THE REFLEX CIRCUITS 



63 



stronger and more rapid; if, on the other hand, they tend to pro- 

 duce antagonistic responses, there will be an inhibition of the 

 response or a delay until one or the other stimulus obtains the 

 mastery. 



Yerkes has given a striking illustration of this method of re- 

 inforcement of stimuli in his experiments on the sense of hearing 

 in frogs. The reflex mechanism of touch, hearing, and vision in 

 the midbrain of the frog is similar to that of Necturus as des- 

 cribed above (Fig. 20). Yerkes found that frogs under labora- 



midbrain 



Fig. 21. 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, B, are of the type shown in Fig. 20, receiving both acoustic 

 and optic impulses. This 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 descend- 

 ing impulses from the cerebral hemisphere through the neuron, D, and, in 

 turn, discharges through the motor tract, S. 



tory conditions do not ordinarily react at all to sounds alone, 

 but that they do react to tactual and visual -stimuli. When 

 these reactions are carefully measured, it is found that the sound 

 of an electric bell occurring simultaneously with a tactual or 

 visual stimulus markedly increases (reinforces) the strength of 

 the reaction. 



The reflex centers of the midbrain are further complicated by 

 the fact that the efferent tract from the sensory centers above 

 the aqueduct of Sylvius is not simple as diagrammed in Fig. 

 20, but it divides into a descending and an ascending path, as 



