758 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. 



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 

 interrupted current acts like a constant current, causing a single twitch 

 at its commencement or at its end, but no contraction during its pas- 

 sage. This limit 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 saw tetanus even with 4,000 shocks a second. 

 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. stimu- 

 lation of ten times this frequency still caused tetanus. 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 dis- 

 cordant 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 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 (Weden- 

 sky). A physical reason for this is the overlapping of the make and 

 break shocks (Erlanger and Garrey) ; and a physiological reason, the 

 alterations of conductivity and excitability, which even very brief 

 currents leave behind them (Sewall), and which we shall have to discuss 

 in another chapter. 



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

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

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

 pended for the metabolism of a muscle during tetanus is greater than 

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

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

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

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

 of most obstinate contraction. Richet tetanized one for over seventy 

 minutes, and another for an hour and a half, before exhaustion came 



