Relations.—The superficial surface of that part of the muscle which covers the lids (the palpebralcs) is connected to the skin by delicate loose cellular tissue entirely destitute of fat. The stronger fibres which form the outer part of the muscles are closely adherent to the integument by cellular tissue more densely woven, and presenting more or less fat. The posterior surface covers, above, the lower part of the frontalis and the corrugator supercilii, with whose fibres it is connected ; internally the corresponding part of the fibro cartilages of the lids, the lachrymal sac, and the inner border of the orbit externally, the outer border of the orbit and part of the tem poral fascia inferiorly, the upper part of the malar bone, the origins of the levator labii superioris prnprius, the part of the levator labii supertoris alxque nasi, and the inferior border of the orbit. At its circumference this muscle corresponds, by its upper half, to the frontal, which it slightly overlaps, and inter nally to the border of the pyramidalis, with which it is connected; externally it is free. Below its border is free, covering the origin, and giving some fibres to the lesser zygomatic; and internally it is separated from the levator labii superioris alaque nasi by cellular tissue, in which runs the facial vein. The central fibres cover the palpebral fascia and the lids, which separate them from the conjunctiva.
Action.—The action of this muscle resem bles that of other sphincters, the curved fibres in contraction approaching the centre; but as in the orbicularis palpebrarum these fibres are fixed at the inner side, it follows that the skin to which the muscle is attached by its anterior surface is drawn towards the nose, and when the muscle is in strong action, becomes cor rugated, presenting folds which converge to wards the inner angle of the eye; above, where the effect of the muscle on the skin is most marked in consequence of its closer connec tion with the integuments, the brow and the skin of the forehead are drawn down by it and its associate the corrugator; the lower fibres when in strong action, draw the cheeks upwards and inwards. Like the other sphinc ters, also, this is a mixed muscle. Those fibres which may be supposed to be voluntary, are the larger and outer ones, which corres pond to the border of the orbit, and are of a red colour. The involuntary fibres are those thin ones which cover the lids, are of a pale colour, like the muscles of organic life, and arise from the palpebral subdivisions of the horizontal tendon. They contract involuntarily while we are awake, in the action of winking, and during sleep in maintaining the lids closed; they also act under the will in closing the lids, particularly the upper. It appears then
that the orbicularis may be divided both ana tomically and physiologically into two sets of fibres; an outer, or orbicularis proper, which is entirely a voluntary muscle, and an inner (the palpebralis) which is both voluntary and involuntary in its action. These fibres may act independently of each other, for in wink ing and during sleep the palpebralis contracts, while the orbicularis is quiescent ; and the orbicularis may contract even strongly, as when we peer with the eyes under the influence of a strong light, while the fibres of the pal pebrales are relaxed. It has been supposed, however, by some, that during sleep the lid is closed simply by the weight of the upper palpebra, and the relaxation of its proper elevator muscle, but this seems in contra diction to the fact that we meet with resistance in endeavouring to unclose the lids of a sleep ing person.
Corrugator supercilii, which is the associate of the orbicularis palpebrarum, has been al ready described, together with the occipito frontalis, which is the antagonist of those muscles. See CRANIUM, MUSCLES OF THE, vol. i. p. 747.
Levator palpebre sup.•rioris (orbito-polpe bral), though situated within the orbit, is nevertheless the direct antagonist of the palbe bralis, and is therefore properly described with these muscles of the face. It is a thin trian gular muscle, which arises by a narrow slen der tendon at the back of the orbit from the inferior surface of the lesser wing of the sphe noid bone, above and in front of the optic foramen ; from this origin the fibres proceed almost horizontally forwards under the roof of the orbit, and gradually spreading and be coming thinner as they advance, curve over the globe of the eye, and are inserted into the upper border and anterior surface of the upper lid.
Relations.—Its upper surface is in contact, behind, with the frontal branch of the ophthal mic nerve, which with some cellular tissue alone separates it from the periosteum of the roof of the orbit; anteriorly with cellular tissue and the palpebral fascia, which separate it from the orbicularis. The lower surface behind rests upon the superior rectus oculi, with which it is connected by cellular tissue, and anteriorly on the conjunctiva and upper lid.
Its action is to raise the upper lid, and to draw it backwards over the globe and under the supra-ciliary ridge. There is no separate muscle to effect the depression of the lower lid, that action being occasioned, as Sir C. Bell ingeniously suggested, by the protrusion of the eyeball.
Nasal region.—The muscles of this region, some of which are common to the upper lip, are, 1. the pyramidalis; 2. the levator labii superioris aleque nasi; 3. the triangularis nasi ; 4. the depressor title nasi.