FACTORS VARYING THE CHARACTER OF THE CONTRACTION 77 



old, these contractions are designated respectively as minimal, maxi- 

 mal and supramaximal. 



The Duration of the Stimulus. In a general way it may be said 

 that the highest contraction is obtained when the stimulus is of long 

 duration, but this rule is applicable only to stimuli of equal intensity 

 and moderate duration. It is evident that an undue prolongation of 

 the excitation must tend to produce fatigue and to lessen the ampli- 

 tude of the reaction, until it finally becomes smaller than the one 

 obtained previously with stimuli of much briefer duration. 



O 10 20 30 <K> SO tO -}0 80 <JO JQO 



FIG. 43. INFLUENCE OF LOAD. 

 This muscle has been successively loaded with 10 gram weights. 



Influence of Load. Provided that the writing lever has been prop- 

 erly counterpoised so that weights may be attached to it without 

 stretching the muscle, the amplitude of the contractions decreases 

 gradually with the increasing load. It is true, however, that a muscle 

 reacts much better when moderately weighted than when no weight is 

 attached to it at all. In other words, the contractility of a muscle 

 may be augmented by subjecting it to a slight tension. 



'if -sts 



FIG. 44. THE CONTRACTION OF FOUR DIFFERENT MUSCLES OF THE TURTLE 



RECORDED UNDER SIMILAR CONDITIONS. 

 1, Palmaris; 2, Gracilis; 3, Omohyoid; 4, Pectoralis Major. 



Character of the Muscle Substance. We have previously seen 

 that the irritability, conductivity, and contractility of muscle tissue 

 differ not only in different animals, but also in muscles of the same 

 animal. Thus, the striated muscles attached to the wings of insects, 

 contract at the rate of 300 times in a second, while those of birds may 

 attain a frequency of 100 in a second. The gastrocnemius muscle of 

 the frog requires 0.1 sec., the hypoglossal muscle of the turtle 0.2-0.3 

 sec., and those used in the retraction of the head of this animal 0.5 sec. 



