CHAP, ii.] THE CONTRACTILE TISSUES. 127 



ceding sections, 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 sec. 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 

 transmuted into what we have called a muscle impulse, which 

 with a greatly diminished velocity (about 3 m. per sec.), 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. f As this impulse wave sweeps along the fibre it 

 initiates 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; 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. 



