|SECTION III||MUSCLES OF THE MOUTH|
The objectives for this section are to prepare the reader to perform the following:
- Identify the four pairs of major muscles of mastication.
- Describe and identify the origin and insertion of each of these muscles of mastication on a skull and be able to palpate each (if possible) on yourself or a partner.
- Describe and demonstrate the function of each of these muscles.
- List other factors that contribute to the position of teeth and movement of the mandible.
- Describe the location and list the functions of the groups of muscles that contribute to facial expression.
The following general terms relate to muscles and will be helpful to know as you read this section:
Anguli [AN gyoo lie]: triangular area or angle of a structure
Depressor: acts to depress or make lower
Insertion (of a muscle of mastication): place of attachment of muscles to the bone that moves, such as muscle attachments on the movable mandible
Labial [LAY bee al]: related to, or toward, the lips; like the labial surface of a tooth
Levator [le VA tor]: acts to raise (compare elevator)
Lingual [LIN gwal]: related to the tongue; for example, the lingual nerve innervates the tongue; the lingual muscle is within the tongue; and the lingual surface of a tooth is the side toward the tongue.
Mental: referring to the chin; the mental foramen is the hole in the mandible where the mental nerve passes out of the mandible to the chin; the mentalis muscle inserts into the chin.1
Orbicularis [or BIK u lar is]: round; compare an orbit
Origin (of a muscle of mastication): is the source, beginning, or fixed proximal end attachment of a muscle as compared to its insertion, which is a muscle’s more movable attachment or distal end.1
Oris: referring to the edge of the mouth; compare oral
Procerus [pro SE rus]: long and slender
Muscles of mastication or chewing move the mandible. They include four pairs of muscles (right and left): masseter, temporalis, medial pterygoid, and lateral pterygoid muscles. These muscles have the major control over the movements of the mandible. Each of these muscles has one end identified as its origin and the other end identified as its insertion. The origin end of each of the muscles of mastication is the source, beginning, or fixed proximal attachment located, in this case, on the bones of the neurocranium that are relatively immovable. The insertion end is the attachment on the movable bone that for each of these muscles is attached to, and moves, the mandible.
There are five different ways in which the mandible moves. We can elevate the mandible (closing the mouth), depress it (opening the mouth), retrude it (retracting or pulling back the mandible), protrude it (protracting or moving the mandible anteriorly), and move it into lateral excursions (moving the mandible sideways, as when chewing).
1. Masseter Muscle
Origin: The masseter arises from the inferior and medial surfaces of the zygomatic arch that is made up of the zygomatic bone, the zygomatic process of the maxillae, and the temporal process of zygomatic bone (seen in Fig. 14-28). From here, it extends inferiorly and posteriorly toward its insertion.
Insertion: The masseter inserts on the inferior lateral surface of the ramus and angle of the mandible (Fig. 14-28).
As you clench your teeth several times, feel the contraction of the masseter by placing a finger on the outside of your cheek posterior to the third molar region. The muscle will produce a noticeable bulge beneath your finger each time. The part felt just inferior to the cheekbone (anterior to the earlobe) is near the origin, and the bulge felt over the angle of the mandible is near the insertion.
2. Temporalis Muscle
The temporalis [tem po RA lis] muscle is a fan-shaped, large but flat muscle with both vertical anterior (and middle) fibers and more horizontal posterior fibers. Vertical and horizontal fibers are shaded darker in Figure 14-29.
Origin: The temporalis arises from the entire temporal fossa (Fig. 14-28) (composed of the squamous part of temporal bone, the greater wing of the sphenoid bone, and the adjacent portions of the frontal and parietal bones). From here, its anterior (and middle) fibers are directed vertically downward while its posterior fibers are directed more horizontally, mostly anteriorly and somewhat inferiorly, passing medial to the zygomatic arch.
Insertion: The temporalis inserts on the coronoid process of the mandible, the medial surface of the anterior border of the ramus, and the temporal crest of the mandible (Fig. 14-30) via one common tendon.
Action: The anterior (and middle) vertical fibers contract to act to elevate the mandible (close the jaw) especially when great power is not required, and the posterior horizontal fibers retrude or pull the mandible posteriorly.13,15,16 This muscle can position the mandible slightly more anteriorly or more posteriorly while also closing the teeth together.
Feel contraction of the origin of the temporalis by placing several fingers above and in front of your ear to feel the vertical fibers contract as you firmly close your teeth together several times. Then, feel the nearly horizontal fibers just above and behind your ears contract as you retrude or pull your mandible posteriorly. This may be more difficult to feel since the bulge is less evident.
3. Medial Pterygoid Muscle
Origin: The medial pterygoid muscle arises mainly from the medial surface of the lateral pterygoid plate and the pterygoid fossa between the medial and lateral pterygoid plates (Fig. 14-33) of the sphenoid bone. (Also, there are fibers attached to the posterior surface of the maxilla and to the adjacent vertical processes of the palatine bones and to the maxillary tuberosity.10) Similar to the masseter, the fibers pass from their origin inferiorly and posteriorly (but laterally) toward their insertion.
Insertion: The medial pterygoid muscle inserts on the medial surface of the mandible in a triangular region at the angle and on the adjacent portions of the ramus just above the angle (Fig. 14-30). Along with the masseter located on the lateral surface, these two muscles serve as a sling with the medial pterygoid attached on the medial side and the masseter attached on the lateral side of the angle of the mandible. They have similar actions.
Action: It elevates the mandible (closes jaw) like the masseter and the anterior (and middle) fibers of the temporalis muscles. Although not as large or powerful, it works together with the larger masseter muscle in helping to apply the power or great force upon closing the teeth together.
Attempt to palpate the insertion of the medial pterygoid muscle in your mouth by bending the head forward to relax the skin on the neck and placing the forefinger medial to the internal angle of the mandible while gently pressing your finger upward and outward. When the teeth are squeezed together, you should feel the bulge of this muscle.