Nervus terminalis (cranial nerve zero) in the adult human.

The presence of an additional cranial nerve (the nervus terminalis or cranial nerve zero) is well documented in many non-human vertebrate species. However, its existence in the adult human has been disputed. The present study focused on the structure and incidence of this nerve in the adult human brain. The nerve was examined post-mortem in 10 adult brains using dissection microscopy, light microscopy, transmission electron microscopy, and immunohistochemistry. In all specimens, the nervus terminalis was identified bilaterally as a microscopic plexus of unmyelinated peripheral nerve fascicles in the subarachnoid space covering the gyrus rectus of the orbital surface of the frontal lobes. The plexus appeared in the region of the cribriform plate of the ethmoid and coursed posteriorly to the vicinity of the olfactory trigone, medial olfactory gyrus, and lamina terminalis. We conclude that the terminal nerve is a common finding in the adult human brain, confirming early light microscopic reports.