Levator Labii Inferioris.
Depressor Labii Infcrioris (Quadratus menti).
Depressor Anguli Oris (Triangularis menti).

Dissection. The Muscles in this region may be dissected by making a vertical incision through the integument from the margin of the lower lip to the chin: a second incision should then be carried along the margin of the lower jaw as far as the angle, and the integument carefully removed in the direction shewn iu fig. 130.

The Levator Labii Inferioris (Levator menti) is to be dissected by everting the lower lip and raising the mucous membrane. It is a small conical muscular fasciculus, which arises from the incisive fossa, external to the symphysis of the lower jaw; its fibres expand downwards and forwards, to be inserted into the integument of the chin.

Relations. On its inner surface, with the buccal mucous membrane; in the median line, it is blended with the muscle of the opposite side; and on its outer side, with the Depressor labii inferioris.

The Depressor Labii Inferioris (Quadratus menti) is a small quadrilateral muscle, situated at the outer side of the preceding. It arises from the external oblique line of the lower jaw, between the symphysis and mental fommen, anil passes obliquely upwards and inwards, to be inserted into the integument of the lower lip, its fibres blending with the Orbicularis, and with those of its fellow of the opposite side. It is continuous with the fibres of the Platysma at its origin.

Relations. By its superficial surface, with part of the Depressor anguli oris, and with the integument, to which it is closely connected. By its deep surface, with the mental vessels and nerves, the mucous membrane of the lower lip, the labial glands and the Levator labii inferioris, with which it is intimately united.

The Depressor Anguli Oris is a triangular muscle, arising, by its broad base, from the external oblique line of the lower jaw; its fibres pass upwards, to be inserted, by a thick and narrow fasciculus, into the angle of the mouth, being continuous with the Orbicularis, Levator anguli oris, and Zygomaticus major.

Relations. By its superficial surface, with the integument. By its deep surface, with the Depressor labii inferioris, the Platysma, and Buccinator.

Nerves. This group of muscles is supplied by the facial nerve.

Actions. The Levator labii inferioris raises the lower lip, and protrudes it forwards; at the same time it wrinkles the integument of the chin. The Depressor labii inferioris draws the lower lip directly downwards and a little outwards. The Depressor anguli oris depresses the angle of the mouth, being the great antagonist, to the Levator anguli oris and Zygomaticus major: acting with those muscles, it will draw the angle of the mouth directly backwards.