Sternocleidomastoid: where does it insert?

Sternocleidomastoid: where does it insert?

• A. Origin: clavicle and manubrium of sternum
• B. Insertion: mastoid process of temporal bone
• C. Action: rotates head, head flexion
• D. Insertion: galea aponeurotica

Answer: (B)

The main idea here is where the sternocleidomastoid attaches to bone. This muscle inserts on the mastoid process of the temporal bone, just behind the ear. Its two heads originate from the clavicle and the manubrium of the sternum, but the insertion is on the mastoid process, which lets it rotate the head to the opposite side and help bend the neck when both sides work together. The other options describe either the origin (where it starts), the action (what it does), or insertions of other muscles (like the galea aponeurotica for the occipitofrontalis), so they don’t fit as the insertion site.