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The article discusses the speed of mental processes and the relationship between blood flow and brain function. The author, Donders, conducted experiments to investigate the metabolism of the brain and the impact of blood flow on mental processes. He found that the loss of blood or suppression of heart consciousness is lost, which suggests that blood is an essential condition for mental processes. Additionally, he discovered that the blood undergoes a change as it nourishes the brain, leading to the formation of carbonic acid and the generation of heat. The author postulates that the heat may have originated from other forms of energy in the brain, such as electromotive action. The article appears in "Acta Psychchgica 30: Attention and Performance II" (1969) and was translated by W. G. Koster.