A broad, thick, square muscle situated on the outer face of the lower jaw. It is largely intersected by tendinous layers and covered by a strong fibrous membrane.

Origin. - From the zygomatic ridge of the upper jaw-bone by a strong broad tendon.

Insertion. - Into the external surface of the upper broad portion of the inferior maxilla or lower jaw.

Action. - It brings the teeth of the lower jaw forcibly into contact with those of the upper in grinding the food. It is the most powerful of the muscles engaged in mastication.