Buccal (outer) surfaces face to left. _A_, Upper tooth, _B_, lower
tooth; _I_, anterior, _I′_, posterior infundibulum, both almost
filled up with cement.
]
The =structure= is quite complex. Two infundibula run vertically through
the entire length of the crown; these become filled with cement. There
are five main divisions of the pulp-cavity and five enamel folds, four
of which are arranged symmetrically, while the fifth is an outgrowth
from the inner side of the antero-internal fold. On the exposed crown of
the unworn tooth the enamel folds form rounded ridges covered with a
thin layer of cement. After the tooth comes into wear the enamel on the
masticatory surface stands out in the form of sharp prominent ridges.
Progressive cementation of the periphery of the tooth takes place, thus
leveling up the irregularities of surface to a considerable extent.
The =lower= or =mandibular cheek teeth= are implanted in the rami of the
mandible, forming two straight rows which diverge behind. The space
between the rows is considerably less than that separating the upper
teeth, especially in the middle of the series. The length of the lower
teeth is about the same as that of the upper set. Their direction is
also similar, but the embedded portions diverge even more, with the
exception of the first and second. The long axis of the first is
vertical; the remainder project downward and backward in a gradually
increasing obliquity. The =buccal= (outer) =surface= has a longitudinal
furrow; the last molar has a secondary, shallower furrow in addition.
The =lingual= (inner) =surface= is uneven, but the grooves are not
regular; there are usually three on the first and last tooth. The
=masticatory= or =table surface= is oblique, sloping upward and inward
in correspondence with the opposing tooth; thus the inner edge is
prominent. The first five have two roots, while the sixth commonly has
three. The width of the lower molars is a little more than half that of
the upper. There are two infundibula, which are open along the inner
face of the tooth until closed by cement. The pulp-cavity has two
principal divisions, and four or three secondary diverticula. The enamel
folds correspond, forming a pattern even more complicated than on the
upper teeth.
[Illustration:
FIG. 253.—SKULL OF TWO-YEAR-OLD COLT, SCULPTURED TO SHOW THE EMBEDDED
PARTS OF THE TEETH.
Both permanent and temporary cheek teeth are shown. The upper first
premolar (“wolf tooth”) is present, but not visible; the lower one
is indicated by the arrow. Temporary chief premolars are numbered
_1_, _2_, _3_; permanent premolars and molars are designated by
Roman numerals; _Dc._, upper temporary canine; _C_, lower permanent
canine, which was not ready to erupt; _Di 2_, _Di 3_, second and
third temporary incisors; _I 1_, first permanent incisor, not quite
ready to erupt.
]