THE MECHANICAL RESPONSE. 



375 



Fig. 209. — Isotonic and arrest curves of an un- 

 loaded muscle, with maximal stimulation 

 and with summation of stimuli. Arrest 

 at x in both cases ; X 2 indicates the point 

 which represents the probable second 

 equilibrium length of the muscle, as ex- 

 cited by the two stimuli. — Modified from 

 Kaiser. 



a second twitch indicated that contractile stress was then still present. The 

 series of twelve curves, reproduced in Fig. 208, were obtained by a similar 

 method, with the exception that before each release of the lever it was arrested 

 at a level about halfway between I and A, and that the muscle had been 

 cooled to a temperature of 0° C. 



These curves show that, although in each muscular element the transi- 

 tion from the unexcited to the excited state may be instantaneous, a 

 certain time is required in order that the contractile stress of the whole 

 may attain its maximum. This 

 time Kaiser refers to as that of 

 the Entwickelung der Verkilrzungs- 

 Jcraft. Thus, although a low tem- 

 perature does not affect the position 

 of a, and accordingly does not in- 

 crease the contractile stress, it may 

 yet enable the muscle to raise a 

 weight to a greater height by 

 postponing the moment at which 

 the contractile stress ceases to act. 

 It having been found that the 

 second equilibrium length of the 

 muscle is independent of the 

 physical conditions under which 

 it may be placed, the next step 

 is to demonstrate its relation to the physiological conditions. Now, 

 the means by which function is evoked in a muscle is excitation. 

 We may regard a muscle when " maximally " stimulated (and not 

 fatigued) as in a definite physiological condition. The condition is 

 altered when a supramaximal response is evoked, as may be done by 

 applying two instantaneous stimuli, both of which are maximal, within 

 a period of one- or two-hundredths of a second, or a submaximal one, by 

 applying a stimulus weaker than that required to produce a maximal 

 contraction. Comparing the 

 curves so obtained with a 

 normal maximal curve of the 

 same muscle, Kaiser found 

 that in the case of supra- 

 maximal stimulation (Fig. 

 209), although the summit 

 of the curve is not much 

 higher, there is a hesitation 

 of the writing-lever when it 

 is arrested at a height which 

 corresponded to A in a maxi- 

 mal contraction, indicating 



that the second equilibrium position of a muscle excited by a summation 

 of stimuli, is at a higher level, say at X 2 . Conversely, when the lever is 

 arrested at different points in the course of a submaximal response 

 (Fig. 210), its sudden fall occurs when it is at a considerably lower level 

 than that of A in the curve of maximal stimulation, indicating that the 

 second equilibrium position of a muscle, submaximally stimulated, is at a 

 lower level, say at A°. It thus appears that the equilibrium length of 

 an excited muscle varies with the degree of action of the exciting cause, 



Fig. 210. — Two isotonic curves of an unloaded 

 muscle, the one with maximal and the other 

 with submaximal stimulation. X and x° the 

 positions representing the second equilibrium 

 length in the two cases. — Two figures com- 

 bined, after Kaiser. 



