VISION. 



501 



which we do not understand the design, and 

 which I have therefore purposely omitted to 

 notice, as being foreign to the object of this 

 treatise. 



In most birds the memhrana nictitans, or third 

 eye-lid, is of considerable size, and consists of a 

 semi-transparent fold of the conjunctiva, lying, 

 when not used, in the inner corner of the eye, 

 with its loose edge nearly vertical : it is repre- 

 sented at N, Fig. 434, covering half the surface 

 of the eye : its motion, like that of a curtain, is 

 horizontal, and is effected by two muscles: the 

 first of which, seen at q, in Fig. 435, is called 

 from its shape the quadratus, and arises from the 

 upper and back part of the sclerotica : its fibres 

 descending in a parallel course towards the optic 



nerve, where they terminate, by a semi-circular 

 edge, in a tubular tendon. This tendon has no 



