234 DIGESTIVE ORGANS spaces containing air. These spaces freely communicate with each other. As the dentine which forms their boundary has not unfrequently the appearance of globular contours, they were named by Czermak the interglobular spaces. In a fresh tooth they are not empty, but are occupied by a soft part of the matrix, which is traversed in the usual manner by the dentine tubes. This matrix is apparently imperfectly calcified dentine, which shrinks up in a dried tooth, and occasions an air-containing space. A layer of small irregular spaces situated in the peripheral part of the dentine in the fang, immediately under the crusta petrosa, and sometimes named the granular layer, is apparently of the same nature as the interglobular spaces. The Enamel is the brilliant white layer which forms a cap on the surface of the crown of a tooth. It is thickest 011 the cutting edge or grinding surface of the crown, and thins away towards the neck, where it disappears. It is not only the hardest part of a tooth, but the hardest tissue in the body, and consists of 9 6 5 per cent, of earthy and of 3 5 per cent, of animal matter. The earthy matter consists almost eutiraly of salts of lime. The great hardness of the enamel admirably adapts it as a covering for the cutting edge, or grind ing surfaces, of the crowns of the teeth. The enamel is composed of microscopic rods, the enamel fibres, or enamel prisms. These rods are set side by side in close contact with each other ; one end of each rod rests on the surface of the dentine, the other reaches the free surface of the crown. The rods do not all lie parallel to each other, for whilst some are straight, others are sinuous, and the latter seem to decussate with each other. The rods are marked by faint transverse lines, and are solid structures in the fully, formed enamel. "When cut across transversely, they are seen to be hexagonal or pentagonal, and about s-^jth inch in diameter. The free surface of the enamel of an unworn tooth is covered by a thin membrane, named the cuticle of the enamel, or Nasmyth s membrane. This membrane can be demonstrated by digesting an unworn tooth in a dilute mineral acid, when it separates as a thin flake from the free surface of the crown. It is a horny membrane, which resists the action of acids. Its deep surface is pitted for the ends of the enamel rods. As the crown of the tooth comes into use, Nas myth s membrane is worn off, and the enamel itself by prolonged use is thinned and worn down. In persons who live on hard food, that requires much mastication, it is not uncommon to find the grinding surface of the crowns of the molar teeth worn down quite flat, and the dentine exposed. The Cement, Crusta Petrosa, or Tooth Bone, forms a thin covering for the surface of the fang of a tooth, and extends upwards to the neck. It is of a yellowish colour, and is usually thickest at the point of the fang ; though in the multifanged teeth it sometimes forms a thickish mass at the point of convergence of the fangs. It possesses the structure of bone, and consists of a lamellated matrix with perforating fibres, lacunae, and canaliculi. The lacunas are irregular in size and mode of arrangement, and vary also in FIG. 14.* 1, Vertical section through the enamel and immediately subjacent dentine; e, enamel rods; d, branched termination of dentine tubes. 2, trans verse section through the enamel rods. 3, transverse section through dentine tubes and matrix. X 300. the number of the canaliculi proceeding from them. Some times the canaliculi anastomose with the branched terminations of the dentine tubes. In the thin cement situated near the neck of the tooth the lacuna? are usually absent. If the jaw with its contained teeth be softened in acid, and sections be made so as to show the teeth in situ, there is no difficulty in recognizing the cellular masses of nucleated protoplasm within the lacumu, which resemble in f d FIG. 15. Section through the socket and fang of a tooth, b, the bony wall of a socket, its lacunae containing the bone corpuscles; /, the fibrous, and r, tin: reti culated portion of the alveolo-deni al periosteum, in which transversely divided vessels, v, v, may be seen ; c, the cement, the lacuna; of which contain thu bono corpuscles ; d, the dentine. X 450. appearance the corresponding structures in the adjacent bone. Haversian canals are only found in the cement when it acquires unusual thickness. In old teeth he cement thickens at the tip of the fang, and often closes up the orifice into the pulp cavity ; the passage of the nerves and vessels into the pulp is thus cut off, and the nutrition of the tooth being at an end, it loosens in its socket and drops out. Osteo-dentine and Yaso-dentine do not exist as normal structures in human teeth, though they occur in various animals. They may appear, however, as abnormalities in the human teeth, and are found on the inner wall of the pulp cavity. Osteo-dentine consists of dentine structure, intermingled with lacunae and canaliculi. If vascular canals, like the Haversian canals of bone, are formed in it, then the name vaso-dentine is applied. The Pulp of the tooth is one of its most important con- Fulp stituents. It is a soft substance occupying the cavity in the dentine, or the pulp cavity, and is destroyed in a macerated and dried tooth. It consists of a very delicate gelatinous connective tissue, in which numerous cells are imbedded. Those which lie at the periphery of the pulp are in contact with the dentine wall, and form a layer, named by Kolliker the membrana eboris. As the cells of this layer play a part in the formation of the dentine similar to that performed by the osteoblast cells in the formation of bone, "VValdeyer has named them odontobtasts. The odontoblasts are elongated in form, and their protoplasm gives off several slender processes ; some enter dentine tubes to form the soft di iitinal fibres already described ; one passes towards the centre of the pulp, to become con nected with more deeply-placed pulp cells ; whilst others are given off laterally to join contiguous cells of the odontoblast layer. The pulp contains the nerves and blood-vessels of the tooth, which pass into the pulp, through the foramen at the point of the fang. The vessels form a beautiful plexus of capillaries. The nerves are sensory branches of the fifth cranial nerve. They enter the pulp as medullated fibres, which divide into very fine non- medullatcd fibres, that form a network in the peripheral portions of the pulp. The pulp of the tooth is the remains of the formative papilla, out of which the dentine or ivory has been produced. In adult teeth changes that lead to the production of osteo-dentine and vaso-dentine may take place in it. Through the dentinal fibres an organic con
nection is preserved between the dentine and the puip, and