j THE TEETH. 971 
proximal and distal surfaces are not in transverse but in oblique planes, sloping strongly back- 
wards and inwards, and converging somewhat internally. A knowledge of this is useful in 
determining the side to which an upper molar belongs, as is the fact that the anterior labial root 
is broader than the posterior (Fig. 654). ; 
As regards the number of cusps (Fig. 652) :-—The first upper molar has four cusps in 
practically all skulls (99 per cent); occasionally, indeed, another, but very rudimentary, cusp is 
present on the lingual side of the antero-internal (proximo-lingual) cusp. The second molar has 
either three or four in an almost equal proportion of Europeans, but more frequently four taking the 
teeth of all nations together. (According to Topinard, four cusps are present in 66 per cent of all 
races, and in 58 per cent of European, Semitic, and Egyptian skulls ; according to Zuckerkand], in 
73°5 per cent of the lower races and 45°6 per cent of Europeans). The third upper molar has three 
cusps much more frequently than four amongst Europeans (four cusps only in 36 per cent, although 
it has four cusps more frequently in certain lower races). It should be remarked that, while there 
are practically always four cusps in the first molar, still there is a tendency to the disappearance 
of the postero-internal (disto-lingual) cusp, which tendency grows more pronounced as we 
pass backwards to the second and third 
molars. The other cusps are practically 
constant. 
The three roots of the upper molars 
(Figs. 650, 651, and 654) are a large inner 
or palatal, sub-cylindrical in shape, and 
two external or labial roots, smaller and 
- 1st molar ——_— 
2nd premolar 
_- 1st premolar —_ 
flattened from before backwards. The Canine uh 
‘ = : {/ -—Canine —. Yh 
palatal fang, which is placed opposite [ (Lateral incisor \ \ Wy 
the posterior labial root, is often united { Central incisor 
to one of the others. The lower part 
of the antrum of Highmore generally 
extends down between the palatal and 
the two labial fangs (Fig. 637, p. 946), 
but the latter project on its floor more 
frequently than the palatal root. In 
the wisdoin tooth the three roots are 
frequently more or less united into a 
single conical process (Fig. 654). 
Lower Molars.— The crowns are 
more massive than those of the upper 
molars, and are elongated antero-pos- 
teriorly (Fig. 653). A crucial groove 
separates the four chief cusps from one 
another; this bifureates behind to en- 
close the fifth cusp, which lies slightly 
to the outer side of the middle of the 
tooth. The number of cusps present in 
the lower molars is as follows :—The 
first has usually five cusps (62 per cent 
of all races, 61 per cent of Europeans) ; 
the second has four cusps, as a rule (five 
Ist molar ——~__ 
Va 2nd premolar — > 
Ist premolar-—~ 
ae | 
Lateral incisor~ 
chi 
| | | Central incisor )| 
Fic. 654.—HOoORIZONTAL SECTIONS THROUGH BOTH THE UPPER 
AND Lower Jaws to show the roots of the teeth. The 
cusps In only 24 per cent of all skulls) ; sections were carried through the bones a short distance 
the lower wisdom tooth has four cusps from the edge of their alveolar borders. The upper figure 
a little more frequently than five (five shows the upper teeth, the lower figure the lower teeth. 
in 46 per cent of all skulls), but like Note the flattened roots of the lower incisors, the two 
the upper wisdom tooth it is extremely root canals in the anterior root of each lower molar, and 
variable. the confluence of the three roots of the upper wisdom 
The roots of the lower molars teeth. 
(Fig. 650), two in number, are flattened ; 
from before backwards, and very wide. The anterior of these has two root canals; the posterior 
but one (Fig. 654). The wisdom tooth has commonly two roots like its fellows ; occasionally the 
two are united. In determining the side to which a lower molar belongs, it should he remembered 
that the lower part of the root is generally curved backwards, and also that the blunter margin 
of the crown (see above) and the fifth cusp, if present, are on the outer side. 
Arrangement of the Teeth in the Jaws.—The teeth are arranged in each jaw 
in a curved row—the dental arch (arcus dentalis)—of approximately a semi-oval 
form (Figs. 652 and 653). The curve formed by the upper teeth, however, is wider 
than that formed by the lower set, so that when the two are brought in contact 
the upper incisors and canines overlap their fellows in front, and the outer cusps 
of the upper premolars and molars overlap the corresponding cusps of the lower 
teeth (Fig. 637, p. 946). It will also be seen that, as a rule, the teeth in one 
jaw are not placed exactly opposite their fellows, but rather opposite the interval 
between two teeth, in the other jaw (Fig. 655). This arrangement is brought 
about largely by the great width of the upper central incisors as compared with 
