544 



COMPARATIVE PHYSIOLOGY. 



white surface and sees the latter green, 

 sidered as the results of exhaustion. 



They may be con- 



OO-ORDINATION OP THE TWO EYES IN VISION. 



As a matter of fact, we are aware that an object may be 

 seen as one either with a single eye or with both. For bi- 

 nocular vision it may be shown that 

 the images formed on the two retinas 

 must fall invariably on corresponding 

 points. 



The position of the latter may be 

 gathered from Fig. 392. It will be no- 

 ticed that the malar side of one eye 

 corresponds to the nasal side of the 

 other, though upper always answers to 

 upper and lower to lower. This may 

 also be made evident if two saucers 

 (representing the fundus of each eye) 

 be laid over each other and marked off, 

 as in the figure. 



That such corresponding points do 

 actually exist may be shown by turnuig 

 one eye so that the image shall not 

 fall, as indicated in the figure. Only 

 now and then, however, is a person to 

 be found who can voluntarily accom- 

 plish this, but it occurs in all kinds of 

 natural or induced squint, as in alcohol- 

 sponds to the nasal side of igm^ owing to partial paralysis of some 

 of the ocular muscles. We are thus 

 naturally led to consider the action of these muscles. 



Ocular Movements. — Upon observing the movements of an 

 individual's eyes, the head being kept stationary, it may be 

 noticed that (1) both eyes may converge ; (2) one diverge and 

 the other turn inward ; (3) both move upward or downward ; 

 (4) these movements may be a'ccompanied by a certain degree 

 of rotation of the eyeball. 



The eye can not be rotated around a horizontal axis without 

 combining this movement with others. To accomplish the 

 above movements it is obvious that certain muscles of the six 

 with which the eye is provided must work in harmony, both as 



Fig. 393. —Diagram to illus- 

 trate corresponding points 

 (after Foster). L, S, left 

 and right eyes; a, ft, e, are 

 points in one eye corre- 

 sponding to ai, ft,, C], in 

 ■ the other. The lower fig- 

 ures are projections of the 

 retina of the right (B) and 

 the left (Z) eye. It may be 

 observed that the malar 

 side of one retina corre- 



