Page:EB1911 - Volume 27.djvu/824

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 Infero-lateral

area

Urethra

Urelel

Posterior surface of prostate

Seminal vesicle

From A. F. Dixon, Cunningham's Tex!-Book of Anatomy. FIG. 4.-The Bladder, Prostate and Seminal Vesicles, viewed from below. Taken from a subject in which the viscera were hardened in situ. The bladder contained but a small amount of fluid. or Mercier's bar joins the orifices of the ureters. The female urethra is only 1% 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 in growths 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 Neuraltube

Notochord Intermediate cell mass

and

Wolfiian duct

and mesonephros avitY

From A. F. Dixon, Cunningham's Tex!-Book of Anatomy. FIG. 5.-Transverse Section through the Body of a Fowl Embryo. types carries away its excretion is well developed. This is the Woljian 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, Wv.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 diict 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 Dronephros; 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 WoL[7'ian body, which projects into the coelom as the now very definite Woljian 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-DUCTIVE SYSTEM.

The metanephros or hind kidney begins as a diverticulum from the dorsal side of the Wolfiian duct close to its opening into the

FIG. 6.-Diagram of the Formation of the Genito-Urinary Apparatus. The first figure is the generalized type, the second the male and the third the female

Suppressed parts are dotted.

specialized arrangements.

Pro. N. Pronephros. N. Nephrostome. M. N. Mesonephros. M. C. Malpighian corpuscle. Mt. N. Metanephros. T. Testis.

B. Bladder. E. Epididymis.

Clo. Cloaca. 0. G. Organ of Giraldés. R. Rectum. V. D. Vas defcrens.

M. D. Mullerian duct. U. M. Uterus masculines. W. D. Wolnian duct. O. Ovary.

Ur. Ureter. Ep. O. Epoiiphoron.

S. H. Sessile hydatid. Par. O. Paroiiphoron. P. H. Pedunculated hydatid. F. 'I'. Fallopian tube. S. G. Sexual gland. U. Uterus.

cloaca (see fig. 6, Mt.N.); this occurs about the fourth week of intra-uterine life, and the diverticulum grows forward (cephalad), dorsal to the hind end of the Wolffian body. In doing this it forms a duct-the metanephric duct or meter-the cephalic end of which enlarges and divides to form the calices of the kidney. From the calices numerous smaller ducts grow into the mesoderm of the hind (caudal) end of the intermediate cell mass and become the collecting Neural

nerve

-ndvdl

Inzesrine

From A. F. Dixon, Cunningl1am's Text-Book of Anatomy. FIG. 7.-Transverse Section through the Body of a Rat Embryo. The position where the germinal epithelium arises is indicated at a. tubes of the kidney. While this is going on another set of tubules, probably in series with the mesonephric tubules, develop independently in the intermediate cell mass and so form all the rest of the tubular system of the kidney. Toward these tubules, at one point, branches from the aorta push their way and invaginate each tube, thus forming the Malpighian corpuscles. By the eighth week the kidney is definitely formed and takes over the excretory work of the mesonephros, which now atrophies; its surface is distinctly lobulated, a condition which persists until after birth.