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 



that, during the 



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 



FIG. 196. Series of "after-loading" curves, with increasing 

 load. After v. Kries. 



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, 

 Anschlagszuckungen 

 (arrested contractions). 



FIG. 



197. v. Kries' apparatus for taking "after-loading" 

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

 lever with its axis at k ; gg, 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 ; m, 

 muscle (Pick's double adductor preparation, with the 

 two groups of muscles used abreast). 



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 



The load is increased 

 before, after each 



as 



