310 SPECIAL SENSES. 



The superior oblique muscle is supplied by a single nerve, the patheticus. When this 

 muscle is paralyzed, the inferior oblique acts without its antagonist, and the eyeball is 

 immovable, as far as the twisting of the globe, just described, is concerned. When 

 the head is moved toward the shoulder, the globe cannot rotate to maintain a position 

 corresponding to that of the other eye, and we have double vision. This point has 

 already been touched upon in connection with the physiology of the nerves of the eye- 

 ball and the situation of corresponding points in the retina. 



Associated Action of the Different Muscles of the Eyeball. It is almost unneces- 

 sary to add, after the description just given of the actions of the individual muscles of 

 the globe, that their contractions may be associated, so as to produce an infinite variety 

 of movements. We have no consciousness, under ordinary circumstances, of the muscular 

 action by which the globe is rotated and twisted in various directions, except that, by an 

 eifort 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 practically parallel. When we 

 look at near objects, the effort of accommodation is attended with the amount of con- 

 vergence 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 illustrative of this combination of action : 



u If the eyes be directed obliquely upward and to the left, the vertical meridians of 

 the two eyes are parallel and inclined from left to right, for the left eye, outward, and for 

 the right eye, inward. The movement of the left eye upward 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 upward and to the left, that is 

 to say, inward, this is moved by the simultaneous action of the superior rectus, the inter- 

 nal rectus, and the inferior oblique." 



We have given the above quotation simply to illustrate a combination of action of 

 three muscles for each eye, the only difference in binocular vision being that in one eye 

 the external rectus is brought into play, while the internal rectus acts upon the opposite 

 side. Reversing this action of the internal 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 inferior oblique, the inferior rectus and the superior oblique, we have the pupil directed 

 downward, and either to the right or left, as the internal or external 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 ths globe, relates to the associated 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. 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 one retina ; for, when the impressions 

 exactly correspond, the two retina? act practically as a single organ. The same is true 

 in deviation^ the globe in the vertical plane. If we suppose, for the sake of argument, 

 that the retina is square, it is evident that a torsion, or twisting of one globe upon an 

 antero-posterior axis 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, 



