136 



INVERTEBRATE PHYSIOLOGY 



again to c-c' as indicated by the envelope d-c, d-c' . The direction of move- 

 ment around the tension-length loop can be found by dimming the trace 

 through the Z-axis by the output of either the length or the tension re- 

 corder. By noting whether the dimmer trace is above or below the brighter, 

 one can immediately determine whether the tension is higher or lower on 

 muscle shortening and so whether the system is doing work on the muscle 

 or the muscle on the system. In the case where the muscle is vibrating the 



40- 



30- 



o 



to 



uj20- 

 10- 



0.1 



c' 



0.3 * 0^5 

 LENGTH 



Fig. 10. Behavior of the longitudinal flight muscle of the bumble bee, Bombus, in 

 isotonic contractions with a weight. The loops are not actual records but are drawn 

 from observations on many preparations. The records were obtained with strain 

 gages giving instantaneous measure of tension and length. Tension was applied to 

 the horizontal plates and length to the vertical plates of a cathode-ray oscillograph 

 to give the dynamic tension-length diagrams shown. Muscles stimulated at 60 per 

 sec. to complete tetanus. See text for explanation of details. 



platform, as in Fig. 10, the tensions are higher on shortening since the 

 muscle is doing work. The area enclosed in the loop represents the work 

 done by the muscle against damping forces in the mechanical system. By 

 introducing more frictional damping, larger loops were obtained. Many 

 dififerent loops are possible within the area enclosed by the curves of active 

 and passive tension, depending upon the load, the damping forces, and the 

 characteristics of the contractile element of the muscle. 



