these are muscular activity and emotional disturbance. Practically all the changes in the heart action that we observe from moment to moment can be explained as being due to one or the other of these causes. There are, however, two additional points to be noted briefly; the first is that after muscular exercise the heart slows down very gradually, not returning to its ordinary resting rate for a half hour to an hour after the exercise is over, depending on how long the exercise was kept up. The explanation of this long-continued rapid beat is found in the great outpouring of waste products as the result of the exercise. We have already learned that the functional metabolism of muscular work involves the oxidation of a large amount of energy-yielding material and therefore brings about the production of large amounts of oxidation products. Their presence in the blood serves as a stimulus to [Illustration: THE MICROSCOPE IS NECESSARY TO UNDERSTAND THE STRUCTURE OF THE BODY In this laboratory the workers are examining blood smears] [Illustration: Photo, Cornell University Medical School MICROPHOTOGRAPH OF BRAIN TISSUE] the nerve center in the brain stem, which acts to quicken the heart, and this keeps the rapid beat going until these products are gradually gotten rid of from the body. Another somewhat similar case is the prolonged rapid heartbeat following a violent emotion. The explanation of this we saw a couple of chapters ago in the outpouring of adrenalin that accompanies emotion. One property of adrenalin, as already noted, is to quicken the heart; so, as long as any adrenalin remains in the blood stream, the heartbeat will be faster than normal.