Page 120 - My FlipBook

94 THE MICRO-ORGANISMS OF THE HUMAN MOUTH.

viridautia, are excluded, because greeu dentine does not occur.
Those which assume a yellow color (Bacteria flavescentia) cannot
be looked upon as the direct cause of the yellow shade of carious
dentine, because the color is confined to the micro-organisms
themselves, the medium on which they are cultivated becoming
very little, if at all, stained. In the case of dentine, the relations
are exactly the opposite ; the dentine becomes stained, while the
micro-organisms remain colorless (white). This is probaV)ly well
known to those who have made a few sections of carious dentine.
The following experiment may serve to furnish an idea of the
manner in which the pigmentation of the carious tooth-tissue
may be brought about.
I inoculated a tube of culture-gelatine with a bacterium ob-
tained from decayed dentine ; almost any bacterium which lique-
fies the gelatine would, however, have served the same purpose.
In about two weeks the gelatine was completely melted and a
white mass of bacteria lay on the bottom of the tube.
At the beginning of the experiment the gelatine had only the
slight yellowish tinge often present in culture gelatine. Soon,
however, a yellowish-l)rown color made its appearance, Avhich
gradually became darker even after all life had disappeared from
the culture, until at the end of ten weeks the whole mass of
melted gelatine had a deep brownish color.
A very old dry culture presented about the color of the Ijlack
spots (so-called caries nigra) often seen on the approximal sur-
faces of teeth where caries once began and then ceased after the
removal of the approximating tooth. Organic matter under-
going decomposition assumes, as is well known, a dark color,
and the same is true of deca^dng dentine. The colors charac-
teristic of decaying dentine do not exist in the very beginning
of the decay, but appear subsequently. The more recent or
acute the decay, the less the discoloration ; the older or more
chronic the decay, the deeper the color.
There is, however, another factor Avhich may play a jiart in
the dirscoloration of dentine, more particularly in teeth contain-
ing dead pul}>s ; the latter sometimes become intensely black,
and it is to these in particular that the following suggestion re-
fers. My attention was some time since called to the fact that