11. _The crusta petrosa_ is the most externally situated, and when the tooth first appears in the mouth, it is entirely coated by this substance, having a somewhat dull and dark appearance, which is only lost when the hard crust is removed by attrition, and the enamel thereby exposed. In amount, the crusta petrosa is second to the ivory, being more in quantity than the enamel. In composition, it is characterized by containing a great amount of animal substance; and, in structure, it is peculiar for exhibiting, under the microscope, numerous corpuscles or cells, from which pores or minute tubes appear to radiate. Within the alveolar cavity, the crusta petrosa, which around the fang becomes of considerable thickness, is of a yellowish white colour; but where, in connexion with the crown of the tooth, it is exposed to the chemical action of the food and air, it presents a darker aspect and looks like an accumulation of tartar, for which indeed it has been mistaken. It fills up the infundibula of the molars of the upper jaw, and lines those of the incisors, being pierced by all the vessels which nourish the teeth. If a tooth be subjected to the action of dilute hydrochloric acid, the enamel will be removed, and the ivory and crusta petrosa be rendered separate and distinct. 12. _The enamel_ of the horse's tooth appears to be unorganized, and to contain no animal matter. Hydrochloric acid entirely dissolves it with a slight effervescence, and though a few and a very few threads remain, these rather seem to be connexions between the ivory and crusta petrosa, than component parts of the enamel itself. That the enamel is an unorganized substance, it may be essential to state, is asserted only of this body in the tooth of the horse; for the enamel of the cow's tooth is of a different nature, since hydrochloric acid does not dissolve it, affect its whiteness, or destroy its form. The enamel is the least of the three components of the tooth; of a whitish semi-transparent shining aspect, it forms a thin crust to the ivory, lying immediately under the crusta petrosa, and extending nearly to the root. 13. _The ivory_ forms the main bulk of the tooth, and though blood vessels can be traced to, but not within its substance, yet it presents numerous pores or minute canals, radiating from the centre to the circumference, which the mind naturally associates with nutrition. These pores cannot be traced to the cells or corpuscles of the crusta petrosa, though in works of high authority, drawings obviously indicating such a connexion, are presented; they are limited to the ivory, and terminate within it. What their exact use may be, is not known, but that they are of service, their presence is sufficient proof. Like the minute canals of bone, they probably convey a serous or colourless fluid, for the nutrition of the part in which they are developed. [Illustration]