36 PRACTICAL PHYSIOLOGY 



its contraction in the same manner as the curves of 

 shortening of the gastrocnemius were obtained in 

 previous experiments. 



Connect two pairs of pin electrodes with a com- 

 mutator, the cross wires of which are removed so 

 that it is used merely as a switch. The commutator 

 is placed in the secondary circuit. One pair of elec- 

 trodes is used to stimulate the muscles close to the 

 lever, the other at the far end of the muscular mass ; 

 the electrodes must be securely fixed into the frog- 

 cork. Describe an abscissa, and mark the point of 

 stimulation as in previous experiments, making use 

 of the stop. Then take two tracings of the contrac- 

 tion of the muscle, firstly when stimulated close to 

 the lever, secondly when stimulated at the further 

 end of the muscle. In the latter case the latency 

 period is prolonged, and the second curve, there- 

 fore, occurs later than the other. The interval 

 between the two curves represents* the time which 

 it has taken for the wave of contraction to pass 

 along the length of the fibres which intervene 

 between the two points which were successively 

 stimulated. Take a tuning-fork tracing, and meas- 

 ure this time, and from it and the length of muscle 

 traversed by the wave calculate the rate of propa- 

 gation of the muscle wave per second. 



It is essential for the success of this experiment 

 that the muscles used should have their fibres run- 

 ning longitudinally and parallel with one another. 

 If the frogs are large the two sartorius muscles 

 may be used with advantage instead of the adduc- 

 tor preparation described. 



