MUSCLE TONE. 469 



muscle. The lowest limit of this range is probably about the 

 number of impulses communicated to human muscles by their 

 nerves, since the tone produced by contracting muscle corresponds 

 to the first overtone of a primary note produced by 19.5 vibra- 

 tions in a second. The number of stimuli required varies with 

 the rate of contraction of the muscle employed, the quick-con- 

 tracting bird's muscle requiring 70 per second, while the excep- 

 tionally slow-moving tortoise muscle only requires 3 per second. 

 According to some, there is a limit to the number of stimuli 

 which will cause tetanus, 3HO per second is named as the maxi- 

 mum for a certain strength of stimulus; with stronger stimuli, 

 even when more frequent, tetanus occurs. It has been shown 

 that many thousand stimuli per second can cause tetanus even 



FIG. 103. 





Tetanus produced by 8 stimulations per second. The more perfect fusion of the single 

 contractions shown toward the end of the curve depends on the altered condition of the 

 muscle. 



with very weak currents. If tetanus be kept up for some second?, 

 and the stimulation be then suddenly stopped, the lever falls 

 rapidly for a certain distance, but the muscle does not quite 

 return to its normal length for some few seconds. This residue 

 contraction is easily overcome by any substantial load. If kept 

 in a state of tetanus by weak stimulation, after some time the 

 muscle commences to relax from fatigue, at first rapidly, then 

 more slowly ; this falling off of the tetanic contraction may be 

 prevented by increasing the stimulus. 



MUSCLE TONE. 



Although the tracing drawn by a lever attached to a muscle 

 in tetanus is straight, and does not show any variation in the 



