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 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 cor- 

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

 presented 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 de- 

 scending in a parallel course towards the optic nerve, where 



433 



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 

 pyramidaliS) 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, which is a magnified 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 



