30 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



Functional activity is lost usually at 45 C. or below. The duration 

 of the contraction shows usually in frogs' muscles a simple relation- 

 ship to the changes of temperature. At low temperatures, 4 or 5 C., 

 the contractions are enormously prolonged, particularly in the phase 

 of relaxation ; but as the temperature is raised the duration of the 

 contractions diminishes, at first rapidly, then more slowly, to a 

 certain point about 18 to 20 C., beyond which it remains more or 

 less constant in spite of the changes in extent of shortening. The 

 relationship between duration of contraction and temperature may 

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

 which the heights of the ordinates represent the relative durations 

 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 

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



