THE URETHRA. 493 



Interior. This pouch, studied internally, exhibits folds and ridges more 

 or less marked, according to its state of plenitude. It also shows, pos- 

 teriorly, the opening of the neck, which communicates with the urethral 

 canal, and a little higher, the orifices of the ureters. These three apertures 

 circumscribe a triangular space, the trigonum vesicse. 



STRUCTURE. Th6 structure of the bladder is very simple. Two mem- 

 branes compose its walls, the internal of which is mucous, and the external 

 muscular. Anteriorly, the latter is covered by the serous investment 

 described above. 



The mucous membrane is pale and thin, and is continuous with that lining 

 the ureters and the urethra. It Lhows some papillae and some simple 

 tubular glands towards the neck. Its epithelium is stratified and tesselated, 

 the superficial cells being very irregular. 



The muscular layer is composed of white fibres, the arrangement of which 

 is very complicated. Certain authorities describe three superposed planes, 

 whose fibres pass in different directions. In the Horse, the walls of whose 

 bladder are very thin, these planes are difficult to demonstrate. The fibres are 

 longitudinal, circular, oblique, spiral, and even twisted towards the fundus 

 of the bladder ; the deep fibres are reticulated. In the posterior region they 

 do not form a sphincter around the neck of the organ, as is generally 

 believed ; the real sphincter is Wilson's muscle, which encircles the 

 membranous portion of the urethral canal. 



Vessels and nerves. The parietes of the bladder receive their blood 

 from several sources. The principal arteries come from the ^esico-prostatic 

 branch of the internal pudic ; the umbilical artery also furnishes ramifications 

 that reach the fundus of the organ. The lymphatics pass to the sublumbar 

 glands. The nerves are furnished by the pelvic or hypogastric plexus, and 

 the inferior branches of the two last sacral pairs ; their twigs are spread 

 more especially between the muscular and mucous layers. 



DEVELOPMENT. The study of the development; of the urinary reservoir 

 is very interesting. It is narrower and more elongated in the foetus than 

 the adult, and is relatively more capacious during the whole period of intra- 

 uterine life. It then occupies the abdominal cavity as far as the umbilical 

 opening, and is flanked by the two umbilical arteries. Its posterior 

 extremity alone enters the pelvis ; the anterior extremity, forming a veritable 

 neck, is continuous with the urachus, just as the neck, properly so called, is 

 continuous with the urethral canal (Fig. 253). At birth, this anterior neck 

 separates from the urachus, and is transformed into a free cul-de-sac ; while 

 the bladder is gradually withdrawn into the pelvic cavity, carrying with it 

 the umbilical arteries, and finishes by acquiring the position it definitively 

 preserves in the adult. 



FUNCTIONS. The part played by the bladder is one of incontestible 

 utility. In permitting the accumulation of the urine and the intermittent 

 expulsion of that excrementitial fluid, it spares animals the disagreeable 

 condition in which they would be placed if the liquid secreted by the 

 kidneys was continually being discharged as it was produced. 



4. Urethra. 



The description of this organ will be given with that of the genital 

 organs ; as in the male it is common to the urinary and generative apparatus ; 

 even in the female it is intimately connected with the latter 



