Having for its function the narrowing of the orifice it surrounds, it acts during suction and in the prehension of food.
Triangularis Oris.—This muscle does not exist in domestic quadrupeds.
Quadratus Menti.—In the pig and the carnivora, it arises from the anterior part of the body of the inferior maxillary bone, and passes at the other end to terminate in the corresponding portion of the lower lip, which it depresses by its contraction.
In the ox and the horse this muscle does not exist; it is replaced for the depression of the lower lip, which it affects in other animals, by supplemental fibres of the buccinator.
The Prominence of the Chin.—Below the lower lip in the horse is situated the so-called prominence of the chin, limited posteriorly by the beard, a depressed region which gives point to the curb of the bridle.
The prominence, which also exists in the ox, is a fibro-muscular pad which blends with the orbicular muscle of the lips, and receives on its superior aspect the insertion of the two muscles (levator menti) by which it is suspended. These arise, above, on each side of the symphysis of the inferior maxillary bone. They raise the lower lip with force, and they are the agents which, as we can sometimes observe in the horse, make it click against the upper lip, suddenly projecting it upwards. This action sometimes becomes a habit, and its continuance constitutes a vice.
A corresponding structure is found in the pig and in the carnivora, but in them it does not produce an external prominence such as we have described.
Buccinator ([Fig. 90], 9; [Figs. 91], [92]).—Further designated by the name of alveolo-labial, this muscle is situated on the lateral portions of the face, in the thickness of the cheeks. It consists of two layers, one superficial and the other deep.
The deep portion arises from the portion of the alveolar border of the superior maxillary bone which corresponds to the molar teeth, and from the anterior border of the ramus of the mandible. Thence it is directed forwards, passes under the superficial layer, and blends with the fibres of the orbicular muscle of the lips. To this part of the buccinator some authors give the name of molar muscle.
The superficial portion is formed by fibres which pass from the alveolar border of the superior maxillary bone to the corresponding border of the opposite bone. It is very highly developed in the herbivora.