154 THE ENGINES OF THE HUMAN BODY 



may be judged from another instance given by Flack 

 and Hill. A soldier with a body-weight of 154 lb. and 

 carrying a pack weighing 68 lb. was found to produce, 

 every five minutes, sufficient heat to raise the temperature 

 of his whole body i° F. If he had been enclosed in a 

 vacuum coat, so that heat could not escape from his 

 body, his temperature at the end of an hour's march 

 would have risen from 98 to no°; in eleven hours 

 more it would have reached boiling-point. Yet men 

 make marches for five hours at a stretch, and at the 

 end of the journey their temperatures are found to be 

 scarcely altered. Muscles, then, are a source of heat ; 

 by setting them in motion the amount of heat pro- 

 duced in the body is increased many times. Shivering, 

 as Dr Hill has pointed out, is one of Nature's means 

 for increasing heat production by setting muscles into 

 involuntary action. We see, too, why a man who is cold 

 beats his arms, rubs his hands, and stamps his feet to 

 warm his body. 



Nature has certain other mechanisms she uses for in- 

 creasing the production of heat in the human body. The 

 organs of digestion, such as the liver, when fully at work, 

 particularly if nourished on a meat diet, are most im- 

 portant sources of heat, so active are the chemical processes 

 which go on in them. The feeling of chill which may 

 accompany a meal is not due to a lessened production of 

 heat, but to the withdrawal of blood from the surface 

 of the body to supply the needs of the organs of digestion. 

 Fats, when freely consumed in the tissues of the body, 

 serve as a particularly good heat-producing fuel. Then 

 there are other means. We can make a fire burn faster 

 or slower by adding certain substances to the fuel. 

 Amongst various substances or drugs which the thyroid 

 gland of the neck throws into the blood of the human 

 body is one which can hasten the rate of combustion in 

 its tissues, and thus increase heat production. Sometimes 

 thyroid extract is taken by people with the object of 

 burning up superfluous stores of fat which have accumu- 

 lated in their bodies. There are probably other substances 



