CHAP, in.] SIGHT. 147 



can carry out by the help of one muscle alone, namely that in 

 which we simply turn the visual axis to the nasal side, employing 

 the internal rectus, and that in which we turn it to the temporal 

 side, employing the external rectus, the visual axis in both cases 

 remaining in the same plane, the visual plane. In order to raise 

 or lower the visual axis in the same vertical plane, without lateral 

 movement, we must use two muscles ; and if we wish to execute an 

 oblique movement combining an up and down with a side to 

 side movement of the visual axis we must employ three of the 

 ocular muscles. These several movements, with the muscles 

 concerned, may be stated as follows, the movement in each case 

 being described with reference to changes in the direction of the 

 visual axis. 



-^ -g j To nasal side. Internal rectus. 



^W) g J To temporal side. External rectus. 



S | Upwards. Superior rectus and inferior oblique. 



53 o Downwards. Inferior rectus and superior oblique. 



C 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 contract- 

 ing 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. 



