Optic Nerve

The optic nerve reaches from the optic chiasm to the eyeball, a distance of about 5 cm. (2 in.). It enters the apex of the orbit through the optic foramen at the upper inner angle, in company with the ophthalmic artery. The artery crosses the under surface of the nerve from its inner to its outer side. The optic nerve has t. as its covering a prolongation of the membranes of the brain. The dura mater when it reaches the foramen splits and gives one layer to form the periosteum lining the orbit and the other to form a fibrous sheath of the nerve. This arrangement prevents pus, forming in the orbit, from passing through the optic foramen into the skull. The arteria centralis retince enters the nerve on its under side and passes through its centre to the interior of the eye. The nerve itself is covered with a fine pial membrane and an arachnoid separating it from the dura, thus forming subdural and subarachnoid spaces. As these membranes and spaces are continuous with those of the brain, hemorrhage or serous effusions occurring within the brain can thus find their way into the sheath of the nerve.

As the nerve enters the eye, it is contracted and forms the optic disk or papilla.

Fig 94.   Diagram illustrating circulation of eyeball. (Leber).

Fig 94. Diagram illustrating circulation of eyeball. (Leber).

It is readily seen with the ophthalmoscope as a round spot somewhat lighter in color than the surrounding eyeground. Coming from a depression or cup in the disk, called the porus opticus, are the retinal arteries and veins. A certain amount of cupping is normal, but if wide and deep, with overhanging edges over which the vessels can be seen to dip, it is indicative of glaucoma.

Sometimes the papilla or disk is swollen, constituting an optic neuritis. In brain tumor this is frequently the case and is called choked disk, or "staining papilla" so named because the circulation was thought to be interfered with owing to the intracerebral pressure being transmitted directly to the nerve. On the subsidence of a severe neuritis the nerve is left in a state of optic atrophy and blindness is the result.

Fig. 95. Normal Human Fundus Oculi, Showing Optic Papilla And Blood Vessels; Also The Macula Lutea.

Fig. 95. Normal Human Fundus Oculi, Showing Optic Papilla And Blood Vessels; Also The Macula Lutea.