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 exercise _in the open air_, till a
slight degree of fatigue be felt; then to rest one hour, and breakfast.
After this rest three hours, "in order that the energies of the
constitution may be concentrated in the work of digestion;" then take
active exercise again for two hours, rest one, and then dine. After dinner
rest for three hours; and afterwards, in summer, take a gentle stroll,
which, with an hour's rest before supper, will constitute the plan of
exercise for the day. In wet or inclement weather, the exercise may be
taken in the house, the windows being opened, "by walking actively
backwards and forwards, as sailors do on ship-board."

We now come to the consideration of _air_. Pure air is as necessary to
existence as good and wholesome food; perhaps more so; for our food has to
undergo a very elaborate change before it is introduced into the mass of
circulating blood, while the air is received at once into the lungs, and
comes into immediate contact with the blood in that important organ. The
effect of the air upon the blood is this: by thrusting out as it were, all
the noxious properties which it has collected in its passage through the
body, it endues it with the peculiar property of vitality, that is, it
enables it to build up, repair, and excite the different functions and