The Human Eye Structure
The human eye is one of the most important organs and probably the most useful of various sense organs of our body.
Only about 1/6th of the eyeball is externally visible. This part is protected by the eyelids.
The eyeball is made up of three concentric coats: (1) the outer, fibrous sclera (also called the white of the eye), (2) the middle blood-vascular choroid and (3) the inner, nervous retina.
The sclera is thick, tough, white and opaque. It maintains the shape of the human eye, protects the inner structures and creates darkness inside the eye.
The centre of the front part of the human eye has a thin, transparent watchglass-like coat called cornea. The rays of light coming from various objects are bent by the cornea to bring them to a focus on the retina.
The middle coat of the human eye, i.e. the choroid is made up mainly of blood-vessels and hence nourishes the eye.
The inner retina consists of numerous nerve cells which receive light rays coming from the outside world, convert them into electrical impulses and transmit them to the brain.
The black of darkbrown (sometimes yellowish, bluish or greenish) part of the eye is called the pupil, which allows the light rays to enter the eye and controls their intensity by changing its own size (i.e. the pupil contracts in bright light and expands in dim light).
Behind the iris is the crystalline lens, which is normally transparent. It is held in place by 72 suspensory ligaments. When this lens becomes opaque, the person is said to suffer from cataract.
From the back of the eyeball starts the optic nerve which conveys the electrical impulses orginating in the retina to the brain.
That part of the brain which is concerned with vision is called the visual cortex. It is here that the impulses comig from the retina are processed and interpreted and the person 'sees'.
The harmonious and coordinated movements of the two eyes are brought about by six pairs of muscles, four of which are called the 'recti muscles' and two are called the obliqui muscle".