LANDRY,, S.O.; SUNY Binghamton.: The Course of the Nasociliary Nerve- A Minor Mystery Solved.

The nasocilary nerve in mammals takes a rather peculiar course. It branches from the ophthalmic division of the trigeminal nerve, and runs through the ethmoid foramen into the cranial fossa of the cranial cavity. Here the nerve pushes upward across the cribriform plate to a point near the midline where it passes through the cribriform plate into the nasal cavity. From this point (the nerve now called the ethmoid nerve) gives branches to the medial region of the nasal cavity, and eventually exits between the nasal bone and nasal cartilage to terminate in the rhinarium. In non-mammalian vertebrates, a distinct nasal cavity, scarcely exists, but in the mammalian line, beginning with Early Triassic therapsids, the presence of an infraorbital foramen (by which the the terminal portion of the maxillary division by-passes the nasal cavity laterally to reach the rostrum) indicates that a medially isolated nasal cavity was present. The mammalian nasociliary nerve is the ophthalmicus profundus branch of the trigeminal, forced to take this circuitous route to the anterior-most midline structures by the isolation of the nasal cavity. The cartilaginous nasal cavity at the tip of the nose and the rhinarium, both appear to be neomorphs in Therian mammals.