464 MANUAL OF PHYSIOLOGY. 



stimulus. A fresh muscle then, at the ordinary temperature with 

 a medium load, will contract more and more as the intensity of 

 the current employed increases. There is a limit to this increase, 

 and with comparatively weak stimulation, an effect is produced 

 which cannot be surpassed by the same muscle, no matter what 

 stimulus be applied. This greatest contraction is the same for all 

 medium stimuli while the muscle is fresh, and is called the maxi- 

 mum contraction, being the greatest shortening which can be pro- 

 duced by a single instantaneous stimulus. 



FIG. 190. 



Pendulum Myograph Tracings showing Summation. 1. Curve of maxi- 

 mum contraction drawn by first stimulus, the exact time of application of 

 which is shown by the small up-strokeof the left hand of the base line. 

 2. Maximum contraction resulting from second simple stimulation given at 

 the moment indicated by the other small up-stroke. 3. Curve drawn as the 

 result of double stimulation sent in at an interval indicated by the distance 

 between the up-strokes, showing summation of stimulus and consequent 

 increase in contraction over the " maximum contraction." 



Summation. Each time a muscle receives an induction shock 

 of medium strength it contracts to its maximum. If a second 

 shock be given while the muscle is in the contracted state, a new 

 maximum contraction is added to the extent of the contraction 

 the muscle was in at the moment of the second stimulation, and 

 if stimulated when the lever is at the apex of the curve the sum 

 of the effect produced will be equal to two maximum contractions. 



If applied in the middle of the period of the ascent or descent 

 of the lever, a second stimulation gives rise to H maximum con- 

 tractions, and so on, in various parts of the curve, a new maxi- 



