What Are Our Eyes Made Of?
The outer shell of the eyeball is made up of three coats: the fibrous tunic, composes of the cornea and sclera. The middle layer, known as the vascular tunic or uveal tunic, consists of the choroid, ciliary body, pigmented epithelium and iris and the innermost is the retina, which gets its oxygenation from the blood vessels of the choroid (posteriorly) as well as the retinal vessels (anteriorly).
The fibrous tunic is tough and elastic. It is opaque at the back, where it is called the sclera, and transparent at the front where it is known as the cornea. The uveal tunic is so called because of its similarity to a grape (Latin: uva). It contains the choroid membrane which supplies blood to the eye. The retina, or innermost coat, is an extremely delicate tissue and is only a fraction of a millimeter thick. Its inner layers carry blood from the central artery to the central vein.
At the centre of the eye is a circular opening, the pupil, and immediately behind it is the crystal line lens, which consists of a great many transparent fibers arranged in sheets like the layers of an onion and enclosed in a clear, elastic membrane, the capsule. The perfectly smooth surface of the lens is curved and it is suspended by a number of delicate threads.
The spaces of the eye are filled with the aqueous humour anteriorly, between the cornea and lens, and the vitreous body, a jelly-like substance, behind the lens, filling the entire posterior cavity. The aqueous humour is a clear watery fluid that is contained in two areas: the anterior chamber between the cornea and the iris, and the posterior chamber between the iris and the lens.
The lens is suspended to the ciliary body by the suspensory ligament (Zonule of Zinn), made up of hundreds of fine transparent fibers which transmit muscular forces to change the shape of the lens for accommodation (focusing). The vitreous body is a clear substance composed of water and proteins, which give it a jelly-like and sticky composition.
The human eye is an organ which reacts to light and pressure. As a sense organ, the mammalian eye allows vision. Human eyes help provide a three dimensional, moving image, normally colored in daylight. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth. The human eye can differentiate between about 10 million colors and is possibly capable of detecting a single photon.
Similar to the eyes of other mammals, the human eye’s non-image-forming photosensitive ganglion cells in the retina receive light signals which affect adjustment of the size of the pupil, regulation and suppression of the hormone melatonin and entrainment of the body clock.