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(also zonule of Zinn), in terrestrial vertebrates, including humans, a round ligament supporting the crystalline lens. The ciliary zonule was first described in 1755 by the German scientist J. Zinn. It consists of dense glycoprotein fibers attached to the basal membrane of the ciliary folds and to the equator of the lens capsule. The fibers of the ciliary zonule are covered by a mucopolysaccharide gel. The gel fills the spaces between the fibers and protects the fibers from the proteolytic enzymes of the anterior chamber of the eye. The gel imparts a membranous appearance to the anterior and posterior surfaces of the ciliary zonule. Tightening or loosening the ciliary zonule by contraction of the ciliary muscle alters the curvature of the lens and thereby effects accommodation.