THE SENSE ORGANS 419 



only skill consists in their ability to separate their victims from 

 their money. 



Sight. Plants and the lower animals respond to light but can 

 hardly be said to " see." The sensation of sight reaches us by 

 way of waves in the ether, which are studied more fully in Physics. 

 These light waves reach us from vast distances and at enormous 

 speed and put us in touch with a wider extent of our surroundings 

 than all the other senses combined. This fact, and its relation to our 

 other activities, make sight the most valued of all our senses. Yet 

 there is hardly an organ that we abuse more than we do our eyes. 



The simplest eyes were mere colored spots connected with special 

 nerves to absorb light and tell its direction. Now we have lenses 

 developed to concentrate light upon these sensitive pigment spots, 

 muscles to adjust both lens and eye and various devices to protect 

 the whole. 



Structure of the Human Eye. The eye is almost spherical in 

 shape, flattened a little from front to rear. The wall of the eye- 

 ball consists of three layers. The outer one is tough and white, 

 called the sclerotic coat, and shows in front as the " white of the 

 eye." The anterior surface of the sclerotic bulges out a little, and 

 becomes transparent in the circular region called the cornea. 



The second coat, called the choroid, is richly supplied with 

 blood vessels and pigment (color) cells which prevent reflection of 

 light inside the eye-ball. This coat shows in front as the iris or 

 " color " of the eye. The iris is provided with muscles which regu- 

 late the size of the center opening, the pupil, according to the 

 amount of light. 



The inner layer is the most delicate and complicated part of the 

 eye and is called the retina. It is really the expanded end of the 

 optic nerve and connects directly with the brain. It also has a dark 

 pigment and though only V of an inch in thickness, it consists 

 of at least seven distinct layers of cells which help in receiving the 

 impression which we call sight. 



The lens of the eye is located just behind the iris and is con- 

 nected to the choroid by delicate muscles which can change its 

 thickness, to adjust for near or distant vision. 



