Human Eye Structure: the Basics
About 80% of the information we receive with the help of the eyes. Structure of the eyes is very complex, let’s just touch the basics.
Parts of the human eye, a pair organ of vision, are:
- eyeball
- optic nerve
- lacrimal gland
- eyelids
- eyeball muscles
Eyeball: the human eyeball has irregular sphere shape, about 2,5 cm diameter. Two eyeballs are located in eye pits of the skull. All people have eyeballs of about the same size. From the moment of birth it increases almost twice.
Optic nerve: with the help of the optic nerve the information about the object is transported to the occipital cortex where the image is being analyzed.
Lacrimal gland: it is located in the upper part of the eye pit. Liquid that it produces makes the conjunctiva (mucous membrane of the eye) wet, that’s why the eyeball surface is always wet.
Eyelids: in the front the eye is covered by superior and interior eyelids. Eyelids are covered by skin from the outside and by conjunctiva from the inside. Eyelids protect the eyes from the negative factors of the environment as well as eyelashes that grow at the edge of the eyelids.
Eyeball muscles: the eyeball moves with the help of 6 muscles. The actions of these muscles are concerted, that’s why eyes’ movement is free and painless.
Let’s consider eyeball anatomy in more detail.
Eyeball consists of membranes that surround transparent contents of the eye (vitreous humor, lens, aqueous humor in anterior and posterior chambers).
There are 3 eyeball membranes:
1. external:
- Sclera (or the white of the eye): it is very tough and fibrous tissue that has a protective function. It provides constancy of the eye form, volume and tone. In human eye the whole sclera is white and contrasts with the iris.
- Cornea: it is a transparent anterior membrane of the eye. Behind the cornea the anterior chamber is located.
Sclera turns into cornea but not all at once. Sclera’s deep layers go first followed by the surface ones, that is why at the crossing place a special spout called limbus appears.
2. middle:
- Iris: this thin membrane is located behind transparent cornea. Iris might have green, grey, blue or brown color. The color of iris depends on the pigment (melanin) contained in the iris tissue, this pigment defines the eye color.
- Ciliary body
- Choroid
3. internal:
- Retina
From the functional point of view eye membranes (or layers of the eye) can be divided into:
1. refractive (or light-refracting):
- cornea
- aqueous humor in anterior and posterior chambers
- lens
- vitreous humor
2. accommodative:
- iris
- pupil
- ciliary body
3. sensory (or receptor):
- retina
Lens: it is a biconvex lens that is located behind the pupil. Light rays come through the human eye lens and refract. Lens can change its curvature.
Vitreous humor: this jellylike mass is located behind the lens.
Pupil: this is a black round opening in the iris that regulates the amount of incoming light with the help of the muscles. Pupil diameter can change from 2 to 8 mm depending on the lighting and central nervous system condition. If the lighting is bright the pupil narrows, if the lighting is poor it widens.
Light rays come through the cornea, anterior chamber, lens, vitreous humor and get to the retina, light-sensitive layer.
Retina has a very complicated structure:
- retina thickness is from 0,05 to 0,5 mm
- it consists of 10 cell layers
- the external retina layer is color- and photoreceptive, it consists of two types of neuroepithelial cells: rods and cones. There are about 7 million cones and about 130 million rods in human retina.
- Cones: play a significant role in the process of sharp vision and color sensation.
- Rods: they help us see in the darkness and poor lightening.
Light enters the eye through the cornea, comes through the aqueous humor in anterior and posterior chambers, lens and vitreous humor. Then light rays getting to the retina provoke complex photochemical process, as a result of it nerve impulse is formed. This nerve impulse is passed to the occipital cortex through the optic nerve. Occipital cortex is a visual center where the visual image of the object is created.