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46 MACROSCOPIC ANAT03n^ OF THE HUMAN TEETH.

erupt them :>t tlic normal |)('ri()(l, /. c. seventecMi to twonty-one year.'< of
ago. Ill oiu'-fourth they erupt at irregular intervals to the thirtieth
vear, and in the remainder they may appear later, or the first, seeond,
third, or all o\' them, may he absent altogether. In one series of forty
adult skulls observed, twelve had one or more absent. The absence and
other erratic peculiarities of these teeth sometimes seem to be hereditary
and can be traced in families through several generations.
This tooth is often reduced in size and may be a mere peg (Fig. 26, a).
It is of very irregular form in civilized races, but is as large and as well
formed as the other molars in most
a b d e races low in the ethnological scale.
The contraction of the jaws through
disuse has much to do with the mal-
development of this tooth, and it is
often so cramped for room as to pro-
duce distressing irritation which ne-
The upper third molar
cessitates its removal. Impaction
and malposition of the third molars render them difficult of extraction
and are the fruitful source of many serious lesions. (See the chapter
on Extraction of Teeth.)
The upper third molar is more or less similar to the other upper
molars when perfect and well developed, but it is very erratic as to form
and structure.
This tooth, when well formed, is of trituberculate form (b), the
disto-lingual cingule being suppressed. This cingule diminishes grad-
ually from the first molar, in which it is well formed, to the second,
Mdiere it is reduced, then to the third, where it is almost or entirely
absent. The oblique ridge thus becomes the posterior marginal ridge
(c), as in the typical trituberculate molar. The three tubercles are
reduced and rounded. The sulci usually degenerate into fissures, as
the formation of this tooth is notoriously faulty. The enlarged mesial
fissures thus become the seat of extensive caries.
The buccal face resembles that of the first and second molars, but is
more rounded.
The lingual face (d) is full and rounded, with but a single lobe, owing
to the reduction or absence of the disto-lingual tubercle.
The mesial face (e) is similar to that of the second molar, but reduced,
and the dwtalface is round and short, as no tooth succeeds it in the rear.
The 7ieck is constricted and tapers toward the conate roots. It is of
a rather rounded triangular shape.
The three roots of the upper molars are, in the third, usually more
blunt, conate, short in form, and may curve backward. In lower races
and sometimes in individuals having strong osseous organizations, the