Now, the first point we may consider settled, as to the mode in which pain expresses itself is in an inherited susceptibility, a lessened power of resistance, and this can only reside in the central nervous system. But, as we have already said, the lessened power of resistance may be acquired, it need not be inherited. Without stopping to dwell very long on this part of our subject, it will suffice to enumerate one or two of the principal efficient agents. And the first and far the most important of these is malnutrition of the nerve tissues, whether accompanied by the signs of anæmia and general constitutional malnutrition or not, the main cause being our civilization, with its excessive nervous wear and tear, no less in the educational period than in the intense competition of mature life. No more striking verification of this fact is needed than the results obtained in the relief of pain by physiological rest, by systematic feeding, especially of certain kinds of food, particularly fatty food. It is the general rule that in these cases there is either an indisposition to take sufficient food, or else that certain necessary ingredients are omitted owing to the patient’s repugnance. In the familiar example of sick-headache, or migraine, the patient invariably ascribes his condition to a disordered stomach, and scrupulously avoids such foods as eggs and milk and fat, which he will tell you always make him bilious. It is the hardest thing in the world to convince him that he has put the cart before the horse, and that the real fact is that the nervous trouble, the neurosis of the ophthalmic division of the fifth, is the cause and not the effect of the gastric disturbance. I am convinced that much of the suffering in the dental branches of the fifth nerve can similarly be traced to the nervous malnutrition of insufficient food, and, in addition, the local condition of the teeth is pathologically influenced by their not getting their proper physiological stimulus in the quantity or character of the food to be chewed.