June 20, 2014
The jaw-closing muscles include the masseter, the temporalis, and the medial pterygoid. The masseter muscle has two portions - a larger superficial portion and a much smaller, deep portion. The origin of the superficial masseter muscle is on the zygomatic process of the maxilla and the inferior border of the zygomatic arch. It has a stong and large tendinous attachment to these structures. The muscle-tendor fibers on the superficial masseter run obliquely inferiorly and posteriorly to the area of insertion on the angle of the mandible and the inferior half of the lateral side of the ramus. The fiber direction is roughly perpendicular to the plane of occlusion.
The deep portion of the masseter muscle also has its origin on the zygomatic process. It then runs vertically down to insert on the ramus of the mandible up to the base of the coronoid process.
The principal function of the masseter is to close the jaws. The deep portion of the masseter muscle, because of its different fiber orientation, also helps to retrude the mandible. The masseter muscle is a powerful elevator of the mandible and is thought to provide the bulk of the power for chewing a bolus. Innervating this muscle is the masseteric nerve.