MOUTH CAVITY. 157 



the odontoblasts forming a continuous layer (Fig. 111). 

 These are long cylindrical cells with the nucleus in the inner 

 half of the cell. They each send one process, seldom more, 

 into the dentine toward the outside. These processes form the 

 fibres in the dentine. There are other processes sent out by 

 the odontoblasts in the direction of the pulp. These branch 

 and surround the pulp elements. The whole pulp is sur- 

 rounded by dentine, which forms the main mass of the tooth. 

 The dentine itself is covered entirely by two other coats, on 

 the crown of the tooth by the enamel, and on the root by the 

 cement. These two coats meet at the neck. 



The dentine (substantia eburnea) is a kind of bone which is 

 distinguished from ordinary bone by the fact that its cells are 

 not situated in cavities of the ground substance. The cell 

 bodies lie on the surface of the pulp close to the dentine, so 

 that the dentine itself contains only their processes, the so- 

 called dental fibres, which lie in the dental canals. These 

 canals begin at the pulp surface of the dentine, and run radially 

 toward its outer surface in a slightly curved direction, like the 

 letter S. At their beginning the canals are 2.55 u in diam- 

 eter, but become narrower as they proceed outward, on account 

 of division. At the outer surface of the dentine they measure 

 only 0.6 ^. They give off throughout their course fine side- 

 branches in every direction, thus joining with neighboring 

 canals. These side-branches are usually 0.3-0.6 ^ in diam- 

 eter. A section cut at right angles to the course ^of the canals 

 shows their relation to the side-branches. Fig. 112 shows that 

 they join not only canals near one another, but also those at 

 some distance from one another. 



The relation of the main dental canals, as well as of the 

 side-branches, is characteristic for different parts of the tooth 

 (Plates IX., X., XL). In the part near the pulp the lateral 

 branches leave the canals at almost a right angle. In the more 

 peripheral parts of the dentine, on the contrary, the angle is 

 acute. In the former position the side-canals are less numerous 

 than in the peripheral parts. 



In the crown of the tooth the main canals take a fairly 



