EYE MOVEMENTS AND THE FOVEA 301 



lence is the converse eye movement we make with each movement of the 

 head when we shake it vigorously in the gesture of 'no'. Whenever this 

 gesture is made in the course of a face-to-face conversation, we should 

 find it most disagreeable if the image of the other person, and the whole 

 visual field, oscillated with our head movements. If the reader will try to 

 obtain this unpleasant experience by shaking his head without letting the 

 eyes turn in their orbits, he will find some difficulty. The very act of fix- 

 ation itself seems to set off any and all eye-muscle reflexes which are 

 needed to compensate for head and body movements and maintain the 

 status quo of the visual field. 



Actually, the eye-movements of the 'no' gesture, and those made auto- 

 matically when the head or body is turned actively or passively in any 

 direction, have their origin in muscles of the neck and in the apparatus 

 of dynamic equilibrium, in the membranous labyrinth of the internal ear. 

 Disturbance of this apparatus will disturb the involuntary eye move- 

 ments, as occurs in vertigo, intoxication, and in artificial situations such 

 as caloric nystagmus — the induction of convection currents in the laby- 

 rinthine endolymph by the instillation of hot or cold water into the 

 external auditory canal. 



These involuntary eye movements in man and other vertebrates are 

 invariably coordinated; that is, the movements of the two eyes are always 

 in the same sense. If a fish turns sharply to the right, the two eyes rotate 

 leftward, the right eye turning toward the snout and the left eye away 

 from the snout. Though the eyes may move independently for the explor- 

 ation of the visual field, it would never do for them to move unharmon- 

 iously if the field is to be kept as nearly constant as possible. Where this 

 is actually impossible of accomplishment, the eyes will still try to hold 

 on to the field, as in the 'optomotor reaction' so often elicited from lab- 

 oratory animals for the study of their vision : 



The animal is placed on a turntable, in the center of a cylinder coaxial 

 therewith. The inside of this cylinder or drum bears a pattern, say, of 

 vertical stripes. If either the cylinder or the turntable is rotated, the 

 visual field is swept past the animal's eyes. If it is the turntable which 

 rotates, the animal's labyrinths are naturally being stimulated and we 

 should expect him to make compensatory movements of the eyes, head, 

 body, or perhaps all three, in the opposite direction. If only the drum 

 rotates, there is then no stimulation of the labyrinths; but still the eyes 

 turn, in the direction the field is moving. This is the optokinetic or 

 optomotor reaction. When the eyes have swung over as far as they can, 



