Chap, hi.] SIGHT. 955 



on to parts of the retina which do not correspond, are the move- 

 ments which our unassisted will cannot carry out. 



In an earlier part of the work (§ 478) 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 de- 

 gree. 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 accom- 

 panied 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 accompaniment 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 coordi- 

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

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

 cate 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 ob- 

 ject, 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 instance. Sup- 

 pose the eyes, to start with, directed for the far distance, and that 

 it is desired to direct attention to a nearer point lying in the vis- 

 ual line of the right eye. In this case no movement 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 internal rectus of the left eye to be 

 thrown into action. But in ordinary movements the contrac- 

 tion of this muscle is always associated with either the external 

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

 right, or the internal rectus 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 

 internal rectus of the left eye is accompanied by a contraction 

 of both the external and the internal rectus of the right eye, 



