126 ELECTRIC CURRENTS IN NERVES. [BOOK i. 



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 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 

 '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 ( 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 meters a second. 

 By means of a special and somewhat complicated apparatus it is 

 ascertained that the current of action travels along an isolated 

 piece of nerve at the same rate. It also, like the contraction, 

 travels in the form of a wave, rising rapidly to a maximum at 

 each point of the nerve and then more gradually declining 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 

 travel down the nerve and reach the muscle as a series of distinct 

 impulses ; and the first changes in the muscle, the molecular 

 changes which, sweeping along the fibre, initiate the change of 

 form, and which we may perhaps speak of as constituting a muscle 

 impulse, also probably form a series the members of which are 

 distinct. It is not until these molecular changes become trans- 

 formed into visible changes of form that any fusion or summation 

 takes place. 



73. Putting together the facts contained in this and the pre- 



