400 A YEAR IN SCIENCE 



A tear gland is located on the outer and upper 

 side of each eye. The fluid secreted by this gland 

 passes over the eye and is carried away by a duct, of 

 Avhich one branch opens on the inner angle of the lower 

 eyelid, and another opens in the upper eyelid. The 

 other end f this duct opens into the nasal passage. 



The eyeball is held in place and moved by six muscles. 

 One end of these muscles is attached to the eyeball, 

 and the other end at the back part of the bony socket 

 into which the eye fits. One set of these muscles turns 

 the eye toward or away from the nose. A second set 

 turns the eyeball upward or downward. The third set 

 of oblique muscles rotates the eyeball. 



Structure. The eye is made up of three layers or 

 coats. The outer coat is opaque and white, forming 

 the " white of the eye." It is known as the sclerotic 

 layer. It is a tough strong coat filled with blood ves- 

 sels. At the front this layer becomes transparent and 

 forms the cornea. 



Inside of the sclerotic layer is the choroid coat, which 

 is colored black. In front it is usually brown or blue 

 and forms the iris. In the center of the iris is an open- 

 ing, the pupil. This is controlled by muscles which 

 open or close it. 



The innermost coat, the retina, covers the back por- 

 tion of the choroid coat. It is a very complex mem- 

 brane, and in it are the endings of the optic nerve 

 which enters the eye at the back. 



Just back of the pupil is the crystalline lens. Between 



