CHAP, in.] SIGHT. 151 



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instance. 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 internal 

 rectus of the left eye to be thrown into action. But in ordinary 

 movements the contraction 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, keeping that eye in lateral equilibrium. And 

 the peculiar oscillating movements seen in the right eye, as well 

 as the sense of effort in the right eye which is felt by the person, 

 support this idea. We need not multiply these instances ; it must 

 be sufficiently obvious that a very large amount of coordination 

 takes place in the daily use of our eyes. 



793. Such a coordination involves the existence of what, to 

 continue the use of a term which we have previously used, we 

 may call a coordinating nervous mechanism. The coordinated 

 efferent impulses issue from one or more of the nuclei of the three 

 cranial nerves concerned, namely the sixth, the fourth, and the 

 third. The afferent visual impulses taking part in the coordina- 

 tion, we have in an earlier part of this book ( 669) traced to the 

 primary visual centres, and thence to the occipital cortex. The 

 volitional impulses themselves are we have seen ( 655) connected 

 in some way or other with an area of the cortex lying in the 

 monkey in the frontal lobe, in the neighbourhood and in front of 

 the precentral fissure (Figs. 125, 126) and probably in man 

 occupying a corresponding position. How are these three factors 

 of the whole nervous action brought to bear the one on the 

 other? When it is remembered how complex and delicately 

 balanced are the movements in question, probably the most 

 intricate and the most delicately balanced of all the movements 

 of the body, it will readily be understood how difficult is the 

 answer to such a question. Stimulation of the cortical areas for 

 movements of the eyes leads as might be expected to bilateral 

 movements, to movements of both eyes ; but, so far as results 

 hitherto obtained shew, the movements are bilateral in a special 

 manner. The most common effect of stimulating the cortical area 

 is a lateral movement of both eyes in the same direction towards 

 the opposite side, a conjugate lateral deviation of both visual 

 axes towards the opposite side. For instance when the cortical 

 area of the left hemisphere is stimulated, the visual axes of both 

 eyes are turned to the right, the external rectus of the right 

 eye and the internal rectus of the left eye being thrown into 



