augmented seeing
The anterior chamber opens as a transparent vault between the corneal endothelium and the iris plane. Aqueous humor fills this space -- a clear, protein-poor fluid continuously produced by the ciliary body and drained through the trabecular meshwork at the iridocorneal angle. The chamber depth averages 3.11 millimeters at its central axis, shallowing toward the periphery where the iris root meets the scleral spur.
This space is not empty but dynamically pressurized, its fluid maintaining the structural integrity of the anterior segment and nourishing the avascular tissues it bathes.
The crystalline lens is a biconvex, avascular, transparent structure suspended behind the iris by the zonular fibers of Zinn. Its anterior surface has a radius of curvature of approximately 10mm in the unaccommodated state, flattening toward the equator. The posterior surface curves more steeply at 6mm radius. Between these surfaces lies a gradient refractive index -- not uniform glass but a living optic whose core (n=1.406) refracts more strongly than its cortex (n=1.386), creating a smooth gradient that reduces spherical aberration beyond what a homogeneous lens of the same shape could achieve.
The lens grows throughout life, adding concentric lamellae like tree rings. At age 20 it weighs 175mg; by 90, it has doubled. This lifelong accretion is the root of presbyopia and, eventually, cataract -- the slow opacification of accumulated protein.
The vitreous is 99% water, yet it holds its shape -- a gel of collagen fibrils and hyaluronic acid that fills the posterior four-fifths of the globe.
Cloquet's canal traces the path where the hyaloid artery once ran during fetal development -- a ghost vessel, its absence now a transparent corridor through the gel from the optic disc to the posterior lens surface.
With age, the vitreous liquefies. The collagen network collapses, forming the floaters that drift across the visual field -- shadows cast on the retina by the scaffolding of our own sight.
The retina is a 0.5mm-thick sheet of neural tissue lining the inner posterior surface of the globe. Ten distinct layers process light into electrochemical signal: photoreceptor outer segments absorb photons; bipolar cells relay graded potentials; ganglion cells fire action potentials along axons that converge at the optic disc and exit the eye as the optic nerve.
Rod photoreceptors number approximately 120 million, distributed across the peripheral retina and mediating scotopic (low-light) vision. Cone photoreceptors -- 6 million -- concentrate in the macula, with peak density at the foveal center.
The fovea centralis is a pit 1.5mm in diameter at the optical center of the retina. Here, the inner retinal layers are displaced laterally, creating a depression that allows light to reach the cone photoreceptors with minimal scattering. The foveal avascular zone -- devoid of blood vessels -- ensures maximum optical clarity at the point of highest acuity.
The optic disc -- the blind spot -- is a 1.76mm oval where ganglion cell axons gather and exit the eye. It contains no photoreceptors; light falling here is not perceived. The central retinal artery and vein enter and exit through the disc, branching across the retinal surface in a pattern unique to each individual.