102 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



behind it a flat colored screen, the iris, in the center of 

 which is a round hole, the pupil, which in the living eye 



is seen to expand 

 X A or contract, accord- 



ing to the intensity 

 of the light (Fig. 

 72). The color of 

 the eye is the color 

 of the iris. The 

 pupil is black, as 

 any hole opening 

 in a dark room is 

 black. 



Linings. — For 

 further examina- 

 tion dissection is 

 necessary. We thus 

 find that the sclerot- 

 ic part is lined with 



Fig. 72. — Section of the eye: O, optic nerve; two membranes. In 



S, sclerotic ; C/i, choroid ; J?, retina ; v, vitre- , . . , 



ous body; Cm, ciliary muscle; Cj\ conjunc- direct contact With 



tiva; C, cornea; /, iris ; L, lens ; *, aqueous f i. crWntir ic a 



humor ;**, ciliary body or zonule of Zinn. mc ^ciuuc is a 



dark brown, almost 

 black, and very vascular coat, the choroid. Its function 

 is to absorb the light as soon as it strikes. The choroid 

 lines the whole sclerotic as far forward as the outer 

 margin of the cornea. It is there split into two layers, 

 the anterior one, as it were, drawn together and thick- 

 ened, forms the iris. The posterior one, also drawn 

 together and plaited, forms the ciliary processes radiat- 

 ing about the back portion of the lens. The innermost 

 lining coat is the retina, the most important of all. This 

 is a deep cup-shaped expansion of the optic nerve. This 

 nerve enters the eye socket at the bottom, comes for- 

 ward through the middle of the fatty cushion, pierces 



