CHAP, ii.] THE BRAIN. 1071 



upon the retina ; or in other words in any one position of the eye 

 only a portion of the external world is visible at the same time. 

 The portion so seen is spoken of as the visual field for that position. 



The image thrown on the retina is an inverted one, so that the 

 top of an actual object is represented by the lower, and the bottom 

 by the upper part 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 move- 

 ments of the head. Moreover we normally 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. 133, ox) (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 on 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 shewn in Fig. 133, where the left-hand visual field 

 and the retinal area concerned are shewn 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 corresponding parts, while the nasal side of the right eye 

 corresponds to the temporal side of the left eye. But the whole 



F. 68 



