EFFECT OF THE STRENGTH OF STIMULUS 485 



ated with the tension under which the muscle contracts. As the tension in- 

 creases from no load up to 100 or 150 grams (for a i-gram muscle), the work 

 increases. But as the tension continues to increase, the work falls off until 

 a point is reached at which the load is not lifted at all. 



CONDITIONS WHICH AFFECT THE IRRITABILITY] OF 



THE MUSCLE AND THE CHARACTER OF THE 



CONTRACTION. 



There are a number of conditions which influence both the irritability 

 of a muscle and the power and character of its contractions. Irritability 

 and contractility may vary independently, but as a rule any condition which 

 decreases one also decreases the other. The most important of these con- 

 ditions are: relation of the muscle to the central nervous system, condition 

 of nutrition, stimulus, temperature, fatigue, drugs, disease, etc. 



Effect of the Strength of Stimulus. A strength of current that is just 

 sufficient to give the contraction of a muscle is called a minimal stimulus. 



FIG. 326. Contraction of the Gastrocnemius Under the Influence of Variation of 

 Strength of Stimulus. The muscle was stimulated by Petzold's inductorium, graduated to 

 show units of current. The figures 6, 7, 8, 9, 10, etc., indicate relative strength of stimulus. 



This is a comparatively weak induction current, one which can scarcely be 

 detected by the tip of the tongue. As the strength of the current is very 

 gradually increased, the height of the contraction curve increases until the 

 maximal stimulus is reached, which produces a contraction of an amplitude 

 beyond which no increase occurs even though the strength of the stimulus be 

 multiplied many fold. The range between the strengths of the minimal and 

 maximal stimuli is very restricted indeed. The absolute strength of a mini- 



