RESPIRATION. 



In the living body the organic nitrogenized principles are 

 in a condition of continual change ; breaking down, and form- 

 ing various excrementitious principles, at the head of which 

 may be placed carbonic acid. It is essential to life that these 

 principles be maintained in their chemical integrity, which 

 requires a supply of fresh matter as food, and above all a 

 supply of oxygen. We put ourselves in the position of ig- 

 noring well-established facts and principles when we assimi- 

 late without reserve the process of the consumption of oxygen 

 and production of carbonic acid by living organic bodies, to 

 simple combustion of sugar or fat. The ancients saw that 

 the breath was warmer than the surrounding air, that in the 

 lungs the air took heat from the body ; and as they knew of 

 no other changes in the air produced by respiration, they as- 

 sumed that its object was simply to cool the blood. Lavoisier 

 discovered that the air, containing oxygen, lost a portion of 

 this principle in respiration, and gained carbonic acid and 

 watery vapor. He saw that this might be imitated by the 

 combustion of hydro-carbons, such as exist in the blood. He 

 called respiration a slow combustion, and regarded as its prin- 

 cipal office the maintenance of animal temperature. When 

 it was shown by analyses of the blood for gases, that oxygen 

 was not consumed in the lungs, but taken up by the circulating 

 fluid, and carried all over the body, and that carbonic acid 

 was brought from all parts by the blood to the lungs, these 

 facts, taken in connection with the fact that the tissues have 

 the property of consuming oxygen and exhaling carbonic 

 acid, led physiologists to change the location of the combus- 

 tive process from the lungs to the tissues. 



We cannot stop at this point. Now it is known that the 

 organic principles of the body, which form the basis of all 

 tissues and organs, are continually undergoing change as a 

 condition of existence; that they do not unite with any 

 substance in definite chemical proportions, but their par- 

 ticles, after a certain period of existence, degenerate into 

 excrementitious substances, and they are regenerated by an 



