VISION. 289 



The Movements of the Eyeballs. In order that we may be 

 enabled to move our eyes so as to see objects in different posi- 

 tions in the visual field, the eyeballs are provided with six little 

 muscles, four recti and two obliques. These muscles are in- 

 nervated by the third, fourth and sixth nerves (see p. 259). The 

 images in the two eyes cannot of course fall on anatomically 

 identical parts of the retinae, but they fall on parts that are 

 physiologically identical. Thus, an object, say on the right of 

 the field of vision, will cause an image to fall on the. nasal side 

 of the right retina and 011 the temporal side of the left retina. 

 We do not, however, see two objects because by experience we 

 have come to learn that these are corresponding points on the 

 retinae. When an object is brought near to the eye, the two 

 eyeballs must converge so as to bring the visual axes on to the 

 corresponding points. This convergence of the eyeballs con- 

 stitutes the third change occurring in the eyes during accom- 

 modation for near vision, the other two being, as we have seen, 

 bulging of the lens and contraction of the pupil. It is interest- 

 ing that these three changes are controlled by the third nerve. 

 If anything happens to throw one of the images on to some 

 other portion of one retina, double vision is the result. This 

 condition of diplopia, as it is called, can be brought about, vol- 

 untarily, by pressing on one eyeball at the edge of the eye, or 

 it may occur as a result of paralysis or incoordinate action of 

 one or. more of the ocular muscles. This occurs in certain in- 

 toxications, such for example, as in that produced by alcohol. 



Just as in the case of errors of refraction, e. g., astigmatism, 

 slight degrees of diplopia may cause symptoms that are more 

 distressing than where marked diplopia exists, because we try 

 to correct for slight errors and the effort causes pain (headache) 

 and fatigue, whereas with extreme errors we do not try to correct 

 but, instead, we learn to disregard entirely the image in one 

 eye. When the incoordination of ocular movement is permanent, 

 as when it is due to shortening of one of the muscles it is called 

 strabismus. This condition is usually congenital, and can often 

 be rectified by surgical operation. 



Judgments of Vision. Besides these purely physiological 



