The Optic Nerve

The optic nerve is located at the posterior portion of the globe and transmits visual impulses from the retina to the brain itself. Only the head of the optic nerve, called the optic disc, can be seen by ophthalmoscopic examination. The optic nerve contains no sensory receptors itself, and therefore its position corresponds to the normal blind spot of the eye. Branching out from the surface of the optic disc are the retinal arterioles and veins, which divide soon after leaving the optic disc and extend out on the surface of the retina to supply the inner one third with nutrients. As the optic disc enters the globe, it goes through a fibrous, sieve like structure, visible on ophthalmoscopic examination, called the lamina cribrosa. When the lamina cribrosa is prominent, it forms the base of a depression in the disc called the physiologic cup. The optic nerve consists of 1 million axons arising from the ganglion cells of the retina. The nerve emerges from the back of the eye through a small circular opening, it extends for 25 to 30mm and travels within the external eye muscle cone to enter the optic foramen, the aperture at the back of the eye’s boney socket, where it travels for 4 to 9mm to pass into the intra-cranial cavity and joins its fellow optic nerve to form the optic chiasm.

The Visual Pathway

As the retinal fibres leave the optic nerves, half of them cross to the opposite side

The fibres that cross are derived from the retinal receptors nasal to the macula. The structure so formed by the mutual crossing of nasal fibres by both optic nerves is the optic chiasm. From the optic chiasm the nasal fibres emanating from the nasal half of the retina of one eye intermingle with the fibres derived from the temporal sector of the retina of the opposite eye, forming a band called the optic tract.

(Fig. 8)

Fibres in the optic tract continue toward a cell station in the brain called the lateral geniculate body, so named because this body in the brain is shaped like a knee (Latin genu). The geniculate body is a relay station. From here, fibres spread out in a fan shaped manner and extend to the parietal and temporal lobe of the brain. They continue to their final destination, the posterior portion of the brain called the occipital lobe in an area denoted as the visual cortex. It is in this area of the brain that it is thought that the first step in conscious recognition of visual impulses takes place.