478 RESPIRATION. 



that it unites with the organic principles of the system, satis- 

 fying the " respiratory sense," and supplying an imperative 

 want which is felt by all animals, and extends to all parts of 

 the organism. After being absorbed, it is lost in the intri- 

 cate processes of nutrition. There is no evidence in favor of 

 the view that oxygen unites directly with carbonaceous mat- 

 ters in the blood which it meets in the lungs, and, by direct 

 union with carbon, forms carbonic acid. 



2. How is carbonic acid produced ~by the system f 

 That carbonic acid makes its appearance in the blood it- 

 self, produced in the red corpuscles, has been abundantly 

 proven by observations already cited; though it is impos- 

 sible to determine to what extent this takes place during 

 life. It is likewise a product of the physiological decompo- 

 sition of the tissues, whence it is absorbed by the blood cir- 

 culating in the capillaries and conveyed by the veins to the 

 right side of the heart. It has been experimentally demon- 

 strated that its production is not immediately dependent 

 upon the absorption of oxygen ; for it will go on in an atmos- 

 phere of hydrogen or of nitrogen. It is most reasonable to 

 consider the carbonic acid thus formed as a product of excre- 

 tion or destructive assimilation, like urea, creatine, or choles- 

 terine. The fact that it may easily be produced artificially, 

 out of the body, does not demonstrate that its formation 

 in the body is as simple as when it is formed by the pro- 

 cess of combustion. We may be able at some future time 

 to produce artificially all the excrementitious principles, 

 as has already been done in the case of urea ; l but we are 

 hardly justified in supposing that the mode of formation 

 of this principle, as one of the phenomena of nutrition, is 

 precisely the same as when it is made by our chemical ma- 

 nipulations. 



1 Woller first formed urea artificially by a union of cyanic acid and am- 

 monia. Since then it has been prepared by chemists by various processes 

 (LEIIMAKN, Physiological Chemistry, Philadelphia, 1855, vol. i., p. 147). 



