THE PHENOMENON OF CONTRACTION. 29 



with great accuracy. When the stimulus is of such a strength as 

 to produce a just visible contraction it is spoken of as a minimal 

 stimulus and the resulting contraction as a minimal contraction. 

 Stimuli of less strength than the minimal are designated as sub- 

 minimal. If one increases gradually the intensity of the electrical 

 current used as a stimulus without altering its duration, beginning 

 with a stimulus sufficient to cause a minimal contraction, the result- 

 ing contractions increase proportionally up to a certain maximum 

 beyond which further increase of stimulus, other conditions remain- 

 ing the same, causes no greater extent of shortening. Contrac- 

 tions between the minimal and the maximal are designated as 

 submaximal.* (See Fig. 9.) 



Effect of Temperature upon the Simple Contraction. Varia- 

 tions in temperature affect both the extent and the duration of the 

 contraction. The relationship is, however, not a simple one in the 

 case of the frog's muscle upon which it has been studied most fre- 

 quently. If we pay attention to the extent of the contraction alone 

 it will be found that at a certain temperature, C., or slightly below, 



Fig. 10. Curve showing the effect of temperature. The temperatures at which the 

 contractions were obtained are indicated on the figure. In this experiment a large resis- 

 tance was introduced into the secondary circuit so that changes in the resistance of the 

 muscle itself due to heating could not affect the strength of the stimulus. 



the muscle loses its irritability entirely. As its temperature is 

 raised a given stimulus, chosen of such a strength as to be maximal 

 for the muscle at room temperatures, causes greater and greater 

 contractions up to a certain maximum, which is reached at about 

 5 to 9 C. As the temperature rises beyond this point the con- 

 tractions decrease somewhat to a minimum that is reached at about 

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



* Fick, " Untersuchungen iiber elektrische Nervenreizung," Braun- 

 schweig, 1864. 



