A joint is the point at which two bones connect. In any moveable joint the essential feature is a sliding of one surface on another. Joined to the sides of the two bones near their ends are ligaments which are strong and inelastic and hold the joints within the joint cavity, and which prevent the joint from exceeding its normal range of motion. But the function of holding the bones together and keeping them in different positions belongs to the controlling muscles. Bones are usually activated by at least two sets of muscles which effect the movements in opposite directions. Normally muscles are under a slight but constant tension known as _tonus_, and the simultaneous pull of muscles on both sides of the joint presses the bone surfaces closer, and keeps the muscle in a state which makes immediate action possible. Joints move by virtue of the contractions of the muscles. Most movements are made not by one muscle alone, but rather by the co-ordinated contraction of various muscles and the simultaneous relaxation of their antagonists. As a result of muscle contractions, a chemical change takes place which produces substances in the muscle that interfere with good muscle action. Ordinarily these waste products are carried away by the circulating blood with sufficient speed to prevent noticeable effects. If, however, the muscle produces these deleterious chemicals faster than the blood stream can carry them away, fatigue results. The earliest manifestation of fatigue is inability to relax, and the second contraction may be initiated before relaxation is complete. The second effect of fatigue is interference with rate and quality of contraction. Only relatively brief periods of relaxation are necessary for complete recovery, but these periods are important. When normal muscles practice on the piano, the fatigue limit is rarely reached, but for the weakened muscles of patients, fatigue must be guarded against by limiting duration of continuous playing and by proper interludes of rest. Ordinary piano-playing offers short rest periods because there is a reflex relaxation after the sound is produced and it requires less muscle energy to keep the key depressed than to depress it. Muscles are excited into contraction by minute bio-electrical impulses which enter through their motor nerves, but the property of contraction is independent of the nerve and can also be accomplished by artificial external stimuli of electricity or mechanical force. The quality of contraction is a function governed by the health and nutrition of the muscle. The nutrition of the muscle depends upon its blood supply, which depends in part upon its warmth. Delicate motions are difficult for cold muscles and artificial warming is advisable before exercise, a fact which assumes greater importance in cold weather.