Many definitions of pain have, very unnecessarily, been offered. Some are even tautological, others imply a hypothesis as to its nature by relating it to strong stimulations.[11] Let us regard it as an internal state which every one knows by experience, and of which consciousness reveals innumerable modes, but which by its generality and its multiplicity of aspect escapes definition. ----- Footnote 11: “Pain is a powerful and prolonged vibration of the conscious nervous centres, resulting from a strong peripheral excitation, and consequently of a sudden change of condition in the nervous centres” (Richet). “It is the most violent stimulation of certain sensorial regions—a stimulation to which contribute the more extended stimulations of other regions” (Wundt). ----- In its primitive form pain is always physical, that is to say, connected with external or internal sensations. Sufficiently precise as regards superficial parts of the body, especially the skin, its localisation is vaguer when it is seated in the deeper parts, the viscera, the instruments of organic life. In the last case, when the pain is of internal and non-peripheral origin, coming from the great sympathetic or the related vagus nerve, it is accompanied by a state of anxiety, of depression, or of anguish, which we shall often encounter, and which frequently causes it to be said that “it seems to the patient that the workings of nature within him are suspended.” For the present, without distinguishing between these two origins, external and internal, we will study the _objective_ characters of physical pain taken in general: first its anatomical and physiological conditions, then the bodily modifications which accompany it and in popular language are called its effects. I. The transmission of painful impressions from the periphery to the cortical centres is far from being determined in all the stages of its course.