MUSCULAR ACTIVITY 97 



maximal contractions. The gradation of response by grading the 

 intensity of stimulation is obviously the means employed in our 

 own bodies to produce graded movements. We realize without 

 difficulty that a powerful contraction requires a great effort (strong 

 stimulation), while a gentle contraction is produced with very 

 little effort (weak stimulation). 



Every skeletal muscle, even the smallest, is made up of a great 

 number of fibers. This must be borne in mind when we attempt 

 to explain the production of graded responses, for when the muscle 

 contracts feebly it may be that all the fibers are contracting, each 

 one feebly, or it may be that a few fibers scattered through the 

 muscle are contracting powerfully while the others are inactive, 

 If the first of these suppositions is correct we must look upon all 

 the fibers as equally sensitive to stimulation, so that all respond 

 feebly to weak stimuli and powerfully to strong ones. If the 

 second view is the true one we must picture the various fibers as 

 differing in sensitiveness over a wide range. Some are aroused 

 by very feeble stimuli, others require stronger ones, and others 

 still stronger ones; but whether aroused by a weak stimulus or a 

 strong one the response of the individual fibers is powerful. Ac- 

 cording to this view the muscle fibers could be compared to the 

 cartridge in a rifle and the stimulus to the pull of the trigger. In 

 some rifles the trigger is harder to pull than in others, but the ex- 

 plosion of the cartridge is as violent in the rifle with the hair 

 trigger as in the stiffer one. 



The first of these theories, the one that makes the gradation of 

 response a gradation within the fibers, is the older and the one that 

 has formerly been generally accepted. The second view, accord- 

 ing to which the gradation of response depends on the number of 

 fibers involved, has been urged only recently, and while many 

 known facts are in accord with it, it cannot be said to be conclu- 

 sively proven. 



The Influence of Temperature on Contraction. If an isolated 

 muscle is cooled down and then stimulated, the contraction will 

 occur much more slowly than at ordinary temperatures. On the 

 other hand, a muscle that is warmed contracts and relaxes more 

 rapidly than one that is at room temperature. This variation in 

 the speed of contraction with change of temperature is in accord 

 with a general chemical law which states that chemical processes 



