Perineum
The perineum
refers to the areas both externally on the surface and internally as a shallow
compartment of the body. The compartmental area of perineum lies inferior to
the pelvic outlet and is separated from the main pelvic cavity by the pelvic
diaphragm. In other words the pelvis is divided into main pelvic cavity above
and perineum below by the pelvic diaphragm.
Extent and boundaries of
Perineum
When viewed from
below with lower limbs abducted, the perineum is diamond-shaped area that
includes mons pubis anteriorly, insides of the thighs laterally and the gluteal
folds and upper end of natal cleft posteriorly and presents following
osseofibrous boundaries:
Anteriorly –
pubic symphysis
Anterolaterally
– ischiopubic ramus
Laterally –
ischial tuberosities
Posterolaterally
– sacrotuberous ligament
Posteriorly –
tip of the coccyx
Division of Perineum
An imaginary
transverse line joining the anterior ends of the ischial tuberosities divides
the perineum into two triangles:
Anal triangle
lies posterior to this line and contains the anus or lower end of anal canal
and ischiorectal fossa.
Urogenital
triangle lies anterior to this line and contains root of the scrotum and
penis in males or the external genitalia in females.
Perineal Body
The mid-point of the imaginary
transverse line joining the ischial tuberosities is the central point of the
perineum and overlies the Perineal body that provides attachment for the
perineal muscles.
The perineal body lies deep to
the skin and subcutaneous tissue in the midline at the junction of anal and
urogenital triangles, behind the vestibule of the vagina or bulb of the penis
and in front of the anus and anal canal. It is attached to the posterior border
of perineal membrane. The pernieal body is variable in size with relatively
little fat deep to the overlying skin.
Structurally perineal body is an irregular
fibromuscular mass, containing both collagenous and elastic fibers and both
skeletal and smooth muscles. It is the site of convergence of several muscles
and receives them in three strata:
Superficial
stratum: bulbospongiosus, superficial transverse perenei and superficial part
of external anal sphincter
Intermediate
stratum: sphincter urethrae and deep transverse perenei, and deep part of
external anal sphincter
Deep stratum: levator
ani
Perineal fascia:
The perineal fascia consists of superficial
and deep layers.
The superficial perineal fascia
again can be divided into superficial fatty layer and deep membranous
layer (as in case of lower part of anterior abdominal wall).
The superficial fatty layer
(Camper fascia) is continuous with the fat in the region of anal triangle (or
ischiorectal fossa), the superficial fascia of the thighs and the fatty
superficial layer of abdominal wall. In male, it is replaced in scrotum with
smooth dartos muscle.
The membranous layer (Colles’
fascia) is attached posteriorly to the posterior end of perineal membrane and
the perineal body. On each side it is attached to the margins of pubic arch and
to the deep fascia i.e. fascia lata of the uppermost part of the thigh on
medial aspect. Anteriorly, the membranous layer is continuous with dartos
fascia and with the similar layer of abdominal wall i.e. Scarpa’s fascia.
The deep perineal fascia
(investing or Gallaider’s fascia) tightly invest the ischiocavenous,
bulbospongiosus and superficial transverse perneal muscles. The deep perineal
fascia is fused with the suspensory ligament of the clitoris in females and the
deep fascia of the abdomen in males.
Urogenital diaphragm and
Perineal membrane:
The urogenital
diaphragm is a triangular musculofascial diaphragm situated in the anterior
part of the perineum in the region of urogenital triangle, thus filling in the
gap of pubic arch. It consists of:
Two muscles
External urethral sphincter and
Deep transverse perineal muscles
Two fasciae
Superior fascial layer
Inferior fascial layer
Thus, we can say that the
urogenital diaphragm is composed of two muscles (sphincter urethrae and deep
transverse perineal muscles) enclosed within the two fascial layers (superior
and inferior) of urogenital diaphragm.
The two layers of fascia fuse
anteriorly and posteriorly with each other and with the membranous layer of the
superficial fascia and the perineal body posteriorly. On each side, the layers
of the diaphragm are attached to the pubic arch.
Structures
piercing the urogenital diaphragm
The urogenital
diaphragm is pierced by urethra in both male and female and by also by vagina
behind the urethra in female.
Actions of
Urogenital diaphragm:
It provides
support to prostate gland or the urinary bladder. Sphincter urethrae exert
voluntary control to micturition. It also constricts the vagina in female.
Perineal Membrane
The inferior
layer of fasica of urogenital diaphragm is known as Perineal membrane. Thus the
perineal membrane is a thin sheet of tough deep fascia stretching between the
two sides of the pubic arch and covers the anterior part of the pelvic outlet.
Immediately superior to the perineal membrane (on its superior surface) runs
the deep transverse perineal mjuscles along the posterior aspect of the membrane.
Above the center of the membrane, the external urethral sphincter surrounds the
urethra.
Perineal Pouches
Superficial Perineal Pouch
The superficial
perineal puch is a potential space between the membranous layer of perineal
fascia and the perineal membrane.
Contents of
superficial perineal pouch in Male:
Root (bulb and
crura) of the penis and the muscles associated with it (ischiocavernous and
bulbospongiosus)
Proximal part of
spongy urethra
Superficial
transverse perineal muscles
Branches of internal
pudendal artery
Posterior scrotal branches of the
perieneal branch
Transverse perineal branches of the
perieneal branch
Posterior
scrotal branch of Perineal branch of the pudendal nerves
Perineal branch
of posterior cutaneous nerve of thigh
Contents of
superficial perineal pouch in Female:
Root (crura) of
the clitoris and muscle associated with it (ischiocavenous)
Bulbs of the
vestibule and surrounding muscle (bulbospongiosus)
Superficial
transverse perineal muscles
Branches of the
internal pudendal artery
Posterior
labial branches of the perieneal branch
Transverse perineal branches of the
perieneal branch
Posterior labial
branch of Perineal branch of the pudendal nerves
Perineal branch
of posterior cutaneous nerve of thigh
Greater
vestibular glands
Deep Perineal Pouch
The deep
perineal pouch is the space between the superior and inferior layers of fascia
of urogenital diaphragm.
Contents of
deep perineal pouch in Males:
Intermediate or
membranous part of male urethra
External
urethral sphincter muscles
Bulbourethral
glands
Deep transverse
perineal muscles
Related vessels
Deep artery of
penis
Dorsal
artery of penis
Artery to bulb
of penis
Urethral artery
Dorsal nerve of
penis
Contents of
deep perineal pouch in Females:
Proximal part
of urethra
External
urethral sphincter muscle
Deep transverse
perineal muscles
Related vessels
and nerves
Deep artery of clitoris
Dorsal artery
of clitoris
Artery to bulb
of vestibule
Urethral artery
Dorsal nerve of
clitoris
REFERENCES:
Following resources are used while preparing this post (readers are strongly recommended to go through them for more details):
Gray's Anatomy
K. L. Moore's Clinically Oriented Anatomy
R. Snell's Clinical Anatomy