350 WELLS'S NATURAL PHILOSOPHY. 



forms the front of the eye-ball. It is firmly united to, or 

 fixed in the sclerotic coat, like the glass in the case of a 

 watch. 



The Cornea is represented at a, Fig. 283, 



•What is the The Choroid coat is a delicate membrane, 

 Choroid Coat? lining the inner surface of the sclerotic cOat, 

 and covered on the interior with a black pigment. 



It is represented at k, Fig. 283. 



What is the Thc Rctiua is a delicate, transparent mem- 

 Retina? brauc wliich spreads over the chief part of the 

 internal surface of the eye-ball, and is situated imme- 

 diately within and close to the choroid coat. 



The position of the Retina is shown at to, Fig. 283. 



How is the re- Thc Tctina is formcd by the expansion of a 

 tina formed? jjervc Called the optic nerve, which proceeds 

 from the back of the eye through the bones of the skull 

 into the brain, and conveys to the brain the impressions 

 made by external objects on the organs of vision. If this 

 nerve were divided, notwithstanding the eye might be in 

 other respects perfect, the sense of sight would be de- 

 stroyed. 



No- 11, Fig. 282, and n, Fig. 283, exhibit the relative position of tho 

 optic nerve. 



What is the ^^ looking into the eye from without, we 

 irisT perceive a flat, circular membrane, which, in 



different eyes, is of a black, blue, or gray color. This 

 membrane is called the Iris, and divides the eye into two 

 very unequal portions. 



The Iria is represented at c d, Fig. 283. 



The Pupil of the eye is the circular black 

 Pupil of the opening in the center of the iris, and is th» 

 '^"^ space through which light is admitted intd 



the interior of the eye. 



The open space between c and d, Fig. 283, represents the pupil. It is, 

 properly speaking, the window of the eye, and appears black, only because 

 the chamber within and behind it is dark. When a small quantity of light 

 enters the eye the pupil widens or expands ; but when a largo quantity enters, 

 it closes or contracts. 



