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."