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Muscles and Fasciae of the Perineum
The perineum is the region below the pelvic diaphragm and has the same skeletal boundaries as the pelvic outlet: anteriorly, the pubic arch and its arcuate ligament; posteriorly the tip of the coccyx; and on each side the inferior pubic and ischial rami, ischial tuberosities and sacrotuberous ligaments. It extends superficially to the skin and its lateral limits are reached at the buttocks and the medial sides of the thighs. The perineum is trapezoidal, but not planar. On the surface of the body the perineum in the male is defined by the scrotum in front, the buttocks behind and the medial sides of the thighs laterally. A transverse line in front of the ischial tuberosities divides the region into two triangular parts: a posterior anal triangle, with its apex at the coccyx, contains the anal canal; an anterior urogenital triangle, with its apex at the pubic symphysis, contains external urogenital structures. The urogenital triangle slopes downwards and backwards, whereas the anal triangle slopes downwards and forwards.
Muscles and Fasciae of the Anal Triangle
The superficial fascia of the region is thick, areolar and contains many fat cells. A pad of fatty tissue extends deeply into the ischiorectal fossa between levator ani and obturator internus on each side.
The deep fascia lines each ischiorectal fossa; it comprises the inferior fascia of the pelvic diaphragm and that part of the obturator fascia below the attachment of levator ani.
The ischiorectal fossa is wedge-shaped, with its base on perineal skin and its thin edge at the junction of obturator internus and levator ani, covered by the obturator and inferior pelvic diaphragmatic fasciae. Medially are sphincter ani externus and the pelvic diaphragmatic fascia; laterally are the ischial tuberosity and the obturator fascia. Posteriorly, the fossa is partly limited by the lower border of gluteus maximus but extends deep to it as far as the sacrotuberous ligament. Anteriorly, the fossa is partly bounded by the posterior aspect of the muscles of the urogenital triangle, but it is prolonged above them as a narrow recess, sometimes reaching as far as the retropubic space. This anterior recess retains the same relation to obturator internus and levator, but is deep (superior) to the urogenital muscles and fascia.
In the lateral wall of the ischiorectal fossa are the internal pudendal vessels and accompanying nerves, enclosed in fascia that forms the pudendal canal. This fascia fuses with the lower part of the obturator fascia, which ascends to blend with the inferior pelvic diaphragmatic fascia and descends to fuse with the falciform process of the sacrotuberous ligament.
Sphincter Ani Externus
Sphincter ani externus surrounds the lowest part of the anal canal and is intimately adherent to skin; above, it overlaps the internal anal sphincter. It has deep, superficial and subcutaneous parts. The deep part exchanges fibres with the puborectal sling above it and, via the perineal body, with deep perineal muscles anterior to it.
Perineal branch of the fourth sacral spinal nerve and rami of the inferior rectal branch of the pudendal nerve.
The anococcygeal ligament is a layered musculotendinous structure in the midline between anorectum and coccyx. With the overlying presacral fascia it forms the postnatal plate, on which sits the terminal rectum.
Muscles and Fasciae of the Male Urogenital Triangle
The muscles of the urogenital region are involved in urination, copulation and general support of the pelvic contents. They may be considered to form two groups:
a superficial group, occupying the superficial perineal space
a deep group, occupying the deep perineal space:
Superficial urogenital muscles Deep perineal muscles
A median bulbospongiosus A sphincter urethrae
Right and left ischiocavernosi Right and left transversi perinei profundi
Right and left transversi perinei
The superficial fascia of this region has a superficial adipose and a deeper membranous layer.
The adipose layer is thick and areolar, and contains variable amounts of fat. It is continuous with the thin dartos muscle of the scrotum, with thicker circumanal subcutaneous areolar tissue, and laterally with the superficial femoral fascia. In the midline it is adherent to skin and the membranous layer.
The perineal body is a pyramidal fibromuscular node located in the midline in the angle between the anal canal and the urogenital apparatus, with the rectovesical (rectovaginal) septum at its apex. Below this, muscles and their fasciae converge and interlace. Described from above downwards they are:
the smooth muscle of internal sphincter ani and the conjoint longitudinal coat of the rectum
the two levatores prostatae (pubovaginales)
the deep and some superficial perineal muscles.
The deep muscles comprise sphincter urethrae, the two deep transversi perinei and the deep part of sphincter ani externus. The superficial muscles comprise bulbospongiosus, the two superficial transversi perinei and the superficial part of sphincter ani externus. The attachments of these muscles to the pubes, ischia and coccyx determine the position of the perineal body and hence of the visceral canals in the pelvic outlet. The perineal raphe is a median ridge in the skin and fasciae overlying the perineal body that runs forwards from the anus; in the male it is continuous with the raphe of the scrotum.
Deep Perineal Fascia
The deep perineal fascia is attached to the ischiopubic rami and to the posterior margin of the perineal membrane and perineal body above the membranous layer. In front it fuses with the suspensory ligament of the phallus and the fasciae of external oblique and the rectus sheath, so that extravasations deep to it are limited to the superficial perineal space.
Superficial Perineal Space
The superficial perineal space lies between the deep perineal fascia and the perineal membrane. It contains the root of the phallus and the muscles associated with it, branches of the internal pudendal vessels and the pudendal nerves. The associated muscles are superficial transversi perinei, bulbospongiosus and ischiocavernosi.
Transversus Perinei Superficialis
Transversus perinei superficialis is a narrow muscular strip which passes more or less transversely across the superficial perineal space anterior to the anus. It is very variable, often feebly developed and sometimes absent. It arises by tendinous fibres from the medial and anterior aspects of the ischial tuberosity and runs medially to its insertion, which is mainly into the perineal body but sometimes also into the ipsilateral bulbospongiosus or sphincter ani externus. In the perineal body its fibres decussate and pass into the contralateral bulbospongiosus or transversus perinei or sphincter ani externus.
Bulbospongiosus (bulbocavernosus) is a midline muscle anterior to the anus. It consists of two symmetrical parts united by a median fibrous raphe. The fibres arise from the perineal body, in which they decussate and continue as the contralateral transversi and sphincter ani externus, and from the median raphe itself.
Bulbospongiosus helps to empty the urethra, after the bladder has emptied; it is relaxed during micturition, usually contracting only at the end of the process, but able to arrest it. It assists in erection, it contracts repeatedly in ejaculation.
Ischiocavernosus covers the crus penis. It is attached by tendinous and muscular fibres to the medial aspect of the ischial tuberosity behind the crus and to the ischial ramus on both sides of the crus. The muscle fibres end in an aponeurosis attached to the sides and under surface of the crus penis.
Compression of the crus penis, maintaining penile erection. These muscles form a triangle on each side, with bulbospongiosus medially, ischiocavernosus laterally, transversus perinei superficialis posteriorly and the perineal membrane in its floor.
The perineal membrane (inferior fascia of the urogenital diaphragm) forms a base for superficial and deep perineal muscles and is sandwiched between them. The triangular membrane stretches almost horizontally across the pubic arch. The membrane is perforated by:
urethra, 23 cm behind the inferior border of the symphysis, the aperture being circular and about 6 mm in diameter
arteries and nerves to the bulb
ducts of the bulbo-urethral glands, near the urethra
deep arteries of the penis, near the pubic arch and halfway along its attached margin
dorsal arteries of the penis, near its apex
posterior scrotal vessels and nerves, near its base.
The fascia on the upper surface of the deep transversi perinei and sphincter urethrae is poorly defined but covers them and is continuous laterally with obturator fascia. It is pierced by the urethra and continuous with prostatic fascia.
Deep Perineal Space
The deep perineal space lies deep to the perineal membrane and contains the membranous part of the urethra, deep transversi perinei and sphincter urethrae, bulbo-urethral glands and proximal parts of their ducts, pudendal vessels and dorsal nerves of the penis, arteries and nerves of the bulb of the penis and a plexus of veins.
Transversus Perinei Profundus
Transversus perinei profundus extends from the medial aspect of the ischial ramus to the perineal body.
The superficial and deep transversi perinei tether the perineal body, and hence the visceral canals, in the median plane.
The sphincter urethrae surrounds not only the lower urethra but also the bladder neck and, between the two, has elements within the prostate.
Compression of the urethra, particularly when the bladder contains fluid. Like bulbospongiosus, it is relaxed during micturition, but it contracts to expel final drops of urine, or of semen, from the membranous urethra.
The axons that supply the perineal muscles originate from cell bodies in the pudendal nerve nucleus and those that supply the pelvic diaphragm from cell bodies related to that nucleus. Muscles that lie at the interface between pelvic diaphragm and perineal muscles, such as the deep sphincters, may be supplied from above, from below, or from both above and below; those at the somatic-visceral interface, which often contain a mixture of skeletal and smooth muscle fibres, may receive both somatic and visceral innervation via pelvic splanchnic nerves.
Muscles and Fasciae of the Female Urogenital Triangle
The female urogenital triangle includes muscles, fasciae and spaces similar to those in the male, with some differences in size and disposition due to the presence of the vagina and female external genitalia.
Transversus Perinei Superficialis
Transversus perinei superficialis is a narrow muscular slip which differs little from the corresponding muscle in the male.
Bulbospongiosus covers the superficial parts of the vestibular bulbs and greater vestibular glands and passes forwards on each side of the vagina to attach to the corpora cavernosa clitoridis; a fascicle crosses over the dorsum of the body of the clitoris. It is attached posteriorly to the perineal body, where its fibres decussate with those of sphincter ani externus and the contralateral transversi perinei.
Constriction of the vaginal orifice and expression of the secretions of the greater vestibular glands. Anterior fibres contribute to erection of the clitoris by compressing its deep dorsal vein.
Ischiocavernosus, being related to a smaller crus clitoris, is smaller than, but otherwise similar to, the corresponding muscle in the male.
Compression of the crus clitoridis, retarding venous return and thus serving to erect the clitoris.
The perineal membrane, being divided into two halves by the vagina and urethra, is less well defined and less tense than in the male. It forms a triangle on each side, between the urogenital outlets and the ischiopubic ramus, but is perforated by structures corresponding to those listed for the male. On each side it is continuous anteriorly with a pubo-urethral ligament, there being no transverse perineal ligament as such in the female.
Sphincter urethrae surrounds more than the middle third of the urethra. It blends above with the smooth muscle of the bladder neck and below with the smooth muscle of the lower urethra and vagina.
Compressor urethrae arises from the ischiopubic rami of each side by a small tendon, from which fibres pass forwards to meet their fellows of the opposite side in a flat band, ventral to the urethra, below sphincter urethrae.
Sphincter urethrovaginalis arises from the perineal body and its fibres pass forwards on either side of the vagina and urethra to meet their contralateral counterparts in a flat band, ventral to the urethra, below compressor urethrae.
The location of sphincter urethrae around the region of highest urethral closing pressure suggests that it plays an important role in the continence of urine. It may be stimulated via a vaginal tampon electrode, and this approach has been used effectively in the treatment of stress incontinence of urine. The direction of the fibres of compressor urethrae and sphincter urethrovaginalis suggests that they produce elongation as well as compression and thus aid continence.
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