378 COORDINATION AND SENSATION 



sional white corpuscle. It has a feeble motion and is 

 slowly added to and withdrawn from the eye. It is sup- 

 plied mainly by the blood vessels in the ciliary processes 

 and finds a place of exit through a small lymph duct at 

 the edge of the cornea (Fig. 159). 



The back portion of the eyeball is filled with a soft, 

 transparent, jelly-like substance, called the vitreous humor. 

 It is in contact with the surface of the retina at the back 

 and with the attachments of the lens in front, being sur- 

 rounded by a thin covering of its own, called the hyaloid 

 membrane. The aqueous and vitreous humors aid in keep- 

 ing the eyeball in shape and also in focusing. 



How we see Objects. To see an object at least four 

 things must happen : 



1. Light must pass from the object into the eye. Objects 

 cannot be seen where there is no light or where, for some 

 reason, it is kept from entering the eye. 



2. The light from the object must be focused (made to 

 form an image) on the retina. In forming the image, an 

 area of the retina is stimulated which corresponds to the 

 form of the object. 



3. Impulses must pass from the retina to the brain, 

 stimulating if to produce the sensations. 



4. The sensations must be so interpreted by the mind 

 as to give an impression of the object. 



Focusing Power of the Eyeball. The eyeball is essen- 

 tially a device for focusing light. All of its transparent 

 portions are directly concerned in this work, and the 

 portions that are not transparent serve to protect and 

 operate these parts and hold them in place. Of chief 

 importance are the crystalline lens and the cornea. 

 Both of these are lenses. The cornea with its inclosed 

 liquid is a plano-convex lens, while the crystalline lens is 



