The =urethra= of the male (Urethra masculina) is the long mucous tube
which extends from the bladder to the glans penis. It passes backward on
the floor of the pelvis, turns around the ischial arch, forming an acute
angle, and passes forward inclosed in the corpus spongiosum. It may,
therefore, be divided into two segments, pelvic and extrapelvic.
The =pelvic part= (Pars pelvina) is four or five inches (ca. 10 to 12
cm.) long. At its origin it is not distinguishable from the neck of the
bladder in size or structure; in fact no line of demarcation exists
between the two. Behind the prostate the tube dilates to a potential
width of two inches or more (ca. 5 to 6 cm.). Near the ischial arch,
between the bulbo-urethral (Cowper’s) glands, it contracts again,
forming the =isthmus urethræ=. 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 inclosed, except at its origin, by
the urethral (Wilson’s) muscle. Beneath this is a rich venous plexus,
forming a sort of erectile tissue.[146]
[Illustration:
FIG. 398.—PELVIC URETHRA AND POSTERIOR PART OF BLADDER OF HORSE SLIT
VENTRALLY AND LAID OPEN.
_e.d._, Ejaculatory ducts.
]
The =extrapelvic part= (Pars cavernosa) passes between the two crura of
the penis and runs along the groove on the ventral surface of the corpus
cavernosum penis, inclosed by the corpus spongiosum 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 urethræ=; 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 1.5 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 3 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= (Orificium urethræ internum); it is closed except
during urination. The terminal opening is the =external urethral
orifice= (Orificium urethræ externum) or meatus urinarius. The two
orifices of the =ejaculatory ducts= are situated dorsally on either side
of the colliculus seminalis, about two inches (ca. 5 cm.) behind the
internal urethral 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 papillæ, 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 papillæ, about an inch (ca.
2.5 cm.) further back 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=
(Glandulæ urethrales). The epithelium is at first like that of the
bladder, then becomes 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 trabeculæ 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 urethra, 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 contraction it plays an important rôle in the
ejaculation of the seminal fluid, and also in evacuating the last of the
urine in micturition.[147]
(2) The =bulbo-cavernosus muscle= (or accelerator urinæ) 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.
Beyond this it diminishes very gradually in thickness, and consists of
fibers which arise on a median ventral raphé and curve around the corpus
spongiosum to end on the tunica albuginea of the corpus cavernosum. Its
action is to empty the extrapelvic part 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 pressure on the dorsal veins.