134 VETERINARY PHYSIOLOGY 



the sensation of yellow is used up, then the parts stimulated 

 by the rest of the spectrum would react to white light and 

 give a complemental colour, and so on through the other 

 substances. 



It has also been suggested that the facts may be explained 

 on the assumption that there are three substances in the retina, 

 one more especially stimulated by the red rays but also acted 

 on by the others, one chiefly stimulated by the green rays, 

 and one chiefly acted on by the blue rays. Such theories, 

 however, do not call for consideration from the ordinary 

 student. 



Colour-blindness. While everyone is colour blind at the 

 periphery of the retina, a certain proportion of people about 

 5 per cent. are unable to distinguish reds and green, even 

 at the centre of the retina. It is impossible to say how far 

 this condition exists in the lower animals. 



(II.) BINOCULAR VISION 



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

 is separate 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. 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 us that this stereoscopic vision indicates solidity. 



Movements of Eyeballs. To secure the harmonious action 

 of the two eyes, it is necessary that they should be freely 

 movable. Each eye in its orbit is a ball and socket joint 

 in which the eyeball moves round every axis (fig. 64). The 

 axis of the eye (a) in man and in monkeys, is set obliquely 

 to the axis of the orbit (&), and the centre of rotation is behind 

 the centre of the ball. The movements are produced by three 

 pairs of muscles. 



