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ANATOMY

deeply than that of the last group. Neutrophile granules are sparingly present in the protoplasm. (5) A very small number (less than 0'5 per cent.) are sharply marked off from the first order by possessing intensely basophile granules unequal in size and ! irregular in distribution; when acted on by certain stains they , exhibit tints characteristically different from those which the same | stains produce in other corpuscles. (6) Lymphocytes (about 23 per cent.) are bodies quite distinct from any of the foregoing orders. They are small, about the size of red corpuscles, and contain a large round nucleus, marked as if made of concentric layers, and staining uniformly. It is surrounded by a thin protoplasmic layer whose reticular structure is easily brought out by basic stains. The discrimination of these various forms and their abnormal modifications is regarded as important in modern pathology (see Ehrlich-Lazarus, Ancemie. Vienna, 1898, pt. 1.) VI. Eliminative Organs.—The urinary and reproductive organs are inseparably united in naan as in all vertebrates. The former consist of the kidneys and their ducts—the ureters, the urinary bladder, and the urethra. The latter, in the male, of the testes, vesimlce seminales, vasa deferentia, and the prostatic and anteprostatic glands. In the female the organs are ovary, Fallopian tube, uterus, and vagina, and the several folds which bound the vaginal opening. There is embryological evidence that the kidneys are specialized organs which have originated at a comparatively late period of vertebrate phylogeny. In the elementary condition of urogenital organs found in segmented invertebrates the sex-gland is only a specialized area of the lining epithelium of the coelom (vol. xx. p. 408), and the sex-products, ova or spermatozoa, arising from the germinal epithelium are shed into the body cavity. A communication between the body cavity and the surrounding medium exists in the form of a segmental tube or nephridium on each side of each segment. These are not only the channels of escape of the sex-products, but are also, in general, excretory organs. In the earlier stages of the lower vertebrates the presence of segmental nephridia can be demonstrated, one portion, a head-kidney or pronephros, arising in some of the anterior segments, and a second, or mesonephros, in the segments at a level farther back. These segmental tubules open internally into the coelom and outwardly into a lateral canal which runs longitudinally and ends posteriorly by opening into the cloaca. The longitudinal tube connected with the pronephros is known as the pronephric or Mullerian duct, that connected with the mesonephros or Wolffian body is named the mesonephric or Wolffian duct. In man, as in all the amniota, there arises at an early period of development a third group of nephridial tubules, the metanephric, which by their aggregation make up the metanephros or kidney. That these are the result of a high degree of specialization is obvious from the fact that, while in the cases of the pronephros and mesonephros the nephridia and the longitudinal ducts arise independently and unite after they have attained a considerable degree of development, in the metanephros the tubules are from the first offsets from the metanephric duct or ureter, which itself arises as an offset from the mesonephric duct. In man there is never any trace of a pronephros, but the pronephric duct is distinct. The mesonephric tubules rapidly lose all traces of their secreting portions, and appear only as a vestigial structure, rudimentary in the female; but they become in the male specialized as channels for the transmission of the sex-products. The kidneys are two bean-shaped granular masses, firm in consistence and reddish-brown in colour, about 4J inches long, and placed obliquely behind the other abdominal viscera—one on each 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, and may even be movable in the abdominal cavity—a condition often productive of serious consequences. The kidney in Kidaeys the foetus is lobulated, but the intervals between the lobes become smoothed out in later years of childhood. Each gland is invested 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 backwards 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 cortical layer, and an inner or medullary mass (Eig. 11). The latter consists of a series of eight to sixteen

Fiq. 11.—Vertical section through the kidney. A, branch of renal artery ; U, ureter. 1, cortical substance with cortical pyramids, and labyrinth substance of tortuous tubes ; 2 and 3, medullary pyramids of straight tubules ; 4, fatty masses around blood-vessels (5); 6, papilla ; 7, pelvis. 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 (calyces), 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 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. Next, each suddenly dips back again as a long straight loop 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. The renal artery, after breaking up into branches