Chewing, or mastication, is one of the important functions carried on in the mouth, and a number of muscles are involved either directly or indirectly in this activity. However, the four muscles that are primarily responsible for the forceful chewing movements of the mandible are classified by most authors as the “muscles of mastication.” These are the masseter, temporalis, lateral pterygoid, and medial pterygoid muscles.
The masseter muscle is a thick, quadrangular muscle that is readily palpable on the side of the jaw. It is described as having a superficial and a deep part, which can be rather easily separated on the posterior aspect of the muscle but are blended together anteriorly. The superficial part arises from the inferior border of the anterior two thirds of the zygomatic arch (zygomatic process of maxilla, zygomatic bone, and zygomatic process of temporal bone) and runs medially and a little posteriorly to insert on the lateral surface of the lower part of the ramus of the mandible. The area of insertion continues to the inferior border of the mandible. The deep portion of the masseter muscle arises from the inner surface of the whole length of the zygomatic arch and runs almost vertically inferiorly to insert on the lateral surface of the coronoid process and upper part of the ramus of the mandible. The deepest fibers frequently blend with the adjacent portion of the temporalis muscle. The masseter muscle is supplied by a masseteric branch from the mandibular division of the trigeminal nerve, which reaches the deep surface of the muscle by passing through the mandibular notch.
The temporalis muscle, spread out broadly on the lateral side of the skull, is a thin sheet, except where its fibers converge toward the tendon of insertion. It arises from the whole temporal fossa (the extensive area between the inferior temporal line and the infratemporal crest) and from the inner surface of the temporal fascia that covers the muscle. The temporalis muscle inserts by means of a thick tendon that passes medial to the zygomatic arch and attaches to the apex and deep surface of the coronoid process of the mandible and the anterior border of the ramus almost as far as the last molar tooth, with some of the fibers frequently becoming continuous with the buccinator muscle. Two or three deep temporal branches of the mandibular nerve enter the deep surface of the temporalis muscle.
The lateral pterygoid muscle is somewhat conical in shape and runs horizontally in the infratemporal fossa. It arises as the fusion of superior and inferior heads.
The superior head attaches to the infratemporal surface of the greater wing of the sphenoid bone, and the inferior head attaches to the lateral surface of the lateral pterygoid plate. The two heads join and form a tendon of insertion that ends on the front of the condylar neck of the mandible and on the anterior aspect of the capsule and articular disk of the temporomandibular joint. A lateral pterygoid nerve from the mandibular branch of the trigeminal enters the deep surface of this muscle.
The medial pterygoid muscle, located medial to the ramus of the mandible, is thick and quadrangular. Its main origin is from the medial surface of the lateral pterygoid plate and from the pyramidal process of the palatine bone between the two pterygoid plates. A small slip of muscle originates from the tuberosity of the maxilla and the adjacent surface of the pyramidal process of the palatine bone. The medial pterygoid muscle inserts on the medial surface of the ramus of the mandible between the mylohyoid groove and the angle. The medial pterygoid nerve from the mandibular runs along the medial side of the muscle to enter it.
The muscles of mastication all pass across the temporomandibular joint, and they are the major muscles producing the movements allowed at this joint. Elevation of the mandible is brought about by the masseter, temporalis, and medial pterygoid. They are able to bring the lower teeth powerfully up against the upper teeth. They also are acting against gravity in most positions of the head in keeping the mouth closed. If they are relaxed, the weight of the jaw can gap the mouth to a small degree. The muscle of mastication that actively opens the mouth is the lateral pterygoid. It does this by pulling the articular disk and condyle of the mandible anteriorly. Other muscles that help in opening the mouth against resistance are the suprahyoid, infrahyoid, and platysma muscles. Protrusion of the jaw is brought about primarily by bilateral contraction of the lateral pterygoid, because in this movement, also, the articular disk and condyle of the mandible are brought anteriorly. The superficial portion of the masseter and the medial pterygoid can give some minor aid in protrusion. Retraction of the mandible is accomplished mostly by the posterior part of the temporalis muscle, some of the fibers of which run nearly horizontally. The digastric and geniohyoid muscles can contribute to retraction when the hyoid bone is anchored.
All the muscles of mastication are employed in the act of chewing, because it involves the four movements of the mandible described above (i.e., elevation, depression, protrusion, and retraction) and at least one of the muscles of mastication is involved in each of these movements. For the most part, chewing is done either on one side or the other, and the condyle of the side on which the chewing is being done remains more or less in position while the condyle of the other side moves back and forth, as in protrusion and retraction. This is combined in proper sequence with slight elevation and depression to bring about the grinding action on the food.
In order that grinding can be carried on efficiently, the food must be kept between the teeth by the tongue on one side and the cheek and lips on the other side. Naturally, the muscular framework of the cheek and lips is important in accomplishing this. The framework of the cheek is formed by the buccinator muscle, which takes its origin from the outer surfaces of the maxilla and mandible in the region of the molar teeth and between the posterior ends of these lines of attachment from the pterygomandibular raphe, by means of which it is continuous with the superior constrictor of the pharynx. From this U-shaped origin, the horizontal fibers of the muscle run anteriorly, apparently to continue into the orbicularis oris muscle, with the superiormost and inferiormost fibers going into the upper and lower lips, respectively, and the intermediate fibers crossing near the corner of the mouth, so that the superior fibers of this intermediate group go into the lower lip and the inferior fibers of the intermediate group go into the upper lip. The buccinator muscle is supplied by the facial nerve. The framework of the lips is formed by the orbicularis oris muscle. In addition to the fibers that appear to be the forward prolongations of the buccinator muscle, fibers come into the orbicularis oris from all of the muscles that insert in the vicinity. The orbicularis oris muscle is also supplied by the facial nerve.