702 SPECIAL SENSES 



consciousness, under ordinary conditions, of the muscular action by 

 which the globe is rotated and twisted in various directions, except 

 that by an effort of the will the line of vision is directed toward differ- 

 ent objects. By a strong effort the axes of the eyes may be converged 

 by contracting both internal recti, and some persons can produce 

 extreme divergence by using both external recti ; but this is abnormal. 



In looking at distant objects the axes of vision are practically paral- 

 lel. In looking at near objects the effort of accommodation is attended 

 with the degree of convergence necessary to bring the visual axes to 

 bear on identical points. In looking around at different objects the 

 head is moved more or less and the globes are rotated in various direc- 

 tions. In the movements of the globes vertically the axes are kept 

 parallel, or at the proper angle, by the internal and external recti, and 

 the superior and inferior recti on the two sides act together. In rotat- 

 ing the globe from one side to the other on a vertical axis, the external 

 rectus on one side acts with the internal rectus on the other. In the 

 movements of torsion on an antero-posterior axis, there must be an 

 associated action of the oblique muscles and the recti. 



An important point, not to be lost sight of in the study of the asso- 

 ciated action of the muscles of the globe, relates to the associated move- 

 ments of the two eyes. Perfect binocular vision is possible only when 

 impressions are made on corresponding points in the retina of each eye. 

 If one eye is deviated in the horizontal plane, the points no longer cor- 

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

 made on one retina; for when the impressions exactly correspond, the 

 two retinae act practically as a single organ % The same is true in devia- 

 tion of the globe in the vertical plane. If it be supposed, for the sake of 

 argument, that the retina is square, it is evident that a torsion or twist- 

 ing of one globe on an antero-posterior axis must be attended with an 

 analogous movement of the other globe in order to bring the visual rays 

 to bear on 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 coincidence of the corresponding points would be disturbed 

 and there would be double vision. Deviation of one eye in the horizon- 

 tal or the vertical plane disturbs the relation of the corresponding 

 points, and a deviation from exact coincidence of action in torsion of 

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

 tion is also disturbed. It is evident, therefore, that the varied move- 

 ments of the globes, by the combined action of the recti and oblique 

 muscles, must correspond for either eye, in the movements of torsion on 

 an antero-posterior axis as well as in movements of rotation on the 

 horizontal or the vertical axis. 



