Oral Cavity
Lips
The lips, when opened, afford entrance of the liquid or food into the mouth. When closed, the lips, in conjunction with the front teeth and jaw, provide an anterior containment system so food or liquid does not escape. Anatomically, the lips can be divided into the upper and lower lip. Box 1.2 includes common anatomical markers of the lips.
Box 1.2 Image Gallery: Lip Anatomy
The orbicularis oris, a four quadrant, sphincter-like muscle with fiber attachments at the oral commissure, is the primary muscle of the lips (Box 1.3). It is a complex muscle composed of fibers from various facial muscles, as well as fibers specific to the lips. Collectively these fibers can result in lip rounding, protruding, opening, or closing. Motor innervation to the orbicularis oris comes from CN VII, which is innervated from the contralateral side; sensory innervation comes from CN V from the contralateral side. The muscular action of the lips results from the combined actions of the orbicularis oris and other muscles in the perioral region. While these perioral muscles play a role in the muscular positioning of the lips for articulation, the important actions for swallowing include opening to allow entrance of food or liquid, and closing, with some degree of contact, to maintain a seal against anterior leakage. Lip rounding or closure, an action needed to ensure anterior bolus containment, should be symmetrical from all four quadrants (upper right, lower right, upper left, lower left).
Box 1.3: Orbicularis Oris Muscle
Clinical Note
- Weakness in the lips may result in drooling during mealtimes. Chronic drooling may be the result of weak lips in conjunction with overproduction or poor management of saliva including reduced saliva swallow frequency.
- An open lip posture may be indicative of lip weakness or poor nasal patency.
- When asymmetrical lip movement is observed, it implies that there is either contralateral cortical damage or ipsilateral nerve damage.
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Muscles of importance that insert into the oral commissure of the lips (where the upper lip and lower lip intersect) include the buccinator, levator anguli oris, depressor anguli oris, zygomaticus major, and risorius. Surrounding perioral muscles that insert into the upper lip include the levator labii superioris, levator labii superioris alaeque nasi, and zygomaticus minor. Perioral muscles that insert into the lower lip include the depressor labii inferioris, mentalis, and platysma.
Cheeks
The cheeks provide lateral containment of food and liquid in the oral cavity. This is partly due to the mere presence of the structure, and partly from active contraction of cheek muscle. This contraction provides support to keep material out of the lateral sulci (cheek pocket) and help increase and direct oral pressure.
The external surface of the cheek is covered with skin; the internal surface is covered with a mucous membrane. The primary muscle of the cheek is the buccinator muscle, which whe