THE MUSCLES OF THE EYE AND EAR. 61 



the eye and its appendages in a Raven, we find that its 

 movements are effected by eleven muscles. The or- 

 hicularis 2:)alpehrai'um by its contraction closes the 

 integamental lids. It is antagonized by a feeble levator 

 "palpehrcB superioris in the upper lid, and a much better 

 developed depressor jjolpebrcB inferioris in the lower 

 lid, which latter is the one which really " closes the 

 eye." 



The third eyelid, or memhrana nictitans {nictito, I 

 wink), is governed in its action by the two muscles, the 

 quadratus and the pyramidalis, but their operation is 

 best seen by a study of Fig. 23. AVhen these muscles 

 cease to act, the nictitating membrane contracts by 

 virtue of its own elasticity, and again becomes hidden 

 within the recess of the inner can thus, above the com- 

 missural point, of the true lids. 



The remainino^ six muscles are devoted to the move- 

 ments of the eyeball itself. Any of the recti, when 

 acting, antagonize the muscle inserted diametrically 

 opposite it ; the two obliques similarly oppose each 

 other, and through the varied traction of the group the 

 line of vision is directed to meet the will of the bird. 

 Owing to the close contact of the bony walls of its 

 chamber, the greater simplicity of the ohliqui, and, 

 finally, the shortness of all these muscles, the mechanism 

 involving the movements of the eyeball is not nearly so 

 complicated a one in the bird, as we find it to be in any 

 Mammalian form. 



