THE MALE URETHRA 499 



The lumen of the tube is hirjiely ol)literated in the inactive condition of the parts. When 

 moderately distended its dimensions in a horse of medium size are as follows: At its origin the 

 diameter is about half an inch (1 to 1.5 cm.). The pelvic dilatation at its widest part measures one 

 and a half to two inches (ca. 3.5 to 5 cm.) transversely, and about an inch (2 to 8 cm.) vertically; it 

 is elliptical in cross-section when fully distended. The isthmus at the ischial arch is a little 

 smaller than the initial part. Beyond this the lumen is about one-half to three-fourths of an 

 inch (ca. 1.5 cm.) in diameter, and is fairly uniform to the glans penis. Here there is a slight 

 fusiform dilatation (Fossa navicularis), beyond which the tube contracts. 



The opening from the bladder into the urethra is termed the internal urethral 

 orifice (Orifieium urethrtr internum); it is closed except during urination. The 

 terminal opening is the external urethral orifice (Orificiimi urethrse externum) or 

 meatus urinarius. The two orifices of the ejaculatory ducts are situated dorsally 

 on either side of the colliculus seminalis, aljout two inches (ca. 5 cm.) Ix'hind the 

 internal tn-ethral orifice; they are slit-like and close together. The small orifice 

 of the uterus masculinus is placed centrally on the colliculus; it is inconstant. The 

 orifices of the prostatic ducts are on two groups of small papillae, placed lateral to 

 the openings of the ejaculatory ducts. The ducts of the bulbo-urethral (Cowper's) 

 glands open on two lateral series of small papilla^, abotit an inch (ca. 2.5 cm.) 

 further l)ack and close to the median line. The small orifices of the urethral 

 glands are situated laterally in the wide pelvic portion. 



Structure. — The mucous membrane contains an unusually large amount of 

 fine elastic fibers, and in its pelvic part there are alveolar glands (Glandule ure- 

 thrales) . The epithelium is at first like that of the bladder, then liecomes cylindrical, 

 and in the terminal part is stratified squamous. In the dorsal wall the membrane 

 forms a median ridge, the urethral crest (Crista urethralis); this terminates about 

 two inches from the internal urethral orifice in a rounded elevation, the colliculus 

 seminalis, on the sides of which the ejaculatory ducts open. 



At the origin of the urethra there is a layer of circular unstriped muscle-fibers 

 outside of the mucous coat. Beyond this the latter is inclosed by a layer of erectile 

 tissue, composed of plexuses of veins supported by trabeculse of elastic and smooth 

 muscular tissue. This is continued by the corpus spongiosum. Outside of the 

 erectile tissue there is a continuation of the intrinsic unstriped muscular coat, 

 consisting of external and internal longitudinal strata, with a layer of circular 

 fibers between them. 



Except at its origin and termination the urethra is provided with a continuous 

 layer of striped muscle, placed outside of the erectile tissue. This is described as 

 consisting of two parts or muscles. 



(1) The urethral (or Wilson's) muscle (M. urethralis) consists of dorsal 

 and ventral layers of transverse fibers on the wide pelvic part of the ure- 

 thra, and covers the bulbo-urethral (Cowper's) glands. It is a compressor of the 

 pelvic part of the urethra and the bulbo-urethral glands. By its forcible contrac- 

 tion it plays an important role in the ejaculation of the seminal fluid, and also in 

 evacuating the last of the lu-ine in micturition.' 



(2) The bulbo-cavernosus muscle (or accelerator urinse) is the continuation 

 of the urethral muscle on the extrapelvic part of the urethra; it extends from the 

 ischial arch to the glans penis. At the root of the penis it is the thickest, and 

 forms a complete layer of circular fibers which inclose the corpus spongiosum and 

 the urethra. Bej^ond this it diminishes very gradually in thickness, and consists 

 of fibers which arise on a median ventral raphe and curve around the corpus 

 spongiosum to end on the tunica albuginea of the corpus cavernosum. Its action 

 is to empty the extrapelvic loart of the urethra. 



The ischio-urethral muscles (Fig. 452) are small bands which arise on the ischial arch and 

 crura of the penis and pass forward to become lost on the ventral layer of the urethral muscle. 

 They may assist in erection of the penis by exerting pre.ssure on the dorsal veins. 



^ The statement often made that it acts as a sphincter of the bladder is a hypothesis of 

 doubtful plausibility. In material hardened in situ, in which the internal urethral orifice is 

 tightly closed, the urethral muscle apparently takes no part in the closure. 



