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The Penis

The penis is a tri-compartment erectile organ whose structural integrity (tunica albuginea), vascular inflow (cavernosal arteries), veno-occlusive mechanism (subtunical plexus compressed against the tunica), and neural input (cavernous and dorsal nerves) must all function for normal erection, micturition, and ejaculation.[1][2][19] For the reconstructive urologist, every one of those substrates is an operative target or pitfall: Peyronie's plaque surgery hinges on tunica mechanics, penile prosthesis on tunica and corporal capacity, post-prostatectomy ED on cavernous-nerve preservation, priapism management on corporal physiology, and urethral reconstruction on Buck's-layer dissection planes. This article focuses on the gross, vascular, and neural anatomy that drive those operations, and on erectile physiology only to the depth required to reason about ED, priapism, and nerve-sparing surgery.

Gross Anatomy and Fascial Layers​

The penis is composed of three erectile bodies: the paired corpora cavernosa (the rigidity engines) and the single midline corpus spongiosum (which surrounds the urethra and expands distally into the glans). Proximal attachments include the crura of the corpora cavernosa to the ischiopubic rami, the bulb of the corpus spongiosum to the perineal membrane, and the ischiocavernosus and bulbospongiosus muscles over each.[5]

External layers (superficial to deep)​

LayerCompositionOperative relevance
SkinThin, mobile, hairless on the shaftThe mobile skin tube is the plane of degloving for circumcision, hypospadias repair, Peyronie's curvature repair, and skin-substitution urethroplasty
Dartos fasciaLoose subcutaneous tissue with smooth-muscle fibers and axial-pattern vessels from the external pudendal artery[4][6]The vascular plane for axial and island skin flaps (Orandi, Quartey, McAninch, Blandy) in substitution urethroplasty
Buck's fasciaDense fibroelastic envelope encircling all three erectile bodies; splits into two leaves ventrally around the corpus spongiosum[5][6][7]Carries the dorsal neurovascular bundle (dorsal artery, dorsal nerves, deep dorsal vein). Dissection under Buck's is the Peyronie's / curvature plane; dissection above Buck's is the circumcision / degloving plane
Tunica albugineaBilayered collagen jacket around each corpus cavernosum[8][9]The rigidity-bearing structure; the plane of corporotomy for penile prosthesis and of plication/incision for Peyronie's correction

The tunica albuginea — the key structural layer​

The tunica is bilayered:[8][9]

  • Inner circular layer — a complete 360° lamina with radiating intracavernous pillars (struts) that augment the septum and confer internal structural support.
  • Outer longitudinal layer — incomplete, ~300° coat; absent ventrally between the 5- and 7-o'clock positions where the corpus spongiosum and urethra lie. This ventral gap is why Peyronie's plaques, when they contract, preferentially bend the penis dorsally — the dorsum has the full bilayer, with less capacity to shorten than the mechanically underbuilt ventrum.

Mechanical properties — collagen-dominant (type I ~95%) with elastic fibers; highly anisotropic (stiffer and stronger longitudinally than circumferentially).[10] The tunica is thicker dorsally than ventrally,[11] which is the structural basis of the ventral-plication principle (Nesbit / 16-dot) — removing tissue from the convex side balances a dorsal plaque's short side to straighten the shaft.

Why dissection planes matter

Three parallel dissection planes drive most open penile surgery:

  1. Above Buck's (subcutaneous / dartos) — degloving for circumcision, phalloplasty skin mobilization, hypospadias.
  2. Under Buck's, on the tunica — Peyronie's plication / incision, curvature correction, venous ligation.
  3. Into the tunica (corporotomy) — penile prosthesis insertion, evacuation of ischemic priapism, Peyronie's plaque excision with graft. Mis-identification of the plane is the mechanism of most iatrogenic dorsal-nerve injury and most implant malpositioning.

Ligamentous support​

The penis is suspended by two ligaments relevant to reconstructive surgery:

  • Fundiform ligament — superficial, from the linea alba to the penile dartos.
  • Suspensory ligament — deeper triangular condensation from the pubic symphysis to the dorsal tunica at the penopubic junction. Partial division is the anatomic step of lengthening procedures; complete division destabilizes the penis and is generally avoided.

Erectile Tissue and Corporal Architecture​

The corpora cavernosa contain a three-dimensional network of endothelium-lined sinusoidal spaces separated by bundles of trabecular smooth muscle that insert at multiple points into the fibrous skeleton.[5][12] The two cavernosa communicate through the midline septum, which is fenestrated proximally and complete distally — a fact exploited by the corporal cross-over technique for unilateral dilator impasse during prosthesis insertion, and by the clinical presentation of unilateral stuttering priapism with bilateral rigidity. The corpus spongiosum has the same sinusoidal architecture but thinner walls and larger sinuses, so it remains compressible during erection and does not occlude the urethra.

The subtunical venular plexus lies between the sinusoids and the tunica albuginea and is the mechanical substrate of veno-occlusion.[14]

Vascular Anatomy​

Arterial inflow​

The dominant supply is from the internal pudendal artery (a branch of the anterior division of the internal iliac). As it enters Alcock's canal it becomes the common penile artery, which divides into three terminal branches on each side:[3][5][13]

ArteryCourseTerritory
Cavernosal (deep) arteryEnters the corpus cavernosum at the hilum; gives off helicine arteries that open into the sinusoidsThe engine of erection — fills the corpus cavernosum
Dorsal penile arteryRuns under Buck's fascia on the dorsum, bracketed by the paired dorsal nerves and the deep dorsal veinSupplies skin, Buck's, and contributes to glans engorgement
Bulbourethral arteryPerineal; enters the bulb of the corpus spongiosumSupplies the corpus spongiosum, urethra, and glans

The three paired vessels have extensive anastomoses, which is the anatomic basis of glans survival after urethroplasty even when the urethral artery is sacrificed, and of partial-penectomy flap design.[5] The external pudendal arteries (from the femoral system) supply penile skin and dartos and provide the axial vessels for distal-based skin flaps.[4][6]

Accessory internal pudendal arteries (seen in ~10–15% of men) run above the pelvic diaphragm and can be dominant suppliers — their inadvertent sacrifice during robotic prostatectomy is an under-recognized cause of post-operative arteriogenic ED.

Venous drainage​

Three drainage systems in parallel:[5][8][14]

  • Superficial — superficial dorsal veins in the dartos, drain to the external pudendal / saphenous system.
  • Intermediate — deep dorsal vein in the midline beneath Buck's fascia, fed by circumflex veins from the spongiosum and emissary veins piercing the tunica from the cavernosa.
  • Deep / proximal — cavernosal and crural veins drain the proximal corpora into the internal pudendal venous system.

The subtunical venular plexus interposed between the sinusoids and the tunica is the hinge of the veno-occlusive mechanism (see below). Venous-ligation / dorsal-vein procedures for venogenic ED target the deep dorsal and circumflex drainage; long-term outcomes are poor, so the operation has largely been abandoned.

Innervation​

Penile innervation follows three systems, arriving in two functional compartments:[5][13][15][16][17][18]

SystemOriginNerveDestinationFunction
ParasympatheticS2–S4Pelvic → pelvic plexus → cavernous nervesHelicine arteries and trabecular smooth muscleErection (NO release)
SympatheticT10–L2Hypogastric → pelvic plexus → cavernous nervesTrabecular smooth muscleDetumescence and ejaculation (α-adrenergic)
SomaticS2–S4Pudendal → dorsal nerves of the penis (sensory) and perineal branches (sensory and motor)Penile skin and glans; ischiocavernosus and bulbospongiosusSensation and emission

The cavernous nerves — the clinically critical bundle​

The cavernous nerves are the largest branches of the inferior hypogastric (pelvic) plexus. They run along the posterolateral aspect of the seminal vesicles and prostate, cross the urogenital diaphragm with the membranous urethra, and ascend to the 1- and 11-o'clock positions on the bulbous urethra before piercing the tunica albuginea alongside the cavernosal artery.[13] They terminate on helicine arterioles and trabecular smooth muscle.

Two clinically critical implications:

  1. Nerve-sparing radical prostatectomy. The cavernous nerves run along the posterolateral edge of the prostate, close to the prostatic capsule. Athermal, meticulous dissection preserves them; cautery, wide resection, or bleeding-control at the apex damages them. Bilateral nerve-sparing drives post-operative erectile-function recovery rates from near-zero toward 60–85% at one year.[26]
  2. Dual route to the corpora. Cavernous nerves reach the urethra and corpora along both a dorsal (along the seminal-vesicle / ejaculatory-duct axis) and a lateral (along the inferior vesical artery, forming the NVB) pathway.[17] Preserving only the lateral NVB does not preserve full erectile innervation — a reason why unilateral nerve-sparing alone produces variable outcomes.

The dorsal nerves — anatomy of the DPN block​

The dorsal nerves travel within Buck's fascia paired with the dorsal arteries and deep dorsal vein. They are notably absent at the 12-o'clock position — the anatomic basis of the dorsal penile nerve block: infiltrating at the 10- and 2-o'clock positions deposits anesthetic precisely where the nerves are, while the midline is avoided to reduce the risk of intravascular (dorsal-vein) injection.[5]

Neurochemical organization​

Proximally, the intracavernous compartment is predominantly autonomic (nitrergic); distally, both extra- and intracavernous compartments are predominantly somatic, with intrapenile anastomoses carrying mixed fibers.[18] nNOS-positive cavernous fibers join the dorsal nerve at the penile hilum — the reason priapism can coexist with preserved somatic sensation and the reason dorsal-nerve block does not abolish reflex erection.[16]

Physiology of Erection​

Erection is a neurovascular event coordinating neural input, smooth-muscle relaxation, and a mechanical veno-occlusive switch.[1][2][19][22][23][24]

Flaccid state​

Tonic α-adrenergic sympathetic output contracts trabecular smooth muscle; cavernosal arteries are constricted, sinusoids are collapsed, and intracavernosal PₒO₂ is low (~35 mmHg).

Tumescence​

Sexual stimulus activates pelvic-nerve parasympathetic output and withdraws sympathetic tone:

  1. NO release from nonadrenergic-noncholinergic (NANC) nerve terminals (neurogenic NO, from nNOS) and endothelium (endothelial NO, from eNOS).
  2. sGC activation → ↑ cGMP in trabecular smooth muscle.
  3. ↓ cytosolic Ca²⁺ → smooth-muscle relaxation of helicine arterioles and trabeculae.
  4. Sinusoids engorge with high-flow arterial blood; the corpora expand against the tunica albuginea.

Neurogenic NO initiates erection; endothelial NO sustains it.[20]

Veno-occlusion — the mechanical switch​

As the sinusoids balloon, the subtunical venular plexus is compressed between the expanded sinusoids and the noncompliant tunica albuginea, shutting off emissary venous outflow. Intracavernosal pressure rises toward, and briefly above, systolic blood pressure; intracavernosal PₒO₂ approaches arterial values.[14][19]

Rigid-erection phase​

Reflex contraction of the ischiocavernosus (and, transiently, bulbospongiosus) muscles at the crura drives intracavernosal pressure above systolic pressure, producing maximal axial rigidity during penetrative activity.

Detumescence​

Sympathetic activation (orgasm, ejaculation, or loss of stimulus) releases norepinephrine, constricting arterioles and contracting trabeculae. PDE5 hydrolyzes cGMP, terminating smooth-muscle relaxation. The subtunical venous channels reopen and venous drainage resumes in three phases (initial pressure rise, slow decline, rapid decline).[19][21][25] PDE5 inhibitors act by potentiating this pathway — they do not independently trigger erection; sexual stimulation and intact neurogenic / endothelial NO release are still required.

Clinical Correlations for the Reconstructive Urologist​

  • Erectile dysfunction mechanisms — treatment drivers. Clinically useful ED phenotyping distinguishes arteriogenic (atherosclerotic inflow disease — often co-present with CAD), venogenic (venous leak) (inadequate veno-occlusion from tunical failure, large venous channels, or Peyronie's), neurogenic (post-prostatectomy, diabetes, SCI), smooth-muscle / endothelial (age-related or systemic vascular disease), and psychogenic. PDE5i efficacy decreases in venogenic and severe neurogenic ED; intracavernosal injection, VED, and penile prosthesis populate the escalation ladder.
  • Peyronie's disease. Focal tunica-albuginea fibrosis that bends the penis toward the plaque. Treatment choice depends on curvature direction, magnitude, erectile function, and disease stability:
    • Plication (Nesbit excision, 16-dot, or tunica plication) shortens the convex side — simple, preserves erectile function, but shortens the penis.
    • Incision/excision with grafting preserves length at the cost of higher ED risk — reserved for curvature >60°, hourglass deformity, or compromised erection.
    • Penile prosthesis with concurrent straightening (modeling, incision ± graft) — for Peyronie's with refractory ED.
  • Penile prosthesis. Corporotomies at the 2- and 10-o'clock position on each side spare the dorsal neurovascular bundle and the ventral urethra / spongiosum (5- to 7-o'clock). Crural perforation is the most dangerous error during dilation; cross-over at the fenestrated proximal septum is the salvage for unilateral impasse.
  • Priapism — ischemic vs nonischemic.
    • Ischemic (low-flow) — trabecular venous stasis; dark aspirate with blood-gas showing low PₒO₂, high pCO₂, acidosis. A compartment-syndrome emergency. Sequence: aspiration → intracavernosal phenylephrine → distal corporoglanular shunt (Al-Ghorab / T-shunt / Ebbehoj) → proximal shunt (Quackels) → immediate-placement penile prosthesis if >24 h.
    • Nonischemic (high-flow) — arterial-cavernosal fistula, typically post-traumatic; arterial-blood-gas aspirate. Can often be observed or selectively embolized.
  • Post-prostatectomy ED. Mediated by cavernous-nerve injury (thermal, traction, sacrifice) and by arterial disruption of accessory internal pudendal arteries. Recovery depends on bilateral nerve sparing, patient age, and baseline function. Early penile rehabilitation with PDE5i is typical, although evidence for long-term benefit is mixed.
  • Dorsal penile nerve block. Infiltrate at 10- and 2-o'clock under Buck's fascia; avoid the midline to reduce intravascular injection. An adjunct for circumcision, frenuloplasty, hypospadias, post-operative analgesia, and neonatal procedures.
  • Penile trauma — fracture of the penis. Forcible bending of an erect penis ruptures the tunica albuginea (typically a transverse laceration of the ventrolateral tunica) and presents as an immediate popping sound, rapid detumescence, swelling, and the characteristic "eggplant deformity." Concomitant urethral injury (~10–20%) is signaled by blood at the meatus, hematuria, or inability to void. Emergent surgical repair with tunical closure — delayed repair has worse ED and curvature outcomes.
  • Urethral reconstruction and flap design. The axial dartos vessels (off the external pudendal) supply skin flaps for substitution urethroplasty; the subcoronal and penile shaft skin are reliable donor territories. Harvest must preserve the dorsal neurovascular bundle and the Buck's-fascia plane.
  • Hypospadias and epispadias. Corporal and glandular asymmetry, urethral-plate location, and tunical thickness drive operation choice (TIP / Snodgrass, Koyanagi, Duckett onlay); Buck's-fascia-plane dissection and axial skin-flap design govern reliable coverage.
  • Lymphatic drainage. Penile skin and prepuce → superficial inguinal nodes; glans and corpora → deep inguinal and external iliac nodes. This split dictates the inguinal-dissection template in penile squamous-cell carcinoma.

Videos​

Penile anatomy — lecture 1
Penile anatomy — lecture 2

References​

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