VISION. 



353 



is found in almost every bird having extensive powers of 

 vision.* The comparative anatomy of the eye offers, in- 

 deed, a great number of special structures of which we do 

 not understand the design, and which I have therefore pur- 

 posely omitted to notice, as being foreign to the object of 

 this treatise. 



In most birds the memhrana nictiians, 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 cor- 

 ner of the eye, with its loose edge nearly vertical: it is re- 

 presented at N, Fig. 434, covering half riie 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 de- 

 scending in a parallel course towards the optic nerve, where 



433 



434 



^Zb 



they terminate, by a semicircular edge, in a tubular tendon. 

 This tendon has no particular attachment, but is employed 

 for the purpose of serving as a loop for the passage of the 

 long tendon of the second muscle (p,) which is called the 

 jjyramidalis, and which arises from the lower and back part 

 of the sclerotica. Its tendon (t,) after passing through the 



• It is shown at m, Fig. 433, wliich is a mag-nified section of the eye of a 

 Goose, c is the cornea; i, the iris; p, the ciliary processes, s, the sclerotic 

 coat, and o, the optic nerve. 



Vol. II. 45 



