the size of the lungs.

The substance of the lungs consists of bronchial tubes, air-cells,
blood-vessels, nerves, and cellular membrane. The bronchial tubes are
merely continuations and subdivisions of the windpipe, and serve to
convey the external air to the air-cells of the lungs. The air-cells
constitute the chief part of the lungs, and are the termination of the
smaller branches of the bronchial tubes. When fully distended, they are
so numerous as in appearance to constitute almost the whole lung. They
are of various sizes, from the twentieth to the hundredth of an inch in
diameter, and are lined with an exceedingly fine, thin membrane, on
which the minute capillary branches of the pulmonary arteries and veins
are copiously ramified. It is while circulating in the small vessels of
this membrane, and there exposed to the air, that the blood undergoes
the change from the venous to the arterial state. So numerous are these
air-cells, that the aggregate extent of their lining membrane in man has
been computed to exceed twenty thousand square inches, or about ten
times the surface of the human body. Some writers place the estimate
considerably higher.

A copious _exhalation_ of moisture takes place in breathing, which
presents a striking analogy to the exhalation from the surface of the
skin already described. In the former as in the latter instance, the
exhalation is carried on by the innumerable minute capillary vessels in
which the small arterial branches terminate in the air-cells. Pulmonary
exhalation is, in fact, one of the chief outlets of waste matter from
the system; and the air we breathe is thus vitiated, not only by the
subtraction of its oxygen and the addition of carbonic acid gas, but
also by animal effluvia, with which it is loaded when returned from the
lungs. In some individuals this last source of impurity is so great as
to render their vicinity offensive, and even insupportable. It is this
which gives the disagreeable, sickening smell to crowded rooms. The air
which is expired from the lungs is rendered offensive by various other
causes. When spirituous liquors are taken into the stomach, for example,
they are absorbed by the veins and mixed with the venous blood, in which
they are carried to the lungs to be expelled from the body. In some
instances, when persons have drank copiously of spirits, their breath
has been so saturated with them as actually to _take fire_ and _burn_.
An instance of this ki