Involved in the Electric Change in Muscle and Nerve. 183 



This quantity lies between 1 and 2 per cent, of the value calculated for the 

 sartorius muscle at 8° C. 



The precise values deduced above are, of course, of no importance as they 

 can be changed considerably by small alterations in the experimental 

 conditions. The interest lies in their order of size, not in their exact values. 

 Consider the case of a nerve. In a previous paper* the production of heat 

 (if any) in the passage of a single impulse along a nerve was shown to be not 

 greater than about 5 x 10~^ calories per gramme. This is more than 100 

 times as large as the heat liberated by the electric currents produced by the 

 nerve. Now the only knoivn accompaninient of the nervous impulse is the electric 

 change, and it is of importance therefore to realise that this electric change 

 involves in itself only a very small liberation of energy, corresponding to a 

 rise of temperature of less than one thousand millionth of a degree, or to the 

 amount of work required to lift the tissue through fifteen millionths of a 

 millimetre. It is natural, therefore, that no direct method should be capable 

 of determining the heat liberated in the passage of a nervous impulse. 



The electric change however, is, in a way, a relatively large effect, being 

 quite easily shown on a galvanometer or electrometer, and capable of directly 

 exciting other ■ tissues (" rheoscopic frog "). Presumably, therefore, it is 

 energetic enough to account for the sudden and temporary change of perme- 

 ability required to initiate the muscle twitch. The facts : (<x) that apart from 

 the electric change there is no known accompaniment of the nervous impulse ; 

 (&) that down to 5x10"^ calories per gramme no production of heat occurs in 

 the passage of an impulse ; (c) that the energy involved in the electric change 

 itself is almost inconceivably small ; and {d) that the electric change is never- 

 theless sufficient to stimulate other tissues ; tend to confirm the belief either 

 that the electric change is the nervous impulse, propagated in some manner 

 at present unknown, or that it is the immediate consequence of some physico- 

 chemical change propagated as a wave with a very small degradation of 

 energy. Unlike the mechanical response of muscle, the electric response of 

 muscle and nerve is accompanied by practically no liberation of energy. 



Sacmniary. 



An expression is given for the heating effect, in a muscle or nerve, of the 

 currents produced by the electric response accompanying the propagated 

 impulse. In a muscle the heat produced is not more than one hundred 

 thousandth part of the energy liberated in a twitch ; in a nerve it is of the 

 order of size of 3'5 x 10~^" calorie or 0'015 erg. per gramme. It is concluded 

 from the smallness of these quantities that no appreciable provision of energy 

 * A. V. Hill, 'Journal of Physiol.,' vol. 43, p. 433 (1912). 



