MUSCLES OF MASTICATION

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.

ORAL STRUCTURES OF THE MOUTH

The oral cavity (mouth) contains various structures that aid in the digestion process of food and also serves as an organ of speech and sensory

Figure 3-15.\Anatomy of muscles of facial expression.

Figure 3-16.\Muscle anatomy of mastication.

Table 3-3.\Muscles of Facial Expression

Table 3-4.\Muscles of Mastication

reception. We receive food in the mouth, reducing it in size, and mixing it with saliva for the digestion process.

CHEEKS

The cheeks are the side walls of the mouth. They are made up of layers of skin, a moist inner lining called mucosa, fat tissue, and certain muscles. The buccinator muscle of the cheeks prevents food from escaping the chewing action of the teeth.

LIPS

The lips are covered externally by skin and internally by the same mucous membranes that line the ^{oral cavity.} They form the anterior border of the mouth. The area of the external lips where the red mucous membrane ends and normal outside skin of the face begins is known as the vermilion border. Figure 3-17 illustrates the anatomy of the lips.

The lips are very sensitive and act as sensory receptors, allowing food and liquids to be placed in the

Figure 3-17.\Anatomy of the lips.

mouth but guarding the oral cavity against the ingestion of excessively hot or cold substances. They also provide a seal for the mouth to keep food and saliva from escaping. The lips help to maintain the position of the teeth and are very important in speech.