ACTION OF THE EYE MUSCLES 549 



that simultaneous contrast is not a delusion of the judgment, but rests upon 

 the action of neighboring spots in the visual apparatus. The state of excita- 

 bility of a retinal spot A, for example, is always dependent upon the physio- 

 logical condition of all the rest of the retina, particularly of the parts adja- 

 cent to the spot A. Thus, if the spot A is being constantly stimulated, its 

 excitability may be raised or lowered merely by changing the strength of the 

 light affecting other parts of the retina. Every increase in the intensity of 

 the stimulus on other parts reduces the excitability of the given spot A, so 

 that the sensation mediated by it is less bright. Every decrease in the stimulus 

 on the rest of the retina changes the condition of A so that the corresponding 

 sensation becomes brighter. The same laws apply, according to Hering, to 

 color contrast. That is, if the spot A is exposed to white light and the rest 

 of the retina to yellow-red light, the excitability of the spot A for yellow red 

 is reduced and the white field appears in the complementary color, etc. Hering 

 believes that the effects of adjacent retinal spots on one another play an 

 essential part also in the production of positive and negative after-images. 



THIRD SECTION- 

 MOVEMENTS OF THE EYE AND VISUAL PERCEPTIONS 



1. ACTION OF THE EYE MUSCLES 



In discussing movements of the eye we assume what is only approximately 

 true, namely, that they take place about a definite point called the center of 

 rotation, also that when the head is erect and the gaze is directed straight 

 forward the two lines of vision are horizontal and parallel throughout (primary 

 position). 



Measurements which have been made to determine the point of origin and 

 the point of insertion of the different muscles, with reference to the center 

 of rotation and the line of vision, as the primary axis, have shown that the 

 three pairs of muscles are not directly antagonistic. The axis of rotation of the 

 superior rectus muscle does not coincide with that of the inferior rectus, nor 

 does that of the external rectus coincide with that of the internal, nor that 

 of the superior oblique with that of the inferior. For the sake of simplicity, 

 however, we shall neglect these differences and assume that each pair of mus- 

 cles rotates the eye about one and the same axis (Volkmann). 



The positions of the assumed common axes for the two eyes are shown in 

 Fig. 240. The line D-D' is the axis assumed to be common to the superior 

 and inferior recti and 00' that for the two oblique muscles. The axis for 

 the external recti would be vertical to the plane of the paper at G. The 

 movement of either eye in the figure caused by the isolated action of each of 

 these muscles may be pictured to oneself by placing the book so that one's 

 own line of vision coincides with the axis of any given muscle, and then 

 imagining the eye in the picture to rotate right or left about the observer's 

 line of vision. 



Fig. 241 represents, according to Hering, approximately the paths which 



