452 THE HUMAN BODY. 



every muscle in contracting has part of the mechanical 

 energy expended by it turned into heat by friction against 

 neighboring parts. Similarly the movements of cilia and 

 of amoeboid cells are for the most part converted in the 

 Body into heat. The muscles and nerves are also the seats 

 of manifestations of electricity, which, though small in 

 amount, for the most part do not leave the Body in that 

 form but are first converted into heat. A certain amount 

 of heat is also carried into the Body with hot foods and 

 drinks. 



The Energy Lost by the Body in Twenty-four Hours, 

 Practically speaking, the Body only loses energy in two- 

 forms; as heat and mechanical work: by applying conduc- 

 tors to different parts of its surface small amounts of elec- 

 tricity can be carried off, but the amount is quite trivial in 

 comparison with the total daily energy expenditure. Dur- 

 ing complete rest, that is when no more work is done than 

 that necessary for the maintenance of life, nearly all the 

 loss takes the form of heat. The absolute amount of this 

 will vary with the surrounding temperature and other con- 

 ditions, but on an average a man loses, during a day of 

 rest, 2700 calories; that is enough to raise 2700 kilograms 

 (5940 Ibs.) of water from to 1 C. (from 32 to 33.8 F.); 

 otherwise expressed, this amount of heat would boil 27 kilos 

 (59.4 Ibs.) of ice-cold water. This does not quite represent 

 all the energy lost by the Body in that time: since a small 

 proportion is lost as mechanical work in moving the clothes 

 and air by the respiratory movements, and even by the 

 beat of the heart, which at each systole pushes out the 

 chest- wall a little and moves the things in contact with it. 

 The working Body liberates and loses much .more energy; 

 part as mechanical work done on external objects, part as 

 increased heat radiated or conducted from the surface, or 

 carried off by the expired air in the quickened respirations. 

 Every one knows that he becomes warmer when he takes 

 exercise, and measurements made on men show that the 

 heat produced and lost in a day of moderate work is about 

 one third greater than that in a day of rest. The follow- 

 ing table gives more accurate numbers 



