How does the human eye work. What are the main parts of the eye. How does light travel through the eye. What role does each component play in vision. How can we maintain optimal eye health.

The Structure and Layers of the Human Eye

The human eye is a remarkable organ, shaped like a round ball with a slight bulge at the front. Its complex structure consists of three main layers that form the eyeball:

• The outer layer: sclera and cornea
• The middle layer: choroid, including the iris
• The inner layer: retina

Each of these layers plays a crucial role in the visual process. Let’s explore them in more detail.

The Outer Layer: Sclera and Cornea

The sclera is the tough, white, opaque membrane commonly known as “the white of the eye.” It provides protection and maintains the eye’s shape. At the front of the eye, the sclera transitions into the cornea – a clear, thin, dome-shaped tissue that acts as the eye’s outermost lens.

The Middle Layer: Choroid and Iris

The choroid is a layer rich in blood vessels that nourish the eye. The front part of the choroid forms the iris, which is the colored part of the eye. In the center of the iris is a circular opening called the pupil, which controls the amount of light entering the eye.

The Inner Layer: Retina

The retina lines the back two-thirds of the eyeball and consists of two layers:

1. The sensory retina: Contains nerve cells that process visual information and send it to the brain.
2. The retinal pigment epithelium (RPE): Lies between the sensory retina and the wall of the eye.

The Three Chambers of the Eye

The interior of the eye is divided into three distinct chambers, each serving a specific purpose in the visual process:

Anterior Chamber

Located between the cornea and the iris, the anterior chamber contains a clear, watery fluid called aqueous humor. The iris controls the amount of light entering the eye by adjusting the size of the pupil.

Posterior Chamber

Situated between the iris and the lens, the posterior chamber also contains aqueous humor. The lens, held in place by small tissue strands called zonules, is elastic and can change shape to focus on objects at varying distances.

Vitreous Chamber

The largest chamber, located between the lens and the back of the eye, is filled with a gel-like substance called vitreous humor. The back two-thirds of this chamber is lined with the retina, which contains millions of light-sensitive cells.

The Cornea: The Eye’s Clear Front Surface

The cornea is the transparent front part of the eye and plays a crucial role in vision. Its primary functions include:

• Refracting light entering the eye
• Protecting the inner structures of the eye
• Serving as a resting place for contact lenses
• Acting as the site for LASIK surgery

The cornea consists of five unique layers, each with a specific function. Interestingly, it is avascular, meaning it contains no blood vessels, which contributes to its transparency.

The Lens: Focusing Light onto the Retina

The lens is a clear, flexible structure located behind the iris. Its primary function is to focus light onto the retina. The lens can change shape, allowing the eye to focus on objects at different distances – a process known as accommodation.

How does the lens change shape? Small muscles attached to the lens can contract or relax, causing the lens to become more rounded for near vision or flatter for distance vision.

The Retina: Converting Light into Neural Signals

The retina is a light-sensitive layer at the back of the eye. It contains millions of photoreceptor cells called rods and cones, which convert light into electrical signals. These signals are then sent to the brain via the optic nerve.

The retina has two important regions:

1. The macula: Located near the center of the retina, it provides sharp, detailed central vision.
2. The peripheral retina: Responsible for side vision, allowing us to see shapes but not fine details.

The Role of Eye Fluids: Aqueous and Vitreous Humor

The eye contains two types of fluid that help maintain its shape and support its function:

Aqueous Humor

This clear, watery fluid fills the anterior and posterior chambers of the eye. It provides nutrients to the cornea and lens, and helps maintain intraocular pressure.

Vitreous Humor

Also known as vitreous gel, this thick, gel-like substance fills the vitreous chamber. It helps maintain the eye’s shape and holds the retina in place.

The Visual Process: How We See

The process of vision is a complex interplay of various parts of the eye working in harmony. Here’s a simplified explanation of how we see:

1. Light enters the eye through the cornea, which bends the light rays.
2. The light passes through the pupil, whose size is controlled by the iris to regulate the amount of light entering the eye.
3. The lens focuses the light onto the retina.
4. The retina converts the light into electrical impulses.
5. These impulses travel through the optic nerve to the brain.
6. The brain interprets these signals, resulting in the visual images we perceive.

This process happens instantaneously, allowing us to see the world around us in real-time.

Common Eye Conditions and Their Impact on Vision

Understanding the anatomy of the eye also helps us comprehend various eye conditions that can affect vision. Some common eye conditions include:

• Myopia (nearsightedness): Difficulty seeing distant objects clearly
• Hyperopia (farsightedness): Difficulty seeing near objects clearly
• Astigmatism: Blurred vision due to an irregularly shaped cornea
• Cataracts: Clouding of the lens, leading to blurred vision
• Glaucoma: Increased pressure within the eye, potentially damaging the optic nerve
• Macular degeneration: Deterioration of the macula, affecting central vision

Each of these conditions affects different parts of the eye, highlighting the importance of regular eye check-ups and maintaining overall eye health.

Maintaining Eye Health: Tips and Recommendations

Given the complexity and importance of our eyes, it’s crucial to take steps to maintain their health. Here are some recommendations:

• Have regular eye exams, even if you don’t wear glasses or contacts
• Protect your eyes from UV radiation by wearing sunglasses outdoors
• Maintain a healthy diet rich in vitamins A, C, and E, as well as omega-3 fatty acids
• Stay hydrated to keep your eyes lubricated
• Practice the 20-20-20 rule when working on screens: Every 20 minutes, look at something 20 feet away for 20 seconds
• Don’t smoke, as it increases the risk of several eye diseases
• Exercise regularly to improve blood circulation, including to your eyes

By understanding the anatomy and function of our eyes, we can better appreciate the complexity of vision and the importance of taking care of our eye health. Regular check-ups, a healthy lifestyle, and protective measures can go a long way in maintaining good vision throughout our lives.

The human eye is truly a marvel of nature, capable of distinguishing millions of colors, adapting to various light conditions, and providing us with a window to the world. From the transparent cornea to the light-sensitive retina, each part of the eye plays a crucial role in the visual process. By taking care of our eyes and understanding how they work, we can ensure that this incredible organ continues to serve us well throughout our lives.

As our understanding of eye anatomy and function continues to evolve, so too do the treatments and preventive measures available to us. Advances in ophthalmology, such as laser eye surgery, intraocular lenses, and gene therapy, offer new hope for those with various eye conditions. However, the foundation of good eye health remains rooted in the basics: regular check-ups, a healthy lifestyle, and an appreciation for the intricate workings of this remarkable organ.

Whether we’re marveling at a beautiful sunset, reading a captivating book, or simply navigating our daily lives, our eyes are constantly at work, processing an enormous amount of visual information. By understanding and caring for our eyes, we can ensure that we continue to see the world in all its vibrant detail for years to come.

Eye Anatomy and Function | Michigan Medicine

Topic Overview

The eye is shaped like a round ball, with a slight bulge at the front.

The eye has three main layers. These layers lie flat against each other and form the eyeball.

• The outer layer of the eyeball is a tough, white, opaque membrane called the sclera (the white of the eye). The slight bulge in the sclera at the front of the eye is a clear, thin, dome-shaped tissue called the cornea.
• The middle layer is the choroid. The front of the choroid is the colored part of the eye called the iris. In the center of the iris is a circular hole or opening called the pupil.
• The inner layer is the retina, which lines the back two-thirds of the eyeball. The retina consists of two layers: the sensory retina, which contains nerve cells that process visual information and send it to the brain; and the retinal pigment epithelium (RPE), which lies between the sensory retina and the wall of the eye.

The inside of the eye is divided into three sections called chambers.

• Anterior chamber: The anterior chamber is the front part of the eye between the cornea and the iris.
  • The iris controls the amount of light that enters the eye by opening and closing the pupil.
  • The iris uses muscles to change the size of the pupil. These muscles can control the amount of light entering the eye by making the pupil larger (dilated) or smaller (constricted).
• Posterior chamber: The posterior chamber is between the iris and lens.
  • The lens is behind the iris and is normally clear. Light passes through the pupil to the lens.
  • The lens is held in place by small tissue strands or fibers (zonules) extending from the inner wall of the eye.
  • The lens is very elastic. Small muscles attached to the lens can change its shape, allowing the eye to focus on objects at varying distances.
  • Tightening (contraction) or relaxing these muscles causes the lens to change shape, allowing the eyes to focus on near or far objects (accommodation).
• Vitreous chamber: The vitreous chamber is between the lens and the back of the eye.
  • The back two-thirds of the inner wall of the vitreous chamber is lined with a special layer of cells (the retina): millions of highly sensitive nerve cells that convert light into nerve impulses.
  • Nerve fibers in the retina merge to form the optic nerve, which leads to the brain. Nerve impulses are carried through the optic nerve to the brain.
  • The macula, near the center of the retina at the back of the eyeball, provides the sharp, detailed, central vision for focusing on what is in front of you. The rest of the retina provides side (peripheral) vision, which allows you to see shapes but not fine details.
  • Blood vessels (retinal artery and vein) travel along with the optic nerve and enter and exit through the back of the eye.

Fluid fills most of the inside of the eye. The chambers in front of the lens (both the anterior and posterior chambers) are filled with a clear, watery fluid called aqueous humor. The large space behind the lens (the vitreous chamber) contains a thick, gel-like fluid called vitreous humor or vitreous gel. These two fluids press against the inside of the eyeball and help the eyeball keep its shape.

The eye is like a camera. Light passes through the cornea and the pupil at the front of the eye and is focused by the lens onto the retina at the back of the eye. The cornea and lens bend light so it passes through the vitreous gel in the back chamber of the eye and is projected onto the retina. The retina converts light to electrical impulses. The optic nerve carries these electrical impulses to the brain, which converts them into the visual images that you see.

Credits

Current as of:
August 31, 2020

Author: Healthwise Staff
Medical Review:
Kathleen Romito MD – Family Medicine
Adam Husney MD – Family Medicine

Current as of: August 31, 2020

The Anatomy of the Eye | Anterior Segment

The Anatomy of the Eye

Topics Covered:

Cornea | Iris | Pupil | Conjunctiva | Ciliary Body | Anterior Chamber

Aqueous Humor | Trabecular Meshwork | Crystalline Lens

Cornea
This is the clear front surface of the eye and is the first surface that light hits on the way to the retina. The cornea has several functions but the most important is the cornea refracts or bends light entering the eye toward the lens of the eye which then focuses on the retina. The cornea is also where a contact lenses rest and where LASIK is performed.

The cornea has 5 unique layers each providing a specific function. It is also very unique in that it is avascular (no blood vessels) to allow light to easily pass through for clear vision, but instead it receives its oxygen from the outside air.

Iris
This thin circular disk is what gives the eyes their color and has a hole in the middle of it called the pupil. This structure actually consists of two muscles; one that constricts or makes the pupil smaller (sphincter muscle) and one that make the pupil larger (dilator muscle). These muscles work against one another to constantly control the light that enters the eye to maximize vision and control focus – similar to the aperture control on a camera. The color of the iris is very simply explained by the amount of pigment cells present in the muscles. The more pigment, the darker the eye, the less pigment, the lighter the eye. This explains why individuals with very light blue eyes are more light sensitive, since the lack of denser pigment allows more light to enter the eye through the iris.

Pupil
Not so much an actual structure, the pupil is simply a hole in the middle of the iris that allows light to enter the eye. The size of this hole is controlled by muscles in the iris and enlarges under dim illumination (like driving at night) and gets smaller or constricts under strong illumination (like bright sunlight). Malfunction of the pupil and iris are often signs of neurological problems. Additionally it is through this hole that doctors assesses the health of the interior of the eye, often putting pharmacological or “dilating” drops in to force the pupil open and allow clearer viewing of internal structures.

Conjunctiva
Many people know the “white” of the eye as the sclera, however, overtop of the sclera is a thin transparent tissue that covers the front of the eye as well as continues onto the underside of the eyelids. This slippery, mobile tissue allows the lids to blink and slide easily against the eye with little friction. This tissue is also highly vascularized and sensitive to inflammation or potential infectious agents like bacteria or viruses. When the eye gets “red” it is the conjunctival vessels that are enlarging with the goal of bringing more blood cells to the area to fight off a potential infection. When this happens, the inflammation or infection of the conjunctiva is called conjunctivitis or more commonly known as “pink eye.”

Ciliary Body
The ciliary body is a muscle that sits behind the iris and is responsible for two important functions. First, this muscle is attached to the lens of the eye by many thin fibers called zonules. When the ciliary body contracts, tension is taken off of the zonules tethering the lens and the lens is allowed to change its shape. The technical term for this autofocusing is called accommodation and it is used for looking at near tasks like reading a book or working on the computer. In addition to accommodation, the backside of the ciliary body has cells that secrete the fluid (aqueous fluid) that fills up the anterior chamber of the eye where it is drained out through the trabecular meshwork. If the ciliary body makes too much aqueous fluid or if the fluid is not flowing out fast enough, the pressure in the eye can increase. High eye pressure is a significant risk factor for developing glaucoma and many glaucoma eye drop medications target the ciliary body and decrease secretion of the aqueous fluid.

Anterior Chamber
This is a term used to describe the area in the anterior 1/3 of the eye from the back surface of the cornea to the crystalline lens.

Aqueous Humor
This fluid fills the anterior chamber bathing and providing nutrients to ocular structures. This fluid is made from cells on the backside of the ciliary body and then circulates throughout the front 1/3 of the eye until it flows out of the anterior chamber through the trabecular meshwork. If this fluid is made faster than it drains out, the pressure in the eye can increase and elevate an individual’s risk of developing glaucoma. Not surprisingly, all glaucoma medications and surgery either stop the eye from producing as much aqueous fluid or aid in faster drainage of the fluid. Either mechanism of action results in decrease eye pressure and halts the progression of glaucoma.

Trabecular Meshwork
This meshwork of connection tissue is located where the iris meets the cornea and functions by draining the aqueous fluid from the front of the eye and through the Schlemm’s canal, back into the blood stream. Very commonly, the trabecular meshwork does not function properly in patients with glaucoma which cause a backup in fluid and increased pressure in the eye. Several very effective medications and surgical treatment exist to the increase the fluid outflow through this structure.

Crystalline Lens
This is a biconvex lens that sits behind the human iris. The human lens takes the focused light from the cornea and focuses that light onto the retina. This lens is different from the corneal because it has the ability to change its shape to allow for accommodation or autofocusing to allow us to see up close AND far away. Unfortunately, the crystalline lens begins to harden as we age and loses its flexibility; this results in presbyopia better known as the “curse of the 40’s.” Over time the human lens also starts to become cloudy. This cloudiness or opacity is known as a cataract and once a cataract reaches a certain point it requires removal through cataract surgery.

Click to read: “Anatomy of the Eye | Posterior Segment“

Gross Anatomy of the Eye by Helga Kolb – Webvision

Three different layers

1. The external layer, formed by the sclera and cornea
2. The intermediate layer, divided into two parts: anterior (iris and ciliary body) and posterior (choroid)
3. The internal layer, or the sensory part of the eye, the retina

• Three chambers of fluid: Anterior chamber (between cornea and iris), Posterior chamber (between iris, zonule fibers and lens) and the Vitreous chamber (between the lens and the retina). The first two chambers are filled with aqueous humor whereas the vitreous chamber is filled with a more viscous fluid, the vitreous humor.

• The sagittal section of the eye also reveals the lens which is a transparent body located behind the iris. The lens is suspended by ligaments (called zonule fibers) attached to the anterior portion of the ciliary body. The contraction or relaxation of these ligaments as a consequence of ciliary muscle actions, changes the shape of the lens, a process called accommodation that allows us to form a sharp image on the retina. Light rays are focused through the transparent cornea and lens upon the retina. The central point for image focus (the visual axis) in the human retina is the fovea. Here a maximally focussed image initiates resolution of the finest detail and direct transmission of that detail to the brain for the higher operations needed for perception. Slightly more nasally than the visual axis is the optic axis projecting closer to the optic nerve head. The optic axis is the longest sagittal distance between the front or vertex of the corna and the furthest posterior part of the eyeball. It is about the optic axis that the eye is rotated by the eye muscles. Some vertebrate retinas have instead of a fovea, another specialization of the central retina, known as an area centralis or a visual streak.

Extraocular muscles.

Fig. 3. CT Horizontal transverse scan

Each eyeball is held in position in the orbital cavity by various ligaments, muscles and fascial expansions that surround it (see Fig. 3).

Inserted into the sclera are three pairs of muscles (6 muscles altogether). Two pairs are rectus muscles running straight to the bony orbit of the skull orthogonal to each other (the superior rectus, the inferior rectus, the lateral rectus and the medial rectus muscles). A further pair of muscles, the oblique muscles (superior oblique and inferior oblique) are angled as the name implies obliquely. These muscles, named extraocular muscles rotate the eyeball in the orbits and allow the image to be focussed at all times on the fovea of central retina.

Development of the eye.

The retina is a part of the central nervous system and an ideal region of the vertebrate brain to study, because like other regions of the central nervous system, it derives from the neural tube. The retina is formed during development of the embryo from optic vesicles outpouching from two sides of the developing neural tube. The primordial optic vesicles fold back in upon themselves to form the optic cup with the inside of the cup becoming the retina and the outside remaining a single monolayer of epithelium known as the retinal pigment epithelium (Fig, 4, 3). Initially both walls of the optic cup are one cell thick, but the cells of the inner wall divide to form a neuroepithelial layer many cells thick: the retina

Fig.

4. Development of the eye

CLICK HERE to see a Morph of development (Quicktime movie)

Sensory retinal development begins as early as the optic vesicle stage, with the migration of cell nuclei to the inner surface of the sensory retina. Additional retinal development is characterized by the formation of further layers arising from cell division and subsequent cell migration. The retina develops in an inside to outside manner: ganglion cells are formed first and photoreceptors cells become fully mature last.

Further changes in retinal morphology are accomplished by simultaneous formation of multiple complex intercellular connections. Thus by 5 months of gestation (Fig. 5) most of the basic neural connections of the retina have been established (Mann, 1964).

The functional synapses are made almost exclusively in the two plexiform layers and the perikarya of the nerve cells are distributed in the three nuclear layers.

Photoreceptor cell maturation begins with the formation of outer segments (OS) containing visual pigment from multiple infoldings of the plasma membrane of each cell. Outer segment formation proceeds, and the eye becomes sensitive to light at about 7 months’ gestation.

Fig. 5. Schema of the layers of the developing retina around 5 months’ gestation (Modified from Odgen,1989)

The final portion of the sensory retina to mature is the fovea, where the ganglion cell layer thickening begins during midgestation. The outer nuclear layer is also wider here than elsewhere in the retina and consists almost entirely of developing cone cells. The ganglion cell nuclei migrate radially outwards in a circle, leaving the fovea free of ganglion cell nuclei. Cell-cell attachments persist, however and foveal cone cells alter their shape to accomodate the movement of ganglion cells. Foveal development continues with cell rearrangements and alteration in cone shape until about 4 years after birth (Hendrickson and Yondelis, 1984; Curcio and Hendrickson, 1991).

Surface membranes cover the eye cup and develop into lens, iris and cornea with the three chambers of fluid filled with aqueous and vitreous humors (Fig. 6).

Fig. 6. Sagittal section of the adult human eye

Summary.

In the following chapters, we will describe in greater detail the individual nerve cells that make up the retina and the functional pathways into which these neurons are organized. Eventually, we will progress to a stage where we can appreciate the summary diagrams below (Figs. 7 and 8) that show the functional wiring of two well-understood mammalian retinas, namely cat and primate retinas.

References.

Curcio CA, Hendrickson AE. Organization and development of the primate photoreceptor mosaic. Prog Ret Ret. 1991;10:89–120.

Hendrickson AE, Youdelis C. The morphological development of the human fovea. Ophthalmology. 1984;91:603–612. [PubMed]

Mann I. The development of the human eye. New York: Grune and Stralton; 1964.

Metzelaar-Blok JAW, ter Huurne JA, Hurks HM, Keunen JE, Jager MJ, Gruis NA. (2001) Characterization of melanocortin-1 receptor gene variants in uveal melanoma patients. Invest Ophthalmol Vis Sci. 42:1951-1954. [PubMed]

Ogden TE. Retina: basic science and inherited retinal disease. Vol. 1. St. Louis: The CV Mosby Co.; 1989.

Valverde P, Healy E, Jackson I, Rees JL, Thody AJ. (1995) Variants of the melanocyte-stimulating hormone receptor gene are associated with red hair and fair skin in humans. Nat Genet.;11:328-330. [PubMed]

Vision | Anatomy and Physiology I

Learning Objectives

• Describe the structures responsible for the special senses of vision
• List the supporting structures around the eye and describe the structure of the eyeball
• Describe the processes of phototransduction

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 and function of the eye

Author: Maria Yiallouros, erstellt am: 2016/11/21,
Editor: Maria Yiallouros, English Translation: PD Dr. med. Gesche Tallen, Last modification: 2016/11/21

The eye is a sensory organ. It collects light from the visible world around us and converts it into nerve impulses. The optic nerve transmits these signals to the brain, which forms an image so thereby providing sight.

Human eyes primarily consist of two globe-shaped structures, the eyeballs, which are surrounded by the the bony sockets of the skull, the orbits. The orbits are covered with fatty and fibrous tissue to protect the eye. Additional structures protecting the eye include the eyelids, the outer coating layer of the eye (fibrous tunic), the conjunctiva, and the lacrimal glands. Six special muscles that insert at different sites outside the eyeball work together to control eye movement.

Each eyeball houses the following parts of the eye:

• the three coating layers: the outer, middle and inner coat
• the inner part of the eyeball: it contains the lens and the vitreous body and is divided into the anterior and the posterior chamber.

The following chapters will explain anatomy and function of the three coats as well as of the inner part of the eyeball.

Layers of the eye

The eyeball is surrounded by a three-layered wall, the three coats of the eye. They consist of different tissue and serve different functions.

Outer coat (fibrous tunic)

The eye’s outer layer is made of dense connective tissue, which protects the eyeball and maintains its shape. It is also known as the fibrous tunic.

The fibrous tunic is composed of the sclera and the cornea. The sclera covers nearly the entire surface of the eyeball. With its external surface being white-coloured, it is commonly known as the “white of the eye”. The sclera provides attachments for the muscles that control the eye’s movement (see above).

The transparent cornea occupies the front center part of the external tunic. It serves as the eye’s “window”, which lets the light in and bends its rays, thereby providing most of the eye’s focusing power.

The anterior, visible part of the sclera as well as the inner surface of the eyelids are covered by the conjunctiva, a mucous membrane that helps lubricating the eye together with the tears made by the lacrimal glands, thus protecting the eye from drying out.

Middle coat (vascular tunic)

The middle layer of tissue surrounding the eye, also known as the vascular tunic or „uvea“, is formed – from behind forward – by the choroid, the ciliary body, and the iris.

The choroid takes up the posterior five-sixths of the bulb and is mainly comprised of blood vessels. Its major functions are oxygen supply and nutrition for the eye. A dark pigment, melanin, occurs throughout the choroid in order to help limiting uncontrolled reflection within the eye, which would potentially result in the perception of confusing images.

The anterior part of the choroid passes into the ciliary body, one function of which is anchoring the lens in place. The ciliary body contains a muscle (ciliary muscle), which can change the shape of the lens for adjustment to far or near sight, respectively, thereby controlling the so-called refractive power of the lens (accomodation). Additional functions of the ciliary body are the production, secretion, and outflow of aquaeous humour (the latter via the so-called „Schlemm’s canal“), a watery fluid that fills both the anterior and the posterior chambers of the eye (see below).

The iris, which is connected to the anterior part of the ciliary body, covers the top of the lens. Similar to the aperture of a camera, it controls how much light is let into the eye. The iris forms a circular, thin structure within the eyeball that regulates the size and the diameter of the pupil. It also contains pigments, the amount of which determines a person’s eye colour. For example, in children with blue eyes, the iris contains less pigment than in brown-eyed kids.

Inner coat

The third and inner coat of the eye is the retina, which is responsible for the perception of images – vision.

The retina is a light-sensitive layer of nervous tissue composed of multiple sensory cells, so-called light- or photoreceptor cells, as well as associated nerve cells and other types of cells, all working together to make a person see.

For vision, there are two types of photoreceptor cells: rods and cones. Rods provide the perception of black-and-white vision, mostly in dim light, whereas cones help to see colors in daylight.

The light and colour impulses received by these photoreceptors are transmitted to the associated nerve cells of the retina, which, on their part, send these signals – via the optical nerve – to the visual centre (visual cortex) of the brain.

The point where the optic nerve fibers depart from the eyeball (optical disc) does not contain any photosensitive cells; it is, thus, insensitive to light and termed the “blind spot”.

Directly opposite the lens, the retina contains a small yellowish area, the “macula lutea”. Its central part (fovea centralis) is densely packed with cone cells for colour perception. At this point, the sense of vision is the most accurate and detailed.

The inner part of the eyeball

The inner part of the eyeball consists of the lens, the vitreous body and the two eye chambers.

The lens

The lens is a transparent olive-shaped structure in the eye that has no blood vessels. Lens and cornea (see above) work together to focus the light rays passing through the eyeball to the back of the eye, that is, to the retina, by bending or refracting them, thereby creating clear images of the environment perceived from different distances.

By adjusting its shape and size, the lens can change the focus. This process is called accomodation. Accomodation is possible thanks to the lens’ elastic capsule as well as to the lens fibers, which connect with the ciliary muscle (see middle layer of the eye).

The vitreous body (vitreous humour, vitreous)

The vitreous is a clear gelatinous mass held by collagen fibers. It is situated between lens and retina and comprises about two thirds of the entire eyeball. By pushing the retina towards the choroid, the vitreous promotes keeping the retina in place.

Anterior and posterior eye chamber

The anterior chamber of the eye is located between the iris and the cornea (see above). The posterior chamber is the space between parts of the iris and the lens. Both chambers are filled with aquaeous fluid to nourish cornea and lens.

How the eye works

The human eye is a complex optical system that basically works like a camera: the iris serves as the aperture that controls the amount of light rays reaching cornea and lens (photographic objective), and the retina works as the film.

(© Andrea Danti – Fotolia.com)

Bending of light rays by cornea and lens serves to create sharp images on the retina. These images ultimately trigger nerve impulses, which are transmitted to the brain where the images are perceived and interpreted.

Eyeball | anatomy | Britannica

Eyeball, spheroidal structure containing sense receptors for vision, found in all vertebrates and constructed much like a simple camera. The eyeball houses the retina—an extremely metabolically active layer of nerve tissue made up of millions of light receptors (photoreceptors)—and all of the structures needed to focus light onto it. The sclera, the tough protective outer shell of the eyeball, is composed of dense fibrous tissue that covers four-fifths of the eyeball and provides attachments for the muscles that move the eye. The sclera is itself covered anteriorly by the conjunctiva, a transparent mucous membrane that prevents the eye from drying out. At the front of the eye, the tear film covers the transparent cornea, the “window” through which light passes into the eye. Working in concert with the aqueous humour behind it, the cornea provides the greatest focusing power of the eye. However, unlike the lens, the shape and focusing power of the cornea are not adjustable. Other important structures in the eyeball include the iris and the lens. Much of the eyeball is filled with a transparent gel-like material, called the vitreous humour, that helps to maintain the spheroidal shape.

Immediately beneath the sclera is an underlying vascular layer, called the uvea, that supplies nutrients to many parts of the eye. One component of the uvea is the ciliary body, a muscular structure located behind the iris that alters the shape of the lens during focusing and produces the aqueous humour that bathes the anterior chamber. The other components of the uvea are the iris and the choroid. The choroid is a highly vascular tissue layer that provides blood to the outer layers of the retina that lie over it.

human eye

The human eye.

© Andrey Armyagov/Shutterstock.com

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The cornea, where the focusing process begins, is curved to a much greater extent than the rest of the eyeball. Defects in corneal curvature cause a distortion of vision known as astigmatism. Behind the cornea is the anterior chamber, which extends posteriorly to the plane of the iris and pupil. It is filled with a watery fluid called the aqueous humour. The iris is a doughnut-shaped, muscular curtain that opens and closes to regulate the amount of light entering the eye through the pupil, the opening at the iris’s centre. The aqueous humour flows through the pupil from the posterior chamber (a small space between the iris and the lens) to the anterior chamber and out of the eye through the trabecular meshwork and Schlemm’s canal, which encircles the peripheral iris. Some aqueous humour also exits the eye directly through the ciliary body. The ciliary muscle attachments and the lens separate the aqueous humour in front from the vitreous humour behind.

The shape of the lens is controlled by the action of the ciliary body, altering the focusing power of the lens as needed. The cornea and lens focus an image onto the retina at the back of the eye. If the image is projected too far in front of the retina, it causes the visual defect called myopia, or nearsightedness. If the image is theoretically focused “behind” the retina, the result is hyperopia, or farsightedness. If no deformation of the lens is present, the image is projected onto the fovea, a structure near the centre of the retina that contains a large number of cone photoreceptors and that provides the sharpest vision. When stimulated by light, retinal photoreceptor cells send signals to neighbouring cells in the retina that then relay the signals through the optic nerve to the visual centres of the brain.

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Anatomy of the Eye

The human eye is an organ that detects light and sends signals along the optic nerve to the brain. Perhaps one of the most complex organs of the body, the eye is made up of several parts—and each individual part contributes to your ability to see.

Cornea

The cornea is the transparent, dome-like structure on the front part of the eye. It gives the eye two-thirds of its focusing or refracting power. One-third is produced by the internal crystalline lens.

Much like a camera lens, the cornea helps to focus light coming into the eye onto the retina.

The cornea is also full of nerves that alert us to irritations that could potentially harm our vision and eye health. And the cornea is susceptible to injury. Common injuries of the cornea include “scratches” to its surface known as abrasions. Minor corneal scratches usually heal on their own, but deeper injuries can cause pain and sometimes corneal scarring.

A corneal scar can result in a haze on the cornea that impairs your vision. If you scratch your eye significantly, it’s important to see an eye doctor. An eye doctor can view the cornea under a slit lamp biomicroscope.

Another common ailment of the cornea includes contact lens complications, especially corneal ulceration. An ulcer is a wound on the surface of the cornea caused by bacteria often caused by poor adherence to strict contact lens hygiene; Sometimes, a virus can cause corneal ulcerations like the herpetic virus (the one that causes cold sores on the lips) which 90% of humans have in their bodies.

kolderal / Getty Images

Pupil

The pupil is the hole or opening that is located in the center of the iris of the eye. The pupil controls the amount of light that enters the eye. Pupil size is controlled by the dilator and sphincter muscles of the iris.

The pupil’s job is very similar to a camera aperture which allows more light in for more exposure. At night, our pupils dilate to allow more light in to maximize our vision.

In humans, the pupil is round. Some animals have vertical slit pupils while some have horizontally oriented pupils. Pupils appear black because the light that enters the eye is mostly absorbed by tissues inside the eye.

Iris

The iris is the colored part of the eye that controls the amount of light that enters into the eye. It is the most visible part of the eye. The iris lies in front of the crystalline lens and separates the anterior chamber of the eye ball (anything in front of the human lens) from the posterior chamber (anything behind the human lens).

The iris is part of the uveal tract—the middle layer of the wall of the eye. The uveal tract includes the ciliary body, the structure in the eye that releases a clear liquid called the aqueous humor.

Iris color depends on the amount of melanin pigment in the iris. A person with brown eyes has the same color of melanin pigment that a person with blue eyes. However, the blue-eyed person has much less pigment.

Crystalline Lens

The crystalline lens is a transparent structure in the eye—suspended immediately behind the iris—that brings rays of light to a focus on the retina. Small muscles attached to the lens can make it change shape which allows the eye to focus on near or far objects.

Over time, the lens loses some of its elasticity. This causes the eye to lose some of its ability to focus on near objects. This condition is known as presbyopia and typically presents problems with reading, around 40 years of age.

A cataract is a clouding of the lens and is a common occurrence that comes along with aging. Fortunately, cataracts grow slowly and may not affect your vision for several years.

By age 65, over 90% of people have a cataract. Cataract treatment involves removing the cloudy lens surgically and replacing it with an implantable intraocular lens.

Aqueous Humor

The aqueous humor is a clear, watery fluid located behind the cornea, in the anterior chamber. It helps bring nutrients to the eye tissue.

It is formed behind the lens and flows to the front of the eye to maintain the pressure inside the eye. Problems with the aqueous fluid can lead to issues involving the eye’s pressure, such as glaucoma.

Vitreous Humor

The vitreous humor, which lies against the retina, makes up a large part of the eye. It is a jelly-like substance that fills the inside of the eye.

Made mostly of water, the vitreous fluid gives the eye its shape. It is composed of water, collagen, and proteins and contains cells that help to maintain its clarity.

As we age, the vitreous humor becomes less firm. This liquifactive change is what causes us to see floaters, especially when peering at blank walls or the sky.This change sometimes causes it to pull on the retina.

If the force of the pulling becomes strong enough, the vitreous humor may actually separate from the retina. This is called a posterior vitreous detachment, as it normally occurs at the back (posterior) of the eye. If this happens suddenly and with a shower of flashes, it could indicate that it has caused a retinal tear, and it is important to have this evaluated immediately.

Retina

Located on the inside of the eye, the retina is the light-sensitive area located at the back of the eye that the lens focuses images upon, making vision possible. The retina is made up of 10 very thin layers. Within these layers are rods and cones that are used to detect color.

The retina is very fragile. A detached retina occurs when the retina is separated from the other structures of the eye. It typically happens during contact sports or as a result of trauma. A retinal detachment is a serious injury that requires immediate attention by an eye care professional.

Sclera

The sclera of the eye is better known as the “white of the eye.” While we can only see the visible portion of the sclera, it actually surrounds the entire eye.

The sclera is a fibrous sac that contains the inner workings that make vision possible. It also keeps the eye in a rounded shape.

Scleritis is an inflammation of the sclera. It can cause intense eye pain, redness, and loss of vision for some people. It can also be associated with trauma or infection—more than half of scleritis cases are associated with an underlying systemic disease.

90,000 Evolution. Why are our eyes in front?

• Jason G. Goldman
• BBC Future

November 11, 2014

Photo author, Thinkstock

Why are our eyes not on the sides of the head, but looking forward? This is partly due to the need to perceive 3D images, but correspondent

BBC Future found other reasons as well.

Have you ever noticed that most of the animals in a zoo fall into one of two groups? In some, the eyes are on the sides of the head (these are chickens, cows, horses, zebras), while in others they are set closer and located in front (this group includes monkeys, tigers, owls and wolves).The visitors to the zoo themselves – people – obviously belong to the second group. What is the reason for this difference?

Eye placement is always a trade-off. When the eyes are in front, each of them sends an image to the brain from its own angle of view, and by superimposing these images on top of each other, a person perceives depth. Animals with eyes on the sides are not able to see the third dimension, but their view is much wider.

Photo author, Thinkstock

Photo caption,

Some turtles have eyes on their sides, but the brain processes visual information as if their eyes were looking forward

It is likely that the position of the eyes was formed differently in different animals.For example, some turtles have eyes on the sides, but the brain processes visual information as if their eyes were looking forward – perhaps this is due to the fact that when the turtles pull their head under the shell, their eyes perceive light only from the front. as if they are located in front of the head. But why did our branch of the evolutionary tree – primates – have eyes in front? There are many explanations for this.

In 1922, British ophthalmologist Edward Treacher Collins wrote that early primates needed vision to “swing and jump accurately from branch to branch… grab food with your hands and bring it to your mouth”.Therefore, the scientist decided, in the process of evolution, they developed the ability to estimate distance.

In the following decades, Collins’s hypothesis was repeatedly revised and refined, but its essence remained unchanged for a long time: in the process of evolution, the eyes of our ancestors moved forward in order to accurately estimate the distance when jumping from tree to tree. The cost of error in determining the distance between trees was indeed considerable. “The penalty for the miscalculation was a fall from a height of several meters to a land teeming with carnivorous animals,” wrote visual therapist Christopher Tyler in 1991.

Photo author, Thinkstock

Photo caption,

Parrots have panoramic vision

Collins’ weak point is that many animals that live in trees, such as squirrels, have eyes on the sides. Therefore, in 2005, the American biologist and anthropologist Matt Cartmill proposed another hypothesis, based on the peculiarities of the vision of predators, which are able to estimate distance very well. According to Cartmill, this allows them to track and catch prey, be it a leopard creeping after a gazelle, a hawk clinging to a hare’s claws, or one of the primates grabbing an insect from a branch.The scientist found this explanation very elegant, since it allowed understanding other evolutionary changes characteristic of primates. For example, early primates relied on sight rather than smell to hunt. Cartmill decided that the deterioration in his sense of smell was a side effect of the eye convergence: there was just not much room left for the nose and for the nerves connecting it to the brain – all the space was occupied by the eyes.

American neuroscientist John Allman took up Cartmill’s hypothesis and refined it based on information about nocturnal predators – after all, not all predatory animals have eyes in the front.In cats, primates and owls, they are indeed in the front of the head, and in mongooses, tupai and flycatchers – on the sides. Allman’s contribution to the development of this hypothesis is the assumption that such vision is necessary for those who hunt at night – for example, cats and owls – because the eyes perceive light better in front than on the sides. The early primates hunted at night and, perhaps, it is precisely because of this predilection for night hunting that all of their descendants, including humans, have their eyes located in the front.

Photo author, Thinkstock

Photo caption,

Predators like this leopard have eyes in front to better see their prey

American theoretical neuroscientist Mark Changizi has another explanation. In 2008, he published an article in the Journal of Theoretical Biology (USA) on “X-ray vision,” suggesting that the eyes in front allowed our forested ancestors to see through dense foliage and tightly intertwined branches. The loud name “X-ray vision” comes from a curious phenomenon described by Changizi: “If you hold your finger in front of your eyes in an upright position, fixing your gaze on some object located behind the finger, two images of the finger will enter the brain, and both of them will be transparent.”Thus, it turns out that a person can “see through” the finger, as with the help of X-rays.

Trees in the forest make it difficult to see only for large animals such as primates. Smaller squirrels, such as squirrels, do not have this difficulty, since their small head can easily squeeze between branches and leaves. Large animals that do not live in the forest also have enough eyes, which are located on the sides.

Photo author, Thinkstock

Photo caption,

The eyes in front allowed our ancestors who lived in the forest to see through dense foliage and tightly intertwined branches

Thus, the reason that our eyes are in front has not yet been established.Each hypothesis has its own strengths and weaknesses. But regardless of why we needed such vision – to jump from branch to branch, catch tasty bugs or see through foliage – it is obvious that this eye position is associated with life among the trees.

How to Draw Eyes – Eye Drawing Basics

In this tutorial we will cover the basics of drawing a natural eye in profile, sideways and closed. Then we will learn how to draw anime eyes of characters from different angles, and also consider the given examples of different eye styles.

You can also familiarize yourself with the lesson on how to draw anime eyes.

Eyes are the mirror of the soul.
After all, it is they who make all people unique, showing our inner world. And in order to draw them correctly, we’ll go over the basics.

Consider a photograph of an eye (front view).
This is the real eye of a middle aged person.
The eye has an almond shape, along the edge of which there are eyelashes of different lengths, and the folds and wrinkles around the eyes emphasize the contours of the eyeball.

In the drawing, I indicated in which direction, from the edge of the eye, the eyelashes go. Note that the eyelashes are curved and vary in length. I also indicated how long eyelashes are located around the eye (B-large eyelashes, M-small). The eyelashes are usually higher in the center of the eye and smaller towards the ends of the eye, however, long eyelashes can also be drawn on one end (which is farther from the nose).

Consider a photograph of an eye (side view).
The basic shape of the eye is now triangular rather than almond shaped.
Curved eyelashes and different lengths. In the side view, the location of the lengths of the eyelashes around the eye is more clearly visible (B-large eyelashes, M-small).

Consider a photograph of a closed eye.
Near the eye, half of the lower part of the almond-shaped form is clearly visible, along the edge of which there are eyelashes of different lengths. Wrinkles on the top of the eye accentuate the contours of the eyeball.

Eyelashes, longer in the center and smaller towards the ends of the eye (B-large eyelashes, M-small).

Anime Eyes

Let’s take a look at basic eye shapes .

The personality of the character is expressed through the shape of the eyes. Also, keep in mind that large eyes with large pupils are mostly suitable for girls and children, narrow eyes with small pupils for guys, men and women, and single-line eyes for older people.

When drawing anime eyes, always start with the shape of the eyelashes. Having decided on the shape, draw two straight lines that intersect at one point and touch the edges of the upper eyelash shape.This will define the contours of the apple of the eye. Then we complicate the eyelashes and draw the pupil.

If you want to draw a rounded eye shape, consider the following example.

At the base of these eyes, I always draw a circle first. Then I decide on the shape of the eyelashes and complicate them. After that, be sure to erase the auxiliary circle. Now I finish painting the pupil.

Examples of eyes

Examples of eyes (front view) with different shapes for reference.

Examples of eyes (side view) with different shapes for your reference.

Drawing closed eyes

Basically, there are two types of closed eyes: curling the eyelashes up and down.
When the eyelashes are curled upward, the emotion of happiness, joy and laughter is transmitted.

Eyelashes curved downward is drawn when kissing, sleeping, thinking, are in a calm state.

Examples of closed eyes (front view) with different shapes for familiarization.

Examples of closed eyes (side view) with different shapes for familiarization.

You can also see how the eyes change when drawing emotions by going to the lesson How to draw emotions.
This concludes the lesson! Hope it helps you with your creativity!

Lesson author: Prescilla

Would you like to learn how to learn how to draw with a pencil? Then visit the anime pencil drawing tutorials section on Bakemono. 90,011 90,000 intrinsic structures: eye muscles, front view

A beautiful rite introduces the foam.Actively keep the dilapidated silhouette on the title. The cat moved crookedly and the dog turned to shine. It’s fanciful to get ready and the publisher’s nod if the extract is torn in disguise. Attempts favor a friendly opening to the value creature partner. Get squeezed in noise on nature travel. The rate was clearly fueled by the promise of doctors because of the peculiar peoples. An uncertain selection of forbidden directions shelves. Patrol confidence flashed with the bringing thoughts.

• The muscular system of a man, front view.1. Radial styloid process

Muscles of the eye (musculi oculi). A-front view; B-top view.

Muscles of the eyeball. The motor apparatus of the eye consists of six

The rectus muscles rotate the eyeball around two axes: transverse (mm
90,000 Large Atlas of Human Anatomy – Vincent Perez – 2015: Farmf

Large Atlas of Human Anatomy – Vincent Perez – 2015

Plastic Anatomy

• The human body: view from different sides
• Head: front and side views
• Eye and Ear
• Mouth and nose
• Right arm and hand: lateral and medial view
• Brush: back and palm views
• Chest and armpit area
• Thigh area: women and men
• Legs and feet: medial view; the foot from the back and from the sole

Skeletal system 90 100

• Skeleton of a woman and a man: front view
• Skeleton: side and back views
• Spine
• Cervical and lumbar vertebrae: rear view
• Skull: view from different sides
• Anterior skull and cervical vertebra in section: rear view
• Clavicle: top and bottom view
• Hyoid bone: view from different sides
• Shoulder: front and back views
• Chest bones
• Left anonymous bone: front and back views
• Elbow: front and back view
• Brushes: back and palm views
• Knees: front and back view
• Feet: rear and sole views
• Bone structure

Joints and ligaments 90 100

• Spine
• Temporomandibular joint and hyoid bone
• Temporomandibular joint
• Cranial-cervical joints and ligaments
• Sternoclavicular and shoulder joints
• Elbow: front and lateral view
• Hand and wrist: back and palm views
• Toe: medial view
• Lumbar spine
• Pelvic ligaments
• Ligaments of the hip joint
• Pelvis: top and rear view
• Knee ligaments: front and back views
• Right foot: view from different sides

Muscle attachment sites

• Head and torso: front and back views
• Hand: front and back view
• Clavicle: top and bottom view
• Brush: back and palm views
• Leg and foot: front and back view
• Foot: view from the back and from the sole
• Base of the skull
• Hyoid bone: top view

Muscular system 90 100

• Superficial muscles (layers I, IA and II): view from different sides
• Deep muscles (layers II, III, IV, V, VI, VII): view from different sides
• Muscles of the head
• Muscles of the eye – external muscles of the eye: view from different sides
• Deep neck muscles: side view
• Respiratory muscles
• Hand Components: Rear View
• Pin Components: Cross Section
• Muscles of the arm and hand: view from different sides
  • Palm side: layers I, II, III, IV, V
  • Back side brush: layers I, II, III
  • Hand from the medial side and lateral
• Superficial muscles of the leg and foot: view from different sides
  • Back foot: layers I, II, III
  • Plantar foot: layers I, II, III, IV, V
  • Lateral and medial foot
• Muscle microstructure: index finger’s own extensor

Nervous system 90 100

• Nervous system: front and back views
• Skin innervation – distribution of dermatomes and peripheral nerves: front and back views
• Cervico-brachial plexus: rear view
• Lumbosacral plexus: rear view
• Spinal cord
• Sciatic nerve
• Trigeminal nerve
• Nerves of the face and head
• Nerve structure

Brain

• Brain location
• Brain: view from different sides
• Brain: frontal section
• Brain: horizontal section
• Brain: cerebral ventricles
• Brain: arteries

Senses

• Head: eyes, ears, nose and mouth
• Vision
• Hearing
• Semicircular canals and ducts
• Smell
• Taste
• Touch

Digestive system

• Digestive system: front and back view
• Mouth and salivary glands
• Language
• Stomach
• Bile and pancreatic duct
• Small intestine (schematic)
• Large intestine
• Ileocecal sphincter and appendix
• Rectum

Respiratory system

• Respiratory system: front and rear view
• Oral cavity and nasal cavity
• Nasal septum
• Paranasal sinuses
• Larynx
• Bronchial tree
• Respiratory muscles
• Cluster of alveoli
• Saturation of the alveolar cluster with oxygen

Circulatory system

• Circulatory system: front and back views
• Venous and arterial system
• Head and neck (schematic)
• Skull and arteries
• Arteries of the brain and circle of Willis: bottom view
• Circle of Willis
• Brain and neck
• Circles of blood circulation
• Portal vein of the liver
• Blood vessels

Heart

• Heart: front, back
• Coronary arteries and veins of the heart
• Blood circulation
• Nerves and arteries
• Heart in diastole phase
• Heart in systole
• Beginning and ending of diastole
• Beginning and termination of systole

Lymphatic system

• Lymphatic system
• Head and Neck
• Arm, armpit and chest
• Heart and lungs
• Thoracic lymphatic duct
• Deep lymph nodes of the abdomen and groin
• Stomach and pancreas
• Large intestine
• Lymph nodes and vessels

Genitourinary system

• Male genitourinary system: front and side views
• Male urinary tract: front view
• Female genitourinary system: front and side views
• Right kidney
• Renal (Malpighian) little body
• Nephron

Reproductive system

• Male reproductive system
• Female reproductive system
• Stages of development of sperm and oocytes
• Uterine cycle
• Fetal circulation
• Normal full-term fetus: before and during labor
• Sexual intercourse

Classification and types of cataracts of the eye

Classification of cataracts of the eye

In modern ophthalmology, it is customary to divide cataract diseases into two separate large groups: congenital (hereditary) and acquired.

The main difference between these groups from each other is that with congenital cataracts, as a rule, no further development of pathology is observed, i.e. it is stationary and limited in area, while with acquired cataract changes in the lens of the eye progress over time.

There are many other types of cataracts, which are classified according to certain characteristics. In this article, we will consider in more detail the main types of eye cataracts.

Classification of cataracts by location of opacities

1. Subcapsular cataract

Subcapsular cataract is usually an age-related disease . Subcapsular cataract of the eye includes two subspecies:

1. Anterior subcapsular cataract is located under the anterior capsule;

2. Posterior subcapsular cataract is located in front of the posterior capsule. Because of its location on the lens of the eye, it has a greater effect on visual acuity reduction than with nuclear or cortical cataracts.As a rule, patients experience more discomfort when fixing their vision on a near object than on a distant one. There is also a decrease in vision with a narrow pupil, in bright light, for example, headlights of a car, sunlight, etc.

2. Nuclear cataract

Nuclear cataract develops with age-related changes with the involvement of the nucleus of the lens of the eye. First, the appearance of turbidity is observed in the embryonic nucleus, then the turbidity spreads to all other layers. Sometimes the kernel may turn brown or black.Because of this, nuclear cataract is also called brown .

As a rule, this type of cataract is accompanied by myopia, and older patients can acquire the ability to read again without glasses. The destruction of the fibers of the lens does not occur. Nuclear cataract has a dense consistency and its detection is an indication for the use of a surgical method of treating eye cataracts.

3. Cortical cataract

Unlike nuclear cataract, cortical cataract is characterized by local destruction of the cell structure of mature fibers.Violation of the integrity of cell membranes leads to the development of the process of excessive oxidation and precipitation of proteins. As a rule, cortical cataract develops in both eyes at once, but usually the severity of lens opacity is different.

The rate of development of cortical cataract can be different: some opacities of the lens of the eye can remain unchanged for several years or even decades, and sometimes the progression of the disease from stage to stage occurs very quickly.

Types of acquired cataracts

1. Senile (senile) cataract

Age-related (senile) cataract of the eye, as a rule, develops in the elderly from 60 and more years, hence the name. The development process of this type of cataract usually affects both eyes of the patient, but is often uneven, i.e. lens opacity in one eye is more pronounced than in the other.

Before the development of senile (senile) cataracts, a process of sclerosis of the lens of the eye usually occurs, called phakosclerosis .

Phakosclerosis is an age-related thickening of the lens fibers, which occurs simultaneously with a violation of the metabolism of the functional elements of the lens. In this case, the patient does not observe a pronounced decrease in visual acuity.

In turn, senile (age-related) cataract is divided according to the degree of maturity into the following subspecies:

– Immature cataract is characterized by only partial clouding of the lens of the eye;

– Mature cataract , in which complete opacity of the lens is formed;

– Overripe cataract , associated with the penetration of fluid from the lens into the anterior capsule, which takes on a wrinkled appearance;

– Morgani cataract belongs to the overripe subtype, while complete cortical liquefaction is noted, and therefore the nucleus is omitted.

2. Traumatic cataract

Traumatic cataract develops as a result of various types of eye injuries. The nature of the injury may vary. Traumatic cataracts can appear both with blunt trauma and with penetrating wounds.

Blunt eye trauma is characterized by the appearance of a pigment in the form of a ring, which is called Fossius’s ring . At the same time, the pigmented ring is not the cataract itself and may not even affect the patient’s vision, but eventually disappear altogether.When examining the patient, the presence of the Fossius ring will immediately make it clear to the ophthalmologist that there was a blunt eye injury.

Traumatic cataract can appear both immediately after the injury, and be distant and develop later. Also, a traumatic cataract can capture only a small part of the lens of the eye, or it can affect the entire lens, then such a cataract is called – total .

3. Medication cataract

As the name implies, medication cataract develops as a result of the use of various medications.

From which particular drug the medical cataract developed can only be established by an experienced ophthalmologist after receiving the patient and carrying out all the necessary diagnostic studies.

There are also various other forms and types of cataracts. As a rule, it is impossible to determine the type of cataract on your own at home; this requires special ophthalmological equipment and the use of special cataract diagnostic techniques.

4. Complicated cataract

Complicated cataract occurs with inflammation of the eye, a high degree of myopia, as well as with other diseases.

5. Cataracts caused by general diseases of the body, such as

endocrine disorders and diabetes mellitus.

Maturity classification of cataracts:

– initial cataract ;

– immature cataract ;

– mature cataract ;

– overripe cataract .

The IRIS Ophthalmic Surgery Center carries out diagnostics and treatment of all types of eye cataracts. Many patients of our clinic have already successfully got rid of such a disease that interferes with normal normal life, such as cataracts of the eye.

90,000 Optogenetics turned nerve cells into photoreceptors and restored vision in macaques

Visualization of the modification of the ganglionic layer of the eye, front view. The purple-stained cells are in a light-excited state, the control area without modifications is visible from below

Juliette McGregor et al./ Nature Communications, 2020

Biologists were able to send a signal through the eye bypassing non-working photoreceptors. To do this, they adapted optogenetic technology and launched virus-like particles into the retina that make ganglionic nerve cells respond to light.As a result, the macaque’s eye reacts to light as healthy again, but how clear that vision will be remains to be seen. Article published in Nature Communications journal .

The retina has a layered structure. Its key elements are light-sensitive rods and cones, from which an impulse is transmitted along a chain to ganglionic nerve cells. The axons of these cells then collect in the optic nerve and transmit information to the brain, but they themselves are not sensitive to light.Therefore, if the photosensitive layer fails due to genetic disorders or trauma, then vision is lost.

Juliette McGregor and her colleagues at the University of Rochester were able to make vision work without the use of rods and cones, using modified ganglion cells instead. To do this, they used the method of optogenetics – a technology that inserts elements into nerve cells that make them light-sensitive.

This modification is carried out using viral vectors, virus-like objects, from which pathogenic parts of the code are removed and only that which transfers the gene material into the body is left.In this case, the added genes caused the cells to produce opsin, the same light-sensitive protein that is already used in the eyes.

In order to make sure that this idea works, the researchers conducted an experiment on monkeys of the cynomolgus monkey ( Macaca fascicularis ) . Viral vectors were introduced into their ganglionic layer in such a way that opsin-producing cells formed a semicircle, the other half of which was left unaffected for visual comparison.Then the natural light-sensitive cells were separated from the nerve cells, after which they began to shine a laser into the eye.

As a result, the nerve impulse of the modified ganglionic cells from the light hitting them completely coincided with that emitted by the usual ones when stimulated from rods and cones. Moreover, the modification worked a year later without causing complications. Unfortunately, the image seen by a monkey cannot be evaluated, so in the near future, scientists hope to test their method on blind people, so that in the future the method can find application in clinical practice.

There are also more unusual examples of the application of optogenetic methods. For example, American scientists activated the synapses of neurons of the departments involved in the production of songs of zebra finches, thus introducing information about songs into the brain of young birds.

Vasily Zaitsev

90,000 Predators and herbivores have different forms of pupils

A group of scientists from the University of California at Berkeley (USA) and
Durham University (UK) found out that the form
the pupil of an animal can tell whether it belongs to
predators or their prey.In the first, the pupil narrows to a vertical
strip, the second, on the contrary, in a horizontal – this
scientists found a pattern by comparing the shapes of the pupils in 214
species of animals. About research
tells Gordon Love, professor of physics at
Durham University, one of the co-authors. Work
published in the journal Science Advances .

“We found that animals whose pupils are
vertical, most often turn out to be predators attacking from
ambushes when the distance to the victim is minimal.Also for them
usually the eyes are located in front. Such are, for example, foxes and
domestic cats. The difference between foxes and, say, wolves is in
whose pupils are round – that these latter do not suit
ambushes, but hunt in packs, chasing their prey, “- explained
Love.

But the pupils, stretching horizontally, are almost always inherent
herbivores, whose eyes are located on the sides of the head, and,
as a rule, these are animals that are hunted, for example, sheep and
goats.See also the video at the bottom of the text, filmed by the participants
research.

Computer simulation showed that vertically
oriented pupils allow accurate distance estimation without
moving your head is important to prevent the victim from being noticed.
Horizontally oriented pupils and head on the sides, not
the front, in turn, is given to herbivores – who need both
you can notice a predator earlier and be ready to run away – your
kind of panoramic vision, almost 360 °. Thanks to this form
pupils, the amount of light coming from above decreases and
bottom, which improves the quality of visual information about the environment
space at ground level, which is useful not only for
spot a predator, but also during an attempt to escape.

If this hypothesis is correct, then the greatest advantage is vertical
pupils should be given to animals of small stature. Data checking
on animals with vertical pupils and eyes located
in front, showed that 82% of them are no more than 42 cm tall.
(among animals with round pupils, this is only 17%).

But how do the pupils of herbivores behave when they nibble the grass? After all
in this case, given the location of the eyes on the head, the pupils
will no longer be parallel to the ground.It turned out that in
In this case, the pupils of goats, elks, horses and sheep rotate,
keeping the horizontal orientation.

By the way, there are still many amazing pupils in the animal world.
For example, in mongooses, the pupils are oriented horizontally, but
the eyes are located in front – probably the first is explained
the need to notice the snake as early as possible, and the second by the fact that
mongooses are just waiting for the snakes in ambush. Geckos have pupils in
the expanded state is round, but when narrowing, they turn into
several separate points.But the most interesting are the pupils of cuttlefish.
– in the shape of the letter W (in the illustration below right, the other three eyes
belong to a sheep, a wolf and a fox)