CHAP, in.] SIGHT. 953 



to the sixth nerve, reaching the external rectus, and when we 

 turn it inwards are confined to the third nerve, reaching the 

 internal rectus ; but in all other movements motor impulses 

 must descend to at least two muscles along different nerve- 

 branches, and in many cases must start from two or even all 

 three of the cranial nuclei just mentioned. Even in movements 

 of one eye there must be, in most cases, more or less coordina- 

 tion of actual motor impulses, in order to secure due efficiency 

 of the movement; by actual motor impulses we mean impulses 

 leading to the contraction of muscular fibres, irrespective of 

 any influences which may at the same time be brought to bear 

 on antagonistic muscles, in order to facilitate or qualify the 

 movement. 



But if this is true in the case of one eye, much more is it 

 true when we use both eyes in binocular vision. 



Two facts about binocular vision strike our attention. The 

 one is that, as may be seen by watching the movements of any 

 person's eyes, the two eyes move together. If the right eye 

 moves to the right, so does also the left, and, if the object 

 looked at be a distant one, exactly to the same extent; if the 

 right eye looks up, the left eye looks up also; and so with 

 regard to other movements. Very few persons are able by a 

 direct effort of the will to move one eye independently of the 

 other ; though by some the power has been acquired. We shall 

 refer immediately to particular movements in which one eye 

 only is moved, while the other remains motionless. The other 

 salient fact is that the movements of the two eyes are limited in 

 certain ways. As we have seen one of the simplest ocular 

 movements is the side to side movement of the visual axis, and 

 one of the commonest binocular movements is the convergence 

 of the visual axes, as when we turn our eyes from something 

 far off to something near, or conversely the change from con- 

 siderable convergence to less convergence as when we turn our 

 eyes from something near to something farther off. In a large 

 number of instances this change to convergence from parallelism, 

 or this increase or decrease of convergence takes place without 

 any change in the visual plane, without any raising or lowering 

 of the visual axes; in such instances the movement is carried 

 out in convergence by the two internal rectus muscles, or in 

 decrease of convergence by the two external rectus muscles; 

 and the only coordination necessary is one which secures that 

 the muscle of one eye should work in harmony with the muscle 

 of the other eye. But even this relatively simple movement is 

 limited in a very marked way. We can bring the visual axes 

 of the two eyes from a condition of parallelism to one of almost 

 any degree of convergence, but we cannot, without artificial 

 assistance, bring them from a condition of parallelism to one of 

 divergence. The stereoscope will enable us to create such a 



