1912.] 



Instability of Cortical Points. 



253 



By an extension response we mean one in which the primary movement 

 is extension as regards the isolated muscles under observation at the elbow. 

 By a flexion response we mean conversely one in which the primary move- 

 ment in these muscles is flexion. It may be added that a flexion response 

 has in our preparation two modes of expressing itself ; it can either cause 

 contraction of the flexor muscle, or it can cause inhibitory relaxation of the 

 extensor muscle — but this latter form will be obvious only if the extensor 

 be in some degree of contraction at the time when the inhibition occurs. 

 Very often the two effects — contraction of flexor and inhibitory relaxation 

 of extensor — result together (reciprocal innervation). But sometimes only 

 the one or the other is observable, and of these the evident one is frequently 

 the inhibitory relaxation. Similarly, an extension response has two means 

 of expressing itself in the preparation — namely, either in contraction of the 

 extensor muscle or in inhibitory relaxation of the flexor muscle — and very 

 often it expresses itself in both of these ways together, but sometimes by one 

 or the other alone. 



For brevity, a cortical point which, under ordinary conditions of stimu- 

 lation, regularly yields a primary flexion response may be termed an " E " 

 point ; and similarly a cortical point which regularly yields a primary 

 extension response may be termed an " E " point. 



III. Changes in the Eesponse of a Cortical Point in Result of 

 Serial Stimulation of it. 



We find that successive stimulations of one and the same cortical point at 

 short intervals of time usually produce, as is well known, marked facilitation 

 and intensification of the response. This is clearly demonstrated when 

 stimulations of two seconds' duration are repeated at two seconds' intervals 

 without change in their intensity or other characters (figs. 2, 3, 4, 5). The 

 responses in the earlier portion of the series tend to exhibit a regularly 

 progressive shortening of the latent period (figs. 4, 5) and an increase in the 

 amplitude and intensity of the muscular effects, and this increase in some 

 cases proceeds through a long series of responses, so that although the opening 

 member of the series is very weak, the last may produce a maximal con- 

 traction of the muscle, or, if that is already present, may maintain it (fig. 4). 



Sometimes, however, the character of the response exhibits changes 

 qualitative as well as quantitative. Thus a point which began by yielding 

 primary extension may come to yield primary flexion in the latter part of 

 the series. Its action undergoes reversal. Such reversal, although taking 

 place gradatim, may in some cases develop very rapidly (fig. 1). 



Similarly an F point yielding primary flexion at the beginning of a aeries 



