. 
: 
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OF THE DOCTRINE OF VITAL AFFINITY. 389 
matter originally introduced from vegetables is applied to the nutritive assimila- 
tion and formation of the living textures, other portions of the same elements, 
previously used in the same process, are continually yielding to the influence, 
previously resisted, of the oxygen of the air, and are forming another set of com- 
pounds, by the process of destructive assimilation, which are ready to take the 
form of crystals; which either already possess, or rapidly tend to, the composi- 
tion of the inorganic matter whence all these compounds originate ;—which are 
poisonous if retained in the body, and for which, therefore, outlets are provided 
in the organs of excretion, so as to justify the striking expression of Cuvisr, that 
all living animal matter, although the depository of force which will compel other 
matter to follow the same course as itself, will soon occupy its own place no 
longer. This is a series of chemical changes quite distinct from anything seen in 
any other circumstances of Nature. And farther, we know, in regard to the power 
exciting these, that it is obedient to certain laws of animal life, to which nothing 
analogous is seen in other chemical operations; nutrition and secretion being 
liable to sudden and important change, as living movements are, by living changes 
in nervous matter, ¢. g., by those which attend certain acts of mind; and farther, 
every such action being liable to diminution or exhaustion by the degree in which 
it is itself exercised. 
It will be observed, that all these are peculiarities in the chemical nature and 
constitution, even of the minutest particles of all living structures; and that 
without reference to them nothing living can be characterised. These phenomena 
are just as distinctly peculiar to living bodies, and characteristic of their living 
state, as the contraction of muscles, whether produced by irritations of their own 
fibres or of the nerves entering them; and they are much more general in all 
classes of living beings. If anything in the economy of living beings demands 
explanation, or is deserving of being made the object of scientific research, it must 
be these, their most essential characteristics. If they are not of suchimportance as 
to demand special investigation,—if it is only the movements of the particles 
concerned either in the development of vegetables, or the varied functions of 
animals, that we ought to regard as peculiar to living bodies,—then Physiology, 
so far as these properties of living bodies are concerned, has no claim to the title 
of a separate science, it is only a branch of Chemistry. But it is just as probable, 
a priori, that the laws of chemistry should undergo a modification in living 
bodies, as that the laws of motion should be made subordinate in certain parts 
__ of living animals, to the vital property of Irritability of muscles, as explained 
by Hatter; and the very peculiar changes which are observed in tracing the 
course of the elements of water, carbonic acid, and ammonia, which are absorbed 
by vegetables, until they pass out in the form of water, carbonic acid, and am- 
monia (or in compounds immediately resolvable into them), in the excretions of 
VOL. XX. PART III. ON 
