THE EYE 



373 



one looks toward the clear sky, he receives a sensation 

 of light, but sees no object. He may also get a sensa- 

 tion of light with the eyelids closed, if he turn the eyes 

 toward the window or some bright light. But how differ- 

 ent when the light from various objects enters the eyes. 

 There is apparently no consciousness of light, but instead 

 a consciousness of the size, form, color, and position of the 

 objects. Seeing is perceiving objects. Stimulation by the 

 light waves is only the means toward this end. The chief 

 problem in the study of sight is that of determining how 

 light waves enable us to become conscious of objects. 



Sense Organs of Sight. The sense organs of sight con- 

 sist mainly of the two eyeballs. Each of these is located 

 in a cavity of the skull bones, called the orbit, where it is 

 held in position by suitable tissues and turned in different 

 directions by a special set of muscles.- A cup-shaped 

 receptacle is provided within the orbit, by layers of fat, 

 and a smooth surface is supplied by a double membrane 

 that lies between the fat and the eyeball. In front the 

 eyeballs are provided with movable coverings, called the 

 eyelids. These are composed of dense layers of connec- 

 tive tissue, covered on the outside by the skin and lined 

 within by a sensitive membrane, called the conjunctiva. 

 At the base of the lids the conjunctiva passes to the eye- 

 ball and forms a firmly attached covering over its front 

 surface. This membrane prevents the passage of foreign 

 materials back of the eyeball, and by its sensitiveness 

 stimulates effort for the removal of irritating substances 

 from beneath the lids. The eyelashes and the eyebrows 

 are also a means of protecting the eyeballs. 



The Eyeball, or globe of the eye, is a device tor focusing 

 light upon a sensitized nervous surface which it incloses 

 and protects. In shape it is nearly spherical, being, about 



