MUSCLE 657 



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. 



FIG. 247. ANALYSIS OF ELECTRICAL TETANUS (REDUCED no ). 



Four curves showing the effect of increasing frequency of stimulation of the 

 frog's gastrocnemius through its nerve. In the lowest curve the frequency is 

 such that the muscle relaxes almost completely between the successive con- 

 tractions. In the uppermost curve, with a frequency more than three times greater, 

 the contractions are almost completely fused. In all the curves the fusion becomes 

 more nearly complete as stimulation goes on, owing to the slower relaxation of* 

 the fatigued muscle. 



The gastrocnemius of the frog requires 30 stimuli a second, the 

 hyoglossus muscle only half that number (Richet). The fre- 

 quency of stimulation necessary for complete tetanus of un- 

 striped 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 stimula- 

 tion below which a given muscle cannot be completely tetanized, 



42 



