AFTER-LOAD 43 



method which very commonly obtains in many of our bodily move- 

 ments, in which the muscle is under a low tension until it commences 

 to contract, and then, only, experiences a rise of tension. We can 

 imitate such a process by the plan of supporting the weight, and only 

 allowing it to act on the muscle when it has already begun to con- 

 tract. This is called the method of after-loading. 



Experiment 3. — To examine the nature of a twitch under such circum- 

 stances arrange the apparatus as for taking a simple twitch. Prepare a 

 gastrocnemius and attach it to the modified simple lever shown in fig. 46. 

 This consists of an ordinary simple lever, but to the frame carrying the axis 

 is fitted a stout brass plate, b, which runs parallel to the writing lever but not 

 vertically under it. The end of this plate has a piece which projects under 

 the lever and carries a screw, s. This can be screwed up so that the tip of 

 the screw can support the metal part of the writing lever at any level. 

 Load the muscle, e.g. with a load of 20.g., which, preferably, should be applied 



Fig. 46. — Arrangement of Simple Levee for Eecording by the Method of 



After-load. 



by a proportionately heavier weight attached nearer to the axis. Screw 

 down the screw so that the whole load is carried by the muscle, and bring 

 the writing lever to a horizontal position. Now record a simple twitch. 

 Next screw up the screw to support the writing lever, so that the writing 

 point is placed at the level of the apex of the curve just taken. Record 

 from this position another curve. It will be found that the muscle still 

 raises the lever. Raise the screw s once again until the level of the 

 writing point is at the summit of this second curve, and again record a twitch. 

 Repeat the process two or three times more. 



Fig. 47 gives a series of curves obtained in this way. They are 

 taken from a gastrocnemius in the manner described. The very 

 striking and highly characteristic feature of these curves is that 

 though the weight is supported at a level which it just reached at 

 the height of a previous contraction, yet it is further raised when the 

 muscle is again stimulated. Under such conditions, then, the muscle 

 contracts to a greater degree than when freely loaded. As was to be 

 expected, the latent period is longer in this second case, and, as the 

 tracings show, becomes still further prolonged as the height of sup- 

 port of the weight is increased. The time measurements show for 

 the four successive curves here reproduced - 01 sec, '035 sec, "042 sec, 



