Orbicularis Oris.
Buccinator.
Risorius.

Dissection. The dissection of these muscles may be considerably facilitated by filling the cavity of the mouth with tow, so as to distend the cheeks and lips; the mouth should then be closed by a few stitches, and the integument carefully removed from the surface.

The Orbicularis Oris is a sphincter muscle, elliptic in form, composed of concentric fibres, which surround the orifice of the mouth. It consists of two thick semicircular planes of muscular fibre, which surround the oral aperture, and interlace on either side with those of the Buccinator and other muscles inserted into this part. On the free margin of the lips the muscular fibres are continued uninterruptedly from one side to the other, forming a roundish fasciculus of fine pale fibres closely approximated. To the outer part of each segment some special fibres are added, by which the lips are connected directly with the maxillary bones and septum of the nose. The additional fibres for the upper segment consist of four bands, two of which (Accessorii Orbicularis superioris) arise from the alveolar border of the superior maxilla, opposite the incisor teeth, and arching outwards on each side, are continuous at the angles of the mouth with the other muscles inserted into this part.

The two remaining muscular slips, called the Naso-labialis, connect the upper lip to the septum of the nose:' as they descend from the septum, an interval is left between them, which corresponds to that left by the divergence of the accessory portions of the Orbicularis above described. It is this interval which forms the depression seen on the surface of the skin beneath the septum of the nose. Those for the lower segment (Accessorii Orbicularis inferior) arise from the inferior maxilla, external to the Levator labii inferioris, near the root of the canine teeth, being separated from each other by a considerable interval; arching outwards to the angles of the mouth, they join the Buccinator and the other muscles attached to this part.

Relations. By its superficial surface, with the integument, to which it is closely connected. By its deep surface, with the mucous membrane, the labial glands, and coronary vessels. By its outer circumference, it is blended with the numerous muscles, which converge to the mouth from various parts of the face. Its inner circumference is free, and covered by mucous membrane.

The Buccinator is a broad, thin muscle, quadrilateral in form, occupying the interval between the jaws at the side of the face. It arises, above, from the external surface of the alveolar process of the upper jaw, between the first molar tooth and the tuberosity; below, from the external surface of the alveolar process of the lower jaw, corresponding to the three last molar teeth; and, behind, from the anterior border of the pterygo-maxillary ligament. The fibres of this muscle converge towards the angle of the mouth, where those occupying its centre intersect each other, the inferior fibres being continuous with the upper segment of the Orbicularis oris; the superior fibres, with the inferior segment; but the upper and lower fibres continue forward uninterruptedly into the corresponding segment of the lip.

Relations. By its superficial surface, behind, with a large mass of fat, which separates it from the ramus of the lower jaw, the Masseter, and a small portion of the Temporal muscle; anteriorly, with the Zygomaticus, Risorius, Levator anguli oris, Depressor anguli oris, and Stenon's duct, which pierces it opposite the second molar tooth of the upper jaw; the transverse facial artery and vein lie parallel with its fibres, and the facial artery and vein cross it from below upwards; it is also crossed by the branches of the facial and buccal nerves. By its internal surface, with the buccal glands and mucous membrane of the mouth.

The Pterygo-maxillary ligament separates the Buccinator muscle from the Superior constrictor of the pharynx. It is a tendinous band, attached by one extremity to the apex of the internal pterygoid plate, and by the other, to the posterior extremity of the internal oblique line of the lower jaw. Its inner surface corresponds to the cavity of the mouth, and is lined by mucous membrane. Its outer surface is separated from the ramus of the jaw by a quantity of adipose tissue. Its posterior border gives attachment to the Superior constrictor of the pharynx; its anterior border, to the fibres of the Buccinator.

The Risorius (Santorini) consists of a delicate bundle of muscular fibres, which arises in the fascia over the Masseter muscle, and passing horizontally forwards, is inserted into the angle of the mouth, joining with the fibres of the Depressor anguli oris. It is placed superficial to the Platysma, and is broadest at its outer extremity. This muscle varies much in its size and form.

Nerves. The Orbicularis oris is supplied by the facial, the Buccinator by the facial and buccal branch of the inferior maxillary nerve.

Actions. The Orbicularis oris is the direct antagonist of all those muscles which converge to the lips from the various parts of the face, its action producing the direct closure of the lips; and its forcible action throwing the integument into wrinkles, on account of the firm connection between the latter and the surface of the muscle. The Buccinators contract and compress the cheeks, so that, during the process of mastication, the food is kept under the immediate pressure of the teeth.