506 THE HUMAN BODY 



producing some heat even when at rest, they produce enormously 

 more when they are active. A characteristic of all machines is 

 that they work more or less wastefully; not all the energy imparted 

 to them appears again as useful work; the part that is lost, more- 

 over, appears always as hec^t. In the Body there is this same in- 

 ability to convert food energy into mechanical energy without 

 there being at the same time a large heat production. 



Studies of the metabolism of the Body must necessarily take 

 into account these two main forms in which the energy of the food 

 is manifested. For physical reasons which need not be considered 

 here all muscular and chemical activities occurring wholly within 

 the Body manifest themselves ultimately to the exterior in the 

 form of heat. The total energy turnover of the Body can be de- 

 termined, therefore, if the external mechanical work and the entire 

 heat output can be measured. Theoretically these should exactly 

 balance the energy content of the ingested food. Metabolism 

 studies are devoted in part to demonstrating that this balance 

 actually exists, and in part to determinations of the individual 

 factors concerned. 



Basal Metabolism. A necessary starting point for any study of 

 energy manifestation in the Body is the determination of the 

 amount liberated when the Body is as inactive as possible. The 

 metabolism which gives rise to this energy represents that which 

 is essential to the life processes. It is known as the basal metab- 

 olism. Its energy all appears in the form of heat. As measured in 

 an adult man of average size, who eats nothing during the day 

 of observation, it amounts to about 1,700 Calories (p. 500). The 

 necessary activities of eating and digesting enough food to main- 

 tain the Body involve an expenditure of about 10 per cent addi- 

 tional energy, bringing the total practical basal metabolism up to 

 about 1,870 Calories per day. Any energy liberation in excess of 

 this amount must represent either actual muscular work or the 

 by-product of heat which always attends it on account of the in- 

 efficiency of the muscles. 



We shall see in the chapter on Heat Regulation (Chap. XXXII) 

 that the Body makes very good use *of this by-product of heat in 

 keeping itself at a proper temperature the year round, and so the 

 extra amounts of food we have to eat on account of the inefficiency 

 of our bodily machines are not wholly wasted after all. 



