The greater part of the tooth consists of dentine. Thc root dentine is covered by a thin layer of cernentum and the dentine of the crown is covered by enamel. Internally, the dentine contains the dental pulp in the pulp chanber. The root of the tooth occupies a socket in the alveolar bone to which it is attached by the connective tissue fibres of the periodontal membrane.

1. The pulp: Thc pup consists of loose connective tissue and carries the blood, lyrmphatic and nerve supply to the tooth. Where it meets the dentine, the surface of thc pulp is covered by a layer of odontoblasts. These are columnar cells with oval nuclei. and each celt has a process that lies within a corresponding tubule in the dentine. Immediately internal to the odontoblast layer there is a narrow cell-free zone.

2. Dentine : Physicaily and chemiçally dentine is very similar to bone, consisting of 30 per cent organic material and water, and 70 per cent inorganic material. As in bone, the organic fraction consists of collagen fibrils embedded in a mucopolysaccharide cetmenting substance, and the inorganic fraction consists mainlty of calcium phosphates in the form of apatite crystals. Unlike bone, however, dentinc contains no cell bodies but only cell processes. those of the odontoblasts. in tbe dentinal tubute. The dentinal tubules are 2 to 3u in diameter and cach runs through the whole thickness of the dentine from the cell body of the odontoblast to the outer surface of the dentine. There are cross -communications between the tubules, containing anastomosing branches of the odontoblast processes. Calcification of the outer surface of the dentine. There are cross-communications between the tubules, containing anastomosing branches of the odontoblast processes. Calcification of the dentine occurs in spherical or globular masses or calcospherites, which coalesce to give a uniformly mineralized issue. Where calcification is incomplete the separate globules can be seen, with the uncalcified or hypocalcified ground substance in between them. Such areas are referred to as interglobular dentine.

3. Enamel : Mature enamel can be studied only in ground sections unless special methods are employed. since it is completely removed by routne his tological decalcification. The inorganic material is an apatite and small organic fraction is mainly of keratinous nature. Enamel consists of rods or prisms in an interprismatic substance that is slightly less mineralized than the rods themselves. Each rod runs from the enamel dentine junction through the whole thickness of the enamel to its surface, following a slightly wavy course. The rods have a "fish-scale" appearance in cross-section, with an average diameter of 4μ.

4. Cementum: Cementum is a modified type of bone that covers the dentine of the tooth root in a thin layer. Two varieties of cementum occur noгmally, acellular and celluar. Acellular or primary cementum, the type first formed, covers the root from the enamel-cementum junction to close to the apex. As the name implies, this thin layer of cementum is homogeneous and contains no cells. Cellular or secondary cementum covers the apical portion of the root. Lacunae containing the cementocytes are present, in a similar manner to the lacunae for osteocyte in bone. The cementocytes are very similar morphologically to osteocytes, though they are usually somewhat larger. The processes of the cementocytes do not radiate in all directions like those of osteocytes, but tend to be directed away from the dentine towards the periodontal membrane. Well-marked incremental lines running parallel with the root surface are seen in the cementum and it is quite normal to find successive increments of both acellular and cellular cementum occurring in any order or distribution. Cementum is continuously deposited throughout life. The principal function of cementum is to give attachment to fibres of

the periodontal membrane.

5. Periodontal membrane: The connective tissue fibres, generally termed the periodontal membrane, constitute a suspensory ligament that attaches the tooth to bony alveolus. Fibres are attached to the cementum, and for the most part run in bundles to the alveolar bone. Those from the cementum nearest to the crown, however, run across the alveolar crest to the cementum of the adjacent tooth, and some also run into the gingiva.

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