maximal break shocks. Compare with the curves from the fre^h 

 muscle. 



191. The preceding experiments will give practice in handling the 

 apparatus. The student should now make a neat record of a single 

 muscle twitch, as before, but introducing also a tuning fork which 

 will make a time curve on the drum so that the duration of the 

 phases of a contraction may be recorded. Turn the drum a siiu/le 

 revolution fairly rapidly by hand instead of clockwork and while 

 revolving stimulate the muscle with a single induced shock or break 

 shock. The writing levers and vibrating tuning fork with writing 

 point attached should be pressed against the drum before turning. 

 The writing points should all be in a vertical line. After the record 

 has been obtained place the lever point of the signal magnet over 

 the point of stimulation as indicated on the record and with the drum 

 stationary, stimulate the muscle to contract. This will give the 

 exact latent period in case the writing points are not exactly over 

 each other. Practice may be necessary to obtain a good record. 



D. EFFECT OF VARIOUS FACTORS IX Ml'SCLE CON- 

 TRACTION. 



192. Influence of repeated stimuli Treppe. Fasten the muscle 

 in a moist chamber and arrange the apparatus for recording contrac- 

 tions on a drum. Connect the muscle through a key with the binding 

 posts on the desk. A current will be made and broken by a revolving 

 key which automatically excludes the break shocks from the induc- 

 torium. Thus the muscle will be stimulated at a certain rate by 

 make induced shocks alone. Record the contractions on a slowly 

 moving drum until nearly fatigued. Allow to rest several minutes 

 and again record the contractions till fatigued. Mark the rate of 

 stimulation on the record. One record for a pair of students will 

 be sufficient. 



193. Repeated stimulation at tlie moment of relaxation. Records 

 can be obtained by means of a special muscle lever. Consult the 

 instructor for directions. 



194. Influence of strength of stimulus on lieif/lit of contraction. 

 Arrange the muscle as in experiment 190 for direct stimulation with 

 single induction shocks. Separate the coils ( connected with one dry 

 cell) to a distance at which both make and break shocks are inef- 

 fective. Then slowly increase the strength of stimulus by moving the 

 coils nearer until the break shock just begins to be effective. Now 



