462 JOHN LINCK ULRICH 



In the investigation of learning in the latch-box problem, it 

 was discovered that when reflex excitability was manifest in 

 rats and there existed bilateral symmetrical development of the 

 reflex extensor thrusts, the production of the neck reflex move- 

 ment to raise the latch was without difficulty produced. Such 

 conditions existing in rats proved to be the best for the produc- 

 tion of the extensor thrust to plunge the plane. The extensor 

 thrust was used on the first trial, and continued to be employed 

 throughout learning. Two rats were obtained that possessed 

 fairly well developed reflex thrusts, only a slight decrease in 

 extensor tone of a limb or a disappearance at times of one fore 

 ipsilateral extensor thrust occurred. With the use of the ex- 

 tensor thrusts of the fore limbs on the first trial and throughout 

 learning, conditions resemble those when the neck reflex is 

 effectively produced on the first trial and during learning. Pos- 

 turing is necessary to produce the extensor thrust, as it is to 

 produce the neck reflex movement. Posture is greatly increased 

 on the first trial when the rat's snout touches the latch or slightly 

 moves it, and is once more increased when the fore limbs touch 

 the upper part of the plane and it gives way. Immediately 

 on posturing, the extensor thrust of one or both fore limbs 

 is produced to plunge the plane. The rat's body is slightly 

 crouched before the plane is neared. The effective performance 

 of every movement is quite apparent. 



In all other rats, in the absence of functionally developed 

 reflex extensor thrusts, posturing before the plane does not 

 take place and progression is continued over the plane as it is 

 neared, or a modified extensor thrust is produced. When 

 progression is continued over the plane, the plane is ascended 

 or stepped on, and when a modified extensor thrust is produced, 

 the fore limbs and the weight of the fore part of the body bring 

 the plane down. Sometimes when the modified thrust is used, 

 the hind quarters may lower in the act of posturing. This is 

 but a feeble attempt to attain posture. The functional devel- 

 opment of the extensor thrusts of the rat's limbs determines the 

 method used, and as was the case with the latch-box problem, 

 all degrees of effectiveness to operate the mechanism of the 

 problem are observable. 



