108 ELECTRIC CURRENTS IN NERVES. [Book i. 



ing 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 direc- 

 tions, 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,' mean- 

 ing 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 com- 

 parable 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 transformed into visible changes of form that 

 any fusion or summation takes place. 



§ 68. Putting together the facts contained in this and the pre- 

 ceding sections, the following maybe taken as a brief approximate 

 history of what takes place in a muscle and nerve when the latter 

 is subjected to a single induction-shock. At the instant that the 

 induced current passes into the nerve, changes occur, of whose 

 nature we know nothing certain except that they cause a ' current 

 of action ' or ' negative variation ' of the ' natural ' nerve current. 

 These changes propagate themselves along the nerve in both 

 directions as a nervous impulse in the form of a wave, having 

 a wave-length of about 18 mm., and a velocity (in frog's nerve) of 

 about 28 m. per sec. Passing down the nerve fibres to the muscle, 

 flowing along the branching and narrowing tracts, the wave at last 



