Anatomy and Functions of the Masticatory Muscles

Masseter Muscle

The masseter is a quadrilateral muscle consisting of three layers:

• Superficial Layer: Arises by a thick aponeurosis from the zygomatic process of the maxilla and the anterior two-thirds of the lower border of the zygomatic arch. It passes downward and backward at a 45-degree angle and inserts into the lower part of the lateral surface of the ramus of the mandible.
• Middle Layer: Arises from the anterior two-thirds of the deep surface and the posterior one-third of the lower border of the zygomatic arch. It passes vertically downward and inserts into the middle part of the ramus.
• Deep Layer: Arises from the deep surface of the zygomatic arch, passes vertically downward, and inserts into the upper part of the ramus and the coronoid process.

Actions of the Masseter

1. Elevates the mandible to close the mouth and occlude the teeth during mastication.
2. Provides the powerful force necessary for efficient chewing.
3. The superficial portion may aid in protruding the mandible.
4. The fibers of the deep portion stabilize the condyle against the articular eminence.

Temporalis Muscle

The temporalis is attached to the medial surface, apex, anterior and posterior borders of the coronoid process, and the anterior border of the ramus of the mandible down to the level of the third molars.

Actions

• Anterior portion: Consists of fibers directed almost vertically.
• Middle portion: Contains fibers that run obliquely across the lateral aspect of the skull.
• Posterior portion: Aligned almost horizontally, coming forward above the ear to join other temporalis fibers as they pass under the zygomatic arch.

Lateral Pterygoid Muscle

This is a short, thick muscle with two heads:

• Upper Head: Arises from the infratemporal surface and infratemporal crest of the greater wing of the sphenoid bone.
• Lower Head: Arises from the lateral surface of the lateral pterygoid plate.

Actions

1. When the right and left inferior lateral pterygoids contract simultaneously, the condyles are pulled down the articular eminence and the mandible is protruded.
2. When this muscle functions with the mandibular depressors, the mandible is lowered and the condyles glide forward and downward on the articular eminence.
3. The superior lateral pterygoid is considerably smaller than the inferior; the inferior lateral pterygoid is active during opening.

Medial Pterygoid Muscle

A thick, quadrilateral muscle with the following characteristics:

• Origin: Medial surface of the lateral pterygoid plate, pyramidal process of the palatine bone, and the maxillary tuberosity. Its fibers pass downward, laterally, and backward.
• Insertion: Attached by a strong tendinous lamina to the postero-inferior part of the medial surfaces of the ramus and the angle of the mandible.

Actions

1. Assists in elevating the mandible.
2. Acts with the lateral pterygoid to protrude the mandible.