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MICROSCOPICAL PHENOMENA OF DECAY. 187

neck of the tooth, which leads to intense pigmentation. Under
the microscope the external margin of a longitudinal section
exhibits a layer consisting of masses of bacteria from which
numerous leptothrix threads radiate. Below it the dentine
appears interspersed with many triangular fissures, with their
bases toward the border and their longer diameter parallel to the
tubules (Fig. 88).
The fissures are almost always filled with micro-organisms,
especially with cocci. I suspect that they are not formed by
Fig. 88.

Appearance of Decay at the Neck of the Tooth.
The fissures are filled with bacteria. Circa 50 : 1.
bacteria, but by the contraction of the external layer of dentine,
and that masses of bacteria, as well as very small food-particles,
enter them afterward. Each fissure thus afl:brds a point of re-
tention or a caries-center ti'om which the infection of the dentine
proceeds.

Specimens stained with picrocarmine frequently exhibit pe-
culiar appearances, which cannot always be easily explained.
Longitudinal sections under three hundred diameters show on
the margin a reddish fringe of leptothrix threads, mingled with