The black
line on the illustration shows the approximate size and location of the site where
the surgeon removes the "notch" from the posterior fossa. If this were a 3-D
graphic, you'd see that the area of removal is not so much right at the back of the head,
but a bit lower than it appears on this particular illustration. The "notch", once removed, is not replaced. It is left out so that the cerebellar tonsils (which are right behind the notch) and brain stem/spinal cord have space to co-exist and also to allow for the free flow of cerebral spinal fluid.