Which is the upper horizontal portion of the ethmoid bone?

The upper horizontal portion of the ethmoid bone is the cribriform plate. This structure is a critical component of the skull, situated at the roof of the nasal cavity and forming part of the floor of the anterior cranial fossa. The cribriform plate contains numerous small foramina (holes) through which the olfactory nerve fibers pass from the nasal cavity to the olfactory bulbs in the brain, making it instrumental in the sense of smell.

The other options represent different anatomical parts of the ethmoid bone. The perpendicular plate forms part of the nasal septum, while the ethmoidal labyrinth refers to a complex of air cells that contribute to the structure of the ethmoid bone and the nasal cavity. The olfactory fossa is a depression on the cribriform plate that houses the olfactory bulbs but is not itself the upper horizontal portion of the ethmoid bone. Understanding these distinctions is essential for grasping the anatomy of the skull and its functions.