368 



THE PROPERTIES OF STRIPED MUSCLE. 



the interval of time between the moment of encounter and that of culmination 

 diminishes. In this case each response begins isotonically, and proceeds 

 isometrically, as seen in Fig. 198. 1 When the lever, on the relaxation of the 

 muscle, leaves the stop, the curve again follows an isotonic course. 



To make clear the bearing of these empirical data, some further explanation 

 is necessary. The reader must first understand that the purpose of both the 

 methods above described is to investigate at any moment in the response, the 

 two functional activities (Leistungsfdhigkeiten) of a muscle the Fahigkeit 

 Verkiirzung zu leisten, and the Fahigkeit Spannung zu erzeugen. These are 

 represented by the slant of the isometric and isotonic curves respectively, at 

 the moment that the lever parts from So or encounters Su. To indicate these 

 times the vertical broken lines have been added to von Kries' figure. The 

 important point in the curve of after -loading is that whereas the isotonic 

 curve rises with an initial inclination which diminishes as the load increases, 

 the isometric has nearly the same initial inclination throughout. This means 

 that " the ability of the muscle to generate tension " is unchanged, while its 

 " ability to produce shortening " diminishes. The relation between these he 

 calls " the apparent extensibility " of the muscle. In the experiment given in 

 Fig. 198, the load which is at first minimal is increased after each observation ; 



but as it is not 

 supported, the lift 

 begins at a lower 

 and lower level. 

 At the moment of 

 arrest, which of 

 course occurs later 

 and later, the length 

 of the muscle is 

 always the same. 

 Its rate of shorten- 

 ing diminishes as 

 in the previous 

 case, but its rate of 

 increase of tension 

 is not constant as 

 before, but dimin- 

 ishes with the rate 

 of shortening ; so 



that the " apparent extensibility " remains nearly the same. The explanation 

 given by v. Kries is as follows : In a muscle, notwithstanding that length 

 and tension remain the same, the relation between the two functional 

 capacities differs according to the "way in which the portion of the 

 response which precedes the transition" has occurred (see p. 369). He 

 supposes that what happens at the beginning of the contraction influences 

 the subsequent state of the muscle by its influence in determining the 

 number of fibres which participate in the process, and surmises that the 

 effect of initial tension (starke Spannung im Moment der Reizung) is to 

 augment the number of acting fibres, and thereby to bring about a con- 

 dition in which the rate of change of tension corresponds to that of change 

 of form. The data seem to me to admit of a simpler explanation. The 

 contractile stress of the muscular fibres at the moment of transition is in each 

 case truly indicated by the rate of augmentation of tension that is, by the 

 initial inclination of the isometric curve, and is accordingly greater in the first 

 series than at the corresponding period in the second. But the reason why 

 this is so, is that the measurements relating to the first series are made at an 



1 v. Kries. Arch. f. PhysioL, Leipzig, 1892, S. 10. 



FIG. 198. Series of " arrested contraction " curves, with increasing 

 load, the length of the muscle being the same at the moment 

 of arrest in each case. After v. Kries. 



