230 VETERINARY PHYSIOLOGY 



and being maintained at this level, till the series of stimuli 

 causing the contraction is removed, or until fatigue causes 

 relaxation of the muscle. This is the condition of " com- 

 plete tetanus" (fig. 114 (3)), (Practical Physiology). 



The rate at which stimuli must follow one another in order 

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

 Anything which increases the duration of a single contrac- 

 tion decreases the 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 

 required (p, 225). 



The red fibres in tetanus give a more powerful con- 

 traction than do the white fibres. 



The reason why a very rapid series of stimuli cause 

 a tetanus in skeletal muscle is that the refractory period is 

 so very short (p. 216). Hence, if the stimuli follow one 

 another sufficiently rapidly they fail to produce a tetanus. 



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 ? 



Taking advantage of the electrical change which accom- 

 panies each muscular contraction (p. 212), and using the 

 string galvanometer (p. 214), it has been shown that, in 

 sustained voluntary contraction, electric variations occur at 

 a more or less definite rate in different muscles, faster in the 

 shorter, and slower in the longer muscles. 



Hence it may safely be concluded that impulses at these 

 various rates pass from the spinal cord to the muscles to 

 produce the sustained contraction of voluntary action. 



Besides change in shape muscle when stimulated undergoes 

 electrical changes (p. 212), changes in elasticity (p. 211), 

 changes in temperature, heat production (p. 246), and chemical 

 changes (p. 254). 



