Transmission-time of Reflexes in Spinal Cord of Frog 53 



45 (fig. 8) and Exp. 53 (fig. 6) the crossed-reflex eflect is very weak 

 throughout the experiment; yet in the one [Exp. 53] the extra delay is 

 approximately equal to the whole same-side reflex delay, whereas in the 

 other [Exp. 45] it is a good deal shorter. Again, in Exp. 55, throughout 

 which there was considerable variation in the length of the delay before 

 the muscle began to respond, and in both kinds of reflexes, the ultimate 

 strength of the effect remained about the same (see fig. 11). On the other 

 hand, when the crossed-reflex effect is variable in strength during an ex- 

 periment [Exp. 47], the extra delay, although it also may vary, does not 

 vary with the strength of the effect. 



Nor does the extra delay in the case of the crossed-reflex response vary, 

 any more than the same-side reflex time does, inversely with the strength of 

 the artificial stimulus evoking it: that is to say, it does not do so when 

 this is once fully above the threshold value, whatever it may do when it is 

 very close to it. Thus, in Exp. 47, in which the stimulus was being gradually 

 weakened, the extra delay was getting shorter, not longer ; and in Exp. 56 R, 

 the extra delay remained very fairly constant, notwithstanding a good deal 

 of variation in the strength of the stimulus, only in the last response, one 

 to an extremely weak stimulus, apparently becoming considerably longer. 

 (As, however, no. response could be obtained with this muscle when the 

 sciatic of its own side was excited by an equally weak stimulus, we do not 

 really know how much should be deducted to find the extra delay in the 

 case of the crossed reflex ; moreover, the effect in this last response was 

 so weak that, for a reason which will appear immediately, I think its 

 significance for giving information about cord delay at all is doubtful.) 

 The same preparation possibly gave direct indication of lengthening of 

 extra cord delay with the weakening of the artificial stimulus when the left 

 gastrocnemius was recording ; for, as with the whole delay in the same-limb 

 reflex, only to a more marked degree, the extra delay was in this case longest 

 in the one response recorded with weakest stimulus, but here at any rate 

 this must have been very near its threshold value (see note to experiment). 



To obtain more information regarding the influence of stimuli very close 

 to threshold strength we must again, as with the same-limb reflex, have 

 recourse to observing what happens in preparations in which there is 

 reason to believe that the threshold resistance of the individual synapses 

 is variable, when these are attacked by the impulses, presumably of equal 

 strength, produced by a single stimulus. So far as Exp. 55 is concerned, 

 all that I have said on p. 25 with regard to the same-limb reflex delay 

 would apply to the crossed-reflex delay, and it is perhaps significant that 

 a long extra delay in attaining the maximal effect was more frequently 

 observed when the stimulus was applied to the nerve of the crossed side 

 than when it was to that of the same side. Thus there were three other 

 records closely resembling fig. 11, D, and none other resembling fig. 11, C. 

 The evasiveness of the threshold under the influence of strychnine is well 

 seen in most of the preparations in which the drug was only beginning to 



