These impurities are eliminated from the body by means of the lungs, the skin, the kidneys and bowels. And in order that this elimination may be sufficient, the circulation must be maintained in all parts of the body. Unless the blood is kept moving, this waste matter will tend to collect somewhere or other and give rise to trouble. The way in which this motion is kept up is by exercise, which squeezes out the fluid like an automatic sponge. If the body was kept perfectly still for weeks, it would became loaded with this impure material, as a room that is shut up is found to collect, in such a surprising manner, dust and dirt. When it reaches the lungs it is purified by means of the air. For the air which we exhale is very different from that which is breathed in, the former being charged with impurities. The drowsy feeling which we experience in a crowded, ill-ventilated room is due entirely to the influence of these toxic gases. The purer the atmosphere we breathe, the more effective it is in carrying off impurities from the blood, so that fresh air and hygiene are essential to health, whilst exercise acts as a valuable adjunct by increasing the respirations. Yet exercise itself, important as it is, needs to be carried out in moderation. For the muscles, whilst fulfilling the vital functions just enumerated, produce a poison of their own, if exertion be too violent or prolonged. The severe cramp from which athletes are liable to suffer is due to an accumulation of this toxin. This poison is eliminated from the system during repose, and especially during sleep. Rest is therefore as requisite as exercise itself, and unless the body gets regular rest and sufficient of it, severe damage may result. The muscles will not only become permeated with their own peculiar poison, but will be so enfeebled as to be unable to assist in discharging the waste matter which is constantly being formed throughout the whole system. These impurities are the source of many of the ills with which mankind is afflicted--headaches, vague pains in various parts, languor, and the great majority of rheumatic troubles. But their worst effect of all is that which they exert on the nervous organisation. [Sidenote: Waste matter and the nervous system.]