CHAPTEE XII 



THERMAL AND CHEMICAL CHANGES IN MUSCLE 



IN muscular contraction there is a transformation of the potential 

 energy of chemical affinity into other forms of energy, especially 

 molar motion and heat. Heat is a form of motion in which there is 

 movement of molecules; in molar motion there is movement of 

 masses. The fact that when a blacksmith hammers a piece of iron 

 it becomes hot is a familiar illustration of the transformation of one 

 mode of movement into the other. Heat is measured in heat-units or 

 calories. One calorie is the energy required to raise the temperature 

 of 1 gramme of water from to 1 C. ; and this in terms of work is 

 equal to 425*5 gramme-metres, that is, the energy required to raise 

 the weight of 425*5 grammes to the height of 1 metre. 



A muscle when uncontracted is nevertheless not at absolute rest. 

 We have already seen that it possesses tonus or tone ; it also possesses 

 what we may call chemical tone; that is, chemical changes are 

 occurring in it, and consequently heat is being produced. But when 

 it contracts, the liberation of energy is increased ; work is done, and 

 more heat is produced; the heat produced represents more of the 

 energy than the work done. The more resistance that is offered to a 

 muscular contraction, the more is the work done relatively increased 

 and the heat diminished. The amount of heat produced is increased 

 by increasing the tension of the muscle. It diminishes as fatigue 

 comes on. On increasing the strength of the stimulus the amount 

 of heat increases faster, proportionately, than the work performed. 



If work is done by a few large contractions, more heat is produced 

 than if the same work is done by a larger number of smaller contrac- 

 tions; that is, more chemical decomposition occurs, and fatigue 

 ensues more rapidly in the first case. This fact is within the personal 

 experience of everyone. If one ascends a tower, the work done is 

 the raising of the weight of one's body to the top of the tower. If 

 the staircase in the tower has a gentle slope, each step being low, 

 far less fatigue is experienced than if one ascended to the same height 

 by a smaller number of steeper steps. 



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