696 O. D. ANDERSON 



only differ in many notable ways from that of another at a 

 given moment of comparison, but that each animal may 

 pursue a strikingly divergent course during different periods 

 of time. Thus the characteristic behavior of a sheep, or a 

 dog, or a person, undergoes a continual reorganization. It 

 responds sensitively to a changing system of stimulations from 

 the environment, both internal (change in internal body chem- 

 istry) and external (changes in the patterns of sound, sight, 

 odor, or touch). Alterations in response to environmental 

 change are a consequence of education or conditioning. Thus, 

 in experiments of this kind, the aim of which is to determine 

 the effect upon the C-R of thyroidectomy or of other physio- 

 logical alterations, it is unquestionably of advantage to study 

 the reactions in one and the same animal in the period prior 

 to and following the operation. The animal is thus his own 

 best control. 



Neither Valkov nor Liddell employed a means of quanti- 

 tatively estimating the vigor or magnitude of the conditioned 

 motor reflex. A method of numerically designating whether 

 a reaction was strong, or medium, or weak, might conceivably 

 have been helpful in the analysis of the consequences of 

 thyroidectomy on the response. Further experiments using 

 the modified Fick work accumulator under their exact condi- 

 tions would, of course, be a way of testing this. 



Finally we may state that the effects of thyroidectomy on 

 the salivary C-R in three dogs and the motor C-R in one 

 dog were briefly as follows. Both the salivary and motor 

 positive responses in all the animals were greatly decreased 

 in efficiency and in magnitude after the operation. Efficiency 

 and magnitude tended to return to the normal level following 

 the chronic administration of thyroid extract in two dogs 

 and following the natural regeneration of thyroid tissue in 

 two others. Decrease in general alertness was shown in all 

 the animals by lengthening of the latent period of the C-R. 

 This was shortened by thyroid administration. 



In all cases the percentage of correct responses to the 

 negative conditioned stimuli increased after thyroidectomy. 



