i GENEEAL PHYSIOLOGY OF MUSCLE 13 



body the tired muscle rapidly recovers with rest, owing to the 

 blood circulation ; but excised muscle, too, is capable of a partial 

 restoration, although it is cut off from the circulating tissue fluids. 

 Fatigue is the effect of two factors which act simultaneously 

 upon contractile protoplasm the consumption of the dynamogenic 

 materials of muscle, and the accumulation of waste matters or 

 decomposition products. Recovery depends on the supply of 

 further nutritive material and removal of the waste products, as 

 we shall presently see in discussing muscular metabolism. 



(d~) The height of the twitch also depends on the form or 

 strength of the stimulus. It is advisable in studying these 

 relations to employ the make or break shocks of an induced 

 current, which can be easily graduated. If a muscle is rhythmic- 

 ally excited by break shocks of gradually increasing strength, 

 it begins to respond only when the stimulus reaches a certain 

 intensity, the so-called threshold of stimulation. If the exciting 

 current is then further strengthened, a series of contractions 

 result that increase in height, step by step, up to a certain point, 

 after which they no longer increase with the strength of the 

 stimulus. Stimulation is therefore distinguished as effective and 

 ineffective according as it produces or does not produce a reaction ; 

 effective stimuli, again, may be minimal, median, maximal, or 

 super -maximal. The gradation of the stimuli alters, moreover, 

 according as the muscle is directly or indirectly excited. When 

 the muscle is directly excited the interval between the minimal 

 and maximal stimulus is greater, but as this interval is very small 

 it requires only a slight increase of the stimulus above the threshold 

 to elicit a maximal contraction. The gradation of the response to 

 an increasing stimulus is not, therefore, easy to demonstrate. 

 Certain muscles, e.g. cardiac muscle, either do not respond at all 

 or respond to each shock by a maximal contraction Bowditch's 

 Law of " all or nothing " (Vol. I. p. 318). 



According to Fick's first researches (1862) on the gradation of 

 response to indirect stimulation of skeletal muscle, the increase 

 in height of the contractions is approximately proportional to the 

 increase in strength of the stimulus ; but Tigerstedt has shown, 

 with direct stimulation of curarised muscles, that with regular 

 increase in the strength of the current the contractions at first 

 increase rapidly, and afterwards more slowly, till they become 

 maximal. The ascending line of the contractions is thus not a 

 straight line but a hyperbola. 



At the maximum height of the muscle twitch obtained on 

 exciting a fresh frog's muscle with a maximal or supermaximal 

 stimulus the muscle shortens by \ of its length, as measured in 

 the resting state. 



(e) The height, duration, and form of the contraction are 

 considerably influenced by the load carried by the muscle, i.e. 



