Naso-labialis is a small subcutaneous slip of fibres, only distinctly seen in strong muscular lips. It is situated on each side of the median depression of the upper lip, and arises from the lower septum of the nose at the back part of the nostril; it proceeds downwards and out wards, and is soon lost in the fibres of the or bicularis. It is an elevator of the middle part of the upper lip, and is considered by some as an attachment of the orbicularis.
Levator labii superioris fig. 134) is a thin, flat, quadrilateral muscle, situated about the middle of the face, and nearly on the same plane with the levator labii superioris alwque nasi. It arises from the malar and upper maxillary bones where they form three-fourths of the lower border of the orbit, by short ten dinous slips ; from this origin the fibres, con verging a little, take a direction downwards and inwards, and are inserted partly super ficially into the skin of the upper lip, and partly into the fibres of the orbicularis, between the insertion of the levator labii superioris alzeque nasi and the lesser zygomatic, with which its fibres are partly covered and con founded.
Relations.—Its anterior surface is covered above by the orbicularis palpebrarum, below by the skin and by the muscles with which its fibres are mingled at its insertion. Its posterior surface covers the infra-orbitar vessels and nerves at their exit from the infra-orbitar fo ramen, which, with some fat and cellular tissue, separates it from the upper part of the levator anguli oris. It covers also part of the trian gularis nasi.
Its action is to raise and draw a little out wards the upper lip.
Zygomaticus minor (3', fig. 134) is a narrow rounded muscle, often wanting. It arises from the external surface of the os make, and fre quently also from the deep fibres of the orbicu laris palpebrarum, by which its origin is co vered ; it proceeds downwards and inwards, and is attached to the skin and orbicularis pal pebrarum above the commissure of the lips, where its fibres are also confounded with those of the levator labii superioris proprius.
Relations.—This muscle is covered in front by the orbicularis palpebrarum and skin ; its posterior surface conceals a part of the levator anguli oris and of the labial vein.
ilction.—It is an associate of the levator labii superioris, and contributes to raise the upper lip and draw it a little outwards.
Zygomaticus major (3, fig. 134), placed to the outer side and a little below the preceding muscle, is of a rounded form, and arises by short tendinous slips from a depression on the posterior part of the outer surface of the os maize, near its lower border. Its fibres proceed downwards and inwards, nearly parallel with those of the lesser zygomatic, but much longer; and expanding a little below, they become con founded with the fibres of the orbicularis oris at their commissure, and with those of the levator labii superioris, levator anguli oris, and depressor anguli oris. Its superficial fibres are attached to the skin.
Relations.—This muscle is surrounded by fat, which separates it from the skin. By its deep surface it rests above on the os malat and the masseter ; below, it is separated by fat from the buccinator and the levator labii supe rioris : it crosses also the labial vein.
Its action carries the commissure of the lips upwards and outwards, and is intermediate between the action of the levator and the buc cinator: it is the antagonist of the levator an guli oris in drawing the lip outwards ; its associate in raising it. When both these mus cles act, the commissure of the lips is directly raised.
Levator anguli oris (musculas caninus): (r, fig. 136).—To expose this, the levator labii superioris must be removed. It is a flat qua drilateral muscle, which arises from the middle of the canine fossa of the upper jaw, and be coming somewhat narrower takes a direction downwards and a little outwards and forwards, to terminate at the commissure of the lips, where its fibres mingle with those of the orbi cularis, the buccinator, and the depressor anguli oris.
.Relations.—Deeply placed above, its ante rior surface is covered by the infra-orbitar ves sels and nerves, and by fat, which separate it from the levator labii superioris and the lesser zygomatic. Below it is covered by the zygo maticus major and the integument. The pos terior surface of this muscle rests on the upper maxillary bone on the mucous membrane of the mouth, and on the buccinator. Its action is to raise the commissure of the lips, and draw it a little inwards. Its action when as sociated with that of the zygomatics has been already explained.