| SECTION III | MUSCLES OF THE MOUTH |
OBJECTIVES
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
A. MUSCLES INVOLVED IN MASTICATION (CHEWING)
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
The masseter [ma SEE ter] muscle (Fig. 14-27) is the most superficial, bulky, and powerful of the muscles of mastication. It is four sided in shape.
FIGURE 14-27. Masseter muscle (shaded red) and fan-shaped temporalis muscle (shaded blue). (Reproduced from Clemente CD, ed. Gray’s anatomy of the human body. 30th ed. Philadelphia, PA: Lea & Febiger, 1985:450, with permission.)
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.
FIGURE 14-28. Human skull, left side, showing muscle attachments. This lateral view shows the origin of the fan-shaped temporalis muscle (within the shallow temporal fossa outlined with a blue dotted line) and the origin of the masseter (light red area on the zygomatic arch) as well as the insertion of the masseter muscle (light red area on the lateral surface of the angle of the mandible). The red arrows indicate the slope of the posterior surface of the articular eminence and the subsequent downward (opening) movement of the mandible when it is pulled forward by both lateral pterygoid muscles.
Insertion: The masseter inserts on the inferior lateral surface of the ramus and angle of the mandible (Fig. 14-28).
Action: It elevates the mandible (closes the mouth) and applies great power in crushing food.13,15,16
LEARNING EXERCISE
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.
FIGURE 14-29. Temporalis muscle (blue and purple). The zygomatic process of the temporal bone and temporal process of the zygomatic bone have been removed. When studying this drawing, you should understand why the contraction of the anterior vertical fibers (purple) of the temporal muscle acts to close the jaw while contraction of the posteriorly horizontal fibers (purple) acts to pull the jaw back or to retract (retrude) the mandible. (Reproduced from Clemente CD, ed. Gray’s anatomy of the human body. 30th ed. Philadelphia, PA: Lea & Febiger, 1985:449, with permission.)
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.
FIGURE 14-30. Mandible, medial surface, with the location of the muscle insertions of the temporalis, medial pterygoid, and lateral pterygoid muscles. The insertion of the temporalis muscle (blue) is located on the anterior medial ridge (temporal crest) of the mandibular ramus. The insertion of the medial pterygoid muscle (green) is on the internal surface of the angle of the mandible. The insertion of the lateral pterygoid muscle (yellow) is on the anterior surface of the neck of the condyle in the pterygoid fovea (as well as the articular disc, which is not shown).
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.
LEARNING EXERCISE
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
The medial pterygoid [TER i goid] muscle is located on the medial surface of the ramus (Figs. 14-31 and 14-32).
FIGURE 14-31. The skull from the inferior view. The medial pterygoid muscle (shaded green) and masseter muscles (red) form a sling that supports the mandible. Also, from this view, it is clear that the lateral pterygoid muscle (yellow) has its origin (on the base of the cranium) more medial than its insertion (on the anterior portion of the neck of the condyle and the articular disc). If this muscle contracts only on the right side as shown by the arrow, that condyle of the mandible moves toward its origin, thus bodily moving the mandible toward the left or opposite side. (Reproduced from Clemente CD, ed. Gray’s anatomy of the human body. 30th ed. Philadelphia, PA: Lea & Febiger, 1985:452, with permission.)
FIGURE 14-32. A lateral view of two heads of the lateral pterygoid muscle (shaded yellow) and the medial pterygoid muscle (shaded green) with the zygomatic arch and the anterior part of the ramus removed. The upper head of the lateral pterygoid muscle has its origin on the infratemporal surface of the sphenoid bone, and the lower head has its origin on the lateral surface of the lateral pterygoid plate of the sphenoid bone (covered by the muscle in this drawing). The insertion of both heads of the lateral pterygoid muscle is on the fovea of the neck of the condyle of the mandible and on the articular disc. Notice the horizontal orientation of the lateral pterygoid fibers in direct contrast to the vertical direction of the medial pterygoid fibers. Simultaneous contraction of both lateral pterygoid muscles guides the condyles (and discs) forward, which causes the mandible to protrude and the mouth to open. Contraction of the medial pterygoid muscle in harmony with the masseter elevates the mandible (closes the mouth). (Reproduced from Clemente CD, ed. Gray’s anatomy of the human body. 30th ed. Philadelphia, PA: Lea & Febiger, 1985:451, with permission.)
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.
FIGURE 14-33. Human skull, inferior surface. As you study this drawing, notice the arrow that connects the insertion of the lateral pterygoid muscle (on the anterior neck of the condyle of the mandible) with its origin (on the lateral surface of the lateral pterygoid plate denoted by a yellow line). When only one lateral pterygoid muscle contracts and pulls the insertion end toward its origin, the mandible moves medially, toward the opposite side, as shown by the second arrow near the anterior part of the mandible. The location of the origin of the medial pterygoid muscle is shaded green on the right side of the drawing in the pterygoid fossa.
FIGURE 14-34. Part of human skull, lateral view, has the lateral wall of the maxilla removed to expose the maxillary sinus. Posterior to the maxilla, note the location of the origin of the two heads of the lateral pterygoid muscle: the lateral surface of the lateral pterygoid plate (shaded yellow) just posterior to the maxilla and the roof of the infratemporal space on the base of the cranium. (Reproduced from Clemente CD, ed. Gray’s anatomy of the human body. 30th ed. Philadelphia, PA: Lea & Febiger, 1985:166, with permission.)
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.
LEARNING EXERCISE
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.
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