Pelvic Floor, Penis, Urethra & Perineum

Penis – Gross Structure & Surface Landmarks

• Composite organ for

  • Micturition (urethral conduit)

  • Copulation and semen conveyance

• Three main segments

  • Root (fixed to perineum)

  • Body/shaft (mobile, pendulous)

  • Glans (expanded terminal cone)

• Orientations (independent of body position)

  • Dorsum → continuous with anterior abdominal wall

  • Ventral → faces scrotum

• Skin

  • Loosely attached everywhere except glans

  • Firmly adherent over glans; allows free movement elsewhere

Root of the Penis

• Constituents

  • Bulb (midline, attached to perineal membrane → forms posterior corpus spongiosum)

  • Right & left crura (attached to ischiopubic rami → form proximal corpora cavernosa)

• Muscular envelopes

  • Bulbospongiosus covers bulb

  • Ischiocavernosus covers each crus → compresses venous outflow, stabilises erection, aids emptying of urethra

Shaft & Erectile Tissues

• Three cylindrical masses surrounded by Buck’s deep fascia

  1. Corpora cavernosa × $2$ (dorsal)

  2. Corpus spongiosum × $1$ (ventral; contains penile urethra)

• Expansions of corpus spongiosum

  • Posterior → bulb

  • Anterior → glans penis

• Posterior taper of each CC forms a crus

Glans, Corona, Neck & Prepuce

• Glans: terminal conical expansion; external urethral meatus opens at apex

• Corona glandis: circular ridge at base of glans; separated from prepuce by coronal sulcus

• Neck: slight constriction proximal to corona

• Prepuce (foreskin): hood-like skin fold continued from neck; posteroinferior attachment via frenulum

• Clinical note: structure removed in circumcision → prepuce

Fascial & Ligamentous Support

• Superficial fascia → continuous with Scarpa (abdomen), Dartos (scrotum), Colles (perineum)

• Deep (Buck) fascia → continuous with external spermatic, deep perineal fascia; anchored to suspensory lig.

• Tunica albuginea → dense fibrous capsule of each erectile body; thinner over corpus spongiosum

• Suspensory ligament (deep) → pubic symphysis ↔ Buck’s fascia; supports erect penis

• Fundiform ligament (superficial) → thickening of Scarpa; sling from linea alba around penis; superficial to suspensory lig.

Vascular Supply

• Arterial branches of internal pudendal artery (common penile trunk)

  • Artery of bulb (bulbourethral) → corpus spongiosum & urethra

  • Deep (cavernosal) arteries → within corpora cavernosa; give helicine arteries → fill sinusoids during erection

  • Dorsal arteries (paired) → skin, glans, circumflex twigs to tunica albuginea

• Erection mechanism

  • Parasympathetic S$2$–S$4$ → relaxation of trabecular smooth muscle; helicine arteries unwind & dilate

  • Rapid sinusoidal filling → compression of emissary veins against tunica (veno-occlusive)

• Detumescence: sympathetic T$11$–L$1$ mediated contraction of trabeculae + reopening of venous channels

Venous Drainage

• Superficial system → skin & prepuce → superficial dorsal vein → superficial external pudendal → great saphenous

• Intermediate system → deep dorsal + circumflex veins → prostatic venous plexus

• Deep system → crural & cavernous veins → internal pudendal vein

Lymphatic Drainage

• Skin of penis → superficial inguinal nodes (upper medial group)

• Glans penis & distal spongy urethra → deep inguinal ± external iliac nodes

• Cavernous bodies & proximal urethra → internal iliac nodes

Innervation of the Penis

• Sensory: dorsal nerve of penis (branch of pudendal, S$2$–S$4$)

• Parasympathetic: cavernous nerves from pelvic plexus (pre-ganglionic S$2$–S$4$) → erection

• Sympathetic: hypogastric & pelvic plexus (T$11$–L$1$) fibres → detumescence, emission

• Somatic motor: pudendal → ischiocavernosus & bulbospongiosus (augment erection/ejaculatory expulsion)

Functional & Clinical Correlates

• Phimosis

  • Tight non-retractile prepuce (congenital/acquired)

  • Complications: smegma retention, balanitis, hygiene difficulty, carcinoma risk

• Erectile dysfunction (ED)

  • Failure to maintain erection; vascular (hypertension, diabetes, smoking), neurogenic or psychogenic

  • Tx: PDE-5 inhibitors ↑cGMP → smooth muscle relaxation

• Priapism

  • Painful erection >$4$ h without sexual stimulus; low-flow veno-occlusive; emergency to prevent ischaemic fibrosis → permanent ED

Male Urethra

• Total length ≈ $(20\,\text{cm}=8\,\text{in})$; conveys urine & semen

• Segments

  1. Prostatic $\approx 3\,\text{cm}$ (widest, most dilatable)

  • Posterior wall: urethral crest → prostatic sinus (openings of $15–20$ ducts)

  • Mid-crest: prostatic utricle (paramesonephric remnant) flanked by ejaculatory duct orifices

  1. Membranous $\approx 1.25\,\text{cm}$ (shortest, least dilatable)

  • Traverses urogenital diaphragm; surrounded by external sphincter

  1. Penile/spongy $\approx 15.75\,\text{cm}$

  • Within bulb & corpus spongiosum

  • Distal navicular fossa (fossa terminalis) inside glans; external meatus is narrowest point (catheterisation caution)

  • Bulbourethral glands open into proximal spongy segment

Female Urethra

• Length $\approx 4\,\text{cm}$; opens into vestibule anterior to vagina

• Superior ½: numerous urethral gland openings; inferior end pierces perineal membrane; surrounded by external sphincter & sphincter urethrae

• Blood supply: internal pudendal & vaginal arteries (+ venous parallels)

• Clinical link: short course predisposes to ascending UTIs

Pelvic Floor (Pelvic Diaphragm)

• Funnel/bowl of skeletal muscle + fascia separating pelvic cavity from perineum; incomplete anteriorly for urethra (± vagina)

• Components (bilateral)

  • Levator ani: puborectalis, pubococcygeus, iliococcygeus

  • Coccygeus (ischiococcygeus)

  • Investing superior & inferior fascial layers

• Above → pelvic viscera; below → perineum & ischio-anal fossa

Levator Ani – Origins & Insertions

• Origin line (antero-posterior)

  • Posterior surface of pubic bodies

  • Tendinous arch (thickened obturator internus fascia)

  • Ischial spine

• Insertions

  • Puborectalis → sling posterior to anorectal junction; maintains anorectal angle

  • Pubococcygeus → anococcygeal body, coccyx, perineal body; pubovaginalis/puboprostaticus fibres clasp vagina or prostate

  • Iliococcygeus → anococcygeal body & coccyx

• Coccygeus → ischial spine → lateral sacrum & coccyx, overlapping sacrospinous ligament

Actions & Significance

• Principal pelvic floor support; resists increases in intra-abdominal pressure (cough, lift, parturition)

• Sphincteric modulation: assists anal, urethral & vaginal closure (continence)

• Pelvic organ support: weakness → prolapse, incontinence

Innervation

• Levator ani: nerve to levator ani (S$4$) + inferior rectal/perineal branches of pudendal (S$2$–S$4$)

• Coccygeus: anterior rami S$4$–S$5$

Perineum – Definition, Boundaries & Triangles

• Region inferior to pelvic diaphragm, external boundary = perineal skin/fascia; diamond-shaped in lithotomy view

• Bony/ligamentous borders

  • Anterior: pubic symphysis

  • Anterolateral: ischiopubic rami

  • Lateral: ischial tuberosities

  • Posterolateral: sacrotuberous ligaments

  • Posterior: coccyx & inferior sacrum

• Imaginary line between ischial tuberosities divides diamond into

  • Anterior urogenital triangle

  • Posterior anal triangle

• Central point = perineal body (fibromuscular node anchoring $8$–$10$ muscles)

Anal Triangle

• Contains anal canal, external anal sphincter & ischio-anal fossae; traversed by pudendal nerve & internal pudendal vessels

Urogenital Diaphragm & Pouches

• Urogenital diaphragm = sphincter urethrae + deep transverse perineal muscle sandwiched between superior & inferior fasciae

• Inferior fascial layer = perineal membrane (key landmark, green in diagrams)

• Deep perineal pouch (above membrane)

  • Male: membranous urethra, sphincter urethrae, bulbourethral glands, deep transverse perineal muscle, internal pudendal vessels branches, dorsal nerves of penis

  • Female: urethra segment, vagina part, sphincter urethrae, deep transverse perineal muscle, vessels + dorsal nerve of clitoris

• Superficial perineal pouch (below membrane, above Colles fascia)

  • Male: root of penis (bulb + crura), bulbospongiosus, ischiocavernosus, superficial transverse perineal muscles, branches of pudendal nerve & internal pudendal artery

  • Female: root of clitoris, bulbs of vestibule, bulbospongiosus, ischiocavernosus, superficial transverse perineal muscles, greater vestibular (Bartholin) glands, neurovascular branches

Superficial Fascia Variants

• Fatty layer (Camper) continuous with ischio-anal fat & thigh superficial fascia; replaced by dartos muscle in scrotum

• Membranous layer (Colles) continuous with Scarpa anteriorly; forms tubular sheath around penis/clitoris; limits spread of extravasated urine

Perineal Musculature – Overview

• Superficial pouch: ischiocavernosus, bulbospongiosus, superficial transverse perineal

• External anal sphincter encircles anal canal; attaches anteriorly to perineal body, posteriorly to anococcygeal ligament (fusion of levator ani & sphincter fibres)

Ligamentous & Fascial Mid-line Structures

• Anococcygeal ligament/body: midline raphe connecting levator ani to coccyx; insertion for EAS, site of muscular interdigitation

• Perineal body: fixation for levator ani, bulbospongiosus, superficial & deep transverse perineal, external anal sphincter

Applied Pelvic Floor Function

• Continence: tonic levator ani maintains anorectal angle; voluntary contraction augments urethral & anal closure

• Obstetrics: stretching during vaginal delivery; episiotomy (perineotomy) may be performed to control perineal tears

• Pelvic organ prolapse & urinary incontinence: rehabilitative Kegel exercises strengthen pelvic diaphragm; surgical meshes anchored to levator ani

Self-Assessment Focus (Condensed)

• Which penile structure excised in circumcision? → Prepuce

• Arterial triad to penis? → bulbourethral, deep (cavernosal), dorsal; all from internal pudendal

• Venous drainage pathways (superficial ↔ saphenous; deep ↔ prostatic plexus; crural ↔ int. pudendal)

• Define phimosis & potential sequelae

• Contrast male vs female urethra (length, parts, curvature, glandular relations, clinical implications)

• Enumerate urethral segments + calibre differences (least dilatable = membranous, danger during catheterisation)

• List muscles of levator ani; differentiate from coccygeus

• Recall perineal boundaries & triangles

• Name muscles constituting urogenital diaphragm

Integrative Connections & Real-World Relevance

• Continence surgeries (e.g., mid-urethral slings) anchor to pelvic diaphragm

• Penile arterial disease mirrors cardiovascular risk: penis as ‘vascular barometer’ for systemic atherosclerosis

• Priapism management teaches microvascular physiology & compartment syndrome principles

• Obstetric injuries to perineal body may disrupt pelvic floor → postpartum incontinence/prolapse