Ethmoid bone This voluminous but featherweight bone comprises much of the roof of the nose, contributing minimally to the floor of the anterior cranial fossa and to the medial walls of the orbits and thus part of the lateral wall of the nose. The perpendicular plate forms part of thebony nasal septum and projects above into the floor of the anterior cranial fossa as the crista galli. The most important point to note is that the olfactory nerves enter the nose through the cribriform plate, the foramina allowing contact with the dura above and the nasal periosteum below. The olfactory nerves, being an extension of the brain, do not regenerate after division as is the case with a peripheral nerve, and post traumatic anosmia will therefore be permanent. The dura and nasal mucoperiosteum are also almost in contact over the roof of the superior ethmoid air sinuses.  Any fracture involving these areas is liable to open the nasal cavity to the subarachnoid space, thus leading to cerebrospinal fluid rhinorrhoea and the opportunity for meningeal infection.