It is found that not only tetanisation, but also CO2 has the power of converting the modified response into normal. Hence it has been suggested that the conversion under tetanisation of modified response to normal, in stale nerve, is due to a hypothetical evolution of CO2 in the nerve during stimulation.[16]
(2) In metals.—I have, however, met with exactly parallel phenomena in metals, where, owing to some molecular modification, the responses became reversed, and where, under continuous stimulation, though here there could be no possibility of the evolution of CO2, they tended again to become normal.
If after mounting a wire in a cell filled with water, it be set aside for too long a time, I have sometimes noticed that it undergoes a certain modification, owing to which its response ceases to be normal and becomes reversed in sign. I have obtained this effect with various metals, for instance lead and tin, and even with the chemically inactive substance—platinum.
Fig. 75.—Abnormal Positive (up) Response in Nerve Converted into Normal (down) Response after Continuous Stimulation T (Waller)
The galvanometer is not dead-beat, and shows after-oscillation.
The subject will be made clearer if we first follow in detail the phenomenon exhibited by modified nerve, giving this abnormal response. The normal responses in nerve are usually represented by ‘down’ and the reversed abnormal responses by ‘up’ curves. In the modified nerve, then, the abnormal responses are ‘up’ instead of the normal ‘down.’ The record of such abnormal response in the modified nerve is shown in [fig. 75]. It will be noticed that in this, the successive responses are undergoing a diminution, or tending towards the normal. After continuous stimulation or tetanisation (T), it will be seen that the abnormal or ‘up’ responses are converted into normal or ‘down.’
I shall now give a record which will exhibit an exactly similar transformation from the abnormal to normal response after continuous stimulation. Here the normal responses are represented by ‘up’ and the abnormal by ‘down’ curves. This record was given by a tin wire, which had been molecularly modified ([fig. 76]). We have at first the abnormal responses; successive responses are undergoing a diminution or tending towards the normal; after continuous stimulation (T), the subsequent responses are seen to have become normal. Another record, obtained with platinum, shows the same phenomenon ([fig. 77]).