Our eyes are in effect our windows on the world – it is through these complex sensory organs that the brain receives much of its information about the outside environment. But how much do we know about eye anatomy and physiology! It is worth detailing the constituent parts, and their functions to show how the eye works as a whole. Starting at the front, from the outside in, the parts are as follows.
The conjunctiva is the mucous membrane which covers the front of the eyeball, and then doubles back to line the inside of the eyelid. It must always be kept moist and is extremely sensitive.
The cornea is the transparent layer in front of the eye, over the pupil and iris, which lets the light. Its continuation is the sclera – the white of the eye – which surrounds the rest of the eyeball, except where the optic nerve enters at the back.
The iris, inside the cornea, controls the amount of light entering the eye by contracting or expanding to enlarge or reduce the size of the pupil. One cell layer of the iris, the Stroma, contains pigment cells which determine eye color. Albino eyes are pink because they lack pigment: most white babies are born with blue eyes, a sign of little pigment, but as more is deposited the eye color often chances.
The pupil is the tiny hole through which light enters the eye. It looks black because the light is not reflected back from the inside of the eye.
Behind the cornea lies the anterior chamber, filled with a clear liquid, the aqueous humor, and behind this is the lens which, by being made less or more convex, focuses light rays on the back of the eyeball. The eye anatomy shows that the lens is held in place by the suspensory ligaments. These are attached in turn to the ciliary body, the muscles of which control the shape of the lens, stretching and elongating it when they contract, to adapt for long distance focusing, and making it rounder and fatter when they relax, for short distance focusing. This ‘accommodation’ to different focusing lengths occurs involuntarily, being controlled by the autonomic nervous system.
Eye anatomy shows that behind the lens eye is filled with the vitreous body, a transparent jelly which keeps the eye’s shape. The black spots that you sometimes see when looking at a pale surface are caused by specks floating in the vitreous body.
The choroid tissue lies inside the sclera around the back of the eye. It contains blood vessels, and its brown or black color absorbs excess light inside the eyeball.
The innermost lining of the back of the eye is the retina, the complex nervous structure which contains the photosensitive cells, the rods, and cones. These cells are densest at the fovea centralis, a small pit or depression in the retina where light has almost unrestricted passage to them. This is the area where vision is most accurate.
The optic nerve, which enters the eyeball at the rear, is a direct extension of the brain. The point at which it enters the eye is called the blind spot since it has no light-sensitive cells.
Eye movement is controlled by six muscles attached to the sclera: four recti muscles, which run from front to back behind the eye, and two oblique muscles, running around the eye. Eyestrains is caused by the contraction of these muscles.
Protection from injury: The eye anatomy reveals that it has a protection mechanism. Being ultra-sensitive, the eyes are protected from injury in several different ways:
- The bony sockets of the skull frame and enclose them.
- The eyebrows collect moisture from the brow and stop it from running into the eyes.
- The eyelids keep the conjunctiva moist by blinking, thus sweeping fluid over the surface at regular intervals, every two to ten seconds, and during sleep, they cover the eyes to prevent evaporation. The lids also prevent injury from foreign bodies by the automatic blink reflex.
- The eyelashes catch particles of dust and grit and may set off the protective blink reflex.
- The lacrimal glands, above and towards the outside of the eyes, produce tears – the salty fluid that cleans and lubricates the eye surface.
- The lacrimal ducts drain the fluid into lacrimal sacs, leading into the nasal passage.