The middle fossa, deeper than the anterior cranial fossa, is narrow medially and widens laterally to the sides of the skull.It is separated from the posterior fossa by the clivus and the petrous crest.ehind the tuberculum sellae sellæ is a deep, depression the sella, turcica containing the fossa, hypophyseos which lodges the, hypophysis and presents on its anterior wall the middle clinoid. processes