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Parkinson was scientist who has disease in which his entire body got shakes , he did not hold pen in his hand , so that his name has given to this disease.
it causes due to lack of dopamine harmone! person who has parkinson can't hold pen properly , his hand shakes continuously . 
A progressive disease of the nervous system marked by tremor, muscular rigidity, and slow, imprecise movement, chiefly affecting middle-aged and elderly people. It is associated with degeneration of the basal ganglia of the brain and a deficiency of the neurotransmitter dopamine.