Human Eye

The human eye is the special sense organ of vision, and more than 50% of the body's sensory receptors are located in the eyes. In the adult, the eyeball measures about 2.5 centimeters in diameter, yet only about one-sixth of it is exposed to the outside air. Its sensitivity is confined to a narrow band of electromagnetic energy: the eye detects visible light at wavelengths from 400 to 700 nanometers.

The wall of the eyeball is built from three concentric layers: the fibrous tunic, the vascular tunic, and the retina. The fibrous tunic forms the outermost coat and divides into the anterior cornea and the posterior sclera. The sclera is an opaque layer that surrounds the posterior five-sixths of the eyeball, while the cornea is a transparent layer continuous with it that occupies the anterior one-sixth. Functionally the most important of the three is the retina, which receives external visual stimuli.

The cornea itself is layered, with non-keratinized stratified squamous epithelium on its outer surface, collagen fibers and fibroblasts in its middle coat, and simple squamous epithelium on its inner surface. Its central region receives oxygen from the outside air. The corneal stroma accounts for around 90% of the thickness of the cornea.

The refractive media of the eye — the cornea, vitreous body, lens and aqueous humor — are the structures that help focus light onto the retina where it can be detected by the photoreceptors.Light first passes through the cornea, the clear dome-shaped front layer of the eye that bends light to help the eye focus, and then through the lens, a clear inner part of the eye that works with the cornea to focus light correctly on the retina.The iris, which is the colored part of the eye, controls how much light enters the eye through the pupil.The lens, a clear inner part of the eye just behind the pupil, changes shape to focus on objects that are close.

The anterior chamber holds aqueous humor, which the eye continually produces and drains through the drainage angle to keep eye pressure constant.Light focused by the cornea and lens passes through the vitreous onto the retina, the light-sensitive tissue lining the back of the eye, where the macula provides detailed central vision and the peripheral retina provides side vision.

The retina is the light-sensitive tissue lining the back of the eye, and it is the nervous layer of the eyeball that receives external visual stimuli. The retina is composed of several layers and contains specialized cells for the initial processing of visual stimuli. The cells in the retina that respond to light stimuli are an example of a specialized receptor known as a photoreceptor, and photoreceptors are neurons that function as specialized receptor cells located in the retina of the eyeball, primarily responsible for the transduction of light stimuli for vision. The photoreceptors, rods and cones, change their membrane potential when stimulated by light energy, which alters the amount of neurotransmitter they release onto bipolar cells in the outer synaptic layer.

Each retina holds about 6 million cones and more than 100 million rods, giving a rod-to-cone ratio of about 20 to 1 except at the fovea centralis.Rod cells have elongated cylindrical outer segments resembling rods, with stacks of membrane-bound discs containing the photopigment rhodopsin, which consists of the protein scotopsin and the chromophore retinal; rods are strongly photosensitive and are located in the outer edges of the retina, where they detect dim light and serve primarily for peripheral and nighttime vision. Cones are weakly photosensitive and are located near the center of the retina, responding to bright light with a primary role in daytime color vision. Cones are important for visual acuity and contribute to high-resolution vision in bright conditions.

Rhodopsin reaches its peak sensitivity at 498 nanometers. The three cone types peak at 564 nm for long-wave cones, 534 nm for medium-wave cones and 420 nm for short-wave cones.Primates use a three-cone trichromatic system that results in full color vision, and the perceived color is a result of the ratio of activity of the three types of cones.

The fovea is a small area at the exact center of the retina that lacks supporting cells and blood vessels and contains only photoreceptors, which is why visual acuity is greatest there. Because the axons of retinal ganglion cells pass through the retina, there are no photoreceptors at the very back of the eye where the optic nerve begins, creating a blind spot in the retina and a corresponding blind spot in the visual field.

The shape change of retinal in the photoreceptors initiates visual transduction in the retina; activation of retinal and the opsin proteins results in activation of a G protein, which changes the membrane potential of the photoreceptor cell so that it releases less neurotransmitter into the outer synaptic layer.

The visual pathway refers to the anatomical structures that convert light energy into electrical action potentials interpretable by the brain, beginning at the retina and terminating at the primary visual cortex.The optic nerves are formed by about one million myelinated axons arising from retinal ganglion cells that converge at the optic disc, and their fibers are myelinated by oligodendrocytes rather than Schwann cells. The optic chiasm is a point of union and decussation of the two optic nerves, where the nasal fibers of each eye cross the midline to join the temporal fibers of the contralateral eye.The lateral geniculate bodies are rounded protuberances on the inferior surface of the diencephalon that house the multi-layered lateral geniculate nuclei, whose six layers contain cells similar to those of the retinal ganglion layer.

The eye is protected from mechanical injury by being enclosed in a bony socket called the orbit, a four-sided pyramid whose floor is formed by parts of the maxilla, zygomatic, and palatine bones and whose roof includes the orbital plate of the frontal bone.The accessory structures of the eye include the eyelids, eyelashes, eyebrows, the lacrimal apparatus, and the extrinsic eye muscles, and the eyelids cover the eyes during sleep and spread lubricating secretions over the eyeball.

Six extraocular muscles attach to the sclera and move the eye up and down, from side to side, and in rotation. The conjunctiva is a mucous membrane that lines the lids and bends back over the surface of the eyeball, with a palpebral portion lining the lids and a bulbar portion covering the white of the eye.

A three-layer tear film — mucous, watery and oily — keeps the front of the eye lubricated. The eye is kept moist by the secretions of the almond-shaped lacrimal glands, which lie under the upper lids and extend inward from the outer corner of each eye, with one portion resting in a depression of the frontal bone and the other projecting into the back of the upper lid.The ducts from each lacrimal gland, three to 12 in number, open into the superior conjunctival fornix, from which tears flow across the eye into the puncta lacrimalia at the margin of each eyelid and then through the lacrimal ducts into the lacrimal sacs and nasolacrimal ducts, which carry them into the nose.

Evaporation of the tears as they flow across the eye is largely prevented by oily and mucous secretions from other glands, including the meibomian, or tarsal, glands, which run through the tarsal plates and release an oil onto the lid margin that acts as a barrier for the tear fluid. The front surface of the eyeball, the cornea, must remain moist, which the eyelids achieve by sweeping the secretions of the lacrimal apparatus and other glands over it during waking hours and covering the eyes during sleep; the blink reflex also prevents injuries from foreign bodies.

Trilobite compound eyes stand as the earliest well-preserved visual surfaces in the fossil record, a status illustrated by Telephina convexa, an animal that swam in the oceans around 450 million years ago. A different form of early vision belonged to Anomalocaris, an early arthropod relative whose giant compound eyes were set on the ends of stalks and built from repeating units known as ommatidia.

Nilsson's calculations assumed a tiny change with each generation towards a more evolved eye. Even under that model, his pessimistic estimate of the time required for an eye to evolve was less than 364,000 years.

• 1851
  Helmholtz's ophthalmoscope and ophthalmometer
  Helmholtz introduces the ophthalmoscope and the ophthalmometer, also known as the keratometer.
• 7 December 1905
  First successful penetrating keratoplasty in a human
  Eduard Zirm performs the first successful penetrating keratoplasty in a human in Olmutz near Prague, on Alois Glogar, a 45-year-old farm labourer who had sustained severe bilateral alkali burns 16 months earlier while cleaning his chicken coop with lime. Donor tissue came from an 11-year-old boy blind from a penetrating scleral injury; two 5-mm buttons were removed by von Hippel trephine, and bilateral transplants were carried out under chloroform anaesthesia with strict asepsis. ^{10Centennial review of corneal transplantation · Clinical & Experimental Ophthalmology (Wiley); hosted by University of Iowa ↗}
• 1911
  Nobel Prize for work on the dioptrics of the eye
  Allvar Gullstrand receives the Nobel Prize in Physiology or Medicine for work on the dioptrics of the eye.
• 1967
  Nobel Prize for visual processes
  The Nobel Prize in Physiology or Medicine is awarded jointly to Ragnar Granit, Haldan Keffer Hartline and George Wald for their discoveries concerning the primary physiological and chemical visual processes in the eye. ^{11The Nobel Prize in Physiology or Medicine 1967 — Granit, Hartline and Wald · Nobel Prize Outreach ↗}
• 1981
  Nobel Prize for visual information processing
  The Nobel Prize in Physiology or Medicine is divided, one half going jointly to David H. Hubel and Torsten N. Wiesel for their discoveries concerning information processing in the visual system. ^{12The Nobel Prize in Physiology or Medicine 1981 — Sperry, Hubel and Wiesel · Nobel Prize Outreach ↗}

Unoperated cataract accounts for a higher proportion of vision impairment in low- and middle-income countries than in high-income countries, where glaucoma and age-related macular degeneration are more common. At least 1 billion of those cases could have been prevented or have yet to be addressed.

Over time cataracts can make vision blurry, hazy, or less colorful, interfere with reading and other everyday activities, and eventually lead to vision loss. Cataract surgery removes the clouded lens and replaces it with an artificial intraocular lens; 9 out of 10 people who have the surgery can see better afterwards.

The most common type of color vision deficiency makes it hard to tell red from green, another type makes blue and yellow look the same, and rare cases involve seeing no color at all.

Eye health data are reported as estimates, and they may show a higher or lower number of eye disease cases than a population actually has, depending on how researchers collected the data. Department of Health and Human Services leads the Healthy People 2030 national public health initiative, and the National Eye Institute leads its vision workgroup.

The CDC's Vision and Eye Health Data Portal provides detailed datasets on eye health, while the National Vision and Eye Health Surveillance System shares estimates on vision loss and blindness in the United States.

Prolonged weightlessness allows bodily fluids, including blood and cerebrospinal fluid, to shift toward the head, a redistribution that can produce swelling of the optic nerve and folds in the retina. The cluster of eye and brain changes observed in astronauts is designated spaceflight associated neuro-ocular syndrome, or SANS. The likelihood of developing SANS rises with the length of a space mission, and the condition remains a leading research priority for scientists preparing for a journey to Mars.