THE PHENOMENON OF CONTRACTION. 31 



tractions decrease somewhat to a minimum that is reached at about 

 15° to 18° C. Beyond this the contractions again increase in 

 extent to a second maximum at about 26° to 30° C, this maxi- 

 mum being in some cases greater, and in others less than the first 

 maximum. Beyond the second maximum the contractions again 

 decrease rather rapidly as the temperature rises until at a certain 

 temperature, 37° C, irritability is entirely lost (Fig. 10). If the tem- 

 perature is raised somewhat beyond this latter point heat rigor makes 

 its appearance, and the muscle may be considered as dead. The re- 

 lationship between temperature and extent of contraction, therefore, 

 may be expressed by a curve such as is represented in Fig. 11, in 

 which there are two maxima and two points at which irritability is 

 lost. The second maximum indicates a fact of general physiological in- 

 terest, — namely, that in all of the tissues of the body there is a certain 

 high temperature at which optimum activity is exhibited, and if the 

 temperature is raised beyond this point functional activity becomes 

 more and more depressed. The point of optimum effect is not iden- 

 tical for the different tissues of the same animal, much less so for 

 those of different animals, but the fact may be emphasized that in 

 no case do protoplasmic tissues withstand a very high temperature. 

 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. 



Efifect 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. It would seem probable 

 that two different contraction processes are initiated by the stimu- 

 lus, one much more rapid than the other. Many other facts in 

 physiology speak for this general view that a muscle may, according 



