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Before we go further, we need to catch up on something that we left unfinished in the previous section: understanding the ethmoid bone. We’ve encountered the various parts of the ethmoid bone, but till now we’ve put off seeing the whole of it.

We’ll do that now, then we’ll come back and look at the openings of the paranasal sinuses. The ethmoid bone is a fragile coalition of parts. The best way to see all of them is to go back to the skull that was divided in the frontal plane.

All of this is the ethmoid bone. This part, the perpendicular plate of the ethmoid, forms a large part of the bony nasal septum. This upward projection is the beginning of the crista galli, which rises up in the floor of the anterior cranial fossa.

On each side of the crista galli are the cribriform plates, which we've seen already from above, and from below. The most lateral part of the ethmoid bone is this paper-thin layer, the lamina papyracea, which forms this part of the medial wall of the orbit.

Between the lamina papyracea and the upper part of the nasal cavity are the ethmoid air cells, as we’ve seen. The superior and middle conchae are also parts of the ethmoid bone. The ethmoid bone is joined to the frontal bone above, the maxillae below, and the central part of the sphenoid bone behind.