Anatomy of the perineum

The perineum is a diamond shaped area and a shallow chamber of the pelvis that lies below the pelvic diaphragm.

At first glance it may seem like a small and insignificant area but it actually plays a key role in micturition, defecation, intercourse, and delivery by supporting the urinary, gastrointestinal and reproductive organs of the pelvis!

Now, let’s start with the outer surface of the perineum, called the perineal region. This region is shaped like a diamond that extends posterolaterally from the inferior surface of the pubic symphysis, to the ischial tuberosities laterally, bounded by the inferior pubic and inferior ischial rami anterolaterally.

From the ischial tuberosities, it continues posteromedially to the sacrum and coccyx, bounded by the sacrotuberous ligaments posterolaterally.

This diamond is divided into two triangles by an imaginary transverse line that extends from the two ischial tuberosities.

These triangles are called the urogenital triangle, which lies anteriorly, and the anal triangle, which lies posteriorly.

Now, in the middle of the imaginary line that connects the ischiopubic rami lies an irregular mass called the perineal body.

This mass forms the central point of the perineum, and is a meeting point for many muscles, including the superficial transverse perineal muscles, deep transverse perineal muscles, levator ani, bulbopongiosus, external anal sphincter, and external urethral sphincter muscles.

Alright, now, deep to the skin of the perineal region, there’s the perineal fascia. This fascia has two layers; the superficial perineal fascia, or colle’s fascia, and the deep perineal fascia.

The superficial perineal fascia also has two layers; a superficial fatty layer and a deep membranous layer. The superficial fatty layer differs between females and males.

In females, the fatty layer forms the substance of the labia majora, which are the larger folds of the vulva, and the mons pubis, which is the rounded mass that lies anterior to the pubic symphysis.

In males, the fatty layer extends into the penis and the scrotum. In the scrotum, it is replaced by a thin layer of smooth muscle, called the dartos muscle.

In both males and females, this fatty layer continues above with the fatty layer of the superficial abdominal fascia in the lower part of the anterior abdominal wall, called Camper fascia.

The superficial fatty layer also continues posteriorly with a fatty mass that surrounds the anal canal, called the ischioanal fat pad.

Next the superficial perineal fascia, also called Colles fascia, has a deep membranous layer which has many attachments to nearby structures.

Posteriorly, it attaches to the posterior margin of the perineal membrane and the perineal body, meaning this layer doesn’t extend into the anal triangle.

Laterally, it attaches to the deep fascia of the upper and medial sides of the thigh, called the fascia lata. The anterior attachment differs between females and males.

In females, the deep membranous layer and the superficial fatty layer extend into the labia majora. In males, it connects with a layer of connective tissue in the penis and the scrotum, called the dartos fascia.

Finally, in both females and males, the deep membranous layer continues above with the membranous layer of the superficial abdominal fascia in the lower part of the anterior abdominal wall, called Scarpa fascia.

Alright, now let’s discuss the deep perineal fascia, also called the investing fascia, which lies beneath deep to the membranous layer of the superficial perineal fascia.

This layer surrounds the ischiocavernosus, bulbospongiosus, and superficial transverse perineal muscles. In both males and females, this layer attaches laterally to the ischiopubic rami.

This fascia is also continuous with many supportive structures in both females and males. In females, it continues anteriorly with a ligament that supports the clitoris, called the suspensory ligament of the clitoris.

In males, it continues with a similar ligament that supports the penis, called the suspensory ligament of the penis.

Finally the deep perineal fascia continues above with the deep fascia that invests the muscles of the abdominal wall.

Okay, now let’s take a deep breath and have a quick quiz! Alright, can you name this fascia?

Now, let’s switch gears and explore the urogenital triangle. The urogenital triangle is covered by a sheet of fascia called the perineal membrane that extends between the ischiopubic rami.

This membrane fills the urogenital hiatus, which is the gap in the anterior part of the pelvic diaphragm.

In both males and females, this membrane is perforated by the urethra. In females, it is also perforated by the vagina.

Finally, the perineal membrane and the ischiopubic rami form a foundation for the male and female erectile bodies, which lie in the superficial part of the urogenital triangle.

Within the urogenital triangle are the perineal pouches, which are two spaces or compartments called the superficial and deep perineal pouches separated by the perineal membrane.

Let’s start with the superficial perineal pouch, which lies between the perineal membrane and Colles fascia.

The contents of this pouch differ between females and males. In males, this pouch contains the root of the penis, which has two parts: the bulb and the crura of the penis.

The bulb of the penis lies in the midline and the crura of the penis lie on each side of the bulb. Each of these parts is associated with one of the superficial muscles of the perineum; the bulbospongiosus muscles cover the bulb of the penis, and the ischiocavernosus muscles cover the crura.

The superficial transverse perineal muscles also lie within the superficial perineal pouch and extend from the ischial tuberosities to the perineal body.

Now, in the female urogenital triangle there are multiple erectile structures. First, there’s the clitoris, which also has paired erectile structures called the crura of the clitoris.

Next are the bulbs of the vestibule, which are paired erectile structures that extend posteriorly from the clitoris lateral to the urethra and the vagina.

Also in the superficial perineal pouch there are the greater vestibular glands, which are two pea-sized glands that lie posterolateral to the vaginal orifice.

Next are the superficial perineal muscles, which are analogous to those in the male perineum. The bulbospongiosus muscles cover the bulbs of the vestibule and vestibular glands.

Then ischiocavernosus muscles cover the crura of the clitoris, and finally the superficial transverse perineal muscles.

The last structures in this pouch are the branches from the internal pudendal artery and pudendal nerve. Both the nerve and the artery give off an inferior rectal branch.

The internal pudendal artery continues further and gives off a perineal branch which supplies the bulb of the penis in males or vestibule in females. The artery ends as the posterior scrotal artery or posterior labial artery.

Like the artery, the internal pudendal nerve also gives off a perineal branch which bifurcates into a deep perineal branch and superficial perineal branch, this branch terminates as the posterior scrotal nerve or posterior labial nerve.

Alright, now let’s switch gears and discuss the deep perineal pouch, which lies between the pelvic diaphragm, and the perineal membrane.