238 COMPARATIVE PHYSIOLOGY OF THE BRAIN 



suited for demonstrating reflexes than the frog with 

 cerebral hemispheres. If the skin on the back of 

 a normal frog is touched, it may or may not croak. 

 Goltz showed that this croaking reflex never fails in a 

 frog whose cerebral hemispheres have been excised 

 (3). In the normal frog, however, touching the skin 

 of the back produces, in addition, another reflex : it 

 shows a tendency to leap away. The normal frog as 

 well as the frog without cerebral hemispheres is a re- 

 flex animal that is, its reactions are chiefly segmen- 

 tal reflexes. But there is this difference between the 

 two : In the animal with cerebral hemispheres the 

 same stimulus can produce more than a single reflex, 

 and this fact adds to the greater complication and 

 capriciousness of the reactions of the animal. On the 

 other hand, the cerebral hemispheres can also restrict 

 the play of the segmental reflexes. The clasping re- 

 flex of the male frog in the act of copulation is a 

 segmental reflex of the arm-segments during the 

 period of heat. It seems that sexual substances de- 

 termine this reflex, since it cannot be shown to exist 

 in animals that are castrated before the period of 

 heat. Now male fro^s that have lost the cerebral 



o 



hemispheres are much more indifferent in the choice 

 of the object they clasp during the period of heat 

 than animals with cerebral hemispheres. 



2. In birds the conditions are different from those 

 which exist in frogs and sharks. We are indebted to 

 Schrader for an exact and, in many respects, classic 

 investigation of the effect of the extirpation of the 



