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Nerve Supply. The external sphincter is supplied by the inferior hæmorrhoidal branch of the pudendal nerve (S. 3. 4.), by the perineal branch of the fourth sacral nerve, and by the deep perineal branch of the pudendal nerve (S. 3. 4.).
Actions. The muscle closes the anal aperture. It is a voluntary muscle.
M. Transversus Perinei Superficialis.-The transversus perinei superficialis is not always present. It consists of a more or less feeble bundle of fibres, which arises from the inferior ramus of the ischium and the fascia over it, and from the base of the fascia inferior of the urogenital diaphragm.
It passes obliquely over the base of the fascia inferior to be inserted into the central point of the perineum.
Nerve-Supply.-Deep perineal branch of pudendal nerve (S. 3. 4.).
Action. The two muscles acting together draw backwards and fix the central point of the perineum.
M. Bulbocavernosus.-The bulbocavernosus (O.T. ejaculator urine), in the male, surrounds the bulb, corpus cavernosum urethra, and root of the penis. It is sometimes separated into two partsposterior (compressor bulbi), and anterior (compressor radicis penis). It arises from the central point of the perineum, and from a median raphe on the under surface of the bulb and corpus cavernosum urethræ.
The muscular fibres pass laterally and forwards and have a triple insertion: from behind forwards, (1) into the inferior surface of the fascia inferior of the urogenital diaphragm; (2) into the dorsal aspect of the corpus cavernosum urethræ; and (3), after encircling the corpora cavernosa penis, into the fascia covering the dorsum of the penis.
The ischiobulbosus, not always present, arises from the ischium,
SPHINCTER ANI EXTERNUS
FIG. 432.-THE MUSCLES OF THE FEMALE PERINEUM
and passes obliquely medially and forwards over the bulbocavernosus, to be inserted into the raphe superficial to that muscle. It belongs to the same stratum as the transversus perinei superficialis and ischiocavernosus.
The compressor hemispheriorum bulbi is frequently absent. It consists of a thin cap-like layer of muscular fibres surrounding the extremity of the bulb under cover of the bulbocavernosus.
M. Bulbocavernosus.-The bulbocavernosus, in the female (O.T. sphincter vagina), is separated into lateral halves by the vaginal and urethral openings. It forms two thin lateral layers covering the bulb of the vestibule, and arises behind the vaginal orifice from the central point of the perineum.
Anteriorly it is inserted into the root of the clitoris, some of its fibres embracing the corpora cavernosa clitoridis so as to reach the dorsum of the clitoris.
Nerve-Supply.-Deep branch of the perineal nerve (pudendal, S. 3. 4.).
Actions. In the male.-The bulbocavernosus contracts the urethra in the emission of urine and semen, and is an accessory muscle in erection of the penis.
In the Female.-The muscle contracts the vaginal orifice, and compresses the bulb of the vestibule of the vagina.
M. Ischiocavernosus.-The ischiocavernosus (O.T. erector penis), in the male, covers the crus penis. It arises from the ischial tuberosity and the sacrotuberous ligament.
Passing forwards, it is inserted by a fascial attachment into the inferior surface of the crus penis, and into the lateral and dorsal aspects of the corpus cavernosum penis.
The ischiocavernosus (O.T. erector clitoridis), in the female, has a similar disposition, but is of much smaller size than in the male.
The pubocavernosus is an occasional slip arising from the pubic ramus, and inserted into the dorsum of the penis. It corresponds to the levator penis of lower animals.
Nerve-Supply. Deep branch of the perineal nerve (pudendal, S. 3. 4.).
FIG. 433.—THE FASCIE OF THE UROGENITAL DIAPHRAGM OF THE PERINEUM,
Diaphragma Urogenitale.-The sphincter urethræ membranacea and the transversus perinei profundus constitute the deeper muscular stratum of the perineum and form the urogenital diaphragm. They lie between two layers of fascia called the fascia inferior, and fascia superior of the urogenital diaphragm (O.T. superficial and deep layers of the triangular ligament).
M. Sphincter Urethra Membranaceæ.-The sphincter of the membranous urethra (O.T. compressor urethra) arises from the inferior part of the pubic ramus, and is directed medially, its fibres radiating so as to enclose the membranous urethra. It is inserted into a median raphe, partly anterior to the urethra, but for the most part posterior to it. The fibres most intimately related to the urethra form a muscular sheath for the canal, and have no bony attachments.
M. Transversus Perinei Profundus.-The transversus perinei profundus consists of a bundle of fibres on each side which arises from the inferior ramus of the ischium just below the sphincter urethræ membranaceæ. It is inserted into a median raphe continuous with that of the sphincter urethræ membranacea. The muscle, in fact, constitutes a separate bundle below and behind the sphincter.
The ischiopubicus is a term applied to a feeble bundle of fibres which, when present, lies above and in front of the sphincter urethra membranaceæ. It arises from
the pubic ramus, and is inserted into a median raphe on the dorsum of the membranous urethra. This muscle is homologous with the compressor venæ dorsalis penis of lower animals.
The sphincter urethræ in the female is smaller than in the male. Its insertion is modified by the relations of the urethra to the vagina. The anterior fibres are continuous with those of the opposite side above the urethra; the intermediate fibres pass between the urethra and vagina, and the posterior fibres are attached, along with the transversus perinei profundus (transversus vagina), into the side of the vagina.
Nerve-Supply.-Deep branch of the perineal nerve (pudendal, S. 3. 4.).
Action. It is a feeble compressor of the membranous urethra, and by no means a sphincter. In the female it has an accessory influence in constricting the vagina.
THE FASCIA OF THE PELVIS.
The extra-peritoneal tissue in the pelvic cavity is of great importance. The hypogastric vessels and their branches, the visceral nerves and plexuses, the ureters, and ductus deferentes, take their course in this tissue outside the peritoneum. It forms in relation to the rectum a thick sheath, for the most part devoid of fat, which encloses the lower part of the rectum completely, down to its termination in the anal canal. It forms a kind of packing for the parts of the bladder uncovered by peritoneum, and is present under the organ in relation to the symphysis pubis and pubo-prostatic ligaments. In the female it forms, in addition, the basis or matrix of the broad ligament, and also occurs as a layer devoid of fat, which loosely connects the anterior surface of the cervix uteri with the base of the bladder.
The cavity of the pelvis minor, in the erect position, resembles a basin tilted forward, with its margin formed by the superior aperture of the pelvis, with a cylindrical wall, and a concave floor, formed by bones, ligaments, and muscles. The deficiencies in the bony walls of the cavity are filled up laterally by the obturator membrane and the sacro-tuberous and sacro-spinous ligaments. Inferiorly and anteriorly, behind the symphysis pubis, the fascia diaphragmatis urogenitalis inferior fills up the pubic arch, and separates the anterior part of the pelvic cavity from the perineum. The inner surface of this osseo-ligamentous chamber is lined by a series of muscles; the piriformis and coccygeus posteriorly, the obturator internus on each side, and the sphincter urethræ membranacea and transversus perinei profundus, inferiorly and anteriorly, on the pelvic surface of the inferior fascia of the urogenital diaphragm.
The pelvic fascia, continuous above with the fascial lining of the abdominal cavity, forms a continuous cylindrical investment for these muscles. On the pelvic surface of the pubis, where muscles are absent, it is merged with the periosteum. It gains an attachment to the spine of the ischium as that projects between the piriformis and obturator internus muscles. Perforations occur in it for the transmission of the obturator nerve and the parietal branches of the hypogastric artery. At the inferior aperture of the pelvis, it is attached to the posterior border or base of the fascia inferior of the urogenital diaphragm, to the ischial ramus and tuberosity, and to the lower edge of the sacro-tuberous ligament. Different names are applied to the fascia in relation to the several muscles which it covers. Posteriorly it constitutes the piriformis fascia laterally it is the obturator fascia, while that part of the sheet of fascia which covers the pelvic surface of the sphincter urethræ membranacea and transversus perinei profundus is known as the fascia diaphragmatis urogenitalis superior.
The disposition of the pelvic fascia is complicated by its relations to (1) the structures which constitute the pelvic floor, and (2) the genito-urinary passages
and the rectum.
The pelvic floor, tense in its anterior part and flexible posteriorly, is formed behind the symphysis pubis by, successively, (1) the fascia of the urogenital diaphragm and the transversus perinei profundus and sphincter muscle of the membranous urethra between them, the latter enclosing the urethra; and the vagina in the female. (2) The perineal body. (3) The levator ani and external sphincter of the anus on each side of the anal canal; (4) the ano-coccygeal body, between the anal canal and the coccyx, containing the main insertions of the levatores ani and external sphincter.
Median pubo-prostatic ligament
FIG. 434.-RELATIONS OF THE PELVIC FASCIA TO THE RECTUM AND PROSTATE.
The levator ani muscle completes the concave floor of the pelvic cavity, sweeping downwards and backwards from its lateral wall, so as to form a muscular diaphragm, with an intra-pelvic and a perineal surface. Its superior concave pelvic surface occupies the lateral part of the pelvic floor. Its inferior convex surface forms the oblique medial wall of the ischio-rectal fossathe, lateral wall of which is formed by the obturator fascia covering the pelvic surface of the obturator internus. In this wall is a fascial sheath containing the pudendal vessels and nerve. The levator ani is covered on both surfaces by pelvic fascia. The anal fascia clothing its perineal surface is thin and unimportant. The fascia covering its intra-pelvic surface is thick and strong. At the origin
of the muscle it is continuous with the general fascial lining of the pelvic cavity, and gives rise to a conspicuous thickening, the tendinous arch (arcus tendineus) of the pelvic fascia, which stretches like a bow-string from the back of the symphysis pubis to the ischial spine. This band is related not so much to the origin of the levator ani muscle, which often extends higher up external to the pelvic fascia, as to the attachments of the fascial investments of the genito-urinary passages, to be described below. There are sometimes additional thickenings of the fascia, branching upwards from the tendinous arch towards the superior aperture of the pelvis. At the insertion of the levator ani, the fascia clothing its pelvic surface is attached to the perineal body, the margin of the anal canal, and the ano-coccygeal body, over which it passes to be continuous, above the raphe of insertion of the levatores ani, with the layer of the opposite side. At the antero
FIG. 435.-RELATIONS OF PELVIC FASCIA TO THE RECTUM AND PROSTATE
inferior border of the muscle the fascia enclosing it become continuous with the superior fascia of the urogenital diaphragm; at its postero-superior border they join the fascia enclosing the coccygeus muscle.
Within the pelvic basin, the walls and floor of which are thus continuously invested by the pelvic fascia, are contained the rectum and bladder, and in the female the uterus, suspended and maintained in position by the peritoneum, extra-peritoneal tissue, and the pelvic vessels and nerves. They are essentially free to distend or collapse, and are not bound down by the pelvic fascia. The rectum in both sexes extends down to the floor of the pelvis, where the anal canal takes its origin. It is invested by the peritoneum and extra-peritoneal tissue, and occupies a special rectal channel; this is lined by pelvic fascia, which gains an attachment to the floor of the pelvis at the margin of the anal canal.
The arrangement of the fascia in relation to the genito-urinary passages is essentially different.
Just as from the perineal aspect the inferior aperture of the pelvis is divisible into two different parts,-a posterior or dorsal part, comprising the ischio-rectal