It’s not that the demon or machine takes the ‘brain’ out of their body. He distrusts his sense experience because he speculates that his body could be controlled by a ‘malicious demon’ - the machine reference was actually something added on later by Gilbert Ryle who in his work ‘the concept of the mind’ (1949) critiqued Descartes notion of the mind body and soul problem, he called it the ‘ghost in the machine’ model to describe Descartes, the body being the machine. Furthermore, it’s not the brain he is concerned with it’s the soul because the brain is a material thing that he is a aware of, he even argues that the soul interacts with the body through the pineal gland of the brain