MULL UCCAPUIVZI fill up osteo-dentine, the result of a modified fication of the dentinal pulp ; and the grown tooth presents three tissues, as shown in fig. 547., in which c is the thick external cement, d the hard dentine, and o the osteo dentine ; sometimes developed in loose stalac titic-shaped nodules.
In the teeth of the Sloth, and its great extinct congener, the Megatherium, the hard dentine is reduced to a thin layer, and the chief bulk of the tooth is made up of a central body of vaso-dentine, and a thick ex ternal crust of cement. Fig. 548. represents a called them, therefore, simple teeth ; but their crowns are originally, and their fangs are always, covered by a thin coat of cement. There is also commonly a small central tract of osteo-dentine in old teeth.
In fig. 7, pl. 122, of my Odontography is given a longitudinal section of a human molar tooth, in which d is the dentine, e the enamel, and c the cement.
The teeth, called by Cuvier compound or complex in Mammalia, differ, as regards their composition, from the preceding, only by the different proportion and disposition of the constituent tissues. Fig. 549. is a longitudinal section of the incisor of a horse ; d is the dentin; e the enamel, and c the cement ; c' is the layer of cement reflected into the deep central depression of the crown ; and s is the coloured mass of tartar and particles of food which fills up that cavity, forming the "mark" of the horse-dealer. The characteristic struc ture of the three tissues is shown in the magnified part of the section,fig..550.
longitudinal section of a lower molar of the Megatherium, of half the natural size : v is the vaso-dentine, d is the, hard dentine, and c is the cement ; p is the base of the wide per sistent pulp-cavity.
The hard dentine is, of course, the firmest tissue of a tooth so composed, and forms the crest of the transverse ridges of the grinding surface, like the enamel plates in the elephant's grinder. It has, consequently, been described to be enamel*, but its relation to that tissue is only one of analogy or function.
The human teeth, and those of the carni• vorous mammals, appear at first sight to be composed of dentine and enamel only, as they were described to be by the Cuviers t, who A very complex tooth may be formed out of two tissues by the way in which these may be interblended, as the result of an original complex disposition of the constituents of the dental matrix.
Certain fishes, and a singular family of gigantic extinct Batrachians, which I have called " Labyrinthodonts,"* exhibit, as the name implies, a remarkable instance of this kind of complexity. Fig. 5.51. is a view of a canine tooth of the Labyinthodon salaman droYdes, of the natural size : and fig. 552. is a slightly magnified view of a transverse section across the part of the crown marked a. At first view, the tooth appears to be of the simple conical kind, with the exterior surface merely striated longitudinally, but every streak is a fissure into which the very thin external layer of cement (c) is reflected into the body of the tooth, following the sinuous wavings of the lobes of dentine (d), which diverge from the central pulp-cavity, a.
The inflected fold of cement c runs straight for about half a line, and then becomes wavy, the waves rapidly increasing in breadth as they recede from the periphery of the tooth ; the first two, three, or four undulations are simple ; then their contour itself becomes broken by smaller or secondary waves ; these become stronger as the fold approaches the centre of the tooth, when it increases in thickness, and finally terminates by a slight dilatation or loop close to the pulp-cavity, from which the free margin of the inflected fold of cement is separated by an extremely thin layer of dentine. The number of the inflected converging folds of dentine is about fifty at the middle of the crown of the tooth figured, but is greater at the base. All the inflected folds of cement at the base of the tooth have the same complicated disposition with increased extent; but, as they approach their termination towards the upper part of the tooth, they also gradually diminish in breadth, and consequently penetrate to a less distance into the substance of the tooth. Hence, in such a section as is delineated (fig. 552.), it will be observed that some of the convoluted folds, as those marked cc, extend near to the centre of the tooth; others, as those marked c', reach only about halfway to the centre ; and those folds, c", which, to use a geological expression, are " cropping out," penetrate to a very short distance into the dentine, and resemble, in their extent and simplicity, the converging folds of cement in the fangs of the tooth of the Ichthyosnurus.