l the twelfth or
thirteenth year, they are gradually pushed out, one by one, by the
permanent teeth. The roots of the milk teeth are much smaller than those
of the second set.

[Illustration: Fig. 48.--Temporary and Permanent Teeth together.

_Temporary teeth:_
A, central incisors;
B lateral incisors;
C, canines;
D, anterior molars;
E, posterior molars

_Permanent teeth:_
F, central incisors;
H, lateral incisors;
K, canines;
L, first bicuspids;
M, second biscuspids;
N, first molars
]

The plan of a gradual succession of teeth is a beautiful provision of
nature, permitting the jaws to increase in size, and preserving the
relative position and regularity of the successive teeth.

[Illustration: Fig. 49.--Showing the Principal Organs of the Thorax and
Abdomen _in situ_. (The principal muscles are seen on the left, and
superficial veins on the right.)]

135. Structure of the Teeth. If we should saw a tooth down through
its center we would find in the interior a cavity. This is the pulp
cavity, which is filled with the dental pulp, a delicate substance
richly supplied with nerves and blood-vessels, which enter the tooth by
small openings at the point of the root. The teeth are thus nourished like
other parts of the body. The exposure of the delicate pulp to the air, due
to the decay of the dentine, gives rise to the pain of toothache.

Surrounding the cavity on all sides is the hard substance known as the
dentine, or tooth ivory. Outside the dentine of the root is a
substance closely resembling bone, called cement. In fact, it is true
bone, but lacks the Haversian canals. The root is held in its socket
by a dense fibrous membrane which surrounds the cement as the periosteum
does bone.

[Illustration: Fig. 50.--Section of Face. (Showing the parotid and
submaxillary glands.)]

The crown of the tooth is not covered by cement, but by the hard
enamel, which forms a strong protection for the exposed part. When
the teeth are first "cut," the surface of the enamel is coated with a
delicate membrane which answers to the Scriptural phrase "the skin of the
teeth." This is worn off in adult life.

136. Insalivation. The thorough mixture of the saliva with the food
is called insalivation. While the food is being chewed, it is
moistened with a fluid called saliva, which flows into the mouth from
six little glands. There are on each side of the mouth three salivary
glands, which secrete the saliv