It consists of four parts: a horizontal or cribriform plate, forming part of the base of the cranium; a perpendicular plate, constituting part of the nasal septum; and two lateral masses or labyrinths.
The ethmoid generally is the most anterior region of the braincase. In early development, two pairs of cartilaginous rods form parallel to the notochord in the cranium: the anterior trabeculae and the posterior parachordals.
In humans, the sphenoid and ethmoid bones contribute to the floor of the braincase. The ethmoid is difficult to observe since it attaches the cranium to the face.
Between the orbits along the anterior portion of the cranium floor, the ethmoid bone composes the roof of the nasal cavity.
The perpendicular plate projects to form the the superior portion of the nasal septum, which then segregates the nasal cavity into two separate but equal chambers. The nasal fossa refers to these separate chambers of the nasal cavity.
The sphenoid and ethmoid regions of the human skull existed as a multiple cartilaginous structures in more primitive vertebrates which would later fuse and ossify
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