Dentistry

Surrounded by the dentine is the "pulp cavity," which is filled by the tooth pulp, a highly vascu lar and nervous mass of branched connective tissue cells, which, in a young tooth, has a layer of epithelial cells, the "odontoblasts," lying close against the wall of the cavity and forming new dentine. Slender processes ("Tomes's fibrils") project from these cells into the dentinal tubes and are probably sensory. A nerve and artery enter the apex of the root of the tooth, but it is not understood how the nerve ends.

Surrounding the dentine where it is not covered by enamel is the "cement" or "crusta petrosa," a thin layer of bone which is only separated from the bony socket by the alveolar periosteum.

Embryology.—The lip is marked off in the mouth by a "lip groove," which, in the case of the lower jaw, grows obliquely downward and backward, and the mass of ectodermal cells bound ing it penetrates for some distance into the surrounding mesoderm below the bottom of the groove. This is known as the "tooth band." On the under surface of this oblique tooth band (still taking the lower jaw), and close to its edge, appear ten thickenings, below each of which the mesoderm rises up into a "dental papilla," and so moulds the thickening into a cap for itself—the "enamel organ." The superficial cells of the dental papilla become the "odontoblasts" and manufacture the dentine, while those cells of the cap (enamel organ) which are on its concave surface and therefore nearest the dental papilla are called "ameloblasts," and form the enamel. The cutting or grinding part of the tooth is first formed, and the crown gradually closes round the dental papilla, so that at last, when the root is formed, the central part of the papilla remains as the pulp cavity surrounded by dentine except at the apex of the root. The roots, however, are formed slowly, and as a rule are not complete until some time after the tooth is cut. The mesoblastic connective tissue surrounding the developing tooth becomes condensed into a fibrous bag called the tooth-sac, and round this the lower jaw grows to form the alveolus. The crusta petrosa covering the root is developed from the tooth-sac. Hence of the various structures which make up a tooth, the en amel is derived from the ectoderm, while the dentine, pulp and crusta petrosa or cement are mesodermal.

Comparative Anatomy.

The details of the teeth vary so greatly in different animals, and, on account of their being the most durable tissues of the body, are so important for classifica tory purposes, that they are dealt with freely in the various zoological articles. All that can be done here is to give a broad

general survey, taking the details of man's dentition as a point of departure.

In some fishes the teeth are continuous over the edges of the jaws with the scales on the surface of the body, and there is no doubt that teeth should be regarded as modified scales which have migrated into the mouth.

In

the Cyclostomata (lampreys and hags) the teeth are horny cones, but beneath them there are papillae of the mesoderm cov ered with ectoderm which resemble the dental papillae and enamel organs although no calcification occurs except in Bdellostoma. In the Elasmobranchii (cartilaginous fishes) the teeth are arranged in several rows, and as those of the front row fall out the hinder row take their place ; sometimes they are triangular and very sharp as in the sharks, sometimes flattened and arranged like a pavement for crushing as in rays. These teeth only represent the crowns of man's teeth, and they are not embedded in sockets except the teeth in the saw of the saw-fish (Pristis), moreover the dentine of which they are largely composed resembles bone and fills up the whole pulp cavity. From its structure it is known as osteodentine.

In the Teleostomi (teleostean and ganoid fishes) there is great variability; sometimes, as in the sturgeon, there are no teeth at all, while in others every bone bounding the mouth, including the branchial arches, bears teeth. As an example of a very full tooth armature the pike's mouth and pharynx may be instanced. Both in the pike and the hake hinged teeth occur; these bend backward during the passage of prey down the throat, but are re-erected by elastic ligaments. As a rule, the dentine of the Teleostomi is osteodentine, but sometimes, as in the hake, it is vascular and is known as vasodentine.

In the Amphibia teeth are not so numerous as in the fishes, though like them they are not confined to the jaws, since vomerine teeth are very constant. The toad is edentulous, while the frog has no teeth in the lower jaw. An extinct order of tailed am phibians, the Stegocephali, are often called labyrinthodonts on account of the complex way in which the enamel is involuted into the interior of the teeth.