302 ADAPTATIONS TO SPACE AND MOTION 



they may periodically jerk to the position of rest and repeat the slow 

 following-movement. If the animal is one which has little or no eye 

 mobility, the optomotor reaction will be given by the head itself or, if 

 this be restrained, by the whole body. This reaction has been much used 

 in late years (quite improperly!) as a test of visual acuity and as a tool 

 for the investigation of color vision and still other matters — the assump- 

 tion being that if the stripes are made so narrow or so much like the 

 intervening spaces that the reaction fails to occur, the width of the stripes 

 and spaces look alike to him however different they may look to us, etc. 



We perform something essentially like the optomotor reaction, in our 

 so-called railroad nystagmus. When watching out of the window of a 

 swift train, we are comfortable enough if we look at distance objects, 

 which seem hardly to move backward at all as we fly along. But if we 

 try to watch the roadbed close beside the train we soon experience a 

 discomfort — our eye muscles are in a turmoil, the eyes constantly jerking 

 ahead and drifting back in a vain effort to stop the flight of the ties 

 under the neighboring track. 



Voluntary eye movements are those made for exploratory purposes. 

 In ourselves, they are conjugated, which means something more than 

 simply coordinated: we are quite incapable of voluntarily moving one 

 eye independently of the other. The two eyes move together in both 

 involuntary and voluntary movements, just as though there were a tie- 

 rod inside the head like that which conjugates the front wheels of an 

 automobile. There has been exactly one case reported, of a human being 

 who could move either eye at will. This was a 28-year-old Australian, 

 described by Sir James Barrett, who could turn either eye outward 20 , 

 or both eyes at once — an amazing feat which he had always been able to 

 do and which "came as natural to him as moving his hand." Our eyes 

 always move in the same sense, in obedience to certain laws which gov- 

 ern the interactions of their muscles (Donders' and Listing's laws), for 

 a change of fixation; but they move in opposite senses — toward or away 

 from each other — for a change in accommodation. These contradictory 

 tendencies are controlled from separate centers in the tegmentum, be- 

 neath the aqueduct of Sylvius (the convergent movements being com- 

 manded by a special center, the nucleus of Perlia) ; but they are smoothly 

 blended without conflict whenever we turn our gaze to a new object 

 which lies both in a new direction and at a new distance. 



The system of involuntary and voluntary eye movements is subject to 

 enormous differences from the human scheme of things, as is hinted in 



