478 ELEMENTARY PHYSIOLOGY less. 



way we come to speak of a nervous impulse as due to 

 the propagation of a " molecular disturbance " along a 

 nerve. But this expression serves rather to hide our 

 ignorance than to explain wliat the impulse really is. 



Electrical Properties of a Nerve. — In the case of a 

 muscle we saw (p. 300) that its entry into a state of (con- 

 tracting) activity was accompanied by an easily recognised 

 change of shape, by chemical changes and by changes of 

 temperature. Of these the first is of course entirely 

 wanting in a nerve when it Ijecomes active, i.e. is conveying 

 an impulse, and the other two kinds of change have not 

 so far been shown to take place in a nerve. But we saw 

 also that the contracting activity of a muscle is accom- 

 panied by an electrical disturbance ; a similar disturbance 

 takes place in a nerve as the impulse sweeps along it, and 

 is indeed the only evidence we possess of any change 

 which accompanies the transmission of that impulse. 

 Hence in a nerve this electrical phenomenon becomes of 

 extreme interest and merits a short consideration. 



If a piece of nerve, as for instance the sciatic nerve 

 (see Fig. 90), from a freshly-decapitated frog, is removed 

 from the body and suitably examined, it is found that 

 each cat end of the nerve is electrically iiegative as compared 

 with any other jjoint of the nerve nearer to its middle or 

 equator. Hence if one end li (Fig. 153), of the nerve and 

 its middle point C are brought into contact with the 

 terminals a,b, of a sensitive galvanometer ^ G, the needle 

 of this instrument is at once deflected in a direction which 

 shows that an electric current is passing (through the 

 galvanometer) from the middle of the nerve to the cut 

 end. If, now, when the needle has come to rest under the 

 influence of this current, the end A of the nerve be 

 stimulated at x, the needle of the galvanometer is seen to 

 swing back towards the position it occupied before it icas 

 deflected by the current from the nerve. This means that as 



1 A galvanometer is .aii instrument used for the detection and 

 ■measurement of electric currents. 



