MAN AND LOCOMOTIVE. 135 



If a visitor from another planet, unfamiliar with such 

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

 motive, he would see several points in common : 



1. Both are warm. 



2. Both move. 



3. Both use fuel (food and coal). 



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



5. Both give off smoke (which is essentially the same in 

 the two, carbon dioxid and water vapor being the chief con- 

 stituents). 



By a closer examination he would find out some of the 

 differences that we have noticed : 



1. That the body does not get hot enough to burn; i.e., the 

 oxidation is relatively slow, and is not combustion. 



2. That the oxidation of the body never produces light. 



3. That the oxidation here is always in the presence of 

 moisture. 



When the breath is held for some time, the carbon dioxid 

 in the expired air may reach 7 or 8 per cent. 



During violent exercise the amount of carbon dioxid given 

 off may be from two to two and a half times as much as when 

 we are at rest. 



The amount of carbon dioxid given off is increased in cold 

 weather, and by taking food ; is from one-fifth to one-fourth 

 less during sleep. 



Oxygen is carried chiefly in the corpuscles, but the carbon 

 dioxid is carried in both plasma and corpuscles. 



The activity of the tissues from their oxidation does not' 

 necessarily mean that the oxidation is direct; that is, that 

 the oxygen is used as soon as it is brought to the tissue. For 

 instance, in the muscles it is believed that the oxygen is stored 

 in some form, probably in combination, so that it can be used 

 when needed, perhaps much more rapidly than could be sup- 



