Overview
A paired, two-headed long muscle of the neck.
- Sternal head – a round fasciculus, tendinous in front, fleshy behind, arising from the upper part of the front of the manubrium of the sternum. It’s fibres travel superiorly, laterally, and posteriorly.
- Clavicular head – arises from the upper, frontal surface of the medial third of the clavicle. It’s fibres are directed almost vertically upward.
The two heads are separated from one another at their origins by a triangular interval (lesser supraclavicular fossa) but gradually blend, below the middle of the neck, into a thick, rounded muscle which is inserted by a strong tendon into the lateral surface of the mastoid process and by a thin aponeurosis into the lateral half of the superior nuchal line of the occipital bone.
Origin
Manubrium and medial portion of the clavicle
Course
It travels obliquely across the side of the neck and inserts at the mastoid process of the temporal bone of the skull by a thin aponeurosis. The sternocleidomastoid is thick and narrow at its centre, and broader and thinner at either end.
Insertion
- Mastoid process of the temporal bone
- Superior nuchal line of the occiput
Actions
- Unilateral contraction: contralateral cervical rotation, ipsilateral cervical flexion
- Bilateral contraction: cervical flexion, elevation of sternum and assists in forced inhalation.
Arterial Supply
- Occipital artery – arises from the external carotid artery
- Superior thyroid artery – arises from the external carotid artery
Innervation
- Motor: Spinal accessory nerve
- Sensory: Cervical plexus
- Proprioceptive: ventral rami of C2-3
Relations
Posteriorly
- Cervical plexus emerges from the lateral border of sternocleidomastoid
- Anterior scalene muscle
- Carotid sheath (common carotid artery, internal jugular vein – between the two heads near point of insertion)
Anteriorly
- External jugular vein
- Platysma
Medially
- Anterior jugular vein
Inferiorly
Anatomical Variants
- Separate cleidomastoid muscle
- Absent mastoid insertion
- Absent clavicular or sternal head
- Fusion with trapezius