Depressor labii inferioris muscle

The depressor labii inferioris muscle in humans is a thin, fleshy, quadrilateral blade that 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: The depressor labii inferioris is absent in carnivores, where the ventral bundle of the buccal part of the buccinator seems to take its place. In equines, it is well-defined as a long, fusiform muscle. The caudal part of the muscle merges with the molar part of the buccinator, while rostrally, it continues with a tendon that blends into the muscular tissue of the lower lip. In ruminants and pigs, the muscle is less well isolated but still quite distinct. 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 inserts into the lower lip via a tendon, which separates there.

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

Functions: It lowers the lower lip, directly when acting with its opposite side, and laterally when acting unilaterally.

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 side to side. It begins by detaching from the molar part of the buccinator at the rostral edge of the mandibular ramus. The fleshy body is extended by an expanded tendon that begins at the mental foramen and ends by dissociating into the upper lip.

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

• In pigs, the depressor labii inferioris muscle is arranged similarly to that in cattle, 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, dissociating near the middle of the lower lip.

• In carnivores, the depressor labii inferioris muscle is absent, with the ventral bundle of the buccal part of the buccinator taking its place.

• In rabbits, the depressor labii inferioris muscle is not very distinct from the buccinator.