MOVEMENTS OF THE EYEBALL. 137 



combination of action of three muscles for each eye, the only 

 difference in binocular vision being that in one eye the ex- 

 ternal rectus is brought into play, while the internal rectus 

 acts upon the opposite side. Reversing this action of the in- 

 ternal and external recti, we have the action which directs 

 the pupil upward and to the right. If we substitute for the 

 superior rectus and the 1 inferior oblique, the inferior rectus 

 and the superior oblique, we have the pupil directed down- 

 ward, and either to the right or left, as the internal or ex- 

 ternal rectus upon either side is brought into action. 



One important point, never to be lost sight of in our 

 study of the associated action of the muscles of the globe, 

 relates to the combined movements of the two eyes. "We 

 have already seen that perfect binocular vision is possible 

 only when impressions are made upon exactly corresponding 

 points in the retina of each eye. 1 If one eye be deviated in 

 the horizontal plane, the points no longer correspond, and 

 there is double vision, the same as if two impressions were 

 made upon onB retina ; for when the impressions exactly corre- 

 spond, the two retinae act practically as a single organ. The 

 same is true in deviation of the globe in the vertical plane. 

 If we suppose, for sake of argument, that the retina is 

 square, it is evident that a torsion, or twisting of one globe 

 upon an antero-posterior axis (Itaddrehung) must be attended 

 with an analogous movement of the other globe, in order to 

 bring the visual rays to bear upon the corresponding points ; 

 in other words, the obliquity of the assumed square of the 

 retina must be exactly the same for the two eyes, or the coin- 

 cidence of the corresponding points would be disturbed, 

 and we should have double vision. When we clearly under- 

 stand that deviation of one eye in the horizontal or the ver- 

 tical plane disturbs the relation of the corresponding points, 

 which is sufficiently easy of comprehension, and that a devia- 

 tion from exact coincidence of action in torsion of the globes, 

 twists, as it were, the corresponding points, so that their rela- 



1 See page 119. 



