100 THE CONTRACTILE TISSUES. 



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

 and the first changes in the muscle, the molecular latent-period changes, 

 also 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. 



71. Putting together the facts contained in this and the preceding sec- 

 tions, the following may be 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 second. 

 Passing down the nerve fibres to the muscle, flowing along the branching 

 and narrowing tracts, the wave at last breaks on the end-plates of the fibres 

 of the muscle. Here it is transmitted into what we may call a muscle 

 impulse, with a shorter, steeper wave, and a greatly diminished velocity 

 (about 3 m. per second). This muscle impulse, of which we know hardly 

 more than that it is marked by a current of action, travels from each end- 

 plate in both directions to the end of the fibre, where it appears to be lost ; 

 at all events, we do not know what becomes of it. As this impulse wave, 

 whose development takes place entirely within the latent period, leaves the 

 end-plate, it is followed by an explosive decomposition of material, leading 

 to a discharge of carbonic acid, to the appearance of some substance or 

 substances with an acid reaction, and probably of other unknown things, 

 with a considerable development of heat. This explosive decomposition gives 

 rise to the visible contraction wave, which travels behind the invisible mus- 

 cle impulse at about the same rate, but with a vastly increased wave-length. 

 The fibre, as the wave passes over it, swells and shortens, and thus brings its 

 two ends nearer together. 



When repeated shocks are given, wave follows wave of nervous impulse, 

 muscle impulse, and visible contraction ; but the last do not keep distinct ; 

 they are fused into the continued shortening which we call tetanus. 



THE NATURE OF THE CHANGES THROUGH WHICH AN ELECTRIC CUR- 

 RENT is ABLE TO GENERATE A NERVOUS IMPULSE. 



Action of the Constant Current. 



72. In the preceding account, the stimulus applied in order to give 

 rise to a nervous impulse has always been supposed to be an induction-shock, 

 single or repeated. This choice of stimulus has been made on account of 

 the almost momentary duration of the induced current. Had we used a 

 current lasting for some considerable time, the problems before us would 

 have become more complex, in consequence of our having to distinguish 

 between the events taking place while the current was passing through the 

 nerve from those which occurred at the moment when the current was 

 thrown into the nerve or at the moment when it was shut off from the nerve. 

 These complications do arise when, instead of employing the induced current 

 as a stimulus, we use a constant current, i. e., when we pass through the nerve 

 (or muscle) a current direct from the battery without the intervention of 

 any induction-coil. 



'Before making the actual experiment, we might, perhaps, naturally sup- 

 pose that the constant current would act as a stimulus throughout the whole 



