CHAP. IL] THE CONTRACTILE TISSUES. 73 



to muscle, 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 

 sections cannot well be shewn 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' undergoes a negative variation during a contraction. 

 There are however difficulties in the case of the nerve similar to 

 those in the case of the muscle, concerning the pre-existence of 

 any such 'natural' currents. It is maintained by many that 

 a nerve in an absolutely natural condition is like a muscle, 

 iso-electric ; hence we may say that in a nerve during the passage 

 of a nervous impulse, as in a muscle during a muscular contraction, 

 a ' current of action ' is developed. 



This ' current of action ' or ' negative variation ' may be shewn 

 either by the galvanometer or by the rheoscopic frog. If the nerve 

 of the 'muscle-nerve preparation' B (see p. 62) be placed in an 

 appropriate manner on a thoroughly irritable nerve A (to which of 

 course no muscle need be attached), i.e. touching say 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 B, 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 

 4 current of action or as a negative variation of a ' pre-existing ' 

 current, is an essential feature of a nervous impulse is shewn by 

 the fact that the degree or 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 shewn (p. 47) how the velocity of the 

 nervous impulse is measured also, and stated that the rate in the 

 nerves of a frog is about 28 metres a second. Bernstein by means 

 of a special and somewhat complicated apparatus finds that the 

 current of action travels along an isolated piece of nerve at the 

 same rate. He also finds that it, like the molecular change in a 

 muscle preceding the contraction, and indeed like the contraction 

 itself, passes over any given spot of the nerve in the form of a 

 wave, rising rapidly to a maximum and then more gradually 

 declining again. He has been able to measure tne length of the 

 wave, and this he finds to be about 18 mm., taking *0007 sec. to 

 pass over any one point. 



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 



