294 



THE SPECIAL SENSES 



FIG. 65. FRONT SECTION OF THE EYEBALL, 

 VIEWED FROM BEHIND, AND SHOWING SlJS- 

 PENSORY LIGAMENT, IRIS, AND PUPIL 



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. 64, i). A front 

 view of it is shown in Fig. 

 63. The iris is pierced in 

 its centre by a round open- 

 ing, called the pupil (P), 

 which is constantly vary- 

 ing 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 mat- 

 ter 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. Their action regulates 

 the size of the pupil ; for when the circular set acts, the open- 

 ing contracts. 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 expan- 

 sion when the illumination is feeble. The suspensory ligament 

 holds the crystalline lens in its place. (Fig. 65.) 



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

 regulating the admission of light to the eye (see Fig. 65). It, 

 however, 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 perfect. On the 

 other hand, when we enter a dark apartment, Such as a cellar, 

 for a short time we can see nothing clearly ; but as soon as the 



65. The admission of light to the eye ? The action of the iris under different "'-nm- 

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



