114 ELECTRO-PHYSIOLOGY CHAP. 



ratio with the stimulation frequency, and at which the muscle in 

 a measure oscillates. The more rapid the sequence of stimuli, the 

 more tense will be the contraction of the muscle, and the smaller 

 the individual rhythmical oscillations, which finally betray 

 themselves only by a slight irregularity of the " tetanus curve " 

 in a tracing, or to the eye by a slight tremor of the shiny 

 surface. 



Finally this " incomplete " fuses into " complete " tetanus, 

 in which visible changes of form can no longer be detected. 

 The muscle reaches its maximum of contraction soon after the 

 commencement of the tetanising excitation, and the summit 

 usually lies in this case much higher than in the (maximal) 

 single contractions ; during the persistence of the intermittent 

 excitation it remains uniformly contracted, and returns rapidly 

 to rest (as a rule) when this is over. In spite of its apparent 

 steadiness, tetanus as follows directly from its origin must be 

 regarded as a discontinuous process, arising out of a summation 

 of single twitches, which are only prevented by the sluggishness 

 of the muscle from expressing themselves in visible mass-move- 

 ments, while as we shall see the internal, molecular changes 

 do clearly and unmistakably reveal their intermittent character. 



The manifold varieties of tetanus forms of contraction are only 

 to be understood when the laws of summation of stimuli under the 

 simplest conditions are familiar to us. Here again we are 

 indebted to Helmholtz (1) for the first fundamental investiga- 

 tion. He led two maximal induction shocks into the nerve of 

 a muscle in rapid succession, by opening two primary circuits 

 behind the same secondary coil, one after the other. If the 

 second excitation fell in the latent period of the first, it pro- 

 duced no effect, and the curve of contraction showed no differ- 

 ence from that traced by the first alone. But if it fell later, 

 the relations of the corresponding curve would be the same as 

 if the second stimulus had ensued during the resting stage of 

 the muscle. " From the point at which the second excitation 

 becomes effective, the twitch behaves as if the contracted state 

 of the muscle at the moment was its natural state, and the 

 second twitch alone induced in it" (Fig. 47). 



Let (a, b, c) be the contraction curve of the first excitation, 

 and (d, e, f) of the second, in their separate working, then the 

 actual curve according to Helmholtz 's law would correspond to 



