130 TROPISMS 



is kept quiet and objects are moved in front of it, the 

 eyes following the moving object. 



it is of interest to separate the nystagmus or compen- 

 satory motions of eyes and head caused by the orienting 

 effect of a moving retina image from those caused by the 

 orienting effect of centrifugal force and this can be done 

 easily in Phrynosoma. 



When the lizard is rotated very slowly on a turntable 

 with its eyes closed, only very slight compensatory 

 motions of the head and body are observed daring rota- 

 tion, while very powerful compensatory motions are pro- 

 duced when the motion of the turntable is suddenly 

 interrupted after a rotation lasting about thirty seconds. 

 When, however, the same experiment is made with 

 the eyes of the lizard open the reverse is observed. The 

 compensatory motions of the animal during rotation are 

 exceedingly vigorous, while the compensatory motions of 

 the animal after the interruption of the rotation are slight. 

 When the eyes of the animal are closed we are dealing 

 only with the geotropic effect of passive rotation ; when 

 the eyes are open the orienting influence of the moving 

 retina image is added algebraically to the orienting effect 

 of centrifugal force upon the ear. These two influences 

 act in the same sense during rotation and therefore are 

 additive ; while after the rotation they act in the opposite 

 sense to each other. When we rotate the body of an 

 animal passively to the right, during rotation the objects 

 have an apparent motion to the left and the eyes and head 

 of the animal are compelled to follow these moving 

 objects, i.e., to the left. The geotropic effect of passive 

 rotation of the animal to the right also causes a. motion 

 of the eyes and head to the left and hence both effects 

 are additive. 



