Perineal region

Muscles of the urogenital triangle

There are several muscles found in the urogenital triangle that are important for reproduction and urinary excretion. Some of these muscles include:

• Superficial transverse perinei: as the name suggests, courses transversely across the superficial perineal space with the anus posterior to it. Each has an attachment on the anteromedial surface of the ischial tuberosity and interlinks at the perineal body with the fibers of its counterpart muscle on the other side.
• Deep transverse perinei: has several attachments in the deep perineal pouch. As it spans the space in a transverse manner (albeit, incompletely), each deep transverse perineal muscle attaches to the ischiopubic rami laterally, and similar to the superficial transverse perinei, interlinks at the perineal body with the fibers of its counterpart muscle on the other side. The muscle has an anterior deficiency. 
• Bulbospongiosus: differs between males and females. In the latter, the muscle is superficial to the bulbs of the vestibule and the associated glands. They subsequently travel lateral to the vagina and insert into the corpora cavernosa of the clitoris along with the body of the glans clitoris itself. However, in the former, the muscle starts as identical halves and fibers decussate in the perineal body and are attached to transverse superficial perinei muscles. The muscle assists with voiding the urethra after micturition and also in expelling semen or vaginal secretions during the ejaculatory process. 
• Ischiocavernosus: is found in both males and females, although it is significantly larger in males. It is attached medially to the ischial tuberosity and ischial rami. The aponeurotic ending of the muscle merges into the crus of the penis (clitoris). The primary action in males is to aid in stabilizing the erect penis; and in females it assists in promoting clitoral erection. 

Female urogenital triangle

The female urogenital triangle contains the mons pubis, labia majora and labia minora, the clitoris, and the vaginal and urethral orifices. The perineal body is attached to the vaginal introitus, as well as the posterior commissure labia majora. In addition to the other muscles of the deep perineal pouch, females also have the urethrovaginalis muscle that surrounds both the urethral and vaginal orifices.

The urogenital diaphragm is almost completely divided by the urethra and vagina into two triangular halves. The membrane is held in place by the pubourethral ligament, which is traversed by the vagina, urethra, Bartholin’s ducts, the labial neurovasculature and the deep dorsal neurovasculature of the clitoris.

If you're tired of reading, take this quiz about the female perineum and see what you've learned so far and if you have any weak spots!

Male urogenital triangle

The male urogenital triangle houses the bulb of the penis, the scrotum and serves as a point of attachment for the penis. The perineal body is continuous with the perineal raphe in the scrotal skin. In males, the bulbourethral ducts, urethra, deep dorsal neurovasculature of the penis and the nerves and vessels of the scrotum all pass over the perineal membrane.

Structural differences between the male and female

The anorectal triangles in males and females only differ only in size, where the female anal triangle is significantly wider than that of the male. This disparity is a direct result of the structural differences between the male and female pelvis. Not only is the transverse diameter (distance between the ischial tuberosities) of the female pelvis wider, but the plane of the pelvic outlet (the anteroposterior distance between the pubic arch and the tip of the coccyx) is also longer. This sexual dimorphism accommodates the passage of a baby through the pelvic cavity during childbirth.

Structure

The anal canal is present in the middle of the anorectal triangle and on each side is ischioanal (ischiorectal) fossa. The ischioanal fossa occupies most of the anal triangle. In a transverse section through the pelvis (in a lithotomy position), it has a horse shoe appearance, while in a coronal section (through a vertically erect individual) each fossa appears roughly pyramidal.

The ischioanal fossa is limited and comprised of the following structures:

• Medially: by the external anal sphincters and the inferomedial part of the sloping levator ani muscle;
• Laterally: by the obturator internus muscle and its fascia. The two muscles of the walls abut and form the apex at the rostrolateral margin; 
• Base: the base of the pyramid is formed by skin; 
• Space between borders: filled with loose adipose tissue.

The fascia of obturator internus continues towards the superficial surface of the levator ani and covers the deep region of the ischioanal fossa. At that point, it is called the deep fascia of the anal triangle. Its counterpart, the superficial fascia of the anal triangle, is a continuation of the subcutaneous fascia of the perineal skin and the skin of the buttocks and thigh.

The release of end products of digestion from the gastrointestinal tract is regulated by the external anal sphincters. This oblong cluster of skeletal muscles circumscribes the anal canal and can be voluntarily constricted in order to restrict the process of defecation or the passage of flatus (gaseous by-product of bacterial activity in the intestines) in situations where it is not appropriate. Cranially, fibers of the muscle integrate with the inferior portion of puborectalis. Other fibers cross anteriorly and blend with the superficial transverse perineal muscles, while others attach to the anococcygeal raphe posteriorly. Caudally, the muscle has anteroposterior attachment from the perineal body to the anococcygeal ligament (and by extension, the coccyx).