SUGGESTIONS FOR LABORATORY WORK 593 



on a drum which can be moved by hand, or driven at various speeds 

 by a clockwork. This apparatus is called a kymograph. Tracings 

 made with it can be preserved by passing the paper bearing them 

 through a solution of shellac in alcohol. 



Skeletal muscles contract only when stimulated. A suitable 

 artificial stimulus is the shock from an induction coil. Induced 

 currents are generated in the secondary coil of an inductorium 

 when, and only when, a current is made or broken in the primary 

 coil. An ordinary dry cell is a good source of current. The strength 

 of the induced current varies with the position of the secondary coil 

 relative to the primary, being greatest when the secondary is di- 

 rectly over the primary, and least when the secondary is with- 

 drawn as far as possible from the primary and turned at right angles 

 to it. 



Mount the frog's gastrocnemius muscle in readiness for obtaining 

 tracings of its contractions. By means of fine copper wires es- 

 tablish a circuit from the secondary coil of the inductorium through 

 the tissue. Be careful to avoid short circuits. 



With the secondary coil in the position of least effectiveness 

 make and break the primary circuit. If the muscle does not re- 

 spond shift the secondary, little by little, toward the position of 

 greatest effectiveness, making and breaking the primary circuit 

 with each shift. As soon as the muscle responds, recording its 

 contraction on the smoked paper, move the drum forward by hand 

 so that new contractions will not be superposed on the first one. 



Continue increasing the strength of the shock, obtaining records 

 of each contraction. 



Thus the relation of contraction height to stimulation strength 

 is shown. 



A current is induced in the secondary at make and at break of 

 the primary current. Determine which is a more powerful stimu- 

 lus, a "make" shock or a "break" shock. 



The inductorium as used in the physiological laboratory is pro- 

 vided with an automatic circuit breaker which can be included in 

 the primary circuit. Make the proper connections for doing this. 

 Now when the circuit is closed shocks are sent into the muscle in 

 rapid succession. 



Set the clockwork of the kymograph to drive the drum at a slow 

 speed. Obtain a tracing showing the response of the muscle to 



