VOLUNTARY MUSCLE. 27 



same stand. The tuning fork with writing point bent back- 

 ward is fastened on a separate stand at such a height that when 

 vibrating its record will come between that of the muscle lever 

 and signal-magnet. The tuning fork is placed in the opposite 

 direction to that of the muscle lever but as close as possible 

 without interference. The screw at the top of the rod support- 

 ing the drum is tightened so that the drum can be spun freely 

 by hand. Two persons are required to perform the experiment. 

 They should practice until they can perform teamwork without 

 difficulty. 



The muscle and signal-magnet levers are adjusted against 

 the surface of the drum, then number one vibrates the tuning 

 fork and applies its writing point to the drum. Number two 

 spins the drum rapidly enough to show distinctly the individual 

 vibrations on the tuning fork, and as soon as the speed is 

 uniform number two stimulates the muscle with a break shock. 

 Number one must be ready to push the kymograph away from 

 the recording points as soon as the contraction and relaxation 

 are finished. Directions for the remainder of the experiment 

 are the same as in Experiment 8. 



Isometric Contraction. Sometimes a muscle tries to lift a 

 load which it cannot budge. The muscle tugs at the weight with 

 an increasing effort, but it cannot shorten, although the tension 

 increases with the effort. Such a muscle maintains a constant 

 length. It is said to undergo an isometric contraction. 



In the preceding experiment the force which opposed the 

 contraction of the muscle was essentially a constant one, so that 

 the muscle tension remained nearly the same. In the present ex- 

 periment the tension is made to increase as contraction proceeds 

 and the change in muscle length is small. It is true that the muscle 

 does not maintain a constant length, but it approaches that con- 

 dition. 



*Experiment 9. Fasten the femur of a gastrocnemius preparation 

 in a flat-jawed clamp which has been fixed in the upper part of 

 the tripod. Connect the tendon by a hook to the spring lever, 

 the latter being supplied with a writing point. Connect the 

 secondary terminals of the inductorium with the binding posts 

 of the clamp and the lever. To avoid poor contacts you should 



