Crystalline Lens, a lenticular transparent body, placed between the aqueous and vitreous humors of the eye in vertebrate animals, at about its anterior third; it is about four lines in diameter and two in thickness in man, and its axis corresponds to the centre of the pupil. The curvature of the lens is in proportion to the density of the medium in which the eye is habitually placed, being very flat in birds of the highest flight, and very convex in aquatic mammals and diving birds; in fishes it is almost spherical. This most important refracting structure of the eye is imbedded in the anterior portion of the vitreous humor, and is enclosed in a membranous capsule, to which it is prevented from adhering by the "liquid of Morgagni." Its structure is complicated, but it consists, when fully formed, of fibres arranged side by side, and united into laminae by serrations of their edges; the fibres originate in cells; the vessels are confined to the capsule, and are derived from the central artery of the retina; when hardened in spirit, it may be split into three sections, composed of concentric laminae; it is made up of 58 parts of water and 42 of soluble albumen; the central parts are the densest, and this property increases with age.

Besides its refractive power, necessary for distinct vision, it is generally believed that a change in its curvatures, by means of the ciliary muscle and the elasticity of its own tissue, is the mechanism by which the eye is adapted to distinct vision at varying distances; besides the anatomical arrangement of the parts, this view is rendered more probable by the development of this muscle in pre-daceous birds which have a great range of vision, and by the loss of this power of adaptation when the lens of the human eye is removed or displaced in the operation for cataract. . - For the diseases of the lens and its capsule, and their treatment, see Cataract.