STRUCTURE OF THE TEETH. 



1553 



apex. Beginning where the enamel ceases, or overlapping the latter to a small 

 extent, as a layer only .02-03 mm. thick, the cement gradually increases in thick- 

 ness until over the root, especially between the fangs of the molars, its depth reaches 

 several millimetres. When well developed the cement usually presents two layers, — 

 an inner, almost homogeneous stratum next the dentine, in which the cement-cells are 

 absent, and an outer supplemental layer which exhibits the appearance of true bone- 

 tissue. The eround-substance of cemen- 



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1311. 



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Granular layer 

 of Tomes 



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U_ 



Cetnentum 



Lacuna 



Ground-section of root of dried tooth including adjacent 

 dentine and cementum. X 300. 



tum differs from that of ordinary bone 

 in containing, according to Bibra, slightly 

 less organic matter and a great number 

 of fibre-bundles that extend vertically to 



the lamellae, corresponding to Sharpey's v ]1, \%>\\^k\X\--V\v''\\\N Dentine 

 fibres. The lacunae are larger than those 

 of bone and vary greatly in their number 

 and form ; their processes, the canaliculi, 

 are unusually long and elaborate. As in 

 bone, so these lymph-spaces contain con- 

 nective-tissue cells, the cement-corpuscles. 

 The lamellae are so disposed that the 

 lacunae lie generally parallel with the long 

 axis of the tooth, their processes extend- 

 ing vertically to the free surface. While 

 connecting with one another by means 

 of the canaliculi, the lacunae very rarely 

 communicate with the dentinal tubules, 

 the latter terminating in blind endings. 

 The union between the outer surface of 

 the cement and the pericementum is in- t\ >|^ 

 timate, since the latter is in fact the alve- ^ ';! 

 olar periosteum from which the cement ' \ 

 was derived ; this close relation is indi- 

 cated by the roughness which the outer 

 surface of the cement presents when 

 macerated. Although at times feebly 



developed under normal conditions, typical Haversian canals are found only in con- 

 ditions of hypertrophy. 



The Alveolar Periosteum. — The periosteum investing the jaws likewise lines 

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

 in place. The name pericementum is often applied to this special part of the peri- 

 osteum, which clothes the alveoli on the one hand and covers the cement on the 

 other, thereby fulfilling the double role of periosteum and root-membrane. The 

 latter consists of tough bundles of fibrous tissue, elastic tissue being almost want- 

 ing, which are prolonged into the penetrating fibres characterizing the cementum 

 on one side and into the fibres of Sharpey of the alveolar wall on the other. The 

 fibrous bundles run almost horizontally in the upper part of the root, but become 

 more oblique towards the apex of the fang. In the latter situation the pericemen- 

 tum loses its dense character and becomes a loose connective tissue through which 

 the blood-vessels and nerves pass to reach the tooth. The less dense portions 

 of the root-membrane between the penetrating bundles of fibrous tissue contain, 

 in addition to the vessels and nerves, irregular groups of epithelial cells which 

 appear as cords or net-works within the connective-tissue stroma. These groups 

 are the remains of the epithelial sheath which surrounded the young tooth during 

 its early development. They have sometimes been described as glands, lymphatics, 

 and other structures, their true nature being unrecognized. At the alveolar mar- 

 gin the pericementum is direcdy continuous with the tissue composing the gum, 

 the fibrous bundles being so disposed immediately beneath the enamel-border that 

 they form an encircling band of dense fibrous tissue, the ligamentum circulare 

 dentis of KoUiker, which aids in maintaining firmer union between the tooth and 



the alveolar wall. 



98 



