158 VETERINARY PHYSIOLOGY 



the retina, is thus made possible. The way in which the 

 more divergent peripheral rays are cut off by the iris prevents 

 this from manifesting itself in vision (p. I'lS). 



(II.) BixocuLAR Vision. 



A. Advantages. 



In most of the lower animals, the field of vision of each 

 eye is sejoarate and distinct at all times, but in the horse and 

 dog the two eyes can be directed forwards so that the fields 

 of vision overlap as they always do in man and in apes. 



Fig. 77. — Corresponding Areas of the two Retina; in Binocular Vision. The 

 upper and outer area of the right retina corresponds to the upper and 

 inner area of the left retina, and the other areas correspond as shown 

 by the shading. In each pair of areas definite points correspond with 

 one another, a — a. 



When in this position, the combined action of the eyes 

 affords a means of determining the distance and solidity 

 of near objects. 



1. Distance of Near Objects. — As an object is approached, 

 the two eyes have to be turned forwards by the internal 

 recti muscles, and by the degree of contraction of these, an 

 estimation of the distance is made. 



2. Solidity of an Object. — If the object is near, a slightly 

 different picture is given on each retina, and experience has 

 taught that this stereoscopic vision indicates solidity. 



B. Single Vision with Two Eyes. 



1. Corresponding Areas of the Two Retinae. — In order that 

 single vision may occur with the two eyes, the eyes must 

 be directed to the same place, as can be done in the horse and 



