The ophthalmic nerve is the smallest of the three divisions of the trigeminal nerve. It arises from the upper medial part of the trigeminal ganglion, enters the cavernous sinus at once, runs forward in the lateral wall of the sinus, and divides into three branches – frontal, lacrimal, and nasociliary – which pass into the orbit of the eye through the superior orbital fissure. The nasociliary usually arises first, then the lacrimal, and the main part of the nerve is continued as the frontal. Near its origin, the ophthalmic nerve gives off a minute branch to the tentorium cerebelli; and, as it runs in the sinus, it receives filaments from the sympathetic plexus around the internal carotid artery, and gives filaments to the oculomotor, trochlear, and abducent nerves – the first two of which lie above it in the sinus.