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Nerve-Supply. The facial and scalp muscles are all innervated by the facial nerve. posterior auricular branch supplies the posterior auricular muscle and occipitalis; the branches into which it breaks up in the parotid gland supply the frontalis, superior and anterior auricular muscles, the several muscles associated with the apertures of the eye, nose, and mouth (including the buccinator), and the platysma.
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, for example, is betrayed by the action of one set of muscles, while grief is accompanied by the contraction of another (opposing) set. Determination or eagerness is 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 Expression of the Emotions in Man and Animals, and Duchenne's Mécanisme de la Physiologie humaine.
The platysma retracts and depresses the angle of the mouth, and depresses the mandible. The epicranius, by its anterior belly, raises the eyebrows; both bellies acting together tighten the skin of the scalp; acting along with the orbicularis oculi, it shifts the scalp backwards and forwards. The corrugator supercilii draws the eyebrow medially and wrinkles the skin of the forehead vertically. The procerus draws downwards the skin between the eyebrows, as in frowning. The upper eyelid is raised by the levator palpebræ superioris. The closure of the lids is effected by the orbicularis oculi, whose fibres also assist in the lowering of the eyebrows, in the protection of the eyeball, and, by pressure on the lacrimal gland, in the secretion of tears. The tarsal part, acting along with the orbicularis oculi, compresses the lacrimal sac and aids in the passage of its contents into the naso-lacrimal duct. The muscles of the ear and nose have quite rudimentary actions. Of the muscles of the mouth, the orbicularis oris has a complex 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 (zygomaticus, quadratus labii superioris), laterally (zygomaticus, risorius, platysma, triangularis, buccinator), and downwards (triangularis, quadratus labii inferioris, platysma). The mentalis muscle 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 PALPEBRE SUPERIORIS
posterior part of the eyeball is the fascia bulbi (O.T. capsule of Tenon), which constitutes a large lymph space or synovial bursa in relation to the posterior part of the eyeball. Anteriorly the capsule is in contact with the conjunctiva, and intervenes between the latter and the eyeball; posteriorly it is pierced by and prolonged along the optic nerve. It is a smooth membrane connected
FIG. 399.-TRANSVERSE VERTICAL SECTION THROUGH THE LEFT ORBIT to the globe of the eye by
BEHIND THE EYEBALL TO SHOW THE ARRANGEMENT OF MUSCLES.
loose areolar tissue. It is pierced by the tendons of the ocular muscles, along which it sends prolongations continuous with the muscular sheaths.
The muscles of the orbit are seven in number: one, the levator palpebræ superioris, belongs to the upper eyelid; the other six are muscles of the eyeball.
M. Levator Palpebræ Superioris. The levator palpebræ superioris lies immediately beneath the orbital periosteum and covers the superior rectus muscle. It has a narrow origin above that muscle from the margin of the optic foramen.
It expands as it passes forwards, to end, in relation to the upper lid, in a membranous expansion which is inserted in a fourfold manner: (1) into the orbicularis oculi and skin of the upper lid, (2) mainly into the superior border of the superior tarsus, (3) into the conjunctiva, and (4) by its edges into the upper border of the margin of the orbital opening.
Nerve-Supply. The muscle is supplied by the superior division of the oculo-motor nerve. Actions. It elevates the upper eyelid and antagonises the action of the orbicularis oculi muscle.
Mm. Recti.-The recti muscles are four in number-superior, inferior, medial, and lateral. They all arise from a membranous ring surrounding the optic foramen, which is separable into two parts-a superior common tendon, giving origin to the superior and medial recti and the superior head of the lateral rectus; and an inferior common tendon, giving origin to the medial and inferior recti and the inferior head of the lateral rectus. The two origins of the lateral rectus muscle are separated by the passage into the orbit of the oculo-motor, naso-ciliary, and abducent nerves. Forming flattened bands which lie in the fat of the orbit around the optic nerve and eyeball, the four muscles end in tendons which pierce the fascia bulbi, and are inserted into the sclera about eight millimetres (three to four lines) behind the margin of the cornea.
The superior and inferior recti are inserted in the vertical plane slightly
medial to the axis of the eyeball; the FIG. 400.-MUSCLES OF THE RIGHT ORBIT (from above). lateral and medial recti in the trans
verse plane of the eyeball; and all are attached in front of the equator of the eyeball.
M. Obliquus Superior.-The obliquus superior arises from the margin of the optic foramen between the rectus superior and rectus medialis. It passes forwards, as a narrow muscular band, medial to the rectus superior, and at the anterior margin of the orbit. forms a narrow tẹndon which passes through a special fibrous pulley (trochlea) attached to the roof of the orbit.
Its direction is then altered, and passing laterally, between the tendon
Abducent nerve of the superior
FIG. 401.-MUSCLES OF THE LEFT ORBIT (from lateral aspect).
rectus and the eyeball, it is inserted into the sclera between the superior and lateral recti, midway between
the margin of the cornea and the entrance of the optic nerve.
M. Obliquus Inferior.-The obliquus inferior arises from the medial side of the floor of the orbit just behind its anterior margin, and lateral to the nasolacrimal groove.
It forms a slender rounded slip, which curls round the inferior rectus tendon,
and passes between the lateral rectus and the eyeball, to be inserted into the sclera between the superior and lateral recti, and farther back than the superior oblique muscle.
M. Orbitalis (O.T. Müller's muscle) is a rudimentary bundle of non-striated muscular fibres bridging across the inferior orbital fissure and infra-orbital groove. It is supplied by fibres from the sympathetic, and may have a slight influence in the protrusion of the eyeball.
FIG. 402.-SCHEMATIC REPRESENTATION OF THE NERVES WHICH TRAVERSE THE CAVITY OF THE RIGHT ORBIT.
Nerve-Supply.-The muscles of the eyeball are supplied by the third, fourth, and sixth cerebral nerves. The trochlear (fourth nerve) supplies the obliquus superior; the abducent (sixth) supplies the rectus lateralis; the oculo-motor (third nerve) supplies the others-recti, superior, inferior, and medialis, and obliquus inferior.
Actions. The six muscles inserted into the eyeball serve to move the longitudinal axis of the eyeball upwards, downwards, medially, and laterally, besides causing a rotation of the eyeball on its own axis. The following table expresses the action of individual muscles. It must be remembered that, while similar movements occur simultaneously in the two eyeballs, the horizontal movements may, by adduction of the muscles of both sides, cause convergence of the axes of the two eyeballs for the purposes of near vision.
The muscles of mastication comprise the masseter, temporal, external and internal pterygoids, and buccinator (described above).
M. Masseter. The masseter is the most superficial. Covered by the parotid gland on the side of the face, it has an origin which is partly tendinous and partly fleshy. It arises in two parts: (1) superficially from the inferior border of the zygomatic arch in its anterior two-thirds, and (2) more deeply from the deep surface of the zygomatic arch in its whole length. The superficial fibres are
directed downwards and backwards towards the angle of the mandible; the deeper fibres are directed vertically downwards.
The muscle is inserted by fleshy and tendinous fibres into the lateral surface of the ramus and angle of the mandible and the coronoid process (Fig. 403). The deepest fibres
blend with the fibres of the subjacent temporal muscle.
The muscle is partially concealed on the face by the parotid gland, accessory parotid gland, and parotid duct; by the external maxillary artery; the branches of the facial nerve; and by the zygomatic and platysma muscles. It conceals the ramus of the mandible, and, at its
anterior border, is
FIG. 403.-MUSCLE-ATTACHMENTS TO THE LATERAL ASPECT OF THE MANDIBLE.
separated from the buccinator muscle by the corpus adiposum bucca.
M. Temporalis.-The temporal muscle is a fan-shaped muscle arising from the whole area of the temporal fossa, as well as from the temporal fascia which covers it. Its converging fibres pass medial to the zygomatic arch.
The muscle is inserted into the deep surface and apex of the coronoid process, and into the anterior border of the ramus of the mandible (Figs. 403 and 404).
The origin of the muscle is concealed by the temporal fascia. As it passes to its insertion the muscle is concealed by the zygomatic arch, the masseter muscle, and the coronoid process of the mandible. It is separated from the external pterygoid in a majority of cases by the internal maxillary artery. The masseteric
nerve and vessels appear at its posterior border; the buccinator nerve and vessels at its anterior border.
M. Pterygoideus Externus.-The external pterygoid muscle is deeply placed under cover of the temporal muscle, in the infra-temporal fossa. It arises by two heads, superior and inferior. The superior head is attached to the infra-temporal surface of the great wing of the sphenoid; the inferior head takes origin from the lateral surface of the lateral pterygoid lamina of the pterygoid process.
The muscle is directed laterally and backwards, to be inserted into (1) the