Now the full effects of muscular movement are far too complicated to be spoken of with any completeness. Three, however, of the simplest and most obvious may be mentioned. Exercise, as everyone knows, is productive first of quickened respiration, quickened circulation, and heat. Just as an engine burns coal in order to produce the steam which moves it, so the muscles consume material in their working, and this consumption of material again gives rise to waste products partly given off (in the form of carbonic acid gas) by the lungs, which therefore are called upon to supply a greater amount of oxygen. Hence the quickened respiration. Similarly, more blood is required to feed the working part, hence the heart is called upon to supply it with greater speed. And thirdly, since exercise produces heat, and heat perspiration, the pores of the skin are called upon to open for the purpose of exudation, and pass out with the sweat many waste products.[1] Exercise, in short, develops not only the particular muscles used, but whatever muscle is used, the heart and the lungs as well (since they have muscles of their own, which are called upon to work), and it gets rid of waste products in the body by means immediately of the skin, and, if certain most important muscles are used, by facilitating the action of the kidneys and bowels. It also, as we have seen above, quickens the co-ordination of brain, nerve and muscle, until with practice many movements become automatic instead of comparatively slow. At the same time it uses up nervous force just as it does muscular force, but only temporarily, since rest and sleep automatically (and nobody yet knows how) restore both.