30 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



relationship between duration of contraction and temperature may 

 therefore be expressed by such a curve as is shown in Fig. 12, in 

 which the height of the ordinates represents the relative duration of 

 the contractions. Muscles from different frogs show considerable 

 minor variations in their reactions to changes in temperature, and 

 we may suppose that these variations depend upon differences in 

 nutritive condition. In this, as in many other respects, the reactions 

 obtained from so-called winter frogs after they have prepared for 

 hibernation are more regular and typical than those obtained in the 

 spring or summer. 



Effect of Veratrin. The alkaloid veratrin exhibits a peculiar 

 and interesting effect upon the contraction of muscle. A muscle 

 taken from an animal that has been veratrinized and stimulated 

 in the usual way by a single stimulus gives a contraction such as 

 is exhibited in the accompanying curve (Fig. 13). Two peculiarities 

 are shown by the curve: (1) The phase of shortening is not altered, 

 but the phase of relaxation is greatly prolonged. (2) The curve 



Fig. 13. Curve showing the effect of veratrin. 



shows two summits, that is, after the first shortening there is a 

 brief relaxation followed by a second, slower contraction. The 

 cause of this second shortening is not known. Biedemann has 

 suggested that it is due to the presence in the muscle of the 

 two kinds of fibers red and pale which were spoken of on p. 

 25, and that the veratrin dissociates their action, but this expla- 

 nation, according to Carvallo and Weiss,* is disproved by the 

 fact that muscles composed entirely of white or red fibers show 

 a similar result from the action of veratrin. Although the explana- 

 tion is not forthcoming, the fact that a single stimulus gives under 

 these conditions two processes of contractions is interesting as an 

 exception to the general rule. It may be added that a curarized 

 frog's muscle, when heated to the point of optimum activity (28 C.) 

 sometimes shows also a double contraction for a single stimulus. 

 The very prolonged relaxation is, however, the most peculiar effect 

 of the veratrin. A somewhat similar effect is produced by the 

 * "Journal de la physiol. et de la path. g6n6rale," 1899. 



