Iris. (ainbow)

So called from its resembling the rainbow in its many colors. Themembrane, stretched vertically in the anteriorpart of the eye, in the aqueous humor, in whichit forms a flat circular partition separating theanterior from the posterior chamber. It is theanterior part of the second tunic, and is perforated by a circular opening called the pupil,which is constantly varying in size, owing to thecontractions of its two sets of muscles. Its posterior surface is covered with a black coatof pigment which continues backward over theciliary body and choroid.

Source: Ophthalmic Dictionary including pronunciation, derivation and definition of the words used in Optometry and Ophthalmology by James J. Lewis, Oph. D. Professor of Optometry in the Northern Illinois College of Ophthalmology and Otology, Chicago.