Leaving for the present the question as to the nature of pain, let us examine some of the modes in which it expresses itself; and as far as practicable I will limit myself to the various pains about the head, for all the varieties are there manifested. The first point which strikes every observing man is the difference of individuals in their susceptibility to pain. It is not merely or even mainly a question of the amount of courage of the patient in bearing pain, but it is far more a question of inherited or acquired sensitiveness. The same amount of injury, as nearly as we can judge, in two differently organized individuals will produce extremely differing degrees of pain. In general it may be stated that the unduly susceptible individual has either inherited a weak nervous constitution as regards pain, or else that some depressing agency has lowered his power of resistance. When I speak of a weak nervous constitution as regards pain, I do not mean that it need be a generally weak physique. Perhaps a more happy word would be unstable. You remember the physicists talk of bodies being in stable equilibrium when after a disturbance they tend to return to their bottom, or centre of gravity; while unstable equilibrium is that state where a little shove off the centre, results in a big tumble. Now, the people who are markedly susceptible to pain, who have recurrences of it, may be said to have a nervous system in a state of unstable equilibrium. In other respects these same individuals may be splendid types of muscular or mental development. The same condition holds good with pain’s first cousin, muscular spasm. The analogue to the sensory crisis of attacks of neuralgia is seen in the muscular convulsions of attacks of epilepsy. And yet some of the greatest men of the world’s history in mental vigor have been epileptics, notably Napoleon Buonaparte and Julius Cæsar. Although at first we may not be able to see any outward manifestation of such attacks of pain as I have spoken of, if they recur sufficiently often they are sure to leave their traces behind. If we prosecute our inquiries in the other direction, to find what has predisposed our patient to recurrences of pain, we find in a large number of cases that his immediate progenitors have suffered from similar or allied manifestations. By allied manifestations I mean such other nervous diseases as epilepsy or chorea (St. Vitus’ dance), or insanity. Moreover, there is one predisposing cause that I believe to have quite peculiar efficacy, and that is the tendency to phthisis. Again and again I have verified the truth that where a member of a tubercular family escapes consumption, he is extraordinarily liable to develop one of the graver neuroses, preferably recurrent attacks of pain.