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OF THE STRUCTURE OF A TOOTH, ETC. 39
it, and we are obliged to use a file in dividing or cutting it. (m)

although not always, disposed in series ; or they appear as if placed
between concentric laminae. They are always absent near the neck of
the tooth, where the cement is thinnest ; they generally appear about the
middle of the fang, and increase in number with the thickness of the
cement towards the apex, where they often present the serial appear-
ance to which we have just referred. They possess the essential
characters of the lacunae of bone. They appear dark by transmitted,
and white by reflected light ; a circumstance due simply to their
being filled with air. In the lacunae of fresh osseous tissue, a delicate-
walled cell, containing a clear fluid and a single nucleus, has been dis-
covered by Virchow. From this included cell, numerous processes are
pent into the canaliculi which branch out on every side from the lacuna.
These processes anastomose with like processes from the included cells
of neighbouring lacunae. In shape the lacuna is generally oval, but
occasionally round or fusiform ; in fact, in this respect they are subject
to great variety. Equally variable is their size ; according to Kolliker,
In the thick layers of
- 005'" to 0-02'", even to 0'03'", in diameter.
cement which are found on old teeth, these recesses are generally of an
elongated form and very irregular. Some lacunae, either separately or
in groups, may be seen to be surrounded by a tissue of more than the
ordinary transparency. This clearer substance is well defined by a
6lightly-waved border, which Kolliker supposes may be in some way
related to the cell from which the lacuna is formed. The canaliculi
which are given off by the lacunae of cement are of unusual number
and length. They are frequently so numerous as to give the idea of a
pencil or plume of processes, which again branch into still finer rays.
The majority of the canaliculi are directed outwards towards the
surface of the tooth, or inwards towards the dentine : some, however,
follow the course of the length of the tooth ; or, where a vascular canal
is present, they take a direction towards its surface. In crossing the
matrix, they give it a finely-striated appearance ; they anastomose freely
with the canaliculi of neighbouring lacunae, and near the surface of the
dentine they may be seen to inosculate with the terminations of the
dentinal tubes. Other cavities may be observed in cement ; some of
them are of a pathological nature. But, besides these, Mr Tomes notices
tubes of occasional occurrence which pass across the cement towards
the surface of the tooth. They equal in size the dentinal tubes, but
have fewer branches. Occasionally the canaliculi enter them. Kolliker,
in referring to what appear to be the same tubuli, says that their
branches are frequently connected with the terminations of the dentinal
canals, and with the canaliculi. On the fangs of deciduous teeth the
cement is thin, and exhibits comparatively few lacunae.] (1)
(m) [Such is the hardness of the enamel, that it emits sparks when
(1) Kolliker, Tomes, Owen, op. cit.