Anterior and Posterior Ethmoidal Nerves
The ophthalmic nerve gives rise to the nasociliary nerve and subsequently the anterior ethmoidal, Posterior Ethmoidal, and infratrochlear nerves. The anterior ethmoidal nerve leaves the orbit to return to the cranial cavity to run above the ethmoid bone and the cribriform plate. It passes through the anterior ethmoid foramen to enter the nasal cavity. Within the nose it divides into a small medial internal branch and a lateral internal nasal branch which exits between the upper nasal cartilage and the nasal bone, and is renamed the external nasal branch. The posterior ethmoidal nerve supplies the mucosa of the sphenoid sinus and the posterior ethmoidal air cells.
The sensory distribution of the three branches of the trigeminal nerve is helpful in understanding a patient's complaint of facial pain or numbness. A quick screen of facial sensation is to touch each side of the forehead, cheek, and chin and to ask if the sensation is "about the same" on both sides. The ophthalmic branch is the afferent limb of the corneal reflex. This reflex is useful in evaluating a comatose patient because it allows examination of CN V, the pons, and CN VII (the efferent limb of the corneal reflex). In an awake patient, testing the corneal reflex provides objective information about the patient's complaint of facia numbness. With the patient looking to one side, lightly touch the edge of the cornea with the corner of a piece of tissue.