A poison is a substance which when introduced into the living body in a relatively small amount will disturb the structure or functional activity. These substances may be formed within the body through faulty metabolism and give rise to endogenous or auto-intoxication. They may result also from faulty elimination, irregular absorption, incomplete chemical transportation, or excessive glandular secretion. They may be introduced from without, exogenous intoxication. The exogenous may be (I) immediate and indiscriminate in their action or they may be (2) remote and selective.

1. The first group includes the caustics and irritants. Their effects are the more marked the greater the concentration, and may be purely local. The poison may, however, be absorbed and give rise to remote effects. In this class belong the salts of the heavy metals, a few vegetable substances, and some animal products.

The effects may vary from a slight reddening to marked necrosis and sloughing. They are brought about by abstracting the water from the tissue, by coagulating the albumins, and forming definite compounds with the elements. The effect depends on various conditions both of the individual and of the poison. If a patient has been addicted to the use of a drug, a dose fatal to others may cause in him very slight disturbance, a condition known as tolerance, and not similar to immunity. Sometimes a very large dose may cause vomiting, and the poison is in that way removed.

2. Many of the first group come secondarily into this class by being absorbed and taken up into the blood. They may unite with the hemoglobin or they may bring about hemolysis, a destruction of the red corpuscles.

When the poison combines with the hemoglobin, forming methemoglobin, the union is so close that the oxygen can no longer be taken up and supplied to the tissue. Death then results from a general asphyxia. Instead of death, cyanosis may develop, this commonly resulting from the use of coal-tar products.