236 



GENERAL SCIENCE 



Cornea 

 trfi- 



Structure of the Eyeball. - - The eyeball has a diameter 

 of about one inch. Except at the front and at the place 

 where the optic nerve enters, the eye is covered with 

 an opaque, white coat called the sclerotic coat (Figure 212). 

 This coat forms the white of the eye "and the transparent 



modification of it at 

 the front of the eye 

 is called the cornea. 

 Inside the sclerotic 

 coat is a second layer 

 called the choroid. 

 This coat contains a 

 network of blood ves- 

 sels and is colored black 

 by pigment cells so that 

 it appears like the in- 

 side of a dark grape 

 skin. The dark sur- 

 face of this coat absorbs the rays of light which would 

 otherwise be reflected and interfere with perfect vision. 

 In albinos this coat has no dark pigment cells, and the 

 vision is therefore imperfect in strong light. 



At the front of the eye the choroid coat is continued 

 as a muscular curtain known as the iris. At the center 

 of the iris is a round opening (the pupil) through which 

 the light enters the eye. The size of the pupil is regu- 

 lated by the involuntary muscles of the iris, of which 

 there are two sets, one circular, and the other radiating. 

 The contraction of the circular muscle fibers decreases the 

 size of the pupil, and the contraction of the radial fibers 

 increases the size of the pupil. The iris is colored by 

 pigment. When we speak of a person's eyes as blue or 

 gray we simply mean that these colors are the predorni- 



FIG. 212. Showing a Section of the 

 Eyeball. 



