IX 



i;\ E 



339 



retina is bent inwards to form the iris (l\-. 141. n a kind of diaphragm 

 surrounding a circular opening the pupil (/>) behind which the lens is 

 situated. 



The inner layer of the choroid is in the Dogfish laden with silvi-ry- 

 looking flakes of guanin : this strongly reflecting layer is the tapetum. 

 A similar layer on the outer side of the choroid is the argentea : it is 



P. 



R 



o,n. 



C. 



v. 



FIG. 141. 



Illustrating the structure of the eye of a vertebrate, a, Aqueous humour : c, conjunctiva ; 

 C, cornea ; ch, choroid; i, iris ; /, lens; o.n, optic nt-rvc; P, pigment layer; p, pupil; /?, retina; 

 r, layer of rods ; s, sclerotic ; v, vitreous body. 



this which gives the shining metallic appearance to the iris as seen from 

 without. 



The eyeball retains its shape owing to its cavity being tensely filled 

 in the portion lying between iris and lens on the one hand and cornea 

 on the other with a watery fluid : the aqueous humour (Fig. 141, a) 

 and in the larger portion lying between lens and retina with a clear jelly 

 the vitreous body (Fig. 141, v) containing a few scattered cells. 



The eyeball is contained within the orbit. Its somewhat flattened 



