham's Text-Book of Anatomy. or Mercier's bar joins the orifices of the ureters. The female urethra is only II- in. 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). Embryology. The excretory organs of the embryo are developed as a series of small tubes in the intermediate cell mass (see fig. 5), 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 anteriorpronephros 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. 6, Pro.N.). From the study of comparative anatomy it is probable that these are mere vestiges. Although the pronephros is rudimentary, the duct which in lower Neural tube Somite l Intermediate cell mass Mesonephros and Wolffian duct r. (v 4 Wolffian duct and Mesonephros Body cavity From A. F. Dixon, Cunningham's Text-Book of Anatomy. types carries away its excretion is well developed. This is the Wolffian duct, which appears in man before the pronephric tubes are formed, and runs longitudinally back in each intermediate cell mass to open into the cloaca (fig. 6, W.D.). In certain parts of its course it is at an early date in very close relation with the skin on the dorsal side of the intermediate cell mass, and many embryologists hold that it is originally ectodermal in origin, and has sunk into the mesoderm secondarily. Others think that it is primarily mesodermal but has gained secondary connexions with the ectoderm. From a morphological point of view, as will be explained in the comparative anatomy section, the former view seems the more likely. When the pronephric tubules disappear, which they, do at an early stage of the embryo's development, the Wolffian duct persists and acts as the drain for another and much more important series of tubules, which are formed in the intermediate cell mass behind the region of the pronephros, and make up the mesonephros or middle kidney (fig. 6, M.N.). There is some doubt as to whether these tubes are strictly homologous and in series with those of the oronephros; but they are certainly of later development. By about the sixth week of intra-uterine life these tubules reach their maximum development and form the Wolffian body, which projects into the coelom as the now very definite Wolffian ridge and acts as the functional excretory organ of the embryo (see fig. 7). When the permanent kidney is formed this organ degenerates and its ultimate fate is discussed in the article on the REPRO-
End of Article: URINARY SYSTEM

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