COMPENSATORY MOVEMENTS 39 



COMPENSATORY MOVEMENTS 



A class of phenomena having certain relations to rheotaxis 

 are the so-called compensatory motions of animals. These 

 may easily be illustrated by a common frog. If a frog be 

 slowly rotated about a vertical axis it will turn its head 

 and may begin to walk opposite the direction of movement. 

 If it is tilted downward in front, the head will be raised, 

 while if it is inclined upward, the head will be lowered. 

 Various combined motions will be responded to by move- 

 ments which tend to keep the head in the same position as 

 before. 



A pigeon when slowly rotated on a horizontal turntable 

 turns its head opposite the direction of movement until 

 it reaches a certain angle with the body when it is suddenly 

 jerked back to its original position. It immediately repeats 

 the previous movement until its head reaches again the 

 maximum angle when it is jerked back again as before. 

 If the head is held during rotations, compensatory motions 

 followed by regular jerking back movements are performed 

 by the eyes. Mammals show similar movements, and mice 

 and several other forms run around on the turntable opposite 

 the direction of rotation. An interesting form of compen- 

 satory movement is shown by the common domestic fowl. 

 Hold an individual in the hands and move it slowly back 

 and forth, up and down, or sidewise. If the fowl is not 

 carried too far the head will keep in almost exactly the same 

 position, the neck being often stretched out to its extreme 

 length before the head follows the movements of the body. 



It was formerly supposed that in vertebrate animals 

 compensatory motions were dependent on the semicircular 

 canals, but it was found that these motions still persisted 

 after the semicircular canals were plugged up or extirpated, 

 or after the nerves supplying them were cut. The otoliths 



