146 



ACTION OF OCULAK MUSCLES. [BOOK m. 



above ( 788) as the ordinary movements of the eye, namely 

 movements of rotation round a vertical and round a horizontal 



obi sup 



FIG. 156. DIAGRAM TO ILLUSTRATE THE ACTIONS OP THE MUSCLES OF THE EYE. 



The eye represented is the left eye seen from above. The thick lines shew, by 

 means of the arrows, the direction in which the several muscles pull, the 

 beginning of each line also indicating the attachment of the muscle. The 

 dotted lines indicate the axis of rotation of the superior and inferior rectus 

 and of the oblique muscles. The axis of rotation of the internal and external 

 rectus being perpendicular to the plane of the paper cannot be shewn, v x 

 represents the visual axis and hx a line at right angles to it. (After Fick.) 



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 rota- 

 tion 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 employ 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 



