202 



ANIMAL MECHANICS. 



it forms a thin elliptical sheet, surrounding the fissure be- 

 tween the eyelids, and covering their surface, and spreading 

 for some distance outwards on the temple, upwards on the 

 forehead, and downwards on the cheek. The fibres may be 

 regarded as fixed at i, the inner angle of the eye, where 

 they terminate either in the tendo palpebrarum or in the 

 superior maxilla or frontal bone. 



The muscle is often considered by anatomists as composed 

 of an external and internal ring, according as the fibres lie 

 upon the eyelids (ciliaris) or lie outside them. The external 

 portion is bounded above by the frontal muscle, FF, with 

 whose fibres the outer fibres of the sphincter are blended ; 

 and the lower portion of the external sphincter sends off cross 

 slips, by which it blends itself with the malar muscles, MM. 



in. ; the fibres of the inner sphincter are elliptical, and theii 

 outer portion corresponds to an ellipse, whose axes are in 

 the proportion of 9 to 5— the radii of curvature at the ex- 

 tremities of the axes being 0.25 in. and 0.45 in. respectively. 

 When the whole orbicularis muscle contracts, its outer 



The frontal and malar 

 muscles are the antagonists 

 of the external portion of 

 the sphincter ; but the an- 

 tagonists of the inner por- 

 tion (ciliaris) are the radi- 

 ating fibres shown in Fig. 

 46, named levator palpebra- 

 rum, uu, levator palpebral 

 superioris, 11, a portion of 

 the orbicularis muscle. 



Fig. 46. 



nearly circular, and have a radii 



us 



The outer fibres of the 

 external sphincter are 

 of curvature equal to 0.89 



