In the case of phosphorus, I think it is now pretty generally believed that its poison has very little effect in the mouth unless there exist a precedent caries of a tooth or its socket. These facts almost suffice to take these agents out of the group of constitutionally acting into that of peripherally irritant causes. In this class of agents, as in the preceding one, the first indication in treatment is the complete removal of the sufferer from their baneful influence. We have now briefly reviewed the main agencies which act constitutionally in the production of pain. It is apparent, to recur to our simple illustration, that they must have their main efficiency in the action they have on the central cell, and not on any modification of the impulses sent to that cell. It cannot be denied that in rare instances these various agents are productive of pain referred to a particular nerve, when we cannot find anything in the nerve itself or in the tissues supplied by it to account for the morbid manifestation. We are, therefore, constrained to believe, at least for the present, that morbid manifestations, sensations of pain, may originate in the cell itself and thence be referred outward. But I would remind you that the whole tendency of modern medical thought is to more accurately localize the starting point of disease, and to circumscribe the area of cases in which such outward cause of disease is unknown. So long as men were satisfied to cover up their ignorance in such vague phrases as “humors of the blood,” “rheumatic diathesis,” etc., etc., few were tempted to carefully examine the local conditions for an explanation. But the last fifty years have seen an enormous change in our attitude of mind to these problems. It is a change which is one of the greatest in the history of the human mind. And while I do not for a moment wish to underrate the great importance of a due regard to the constitutional causes of pain, especially of the malnutrition of the nerve cells, I believe that in the main they must be classed as predisposing causes and not as efficient ones. When we come to the question of why pain is located or referred to a particular nerve, I believe the answer in the overwhelming majority of cases will be because there is some peripheral abnormality in that nerve or in some other nerve with which it is intimately associated; for we have to recognize in the philosophy of pain the same fact that we do in the philosophy of the human mind, namely, that our ideas are so closely associated that one thought will almost necessarily suggest another. Just as, if we have always been accustomed to see Smith and Jones together, we can hardly think of Smith without Jones also putting his nose in; so in feeling sensations, certain ones get so closely intertwined that one will almost inevitably causes the other. This, then, leads us naturally to the second great division of our subject, and that is the influence of peripheral