THE PLAXT. 45 



ceptible to our senses. In combustion (or burning) 

 the same changes take place with much greater 

 rapidity, and the same amount of heat, being con- 

 centrated, or brought out in a far shorter time, it 

 becomes intense, and therefore apparent. In the 

 lungs and blood-vessels of animals the same law 

 holds true. The blood contains matters belonging 

 to this carbonaceous class, and they undergo, during 

 its circulation, the changes which have been de- 

 scribed under the head of combustion and decay. 

 Their hydrogen and oxygen unite, and form the 

 moisture of the breath, while their carbon is com- 

 bined with the oxygen of the air drawn into the 

 lungs, and is thrown out as carbonic acid. The 

 same consequence heat results in this, as in the 

 other cases, and this heat is produced with sufficient 

 rapidity for the necessities of the animal. When he 

 exercises violently, his blood circulates with in- 

 creased rapidity, thus carrying carbon more rapidly 

 to the "lungs. The breath also becomes quicker, 

 thus supplying increased quantities of oxygen. In 

 this way the decomposition becomes more rapid, 

 and the animal is heated in proportion. 



Thus we see that food has another function be- 

 sides that of forming animal matter, namely to sup- 

 ply heat. When the food does not contain a suffi- 

 cient quantity of starch, sugar, etc., to answer the 

 demands of the system, the animal's own fat is car- 

 ried to the lungs, and there used in the production 

 of heat. This important fact will be referred to 

 again. 



