INTEGRATION OF MOVEMENTS IN LEARNING IN THE RAT 423 



the development of a greater interaction of the parts of an or- 

 ganism. Any undeveloped condition that may exist in the 

 external senses can not be revealed by the behavior of rats, for 

 it is not at all times observable what external senses can be sup- 

 posed really to direct progression. At times it is not certain that 

 the movements produced are at all evoked by excitations in an 

 external sense, but appear because the reflex mechanism for 

 progression is undeveloped. Only when a symmetrical develop- 

 ment of the extensor thrusts exists, is it observable that move- 

 ments are directive and that the senses are at all concerned in 

 producing them. Moreover, in such a developed condition of 

 the thrust, the interaction of the external senses with the mech- 

 anism for the production of reflex excitability is explicit, for when 

 rhythmic extension and flexion of the limbs is evoked by the 

 test for them, and reflex excitability is manifest, the relation of 

 one to the other of the reflex mechanisms is reciprocal. An ex- 

 isting undeveloped condition of the thrusts or of the mechanism 

 for the production of reflex excitability, reveals a want of inter- 

 action of body parts which results in the performance of a num- 

 ber of movements. Sometimes it appears that the numerous 

 movements made are the direct result of an incomplete interac- 

 tion of body parts of the rats, and not due to any specific sensory 

 excitations. The term " random" is then not inappropriately 

 applied to movements when they can not, for a time, be effectively 

 produced, but not for any other reason, such as the rat produces 

 them; for when movements are at all directive on the first trial, 

 it appears that a spread of excitations from numerous senses in- 

 crease the extensor tone of integrated reflexes, and all movements, 

 inclusive of posture, are effectively performed. When the ex- 

 tensor tone of the reflexes can not reach a certain point, then an 

 excitation in the external sense of touch, of vision, or of olfaction 

 does not produce effective responses and direct progression. The 

 external sense, touch, does not then produce the complex integrated 

 movement of the neck reflex. The results obtained and presented 

 in tables 1 and 2, indicate this very clearly. Before this neck reflex 

 movement can be produced, it is necessary that many sensory 

 excitations be aroused by manifold sensory stimuli, or in other 



