INTEGRATION OF MOVEMENTS IN LEARNING IN THE RAT 415 



another, or when an ipsilateral fore reflex thrust can alone be 

 evoked. These fluctuations occur after repeated tests, or at 

 greater intervals from day to day, and consist in &n increase or 

 in a decrease in the extensor tone of the limbs, at times amounting 

 to a total disappearance of one or more of the reflex thrusts, or to 

 a reappearance of all thrusts when one or more are absent. In 

 some rats the reflex thrusts may not fluctuate for several days, 

 all remaining vigorous and strong, when suddenly they fluctuate. 

 Only in a few rats is the stability of the reflex thrusts fairly con- 

 stant, either after immediate successive tests at one time, or 

 after daily tests. Of all the rats tested, only 2 of the 300 or 

 more had developed reflex thrusts, and fluctuation of them did 

 not take place to any great extent. Slight decreases in extensor 

 tone were alone noticed and then at the end of learning. Fluctu- 

 ations of the reflex extensor thrusts, then, are the general rule 

 in rats and may be said to be more evident in the fore limb thrusts 

 than in the hind limb thrusts. 



What conditions this fluctuation of the reflex thrusts cannot 

 be considered at this point, but a few remarks will have some 

 bearing on observations to be noted later. There are indications 

 that environmental influences condition in a measure these 

 fluctuations. In the daily investigation of them, giving only one 

 test for each limb, it was early observed that marked changes in 

 the environment seemed to increase not only the degree of, but 

 also the frequency of the fluctuations. Removing the rat from 

 the living cage to the problem often produced in many rats rapid 

 and marked fluctuations. Sometimes the extensor tone of the 

 limbs was particularly strong when a rat was first handled or 

 when put for the first time in the problem, and sometimes gradu- 

 ally, sometimes suddenly, when the new situation was about 

 learned, a decrease occurred in the extensor tone and some reflex 

 thrusts disappeared. This fluctuation may be due to the fact 

 that in a new environment reflex excitability, which is aroused 

 because of a change, increases the tonic reflexes of the body in- 

 cluding those of the limbs. Decided changes in temperature, such 

 as a rapid rise or fall, have occasionally been noted to produce 

 fluctuations in the extensor thrusts. Decrease in the extensor 



