Cleidooccipitalis
Neck
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The cleidooccipitalis is basically a rare extra “strap” in the side and back of the neck. It is usually described as an accessory slip of the trapezius–sternocleidomastoid region, and it matters clinically because it can cross right where the supraclavicular nerves run.
Function
May assist neck stabilization, lateral flexion of the head and neck, and tension control between the clavicle, upper trapezius, and occipital region. Because it is a variant muscle, its exact action depends on its attachment pattern.
Origin
Most commonly described from the medial or middle portion of the clavicle, or from the anterior border of the cleido-occipital portion of the trapezius in some reports.
Insertion
Usually toward the cervical/occipital portion of the trapezius or the occipital region; some reported slips blend into the upper trapezius, while others insert onto the clavicle when the muscle is treated as a separated occipital portion of trapezius.
Innervation
Most likely the spinal accessory nerve (CN XI), because it is usually related to the trapezius/sternocleidomastoid complex. Cervical nerve contributions are possible depending on the variant pattern.