One of the most essential characteristics of living material—indeed, according to many, its most essential characteristic—is the fact that it is irritable. A living cell responds to sudden environmental changes, and typically a cell of a given sort responds in a definite and particular way no matter what the nature of the stimulation. Thus when a muscle cell is exposed to sudden heat, to sudden cold, to a sharp mechanical impact, to ultraviolet radiation, or to an electric current, it shortens. (...) Similarly, a nerve fiber responds in its own characteristic way, and this response is independent of the nature of the stimulation. In general, mechanical impacts, ultraviolet radiation, electric shocks, and certain types of chemical substances are stimulating agents for many diverse types of protoplasm and they serve to arouse it to activity. The response of protoplasm to stimulation may be prevented by anesthetics. These are substances which nullify the effect of a stimulus without killing or seriously injuring the protoplasm. The remarkable fact is that there are general anesthetics, substances which in much the same concentration can prevent protoplasmic activity in widely different protoplasmic systems. Many fat solvents, such as ether, are general anesthetics. Magnesium salts also behave as anesthetics for many different types of cells. (shrink)