THE TISSUES 59 



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 Q{ " complete tetanus" (Fig. 26, 3). 

 (Practical Physiology, Chap. V.) 



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. 55). D'Arsonval has shown that an alternating 

 current with very frequent interruptions of about 1,000,000 

 per second causes no contraction. 



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. (Practical Physiology, 

 Chap. V.) 



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. 



