nerve-endings or in free sensory nerve-endings, or on the bloodvessels. In the papillæ of the mucosa are found corpuscles of Krause. (See p. 154.) The nerves terminating in free sensory endings are the dendrites of sensory neurones (medullated sensory nerves), which, while yet medullated, branch and form plexuses with large meshes, situated in the submucosa and deeper portion of the mucosa. The medullated branches of the nerve-fibers constituting these plexuses proceed toward the epithelium, dividing further in their course. Immediately under the epithelium the medullated branches lose their medullary sheaths, divide further, and form the subepithelial plexuses. The nonmedullated branches enter the epithelium, where they form telodendria (end-brushes), the terminal branches of which surround the epithelial cells, between which they end either in very fine granules or in small groups of such, or, again, in variously shaped end-discs. (See Fig. 130.) The bloodvessels are richly supplied with vasomotor nerves, the neuraxes of sympathetic neurones, which terminate on the muscle-cells of the vessels. In the adventitia are also found free sensory nerveendings. (See Fig. 170.)

1. THE TEETH.

Structure of the Adult Tooth.-The adult tooth is made up of three substances-the enamel, the dentin, and the cementum. The latter covers that part of the tooth within the alveolar process of the jaw and also the root of the tooth. The enamel caps that part of the tooth projecting into the oral cavity, the crown of the tooth. The neck of the tooth is the region where the cementum and enamel come in contact. The greater part of the tooth consists of dentin, which is present in the crown as well as in the root. All the substances of the tooth just mentioned become very hard from the deposition of lime-salts. Every tooth contains a cavity surrounded by dentin, the pulp cavity, or dental cavity. This is filled with a soft tissue, the pulp, consisting of white fibrous tissue, vessels, and nerves. That part of the pulp cavity lying in the axis of the fang is called the root-canal; by an opening in the latter (foramen apicis dentis) the pulp is connected with the periosteal connective tissue of the dental alveolus.

The enamel is a very hard substance, the hardest in the body, and may be compared to quartz. In uninjured teeth the enamel is covered by an exceedingly thin, structureless enamel membrane (cuticula dentis). The enamel contains very little organic substance (from 3% to 5%), in consequence of which it is soluble in acids with scarcely any residue. The elements composing it are prismatic columns, the enamel prisms, which occupy the whole thickness of the enamel from the superficial membrane to the dentin. These are not thicker at the surface of the tooth than at the dentin, and in transverse section show a hexagonal or polygonal shape. They are

joined to each other by a cement-substance which is somewhat more resistant than the substance of the prisms themselves. In the adult they are entirely homogeneous, but in embryos and even in the new-born they show a (fibrillar) longitudinal striation. In their course through the thickness of the enamel they change their

Pulp cavity. (Berten).

Dentin.

The dentin is, next to the enamel, the hardest tissue of the tooth. After its decalcification a substance is left which yields gelatin. The dentin is permeated by a system. of canals having usually a transverse direction, the so-called dentinal tubules, which are from 1.3 to 4.5 in diameter. These originate in the pulp cavity, and during their course become slightly curved, like the letter S. At their outer third they branch and become constantly smaller. As a rule, they extend as far as the enamel, although a few now and then even cross the boundary of the enamel. They never reach the cement, but leave here a free zone in which the ground-substance of the dentin alone is present. The dentinal tubules possess sheaths, the sheaths of Neumann, which may be isolated, analogous to the sheaths of the canaliculi of bone, and which contain throughout their entire length filiform prolonga

Cementum.

Fig. 174.-Scheme of a longitudinal section through a human tooth. In the enamel are seen the lines of Retzius."

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tions of certain pulp-cells (odontoblasts), the dentinal fibers. According to v. Ebner, the ground-substance of the dentin consists of bundles of connective-tissue fibrils, which in the root run parallel to the long axis of the tooth, and in the crown have a direction at right angles to that of the dentinal tubules. The S-shaped curves of the latter give rise to the lines of Schräger in the dentin, which are visible with an ordinary magnifying glass. Peculiar, irregularly branched spaces are often seen in the dentin. These are the interglobular spaces, and represent areas in which calcification has not. taken place.

The cementum is closely adherent to the dentin, and consists of

Fig. 175. A portion of a ground tooth from man, showing enamel and dentin; X 170. Technic No. 225.

bone tissue, the parallel lamella of which contain, as a rule, no Haversian canals. There occur, however, cement lamellæ, which in places lose their bone-cells. A peculiarity of the cementum is the presence of a large number of Sharpey's fibers, which are especially abundant in those areas containing no bone-corpuscles. These fibers are usually found in an uncalcified condition.

The tooth-pulp is a tissue resembling embryonic connective tissue, consisting of connective-tissue fibrils, branched connectivetissue cells, and a semifluid, interfibrillar ground-substance. It is characteristic of this tissue that the fibrils never join to form connective-tissue fibers. At the surface of the pulp is a continuous

layer of cells, the odontoblasts. These are columnar cells with basal nuclei and two or three processes extending into the canaliculi of the dentin, forming here the dentinal fibers already described. As a rule, the odontoblasts also send a single fiber into the pulp. These may intertwine and give rise to a network within its substance.

The tooth is joined to the periosteum of the alveolus by a reduplication of the latter over its root, the dental periosteum or peridental membrane. This consists of bundles of connective tissue (elastic fibers are here absent) directly continuous with Sharpey's fibers in the cementum. At the neck of the tooth the peridental membrane disappears in the submucosa of the gum; in

Fig. 176.-A, Longitudinal section through a human molar from the center of the enamel layer, decalcified with dilute hydrochloric acid; B, tangential, C, radiate, and D, transverse section through the dentin of a human tooth, showing the fibrillar structure of the ground-substance (taken from v. Ebner, 91): a and b, Two layers in which the direction of the enamel prisms changes; in c is seen a dentinal fiber with its sheath; e, groups of fibrils; d, dentinal tubules.

the former are found here and there peculiar masses of epithelial cells representing the remains of the enamel organs.

The tooth-pulp has a rich blood supply. A small artery enters the pulp cavity through the apical foramen, which, as it passes up through the pulp, gives off numerous smaller branches which end in a capillary network situated under the layer of odontoblasts. Numerous medullated nerve-fibers (dendrites of sensory neurones) enter the pulp cavity through the apical foramen. Some of these lose their medullary sheaths soon after entering, or just before entering, the pulp, and divide into long, fine, varicose fibers which interlace to form a loose plexus under the odontoblasts. Other

medullated fibers, grouped into small bundles, ascend in the pulp for variable distances; the nerve-fibers of the bundles then separate and as single fibers approach the superficial portion of the pulp, and, after losing their medullary sheaths, divide into fine varicose fibers forming under the odontoblasts a plexus continuous with the plexus above mentioned. From the varicose nerve-fibers of this plexus small terminal branches are given off which terminate between the odontoblasts, or pass through the layer of odontoblasts, to end between these and the dentin (Retzius, 94; Huber, 98). Medullated nerve-fibers also terminate in free endings in the peridental membrane.

Cementum.

Development of the Teeth.-In the second month of fetal life the first traces of the teeth are seen in the development of a groove along the inner edge of the fetal jaw, the dentinal or enamel groove. From the floor of the latter an epithelial ridge. is formed constituting the anlage of the enamel organs and known as the dentinal ridge, or enamel ledge. At those points at which the milk-teeth later appear, the enamel ledge develops solid protuberances corresponding in number to the temporary teeth. These are known as the dentinal bulbs or enamel germs. In their first stage of development the enamel germs are knoblike, but later their bases spread, and they become. flattened and finally cupshaped by the pushing up into them of connectivetissue projections, the dentinal papille. At the same time they gradually sink deeper into the underlying tissue, but still remain connected, by means of a thin cord, with the epithelium of the enamel ledge, which now lies on the inner side of the enamel germs. The enamel germs now differentiate into enamel organs. In this stage they consist of an outer layer of columnar epithelial cells, which are to be regarded as a direct continuation of the basal cells from the epithelium of the oral mucous membrane, or still better, of the enamel ledge; the epithelium in the interior of the organ is derived from the stratum Malpighii of the oral epithelium. The cells of this layer, however, undergo a change in shape and structure, in that an increased quantity of lymph-plasma or intercellular substance collects in the interspinous spaces between the cells, pushing the cells apart, and allowing their processes to

Fig. 177.-Cross-section of human tooth, showing cement and dentin; X 212. Technic No. 225 (vid. also Technic 152). At a are seen small interglobular spaces (Tomes' granular layer).