452 THE MUSCULAR SYSTEM. 



Nerve-Supply. The facial and scalp muscles are all innervated by the facial nerve. The 

 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 accom- 

 panied 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 Mecanisme 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 back- 

 wards 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 palpebrse 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 Fasciae 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 



posterior part of the eyeball 



LEVATOR PALPEBRSE SUPERIORIS ,, ... , ., . /^ m 



RECTUS SUPERIOR is the fascia bulbi (O.T. cap- 



OBLIQUUS SUPERIOR su l e O f Tenon), which COU- 



RECTUS MEDIALIS 



or synovial bursa in relation 

 to the posterior part of the 

 eyeball Anteriorly the cap- 



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 ^ ^ o-lobe of the eve bv 



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 palpebra? 

 superioris, belongs to the upper eyelid ; the other six are muscles of the eyeball. 



M. Levator Palpebrse Superioris. The levator palpebrae 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. 



