258 Physiology. 



an object passes through the cornea, aqueous humor, lens, 

 and vitreous humor, and the rays are so refracted as to 

 form an inverted image on the retina. 



The Blind Spot. Light falling on the optic nerve itself has no effect 

 in giving a sensation of light. If the light falls on the spot where the 

 optic nerve enters the eyeball, we see nothing. Hence, this spot is 

 called the blind spot. 



Experiment illustrating the Blind Spot. At the left (as looked at 

 by the class) of a long blackboard make a bright circular spot, three 

 inches in diameter, with white or yellow crayon. Beginning at the 

 right of this write the figures I, 2, 3, etc., along the whole length of the 

 board, about eight inches apart. Let each pupil close the right eye and 

 look at the bright spot. Then let each read the figures, passing slowly 

 from one to another, at the same time noticing, whether the bright 

 spot can be seen. To succeed in this the eye must not be allowed to 

 waver. Have the pupils tell when the bright spot disappears, then 

 read on, and note when the spot reappears. 



Another Experiment. In this experiment shut the right eye, and 

 be careful not to let the left eye waver. 



>fc Read this line slowly. Can you see the star all the time ? If the 

 star does not disappear before reaching the end of the line, let the eye 

 travel on across the right-hand page, or hold the book nearer the face. 

 In the human eye the optic nerve enters the eye not in the center, but 

 nearer the nose, so that in turning the left eye toward the right at the 

 proper angle, the image of the star falls upon the spot where the optic 

 nerve enters. As this spot is insensitive to light, the star no longer 

 appears. 



The Ojftic Nerve not Sensitive. The optic nerve, while capable 

 of carrying nerve impulses that cause sensations of light, is not itself 

 sensitive to light. If the optic nerve be cut, it does not give pain, but 

 gives the sensation of a flash of light. 



Sympathy between the Two Eyes. While most of the 

 fibers from each optic nerve cross to the other side of the 

 brain, some fibers go to the same side of the brain. We 

 can therefore better understand the close sympathy be- 

 tween the two eyes. 



