88 PHYSIOLOGY FOR DENTAL STUDENTS. 



poses so large a proportion of the diet of peoples living in cold 

 regions, and why it is cut down in the diet of those who dwell 

 near the tropics. Individuals maintained on a low protein diet 

 may suffer intensely from the cold. 



If we add to the basal heat production of 1,680 C. another 

 168 C. (or 10 per cent) on account of food, the total 1,848 C. 

 nevertheless falls far short of that which we know must be liber- 

 ated when we calculate the available energy of the diet. What 

 becomes of the extra fuel? The answer is that it is used for 

 muscular work. Thus it has been found that if the observed 

 person, instead of lying down in the calorimeter, is made to sit 

 in a chair, the heat production is raised by 8 per cent, or if 

 he performs such movements as would be necessary for ordinary 

 work (writing at a desk), it may rise 29 per cent, that is to say, 

 to 90 C. per hour. Allowing 8 hours for sleep and 16 hours for 

 work, we can thus account for 2,168 C., the remaining 300 odd 

 C. which is required to bring the total to that which we know, 

 from statistical tables of the diets of such workers, to be the 

 actual daily expenditure, being due to the exercise of walking. 

 If the exercise be more strenuous, still more calories will be ex- 

 pended; thus, to ascend a hill of 1,650 feet at the rate of 2.7 

 miles an hour requires 407 extra calories. Field workers may 

 expend, in 24 hours, almost twice as many calories as those en- 

 gaged in sedentary occupations. 



Another factor which controls the energy output is the cool- 

 ing influence of the atmosphere. When this is marked, more 

 heat must be liberated in order to maintain the body temperature 

 (see p. 135). In other words, the necessary heat loss must be 

 compensated by an increased heat production, just as we must 

 burn more coal to keep the house at a given temperature on a 

 cold, than on a warm, day. This adjustment of energy liberation 

 to the rate of cooling at the surface of the body explains, among 

 other things, why it should be that small animals give out much 

 more energy, per unit of body weight, than those that aiv larger. 

 The small animal has relatively the greater surface area, just 

 as two cubes of equal weight when brought together have a com- 

 bined weight which is double that of either cube, but a surface 



