The American encyclopedia and dictionary of ophthalmology Edited by Casey AWood, assisted by a large staff of collaborators . is at the posteriorpole. Each surface shows a zone of maximum thickness occupying aposition concentric with the equator toward the periphery of thelens. The lens substance is formed by the invagination of the ectoderm,and is, consequently, an epithelial structure—the capsule being de-rived from the mesoderm. For this reason the lens grows (by con-version of the marginal cells of the anterior capsule into younglenticular fibres) larger, heavier, thicker, and harder, as a
The American encyclopedia and dictionary of ophthalmology Edited by Casey AWood, assisted by a large staff of collaborators . is at the posteriorpole. Each surface shows a zone of maximum thickness occupying aposition concentric with the equator toward the periphery of thelens. The lens substance is formed by the invagination of the ectoderm,and is, consequently, an epithelial structure—the capsule being de-rived from the mesoderm. For this reason the lens grows (by con-version of the marginal cells of the anterior capsule into younglenticular fibres) larger, heavier, thicker, and harder, as age ad-vances. Lens substance in process of formation can be found at allages of life. The hardening process begins in childhood, and affects chiefly thecentral portions, so that in the adult a distinct condensation of thetissues occurs, and a nucleus can be demonstrated. Later the differ- ANATOMY (GROSS) OF THE HUMAN EYE 379 enee between the cortex and nucleus can generally be demonstratedwith the mirror. In old age, the whole lens increases so much inhardness that changes in its form are with difficulty brought about. Parts at Equator of Lens Showing the Growth of Its Epithelial Cells IntoLens Fibres. (Schultze.) by the ciliary muscle, and the loss of accommodative power is verymarked. Moreover, the denser nucleus reflects more light, and theoutline of the lens is seen through the pupil, which does not appearas black as in youth. This appearance generally deepens, until, byreflected light, it may show a distinct opalescence, not uncommonlygiving rise to a suspicion of cataract, even when no tiiie opacity ispresent. The adult lens is about 9 mm. in its transverse diameter, and isfrom 4 to 6 mm. thick. The posterior surface is somewhat flattened;the anterior surface has more the outline of a sphere. These, andother measurements vary with the state of the accommodation, withage, and with stature—very tall individuals having large specific gravity of the lens is gre
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectophthalmology, bookye
