Internal Pterygoid.
External Pterygoid.

The Internal Pterygoid is a thick quadrilateral muscle, and resembles the Masseter in form, structure, and in the direction of its fibres. It arises from the pterygoid fossa, its fibres being attached to the inner surface of the external ptery-goid plate of the sphenoid, and to the grooved surface of the tuberosity of the palate bone; its fibres descend downwards, outwards, and backwards, to be inserted, by strong tendinous laminae, into the lower and back part of the inner side of the ramus and angle of the lower jaw.

Relations. By its external surface, with the ramus of the lower jaw, from which it is separated at its upper part by the External Pterygoid, the internal lateral ligament, the internal maxillary artery, and the superior dental vessels and nerves. By its internal surface, with the Tensor palati, being separated from the Superior constrictor of the pharynx by a cellular interval.

The External Pterygoid is a short thick muscle, somewhat conical in form, being broader at its origin than at its insertion. The two extremities of the muscle are tendinous, the intervening portion being fleshy. It arises by two heads, separated by a cellular interval. The upper head is attached to the pterygoid ridge on the great ala of the sphenoid, and the portion of bone included between it and the base of the external pterygoid plate; the other, the larger fasciculus, from the outer surface of the external pterygoid plate, and part of the tuberosity of the palate bone. From this origin, its fibres proceed horizontally backwards and outwards, to be inserted into a depression on the anterior part of the neck of the condyle of the lower jaw, and into the corresponding part of the interarticular fibro-cartilage.

Relations. By its external surface, with the ramus of the lower jaw, the internal maxillary artery, which crosses it, the tendon of the Temporal muscle, and the Masseter. By its internal surface, it rests against the upper part of the Internal pterygoid, the internal lateral ligament, the middle meningeal artery, and inferior maxillary nerve; by its upper border it is in relation with the temporal and masseteric branches of the inferior maxillary nerve.

Nerves. These muscles are supplied by the inferior maxillary nerve.

Actions. The Temporal, Masseter, and Internal pterygoid raise the lower jaw against the upper with great force. The two latter muscles, from the obliquity in the direction of their fibres, assist the External pterygoid in drawing the lower jaw forwards upon the upper, the jaw being drawn back again by the deep fibres of the Masseter, and posterior fibres of the Temporal. The External pterygoid muscles are the direct agents in the trituration of the food, drawing the lower jaw directly forwards, so as to make the lower teeth project beyond the upper. If the muscle of one side acts, the corresponding side of the jaw is drawn forwards, and the other condyle remaining fixed, the symphysis deviates to the opposite side. The alternation of these movements on the two sides, produces trituration.