RESPIRATION. 109 



entirely of coal, and then start a fire in it. In that case it 

 would produce heat not merely by burning in one place 

 within, but would be burning throughout the whole of its 

 substance. This is the case with the body. 



Oxidation in Tissue the Source of Heat in the Body. - 



We have seen that the muscles constitute nearly half of 

 the weight of the body. We know, too, that they are more 

 active than most of the tissues. We would now naturally 

 infer, as indeed is the fact, that they are the chief source 

 of the heat produced in our bodies. 



The tissues of the body are oxidizing all the time. But 

 when they are in vigorous action they oxidize very much 

 more rapidly. 



Next to the muscles, in importance as a heat producer, 

 is the liver, which is the largest gland in the body, and, as 

 we shall soon see, one of the most active. The blood, as 

 it leaves the liver by the hepatic vein, is hotter than 

 anywhere else in the body. 



How the Body is like a Locomotive. But it will be 

 better to compare the body to a locomotive, as we produce 

 not only heat, but motion as well. 



If a visitor from another planet, unfamiliar with such 

 creatures as we are, were to observe closely a man and a 

 locomotive, he would see several points in common : 



1. Both are warm. 



2. Both move. 



3. Both use fuel (food or coal). 



4. Both take in air, and (if it were a winter day) 



5. Both give off "steam " (which .is essentially the same 

 in the two, carbon dioxid and water vapor being the chief 

 constituents). 



