Vision
We live primarily in visual world, and sight is our dominant sense. Although we can rely on our eyes to bring us many of the sights of the external world, they are not able to reveal everything. We see only objects that emit or are illuminated by light waves in our receptive range, which is only 1/70 of the entire electromagnetic spectrum.
Eyeball
It’s divided into three cavities: anterior chamber (the region between the cornea and iris), posterior chamber (between the iris and the lens) and vitreous chamber (occupies 80% of the eyeball – the entire space behind the lens).
The first two chambers, anterior and posterior, are both filled with aqueous humor, a thin, watery fluid that is essentially an ultra filtrate of blood similar to cerebrospinal fluid. It’s largely responsible for maintaining a constant pressure within the eyeball. It also provides oxygen, glucose and amino acids for the lens and cornea.
The vitreous chamber is filled with vitreous humor; a gelatinous substance, which is, actually modified connective tissue. It protects eyeball from collapsing as a result of external pressure. The wall of the eyeball consists of three layers: supporting, vascular and retinal.
Supporting layer consists mainly of though fibrous connective tissue. The posterior segment, which makes up 5/6 of the tough outer layer, is the opaque white sclera (Gr. skleros, hard), which gives the eyeball its shape and protects the delicate inner layers.
The anterior segment of the supporting layer is the transparent cornea. It bulges slightly. The light enters the eye through the cornea. The sclera and cornea are continuous. The cornea of this supporting layer contains no blood vessels.
Because the middle layer of the eyeball contains many blood vessels, it is called the vascular layer. The dark color of the middle layer is produced by pigments that help to lightproof the wall of the eye by absorbing stray light and reducing reflections. The posterior 2/3 of the vascular layer consists of a thin membrane called the choroids, which is essentially a layer of blood vessels and connective tissue between the sclera and the retina.
The vascular layer becomes thickened toward the anterior portion to form the ciliary body.