There is nothing more difficult in toxicology than to give a satisfactory classification of poisons, insomuch that some have fallen back on the no-classification, or natural history system, and grouped poisons as mineral, vegetable, and animal, according to their source. In despair of achieving anything better, a modification of the old and well-known system is here followed, poisons being classed as Corrosives, Irritants (Simple and Specific), and Neurotics; the latter group is, however, further subdivided, something in the same way as that adopted by Dr. Guy. The group of corrosives should comprehend all which by contact destroy the bodily textures, and so occasion death. These same substances, when diluted, may be incapable of destroying the tissues directly, but may do so by setting up inflammation; these with certain others having like effects, would form the group of simple irritants. They kill by virtue of their secondary effects on the constitution. But some substances, like arsenic, are not only capable of inducing local inflammations, with their secondary effects, but are also possessed of certain specific and well marked properties differing in each case. These are specific irritants.

Neurotics comprehend all poisons whose effects are mostly referable to the nervous system, necessarily a most diverse group, which we are not yet in a position to minutely analyze. Some, however, act mainly on the brain, some on the spinal cord, some on certain nerves only, or on the vasomotor system of nerves; some act it is hardly possible to tell how.

There was an old group of septic poisons; to this might still be referred certain noxious gases, such as sulphuretted hydrogen; or were it made to include all poisons acting directly on the blood, it would include the still more dangerous gas, carbonic oxide.

The following table exhibits these subdivisions, and some of the poisons contained in each:

Irritant poisons give rise to pain in the stomach and bowels, faintness and sickness, and purging with tenesmus. The evacuations are often tinged with blood, the pulse is feeble and irregular, and the skin cold. Many of the substances of this class from irritating the tissues with which they come in contact, produce a severe burning sensation in the mouth and oesophagus, as well as in the stomach. The degree of local destructive action produced will of course vary in proportion to the amount of the vehicle with which the noxious agent may be diluted. Irritants cause death by inducing collapse or convulsions, or by exciting severe inflammation; or, after a variable interval, by leading to stricture of the oesophagus. The diseases which most resemble the action of irritants are, malignant cholera, severe diarrhoea, colic, gastritis, enteritis, rupture of the stomach or intestines, and obstruction of the bowels, mechanical or otherwise.

The symptoms of apoplexy, epilepsy, and urÆmia bear a resemblance to those caused by some of the poisons of the neurotic class. Others give rise to delirum with spectral illusions or convulsions. Sometimes there is tetanus, sometimes coma or syncope. Diseases of the brain and spinal cord, likely to be confounded with these, are often very insidious in their progress, and hence may suddenly give rise to suspicious symptoms. The history, mode of attack, &c., will generally negative any suspicion of poisoning.