What are the two iris muscles and their histological location?

• A. Sphincter pupillae — circular muscle in the iris stroma near the pupil; Dilator pupillae — radial muscle in the iris posterior epithelium.
• B. Sphincter pupillae — radial muscle in the iris posterior epithelium; Dilator pupillae — circular muscle in the iris stroma near the pupil.
• C. Ciliary muscle — circular muscle in the iris stroma; Dilator pupillae — radial muscle in the iris posterior epithelium.
• D. Sphincter pupillae — circular muscle in the iris pigment epithelium; Dilator pupillae — radial muscle in the iris stroma.

Answer: (A)

Two iris muscles regulate pupil size: the sphincter pupillae, a circular smooth muscle that forms a ring around the pupil, and the dilator pupillae, a radial smooth muscle.

Histologically, the sphincter pupillae sits in the iris stroma right near the pupil. When it contracts, it narrows the pupil. The dilator pupillae has its origins in the posterior pigmented epithelium of the iris, and its fibers run radially through the iris stroma to the anterior border, widening the pupil when it contracts. This combination—circular muscle in the stroma near the pupil for constriction, and radial muscle from the posterior epithelium for dilation—is the correct description. Other descriptions misplace one muscle in the wrong layer or identify the wrong tissue arrangement, which doesn’t match histology.