The optic nerve consists of about 1.2 million very fine retinal ganglion cell fibres. The nerve within the orbit is slightly sinuous to allow for eye movements and is covered by extensions of the three layers of the meninges and a fine layer of cerebrospinal fluid.

The two optic nerves form a flattened cross, the optic chiasm, which overlies the tuberculum and diaphragma sellae to form the anterior part of the floor of the third ventricle. The nasal retinal fibres decussate at the chiasm so that left eye nasal fibres and right eye temporal fibres form the right optic tract and vice versa. As light enters the eye through the pupil, the nasal retina is ‘facing’ the temporal visual field, and vice versa. Pituitary tumours are the commonest cause of compression of the chiasm, resulting in bitemporal hemianopia.

The optic tract is a cylindrical bundle of nerve fibres continuing backwards in the line of the optic nerve of the opposite side, dividing into a lateral and a medial root. The larger lateral root ends in the lateral geniculate body. The pupillary reflex fibres travel on to the superior colliculus.

The optic radiation sweeps from the lateral geniculate body to the occipital cortex through the posterior end of the internal capsule, the lower fibres (serving the superior visual field) looping adjacent to the temporal lobe, while the upper fibres (serving the interior visual field) travel next to
the parietal lobe.

CLINICAL CORNER