Electro-physiology and electro-therapeutics : showing the best methods for the medical uses of electricity / By Alfred C. Garratt.
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Indeed, here, according to my experience, is one of the most marvellous and valuable effects of all the medical uses of elec- tricity. Even as long ago as in Volta's day, it was known that the prolonged action of a steady current in the same direction, — say for an hour or so, — on the nerves of a mutilated frog rendered them so that contractions were no longer produced, either at the making or breaking of that current. But the instant it was exposed even to the same current reversed in direction, the frog showed very violent contractions and agi- tations, even stronger than at first. Therefore, the continuous action of any moderate galvanic current, we may then safely conclude, does not produce any sort of disorganization of the nerve, as has been so frequently taught, but that it rather modifies it, to a certain degree, so that it obtains a new susceptibility. If the nerve is sick or diminished in its physiological irritability and power, this cur- rent can increase and restore that; if the nerve is already alive and in health, it can thus be exalted above the normal. When, by a protracted duration of the same even directed stream, the nerve gets accustomed to this new state, it may remain so for a considerably shorter or longer time, even after the cause that has produced this changed state is removed. For we notice that, if we suspend the action of such current for a few moments, and then apply it again in the same direction, we find it produces no sort of effect, because it finds the nerve in the same condition still which that current tended to produce on it. It is no longer the same, however, if now the direction of the current is changed; for the more the nerve has become accustomed to the first cur- rent direction, within certain bounds, all the less will it tolerate the action of the second, or reversed. But what this exact modification is, we can only infer. It clearly proves, that the modification produced by each direct and inverse current, and by each closing and opening current, is, in fact and effect, very dif- ferent. Therefore M. Nobili calls the first direct alteration, which wo understand is produced by the direct or down-running cur- rent, while the second ho terms the inverse alteration, which is likewise produced by the inverse or np-running current.