I confess that strikes me as a very excellent description of what surgeons usually term caries, namely, a molecular death of bone tissue. Nor can I see any difference in the essential nature of the two processes, if you make due allowance for the morphological modifications of tooth structure from bone structure, and the different environment under which the process takes place. If I were asked to define dental caries, I should say it was a molecular death of the tooth structures, especially the dentine, due to the action of micro-organisms; that in the course of the process lactic or other acid is developed, which decalcifies the teeth, is doubtless true, but the very presence of acid fermentation in a normally alkaline cavity necessitates the assumption of the action of micro-organisms. I would remind you that the conditions favorable to the activity of such organisms are all apt to be present. They are: 1st. The presence of the micro-organism. 2d. The existence of a suitable pabulum. 3d. A certain degree of moisture. 4th. A certain degree of warmth. 5th. A certain amount of oxygen or air. 6th. A lessening of the resisting power of the tissues affected, as compared with health. All of these first five conditions are notoriously present in the mouth. That we do not oftener suffer from their effects is due to the absence of our sixth element, the lessening of the resistance of the tissues. Thus, in health, those organisms which flourish best in an acid secretion have their baneful activity held in check by the alkaline saliva as well as the resisting power of the dense enamel. But once let the alkalinity of the saliva be lessened, or changed to an acid reaction, or let ever so small an abrasion occur in the enamel, and the myriads of these agents find a foothold for starting the morbid train of symptoms. Similarly, even in the tissues themselves, the enamel, no less than the dentine, suffers from those predisposing causes of constitutional malnutrition, which are so important in their effect on the central cell, and which act in lessening the normal resistance of the periphery.