Specific exercises must be directed to central and harmonious effects. For example, expanding the chest and extending the balls of the feet downward as far as possible co-ordinates the parts that are used in standing, though in a different way. It gives extension to the parts; and to extend muscles is often the best way to bring activity into them. Formerly a horse was fed in a high trough in order to make him hold his head high, but no horse carries his head so high or has such a beautiful arch to the neck as the wild horse, that feeds on the ground. Weak muscles may often be improved by giving them extension. This eliminates constrictions and brings more rhythm or balanced activity in opposition to other muscles or in union with them. The co-ordination must be felt. When there are co-ordinations there will be a sense of satisfaction in the vital organs. The exercises will not weary. They will not be a strain or tax the strength. They accumulate vitality rather than waste it. Co-ordination must especially be studied and used consciously and deliberatively with reference to the chest. In the start of every exercise there should be, as has been said before, something of an increase of activity in the chest and the breath. 10. Practice all exercises as rhythmically as possible. Rhythm and co-ordination are the deepest lessons of life and are necessary to each other. Activity and passivity must alternate in proportion as far as possible in all exercise. Observe also that the active exertion of an exercise should determine the amount of the reaction. We should go as slowly in the recoil or eccentric contraction as we do in the concentric contraction. Nature is always rhythmic. Notice the beating of the heart, going on constantly for eighty or a hundred years. It acts and then re-acts. Observe, too, the rhythm of the peristaltic action of the stomach. An exercise must obey this universal law of nature. Jerks should never be permitted; but all be easy and gradual. Even the surrender of a movement should be gradual. The eccentric action which results is more important in many cases than the concentric. For example, in the diaphragm we make voice by an eccentric action of the inspiratory muscles. We take breath by a concentric action of the diaphragm, we give out breath in making voice by eccentric contraction. Rhythm, therefore, means primarily that there should be a rest after each exercise. If we feel very weary we should especially emphasize this rest. It is lack of this rest that causes strain and weariness and makes a person nervous. The normal effect of the exercises when practiced rhythmically, is to eliminate fatigue, correct nervousness and weakness. Rhythmic movements accomplish ten times more than unrhythmic ones, even if unrhythmic movements do not produce unhealthy and abnormal results.