Before revision: Causes 300x300px| Naturally dilated pupil (measuring 9mm) as a result of dim lighting There are two types of muscle that control the size of the iris: the iris sphincter, composed of circularly arranged muscle fibers, and the iris dilator, composed of radially arranged muscle fibers.
After revision: Causes 300x300px| Both pupils are dilated (mydriasis) because of hyper-activation of the sympathetic system caused by the psychedelic drug LSD There are two types of muscle that control the size of the iris: the iris sphincter, composed of circularly arranged muscle fibers, and the iris dilator, composed of radially arranged muscle fibers.