956 COORDINATION OF OCULAR MOVEMENTS. [Book hi. 



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, sup- 

 port 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. 



§ 595. 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 coor- 

 dinated 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 coordination, we have in an earlier part of this book 

 (§ 496) traced to the primary visual centres, and thence to the 

 occipital cortex. The volitional impulses themselves are we 

 have seen (§ 484) 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. 122, 

 123) 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 area for movements of the eyes leads as might be ex- 

 pected to bilateral movements, to movements of both eyes. The 

 most common effect of stimulating the cortical area is a lateral 

 movement of both eyes in the same direction towards the oppo- 

 site 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 contraction by impulses 

 passing down the right sixth nerve and left third nerve ; the 

 efferent impulses therefore cross in the case of one nerve but 

 not in the case of the other. Similarly, when the right hemis- 

 phere is stimulated, impulses pass down the right third nerve 

 and left sixth nerve. 



Though these lateral movements are those most easily pro- 

 duced, other movements, conjugate raising or lowering of both 

 eyeballs, oblique movements, and even movements of convergence 

 have been obtained. And, were our means of stimulation ade- 

 quately discriminating, it would probably be found that each of 

 the several ocular movements might be called forth by stimu- 

 lating the appropriate cortical focus. Like movements of the 

 eyeballs may also be obtained by stimulating not the frontal 

 motor region, but the occipital region (§ 498). These latter 



