Paired triangular space on lateral neck, behind SCM. Divided by inferior belly of omohyoid into occipital triangle (above, larger) + subclavian triangle (below, smaller).

Boundaries

Floor Muscles (Superficial → Deep)

Occipital Triangle (Above Omohyoid)

Nerves:

• Accessory nerve (CN XI) — crosses obliquely from Erb’s point to deep surface of trapezius
• Cutaneous cervical plexus branches — all emerge at Erb’s point (junction of upper 1/3 + lower 2/3 of posterior border of SCM)

Vessels: occipital artery (apex, briefly); transverse cervical artery (lower); external jugular vein (crosses SCM in superficial fascia)

Lymph nodes: occipital nodes (apex, drain back of scalp); lymphadenopathy here = scalp infections, rubella

Subclavian Triangle (Below Omohyoid)

Arteries:

Veins:

Nerves:

Arrangement at base (medial → lateral): scalenus anterior → phrenic nerve (on ant. surface of scalenus ant.) → subclavian vein → subclavian artery → brachial plexus trunks (between scalenes)

Lymphatics: supraclavicular nodes along transverse cervical vessels; Left supraclavicular = Virchow’s node (drains via thoracic duct — metastasis from abdominal malignancy = Troisier’s sign)

Cervical Plexus (C1–C4)

• Ventral rami C1–C4; deep to IJV + SCM; on scalenus medius + levator scapulae

Deep (motor) branches:

Mnemonic: “C3, 4, 5 keep the diaphragm alive”

Phrenic nerve: descends on scalenus anterior (under prevertebral fascia) → behind subclavian vein → thorax; injury = ipsilateral hemidiaphragm paralysis; referred pain = shoulder tip (C4 dermatome); palsy causes: lung carcinoma, mediastinal masses, aortic aneurysm; hiccup = phrenic irritation

Ansa cervicalis: superior root = C1 fibres with CN XII; inferior root = C2+C3; lies superficial to carotid sheath; supplies all infrahyoid strap muscles except thyrohyoid (C1 via hypoglossal nerve directly)

Accessory Nerve (CN XI) in Detail

Winging of scapula:

External Jugular Vein

• Formation: posterior auricular vein + posterior division of retromandibular vein (behind auricle)
• Crosses SCM in superficial fascia → pierces investing fascia → drains into subclavian vein
• Distension at 45° above clavicle = raised JVP; used for emergency IV access

Applied Anatomy

Accessory nerve injury — most commonly injured nerve in posterior triangle (runs superficially)

• Causes: lymph node biopsy, radical neck dissection, penetrating trauma
• Effects: trapezius paralysis → shoulder drooping, lateral winging of inferior angle, inability to abduct arm >90°; SCM paralysis → weakness of head rotation to opposite side

Cervical lymphadenopathy — posterior triangle nodes drain thoracic + abdominal viscera via thoracic duct; Virchow’s node (left supraclavicular) = stomach, colon, lung, breast malignancy; also TB, lymphoma, EBV

Brachial plexus:

• Supraclavicular block — trunks compactly arranged in subclavian triangle → reliable upper limb anaesthesia
• Erb’s palsy (C5, C6) — waiter’s tip (violent traction/shoulder depression — motorcycle accidents)
• Klumpke’s palsy (C8, T1) — claw hand (violent upward arm traction)

Cervical rib / Thoracic outlet syndrome — C7 accessory rib → elevates lower brachial trunk + compresses subclavian artery → neurological (C8, T1 distribution tingling, intrinsic hand weakness) + vascular (↓radial pulse, Raynaud’s)

Subclavian artery: subclavian steal syndrome — stenosis proximal to vertebral artery origin → retrograde vertebral flow → vertebrobasilar insufficiency on exercising ipsilateral arm

A paired triangular space on the lateral aspect of the neck, situated behind sternocleidomastoid. Divided by the inferior belly of omohyoid into the occipital triangle (above, larger) and subclavian triangle (below, smaller).

Boundaries

Subdivisions

The posterior triangle is divided by the inferior belly of omohyoid, which crosses it obliquely, into:

The omohyoid is enclosed in a fascial sling derived from the investing layer of deep cervical fascia, tethering it to the clavicle.

Roof and Floor

Roof

Formed from superficial to deep by:

1. Skin
2. Superficial fascia (containing platysma in lower part, cutaneous nerves, external jugular vein)
3. Investing layer of deep cervical fascia — the true roof

Platysma covers only the lower part (subclavian triangle); absent over the upper triangle.

Floor

Formed by the prevertebral layer of deep cervical fascia covering (from above downward):

The investing fascia (roof) splits to enclose SCM anteriorly and trapezius posteriorly. The thick prevertebral fascia (floor) resists spread of infection into the triangle — infections track along it to the axilla.

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Contents of Occipital Triangle

The larger upper subdivision, above the inferior belly of omohyoid.

Floor muscles: Splenius capitis (above), levator scapulae (middle), scalenus medius and posterior (below).

Nerves

• Accessory nerve (CN XI): Most important content — crosses obliquely from Erb’s point to exit deep to trapezius
• Cutaneous branches of cervical plexus — all emerge at Erb’s point

Vessels

• Occipital artery (from ECA): crosses the apex briefly before entering the scalp
• Transverse cervical artery (from thyrocervical trunk): passes through lower part
• External jugular vein: crosses SCM in superficial fascia at the junction of the two triangles

Lymph Nodes

• Occipital lymph nodes: at the apex, drain the back of the scalp
• Lymphadenopathy here: scalp infections, rubella (classical — posterior auricular + occipital nodes)

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Contents of Subclavian Triangle

The smaller lower subdivision, below the inferior belly of omohyoid.

Floor: First rib, scalenus medius, scalenus anterior (medial wall).

Arteries

Veins

Nerves

Arrangement at Base of Subclavian Triangle (Medial → Lateral)

1. Scalenus anterior (medial wall)
2. Phrenic nerve (on anterior surface of scalenus anterior)
3. Subclavian vein (anterior to scalenus anterior)
4. Subclavian artery — 3rd part (lateral to scalenus anterior)
5. Brachial plexus trunks (above and behind artery, between scalenes)

Lymphatics

• Supraclavicular lymph nodes along transverse cervical vessels
• Left supraclavicular node = Virchow’s node (drains via thoracic duct — metastasis from abdominal malignancy)

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Nerves of the Posterior Triangle

Accessory Nerve (CN XI)

• Entry: Emerges from deep surface of SCM at Erb’s point — junction of upper 1/3 and lower 2/3 of posterior border of SCM
• Course: Runs obliquely downward and backward on the floor (levator scapulae), covered by investing fascia; accompanied by C3, C4 branches (proprioceptive)
• Exit: Passes deep to anterior border of trapezius, ~5 cm above clavicle

Erb’s point (nerve point of neck): Junction of upper 1/3 and lower 2/3 of posterior border of SCM — site where CN XI enters, and all 4 cutaneous cervical plexus branches emerge.

Cutaneous Branches of Cervical Plexus

All emerge at Erb’s point:

Brachial Plexus Trunks

Upper, middle, and lower trunks emerge between scalenus anterior and scalenus medius in the subclavian triangle.

• Upper trunk (C5+C6): gives suprascapular nerve and nerve to subclavius
• Long thoracic nerve (C5,6,7): descends on outer surface of serratus anterior

Other Nerves

• Dorsal scapular nerve (C5): Pierces scalenus medius; supplies levator scapulae and rhomboids
• Phrenic nerve (C3,4,5): Descends on scalenus anterior, enters thorax — see Cervical Plexus below

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Cervical Plexus

Formation: Ventral rami of C1, C2, C3, C4, deep to internal jugular vein and SCM, on scalenus medius and levator scapulae. Each nerve (except C1) forms loops with adjacent segments.

Superficial (Cutaneous) Branches

See table above — all emerge at Erb’s point.

Deep (Motor) Branches

Phrenic Nerve

• Roots: C3, C4, C5
• Course: Descends on scalenus anterior (under prevertebral fascia) → behind subclavian vein → superior thoracic aperture → thorax
• Clinical:
  • Injury → ipsilateral diaphragm paralysis (raised hemidiaphragm on X-ray)
  • Referred pain from diaphragm → tip of shoulder (C4 dermatome)
  • Causes of palsy: lung carcinoma, mediastinal masses, aortic aneurysm
  • Hiccup = phrenic nerve irritation

Mnemonic: “C3, 4, 5 keep the diaphragm alive”

Ansa Cervicalis

• Superior root (descendens hypoglossi): C1 fibres travelling with CN XII
• Inferior root (descendens cervicalis): C2 + C3 fibres
• Lies superficial to carotid sheath
• Supplies all infrahyoid strap muscles except thyrohyoid (supplied by C1 via hypoglossal nerve directly)

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Accessory Nerve — Course in Detail

The spinal root of CN XI (relevant to the posterior triangle) supplies SCM and trapezius.

Winging of Scapula — Differentiation

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Vessels

Arteries

Subclavian artery — 3rd part (only part in posterior triangle):

• Extends from lateral border of scalenus anterior to outer border of 1st rib
• Lies on scalenus medius; brachial plexus trunks above and behind; subclavian vein anterior and inferior
• Gives no branches here (all branches from 1st or 2nd part)

Transverse cervical artery (thyrocervical trunk → subclavian 1st part):

• Crosses triangle horizontally, superficial to brachial plexus
• Divides into superficial and deep branches → trapezius and rhomboids

Suprascapular artery (thyrocervical trunk):

• Crosses anterior to brachial plexus; accompanies suprascapular nerve to supraspinous fossa

Veins

External Jugular Vein (EJV):

• Formation: Union of posterior auricular vein and posterior division of retromandibular vein (behind the auricle)
• Course: Superficial fascia → crosses SCM obliquely → pierces investing fascia (roof of posterior triangle) → drains into subclavian vein
• Tributaries: Posterior external jugular vein, transverse cervical vein, suprascapular vein, anterior jugular vein (via jugular venous arch)
• Clinical: Distension at 45° = raised JVP; used for emergency venous access and CVP estimation

Lymph Nodes

Virchow’s node / Troisier’s sign: Left supraclavicular lymphadenopathy — metastasis from abdominal malignancy (especially stomach), draining via the thoracic duct.

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Applied Anatomy

Accessory Nerve Injury

• Most commonly injured nerve in the posterior triangle — runs superficially
• Causes: Lymph node biopsy, radical neck dissection, penetrating trauma
• Effects:
  • Trapezius paralysis → shoulder drooping, winging of scapula (inferior angle rotates laterally), inability to abduct arm beyond 90°
  • SCM paralysis → weakness of head rotation to opposite side

Cervical Lymphadenopathy

• Posterior triangle nodes drain thoracic and abdominal viscera via the thoracic duct
• Virchow’s node (left supraclavicular): stomach, colon, lung, breast malignancy — Troisier’s sign
• Also seen in: tuberculosis, lymphoma, infectious mononucleosis, metastatic carcinoma

Brachial Plexus

• Supraclavicular block: Trunks compactly arranged in subclavian triangle → reliable upper limb anaesthesia via interscalene approach
• Brachial plexus injuries:
  • Upper trunk (C5,6) — Erb’s palsy: waiter’s tip position (from violent traction/shoulder depression — motorcycle accidents)
  • Lower trunk (C8,T1) — Klumpke’s palsy: claw hand (from violent upward traction of arm)

Cervical Rib and Thoracic Outlet Syndrome

• Accessory rib from C7 vertebra elevates lower trunk of brachial plexus and compresses subclavian artery
• Neurological: Tingling/numbness in medial forearm and hand (C8, T1 distribution); weakness of intrinsic hand muscles
• Vascular: Diminished radial pulse, Raynaud’s phenomenon
• Treatment: Surgical resection

External Jugular Vein

• Visible at surface during raised intrathoracic pressure (Valsalva, coughing) or elevated JVP
• Distension above clavicle at 45° head elevation = raised JVP (normal: not visible above clavicle)
• Used for emergency IV access

Subclavian Artery

• Subclavian steal syndrome: Stenosis proximal to vertebral artery origin → retrograde vertebral artery flow → vertebrobasilar insufficiency on exercising ipsilateral arm
• Subclavian aneurysm: Pulsatile supraclavicular swelling

Subclavian Vein Cannulation

• Accessible below clavicle at base of subclavian triangle
• Risks: Pneumothorax, subclavian artery puncture, brachial plexus injury, air embolism

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