THE HUMAN BODY AS AN ENGINE. 497 



setting free a definite quantity of heat for every pound of fuel so burned. 

 So, in exactly the same way, oxygen, which has been taken up by the 

 blood from the air in the lungs, unites with carbon and hydrogen in the 

 tissues of the body and forms carbonic acid and water, yielding pre- 

 cisely the same amount of heat as though the combustion had occurred 

 in a furnace. This idea of food, that it is literally fuel, is a very sug- 

 gestive one. And as fuels differ in the quantity of ash contained and 

 the amount of heat produced, so food materials differ in the quantity of 

 undigestible residue and in their heat-producing power. 



Remembering the analogy of the steam engine, let us now inquire 

 what becomes of the energy supplied to the body in the fuel foods eaten, 

 and which is turned into heat by this process of combustion constantly 

 going on. 



1. A large amount of heat is constantly being expended in keep- 

 ing the body warm. Like the locomotive, the body is warmer than the 

 surrounding air, and is constantly losing heat to the atmosphere. Un- 

 like the locomotive, however, the body has a nearly uniform tempera- 

 ture throughout, namely, 98 degrees Fahr. The delicate regulation 

 of temperature which is automatically maintained in the animal body 

 is one of the wonders of physiology. 



2. A second portion of energy is required to do the mechanical 

 work of the body. When a locomotive hauls a loaded train up grade, 

 or steams up grade alone, it is doing work in proportion to the total 

 weight and the height to which it is carried. So when a man walks 

 up hill or climbs a ladder he is lifting his body against the force of 

 gravity, and hence doing work. If his weight be 200 pounds he is 

 doing twice as much work as though he weighed only 100 pounds. If 

 a man weighing 150 pounds climbs Bunker Hill Monument (220 feet), 

 33,000 foot-pounds of work will then be done; and if he succeeds in 

 making the ascent in one minute, he would be doing work at the rate 

 of one full horse power for that minute. If he climbs a mountain two 

 miles high in three hours and twelve minutes he would be doing work 

 in so lifting his body at the rate of one quarter of a horse power. This 

 is, of course, a faster rate of work than an average man could maintain. 

 In all the functions of daily life the body is necessarily doing some 

 mechanical work. Even dressing and eating require a certain expendi- 

 ture of energy, and in ordinary business and manual labor the amount 

 of mechanical work done is considerable. Moreover, a large amount of 

 work is done by the heart in pumping the blood through the circu- 

 latory system, and by the chest in respiration. This, then, the internal 

 and external work done, as in the locomotive, represents the second 

 portion of energy derived from the food eaten. 



3. The warm air, carbonic acid gas and water vapor passing away 

 from the lungs in respiration carry with them a large amount of heat. 



