Coats of the eyeball

1. FIBROUS COAT
It is a dense strong wall which protects the intraocular contents. Anterior 1/ 6th of this fibrous coat is transparent and is called cornea. Posterior 5/ 6th opaque part is called sclera.

Cornea
It is the clear front surface of the eye. It lies directly in front of the iris and pupil, and it allows light to enter the eye. It is set into the sclera like a watch glass. Junction of the cornea and sclera is called limbus.
Functions
It allows light to enter the eye for vision
It provides approximately 65% to 75% of the focusing power of the eye.

Conjunctiva
It is a thin mucous membrane firmly attached at the limbus and lines the inside of the eyelids and covers the sclera (the white of the eye). The conjunctiva is highly vascularized, with many micro blood vessels.
Parts
Bulbar conjunctiva, covers the anterior part of the sclera (the “white” of the eye).
Palpebral conjunctiva, covers the inner surface of both the upper and lower eyelids. (Another term for the palpebral conjunctiva is tarsal conjunctiva.)
Functions
It keeps the front surface of the eye moist and lubricated.
It protects the eye from dust, debris and infection-causing microorganisms.

2. VASCULAR COAT (UVEA)

It supplies nutrition to various structures of the eyeball. It consists of three parts, from anterior to posterior, which are:
Iris
It is the thin, circular structure surrounding the pupil made of connective tissue and smooth muscles.
Functions
It determines colour of the eyes.
It facilitates accommodation.

Pupil
The pupil is the opening in the center of the iris. Typically, the pupils appear perfectly round, equal in size and black in color. The black color is because light that passes through the pupil is absorbed by the retina and is not reflected back. Pupil measures from 2 to 4 mm in diameter in bright light to 4 to 8 mm in the dark.
Function
It controls the amount of light entering the eye.

Ciliary body
It is a ring of smooth muscular tissue surrounding the iris connecting it to the choroid.
Parts
Ciliary muscle
Its contraction and relaxation modifies shape of the crystalline lens for an easy light focusing. The ciliary muscle is connected to the lens by a series of very thin, radially-arranged fibers called the ciliary zonules which hold the lens in place in the eye ball.
Ciliary processes
Contain cells involved in the production of the aqueous humor.
Functions
It facilitates accommodation
It produces Aqueous humor
It Holds crystalline lens in place

Choroid
It is a thin layer made up of blood vessels sandwiched between sclera and retina. It makes about 85% of ocular circulation.
Functions
Providing nutrients for the retina, macula and optic nerve.
Regulating the temperature of the retina.
Helping control pressure within the eye.
Absorbing light and limiting reflections within the eye that could interfere vision

3. NERVOUS COAT (RETINA).
The retina is a thin, semitransparent, multilayered sheet of neural tissue that lines the inner aspect of the posterior two-thirds of the wall of the eye ball. It is made up of two types of photosensitive cells.
Rods detect motion, provide black-and-white vision and function well in dim light and they are scattered throughout the retina.
Cones are responsible for central vision and color vision and perform best in medium and bright light. Cones are concentrated in a small central area of the retina called the Macula. At the center of the macula is a small depression called the Fovea containing only cone photoreceptors and is the point in the retina responsible for maximum visual acuity and color vision
Function
Photoreceptor cells take light focused by the cornea and lens and convert it into electrical signals which are transmitted to visual centers in the brain by way of the optic nerve.