MUSCLE 571 



there may be great variations in the length of contraction. 

 In the frog, for instance, the hyoglossus muscle contracts 

 much more slowly than the gastrocnemius. The wave of 

 contraction, which in frogs' striped muscle lasts only about 

 07 second at any point, may last a second in the forceps 

 muscle of the crayfish, though only half as long in the 

 muscles of the tail. In the muscles of the tortoise the con- 

 traction is also very slow. The muscles of the arm of man 

 contract more quickly than those of the leg. 



Summation of Stimuli and Superposition of Contractions. 

 Hitherto we have considered a single muscular contraction 



as arising from a single 

 stimulus, and we have 

 assumed that the muscle 

 has completed its curve 

 and come back to its 

 original length before the 

 next stimulus was thrown 

 in. We have now to in- 

 FIG. 179. SUPERPOSITION OF CONTRAC- quire what happens when 



a second stimulus acts 



I is the curve when only one stimulus is , . . 



thrown in ; 2, when a second stimulus acts Upon the mUSCie during 



fts ^ximumteTght 1 ; 1 "" 6 ' *" neady *""*** the contraction caused by 



a first stimulus, or during 



the latent period before the contraction has actually begun ; 

 and what happens when a whole series of rapidly-succeeding 

 stimuli are thrown into the muscle. 



First let us take two stimuli separated by a smaller 

 interval than the latent period (p. 559). If they are both 

 maximal (i.e., if each by itself would produce the greatest 

 amount of contraction of which the muscle is capable when 

 excited by a single stimulus), the second has no effect what- 

 ever, the contraction is precisely the same as if it had never 

 acted. But if they are less than maximal, the contraction, 

 although it is a single contraction, is greater than would 

 have been due to the first stimulus alone ; in other words, 

 the stimuli have been summed or added to each other during 

 the latent period so as to produce a single result. 



Next let us consider the case of two stimuli separated by 



