THE NERVE CURRENTS. 225 



other ; and the effect is much increased by doubling the piece of nerve, 

 and applying both cut ends to one cushion, and the centre of the loop 

 to the other. The nerve current is, however, more difficult to detect 

 than the muscular current, being many times weaker ; but, as in the 

 case of that current, it must be remembered that only a portion of the 

 proper current of the nerve operated upon, or, as believed by Du Bois- 

 Reymond, only a secondary derived current, can be made to pass 

 through the circuit of the galvanometer. The nerve current ceases in 

 the dead nerve. Budge alone regards it as an artificial current, of 

 doubtful existence in the living nerve; but its presence is, by others, 

 universally admitted. To explain this electrical condition of the liv- 

 ing quiescent nerve, its ultimate molecules have been supposed, as in 

 the case of muscle, to be either single molecules with a peripolar ar- 

 rangement, that is to say, with an equatorial positive band all round 

 them, and with the two extremities negative (Diagram D); or else to 

 be composed of a series of double molecules, having their correspond- 

 ing poles placed towards each other (Diagram F, a). When a nerve 

 is excited to action, its normal current, like that of muscles, undergoes 

 a diminution, and this takes place whether the stimulus be galvanic, 

 or mechanical, or chemical, such as salt and strychnia. This is most 

 evident, when interrupted electrical currents are used to stimulate the 

 nerve, and the muscles are tetanized. According to Du Bois-Reymond, 

 the current may even be reversed; and a portion of a nerve so altered, 

 has its cut ends neutral or positive to the longitudinal surface, instead 

 of negative. In this condition, the conducting power of the nerve is 

 lessened. The nerve regains its normal conditions, on being placed 

 for a time between pieces of muscle. 



The resemblances between nerve and muscle, in regard to their 

 electrical currents, however, are not complete ; for the electrical condi- 

 tion of a nerve is capable of being altered in a peculiar manner, by the 

 application of a continuous galvanic current, which we shall here speak 

 of as the exciting current, to a distant portion of the nerve ; for, in 

 such case, the normal current is altogether changed, according to, and 

 in obedience with, the direction of the exciting current. If, for ex- 

 ample, a portion of a nerve. Diagram E, 1, a, is connected with the 

 galvanometer, by being placed on the moist cushions of the apparatus, 

 represented in Diagram A, the normal nerve current passes within the 

 portion of nerve, a, in the direction of the arrow, i. e., from the cut 

 end to the surface of the nerve. When now a constant exciting cur- 

 rent is applied to another part of the nerve, by means of a galvanic 

 cell B, in a direction, as marked by the arrow, 6, corresponding with 

 that of the nerve current in the part a, then the strength of this latter 

 current is increased ; but if, as shown in 2, the direction of the ex- 

 citing current at b is opposed to that of the normal current in a, then 

 the strength of the latter is diminished. Furthermore, if, as in 3, a 

 portion of nerve is placed on the cushions, with the points of contact 

 equidistant from its centre, so that the normal nerve currents passing 

 from the middle or equator towards the ends or poles of the nerve coun- 

 terbalance each other, as is well known, no effect is produced on the 

 galvanometer needle ; but when an exciting current is passed, as from 



15 



