104 



It will be noticed that the latent period increases as the 

 load increases ; the period of contraction also tends to increase ; 

 the period of relaxation is at first decreased but -with heavier 

 loads is increased ; the heights of the contraction diminish 

 progressively as the load increases. 



349. After Load. In the ordinary study of muscle 

 twitch, the contractions have occurred whilst the load on the 

 muscle was as nearly as possible constant. There is, how- 

 ever, a method which is exemplified in many 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. This is called the method of after loading. Arrange 

 the apparatus for taking a simple twitch with a nerve-muscle 

 preparation. Load the muscle with a weight of 20 grams 

 attached to the pully of the muscle lever. Have the screw of 

 the muscle lever adjusted so that the muscle itself bears no 

 weight. Now lower the screw so that the whole load is 

 carried by the muscle and bring the writing tip of the lever 

 so that it will write horizontally upon the drum. Get a trac- 

 ing of the muscle curve, using the break shock only. Now 

 raise the screw to support the writing lever, so that the writ- 

 ing 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 

 again until the level of the writing point is at the summit of 

 this second curve, and again get tracing. Repeat this pro- 

 cess until no further result is obtained. 



The important and 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, it is still fur- 

 ther raised when the muscle is again stimulated. Under such 

 conditions, the muscle contracts to a greater degree than 

 when freely loaded. The latent period is increased as the 

 height of support of the weight is increased. This is ac- 

 counted for by the fact that the muscle is taking in any " slack " 

 there may be ; also that it is gradually increasing its tension 

 until it is able to lift the load. The first portion of such a 

 twitch is isometric ; but beyond a certain point it suddenly 

 becomes isotonic, and its shortening is then registered. 



