i SIHAT : Nose

Nose                                                           

The nose is a part of the body in humans and vertebrate animals to smell or 

breathe the air, there is on the face above the mouth.

i SIHAT : Ear ( telinga )

Ear

Human ear

The ear is the organ that detects sound. It not only receives sound, but also aids inbalance and body position. The ear is part of the auditory system.

The word "ear" may be used correctly to describe the entire organ or just the visible portion. In most mammals, the visible ear is a flap of tissue that is also called the pinnaand is the first of many steps in hearing. In humans, the pinna is often called the auricle.Vertebrates have a pair of ears placed somewhat symmetrically on opposite sides of the head. This arrangement aids in the ability to localize sound sources.

Introduction to ears and hearing

Audition is the scientific name for the sense of sound. Sound is a form of energy that moves through air, water, and other matter, inwaves of pressure. Sound is the means of auditory communication, including frog calls, bird songs and spoken language. Although the ear is the vertebrate sense organ that recognizes sound, it is the brain and central nervous system that "hears". Sound waves are perceived by the brain through the firing of nerve cells in the auditory portion of the central nervous system. The ear changes sound pressure waves from the outside world into a signal of nerve impulses sent to the brain.

The outer part of the ear collects sound. That sound pressure is amplified through the middle portion of the ear and, in land animals, passed from the medium of air into a liquid medium. The change from air to liquid occurs because air surrounds the head and is contained in the ear canal and middle ear, but not in the inner ear. The inner ear is hollow, embedded in the temporal bone, the densest bone of the body. The hollow channels of the inner ear are filled with liquid, and contain a sensory epithelium that is studded with hair cells. The microscopic "hairs" of these cells are structural protein filaments that project out into the fluid. The hair cells are mechanoreceptors that release a chemical neurotransmitter when stimulated. Sound waves moving through fluid push the filaments; if the filaments bend over enough it causes the hair cells to fire. In this way sound waves are transformed into nerve impulses. Invision, the rods and cones of the retina play a similar role with light as the hair cells do with sound. The nerve impulses travel from the left and right ears through the eighth cranial nerve to both sides of the brain stem and up to the portion of the cerebral cortex dedicated to sound. This auditory part of the cerebral cortex is in the temporal lobe.

The part of the ear that is dedicated to sensing balance and position also sends impulses through the eighth cranial nerve, the VIIIth nerve's Vestibular Portion. Those impulses are sent to the vestibular portion of the central nervous system. The human ear can generally hear sounds with frequencies between 20 Hz and 20 kHz (the audio range). Although the sensation of hearing requires an intact and functioning auditory portion of the central nervous system as well as a working ear, human deafness (extreme insensitivity to sound) most commonly occurs because of abnormalities of the inner ear, rather than the nerves or tracts of the central auditory system.

Mammalian ear

The shape of outer ear of mammals varies widely across species. However the inner workings of mammalian ears (including humans') are very similar.

The outer ear is the most external portion of the ear. The outer ear includes the pinna(also called auricle), the ear canal, and the very most superficial layer of the ear drum (also called the tympanic membrane). In humans, and almost all vertebrates, the only visible portion of the ear is the outer ear. The word "ear" may properly refer to the pinna (the flesh covered cartilage appendage on either side of the head). This portion of the ear is very vital for hearing. The outer ear does help get sound (and imposes filtering), but the ear canal is very important. Unless the canal is open, hearing will be dampened. Ear wax (cerumen) is produced by glands in the skin of the outer portion of the ear canal. This outer ear canal skin is applied to cartilage; the thinner skin of the deep canal lies on the bone of the skull. Only the thicker cerumen-producing ear canal skin has hairs. The outer ear ends at the most superficial layer of the tympanic membrane. The tympanic membrane is commonly called the ear drum. The pinna helps direct sound through the ear canal to the tympanic membrane (eardrum).

The framework of the auricle consists of a single piece of yellow fibrocartilage with a complicated relief on the anterior, concave side and a fairly smooth configuration on the posterior, convex side. The Darwinian tubercle, which is present in some people, lies in the descending part of the helix and corresponds to the true ear tip of the long-eared mammals. The lobule merely contains subcutaneous tissue.^[2] In some animals with mobile pinnae (like the horse), each pinna can be aimed independently to better receive the sound. For these animals, the pinnae help localize the direction of the sound source. Human beings localize sound within the central nervous system, by comparing arrival-time differences and loudness from each ear, in brain circuits that are connected to both ears. This process is commonly referred to as EPS, or Echo Positioning System.

The complex geometry of ridges on the inner surface of some mammalian ears helps to sharply focus echolocation signals, and any sound produced by the prey. These ridges can be regarded as the acoustic equivalent of a fresnel lens, and may be seen in a large variety of unrelated animals such as the bat, aye-aye, lesser galago, bat-eared fox, mouse lemur and others.

Middle ear
The middle ear, an air-filled cavity behind the ear drum (tympanic membrane), includes the three ear bones or ossicles: the malleus (or hammer), incus (or anvil), and stapes (or stirrup). The opening of the Eustachian tube is also within the middle ear. The malleus has a long process (the manubrium, or handle) that is attached to the mobile portion of the eardrum. The incus is the bridge between the malleus and stapes. The stapes is the smallest named bone in the human body. The three bones are arranged so that movement of the tympanic membrane causes movement of the malleus, which causes movement of the incus, which causes movement of the stapes. When the stapes footplate pushes on the oval window, it causes movement of fluid within the cochlea (a portion of the inner ear).

In humans and other land animals the middle ear (like the ear canal) is normally filled with air. Unlike the open ear canal, however, the air of the middle ear is not in direct contact with the atmosphere outside the body. The Eustachian tube connects from the chamber of the middle ear to the back of the nasopharynx. The middle ear is very much like a specialized paranasal sinus, called the tympanic cavity; it, like the paranasal sinuses, is a hollow mucosa-lined cavity in the skull that is ventilated through the nose. The mastoid portion of the human temporal bone, which can be felt as a bump in the skull behind the pinna, also contains air, which is ventilated through the middle ear.

Normally, the Eustachian tube is collapsed, but it gapes open both with swallowing and with positive pressure. When taking off in an airplane, the surrounding air pressure goes from higher (on the ground) to lower (in the sky). The air in the middle ear expands as the plane gains altitude, and pushes its way into the back of the nose and mouth. On the way down, the volume of air in the middle ear shrinks, and a slight vacuum is produced. Active opening of the Eustachian tube is required to equalize the pressure between the middle ear and the surrounding atmosphere as the plane descends. The diver also experiences this change in pressure, but with greater rates of pressure change; active opening of the Eustachian tube is required more frequently as the diver goes deeper into higher pressure.

The arrangement of the tympanic membrane and ossicles works to efficiently couple the sound from the opening of the ear canal to the cochlea. There are several simple mechanisms that combine to increase the sound pressure. The first is the "hydraulic principle". The surface area of the tympanic membrane is many times that of the stapes footplate. Sound energy strikes the tympanic membrane and is concentrated to the smaller footplate. A second mechanism is the "lever principle". The dimensions of the articulating ear ossicles lead to an increase in the force applied to the stapes footplate compared with that applied to the malleus. A third mechanism channels the sound pressure to one end of the cochlea, and protects the other end from being struck by sound waves. In humans, this is called "round window protection", and will be more fully discussed in the next section.

Abnormalities such as impacted ear wax (occlusion of the external ear canal), fixed or missing ossicles, or holes in the tympanic membrane generally produce conductive hearing loss. Conductive hearing loss may also result from middle ear inflammation causing fluid build-up in the normally air-filled space. Tympanoplasty is the general name of the operation to repair the middle ear's tympanic membrane and ossicles. Grafts from muscle fascia are ordinarily used to rebuild an intact ear drum. Sometimes artificial ear bones are placed to substitute for damaged ones, or a disrupted ossicular chain is rebuilt in order to conduct sound effectively.

Inner ear: cochlea, vestibule, and semicircular canals

The inner ear includes both the organ of hearing (the cochlea) and a sense organ that is attuned to the effects of both gravity and motion (labyrinth or vestibular apparatus). The balance portion of the inner ear consists of three semicircular canals and the vestibule. The inner ear is encased in the hardest bone of the body. Within this ivory hard bone, there are fluid-filled hollows. Within the cochlea are three fluid filled spaces: the scala tympani, the scala vestibuli and the scala media. The eighth cranial nerve comes from the brain stem to enter the inner ear. When sound strikes the ear drum, the movement is transferred to the footplate of the stapes, which presses it into one of its fluid-filled ducts through the oval window of cochlea . The fluid inside this duct is moved, flowing against the receptor cells of the Organ of Corti, which fire. These stimulate the spiral ganglion, which sends information through the auditory portion of the eighth cranial nerve to the brain.

Hair cells are also the receptor cells involved in balance, although the hair cells of the auditory and vestibular systems of the ear are not identical. Vestibular hair cells are stimulated by movement of fluid in the semicircular canals and the utricle and saccule. Firing of vestibular hair cells stimulates the Vestibular portion of the eighth cranial nerve.

Vestigial structures

It has long been known that humans, and indeed primates such as the orangutan and chimpanzee have ear muscles that are minimally developed and non-functional, yet still large enough to be easily identifiable.^[14] These undeveloped muscles arevestigial structures. A muscle that cannot move the ear, for whatever reason, can no longer be said to have any biological function. This serves as evidence ofhomology between related species. In humans there is variability in these muscles, such that some people are able to move their ears in various directions, and it has been said that it may be possible for others to gain such movement by repeated trials.^[14] In such primates the inability to move the ear is compensated mainly by the ability to turn the head on a horizontal plane, an ability which is not common to most monkeys—a function once provided by one structure is now replaced by another.

The outer structure of the ear also shows some vestigial features, such as the node or point on the helix of the ear known as Darwin's tubercle which is found in around 10% of the population, this feature is labelled (a) in the accompanying figure.

References
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i SEHAT : Nose

The Human Nose
The visible part of the human nose is the protruding part of the face that bears thenostrils. The shape of the nose is determined by the ethmoid bone and the nasal septum, which consists mostly of cartilage and which separates the nostrils. On average the nose of a male is larger than that of a female.^[1]

The nose has an area of specialised cells which are responsible for smelling (part of theolfactory system). Another function of the nose is the conditioning of inhaled air, warming it and making it more humid. Hairs inside the nose prevent large particles from entering the lungs. Sneezing is usually caused by foreign particles irritating the nasal mucosa, but can more rarely be caused by sudden exposure to bright light (called thephotic sneeze reflex) or touching the external auditory canal. Sneezing is a means oftransmitting infections because it creates aerosols in which the droplets can harbourmicrobes.

Anatomy
The nasal root is the top of the nose, forming an indentation at the suture where the nasal bones meet the frontal bone. The anterior nasal spine is the thin projection of bone at the midline on the lower nasal margin, holding the cartilaginous center of the nose.^[2]Adult humans have nasal hairs in the anterior nasal passage.

Related medical conditions
One of the most common medical conditions involving the nose are nosebleeds (in medicine: epistaxis). Most of them occur in Kiesselbach's area (synonym: Little's area). Nasal congestion is a common symptom of infections or other inflammations of the nasal lining (rhinitis), such as in allergic rhinitis or vasomotor rhinitis (resulting from nasal spray abuse). Most of these conditions also cause anosmia, which is the medical term for a loss of smell. This may also occur in other conditions, for example following trauma, in Kallmann syndrome or Parkinson's disease.

Nose-picking is a common, mildly taboo habit. Medical risks include the spread of infections, nosebleeds and, rarely, self-induced perforation of the nasal septum. The wiping of the nose with the hand, commonly referred to as the "allergic salute", is also mildly taboo and can result in the spreading of infections as well. Habitual as well as fast or rough nose wiping may also result in a crease (known as a transverse nasal crease or groove) running across the nose, and can lead to permanent physical deformity observable in childhood and adulthood.^[3][4][5][6]

Nose fetishism (or nasophilia) is the sexual fetish (or paraphilia) for the nose. The psychiatric condition of extreme nose picking is termed rhinotillexomania.

The nose is a common site of foreign bodies. The nose is susceptible to frostbite. Nasal flaring is a sign of respiratory distress that involves widening of the nostrils on inspiration.

Because of the special nature of the blood supply to the human nose and surrounding area, it is possible for retrograde infections from the nasal area to spread to the brain. For this reason, the area from the corners of the mouth to the bridge of the nose, including the nose and maxilla, is known to doctors as the danger triangle of the face.

A rhinoplasty plastic surgery for aesthetic surgery Specific systemic diseases, infections or other conditions that may result in destruction of part of the nose (for example, the nasal bridge, or nasal septal perforation) are rhinophyma, skin cancer (for example,basal cell carcinoma), Wegener's granulomatosis, systemic lupus erythematosus, rheumatoid arthritis, tuberculosis, syphilis, leprosyand exposure to cocaine, chromium or toxins. The nose may be stimulated to grow in acromegaly.

Samter's triad is the simultaneous occurrence in a patient of asthma, nasal polyps and aspirin sensitivity.

Culture

See also: Aquiline nose

Some people choose to get rhinoplasty to change the aesthetic appearance of their nose. Nose piercings are also common, such as nostril, septum or bridge.

In New Zealand, nose pressing ("hongi") is a traditional greeting amongst Maori people. However it is now generally confined to certain traditional celebrations.

The Mimizuka monument enshrines the mutilated noses of at least 38,000 Koreans killed during the Japanese invasions of Korea from 1592 to 1598.^[7]

A sexual fetish (or paraphilia) for the nose is called nasophilia.

Jewish people are often portrayed as having large noses in antisemitic propaganda.^[8] The term "Jewish nose" is controversial.^[9] In China and some other locations in East Asia, Westerners (Caucasians) are often designated as having "big" or "high" noses, due to the differences in shape and proportion between noses typical of Caucasians and those typical of Chinese.^[10]

Neanderthals
Clive Finlayson of the Gibraltar Museum said the large Neanderthal noses were an adaption to the cold,^[11] Todd C. Rae of theAmerican Museum of Natural History said primate and arctic animal studies have shown sinus size reduction in areas of extreme cold rather than enlargement in accordance with Allen's rule.^[12] Therefore, Todd C. Rae concludes that the design of the large and prognathic Neanderthal nose was evolved for the hotter climate of the Middle East and was kept when the Neanderthals entered Europe.^[12]

Miquel Hernández of the Department of Animal Biology at the University of Barcelona said the "high and narrow nose of Eskimos" and "Neanderthals" is an "adaption to a cold and dry environment", since it contributes to warming and moisturizing the air and the "recovery of heat and moisture from expired air".^[13]

Humans

An article published in the speculative journal Medical Hypotheses suggested that the nose is an alteration of the angle of skull followinghuman skeletal changes due to bipedalism. This changed the shape of the skull base causing, together with change in diet, a knock-on morphological reduction in the relative size of the maxillary and mandible and through this a "squeezing" of the protrusion of the most anterior parts of the face more forward and so increasing nose prominence and modifying its shape.^[14]

The aquatic ape hypothesis relates the nose to a hypothesized period of aquatic adaptation in which the downward-facing nostrils and flexible philtrum prevented water from entering the nasal cavities.^[15] The theory is not generally accepted by mainstream scholars ofhuman evolution.^[16]

Neoteny

Main article: Neoteny

Stephen Jay Gould has noted that larger noses are less neotenous, especially the large "Grecian" nose.^[17] Women have smaller noses than men due to not having "increased secretion of testosterone in adolescence"^[1] and smaller noses are considered more attractive on women.^[18] Werner syndrome, a condition that causes the appearance of premature aging, causes a "bird-like" appearance due to pinching of the nose^[19] while, conversely, Down syndrome, a neotenizing condition,^[20] causes flattening of the nose.^[21]

See also

• Empty nose syndrome, a nose crippled by excessive resection of the inferior and/or middle turbinates of the nose
• Jala neti, an Ayurvedic technique of nasal cleansing
• Nasal administration
• Nasal sebum, the greasy substance on the outer part of the nose
• Sròn, the Scottish Gaelic word for nose and the name of some hills in the Scottish Highlands
• Nasion the curve part at top of nose, between eyebrows

References

Wikipidia English

i SIHAT : Eye

Eye


The eye is the organ of vision, meaning that allows a living being to capture light and then analyze and interact with its environment.
In the animal world, there are at least forty types of visual organs called 'eyes'. This diversity raises the question of the origin of visual perception. The simplest eyes are barely able to detect the difference between light and darkness while the eyes of the most complex, as the human eye can distinguish shapes and colors.

human eye

The image formation

Any mechanism forming an image should be able to perceive differences in intensity between the different directions of incidence of light. The eye should be able to detect light, identify its direction, and establish a hierarchical relationship between the signals from different directions.

Cross section of a compound eye of dragonflies

The perception of light in the eye is done with pigments, composed of two covalently linked parts: one part protein, opsin, and a lipid moiety derived from vitamin A, the chromophore. The pigment is disposed in the photoreceptor cell membrane, and is composed of 7 transmembrane helices arranged in a circle in the membrane around the chromophore. It is the absorption of a photon by the chromophore, allowing the passage of 11-cis configuration of the chromophore to all-trans configuration, which allows the sensitivity to light. Once the pigment excited, opsin allows the activation of a G protein via one of its cytoplasmic loops, which then triggers the cellular response.

Perception of management requires focus light rays from the same direction in space on a small number of photoreceptors in the retina, which should be grouped spatially. There are many ways to combine the light rays from one direction in the animal world, emerged independently during evolution. However, we can divide the methods into three broad strategies: light rays not originating from the right direction are eliminated by shading of another structure of the eye onto the retina, the rays from one direction are curved and oriented to the same point of the retina by refraction, or rays are directed onto the photoreceptor by reflection on a concave mirror arranged behind the retina. Thus, each photoreceptor or photoreceptors group detects light from one direction.

Finally, the comparison of light intensities from the same direction in space requires an integration of electrical signals provided by the photoreceptor neurons. This integration is downstream of the retina. The signal received by the brain is never absolute, and only the difference in perceived intensity between photoreceptors is retained, and not the total level of intensity. This allows the eye to adjust to ambient light. Indeed, under conditions of high light, even a difference in intensity between two receptors appear smaller, which reduces image quality.

Optical characteristics of the eye

The eyes may be more or less successful and all have unique characteristics. The different eyes of the animal world have very different optical characteristics, often related to lifestyle of the animal. The human eye can differentiate about 8 million tones in color.

Multiple eyes of a spider (Maevia inclemens)

Sensitivity

The sensitivity of the eye is the minimum amount of light that is capable of perceiving. The sensitivity depends mainly on the size of the eye, but also on its geometry and in particular the presence of other structures ombrageantes decreasing the amount of incident light. Furthermore, the sensitivity of the eye is often modulated by the animal, for example by the presence of a diaphragm in mammals varying the amount of light admitted.

Resolution

The resolution is the smallest perceptible difference in angle between two incident rays. It therefore reflects the precision of the image that the eye is capable of forming, and the amount of detail that the eye will be able to perceive. It depends on the type of optical system for forming the image and performance. It is particularly limited by the phenomenon of diffraction of light in the case of images formed by refraction. It also depends on the number of photoreceptors: the resolution is equal to the angle between the center of two adjacent receivers. However, we observe that this is rarely the density of photoreceptors that is limiting, but more often the optical system used. This shows a very fine adjustment of the number of photoreceptors in the optical system, to minimize the loss of resolution. Finally, the resolution is often not the same throughout the retina, and peripheral parts often have a lower resolution than the center of the retina.

Diversity of eyes in the animal world

Compound eyes of a fly (Holcocephala fusca)

Eye of a viper (Vipera berus)

Eye of Squid

Cat Eyes

Detecting light

In all animals, the eyes detect light through opsins. However, the specialized nerve cells in the sensitivity to light, the photoreceptor cells are very diverse. There are two main types of photoreceptors: rhabdomériques receptors and receptors ciliates.

Receptors rhabdomériques

Rhabdomériques receptors, or rhabdoms, photoreceptor cells are characterized by the presence of membrane microvilli on the receiving carrier molecules opsins, allowing an increase of the surface of light perception. These receptors are present in all living things, but are found preferentially in protostomes. Some of these receptors have changed function during evolution, and no longer participate in the functioning of the eye, but may play a role in the synchronization of circadian rhythms, for example.

During excitation of the opsin in rhabdomériques receptors, G protein-activated in turn triggers the activation of the phosphatidylinositol membrane, and releases a second messenger, inositol triphosphate. The activation of this second messenger results in the opening of sodium channels and thus depolarization of the plasma membrane.

Forming an image

There are two main types of eyes in the animal world, each appeared many times independently during evolution. In both types, the image may be formed either by shading or by refraction or by reflection.

The simple eyes or camérulaires

The eyes have only a single chamber of photoreceptors, and oppose it in the compound eyes. The image can be formed by shading as in the nautilus, by refraction as in vertebrates or by reflection as in the scallop Jacques2.The nautilus is the only example of an animal with a single eye operated by shading. This eye, then functioning as a pinhole, then qualified eye pinhole (pinhole eye in English). It consists of a concave retina of photoreceptor cells surrounded by a layer of pigmented cells preventing the entry of the light except at a small diameter hole (pinhole) facing the retina. Thus, the rays from one direction only excite a small number of photoreceptors, which are grouped on the retina. The system allows to identify the direction of light rays and thus form an image. However, the only way to increase the resolution of the image in this system is to reduce the size of the pinhole allowing the entry of light, and therefore to reduce the amount of light admitted, that is to ie the sensitivity of the eye. The aperture size can vary from 0.4 to 2.8 mm3, which allows the nautilus to favor sensitivity or resolution within the environmental conditions.

Eye of nautilus

In vertebrates and some molluscs, the image is formed by refraction through the provision of material transparent to high refractive index front of the retina. This structure helps deflect the light rays and focus all the rays from one direction over a limited area of the retina and thus form an image. This is the lens that acts as a refractive structure in fish and molluscs. The lens is generally spherical in aquatic environments. The lenses of fish and cephalopods are characterized by an increasing gradient of refractive index from the outside inwards (Mathésienne lens), which allows a correct focusing of the light rays. However, some annelids and gastropods have homogeneous lenses, and vision remains relatively unclear. The lens Mathésienne appeared independently in vertebrates and cephalopods. In terrestrial vertebrates, the lens has lost some of its power refractory, and the cornea is responsible for 2/3 of the refraction of light. Some insect larvae also have simple eyes with corneal refraction, as the larva of the beetle Cicindela.

Frogeye

The eyes of the Saint-Jacques shell form an image by reflection. A concave reflective layer is placed behind the retina and acts as a mirror. Rays from one direction and are reflected differently depending on their impact on the mirror and concentrated on a small number of photoreceptors, allowing the formation of an image. There are also structures containing photosensitive mirror in some rotifers, flatworms and copepods, but the size of these structures is not sufficient to allow formation of an image.

The many eyes of this shell ajar St. Jacques are visible in the form of bright spots on the mantle edge.

The compound eyes

The compound eyes of arthropods (especially in insects and crustaceans) consist of a set of receivers (up to 30 000 in some beetles) sensitive to light which are called ommatidia. More commonly called the compound eye: compound eye. For copepods ago, in most cases, an odd look, median, which corresponds to the eye of the Nauplius larva. It is then commonly called eye nauplien.

Color perception

Some mammals such as cats or some owls are night vision. The visible spectral band varies among species. And some mammals (rats), birds (hummingbirds, swallows, pigeons ...), arthropods (lobsters, bees ...), reptiles (gecko, turtle ...) and fish (truite. ..) seem to see ultraviolet rays.

Some snakes can "see" in the infrared, but with their sensory pits. Color vision is also different depending on the species or individuals.

Are two types of photoreceptors in the retina of the human eye: rods and cones. The cones are responsible for color vision and daytime vision, they are of three types, each responsive to more red, green or blue, while the rods are responsible for the vision of light intensity and night vision. The latter being more reactive, they are used more in the reflexes and peripheral vision in the eye.

The problems of color vision, or dyschromatopsia are often grouped under the term colorblindness. The total absence of color vision is called achromatopsia.

Number and position of eyes in animals

In predators such as cats or birds of prey, the eyes are placed one beside the other which allows, in binocular vision, to better perceive the distances of prey located in front of them, in contrast, in prey such as rabbits or mice, the eyes are usually placed on either side of the head which can cover a wider field of view and to better detect the presence of a hazard in the environment.

Origin and evolution of the eye

The diversity of organisms and types of vision is, as already emphasized in Charles Darwin's Origin of Species, an intellectual challenge for the proponents of evolution. For this reason, the evolution of the eye has long been a subject of controversy between advocates of evolution and the creationists, they regard the eye as too perfect to have evolved according to the mechanisms proposed by the theory of evolution [ref. required].

There are many similarities in the functioning of the eyes of various species, such as how visual stimuli are transmitted to receivers on the central nervous system. These similarities are very numerous in amniotes. The eye of these animals derive ancestral species of the order of Captorhinidae missing there are 300 million years5.

We [who?] Was long thought that the different forms of eyes had developed independently of one species from various origins (known as development paraphyletic). However the discovery of the existence of the Pax6 gene, conserved throughout the animal kingdom and controlling eye development, has recently challenged this idea, suggesting monophyly of the eye. Now considered a primitive eye composed of a few cells developed uniquely in the animal kingdom, and afterward have diversified during the Cambrian to form at least 40 times independently of structures capable of forming images6.

Theoretical model of the evolution of the vertebrate eye.

Notes and references

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