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 oneto 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 organs of the body. If therefore this air, which we inhale every instant, be not pure, the whole mass of blood is very soon contaminated, and the frame, in some part or other speedily experiences the bad effects. This will explain to us the almost miraculous benefits which are obtained by change of air, as well as the decided advantages of a free and copious ventilation. The prejudices against a free circulation of air, especially in the sick chamber, are productive of great evil. The rule as regards this is plain and simple: admit as much fresh air as you can; provided it does not blow in upon you in a stream, and provided you are not in a state of profuse perspiration at the time; for in accordance with the Spanish proverb
but if the whole of the body be exposed at once to a cold atmosphere, no bad consequences need be anticipated.
A great deal has been said about the necessary quantity of sleep; that is, how long one ought to indulge in sleeping. This question, like many others, cannot be reduced to mathematical precision; for much must depend upon habit, constitution, and the nature and duration of our occupations. A person in good health, whose mental and physical occupations are not particularly laborious, will find seven or eight hours' sleep quite sufficient to refresh his frame. Those whose constitutions are debilitated, or whose occupations are studious or laborious, require rather more; but the best rule in all eases is to sleep till you are refreshed, and then get up. If you feel inclined for a snug nap after dinner, indulge in it; but do not let it exceed half an hour; if you do, you will be dull and uncomfortable afterwards, instead of brisk and lively.