MECHANICAL PHENOMENA OF MUSCULAR CONTRACTION 757 



The meaning of a complete tetanus is readily grasped if, beginning 

 with a series of shocks of such rapidity that the muscle can just 

 completely relax in the intervals between successive stimuli, we 

 gradually increase the frequency (p. 816). As this is done, the 

 ripples on the curve become smaller and smaller, and at last fade 

 out altogether. The maximum height of the contraction is greater 

 than that produced by the strongest single stimulus ; and even after 

 complete fusion has been attained, a further increase of the fre- 

 quency of stimulation may cause the curve still to rise. 



Fig. 267. Analysis of Electrical Tetanus (reduced to f ). 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 contractions. 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. 



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 



