IgQ DR ALISON ON THE PRINCIPLE OF VITAL AFFINITY. 



compounds, in Avliich the characteristic predominance of carbon is not perceived ; 

 because they are tliose wliich are formed in circumstances wlicre the vital affini- 

 ties are losing their power, and where a step has been made towards that final 

 dissolution of organic compounds, when the oxygen is to resume its power over 

 the carbon, and this is to revert, directly or indirectly, to the condition of car- 

 bonic acid. This general principle as to the respective offices of carbon and oxy- 

 gen in living bodies, — the one the main agent in nourishing and supporting living 

 structures, the other in maintaining the excretions 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 com- 

 pared 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 albuminous sub- 

 stances. 



in the excretions, 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 farther than the circulating blood ; and that, although 

 its action there is essential to all the metamorphoses which are there accom- 

 plished, yet all the combinations into which it actually enters, are destined to 

 immediate separation from the living body, — being, in fact, the media by which 

 all living bodies, at all periods of their existence, are continually resolving them- 

 selves into the inanimate elements from which they sprung. This principle Avill 

 be l^etter illustrated, however, by a review of the leading facts lately ascertained 

 as to the formation of the other compounds peculiar to organized bodies, and the 

 excretions of animals. 



Edinblroh, April 1346. 



