IX 



EYE 



339 



retina is bent inwards to form the iris (Fig. 141, i) a kind of diaphragm 

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

 situated. 



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

 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 



Fig. 141. 



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

 C, cornea ; ch, choroid ; i, iris ; I, lens ; o.m, optic nerve ; P, pigment layer ; p, pupil ; R, retina ; 

 f, 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 



