The eye is spherical in shape. The wall of the eye is composed of three layers or coats— an outer sclera, a middle choroid and an inner retina. The outermost layer or sclerotic coat is fibrous in texture. It preserves the form of the eyeball and protects the more delicate
vascular and nervous coats within.
The sclerotic coat is transparent in front, where it forms the cornea and is commonly called the white of the eye.
The second layer is the choroid which is a thin, black membrane with a net work of capillaries. Behind the cornea and in front of the lens, the choroid forms the iris. The iris has a circular opening at the centre, called the pupil. The iris may be of different colours (brown, black, gray or green). Behind the iris, the choroid forms the ciliary body with Ciliary muscles. Attached to the ciliary muscles are the suspensory ligaments. The crystalline, biconvex lens is held in position by the suspensory ligaments.
The third layer that lies inner to the choroid is the retina. The retinal layer is made up of two types of light sensitive cells called rods and cones. Cones are concentrated in the region fovea centralis which is at the back of the retina. This region, where the vision is acute, is also called the yellow spot. Nerve fibres arise from the rods and cones, join together forming the optic nerve and leave the eyes at the rear portion. The point where the optic nerve emerges from the eye is called the blind spot because no image is formed here. The iris and the lens divide the cavity of the eyeball into two unequal chambers. The small, anterior chamber in front of the lens is called the aqueous chamber. The large, posterior chamber that lies behind the lens is known as the vitreous chamber.
The two chambers are filled with watery transparent aqueous humour and jelly-like vitreous humour respectively.