862 THE GENERATIVE APPARATUS. 



extends to its curve over the iscliial arch, where it gradually contracts. 

 After this it preserves the same reduced dimensions throughout its course, 

 though these dimensions may be increased during the passage of the urine 

 or semen. There is, however, behind the urethral tube a small oval 

 dilatation, named the fossa navicularis (Fig. 403). Even throughout its 

 extrapelvic portion, the inner surface of the urethra offers, near the neck of 

 the bladder, and on its upper wall, the excretory orifices of the prostate gland, 

 and which form two lateral lines of minute perforated tubercles. Between 

 these two lines is found the urethral ridge or veru montanum (caput gal- 

 linaginus), a little eminence elongated from before to behind, on the sides of 

 which the ejaculatory ducts open. Behind this are the excretory orifices 

 of Cowper's glands. 



Relations. The intrapelvic portion of the urethra is in relation, above, 

 with the prostate, which adheres closely to it, and with the rectum, to 

 which it is united by the abundant loose connective tissue in this part of the 

 pelvis ; below, it lies on the internal obturator muscle ; laterally, it is related 

 to the muscles and ligamentous or aponeurotic expansions that close in the 

 sides of the pelvis. Without the pelvic cavity, the urethra is united in the 

 most intimate manner to the corpora cavernosa, which embrace its anterior 

 border. By its posterior border, it is related to the suspensory ligament of 

 the penis. 



STRUCTURE. The urethra is composed of : 1, Mucous membrane ; 2, An 

 erectile envelope ; 3, Muscles ; 4, Vessels and nerves ; and, 5, we will add 

 some remarks concerning the perineal aponeuroses, which are in immediate 

 relations with this canal. 



1. Mucous MEMBRANE. This is rather delicate, and forms the lining of 

 the canal ; it is continuous, posteriorly, with that of the bladder, and in 

 front with the integument enveloping the head (glans) of the penis ; it is also 

 prolonged into the excretory ducts of the glands annexed to the urethra and 

 the ejaculatory ducts. It has longitudinal folds, and is always in contact 

 with itself, except during the passage of urine or semen ; it has scarcely any 

 papillae, only a few being found near the anterior extremity of the canal. 



The epithelium of this membrane is stratified and cylindrical, but at the 

 portion furnished with papillae it becomes pavemental. 



2. ERECTILE ENVELOPE. This envelope, lying outside the mucous mem- 

 brane, does not cover the intrapelvic portion of the canal. It commences a 

 little above the ischial contour, behind Cowper's glands, by a very thick 

 bulging portion, named the bulb of the urethra. In front, it terminates by 

 another bulbous enlargement, into which the anterior extremities of the 

 corpora cavernosa enter, named the head of the penis (glans penis). 



The tissue composing this envelope has the same organisation as other 

 erectile apparatus, being a network of communicating cavities separated by 

 elastic septa, the latter showing in their structure some contractile elements. 



3. MUSCLES. Behind the prostate gland, the mucous membrane of the 

 urethra is covered by a fleshy layer of circular fibres, forming Wilson's 

 muscle. Another muscular envelope, constituting the bulbo-cavernous or 

 accelerator, also covers the erectile tissue of the urethra, accompanying it to 

 near the glans, where it gradually disappears. To these two principal 

 muscles of the urethra are added two pairs of secondary fasciculi the 

 ischio-urethral and transversus perinei. This is the description of the 

 muscular apparatus : 



a. Wilson's muscle. This may be described as a single muscle com- 

 posed of two portions, an inferior and superior. Both are formed by 



