INTEGRATION OF MOVEMENTS IN LEARNING IN THE RAT 459 



of the ease with which the plane can be plunged, all rats, even 

 those manifesting hyponormal reflex excitability, can solve the 

 inclined plane problem. Consequently, different results and 

 additional facts in relation to reflex excitability may be expected. 



Considerable mention of the cumulative effect produced by all 

 movements has been made to emphasize its importance, for in 

 the inclined-plane problem this effect is significant in learning. 

 It will be remembered that an interaction of body parts for the 

 production of the neck reflex movement and those accessory to 

 it appeared when this movement was first produced. It is 

 apparent that plunging of the plane ought to produce the same 

 effect in initiating interaction of body parts for learning the 

 inclined-plane problem, but this effect does not appear in all 

 rats at the same time. Interaction is readily observed to be 

 initiated in rats when reflex excitability is manifest and an 

 additional cumulative effect is brought about by the plane 

 being plunged, for at this time sensory excitations aroused by the 

 plane going down, produce responses which have a profound 

 effect on all movements in succeeding trials. When, however, 

 excitability is hyponormal, and no cumulative effect on tonic 

 reflexes occur, the going down of the plane scarcely produces 

 any additional cumulative effect on these reflexes and the initiat- 

 ing of interaction to learn the problem. Several successive 

 plungings of the plane are required before learning is at all 

 perceptible. 



Preliminary work with the inclined plane problem revealed 

 the fact that usually the extensor thrust of the fore limbs or 

 some modified form of it is used to plunge the plane. In some 

 instances the plane was ascended, but this method was not 

 continued for more than a trial or two. The exclusive use of 

 the extensor thrust of the fore thrust proved to be the method 

 most effectively used. When the extensor thrust was not ex- 

 clusively used, no one method was as a rule consistently em- 

 ployed throughout learning, but generally at the end of learning 

 one method was adhered to. 



Two conditions seemed to determine this inconstancy of 

 method. Either the mechanism of the plane was at fault, too 



