136 SPECIAL SENSES. 



du.ce an infinite variety of movements. We have no con- 

 sciousness, under ordinary circumstances, of the muscular 

 action by which the globe is rotated and twisted in various 

 directions, except that, by an effort of the will, we direct the 

 visual line toward different objects. By a strong effort, we 

 can make the eyes converge 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 prac- 

 tically parallel. When we look at near objects, the effort of 

 accommodation is attended with the amount of convergence 

 necessary to bring the visual axes to bear upon identical 

 points. In looking around at different objects, we move the 

 head more or less, rotating and twisting the globes in various 

 directions. 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 upon 

 the two sides act together. In rotating the globe from one 

 side to the other, upon a vertical axis, the external rectus 

 upon one side acts with the internal rectus upon the other. 

 In the movements of torsion upon an antero-posterior axis, 

 there must be an associated action of the oblique muscles 

 and the recti. We quote from Longet the following, as illus- 

 trative of this combination of action : 



" If the eyes be directed obliquely upward and to the left, 

 the vertical meridians of the two eyes are parallel and in- 

 clined from left to right, for the left eye, outward, and for 

 the right eye, inward. The movement of the left eye up- 

 ward and to the left, or outward, necessitates a contraction of 

 the superior rectus, the external rectus, and the inferior 

 oblique muscles. As regards the right eye, also directed up- 

 ward and to the left, that is to say, inward, this is moved by 

 the simultaneous action of the superior rectus, the internal 

 rectus, and the inferior oblique." 



We have given the above quotation simply to illustrate a 



1 LONGET, Traite de physiologic, Paris, 1869, tome ii., p. 928. 



