On each side of the face in the parotid region, the masseter is one of the muscles of mastication. It is quadrilateral in shape and consists of three layers. The layers are fused anteriorly but diverge posteriorly.
The most pronounced layer is superficial. It originates from the anterior two-thirds of the inferior border of the zygomatic arch up to the zygomatic process of the maxilla. Initially, it is aponeurotic and passes obliquely in an inferomedial direction. It inserts into the inferior border of the external surface of the angle of the mandible. Its insertion extends anteriorly to the junction of the ramus with the body of the mandible.
The intermediate part of masseter originates from the middle third of the zygomatic arch. It blends with the superficial part at the mandibular insertion and anteriorly. The posterior divergence of fibres between the superficial and deep parts of masseter permits the entry of its blood supply - the masseteric artery - which is a branch of either the superficial temporal or transverse facial arteries.
The deep part of masseter has a similar course to the intermediate part but is separated from it by the masseteric nerve. Similar to the artery, the nerve enters posteriorly where the layers diverge.
The masseteric nerve provides innervation. It is a branch of the anterior division of the mandibular nerve (V3).
Masseter acts to close the jaw by moving the ramus of the mandible anterosuperiorly relative to the zygoma.