1924 HUMAN ANATOMY. 



per's glands open by slit-like orifices on the posterior wall or floor of this part of the 

 urethra. The terminal dilatation, the navicular fossa (fossa navicularis urethrae), 

 occurs at the extreme distal end of the canal within the glans and opens onto the 

 surface by a vertical slit-like aperture, the exteryial urethral orifice (orificium urethrae 

 externum) or meatus, the most contracted and least distensible part of the entire pas- 

 sage. Since the lateral walls of the navicular fossa are in apposition e.xcept during 

 the passage of fluid, its lumen appears as a vertical slit on cross-section (Fig. 1674) ; 

 beyond the fossa, however, the anterior and posterior walls come into contact, and 

 hence the lumen is here represented by a transverse cleft (Fig. 1674, C), which in the 

 region of the bulb is replaced by one of irregularly stellate outline. 



The female urethra — about 3.5 cm. (i^ in.) in length — is much shorter than 

 the canal in the male and embryologically corresponds to the portion of the latter that 

 lies between the internal urethral orifice and the openings of the ejaculatory ducts. 

 Except at its beginning, the canal is firmly united behind with the anterior vaginal 

 wall, the downward and forward curve of which it closely follows until near its termi- 

 nation, where it turns more sharply forward (Fig. 1622}, In consequence, the lower 

 part of the urethro-v-aginal septum is somewhat thicker below than above. With the 

 exception of a slight spindle-form dilatation about the middle of its course, the lumen 

 of the female urethra is fairly uniform, with a diameter of about 7.5 mm. during 

 physiological distention ; except during the passage of fluid, however, its walls are in 

 contact and the mucous membrane is thrown into slight longitudinal folds. One of 

 these on the upper half of the posterior wall, known as the urethral crest, is more 

 conspicuous, ineffaceable, and continuous with the apex of the vesical trigone ; it cor- 

 responds, therefore, with the similar ridge in the male urethra. The position of its 

 termination below, on the roof of the vestibule, is marked by a low, corrugated, coni- 

 cal elevation or papilla which surrounds the external urethral orifice and lies from 

 1 . 5-2 cm. below the subpubic border. The urethral orifice, usually a small sagittal 

 slit about 5 mm. in length, is subject to much variation in size and shape, being at 

 times triangular, crescentic, cruciate, or stellate in form. On the papilla, on either 

 side of the mid-line and close to the posterior margin of the urethral orifice, lie the 

 minute openings of the paraurethral ducts, or tubes of Skene, from 1-2 cm. long, 

 which are the excretory passages of small groups of tubular glands situated without 

 the wall of the urethra. These ducts, regarded as the homologues of the prostatic 

 ducts that open into the grooves at the sides of the urethral crest, sometimes open 

 directly onto the posterior urethral wall just within the orificium externum. 



Structure. — The Male Urethra. — The wall of this canal consists of a mucous 

 membrane containing a rich venous plexus and supplemented in the prostatic and 

 membranous portions by considerable tracts of muscular tissue. The mucous mem- 

 brane, which possesses an unusual amount of fine elastic fibres, is clothed with an 

 epithelium that varies in different parts of the canal. Throughout the upper two- 

 thirds of the prostatic portion it resembles that of the bladder, belonging to the 

 transitional variety ; on approaching the pars membranacea the epithelium becomes 

 columnar in type, usually being simple, but in places suggesting a stratified arrange- 

 ment on account of the presence of small reserve cells ' between the outer ends of the 

 chief epithelial elements. This variety is continued through the cavernous portion as 

 far as the navicular fossa, where the epithelium becomes stratified squamous in type, 

 and at the external orifice is directly continuous with the epidermis covering the glans. 

 The deeper parts of the mucosa contain a rich venous plexus, and in places, notably 

 in the urethral crest, assume the character of erectile tissue. The constriction of the 

 external orifice is due to a ring of fibro-elastic tissue prolonged from the envelope 

 and septa of <"he cavernous tissue of the glans. 



The muscular tissiie associated with the male urethra includes intrinsic and ex- 

 trinsic fibres, the former being involuntary in character and directly incorporated 

 with the wall of the canal and the latter being accessory bands of striped muscle de- 

 rived from structures surrounding the duct. The intrinsic musculature consists of an 

 inner longitudinal and an outer circular layer, of which the former is thinner but 

 more widely distributed, extending from the internal urethral orifice (where it is con- 

 tinuous with the superficial layer of the muscle of the vesical trigone) as far forward as 



* Herzog : Archiv f. mikro. Anat. u. Entwick., Bd. Ixiii., 1904. 



