The organs of respiration are the larynx, the trachea, or windpipe,
bronchia, and the lungs.
The air is expelled from the lungs principally by the action of the
muscles of respiration; and when these relax, the lungs expand by virtue
of their own elasticity. This may be exemplified by means of a sponge,
which may be compressed into a small compass by the hand, but, upon
opening the hand, the sponge returns to its natural size, and all its
cavities become filled with air. The purification of the blood in the
lungs is of vital importance, and indispensably necessary to the due
performance of all the functions; for if they be in a diseased
state,--either tuberculous, or having adhesions to the pleura, their
function will be impaired; the blood will appear black; loaded with
carbon; and the phlebotomizer will have the very best (worst) excuse for
taking away a few quarts with a view of purifying the remainder! The
trachea, or windpipe, after dividing into smaller branches, called
_bronchia_, again subdivides into innumerable other branches, the
extremities of which are composed of an infinite number of small cells,
which, with the ramifications of veins, arteries, nerves, and connecting
membranes, make up the whole mass or substance of the lungs. The
internal surface of the windpipe, bronchia, and air-cells, is lined with
a delicate membrane, highly organized with blood-vessels, &c. The whole
is invested with a thin, transparent membrane--a continuation of that
lining the chest, named _pleura_. It also covers the diaphragm, and, by
a duplication of its folds, forms a separation between the lobes of the
lungs.