Anatomy of the eye
Although the basic principles of human vision are fairly simple, when you go into things, they appear a lot more complex. This applies particularly to the eye which is one or our most highly developed organs.
The eye - or eyeball - is a hollow sphere about 25 mm in diameter and weighing about 8 grams. It is formed of 3 envelopes, or tunicas, surrounding a gelatinous substance called the vitreous. The three tunicas, that you can see below in points 2, 3 and 4, are called the external tunica, the tunica media and the inner tunica.
Below you will find a full anatomical section of the eye and its different components, each with its role to play in the eye’s basic mission: to convert light into images.
Click on each item below. Each component of the eye will be shown and explained to you.
The eye muscles are connected by one end at the back of the eye-socket and by the other end to the external part of the eye. There are 4 rectus eye muscles which are connected above, below and on both sides of the eye, which enables the eye to turn up and down and from side to side. There are also 2 oblique eye muscles which take care of slanting vision upwards and downwards.
The fibrous or sclerotic wall or sclera forms the outer layer of the eye to which the eye muscles are at-tached. It is a solid, white and opaque membrane composed of connective or fibrous tissues which gives the eyeball solidity and protects the inner parts of the eye. The membrane surrounds the largest part of the eyeball and in the front transforms itself into the cornea.
The vascular layer or choroid is inside the sclera. This is a vascularised membrane which among other things is responsible for supplying the retina’s rods and cones.
At about the place where the fibrous wall turns into the cornea, the vascular layer transforms itself into the iris, in the centre of which the pupil is located.
The retina is a very fine membrane, light sensitive, which covers the inner part of the eye. The retina extends from the place where the optic nerve penetrates into the eyeball to the place where the chor-oid transforms itself into the iris.
The retina is made up of different layers: the outer layer or pigmentary epithelium is dark due to the pigments present. Under the pigmentary epithelium are the ends of the photoreceptor’s sensorial cells. There are two sorts: the cones - approximately 6 million per eye - need light to work properly and enable details and colours to be detected. The rods need very little light and detect contours (rough) in the dark. The retina receives the light, the rods and cones transform the luminous stimuli into nervous stimuli which are transmitted to the brain via the optic nerve. This is where the image is finally interpreted.
The optic nerve conducts the nervous stimuli, formed by the retina, towards the brain. There the image is interpreted. Inside the optic nerve there are about one million nerve fibres. Each fibre is capable of simultaneously transmitting different signals to the brain. More than 50% of all the nerve impulses that reach the brain come from the eyes. The place where the optic nerve is connected to the eyeball is the head of the optic nerve also called a blind spot or papilla.
The blind spot or papilla is approximately 1.5 millimetres in size and is in the place where the optic nerve is connected to the eye. Here, there is no retina, therefore no rods or cones, which explains why the light that reaches the blind spot is not transformed into nerve impulses and therefore not detected, hence its name “blind spot”.
The yellow spot or macula lutea is behind, in the centre of the retina, just behind the papilla. It is tiny, but nevertheless contains a large concentration of cones, which explains why it is the most sensitive part of the retina. The macula enables us to detect details clearly. Around the macula is the periphery of the retina. Within this, the number of cones decreases as you go further away from the macula.
The blood vessels are responsible for supplying the eye.
The vitreous is a gelified and clear liquid substance, surrounded by a fine membrane, which fills the centre of the eye and presses all the inner components of the eye against the outer wall.
The ciliary body is in the place where the choroid transforms itself into the iris. It forms a thickening there which contains a network of muscles enabling the curvature of the crystalline lens to be changed to make vision clear.
The posterior chamber of the eye secretes the aqueous humor, a clear substance that fulfils a nutritive function for the cornea, lens and vitreous. This space is behind the iris, in the place where the crystal-line lens is suspended. The aqueous humor circulates along the crystalline lens going through the pupil up to the anterior chamber of the eye.
The anterior chamber of the eye is the space between the cornea and the iris. The pressure in the eye remains constant due to the continuous supply and evacuation of aqueous humor.
The pupil is a dark opening in the middle of the iris. Light penetrates into the eye through the pupil. When light is strong, the pupil contracts to protect the retina. In semi-darkness, the pupil dilates to en-able maximum light absorption.
The pupil therefore works like the aperture of a camera: the diameter of its opening automatically ad-justs to the light intensity detected.
The iris is the coloured membrane that is stretched at the front of the eye behind the cornea. In the centre is an opening, the pupil. Due to the dilation and contraction of the pupil’s opening, the iris de-termines the amount of light that penetrates into the eye. If the eye is exposed to too much light, the iris’s circular muscle contracts and the pupil shrinks. In reduced light the pupil dilates. The colour of the iris is determined by the number of pigmentary cells. If there is a large number of pigmentary cells the iris is brown, if there are not many, the iris is blue or grey.
The cornea is the transparent, clear and convex membrane which is in the front of the eye. It absorbs light over a large surface and reflects it to the pupil. The cornea is responsible for two thirds of the total refraction of light rays which are deflected towards the retina. The cornea contains a lot of nerve ends and is therefore very sensitive. The blinking of tears above the cornea enables preventing it from drying out.
The crystalline lens or ocular lens is a transparent and flexible lens, suspended just behind the pupil. Together with the cornea, the crystalline lens is responsible for concentrating and reflecting the light rays that penetrate into the eye to project them exactly onto the retina. This is called refraction. As the crystalline lens is elastic, it can bulge or flatten itself depending on the distance to the object looked at and this being due to an internal eye muscle, the accommodating muscle. This enables both an object far away from the retina and an object close to the retina to be detected clearly.