r end of the mouth,
between the root of the tongue, and the passage into the stomach: its
upper part is termed the larynx; it forms the projection in the fore
part of the neck, which is more prominent in the male than the
female: its opening is called the glottis, and is covered with a
small valve, or lid, called the epiglottis, which is open while we
breathe, but shuts when we swallow any thing, to prevent its getting
into the lungs: sometimes, however, particularly when we attempt to
speak at the time we swallow, a small portion of our food or drink
gets into the larynx, and excites violent coughing until it is thrown
back again.

The windpipe is composed of cartilaginous rings, covered with
membrane, which keep it open: after having run downwards for the
space of a few inches, it divides into two great branches, each of
which is subdivided into a vast number of ramifications, ultimately
terminating in little vesicles, which, when distended with air, make
up the greatest part of the bulk of the lungs.

The cavity in which the lungs are contained is called the thorax, or
chest: and is bounded by the ribs, and backbone or spine, and
separated from the abdomen by a muscular membrane, called the
diaphragm. The thorax, by the action of the diaphragm and intercostal
muscles, is alternately enlarged and diminished. Suppose then the
thorax to be in its least state; if it become larger, a vacuum will
be formed, into which the external air will descend by its weight,
filling and distending the vesicles of the lungs.

The thorax, thus dilated, is brought back to its former magnitude,
principally by the relaxation of the muscles, which distended it, and
the natural elasticity of the parts, aided by the contraction of the
abdominal muscles; the thorax being thus diminished, a quantity of
air is expelled from the lungs. The muscles which distend the thorax
beginning again to act, the air reenters; and this alternate dilatation
and contraction, is called respiration. The entrance of the air into
the lungs, is termed inspiration, and its expulsion, expiration.

To form a more accurate idea of the manner in which respiration is
performed, let us suppose this room to be filled with water. On
enlarging the thorax, in the manner before mentioned, the water by
its weight would rush in, and fill the newly formed void; and, upon
the diminution of the capacity of the thorax, a part of this water
would be expelled. Just in the same manne