482 JOHN LINCK ULRICH 



variety of methods to plunge the plane occur until responses 

 become more directive and effective. 



The facilitation of direct progression in one common path 

 from the entrance box to the plane is a slow process, even in 

 rats with fairly well developed reflex thrusts. It was shown in 

 table 11, that unobservable decreases in the extensor tone of 

 the fore limbs were supposed to account for the variations in 

 direction to the plane, and this view can now be supported by 

 observations of other rats. When the reflex extensor thrusts are 

 asymmetrically developed, the direction of progression to the 

 plane is to a great extent conditioned by the stronger extensor 

 tone existing in the two ipsilateral fore thrusts when a rat's 

 head is bent to the right or to the left of the midline of the body. 

 In many cases, however, the direction to the plane is apparently 

 conditioned by the position of the operator. When, for in- 

 stance, the operator stands to one side of the problem, progres- 

 sion to the plane takes place usually on the side of the problem 

 more distant to the operator; that is, if the position of the oper- 

 ator is constantly to the left of the problem, progression to the 

 plane will generally be to the right of the problem to the plane. 

 And when the operator is directly in back of the problem, pro- 

 gression is then conditioned by the development of the rat's 

 extensor thrusts and the path so determined is illustrated in 

 the diagrams figure 5, A, B, and C. 



In rats in which a right ipsilateral fore reflex thrust exists 

 or in which the extensor tone of the right is stronger than that of 

 the left, progression is to the left of the problem as is traced in 

 diagram A. The reverse is the case when a predominant left 

 ipsilateral fore thrust is evoked, for progression is as in diagram 

 B, to the right of the problem. Certain exceptions to this rule 

 must be made, for some tunes progression to the plane, when 

 the extensor tone of the left ipsilateral fore thrust is stronger 

 than the right, is to the left of the plane, instead of to the right. 

 This variation is due to the right fore thrust or neural center of 

 the cord, influencing the left thrust or center of the cord; for, 

 during learning, the extensor tone of the left ipsilateral fore 

 thrust may become stronger and progression is no longer direct, 



