THE EFFECT OF INERTIA 37 



effected at some speed. That the mode of action of this factor may 

 be made more clear, let us consider what happens during the twitch 

 of a muscle to which a weight is directly attached. The force exerted 

 by the muscle during its contraction acts directly on the weight, 

 which it sets in motion, and produces an acceleration in it directly 

 proportional to the force and inversely proportional to the mass 

 moved. The result is that the weight gains a certain amount of 

 kinetic energy by virtue of which it will continue to move upwards, 

 even though the muscle force ceases to act upon it, until that kinetic 

 energy is neutralised by the constantly acting force of gravity. This 

 is exactly the condition, then, when the force of the contraction begins 

 to diminish ; and if, as is usually the case, we are actually recording 

 the movement of the weight, the record of the true alteration of length 

 is distorted by the operation of this acceleration. But the acceleration 

 introduces another alteration which is even of greater influence, for 

 as the acceleration upwards increases, so more and more of the 

 weight ceases to pull directly upon the muscle, i.e. its tension 

 diminishes. The result of a diminution in tension is that the elastic 

 force of the muscle comes into play and produces a shortening which 

 thus interferes with the shortening process we are attempting to 

 record. The greater the load pulling upon the muscle the more will 

 the action of acceleration interfere with the record. In the second 

 process, that of relaxation, acceleration again comes into play. At 

 first the tension diminishes because the weight does not follow the 

 rapid relaxation instantaneously, but with a certain delay. When the 

 relaxation is, however, becoming slower, the weight is moving down- 

 wards with a certain velocity imparted to it by the action of gravity, 

 and a time is reached when the weight is moving downwards with a 

 velocity greater than the rate of lengthening of the muscle. The 

 result is it acts upon the muscle and the tension begins to rise, 

 increasing until it is equal to the weight plus that force necessary to 

 stop the acceleration. The muscle is therefore stretched beyond its 

 initial length, and then, when the acceleration of the weight is stopped, 

 the excess of tension over load acts upon the weight which is once 

 more lifted, acceleration imparted to it, and the whole process is once 

 more repeated, until with a few more oscillations the weight at last 

 comes into equilibrium. 



These final oscillations are to be observed in all tracings, and 

 fig. 41 is reproduced especially in order to show them. The muscle 

 recording was the double hyoglossus and the magnification five times. 

 The only load attached to the muscle was the recording lever, in 

 this case made of a light straw of about 25 cm. in length. The 

 preparation was the same one that had been previously employed 



