STERNOCLEIDOMASTOID (STERNOMASTOID) MUSCLE
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Sternocleidomastoid Muscle
- Also called sternomastoid.
- It is a long, flat strap of muscle that extends obliquely across the side of the neck.
- It is enclosed within the investing layer of deep cervical fascia.
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Origin
1. Sternal Head
- Tendinous.
- Arises from the superolateral part of the front of the manubrium sterni.
2. Clavicular Head
- Musculotendinous.
- Arises from the medial one-third of the superior surface of the clavicle.
- It passes deep to the sternal head.
- The two heads blend below the middle of the neck.
- Between the two heads is a small triangular depression called the lesser supraclavicular fossa.
- It overlies the internal jugular vein.
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Insertion
- By a thick tendon into the lateral surface of the mastoid process, from its tip to the superior border.
- By a thin aponeurosis into the lateral two-thirds of the superior nuchal line of the occipital bone.
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Nerve Supply
1. Spinal Accessory Nerve
- Provides the motor supply.
- It passes through the muscle.
2. Branches from the Ventral Rami of C2 and C3
- Provide proprioceptive supply.
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Blood Supply
Arterial Supply
- One branch from the superior thyroid artery.
- One branch from the suprascapular artery.
- Two branches from the occipital artery.
Venous Supply
- Veins follow the arteries.
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Actions
When One Muscle Contracts
- Turns the chin to the opposite side.
- Tilts the head towards the shoulder of the same side.
When Both Muscles Contract Together
- Draws the head forwards, as in:
- Eating.
- Lifting the head from a pillow.
- With the longus colli, flexes the neck against resistance.
- Helps in forced inspiration.
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Relations of Sternocleidomastoid
- Sternocleidomastoid is enclosed in the investing layer of deep cervical fascia.
- It is pierced by:
- Accessory nerve.
- Four sternocleidomastoid arteries.
A. Superficial Relations
- Skin
- Superficial fascia
- Superficial lamina of the deep cervical fascia.
- Platysma
- External jugular vein
- Superficial cervical lymph nodes lie along the vein.
- Cutaneous nerves
- Great auricular nerve.
- Transverse or anterior cutaneous nerve.
- Medial supraclavicular nerves.
- Lesser occipital nerve.
- Parotid gland
B. Deep Relations
1. Bones and Joints
- Mastoid process.
- Sternoclavicular joint.
2. Carotid Sheath
3. Muscles
- Sternohyoid
- Sternothyroid
- Omohyoid
- Three scaleni
- Levator scapulae
- Splenius capitis
- Longissimus capitis
- Posterior belly of digastric.
4. Arteries
- Common carotid artery
- Internal carotid artery
- External carotid artery
- Sternocleidomastoid arteries:
- Two from the occipital artery.
- One from the superior thyroid artery.
- One from the suprascapular artery.
- Occipital artery
- Subclavian artery
- Suprascapular artery
- Transverse cervical artery.
5. Veins
- Internal jugular vein
- Anterior jugular vein
- Facial vein
- Lingual vein.
6. Nerves
- Vagus nerve (X)
- Parts of IX, XI, XII
- Spinal root of XI leaves the SCM at the middle of its posterior border to lie in the posterior triangle.
- Cervical plexus
- Upper part of brachial plexus
- Phrenic nerve
- Ansa cervicalis
7. Lymph Nodes
- Superficial and deep cervical lymph nodes.
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Clinical Anatomy
External Jugular Vein
- The fascia of the supraclavicular space is pierced by the external jugular vein to drain into the subclavian vein.
Torticollis
- Torticollis is a deformity in which:
- The head is bent to one side.
- The chin points to the other side.
- It results from spasm or contracture of the muscles supplied by the spinal accessory nerve, namely:
- Sternocleidomastoid.
- Trapezius.
- There are many varieties of torticollis depending on the causes.
- Common types are:
1. Rheumatic Torticollis
- Due to exposure to cold or draught.
2. Reflex Torticollis
- Due to inflamed or suppurating cervical lymph nodes.
- These irritate the spinal accessory nerve.
3. Congenital Torticollis
Wry Neck
- Wry neck → shortening of the muscle fibres due to intravascular clotting of veins within the muscle.
- It usually occurs during difficult delivery of the baby.
