a . STRUCTURE OF THE TEETH. 973 
are flattened from side to side. The second molars of the upper jaw have four, 
those of the lower jaw five cusps each. In every case the second are much larger 
than the first molars. The cusps are sharper and are separated by deeper fissures 
or fosse than those of the per- 
manent teeth, whilst the roots 
of the milk molars, except for 
their greater divergence, agree Banca ise 
with those of the permanent set. Centralincisor | 
1st molar 
2nd inolar 
Canine 
\ crowh 
The marked constriction at the 
neck of the milk teeth (Fig. 656) is 
due to a great thickening of the cap 
of enamel on the Crown, and its 
abrupt termination as the neck is 
reached. The enamel, too, is much 
whiter as a rule than in the per- 
manent teeth. It should be added 
that the labial surface of the canines 
and molars departs very markedly 
from the vertical ; it slopes strongly 
inwards towards the mouth cavity 
as it approaches the grinding surface Lateral incisor 
of the crown, which latter is, as a Canine 
result, much reduced in width. 
——2nd molar 
2nd molar 
2nd molar 
~~ erown 
The divergence of the fangs in ° era re 
the milk molars allows the crowns 
of the permanent premolars to fit FIG. 656.—THE MILK TEETH OF THE LEFT SIDE. 
in between them before the milk The masticating surfaces of the two upper molars are shown above. 
| molars are shed. In the second row the upper teeth are viewed from the outer 
or labial side. In the third row the lower teeth are shown 
in a similar manner; and below are the masticating surfaces 
of the two lower molars. In the specimen from which the 
, ; first upper molar was drawn the two outer or buccal cusps 
As mentioned above, the teeth were not distinctly separated, as is often the case. 
are composed of three special 
tissues, enamel, dentine, and crusta petrosa, in addition to the pulp which occupies the 
tooth cavity. The chief mass of the tooth is formed of dentine, which surrounds the 
pulp cavity and extends from crown to root ; outside this is a covering of enamel on the 
crown, and a layer of crusta petrosa or cement on the root. 
The enamel (substantia adamantina) is the dense, white, glistening layer which 
forms a cap, thickest over the cusps, for the portion of each tooth projecting above the 
gum (Fig. 649). At the neck it ceases gradually, being here slightly overlapped by the 
crusta petrosa. 
STRUCTURE OF THE TEETH. 
It is composed chiefly of phosphate and carbonate of lime (phosphate of calcium 89°82 per 
| cent, carbonate of calcium 4°37 per cent, magnesium phosphate 1°34 per cent, a trace of calcium 
| fluoride, other salts ‘88 per cent), and has generally been considered to contain about 3°6 per cent 
| of organie substance ; but this Tomes has recently shown to be inaccurate: “That which has 
heretofore been set down as organic matter is simply water combined with the lime salts. 
| Enamel is to be regarded as an inorganic substance composed of lime salts, which have been 
. deposited in particular patterns and formed under the influence of organic tissues, which have 
| themselves disappeared during its formation.” 
| Enamel consists of calcified microscopic prisms (prismata adamantina), radiating from 
the surface of the dentine, on which their inner ends lie, to the surface of the crown, 
on which they terminate by free ends. These prisims are hexagonal in shape, solid, and 
of considerable length, for most of them reach from the dentine to the surface of the crown 
without interruption. The prisms, which are calcified themselves, are held together by the 
smallest possible amount of calcified matrix (Tomes). In old teeth the cap of enamel is 
often worn away over the cusps, the dentine is then exposed, and is easily recognised by 
its yellowish colour, which contrasts strongly with the whiteness of the enamel. 
Whilst adjacent enamel prisms are in general parallel to one another, they do not 
usually take a straight, but rather a wavy course, and in alternate layers they are often 
inclined in opposite directions, thus giving rise to certain radial striations seen by 
reflected light (Schreger’s lines). Certain other pigmented lines, more or less parallel to 
the surface, are also seen in the enamel (brown striz of Retzius). They are due to true 
pigmentation (Williams), and mark the lines of deposit of the enamel during its develop- 
ment. The enamel prisms are more or less tubular in certain animals—viz. in all 
marsupials except the wombat, in the hyrax, certain insectivora, and certain rodents. 
