BINOCULAR VISION. 593 



nistic oblique muscle when simple elevation or depression is per- 

 formed. 



Thus pure abduction or adduction only requires the unaided 

 action of the internal or external recti, while direct depression 

 of the eye requires the combined action of the inferior rectus 

 and superior oblique, and direct elevation requires the superior 

 rectus and inferior oblique to act together. The various oblique 

 movements are accomplished by various combined coordinations 

 of movements of the different muscles. 



The diagram shows the directions toward which the different 

 muscles tend to draw the centre of the cornea from the straight 

 position. 



From this it is obvious that the commonest movements of the 

 eye require the cooperation of different muscles. 



In the ordinary movements of the two eyes more than this is 

 necessary. The two eyes must move in the same direction at the 

 same time, now to the right, now to the left, so that while the 

 external rectus moves the right eye to the right side, the internal 

 rectus moves the other eye in the same direction. We say, then, 

 that the coordination of the movements of the muscles of the 

 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 

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