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38 HUNTER ON THE TEETH. —
hardest and sharpest saw will scarce make an impression upon
In the Ox the proportions of the several ingredients were
Phosphate of Lime and Fluoride of Calcium - 58-73
Carbonate of Lime - - - - 7-22
Phosphate of Magnesia - 0"99
Salts - - - - - - 0-82
Cartilage - - - - - 31 31
Fat - - - - - - 0-93
100-00
The cement may be readily deprived of its earthy salts by the action
of acids. The organic residue yields ordinary gelatine on boiling.
Cement, we have already said, is a layer of true bone, with but little
modification. But it is only in situations in which it attains great
thickness {e.g., the teeth of the Horse, Sloth, Ruminants), that it exhibits
canals for the passage of vessels (Haversian canals). In the human
tooth the existence of Haversian canals is to be considered exceptional,
although they are occasionally to be met with in situations where the
cement is of unusual thickness. When, for instance, the fangs of a
molar are united by cement, a vascular canal is not unfrequently found
traversing the medium of union (Tomes.) Their existence, therefore,
in the human tooth is not to be regarded ai necessarily an indication of
morbid action, although they are most frequently to be noticed in the
thickened cement of old teeth and in cases of dental exostosis. When
present, they penetrate from without ; in some instances they branch
once or twice, and anastomose, or end by blind extremities, or a canal
may end by a dilatation, " as though occupied by a vessel that had
turned upon itself and gone out by the same channel through which it
•
entered." (1) Their diameter is too narrow (0-003'" to 0-01"', Kolliker)
to admit the presence of marrow as well as of blood-vessels. As in bone,
the vascular canal in cement is surrounded by concentric laminae.
Cement, like other osseous tissues, is composed of a matrix, in which are
imbedded more or less numerous microscopic cells {corpuscles or lacuna),
from which proceed, in various directions, fine, delicate ray-like canals
(canaliculi), which by anastomosing constitute a system of tubes for the
convection of nutrient fluid through the tissue. The matrix or basal
substance usually appears to consist of lamellae, concentrically placed,
the centre of the tooth being the common centre. According to
Kolliker, the basal substance is sometimes granular, sometimes striated
in the transverse direction, sometimes more amorphous, frequently
lamellated. Mr Tomes describes it as granular, and likens it to a mass
of coherent fig-seeds or to oolite. The cells or lacuna: are scattered
through the matrix with some degree of regularity, being frequently,
(1) Tomes, Lectures on Dental Physiology and Surgery, p. 58 ; 1848.