order to eliminate the errors that might be inherent in personal observation, it is desirable that all the data for this determination should be furnished automatically in records made by the plant itself. Successive records, therefore, should enable us to determine with equal accuracy not only the normal velocity but also its variation under given changed conditions.
And here the preliminary questions arise: With what degree of accuracy can we determine the normal velocity of transmission? And how far may we depend on the constancy of this velocity, in successive experiments, under normal conditions? As regards these points, some mis-givings might naturally arise. For the factors calculated to interfere with this constancy will in all probability prove to be numerous. First, we may have the variation of excitability at the point of application induced by the stimulus itself. We have, therefore, to find out what is the maximum intensity of stimulus that may be employed without causing fatigue or other deleterious changes in the tissue. Another point to be remembered is the question already discussed in previous chapters of our ability to apply stimuli, in successive experiments, of identical intensity and duration. Unless this can be secured we cannot look for consistent results, inasmuch as the velocity of transmission may to some extent be dependent on the intensity of stimulus. Likewise, if the transmission of excitation should prove to be due to the transmission of a protoplasmic change, it is easy to see that we must allow the tissue a definite time for protoplasmic recovery after each application of stimulus, without which interval consistency of results could hardly be expected.
It was only after a long course of investigations—some of which will be described in the course of the present chapter—that I was able to analyse and provide against these several sources of variation. But even after this, I was by no means prepared for the very great consistency of the results which it has been my good fortune to obtain.
