air, it having been in contact with the warm air
passages. It is also loaded with aqueous vapor, imparted to it like
the heat, not in the depth of the lungs, but in the upper air passages.

Expired air contains, besides carbon dioxid, various impurities, many of
an unknown nature, and all in small amounts. When the expired air is
condensed in a cold receiver, the aqueous product is found to contain
organic matter, which, from the presence of _micro-organisms_,
introduced in the inspired air, is apt to putrefy rapidly. Some of these
organic substances are probably poisonous, either so in themselves, as
produced in some manner in the breathing apparatus, or poisonous as being
the products of decomposition. For it is known that various animal
substances give rise, by decomposition, to distinct poisonous products
known as _ptomaines_. It is possible that some of the constituents of the
expired air are of an allied nature. See under "Bacteria" (Chapter XIV).

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.