THE SPECIAL SENSES. 133 



circle round the iris. When these contract they narrow the 

 pupil. In the other layer the fibres run from the outer 

 border of the iris to the free margin of the pupil. When 

 these act they pull back the curtain and dilate the pupil. 

 No light can enter the eye excepting through the pupil, so 

 that the amount of light admitted is regulated by the iris. 



FIG. 40. Section of Eye-ball. 



If the light is very bright, the pupil becomes small : if it is 

 dim, it enlarges. 



This ring-like curtain, which is drawn and withdrawn 

 according to the intensity of the light, varies in color in 

 different persons. The color of one's eye is the color of the 

 iris. Blue, gray, black or brown, with the varying shades 

 between, are all due to the color of the iris, except that in 

 some persons the pupil is always large and gives the 0&** & 

 dark appearance. 



