REGULATION OF HEAT. 197 



body at about the temperature of the blood, and is always 

 saturated with moisture. No inverse proportion, therefore, 

 exists between the loss of heat by radiation and conduction on 

 the one hand, and by evaporation on the other. The colder 

 the air, for example, the greater will be the loss in all ways. 

 Neither is the quantity of blood which is exposed to the cool- 

 ing influence of the air diminished or increased, so far as is 

 known, in accordance with any need in relation to temperature. 

 It is true that by varying the number and depth of the respi- 

 rations, the quantity of heat given off by the lungs may be 

 made, to some extent, to vary also. But the respiratory pas- 

 sages, while they must be considered important means by which 

 heat is lost, are altogether subordinate in the power of regu- 

 lating the temperature, to the skin. 



It may seem to have been assumed, in the foregoing pages, 

 that the only regulating apparatus for temperature required 

 by the human body is one that shall, more or less, produce a 

 cooling effect ; and as if the amount of heat produced were 

 always, therefore, in excess of that which is required. Such an 

 assumption would be incorrect. We have the power of regu- 

 lating the production of heat, as well as its loss. 



In food we have a means for elevating our temperature. It 

 is the fuel, indeed, on which animal heat ultimately depends 

 altogether. Thus, when more heat is wanted, we instinctively 

 take more food, and take such kinds of it as are good for com- 

 bustion ; while everyday experience shows the different power 

 of resisting cold possessed by the well-fed and by the starved. 



In northern regions, again, and in the colder seasons of more 

 southern climes, the quantity of food consumed is (speaking 

 very generally) greater than that consumed by the same men 

 or animals in opposite conditions of climate and seasons. And 

 the food which appears naturally adapted to the inhabitants 

 of the coldest climates, such as the several fatty and oily sub- 

 stances, abounds in carbon and hydrogen, and is fitted to com- 

 bine with the large quantities of oxygen which, breathing cold 

 dense air, they absorb from their lungs. 



In exercise, again, we have an important means of raising 

 the temperature of our bodies (p. 190). 



The influence of external coverings for the body must not be 

 unnoticed. In warm-blooded animals, they are always adapted, 

 among other purposes, to the maintenance of uniform temper- 

 ature ; and man adapts for himself such as are, for the same 

 purpose, fitted to the various climates to which he is exposed. 

 By their means, and by his command over food and fire, he 

 maintains his temperature on all accessible parts of the sur- 

 face of the earth. 



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