37 6 THE PROPERTIES OF STRIPED MUSCLE. 



so that A is the expression of the capacity of the muscle to respond to a 

 maximal instantaneous stimulation, and we are justified in designating 

 the contractile stress which determines the change from the first to 

 the second equilibrium position as "physiological, although it acts as 

 if it were "elastic." Kaiser gives the name of Verkurzmigskraft to 



this contractile stress. . 



The preceding summary will be sufficient to give the reader an idea 

 of the scope of Kaiser's research, a main purpose of which was to 

 elucidate the relation between the length of a muscle, when allowed to 

 shorten freely in response to a single maximal stimulation, and the 

 isotonic curve of such a contraction. The experimental data obtained, 

 lead him to reject the theory that the single contraction is a dud 

 process. Following out Weber's original idea, he thinks of the " forces " 

 operative in the process as elastic, and accounts for all that happens by 

 supposing that shortening expresses transition from the "natural" 

 unexcited condition to the " natural " excited condition ; and that 

 lengthening is a process which is simply the reverse of the other. The 

 first is the direct response to excitation, the second a necessary con- 

 sequence of the first. The proof that the stage of relaxation is a mere 



consequence of that of contraction, 

 he finds in the fact that temperature, 

 which has so marked an influence 

 on the first part — that in which the 

 muscle is actively contracting — has 

 no effect on the second, for the curve 

 drawn by the same muscle in relax- 

 ing is always precisely the same, 

 provided that it is unloaded and 

 that the muscle at the starting-point 

 of relaxation is of its natural length 

 when excited, i.e., that its un- 

 attached end has to accomplish, 

 in transition from the excited to the unexcited state, the distance 

 above designated as l-X. 1 



Before concluding the consideration of the equilibrium length of 

 excited muscle, it may be well once more to refer to the difference 

 between the maximum degree of contraction which is evoked by a 

 single instantaneous stimulation, and that more intense state of excita- 

 tion which exists in tetanus. 



When a lightly loaded muscle is excited by maximal instantaneous 

 excitations which succeed one another with sufficient frequency (100 per 

 second), the contraction being recorded isotonically, the lever rapidly 



1 To the reader who consults Kaiser's papers it may be of use to note that he employs 

 the terms erster and ziveiter Fusspunkt to designate the position of the unattached end of 

 the unextended muscle in the unexcited and excited states respectively. I have in the text 

 made no reference to Kaiser's theoretical explanation of the isotonic curve. He attributes 

 the descent of the lever at the moment indicated by x in Fig. 204, not to the rebound of the 

 lever against the stop, but to a downwards impulse of the muscle, and connects this 

 with the theory that so much of the ascent of the lever as is above X is due, not to the 

 action of the lever, but to the action on it of the muscle, which, after attaining its 

 equilibrium length x, shortens, by virtue of its previous motion, to a length less than 

 its equilibrium length. Actual experiment does not favour this conclusion, for Schenck 

 has shown that if the lever is supported at the height of x, before the muscle is excited, 

 it remains motionless (Arcli.f. d. ges. Physiol., Bonn, Bd. lxv. S. 325). This being the 

 ease, it is clear that, so far as concerns the part of the isotonic curve which is above X, the 

 lever is not actuated by the muscle. 



Fig. 211. — Isotonic curve and release curve, 

 after arrest, of an unloaded muscle. 

 — After Kaiser. 



