AVES BIRDS. 



618 



The third eye-lid, or nictitating membrane, is represented in 

 fig. 280, A, e ; the upper and the lower eye-lids having been divided 

 through the middle, and turned back to display it: it is necessarily, 

 to a certain extent, transparent, for birds sometimes look through it ; 

 as for instance, when the eagle looks at the sun :* it is, therefore, of a 

 membranous texture ; and a Fig. 280. 



most admirable and peculiar 

 muscular apparatus is given, 

 by which its movements are 

 effected. This is placed at 

 the back of the eye-ball, and 

 may easily be displayed by 

 turning aside the recti and 

 obliqui muscles, as in Jig. 

 280, B. Two muscles are 

 then perceived arising from 

 the globe of the eye, taking 

 their origin from the outside 

 of the sclerotic coat : one of 

 these (c), named the quad- 

 ratus membrane nictitantis, 

 arising from near the upper 

 aspect of the eye, descends 

 towards the optic nerve ; but 

 instead of being inserted into 

 anything, as muscles usually 

 are, it terminates in a most remarkable manner, ending in a tendi- 

 nous sheath or pully, through which the tendon of the next muscle 

 passes as it winds around the optic nerve. The second muscle (d) 9 

 called the pyramidalis memb. nictitantis, arises from the inner 

 aspect of the eye-ball ; and its fibres are collected into a long, 

 slender tendon, which, as it turns round the optic nerve, passes 

 through the tendinous sheath formed by the quadratus, as a rope 

 through a pully, and then is continued in a cellular sheath formed 

 by the sclerotic, underneath the eye, to the lower angle of the third 

 eye-lid, into which it is inserted. The reader will at once perceive 

 how beautifully these two muscles, acting simultaneously, cause the 

 nictitating membrane to sweep over the cornea, which returns again 

 into the inner canthus of the eye by its own elasticity. 



(682.) Being thus provided with moveable eye-lids, a lacrymal 



* Cuv. Lemons, d'Anat. Comp. torn. ii. p. 431. 



