Anterior Chamber and Vitreous Gel Humor

The anterior chamber and its fluid, the aqueous humor, occupy a space between the cornea and the lens (Fig. 2B). The size and volume of the eye's anterior chamber decrease with age, mainly due to thickening of the lens. This growth occasionally exerts pressure on the canal of Schlemm (Fig. 2B), an outflow channel at the junction between the iris and the cornea, causing decreased flow and increased pressure [intraocular pressure (IOP)] of the aqueous humor. In normal aging, the increase in IOP is small and steady. Severe obstruction of the canal of Schlemm caused by degenerative changes in the endothelial cells of the trabecular sheets and

FIGURE 3 Changes in visual accommodation in humans with age. Note that the decline occurs throughout life, resulting in presbyopia in the early 50s. Source: From Ref. 36.

FIGURE 4 Changes with age in pupillary diameter and area, measured in light-adapted and dark-adapted individuals. Note the more pronounced effects in dark-adapted eyes. Source: From Ref. 13, based on the original data of Ref. 41.

FIGURE 3 Changes in visual accommodation in humans with age. Note that the decline occurs throughout life, resulting in presbyopia in the early 50s. Source: From Ref. 36.

FIGURE 4 Changes with age in pupillary diameter and area, measured in light-adapted and dark-adapted individuals. Note the more pronounced effects in dark-adapted eyes. Source: From Ref. 13, based on the original data of Ref. 41.

meshwork leads to markedly elevated IOP (>22 mmHg) and the serious eye disease glaucoma (7,8,10) (see below).

The vitreous humor, also called vitreous gel, is a mass of gellike substance filling the eye's posterior chamber. It gives the eye globe its shape and internal mechanical support (Fig. 2B). With age, the vitreous humor loses its gel-like structure and support, becoming more fluid and pigmented. The increasing inhomo-geneity in its gel structure, a process called syneresis, can lead to vitreous collapse or its detachment from the retina; during this process, often vitreous floaters (inclusion bodies) are released in the process, which are responsible for occasional visual flashes. These physical changes in the vitreous humor may also be due to aging changes in its collagenous fibrous skeleton, which has attachments to the retina, particularly in the vitreous gel base near the periphery. These attachments change with age, moving posteriorly and decreasing in number (9,10,12).

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