Active exercise is further divided into exercise of effort and exercise of endurance. Under exercise of effort are classed all gymnastic feats. The primary object of a gymnastic training or education is to produce a symmetric development of the entire body, while, on the other hand, the training necessary to execute gymnastic feats produces an overdevelopment of one part of the body at the expense of the rest, as is seen in the arm of the blacksmith and the leg of the danseuse. All exercises of effort, whether of strength, skill, or speed, demand and cultivate mental concentration, a rapid response of the muscle to the orders of the will, develop the power to accomplish complicated coördinations, and the knowledge of how these difficult movements may be performed with the least expenditure of nerve and muscle force. Exercising a muscle develops it up to its physiologic capacity, but if a muscle is habitually overworked, pathologic results occur, and instead of a quick, sharp contraction of the muscle, the contractions will be weak and uncertain, and, if carried too far, the muscle may eventually atrophy from overwork. Exercises of endurance include walking, running, swimming, and rowing—the range of movement in these is much more limited than in exercises of effort. In these, each movement is well within the individual’s powers, yet, by increasing the rapidity of the movements, or by their prolonged continuance, the total amount of muscular work accomplished may be very great. Normally, the contraction and relaxation of the muscles are comparatively slow, so that the poisonous waste matter producing fatigue is continually being removed from the tissues, and not allowed to accumulate; whereas, in exercises of effort, there is no time allowed for the scavengers to work, and fatigue of the most active muscles sets in rapidly. Fatigue may appear in several forms, depending on the character of the exercise which produced it. When the exercise is sufficiently active, the amount of waste matter thrown into the circulation is greater than can be eliminated by the lungs; breathlessness and palpitation of the heart result; so soon as the equilibrium between waste production and elimination is established, the individual is said to have gotten his second wind. Or, again, a slow pace, too long kept up, will produce exhaustion, so that the products of tissue waste accumulate, the beat of the heart is fast, irregular and weak, the nervous system becomes stupefied, and the muscles fail, to respond to the normal physiologic stimulus. This is a form of fatigue not infrequently found among zealous housewives, in which the demands made upon the nervous system by continual and carking family cares, added to the very strenuous work of the household, exhausts both nervous and muscular systems.