Identify the origin, insertion, and primary actions of the sternocleidomastoid.

Identify the origin, insertion, and primary actions of the sternocleidomastoid.

• A. Origin: manubrium; Insertion: zygomatic process; Actions: flexes head.
• B. Origin: occipital; Insertion: medial clavicle; Action: rotates head.
• C. Origin: manubrium of sternum and medial clavicle; Insertion: mastoid process of the temporal bone; Actions: unilateral contraction tilts and rotates head; bilateral contraction flexes the neck.
• D. Origin: scapula; Insertion: mastoid; Action: rotates trunk.

Answer: (C)

The attachments of the sternocleidomastoid determine how it moves the head and neck: it originates from the sternum (manubrium) and the medial end of the clavicle, and it inserts at the mastoid process of the temporal bone. When both sides contract, the neck flexes forward; when it contracts on one side, it tilts the head toward the same side and rotates the face to the opposite side. This combination of origins, insertion, and actions is exactly what the correct option describes. The other options mix up the attachments or the actions—for example, placing the insertion at a facial bone like the zygomatic process or placing an origin at the scapula or the occipital region—so they don’t match the actual anatomy or movement pattern of the sternocleidomastoid.

The attachments of the sternocleidomastoid determine how it moves the head and neck: it originates from the sternum (manubrium) and the medial end of the clavicle, and it inserts at the mastoid process of the temporal bone. When both sides contract, the neck flexes forward; when it contracts on one side, it tilts the head toward the same side and rotates the face to the opposite side. This combination of origins, insertion, and actions is exactly what the correct option describes.

The other options mix up the attachments or the actions—for example, placing the insertion at a facial bone like the zygomatic process or placing an origin at the scapula or the occipital region—so they don’t match the actual anatomy or movement pattern of the sternocleidomastoid.