REGULATING THE HEAT 153 



surface of the body. We shall see that she can alter and 

 control both combustion chambers and skin surfaces. 



A driver regulates the rate of combustion in his engine 

 by turning the throttle, valve on or off. The steeper the 

 hill in front of him, the wider he opens the throttle and 

 the greater becomes the consumption of petrol and of 

 air. The engine becomes hot ; it may become over- 

 heated and unworkable in spite of the means he adopts 

 to keep it cool. He may bring the engine to a stand- 

 still by closing the throttle ; combustion then ceases ; 

 the engine quickly cools to the temperature of the 

 surrounding air. Except at death the countless throttles 

 which control combustion in the human machine are 

 never turned off; they are so regulated that when the 

 body is at rest combustion still goes on at a certain rate, 

 for we have seen that the human engines never stop ; 

 they have always their "steam up." The manner in 

 which the throttles are set and controlled to maintain a 

 regulation rate of combustion when the human machine 

 is at rest is a mystery of life which has not been dis- 

 covered, but we do know a little of the mechanisms by 

 which the regulation speed may be altered so that the 

 rate of combustion may be raised or lowered according 

 to the needs of the body. The will can indirectly turn 

 on the throttle valves of the combustion chamber when 

 it sets the muscles to move the body and perform work. 

 Dr Leonard Hill 1 found, when he kept his body at rest, 

 that he gave off in his breath 301 c.c. of C0 2 per minute, 

 but when he went swimming the rate of combustion 

 was increased more than twelvefold ; he then gave off 

 3804 c.c. of C0 2 per minute. We cannot move a muscle 

 in our bodies without raising their temperature, however 

 small the rise may be ; at the end of a long race the heat 

 of the runner's body may have risen from 98 to 105 F. ; 

 even Nature's mechanism for regulating temperature may 

 fail when overtaxed. Still, how perfect it usually is 



1 A Textbook of Physiology, by Martin Flack and Leonard Hill. The reader is 

 referred to this excellent work for original data connected with respiration and 

 temperature. 



