290 Dr. Alison on the Principle of Vital Affinity. 



and where a step has been made towards that final dissolu- 

 tion of organic compounds, when the oxygen is to resume its 

 power over the carbon, and this is to revert, directly or indi- 

 rectly, to the condition of carbonic acid. This general prin- 

 ciple as to the respective offices of carbon and oxygen in liv- 

 ing bodies, — the one the main agent in nourishing and sup- 

 porting living structures, the other in maintaining the excre- 

 tions by which these structures are continually restored to 

 the inorganic world, — we shall find to be applicable, not 

 only to the excretion of carbonic acid and water by the skin 

 and lungs, as compared with the amylaceous compounds 

 taken into animal bodies, but likewise to the excretions by 

 the liver and kidneys, as compared with the two other great 

 constituents of the food of animals, viz., the oily and the al- 

 buminous substances. 



Oxygen, in its elementary state, although indispensable to 

 all living action, — although a condition of vitality equally 

 universal as heat, — yet hardly enters, if it enters at all, into 

 any of the combinations which are due to the vital affinities. 

 Although taken into the interior of every living being, it ap- 

 pears to comport itself there almost, if not entirely, as it does 

 in acting on dead matter. The expression of Liebig, that 

 the action of the oxygen of the air in living bodies is destruc- 

 tive, is perhaps fitted to convey an erroneous idea, but we 

 are certain that its chief, if not its sole, action in the animal 

 economy, is on those portions of matter which have no vital 

 properties ; either because they are redundant, — not required 

 for the nourishment of the tissues, — or because they have 

 been re-absorbed from them, having lost their vital affinities ; 

 and with these it unites, only to carry them off in the excre- 

 tions, particularly in the great excretion by the lungs. We 

 now know that the speculation as to the connection of the 

 oxygen of the air with vital action, long and ably maintained 

 by the late Mr Ellis, viz., that its sole use is to dissolve and 

 carry off excreted carbon, and therefore that in the bodies of 

 animals it goes no farther than the lungs, was erroneous ; 

 but we may assert with much confidence, that it goes no far- 

 ther than the circulating blood ; and that, although its action 



