264 OUR BODIES AND HOW WE LIVE 



amount of light ; and the lens, with the greatest exactness, 

 has focused the rays on the retina. The dark surface of 

 the middle coat acts to absorb the excess of light, the 

 entrance of which would disturb accurate vision. 



As a result, an exact but inverted image is formed on the 

 retina. The impression is carried to the brain by the fibers 

 of the optic nerve, which are spread out on this wonderfully 

 sensitive membrane. 



Experiment 78. To show the blind spot. 1 The retina is not 

 sensitive where the optic nerve enters the eyeball. This is called 

 the " blind spot." Put two ink bottles, about two feet apart, on a 

 table covered with white paper. Close the left eye and fix the right 

 steadily on the left-hand inkstand, gradually varying the distance from 

 the eye to the ink bottle. At a certain distance the right-hand bottle 

 will disappear, but nearer or farther than that it will be plainly seen. 



364. The Muscles of the Eye. The eyeball is rolled and 

 moved about by six muscles. They spring from the back 

 part of the bony orbits and are fastened to the front part of 

 the eyeball by means of tendons. Four of these muscles 

 (the recti, or straight, muscles) move the eye up or down, 

 and to the right or left. The other two (the oblique) are 

 so fastened that they rotate the eyeball in one direction 

 or another. If the eye muscles are not properly balanced 

 in their action, squinting results. 



1 The location of the blind spot may be shown in the following simple 

 manner. The left eye being shut, let the right eye be fixed on the cross 

 below. When the book is held at arm's length both cross and round spot 



will be visible ; but if the book be brought to about eight inches from the 

 eye, the gaze being kept steadily upon the cross, the round spot will at first 

 disappear, but as the book is brought still nearer both cross and round spot 

 will again be seen. 



