i GENEEAL PHYSIOLOGY OF MUSCLE 17 



conditions the movements of our body are not the effects of 

 simple muscular contraction, due to isolated and instantaneous 

 stimulations, but almost invariably result from a series of rapidly 

 succeeding stimuli, which produce in the muscle the state of 

 permanent and apparently uniform contraction known as tetanus. 



Volta (1792) was the first who recognised that frequently 

 repeated stimuli were able to produce persistent contraction in 

 muscle. Matteucci (1838) first termed this state of contraction 

 tetanus, and the interrupted currents which produce it, tetanising 

 currents. Helmholtz (1854) first demonstrated that tetanus of 

 the skeletal muscles is the effect of the summation and fusion of 

 a rapid succession of simple contractions. 



On sending two shocks from an induced current into the 

 nerve of a muscle at very brief intervals, so that the second 

 stimulus falls 011 the muscle during the period of latent excitation, 

 the resulting curve does not differ from that produced by a single 

 shock if the current is maximal, but if, on the contrary, the 



Fin. 11. Diagrammatic superposition of two contractions. (Helmholtz.) The curves a b c and 

 </ i: / represent two distinct contractions excited by two shocks rr'. The curve a g h i k 

 represents the superposition and fusion of the two preceding, as if the contractions <l ef rose 

 from the abscissa line g i, and not from d /. 



current is moderate or hardly effective, the height of the curve 

 is different. Accordingly, two shocks of medium strength act 

 in this case like a single maximal or supra -maximal stimulus, 

 showing that there is summation of the two excitations (Helm- 

 holtz). In the crab's muscles it is possible also to observe latent 

 summation of several shocks, each of which is ineffective in itself, 

 that is, incapable of producing any visible sign of excitation 

 (Richet). 



If the interval between two stimuli is such that the second 

 induction shock falls on the muscle during the contraction 

 induced by the first, the second shock is superposed upon the 

 former, as if the muscle were at the moment of its application in 

 the natural state of rest (Helmholtz). In this case, accordingly, 

 the two contractions fuse into a single one of greater height and 

 duration (Fig. 11). 



If the interval between the two stimulations is such that the 

 second contraction is sent in when the muscle is at the height of 

 the contraction produced by the first, the fusion of the two will be 

 maximal, i.e. almost double that of the simple twitch. This 



VOL. in c 



