Are Nervous Diseases Increasing? The direct use of the brain and nervous system has much less to do with the production of its diseases or even its serious disturbances than is usually believed. Most of these, as we have seen, are due either to the poisons of disease or alcohol, or to the fatigue-poisons, or other poisons, produced in the stomach, the liver, the muscles, or other parts of the body. The worst results of brain-work are due to the extent to which it deprives us of proper exercise and fresh air. Good, vigorous mental activity,--hard brain work, in fact,--when you are in good condition, is, if not overdone, as healthful and almost as invigorating as physical exercise or hearty play. We often hear it said that the rush and hurry of our modern strenuous life is increasing the number of mental diseases and nervous breakdowns. But there is no evidence that the strain of civilization upon our brains and nervous systems is damaging them, or that either nervous diseases or insanity are more frequent now than they used to be one hundred or five hundred years ago. In fact, all the evidence that we have points in exactly the opposite direction; for, as we have seen, most of these brain and nerve diseases are due to infectious diseases, bad food, and bad living conditions generally, all of which the progress of modern civilization is rapidly lessening and preventing. We are collecting our insane in modern hospitals and comfortable homes, instead of letting them wander in rags about the country, and this makes them live longer and seem more numerous. But the poorest and least highly civilized classes and races have much more insanity among them than those who live under more favorable conditions. FOOTNOTES: [27] Some of these coal-tar remedies are _Acetanilid_, and _Antipyrin_, and _Phenacetin_. CHAPTER XXII EXERCISE AND GROWTH Fatigue as a Danger Signal. The chief use of exercise in childhood, whether of body or mind, is to make us grow; but it can do this only by being kept within limits. Within these limits it will increase the vigor of the heart, expand the lungs, clear the brain, deepen sleep, and improve the appetite. Beyond these limits it stunts the body, dulls the brain, overstrains the heart, and spoils the appetite. How are we going to tell when these limits are being reached? Nature has provided a danger signal--fatigue, or "tiredness." Fatigue is due, not to complete exhaustion, but to poisoning of the muscle, or nerve, by its own waste substances. If the fatigue is general, or "all over," it is from these waste substances piling up in the blood faster than the lungs, skin, and kidneys can get rid of them. In other words, fatigue is a form of self-poisoning.