METABOLISM 235 



general terms, of various nerve-centers under the supreme dictation of 

 one chief center, and of tissues and organs all over the body so coordinated 

 that they bring about the result required. In infants (and in poikilo- 

 therms) the apparatus is not well developed. 



We can make the working of the heat-regulating mechanism clearer 

 if we discuss its two phases (the control of heat-production and that of 

 heat-expense) separately. 



THE MEANS OF REGULATING HEAT-PRODUCTION are chiefly the 

 increasing or decreasing of metabolism and of muscular activity; only in 

 indirect ways can glandular activity be varied. In man these means are 

 partly voluntary, although the "sensations" underlying the voluntary 

 acts required are purely physiological. In cold weather animals 

 "naturally," as we say, eat somewhat more food than on warm days. 

 Moreover, the human appetite then tends to demand foods which are 

 large producers of heat and energy in winter beefsteak and potatoes 

 and bacon and hot rich soups; in summer, on the other hand, salads, 

 ices, fruits, and "plain living." These same tendencies are seen in 

 whole racial diets. We find the dwellers of the far North eating much 

 fat (combustion-equivalent, 9.3), while those of the Tropics live on fruits 

 and cereals containing much liquid and waste cellulose. It is only 

 from habit that perhaps most people eat nearly as much in warm weather 

 as in cold, for the actual body-demand is much less. It is part of the 

 heat-regulating arrangements (but how brought about is unknown), 

 that fats are actually distasteful on a very warm day. 



Besides tending to limit the general metabolism by thus decreasing 

 its fuel, the organism automatically inclines to lessen that large percentage 

 of heat and energy which the muscles give out. Exertion tends to be 

 irksome in warm weather, partly it is true because the abundant sweat 

 so occasioned is a source of much discomfort, but also because muscular 

 exertion is unnatural in great heat and rest not only strongly desired but 

 also based in physiological conditions which only the will can overcome. 

 The same is true, but to a less extent, perhaps, in regard to mental 

 labor, although the nervous system as compared with the muscles 

 produces but little heat. Muscular contractions, however, always tend 

 to be proportionate to mental activity. Sleep, as we shall see, tends 

 to increase heat-loss as well as to limit heat-production, and there is 

 a natural tendency to sleep when the temperature is high. 



In the opposite direction corresponding influences are at work. These 

 increase heat-production in cold surroundings or when heat-loss (ther- 

 molysis) is excessive. Under such circumstances one eats more and 

 "heavier" food and takes less liquid than in the opposite condition of 

 environment. Observing the diet of lumbermen in the northern woods 

 in winter, one is almost surprised at the large amounts of bread, butter, 

 baked-beans, bacon, salt-pork, and very hot tea that are consumed, 

 and these diets often reach 6000 calories. Severe muscular exertion, 

 cold, and wet combine to make the demand for fuel apparently exces- 

 sive, the last two increasing the loss of heat to a high proportion, while 



