MUSCLE 573 



maximum height of the contraction is greater than that pro- 

 duced by the strongest single stimulus ; and even after com- 

 plete fusion has been attained, a further increase of the 

 frequency of stimulation may cause the curve still to rise. 



It is evident from what has been said that the frequency 

 of stimulation necessary for complete tetanus will depend 

 upon the rapidity with which the muscle relaxes ; and 

 everything which diminishes this rapidity will lessen the 

 necessary frequency of stimulation. A fatigued muscle may 

 be tetanized by a smaller number of stimuli per second 

 than a fresh muscle, and a cooled by a smaller number than 

 a heated muscle. The striped muscles of insects, which 

 can contract a million times in an hour, require 300 stimuli 

 per second for complete tetanus, those of birds 100, of man 

 40, the torpid muscles of the tortoise only 3. The pale 

 muscles of the rabbit need 20 to 40 excitations a second, the 

 red 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 hyoglossus muscle only half that number (Richet). 



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

 tion below which a given muscle cannot be completely tetanized, 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 appar- 

 ently 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 commencement 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 without any harmony of result, 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 a novel and ingenious method 



