THE MECHANICAL RESPONSE. 371 



last set of experiments and the preceding inertia experiment needs scarcely 

 to be indicated. When the jerk method was used, the rise of tension was 

 abrupt and forcible. In the contractions with the equilibrated bar, it rose 

 and subsided gradually. 



Schenck also made experiments with arrested contractions, for 

 the purpose of comparing the effects of the arrest on loaded and unloaded 

 muscles, 1 and found that similar differences present themselves. If 

 the contraction of a lightly loaded muscle is arrested when already half 

 accomplished, it gives way so promptly that the writer at once leaves 

 the stop in the same way as in Fig. 200. The more the load is in- 

 creased, the less the relaxation is hastened, until eventually the writer 

 is held against the stop during the whole period of iso tonic shortening, 

 or even longer. In one case the tension is augmented at the moment 

 of arrest, from a very small amount to one which is fifty or a hundred 

 times as great ; in the other, from an amount already large to another a 

 little larger. 2 As has already been stated, cold has a similar effect to 

 load, so that the same muscle which at 20 C. promptly relaxes during 

 the period of arrest, holds the writer against the stop during the whole 



FIG. 203. Isotonic (dotted line) and jerk curves (continuous line) ; z, position of 

 point of writing-lever Avhen jerk took place. After Schenck. 



period of isometric contraction, when the temperature approaches C. 

 (Fig. 199). 



Schenck sums up his conclusions as to the influence of augmentation 

 of tension on the isotonic contraction of a lightly loaded muscle as 

 follows : 3 Sudden and extensive alterations of tension hasten relaxa- 

 tion; changes of longer duration, which develop slowly and are of 

 small amount, tend to increase contraction and to defer relaxation. Of 

 the two effects, the former follows the change of tension immediately, 

 the other often not until a little later. 



It can scarcely be questioned that this statement is in accordance 

 with the experimental data which have been given ; but when we 

 pass from the observed facts to their explanation, we find ourselves 

 in considerable perplexity. Schenck finds the link which connects 

 the hastening of relaxation (verfrulite Ersclilaffung\ with the conditions 

 which produce it, in the paradox of inhibitory excitation. He imagines 



* Schenck, Arch. f. d. ges. PhysioL, 1894, Bd. Ivii. S. 608. 



2 It is here again to be noted that the length of the muscle at the time that the arrest 

 was applied in these experiments was not the same. 



3 Schenck, Arch.f. d. ges. PhysioL, Bonn, Bd. Ixi. S. 85, 103. 



