THE CEMENTUM. 



Dentine 



Granule layer 



The intertubular dentine-matrix resembles that of bone in being com- 

 posed of bundles of extremely delicate fibrous fibrillae that swell on treatment 

 with water containing acids or alkalies and yield gelatin after prolonged 

 boiling in water. The disposition of the bundles of fibrillae, more regular in 

 dentine than in bone, is chiefly longitudinal and parallel to the primary sur- 

 faces of the dentine; additional bundles run obliquely crosswise in the layers 

 of dentine. The bundles of fibrillae, from 2-3 /* in diameter, appear in trans- 

 verse sections as small punctated fields. The fibrillae are knit together by 

 the calcified ground-substance, in which the lime-salts are deposited in the 

 form of minute spheres, the interstices between the spherules being later 

 filled and calcification thus completed. When, as often happens, the calci- 

 fication is imperfect, irregular clefts, the interglobular spaces, result. These 

 spaces are bounded by the spherules, or dentine-globules, of calcareous 

 material and are of irregular form and uncertain extent, being, however, 

 usually largest in the crown. The 

 junction of the dentine and cement- 

 urn is always marked by a zone of 

 closely placed interglobular spaces 

 of small size ; under low magnifica- 

 tion in ground sections these spaces 

 appear as dark granules, hence the 

 zone is called the granule layer 

 of Tomes. 



The Cementum. The ce- 

 mentum, the crusta petrosa of the 

 older writers, forms an investment 

 of modified bone that covers the 

 outer surface of the dentine from 

 the neck to the apex of the tooth. 

 Beginning where the enamel ends, 

 or overlapping the latter to a slight 

 extent, the cementum gradually 

 increases in thickness until over the 

 root, especially between the fangs 

 of the molars, it forms a layer 

 several millimeters thick. The 

 matrix of the cementum differs 

 from that of ordinary bone in con- 

 taining slightly less organic matter 

 and a greater number of fibre-bundles that run vertically to the bone-lamellae. 

 These bundles correspond to the fibres of Sharpey in other situations. The 

 lacunae are larger than those of ordinary bone and the canaliculi are unusually 

 long and elaborate. As in bone, so in the cementum these lymph-spaces 

 contain connective tissue elements, the cementum-cells. Although connect- 

 ing with one another by means of the canaliculi, the lacunae seldom commu- 

 nicate with the dentinal tubules, the latter commonly ending in loops or 

 blind expansions. The outer surface of the cementum is intimately attached 

 to the surrounding alveolar periosteum, the so-called pericementum, since 

 from the latter the cementum is derived. Typical Haversian canals are 

 found in cementum only when this layer is hypertrophied. 



The Alveolar Periosteum. The periosteum investing the jaws also 

 lines the sockets receiving the roots of the teeth, which are by this means 

 securely held in place. The name, pericementum or peridental membrane, 



Cementum 



Lacuna 



FIG. 175. Ground section of root of dried tooth in- 

 cluding adjoining dentine and cementum. X 300. 



