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Ophthalmology A Short Textbook - part 4

Những sợi thứ cấp mới thay các sợi chủ yếu về phía trung tâm của ống kính. Hình thành của một hạt nhân bào thai bao quanh hạt nhân phôi là hoàn tất khi sinh. Sợi hình thành tại đường xích đạo, tiếp tục trong suốt cuộc đời, sản xuất hạt nhân trẻ con trong | 168 7 Lens of cornea surface of the iris Fig. 7.4 The various density zones 1 -4 created as the lens develops are discernible as discontinuity zones. from the aqueous humor into the lens as well as passive diffusion through the posterior lens capsule. Maintaining this equilibrium homeostasis is essential for the transparency of the lens and is closely related to the water balance. The water content of the lens is normally stable and in equilibrium with the surrounding aqueous humor. The water content of the lens decreases with age whereas the content of insoluble lens proteins albuminoid increases. The lens becomes harder less elastic see Loss of accommodation and less transparent. A decrease in the transparency of the lens with age is as unavoidable as wrinkles in the skin or gray hair. Manifestly reduced transparency is present in 95 of all persons over the age of 65 although individual exceptions are not uncommon. The central portion or nucleus of the lens becomes sclerosed and slightly yellowish with age. 7.2 Examination Methods Cataracts Retroillumination of the lens Briickner s test is the quickest preliminary examination method for lens opacities Cataracts see section 7.4 . Under a light source or ophthalmoscope set to 10 diopters opacities will appear black in the red pupil Fig. 7.5 . The lens can be examined in greater detail and in three dimensions under focal illumination with a slit lamp with the pupil maximally dilated. The extent type location and density of opacities and their relation to the visual axis may be evaluated. Mature lens opaci- Lang Ophthalmology 2000 Thieme All rights reserved. Usage subject to terms and conditions of .