GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 63 



in some insects one three-hundredth of a second, and in the turtle one second. 

 Pale muscles have a shorter period than the red. 



Influences Modifying the Effect of the Stimulus. The contraction 

 process in its entirely as well as in its individual parts is considerably modi- 

 fied by both external and internal conditions, among which may be mentioned 

 the following: 



i. Character 0} the Stimulus. As the contraction is the response of the 

 muscle to a stimulus, it may be inferred that the vigor of the former is 

 proportional, within limits, to the strength of the latter. Thus using 

 as a stimulus the single induced current, it has been found that if the 

 strength of the current is progressively increased, the height of the con- 

 traction will correspondingly increase until a certain maximum height is 

 attained (Fig. 25, A, a b); then notwithstanding a continued increase 

 in the strength of the stimulus, this height will not be exceeded for some 

 time. But if the strength of the stimulus be yet further increased, there 



A. B. 



FIG. 25. TRACING SHOWING THE EFFECTS OF A GRADUAL INCREASE IN THE STRENGTH 

 OF THE STIMULUS ON THE HEIGHT OF THE CONTRACTION, a. Minimal contraction; a b. pro- 

 gressive increase in the height; b c. first maximum (a number of contractions have been omitted 

 for economy of space) ; d e. second maximum. 



comes a moment when the contractions again increase in vigor and a 

 second maximum height is attained (Fig. 25, B, d e). Beyond this no 

 further increase in height is observed. The second maximum has been 

 attributed to the presence in the muscle of two different substances 

 differently affected by changes in temperature, by fatigue and by 

 various drugs. 



It has also been shown that the rate at which the muscle is stimulated 

 with a given stimulus of uniform strength will influence the char- 

 acter of the contraction process. If the intervals between the successive 

 stimulations be such as permit the muscle to recover from the effects of 

 the contraction, it may contract as many as a thousand times without 

 showing any particular variation from the normal form; but if the 

 intervals are shorter than that just stated it is found that from the 

 beginning of the stimulation each succeeding contraction slightly exceeds 

 in height the preceding contraction, until a certain maximum is reached 

 and maintained, indicating that for some reason the irritability and the 

 energy of the contraction have been increased. This gradual increase 



