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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



nerve gives a single twitch at the moment that the current enters the 

 nerve, and then remains quiet ;' and thus we meet with the remarkable fact 

 that an electric current, though irritating a nerve at the moment that it 

 enters it, can flow through the nerve continuously without exciting it. Fur- 

 ther, although the current while flowing through the nerve does not excite 

 it, a sudden withdrawal of the current from the nerve irritates it, and causes 

 the muscle connected with it to contract. It is our custom to speak of 

 closing, or making, the circuit when we complete the circuit and let the 

 current flow through the nerve, and of opening, or breaking, the circuit 

 when we withdraw the current from the nerve. Since the closing of the 

 circuit acts as a sudden irritant to the nerve, we speak of this irritant as 

 a " making " or " closing " shock, and the corresponding contraction of the 

 muscles as a making or closing contraction ; similarly we speak of the effect 

 of opening the circuit as an " opening " or " breaking " shock, and the result- 

 ing contraction as an opening or breaking contraction. As we shall see later, 

 the making contraction excited by the direct battery current is stronger than the 

 breaking contraction : the explanation of this must be deferred (see page 53). 



(a) Effect of the Rate at which an Irritant is Applied, Illustrated by the Elec- 

 tric Current. As has been said, an electric current of constant medium strength 



FIG. 9. Rheonome. 



does not irritate a nerve while flowing through it, but the nerve is irritated at 

 the instant that the current enters it, and at the instant that the current leaves 

 it. Is it the change of condition to which the nerve is subjected, or is it the 

 suddenness of the change, which produces the excitation ? Would it be possi- 

 ble to turn an electric current into a nerve and remove it from a nerve so 

 slowly that it would not act as an irritant ? 



The experiment has been tried, and it has been found that if the nerve be 

 subjected to an electric current the strength of which is increased or decreased 

 very gradually, no change occurs in the nerve sufficient to cause a contraction 

 of the muscle. In this experiment, instead of using the ordinary key, we close 

 and open the circuit by means of a rheonome (see Fig. 9). 



This instrument contains a fluid resistance, which can be altered at will, thereby per- 

 mitting a greater or less strength of current to pass from the battery into the circuit 



