ANIMAL MECHANICS. 



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fibres, being circular, contract symetrically towards the centre ; 

 but the elliptical fibres, being attached to the tendo palpebra- 

 rum on the inner side, on contracting draw with them the 

 entire orbicular muscle towards the nose; the force with which 

 this is done varying as the curvature of the fibres at the 

 outer side of the eye. 



The motor nerves of the orbicularis muscle are derived 

 from the temporal branch of the facial nerve (7th pair). These 

 same nerves furnish the motor supply of the frontalis muscle. 

 The other antagonist of the orbicularis muscle, viz., levator 

 palpebrce, derives its motor supply from the 3rd pair of nerves 

 (motores oculorum). There would thus seem to be a more 

 complete antagonism between the levatores palpebrarum and 

 the orbicularis internus than between the frontalis and orbicu- 

 laris externus. 



The mouth furnishes us with an extremely complex but 

 most beautiful example of sphincter and radiating muscles, 

 whose varied action produces an infinite variety in the shape 

 and expression of the lips. 



The orbicular muscle of the mouth, like that of the eye, is 

 elliptical, and constitutes the only force available for closing 

 the lips. The radiate muscles of the mouth are eighteen in 

 number, nine at each side, placed symmetrically in relation to 

 those of the opposite side. The nine muscles at each side of 

 the mouth are arranged in two sheets, an outer and an 

 inner ; the outer sheet consists of six muscles, and the inner 

 sheet consists of three muscles, which overlap the muscles ot 

 the outer sheet at the angles of the mouth, where the curva- 

 ture of the orbicular muscle is greatest. 



These muscles are — 



