944 MOVEMENTS OF THE EYEBALL. [BOOK in. 



right side, and the left side to the left side. But when we turn 

 to the structure of the retina we find that the left or nasal side 

 of the right eye, since it contains the entrance of the optic 

 nerve, is comparable with, not the left, but the right or nasal 

 side of the left eye, and in like manner the right or temporal side 

 of the right eye is comparable with the left or temporal side of 

 the left eye. Hence, considered in relation to the structure 

 of the retina, the corresponding points appear to be reversed 

 from side to side, though not from top to bottom. While 

 the upper half of the retina of the left eye corresponds to the 

 upper half of the retina of the right eye, and the lower to 

 the lower, the nasal side of the left eye corresponds with the 

 temporal side of the right, and the temporal of the left with 

 the nasal side of the right. 



It will be observed that in each eye a vertical plane through 

 the visual axis (v. I. in Fig. 158) cuts the retina in a vertical 

 line v. m., which divides the retina into two lateral, temporal 

 and nasal, halves, each temporal and each nasal half correspond- 

 ing with the nasal and temporal half respectively of the other 

 eye. When the visual axes of the two eyes are parallel, the 

 two vertical planes in question are parallel to the median plane 

 and to each other. Further, a horizontal plane drawn through 

 the visual axis at right angles to the above vertical plane cuts 

 the retina in a horizontal line h. m. ; and this also divides the 

 retina into two halves, an upper and lower half, the upper and 

 the lower halves of both retinas being corresponding. These 

 two lines, each of which may be considered as a series of 

 corresponding points, are sometimes spoken of as lines of 

 separation. 



The blending of the two sensations into one occurs, we 

 repeat, only when the two images of an object fall on corre- 

 sponding points of the two retinas. Hence it is obvious that in 

 single vision with two eyes the ordinary movements of the eye- 

 balls must be such as to bring the visual axes to converge at 

 the object looked at so that the two images may fall on corre- 

 sponding points. When the visual axes do not so converge, and 

 when therefore the images do not fall on corresponding points, 

 the two sensations are not blended into one perception, and 

 vision becomes double. It is therefore important to study in 

 some detail the movements of the eyeballs, by means of which, 

 in ordinary vision, the relative positions of the two retinas are 

 so carefully adjusted that we habitually see objects single not 

 double. 



590. The movements of the Eyeball. As we have said, the 

 movements of the eyeball are movements of rotation round an 

 immobile centre, the centre of rotation ; but these movements 

 are limited in a particular way, and it is necessary to pay atten- 

 tion to their characters and limitation. 



