The mind and the body are usually seen as opposites. Opposites are often seen as being "at war"--for instance, heat and cold try to destroy each other. If you spend an hour reading, you help your mind but make your body fatter; if you spend all your time exercising, you help your body but hurt your mind. The neck connects the head (mind) and the body (body). A no-man's-land is the region between two armies that neither army controls. The neck isn't controlled by either the head or the body, so it is like a no-man's-land or border town between them.