Expert: John Locke - 6/3/2007

Question
Hi, I was wondering if you could help me on a hopefully easy question: What is the average length of a Living Human Optic Nerve?

Answer
Thanks for using AllExperts, Daniel.

Your question is straightforward but that does not necessarily mean that it is easy: the dimensions of all different body parts necessarily differ from person to person. The dimensions are genetically controlled to a great extent, though nongenetic factors play a role as well (think of stretching a muscle to make it longer). Although the dimensions of the body are all different, they are generally all in proportion.

For instance, armspan is always roughly equal to one's height. One's hands usually extend to below the tops of one's hips. The bellybutton falls roughly a third of the way up the torso from the point where the legs join, and so forth. There are literally hundreds of these kinds of relationships for the human body, which any good art textbook will describe.

The optic nerve has a similar relationship (that is, the nerve is always just long enough to extend from the back of the eye to the brain, regardless of the size of one's head). From what I have been able to find, the optic nerve is 20-30 mm in length from the back of the eye to the optic chiasm, which is the point where the two optic nerves join. Beyond that, the optic nerves are no longer considered separate entities: two nerves (called the optic tracts) extend to the back of the brain from the optic chiasm, but these are considered distinct from the optic nerves themselves.

Here's Gray's reference on the subject:
http://www.bartleby.com/107/197.html