FOECED MOVEMENTS 29 



dog is operated on the other, e.g., the right hemisphere, 

 circus motions to the right appear. 



We do not wish to exhaust the chapter on forced move- 

 ments but may perhaps for the sake of completeness point 

 out the following facts. We have seen that if one cerebral 

 hemisphere is injured the dog shows a tendency to circus 

 movements to the operated side. When both hemispheres 

 are injured, e.g., both occipital lobes are removed, the dog 

 can hardly be induced to move forward and it is impos- 

 sible to cause it to go downstairs, while it is willing to go 

 upstairs. Its front legs are extended and its head is 

 raised high, giving the impression as if such a dog had 

 a tendency to move backward rather than forward or that 

 the forward movement was difficult. If the two anterior 

 halves of the cerebral hemispheres are removed the re- 

 verse happens. The animal runs incessantly as if driven 

 by a mad impulse ; its head is bent down and it is in every 

 respect the converse of the animal operated in the occipi- 

 tal lobes. These two types of forced movements corre- 

 spond to the morphological polarity tail-head. This 

 corresponds to the idea of a projection of the surface 

 elements upon the brain either directly or by crossing. 



These three types of forced movements: the circus 

 motions, the tendency to go backward, and the irresistible 

 tendency to move forward will appear in the form of the 

 tropistic reactions to be described in this volume. 



Since we shall deal in this volume chiefly with inverte- 

 brates, it may be of importance to show that forced move- 

 ments can also be produced in this group of animals 

 by lesion of one side of the cerebral ganglion, and that 

 these forced movements depend also upon the fact that 

 as a consequence of the operation the tension of sym- 

 metrical muscles (which is equal under normal condi- 



