ADVANCED EXPERIMENTAL PHYSIOLOGY 295 



contraction, rotate the drum and stimulate. The lever will be raised a 

 less distance than before, but still it will be raised above the level of 

 the previous single contraction. Repeat this process of supporting the 

 muscle at the level of the top of the previous contraction until the 

 muscle is no longer able to lift the lever off the after-loading screw. It 

 will be found that the highest point reached by a series of single con- 

 tractions taken in this way will be of about the same height as that of 

 the tetanus (Fig. 175). 



The reason why the single isotonic contraction of a weighted muscle 

 is not so high as a tetanus curve seems to be as follows. The electro- 

 chemical change in muscle begins directly the stimulus reaches the 

 muscle and has already culminated when the change in form is begin- 

 ning ; that is, the change in form being a relatively slow process 

 continues after its real cause, the electro-chemical change, has ceased. 

 It is quite at the very beginning of the period of shortening, that the 

 process, by which chemical energy is transformed into muscular force, 

 is at its maximum, and then it rapidly declines. Consequently, if the 

 inertia of the mass to be moved is great and is not overcome by the 

 muscular force as rapidly as this force develops, the weight lags 

 behind, the change in form, therefore, can only take place more slowly, 

 and the muscle has ceased to pull at the weight before it has had time 

 to accomplish its maximal shortening. In complete tetanus, however, 

 before the pull of the muscle in response to one stimulus is at an end, 

 the muscle again begins to pull in response to a second stimulus and so 

 on, consequently the muscle has ample time to undergo its greatest 

 shortening in spite of the inertia of the weight. 



That it really is the inertia of the load which is the principal cause 

 of the difference in height between these two curves can be shown by 

 the fact that a muscle, weighted only by a very light lever, will, in 

 response to a single maximal stimulus, shorten almost as much as it can 

 do in tetanus. 



In the case of the muscle with its load supported until later and 

 later into its period of shortening, it is clear that the more the period 

 of shortening is passed through by the muscle in its unloaded condition, 

 the less will the inertia of the weight be able to cause delay in the 

 process of shortening, and, consequently, the muscle in the same time 

 will be able to undergo more and more nearly its full shortening. 



It follows, too, from what has been said, that, if it were possible to 

 slow down or prolong the chemical changes in muscle in response to a 

 stimulus, and consequently to prolong the period .during which the 

 muscle exerted an active pull, it would enable the muscle to undergo 

 more nearly its complete shortening, by providing a longer period 



