The Brainstem is the lowest and most primitive part of the brain, with this structure being found in almost all vertebrates.  It is connected directly to the spinal cord. Almost all the nerves that control movement and the sense of touch run through the brain stem. More importantly, it regulates heart and lung function, maintaining heart beat and breathing autonomously even when a human loses consciousness or despite any attempt to control these functions consciously. Indeed, the brain stem appears to control consciousness and sleep.

Brainstem at Wikipedia

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