Muscular dystrophy is a hereditary muscular disease characterized by the breakdown of muscular tissue. Myositis can be caused by abnormalities of the body’s defence system (polymyositis and dermatomyositis). Muscular disorders can also be caused by conditions of the nervous system. In myasthenia gravis the junction of nerve and muscle is affected. Myotonia is a muscular condition associated with delayed muscular relaxation. In congenital myotonia, usually manifesting itself in childhood, the eyes cannot be opened when they are tightly shut, and the hand is not released when grasped. Myatonia atrophicans begins between the ages of 20 and 30; as well as the above-mentioned characteristics muscular tissue decay also sets in, particularly in the lower arms, neck and face. Both forms of the disease are hereditary. The congenital form usually runs a favourable course, and may clear up spontaneously in later life; myatonia atrophicans progresses slowly, and can cause invalidity.