The jaw has four distinct movements. It can be moved directly forward or backward; up and down, a pure hinge motion; a rotary movement on a vertical axis through one of the condyles; and rotation on a transverse axis passing from side to side through the mandibular or inferior dental foramina. The muscles of mastication are the temporal, masseter, and pterygoids; these are supplied by the motor branch of the fifth nerve. To these we may add the buccinator, which is supplied by the seventh nerve, and the depressors of the jaw, - the digastric, geniohyoid, geniohyoglos-sus, mylohyoid, and platysma. The posterior belly of the digastric receives its nerve supply from the facial; its anterior belly from the mylohyoid branch of the inferior dental from the fifth. The mylohyoid is supplied by the mylohyoid branch of the inferior dental. The geniohyoid and genio-hyoglossus are supplied by the hypoglossal nerve. The platysma is supplied by the inframandibular branch of the facial nerve. The upward movement is produced mainly by the masseter and temporal muscles. It is the principal movement in carnivorous animals; therefore, these muscles in them are well developed, and the joint is a pure hinge joint. The internal pterygoid and buccinator likewise aid in closing the mouth; the depressors already mentioned open it. The lateral or rotary movement around a vertical axis passing through one condyle is used in chewing; therefore, we find the muscles most concerned, the pterygoids, best developed in herbivorous animals, or those which chew the cud. The external pterygoid is especially efficient in pulling the jaw forward; superficial fibres of the masseter help in this. The posterior fibres of the temporal muscle pull the jaw back, as do likewise the depressor muscles of the jaw. In this rotary movement one condyle remains back in its socket while the other is brought forward on the eminentia articularis.

The up-and-down movement of the jaws, when limited in extent, is a pure hinge movement without any anteroposterior displacement, and takes place between the condyle and the interarticular cartilage (Fig. 74). The anteroposterior movement is necessarily accompanied by a slight descent of the jaw, as the condyle glides from the glenoid cavity (Fig. 75) onto the eminentia articularis. It goes nearly, but not quite, to the highest point of the articular eminence. If the jaws are kept closed during this anteroposterior movement, some of the teeth of the upper and lower jaws will still be in contact, the number varying in different individuals. The last molar teeth are usually higher than those in front,

Fig. 74.   Illustrating up and down or pure hinge motion or the mandible.

Fig. 74. - Illustrating up-and-down or pure hinge motion or the mandible.

Fig  75.   Illustrating direct anteroposterior movement of the mandible. The condyle is resting on the eminentia articularis.  so that as they glide forward the last lower molars strike the second upper ones. The incisors likewise can be kept in contact as the jaw moves backward and forward. It is this movement in the rodent animals which keeps their edges sharp. In chewing, the jaw is depressed, the teeth separated, and the food held between them by the tongue and buccinator muscle. The teeth are then approximated by the lower jaw closing and the condyle sliding upward and backward from the eminentia articularis into the glenoid cavity, carrying with it the articular cartilage.

Fig. 75

Illustrating direct anteroposterior movement of the mandible. The condyle is resting on the eminentia articularis. so that as they glide forward the last lower molars strike the second upper ones. The incisors likewise can be kept in contact as the jaw moves backward and forward. It is this movement in the rodent animals which keeps their edges sharp. In chewing, the jaw is depressed, the teeth separated, and the food held between them by the tongue and buccinator muscle. The teeth are then approximated by the lower jaw closing and the condyle sliding upward and backward from the eminentia articularis into the glenoid cavity, carrying with it the articular cartilage.

The hinge motion takes place between the condyle and the interarticular cartilage. The anteroposterior motion takes place between the interarticular cartilage and the eminentia articularis: the cartilage is carried forward with the mandible. A rotary movement occurs when, in chewing, the condyle of one side remains in the glenoid cavity while that of the other rises on the articular eminence. The radius of rotation is a line passing from one condyle to the other. In widely opening the mouth, as in yawning, the condyles are tilted forward while the angles of the mandible are carried somewhat backward. As the axis of this motion passes from side to side through the mandibular foramina, this portion of the bone moves but little, and the inferior dental vessels and nerve are not put on the stretch.

Fig. 76.   Dislocation of the lower jaw; the zygoma and part of the masseter muscle have been cut away.

Fig. 76. - Dislocation of the lower jaw; the zygoma and part of the masseter muscle have been cut away.