440 MAN BECOMES THE CONQUEROR OF THE WORLD 



is able to perceive as low as thirty vibrations per second and as 

 high as almost thirty thousand. 



Seeing. The organ of vision, the eye, is almost spherical, and 

 fits into a socket of bone, the orbit. A stalklike structure, the optic 

 nerve, connects the eye with the brain. Free movement is made 

 possible by means of six little muscles which are attached to 



the outer coat of the 

 eyeball, and to the 

 bony wall around 

 the eye. 



The wall of the 

 eyeball is made up 

 of three coats. An 

 outer tough white 

 coat of connective 

 tissue is called the 

 sclerotic (skle-rot'ik) 

 coat. In front, where 

 the eye bulges out 

 a little, this outer 

 coat is replaced by a transparent tough layer called the cornea. 

 A second coat, the choroid (ko'roid), is supplied with blood vessels 

 and cells which contain pigments. The iris is part of this coat 

 which we see through the cornea as the colored part of the eye. 

 In the center of the iris is a small circular hole, the pupil. The 

 iris is under the control of muscles, and may be adjusted to varying 

 amounts of light, the hole becoming larger in dim light, and 

 smaller in bright light. The inmost layer of the eye is called the 

 retina (ret'i-nd). This is, perhaps, the most delicate layer in the 

 entire body. Despite the fact that the retina is less than -^ of 

 an inch in thickness, there are several layers of cells in its com- 

 position. The optic nerve enters the eye from behind and spreads 

 out over the surface of the retina. Its finest fibers are ultimately 

 connected with numerous elongated cells, which are stimulated 

 by light. The retina is dark purple in color, this color being due 

 to a layer of cells next to the choroid coat. This accounts for the 

 black appearance of the pupil of the eye, when we look through 



What happens to the eye when we pass from a brightly 

 lighted room into a dark room ? 



