VISION. 447 



form of the sclerotica, this membrane in birds is 

 strengthened by a circle of bony plates, which 

 occupy the fore-part, and are lodged between the 

 two layers of which it consists. These plates vary 

 in number from fifteen to twenty ; and they lie 

 close together, their edges successively overlapping 

 each other. There is manifest design in this ar- 

 rangement ; for it is clear that a ring formed of a 

 number of separate plates is better fitted to resist 

 fracture, than an entire bony circle of the same 

 thickness. 



There is a dark-coloured membrane, called the 

 Marsupium, situated in the vitreous humour, the 

 use of which is unknown, though it appears to be 

 of some importance, as it is found in almost every 

 bird having extensive powers of vision.* The com- 

 parative anatomy of the eye offers, indeed, a great 

 number of special structures of 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 membrana 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 represented 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 qua- 

 drains, and arises from the upper and back part of 



* It is shown at m, Fig. 433, which is a magnified section of the 

 eve of a Goose, c is the cornea; i, the iris; p, the ciHary pro- 

 cesses; s, the sclerotic coat, and o, the optic nerve. 



