160 ANIMAL HEAT 



V is the observed volume; V, desired volume at C. and 

 760 mm. Hg; P, observed pressure; and t, mean temperature. 

 The value of 760 (1 + 0.003665) is obtained from standard 

 tables. A liter of dry air at C. weighs 0.001293 kilogram. 



The measurement of the aqueous vapor of the air before 

 entering and after leaving the calorimeter gives the data for 

 the estimation of the heat lost through evaporation. If it 

 is found that the total quantity of water evaporated from the 

 animal is 100 grams, it is only necessary to multiply by 582 

 (since it requires this number of calories to evaporate 1 gram 

 of water) in order to obtain the heat in kilogram degrees that 

 is lost by evaporation. The principal part of the total heat 

 produced by the body is generated by muscular activity. 

 Subsidiary sources are the chemical action going on during 

 digestion, friction of muscles, blood, warm foods, sun's rays, etc. 



Throughout life the body maintains a constant temperature, 

 so that there is a regulation of heat produced and heat dissi- 

 pated. The production of heat is known technically as ther mo- 

 genesis; the dissipation of heat, as thermolysis; and the regulation 

 of the relations between them, as thermotaxis. It is evident 

 that if thermogenesis and thermolysis vary together, the body 

 temperature will remain unchanged; but an increase in thermo- 

 genesis with a constant or decreased thermolysis will raise 

 the body temperature. Further, a decrease in thermogenesis 

 with constant or increased thermolysis will lower the body 

 temperature. The production of heat probably takes place 

 in all the tissues of the body, since they all undergo oxidative 

 changes; but the muscles are the main source of the heat not 

 only when active, but when at rest. During activity the greater 

 part of their chemical energy is liberated as heat, only one- 

 fifth appearing as mechanical energy. The work of the heart 

 is entirely converted to heat, forming less than 5 per cent, 

 of the total amount produced in the body. It is known that 

 when a muscle is separated from the central nervous system 

 it continues to produce heat, but much less than before. Specific 

 thermogenic fibers have not been isolated. It has been claimed 

 that the act of shivering has as its only purpose the production 

 of heat, so that if the muscular contractions of shivering are 

 brought about by impulses passing over ordinary motor nerves, 

 they must have a specific thermogenic function. The follow- 



