Iridology (also known as iridodiagnosis is an alternative medicine technique whose proponents believe that patterns, colors, and other characteristics of the iris can be examined to determine information about a patient's systemic health. Practitioners match their observations to iris charts which divide the iris into zones corresponding to specific parts of the human body. Iridologists see the eyes as "windows" into the body's state of health.
Official description (Wiki)
Here are Iridology eye charts labeling the portions of both the left & right irises.
Left Eye Chart
Right Eye Chart
Not sure about the possible meaning of the letter 'Z,' however a naturopath that practices Iridology might be able to help.
[edit on 28/2/09 by Evasius]