The Anatomy of the Perineum
The perineum is the region between the thighs inferior to the pelvic diaphragm. The boundaries of this region are the same as that for the pelvic outlet, namely the pubic symphysis, ischiopubic rami, sacrotuberous ligaments, and coccyx. The perineum has a roof formed by the pelvic diaphragm and a floor of fascia and skin. It also contains the muscles and neurovasculature associated with urogenital structures and the anus.
When the legs are abducted, the perineum forms a diamond shape that can be divided into two triangles by drawing a line connecting the ischial tuberosities: the urogenital triangle and the anal triangle.
Urogenital triangle: The urogenital triangle makes up the anterior portion of the perineum. The female urogenital triangle is home to the opening of the vagina, the urethra, and the clitoris.
The perineal fascia of the urogenital triangle includes superficial and deep layers. The superficial perineal fascia has a fatty layer and a deeper membranous layer (Colles fascia). The fatty layer makes up the thickened areas of the labia majora and mons pubis. In males, the fatty layer is much thinner and is absent in the penis and scrotum.
Anal triangle: The anal triangle contains the anus and the ischioanal fossae, which are two wedge-shaped spaces between the skin around the anal canal and the pelvic diaphragm. They contain fat and loose connective tissue, which helps support the anal canal but is pliable enough to allow for expansion during bowel movements.
Hemorrhoids are painful, swollen veins in the anal canal or anus. Internal hemorrhoids occur in the anal canal, but they may not be painful because the rectum doesn’t have many nerves that sense pain. External hemorrhoids affect the anus and are easy to see and palpate by inspecting the anus. More sensory nerves are located around the anus, so external hemorrhoids may be quite painful. They may also result in itching and bleeding. Topical medications may help shrink hemorrhoids, and dietary changes may prevent their occurrence; however, surgery may be indicated when reoccurrences are frequent.
The anal region is innervated by the inferior hypogastric plexuses and the inferior rectal nerve. The blood supply to the anal area is provided by the superior and inferior rectal arteries. Blood is retuned by their accompanying veins. Lymph drains into the pararectal nodes and superficial inguinal nodes.
The male perineum
The male perineum includes the penis, scrotum, and the perineal muscles in the urogenital triangle, along with the anal triangle.
The scrotum is the fibromuscular sac that houses the testes. It’s located behind and below the penis. On the surface, you can see the scrotal raphe, which is a ridge that runs along the midline of the scrotum. It continues on the penis as the penile raphe. Internally, the scrotum is divided by a septum of the scrotum.
The female perineum
The female perineum includes the external genitalia, orifices of the urethra and vagina, and the perineal muscles. Pelvic examinations begin with the inspection of these areas.
The external genitalia are collectively referred to as the vulva. They include the mons pubis, the labia majora, the labia minora, and the clitoris:
Mons pubis: This rounded, fatty eminence is anterior to the pubic symphysis.
Labia majora: These prominent folds of fat and skin help to protect the orifices of the urethra and vagina. The labia majora are separated by the pudendal cleft.
Labia minora: Inside the pudendal cleft you find these two folds of skin, but they don’t contain fat. The anterior portions of the labia minora unite to form laminae that become the frenulum and prepuce of the clitoris.
Clitoris: This small erectile organ corresponds with the male penis. It’s located where the labia minor meet at the front of the vulva. The root of the clitoris is made up of the bulb of the vestibule and the left and right crura of the clitoris. The body of the clitoris contains two corpora cavernosa and ischiocavernosus muscles. The glans of the clitoris is a mass of erectile tissue that has many nerve endings. The blood supply is provided by the branches of the internal pudendal arteries and veins.
Vestibule: This feature is a space between the labia minora. You can find the external urethral orifice there, along with the vaginal orifice. Greater vestibular glands and lesser vestibular glands lie on either side of the vestibule posterior to the vaginal orifice. The vestibular glands secrete slippery fluids during sexual arousal.
The anterior vulva is innervated by anterior labial nerves (branches of the ilioguinal and genital branch of the genitofemoral nerves). Posterior (labial) nerves supply the labia minora, vagina, and clitoris (via a branch called the dorsal nerve of the clitoris). Blood flow to the vulva is provided by external and internal pudendal arteries. Blood is returned by the labial and internal pudendal veins. Lymph drains into the superficial inguinal lymph nodes, deep inguinal nodes, and internal iliac nodes.