#Reversed response due to molecular modification and its transformation into normal after continuous stimulation (1) in nerve.#--Reference has already been made to the fact that a nerve which, when fresh, exhibited the normal negative response, will often, if kept for some time in preservative saline, undergo a molecular modification, after which it gives a positive variation. Thus while the response given by fresh nerve is _normal_ or negative, a stale nerve gives _modified_, i.e. reversed or positive, response. This peculiar modification does not always occur, yet is too frequent to be considered abnormal. Again, when such a nerve is subjected to tetanisation or continuous stimulation, this modified response tends once more to become normal. It is found that not only tetanisation, but also CO_2 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 CO_2 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 CO_2, 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. [Illustration: 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.'