Structure
The penis has a fixed root in the perineum and a free body, both built from three erectile bodies bound together by fascia.
Root
The root lies in the superficial perineal pouch and has three parts:
- Two crura: the tapered back ends of the corpora cavernosa, each attached to the ischiopubic ramus and covered by ischiocavernosus.
- Bulb of the penis: the expanded back end of the corpus spongiosum, attached to the perineal membrane, covered by bulbospongiosus and entered by the urethra from above.
Body
In the body, the two corpora cavernosa lie side by side on the dorsal surface, separated by an incomplete septum. Each is wrapped in a thick fibrous tunica albuginea. The corpus spongiosum lies in the groove beneath them and carries the spongy urethra. It has a thinner tunica, so it stays compressible during erection and semen can pass. Distally it expands into the glans penis, whose raised rim is the corona and whose tip carries the external urethral orifice. The prepuce (foreskin) is a fold of skin covering the glans, tethered on the underside by the frenulum.
Fascia and ligaments
The deep fascia of the penis (Buck's fascia) encloses all three corpora together with the deep dorsal vein and the dorsal arteries and nerves. Superficial to it lies loose, fat-free superficial fascia, continuous with the membranous layer of the perineum (Colles' fascia) and of the abdominal wall (Scarpa's fascia). The suspensory ligament runs from the pubic symphysis to the deep fascia; the more superficial fundiform ligament descends from the linea alba.
Blood supply and lymphatics
The erectile tissue and glans are supplied by branches of the internal pudendal artery, while the skin is supplied by the external pudendal arteries from the femoral artery.
| Artery | Course | Supplies |
|---|---|---|
| Artery of the bulb | Pierces the perineal membrane | Bulb and corpus spongiosum, bulbourethral gland |
| Deep artery of the penis | Runs in the centre of each corpus cavernosum | Erectile tissue, via the coiled helicine arteries |
| Dorsal artery of the penis | Runs on the dorsum deep to Buck's fascia | Glans, fascia and skin |
On the dorsum, the structures deep to Buck's fascia lie in order from the midline outwards: the single deep dorsal vein, then a dorsal artery and a dorsal nerve on each side. The deep dorsal vein passes under the pubic symphysis into the prostatic venous plexus. The superficial dorsal vein, above Buck's fascia, drains to the superficial external pudendal vein.
Lymph from the skin drains to the superficial inguinal nodes; from the glans to the deep inguinal and external iliac nodes; and from the erectile tissue and spongy urethra to the internal iliac nodes.
Innervation and erection
Sensation comes through the pudendal nerve, while erection and ejaculation are controlled by autonomic nerves from the pelvic plexus.
The dorsal nerve of the penis, the terminal branch of the pudendal nerve (S2 to S4), supplies the skin of the shaft and the richly sensitive glans. The ilioinguinal nerve supplies skin at the root.
The cavernous nerves leave the inferior hypogastric (pelvic) plexus and run along the posterolateral aspect of the prostate before piercing the perineal membrane.
- Erection: parasympathetic fibres from the pelvic splanchnic nerves (S2 to S4) relax the smooth muscle of the helicine arteries and erectile tissue. The spaces fill with blood, and compression of the draining veins against the tunica albuginea traps it.
- Emission: sympathetic fibres (L1, L2) contract the ductus deferens, seminal glands and prostate and close the bladder neck.
- Ejaculation: the pudendal nerve drives rhythmic contraction of bulbospongiosus to expel semen.
Clinical relevance
The fascial layers and neurovascular arrangement determine how penile injuries present and how surgery and anaesthesia are performed.
- Penile fracture: rupture of the tunica albuginea of a corpus cavernosum during erection. Bruising confined within Buck's fascia stays on the shaft; if Buck's fascia tears, blood spreads under Colles' fascia into the scrotum and perineum in a butterfly pattern.
- Rupture of the bulbar urethra: urine extravasates deep to the membranous fascia into the scrotum, penis and lower anterior abdominal wall, but not the thigh, where the fascia fuses with fascia lata.
- Dorsal penile nerve block: local anaesthetic is placed deep to Buck's fascia at the base on each side of the midline. Adrenaline is avoided.
- Erectile dysfunction after radical prostatectomy: the cavernous nerves in the posterolateral neurovascular bundles are at risk; nerve-sparing surgery aims to preserve them.
- Priapism: a prolonged painful erection, usually from failure of venous outflow, which risks ischaemic damage.
- Hypospadias: the urethral opening lies on the underside of the penis from incomplete fusion of the urethral folds.