The _Mouth_ is an irregular cavity, situated between the upper and the
lower jaw, and contains the organs of mastication. It is bounded by the
lips in front, by the cheeks at the sides, by the roof of the mouth and
teeth of the upper jaw above, and behind and beneath by the teeth of the
lower jaw, soft parts, and palate. The soft palate is a sort of pendulum
attached only at one of its extremities, while the other involuntarily
opens and closes the passage from the mouth to the pharynx. The interior
of the mouth, as well as other portions of the alimentary canal, is
lined with a delicate tissue, called _mucous membrane_.

The _Teeth_ are firmly inserted in the alveoli or sockets, of the upper
and the lower jaw. The first set, twenty in number, are temporary, and
appear during infancy. They are replaced by permanent teeth, of which
there are sixteen in each jaw; four incisors, or front teeth, four
cuspids, or eye teeth, four bicuspids, or grinders, and four molars, or
large grinders. Each tooth is divided into the crown, body, and root.
The _crown_ is the grinding surface; the _body_, the part projecting
from the jaw, is the seat of sensation and nutrition; the _root_ is that
portion of the tooth which is inserted in the alveolus. The teeth are
composed of dentine, or ivory, and enamel. The ivory forms the greater
portion of the body and root, while the enamel covers the exposed
surface. The small white cords communicating with the teeth are the
nerves.

The _Tongue_ is a flat oval organ, the base of which is attached to the
os hyoides, while the apex, the most sensitive part of the body, is
free. Its surface is covered with a membrane, which, at the sides and
lower part, is continuous with the lining of the mouth. On the lower
surface of the tongue, this membrane is thin and smooth, but on the