328 DR ALISON'S OBSERVATIONS ON 



sen, ■w'hich they give up in theii' passage through the capillary vessels. Here the 

 current of oxygen meets ivUh the compoumls 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 fi'om the living tissues. I believe the true reason to be, that this is an ex- 

 emplification of a general principle of essential importance, which has been par- 

 tially stated, but never, so fai- 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 

 imder the otlier 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 commimicated, 

 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 ft-om them, i. e., 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, i. e., the pecu- 

 liarities of the Excretions ; first, 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 Pkout says, that " the precise use of its 

 constant evolution Ave 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 

 tliose portions of the ingesta which are not applied to the maintenance of the or- 



