THE HEAT OF THE BODY 267 



a local excess of heat. A group of muscles, for instance, 

 by their contraction, produce heat. A gland, by the act 

 of secretion, does the same. But the tide of blood, flow- 

 ing swiftly through the system and bathing every part, 

 tends to equalize the heat throughout. 



393. The Sources of Animal Heat are of two sorts, direct 

 and indirect. The great source of heat is the combustion 

 of food, that is, the oxidation which takes place in the 

 living cells in all parts of the body. This is equal to the 

 amount of heat which would be given off by the burning 

 in the open air of the same quantity of food which is con- 

 sumed in the body. The muscles and the glands are the 

 parts in which the greatest amount of oxidation takes 

 place. Some little heat is also received by the body in 

 hot foods and drinks. 



Heat is produced indirectly by the transformation of 

 other forms of energy. Friction of one part upon another 

 as of the blood along the walls of the blood vessels 

 becomes heat. All mechanical work, all nervous activity, 

 and the slight manifestations of electricity within the 

 body liberate heat. 



394. Regulation of Temperature is accomplished in two 

 ways; viz. by variation in the loss of heat and by varia- 

 tion in its production. 



Variation in loss of heat. Everything which leaves the 

 body carries away a portion of its heat. The expired air, 

 the perspiration, the excretions, are sources of loss ; and 

 radiation and conduction of heat from the surface are con- 

 tinually going on, as well as evaporation from the skin and 

 the lungs. The skin is the chief regulator of loss. By 

 clothing the body a portion of the loss by radiation is pre- 

 vented, and we have already seen ( 376) how the secre- 

 tion of sweat, and hence the cooling of the surface by 



