Fig. 435.—Fish Scale (Sole).
Of the harder outgrowths of the dermal structures, the teeth afford the chief example among animals. The rough anatomy of the tooth in mankind consists of a crown, that projects from the gum; a root, or fangs, fixed in a socket of the jawbone, and a short intermediary neck. Each tooth is supplied with an artery and nerve, and has a central cavity filled with a soft, vascular, sensitive substance, the pulp. On making a vertical section of a tooth, we recognise the several structures in the order of, pulp, crusta, petrosa, dentine, and enamel. A section through a human molar tooth (shown in [Fig. 436]) will convey some idea of the arrangement of the denser structures referred to above.
Fig. 436.—Sections of Human Molar Tooth (magnified 50 diameters). 1. Vertical section; 2. Horizontal section.
Blandin was the first to demonstrate that teeth are developed in the mucous membrane, similar to that of hair and nails. Teeth are formed in grooves of the mucous membrane, and subsequently converted into closed sacs by a process of involution, and their final adhesion to the jaw is a later process. It is very generally conceded that teeth belong to the muco-dermoid, and not to the periosteal, series of tissues; that, instead of standing in close relation to the endo-skeleton, they are part of the dermal or exo-skeleton; their true analogues being the hair, and some other epidermic appendages. Huxley proved that, although teeth are developed in two ways, they are mere varieties of the usual mode in the animal kingdom. In the first, which is typified by the mackerel and the frog, the pulp is never free, but from the first is inclosed within the capsule, seeming to sink down as fast as it grows. In the other, the pulp projects freely at one period above the surface of the mucous membrane, becoming subsequently included within a capsule formed by the involution of the latter; this occurs in the human subject. The skate offers a sort of intermediate structure.
Fig. 437.
1. Section of a cusp of the posterior molar of a child. The inner outline represents it before the addition of acetic acid—the outer afterwards, when Nasmyth’s membrane g is seen raised up in folds; f. the enamel organ; c. the dentine; the central portion being filled with pulp. 2. Edge of the pulp of a molar cusp, showing the first rudiment of the dentine, commencing in a perfectly transparent layer between the nuclei of the pulp and the membrana preformativa. 3. Nasmyth’s membrane detached from the subjacent enamel by acetic acid. 4. Stellate-cells of the enamel organ. 5. Tooth of frog, acted on by dilute hydrochloric acid, so as to dissolve out the enamel and free Nasmyth’s membrane. The structure of the dentine e is rendered indistinct. At the base, Nasmyth’s membrane is continued over the bony substance at z, in which the nuclei of the lacunæ are visible. (After Huxley.) 6. Decalcified tooth-structure; a. the dentine; b. enamel organ; c. enamel; d. Nasmyth’s membrane.
The enamel forms a continuous layer, and invests the crown of the tooth; it is thickest upon the masticating surface, and decreases towards the neck, where it usually terminates. The external surface of the enamel appears smooth, but is always marked by delicate elevations and transverse ridges, and covered by a fine membrane (Nasmyth’s membrane), containing calcareous matter. This membrane is separable after being subjected to hydrochloric acid; it then appears like a network of areolar tissue, shown in [Fig. 438], No. 6; Huxley’s “calcified membrana,” which commence at the pulp cavity, and pass up to the enamel.
