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148 THE MICRO-ORGANISMS OF THE HUMAN MOUTH.
The structure of cement is similar to that of bone, from which
it diifers, however, particularly in the absence of Haversian
canals, which, if present in the cement, would naturally render
it far more susceptible to the action of external agents than it in
reality is.
The relation of Sharpey's fibers to the progress of decay iu
the cement is very significant ; they may accordingly claim espe-
cial mention. Without concerning ourselves about the genesis
of these so-called penetrating fibers, or fibers of Sharpey, it
may suffice to say that they indicate the presence of tubules or
canals running perpendicular to the longitudinal axis of the tooth,
present in all parts of the cement, but particularly numerous at
the neck of the tooth. In case of decay, the micro-organisms are
seen to penetrate these canals very much as they enter the den-
tinal tubules. They thereby facilitate the invasion of bacteria
into the interior of the cement, often making the course of
cement-decay very similar to that of dentine-decay.

Chemical Composition of the Hard Dental Substance.
Numerous analyses of the hard tooth-substance have been made
by dift'erent chemists. Some of the older ones, however, are un-
reliable, and the results obtained are at variance with each other.
Dentine contains a quantity of organic substance which is
subject to slight variations in different teeth ; in very hard den-
tine it amounts to about 26 per cent., in very soft to 28.5 per
cent. These variations, however, stand in no direct proportion
to the hardness of the dentine ; that is to say, we find much
greater differences in the density of the dentine of different
teeth than we do in the percentage of lime-salts which they con-
tain. They probably do not relate to the normally formed
intertubular substance, but depend upon the composition of the
entire mass of dentine, including the tubules, interglobular
spaces, etc. For example, a tooth with wnde tubules and large
interglobular spaces would, on analysis, give a higher percentage
of organic matter than a tooth with narrow tubules and smaller
interglobular spaces, even though the intertubular substance
should have exactly the same composition in both cases.
In like manner, differences in the proportion of organic matter