sential as wholesome food; for the air, by coming into immediate
contact with the blood, enters at once into the constitution. If therefore
the air be bad, every part of the body, whether near the heart or far from
it, must participate in the evil which is produced.

It is on this account that exercise _in the open air_ is so materially
beneficial to digestion. If the blood be not properly prepared by the
action of good air, how can the arteries of the stomach secrete good
gastric juice? Then, we have a mechanical effect besides. By exercise the
circulation of the blood is rendered more energetic and regular. Every
artery, muscle, and gland is excited into action, and the work of
existence goes on with spirit. The muscles press the blood-vessels, and
squeeze the glands, so that none of them can be idle; so that, in short,
every organ thus influenced must be in action. The consequence of all this
is, that every function is well performed. The stomach digests readily,
the liver pours out its bile freely, the bowels act regularly, and much
superfluous heat is thrown out by perspiration. These are all very
important operations, and in proportion to the perfection with which they
are performed will be the health and comfort of the individual.

There is another process accomplished by exercise, which more immediately
concerns the nervous system. "Many people," says Mr. Abernethy, "who are
extremely irritable and hypochondriacal, and are constantly obliged to
take medicines to regulate their bowels while they live an inactive life,
no longer suffer from nervous irritation, or require aperient medicines
when they use exercise to a degree that would be excessive in ordinary
constitutions." This leads us to infer that the superfluous energy of
the nerves is exhausted by the exercise of the body, and that as the
abstraction of blood mitigates inflammations, in like manner does the
abstraction of nervous irritability restore tranquillity to the system.
This of course applies only to a state of high nervous irritation; but
exercise is equally beneficial when the constitution is much weakened, by
producing throughout the whole frame that energetic action which has been
already explained.

A debilitated frame ought never to take so much exercise as to cause
fatigue, neither ought exercise to be taken immediately _before_ nor
immediately _after_ a full meal. Mr. Abernethy's prescription is a very
good one--to rise early and use active