The relative positions of the various structures that enter into the composition of the incisor tooth, will be better understood by reference to the accompanying wood-cut, which represents one of those members divided down its centre:--_a_ indicates the crusta petrosa, which can be traced to enclose the organ, and to dip down the infundibulum B. The crusta petrosa is shown to be thicker at the root of the fang, and at the base of the infundibulum, than at other parts. _b_ denotes the ivory, which forms the principal and central portion of the tooth, in the middle of which is the darker space marked by the letter _d_. This last is intended to represent the cavity of the pulp, which, in the young incisor is, as in the diagram, of great length. Between the ivory _b_, and the crusta petrosa _a_, is a white line _c_, which indicates the position of the enamel, and will be seen to cover the upper surface of the tooth dipping into the infundibulum, but not to extend quite to the root. 14. _The uses of the ivory, enamel, and crusta petrosa_ must be now noticed. The ivory is less dense than the enamel, and harder than the crusta petrosa. On the external surface of the incisor teeth, the crusta petrosa is, by the attrition to which this part must be subjected during the gathering of the food, soon sufficiently removed to expose a portion of the enamel: as the years of the animal increase, the outer coating is almost worn away, and not being reproduced, little of the crusta petrosa will be found on the nippers of very old horses. The ivory, however, is always nearly on a level with the enamel, notwithstanding the greater attrition which the substance forming the principal portion of the table of the tooth must necessarily endure. A sufficient indentation of the ivory, nevertheless, can be observed, to render prominent the ridge of enamel, and to indicate that the last-named material is endowed with the greatest power of resistance. The enamel, in fact, is as hard as flint, and by striking it against a steel, fire can be drawn forth. The three structures, therefore, vary in hardness, and in an opposite direction they contrast to one another in toughness. The ivory is sometimes fractured, but not frequently. I have never seen the crusta petrosa of a living tooth exhibit such an injury; but the enamel is rarely inspected without its being discovered to be more or less in a ragged, chipped, and broken condition, especially at the anterior edge of the table of the incisors.