_Figure_ 1 represents the table of one of the molars taken from the upper jaw. _Figure_ 2 the table of a molar extracted from the lower jaw. The difference of size between the two organs is thus well marked, as also the difference of shape. The white line indicates the enamel, and the variegated substance which is encircled by it, denotes the comparative quantity and position of the ivory. The crusta petrosa is represented by the somewhat darker shade situated on the exterior of the enamel. In figure 1, however, the reader will observe two patches enclosed by white lines. These are the representatives of the two infundibula which exist in all of the molars of the upper jaw, but which are not developed in the lower teeth. The similar direction of the lines will enable the reader to connect them with the crusta petrosa, which substance, save in the newly-cut tooth, generally fills up these cavities, although the enamel lining extends almost to the root. The intricacy of the arrangement is, by this diagram, made apparent, and the purpose rendered plain. Without such a provision, the first meal of the animal consumed would probably shatter the instruments of mastication into innumerable atoms; but girded round, and bound up with the tough and resistant crusta petrosa, however hard may be the fibre the animal chews--however much of grit or sand may be contained in it--or however great may be the force by which the molars are pressed together, and made to pass at the same time from side to side--not a particle of the brittle enamel is fractured. Like a thin layer of glass, guarded between two pieces of wood, it performs its office in security; and as the necessity for the provision, towards which I have directed attention, is obvious, so is it the more strange that the thick coating of the crusta petrosa around the molars should by authors have been mistaken for an accumulation of tartar. The mistake was certainly extraordinary, as the crusta petrosa in the horse's tooth is of physiological importance, and will presently be shown to merit the attention of the pathologist. I am aware, when making this statement, that contradiction is offered to the opinions of many and deservedly esteemed authorities; and while I regret the necessity of differing with such writers, I also lament that my opinion as to the supposed use of the outer membrane of the newly-developed organ, does not coincide with theirs. The nature of this treatise will not allow me to enter into the subject of the development of the teeth; but I may here say, that the outer membrane has been generally stated to secrete the enamel. It is that membrane which, thickening with the growth of the tooth, becomes the crusta petrosa; but with regard to its secretive powers, I do not imagine it is endowed with any function of that description, or that it is in any way concerned in the production of the enamel. My reasons for making such