What is "hindbrains"

Usage examples of "hindbrains".

Then the Britannia was finally clear and outbound tourists rushed past, laughing excitedly and squealing as they stepped off the edge of the platform into what their hindbrains insisted was a hundred-foot sheer drop to the floor below.

Then the Britannia was finally clear and outbound tourists rushed past, laughing excitedly and squealing as they stepped off the edge of the platform into what their hindbrains insisted was a ­hundred-­foot sheer drop to the floor below.