378 THE MUSCULAR SYSTEM. 
the orbicularis oris muscle. The middle fibres of the muscle decussate at the angle 
of the mouth, so as to pass, the lower set to the upper lip, the upper set to the lower 
lip. The buecinator is covered on its deep surface by the mucous membrane of the 
mouth. Superficially it is concealed by the muscles above mentioned, which con- 
verge to the angle of the mouth; it is separated from the masseter by the suctorial 
pad of fat; it is pierced by the duct of the parotid gland, and by branches of the 
buecal nerve. 
NERVE SUPPLY. 
The facial and scalp muscles are all innervated by the facial nerve. The posterior auricular 
branch supplies the retrahens aurem and occipitalis; the branches forming the pes anserinus 
supply the frontalis, attollens, and attrahens aurem, the several muscles associated with the 
apertures of the eye, nose, and mouth (including the buccinator), and the platysma myoides. 
ACTIONS. 
The almost infinite variety of facial expression is produced partly by the action of these 
muscles, partly by their inactivity, or by the action of antagonising muscles (antithesis). On the 
one hand joy is, for example, betrayed by the action of one set of muscles, while grief is accom- 
panied by the contraction of another (opposing) set. Determination or eagerness 1s accompanied 
by a fixed expression due to a combination of muscles acting together; despair, on the other 
hand, is expressed by a relaxation of muscular action. For a philosophical account of the action 
of the facial muscles, the student should consult Darwin’s Hapression of the Emotions in Man and 
Animals, and Duchenne’s Mechanisme de la Physiologie humaine. 
The platysma myoides retracts and depresses the angle of the mouth, and depresses the lower 
jaw. The occipito-frontalis, by its anterior belly, raises the eyebrows; both bellies acting 
together tighten the skin of the scalp; acting along with the orbicularis palpebrarum, it shifts 
the scalp backwards and forwards. 
The corrugator supercilii draws inwards the eyebrow and vertically wrinkles the forehead. 
The pyramidalis nasi draws downwards the skin between the eyebrows, as in frowning. The 
upper eyelid is raised by the levator palpebre superioris. The closure of the lids is effected by 
the orbicularis palpebrarum, whose fibres also assist in the lowering of the eyebrows, in the pro- 
tection of the eyeball, and, by pressure on the lachrymal gland, in the secretion of tears. The 
tensor tarsi, acting along with the orbicularis palpebrarum, compresses the lachrymal sac and 
aids in the evacuation of its contents. The muscles of the ear and nose have quite rudimentary 
actions expressed by their names. Of the muscles of the mouth, the orbicularis oris has a com- 
plex action, depending on the degree of contraction of its component parts. It causes compression 
and closure of the lips in various ways, tightening the lips over the teeth, contracting them as in 
osculation, or causing pouting or protrusion of one or the other. The accessory muscles of the 
lips draw them upwards (zygomatici, levator anguli oris, levator labii superioris aleeque nasi, 
levator labii superioris), outwards (zygomaticus major, risorius, platysma, depressor anguli oris, 
buccinator), and downwards (depressor anguli oris, depressor labii inferioris, platysma). The 
levator menti elevates the skin of the chin and protrudes the lower lip. The buccinator retracts 
the angles of the mouth, flattens the cheeks, and brings them in contact with the teeth. 
THE FASCIA AND MUSCLES OF THE ORBIT. 
The eyeball with its muscles, vessels and nerves, is lodged in a mass of soft and 
yielding fat which entirely fills up the cavity of the orbit. Surrounding the 
LEVATOR PALPEBR® SUPERIORIS posterior part of the eyeball 
\ Rectus SUPERIOR is the capsule of Tenon, which 
eee een ee manus COuSstitutes a large lymph 
"ss Space or synovial - bursa in 
relation to the eyeball. An- 
teriorly the capsule is in con- 
tact with the conjunctiva, 
which intervenes between the 
latter and the eyeball; pos- 
teriorly it is pierced by and | 
prolonged along the optic 
nerve. It is a smooth mem- 
brane connected to the globe 
Fic. 276.—TRANSVERSE VERTICAL SECTION THROUGH THE ORBIT of the eye by loose areolar 
BEHIND THE EYE-BALL TO SHOW THE ARRANGEMENT OF MUSCLES. tissue. It 18 pierced by the . 
tendons of the ocular muscles, 
along which it sends prolongations continuous with the muscular sheaths. 
RECTUS EXTERNUS — 
OBLIQUUS INFERIOR 
RECTUS INFERIOR 
