Hiccough is a sudden jerking inspiration due to the spasmodic
contraction of the diaphragm and of the glottis, causing the air to rush
suddenly through the larynx, and produce this peculiar sound. Snoring
is caused by vibration of the soft palate during sleep, and is habitual
with some, although it occurs with many when the system is unusually
exhausted and relaxed.

Laughing consists of a series of short, rapid, spasmodic expirations
which cause the peculiar sounds, with characteristic movements of the
facial muscles. Crying, caused by emotional states, consists of
sudden jerky expirations with long inspirations, with facial movements
indicative of distress. In sobbing, which often follows
long-continued crying, there is a rapid series of convulsive inspirations,
with sudden involuntary contractions of the diaphragm. Laughter, and
sometimes sobbing, like yawning, may be the result of involuntary
imitation.

Experiment 112. _Simple Apparatus to Illustrate the Movements of
the Lungs in the Chest_.--T is a bottle from which the bottom has been
removed; D, a flexible and elastic membrane tied on the bottle, and
capable of being pulled out by the string S, so as to increase the
capacity of the bottle. L is a thin elastic bag representing the lungs.
It communicates with the external air by a glass tube fitted air-tight
through a cork in the neck of the bottle. When D is drawn down, the
pressure of the external air causes L to expand. When the string is let
go, L contracts again, by virtue of its elasticity.

[Illustration: Fig. 93.]

Coughing is produced by irritation in the upper part of the windpipe
and larynx. A deep breath is drawn, the opening of the windpipe is closed,
and immediately is burst open with a violent effort which sends a blast of
air through the upper air passages. The object is to dislodge and expel
any mucus or foreign matter that is irritating the air passages.

Sneezing is like coughing; the tongue is raised against the soft
palate, so the air is forced through the nasal passages. It is caused by
an irritation of the nostrils or eyes. In the beginning of a cold in the
head, for instance, the cold air irritates the inflamed mucous membrane of
the nose, and causes repeated attacks of sneezing.

217. How the Atmosphere is Made Impure. The air around us is
constantly being made impure in a great variety of ways. The combustion of
fuel, the respiration of men and animals, the exh