ANA TOMY OF BIRDS 



267 



om/i, which nearly surrounds the eye, and whose chief office is to 

 lift the lower lid ; the latter has a small distinct depressor muscle. 

 Birds have no true hairs, hut in some kinds modified filiform feathers 

 answer to eyelashes. When wide open the orifice of the lids is 

 circular, that is, without the inner and outer corners {canthi) of 

 almond-eyed creatures like man. There is a third inner eyelid, 

 highly developed and of beautiful mechanism : this is the rdditating 

 membrane, or "winker" (nictito, I wink), a delicate, elastic, trans- 

 lucent, pearly-white fold of the conjunctiva. While the other lids 

 move vertically and 

 have a horizontal com- 

 missure, the winker 

 sweeps horizontally or 

 obliquely across the ball, 

 from the side next the 

 beak to the opposite. 

 If we menace a bird's 

 eye with the finger, it 

 is curious to see the 

 Avinker rush out of the 

 corner to protect the 

 ball. Owls habitually sit 

 in the daytime with this 

 curtain shading the eyes 

 from the glare of light ; 

 and doubtless the eagle 

 throws the same screen 

 over its sight when soar- 

 in"" towards the sun. I^''G- si.— Right eyeball, seen from beliind, showing 

 __ ° . . , ' the muscles : a, rectus superior ; b, rectus externus ; c, 

 When not in action, the rectus inferior; d, rectus internus ; c, obliquus superior; 

 wmL-Av li'p« ^iirlprl nn in ■''C"°* lettered), obliquus inferior; 3, quadratus ; h, pyra- 

 uiiiKei liess cillieu up lu niidalis, with its tendon, k, passing through a pulley in 

 the corner of the eve the quadratus (as shown by the dotted line) to keep it otf 

 , ' the optic nerve, i, then passing around the edge of the 

 like those patent window bail to its insertion in the nictitating membrane. 



shades which stay up of 



themselves till pulled down. The ingenious mechanism of the 

 movement of the winker across the lid may be understood with the 

 help of Fig. 81, which represents the hack of the eyeball. The 

 winker lies in front, on the left hand of the picture, and is to be 

 pulled across the front by the slender tendon, k, of the pyramidcdis 

 muscle, h. As h contracts it pulls on h, and k, winding round to 

 the front, pulls the winker to the right hand. But i is the 

 optic nerve, entering the ball ; k would press upon it, were it 

 not fended off by passing, as seen by the dotted line, through a 

 pulley in the end of the quadruius muscle, g. The harder h pulls, 

 the harder does g also pull, their consentaneous action at once 



