sentences:  Emetics can be divided into two categories, those which produce their effect by acting on the vomiting centre in the medulla, and those which act directly on the stomach itself.Some emetics, such as ipecac, fall into both categories; they initially act directly on the stomach, while their further and more vigorous effect occurs by stimulation of the medullary centre.Salt water and mustard water, which act directly on the stomach, have been used since ancient times as emetics.