THE SENSES 145 



can become larger or smaller so as to regulate the amount 

 of light which enters the eye. In a bright light it becomes 

 small and shuts out some of the rays. In a dim light it 

 becomes large, and admits all the light it can. Notice the 

 pupil of a cat's eye. In the middle of the day it is a 

 narrow slit, but in the evening it is almost round, and 

 admits more light than the pupil of a man's eye. A cat 

 can see well when it is so dark that we cannot see at all. 



Behind the pupil is a clear body, shaped like two saucers 

 put together by their edges, or like a magnifying glass. 

 It is called a lens. A magnifying glass brings rays of 

 light together into one bright spot. The lens of the eye 

 brings together the rays from an object, and they form a 

 picture upon the nerves in the back part of the eyeball, 

 like a picture in a photographer's camera. The nerves 

 carry the impression of the picture to the back part of the 

 brain and so produce sight. 



285. Movements of the eyes. The eyes can be turned 

 in any direction we wish by means of muscles. Some- 

 times the eyes will not turn together, but while one looks 

 at one object, the other looks somewhere else, making the 

 person cross-eyed. A cross-eyed person usually sees with 

 only one eye. If the eye is treated before a child has 

 grown, it can be cured. 



286. Coverings of the eyes. The eyeball lies upon a 

 bed of fat in a bony case. It is covered in front by two 

 lids of flesh. These can be shut so as to protect the eye 

 from dust or injuries. Whenever anything is about to 

 enter the eye, it causes the lids to close so as to shut it 

 out. We cannot help winking when something is about 

 to strike the eye. When we are sleepy, we cannot keep 

 the lids from falling together. 



Hairs grow from the edges of the lids. They curl 

 ov. PHYSIOL. (INTER.) 10 



