42 



ADVANCED LESSONS IN PRACTICAL PHYSIOLOGY 



(a) Moment of stimulation as indicated by the signal, 

 (6) Point at which the muscle just begins to shorten, 



(c) Point at which the muscle shows its greatest degree of short- 

 ening, and 



(d) Point at which the writing lever again reaches the abscissa. 



FIG. 19. A MUSCLE TWITCH. 



M, Make shock recorded by magnetic signal connected with primary circuit. Time in 

 T^n sec.; L, latent period; C, period of contraction; R, period of relaxation. 



Determine the duration of: (a) The latent period, (6) the period 

 of contraction, and (c) the period of relaxation. Note that the muscle 

 does not relax properly if the stimulus is too strong, and that the length 

 of the curve increases after a time, owing to fatigue. 



2. Summation of Contractions. Remove the tuning-fork. Use a 

 rapid speed of drum and again stimulate the muscle with a make and 



FIG. 20. SUMMATION OF CONTRACTIONS. 



M and B, Make and break shocks indicated by an electromagnetic signal. Time 

 in jfa sec. As the break contraction occurs during the period of relaxation of the make 

 contraction, it is added to the first. 



break shock, but in such a manner that the break stimulus strikes the 

 muscle during its period of relaxation. A second contraction then re- 

 sults which is added to the first, thereby rendering it higher than the 

 first. 



3. Fusion of Contractions. Reduce the speed of the drum. Repeat 

 the previous experiment, gradually increasing the rate of stimulation 

 until the contractions resulting from the separate stimuli are partially 



