188 DR ALISON ON THE PRINCIPLE OF VITAL AFFINITY. 
compounds, in which the characteristic predominance of carbon is not perceived ; 
because they are those which are formed in circumstances where 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. 
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 affini- 
ties. Although taken into the interior of every living being, it appears to com- 
port itself there almost, if not entirely, as it does in acting on dead matter. The 
expression of Lizpiac, that the action of the oxygen of the air in living bodies is des- 
iructive, 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 novital 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 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 Extis, 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 will 
be better 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. 
Epinsureu, April 1846. 

