The sternal head of the sternocleidomastoid muscle (also known as the sternocleidomastoideus), along with the clavicular head, assist one another to stretch the cervical portion of the spinal column and the head at the atlanto-occipital joint. It's part of a long neck muscle called the sternocleidomastoid that goes from the mastoid process at the mastoid bone to the sternum and clavicle. This muscle rotates the head and neck.