CHANGES IN A MUSCLE DUKING CONTRACTION. 99 



piece of nerve removed from the body exhibits nearly the same electric 

 phenomena as a piece of muscle. It has an equator which is electrically 

 positive relatively to the two cut ends. In fact, the diagram Fig. 30, and 

 the description which was given in 66 of the electric changes in muscle 

 may be applied almost as well to a nerve, except that the currents are in 

 all cases much more feeble in the case of nerves than of muscles, and the 

 special currents from the circumference to the centre of the transverse sec- 

 tions cannot well be shown in a slender nerve ; indeed, it is doubtful if they 

 exist at all. 



During the passage of a nervous impulse the "natural nerve current" 

 undergoes a negative variation, just as the " natural muscle current " under- 

 goes a negative variation during a contraction. There are, moreover, reasons 

 in the case of the nerve, as in the case of the muscle, which lead us to doubt 

 the pre-existence of any such " natural " currents. A nerve in an absolutely 

 natural condition appears to be, like a muscle, isoelectric ; hence we may 

 say that in a nerve during the passage of a nervous impulse, as in a mus- 

 cle during a muscular contraction, a " current of action " is developed. 



This " current of action " or " negative variation " may be shown either 

 by a galvanometer or by the rheoscopic frog. If the nerve of the " muscle 

 nerve preparation," B (see 67) be placed in an appropriate manner on a 

 thoroughly irritable nerve, A (to which, of course, no muscle need be attached), 

 touching for instance the equator and one end of the nerve, then single 

 induction-shocks sent into the far end of A will cause single spasms in the 

 muscle of J5, while tetanization of A, i. e. } rapidly repeated shocks sent into 

 A, will cause tetanus of the muscle of B. 



That this current, whether it be regarded as an independent " current of 

 action " or as a negative variation of a " pre-existing " current, is an essential 

 feature of a nervous impulse is shown by the fact that the degree of intensity 

 of the one varies with that of the other. They both travel, too, at the same 

 rate. In describing the muscle-curve, and the method of measuring the 

 muscular latent period, we have incidentally shown ( 46) how at the same 

 time the velocity of the nervous impulse may be measured, and stated that 

 the rate in the nerves of a frog is about 28 metres per second. By means of 

 a special and somewhat complicated apparatus it is ascertained that the cur- 

 rent of action travels along an isolated piece of nerve at the same rate. It 

 also, like the molecular change in a muscle preceding the contraction, and 

 indeed like the contraction itself, travels in the form of a wave, rising rapidly 

 to a maximum at each point of the nerve and then more gradually declin- 

 ing again. The length of the wave may by special means be measured, and 

 is found to be about 18 mm. 



When an isolated piece of nerve is stimulated in the middle, the current 

 of action is propagated equally well in both directions, and that whether the 

 nerve be a chiefly sensory or a chiefly motor nerve, or indeed if it be a nerve- 

 root composed exclusively of motor or of sensory fibres. Taking the current 

 of action as the token of a nervous impulse, we infer from this that when a 

 nerve fibre is stimulated artificially at any part of its course, the nervous 

 impulse set going travels in both directions. 



We used just now the phrase " tetanization of a nerve," meaning the 

 application to a nerve of rapidly repeated shocks such as would produce 

 tetanus in the muscle to which the nerve was attached, and we shall have 

 frequent occasion to employ the phrase. It must, however, be understood 

 that there is in the nerve, in an ordinary way, no summation of nervous 

 impulses comparable to the summation of muscular contractions. Putting 

 aside certain cases which we cannot discuss here, we may say that the series 

 of shocks sent in at the far end of the nerve start a series of impulses ; these 



