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98 DENTAL HISTOLOGY AND OPERATIVE DENTISTRY.

is possible in the bono at least, therefore, to have degenerative and re-
generative changes, <»r the removal of part of the calcium salts and their
replacement through the agency of the formative tissue ; while in the
enamel no such regenerative change is possible, as the formative tissue
disap[)eared when the tissue was completed and before the eruption of
the tooth.
The enamel is the hardest of human tissues. Chemicallv it is com-
posed of the ]ihosphates and carbonates of calcium and magnesium and
a very small amount of the fluorids, water, also a very small amount
of organic matter if any.^ The enamel in the natural condition, bathed
in the fluids of the mouth, contains a considerable amount of water.
If dried at a little above the boiling-point of water, it gives up part
of it and shrinks considerably, so as to crack in fine checks. If heated
almost to redness, it suddenly gives off from 3 to 5 per cent, (of the
dry weight) of water with almost explosive violence. These facts were
demonstrated some years ago by Charles Tomes," and account for most
of what was formerly recorded as organic matter in old analyses.
If we observe under the microscope the action of acids upon thin
sections of enamel, when the inorganic salts are entirely removed, the
structure of the tissue vanishes, there being no trace of organic matrix
left as in the case of bone or dentin. In the growth of l)one and
dentin the formative tissue produces first an organic matrix (see Chap.
II., p. 74) in the form of the tissue, and into this the inorganic salts are
deposited, combining with the organic substances of the matrix. This
union is comparatively weak, however, for by the action of acids the
combination is broken up and the inorganic salts are dissolved ; or by
heat the organic matter is removed, and in either case the form of the
tissue will be maintained.
In the case of the enamel, the formative organ produces organic
substances containing inorganic salts, and the substances are arranged
in the form of the tissue after the manner of a matrix ; but finally
under the action of the formative organ all of the organic matter is
removed and substituted by inorganic salts, whatever organic matter is
' Von Bibra gives the following- analysis of enamel
:
Calcinm phosphate and lluorid 89.82
Calcium carbonate 4.37
Magnesium phosphate 1 -34
88
Other salts
Cartilage 3.39
Fat 20
Total organic 3.59
" inorganic 96.41
* Journal of Phy.tioloyii, 1896.