MYOdltAM 187 



and relaxation take place at first slowly, then faster, and 

 finally slowly again. The entire time involved may be put at 

 an average of T V of a second. The time relations vary with 

 the nature of the muscle and the conditions under which it 

 works. Finer methods of determination have reduced the 

 latent period to a mechanical and an electrical latent period 

 of 0.004 and 0.001 second respectively. The latent period of 

 the motor end plates is about 0.002 to 0.003 of a second. When 

 the striated muscle of a frog is submitted to a series of equal 

 induction shocks at a regular rate, a series of contractions are 

 recorded, of which the first four or five may fall in height, 

 and are known as the introductory contractions. After this 

 there is an increase in the height regularly to a maximum which 

 forms the "treppe" or staircase. Following this again the 

 contractions lose in height until they disappear, which is known 

 as fatigue. 



The staircase contractions result from the fact that each stimu- 

 lation occasions a heightened irritability, so that the succeed- 

 ing stimuli become more effective. Fatigue is caused by the 

 accumulation of waste products and to the consumption of 

 available nutriment. When the rate of stimulation is increased, 

 the separate muscular contractions may not come down to 

 the base-line, giving rise to the phenomena of contracture. This 

 condition may be explained by the fact that as the muscle 

 contracts it becomes fatigued, resulting in a prolongation of 

 the relaxation. A second stimulus reaches the muscle before 

 the contraction resulting from the first is over. There is a 

 summation of contractions. Double contractures are given by 

 muscles composed of two distinct kinds of fibers, which react 

 differently to the same stimulus (Fig. 11). 



The pale fibers react quickly and are soon fatigued, while 

 the dark fibers have greater endurance. The summation of 

 contractions of the pale fibers occurs first and quickly raises 

 the base line. As they become fatigued, the base line gradually 

 sinks until the dark fibers in a similar manner raise it again. 

 When both dark and pale fibers are fatigued, separate contrac- 

 tions, and relaxations no longer take place, and the base-line 

 gradually sinks. With still more frequent rates of stimulation 

 the staircase and contracture effects are merged into one another, 

 giving rise to a very great height of contraction. As the curve 



