BINOCULAR VISION. 785 



visual axes may be brought to diverge. Similarly if a distant object be 

 looked at with a prism before one eye, and the image of the object be kept 

 carefully single, while the prism is turned very slowly up or down, then on 

 suddenly removing the prism a double image is for a moment seen ; show- 

 ing that the eye before which the prism was placed has moved in disaccord- 

 ance with the other. The double image, however, in a few seconds after the 

 removal of the prism becomes single, on account of the eyes coming into 

 accordance. 



It is only when loss of coordination occurs, as in various diseases and in 

 alcoholic or other poisoning, that the movements of the two eyes cease to 

 agree with each other. It is evident, then, that when we look at an object 

 to the right, since we thereby abduct the right eye and adduct the left, we 

 throw into action the rectus externus of the right eye and the rectus intern us 

 of the left, and similarly when we look to the left we use the rectus externus 

 of the left and the rectus internus of the right eye. On the other hand, 

 when we look at a near object, and therefore converge the visual axes, we 

 use the recti interni of both eyes ; and when we 1 look at a distant object, and 

 bring the axes from converging toward parallelism, we use the recti externi 

 of both eyes. In the various movements of the eye there is therefore, so to 

 speak, the most delicate picking and choosing of the muscular instruments. 

 Bearing this in mind, it cannot be wondered at that the various movements 

 of the eye are dependent for their causation on visual sensations. In order 

 to move our eyes we must either look at or for an object ; when we wish to 

 converge our axes we look at some near object, real or imaginary, and the 

 convergence of the axes is usually accompanied by all the conditions of near 

 vision, such as increased accommodation and contraction of the pupil. And 

 so with other movements. The close association of the movements of the 

 eye may be illustrated by the following case: Suppose the eyes, to start with, 

 accommodated for the far distance, and that it is desired to direct attention to a 

 nearer point lying in the visual line of the right eye. In this case no move- 

 ment of the right eye is required ; all that is necessary is for the left eye to 

 be turned to the right, that is, for the rectus internus of the left eye to be 

 thrown into action. But in ordinary movements the contraction of this 

 muscle is always associated with either the rectus externus of the right eye, 

 as when both eyes are turned to the right, or the rectus internus of that eye, 

 as in convergence ; the muscle is quite unaccustomed to act alone. This 

 would lead us to suppose that in. the case in question the contraction of the 

 rectus internus of the left eye is accompanied by a contraction of both recti 

 externus and internus of the right eye, keeping that eye in lateral equilib- 

 rium. And the peculiar oscillating movements seen in the right eye, as well 

 as the sense of efforts in the right eye which is felt by the person, show this 

 to be the case. 



685. Such a complex coordination requires for its carrying out a dis- 

 tinct nervous machinery, and we have reasons for thinking that such a 

 machinery exists in certain parts of the corpora quadrigemina or in the 

 underlying structures. In the nates there appears to be a common centre 

 for both eyes, stimulation of the right side producing movements of both 

 eyes to the left, of the left side movements to the right; while stimula- 

 tion in the middle line behind causes a downward movement of both 

 eyes with convergence of the axes and in the front an upward move- 

 ment with return to parallelism, both accompanied by the naturally asso- 

 ciated movements of the pupil. Stimulation of various parts of the nates 

 causes various movements, depending on the position of the spot stimu- 

 lated. After an incision in the middle line, stimulation of the nervous 

 centre on one side produces movements in the eye of the same side only. 



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