THE HEAT OF THE BODY 485 



which is in direct contact with external bodies, usually colder than 

 itself, is cooler than the internal organs; its temperature in health 

 is from 36 to 37 C. (96.8-98.5 F.), being warmer in more pro- 

 tected parts, as the hollow of the armpit. In internal organs, as 

 the liver and brain, the temperature is somewhat higher. In the 

 lungs there is loss of the heat carried out by the expired air and 

 that used up in evaporating the water carried out in the breath, so 

 the blood returned to the heart by the pulmonary veins is slightly 

 colder than that carried from the right side of the heart to the 

 lungs. 



The Sources of Animal Heat. Apart from heat received from 

 its surroundings in hot food and drink the sources of heat in the 

 Body are twofold direct and indirect. Heat is directly pro- 

 duced wherever oxidation is taking place; the living tissues at 

 rest produce heat as one result of the chemical changes supplying 

 them with energy for the maintenance of their vitality : and when- 

 ever an organ is active and its chemical metamorphoses are in- 

 creased it becomes hotter: a secreting gland or a contracting 

 muscle is warmer than a resting one, and the venous blood leav- 

 ing noticeably warmer than the arterial supplied to it. Indirectly, 

 heat is developed within the Body by the transformation of other 

 forms of energy: mainly mechanical work. All movements of 

 parts of the Body which do not move it in space or move ex- 

 ternal objects, are transformed into heat within it; and the energy 

 they represent is lost in that form. Every cardiac contraction 

 sets the blood in movement, and this motion is for the most part 

 turned into heat within the Body by friction within the blood- 

 vessels. The same transformation of energy occurs with respect 

 to the movements of the alimentary canal, except in so far as they 

 expel matters from the Body; and 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 Maintenance of a Uniform Temperature. Obviously if 

 the Body is to preserve the same temperature during any period 

 of time the production of heat within it must exactly balance the 

 loss of heat from it during that time. In ourselves this balance is 

 actually maintained within narrow limits of fluctuation through- 



