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 It would be even more fruitless to attempt an arrangement of poisons according to their medium of action; for no sure criterion is known, by which a poison acting through direct transmission of an impulse along the nerves can be distinguished from one that acts by entering the blood.

Neither is the embarrassment of the toxicologist materially less, if he attempts to classify poisons according to the symptoms they induce in man. This is the principle now generally followed, and which in common with others I shall pursue. But the reader will be at no loss to discover that the partitions which separate the classes are exceedingly slight, and that very many poisons might be arranged without impropriety in either of two classes.

The preceding statements show the impossibility of founding a good system of arrangement on the only basis which can be acknowledged philosophical and practical; and consequently, that, as the science of toxicology now stands, we must altogether despair of forming one that shall be even moderately satisfactory.

On the whole I see no reason for deviating from the classification adopted in the first edition of the present work, being a modification of that previously followed by Professor Orfila. In this classification poisons are divided into irritants, narcotics, and narcotic-acrids.

The class of irritants includes all poisons whose sole or predominating symptoms are those of irritation or inflammation; the narcotics those which produce stupor, delirium, spasms, paralysis, and other affections of the brain and nervous system; and the narcotico-acrids those which cause sometimes irritation, sometimes narcotism, sometimes both together. Some writers still adopt a fourth class, called septics, because they give rise to putrefaction in the living body. But modern physiology will scarcely sanction the continuance of such a class of poisons. For assuredly no substance can cause putrefaction in the living body.

CHAPTER II.

CLASS FIRST.

ON IRRITANT POISONS GENERALLY.

The class of irritant poisons comprehends all whose sole or predominant action consists in exciting irritation or inflammation. That is, it comprises both those which have a purely local, irritating action, and likewise many which also act remotely, but whose most prominent feature of action still is the inflammation they excite wherever they are applied.

This subject will be introduced with an account of the general symptoms and morbid appearances caused by the irritants, and a comparison of these with the symptoms and morbid appearances of the natural diseases which are chiefly liable to be confounded with irritant poisoning, or mistaken for it.