Lower incisive muscle

There are in principle two incisive muscles on each side, one within the upper lip (M. incisivus superior) and the other in the lower lip (M.incisivus inferior). They are located under the labial mucosa. They originate near the incisive alveoli of each jaw, and end on the deep surface of the orbicular muscle of the mouth, near the labial commissure. The superior is always more easily separable than the lower. It is particularly developed in Equidae, where it brings the upper lip back against the teeth, that is very protractile in these animals. The incisive muscles tighten the lateral part of the lips and protrude their median part, while pulling the commissures forward.