690 THE BRAIN. 



of the retinal image ; similarly the actual left-hand side of the retinal image 

 corresponds to the right-hand side of the actual object, and the right-hand 

 side to the left-hand side. Hence the right-hand half of the visual field 

 corresponds to the left-hand side of the retina, and the left-hand half to the 

 right-hand side. 



The eye can be moved in various directions, and since in the visual field 

 the portion of external nature which can be seen at the same time differs 

 with each different position, a large range of vision is thus secured ; and this 

 can be further increased by movements of the head. Moreover, we nor- 

 mally make use of two eyes ; our normal vision is binocular, and the visual 

 field of the right eye differs from that of the left eye. There is one striking 

 difference which must always be borne in mind. A section carried through 

 the eye in a vertical and front-to-back plane, through what we shall learn 

 to call the optic axis (Fig. 156, o. x.~) (the exact details of the plane may be 

 left for the present), will divide the retina into two lateral halves, and in 

 each retina one-half will be on the nasal side next to the nose, and the other 

 half will be on the malar or temporal side, next to the cheek or temple. It 

 must be remembered that the nasal halves and temporal halves of the two 

 retinas do not occupy corresponding positions in space. The temporal half 

 of the left retina is on the left side of its own eye, whereas the temporal half 

 of the right retina is not on the left, but on the right, side of its eye ; and 

 so with the nasal halves. Now, in the right eye, the right-hand side of the 

 visual field corresponds to the nasal half of the retina, and the left-hand side 

 of the visual field to the temporal half of the retina, whereas in the left eye 

 the right-hand side of the visual field corresponds to the temporal half of 

 the retina, and the left-hand side to the nasal half. This is shown in Fig. 

 156, where the left-hand visual field and the retinal area concerned are 

 shown shaded in each eye. 



When we look at an object with the two eyes, though two retinal images 

 are produced, one in one eye and one in the other, we perceive one object 

 only, not two. This is the essential fact of binocular vision ; when certain 

 parts of each retina are stimulated at the same time we are conscious of one 

 sensation only, not two ; and the parts of the two retinas which, stimulated 

 at the same time, give rise to one sensation are spoken of as " corresponding 

 parts." From the structure and relations of the two eyes it follows that the 

 temporal side of the right, and the nasal side of the left, eye are such cor- 

 responding parts, while the nasal side of the right eye corresponds to the 

 temporal side of the left eye. But the whole of each retina is not employed 

 in binocular vision. Owing to the position of the two eyes in relation to the 

 nose, it comes about that an object held very much on one side, to the left- 

 hand side for instance, while it is capable of producing an image on the 

 extreme nasal side of the left eye, and can be seen, therefore, by that eye, 

 cannot produce an image on the temporal side of the right eye ; the nose 

 blocks the way. It is, therefore, not seen by the right eye, and the vision 

 of it is monocular by the left eye only. In Fig. 156 it may be seen that the 

 left visual field of the left eye (L. F. _L.) extends more to the left, and is 

 larger than the left visual field of the right eye (L. F. #.), and that the right 

 retinal area, corresponding to the left visuaffield, extends further along the 

 nasal side of the left side (a') than it does along the temporal side of the 

 right eye (a'), the difference being due to the presence of the nose (f\). 

 And similar conditions obtain with regard to the extreme right-hand side 

 of the visual field. 



580. After these preliminary statements, we may now turn to con- 

 sider some anatomical facts concerning the ending of the optic nerve in the 

 brain. 



