58 HUMAN PHYSIOLOGY 



How is this continued contraction of muscles produced ? 

 To understand this it is necessary to study the influence of 

 a series of stimuli on muscle. 



If, to a frog's muscle which takes T Vth of a second to 

 contract and relax, stimuli at the rate of five per second 

 are applied, it is found that a series of simple contractions, 

 each with an interval of T Vth of a second between them 

 are produced (Fig. 26, 1). If the stimuli follow one another 

 a b c at the rate of ten per second, a 



series of simple contractions is still 

 produced, but now with no interval 

 between them. 



r If stimuli be sent more rapidly 

 to the muscle, say at the rate 

 of twelve per second, the second 

 stimulus will cause a contraction 

 before the contraction due to the 

 irst stimulus has entirely passed 

 >ff (Fig. 26, 2). The second con- 

 raction will thus be superimposed 

 >n the first, and it is found that 

 he second contraction is more 

 jomplete than the first, and the 

 hird than the second. But while 



\ 



A 



Fio. 26. Effect of a series of 

 Stimuli on Skeletal Muscle. 

 (See text.) 



the second contraction is 



markedly greater than the first, the 



third is not so markedly greater than the second, and each 

 succeeding stimulus causes a less and less increase in the 

 degree of contraction until, after a certain number, no 

 further increase takes place, and the degree of contraction 

 I is simply maintained. 



When the contractions follow one another at such a rate 

 that the relaxation period of the first contraction has begun, 

 but is not completed, before the second contraction takes 

 place, a lever attached to the muscle, and made to write on 

 a moving surface, produces a toothed line. The contraction 

 is not uniform, but is made up of alternate shortenings and 

 lengthenings of the muscle. This constitutes "incomplete 



If the second stimulus follows the first so rapidly that the 

 contraction period has not given place to relaxation, then 



