eing regularly poured into the venous blood just before it
reaches the right side of the heart, and the second by the important
process of _respiration_, which takes place in the air-cells of the
lungs. The venous blood, having arrived at the right side of the heart,
is propelled by the contraction of that organ into a large artery,
leading directly, by separate branches, to the two lungs, and hence
called the _pulmonary_ artery. In the innumerable branches of this
artery expanding themselves throughout the substance of the lungs, the
dark blood is subjected to the contact of the air inhaled in breathing,
and a change in the composition both of the blood and of the inhaled air
takes place, in consequence of which the former is found to have
reassumed its florid or arterial hue, and to have regained its power of
supporting life. The blood then enters minute venous ramifications,
which gradually coalesce into larger branches, and at last terminate in
four large trunks in the left side of the heart, whence the blood, in
its arterial form, is again distributed over the body, to pursue the
same course and undergo the same change as before.

It will be perceived that there are two distinct circulations, each of
which is carried on by its own system of vessels. The one is from the
_left_ side of the _heart_ to _every part of the body_, and back to the
_right_ side of the _heart_. The other is from the _right_ side of the
_heart_ to the _lungs_, and back to the _left_ side of the _heart_. The
former has for its object nutrition and the maintenance of life; and the
latter, the restoration of the deteriorated blood, and the
_animalization_ or _assimilation_ of the _chyle_ from which the _blood_
is formed. This process has already been referred to as the _completion
of digestion_; for _chyle_ is not fitted to nourish the system until, by
its exposure to the atmospheric air in the lungs, it is converted into
_arterial blood_.

As the food can not become a part of the living animal, or the venous
blood regain its lost properties until they have undergone the
requisite changes in the air-cells of the lungs, the function of
respiration by which these are effected is one of pre-eminent importance
in the animal economy, and well deserves the most careful examination.
The term respiration is frequently restricted to the mere inhalation and
expiration of air from the lungs, but more generally it is employed to
designate the whole series