Fundamentals of Dental Technology

The muscles of the head can be classified into two groups, muscles of facial expression and muscles of mastication. How muscles work and function depends on the action of each muscle (movement), the type of joint it is associated with, and the way the muscle is attached on either side of the joint. Muscles are usually attached to two places: one end being joined to an immovable or fixed portion, and the other end being joined to a movable portion on the other side of a joint. The immovable portion is called the origin of the muscle, and the movable portion is called the insertion. When muscles of the head contract, the insertion end is pulled toward the origin.

The muscles that are underneath the skin of the face are responsible for helping communicate our feelings through facial expression. The muscles of the mouth help us express surprise, sadness, anger, fear, and pain. Table 3-3 lists the muscles of facial expression and figure 3-15 illustrates these muscles.

Muscle	Origin	Insertion	Description
Orbicularis oris	Encircles the mouth (no attachement to bone)	Corners of the mouth	Located between the skin and mucous membranes of the lips. Makes lips close and pucker.
Buccinator	Alveolar process of maxilla and mandible	Orbicularis oris at the corner of the mouth	Located in the walls of the cheeks, holds food in contact with teeth when chewing, and assists in blowing air out of the mouth.
Mentalis	Mandible	Skin of chin	Raises and wrinkles the skin of the chin and decreases and protrudes the lower lip.
Zygomaticus Major	Zygomatic bone	Orbicularis oris (angle of the mouth)	Raises the corner of the mouth when smiling.

Mastication is defined as the process of chewing food in preparation for swallowing and digestion. Four pairs of muscles in the mandible make chewing movements possible. These muscles can be grouped into two different functions. The first group includes three pairs of muscles that elevate the mandible to close the mouth as in biting down. The last group includes one pair that can depress the mandible (open the mouth), make grinding actions side to side, and can make the mandible go forward in a protruding motion. Table 3-4 lists the muscles of mastication and figure 3-16 illustrates these muscles.

Muscle	Origin	Insertion	Description
Masseter	Zygomatic arch	Mandible (external surface)	Closes jaw; flat, thick muscle
Temporalis	Temporal bone	Coronoid process at the anterior border of the ramus	Closes jaw; fan-shaped
Medial pterygoid	Sphenoid, palatine, and maxillary bones	Inner (medial) surface of the ramus	Closes jaw; parallels masseter muscle
Lateral pterygoid	Sphenoid bone	Anterior surface of mandibular condyle	Opens jaw; allows grinding action side to side, and protrudes the mandible