MUSCLE 



655 



calcium, have an action on muscle similar to that of veratrine. 

 Sometimes the curve shows a peak (Fig. 243), due to a rapid descent 

 of the lever for a certain distance. This is followed by a slow 

 relaxation. The peak appears to be analogous to the initial con- 

 traction when a strong voltaic current is passed through a muscle, 

 and the rest of the curve to the tonic contraction. 



(e) The individuality of the muscle itself has an influence on the 

 muscle-curve. Not only do the muscles of different animals vary 

 in the rapidity of contraction, but there are also differences in the 

 skeletal muscles of the same animal. 



In the rabbit there are two kinds of striped muscle, the red and 

 the pale (the semitendinosus is a red, and the adductor magnus a 

 pale muscle), and the contraction of the former is markedly slower 

 than that of the latter. In many fishes and birds, and in some 

 insects, a similar difference of colour and structure is present, 

 although a physiological distinction has not here been worked out. 



Even where there is no distinct histological difference, there may 

 be great variations in the length of contraction. In the frog, for 



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FIG. 245. VERATRINE CURVE COMPARED WITH NORMAL : FROG'S 

 GASTROCNEMIUS. 



The tuning-fork marks hundredths of a second. Between i and 2 a portion 

 of the tracing corresponding to one and a half seconds has been cut out, and 

 between 2 and 3 a portion corresponding to one second. The veratrine curve 

 does not show a peak. At 3 it has not yet fallen to the base-line. 



instance, the hyoglossus muscle contracts much more slowly than 

 the gastrocnemius. The wave of contraction, which in frogs' 

 striped muscle lasts only about 0*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 

 contraction 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 inquire what happens 

 when a second stimulus acts upon the muscle during the contraction 

 caused by a first stimulus, or during the latent period before the con- 

 traction 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. 642). If they are both maximal i.e., 

 if each by itself would produce the greatest amount of contraction 



