FORCED MOVEMENTS 27 



body toward the lei't. These rolling motions are another 

 well-known type of forced movements. When such an 

 animal swims slowly, it will roll more than a normal fish, 

 but it will not roll completely around its longitudinal axis. 

 These are the same motions which were observed in dogs 

 by Magendie and Flourens 155 after an operation in the 

 medulla or pons. We can state, therefore, that the rolling 

 motions are caused by the weakening of one group of 

 (associated) muscles while their antagonists are not 

 weakened. 



It is of interest to consider the nature of forced move- 

 ments after injury of the cerebral hemispheres in a dog. 

 When in a dog one of the cerebral hemispheres is injured 

 the animal immediately after the operation no longer 

 moves in a perfectly straight line, but deviates from the 

 straight line toward that side where the brain is in- 

 jured. 178 When the left hemisphere is injured circus 

 motions toward the left ensue. If one offers a dog which 

 was operated in the left cerebral hemisphere a piece of 

 meat, removing it as fast as the dog approaches, the 

 dog will move at first a certain distance in a straight line ; 

 it will then suddenly turn to the left and describe a com- 

 plete circle, moving afterward for a little while in a 

 straight line toward the meat and turning again through 

 an angle of 360° to the left, and so on. 284 The explanation 

 is the same as for the foregoing cases. The lesion of the 

 left cerebral hemisphere caused a weakening or partial 

 paralysis of the muscles which turn the body to the right. 

 Hence the animal will, when following the meat, deviate 

 to the left, and this causes a displacement of the retina 

 image in the same direction and an apparent motion of the 

 object to the right. We shall see in a later chapter on 



