546 BINOCULAR VISION [BOOK in. 



moment seen; shewing that the eye before which the prism was 

 placed had moved in disaccordance 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 internus 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 

 look at a distant object, and bring the axes from convergence 

 towards 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, directed 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 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 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 equi- 

 librium. 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, shew this to be the case. 



Such a complex coordination requires for its carrying out 

 a distinct nervous machinery; and we have reasons for thinking 

 that such a machinery exists in certain parts of the corpora quadri- 

 gemina or in the underlying structures. In the nates, there 

 appears to be a common centre for both eyes, stimulation of the 



