Orbicularis oris muscle

The orbicularis oris muscle is an elongated fleshy ring placed around the oral cleft, in the thickness of the lips. 

Conformation: This muscle has a labial part and a marginal part, which is continuous with the neighboring muscles, these two parts are distinct in all species. This muscle is always divisible into two parts, one for each lip, formed of transverse bundles which, facing the angles of the mouth, are continuous or intersect with those of the other lip. In carnivores, ruminants and rabbits, the part that occupies the upper lip is interrupted on the median plane by a raphe, from which the fleshy bundles are organized on either side. In beef and carnivores a similar raphe may also exist in the lower lip. Above the upper lip, a few marginal bundles will attach to the nasal septum, in men this part corresponds to the depressor muscle of the nasal septum. The thickness of this muscle depends on that of the lips. It is strong and thin in pigs and carnivores, but thick and meaty in humans, horses and ruminants.

Insertions: This muscle has no bone insertion. But it receives at its periphery the terminal fibers of most of the neighboring muscles. The most important attachment is that of the buccinator, near the corner of the lips. Many fibers of the orbicularis oris muscle are inserted deep into the skin or lining of the lips.

Relations: The superficial side responds to the skin and its glands and adheres to them intimately, except in certain points, where expansions of neighboring muscles end. The deep face responds to the incisive muscles, the oral mucosa and the labial salivary glands; in the lower lip, it additionally comes into contact with the mental muscle.

Functions: Constrictor of the oral orifice, this muscle participates, in a variable way depending on the species, in the prehension of food, in sucking and even in chewing.

Vessels and nerves: labial, inferior alveolar and even, in equines, major palatine arteries. The veins are satellites and the lymphatics drained by the mandibular lymph nodes. The motor nerves come from the facial; each lateral half is innervated independently by the facial on the same side. The sensory fibers come from the infra-orbital and inferior alveolar nerves.

Variations: In equines, the orbicularis oris muscle is fleshy and relatively thick. On the upper lip, it is separated from the skin by the thin median and fibrous expansion common to the two levator labii superioris muscles. Its development appears to be linked to the great mobility of the lips and their active participation in grasping food.