odontoblast is a somewhat pyramidal, nucleated cell, giving off from its base a dentinal fibre, and continuous with a nerve fibril at its apex. The bases of these cells are closely applied to the inner surface of the dentine. The dental canal, running longitudinally in the bone of the jaw beneath the alveoli for the teeth, contains the branches of the dental artery, vein, and nerve (ƒ), with connective tissue and lymphatic channels. The artery, vein, and nerve, all send branches upwards, which enter the pulp cavity of each tooth.

DEVELOPMENT OF THE TEETH.

The teeth are developed in two divisions: (1,) the temporary or milk teeth; (2,) the permanent teeth, the latter replacing the former.

The temporary teeth are developed in connection with primary down-growths of the epithelium of the foetal jaw into the connective tissue beneath. The permanent are developed in connection with secondary growths from the necks of the primary ones.* Each tooth owes its enamel to the epiblast; its dentine, crusta petrosa, and pulp to the mesoblast.

The milk teeth begin to develop during intra-uterine life. The gum at this time consists of a layer of stratified, squamous epithelium, with connective tissue below it.

The first stage in the development of the tooth consists in a thickening of the epithelium along a line extending the whole length of the alveolus of the jaw-a line which is therefore somewhat horse-shoe shaped. The thickening above, i.e., that seen from the mouth, is termed the dental ridge; that encroaching on the connective tissue below, the common enamel germ. These two projections, above and below respectively, are seen in Fig. 105, 1. It will be understood that the section

* Or from the common enamel germ in close proximity to the neck. It should be noted, however, that the three permanent molars are developed independently from a backward prolongation of the common enamel germ, unconnected (in vertical transverse section) with the epithelium of the gum.

The first down-growth of epithelium becomes divided into two, of which the outer has been named the labio-dental strand, and the inner, the dental lamina, or common dental germ. It is the latter from which the enamel of the teeth is developed, and which is here termed the common enamel germ.

from which the figure is taken is vertical, and transverse to the line of thickening. It is of the greatest importance to realise distinctly the position of this line. It might be represented by supposing the points of eruption of the teeth along the edge of the alveolus, united by a line passing through each of them.

Let us consider for a moment the nature of the epithelium in this region. It consists of a stratum corneum at the surface, which, however, is not always well marked, and a stratum Malpighii beneath it; the latter consisting of more or less polygonal cells, the lowest of which are smaller and more cubical or columnar, and constitute the germinal layer. The common enamel germ consists of a down-growth of both these kinds of cells-namely, the ordinary cells of the Malpighian stratum, and the layer of cubical germinal cells investing the others below.

The second stage in the development of the tooth is shown in Fig. 105, 2, and consists in the formation of the special enamel germ. This consists of a renewed down-growth of epithelium in the form of a series of flask-shaped bodies from the common enamel germ. These flask-shaped down-growths occur only, however, where the teeth are to be developed. This downgrowth is therefore an interrupted, and not a continuous one, as contrasted with that of the common enamel germ. As shown in the figure, each special enamel germ consists of two kinds of cells in the centre, the ordinary polygonal ones of the stratum Malpighii; and surrounding them, the cubical cells, continuous with the germinal layer. Each enamel germ possesses a body and a more or less constricted neck, from the latter of which a small projection may be seen the rudiment of the permanent tooth (e). The fibrous tissue surrounding the special enamel germ tends to become arranged in a laminated manner conformable to its surface. This laminated tissue, more marked in subsequent stages, is called the sac of the developing tooth.

The third stage in the development of the tooth is shown at Fig. 105, 3, in which the commencement of a connective tissue papilla, and a corresponding invagination of the enamel germ in its lower part, is seen. With the exception of being invaginated from below, however, the enamel germ remains in much the same condition as in the previous stage.

The fourth stage is characterised by a further development of the connective tissue papilla (the future pulp) with a correspondingly increased invagination of the flask (Fig. 105, 4). But the central cells of the invaginated flask have now undergone a certain change of a degenerative type. They have become branched, mucoid cells, forming a more or less. delicate network by the anastomosis of their branches. This stage is shown also in Fig. 106; though there the developing tooth is a permanent and not a temporary one. The structure of the enamel organ, as it is now called, is the same in either case. It should be especially noted that it is only the central cells which thus degenerate, with the result that between the central degenerated cells and the peripheral cubical cells (continuous above with the germinal layer), we have a narrow layer left of undifferentiated epithelium. Starting from the centre of the enamel organ and passing to the periphery, we thus have at this stage, three layers of cells-degenerated, undifferentiated, and cubical (or columnar). But a section taken through the entire thickness of the enamel organ at any point will show five: (1,) cubical; (2,) undifferentiated; (3,) degenerated; (4,) undifferentiated; (5,) cubical.

The papilla is composed of embryonic tissue, in which the fibrous or formed element is scanty, and the cellular predominates. In fact it consists almost entirely of small, round, or oval cells, with large nuclei and little peri-nuclear protoplasm.

The fifth stage is represented in Fig. 107. It is associated with the commencement of the formation of the enamel and dentine.

The inner layer of cubical cells of the enamel organ, i.e., those immediately surrounding the connective tissue papilla, is the functionally active part of the enamel organ. These cells increase in length till they form a very regular layer of palisades, the nuclei remaining in their external part (b). The rest of the organ gradually atrophies, and remains as a thin cap to the tooth (Nasmyth's membrane) for a short time, but finally disappears. The figure shows the increase in the length of the inner cubical cells to form what are now termed enamel cells, and the commencing atrophy of the rest of the organ.

But when this stage has been reached in connection with the enamel organ, the connective tissue papilla has also become