28 TROPISMS 



the orienting effect of moving retina images that this 

 deviation of the retina image to the left causes a forced 

 motion of the animal to the right which compensates its 

 tendency to deviate to the left due to the effect of the 

 brain lesion. Hence the animal approaches the meat in 

 an approximately straight line. But it does so with diffi- 

 culty and sooner or later tiring of this effort it moves 

 in the usual automatic way, whereby equal impulses reach 

 the muscles on both sides. This results in a complete 

 circus movement to the left on account of the weakening 

 (caused by the operation) of muscles which turn the body 

 to the right. The retina image of the meat again induces 

 a straight motion and the whole process described is 

 repeated. When the injury to the brain was less severe 

 the animal may follow the meat for long distances without 

 turning in a circle. 



When such a dog is offered simultaneously two pieces 

 of meat, one in front of the left, the other in front of 

 the right eye, it invariably moves toward the one on the 

 left side. The equal flow of impulses caused by the sym- 

 metrically located pieces of meat results in a stronger 

 contraction in the muscles on the left than on the right 

 side of the body, since as a consequence of the lesion the 

 tension of the former muscles is greater than that of the 

 latter. When two pieces of meat are simultaneously 

 offered to the dog, but both pieces are in front of the left 

 eye, the dog tries to get the piece nearest to its mouth, 

 but its effort carries it a little too far to the left and then 

 it takes the other piece of meat which is situated farther 

 to the left. 284 



Some time after the operation these disturbances may 

 become less and may ultimately disappear. If now the 



