EYE MOVEMENTS 495 



rectus and becomes attached to the eyeball. The inferior arises from the 

 nasal side of the orbit, just within its lower margin. It passes outwards 

 and backwards beneath the inferior oblique to become attached to the 

 eyeball nearly opposite to the attachment of the superior oblique. On con- 

 traction of the superior oblique the upper side of the eye is rotated towards 

 the nose ; at the same^time the pupil is directed slightly downwards and out- 



SUP. OBLIQUE 



FIG. 246. Diagram shewing the directions in which the different external eye 

 muscles rotate the eyeball. 



wards. The inferior oblique also causes rotation about the visual axes, but 

 in the opposite direction ; it at the same time produces upward and outward 

 movement of the pupil. The function of these two small muscles appears to 

 be to prevent the eyes from rotating about their visual axes, and in particular 

 to prevent the rotation inwards which is associated with the contraction 

 of the superior and inferior recti. For this purpose the superior rectus is 

 associated with the inferior oblique and vice versa. In this action the oblique 

 muscles appear to be very efficient ; for if the eye is first fatigued by looking 

 at a brilliant line of light, e.g. a long straight electric lamp filament, and is 

 then directed upwards or downwards at a white surface, the after image thus 

 produced is always found to keep its vertical direction. 



On tilting the head suddenly about a transverse axis, it is found that the 

 eyes rotate in the opposite direction, so that in fact the image formed on 

 the retina shall still keep in the same apparent meridian. This rotation 

 is called compensatory, and is largely effected by the "oblique muscles. 



CO-ORDINATED MOVEMENTS OF THE EYES 



The notable feature of the eye movements is the close association which 

 exists between the muscles of the two eyes. For so perfectly has this mechan- 

 ism been developed that the eyes are able to glance rapidly from place to place 

 without there being any obvious doubling of the images. The eye movements 

 are therefore of such a kind that the image of an object conveys a single 

 impression to consciousness. But objects vary in the distance at which 

 they are placed and therefore, beside movements of the eyes in which the 

 visual axes remain parallel, there are also movements in which there is 

 a certain amount of convergence. In the latter case there is usually some 



