Irritability and contractility are general properties of the organism. These properties are illustrated by the irritability of an _Amœba_ or _Paramœcium_ to stimuli of many kinds; by the movements of the pseudopodia of the former animal, or of the cilia of the latter; by the nervous irritability of the higher animal, and the contraction of its muscles when they are stimulated. They are among the fundamental properties or functions of living protoplasm, and their study is of paramount interest, and carries us to the very centre of the problem of the activities of the organism. Naturally physiologists have never ceased to attempt to describe irritability and contractility in terms of physics, but though we may be quite certain that the things that do occur in these phenomena are controlled physico-chemical reactions, it must be remembered that what we positively know about their precise nature is exceedingly little. What is the nature of a nervous impulse? When a receptor organ is stimulated, as, for instance, when light impinges on the cone cells of the retina, or when the nerve-endings in a “heat-spot” in the skin are warmed, or when the wires conveying an electric current are laid on a naked nerve, an impulse is set up in the nerve proceeding from the place stimulated, and we must suppose that approximately the same amount of energy moves along the nerve as was communicated to the receptor or the nerve itself by a stimulus of minimal strength. How does it so move? Several facts of capital importance result from the experimental work. (1) The impulse travels with a velocity variable within certain limits, say from 8 to 30 metres per second; (2) it travels faster if the temperature is raised (up to a certain limit); (3) it is difficult to demonstrate that the passage of this impulse is accompanied by definite chemical changes in the nerve substance: it is stated that carbon dioxide is produced, but this is not certainly proved; (4) an electric current is produced in the nerve as the result of stimulation; (5) no heat is produced, or at least the rise of temperature, if it occurs, is less than 0.0002° C.