ACTION OF THE EYE MUSCLES 



551 



A. LIMITS OF THE EYE MOVEMENTS 



Helmholtz, Aubert, Hering and others have determined how far the eye 

 can be moved in the different directions by means of its muscles, and have 

 thus mapped out the limits of the field of vision. With parallel lines of 

 vision the monocular fields of the two eyes, projected upon a distant plane, 

 have the positions represented in Fig. 242. The point ra represents a very 

 distant fixation point. The two monocular fields do not cover each other. 

 The parts accessible only to the left eye are ruled and are designated by the 

 letter /, those accessible to the right eye only are horizontally ruled and are 

 designated by the letter r. 



However it would not be correct to suppose that the unruled part of the 

 monocular field common to the two eyes is in fact the binocular field. On 



FIG. 241. The paths described by the line of vision as the result of rotation by the separate eye 



muscles, after Hering. 



the contrary the two lines of vision cannot be directed at the same time to 

 every point in the outer space to which each line of vision can be directed 

 alone. The space surrounded by the line a a in Fig. 242 represents the binocu- 

 lar field for distant vision. We see how small is the binocular field common 

 to the two lines of vision; it is much smaller than the field common to both 

 visual axes also for near vision. 



