THE SENSE OF SIGHT. 289 



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 

 room. We see, what we have all noticed in watching the eyes of a cat, 

 that when subject to a bright light the pupil is small, but with less light 

 the pupil is larger. The iris has circular muscle fibers that reduce the 

 pupil when there is too much light for the eye, and when the light is 

 feeble the pupil opens wider. 



The Refracting Media of the Eye. The media that 

 refract the rays of light to form the images on the retina 

 are the cornea, the aqueous humor, the crystalline lens, and 

 the vitreous humor. The cornea has already been described. 



The Aqueous Humor. In looking at the entire eye it 

 is not easy to realize that there is a space between the cor- 

 nea and the iris. In this space is the clear, watery aque- 

 ous humor. 



The Vitreous Humor. All but the front part of the 

 space within the coats of the eye is filled with a clear, 

 jellylike substance, the vitreous humor. 



The Crystalline Lens. Just back of the iris is a double- 

 convex lens, clear as crystal, and of about the consistency 

 of a gumdrop. It is less convex on the front surface. 



The Lens Capsule. The lens is completely enveloped 

 in a thin, transparent membrane called the lens capsule. 



The Hyaloid Membrane. A thin membrane, the hya- 

 loid membrane, lines the inner surface of the retina. As 

 it continues forward toward the lens capsule it is called 

 the suspensory ligament. 



