Transiiiission-time of Reflexes in Spinal Cord of Frog 15 



On deducting from the measured time the time which was, according to 

 what has just been said, assumed to have been spent in traversing, in each 

 response in each experiment, the kno\\Ti length of nerve, we find that the 

 time spent in the cord itself in the same-limb reflex, the time taken, as it 

 seems to me, for the impulse to pass from the branched ultimate endings of 

 the several afferent fibres concerned, each across a synapse, to their respective 

 motor cells, may be, though it only once was so in my experiments, as short 

 as 12(7 in the normal frog's spinal cord, but that it is more frequently some- 

 thing between 14cr and 21cr. If one maj^ use an analogy, which is possibly 

 something more than an analogy, this is the time taken for the endings of 

 the aflerent fibre or fibres, when sufficiently charged, to discharge themselves 

 across the gap to the oppositely charged, or uncharged motor cell. 



With regard to the influence of strength of stimulus on such time of 

 discharge, if such it be, it is not easy to make a definite statement, mainly 

 on account of the difficulty in getting the reflex effect (without the use of 

 drugs) sufficiently often in one preparation.^ In two experiments only, the 

 details of which have already been given in tabular form, did the reflex effects, 

 in response to different strengths of stimulus, give a definite result. Exp. 58 

 shows that stimuli three or four times as strong as one which cannot be far 

 from just producing the reffex effect, ma}^ produce it in almost exactly 

 the same time. In Exp. 37 the reflex times were more varied, but did not 

 consistentl}^ vary inverselj^ with the strength of the induction current. It 

 is, however, possible and even probable that there is for each preparation 

 some strength of stimulus, not quite weak enough to be wholly ineffectual, 

 with which the reflex time is longer, although the fact that it is not until the 

 experiment is over and the photographs developed that one knows exactly 

 what has happened creates a difficulty in deciding this point experimentally. 



The strength of the reflex effect, as indicated by the steepness of the 

 rise in the muscle record, is almost always less, and very considerably less, 

 than that produced directly by the stimulation of the motor part of the 

 nerve. Only with one undrugged preparation have I obtained records 

 which indicate that the effect of the central stimulus was not much less 

 than that of the artificial stimulus to the motor nerve. In this experiment 

 all the four records of the response, when the intact nerve was stimu- 

 lated, showed this. In the first response (to an artiHcial stimulus of 5000 

 units) the reflex effect was strongest; its strength ma}^ be judged by 

 comparing the steepness of the rise in the curves representing the reflex 

 and the direct response in fig. 3. In the second record it was nearly but 

 not quite as strong ; in the third and fourth it was decidedly weaker (but 

 about the same for each); the artificial stimulus used to obtain the last 

 three records was twice as strong as that used to obtain the first. 



^ This is a ditiiculty wliiili I liavi- now overcome by using wholly tloxor mviscles (see 

 footnote, p. 30) to indicu'to cord delay, and by keeping the preparation at a low tempera- 

 ture. The results fully confirm those obtained from the two exiieriments referred to in the 

 text. [November IQOf.] 



