The term facial expression in the chapter title may be a bit misleading because the muscles included in this chapter are located around the ears, scalp, neck, eyes, nose, and mouth. However, although some of them are not located in an area normally considered the face, they are located in areas that can physically display some kind of emotion or attentiveness. All these muscles are innervated by the seventh (VII) cranial nerve (facial). Although most of the muscles are mentioned, only the muscles around the oral cavity are discussed in great detail. These are the muscles you will be most concerned with in the dental office, and they are responsible for some functions related to speech and mastication.
The muscles around the ears are not well developed in humans. However, in lower forms of animals, they are better developed, and the ears can be easily moved and repositioned to better catch sounds. There are three pairs of ear muscles (Fig. 30-1).
The posterior auricular muscle arises from the superior nuchal line of the occipital bone and the mastoid area. The fibers run forward to insert into the posterior part of the ear. The action of this muscle pulls the ear back. This is probably the best developed of the ear muscles.
The occipitofrontalis (epicranius) is a paired muscle with groups of fibers in front and back connected by a broad flat band of fascia. The anterior and posterior groups of muscle fibers take their origin from connective tissue of the scalp. This kind of attachment allows for either forward or backward movement of the scalp. The forward movement results in a frown or a squint, and the backward movement raises the forehead skin as in surprise (see Fig. 30-1).
There is some disagreement as to which end of the platysma is the origin and which is the insertion. The upper end of the fibers attaches to the inferior border of the mandible, near the angles of the mouth and the skin of the face in that area. They pass down in a broad flat sheet to end in the skin of the chest area just below the clavicle. The muscle lies just below the skin of the neck; thus it moves the skin over the neck quite noticeably when it contracts, pulling the corners of the mouth down or the skin of the upper pectoral region up (see Fig. 30-1).