Foramen Magnum

The foramen magnum  is a large opening in the occipital bone of the cranium. It is one of the several oval or circular apertures in the base of the skull (the foramina), through which the medulla oblongata (an extension of the spinal cord) enters and exits the skull vault. (from wikipedia)

here is the key to the above illustration, but I think that this website has a better tutorial

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