MUSCLE TONE. 467 



there is a limit to the number of stimuli which will cause tetanus, 

 360 per second is named as the maximum 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 with very weak currents. If 

 tetanus be kept up for some seconds, 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 stimula- 

 tion, _after some time the muscle commences to relax from fatigue, 



FIG. 193. 



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

 of the single contractions shown towards the end of the curve depends on 

 the altered condition of the muscle. 



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 

 tension of the tetauized muscle, some variation in tension must 

 occur, since a low humming sound is produced during contraction. 

 This muscle-tone can be heard by applying the ear firmly over any 

 large muscle (biceps) while in tetanus, or by throwing the mus- 

 cles attached to the Eustachean tube into action, as in swallow- 

 ing, or during spasm of the muscles in mastication. 



The number of vibrations of the muscle-sound has been esti- 

 mated to be from 18-20 for the human skeletal muscles. This 



