Relations.—It is covered by the mucous membrane of the upper lip, by the orbicularis oris, and by the levator labii superioris almque nasi ; it covers the myrtiform fossa of the upper jaw : its inner border is separated from its fellow by the frsrnum.
A dilator alas nasi is described by Bourgery as a little triangular muscle, consisting of fibres placed underneath the skin lying on the outside of the ala nasi, from the posterior part of whose cartilages the fibres arise by a narrow point, and then radiate upwards, outwards, and downwards, to be mingled with the fibres of the elevators of the lip, the orbicularis, and the naso-labial, all being attached to the skin. This muscle, according to Bourgery, directly draws the ala outwards, and is consequently a dilator of the nostril.
The labial region presents in the centre, 1. a sphincter (the orbicularis oris), with which are associated two muscles on each side, the depressor labii superioris and the levator labii inferioris: all these are contractors or com pressors of the lips: 2. a number of anta gonist muscles or dilators, which comprise many muscles, which on each side radiate from the lips, or from their commissure at different angles. They are, above, the levator labii superioris proprius and the zygomaticus minor; below, the depressor labii inferioris at the commissure, the buccinator, the levator anguli oris, and the depressor anguli oris. By some anatomists the muscles of this region of the face are divided into, 1. the sphincter, and, 2. the elevators and depressors of the lips.
Orbicularis or sphincter oris (labial, Chauss. and Dum.) (o o, fig. 134) is a thick oval muscle, placed transversely around the aper ture of the mouth, which varies in size in dif ferent persons, but bears no relation to the size of the buccal cavity. It extends above from the free border of the upper lip to the nostrils, and inferiorly from the free border of the lower lip to the depression above the chin. Its fibres, arranged in successive layers, consist of two semi-elliptical halves, one superior, the other inferior, which are on each side united externally to the commissure of the lips by decussating each other, and mingle also at their circumference with the dilators which are attached to it. These fibres are concentric,
with their curve towards the lips; the most central run nearly in a horizontal direction along the borders of the lips, and take a di rection forwards, which gives the prominence to the lips which is so remarkable in the Negro. The outer fibres are more curved, and receive between their layers the extensors of the lips, which are attached around them. This is the only muscle of the face which has no attachment to bone.
lielations.—The anterior surface is closely connected with the thick skin which covers it. The posterior surface and free border is covered with the mucous membrane of the mouth, from which it is only separated in places by the labial glands, by the coronary vessels, and by numerous nerves. Its outer border or circum ference receives the antagonist muscles which are attached around it.
.Actions.—The orbicularis enjoys a very va ried and extensive motion, and possesses the remarkable power of either acting as a whole or in parts. Its simple use is to close the mouth, in correspondence with the elevation of the lower jaw, by bringing the red borders of the lips in contact, or by pressing them to gether firmly. But the upper or lower labial fibres can act separately, or the fibres at either commissure, or the fibres of one side may con tract, while the others are quiescent, so that different parts of the lips may be moved by different portions of the muscle, which is made in this way to antagonize in turn the different muscles which are attached around.
The lips may be thrown forward by the con traction of the labial and commissural fibres forming in strong action a circular projection, as in the action of whistling, or, when more relaxed, in blowing. By the contraction of the inner labial fibres the lips may, on the contrary, be turned inwards so as to cover the teeth. The play of the mouth, however, which contributes in so eminent a degree to the expression of the face, depends not only on the orbicularis, but upon its association with the different muscles which are attached around it.