The Penis is a male organ that is used for reproduction and urination which is located within the urogenital triangle between the perineal membrane superiorly and deep perineal fascia inferiorly. Its composition divides it into 3 parts: the glans, body, and root. Dorsally, it is composed of the paired corpora cavernosa and ventrally the corpus spongiosum which contains the urethra. The anatomical positions of the penis are according to the erect penis with the dorsal direction being closer to the head and ventral direction being closer to the feet. The tunica albuginea is a fibrous sheet that encapsulates the cavernous body. Buck’s fascia, also known as the deep fascia of the penis is superficial to the tunica albuginea and is the continuation of the deep perineal fascia which covers the corpora cavernosa and corpus spongiosum together. The spongy urethra is contained within the corpus spongiosum. The crura of the penis are between the 2 corpora cavernosa in the median plane externally. The corpora cavernosa are separated by the septum of the penis internally. The attachment of the penis consists of the bulb, crura, and ischiocavernosus, and bulbospongiosus muscles, which collectively make up the root of the penis. The bulb of the penis and crura contain erectile tissue. The middle portion of the penis is suspended and known as the body of the penis. The body of the penis lacks muscles and consists mostly of corpora cavernosa, corpus spongiosum, and spongy urethra with supporting skin, connective tissue, and blood, lymph, and fascia. The distal part of the penis is known as the glans penis. The proximal portion of the glans is termed the corona. The neck of the corona glans separates it from the body of the penis. The tip of the glans contains the opening to the urethra, known as the meatus. There is the loose connective tissue between the thin skin of the penis and the tunica albuginea. The prepuce or foreskin is the second skin layer that covers the glans penis. The frenulum of the prepuce connects it to the urethral surface of the penis glans. The suspensory ligament of the penis is part of the deep fascia that arises from the anterior surface of the pubic symphysis. The fundiform ligament of the penis descends in the midline from the linea alba anterior to the pubic symphysis and splits to surround the penis and then unites and blends inferiorly with the dartos fascia forming the scrotal septum.[rx][rx][rx][rx]
Structure and Function of The Penis
The 2 main functions of the penis are intercourse and urination. For intercourse, the penis must be erect. Erection occurs via parasympathetic innervation which allows engorgement of the corporal bodies. The bulbospongiosus and ischiocavernosus muscle compress the veins to prevent them from draining blood from the corpora cavernosa. The erectile tissue of the cavernosum becomes firm and enlarged when the erection occurs. The emission phase is due to the sympathetic response and occurs when semen exits the vas deferens to the ejaculatory ducts where fluid from the seminal glands are added. They then continue onto the prostatic urethra where the prostatic fluid is added. The ejaculation phase is due to the sympathetic response and is when semen exits from the urethra through the external urethral orifice. After ejaculation, the penis becomes flaccid again due to the sympathetic stimulation and constriction of the coiled helicine arteries and relaxation of the bulbospongiosus and ischiocavernosus muscles allowing blood to drain from the cavernous spaces.[rx][rx][rx]
Blood Supply and Lymphatics of The Penis
The branches of the internal pudendal arteries are the main supplier of blood to the penis. The dorsal arteries of the penis run along the dorsal vein between the corpora cavernosa to supply the fibrous tissue surrounding the corpora cavernosa, corpus spongiosum, spongy urethra, and penile skin. The deep arteries of the penis run in the center of each corpora cavernosa to supply the erectile tissue of the penis. The helicine arteries of the penis are branches of the deep arteries of the penis which are coiled when the penis is flaccid. The arteries of the bulb of the penis supply the bulbous part of the corpus spongiosum, urethra, and bulbourethral gland. The superficial and deep branches of the external pudendal arteries supply the penile skin.
The deep dorsal vein of the penis receives blood from the venous plexus which receives drained blood from the cavernous spaces. The superficial dorsal vein drains blood from the skin and subcutaneous tissue of the penis.
The skin of the penis and all of the perineum excluding the glans penis drain lymph to the superficial inguinal nodes. The lymphatics of the intermediate and proximal parts of the urethra and cavernous bodies drain into the internal iliac lymph nodes. The distal spongy urethra and glans penis drain to the deep inguinal nodes.
Nerves Supply of The Penis
The penis innervation is from spinal cord segments of the S2 to S4. These cord segments pass through the pelvic splanchnic nerves. The spinal ganglia of S2 to S4 passes through the pudendal nerves. The dorsal nerve provides sympathetic plus sensory innervation to the penis. The pudendal nerve branches to become the dorsal nerve of the penis. The nerve runs next to the dorsal artery and supplies the skin and glans penis also. The glans penis has a copious amount of sensory nerve endings. The root of the penis has innervation from the ilioinguinal nerve. The cavernous nerves have parasympathetic fibers that innervate the helicine arteries which supply erectile tissue.
Muscles of Supply of The Penis
The bulbospongiosus muscles cross the pelvic outlet intersecting with the superficial transverse perineal muscles. These structures support the perineal body and pelvic viscera. The bulbospongiosus muscles compress the bulb of the penis and the corpus spongiosum which can aid in emptying the spongy urethra. The anterior fibers of the muscle surround the proximal part of the body of the penis. These fibers can increase the pressure on the erectile tissue in the root of the penis for erections. The bulbospongiosus muscle can compress the deep dorsal veins to prevent drainage of the cavernous spaces and help promote erection of the penis. The root of the penis contains the crura which is encircled by the ischiocavernosus muscle. The muscle can push blood to the distal parts of the corpora cavernosa and help with erections. The muscle can also lead to compression of the deep dorsal vein of the penis as it is leaving the crus of the penis and restrict venous outflow to help with erections.
Functions of The Penis
The other function of the penis is to drain urine from the body. Urine starts draining from the bladder into the bladder neck where the internal urethral sphincter will relax to allow urine to pass via involuntary control of the parasympathetic system. Urine will then pass into the prostatic urethra and the membranous urethra. The external sphincter will relax to allow urine to pass from the membranous urethra into the spongy penile urethra, then into the navicular fossa and out of the urethra through the external urethral orifice.
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