THE TISSUES 57 



contraction period has not given place to relaxation, then the 

 second contraction will be superimposed on the first, the third 

 on the second, and so on continuously and smoothly without any 

 slight relaxations, and thus the lever will describe a smooth 

 line, rising at first rapidly, then more slowly, till a maximum 

 is reached, and being maintained at this till the series of stimuli 

 causing the contraction is removed, or until fatigue causes 

 relaxation of the muscle. This is the condition of " complete 

 tetanus" (fig. 27,3). (Practical Physiology.) 



The rate at which stimuli must follow one another in order 

 to produce a tetanus depends on a large number of factors. 

 Anything which increases the duration of a single contraction 

 renders a smaller number of stimuli per second sufficient to 

 produce a tetanus, and thus all the various factors modifying 

 a single muscular contraction, modify the number of stimuli 

 necessary to produce a tetanus (see p. 53). D' Arson val has 

 shown that an alternating current with very frequent in- 

 terruptions of about 1,000,000 per second causes no con- 

 traction. 



Every voluntary contraction of any group of the muscles is 

 probably of the nature of a tetanus; and the question thus 

 arises, at what rate do the stimuli which cause such a tetanus 

 pass from the spinal cord to the muscles ? 



In a tracing of a continued voluntary contraction, indica- 

 tions of about ten variations per second are to be seen, while 

 the rate of the clonic tremor of the leg which may be produced 

 during fatigue by supporting the weight of the leg on the toes 

 is about ten, backward and forward movement, per second, and 

 in various morbid muscular spasms the rate is about the 

 same. 



All this would seem to indicate that the number of stimuli 

 which pass to human muscle from the central nervous system 

 is probably about ten per second. 



It has, however, been found that passing a strong galvanic 

 current into a muscle may lead to rhythmic contraction, and 

 hence it is possible that the contractions of muscle induced by 

 the central nervous system may be caused by a continued 

 discharge from the nerve centres. 



