How does the eye function. What are the main anatomical structures of the eye. How do extraocular muscles control eye movement. What are the three layers of the eyeball. How does the iris regulate light entering the eye.

The Eye’s Location and Protective Structures

The eyes, our organs of vision, are situated within the bony orbits of the skull. These orbits serve a dual purpose: they protect the delicate eyeballs and anchor the surrounding soft tissues. Let’s explore the protective structures that safeguard our eyes:

• Eyelids: These movable folds of skin, adorned with lashes at their edges, act as a barrier against foreign particles.
• Palpebral conjunctiva: A thin membrane lining the inner surface of the eyelids.
• Conjunctiva: This membrane extends over the white part of the eye (sclera), connecting the eyelids to the eyeball.
• Lacrimal gland: Located beneath the lateral edges of the nose, it produces tears that flow through the lacrimal duct to the medial corner of the eye.

How do tears contribute to eye protection? Tears wash over the conjunctiva, effectively removing foreign particles and maintaining eye hygiene.

Extraocular Muscles: The Eye’s Movement Controllers

The eye’s ability to move within its orbit is facilitated by six extraocular muscles. These muscles originate from the orbital bones and insert into the eyeball’s surface. Let’s examine the primary muscles responsible for eye movement:

1. Superior rectus
2. Medial rectus
3. Inferior rectus
4. Lateral rectus
5. Superior oblique
6. Inferior oblique

How do these muscles control eye movement? When each muscle contracts, it pulls the eye in its direction. For instance, contraction of the superior rectus rotates the eye upward.

The Unique Role of Oblique Muscles

The superior oblique muscle has a distinctive feature. Its tendon passes through a cartilaginous structure called the trochlea, resulting in medial eye rotation upon contraction. Conversely, the inferior oblique muscle originates from the orbit’s floor and inserts into the eye’s inferolateral surface, causing lateral rotation when contracted.

Why are these rotations necessary? The eye isn’t perfectly aligned on the sagittal plane, so these rotations compensate for the angles at which the rectus muscles pull, ensuring precise eye movements.

Innervation of Extraocular Muscles

The extraocular muscles receive innervation from three cranial nerves:

• Abducens nerve: Innervates the lateral rectus
• Trochlear nerve: Innervates the superior oblique
• Oculomotor nerve: Innervates all other extraocular muscles and the levator palpebrae superioris

How does the brain coordinate eye movements? The motor nuclei of these cranial nerves connect to the brain stem, which orchestrates the complex symphony of eye movements.

The Three Layers of the Eye

The eyeball itself is a hollow sphere composed of three distinct layers of tissue. Let’s explore each layer in detail:

1. Fibrous Tunic: The Protective Outer Layer

The fibrous tunic forms the outermost layer of the eye and consists of two main components:

• Sclera: The white, opaque part that covers about five-sixths of the eye’s surface
• Cornea: The transparent anterior portion that allows light to enter the eye

How does the cornea contribute to vision? Its transparency and curved shape help to focus incoming light onto the retina.

2. Vascular Tunic: The Middle Layer

The vascular tunic, or uvea, is the middle layer of the eye and comprises three structures:

• Choroid: A highly vascularized connective tissue layer that supplies blood to the eye
• Ciliary body: A muscular structure attached to the lens by zonule fibers
• Iris: The colored part of the eye, visible in the anterior portion

How does the ciliary body affect vision? It plays a crucial role in accommodation by altering the shape of the lens, allowing us to focus on objects at varying distances.

3. Neural Tunic: The Inner Layer

The innermost layer of the eye is the neural tunic, also known as the retina. This layer contains the photoreceptive nervous tissue responsible for converting light into neural signals.

The Iris and Pupil: Guardians of Light Entry

The iris, a smooth muscle structure, plays a vital role in regulating the amount of light that enters the eye. It achieves this by controlling the size of the pupil, the central opening through which light passes.

How does the iris respond to different light conditions?

• In bright light: The iris constricts the pupil, reducing the amount of light entering the eye
• In dim light: The iris dilates the pupil, allowing more light to enter the eye

This adaptive mechanism helps maintain optimal visual acuity across varying light conditions and protects the sensitive retina from potential light damage.

The Eye’s Cavities: Anterior and Posterior

The eye is divided into two main cavities:

1. Anterior cavity: Located in the front portion of the eye
2. Posterior cavity: Occupies the back portion of the eye

What is the significance of these cavities? They contain fluids that help maintain the eye’s shape and contribute to its proper functioning.

Unique Features of Human Eyes

Human eyes possess certain characteristics that set them apart from those of many other species. One notable feature is the extensive visibility of the “white of the eye” or sclera.

Why is this feature significant? The prominent sclera in humans is believed to play a role in non-verbal communication, allowing for easier detection of gaze direction and enhancing social interactions.

The Process of Vision: From Light to Perception

Vision, our special sense of sight, relies on the transduction of light stimuli received through the eyes. This complex process involves several steps:

1. Light enters the eye through the cornea
2. The iris adjusts the pupil size to regulate light entry
3. The lens focuses light onto the retina
4. Photoreceptors in the retina convert light into electrical signals
5. These signals are transmitted to the brain via the optic nerve
6. The brain processes these signals, resulting in visual perception

How do different parts of the eye contribute to this process? Each structure plays a crucial role, from the cornea’s initial refraction of light to the retina’s complex signal transduction.

The Role of Photoreceptors

The retina contains two main types of photoreceptors:

• Rods: Responsible for vision in low light conditions and peripheral vision
• Cones: Enable color vision and high-acuity vision in bright light

How do these photoreceptors differ in their function? Rods are more sensitive to light but provide less detailed vision, while cones require more light but offer higher resolution and color perception.

Accommodation: Focusing on Near and Far Objects

Accommodation is the process by which the eye adjusts its focusing power to maintain a clear image of objects at different distances. This is primarily achieved through changes in the shape of the lens.

How does accommodation work?

1. For near objects: The ciliary muscles contract, allowing the lens to become more convex
2. For distant objects: The ciliary muscles relax, and the lens flattens due to the tension of the zonule fibers

These adjustments alter the refractive power of the lens, enabling us to focus on objects at varying distances.

Common Vision Problems and Their Causes

Several vision problems can arise due to irregularities in the eye’s structure or function:

• Myopia (nearsightedness): Difficulty seeing distant objects clearly
• Hyperopia (farsightedness): Difficulty seeing near objects clearly
• Astigmatism: Blurred vision due to irregular corneal curvature
• Presbyopia: Age-related difficulty focusing on near objects

What causes these vision problems? They often result from abnormalities in the eye’s shape, lens flexibility, or the aging process affecting the ciliary muscles.

Corrective Measures for Vision Problems

Various methods exist to correct vision problems:

• Eyeglasses: Correct refractive errors by bending light before it enters the eye
• Contact lenses: Similar to eyeglasses but sit directly on the eye’s surface
• Laser eye surgery: Reshapes the cornea to correct refractive errors
• Intraocular lenses: Artificial lenses implanted in the eye, often used in cataract surgery

How do these corrective measures work? They aim to adjust the way light enters and focuses within the eye, compensating for the specific vision problem.

The Eye’s Defense Mechanisms

The eye has several built-in mechanisms to protect itself from damage:

1. Blink reflex: Rapidly closes the eyelid in response to threats
2. Tear production: Lubricates the eye and washes away irritants
3. Pupillary light reflex: Constricts the pupil in bright light to prevent retinal damage
4. Corneal reflex: Causes the eye to close when the cornea is touched

How do these mechanisms enhance eye protection? They work together to shield the eye from physical damage, maintain its moisture, and regulate light exposure.

The Eye’s Role in Circadian Rhythms

Beyond its primary function in vision, the eye plays a crucial role in regulating our body’s circadian rhythms. Specialized photosensitive retinal ganglion cells in the eye detect overall light levels and transmit this information to the brain’s suprachiasmatic nucleus, our central circadian pacemaker.

How does this affect our daily lives? This light-sensing mechanism helps synchronize our internal biological clock with the external day-night cycle, influencing sleep patterns, hormone release, and various physiological processes.

Evolution of the Eye

The complex structure of the human eye is the result of millions of years of evolution. From simple light-sensitive spots in early organisms to the sophisticated eyes of vertebrates, the evolution of the eye demonstrates nature’s remarkable ability to develop increasingly complex and efficient structures.

What are some evolutionary milestones in eye development?

• Development of pigmented eyespots in simple organisms
• Formation of primitive lenses to focus light
• Evolution of more complex retinal structures
• Development of color vision in some species

This evolutionary journey has resulted in the diverse array of eye types seen across the animal kingdom, each adapted to its specific environmental needs.

Maintaining Eye Health

Proper eye care is essential for maintaining good vision and overall eye health throughout life. Here are some key practices for preserving eye health:

• Regular eye exams: Help detect vision problems and eye diseases early
• Proper nutrition: Consuming foods rich in vitamins A, C, E, and omega-3 fatty acids
• UV protection: Wearing sunglasses to shield eyes from harmful ultraviolet rays
• Digital eye strain prevention: Following the 20-20-20 rule (every 20 minutes, look at something 20 feet away for 20 seconds)
• Adequate sleep: Allows eyes to rest and repair

How do these practices contribute to eye health? They help prevent eye strain, protect against damage from environmental factors, and support the eye’s natural maintenance processes.

The Future of Eye Care and Vision Technology

Advancements in medical science and technology are continuously shaping the future of eye care and vision enhancement. Some exciting developments include:

1. Gene therapy for inherited eye diseases
2. Artificial retinas for certain types of blindness
3. Advanced contact lenses with built-in displays
4. Stem cell treatments for retinal regeneration
5. AI-powered diagnostic tools for early disease detection

How might these advancements impact vision care? They have the potential to treat previously incurable conditions, enhance visual capabilities beyond natural limits, and revolutionize the way we diagnose and manage eye health.

As our understanding of the eye’s intricate anatomy and function continues to grow, so too does our ability to care for and enhance this remarkable organ. From its complex structure to its vital role in our daily lives, the eye remains a testament to the wonders of biological evolution and a frontier for future medical innovations.

Vision | Anatomy and Physiology I

Vision is the special sense of sight that is based on the transduction of light stimuli received through the eyes. The eyes are located within either orbit in the skull. The bony orbits surround the eyeballs, protecting them and anchoring the soft tissues of the eye (Figure 1). The eyelids, with lashes at their leading edges, help to protect the eye from abrasions by blocking particles that may land on the surface of the eye. The inner surface of each lid is a thin membrane known as the palpebral conjunctiva. The conjunctiva extends over the white areas of the eye (the sclera), connecting the eyelids to the eyeball. Tears are produced by the lacrimal gland, located beneath the lateral edges of the nose. Tears produced by this gland flow through the lacrimal duct to the medial corner of the eye, where the tears flow over the conjunctiva, washing away foreign particles.

Figure 1.  The Eye in the Orbit The eye is located within the orbit and surrounded by soft tissues that protect and support its function. The orbit is surrounded by cranial bones of the skull.

Movement of the eye within the orbit is accomplished by the contraction of six extraocular muscles that originate from the bones of the orbit and insert into the surface of the eyeball (Figure 2). Four of the muscles are arranged at the cardinal points around the eye and are named for those locations. They are the superior rectus, medial rectus, inferior rectus, and lateral rectus. When each of these muscles contract, the eye to moves toward the contracting muscle. For example, when the superior rectus contracts, the eye rotates to look up.

Figure 2. Extraocular Muscles The extraocular muscles move the eye within the orbit.

The superior oblique originates at the posterior orbit, near the origin of the four rectus muscles. However, the tendon of the oblique muscles threads through a pulley-like piece of cartilage known as the trochlea. The tendon inserts obliquely into the superior surface of the eye. The angle of the tendon through the trochlea means that contraction of the superior oblique rotates the eye medially.

The inferior oblique muscle originates from the floor of the orbit and inserts into the inferolateral surface of the eye. When it contracts, it laterally rotates the eye, in opposition to the superior oblique. Rotation of the eye by the two oblique muscles is necessary because the eye is not perfectly aligned on the sagittal plane.

When the eye looks up or down, the eye must also rotate slightly to compensate for the superior rectus pulling at approximately a 20-degree angle, rather than straight up. The same is true for the inferior rectus, which is compensated by contraction of the inferior oblique. A seventh muscle in the orbit is the levator palpebrae superioris, which is responsible for elevating and retracting the upper eyelid, a movement that usually occurs in concert with elevation of the eye by the superior rectus (see Figure 1). The extraocular muscles are innervated by three cranial nerves. The lateral rectus, which causes abduction of the eye, is innervated by the abducens nerve. The superior oblique is innervated by the trochlear nerve. All of the other muscles are innervated by the oculomotor nerve, as is the levator palpebrae superioris. The motor nuclei of these cranial nerves connect to the brain stem, which coordinates eye movements.

The eye itself is a hollow sphere composed of three layers of tissue. The outermost layer is the fibrous tunic, which includes the white sclera and clear cornea. The sclera accounts for five sixths of the surface of the eye, most of which is not visible, though humans are unique compared with many other species in having so much of the “white of the eye” visible (Figure 3). The transparent cornea covers the anterior tip of the eye and allows light to enter the eye.

The middle layer of the eye is the vascular tunic, which is mostly composed of the choroid, ciliary body, and iris. The choroid is a layer of highly vascularized connective tissue that provides a blood supply to the eyeball. The choroid is posterior to the ciliary body, a muscular structure that is attached to the lens by zonule fibers. These two structures bend the lens, allowing it to focus light on the back of the eye. Overlaying the ciliary body, and visible in the anterior eye, is the iris—the colored part of the eye. The iris is a smooth muscle that opens or closes the pupil, which is the hole at the center of the eye that allows light to enter. The iris constricts the pupil in response to bright light and dilates the pupil in response to dim light.

The innermost layer of the eye is the neural tunic, or retina, which contains the nervous tissue responsible for photoreception. The eye is also divided into two cavities: the anterior cavity and the posterior cavity. The anterior cavity is the space between the cornea and lens, including the iris and ciliary body. It is filled with a watery fluid called the aqueous humor. The posterior cavity is the space behind the lens that extends to the posterior side of the interior eyeball, where the retina is located. The posterior cavity is filled with a more viscous fluid called the vitreous humor. The retina is composed of several layers and contains specialized cells for the initial processing of visual stimuli. The photoreceptors (rods and cones) change their membrane potential when stimulated by light energy. The change in membrane potential alters the amount of neurotransmitter that the photoreceptor cells release onto bipolar cells in the outer synaptic layer. It is the bipolar cell in the retina that connects a photoreceptor to a retinal ganglion cell (RGC) in the inner synaptic layer. There, amacrine cells additionally contribute to retinal processing before an action potential is produced by the RGC. The axons of RGCs, which lie at the innermost layer of the retina, collect at the optic disc and leave the eye as the optic nerve (see Figure 3). Because these axons pass through the retina, there are no photoreceptors at the very back of the eye, where the optic nerve begins. This creates a “blind spot” in the retina, and a corresponding blind spot in our visual field.

Note that the photoreceptors in the retina (rods and cones) are located behind the axons, RGCs, bipolar cells, and retinal blood vessels. A significant amount of light is absorbed by these structures before the light reaches the photoreceptor cells. However, at the exact center of the retina is a small area known as the fovea. At the fovea, the retina lacks the supporting cells and blood vessels, and only contains photoreceptors. Therefore, visual acuity, or the sharpness of vision, is greatest at the fovea. This is because the fovea is where the least amount of incoming light is absorbed by other retinal structures (see Figure 3).

Figure 3. Structure of the Eye The sphere of the eye can be divided into anterior and posterior chambers. The wall of the eye is composed of three layers: the fibrous tunic, vascular tunic, and neural tunic. Within the neural tunic is the retina, with three layers of cells and two synaptic layers in between. The center of the retina has a small indentation known as the fovea.

As one moves in either direction from this central point of the retina, visual acuity drops significantly. In addition, each photoreceptor cell of the fovea is connected to a single RGC. Therefore, this RGC does not have to integrate inputs from multiple photoreceptors, which reduces the accuracy of visual transduction. Toward the edges of the retina, several photoreceptors converge on RGCs (through the bipolar cells) up to a ratio of 50 to 1.

The difference in visual acuity between the fovea and peripheral retina is easily evidenced by looking directly at a word in the middle of this paragraph. The visual stimulus in the middle of the field of view falls on the fovea and is in the sharpest focus. Without moving your eyes off that word, notice that words at the beginning or end of the paragraph are not in focus. The images in your peripheral vision are focused by the peripheral retina, and have vague, blurry edges and words that are not as clearly identified. As a result, a large part of the neural function of the eyes is concerned with moving the eyes and head so that important visual stimuli are centered on the fovea. Light falling on the retina causes chemical changes to pigment molecules in the photoreceptors, ultimately leading to a change in the activity of the RGCs.

Photoreceptor cells have two parts, the inner segment and the outer segment (Figure 4). The inner segment contains the nucleus and other common organelles of a cell, whereas the outer segment is a specialized region in which photoreception takes place. There are two types of photoreceptors—rods and cones—which differ in the shape of their outer segment. The rod-shaped outer segments of the rod photoreceptor contain a stack of membrane-bound discs that contain the photosensitive pigment rhodopsin. The cone-shaped outer segments of the cone photoreceptor contain their photosensitive pigments in infoldings of the cell membrane. There are three cone photopigments, called opsins, which are each sensitive to a particular wavelength of light. The wavelength of visible light determines its color. The pigments in human eyes are specialized in perceiving three different primary colors: red, green, and blue.

Figure 4. Photoreceptor (a) All photoreceptors have inner segments containing the nucleus and other important organelles and outer segments with membrane arrays containing the photosensitive opsin molecules. Rod outer segments are long columnar shapes with stacks of membrane-bound discs that contain the rhodopsin pigment. Cone outer segments are short, tapered shapes with folds of membrane in place of the discs in the rods. (b) Tissue of the retina shows a dense layer of nuclei of the rods and cones. LM × 800. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

At the molecular level, visual stimuli cause changes in the photopigment molecule that lead to changes in membrane potential of the photoreceptor cell. A single unit of light is called a photon, which is described in physics as a packet of energy with properties of both a particle and a wave. The energy of a photon is represented by its wavelength, with each wavelength of visible light corresponding to a particular color. Visible light is electromagnetic radiation with a wavelength between 380 and 720 nm. Longer wavelengths of less than 380 nm fall into the infrared range, whereas shorter wavelengths of more than 720 nm fall into the ultraviolet range. Light with a wavelength of 380 nm is blue whereas light with a wavelength of 720 nm is dark red. All other colors fall between red and blue at various points along the wavelength scale.

Opsin pigments are actually transmembrane proteins that contain a cofactor known as retinal. Retinal is a hydrocarbon molecule related to vitamin A. When a photon hits retinal, the long hydrocarbon chain of the molecule is biochemically altered. Specifically, photons cause some of the double-bonded carbons within the chain to switch from a cis to a trans conformation. This process is called photoisomerization. Before interacting with a photon, retinal’s flexible double-bonded carbons are in the cis conformation. This molecule is referred to as 11-cis-retinal. A photon interacting with the molecule causes the flexible double-bonded carbons to change to the trans– conformation, forming all-trans-retinal, which has a straight hydrocarbon chain (Figure 5).

Figure 5. Retinal Isomers The retinal molecule has two isomers, (a) one before a photon interacts with it and (b) one that is altered through photoisomerization.

The shape change of retinal in the photoreceptors initiates visual transduction in the retina. Activation of retinal and the opsin proteins result in activation of a G protein. The G protein changes the membrane potential of the photoreceptor cell, which then releases less neurotransmitter into the outer synaptic layer of the retina. Until the retinal molecule is changed back to the 11-cis-retinal shape, the opsin cannot respond to light energy, which is called bleaching. When a large group of photopigments is bleached, the retina will send information as if opposing visual information is being perceived. After a bright flash of light, afterimages are usually seen in negative. The photoisomerization is reversed by a series of enzymatic changes so that the retinal responds to more light energy.

Figure 6. Comparison of Color Sensitivity of Photopigments Comparing the peak sensitivity and absorbance spectra of the four photopigments suggests that they are most sensitive to particular wavelengths.

The opsins are sensitive to limited wavelengths of light. Rhodopsin, the photopigment in rods, is most sensitive to light at a wavelength of 498 nm. The three color opsins have peak sensitivities of 564 nm, 534 nm, and 420 nm corresponding roughly to the primary colors of red, green, and blue (Figure 6). The absorbance of rhodopsin in the rods is much more sensitive than in the cone opsins; specifically, rods are sensitive to vision in low light conditions, and cones are sensitive to brighter conditions.

In normal sunlight, rhodopsin will be constantly bleached while the cones are active. In a darkened room, there is not enough light to activate cone opsins, and vision is entirely dependent on rods. Rods are so sensitive to light that a single photon can result in an action potential from a rod’s corresponding RGC.

The three types of cone opsins, being sensitive to different wavelengths of light, provide us with color vision. By comparing the activity of the three different cones, the brain can extract color information from visual stimuli. For example, a bright blue light that has a wavelength of approximately 450 nm would activate the “red” cones minimally, the “green” cones marginally, and the “blue” cones predominantly. The relative activation of the three different cones is calculated by the brain, which perceives the color as blue. However, cones cannot react to low-intensity light, and rods do not sense the color of light. Therefore, our low-light vision is—in essence—in grayscale. In other words, in a dark room, everything appears as a shade of gray. If you think that you can see colors in the dark, it is most likely because your brain knows what color something is and is relying on that memory.

Watch this video to learn more about a transverse section through the brain that depicts the visual pathway from the eye to the occipital cortex.

The first half of the pathway is the projection from the RGCs through the optic nerve to the lateral geniculate nucleus in the thalamus on either side. This first fiber in the pathway synapses on a thalamic cell that then projects to the visual cortex in the occipital lobe where “seeing,” or visual perception, takes place. This video gives an abbreviated overview of the visual system by concentrating on the pathway from the eyes to the occipital lobe. The video makes the statement (at 0:45) that “specialized cells in the retina called ganglion cells convert the light rays into electrical signals.” What aspect of retinal processing is simplified by that statement? Explain your answer.

Sensory Nerves

Once any sensory cell transduces a stimulus into a nerve impulse, that impulse has to travel along axons to reach the CNS. In many of the special senses, the axons leaving the sensory receptors have a topographical arrangement, meaning that the location of the sensory receptor relates to the location of the axon in the nerve. For example, in the retina, axons from RGCs in the fovea are located at the center of the optic nerve, where they are surrounded by axons from the more peripheral RGCs.

Spinal Nerves

Generally, spinal nerves contain afferent axons from sensory receptors in the periphery, such as from the skin, mixed with efferent axons travelling to the muscles or other effector organs. As the spinal nerve nears the spinal cord, it splits into dorsal and ventral roots. The dorsal root contains only the axons of sensory neurons, whereas the ventral roots contain only the axons of the motor neurons. Some of the branches will synapse with local neurons in the dorsal root ganglion, posterior (dorsal) horn, or even the anterior (ventral) horn, at the level of the spinal cord where they enter. Other branches will travel a short distance up or down the spine to interact with neurons at other levels of the spinal cord. A branch may also turn into the posterior (dorsal) column of the white matter to connect with the brain. For the sake of convenience, we will use the terms ventral and dorsal in reference to structures within the spinal cord that are part of these pathways. This will help to underscore the relationships between the different components. Typically, spinal nerve systems that connect to the brain are contralateral, in that the right side of the body is connected to the left side of the brain and the left side of the body to the right side of the brain.

Cranial Nerves

Cranial nerves convey specific sensory information from the head and neck directly to the brain. For sensations below the neck, the right side of the body is connected to the left side of the brain and the left side of the body to the right side of the brain. Whereas spinal information is contralateral, cranial nerve systems are mostly ipsilateral, meaning that a cranial nerve on the right side of the head is connected to the right side of the brain. Some cranial nerves contain only sensory axons, such as the olfactory, optic, and vestibulocochlear nerves. Other cranial nerves contain both sensory and motor axons, including the trigeminal, facial, glossopharyngeal, and vagus nerves (however, the vagus nerve is not associated with the somatic nervous system). The general senses of somatosensation for the face travel through the trigeminal system.

Anatomy of the Eye | Kellogg Eye Center

• Choroid
  Layer containing blood vessels that lines the back of the eye and is located between the retina (the inner light-sensitive layer) and the sclera (the outer white eye wall).  
• Ciliary Body
  Structure containing muscle and is located behind the iris, which focuses the lens.
• Cornea
  The clear front window of the eye which transmits and focuses (i.e., sharpness or clarity) light into the eye. Corrective laser surgery reshapes the cornea, changing the focus.
• Fovea
  The center of the macula which provides the sharp vision.
• Iris
  The colored part of the eye which helps regulate the amount of light entering the eye. When there is bright light, the iris closes the pupil to let in less light. And when there is low light, the iris opens up the pupil to let in more light.
• Lens
  Focuses light rays onto the retina. The lens is transparent, and can be replaced if necessary. Our lens deteriorates as we age, resulting in the need for reading glasses. Intraocular lenses are used to replace lenses clouded by cataracts.
• Macula
  The area in the retina that contains special light-sensitive cells. In the macula these light-sensitive cells allow us to see fine details clearly in the center of our visual field. The deterioration of the macula is a common condition as we get older (age related macular degeneration or ARMD).
• Optic Nerve
  A bundle of more than a million nerve fibers carrying visual messages from the retina to the brain. (In order to see, we must have light and our eyes must be connected to the brain.) Your brain actually controls what you see, since it combines images. The retina sees images upside down but the brain turns images right side up. This reversal of the images that we see is much like a mirror in a camera. Glaucoma is one of the most common eye conditions related to optic nerve damage.
• Pupil
  The dark center opening in the middle of the iris. The pupil changes size to adjust for the amount of light available (smaller for bright light and larger for low light). This opening and closing of light into the eye is much like the aperture in most 35 mm cameras which lets in more or less light depending upon the conditions.
• Retina
  The nerve layer lining the back of the eye. The retina senses light and creates electrical impulses that are sent through the optic nerve to the brain.
• Sclera
  The white outer coat of the eye, surrounding the iris.
• Vitreous Humor
  The, clear, gelatinous substance filling the central cavity of the eye.

The five senses include sight, sound, taste, hearing and touch. Sight, like the other senses is closely related to other parts of our anatomy. The eye is connected to the brain and dependent upon the brain to interpret what we see.

How we see depends upon the transfer of light. Light passes through the front of the eye (cornea) to the lens. The cornea and the lens help to focus the light rays onto the back of the eye (retina). The cells in the retina absorb and convert the light to electrochemical impulses which are transferred along the optic nerve and then to the brain.

The eye works much the same as a camera. The shutter of a camera can close or open depending upon the amount of light needed to expose the film in the back of the camera. The eye, like the camera shutter, operates in the same way. The iris and the pupil control how much light to let into the back of the eye. When it is very dark, our pupils are very large, letting in more light. The lens of a camera is able to focus on objects far away and up close with the help of mirrors and other mechanical devices. The lens of the eye helps us to focus but sometimes needs some additional help in order to focus clearly. Glasses, contact lenses, and artificial lenses all help us to see more clearly. 

And an eye like an eagle

Olga Nesterenko,
Moscow Zoo
Chemistry and Life No. 6, 2017

It seems to us that animals see the world in much the same way as we do. In fact, their perception is very different from human. Even in birds – warm-blooded terrestrial vertebrates like us – the sense organs work differently than in humans.

Vision plays an important role in the life of birds. Anyone who can fly needs to navigate in flight, notice food in time, often at a great distance, or a predator (which, perhaps, can also fly and is approaching rapidly). So how is bird vision different from human vision?

To begin with, we note that the eyes of birds are very large. So, in an ostrich, their axial length is twice that of a human eye – 50 mm, almost like tennis balls! In herbivorous birds, the eyes make up 0.2-0.6% of the body weight, and in predatory, owls and other birds looking for prey from afar, the mass of the eyes can be two to three times the mass of the brain and reaches 3-4% of the body weight, in owls – up to 5%. For comparison: in an adult, the weight of the eyes is approximately 0.02% of the body weight, or 1% of the head weight. And, for example, in a starling, 15% of the mass of the head falls on the eyes, in owls – up to a third.

Visual acuity in birds is much higher than in humans – 4-5 times, in some species, probably up to 8. Carrion-eating vultures see the corpse of an ungulate animal 3-4 km away. Eagles notice prey from a distance of about 3 km, large species of falcons – from a distance of up to 1 km. And a kestrel falcon flying at a height of 10–40 m sees not only mice, but even insects in the grass.

What features of the structure of the eyes provide such visual acuity? One factor is size: large eyes produce large images on the retina. In addition, the bird’s retina has a high density of photoreceptors. People in the zone of maximum density have 150,000–240,000 photoreceptors per mm ² , in the house sparrow – 400,000, in the common buzzard – up to a million. In addition, good image resolution is determined by the ratio of the number of nerve ganglia to receptors. (If multiple receptors are connected to the same ganglion, resolution is reduced.) This ratio is much higher in birds than in humans. For example, in the white wagtail, there are about 100,000 ganglion cells per 120,000 photoreceptors.

Like mammals, birds have an area in their retina called the fovea, a depression in the middle of the macula. In the fovea, due to the high density of receptors, visual acuity is the highest. But it is interesting that 54% of bird species – raptors, kingfishers, hummingbirds, swallows, etc. – have another area with the highest visual acuity to improve lateral vision. It is more difficult for swifts to forage than for swallows, also because they have only one area of ​​sharp vision: swifts see well only forward, and their methods of catching insects on the fly are less diverse.

The eyes of most birds are fairly far apart. The field of view of each eye is 150–170°, but the overlap of the fields of both eyes (field of binocular vision) is only 20–30° in many birds. But a flying bird can see what is happening in front of it, from the sides, behind and even below (Fig. 1). For example, the large and protruding eyes of the American woodcock Scolopax minor are set high on a narrow head, and their field of view reaches 360° in the horizontal plane and 180° in the vertical plane. The woodcock has a field of binocular vision not only in front, but also behind! A very useful quality: a feeding woodcock sticks its beak into soft ground, looking for earthworms, insects, their larvae and other suitable food, while also seeing what is happening around. The large eyes of nightjars are slightly shifted back, their field of vision is also about 360 °. A wide field of view is characteristic of pigeons, ducks and many other birds.

In herons and bitterns, the field of binocular vision is shifted down, under the beak: it is narrow in the horizontal plane, but extended vertically, up to 170°. Such a bird, when holding its beak horizontally, can see its own paws with binocular vision. And even raising her beak up (as bitterns do when waiting for prey in reeds and disguising themselves due to vertical stripes on plumage), she is able to look down, notice small living creatures floating in the water and catch them with accurate throws. After all, binocular vision allows you to determine the distance to objects.

For many birds, it is more important not to have a large field of view, but to have good binocular vision, with both eyes at once. These are primarily birds of prey and owls, since they need to estimate the distance to prey. Their eyes are close-set, and the intersection of the visual fields is quite wide. At the same time, the narrow overall field of view is compensated by the mobility of the neck. Of all bird species, binocular vision is best developed in owls, and they can turn their heads 270 °.

To focus the eyes on an object during rapid movement (intrinsic, or object, or total), good accommodation of the lens is needed, that is, the ability to quickly and very quickly change its curvature. The eyes of birds are equipped with a special muscle that changes the shape of the lens more effectively than in mammals. This ability is especially developed in birds that catch prey under water – cormorants, kingfishers. In cormorants, the ability to accommodate is 40-50 diopters, and in humans 14-15, although some species, such as chickens and pigeons, have only 8-12 diopters. Diving birds are also helped to see under water by a transparent third eyelid that covers the eye – a kind of goggles for scuba diving.

Everyone must have noticed how brightly colored many birds are. Some species – tap dances, linnets, robins, in general, are not brightly colored, have areas of bright plumage. In others, bright body parts appear during the mating season, for example, male frigatebirds inflate a red throat sac, and in puffins, the beak becomes bright orange. Thus, even by the coloration of birds, it can be seen that they have well-developed color vision, unlike most mammals, among which there are no such elegant creatures. In mammals, primates are the best at distinguishing colors, but birds are even ahead of them, including humans. This is due to some features of the structure of the eyes.

There are two main types of photoreceptors in the retina of mammals and birds – rods and cones. Rods provide night vision, in the eyes of owls they predominate. Cones are responsible for daytime vision and color discrimination. Primates have three types (they perceive red, green and blue colors known to all ophthalmologists and color correctors), other mammals have only two. Birds have four types of cones with different visual pigments – red, green, blue, and violet/ultraviolet. And the more varieties of cones, the more shades the eye distinguishes (Fig. 2).

Unlike mammals, each cone of birds contains another drop of colored oil. These drops act as filters – they cut off part of the spectrum perceived by a particular cone, thereby reducing the overlap of reactions between cones containing different pigments, and increasing the number of colors that birds can distinguish. Six types of oil droplets have been identified in cones; five of them are mixtures of carotenoids that absorb waves of different lengths and intensities, and in the sixth type there are no pigments. The exact composition and color of the droplets vary from species to species: they may provide a fine-tuning of vision so that its capabilities are best suited to habitat and feeding behavior.

The fourth type of cone allows many birds to see ultraviolet color, invisible to humans. The list of species for which this ability has been experimentally proven has grown greatly in the last 35 years. These are, for example, ratites, waders, gulls, auks, trogons, parrots and passerines. Experiments have shown that areas of plumage displayed by birds during courtship are often ultraviolet in color. To the human eye, about 60% of bird species do not have sexual dimorphism, that is, males and females are outwardly indistinguishable, but the birds themselves may not think so. Of course, it is impossible to show people how birds see each other, but you can roughly imagine this from photographs where the ultraviolet areas are tinted with a conventional color (Fig. 3).

The ability to see ultraviolet color helps birds find food. Fruits and berries have been shown to reflect ultraviolet rays, making them more visible to many birds. And kestrels, perhaps, see the paths of voles: they are marked with urine and excrement, which reflect ultraviolet and due to this become visible to a bird of prey.

However, having the best perception of color among terrestrial vertebrates, birds lose it at dusk. To distinguish colors, birds need 5 to 20 times more light than humans.

But that’s not all. Birds have other abilities that are inaccessible to us. So, they see fast movements much better than people. We do not notice flickering at a speed greater than 50 Hz (for example, the glow of a fluorescent lamp seems to us to be continuous). The temporal resolution of vision in birds is much higher: they can notice more than 100 changes per second, for example, in the pied flycatcher – 146 Hz (Jannika E. Boström et al. Ultra-Rapid Vision in Birds // PLoS ONE , 2016, 11(3): e0151099, doi: 10. 1371/journal.pone.0151099). This makes it easier for small birds to hunt insects, but, perhaps, makes life in captivity unbearable: the lamps in the room, according to a person, normally luminous, flicker disgustingly for a bird. Birds are also able to see very slow movement, such as the movement of the sun and stars across the sky, inaccessible to our naked eyes. It is assumed that this helps them navigate during flights.

Colors and shades unknown to us; circular review; switching modes from “binoculars” to “loupe”; the fastest movements are clearly visible, as if in slow motion… It’s hard for us to even imagine how birds perceive the world. One can only admire their capabilities!

description of fish, where it is found, species, how long it lives

Flounder fish is a representative of the class “Ray-finned” and belongs to the order “Flounder-like”. The family consists of 6 dozen different fish, which differ in the characteristic body shape.

Contents

• 1 Flounder fish: description
  • 1. 1 Appearance
  • 1.2 Character and lifestyle
  • 1.3 How long does a flounder live
  • 1.4 Sexual dimorph unit
• 2 Species of flounder
• 3 Natural habitats
• 4 What flounder eats
• 5 Reproduction and offspring
• 6 Natural enemies
• 7 Population and species status

900 59 8 Commercial value of flounder

• 9 Flounder in cooking
• 10 Useful properties of flounder meat

Flounder fish: description

Due to the fact that the eyes of this detachment are located on the right side, they are also called “Right-sided flounders”. It is also known that there are forms of flounder in which the eyes are located on the left side of the head. The symmetrically located ventral fins have a rather narrow base.

All varieties of flounder are characterized by common data, such as:

• The presence of a flat body.
• Elongated dorsal and anal fins with many rays.
• Asymmetrical head.
• Closely spaced, bulging eyes that can function independently.
• Lateral line that runs between the eyes.
• Oblique mouth and fairly sharp teeth.
• Fairly short tail.
• The opposite side is distinguished by a light coloration and strong, rough skin.

Flounder eggs differ in that they do not have a fat drop, so they are in suspension in the water column, where the process of development of future offspring takes place. Five species of flounder prefer spawning in the near-bottom area.

An interesting moment! The ability to mimic this family allows them to skillfully disguise, regardless of the complexity of the background color. At the same time, the Kambalovye are not inferior in their camouflage capabilities even to a chameleon.

Appearance

All varieties of flounder prefer to lead a benthic lifestyle at considerable depths. Their characteristic distinguishing feature is the presence of a thin, oval or diamond-shaped body, as if flattened from the sides.

River flounder (Platichthys flesus) is a term used to describe Star, Black Sea (Kalkan) and Polar flounders.

• Star flounder differs in that it is characterized by a left-sided arrangement of eyes. At the same time, the fish has a dark, greenish or brown body color, with wide black stripes on the fins and studded star-shaped plates. This is characteristic of the side where the eyes are located. The fish grows up to 60 cm in length and can weigh up to 4 kilograms.
• Black Sea Kalkan . Represents a left-handed flounder. On a round body, you can see a lot of bumpy spikes that are randomly scattered throughout the body on the left side. The main body color is brown-olive. The flounder grows in length more than one meter, gaining weight up to 20 kilograms.
• Arctic flounder . It is a cold-resistant representative of a large family. It has an elongated oval body. The body color is monophonic and made in dark brown tones, while the color of the fins has a brown tint.

Varieties of sea flounder thrive in salt water, in addition, they differ in that they have a wide scatter of data regarding their size, body shape, fin color, and eye position.

• Common flounder has a brown-green base color with reddish or orange spots. Adult specimens can weigh all 7 kilograms and grow up to 1 meter in length. The species has excellent opportunities in terms of mimicry.
• The white-bellied southern and northern flounders represent bottom-dwelling marine fish. They can grow up to half a meter in length. A characteristic feature of this species is the presence of a bifurcated lateral line, an arcuate shape. The lower part of the body of the fish has a milky color, and the upper part is brown or wheat-brown.
• Yellowfin flounder is a cold-loving species. The body is rounded, covered with scales with tiny spines. The fins are distinguished by a yellow-golden hue. Adult specimens grow up to 0.5 meters in length and weigh no more than 1 kilogram.
• Halibut . The variety is represented by 5 species. Some of them have a length of 4.5 meters and a weight of 350 kilograms. The arrow-toothed halibut is considered the smallest representative, the length of which is not more than 0.8 meters and weighs about 8 kilograms.

The name Far Eastern flounder is applied to individual subspecies representing the so-called flatfish.

This fish came off Picasso’s painting – Tropical flounder botus

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Character and lifestyle

These fish prefer to lead an isolated bottom lifestyle, while they can very skillfully disguise themselves depending on the nature of the surrounding landscape. They spend most of their lives in a supine position either on the surface of the ground or in the thickness of the ground, burrowing into it almost completely, with the exception of the eyes, which are always on the surface. This type of life activity allows the fish not only to reliably hide from their enemies, but also to get food for themselves, attacking their prey from a kind of ambush.

At first glance, this fish seems to be slow and clumsy, because it is mainly seen moving slowly in the water column, at a considerable distance from the bottom. In fact, flounder, if necessary, can demonstrate their skills, instantly gaining decent speed.

In case of danger, the fish makes an instant leap forward several meters due to a very powerful jet of water, which is formed by the gill cover. This jet literally “shoots” into the bottom area. As a result, a cloud of turbidity is formed, which gradually settles to the bottom. During this time, the fish manages to grab its prey or get away from the predator.

How long does a flounder live

A flounder can live, if favorable conditions are created, for at least 30 years. Being in the natural environment, the flounder rarely lives to such a venerable age, since the bulk falls into industrial fishing nets.

Sexual dimorphism

Male flounders can be distinguished from females in several ways. First, they are smaller in size and weight, and secondly, they have a greater distance between the eyes. In addition, they have longer rays of the pectoral and dorsal fins.

Species of flounder

Currently known 60 species of flounder are represented by 23 genera:

• The genus of spiny flounder includes Spiny flounder and Nadezhny flounder.
• The genus of arrow-toothed halibut is represented by the Asian arrow-toothed halibut and the American arrow-toothed halibut.
• The genus of sharp-headed flounder includes Gertsenstein’s flounder and sharp-headed flounder.
• The Warty flounder genus consists of one species called the Warty flounder.
• The Eopsetta genus is represented by the Far Eastern flounder (Grigoriev’s flounder) and Jordan’s flounder.
• The genus “Long flounder” consists of such species as the Red flounder and the Far East long flounder (Steller’s smallmouth).
• The halibut flounder genus consists of such species as Japanese halibut flounder (Japanese ruff flounder), Northern halibut flounder, Ruff flounder and Bering Sea halibut flounder.
• The genus “Halibut” (whitebark halibut) consists of the Atlantic halibut and the Pacific halibut.
• The genus of two-colored flounder and two-lined flounder is represented by such species as white-bellied flounder and northern two-lined flounder.
• The genus Limandy includes several more species and is represented by the yellowfin flounder, the yellow-tailed flounder, the Ershovatka, the long-snouted estuary and the Sakhalin flounder.
• The genus “Polar flounder” is represented by one species – Naked flounder.
• Genus “Oregon flounders”.
• The genus “Smallmouth flounder” is represented by some species, including the small-headed flounder and the Pacific smallmouth.
• The genus “River flounder” is also represented by several species, including the Starry flounder.
• The genus “Sea flounder” includes several species, including the species Yellow sea flounder.
• The genus Hardhead flounder includes the species Horned flounder.
• A genus of spotted flounders.
• The genus of winter flounder includes species such as the Yellowstriped flounder, Schrenk flounder and Japanese flounder.

Attention should be paid to the genus Dexists and Embassichts, which includes the Deep Sea Embassicht, as well as the genus Isopsets, Veraspets, Tanakiuses, Psalmodiscuses, Parophrises and Black Halibuts.

Interesting to know! Halibuts are considered the largest representatives of the family that live in the waters of the Pacific and Atlantic oceans. Halibuts live for about half a century, inhabiting great depths.

Natural habitats

Some species have a preference for the northern latitudes of the Pacific depths, with many of them found in the Sea of ​​Japan, Bering, Okhotsk and Chukchi Seas. For freshwater forms, characteristic habitats are lagoons, lower reaches of rivers and bays. For some species, distribution within the northern latitudes of the Atlantic, as well as within the waters of the Black, Baltic and Mediterranean seas, is considered characteristic. Some species feel great not only in the marine environment, but also in the lower reaches of such rivers as the Southern Bug, Dnieper and Dniester. Due to the fact that the Sea of ​​Azov has recently become too salty, and the inflowing rivers have become noticeably shallow, the Black Sea flounder Kalkan preferred to migrate to the mouth of the Don River. Cold-resistant species, as a rule, live in arctic latitudes, occurring in the waters of the Kara, Barents, White, Bering and Okhotsk seas. At the same time, they swim in such rivers as the Yenisei, Ob, Kare, Tugura, where they prefer places with soft silty soils.

Marine varieties of flounder inhabit weakly and strongly saline waters, living at depths up to 200 meters inclusive. Many of the species are considered valuable commercial fish, while they are found in the waters of the Eastern Atlantic, as well as in such seas as the Mediterranean, Barents, White and Baltic Seas. The southern white-bellied flounder is characterized by distribution in the waters of the Sea of ​​Japan, and some northern subspecies are limited to the waters of the Okhotsk, Kamchatka and Bering Seas.

An interesting moment! Flatfish are rich in species diversity and great biological flexibility, which allowed them to spread throughout the territories located along the entire Eurasian continent, as well as in the waters of inland seas.

Yellowfin flounder is widespread in the waters of the Sea of ​​Japan, the Sea of ​​Okhotsk and the Bering Sea. Large populations of this species are distributed within Sakhalin and the western waters of Kamchatka, where they live at depths up to 80 meters, preferring the near-bottom area. For halibut fish, characteristic habitats are the waters of the Arctic Ocean and the Pacific Ocean, including the waters of the Barents Sea, the Bering Sea, the Sea of ​​Okhotsk and the Sea of ​​Japan.

What flounder eats

These fish go hunting either at dusk, or at night, or in the morning. Flounder is considered a predatory fish, so its diet is represented by animal food objects. At a young age, individuals feed on benthos, amphipods, various worms, larvae, crustaceans and caviar. As they mature, the flounder moves on to larger worms, brittle stars, echinoderms, as well as small fish, many invertebrates and crustaceans. Shrimps and medium-sized capelin are especially popular with representatives of this family.

Due to the peculiarities of the body structure, which is associated with the lateral position of the head, this fish easily gets various mollusks from the thickness of the sea or river bottom. At the same time, it should be noted that the jaws of the flounder are strong and powerful, so the fish can easily deal even with crab shells. Due to the fact that the family feeds on food objects with a high content of proteins, the fish is of great value.

Reproduction and offspring

The periods of spawning largely depend on the living conditions, which is associated with the periods of the onset of spring, warming of the water column, etc. As a rule, many species spawn between the first decade of February and the month of May, although there are some exceptions.

Large rhombus (turbo) prefers to spawn in the waters of the Baltic and North Seas, starting from April and including August. As for the polar flounder, the process of its spawning takes place in the water column covered with ice, and the period occupies a segment from December to January inclusive.

Depending on the species, the ability to reproduce occurs after a minimum of 3 years of life. Many species are characterized by a high degree of fertility, so the number of eggs in a clutch can range from several hundred thousand to several million pieces. As a rule, the period of development of eggs takes a couple of weeks no more. To spawn, the flounder chooses places with sufficient depth and a sandy bottom.

An important point! The flounder fry born into the world have the usual body shape, like for a fish, since they are characterized by symmetrically developed sides, since the eyes are located classically, on both sides of the head. Flounder fry eat zooplankton and other small food items.

Some species prefer areas up to 50 meters deep, which is due to the excellent buoyancy of the eggs, as well as the absence of the need to attach them to any base.

Giant turbot fish underwater. Giant halibut/flounder underwater.

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Natural enemies

Thanks to its abilities, the flounder fish changes its body color depending on the nature of the color of the bottom of the reservoir. This allows the fish to be unnoticed by many predators. This does not mean at all that the fish is completely devoid of natural enemies. The most dangerous for her are halibut, eel and man. The fish has quite tasty, delicious meat, therefore it is caught in large quantities in all areas of the oceans.

Population and species status

At the moment there are many problems associated with daily fishing for many valuable fish species. The main problem is associated with overfishing of easily accessible and, especially, small species. This problem has deep roots and is related to multi-species fisheries. Unfortunately, we have not yet been able to solve it. The total number of flounder depends on many natural factors, so experts note some cyclicality, characteristic of both the conditions of decline and the conditions of increase in the population.

Such factors include the constant negative impact of humans associated with their life, including the fact that humans exert enormous pressure as a result of commercial, uncontrolled fishing. As an example, we can take the Arnoglos Mediterranean flounder and the Kessler flounder, which are on the verge of extinction, so their total number is extremely small.

Commercial value of flounder

Flounder belongs to the category of valuable commercial fish. It is caught in large volumes in the Black and Baltic Seas, and flounder kalkan and turbot flounder – in the waters of the Mediterranean Sea. In the caught fish, the body color is slightly greenish in color, while the meat is white. Regardless of the method of preparation, all flounder dishes are easily absorbed by the human body. Often the meat of this fish is used as a preparation of dietary dishes.

Flounder in cooking

Flounder meat is rich in protein, while the amount of fat is negligible, which allows you to include the meat of this fish in the diet. The energy value of fresh fish is at the level of 90 kcal per 100 grams of meat, and boiled flounder – 103 kcal per 100 g of product. The largest number of calories in fried fish is about 223 kcal, so fried flounder is not suitable as a dietary dish, since there is a high probability of gaining extra pounds.

Flounder meat is quite tasty, but if cooked incorrectly, you can get a dish with a specific smell. To prevent this from happening, you need to remove the skin from the flounder, cut off the head and get rid of the insides.

Useful properties of flounder meat

This fish has a huge potential for useful characteristics, therefore it is an indispensable product for preparing dietary dishes. A large amount of protein, which is easily absorbed by the human body and which is so necessary for human life, is only a small fraction of the positive qualities. Its meat contains a sufficient amount of omega-3 fatty acids, which allow the body to fight malignant tumors. In addition to fatty acids, meat is rich in phosphorus, nicotinic and pantothenic acids, riboflavin, thiamine, pyridoxine and various salts. Fish also contains a whole complex of vitamins of group “B”, as well as vitamins “D”, “E” and “A”, the effect of which on the human body is purely positive.

Due to the presence of amino acids in flounder, eating flounder dishes can reduce cholesterol levels in human blood.