694 



O. D. AXDKKSO.X 



lengthened and alertness is lost. Therefore, the 100 per cent 

 efficiency viewed as a reaction to a negative stimulus really 

 means no reaction at all, or, in other words, apathy. 



It has been pointed out above that the increased positive 

 efficiency resulting from injecting the normal dog with thyroid 

 extract is due to a heightened excitability of the nervous 

 system. In the present section, we have found that efficiency 

 of the positive C-R decreases after removal of the gland. 

 This is interpreted as due to a state of lessened excitability. 

 Under the thyroidless state we have the negative C-R in- 

 creasing in efficiency! This negative increase, then, (fre- 

 quently going up to 100 per cent), is actually a lowering 

 of sensitivity or excitability to the stimulus. 



The results do not agree in all respects with the findings 

 of Valkov. In his experiment, the salivary C-R was unstable, 

 and in the present work the salivary reflex is aroused by the 

 usual stimuli only with great difficulty, and in the great 

 majority of cases it cannot be aroused at all. Our results 

 are in definite disagreement with his concerning the forma- 

 tion of the conditioned reflexes. Valkov points out that both 

 the motor and salivary C-R could be established as well in 

 the thyroidless dog as in the normal one, the motor reflex 

 being normally maintained after its establishment. We were 

 quite unable to form the salivary C-R in such an operated 

 dog, and could develop only the weakest sort of motor C-R, 

 which was maintained only very poorly. 



The results are also different from those obtained by Liddel] 

 in the sheep and goat. 11 The thyroidectomized animals formed 

 and maintained the motor conditioned reflex equally as well 

 as the normal controls. They could also form conditioned 

 differentiations equally well. The disagreement between these 

 results and those of the present experiment may be apparent 

 rather than real. The two investigations are not comparable 

 in a fundamental respect, namely in the use of two widely 

 different animal forms, the sheep and goat on one hand and 

 the dog on the other. One would naturally expect the behavior 



11 The present author worked with some of the animals in Liddell's experiments. 



