em to cutting off and
separating the food (_D_, Fig. 65). The back teeth are broad and
irregular, having surfaces that are adapted to crushing and grinding.

Each tooth is composed mainly of a bone-like substance, called _dentine_,
which surrounds a central space, containing blood vessels and nerves,
known as the _pulp cavity_. It is set in a depression in the jaw where it
is held firmly in place by a bony substance, known as _cement_. The part
of the tooth exposed above the gum is the _crown_, the part surrounded by
the gum is the _neck_, and the part which penetrates into the jaw is the
_root_ (_A_, Fig. 65). A hard, protective material, called _enamel_,
covers the exposed surface of the tooth.

The teeth which first appear are known as the _temporary_, or milk, teeth
and are twenty in number, ten in each jaw. They usually begin to appear
about the sixth month, and they disappear from the mouth at intervals from
the sixth to the thirteenth year. As they leave, teeth of the second, or
_permanent_, set take their place. This set has thirty-two teeth of four
different kinds arranged in the two jaws as follows:

In front, above and below, are four chisel-shaped teeth, known as the
_incisors_. Next to these on either side is a tooth longer and thicker
than the incisors, called the _canine_. Back of these are two short,
rounded and double pointed teeth, the _bicuspids_, and back of the
bicuspids are three heavy teeth with irregular grinding surfaces, called
the _molars_ (_B_ and _C_, Fig. 65). Since the molar farthest back in each
jaw is usually not cut until maturity, it is called a _wisdom_ tooth. The
molars are known as the superadded permanent teeth because they do not
take the place of milk teeth, but form farther back as the jaw grows in
length.

[Fig. 66]

Fig. 66--*Diagram* showing directions of muscular fibers in tongue.

2. _The Tongue._--The tongue is a muscular organ whose fibers extend
through it in several directions (Fig. 66). Its structure adapts it to a
variety of movements. During mastication the tongue transfers the food
from one part of the mouth to another, and, with the aid of the cheeks,
holds the food between the rows of teeth. (By an outward pressure from the
tongue and an inward pressure from the cheek the food is kept between the
grinding surfaces.) The tongue has functions in addition to these and is a
most useful organ.

3. _The Muscles of Masticati