The medicinal plants used as sources for extracts or pure products for therapeutic use represent a rapidly expanding area. Many traditional remedies are in therapeutic use, including drugs as such, or extracts prepared in accordance with the pharmacopeia of the country where they are used or according to the requirements of use in the pharmaceutical forms for which they are intended. Many fresh drugs in the course of crushing undergo numerous chemical changes, hydrolyses, or oxidations, related to the release of enzymes from the plant cells. There are machines for prechopping or mineing drugs and grinding machines proper. Movement of the liquid can be ensured by keeping the drug and cycling the liquid with pumps or with mechanical agitators, which mix solvent and drug continuously, or by means of ultrasound waves. In the percolation process, as already mentioned, the drug must as a rule is moistened with the extraction solvent before being placed in the percolator.