THE MALE URETHRA : 585 
ischial arch, between the bulbo-urethral glands, it contracts again, forming the 
isthmus urethre. It is related dorsally to the rectum and the prostate, ventrally 
to the internal obturator muscles, and laterally to the bulbo-urethral glands. It 
is enclosed, except at its origin, by the urethral muscle.* 
The extrapelvic part (Pars externa) passes between the two crura of the penis 
and runs along the groove on the ventral surface of the corpus cavernosum penis, 
enclosed by the corpus cavernosum urethre and the bulbo-cavernosus muscle. It 
passes through the glans penis and projects forward about an inch in the fossa 
glandis as a free tube, the processus urethre; this part is covered by a delicate 
integument, under which there is a thin layer of erectile tissue. 
The lumen of the tube is largely obliterated 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 1.5cm.). 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 3 em.) 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 em.) 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 (Orifictum urethre internum); it is closed except during urination. The 
terminal opening is the external urethral orifice (Orificium urethre externum) or 
meatus urinarius. The colliculus seminalis is a rounded prominence, situated 
medially on the dorsal wall, about two inches (ca. 5 em.) behind the internal ure- 
thral orifice. On either side of the colliculus there is a small diverticulum, in which 
the ductus deferens and the duct of the vesicula seminalis open. The smali orifice 
of the uterus masculinus is placed centrally on the colliculus; it isinconstant. The 
orifices of the prostatic ducts are on two groups of small papillee, placed lateral to 
the ejaculatory openings. The ducts of the bulbo-urethral giands open on two 
lateral series of small papillee, about an inch (ca. 2.5 cm.) further back and close to 
the median line. The small orifices of the lateral urethral glands are situated 
laterally in the wide pelvic portion. 
Structure.—The mucous membrane contains a large amount of fine elastic 
fibers, and in its pelvic part there are tubulo-alveolar urethral glands (Glandule 
urethrales).2 The epithelium is at first like that of the bladder, then becomes 
eylindrical, and in the terminal part is stratified squamous. In the dorsal wall the 
membrane forms a median ridge, the urethral crest (Crista urethralis); this termi- 
nates about two inches from the internal urethral orifice in the colliculus seminalis. 
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 enclosed by a layer of erectile 
tissue (Stratum cavernosum), which contains plexuses of veins supported by 
trabeculz of elastic and unstriped muscular tissue; in its peripheral part there are 
numerous small arteries. There is a slight thickening of the erectile tissue at the 
ischial arch, producing an enlargement known as the bulb of the urethra (Bulbus 
urethre). This is continued by the corpus cavernosum urethre. 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. 
1It has been customary to divide the pelvic part of the urethra into prostatic and mem- 
branous parts. These terms apply well in human anatomy, but have no special value in com- 
parative anatomy. In the horse a prostatic part hardly exists, unless we assume that it and the 
neck of the bladder together are only about an inch in length. There is no membranous part in 
the sense in which that term is used in regard to man, since the tube has a continuous envelop of 
erectile tissue. 
* Two sets of glands can be distinguished in the pelvic part of the urethra. Two rows of dor- 
sal glands occur close to the median plane; their ducts open into those of the bulbo-urethral glands. 
A series of lateral glands extends on either side from a point near the prostatic ducts to the end of 
the pelvic urethra; their ducts open laterally, as mentioned above. 
