Perineum: Boundaries, divisions, perineal fascia and perineal pouches

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

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