148 COORDINATION OF OCULAR MOVEMENTS. [BOOK in. 



792. The co-ordination of the movements of the eyes. The 

 external rectus is governed by the sixth nerve, nervus abducens, 

 the nucleus of which, as we have seen ( 620), lies in the floor of 

 the fourth ventricle in a position indicated by the eminentia teres. 

 The superior oblique muscle is governed by the fourth nerve, 

 nervus trochlearis, the nucleus of which ( 622) lies in the floor 

 of the aqueduct, in the region of the posterior corpus quadrigemi- 

 num. All the other ocular muscles are governed by the third 

 nerve, the nucleus of which lies in the floor of the aqueduct in 

 the region of the anterior corpus quadrigeminum ; as we have 

 said ( 726), the fibres of the third nerve going to these ocular 

 muscles seem to be more especially connected with the hind part 

 of the nucleus. 



From what has been said above it is obvious that, even in the 

 movements of one eye, a coordination of the motor nervous 

 impulses must in most cases take place. When we turn the 

 visual axis outwards the motor impulses are confined 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 mention- 

 ed. Even in movements of one eye there must be, in most cases, 

 more or less coordination 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 re- 

 mains 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 considerable convergence to less convergence as when we 



