The most obvious and at the same time the most simple and practical classification of causes of disease is the division into Predisposing and Exciting. This classification includes at. once nearly all that can be said on the subject of causation. Various subdivisions are for convenience employed, but they all relate as a matter of necessity to the two primary divisions, for example, extrinsic and intrinsic causes, in other words, causes acting from without and those which arise within the animal body and which may either increase the tendency to acquire the disease or may actually cause its development. Specific causes occupy a position which in some degree disconnects them from ordinary causes whether predisposing or exciting. The term specific indicates that there is something peculiar to be taken into account, and at the present day the term is limited to those causes of disease which include some particular micro-organism or some specific poison, which may be either extrinsic or intrinsic.



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