THE CHARACTER OF THE CONTRACTION OF MUSCLE 73 



tervals. Hence, the height of a tetanic contraction must always ex- 

 ceed that of a twitch and its summit must be attained more quickly, 

 provided, of course, that the same strength of stimulus is employed in 

 both cases. Having reached its maximal degree of shortening, the 

 muscle remains in the contracted condition until the stimuli are made 

 to cease. It need not surprise us, however, to find that the continued 

 activity of the muscle leads to a destruction of material which eventu- 

 ally causes it to become fatigued. This phenomenon is indicated 

 in the curve by a gradual decline of the lever which becomes the greater, 

 the longer the duration of the stimulation. Eventually, therefore, the 

 muscle must return into the position of complete relaxation in spite 

 of the continuance of the stimulation. Under ordinary conditions, 

 however, the stimuli are sent into a muscle only for a relatively short 

 period of time, but naturally, even the briefest tetanus is longer than 

 a simple twitch. 



Fig. 40. — Tetanic Contraction. 

 Recorded by means of Neef s automatic interrupter. Time in seconds. The de- 

 cline of the curve is an indication of fatigue. 



Whether or no a muscle will become greatly fatigued depends, of 

 course, upon its condition at the time of experimentation and upon 

 the strength and duration of the stimulation. Thus, an already 

 somewhat fatigued muscle requires a much smaller number of stimuli 

 to be tetanized than one just freshly prepared. The same is true 

 of a cooled muscle as against one which is kept at the temperature 

 of the room. It is evident, therefore, that the number of stimuli 

 which are necessary to tetanize a muscle completely, differ very widely. 

 Ordinarily a frog's gastrocnemius necessitates about 20-30 in a second, 

 and smooth muscle one in every 5-7 seconds. 



Voluntary Contractions. — Inasmuch as our skeletal muscles con- 

 tract normally in consequence of an influx of stimuli from the cerebral 

 cortex, their reactions may be of almost any length, until they are 

 finally cut short by fatigue. We have seen that a frog's gastrocnemius 

 completes its contraction in about 0.1 second. Contractions of such 

 brevity are not given by mammalian muscles, because even such seem- 

 ingly instantaneous movements as the closure of the eyelids or the 

 trilling motion of the fingers, cannot be executed in a shorter time 



