592 MANUAL OF PHYSIOLOGY. 



eyeball is so arranged, that the contractions of the external and 

 internal recti of opposite sides always occur in association, and 

 we call these " associated movements." This association of move- 

 ment has been acquired by the habit of voluntarily directing the 

 two eyes at the same object, and has gradually become involun- 

 tary, for few persons have the power of exerting voluntary con- 

 trol over the muscles of one or other eye alone. 



BINOCULAR VISION. 



When we look at an object with both eyes we have a separate 

 image thrown upon each retina, and therefore two sets of im- 

 pulses are sent to the sensorium, one from the right and one from 

 the left eye. Yet we are only conscious of the occurrence of one 

 stimulation. The reason of this is, that experience has taught 

 us that similar images thrown upon some certain parts of the two 

 retinae correspond to the same object, and in our minds we fuse 

 the sensations caused by the two images so that they produce but 

 one idea. 



These points of the retina which are thus habitually stimulated 

 by the same objects are called " corresponding points." 



Besides being of great use in making up for such deficien- 

 cies as the blind spots (which are not corresponding points) 

 binocular vision is useful for the following purposes : 



To judge of distance. When using one eye only, some knowl- 

 edge of distance may be gathered by the force employed to ac- 

 commodate, but a much more accurate judgment can be made 

 when both eyes are used and the muscular sense of the ocular 

 muscles, employed in converging the eyeballs for near objects, 

 can be used as evidence of their distance. 



In judging of size, in the same way, with one eye, we can only 

 have an idea of the apparent size of an object, which will vary 

 with its distance. With a knowledge of the apparent size and 

 of the distance such as is gained by binocular vision, we can 

 come to a fairly accurate conclusion as to the size of an object. 



To judge of the relative distances of objects so as to see depth 

 in the picture before our eyes, binocular vision is necessary. If 



