292 PRACTICAL PHYSIOLOGY 



will be the same. Let the drum revolve, open the Du Bois key, and 

 record a single isometric contraction. 



Now remove the muscle and attach to the lever at the same spot a 

 thread which passes over a pulley held by the muscle-clamp. Tie on to 

 the free end of the thread a weight of 20 grms. ; this will raise the lever 

 a certain amount ; draw a line across the isometric curve on this level 



FIG. 174. Comparison of an isotonic and isometric contraction of a single gracilis 

 and sernimembraiiosus preparation. For the isotonic curve 50 grms. were hung at 

 the axis of the lever, the actual load on the muscle beinar a third of this, and the 

 magnification was 5. For the isometric curve the magnification was 15, and the 

 muscle started to contract under an initial tension of 10 grms. Time marker, 100 per 

 sec. (A.P.B.) 



parallel to the abscissa line. Repeat the same process with weights of 

 50, 75, 100 grms., and so on until the level of the top of the curve is 

 reached. In this way the tension or resistance which the muscle has 

 overcome is measured. 



Fig. 174, obtained by this method, shows the main differences between 

 an isometric and isotonic curve ; these are, that in an isometric curve 

 the highest point is sooner reached and the curve has a flatter top. In 

 other words, a muscle reaches its maximal tension sooner and maintains 

 it longer than its maximal shortening. 



The effect of initial tension on an isometric curve may also be investi- 

 gated. With the lever horizontal, and the muscle-clamp adjusted so 

 that the muscle is under no initial tension, a zero-abscissa line is 

 drawn. The muscle-clamp is now raised so as to slightly elevate the 

 writing point ; by raising the lever the wire axis has undergone torsion, 

 and the muscle is tinder an initial tension. Describe a fresh abscissa 

 line at this level, mark the point of stimulation, and record a single 

 isometric twitch. By removing the muscle and substituting the pulley 

 and weights, the value of the initial tension and of the various parts of 

 the curve in grms. may, as before, be estimated. It will be found that 

 the effect of an initial tension is to increase slightly the latent period 

 and total time of the twitch, but a more important result is that it 

 increases the maximal tension or resistance overcome by the muscle 

 when stimulated. 



