SPECIAL SENSES 371 



keep out dust and slightly to shade the eye. The lids are 

 lined with a mucous membrane which furnishes moisture. 

 This acts as a lubricant so that the eyelids close over the eyes 

 without friction. This membrane is also deflected over 

 the surface of the eye. On the upper outer side of the eye is 

 situated a gland, the lachrymal gland. This secretes a 

 salty fluid which also serves to keep the eye moist and clean 

 of dust. At the inner angle of the eye, next the nose, a tiny 

 duct ordinarily carries into the nasal passage any excess 

 of this secretion. If the membrane covering the eye is 

 irritated or strong emotion is aroused there is such a copi- 

 ous flow of this secretion that the tear duct fails to carry 

 it off and it overflows the edges of the eyelids as tears. 



The orbit of the eye is lined with a fatty layer which forms 

 a sort of cushion for the eyeball. The optic nerve (fig. 185,0) 

 enters the eyeball from behind, through an opening in the 

 socket. This nerve is composed of fibers which pass to 

 the visual centers of the brain. 



The eyeball is held firmly in place and moved from side 

 to side or up and down by six strong muscles. These muscles 

 are controlled by motor nerves from the third, fourth and 

 sixth cranial nerves. The- muscles of the eyeballs contract 

 and relax in pairs so that the eyes move together. 



The eyeball consists of three concentric coats surround- 

 ing and enclosing certain transparent substances through 

 which the light passes to the inner coat of the eye. On this 

 inner coat the sensory surface is found. 



The outer coat is the sclerotic coat (fig. 185,8), commonly 

 known as the white of the eye. It is formed of very tough 

 dense connective tissue, and serves as the main protecting 

 coat. In the front part of the eye, the sclerotic is trans- 

 parent and bulges slightly outward. This part is known 

 as the cornea (fig. 185, C). Within the sclerotic coat and 

 closely applied to it is the choroid coat (Ch). This is a con- 

 nective tissue layer through which ramify the blood-vessels of 



