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132 DENTAL HISTOLOGY AND OrKRATIVE DKNTISTIIY.
also very sensitive to triuiniatic and olK-niiciil irritations, even wiieii
these are conveyed to it tliroii Huber has suggested ' that this transmission may be ac^complislicd by
tiie traumatic or chemical action upon the fibrils setting up metabolic
changes in the odontoblastic cells, which act as stimuli to the sensorv
nerves ending between the cells of that layer.

Cementum.
The cementum covers the surface of the dentin apically from the
border of the enamel, lapping slightly over the enamel at the gingival
margin (Fig. 114). It forms a layer, thickest in the apical region and

Fig. 114.

D.

Gingival border of enamel, showiiiL; tile (•(•iiientum overlapping it: £•, enamel ; C, cementum ; D,
dentin. (About 40 X.)

between the roots of bicuspids and molars, and becoming thinner as the
gingival line is approached. The cementum resembles sub})eriosteal
bone in structure, but differs from it in the character and arrangement
of the lacunae and in the absence of Haversian systems ; the layers, or
lamellse, of the cementum also are less uniform in character than those
of bone.
The function of the cementum is to furnish attachment for the
fibers of the peridental membrane which holds the tooth in its position.
The surrounding tissues are never in ])hysiologic connection with the
outer surface of the dentin, except to form cementum over it or to
remove its substance by absorption ; and when absorption of the dentin
' Devtal Co,s?Ho,