This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol2", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
Emetics are medicines which cause vomiting, in certain doses, as an ordinary result, and in the healthy state of the stomach. A large quantity of almost any liquid may provoke vomiting by over-distension of the stomach. The nauseous taste of certain substances, or offensive associations in relation to them in the mind of the patient, or the idiosyncrasy of particular individuals, may occasionally lead to the same result from medicines which ordinarily have no such effect. Lastly, in an irritated state of the stomach, almost anything introduced into it will give rise to vomiting, sometimes even a teaspoonful of water. Substances, however, acting in these several methods, are not considered as emetics; and it is thus obvious, that each of the conditions mentioned in the definition is essential.