SKELETAL MUSCLE 71 



indicates the shortening of the muscle. Occasionally 

 the reverse is the case. To translate the curve and its 

 subdivisions into terms of actual time it is only necessary 

 to have a tuning fork with a known rate of vibration 

 which leaves its wavy tracing upon the drum at the same 

 time that the muscle is recording. 



If our tuning fork makes 100 vibrations per second, it 

 is likely that the curve described by the lever (moved by 

 a fresh muscle to which one stimulus has been given) 

 will stretch over about 10 of the small waves. The 

 conclusion is that a simple contraction can be executed in 

 Ho second. If this is true 10 stimuli, with uniform 

 intervals, might be given in a second with the result that 

 the muscle would contract 10 times and drop the lever 

 to the base-line after each contraction. As a matter of 

 fact some fusion would probably occur because, in a series 

 of repeated simple contractions, the duration of individual 

 ones tends to increase. Human muscle shows little or 

 no superiority to the frog's in its capacity for rapidly 

 repeated movements. The best that the trained pianist 

 or typewriter can do with one finger is to press a key 10 

 or 11 times in a second. We are far inferior to flying 

 insects whose wings may beat 300 times or more in the 

 same brief interval. 



Summation. If a second stimulus takes effect upon a 

 muscle which has not completed its relaxation, after 

 answering to a first, it may spring from its partially 

 contracted condition to perform a fresh act. In the 

 tracing the second curve seems to be mounted upon the 

 first and reaches a greater height. The second stimulus 

 may be so timed as to make the later elevation rise from 

 the very peak of the earlier one. Two stimuli of mod- 

 erate intensity may be more efficient in forcing up the 

 height of contraction than one stimulus, however strong, 

 can possibly be. At least this is true when a muscle 

 raises any considerable weight. 



Tetanic Contractions. As it is possible to follow one 

 stimulus with another after such a short interval that 



