970 THE DIGESTIVE SYSTEM. 
flattened from before backwards. One is placed anteriorly, the other posteriorly, 
and both are usually recurved in their lower portions (Fig. 650). As in the corre- 
Central incisor 
Ye Lateral incisor 
J 
/ Canine 
vist premolar 
\ 
. 
TH WudAy 
Fic. 653.—THe LOWER PERMANENT TEETH, viewed from above. 
sponding teeth of the upper jaw, the roots of the lower wisdom teeth are often more 
or less*united into a single mass. 
The chief characters of the upper and lower molars may be summarised thus :— 
Molars. 
1st Upper. | and Upper. 3rd Upper. 
Upper, | Cusps . : 4 3 or 4 3 or 4 | 
| Roots 3 7 | 3 5 3 (or 1) | 
f | ist Lower. 2nd Lower. | srd Toe | 
Lower Cusps : : ee 5 | lay 4 ee | 4 or ey | 
RGOtS | aM | of iii | Port)? 4 
The molars diminish in size from before backwards. This remark applies particularly to the 
wisdom teeth, which are extremely variable in form and position among civilised races. The 
long axis of the upper molars has a general direction downwards and outwards ; whilst that of 
the lower molars, which the former partly overlap, slopes upwards and inwards, with the result 
that the outer cusps of the lower molars lie in the groove separating the inner from the outer 
cusps of the upper teeth (Fig. 637, p. 946). As a further result of this overlapping, the outer 
edge of the crown is sharp and the inner edge rounded in the upper molars; whilst the inner 
edge is sharp and the outer edge rounded in the lower set. The cause of this is obvious. The 
outer margins of the upper molars overlap their fellows on the buccal side, whilst the inner 
margins of the lower molars overlap their fellows on the lingual side ; these margins, therefore, 
are subject to comparatively little attrition, and consequently remain sharp. The other margin 
of each tooth, on the other hand, strikes against the groove on the crown of the opposing tooth, 
and consequently becomes worn and round. 
The fissures which separate the cusps on the grinding surfaces of the molar teeth are generally 
continued as faint grooves on the labial and lingual surfaces. 
Upper Molars.—The crowns, as already stated, are rhomboidal in shape, and when viewing 
their grinding surfaces, as in Fig. 652, if the planes of separation between them be prolonged, 
they would strike the middle line near the back part of the hard palate ; in other words, their 
