A group of very delicate muscles controlling the sensitive movements of the wings and septum of the nose (m. compressor narium, m. depressor alœ nasi, m. levator alœ nasi, anterior and posterior, and m. depressor septi) have their points of attachment around the nasal alœ (just above the upper incisor and canine teeth). There is a great wealth of muscles surrounding the mouth; no animal, not even the anthropoid ape, is equipped with so many muscles; it is due to them that the human mouth is able to assume such a great variety of positions. The greater number of these muscles are arranged like radii around the mouth; and there is one which, unlike the rest, surrounds the oral aperture like a ring.
The radiating muscles, descending from the sides of the nose down along the chin are: the levator muscle of the upper lip (m. levator labii superioris, starting from the bony margin below the infraorbital foramen); the levator muscle of the angle of the mouth (m. levator anguli oris, starting from the fossa of the upper maxilla); the large and small zygomatic muscles (starting from the anterior surface of the malar bones); the risorial muscle (m. risorius), the smallest of all the facial muscles, which has its origin in the soft surface tissues (aponeurosis parotido-masseterica); the depressor muscle of the mouth angle (m. depressor anguli oris, or m. triangularis) originating on the lower margin of the maxilla; the depressor muscle of the lower lip or quadratus muscle of the chin (m. quadratus labii inferioris or quadratus menti, also originating on the lower maxilla); the levator muscle of the chin (m. levator menti) between the two musculi quadrati, also has its origin in the lower maxilla; the buccinator muscle, hidden beneath the preceding, has its origin behind the molar teeth in the alveolar process of the two maxillæ, and extends horizontally, terminating in the two lips, in such a manner that its two fasciæ; partly cross, so that the upper fasciæ of the muscle starting from the mandible extend to the upper lip, and the lower fasciæ of the muscle starting from the maxilla extend to the lower lip. Consequently the contraction of this muscle stretches the angles of the mouth in a horizontal direction only; it is the most voluntary of all the muscles, and plays a greater part than the others in forced laughter; in consequence it robs this movement of its characteristic charm.
Lastly we must note the orbicular muscle of the lips (m. orbicularis oris or sphincter oris), which constitutes the fleshy part of the lips and surrounds the oral aperture like a ring.
The contraction of these muscles produces antagonistic motorial action; for instance, the orbicular muscle tends to close the mouth into a circular orifice; the various muscles which radiate from the corners of the mouth (especially the buccinator) tend, on the contrary, to enlarge and stretch it in a transverse direction; certain muscles tend to raise the mouth, and others to lower it. Accordingly, there results a play between the muscles of expression and upon their continual antagonism depend the changing expressions of the human countenance.
Here are a few of the principal facial expressions, described in a masterly manner, and for the first time, by Charles Darwin:[44]
Expression of Sorrow.—The muscles that are principally brought into play are the superciliary, the frontal and the triangular or depressor muscles of the lips; the eyebrows are furrowed, being drawn upward by the action of the frontal muscle; this, however, cannot contract completely because drawn downward laterally by the superciliary muscles, and hence the forehead wrinkles only at its middle point and together with the slanting eyebrows assumes a shape that suggests three sides of a quadrilateral.
Fig. 121.—A six-year-old boy.
Fig. 122.—The same, eleven years old.