398 DR ALISON’S OBSERVATIONS ON 
gen, which they give up in their passage through the capillary vessels. Here the 
current of oxygen meets with the compounds produced by the transformation of the 
tissues, and combines with their carbon to form carbonic acid, and with their hy- 
drogen to form water.”—(Animal Chemistry, p. 60.) But neither author has 
stated as clearly as I think may be done, on what principle it is that the oxygen 
changes its mode of action when it meets with these products of the transforma- 
tion of the tissues ; or, in simpler language, with the matters that have been ab- 
sorbed from the living tissues. I believe the true reason to be, that this is an ex- 
emplification cf a general principle of essential importance, which has been par- 
tially stated, but never, so far as I know, fully developed, viz., that all vital 
affinities are of transient duration only ; and that those which actuate the matter 
of animal bodies especially, soon fail of efficacy, and at the temperature, and 
under the other conditions there present, give place to simply chemical affinities, 
which determine the formation of a very different set of compounds ; therefore, 
that as long as the oxygen is passing along the arteries, and is in contact with 
albuminous matter, to which vital properties have been recently communicated, 
and which are actuated by vital affinities, it has little power to affect them ; but 
when it meets with the same compounds in the substance of the textures, or 
already absorbed from them, 2. ¢., with albuminous or other animal matter, which, 
according to the expression often, but vaguely, used, has become effete, or has lost 
its vital properties, it can act on them in the living body in like manner as it 
does, at the same temperature, in the dead body. 
But, in order to establish this point, it is necessary to enter on the second 
part of our inquiry into the chemical changes of animal bodies, 7. ¢., the pecu- 
liarities of the Excretions ; jirst, of the greatest and most general of all the excre- 
tions from living bodies, the carbonic acid thrown off from the respiratory organs, 
both of animals and plants, of which Dr Prout says, that “ the precise use of its 
constant evolution we know not,”—and then, of the other excretions from animal 
bodies. Until we have precise knowledge of the purpose which is served, and of 
the laws which are obeyed, by the matters which are continually expelled from 
living bodies, it is obvious that our notions in regard to vital affinities must be 
very unsatisfactory. In entering on this subject, I assume it as ascertained that 
all the matters, peculiar to the excretions from the living body, pre-exist in the 
blood, and are only eliminated from the blood at the organs where they appear; 
so that any chemical changes necessary for their formation, take place either in 
the cells of the textures, or in the circulating blood, or both, not in the glands 
which separate them, at least not externally to the vessels of those glands. 
The first idea that must occur to every one who considers that large quantities 
of extraneous matter enter into every living body, different from those that can be 
traced in any of its textures, is, that the excretions from living bodies are simply ~ 
those portions of the ingesta which are not applied to the maintenance of the or- 

