The Perineum

The perineum is diamond shaped, being bound

• anterolaterally by the conjoined rami of pubis and ischium

• laterally by the ischial tuberosities

• posterolaterally by the sacrotuberous ligament

• on the anterior extremity by the pubic symphysis

• on the posterior extremity by the coccyx

It can be divided into :

• an anterior or urogenital triangle

• a posterior or anal triangle

The Male Urogenital Triangle

The Perineal Membrane

The perineal membrane is a triangular sheet of fibrous tissue extending between the conjoined pubic and ischial rami on either side.

Its apex lies close to the pubic symphysis.

Its base stretches across between the ischial tuberosities.

The Penis

The penis has dorsal and ventral surfaces. These terms refer to when the penis is erect.

It consists of three elongated masses of erectile tissue – each consists of spongy fibroelastic tissue with a tough fibrous envelope and blood in its interstices.

The two corpus cavernosa are attached to the angle between the lateral part of the inferior surface of the perineal membrane and the conjoined ischiopubic rami. The posterior part of each is called the crus. In the free part of the penis their interstices are fused. The anterior ends fit into a depression in the proximal surface of the glans penis.

The glans penis is the enlarged distal end of the corpus spongiosum. The corpus spongiosum is a thin cylinder lying in a groove on the undersurface of the fused cavernosa. It is attached to the perineal membrane in the midline. Its posterior end is somewhat dilated – the bulbus spongiosus.

The skin of the penis contains no fat. Distally it forms a double layer called the prepuce.

The urethra pierces the perineal membrane to run in the bulb and corpus spongiosum as the penile urethra. This open to the exterior through the external urethral orifice on the glans penis.

Arterial blood enters the mass of erectile tissue from above. The arteries are branches of the internal pudendal artery. The veins of the penis drain into the prostatic venous plexus.

The muscles of the urogenital triangle

The free surface of each corpus cavernosum is covered by the ischiocavernosus. Corresponding to this on the bulb and corpus spongiosum is the bulbospongiosus. These may help compress the veins draining the erectile tissue helping to maintain the erect state of the penis.

The Perineal Body is a mass of fibrous tissue in front of the anal canal. It is continuous with the perineal membrane. Attached to it are the superficial and deep transverse perineal muscles originating from the ischial tuberosities. These muscles help support the pelvic floor.

The Perineal Membrane represents the fascial layer between the superficial and middle muscular layers of the body wall. Deep to this is the sphincter urethrae. This surrounds the membranous part of the urethra (1cm long) between the prostate above and the perineal membrane below. It is under voluntary control.

Bulbourethral glands

Lie above the perineal membrane

Their secretion descends through the urethra to lubricate the glans penis.

The Female Urogenital Triangle

Resembles the male in its basic form.

However the perineal membrane is pierced in the midline by the vagina.

The bulbs of the vestibule lie on either side of the vagina and are attached to the perineal membrane. They come together anterior to the vagina and are continuous with the glans clitoris. Their structure corresponds to the bulb and corpus spongiosum in the male.

The corpus cavenosa are much smaller in the female. They come together to form the dorsal shaft of the clitoris.

The greater vestibular glands lie below the perineal membrane on either side of the vagina. There is a single duct from each. They lubricate the vagina.

The female urethra is short. It opens on a small elevation in front of the vagina.

The female external genitalia

The mons pubis is a mass of fibro-fatty tisue covered by hairy skin in front of the pubic bones. Extending backwards on either side are the labia majora. They are similar in structure to the mons pubis. The labia minora are a pair of thin folds of hairless skin which possess numerous sebaceous glands and are attached anteriorly to the clitoris which consists of erectile tissue.

The vestibule of the vagina or vulva is the space between the labia minora. The lower urethral opening is on top of a small elevation in front of the vaginal orifice. The vaginal orifice can be narrowed by a membrane called the hymen.

The Anal Triangle

The anal canal runs downwards and backwards from its beginning where it perforates the levator ani to the skin.

It is surrounded by its three sphincter muscles.

On either side of these is the ischiorectal fossa. This is bouneded:

• above by the levator ani

• laterally by obturator fascia

• below by the skin and supoerficial fascia

It extends forwards as far as the posterior edge of the perineal membrane

It is wedge-shaped

It is filled with fatty connective tissue, continuous with the superficial fascia.

The Pudendal Canal runs forwards in the lateral part of the ischiorectal fossa. It lies medial to the obturator internus and below levator ani and is delineated off from the rest of the ischiorectal fossa by a sheet of connective tissue. It continues forward into the urogenital part of the perineum by passing deep to the lateral part of the perineal membrane.

Category: Anatomy Notes