Foramen magnum

opening in the occipital bone of the cranium

The foramen magnum is one of several holes in the base of the skull. Normally the spinal cord is attached to it. The Foramen Magnum is the largest aperture in the primate body. This hole is located in the base of the human skull as part of the occipital bone, back of the monkey skull and in between those locations for apes. It is directly correlated with locomotion ( Humans are bipedal, monkeys quadrupedal and apes are knuckle walkers. )

Schematic of the foramen magnum
An opened skull. The foramen magnum is the large hole in the center; labelled '13'