460 



JOHN LINCK ULRICH 



great a pressure was required to plunge it, or the employment 

 of more than one method was due to the undeveloped condition 

 of the reflex extensor thrusts. Additional observations pointed 

 to the fact that the mechanism of the plane used in this pre- 

 liminary work was at fault, and when an adequate plane was 

 constructed, it was seen that in a measure the functional condi- 

 tion of the extensor thrusts determined the method used. The 

 inadequacy of the plane used led to the construction of the 

 plane seen in figure 4. This plane produced many more effec- 

 tive extensor thrusts than were produced with any other plane. 

 The necessity of adjusting the working mechanism of every 



FIG. 4. THE PLANE 



problem was previously seen to be of importance when investi- 

 gating learning in the latch-box problem, when the height of 

 the latch, and the depth of the notch in which the latch was 

 placed, had to be determined before there could be produced 

 the neck reflex movement adequate to raise the latch. With 

 the inclined-plane problem, this adjustment of the mechanism 

 of the problem was found to be even more essential, for if the 

 plane did not descend readily, other methods, stepping on the 

 plane, ascending it, or partly ascending it, occurred to plunge 

 it. When the extensor thrust was not used, the weight of the 

 body helped to bring down the plane. Consequently it was 

 impossible to discover the most effective method. When the 



