The lateral pterygoid is the big exception in this muscle group in that its fibers run horizontally rather than vertically and one of the two heads of the muscle, does not insert into bone, but rather inserts into the articular cartilage of the TMJ. Like the medial pterygoid and the masseter it has two heads. The superior head originates on the sphenoid bone facing the infratemporal fossa and inserts in the articular cartilage of the temporomandibular joint. Its action is to pull the cartilage so that it stays atop the condylar head as the mandible is protruded. The inferior head originates on the lateral pterygoid plate and inserts into a hollow immediately inferior to the condylar head. The action of this muscle is to move the mandible anteriorly protruding it but because the gliding motion of the mandible is down the articular eminence the ultimate motion is depressing the mandible as it opens wider as it moves anteriorly. Unilateral contractions of the lateral pterygoids will move that side anteriorly but also cause rotation of the mandible in the contralateral joint.