MECHANICAL PHENOMENA OF MUSCULAR ^CONTRACTION 729 



important. When a small quantity of this aubslaice is injected below 

 the skin of a frog, spasms of the voluntary muscles, well marked in the 

 limbs, come on in a few minutes. These are attended with great 

 stiffness of movement, for while the animal can contract the extensor 

 muscles of its legs so as to make a spring, they relax very slowly, and 

 some time elapses before it can spring again. If it be killed before the 

 reflexes are completely gone, the peculiar alterations in the form of the 

 muscle-curve caused by veratrine will be most marked. The poisoned 



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Fig. 257. Veratrine Curve compared with Normal: Frog's Gastrocnetnius. 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. 



muscle, stimulated directly or through its nerve, contracts as rapidly 

 as a normal muscle, while the height of the curve is about the same, 

 but the relaxation is enormously prolonged (Fig. 257). This effect 

 seems to be to a considerable degree dependent on temperature, and 

 it may temporarily disappear when the muscle is made to contract 

 several times without pause. Barium salts, and, in a less degree, those 

 of strontium and calcium, have an action on muscle similar to that of 

 veratrine. Sometimes the curve shows a peak (Fig. 258), due to a 

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

 a slow relaxation. The 

 peak appears to be analo - 

 gous 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 pj g 258. Veratrine Curve: Frog's Gastrocnemius. 

 the muscle itself has an The curve shows a peak, the lever falling a little 

 influence on the muscle- before the sustained contraction begins. 

 curve. Not only do the 



muscles of different animals vary in the rapidity of contraction, but 

 there are also differences between 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. 



Even where there is no distinct histological difference, there may be 

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

 the hyoglossus muscle contracts much more slowly than the gastroc- 



