GENERAL MUSCLE PHYSIOLOGY 191 



wave in the cross-section of a fibre is of course less than the 

 duration of contraction of the whole muscle; it is about 0.05 

 to 0.09 second in the frog-muscle. The length of the con- 

 traction wave in the frog-muscle is 200-380 mm. 



In striated muscles, except in the cardiac muscle, the 

 contraction does not pass from one fibre to another as it does 

 in the smooth muscles. 



Superposition of twitches. Tetanus. If a muscle is 

 stimulated by many single stimuli which follow each other 

 so fast that the interval between two successive stimulations 

 is less than the duration of the contraction, the individual 

 twitches called forth by the individual stimulations combine. 

 But the increase in contraction which each successive 

 stimulation produces is smaller than that of the preceding. 

 Finally a maximum contraction is reached which cannot be 

 surpassed by the succeeding stimulations. If the interval 

 between the stimulations is small enough, a lasting contrac- 

 tion is produced by the combination of the twitches, which 

 is called tetanus. In a frog-muscle tetanus is produced at 

 room temperature when about 20 stimulations per second are 

 sent into the muscle. 



In a muscle without any load, the height of a tetanic con- 

 traction may be 80$ of the length of the fibre. In a loaded 

 muscle it is less in proportion as the load is greater. 



It is difficult to tetanize the cardiac muscle (see page 64). 



The voluntary muscle contraction is also tetanic. This 

 is apparent from the variations frequently seen in the con- 

 traction of a voluntarily contracted muscle which can be 

 graphically registered by recording the thickening of the 

 muscle. There are about 8 to 12 oscillations in one second. 



Muscle-sound. If an artificially stimulated muscle is connected 

 with the ear by means of a sound-conductor, a sound is heard 

 which corresponds to the number of oscillations. From volun- 

 tarily contracted muscles a sound is also heard (19 vibrations per 

 second), but it is doubtful whether this sound is produced by the 

 oscillatory stimulation of the muscle during voluntary contraction, 

 for a sound is also heard during a single twitch (see first cardiac 

 sound, page 68). 



