Chap, ii.] THE CONTRACTILE TISSUES. 109 



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. 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. 



