Sternocleidomastoid muscle

The sternocleidomastoid (SCM) is a muscle of the neck. It has two heads that meld to form one insertion. SCM, along with the trapezius muscle, is invested by the superficial layer of the deep cervical fascia, which splits around it. SCM divides the neck into anatomical anterior and posterior triangles. 

Summary

• origin: manubrium, medial clavicle
• insertion: mastoid process, superior nuchal line
• innervation: accessory nerve (CN XI)
• action: lateral flexion, flexion and rotation of the head on the neck

Gross anatomy

Origin 

• sternal head (tendinosus): manubrium of sternum
• clavicular head (fleshy): medial part of clavicle 

Insertion

• mastoid process of temporal bone 
• lateral half of the superior nuchal line of the occipital bone

Relations

• upper half
  • posteriorly: cervical plexus, which emerges from behind the lateral border of SCM with some branches (lesser occipital, greater auricular and transverse cervical nerves) coursing anteriorly
  • anteriorly: platysma muscle, external jugular vein
• lower half
  • posteriorly: anterior scalene muscle, common carotid artery, internal jugular vein, carotid sheath
  • anteriorly: platysma

Blood supply

• arterial supply: superior thyroid and occipital arteries

Innervation

• spinal part (C2, C3) of the accessory nerve (cranial nerve XI)

Action 

• when acting unilaterally: lateral flexion, rotates head to opposite side
• when acting bilaterally: flexes neck, draws head ventrally and elevates chin, draws sternum superiorly in deep inspiration

Variant anatomy

• absent mastoid insertion (occipital insertion only)
• absent clavicular or sternal head
• fusion with trapezius muscle
• additional clavicular head
• separate muscle bellies for sternomastoid and cleidomastoid
• separate muscle bellies for cleidooccipital and sternocleidomastoid

Related pathology

• fibromatosis colli