PHYSIOLOGY OF MUSCLE TISSUE. 59 



is similar in many respects to that of the striated muscle, except that 

 the duration of the former is considerably longer than that of the 

 latter. 



Action of Successive Stimuli. If a series of successive stimuli 

 be applied to a muscle, the effect will be different according to the 

 rapidity with which they follow one another. If the second stimulus 

 be applied at the termination of the contraction due to the first 

 stimulus, a second contraction follows, similar in all respects to the 

 first. A third stimulus produces a third contraction, and so on until 

 the muscle becomes exhausted. If the second stimulus be applied 

 during either of the two periods of the first contraction, the effects 

 of the two stimuli will be added together and the second contraction 

 will add itself to the first. The maximum contraction is obtained 

 when the second stimulus is applied ^ of a second after the first. 



Tetanus. When a series of stimuli are applied to a muscle, fol- 

 lowing one another with median rapidity, the muscle does not get 

 time to relax in the intervals of stimulation, but remains in a state 

 of vibratory contraction, which may be regarded as incipient tetanus, 

 or clonus. As the stimulation increases in frequency, the vibrations 

 become invisible, being completely fused together. There is, never- 

 theless, during the tetanic condition a series of continuous contrac- 

 tions and relaxations taking place. After a varying length of time 

 the muscle becomes fatigued, and notwithstanding the stimulation, 

 begins slowly to elongate. The number of stimuli necessary a second 

 for the production of tetanus varies in different animals e. g., 2 to 3 

 for muscles of the tortoise; 10 for muscles of the rabbit; 15 to 20 

 for the frog ; 70 to 80 for birds ; 330 to 340 for insects. 



A voluntary contraction in man may be regarded as a state of 

 tetanus, for if the curve of a voluntary movement be examined, it 

 will be found to consist of intermittent vibrations. The simplest 

 voluntary movement of a muscle, however rapidly it may take place, 

 last longer than a single muscular contraction due to an induction 

 shock. The most rapid voluntary contraction is the result of from 

 2.5 to 4 stimulations a second, and has a duration of from 0.041 to 

 0.064 of a second. A continuous voluntary contraction is an incom- 

 plete tetanus. The number of stimuli sent to the muscle is on the 

 average, 16 to 18 for rapid contractions, 8 to 12 for slow con- 

 tractions. 



