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evidenced by the definitions quoted in our paper, as given by 
various modern pathologists. 
These may be shortly summed up in such a phrase as “vital 
processes under abnormal external conditions.” 
In the present paper are considered those external abnormal 
conditions supplied by the presence of micro-organisms. 
The subject must be approached from two points of view — (1) the 
nature and function of the altering agent, represented by the micro- 
organisms ; and (2) the nature of the alterations and reactions set 
up in the elements acted upon, represented by the tissue changes. 
It has now been proved experimentally that micro-organisms act 
upon complex nitrogenous bodies, as do several classes higher in the 
organic scale, e.g., animals and insectivorous plants, by a process 
very nearly allied to true digestion. Their function from a chemi- 
cal point of view, as Frankland and others have pointed out, is 
analytical and not synthetical, and by this difference they are at 
once widely separated from the majority of the more specialised 
members of the vegetable kingdom. 
In carrying on their analytical functions the same sequence 
occurs as in the digestion of albuminoid matters in the animal 
digestive apparatus, and experiment shows that their action upon 
an insoluble albumen, such as fibrin, produces a soluble globulin 
and other bye-products identical with those evolved in tryptic 
digestion. Further, Pasteur has recently expressed the opinion, 
basing his view on numerous analogies, that perfect digestion in 
animals cannot take place without the presence of micro-organisms ; 
and Duclaux states that the process of caseation in tubercle is due 
to, or at least advances pari passu with, the production of a ferment 
which acts upon milk in the same way as pancreatic juice or 
trypsin. We may consider then, that it is after the production of 
these ferments, and after they have exerted their digestive action 
upon the tissues, that micro-organisms can obtain food material 
in an assimilable form, which they can utilise in their growth and 
development. 
The same soluble products of the ordinary processes of digestion 
are equally well adapted for the nutrition of the animal. Digestion, 
then, from this point of view, is a process of fermentation, in which 
the ferment-producers do not utilise the whole of the material on 
