Anatomy of the eye
The eye is the organ that allows us to see. The eyeball itself is a sphere spanning approximately 24 mm in diameter. It is suspended in the bony socket by muscles controlling its movements, and is partially cushioned by a thick layer of fatty tissue within the skull that protects it during movement.
The eyes move symmetrically (in the same direction at the same time). These symmetrical movements are made possible through the coordination of the extra ocular muscles (muscles outside the eye).
Since the eyes are paired structures, the brain receives two slightly different images that overlap with one another. Interpretation of the different images is possible via coordinated eye movements achieved by complex neural mechanisms. Humans are also able to perceive three-dimensional images because they possess binocular vision, which enables the perception of depth and distance.
The eyeball consists of three main components:
The tunics, which are three layers that make up the wall of the eyeball
The optical components, also known as the retractile media components, which admit and focus light
The neural components, which consist of the retina and the optic nerve. The retina is also part of the inner tunic
Layers (tunics) of the eye
The tunics of the eye consist of the following three layers:
Tunica fibroses refers to the outer fibrous layer of the eye. This includes the sclera and the cornea, which are continuous with one another.
Sclera: The sclera is the white part of the eye, and covers most of the eye surface. It is made up of a dense tissue which has a rich supply of blood vessels and nerves, and provides attachment for the external muscles of the eye. The sclera tends to have a slight blue tinge during childhood because of its thinness. It also can appear yellow in the elderly due to the accumulation of a pigment associated with age-related wear and tear in the tissue.
Cornea: The cornea allows light to enter the eye, and can be thought of as being part of the modified sclera.
Tunica vasculosa refers to the middle vascular layer. This is also called the uvea. The uvea is made up of the choroid, ciliary body, and iris.
Ciliary body: The ciliary body forms a muscular ring around the lens. It secrets a fluid called the aqueous humour, and supports the iris and lens. The ciliary muscle, which is a smooth muscle responsible for lens accommodation, is contained within the ciliary body. Contraction of the ciliary muscle enables the lens to focus light onto the retina by changing its shape.
Iris: The iris is an adjustable thin muscle controlling pupil diameter. It consists of two layers – one that blocks stray light from reaching the retina, and another containing cells called chromatophores which contain a substance called melanin. The concentrations of melanin within these chromatophores give rise to eye colour. High concentrations of melanin give the iris a black or brown colour. When there is scarce melanin, light reflects from the epithelium of the posterior pigment, giving the iris a blue, green, or grey colour.