(a) Control horse. Normal portion of the posterior surface of the pupillary margin of the iris with both the dilator muscle (arrow) and sphincter muscle present (asterisk). Masson’s trichrome. Bar = 200 μm. (b) Horse B. Posterior surface of the iris with thin dilator muscle (arrow); the sphincter muscle was absent. Note also the poorly pigmented and disorganized epithelium on the posterior surface of the iris. Masson’s trichrome. Bar = 200 μm.
