Tentorium and incisura in situ (case of pituitary adenoma): Uncal herniation is somewhat more complex than tonsillar herniation. This photograph is a view of the anterior and middle fossae with tentorium intact. The cerebrum has been removed; brain stem and cerebellum lie beneath the tentorium within the posterior fossa. (In passing, note the pituitary adenoma compressing the optic chiasm; the subject of primary interest, however, is the tentorium.) The tentorial opening provides for anatomic continuity between structures in the posterior fossa and structures lying above the tentorium (i.e., supratentorial). This opening or notch is called the incisura of the tentorium. In uncal herniation the free edge of the tentorium around the incisura plays an analogous role to the foramen magnum in cerebellar tonsillar herniation.