The muscles in the oral group move the lips and cheek. They include the orbicularis oris and buccinator muscles, and a lower and upper group of muscles (Fig. 8.59). Many of these muscles intersect just lateral to the corner of the mouth on each side at a structure termed the modiolus.
The orbicularis oris is a complex muscle consisting of fibers that completely encircle the mouth (Fig. 8.59). Its function is apparent when pursing the lips, as occurs during whistling. Some of its fibers originate near the midline from the maxilla superiorly and the mandible inferiorly, whereas other fibers are derived from both the buccinator, in the cheek, and the numerous other muscles acting on the lips. It inserts into the skin and mucous membrane of the lips, and into itself.
Contraction of the orbicularis oris narrows the mouth and closes the lips.
The buccinator forms the muscular component of the cheek and is used every time air expanding the cheeks is forcefully expelled (Figs. 8.59 and 8.60). It is in the space between the mandible and the maxilla, deep to the other facial muscles in the area.
The buccinator arises from the posterior part of the maxilla and mandible opposite the molar teeth and the pterygomandibular raphe, which is a tendinous band between the pterygoid hamulus superiorly and the mandible inferiorly and is a point of attachment for the buccinator and superior pharyngeal constrictor muscles.
The fibers of the buccinator pass toward the corner of the mouth to insert into the lips, blending with fibers from the orbicularis oris in a unique fashion. Central fibers of the buccinator cross so that lower fibers enter the upper lip and upper fibers enter the lower lip (Fig. 8.60). The highest and lowest fibers of the buccinator do not cross and enter the upper and lower lips, respectively.
Contraction of the buccinator presses the cheek against the teeth. This keeps the cheek taut and aids in mastication by preventing food from accumulating between the teeth and the cheek. The muscle also assists in the forceful expulsion of air from the cheeks.
Lower group of oral muscles
The muscles in the lower group consist of the depressor anguli oris, depressor labii inferioris. and mentalis (Fig. 8.59).
The depressor anguli oris is active during frowning.
It arises along the side of the mandible below the canine, premolar, and first molar teeth and inserts into skin and the upper part of the orbicularis oris near the corner of the mouth. It depresses the corner of the mouth.
The depressor labii inferioris arises from the front of the mandible, deep to the depressor anguli oris. Its fibers move superiorly and medially, some merging with fibers from the same muscle on the opposite side and fibers from the orbicularis oris before inserting into the lower lip. It depresses the lower lip and moves it laterally.
The mentalis helps position the lip when drinking from a cup or when pouting. It is the deepest muscle of the lower group arising from the mandible just inferior to the incisor teeth, with its fibers passing downward and medially to insert into the skin of the chin. It raises and protrudes the lower lip as it wrinkles the skin of the chin.
Upper group of oral muscles
The muscles of the upper group of oral muscles consist of the risorius, zygomaticus major, zygomaticus minor, levator labii superioris, levator labii superioris alaeque nasi, and levator anguli oris (Fig. 8.59).
The risorius helps produce a grin (Fig. 8.59). It is a thin, superficial muscle that extends laterally from the corner of the mouth in a slightly upward direction. Contraction of its fibers pulls the corner of the mouth laterally and upward.