Chapter 13: The Perineum
The perineum is the part of the pelvic cavity below the pelvic diaphragm. It is a
diamond-shaped space between the pubic symphysis and the coccyx. It is bounded
anterolaterally by the ischiopubic rami, laterally by the ischial tuberosities and posterolaterally
by the lower borders of the sacrotuberous ligaments. It is divided by a line joining the ischial
tuberosities into an anal triangle posteriorly and a urogenital triangle anteriorly.
The anal triangle contains the canal in the midline and the ischiorectal fossae
laterally.
The ischiorectal fossa is a wedge-shaped space, lying with its base inferiorly between
the ischium and the anal canal. It is bounded superomedially by levator ani attached to the
obturator fascia above, and blending with the external anal sphincter below. Laterally it is
bounded by fascia on obturator internus, and inferiorly by perineal skin. Posteriorly it extends
between the sacrotuberous and sacrospinous ligaments deep to gluteus maximus; anteriorly
it extends between levator ani and the superior fascia of the urogenital diaphragm.
The two fossae communicate with each other round the anal canal and are separated
by the anococcygeal body, the anal canal and the perineal body. Each fossa contains:
(i) a mass of loose fatty tissue;
(ii) the internal pudendal vessels and the pudendal nerve which lie in the lateral wall
within a sheath, the pudendal canal, formed by a split in the fascia covering obturator
internus. The canal lies about 3 cm above the ischial tuberosity.
(iii) the inferior rectal nerve and vessels which pass medially across its roof;
(iv) scrotal nerves and vessels;
(v) perineal branch of the 4th sacral nerve which crosses the fossa and supplies the
peri-anal skin and the external sphincter muscle.
Infection of the glands of the anal canal may spread through the anal wall into the
ischiorectal space to produce large abscesses. Should they discharge externally through the
skin, a chronic fistula between the anal canal and the skin may occur (fistula-in-ano) which
will demand surgical treatment.
The urogenital triangle is divided by the inferior layer (perineal membrane) of the
urogenital diaphragm into superficial and deep compartments (pouches).
The urogenital diaphragm is a triangular double layer of fascia (stronger in the male
than the female) stretching across the pubic arch between its attachments to the ischiopubic
rami. It has a free posterior border, the centre of which is attached to a fibrous mass, the
perineal body. It is pierced by the urethra, vessels and nerves passing to structures in the
superficial perineal pouch and, in the female, by the vagina also. Its inferior layer (perineal
membrane) gives attachment to the bulb and crura of the penis or clitoris. The superior (deep)
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layer is continuous with the pelvic fascia where the viscera pass through between the levatores
ani. It fuses posteriorly with the posterior border of the perineal membrane. The superficial
and deep perineal pouches differ markedly in the two sexes and are considered separately.
The Male Urogenital Triangle
The superficial perineal pouch
This lies between the membranous layer of the superficial fascia and the inferior layer
of the urogenital diaphragm. The superficial fascia is attached posteriorly to the free border
of the diaphragm, laterally to the ischiopubic rami and anteriorly it is continuous between the
pubic tubercles with the membranous layer of superficial fascia of the anterior abdominal
wall. It provides inferiorly a fascial sheath around the penis and scrotum. The pouch contains
the root and body of the penis, the superficial perineal muscles, the scrotal vessels and nerves.
The testis and spermatic cord lying within the scrotum are in a space continuous with the
superficial pouch.
Leakage of urine into the superficial perineal pouch spreads firstly into the subfascial
tissues of the scrotum and penis and then ascends the anterior abdominal wall deep to the
superficial fascia. This may occur in cases of rupture of the spongy urethra after direct trauma
or after pelvic fractures.
The penis
The penis comprises three longitudinal cylinders of erectile tissue, the central corpus
spongiosum and the two lateral corpora cavernosa, all covered by fascia and skin. It has a root
attaching it to the perineal membrane, and a pendulous cylindrical body. The corpus
spongiosum lies inferiorly (ventral) and is expanded to form the bulb of the penis posteriorly
and the glans penis anteriorly. The bulb is attached to the perineal membrane and is covered
by the bulbospongiosus muscle. The corpus spongiosum is traversed by the urethra which
opens at the external urethral orifice on the apex of the glans. The two corpora cavernosa
are united dorsally and their anterior extremities are embedded in the glans. Posteriorly,
beneath the symphysis pubic they diverge and form the crura of the penis. The crura are
covered by the ischiocavernosus muscles. The membranous layer of superficial fascia
surrounds the body of the penis and fuses with the corpora just behind the glans. It gives
attachment to the short suspensory ligament of the penis which descends from the front of the
symphysis pubis. This layer of superficial fascia is devoid of fat, consists of loose areolar
tissue and contains superficial vessels and nerves.
The skin over the body of the penis is thin and hairless and is prolonged forwards as
a fold, the prepuce, over the glans. The glans is covered with thin skin. Ventrally there is a
narrow skin fold, the frenulum, between the glans and the prepuce. This contains a small
artery which has to be ligated in excision of the prepuce (circumcision).
Blood supply
By branches of the internal pudendal artery. The artery to the bulb supplies the bulb,
corpus spongiosum and the glans; the deep artery of the penis, the corpora cavernosa; and the
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dorsal artery of the penis, the skin. The veins correspond to the arteries; the corpora drain to
the internal pudendal vein and also through the deep dorsal vein of the penis to the prostatic
plexus.
Lymph drainage
To the upper superficial inguinal nodes.
Nerve supply
The skin is supplied by the dorsal nerve of the penis and the ilio-inguinal nerve. The
skin over the glans is very sensitive. Parasympathetic vasodilatator nerves from the pelvic
splanchnic supply the erectile tissue.
Histology
Each of the three corpora is surrounded by a thick fibrous coat (the tunica albuginea)
and the two corpora cavernosa are separated by an incomplete fibrous septum. From the
tunica and the septum numerous fibro-elastic trabeculae arise, subdividing the cavities of the
corpora into blood-filled cavernous spaces lined by flattened endothelium. The tortuous
terminal branches of the penile arteries open directly into these spaces. Erection of the penis
is caused by an increased blood flow in the penile arteries distending the cavernous tissue coincident with a reduced venous drainage.
Embryology
The penis forms from a swelling, the genital tubercle, lying cranial to the urogenital
opening. The penile urethra develops from a ventral groove on the penis.
The scrotum
This is a pouch of thin, rugose skin enclosing the two testes and spermatic cords, and
separating them is a midline septum. Its walls contain a layer of smooth muscle (the dartos).
It develops from the fusion of two lateral scrotal swellings.
Superficial muscles of the perineum
The superficial muscles lies in the superficial perineal pouch. They are supplied by the
perineal branch of the pudendal nerve.
(i) Bulbospongiosus - covers the bulb of the penis. The two halves of the muscle arise
from the perineal body and from a common midline raphe on the under surface of the bulb.
The fibres wind around the penis to be attached to the dorsal surface.
(ii) Ischiocavernosus - these two muscles cover the crura of the penis. Posteriorly
each is attached to the ischiopubic ramus, anterior to the corpus cavernosum. Bulbospongiosus
and ischiocavernosus constrict the corpora, playing a subsidiary role in erection and
micturition but a major role in ejaculation.
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(iii) Superficial transverse perineal muscle is a scarcely definable transverse bundle
running along the posterior edge of the perineal membrane.
The deep perineal pouch
This lies between the superior and the inferior layers of the urogenital diaphragm and
below levator ani. It is a closed space, filled mainly by the sphincter urethrae muscle
surrounding the membranous urethra. The bulbo-urethral glands and ducts, the internal
pudendal vessels and pudendal nerves and their branches, lie in the pouch and leave it by
piercing the perineal membrane.
The sphincter urethrae muscle
This is a broad sheet of striated muscle surrounding the membranous urethra, attached
on each side to the ischiopubic ramus. Urinary continence is largely dependent on the action
of this muscle which is supplied by the perineal branch of the pudendal nerve.
The Female Urogenital Triangle
As in the male, the perineal membrane divides the triangle into superficial and deep
pouches but these are almost completely divided again by the passage of the vaginal canal.
The superficial pouch contains the crura of the clitoris, the bulb of the vestibule, the
greater vestibular glands, superficial perineal muscles and the terminal parts of the urethra and
vagina. The sphincter urethrae which lies in the deep pouch also gains attachment to the
vaginal wall. All these tissues may be temporarily distorted during the passage of the child
in parturition.
Female external genitalia comprise the mons pubis, a fatty elevation over the
symphysis, and two pairs of skin folds, an outer labia majora and an inner labia minora
surrounding a median cleft, the vestibule, into which opens the urethra and posteriorly the
vagina. Anterior to the urethral opening is the clitoris, a small sensitive mass of erectile tissue
attached to the ischiopubic rami. It is homologous with the penis and has a very similar
structure, possessing two corpora cavernosa capped by a diminutive glans. A fibrous mass,
the perineal body, separates the vagina and anal canal, it is commonly damaged during
childbirth. In the superficial pouch and alongside the vagina are the vestibular bulb and the
greater vestibular glands.
(i) The vestibular bulb comprises two elongated masses of erectile tissue lying on the
perineal membrane, one on each side of the vaginal opening. They are united anteriorly by
a median commissure and are covered by the bulbospongiosus muscle.
(ii) The two greater vestibular glands lie behind the bulb, one on each side of the
vaginal opening.
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The Urethra
The male urethra
This is an S-shaped tube about 20 cm long extending from the internal (vesical)
urethral orifice through the prostate, deep perineal pouch and corpus spongiosum to the
external urethral orifice. It is divided into prostatic, membranous and spongy parts.
(i) The prostatic urethra is the widest and most dilatable part. It is about 3 cm long
and descends through the gland in a slightly forward direction. The upper part of its posterior
wall is marked by a fusiform elevation, the urethral crest which makes the urethral cavity
C-shaped on transverse section. On the summit of the crest is a small pit, the prostatic
utricle, on each side of which open the ejaculatory ducts. Into the groove on the sides of the
crest open the 20-30 prostatic ducts.
(ii) The membranous urethra is narrow and more rigid. About 1 cm in length, it
descends through the deep perineal pouch, about 3 cm behind the symphysis pubis,
surrounded by the sphincter urethrae muscle. Two bulbourethral glands lie posteriorly to it
embedded in the sphincter muscle. They secrete a lubricant fluid along ducts which pierce the
perineal membrane and enter the spongy urethra.
(iii) The spongy urethra is about 16 cm long. It traverses the whole length of the
corpus spongiosum. Its slit-like lumen is narrowest at the external urethral orifice. It is dilated
posteriorly in the bulb forming the intrabulbar fossa and anteriorly in the glans forming the
navicular fossa.
Blood and nerve supply
This comes from the inferior vesical and branches of the internal pudendal artery. Its
veins drain to the prostatic venous plexus and the internal pudendal vein. The mucous
membrane is supplied mainly by the pudendal nerve.
Lymph drainage
This is from the prostatic and membranous parts to the internal iliac nodes and from
the spongy part to the superficial inguinal nodes.
Histology
It possesses muscular, submucous and mucous coats. The mucous membrane is very
vascular. In the prostatic part it is lined with transitional squamous epithelium. It possesses
numerous mucous glands.
Embryology
The prostatic and membranous parts of the urethra are developed from the urogenital
sinus; the spongy part by the fusion of the lips of the urethral groove on the ventral surface
of the penis.
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The female urethra
This is about 3 cm long. It descends from the neck of the bladder through the deep
perineal pouch where it is surrounded by the sphincter muscle, the superficial pouch and
opens into the vestibule. The external urethral orifice lies between clitoris and the vaginal
opening. It is similar to the proximal parts of the male urethra in structure and develops from
the urogenital sinus.
Vessels and Nerves of the Perineum
The pudendal nerve
The pudendal nerve is a branch of the sacral plexus. It leaves the pelvis through the
lower part of the greater sciatic foramen, crosses the ischial spine and enters the perineum
through the lesser sciatic foramen. In the perineum it passes forwards within the pudendal
canal where it ends by dividing into the perineal nerve and the dorsal nerve of the penis
(clitoris).
Branches
(i) Inferior rectal nerve - arises in the posterior part of the pudendal canal. It passes
medially across the roof of the ischiorectal fossa to reach and supply levator ani, the external
anal sphincter and the peri-anal skin.
(ii) Dorsal nerve of the penis - runs forwards on the medial side of the ischiopubic
ramus into the deep perineal pouch and pierces the perineal membrane to gain the dorsum of
the penis. It supplies the glans and skin of the penis and carries parasympathetic fibres to the
corpora.
(iii) Perineal nerve - runs forwards into the superficial pouch. It supplies the perineal
muscles and, via its scrotal (labial) branch, the perineal and scrotal (labial) skin.
The internal pudendal artery
This is a branch of the internal iliac artery. It leaves the pelvis and enters the perineum
alongside the pudendal nerve, and its branches to a large extent correspond to those of the
nerve. It ends in the deep perineal pouch by dividing into deep and dorsal arteries of the penis
(clitoris) and the artery to the bulb.
Branches
(i) Inferior rectal artery - accompanies the inferior rectal nerve and supplies the
lower part of the anal canal.
(ii) Scrotal (labial) branches.
(iii) Artery to the bulb - arises in the deep perineal pouch. It pierces the perineal
membrane supplying the corpus spongiosum.
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(iv) Deep artery of the penis - supplies the corpus cavernosum.
(v) Dorsal artery of the penis - pierces the perineal membrane and passes along the
dorsum of the penis supplying the prepuce and glans.
Lymph drainage
From the perineal skin to the superficial and deep inguinal nodes.
Cutaneous nerves
Perineal skin is supplied by the perineal branch of the posterior femoral cutaneous
nerve and the scrotal (labial) branches of the perineal nerve. Injection of local anaesthetic into
the pudendal nerves in the pudendal canals will anaesthetise the skin and deeper tissues of
the perineum, and is performed as an aid to obstetric procedures.
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