(Chapter 12) Footnote 36:

It is not clear which foramen Vesalius is describing here. If, as both editions of the Fabrica have it, there is one on each side, this would be the slit for the nasociliary nerve. But if, as Vesalius says here, it transmits a vein instead of a nerve, utrinque could be a misprint carried down from the line above and the reference would be to the foramen caecum, labeled h in fig. 3. When the foramen caecum is patent it usually transmits a small vein from the nasal cavity to the superior sagittal sinus (Williams et al., Gray’s Anatomy (1995), p. 1582).