BIRD’S-EYE VIEWS, 507 
has also a smooth oval plate of cartilage to stiffen it. The 
upper one is raised by a very small muscle, called levator 
palpebræ superioris; arising from the rim of the bony orbit, 
md running to the edge of the lid. There is no special 
lowering muscle ; it is depressed by the action of part of 
mother muscle, the orbicularis oculi, that 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 
tireular; there are no well marked corners or angles (canthi) 
in front or behind, as in man and most mammals. Birds 
have no true eyelashes, but some kinds have two series of 
short modified feathers Fig. 1.* 
along the edges of the 
eyelids, that may be 
‘onsidered to corres- 
pond to the hairs found 
m this situation in 
mammalia, 
Now let us separate 
the lids ana look at 
the eye. Not yet! “In 
the twinkling of an 

titans yond is the nictitating membrane aren 
a Gi. a very curious structure, both ip its movements 
‘ons. It is a very thin, delicate, elastic membrane, 
hi, or nearly so, of a delicate pearly-white color. 
le other two lids move verti ally, and have a hori- 

bd 
8.1 yj is 
ead eyeball, seen from behind, showing the: muscles. æ, rectus superior; 
Xter 
Ns inferi nus; c, rectu inferior; d, rectus in us; e, obliquus superior; f, obli- 

