SEC. IV 



ANATOMY OF BIRDS 



267 



oculi, 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, but 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 nictitating 

 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 

 winker rush out of the 

 corner to protect ^;he 

 ball. Owls habitually sit 

 in the daytime with this 

 citttain shading the eyes 

 from the glare of light ; 

 and doubtless the eagle 

 throws the same screen 

 over its sight when soar- 

 ins towards the sun. P'"- si.— Right eyetall, seen from behind, showing 

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



When not in action, the rectus inferior ; d, rectus intemus ; e, obliquus superior ; 



' ^ 1 . 1 J • /^not lettered), obliquus inferior ; g, quadratus ; h, pyra- 



Winker lies CUriea up in midaUs, with its tendon, h, passing through a pulley in 

 tVip onv-npr nf fho AVP the quadratus (as shown by the dotted line) to keep it off 

 tne corner 01 Wie eye, ^^^ ^^^^ ^^^^^^ ^^^^^^ 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 back 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 pyramidalis 

 muscle, h. As h contracts it pulls on k, 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 quadratus muscle, g. The harder h pulls, 

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



