Illustration of the anatomy of the front of the eye. At centre is the pupil, the hole that allows light into the eye. It is surrounded by the iris, a coloured muscular ring that regulates the amount of light that enters the pupil. The long posterior ciliary arteries (1) and the anterior ciliary arteries (2, 3 and 4) supply blood to the sclera (white of the eye) and the muscles of the iris. From Traite d'Anatomie Humaine (1930) by French anatomist Leo Testut (1849-1925).