bird's-eye views. 



^ 



has also a smooth oval plate of cartilage to stiffen it. The 

 «pper one is raised by a very small muscle, called levator 

 palpebrce sujjerioi-is, arising from the rim of the bony orbit, 

 -and running to the edge of the lid. There is no special 

 lowering muscle ; it is depressed by the action of part of 

 another muscle, the orblcidaris ocnli, thnt nearly surrounds 

 the eye, the chief office of which is to pull up the lower lid. 

 The latter has a small distinct muscle for its depression. 



A bird's eye, when wide open, appears almost perfectly 

 ■circular ; there are no well marked corners or angles {ccoif hi) 

 in front or behind, as in man and most mammals. Birds 

 have no true eyelashes, ])ut some kinds have two series of 

 short modified feathers 

 a,long the edges of the 

 eyelids, that may l)c 

 considered to corres- 

 pond to the hairs foimd 

 in this situation in 

 mammalia. 



Now let us separate 

 the lids and look at 

 the eye. Not yet ! "In 

 the twinkling of an 

 eye" a third lid is dis- 

 closed inside the other 

 two, throwing a veil 

 over the ball. This 

 third inner eyelid is the nictitating membrane {membrana 

 nictitcms) , a very curious structure, both in its movements 

 and functions. It is a very thin, delicate, elastic membrane, 

 transparent, or nearly so, of a delicate pearly-white color. 

 While the other two lids move vertically, and have a hori- 



* Fig. 1, right eyeball, seen from behind, showing the muscles. «, rectus superior; 

 b, rectus externus ; c, rectus inferior ; d, rectus internus ; e, obliquus superior ; /", obll* 

 quus inferior; g, quadratus; h, pyramidalis, with its tendon, /.-, passing through pulley 

 in quadratus (as shown by dotted line) to keep it from pressing on optic nerve /, then 

 running to the edge of the ball, around which it passes. 



