732 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES 



muscles only 10 to 20; the tail muscles of the crayfish 40, but the 

 muscles of the claw only 6 in winter and 20 in summer. The 

 gastrocnemius of the frog requires 30 stimuli a second, the hyo- 

 glossus muscle only half that number (Richet). The frequency of 

 stimulation necessary for complete tetanus of unstriped muscle 

 is much less than for striped muscle. Smooth tetanus of a band of 

 muscle from the frog's stomach was obtained with strong opening 

 induction shocks at the rate of I in 5 seconds. 



We see, then, that there is a lower limit of frequency of stimulation 

 below which a given muscle cannot be completely tetanized. There 

 appears also to be an upper limit beyond which a series of stimuli 

 becomes too rapid to produce complete tetanus, and at which an inter- 

 rupted current acts like a constant current, causing a single twitch at 

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

 This limit undoubtedly does not depend upon the frequency of stimula- 

 tion 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-muscle 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 condi- 

 tions. If the rate of excitation 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 

 nave to discuss in another chapter (p. 759). (See also p. 761.) 



It is only while the actual shortening is taking place that a tetanized 

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

 the contraction no work is done, energy is nevertheless being ex- 

 pended, ior the metabolism of a muscle during tetanus is greater than 



unng rest, and, among other changes, lactic acid is produced. There 

 are great differences m the ease with which different muscles can be 

 exhausted by tetanus. For example, the muscles which close the 

 lorceps of the crayfish or lobster have, as everyone knows, the power 



t most obstinate contraction. Richet tetanized one for over seventy 



