66 TEXT-BOOK OF PHYSIOLOGY 



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 

 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. 



FIG. 25. TEACING 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. 



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. I|_t_he intervals between the successive 

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

 i -fcr^cohtraction, it may contract as many as a thousand times witKout 

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

 / intervals_are shorter than that just stated it is found that from the 

 i beginniiig~6f 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 

 i energy of the contraction have been increased. This gradual increase 

 I in the height oj: the contraction has been termed the staircase effect, or 

 \the treppe. In the beginning of the period of stimulation there is some- 

 times observed a decrease in the height of the contraction following 

 several stimulations before the staircase effect develops, indicating 

 a temporary decrease in the irritability. These staircase contractions 

 have been observed in the muscle of both warm-blooded and cold- 

 blooded animals. The cause for this increase in irritability upon which 

 the effect depends is attributed to the presence of certain chemic sub- 

 stances in the muscle arising as a result of. its katabolism, such as 

 carbon dioxid, mono-potassium phosphate, and paralactic acid. These 

 compounds, when present in small amounts or in larger amounts for a 



