THE SENSES 



55* 



vision out of the ' tail * of the eye and limited vision out of the 

 tail of both eyes simultaneously employed (Fig. 168). When the 

 attention of a horse is drawn to a distant object, either through 

 interest or alarm, it is viewed with both eyes, the head being held 

 very high, inclining to the horizontal, the ears ' pricked ' and 

 turned to the front. In this position it is evident the most sen- 

 sitive area of the retina is exposed, and it is assumed that, like the 

 fovea in man, the retina at this part is thinned out, and only cones 

 exist. A horse can see an object on the ground immediately 

 under his nose, and is able to see when grazing ; this is because 

 his face narrows below the eyes. 

 When looking at a near object on 

 the ground, he prefers to get his head 

 low down in order to see it, and 

 views it with one eye, the head being 

 slightly twisted. Ordinary equine vision 

 is monocular, yet the right eye blinks 

 when an attempt is made to strike 

 the left, though it cannot possibly 

 see what is going on ; and in the same 

 way the right pupil contracts when 

 the left is exposed to sunlight, though 

 not to the same degree. 



In man binocular vision is perfect, 

 and the explanation afforded is that 

 any part of one retina corresponds to 

 the same part of its fellow ; so that if 

 the retinas be laid over one another, 

 the left portion of one will lie exactly 

 over the left portion of the other, and 

 their upper and lower parts will equally 

 correspond ; but the temporal side of 

 one eye does not correspond to the 

 temporal side of its fellow, but to 

 the nasal side. In Fig. 169 the two 



circles represent the two retinas divided into quadrants, L 

 being the left and R the right eye ; a and c in the left eye corre- 

 spond to a' c' in the right eye, and b and d in the left correspond 

 to b' and d' in the right eye ; but the optic nerve is in the left 

 segment of one eye, and the right segment of the other. W T hen 

 the two images of an object fall on corresponding points of the 

 retina? of man, vision is binocular, and only one object is seen ; 

 thus, if the rays fall on the right side of one retina, they must 

 fall on the right side of its fellow. This is shown in Fig. 169: 

 vl from X to X' and X to X, are the two visual axes ; if the object 

 Y X Z be looked at, Z in each case falls to the left of the visual 



Fig. 168. — Diagram illus- 

 trating the Extent to 

 which a Horse can see 

 behind Him. 



With the head straight to the 

 front he can see out of the 

 1 tail ' of both eyes. By 

 the least inclination of the 

 head, as in Fig. 168, a large 

 visual field behind him may 

 be covered. 



