The posterior surface of the iris, facing the surface of the lens (+), is lined with two layers of epithelial cells (E).

The inner or superficial layer is composed of large cuboidal and heavily pigmented cells. Underlying it, the epithelial cells are smaller and less pigmented.

These smaller cells, considered myoepithelial cells, have long non-pigmented spindle-shaped processes at the base of the epithelium and are radially arranged in the iris. Bundles of such contractile processes form the dilator of the pupil. These contractile cells are not distinct in the present field but are clearly seen in Figure 18.17, where they form a distinct acidophilic layer under the pigmented epithelium.

The core of the iris is composed of loose connective tissue (*) plus smooth muscle fibres (M), seen here in cross section. This muscle, the constrictor of the pupil, forms a flat annular rim at the edge of the iris close to the pupil.

The anterior surface of the iris is not lined with an epithelium but is discontinuous and delimited only by connective tissue cells (arrows).

Stain: H–­­E
Magnification: ×250