254 Physiology. 



The Choroid Coat. The middle coat of the eye is the 

 choroid. It is thinner than the sclerotic and of much more 

 delicate structure. It is full of blood tubes, and has an 

 inner lining of dark color to prevent the reflection of light 

 in the eye, just as most optical instruments are painted 

 black on the inside. 



The Retina. The retina is an expanded part of the 

 optic nerve and forms an inner coat that lines all but the 

 front part of the eye. It is a thin, translucent film, some- 

 what like the film that forms over the white of an egg when 

 it is first dropped into hot water. It is very delicate and 

 easily torn. The retina is the only part of the eye that is 

 sensitive to light, and on it the images must be formed to 

 produce distinct vision. 



The Cornea. The clear front part of the eye is the 

 cornea. It is a continuation of the sclerotic coat and is 

 more bulging than the rest of the front of the eye, as can 

 be seen by taking a side view of the eye, or by noticing 

 some one who closes the eyelids and rolls the eyes about. 



The Iris. This is the part that gives the color to the 

 eye, or if the pigment that gives the color is lacking, the 

 blood gives the pink color seen in albinos. The iris is a 

 forward continuation of the choroid coat. 



The Pupil. Most of the light that passes through the 

 transparent cornea is stopped by the opaque iris. But in 

 the center of the iris is a round hole through which light 

 enters the interior of the eye. The pupil looks dark be- 

 cause it is the only opening into a dark room. 



Regulation of the Amount of Light admitted into the Eye. 

 Hold a hand glass between the face and a well-lighted window. Note 

 the size of the pupils. Quickly turn toward the darkest part of the 



