CHAP, in.] SIGHT. 951 



ing through the centre of rotation of the eyeball parallel to the 

 median plane of the head when the head is vertical ; this there- 

 fore is at right angles to the visual axis, and so differs from the 

 other two. 



When we compare the movements thus effected by these 

 several pairs of muscles with the movements which we described 

 above ( 590) as the ordinary movements of the eye, namely 

 movements of rotation round a vertical and round a horizontal 

 axis both at right angles to the visual axis, we see that it is only 

 the movements round the vertical axis which can be carried out 

 by one pair of muscles acting alone, the particular pair being 

 the internal and external rectus. Neither the horizontal axis of 

 rotation of the inferior and the superior rectus, nor that of the 

 oblique muscles, is placed exactly at right angles to the visual 

 axis; each of them makes "an oblique angle with that axis. 

 Hence when in carrying out the ordinary movements of the eye 

 we rotate the eyeball round the horizontal axis, we do not em- 

 ploy either of these pairs of muscles alone, but combine them, 

 making use of one muscle of one pair with one of the other. 

 The superior and inferior rectus in moving the visual axis up 

 and down also turn it somewhat inwards, to the nasal side ; but 

 this is corrected if the oblique muscles act at the same time ; 

 and it is found that the rectus superior acting with the inferior 

 oblique moves the visual axis directly upwards, and the rectus 

 inferior acting with the superior oblique directly downwards in 

 a vertical direction ; that is to say the two combinations rotate 

 the eyeball round a horizontal axis at right angles to the visual 

 axis. 



Hence there are only two movements of the eyeball which 

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

 in which we simply turn the visual axis to the nasal side, em- 

 ploying 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 direc- 

 tion of the visual axis. 



To nasal side. Internal rectus. 



'Sa 5 



To temporal side. External rectus. 



S 1 Upwards. Superior rectus and inferior oblique. 



Si o Downwards. Inferior rectus and superior oblique. 



2 L 



