iscuous crowd--exhalations offensive,
to a certain extent, from the most healthy individuals; but when
arising from a living mass of skin and lungs in all stages of
evaporation, disease, and putridity, they are in the highest degree
deleterious and loathsome.--BIRNAN.

Respiration is usually defined as the process by which air is taken into
the lungs and expelled from them. It explains the changes that take
place in these organs, in the conversion of _chyle_ and _venous_, or
worn-out blood, into _arterial_ or nutrient blood. In order to be
clearly understood, I must premise a few observations on the
circulation of the blood.[11] The blood circulating through the body is
of two different kinds; the one _red_ or _arterial_, and the other
_dark_ or _venous_ blood. The former alone is capable of affording
nourishment and supporting life. It is distributed from the _left_ side
of the heart all over the body by means of a great _artery_, which
subdivides in its course, and ultimately terminates in myriads of very
minute ramifications closely interwoven with, and in reality
constituting a part of, the texture of every living part. On reaching
this extreme point of its course, the blood passes into equally minute
ramifications of the _veins_, which in their turn gradually coalesce,
and form larger and larger trunks, till they at last terminate in two
large veins, by which the whole current of the venous blood is brought
back in a direction contrary to that of the blood in the arteries, and
poured into the _right_ side of the heart. On examining the quality of
the blood in the arteries and veins, it is found to have undergone a
great change in its passage from the one to the other. The florid hue
which distinguished it in the arteries has disappeared, and given place
to the dark color characteristic of venous blood. Its properties, too,
have changed, and it is now no longer capable of sustaining life.

[11] Taken, with slight alterations, from the description of Dr. A.
Combe.

Two conditions are essential to the reconversion of venous into arterial
blood, and to the restoration of its vital properties. The first is an
adequate provision of _new materials_ from the _food_ to supply the
place of those which have been expended in nutrition, and the second is
the free exposure of the _venous blood_ to the _atmospheric air_. The
first condition is fulfilled by the chyle, or nutrient portion of the
food, b