Depressor labii inferioris muscle

The depressor labii inferioris muscle corresponds in humans to a thin fleshy quadrilateral blade which originates on the mandible, following an oblique line located ventrally to the mental foramen. In domestic mammals, this muscle is considered a dependency of the buccinator, whose ventral edge it runs against the molar part of the mandible.

Conformation: It is lacking in carnivores, where the ventral bundle of the buccal part of the buccinator seems to hold in place. In equines, it is best individualized, it is a long fusiform muscle. Its caudal part merges with the molar part of the buccinator; Rostrally the depressor labii inferioris muscle continues with a tendon which plunges into the muscular tissue of the lower lip. In ruminants and pigs, it is less well isolatable but very clear. It is very indistinct in rabbits.

Insertions: The depressor labii inferioris muscle originates in common with the buccinator on the molar part of the mandible. It ends in the lower lip with its tendon, which dissociates there.

Relations: Covered in its caudal third by the masseter, this muscle is covered more rostrally by the facial expansion of the platysma and accompanied by the ventral buccal branch of the facial nerve, as well as by the inferior labial artery and the lower molar glands. It covers the mandible and the mental bundle of the inferior alveolar nerve.

Functions: It lowers the lower lip, directly if it acts with its opposite, drawing it to the side when the action is unilateral.

Vessels and nerves: The depressor labii inferioris muscle is supplied by the buccal, inferior labial and inferior alveolar arteries. It is innervated by the ventral buccal branch of the facial nerve.

Variation:

In equines, the depressor labii inferioris muscle is very distinct, fusiform, long, narrow and flattened from one side to the other. It begins by detaching from the molar part of the buccinator, on the rostral edge of the mandibular ramus. The fleshy body is extended by an expanded tendon which begins at the mental foramen and ends by dissociation in the upper lip.

In beef, the depressor labii inferioris muscle is broad and thick, it merges almost along its entire length with the buccinator muscle whose bundles of the oral part bend forward to merge with it. Only its rostral quarter can be isolated, it continues with a tendon whose fibers will be lost on the side of the tuft of the chin.

In pigs, the depressor labii inferioris muscle is arranged approximately as in beef, but it cannot be isolated from the buccinator, from which it is distinguished mainly by the orientation of the fibers and the volume. Only the terminal tendon is free, which dissociates up to near the middle of the lower lip.

In carnivores, the depressor labii inferioris muscle is missing, where the ventral bundle of the buccal part of the buccinator seems to hold in place.

In the rabbit, the depressor labii inferioris muscle isn’t very distinct from the buccinator.