256 THE BODY AT WORK 



any mechanical contrivance. In the boiler of a steam-engine 

 heat is applied to water until its molecules cannot remain in 

 so close a state of aggregation. Their orbits are greatly in- 

 creased. The cause of the thrust given to the piston of an 

 engine is the increased amplitude of movement of the mole- 

 cules of steam behind it. In a combustion engine a mix- 

 ture of petrol and air is ignited. Energy is set free by the 

 resolution of unstable petrol into stable water and carbonic 

 acid. This energy heats the gases, causing them to expand. 

 Waste of energy as heat is inevitable in a machine which de- 

 pends for its motive-power upon the translation of molecules. 

 The source of muscular force (if it be not intramolecular 

 change) is certainly not, directly, increased amplitude of 

 molecular swing. 



But we must not conclude, as we are tempted to do, that 

 muscle is capable ot liberating as mechanical work the whole 

 of the energy supplied to it in food, seeing that its activity is 

 always accompanied by evolution of more heat than can be 

 attributed to friction. If the bulb of a thermometer be in- 

 serted into a group of muscles, the instrument shows a marked 

 rise of temperature when the muscles contract. Even though 

 the temperature of the chamber in which an animal is placed 

 is equal to its own, the animal makes more heat if compelled 

 to work, notwithstanding the fact that the consequent rise of 

 its body-temperature may prove fatal. 



Nor can muscles dispense unlimited heat without doing 

 mechanical work. If I am too cold, the obvious means of 

 getting warm is jumping about. There appears to be a level 

 of heat-production which cannot be exceeded without move- 

 ment. When more heat is called for than quiescent muscles 

 can produce, they exhibit flickering contractions, shivering, 

 without moving the limbs. The signal for increased pro- 

 duction is given by the skin. The skin is sensible of the amount 

 of heat which is being lost. Exposure to cold air makes one 

 shiver, by suddenly withdrawing heat. But an increase of 

 temperature in the blood behind the skin has an exactly similar 

 effect. In the first stage of fever, when the temperature of 

 the body has risen two or three degrees, and before the system 

 has become accustomed to this state of affairs, the skin an- 

 nounces to the muscles that heat is being rapidly lost. A 



