CRYSTAL CLASS and CRYSTAL SYS TEM. See CRYSTALLOGRAPHY. CRYS'TALLIN. See GLOBULIN. CRYSTALLINE LENS (Fr.
Lat. crystailinus, Gk. npvara2.2,1p6g, krystallinos, from rp;fcFaD,onc, krystallos, crystal, from Kpvc7u sew, krystancin, to freeze, from kinkic, kryos, frost). A biconvex, transparent, solid body, situated immediately behind the pupil of the eye, and imbedded in the vitreous humor. Through it the rays of light from any object must pass to reach the retina. The crystal line lens is more convex on its posterior than on the anterior surface, and its shape andeonsist eney vary at different periods of life. In early youth it is nearly spherical and soft ; as age ad vances it becomes flattened and firm. In the adult human being it measures three-eighths of an inch transversely, and one-sixth of an inch in antero-posterior diameter. The lens is re tained in position by a capsule of equal trans parency, composed of tissue exactly similar to the elastic layer of the cornea. The lens has no vascular connection with its capsule, but is nour ished by means of a very delicate layer of nu cleated cells on its surface, which absorb nourish ment from the capsule.
An increase in the refracting power of the eye for the purpose of near vision is called 'accommo dation.' The mechanism of accommodation is as
follows: The ciliary muscle contracts, drawing forward the choroid and ciliary processes and re laxing the zonula. The lens, which had been flattened by the tension of the zonula, assumes. through its own elasticity, a more spherical shape. The posterior surface of the lens alters but little in shape. being fixed rather firmly in place: but the anterior surface becomes more con vex, and thus its refracting power is increased. The eye can sec objects accurately at every dis tance between the 'far point' (the most distant point of distinct vision for that eye) and the 'near point' The 'near point' is situated at. that place at which the eye can begin to see clearly the fine print on a page' held close to the eye and then moved slowly from it. Between the 'near point' and the eye vision is indistinct, hecausethe ciliary muscle cannot. by any effort, produce the amount of convexity of the lens requisite for so short a. distance. The term 'amplitude of ac commodation' denotes the amount of accommoda tive effort it makes in order to adapt itself from its 'far point' to its 'near point.'