Think of an Eye
In this macro close-up of a human eye, two major elements are visible: the pupil and the iris. The pupil, of course, is the central aperture through which light passes to strike the retina. It appears dark because light rays entering the pupil are either absorbed by tissues inside the eye directly or absorbed after diffusely bouncing around inside the eye, with very little of the light escaping the same way it came in.
The pupil is a marvel, but it’s really just a hole. The work of adjusting the size of that hole – and thus the amount of light allowed inside the eye – falls to the iris. If the pupil is the aperture, the iris is the diaphragm that serves as the aperture stop.
The iris consists of two layers: the front pigmented fibrovascular (a combination of fibrous and vascular tissues), known as the stroma, and a two-cell-thick layer of underlying pigmented epithelial cells. In fact, pigment granules are found in dark colored eyes; blue eyes and those of albinos lack pigment.
Two types of muscle occupy the stroma: a sphincter that contracts the pupil in a circular motion to reduce incoming light and a set of dilator muscles that pull the pupil open, like a ring of grabbing hands, to allow in more light.