REFLEX AND CONDITIONED ACTION 49 



Even in the primitive brain of Necturus the reflex 

 centers of the midbrain are complicated by the fact 

 that the efferent tract from the midbrain roof divides 

 into a descending and an ascending path. The latter 



midbrain _ cerebral hemisphere 



ner/s, 



Fig. 4. — Diagram of some conduction paths In the brain of Necturus, 

 seen in longitudinal section. From the medulla oblongata an acoustic 

 impulse may be carried forward through the neuron A to the midbrain, 

 whose neurons, 5, are of the type shown in Fig. 3, receiving both acoustic 

 and optic Impulses. The neuron B may discharge downward through the 

 tract S to the motor nuclei of the III, V, VII, etc., nerves, or it may dis- 

 charge upward to a neuron of the thalamus, C, which also receives de- 

 scending impulses from the cerebral hemisphere through the neuron D 

 and in turn discharges through the motor tract, S, 



ends in the thalamus, where associations of a still 

 higher order are effected through the thalamic 

 neurons (Fig. 4, C). Here again there are no specific 

 "nuclei" in the thalamus, merely ^an undifferentiated 

 field where still different kinds of nervous impulses 

 from the cerebral hemispheres may play upon the 

 apparatus of correlation and so change the dynamic 

 equilibrium of the reacting system as a whole. 



In these generalized Amphibia we are evidently 



