THE SPECIAL SENSES. 205 



64. The Iris. Continuous with the choroid, in the front 

 part of the globe of the eye, is a thin, circular curtain, 

 which occasions the brown, blue, or gray color of the eye 

 in different individuals. On account of the varieties of its 

 color, this membrane has received the name Iris, which is 

 the Greek word for "rainbow" (see Fig. 48, i). A front 

 view of it is shown in Fig. 47. The iris is pierced in its 

 centre by a round opening, called the pupil (P), which is 

 constantly varying in size. In olden times it was spoken 

 of as the " apple of the eye." The hinder surface of the 

 iris, except in albinos, has a layer of dark coloring matter 

 resembling that of the choroid. The iris is a muscular 

 organ, and contains two distinct sets of fibres; one of which 

 is circular, while the other radiates outward from the pupil. 

 The action of these sets of fibres regulates the size of the 

 pupil ; for when the circular set acts, the pupil contracts, 

 and when the other set acts, the opening expands. Their 

 action is involuntary, and depends on the reflex system of 

 nerves, which causes the contraction of the pupil when a 

 strong light falls upon the eye, and its expansion when 

 the illumination is feeble. 



65. The iris, accordingly, serves a very useful purpose 

 in regulating the admission of light to the eye. It, how- 

 ever, does not act instantaneously; and hence, when we 

 pass quickly from a dark room into the bright sunlight, 

 the vision is at first confused by the glare of light, but as 

 soon as the pupil contracts, the ability to see becomes per- 

 fect. On the other hand, when we enter a dark apart- 

 ment, such as a cellar, for a short time we can see nothing 

 clearly ; but as soon as the pupil expands and admits more 

 light, we are enabled to distinguish the surrounding ob- 

 jects. Animals of the cat species, and others which prowl 

 around after nightfall, are enabled to see in the dark by 



64. What is the iris ? Its construction ? How is the size of the pupil regu- 

 lated ? 



65. The admission of light to the eye ? The action of the iris under different 

 circumstances'? The lustre of the eye, how affected in youth and old age ? 



