3o8 THE BIOLOGY OF THE FROG chap. 



Compensatory Motions. — If a frog is rotated on a circu- 

 lar disk with its head pointing away from the center, the head 

 of the animal will turn opposite the direction of rotation. 

 The turning of the head is often followed by locomotion in 

 the same direction, so that the frog keeps circling around 

 in one direction while the disk is rotating in another. If 

 the frog is placed on a board, and tilted either up or down, 

 the head is turned opposite the direction of motion. Com- 

 bined movements of rotation and tilting are followed by 

 corresponding combined movements of the head, the result 

 being in all cases to keep the head as nearly as possible in 

 its original position. Such movements are therefore called 

 compensatory motions. They take place in response to very 

 slight changes in the position of the body, and they occur 

 with remarkable regularity, apparently independently of the 

 animal's volition. They are performed after the destruction 

 of all parts of the brain in front of the medulla, or even after 

 the removal of the anterior part of the latter organ, provided 

 the injury does not extend so far back as the trigeminus 

 group of nerves. If the semicircular canals are destroyed 

 or the auditory nerves cut, compensatory motions, according 

 to Schrader, are no longer performed, but Steiner finds that 

 compensatory motions of the head at least still take place. 



The Functions of the Brain. — The brain is the great 

 center of communication between the principal organs of 

 sense and the rest of the body ; through it are effected the 

 numerous coordinations between a great variety of stimuli, 

 sights, sounds, odors, etc., and the appropriate muscular 

 actions which enable the animal to adjust itself to the en- 

 vironment. The number and nature of the connections 

 established in the central nervous system determine to a large 

 degree the character of the animal's instincts, and the possi- 

 bilities and the limits of the development of its intelligence. 



