ween the air, the blood, and the tissues.

The respiratory organs in mammals are the mouth, nose, larynx,
trachea, bronchial tubes, and lung-tissue or air-cells proper. The
windpipe is made up of cartilaginous rings completed by membrane,
muscle, etc. (behind). The bronchial tubes are the continuation of the
windpipe, and branch tree-like until they become very fine. The
air-cells are built round these latter. The lung-tissue is highly
elastic. The lungs are made up of an elastic membrane, covered with
flat cells, and very abundantly supplied with a mesh-work of the
finest blood-vessels. The whole of the respiratory tract as far as the
air-cells is lined by mucous membrane.

The air consists essentially of 21 parts of oxygen and 79 parts of
nitrogen, with a variable quantity of watery vapor. Only a small
portion of the total oxygen of the air is removed before it is
exhaled. The respiratory act consists of (1) inspiration, and (2)
expiration; the latter is of a little longer duration than the former.
The rate of breathing in man is from 14 to 18 per minute, in the
resting state, or about one respiration to three or four heart-beats.
The quantity of air inspired depends on (1) the size of the thorax,
and (2) the extent of its movements. These are effected solely by
muscular contractions, and give rise to an increase in all the
diameters of the thorax. The lungs are closely applied (but not
attached) to the inside of the chest wall, and remain so under all
circumstances. When the chest cavity is enlarged by inspiration, the
air, pressing down into the elastic lungs, expands them as much as
possible, that is, as much as the chest walls will allow; but the
lungs are never at any time either filled with or emptied of air to
their utmost capacity. At most, the amount of expansion is very
moderate.

_The Quantity of Air in the Lungs._

1. The quantity of air inspired in quiet breathing is about 20-30
cubic inches.

2. The quantity that can be added to this by a deep inspiration is
about 100 cubic inches.

3. The quantity that can be expelled by a forcible expiration is about
100 cubic inches.

4. The quantity that cannot be expelled at all is about 100 cubic
inches.

The above are named: (1) The tidal air; (2) complemental air; (3)
supplemental air; (4) residual air. The quantity that can be expelled
by the most forcible expiration after the most forcible inspiration,
that is, the air that can be moved, indicatin