ight and by the elastic reaction of the surrounding parts.

*The Diaphragm.*--Another means of varying the thoracic space is found in
an organ known as the diaphragm. This is the dome-shaped, _movable
partition_ which separates the thoracic cavity from the cavity of the
abdomen. The edges of the diaphragm are firmly attached to the walls of
the trunk, and the center is supported by the pericardium and the pleura.
The outer margin is muscular, but the central portion consists of a strong
sheet of connective tissue. By the contraction of its muscles the
diaphragm is pulled down, thereby increasing the thoracic cavity. By
raising the diaphragm the thoracic cavity is diminished.

The diaphragm, however, is not raised by the contraction of its own
muscles, but _is pushed up_ by the organs beneath. By the elastic reaction
of the abdominal walls (after their having been pushed out by the lowering
of the diaphragm), pressure is exerted on the organs of the abdomen and
these in turn press against the diaphragm. This crowds it into the
thoracic space. In forced expirations the muscles in the abdominal walls
contract to push up the diaphragm.

*Interchange of Gases in the Lungs.*--During each inspiration the air from
the outside fills the entire system of bronchial tubes, but the alveoli
are largely filled, at the same time, by the air which the last expiratory
effort has left in the passages. By the action of currents and eddies and
by the rapid diffusion of gas particles, the air from the outside mixes
with that in the alveoli and comes in contact with the membranous walls.
Here the oxygen, after being dissolved by the moisture in the membrane,
diffuses into the blood. The carbon dioxide, on the other hand, being in
excess in the blood, diffuses toward the air in the alveoli. The
interchange of gases at the lungs, however, is not fully understood, and
it is possible that other forces than osmosis play a part.

[Fig. 43]

Fig. 43--*Diagram* illustrating lung capacity.

*Capacity of the Lungs.*--The air which passes into and from the lungs in
ordinary breathing, called the _tidal_ air, is but a small part of the
whole amount of air which the lungs contain. Even after a forced
expiration the lungs are almost half full; the air which remains is called
the _residual_ air. The air which is expelled from the lungs by a forced
expiration, less the tidal air, is called the _reserve_,