Chewing muscles. Illustration of the human skull with part of the jaw cut away to reveal the muscles of chewing (mastication). The face of the skull (at left) shows the orbit of the eye (round, centre left) above the protruding zygomatic (cheek) bone. The teeth (white) are in the maxilla (upper jaw) and mandible (lower jaw). Part of the zygomatic bone and mandible have been cut away to show the position of the pterygoid muscles (pink). The lower muscle is the medial pterygoid, above it are the two heads of the lateral pterygoid. These two muscles work together to produce the up and down and side to side action of chewing.