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 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.”