. Mastication and insalivation, which are
largely mechanical, prepare the food for certain chemical processes by
which it is dissolved. The first of these occurs in the stomach and to
this organ the food is transferred from the mouth. The chief organs
concerned in deglutition are the tongue, the pharynx, and the esophagus.

*The Pharynx* is a round and somewhat cone-shaped cavity, about four and
one half inches in length, which lies just back of the nostrils, mouth,
and larynx. It is remarkable for its openings, seven in number, by means
of which it communicates with other cavities and tubes of the body. One of
these openings is into the mouth, one into the esophagus, one into the
larynx, and one into each of the nostrils, while two small tubes (the
eustachian) pass from the upper part of the pharynx to the middle ears.

The pharynx is the part of the food canal that is crossed by the
passageway for the air. To keep the food from passing out of its natural
channel, the openings into the air passages have to be carefully guarded.
This is accomplished through the soft palate and epiglottis, which are
operated somewhat as valves. The muscular coat of the pharynx is made up
of a series of overlapping muscles which, by their contractions, draw the
sides together and diminish the cavity. The mucous membrane lining the
pharynx is smooth, like that of the mouth, being covered with a layer of
flat epithelial cells.

*The Esophagus*, or gullet, is a tube eight or nine inches long,
connecting the pharynx with the stomach. It lies for the most part in the
thoracic cavity and consists chiefly of a thick mucous lining surrounded
by a heavy coat of muscle. The muscular coat is composed of two layers--an
inner layer whose fibers encircle the tube and an outer layer whose fibers
run lengthwise.

*Steps in Deglutition.*--The process of deglutition varies with the kind of
food. With bulky food it consists of three steps, or stages, as follows:
1. By the contraction of the muscles of the cheeks, the food ball, or
bolus, is pressed into the center of the mouth and upon the upper surface
of the tongue. Then the tongue, by an upward and backward movement, pushes
the food under the soft palate and into the pharynx.

2. As the food passes from the mouth, the pharynx is drawn up to receive
it. At the same time the soft palate is pushed upward and backward,
closing the opening into the upper pharynx, while the epiglottis is made
to close the