54 VETERINARY PHYSIOLOGY 



forth any response. TMs condition is probably caused by the 

 accumulation of the products of activity in the muscle acting as 

 poisons upon its protoplasm, for the same phenomena may be 

 induced by the application of dilute acids and certain other 

 drugs, and may be removed for a time by washing out the 

 muscle with salt solutions (fig. 24). (Practical Physiology.} 



(2) Temperature. If a muscle be warmed above the normal 

 temperature of the animal from which it is taken, all the phases 

 of contraction become more rapid, and the contraction is at first 

 increased in extent, but subsequently decreased in force. If, 



on the other hand, a muscle be 

 cooled, the various periods are pro- 

 longed. At first the contraction 

 becomes greater and more power- 

 ful, but as the cooling process goes 

 1 ori it becomes less and less, until 



FIG. 24.-Iuflence of continued fina jl y tne most powerful stimuli 

 Exercise on Skeletal Muscle ~ ,. . ,, 



(i) the first trace ; (2) a trace produce no effect. Coolmg has thus 

 after moderate exercise ; (3) a practically the same effect as fatigue 

 trace when fatigue has been (fi g- 24). (Practical Physiology.) 



(3) Many drugs modify muscular 



contractions, e.g. veratrin enormously prolongs the relaxation 

 period. (Practical Physiology.) 



5. Strength of Stimulus. A stimulus must have a certain 

 intensity to cause a contraction. The precise strength of this 

 minimum stimulus depends upon the condition of the muscle. 

 The application of stronger and stronger stimuli causes the 

 muscular contraction to become more and more rapid, more 

 and more complete, and more and more powerful. But increase 

 in the contraction is not proportionate to the increase in the 

 stimulus. If the stimulus is steadily increased, the increase in 

 contraction becomes less and less. This may be represented 

 diagrammatically in the accompanying figure, where the con- 

 tinuous lines represent the strength of the stimuli and the 

 dotted lines the extent of the contractions (fig. 25). 



After a certain strength of stimulus has been reached, 

 further increase of the stimulus does not cause any increase 

 in the muscular contraction. This smallest stimulus which 

 causes the maximum muscular contraction is called the 

 optimum stimulus. 



