THE PHENOMENON OF CONTRACTION. 31 



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 reac- 

 tions 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 a veratrinized animal 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 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 sug- 

 gested 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. 19, and that 



Fig. 13. Curve showing the effect of veratrin. 



the veratrin dissociates their action, but this explanation, ac- 

 cording 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. It would seem more probable, 



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



