THE MECHANICAL RESPONSE. 



367 



the curves culminate about the same time, but the inclination of the rise, i.e. 

 the rate of shortening, diminishes as the load increases. The changes of 

 tension are shown in the curves of the upper series. In each case the 

 horizontal part of 



the tracing indicates 

 during 



the 



Fig. 196. 



-Series of "after-loading" curves, with increasing 

 load. — After v. Kiies. 



that, 



time to which it 

 corresponds, the 



muscle is working 

 isotonically. Curve 

 5 of this series is 

 identical with 

 Helmholtz's " energy 

 curve" (see p. 362). 

 In this case it is 

 evident that, as the 

 load increases the 

 rate of increase of 

 length lessens, while 

 the tension remains 

 the same. 



Fig. 197 shows 



the simple mechanism with the aid of which these curves are recorded. 

 Before the muscle begins to shorten, the descent of the load is arrested by 

 the stop So, the height of which is so adjusted that until the moment of 

 excitation there shall 

 be no tension. The 

 stop Su in this case is 

 not used. When the 

 muscle relaxes, and the 

 prolongation of the lever 

 again strikes So, the 

 curve again becomes 

 isometric. 



In another series of 

 experiments the same 

 contrivance was em- 

 ployed to produce 

 sudden increase of ten- 

 sion at a later period 

 during the response, 

 the stop Su being now 

 brought into play, the 

 stop So not being used. 

 The contractions re- 

 corded are of the kind 

 called by v. Kries, 

 Ansclilagszuckungen 

 (arrested contractions). 

 The load is increased 

 as before, after each 

 observation, and conse- 

 quently each successive curve begins at a lower level. In order that the 

 length of the muscle may be the same at the moment of arrest in each case, 

 the stop Su must remain throughout at the same height. It is therefore 

 encountered by the prolongation of the shortening lever later each time, and 



Fio. 197. — v. Kvies' apparatus for taking "after-loading" 

 curves and " arrested-contraction " curves, j, isometric 

 lever with its axis at k ; g g, isotonic lever with its axis 

 at a, 30 mm. to the right of g ; So and Su, screws held 

 by two horizontal arms of a support, Q 2 and Q ; in, 

 muscle (Fick's double adductor preparation, with the 

 two groups of muscles used abreast). 



