| 
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1 
432 The Effect of some Local Anesthetics on Nerve. 
response is attributed by him to step-growth in the number of nerve fibres 
reached by the stimulus as it is increased in intensity, and his explanation is 
borne out by our records of slow restoration of the muscle twitch. 
Since diffusion is slow (supra, p. 423), the superficial nerve fibres must 
recover from the effects of the drug appreciably earlier than the deeper — 
ones, and since partial blocking of individual fibres is excluded, and the 
stimulus applied to the nerve trunk is constant, the step increments in 
response may fairly be assigned to step increments in the number of nerve 
fibres that conduct the nerve stimuli to the muscle. 
It is of course obvious that, if (with constant total load) the number of 
muscle fibres that enter into contraction increases, the amplitude of con- 
traction must increase, since the load per contracting fibre is correspondingly 
diminished. 
VI. CONCLUSIONS. 
1. Tested by their effect on the conductivity of frog’s nerve, stovaine, its 
homologues, and Fourneau’s new salt are more active local anesthetics than 
is cocaine. 
2. Methyl] stovaine, amy] stovaine and Fourneau’s salt appear to offer some 
particular advantage over the others. | 
3. Judged by muscular response, the anesthetic block in individual nerve 
fibres, to impulses evoked by maximal single stimuli, throughout a wide range 
of intensity, is complete or zero. 
4, This block is usually also,* within the same range, complete for impulses 
evoked by tetanising stimuli. 
5. The block, when not complete for tetanising stimuli, is largely resistant 
to summation (cf Wedenski). 
6. From the effects of the local anesthetics in question, it appears that the 
amplitude of a motor nerve impulse is, within normal limits of stimulation, 
substantially maximal or zero (¢f. Gotch). 
7. Ceteris paribus, the amplitude of a muscle twitch depends on the number 
of fibres involved (cf. Griitzner, Gotch, and Keith Lucas). 
* Vide pp. 423, 424. 
