Pupil, pupil dilatation, pupil constriction
The lens of the human eye is connected to its suspensory apparatus, the ciliar body of the ciliar muscle, by so-called zonular fibers. The iris is on front of the lens, with its circular (see inner ring) and radial (see outer ring) muscle fibers. This muscular structure creates a round hole, which is the pupil. The iris contains a sphincter muscle (parasympathetically innervated) and a dilatator muscle (sympathetically innervated). Contraction of the dilatator dilates the pupil (mydriasis), contraction of the sphincter constricts it (miosis). The pupil width can be max. 8mm, min. 1,5mm (miosis), on average it's 3mm.