142 RESPIRATION A KIND OF COMBUSTION, 



creased quantity which, as will be remembered, is 

 found in the air after it has passed through the lungs. 

 The lungs are full of little cavities, so that the blood 

 may come in contact with as much of the air as pos- 

 sible at once, and absorb large quantities of oxygen. 



c. Another result of this decomposition or burning, 

 is water; so that we have here carbonic acid and wa- 

 ter for the final product, as in the ordinary burning of 

 wood or coal. We do not understand how it happens, 

 but the same effect seems to be produced in the lungs 

 as when carbon is actually burned by a flame; its 

 uniting with oxygen and forming carbonic acid, heats 

 the body as an internal flame would do. 



Every person knows how difficult it is for a hungry 

 man to keep warm in cold weather, and how soon a 

 full meal restores the animal heat. The quicker we 

 breathe, the more food or starch is burned; thus strong 

 exertions always heat us, because they compel us to 

 breathe faster. The larger portion of the starch, then, 

 which is received with our food, passes off in the 

 shape of carbonic acid and water. 



In warm weather our appetites are less than in 

 cold, because the outward temperature is such as re- 

 quires less action of the lungs to retain the warmth 

 of the body, and consequently involves a smaller con- 

 sumption of food. Nothing reduces the flesh and 

 strength so rapidly as cold and hunger combined, for 

 then all the resources of the body are most speedily 

 exhausted. Deprivation of food, w^hile the tempera- 

 ture of the air corresponds nearly with that of the 

 body, may be borne with comparative impunity, and 

 with little emaciation, for a period that would in the 

 first case have been fatal. 



There are other substances in our ordinary food, 

 which may serve the same purpose as starch, in keep- 

 ing up the heat of the body. 



a. One of these is sugar, as indeed might be ex- 



