Key muscle of the neck; extends obliquely across the side of the neck, dividing it into posterior triangle (behind) and anterior triangle (in front). Most important surface landmark on the side of the neck.

Origin (Two Heads)

Small triangular gap between two heads above SC joint = lesser supraclavicular fossa — contains terminal internal jugular vein → accessible by needle/catheter

Insertion

• Thick tendon → lateral surface of mastoid process (tip to base)
• Thin aponeurosis → lateral half of superior nuchal line of occipital bone

Nerve Supply

Arterial Supply (3 segments)

Segmental supply essential for planning muscle flap in reconstructive surgery

Actions

Relations

Enclosed in: investing layer of deep cervical fascia; pierced by spinal accessory nerve + 4 sternomastoid arteries

Superficial to SCM (anterior → posterior):

1. Great auricular nerve
2. External jugular vein
3. Transverse cervical nerve

Deep relations:

Nerve Point of the Neck (Erb’s Point)

• Junction of upper + middle thirds of posterior border of SCM
• Where 4 cutaneous branches of cervical plexus + spinal accessory nerve emerge
• Used for cervical plexus nerve block — LA at this point blocks all 4 cutaneous nerves

Applied Anatomy

Torticollis (wry neck) — head bent to one side; chin to opposite side; due to spasm/contracture/paralysis of SCM (+ trapezius)

Sternomastoid tumour — swelling in middle 1/3 of SCM due to oedema + ischaemic necrosis after birth trauma; presents in neonates/infants; usually resolves spontaneously; may lead to congenital torticollis

Lesser supraclavicular fossa — between two SCM heads → used for internal jugular vein cannulation (central venous access)

Subclavian artery compression — pulsations palpable by pressing posteriorly above clavicle at posterior border of SCM; haemorrhage from upper limb arteries = arrested by pressing subclavian artery onto 1st rib at lower end of SCM

Spinal accessory nerve injury — most commonly during lymph node biopsy/removal in posterior triangle; result = trapezius paralysis (shoulder drooping, winging of scapula); SCM is spared (accessory nerve supplies SCM before entering posterior triangle)

The sternocleidomastoid (SCM) is the key muscle of the neck. It extends obliquely across the side of the neck, dividing it into the posterior triangle (behind) and the anterior triangle (in front). It is the most important surface landmark on the side of the neck.

---

Origin

Arises by two heads:

A small triangular gap exists between the two heads above the sternoclavicular joint — the lesser supraclavicular fossa. It contains the terminal part of the internal jugular vein, which can be accessed here by needle or catheter.

---

Insertion

• Thick tendon → lateral surface of the mastoid process (tip to base)
• Thin aponeurosis → lateral half of the superior nuchal line of the occipital bone

---

Nerve Supply

---

Arterial Supply (3 segments)

Knowledge of segmental supply is essential for planning muscle flap in reconstructive surgery.

---

Actions

---

Relations

Enclosure

Enclosed in the investing layer of deep cervical fascia. Pierced by: spinal accessory nerve + 4 sternomastoid arteries.

Superficial Relations

• Skin
• Platysma
• Cutaneous nerves (superficially crossing SCM from above downwards):
  1. Great auricular nerve
  2. Transverse cervical nerve
  3. Medial supraclavicular nerve
• External jugular vein
• Superficial cervical lymph nodes
• Parotid gland (upper part)

Deep Relations

---

Key Relations on the Superficial Surface of SCM

The following structures cross the SCM superficially in order from above downwards:

1. Great auricular nerve
2. External jugular vein
3. Transverse cervical nerve

---

Nerve Point of the Neck

The junction of the upper and middle thirds of the posterior border of SCM — where four cutaneous branches of the cervical plexus and the spinal accessory nerve all emerge.

Used for cervical plexus nerve block — local anaesthetic injected at this point blocks all four cutaneous nerves.

---

Clinical Correlations

Torticollis (Wry Neck)

Head bent to one side; chin points to the opposite side. Due to spasm/contracture/paralysis of SCM (and trapezius) supplied by spinal accessory nerve.

Sternomastoid Tumour

Swelling in the middle third of SCM due to oedema and ischaemic necrosis following birth trauma. Presents in neonates/infants. Usually resolves spontaneously; may lead to congenital torticollis.

Lesser Supraclavicular Fossa

Between the two heads of SCM — used for internal jugular vein cannulation (central venous access).

Subclavian Artery Compression

Pulsations of the subclavian artery can be felt by pressing posteriorly above the clavicle at the posterior border of SCM. Haemorrhage from upper limb arteries can be arrested by pressing the subclavian artery onto the first rib at the lower end of SCM.

Spinal Accessory Nerve Injury

Most commonly injured during removal/biopsy of lymph nodes in the posterior triangle. Result: Trapezius paralysis — inability to elevate shoulder above 90°; shoulder drooping and winging of scapula. SCM is spared because the spinal accessory nerve supplies SCM before entering the posterior triangle.

Diagram Placeholder

Diagram content will be added later.

• Placeholder 1
• Placeholder 2
• Placeholder 3

Previous Year Questions

1. Placeholder question
2. Placeholder question
3. Placeholder question
4. Placeholder question
5. Placeholder question