Anatomy is so much easier when we compartmentalize things! For this reason, the perineum, can be divided into two triangles, the anterior urogenital triangle and the posterior anal triangle.
The urogenital triangle can be further divided into a superficial pouch and a deep pouch by a layer of deep fascia called the perineal membrane. In females, the urogenital triangle houses the female external genitalia and the perineal muscles.
Let’s start with talking about the female external genitalia, also called the vulva, which includes the mons pubis, the paired labia majora, labia minora, and the clitoris. As well as deeper structures - the bulbs of the vestibule, the greater vestibular glands, and the lesser vestibular glands.
The mons pubis is a rounded, fatty eminence that lies anterior to the pubic symphysis, pubic tubercles, and superior pubic rami. This eminence has a coarse pubic hair that grows after puberty.
Now, from the mons pubis, two folds of skin called the labia majora arise and extend posteriorly towards the anus. These labia meet anteriorly to form the anterior commissure, and posteriorly to form the posterior commissure.
Also, the labia majora is the termination point of the round ligament of the uterus which is the embryonic remnant of the gubernaculum in females.
All right, now the labia majora lie on each side of a central depression called the pudendal cleft, which houses the labia minora and the vestibule of the vagina.
The labia minora are paired hairless folds of skin that contain spongy connective tissue, erectile tissue, and many blood vessels. These labia unite posteriorly to form a fold of skin called the frenulum of the labia minora or the fourchette.
Anteriorly, each labium divides into medial and lateral folds. The medial folds unite below the clitoris, forming a pointy extension called the frenulum of the clitoris. The lateral folds unite to form a fold of skin called the prepuce or foreskin of the clitoris. This fold covers the clitoris which is the main female erectile organ and counterpart to the penis in males.
The clitoris has three parts, called the root, the body, and the glans clitoris. The root of the clitoris consists of two crura, that attach to the inferior pubic rami and the perineal membrane deep to the labia. These crura consist of erectile tissue called the corpora cavernosa, which extends into the body at the angle of the clitoris.
Up to this point, the clitoris seems pretty similar to the penis, right? Well, the glans clitoris, unlike the glans penis, doesn’t have an external urethral opening. This means that the clitoris doesn’t have a urinary function and functions solely as an organ of sexual arousal. The glans clitoris is the most highly innervated portion of the clitoris and the only portion that can be seen externally.
All right, now the labia minora surrounds an area called the vestibule of the vagina where the orifices of the urethra and vagina and the ducts of the greater vestibular glands open. The urethra opens through the external urethral orifice, which lies posterior to the clitoris and anterior to the vaginal orifice.
Lateral and deep to the vaginal orifice are the bulbs of the vestibule, which are paired, elongated masses of erectile tissue. A comparable structure in males is the bulb of the penis. These bulbs are covered inferiorly and laterally by the bulbospongiosus muscles which we will discuss further in just a bit.
Next are the greater vestibular glands, which are paired, round glands that lie posterolateral to the vaginal orifice. These glands are also partially covered by the bulbospongiosus muscles. During sexual arousal, they secrete mucus into tiny ducts that open on each side of the vaginal orifice.
Okay, now let’s look at the blood supply and innervation of the vulva. Arterial blood supply of the vulva comes from the external and the internal pudendal arteries. These branches include the anterior labial arteries, which arise from the external pudendal artery, to supply the anterior part of the labia majora.
Next are the posterior labial arteries, which arise from the perineal artery, which is a branch of the internal pudendal artery. The posterior labial arteries supply the labia minora and the posterior part of the labia majora.
Next are the labial veins which drain the vulva. These veins then drain into the internal pudendal veins, which drain into the internal iliac veins.
Finally, let’s talk about the sensory innervation of the vulva. The mons pubis and the anterior part of the labia majora are innervated by the anterior labial nerves, which arise from the ilioinguinal nerve. The anterior labia majora are also innervated by the genital branch of the genitofemoral nerve.
The posterior part of the labia majora and the labia minora are innervated by the posterior labial nerves, which arise from the superficial perineal nerve, which, in turn, is a branch of the pudendal nerve.
All right, now the blood supply and innervation of the clitoris is a bit special. The arterial blood supply comes mainly from branches of the internal pudendal arteries. These branches include the deep artery of the clitoris, which supplies the corpora cavernosa of the clitoris, and the dorsal artery of the clitoris, which runs on the dorsum of the clitoris to supply the glans clitoris.