The heart is often overstrained, and at times has been ruptured during violent exertion, as in lifting an immense weight. The various forms of heart-disease are common with those whose occupations involve severe muscular effort, as professional athletes and oarsmen. Hæmorrhages of various kinds, especially from the lungs, or rupture of blood-vessels in the brain, are not uncommon results of over-exertion. Excessive repetition of muscular movements may lead to permanent contractions of the parts involved. Thus sailors, mechanics, and others frequently develop a rigidity of the tendons of the hand which prevents the full extension of the fingers. So stenographers, telegraphers and writers occasionally suffer from permanent contractions of certain muscles of the arm, known as writer’s cramp, due to their excessive use. But the accidents which now and then may result from severe physical exertion, should discourage no one from securing the benefits which accrue from moderate and reasonable exercise. 86. Muscular Fatigue. We all know how tiresome it is to hold the arm outstretched horizontally even for a few moments. A single muscle, the deltoid, in this case does most of the work. Even in a vigorous man, this muscle can act no longer than four to six minutes before the arm drops helpless. We may prolong the period by a strong effort of the will, but a time soon comes when by no possible effort are we able to hold out the arm. The muscle is said to be fatigued. It has by no means lost its contractile power, for if we apply a strong electric stimulus to it, the fatigue seems to disappear. Thus we see the functional power of a muscle has a definite limit, and in fatigue that limit is reached. Illustration: Fig. 42.—A Well-Equipped Gymnasium. (From a photograph.) The strength of the muscle, its physical condition, the work it has done, and the mental condition of the individual, all modify the state of fatigue. In those difficult acts which involve a special effort of the will, the matter of nerve exhaustion is largely concerned. Thus, the incessant movements in St. Vitus’ dance result in comparatively little fatigue, because there is no association of the brain with the muscular action. If a strong man should attempt to perform voluntarily the same movements, he would soon have to rest. None of the movements which are performed independently of the will, as the heart-beats and breathing movements, ever involve the sensation of fatigue. As a result of fatigue the normal irritability of muscular tissue becomes weakened, and its force of contraction is lessened. There is, also, often noticed in fatigue a peculiar tremor of the muscles, rendering their movements uncertain. The stiffness of the muscles which comes on during severe exercise, or the day after, are familiar results of fatigue.