150 COORDINATION OF OCULAR MOVEMENTS. [BOOK in. 



be kept carefully single while the prism is turned very slowly 

 from the horizontal to the vertical position, then on suddenly 

 removing the prism a double image is for a moment seen this 

 shews that the eye before which the prism was placed had 

 moved in disaccordauce with the other. The double image, 

 however, immediately after the removal of the prism, becomes 

 single on account of the eyes coming into accordance. 



When we examine all the various movements of the eyes 

 which we are capable of making by a direct effort of the will, we 

 find that they are all of such a kind that through them the two 

 images of an external object are brought upon corresponding parts 

 of the two retinas ; conversely the movements which could be 

 ^effected by the contractions of this or that ocular muscle, but the 

 effect of which would be to bring the two images on to parts of 

 the retina which do not correspond, are the movements which 

 our unassisted will cannot carry out. 



In an earlier part of the work ( 643) we insisted at some 

 length on the important share taken by sensations, or at least by 

 afferent impulses, in the coordination of motor impulses ; and the 

 movements of the eye illustrate this in a very marked degree. 

 All the various movements of the eye are dependent on visual 

 sensations. The issue of each efferent motor volitional impulse is 

 dependent on afferent visual impulses. 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 

 constriction of the pupil. And so with other ocular movements. 

 Above all, the careful selection of this or that ocular muscle, the 

 extent to which it is to be thrown into contraction, its accompani- 

 ment by the contraction of other ocular muscles and the due 

 coordination of all the several contractions all these things are so 

 determined by visual sensations that the two images of each object 

 looked at fall on corresponding parts of the two retinas. 



A little reflection will shew how large an amount of co- 

 ordination must thus take place in daily life, how in the various 

 movements of the eye there must be, so to speak, the most 

 delicate picking and choosing of the muscular instruments. 

 When we look at an object to the right, since we thereby turn 

 the right eye to the temporal side, and the left eye to the nasal 

 side, we throw into action the external rectus of the right eye 

 and the internal rectus of the left ; and similarly when we look 

 to the left we use the external rectus of the left and the internal 

 rectus of the right eye. On the other hand when we look at a 

 near object, and therefore converge the visual axes, we use the 

 internal rectus of both eyes ; and when we look at a distant 

 object, and bring the axes from convergence towards parallelism, 

 we use the external rectus of both eyes. Or to take another 



