ons. In diseases
involving the lungs, bronchial tubes, and the pleura, the rate may be
alarmingly increased, and the pulse is quickened in proportion.

210. The Mechanism of Breathing. The chest is a chamber with bony
walls, the ribs connecting in front with the breastbone, and behind with
the spine. The spaces between the ribs are occupied by the intercostal
muscles, while large muscles clothe the entire chest. The diaphragm serves
as a movable floor to the chest, which is an air-tight chamber with
movable walls and floor. In this chamber are suspended the lungs, the air
cells of which communicate with the outside through the bronchial
passages, but have no connection with the chest cavity. The thin space
between the lungs and the rib walls, called the pleural cavity, is in
health a vacuum.

Now, when the diaphragm contracts, it descends and thus increases the
depth of the chest cavity. A quantity of air is now drawn into the lungs
and causes them to expand, thus filling up the increased space. As soon as
the diaphragm relaxes, returning to its arched position and reducing the
size of the chest cavity, the air is driven from the lungs, which then
diminish in size. After a short pause, the diaphragm again contracts, and
the same round of operation is constantly repeated.

The walls of the chest being movable, by the contractions of the
intercostals and other muscles, the ribs are raised and the breastbone
pushed forward. The chest cavity is thus enlarged from side to side and
from behind forwards. Thus, by the simultaneous descent of the diaphragm
and the elevation of the ribs, the cavity of the chest is increased in
three directions,--downwards, side-ways, and from behind forwards.

It is thus evident that inspiration is due to a series of muscular
contractions. As soon as the contractions cease, the elastic lung
tissue resumes its original position, just as an extended rubber band
recovers itself. As a result, the original size of the chest cavity is
restored, and the inhaled air is driven from the lungs. Expiration may
then be regarded as the result of an elastic recoil, and not of active
muscular contractions.

[Illustration: Fig. 91.--Diagrammatic Section of the Trunk. (Showing the
expansion of the chest and the movement of the ribs by action of the
lungs.) [The dotted lines indicate the position during inspiration.]]

211. Varieties of Breathing. This is the mechanism of quiet, normal
respiration. When the r