952 COOEDINATION OF OCULAK MOVEMENTS. [BOOK in. 



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Upwards and to Superior rectus, internal rectus and 



nasal side. inferior oblique. 



Downwards and to Inferior rectus, internal rectus and 



nasal side. superior oblique. 



Upwards and to Superior rectus, external rectus and 



temporal side. inferior oblique. 



Downwards and to Inferior rectus, external rectus and 



temporal side. superior oblique. 



The fact that in our ordinary movements of the eye we do 

 thus combine the actions of muscles, and the advantages of such 

 a combination are further shewn in connection with that swivel 

 rotation of the eye round the visual axis itself, which, as we 

 have seen, is wholly avoided in many of our movements and 

 which we cannot carry out by a direct effort of the will. The 

 superior rectus acting by itself, owing to the position of its 

 insertion in reference to the direction of the fibres, not only 

 turns the visual axis inwards while directing it upwards, but 

 also to a slight extent rotates the eye round the visual axis , 

 and the inferior rectus as well as both the oblique muscles in 

 like manner tend in contracting to give the eyeball such a 

 swivel rotation. This tendency of the superior rectus like its 

 tendency to turn the visual axis inwards is counteracted by the 

 inferior oblique, the swivel rotation of the latter being contrary 

 in direction to that of the former ; and the like tendency of the 

 inferior rectus is in like manner counteracted by the superior 

 oblique. Thus the movements, in carrying out which these 

 muscles are combined, are rendered free from the swivel rotation 

 element. On the other hand this tendency of the muscles in 

 question is utilized in the particular movements in which the 

 swivel rotation does take place. 



594. The coordination of the movements of the eyes. The 

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

 the nucleus of which lies in the floor of the fourth ventricle in 

 a position indicated by the emiuentia teres. The superior 

 oblique muscle is governed by the fourth nerve, nervus troch- 

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

 in the region of the posterior corpus quadrigeminum. 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 ( 539), 

 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, it is true, confined 



