152 THE ENGINES OF THE HUMAN BODY 



have to be compounded and stored as a living mixture 

 before they can be used by the cylinders and thus turned 

 into work and heat. We know that this mixture is con- 

 sumed or burned ; when muscular cylinders are working 

 hard it is consumed at ten times or even twelve times the 

 rate it is used up when the engines are at rest. Clearly 

 one way of warming the body is to set the muscles 

 at work ; that quickens combustion — as every one of us 

 already knows. We have seen, too, how the chief product 

 of combustion (C0 2 ) is carried away from muscles in the 

 blood of the veins and ultimately discharged into the 

 respiratory chambers of the lungs, being finally expelled 

 from the body with the breath. 



If muscles represent the most extensive system of 

 furnaces in the body, they are by no means the ones 

 which give out the fiercest heat. The liver, we shall 

 find, is a vast chemical factory busy with processes in 

 which a great deal of heat is produced. Indeed, all the 

 organs concerned in the digestion of food are also pro- 

 ducers of heat ; so are the kidneys, and also the lungs. 

 When we work our brains very hard we can feel that 

 our heads become hot. Every one of the countless 

 myriads of living units, fitted together so as to form a 

 single human machine, is a microscopic slow-combustion 

 stove. 



We are now in a position to realise what a marvellous 

 thing it is that the human body can maintain its countless 

 fires burning at the exact rate which keeps the temperature 

 of the whole machine just above 98 F. in all weathers 

 and in all climates. We know how difficult it is to keep 

 a room at a comfortable temperature. If it becomes too 

 cold, we warm it by making the fire up or by shutting 

 doors and windows. If a room becomes too warm, we 

 let the fire burn low or open windows and doors to 

 permit draughts of fresh air to flood and cool it. Nature 

 adopts similar means to regulate the temperature of the 

 body ; she may maintain it at the same warmth by raising 

 or lowering the rate of combustion, or by increasing or 

 decreasing the rate at which the heat escapes from the 



