634 THE POTENTIAL ENERGY OF FOOD. [BOOK n. 



If we translate the units of heat into units of work, the 

 2,310,000 gramme-degree, or 2,310 kilogramme-degree calories 

 will give us about 980,000, or, in round numbers, somewhere 

 about one million kilogramme-meters. 



We may, in passing, call attention to the fact that the pro- 

 teids supply a relatively small part of the total energy, and 

 that the share contributed by the large mass of carbohydrates 

 is not much greater than that belonging to the much smaller 

 quantity of fat. In the average diet obtained by the statistical 

 method, in which the data are largely drawn from public insti- 

 tutions, the (cheaper) carbohydrates are still further increased 

 at the expense of the (dearer) fats, a change which may tend 

 to reduce somewhat the total energy ; but this does not mate- 

 rially affect the broad result just given. 



The Expenditure. 



422. There are two ways only in which energy is set 

 free from the body : mechanical labour and heat. The body 

 loses energy in producing muscular work, as in locomotion and 

 in other kinds of labour, in the movements of the air in respira- 

 tion and speech, and, though to a hardly recognizable extent, 

 in the movements of the air or contiguous bodies by the pulsa- 

 tions of the vascular system. The body loses energy in the 

 form of heat by conduction and radiation, by respiration and 

 perspiration, and by the warming of the urine and fseces. All 

 the internal work of the body, all the mechanical labour of the 

 internal muscular mechanisms with their accompanying fric- 

 tion, all the molecular labour of the nervous and other tissues, 

 is converted into heat before it leaves the body. The most 

 intense mental action, unaccompanied by any muscular mani- 

 festations, the most energetic action of the heart or of the 

 bowels, with the slight exceptions mentioned above, the busiest 

 activity of the secreting or metabolic tissues, all these end sim- 

 ply in augmenting the expenditure in the form of heat. 



A normal daily expenditure in the way of mechanical labour 

 can be easily determined by observation. Whether the work 

 take on the form of walking, or of driving a machine, or of 

 any kind of muscular toil, a good day's work may be put down 

 at about 150,000 kilogramme-meters. 



The normal daily expenditure in the way of heat cannot be 

 so readily determined. Direct calorimetric observations on 

 living structures are in all cases attended with many difficul- 

 ties and subject to many sources of error. These are very 

 great when the observations are made on the whole body, even 

 in the case of small animals ; and observations made by placing 

 a part only of the body, an arm or leg for example, in the calo- 



