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Mammals A mammal () is a vertebrate animal of the class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three middle e ...
normally have a pair of eyes. Although
mammalian vision Mammalian vision is the process of mammals perceiving light, analyzing it and forming subjective sensations, on the basis of which the animal's idea of the spatial structure of the external world is formed. Responsible for this process in mammals is ...
is not as excellent as bird vision, it is at least dichromatic for most of mammalian species, with certain families (such as
Hominidae The Hominidae (), whose members are known as the great apes or hominids (), are a taxonomic Family (biology), family of primates that includes eight Neontology#Extant taxa versus extinct taxa, extant species in four Genus, genera: ''Orangutan ...
) possessing a trichromatic color perception. The dimensions of the eyeball vary only 1–2 mm among humans. The vertical axis is 24 mm, while the transverse is larger. At birth it is generally 16–17 mm, enlarging to 22.5–23 mm by three years of age. Between birth and age 13 the eye attains its mature size. It weighs 7.5 grams and its volume is roughly 6.5 ml. Along a line through the nodal (central) point of the eye is the optic axis, which is at a slight slant of five degrees toward the nose from the visual axis (i.e., the section going towards the focused point of the fovea).


Three layers

The structure of the
mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...
ian eye has a laminar organization that can be divided into three main layers or ''tunics'' whose names reflect their basic functions: the fibrous tunic, the vascular tunic, and the nervous tunic. * The fibrous tunic, also known as the ''tunica fibrosa oculi'', is the outer layer of the eyeball consisting of the
cornea The cornea is the transparency (optics), transparent front part of the eyeball which covers the Iris (anatomy), iris, pupil, and Anterior chamber of eyeball, anterior chamber. Along with the anterior chamber and Lens (anatomy), lens, the cornea ...
and
sclera The sclera, also known as the white of the eye or, in older literature, as the tunica albuginea oculi, is the opaque, fibrous, protective outer layer of the eye containing mainly collagen and some crucial elastic fiber. In the development of t ...
.Cline D; Hofstetter HW; Griffin JR. ''Dictionary of Visual Science''. 4th ed. Butterworth-Heinemann, Boston 1997. The sclera gives the eye most of its white color. It consists of dense
connective tissue Connective tissue is one of the four primary types of animal tissue, a group of cells that are similar in structure, along with epithelial tissue, muscle tissue, and nervous tissue. It develops mostly from the mesenchyme, derived from the mesod ...
filled with the protein
collagen Collagen () is the main structural protein in the extracellular matrix of the connective tissues of many animals. It is the most abundant protein in mammals, making up 25% to 35% of protein content. Amino acids are bound together to form a trip ...
to both protect the inner components of the eye and maintain its shape. * The vascular tunic, also known as the ''tunica vasculosa oculi'' or the "uvea", is the middle vascularized layer which includes the iris, ciliary body, and choroid.Cassin, B. and Solomon, S. ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing Company, 1990."Medline Encyclopedia: Eye."
Accessed October 25, 2006.
The choroid contains
blood vessel Blood vessels are the tubular structures of a circulatory system that transport blood throughout many Animal, animals’ bodies. Blood vessels transport blood cells, nutrients, and oxygen to most of the Tissue (biology), tissues of a Body (bi ...
s that supply the retinal cells with necessary
oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
and remove the waste products of respiration. The choroid gives the inner eye a dark color, which prevents disruptive reflections within the eye. The iris is seen rather than the cornea when looking straight in one's eye due to the latter's transparency, the pupil (central aperture of iris) is black because there is no light reflected out of the interior eye. If an ophthalmoscope is used, one can see the fundus, as well as vessels (which supply additional blood flow to the retina) especially those crossing the optic disk—the point where the optic nerve fibers depart from the eyeball—among others"eye, human."Encyclopædia Britannica. 2008. Encyclopædia Britannica 2006 Ultimate Reference Suite DVD 5 Apr. 2008 * The nervous tunic, also known as the ''tunica nervosa oculi'', is the inner sensory layer which includes the
retina The retina (; or retinas) is the innermost, photosensitivity, light-sensitive layer of tissue (biology), tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a focus (optics), focused two-dimensional ...
. ** Contributing to vision, the retina contains the photosensitive rod and
cone cell Cone cells or cones are photoreceptor cells in the retina of the vertebrate eye. Cones are active in daylight conditions and enable photopic vision, as opposed to rod cells, which are active in dim light and enable scotopic vision. Most v ...
s and associated neurons. To maximise vision and light absorption, the retina is a relatively smooth (but curved) layer. It has two points at which it is different; the fovea and optic disc. The fovea is a dip in the retina directly opposite the lens, which is densely packed with cone cells. It is largely responsible for color vision in humans, and enables high acuity, such as is necessary in reading. The optic disc, sometimes referred to as the anatomical blind spot, is a point on the
retina The retina (; or retinas) is the innermost, photosensitivity, light-sensitive layer of tissue (biology), tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a focus (optics), focused two-dimensional ...
where the
optic nerve In neuroanatomy, the optic nerve, also known as the second cranial nerve, cranial nerve II, or simply CN II, is a paired cranial nerve that transmits visual system, visual information from the retina to the brain. In humans, the optic nerve i ...
pierces the retina to connect to the nerve cells on its inside. No photosensitive cells exist at this point, it is thus " blind". Continuous with the retina are the ciliary epithelium and the posterior epithelium of the iris. ** In addition to the rods and cones, a small proportion (about 1-2% in humans) of the ganglion cells in the retina are themselves photosensitive through the pigment melanopsin. They are generally most excitable by blue light, about 470–485 nm. Their information is sent to the SCN (suprachiasmatic nuclei), not to the visual center, through the
retinohypothalamic tract In neuroanatomy, the retinohypothalamic tract (RHT) is a photic neural input pathway involved in the circadian rhythms of mammals. The origin of the retinohypothalamic tract is the intrinsically photosensitive retinal ganglion cells (ipRGC), ...
which is formed as melanopsin-sensitive axons exit the optic nerve. It is primarily these light signals which regulate
circadian rhythm A circadian rhythm (), or circadian cycle, is a natural oscillation that repeats roughly every 24 hours. Circadian rhythms can refer to any process that originates within an organism (i.e., Endogeny (biology), endogenous) and responds to the env ...
s in mammals and several other animals. Many, but not all, totally blind individuals have their circadian rhythms adjusted daily in this way. The ipRGCs have other functions as well, such as signaling the need for changing the diameter of the pupil in changing light conditions.


Anterior and posterior segments

The mammalian eye can also be divided into two main segments: the anterior segment and the posterior segment. The human eye is not a plain sphere but is like two spheres combined, a smaller, more sharply curved one and a larger lesser curved sphere. The former, the anterior segment is the front sixth of the eye that includes the structures in front of the vitreous humour: the
cornea The cornea is the transparency (optics), transparent front part of the eyeball which covers the Iris (anatomy), iris, pupil, and Anterior chamber of eyeball, anterior chamber. Along with the anterior chamber and Lens (anatomy), lens, the cornea ...
, iris, ciliary body, and
lens A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'') ...
."Departments. Anterior segment."
Cantabrian Institute of Ophthalmology.
Within the anterior segment are two fluid-filled spaces: * the anterior chamber between the posterior surface of the cornea (i.e. the corneal endothelium) and the iris. * the posterior chamber between the iris and the front face of the vitreous. Aqueous humor fills these spaces within the anterior segment and provides nutrients to the surrounding structures. Some
ophthalmologist Ophthalmology (, ) is the branch of medicine that deals with the diagnosis, treatment, and surgery of eye diseases and disorders. An ophthalmologist is a physician who undergoes subspecialty training in medical and surgical eye care. Following a ...
s specialize in the treatment and management of anterior segment disorders and diseases. The posterior segment is the back five-sixths of the eye that includes the anterior hyaloid membrane and all of the optical structures behind it: the vitreous humor,
retina The retina (; or retinas) is the innermost, photosensitivity, light-sensitive layer of tissue (biology), tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a focus (optics), focused two-dimensional ...
, choroid, and
optic nerve In neuroanatomy, the optic nerve, also known as the second cranial nerve, cranial nerve II, or simply CN II, is a paired cranial nerve that transmits visual system, visual information from the retina to the brain. In humans, the optic nerve i ...
. The radii of the anterior and posterior sections are 8 mm and 12 mm, respectively. The point of junction is called the limbus. On the other side of the lens is the second humour, the aqueous humour, which is bounded on all sides by the
lens A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'') ...
, the ciliary body, suspensory ligaments and by the retina. It lets light through without refraction, helps maintain the shape of the eye and suspends the delicate lens. In some animals, the retina contains a reflective layer (the tapetum lucidum) which increases the amount of light each photosensitive cell perceives, allowing the animal to see better under low light conditions. The tapetum lucidum, in animals that have it, can produce eyeshine, for example as seen in cat eyes at night. Red-eye effect, a reflection of red blood vessels, appears in the eyes of humans and other animals that have no tapetum lucidum, hence no eyeshine, and rarely in animals that have a tapetum lucidum. The red-eye effect is a photographic effect, not seen in nature. Some
ophthalmologist Ophthalmology (, ) is the branch of medicine that deals with the diagnosis, treatment, and surgery of eye diseases and disorders. An ophthalmologist is a physician who undergoes subspecialty training in medical and surgical eye care. Following a ...
s specialise in this segment.


Extraocular anatomy

Lying over the sclera and the interior of the eyelids is a transparent membrane called the conjunctiva. It helps lubricate the eye by producing mucus and tears. It also contributes to immune surveillance and helps to prevent the entrance of
microbes A microorganism, or microbe, is an organism of microscopic size, which may exist in its single-celled form or as a colony of cells. The possible existence of unseen microbial life was suspected from antiquity, with an early attestation in ...
into the eye. In many animals, including humans, eyelids wipe the eye and prevent dehydration. They spread tears on the eyes, which contains substances which help fight bacterial infection as part of the
immune system The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to bacteria, as well as Tumor immunology, cancer cells, Parasitic worm, parasitic ...
. Some species have a nictitating membrane for further protection. Some aquatic animals have a second eyelid in each eye which refracts the light and helps them see clearly both above and below water. Most creatures will automatically react to a threat to its eyes (such as an object moving straight at the eye, or a bright light) by covering the eyes, and/or by turning the eyes away from the threat. Blinking the eyes is, of course, also a reflex. In many animals, including humans, eyelashes prevent fine particles from entering the eye. Fine particles can be bacteria, but also simple dust which can cause irritation of the eye, and lead to tears and subsequent blurred vision. In humans, the eyelids are also used in
nonverbal communication Nonverbal communication is the transmission of messages or signals through a nonverbal platform such as eye contact (oculesics), body language (kinesics), social distance (proxemics), touch (Haptic communication, haptics), voice (prosody (lingui ...
, and the lashes are sexually and culturally significant. Voluntary blinking of the eyelids in succession while making eye contact is considered a feminine flirtatious behaviour. Long and prominent lashes are seen as a sign of
beauty Beauty is commonly described as a feature of objects that makes them pleasure, pleasurable to perceive. Such objects include landscapes, sunsets, humans and works of art. Beauty, art and taste are the main subjects of aesthetics, one of the fie ...
and are almost universally coveted for emphasizing the eyes and drawing attention to them. In many species, the eyes are inset in the portion of the skull known as the
orbit In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an ...
s or eyesockets. This placement of the eyes helps to protect them from injury. For some, the focal fields of the two eyes overlap, providing them with binocular vision. Although most animals have some degree of binocular vision the amount of overlap largely depends on behavioural requirements. In humans, the eyebrows redirect flowing substances (such as rainwater or sweat) away from the eye.


Function of the mammalian eye

The structure of the mammalian eye owes itself completely to the task of focusing
light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
onto the
retina The retina (; or retinas) is the innermost, photosensitivity, light-sensitive layer of tissue (biology), tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a focus (optics), focused two-dimensional ...
. This light causes
chemical A chemical substance is a unique form of matter with constant chemical composition and characteristic properties. Chemical substances may take the form of a single element or chemical compounds. If two or more chemical substances can be combin ...
changes in the photosensitive cells of the retina, the products of which trigger
nerve impulse An action potential (also known as a nerve impulse or "spike" when in a neuron) is a series of quick changes in voltage across a cell membrane. An action potential occurs when the membrane potential of a specific Cell (biology), cell rapidly ri ...
s which travel to the brain. In the mammalian eye, light enters the pupil and is focused on the retina by the lens. Light-sensitive nerve cells called rods (for brightness), cones (for color) and non-imaging ipRGC ( intrinsically photosensitive retinal ganglion cells) react to the light. They interact with each other and send messages to the brain. The rods and cones enable vision. The ipRGCs enable entrainment to the Earth's 24-hour cycle, resizing of the pupil and acute suppression of the pineal hormone melatonin.


Retina

The retina contains three forms of photosensitive cells, two of them important to vision, rods and cones, in addition to the subset of ganglion cells involved in adjusting
circadian rhythm A circadian rhythm (), or circadian cycle, is a natural oscillation that repeats roughly every 24 hours. Circadian rhythms can refer to any process that originates within an organism (i.e., Endogeny (biology), endogenous) and responds to the env ...
s and pupil size but probably not involved in vision. Though structurally and metabolically similar, the functions of rods and cones are quite different. Rod cells are highly sensitive to light, allowing them to respond in dim light and dark conditions; however, they cannot detect color differences. These are the cells that allow humans and other animals to see by moonlight, or with very little available light (as in a dark room). Cone cells, conversely, need high light intensities to respond and have high visual acuity. Different cone cells respond to different
wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
s of light, which allows an organism to see color. The shift from cone vision to rod vision is why the darker conditions become, the less color objects seem to have. The differences between rods and cones are useful; apart from enabling sight in both dim and light conditions, they have further advantages. The fovea, directly behind the lens, consists of mostly densely packed cone cells. The fovea gives humans a highly detailed central vision, allowing reading, bird watching, or any other task which primarily requires staring at things. Its requirement for high intensity light does cause problems for
astronomer An astronomer is a scientist in the field of astronomy who focuses on a specific question or field outside the scope of Earth. Astronomers observe astronomical objects, such as stars, planets, natural satellite, moons, comets and galaxy, galax ...
s, as they cannot see dim stars, or other celestial objects, using central vision because the light from these is not enough to stimulate cone cells. Because cone cells are all that exist directly in the fovea, astronomers have to look at stars through the "corner of their eyes" ( averted vision) where rods also exist, and where the light ''is'' sufficient to stimulate cells, allowing an individual to observe faint objects. Rods and cones are both photosensitive, but respond in different ways to different frequencies of light. They contain different pigmented photoreceptor
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
s. Rod cells contain the protein
rhodopsin Rhodopsin, also known as visual purple, is a protein encoded by the ''RHO'' gene and a G-protein-coupled receptor (GPCR). It is a light-sensitive receptor protein that triggers visual phototransduction in rod cells. Rhodopsin mediates dim ...
and cone cells contain different proteins for each color-range. The process through which these proteins go is quite similar — upon being subjected to
electromagnetic radiation In physics, electromagnetic radiation (EMR) is a self-propagating wave of the electromagnetic field that carries momentum and radiant energy through space. It encompasses a broad spectrum, classified by frequency or its inverse, wavelength ...
of a particular wavelength and intensity, the protein breaks down into two constituent products. Rhodopsin, of rods, breaks down into
opsin Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become retinylidene proteins, but are usually still called opsins regardless. Most pro ...
and
retinal Retinal (also known as retinaldehyde) is a polyene chromophore. Retinal, bound to proteins called opsins, is the chemical basis of visual phototransduction, the light-detection stage of visual perception (vision). Some microorganisms use ret ...
; iodopsin of cones breaks down into photopsin and retinal. The breakdown results in the activation of Transducin and this activates cyclic GMP Phosphodiesterase, which lowers the number of open Cyclic nucleotide-gated ion channels on the
cell membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...
, which leads to hyperpolarization; this hyperpolarization of the cell leads to decreased release of transmitter molecules at the
synapse In the nervous system, a synapse is a structure that allows a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or a target effector cell. Synapses can be classified as either chemical or electrical, depending o ...
. Differences between the rhodopsin and the iodopsins is the reason why cones and rods enable organisms to see in dark and light conditions — each of the photoreceptor proteins requires a different light intensity to break down into the constituent products. Further, synaptic convergence means that several rod cells are connected to a single bipolar cell, which then connects to a single ganglion cell by which information is relayed to the
visual cortex The visual cortex of the brain is the area of the cerebral cortex that processes visual information. It is located in the occipital lobe. Sensory input originating from the eyes travels through the lateral geniculate nucleus in the thalam ...
. This convergence is in direct contrast to the situation with cones, where each cone cell is connected to a single bipolar cell. This divergence results in the high visual acuity, or the high ability to distinguish detail, of cone cells compared to rods. If a ray of light were to reach just one rod cell, the cell's response may not be enough to hyperpolarize the connected bipolar cell. But because several "converge" onto a bipolar cell, enough transmitter molecules reach the
synapse In the nervous system, a synapse is a structure that allows a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or a target effector cell. Synapses can be classified as either chemical or electrical, depending o ...
s of the bipolar cell to hyperpolarize it. Furthermore, color is distinguishable due to the different iodopsins of
cone cell Cone cells or cones are photoreceptor cells in the retina of the vertebrate eye. Cones are active in daylight conditions and enable photopic vision, as opposed to rod cells, which are active in dim light and enable scotopic vision. Most v ...
s; there are three different kinds, in normal human vision, which is why we need three different primary colors to make a color space. A small percentage of the ganglion cells in the retina contain melanopsin and, thus, are themselves photosensitive. The light information from these cells is not involved in vision and it reaches the brain not directly via the optic nerve but via the
retinohypothalamic tract In neuroanatomy, the retinohypothalamic tract (RHT) is a photic neural input pathway involved in the circadian rhythms of mammals. The origin of the retinohypothalamic tract is the intrinsically photosensitive retinal ganglion cells (ipRGC), ...
, the RHT. By way of this light information, the body clock's inherent approximate 24-hour cycling is adjusted daily to nature's light/dark cycle. Signals from these photosensitive ganglion cells have at least two other roles in addition. They exercise control over the size of the pupil, and they lead to acute suppression of melatonin secretion by the pineal gland.


Accommodation

The purpose of the optics of the mammalian eye is to bring a clear image of the visual world onto the retina. Because of limited depth of field of the mammalian eye, an object at one distance from the eye might project a clear image, while an object either closer to or further from the eye will not. To make images clear for objects at different distances from the eye, its optical power needs to be changed. This is accomplished mainly by changing the curvature of the lens. For distant objects, the lens needs to be made flatter; for near objects the lens needs to be made thicker and more rounded. Water in the eye can alter the optical properties of the eye and blur vision. It can also wash away the tear fluid—along with it the protective lipid layer—and can alter corneal physiology, due to osmosis, osmotic differences between tear fluid and freshwater. Osmotic effects are made apparent when swimming in freshwater pools, because the osmotic gradient draws water from the pool into the corneal tissue (the pool water is Tonicity#Hypotonicity, hypotonic), causing edema, and subsequently leaving the swimmer with "cloudy" or "misty" vision for a short period thereafter. The edema can be reversed by irrigating the eye with Tonicity#Hypertonic solution, hypertonic saline (medicine), saline which osmotically draws the excess water out of the eye.


References

{{vision in animals Vision by taxon Mammal anatomy Sensory organs