THE MECHANICAL RESPONSE. 369 



earlier stage in the response than those relating to the second ; and, con- 

 sequently, that the diminution of the rate of augmentation of tension in that 

 series means nothing more than that the contractile stress of the muscle has 

 already passed its maximum. 



v. Kries also made experiments in which the load (and therefore the 

 initial length of the muscle) was constant, but in which its length at the time 

 the stop was applied, varied. A method founded on the same principle was 

 adopted by Schenck in investigating the influence of tension on the time- 

 relations of the excitatory process. His experiment is as follows : — A muscle 

 is made to contract isotonically with a small load. It draws the curve 

 indicated by the dotted line in the figure (Figs. 199 and 200). Immediately 



Fig. 199. — Isotonic curve (dotted line) and arrest curve (continuous 

 line) of the same muscle as in Fig. 200, cooled to 6° C. 



Fig. 200. — Isotonic curve (dotted line) and arrest curve (continuous 

 line) of a muscle warmed to 31° "5 C. (double adductor preparation, 

 the two groups of muscles arranged as shown in Fig. 189), the arrest 

 taking place when the point of the writing-lever was at z. — After 

 Schenck. 



afterwards a second curve is drawn, with the stop so placed as to arrest the 

 shortening of the muscle half-way. He found that if the observation is made 

 at a low temperature, e.g. 6° C. (Fig. 199), the period of contraction is pro- 

 longed; if at a high temperature, e.g. 31° "5 C. (Fig. 200), it is abbreviated, as 

 compared with an unarrested isotonic contraction at the same temperature. At 

 an intermediate temperature the fall of the arrested curve coincides with that 

 of the isotonic curve. 1 



Schenck explains these apparently inconsistent results by assuming 

 that tension has a twofold action — that it "strengthens and pro- 

 tracts the process of contraction," and that at the same time it 

 " hastens relaxation." " In the warm muscle, the second of these actions 

 predominates; in the cold, the first." 2 He also found that the first 

 effect is the more pronounced the less the muscle is loaded, and connects 

 this with the fact, to be referred to in a later section, that the result 

 of increasing the load in isotonic contractions is to increase the pro- 

 duction of heat. It is to be noted that Schenck applies the arrest when 

 the muscle is half-way between its natural unexcited length and that 



1 Schenck, Arch. f. d. ges. Phi/sioL, Bonn, 1894, Bd. lv. S. 629. 

 - Schenck, ibid., 1894,' Bd. Ivii. S. 600. 



VOL. II. — 24 



