658 A MANUAL OF PHYSIOLOGY 



and the question arises whether there is also an upper limit beyond 

 which a series of stimuli becomes too rapid to produce complete 

 tetanus, or, indeed, to cause contraction at all. We may be certain 

 that every stimulus requires a finite time to produce an effect, and 

 it is possible that if the duration of each shock were reduced below a 

 certain minimum, without lessening at the same time the interval 

 between successive excitations, no contraction would be caused by 

 any or all of the stimuli in the series. But above this minimum 

 there apparently lies a frequency of stimulation at least, when 

 the interval between the stimuli is reduced exactly in the same 

 proportion as the duration at which an interrupted current comes 

 to act like a constant current, causing a single twitch at its com- 

 mencement or at its end, but no contraction during its passage. 



As to this last limit, on the fixing of which much labour has been 

 expended, it undoubtedly does not depend upon the frequency of 

 stimulation alone ; the intensity of the individual excitations, the 

 temperature of the muscle, and probably other factors, affect it. 

 For Bernstein found that with moderate strength of stimulus 

 tetanus failed at about 250 per second, and was replaced by an 

 initial contraction ; with strong stimuli at more than 1,700 per 

 second, tetanus could still be obtained. Kronecker and Stirling, 

 stimulating the muscle by induced currents set up in a coil by the 

 longitudinal vibrations of a magnetized bar of iron, saw tetanus 

 even with the utmost frequency attainable, 4,000 shocks a second, 

 according to Roth;; while v. Kries in a cooled muscle found tetanus 

 replaced by the simple initial twitch at 100 stimuli per second, 

 although in a muscle at 38 C. stimulation of ten times this frequency 

 still caused tetanus. Recently Einthoven, exciting the nerve of a 

 frog's nerve-musle preparation with extremely frequent oscillatory 

 condenser discharges, observed tetanus up to even a million vibrations 

 a second, if the current intensity was at the same time very greatly 

 increased (to more than 16,000 times the intensity needed with a 

 constant current). These results are not really so discordant as 

 they appear ; for it is known that with electrical stimulation the 

 number of excitations is not necessarily the same as the nominal 

 number of shocks. By applying a telephone to a muscle excited 

 through its motor nerve, it has been shown that the pitch of the 

 note produced by the tetanized muscle corresponds exactly to the 

 rate of excitation up to a certain frequency. This frequency is 

 about 200 per second for frog's and about 1,000 per second for 

 mammalian muscle under the best conditions. If the rate of ex- 

 citation is still further increased, there is no corresponding increase 

 in the pitch. Therefore, some of the stimuli are now producing no 

 effect ' falling flat,' so to speak (Wedensky). One reason for this 

 is that even very brief currents leave alterations of conductivity 

 and excitability behind them (Sewall), which we shall have to 

 discuss in another chapter (p. 683). (See also p. 685.) 



It is only while the actual shortening is taking place that a tetan- 

 ized muscle can do external work. But, although during the 

 maintenance of the contraction no work is done, energy is neverthe- 

 less being expended, for the metabolism of a muscle during tetanus 

 is greater than during rest. Among other changes, the carbon 

 dioxide given off is increased, and lactic acid produced. } And upon 

 the whole a muscle is more quickly exhausted by tetanus than by 

 successive single contractions, although there are great differences 

 between different muscles. For example, the muscles which close 

 the forceps of the crayfish or lobster have, as everyone knows, the 



