48 



EXPERIMENTAL PHYSIOLOGY 



II. The Influence of Load. — The effect of load upon the characters 

 of a simple twitch are to be studied in three ways: (1) where the 

 load is applied by the isotonic method ; (2) where it is applied 

 directly to the muscle ; (3) where the load only acts on the muscle 

 when it begins to contract. 



Experiment 5. — Arrange the apparatus as for a simple twitch. Prepare 

 a muscle and fix it in the isotonic lever as in fig. 4'2. The writing lever 

 should be made as light as possible. The ordinary crank myograph lever 

 could also be employed, in which case the loads are to be applied at a point 

 near to the axis. First record a simple twitch with the muscle weighted by 

 the lever only. Then hang on a weight to the pulley by a thread. This 

 extends the muscle, and the writing point must therefore be brought back to 

 its original level by raising the upper support of the muscle. Now record a 

 second twitch. Increase the weight and bring the lever once more to its 

 original level and record a third contraction. This may also be repeated 

 until the amount of the contraction becomes very small. 



Fig. 51 shows a tracing obtained in this way. The muscle was 

 the hyoglossus. The loads in grams additional to the weight of the 



Fig. 51. — Twitches of a Hyoglossus with Different Loads. Time Tracing, 

 200 per sec Magnification, 3. 



lever are written over the curves. Fig. 52 is a similar figure taken 

 from a gastrocnemius. 



Fig. 52. — Twitches of a Gastrocnemius with Different Loads. 

 Magnification, 5. 



The following effects are to be noticed : — 



1. The latent period increases as the load increases. 



2. The length of the period of contraction also tends to increase. 



