206 ELEMENTARY PHYSIOLOGY less. 



Production of Heat. — The temperature of the body is 

 kept constant by that carefully adjusted variation in loss 

 of heat from its surface which has been described in the 

 preceding section. But now we may point out that there 

 is another way by which this constancy might be attained, 

 namely by alteriny the production of heat taking place in 

 the body, in correspondence to the changes of the sur- 

 rounding temperature ; just as the temperature of a room 

 may be regulated by putting out or increasing the fire as 

 well as by opening or closing its windows. The question 

 thus raised is very interesting, but it is also very abstruse, 

 and we must not do more than just touch upon it. 



All oxidation in the body involves the consumption of 

 oxygen, the production of carbonic acid and the genera- 

 tion of an exactly corresponding quantity of heat. We 

 may therefore take the difference in the amount of oxygen 

 used up (and of carbonic acid produced) at different times 

 as a measure of the amount of heat being produced in the 

 body during the same periods. Working in this way it is 

 found that when a warm-blooded animal is exposed to 

 cold, as when it is put into a chamber which is cooled, 

 it uses up more oxygen and gives off more carbonic acid 

 than when put into a warm chamber. But this can only 

 mean that in the cooler surroundings the animal makes 

 more heat than when the surroundings are warm. Perhaps 

 the most evident instance of increased heat production, 

 in response to unusual heat loss, is that of shivering. 

 When, owing to lowering of the external temperature 

 or to insufficient clothing, the ordinary sources of heat 

 fail to maintain the temperature of the body, impulses 

 pass from the brain to the muscles which cause them to 

 contract rhythmically. In other words the subject shivers. 

 Muscular contraction, as we have already seen, involves 

 oxidation (see p. 203), and oxidation is accompanied by 

 heat production. Again we may point out, as tending to 

 the same conclusion, that our desire for food is greater, 

 on the whole, in the cooler winter time than in the 



