sternocleidomastoid muscle

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

Summary

Gross anatomy

Origin 
  • sternal head (round and tendinous): upper part of anterior surface of manubrium of sternum
  • clavicular head (thin and broader fleshy): upper surface of medial third of clavicle 
Course
  • sternal head passes over the sternoclavicular joint and joins the clavicular head halfway up the neck 
  • common muscle belly then courses posterolaterally to its insertion 
Insertion
Relations

Arterial supply

Innervation

Action 

  • when acting unilaterally: lateral flexion of neck to same side, 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 cleido-occipital and sternocleidomastoid
  • doubles or quadrupled unilaterally or bilaterally 
  • absent unilaterally or bilaterally 

Related pathology