Posterior superior view of the frontal and ethmoid bones. There are two parts of the frontal bone. The larger vertically directed squamous portion forms the forehead and anterior part of the neurocranium, and the horizontal orbital part forms the roof of the orbit and most of the floor of the anterior cranial fossa. The remainder of this floor is contributed by the lesser wings and planum sphenoidale of the sphenoid bone. The ethmoid notch receives the cribriform plate of the ethmoid bone and the adjacent portions of the frontal bone form the roof of the ethmoid air cells or sinuses. The cribriform plate forms the roof of the nasal cavity and features multiple sieve-like openings for the neurofilaments of the olfactory nerve (CNI) that extend from the olfactory epithelium in the nasal cavity to the olfactory bulbs. Projecting superiorly in the midline is the crista galli for attachment to the falx cerebri. The ethmoidal labyrinth is a paired structure that extends inferiorly and consists of anterior, middle, and posterior air cells, the orbital plate laterally, and the superior and middle turbinates. The orbital plate is a very thin plate of bone that covers ethmoid air cells on the orbital side. This plate separates the orbit from the ethmoid sinuses and nasal cavity and forms most of the medial wall of the orbit. (Image courtesy of AL Rhoton, Jr.)