IRRITABILITY. 455 



1. The load ; 2. The temperature ; 3. Previous stimulation ; 4. The 

 character of the muscle itself; and 5. Drugs. 



1. Effect of the Load. The extent to which a muscle contracts 

 is increased up to a certain point with the increase of the load ; 

 but when this point is reached it diminishes, and if the load is 

 sufficiently increased, its power to lift it at all ceases. 



2. Effect of Temperature. Up to a certain point cold increases 

 the contraction ; beyond this point it diminishes it ; moderate 

 warmth also increases the height of the contraction, but excessive 

 heat (exceeding 40 C.) coagulates the proteids of the muscle, 

 producing heat-rigor. 



3. Effect of Previous Stimulation. When a muscle is stimu- 

 lated, each curve is a little higher than the preceding one for a 

 time, the curve being called a staircase ; then, as the stimulation 

 is continued, the curve falls, and finally there is no response to 

 the stimulation the muscle is fatigued. 



Cause of Fatigue of Muscles. This is explained in the case of 

 the muscle-nerve preparation by the fact that the destruction or 

 katabolism of the muscular tissue predominates over its anabolism 

 or building up, and after a time its contractile power is lost. 



FIG. 264. Myogram of muscle poisoned with veratrin and that of a normal 

 muscle : a, myogram from a normal gastrocnemius muscle of a frog the waves at 

 the close are due to the recoil of the recording lever ; b, myogram from a gastroc- 

 nemius muscle poisoned with veratrin, recorded at the same part of the drum 

 (Lombard). 



Fatigue also occurs as a result of the accumulation of the prod- 

 ucts of contraction, sarcolactic acid and acid potassium phosphate, 

 which to a certain extent act to prevent contraction, and when 

 these are removed by the circulating blood during a period of rest 

 following muscular activity, the contractile power is restored. 



In the fatigue of muscles which follows their use as the result 

 of volition, the cause is chiefly in the nerve-cells in which the 

 voluntary impulses are generated. 



4. Effect Due to the Character of the Muscle. Some muscles 

 contract more slowly than others, even in the same individual ; as, 



