Friday, April 16, 2021

Human Anatomy:The External Genitilia

A free Educational Medical Resource from Medika Life

Female External Genitalia

The external genitalia are the accessory structures of the female reproductive system that are external to the vagina. They are also referred to as the vulva or pudendum. The external genitalia include the labia majora, mons pubis, labia minora, clitoris, and glands within the vestibule.

The clitoris is an erectile organ, similar to the male penis, that responds to sexual stimulation. Posterior to the clitoris, the urethra, vagina, paraurethral glands and greater vestibular glands open into the vestibule.

The vulva (pudendum) refers to the external female genitalia. Its functions are threefold:

  • Acts as sensory tissue during sexual intercourse
  • Assists in micturition by directing the flow of urine
  • Protects the internal female reproductive tract from infection.

Structures of the Vulva

The vulva is a collective term for several anatomical structures:

  • Mons pubis – a subcutaneous fat pad located anterior to the pubic symphysis. It formed by the fusion of the labia majora.
  • Labia majora – two hair-bearing external skin folds.
    • They extend from the mons pubis posteriorly to the posterior commissure (a depression overlying the perineal body).
    • Embryologically derived from labioscrotal swellings
  • Labia minora – two hairless folds of skin, which lie within the labia majora.
    • They fuse anteriorly to form the hood of the clitoris and extend posteriorly either side of the vaginal opening.
    • They merge posteriorly, creating a fold of skin known as the fourchette.
    • Embryologically derived from urethral folds
  • Vestibule – the area enclosed by the labia minora. It contains the openings of the vagina (external vaginal orifice, vaginal introitus) and urethra.
  • Bartholin’s glands – secrete lubricating mucus from small ducts during sexual arousal. They are located either side of the vaginal orifice.
  • Clitoris – located under the clitoral hood. It is formed of erectile corpora cavernosa tissue, which becomes engorged with blood during sexual stimulation.
    • Embryologically derived from the genital tubercle

Vascular Supply and Lymphatics

The arterial supply to the vulva is from the paired internal and external pudendal arteries (branches of the internal iliac).

Venous drainage is achieved via the pudendal veins, with smaller labial veins contributing as tributaries.

Lymph drains to the nearby superficial inguinal lymph nodes.

Male External Genitalia

The penis is an external organ of the male reproductive system. It has two main functions:

  • Sexual intercourse – During erotic stimulation, the penis undergoes erection, becoming engorged with blood. Following emission, (mixing of the components of semen in the prostatic urethra) ejaculation can occur, whereby semen moves out of the urethra through the external urethral orifice. Finally, the penis undergoes remission, returning to a flaccid state.
  • Micturition –The penis also has an important urinary role. It contains the urethra, which carries urine from the bladder to the external urethral orifice, where it is expelled from the body.

Structure of the Penis

The penis can be anatomically divided into three parts:

  • Root – the most proximal, fixed part of the penis. It is located in the superficial perineal pouch of the pelvic floor, and is not visible externally. The root contains three erectile tissues (two crura and bulb of the penis), and two muscles (ischiocavernosus and bulbospongiosus).
  • Body – the free part of the penis, located between the root and glans. It is suspended from the pubic symphysis. It is composed of three cylinders of erectile tissue – two corpora cavernosa, and the corpus spongiosum.
  • Glans –the most distal part of the of penis. It is conical in shape, and is formed by the distal expansion of the corpus spongiosum. This contains the opening of the urethra, termed the external urethral orifice.

Erectile Tissues

The erectile tissues fill with blood during sexual arousal, producing an erection. The root and body of the penis are spanned by three masses of erectile tissue.

In the root, these tissues are known as the left and right crura, and the bulb of the penis. The bulb is situated in the midline of the penile root, and is traversed by the urethra. The left and right crura are located laterally; attached to the ipsilateral ischial ramus, and covered by the paired ischiocavernosal muscles.

The erectile tissues continue into the body of the penis. The left and right crura continue anteriorly into the dorsal part of the penis – they form the two corpora cavernosa. They are separated by the septum of the penis, although often incompletely. The bulb forms the corpus spongiosum, which lies ventrally. The male urethra runs through the corpus spongiosum – to prevent it becoming occluded during erection the corpus spongiosum fills to a reduced pressure.

Distally, the corpus spongiosum expands to form the glans penis.

Muscles

There are four muscles located in the root of the penis:

  • Bulbospongiosus (x2) – associated with the bulb of the penis. It contracts to empty the spongy urethra of any residual semen and urine. The anterior fibres also aid in maintaining erection by increasing the pressure in the bulb of the penis.
  • Ischiocavernosus (x2) – surrounds the left and right crura of the penis. It contracts to force blood from the cavernous spaces in the crura into the corpus cavernosa – this helps maintain erection.

Fascial Coverings

Each mass of erectile tissue has two fascial coverings. The most superficial layer, immediately under the skin, is the external fascia of Colles (which is in continuity with the fascia of Scarpa which covers the abdominal wall).

A deeper stratum is the deep fascia of the penis (also known as Buck’s fascia). This is a continuation of the deep perineal fascia, and forms a strong membranous covering which holds all three erectile tissues together.

Underneath the deep fascia is the strong fascia called tunica albuginea, forming an individual capsule around each cavernous body and fused in the midline. The incomplete septum between the two corpora is comprised of tunica albuginea.

Ligaments

The root of the penis is supported by two ligaments, which attach it to the surrounding structures:

  • Suspensory ligament – a condensation of deep fascia. It connects the erectile bodies of the penis to the pubic symphysis.
  • Fundiform ligament – a condensation of abdominal subcutaneous tissue. It runs down from the linea alba, surrounding the penis like a sling, and attaching to the pubic symphysis.

Skin

The skin of the penis is more heavily pigmented than that of the rest of the body. It is connected to the underlying fascias by loose connective tissue.

The prepuce (foreskin) is a double layer of skin and fascia, located at the neck of the glans. It covers the glans to a variable extent. The prepuce is connected to the surface of the glans by the frenulum, a median fold of skin on the ventral surface of the penis. The potential space between the glans and prepuce is termed the preputial sac.

Neurovascular Supply

The penis receives arterial supply from three sources:

  • Dorsal arteries of the penis
  • Deep arteries of the penis
  • Bulbourethral artery

These arteries are all branches of the internal pudendal artery. This vessel arises from the anterior division of the internal iliac artery.

Venous blood is drained from the penis by paired veins. The cavernous spaces are drained by the deep dorsal vein of the penis – this empties into the prostatic venous plexus. The superficial dorsal veins drain the superficial structures of the penis, such as the skin and cutaneous tissues.

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