CHAPTER XIV 



FORCED MOVEMENTS CAUSED BY MOVING 

 RETINA IMAGES: RHEOTROPISM: 

 ANEMOTROPISM 



The experiments on forced movements show that we 

 Lave three groups of forced movements, (1) right to left 

 and left to right (circus movements) ; (2) forward move- 

 ment, and (3) backward movement. The latter is not 

 always possible. A fourth group, the rolling motions 

 around the longitudinal axis may be omitted here in order 

 to simplify the discussion. 



The forced movements, called forth by the galvanic 

 current, supported the idea that the nervous elements 

 determining these motions must have a definite orientation 

 and that this orientation bears some simple relation to 

 the direction of motion caused by their activity. The 

 experiments on the effect of blackening different parts 

 of the eye indicate that the different parts of the retinae of 

 positively heliotropic insects are connected in a simple 

 way with the main centers of the three types of forced 

 movements: namely, the left eye is connected with the 

 brain center causing motions from right to left (and the 

 right eye with the center for the opposite motion) ; the 

 lower halves of the retina with the forward movements, 

 the upper halves with the backward movements. 



We know through the work of Ewald Hering that each 

 illuminated element on the human retina determines a 

 definite motion of the two eyes which move as if they were 

 a single organ, and that this motion is a function of the 



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