The eyeball is a spherical structure. It is about 2.5cm in diameter. An eyeball is composed of the three layers- the outer sclerotic layer, the middle choroid and the inner retina.
The Sclerotic Layer or Sclera
Sclerotic layer or sclera is the outer tough coat of the eyeball made up of mainly collagen fibres. The sclerotic region can be divided into two regions- the posterior region called the sclera, and the outer region called the cornea.
The sclera is the white coat of the dense fibrous tissues which covers the whole eyeball except cornea. It is also known as the white of the eye. Sclera gives shape to the eyeball. It also protects the inner parts of the eye.
The cornea is a transparent fibrous coat. Iris can be seen through this transparent fibrous coat. The cornea receives the nourishment from tears and aqueous humour. The outer layer of the cornea is covered by a thin epithelial layer.
The Choroid Layer
The choroid is the middle layer of the eyeball. The choroid is composed of three parts- choroid, ciliary body and the iris.
The choroid is the thin and dark brown coloured membrane. It lines most of the inner surface of the sclera.
It contains a large number of blood vessels and pigments. The choroid layer is responsible for the absorption of light rays. Many blood vessels help in nourishment of the retina.
The word iris is derived from a Latin word irid which means coloured circle. Iris is the coloured part of the choroid around the pupil. There is a hole in the centre of the iris which is known as the pupil. The light enters inside the eyeball through the pupil. The iris contains two types of muscles: radial and circular muscles.
The contraction of these muscles constricts the pupil which regulates the amount of light entering the eyeball.
In the case of bright light, the circular eye muscles contract and the size of the pupil is decreased. This process is called constriction.
In the case of dim light, the radial muscles contracts and the size of the pupil is increased. This process is called dilation.
The retina is the third layer of the eyeball. It is located in the posterior part of the eye. It is a light-sensitive layer. It contains two types of light-sensitive cells – rods and cones.
The rods cells are very sensitive to dim light and they do not respond to colour. Rod contains the pigment rhodopsin. Rhodopsin is also known as visual purple. The rod cells are distributed throughout the retina.
The cones cells are sensitive to bright light. The cone cells are responsible for coloured vision. Cones contain pigment iodopsin. Cone cells are confined to the yellow spot.
The lens is located in the eye and it is divided into two parts by a biconvex lens. The biconvex lens is covered by a transparent crystalline body. The lens of the eye just lies behind the pupil and iris. It is softer at the front than at the back. It is slightly yellow in colour. It contains transparent lens fibres and the elastic lens capsule. Lens capsule is made up of glycoprotein. Blood vessels are absent in lens. The lens is held on its position by suspensory ligaments which are attached to the ciliary body.
The eyeball is divided into two cavities- the anterior cavity called the aqueous chamber and the posterior cavity called the vitreous chamber.
The anterior cavity is situated between the lens and the cornea. It is filled with aqueous humour which is a thin and watery fluid. It protects the lens from physical shock and keeps the lens moist.
The posterior cavity is situated between the lens and the retina. It is filled with a jelly-like substance called the vitreous humour. It is a very large cavity. It prevents eyeball from collapsing and supports the retina.