ELEMENTAEY EXPERIMENTAL PHYSIOLOGY 27 



of relaxation, and lasts about T ^ths of a sec. Relaxation is a passive 

 process brought about by the falling lever and weight doing the same 

 amount of work on the muscle as the muscle during its period of 

 shortening has done in raising the lever and weight to a certain 

 height. 



The muscle-curve, although roughly a magnified record of the change 

 in length of the muscle, is deformed by certain errors of instrumental 

 origin, which it is necessary to mention in order to avoid, so far as 

 they are preventible. The most important are the mass and length of 

 the lever and the disposition of the weight along it. They affect all 

 parts of the curve. The weight of the lever tends to prevent the 

 muscle from beginning to raise it (inertia of position) and so lengthens 

 the latent period ; therefore the lever should be as light as possible. 

 During the stage of shortening the lever, when once in motion, tends 

 to be carried on by its own momentum after the muscle has ceased to 



FIG. 34. Single contraction of gastrocnemius. Muscle loaded only with a rather heavy 

 lever. Magnification, 5. Temp., 15 C. Time marker, 100 per sec. (A.P.B.) 



pull on it (inertia of motion), and so makes the muscle appear to have 

 shortened more than it really has. For the same reason, during the 

 period of shortening, the tension on the muscle is not uniform, but 

 becomes less as the lever undergoes acceleration ; during the relaxation 

 exactly the opposite happens, a heavy lever as it falls again undergoes 

 acceleration and increases the tension on the muscle throughout the 

 relaxation and may even stretch it beyond its original resting length 

 (Fig. 34). In order to reduce these errors the lever again should be 

 as light as possible. 



On the other hand, to attach to the muscle no other weight than 

 that of a very light lever would introduce fallacies. For, unless the 

 muscle is sufficiently weighted to keep it taut, there may be, when the 

 muscle begins to contract, a certain amount of ' slack ' to be taken in 

 which would cause an apparent lengthening of the latent period. 



