blood of a tired animal is injected into the vessels of one that is rested, the muscles of the latter exhibit the phenomena of fatigue. Evidently muscle is self-protective. During activity it prepares a “fatigue-substance” which poisons its own nerve-endings, making them worse conductors from nerve to muscle of the commands which descend from the brain. Not only does the fatigue-substance dull the nerve-endings in the particular muscle which has contracted, but, being distributed by the blood to the whole body, it produces a general effect. If the legs have been severely worked, they exhibit fatigue in the highest degree; but after a long walk the arms also are less ready and less capable than the state of their nutrition warrants. The condition of stiffness experienced for a day or two after excessive exercise is due to various causes in combination. The fact that it may be remedied by encouraging the circulation through the muscles most affected, as by hot baths and massage, tempts us to assign it also in large measure to accumulation of products of action; but the means taken to reduce stiffness favour the nutrition of the muscles both by giving them more food and by carrying off their waste. Equally remarkable with the self-protective disposition of muscle, which forbids it to give, except at the instance of increasingly urgent messages from the central nervous system, more than a part of the work of which it is capable, is its preparation for meeting an increased demand. It grows with use. Running increases the girth of the leg by developing especially the muscles of the calf. Raising weights enlarges the muscles of the shoulder and arm. Use-growth may reach inconvenient proportions. Nothing is more noticeable during the training of young athletes, whose nutritive responsiveness is at its height, than their liability to pass through a stage in which they are “muscle-bound.” Their legs grow bigger, but their pace falls off.