I 



THE PRINCIPLE OF VITAL AFFINITY. 331 



excluded from the new combinations which are foiined in a living body, or re- 

 jected from the selections which are there made. 



Now, if we consider it as ascertained, that a part of all the aliments taken 

 into a living animal body, combines immediately with the oxygen of the air, in 

 the blood, and is thrown off by the excretions in the form of water, carbonic acid, 

 and ammonia, — or in forms which tend towards, and quickly resolve themselves 

 into, these compounds,— we see a distinct confirmation of what was formerly 

 stated, as to the nature of vital aflBnity, viz., that it does not, properly speaking, 

 supersede ordinary chemical affinities, but is merely superadded to them ; so that 

 chemical compounds, taken into animal bodies, are subjected to these attractions 

 as well as others, and are divided between the substances thus acting upon them, 

 in proportions varying probably, as in other cases, according to the strength of the 

 affinities and the quantities of matter exerting them. This, indeed, appears suf- 

 ficiently demonstrated by the effect of exercise (already considered) on the excre- 

 tions by the skin and lungs, on the one hand, and on the deposition of fat or of 

 albuminous compounds, on the other ; we know, that, as the quantity of carbonic 

 acid and water thrown off are increased by that cause, the quantity of fat de- 

 posited from the blood is diminished, — implying that, by the increased quantity 

 of oxygen presented to them by the blood, portions of the carbon and hydrogen 

 of the ingesta, which would otherwise have been subjected to the vital affinity 

 which forms fat, have yielded to the simply chemical affinity which disposes them 

 to unite with oxygen and pass off; and again, it is at least highly probable, that, 

 under this increased supply of oxygen, increasing, by a simply chemical attrac- 

 tion, the proportion of carbon and hydrogen which escape from the ingesta, the 

 effect of the vital affinity by which the remaining elements of the ingesta combine 

 to form albuminous matter, is likewise increased. 



But we have next to consider the evidence for the existence, and the object 

 and importance of another and totally distinct source, long believed to contribute 

 to the formation of the excretions, viz., matter which has formed part of the tex- 

 tures of the living body, and been re-absorbed from them., with the intention of 

 being thrown out of the body ; i. e., the dependence of excretion on what Dr Prout 

 calls " destructive assimilation." 



The mixture, of this matter with the blood appears to be necessary for all 

 the changes there, from which the different excreted fluids result ; or, it may be 

 supposed not merely to escape itself, but to act as a ferment, promoting these 

 changes, and thereby determining the entrance into these combinations, and the 

 expulsion from the body, of the portions of the ingesta which are not required 

 for nutrition. 



The term efete matter has been very generally employed in discussions on 

 this subject ; but it does not appear to me, that any very definite idea has been 

 annexed to the term, nor that any principle has been pointed out to explain how 



