Sclerotic layer or sclera is the outer tough coat of the eyeball made up of mainly collagen fibres. It can be divided into two regions the posterior region called sclera, and the anterior region called cornea.
The sclera is a white coat of dense fibrous tissue that covers all the eyeball except the cornea. It is also known as the white of the eye. The sclera gives shape to the eyeball and also protects its inner parts.
The cornea is a transparent, fibrous coat through which the iris can be seen. The outer surface of the cornea is covered by an epithelial layer which is continuous with the epithelial layer of conjunctiva. The cornea receives nourishment from tears and aqueous humour.
2. The Choroid Layer:
The choroid layer is the middle layer of the eyeball and is composed of three parts—choroid, ciliary body and iris.
The choroid is a thin, dark brown membrane that lines most of the inner surface of the sclera. It contains several blood vessels and a large amount of pigment. The choroid absorbs light rays so that they are not reflected within the eyeball. The numerous blood vessels nourish the retina.
The iris (irid. coloured circle) is a coloured part of choroid around the pupil. It is a coloured part seen through the cornea. There is a hole (round window) in the centre of the iris, known as the pupil. The light enters the eyeball through the pupil. The iris contains radial and circular muscles.
The contraction of these muscles constricts the pupil which regulates the amount of light entering the eyeball.
i. In case of bright light, the circular eye muscles contract and the size of the pupil is decreased (constriction).
ii. In case of dim light, the radial muscles contract and the size of the pupil is increased (dilation).
3. The Retina:
The retina is the third inner layer of the eye. It is located only in the posterior part of the eye. It is the light-sensitive layer. It contains light-sensitive cells called rods and cones.
The rod cells are sensitive to dim light. They do not respond to colour. Rods contain pigment rhodopsin or visual purple. The rod cells are distributed throughout the retina.
The cone cells are sensitive to bright light. The cones are responsible for colour vision. Cones contain pigment iodopsin. Cone cells are mostly confined to the yellow spot or fovea centralis or macula.
The main body of the eye is divided into two parts by a biconvex lens which is a transparent crystalline body. The lens lies just behind the pupil and iris. It is flatter at the front than at the back and is soft and slightly yellow in colour.
It contains transparent lens fibres and an elastic lens capsule made of glycoprotein. There is no blood vessel in the lens. The lens is held in position by the suspensory ligaments which attach it to the ciliary body.
The eyeball is divided into two cavities: the anterior cavity (aqueous chamber) and posterior cavity (vitreous chamber).
The anterior cavity (between the lens and cornea) is filled with a fluid called the aqueous humour. It is a thin and watery fluid. It keeps the lens moist and protects it from the physical shock.
The posterior cavity is called the vitreous chamber. It is a large cavity. It lies between the lens and the retina. It contains a jelly-like substance called the vitreous humour. The vitreous humour prevents the eyeball from collapsing and supports the retina.