238 THE ORGANS 



The ENAMEL covers the exposed part or crown of the tooth and is 

 the hardest substance in the body. It is thickest over the crown and 

 gradually decreases in thickness along the sides of the tooth until it 

 reaches the neck where it stops. It contains little more than a trace 

 of organic substance, its chemical composition being, according to von 

 Bibra: 



Organic matter, 3 . 59 



Calcium phosphate and fluorid, 89.82 



Calcium carbonate, 4.37 



Magnesium phosphate, i . 34 



Other salts, 0.88 



It consists of long six-sided prisms 3 to 6/^ in diameter — enamel fibres 

 or enamel prisons (Fig. 141, Sp) — which take a sHghtly wavy course 

 through the entire thickness of the enamel. The prisms are attached 

 to one another by a small amount of cement substance, and are 

 grouped into bundles, the prisms of each bundle being parallel, but 

 the bundles themselves frequently crossing one another at acute 

 angles. In the human adult the prisms are homogeneous; in the em- 

 bryo they show a longitudinal fibrillation. Rather indistinct parallel 

 lines (the Imes of Retzins) cross the enamel prisms. They probably 

 represent the deposition in layers of the lime salts, although they are 

 considered by some as artefacts. The enamel is covered by a very 

 thin apparently structureless membrane, the cuticula dentis. 



The CEiviENTUM (Fig. 140, C) covers the dentine of the root in a 

 manner similar to that in which the enamel covers the dentine of the 

 crown (Fig. 137, i and 3). It forms a thin layer at the neck, but in- 

 creases in thickness as the deeper part of the root is reached. Cemen- 

 tumis hone tissue. It contains /acz/;z«and hone cells. These vary much 

 more in size and shape than do those of bone, are very irregularly 

 distributed and may be absent from considerable areas. From the 

 lacunae radiate canalicuH, but there is no distinct lamellation and no 

 Haversian systems excepting in the large teeth of the larger mammaUa, 

 and in the teeth of the aged, where they may be present. Channels, 

 similar to Volkmann's canals in bone, not surrounded by concentric 

 lamellae, but serving for the passage of blood-vessels, are quite fre- 

 quent in the thicker portions of the cementum. The ground sub- 

 stance of the cementum is continuous with that of the dentine and 

 many canahcuK of the former open into the interglobular spaces 

 of the latter. Many uncalcilied Sharpey's fibres penetrate the 

 cementum. 



