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78 PATHOLOGY OF THE HAED TISSUES OF THE TEETH.
Figure 79 is a photograph from a central incisor that has
been split mesio-distally through a white spot upon its distal
surface but little larger that the one shown in Figure 78. In this
we see the penetration into the enamel in the form of a some-
what flattened cone, or a cone with a broad base on the surface
of the enamel, and the point just reaching through to the dento-
enamel junction. As we will see quite distinctly later, in caries
of the enamel more highly magnified, the solution of the cement-
ing substance between the enamel rods follows directly the length
of the enamel rods and continues spreading upon the surface
from a spot of much smaller beginning than that now seen. At
the central beginning point, or nidus, the effect of the acid has
reached through the enamel to the dentin and is beginning to
dissolve it, while the depth of penetration is less and less about
that central point in every direction, until it runs out to quite
a thin edge at the surface of the enamel. This is characteristic
of beginning decays in enamel upon all of the smooth surfaces
of the teeth. They begin at a central nidus, or beginning point,
and spread sometimes in every direction, but generally spread
most in some particular direction from that beginning point,
as will be described later. Not infrequently decay begins at
several points close together in a more or less even row, which
takes some particular direction.
In Figure 80 we have another illustration similar to that
in Figure 79, also a photograph from a lateral incisor, cut mesio-
distally, with a cavity in the distal surface. This was a very
white decay and shows to advantage. It will be seen that there
has been considerable spreading both ways from the central
nidus, and that the central portion has just penetrated to the
dento-enamel junction and is quite a little in advance of the
general conical form of the invasion of the enamel. Decays
occur, however, that show very little or none of this spreading
upon the surface, even upon the proximal surfaces of the teeth,
as will be seen from an examination of Figure 81. The tooth is
a little more enlarged in order to show this spot to better advan-
tage. Here it will be seen that a cavity has formed in the enamel.
Some of the enamel rods have fallen out and the effect of the
acid has passed entirely through the enamel and begun to dis-
solve the calcium salts from the dentin, and yet the opening,
or the area, of the enamel affected, as seen in this dimension
(the tooth having been cut mesio-distally from occlusal to gingi-
val), is no larger than the shaft of a large pin. The enamel rods
have been exactly followed lengthwise, and the enamel about