determine whether the leaky valves are on the right side or the left side. Thus an accurate diagnosis of imperfect valves can be obtained. Of course the heart will not work well if its valves are not tight any more than will an ordinary pump, so that persons suffering from this trouble cannot have as good a circulation as those whose valves have nothing the matter with them. It is true that in most cases of imperfect valve action there is a compensation in the form of an increase in the size and strength of the heart muscle, so that the circulation is maintained in spite of poor valve action by harder work on the part of the heart. It is evident that in a case of this kind exceptional strains on the heart are more dangerous than if the heart is normal to begin with, so that persons with faulty valve action must avoid physical strains, such as sharp running after street cars or trains, which would be borne with impunity by ordinary individuals. Since faulty valves are a frequent result of acute rheumatism, which in turn comes from pus pockets, and since no way is known to cure a defective valve, once the trouble has developed, it is evident that prevention is of the utmost importance. Physical efficiency is very seriously hampered by poor heart action. One feature of the heart action with which we are all perfectly familiar is that both the rate and the vigor of the heartbeat vary greatly from time to time. When one is lying quietly, the heartbeat is at its lowest point. It becomes more rapid as one sits up, still more rapid upon standing, increasing still more with the taking of any form of muscular exercise, and in vigorous muscular exercise attains its greatest rapidity and force. The rate in this latter case may be fully double that of the quiet standing position, and, as the vigorous thumping tells us, the force is also very much increased. As we saw in Chapter VII the heart muscle works automatically, contracting and relaxing without being stimulated through the nervous system. The _variations_ in rate and force, however, are the result of nervous action. The heart muscle, as we have already seen, is under the same sort of nervous control as the smooth muscles and glands. It has passing to it two sets of nerves, one to slow it down, the other to speed it up. Both these sets of nerves arise from centers in the brain stem, and both these centers appear to be discharging continuously. So it works out that the heart muscle is under the constant influence of two opposing sets of nerves, and its actual rate and vigor depend upon the balance between them. This has the effect of making the heart extremely responsive to nervous influences. The slightest relaxation on the part of the nerves whose function is to cause slowing will lead to a prompt increase of rate, since the nerves that tend to cause increase are active all the time. Various things may bring about changes in the nervous balance governing the heart; chief of