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CEMENTUM. 133

has occurred on the surfiice of a root it is never repaired except by the
formation of cementum to fill up the cavity and reattach the membrane.
The cementum is intermittently formed during the functioning of
the tooth, being added layer after layer over the entire surface of the
root, the difference in thickness of the tissue in the gingival and apical
portions being chiefly, though not entirely, due to the difference in
thickness of each layer in the two positions (Figs. 114, 115). The
cementum on the roots of newly erupted teeth is thin, and on the roots
of teeth of old persons is thick. This continued formation of cementum

Fig. 115.

Cementum near tbe apex of the root : Gl, granular layer of Tomes ; i, lacuufe , /;, point at which
fibers were cut off and reattached. (About 54 X.)
is due to the necessity for change and reattachment of the fibers of the
membrane.
In the gingival portions, where the cementum is thin, the tissue is
clear and a])parently structureless, and usually contains no lacuna?
while in the apical half and between the roots the lacunte are numerous.
In general, wherever the lamellie are thin, the lacunae are absent ; but
where the lamellae are thick they are found. The canalicuH which
radiate from the lacunae are not as regular as in the case of the lacunie
of bone. Sometimes they are numerous, sometimes few ; they may
extend from a lacuna in all directions, or they may be confined to one
side, usually the side toward the surface of the cementum (Fig. 116).