Modified version of diagram by
On one hand, there is a clear boundary between the brain and the body. Reach into a cracked-open skull, peel away its linings and you can pluck a brain right out of a head. And in school diagrams, it’s clear where the brain stops and the body begins. The brain is the worm-like ball of pink, sometimes with a tiny tail at the bottom, sometimes not, floating in the middle of the head. So in one sense, yes, in a diagram-sense, there is a clear boundary between the brain and the body, but that boundary is an illusion. Of all the human organs, the brain is the most difficult to contain.