We probably have more positive and reliable information concerning the action of Arsenicum than of any other remedial agent. Our knowledge concerning its action and uses, comes from homeopathic provings, thousands of cases of accidental and intentional poisoning, and from extensive experimental studies. Only a brief summary of its effects can here be given.

"A certain degree of resistance to arsenic may be produced; the reaction is, however, non-specific. The drug is fixed by the nucleus with which it enters into a very firm combination. Arsenicum kills cells and stimulates autolysis. The immunity against arsenic depends entirely upon a reduction of absorption in the intestines; the longer the drug is taken, the less appears in the urine and the more in the feces. If it be injected subcutaneously no immunity or resistance is produced." - Wells. "All arsenical combinations which are capable of reacting chemically are pharmacologically active, producing effects which in the last instance are due to the action of the anion AsO3 or AsO4." - Halsey.

The pharmacological action of Arsenicum shows that it affects especially the following tissues:

(1) Mucous Membranes

Practically all of these are involved; symptoms common to all structures of this type are: inflammation with burning pains; acrid discharges; edematous conditions due to increased permeability of the capillary walls; ulceration, and bloody discharges may ensue if larger doses have been taken over a longer period of time. Degeneration and exfoliation of intestinal epithelium may occur. In the nose a rhinitis is an early symptom of arsenical intoxication and in the throat a severe angina may develop. In general it may be said, that many of the disturbances of mucous membranes accompanying the more common systemic and local diseases, such as influenza and coryza, are closely counterfeited by the action of Arsenicum.

(2) The Stomach

In addition to the above symptoms, due to the irritating action of arsenic upon mucous surfaces, there are other effects produced in the stomach, due to the differentiated form of this tissue. Nausea and vomiting are the most important of these, and they may follow from the hypodermic use of the drug, as well as from its oral administration. Arsenicum is capable of producing all of the phenomena, pathological and symptomatic, of a severe attack of gastritis. The red and swollen mucous membrane of the stomach may contain hemorrhagic areas and its epithelial coat may undergo fatty infiltration. An intense thirst accompanies these conditions.

(3) The Large Intestines

Changes observed upon mucous surfaces in general are produced in the large intestine, but in addition to these, a few other reactions are seen. If the dose of the drug has been sufficiently large, an enteritis results. A capillary paralysis causes "a profuse transudation of serous fluid into the intestines, whose epithelium, being here and there in a state of fatty degeneration, is raised up, and with the masses of the exudate may form a pseudomembrane. A profuse watery diarrhea results, the stools containing shreds of mucous membrane and at times blood. As the mucous membrane of the intestine is directly injured as a result of the stasis, and probably in part also by the arsenic excreted through it, it is not able to resist the attacks of the bacteria to which it is constantly exposed, and parts of it succumb to a rapid destruction, so that ulcers may be formed (toxic autolysis). Necroses, therefore, are likely to be more extensive and severe, in the large intestine than in the small intestine, which contains relatively few bacteria." - Hal-sey.

The stools are painful, the rectum burns, and the typical rice water discharges of Asiatic cholera, together with the general pathology of this condition, may be so closely simulated by the action of arsenic, as to make a clinical distinction between the two conditions exceedingly difficult.

(4) The Skin

The irritating action of arsenic upon the skin results in inflammation. Its local application in paste form, causes all of the attendant effects of an acute dermatitis, with fatty degeneration of the epithelium. A severe reaction is manifested by necrosis and ulceration. These symptoms do not immediately make their appearance, since arsenic does not possess the corrosive action of the strong acids. Its local effects are more pronounced when a raw or epithelial surface is involved, than when applied to the skin. Precipitation of protoplasm does not occur, all of the reactions of the skin to the poison being due, instead, to the severe inflammation it establishes. Practically all of the manifestations of many skin diseases, may find their effects duplicated by the action of arsenic. Thus, measles, scarlatina, herpes zoster, erysipelas and eczema may resemble the different forms of skin symptoms, which arsenic is capable of producing. Dead tissue is apparently not attacked by arsenic; it seems to form combinations with living cells only. An "arsenic melanosis" is one of the rarer forms of skin manifestations produced by arsenic; Cushney states that "it seems to be due to the deposition not of an arsenical compound, but of some organic product in the deeper layers of the corium."

(5) The Circulation

The blood pressure is not appreciably influenced by ordinary doses, but a considerable fall in the pressure results from large doses, given intravenously. In chronic poisoning the drug injures the vessels, causing edema by transfusion, and this may cause a lowering of the blood pressure. Fatty degeneration of the heart and arterial system, has also been produced by the prolonged use of arsenic. During attacks of nausea and vomiting, the arterial blood pressure may appreciably fall and the pulse become small and weak. Capillary paralysis may also ensue in severe cases of poisoning, resulting in serous exudates and hemorrhage.