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186 THE MICRO-ORGANISMS OF THE HUMAN MODTE.

Fig. 87 represents an appcarauee frequently observed in cross-
sections. A number of adjacent tubules are enlarged to such an
extent that their walls finally touch or even flatten one another.
In such cases they form live- to six-sided prisms. The question
arises : AVhat has become of the intertubular substance ?
A slight compression of the intervening substance may doubt-
less be caused by the pressure from within the tubules, but its
total compression, as shown in Fig. 87, is altogether inconceiva-
ble. Xow, since the micro-organisms cannot penetrate into the
intertubular substance through the intact dentinal sheath, the
above occurrence can be explained only by the liA'pothesis that

Fig. 86. Fig. 87.

Cross-Section of Decayed
Dentine.
Cross-Section of De-
The tubules through reciprocal
cayed Dentine.
pressure have assumed the
Showing the distention of
shape of 5-3 sided prisms.
the tubuli and formation
of liquefaetion-foei.
400 : 1.
they form a pepsine-like diti'asible ferment, which dissolves the
intervening substance, while Xeumann's sheath remains intact.
I am not, however, at all sure that I have hit upon the right
explanation of this phenomenon.
In the deeper zones of decayed dentine the bacteria lie ex-
clusively in the tubules and their ramifications, and a direct in-
vasion of bacteria into the intertubular substance only excep-
tionally takes place ; the dissolution of the latter progresses from
the surface or from the lumen of the tubules.
At the neck of the tooth the external layer of dentine is either
devoid of tubules, or they are so narrow that the entrance of the
bacteria is greatly impeded. Caries at the neck of the tooth con-
sequently presents phenomena which differ somewhat from those
of other parts. This applies especially to chronic dry caries at the