At all events, these substances have an injurious action, for an atmosphere containing simply one per cent of pure carbon dioxid has very little hurtful effect on the animal economy, but an atmosphere in which the carbon dioxid has been raised one per cent by breathing is highly injurious.

The quantity of oxygen removed from the air by the breathing of an adult person at rest amounts daily to about 18 cubic feet.  About the same amount of carbon dioxid is expelled, and this could be represented by a piece of pure charcoal weighing 9 ounces.  The quantity of carbon dioxid, however, varies with the age, and is increased also by external cold and by exercise, and is affected by the kind of food.  The amount of water, exhaled as vapor, varies from 6 to 20 ounces daily.  The average daily quantity is about one-half a pint.

216.  Modified Respiratory Movements.  The respiratory column of air is often used in a mechanical way to expel bodies from the upper air passages.  There are also, in order to secure special ends, a number of modified movements not distinctly respiratory.  The following peculiar respiratory acts call for a few words of explanation.

A sigh is a rapid and generally audible expiration, due to the elastic recoil of the lungs and chest walls.  It is often caused by depressing emotions.  Yawning is a deep inspiration with a stretching of the muscles of the face and mouth, and is usually excited by fatigue or drowsiness, but often occurs from a sort of contagion.

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.]