These nerves are derived from two thick bands which wind round the crura cerebri in their course from their deep origin in the corpora quadrigemina.

On reaching the inferior surface of the cranium, the two optic bands combine to form the commissure or chiasma of the optic nerves, which is lodged in a depression at the base of the cranium - the optic fossa.

It is important to understand the behaviour of these nerves on reaching the commissure. To look at, it would almost seem as if the nerves proceeding from the optic tracts had crossed in their course, and had gone to the eye on the side opposite to that on which they first appear before the chiasma is reached, but this is not exactly the case.

As a matter of fact, the great bulk of the nerves do cross, but a certain number of filaments continue on the original side and pass into the eye on that side. We find, therefore, that fibres from the left side mingle with fibres from the right, and together form the right optic nerve, and vice versa. It should also be noticed that some of the fibres are believed to cross from right to left in the optic chiasma, and to pass backward through the opposite optic tract to the brain.

When the nerve leaves the commissure it passes out of the cranium through the optic foramen, and reaches the orbital cavity to pierce the sclerotic coat of the lower part of the globe of the eye, and after passing through the choroid coat, opens out and forms a thin nervous expansion termed the retina.

The function of the optic nerve is to transmit to the brain the impressions made upon it by external objects, or, in other words, it is the nerve of sight.