The muscle whose name lists its three attachments: breastbone, collarbone and the bony lump behind the ear
Sternocleidomastoid — sterno-, cleido-, mastoid.
Anatomy · Muscles
Sternocleidomastoid
The landmark muscle that splits the neck into triangles
Neck · superficial lateral muscle
A thick, strap-like muscle running obliquely across each side of the neck from the breastbone and collarbone up to the bony lump behind the ear. It turns the head to the opposite side and tilts it towards the shoulder on its own side, and it stands out like a rope when the head is turned. Surgeons and doctors use it as the main landmark for dividing the neck into anterior and posterior triangles.
How to spot them
- ▸A visible oblique ridge from the top of the sternum and inner collarbone up behind the ear to the mastoid process
- ▸Two heads below, sternal and clavicular, with a small gap between them just above the collarbone
- ▸Supplied by the accessory nerve (cranial nerve XI), with position-sense fibres from the cervical plexus
- ▸The external jugular vein crosses its surface; the carotid sheath lies deep to its anterior border
What to know
Contracts on one side to turn the face to the opposite shoulder while tilting the head to its own side
Sternocleidomastoid — unilateral action: rotation to the opposite side.
Its anterior border divides the side of the neck into two triangles, one in front and one behind
Sternocleidomastoid — the anterior and posterior triangles of the neck.
A newborn is found with the head tilted to one side and a firm lump in the muscle, sometimes after a difficult birth
Sternocleidomastoid — congenital muscular torticollis (wry neck), the sternomastoid tumour.
Motor supply from the accessory nerve, which crosses the posterior triangle, a nerve shared with a big muscle of the upper back
Sternocleidomastoid — cranial nerve XI, shared with trapezius.
Acts as an accessory muscle of inspiration, lifting the top of the ribcage when a person is struggling for breath
Sternocleidomastoid — with the neck fixed, it raises the sternum.
A single muscle with a sternal and a clavicular head, both ending on the mastoid process and the superior nuchal line
Sternocleidomastoid — sternal head from the manubrium, clavicular head from the medial third of the clavicle.
Key ideas
- Together the two muscles form a V that flexes the head forward against gravity; a person lying flat who lifts the head against a hand on the forehead shows both.
- Being so superficial and so consistently placed, it is the surface landmark for the carotid pulse, the internal jugular vein and the carotid sheath just beneath its anterior edge.
Quick-fire facts
- •Origin: sternal head from the manubrium of the sternum; clavicular head from the medial third of the clavicle
- •Insertion: mastoid process of the temporal bone and the lateral half of the superior nuchal line of the occipital bone
- •Action: one side rotates the face to the opposite side and tilts the head to the same side; both sides together flex the neck or extend the head at the atlanto-occipital joint, and help to raise the sternum in forced inspiration
- •Nerve supply: accessory nerve (CN XI) for movement, with branches of C2 and C3 carrying position sense
- •Blood supply: branches of the occipital artery above, the superior thyroid artery in the middle and the suprascapular artery below
- •The gap between its two heads above the collarbone is the lesser supraclavicular fossa
- •The external jugular vein crosses its outer surface; the great auricular nerve winds round its posterior border
- •Injury of the accessory nerve during lymph node biopsy in the posterior triangle weakens both this muscle and the trapezius
- •Torticollis, or wry neck, may be congenital, from shortening or fibrosis of this muscle, or acquired
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Sources
- TeachMeAnatomy — Sternocleidomastoid