The development by means of exercises of a strong muscular system has received much attention during recent years. Our ancestors cultivated strength and agility in certain movements without paying much attention to the muscles by which the movements were performed. It is fashionable nowadays to lay stress upon the importance of maintaining an abundant musculature, because of its relation to general “fitness.” The balance between muscular activity and the organic functions which is observed by everyone who takes an active holiday proves beyond doubt that the nutritive condition of the various glands and of the heart and bloodvessels is in some degree dependent upon the condition of the muscles. Possibly they secrete into the blood other “messengers” in addition to fatigue-substance—messengers whose call wakes up the organs of digestion. The man who is so fortunate as to be able to use his muscles in the open air has no need of exercises in his bathroom. Failing out-of-door opportunities, much can be done by the systematic use of the various muscles working against resistance. It is alleged, and we are not disposed to dispute the justice of the contention, that movements made with the fullest degree of mental concurrence have a more rapid effect upon the growth of muscle than actions more or less unconscious. Muscle and nerve are parts of a single mechanism. It may be that fixing the attention on an exercise, and watching its performance in a looking-glass, aids the nutrition of muscles by increasing the influence of their nerves, possibly by improving the nutrition of their nerve-centre. Unfortunately, this is one of many theories which hardly come within the reach of a control experiment. Could one concentrate attention on the movements of the right arm, then absent-mindedly repeat them with equal vigour with the left, it might be possible to ascertain whether there is anything in this idea. Two other contentions with regard to the best way of performing movements, with a view to the promotion of muscular growth, appear to be justified by their results. Working against a moderate or light load is said to be more effective than putting muscles to a severe strain. A small number of maximal contractions, it is said, induce more rapid growth than many partial shortenings. According to this scheme, when a particular muscle needs strengthening, because in a certain action it is to be the chief performer, it is made to bring its two ends as near together as the plan of its attachments allows. Maximal shortening is apparently favourable to blood-supply and otherwise promotes nutrition.