126 TROPISMS 



When an animal, e.g., a shark or a pigeon, is rotated on 

 a turntable, during rotation a nystagmus is observed in the 

 motions of the eyes and sometimes also of the head. If 

 the rotation is not too rapid the eyes move slowly in the 

 same plane but in an opposite direction from the rotation 

 of the turntable, until they form a maximum angle with 

 their normal position in the head; then they rapidly swing- 

 back and the whole phenomenon is repeated. This phe- 

 nomenon is called nystagmus. It depends upon the nerve 

 endings in the semicircular canals, but is not dependent 

 upon the motion or pressure of the lymph in the 

 canals, 290 . 319 - 320 since the cutting out of the canals in 

 the shark or the plugging up of the canals in the pigeon 141 

 leaves the phenomenon unaltered. When after some rota- 

 tion the motion of the turntable suddenly stops, a nystag- 

 mus of the eyes or head in the same plane but in the oppo- 

 site direction as during the rotation is observed. 



Maxwell"'"' 4 has shown that if Phrynosoma is rotated 

 on a horizontal plane with constant velocity and the eyes 

 of the animal are closed, compensatory motions of the head 

 are produced as soon as the angular velocity exceeds a 

 certain value which was 8 seconds for a rotation through 

 an angle of 45°. 



