The penis along with the scrotum forms the external male genitalia. It develops from tissue in the urogenital sinus and the same structure becomes the clitoris in females. It is located at the anterior base of the perineum. It forms part of the organs of reproduction in men along with the testes, prostate and bulbourethral gland. The functions of the penis are for the performance of sexual intercourse, a route for ejaculation and micturation. Here we will consider the anatomy of the penis and bulbourethral glands.
In the anatomical position the penis lies erect. It begins in a fixed position in the urogenital triangle and then becomes free anteriorly. It can be divided into the root (fixed) and body (free) and is formed of three distinct structures : The Corpus Cavernosa, the Corpus Spongiosum and the Glans Penis.
The Root of the penis lies within the urogenital triangle and is formed from the crura of each cavernosa and the bulb of the penis which is fixed to the perineal membrane.
The body of the penis is the external part of the penis which is entirely covered with skin.
Suspensory ligament: from pubic symphysis to the base of the penis
Fundiform ligament: from the linea alba and anterior abdominal wall and runs around each side of the penis to rejoin below it.
There are two pairs of muscles at the root of the penis
- Bulbospongiosus – contracts around the bulb of the penis to empty residue present.
- Ischiocavernosus – covers the crura of the penis. It can contract to force blood upward into the distal corpus cavernosa to maintain erections.
· Begins as two “crura” (legs), each of which is attached to the inferior surface of the pubic arch.
· The crura converge at the pubic symphysis where they become part of the body of the penis. They form the dorsal side and main bulk of the body of the penis and erectile tissues.
· Each cavernosa contains an artery (cavernous artery) from which blood flows into each cavernosa during erection.
· The insides are formed of interconnected sinusoids separated by smooth muscle trabeculae.
· The function of the cavernosa is to erect the penis for intercourse.
· Begins as the bulb of the penis. It is anchored to the perineal membrane.
· Forms the ventral side of the penis.
· Contains the bulbar and penile urethra.
· The main action is to maintain patency of the urethra whilst erect and fills to a lesser pressure than the cavernosa.
· Forms the distal end of the penis and is contiguous with the corpus spongiosum.
· Distal to the base it bulges outward to form two “corona”. These converge to form the midline raphe and the urethral meatus.
· The skin which was tightly adherent to the penis now loosely covers the glans as a double fold i.e. the prepuce (foreskin).
Fascial Layers (superficial to deep)
2. Dartos Fascia (superficial penile fascia) – continuous with Scarpa’s Fascia.
3. Bucks Fascia – Continuous with the Suspensory ligament and external spermatic fascia of scrotum.
4. Tunica Albuginea.
Fasical Layers - Silent Diseases Batter Testicles / Skin, Dartos, Bucks, Tunica.
Blood supply and Innervation of the Penis
Arterial supply: Internal pudendal artery via the internal iliac. It separates into two dorsal vessels (deep and superficial) in the cavernosa, and the bulbourethral artery in the spongiosum.
Venous Supply – The Skin is drained by the superficial dorsal vein, whilst the deep veins drains the erectile tissues.
Somatic innervations: Via The dorsal nerve of the penis , a branch of the internal pudendal nerve (S2-4).
Parasympathetic innervations: Via the prostatic plexus. This transmits the changes in vascular tone to create erections
Lymphatics – Via the deep inguinal nodes.
Derived from the urogenital sinus (along with the bladder and prostate) they are also known as Cowper’s Glands and lie within the fibers of the external urethral sphincter. They lie posteriolateral to the membranous urethra just above the bulb of the penis (the root).
The ducts of the bulbourethral glands open up into the spongey urethra just below the membranos urethra.
The bulbourethral glands are lined with columnar epithelium and produce a mucinus substance that acts as a lubricant for sexual arousal. It has a high pH and is also thought to neutralise the acidity of the vaginal mucosa.
Blood supply – Small branches from the internal pudendal artery
Innervation : Via the prostatic plexus (autonomic) – See physiology of erection and ejaculation below
Lymphatic drainage - internal and external iliac lymph nodes.
· Erection is a vascular event which is triggered by parasympathetic nerve fibres from S2-4.
· Cavernous arteries relax allowing blood to fill each cavernosa
· Arteries are branches of the internal pudendal artery
· The caverns are drained by the deep dorsal vein into the prostatic plexus
Erection and ejaculation are two distinct actions . Erection is controlled by the parasympathetic nervous system whilst ejacutation is sympathetically modulated.
Mnemonic – Parasympathetic Points (erection) , Sympathetic Shoots (ejaculation)
· Erectile dysfunction
A Disorder of the partial or complete inability to initiate or maintain an erection. It may be secondary to vascular disease (atherosclerosis) or nervous dysfunction (diabetes, Parkinson’s disease, multiple sclerosis or post prostatectomy ).
Management of erectile dysfunction is complex but initially involves lifestyle modification followed by pharmacological measures. Surgical intervention is not commonly used and only carried out in specialist centres.
· Peyronies Disease – A Fibro-inflammatory condition of the tunica albuginea which leads to abnormal curvature of the penis and consequently, painful erections and difficulty in intercourse.
· Squamous cell carcinoma of the penis - A rare but serious malignancy of the penis, treated by partial or complete penectomy.
· Phimosis– the inability to retract the foreskin over the glans
· Paraphimosis – Inability replace the foreskin back over the glans, can commonly occur iatrogenically after catheterisation but can also occur as a result of phimosis after intercourse or masturbation.
· Balanitis Xerotica Obliterans (BXO) - A chronic sclerotic disease of the foreskin leading to phimosis and later meatal stenosis. Manageed by Circumcision
· Hypospadias - Congenital defect where the urethra enters on the ventral side of the penis instead of the glans.
· Penile fracture – traumatic rupture of the corpus cavernosum. An uncommon but true urological emergency, most cases occur during intercourse.
Via a branch of the internal pudendal artery
Deep and superficial dorsal vein complexes
Branches of S2-4 via the cavernous nerves
Disorders of the penis constitute only a small proportion of casework in clinical practice but they can be highly distressing and/or embarrassing for the patient.