CH. xill.] CHANGES IN MUSCLE. 153 



muscle A is produced. This is called secondary contraction. It may 

 be either a secondary twitch or secondary tetanus, according as to 

 whether the muscle B is made to contract singly or tetanically. 



3. Secondary contraction from the heart. If an excised but still 

 beating frog's heart is used instead of muscle B, and the nerve of 

 A laid across it, each heart's beat accompanied as it is by an 

 electrical variation, will stimulate the nerve and cause a twitch 

 in the rheoscopic muscle A. 



CHAPTER XIII. 



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 i gramme of water 

 from o to i 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 i metre. 



A muscle when uncontracted is nevertheless not at absolute rest. 

 We have already seen that it possesses ton us or tone ; it also 

 possesses what we may call chemical tone ; that is, chemical 

 changes are occurring in it, and consequently heat is being pro- 

 duced. 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 pro- 

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



