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A MANUAL OF PHYSIOLOGY 



tions in the form of the muscle-curve caused by veratria 

 will be most marked. The poisoned 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. 178). 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 

 veratria. Sometimes the curve shows a peak (Fig. 177), 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 contraction when a strong voltaic 



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FIG. 178. VERATRIA CURVE COMPARED WITH NORMAL. 



(Frog's gastrocnemius.) The turning-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 veratria curve 

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



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, 



