Structure of an Eye

Human eyes are situated in an eye orbit lateral to the nose.

• These are photosensitive organs. 
• Each eye is a hollow ball-like round (nearly spherical) called an eyeball. Each eyeball is situated in the skull's notch of the lacrymal bone. It is called "Eye-orbit". 

1.0Structure of Human Eye

The Protective Structures of the Eye  

• The two orbits, often called "sockets," that protect the human eyes are located at the front of the skull. 
• Each orbit has a wider opening at the front, narrowing to a small opening at the back where the optic nerve exits to connect through the visual pathways and the brain. 
• The human eye is approximately 24 millimetres (0.94 inches) in diameter and occupies about 25% of the orbit's volume, leaving space for the extraocular muscles, blood vessels, nerves, orbital fat, and connective tissue surrounding and supporting the eye. While the orbit encloses and supports most of the eye, the cornea and part of the anterior globe extend slightly beyond the orbital rims, protected by the eyelids. 
• Each eyelid has a row of eyelashes highly sensitive to touch or particles near the eye. When stimulated, the eyelashes trigger the blink reflex. The eyelids also contain glands responsible for maintaining the tear layer. 
• The globe itself is primarily composed of and protected by the sclera, a thick, opaque white tissue that extends from the edges of the clear cornea at the front of the eye (the "limbus") to the optic nerve at the back, covering 95% of the eye's surface area.

Internal Structure of Eye Ball

The Cornea

• The cornea is a unique biological tissue that is transparent to light and contains no blood vessels. This small, clear dome at the front of the eye measures approximately 11 mm (0.43 in) in diameter and 500 μm thick at the centre, thickening to about 700 μm at the edges. At the very edge of the cornea, transparency gradually diminishes over a 1-mm (0.04-in) range in an area known as the "limbus," where the cornea integrates into the opaque sclera. 
• The cornea is more curved than the rest of the eyeball, with an average radius of curvature of 7.7 mm (0.3 in), compared to approximately 12 mm (0.5 in) for the rest of the globe. The cornea's main role is to transmit and focus light into the eye, and its structures are meticulously organized to perform this function effectively.
• Approximately 90% of the cornea comprises evenly spaced collagen fibrils arranged in a crisscross pattern that covers the entire cornea. This layer, known as the "stroma," provides transparency and strength. The remaining 10% of the cornea consists of four additional layers: the epithelium and Bowman's layer at the front, Descemet's membrane and the endothelium at the back.
• The corneal epithelium, similar to the skin's epithelium, acts as a barrier against bacteria and other pathogens. It also helps maintain the stroma's proper hydration level by preventing fluid from entering through its tight cell junctions and pumping out a small portion of the fluid. Bowman's layer, a very thin (12 μm) membrane located right beneath the epithelium, is found only in primates among mammals. Its exact function is not entirely known, but it may aid in protecting the stroma.

The Aqueous Humour

• The liquid that occupies the space between the cornea and the front surface of the crystalline lens in the eye is known as aqueous humour. It is generated by the ciliary body, located behind the base of the iris and extending towards the back along the inner globe up to the front portion of the retina.
• This fluid reaches the anterior chamber by flowing between the crystalline lens and the iris through the pupil.
• Aqueous humour has two primary functions: it provides nutrients to the cornea and forms part of the optical pathway of the eye. 
• It is a fortified blood plasma that circulates in the anterior chamber, delivering nutrients to both the cornea and the crystalline lens. 

The Iris 

• The iris, seen through the cornea, imparts color to the eye. While all irises possess a dark-pigmented posterior layer, the differing pigment levels in the anterior or stromal layer create various hues.
• A "blue" eye occurs because the iris stroma selectively absorbs long-wavelength light and reflects short-wavelength (blue) light from the posterior layer. In contrast, a "brown" eye absorbs almost all visible wavelengths in the iris stroma, leaving little light to be reflected.
• The main role of the iris is to manage the amount of light that enters the eye by adjusting the size of the pupil. Within the iris are two sets of muscles that work in opposition: the sphincter muscles contract to narrow the pupil, while the dilator muscles expand it.
• Parasympathetic nerves innervate the sphincter muscles, while sympathetic nerves innervate the dilator muscles. During "fight or flight" situations, the sympathetic system dominates, causing the pupils to dilate in response to perceived danger. 
• Although few conditions directly affect the iris, abnormal pupil responses to light or accommodation can result from neural pathway lesions or direct trauma to the iris. 

The Crystalline Lens and Ciliary Muscle 

• The crystalline lens, like the cornea, is a transparent structure. However, unlike the cornea, it can change shape to adjust the refracting power applied to incoming light. This transparency is maintained by the regular arrangement of elongated fibre cells within the lens. These cells originate at the lens's equator and grow toward the anterior and posterior portions, meeting at the central sutures. 
• As they elongate, they incorporate crystallins, giving the lens a higher refractive index than the aqueous and vitreous humors. The refractive index of the lens ranges from about 1.406 in the centre to about 1.386 in the peripheral regions due to the fibre cells near the surface having a lower refractive index than deeper cells. 
• The lens is enveloped by a flexible extracellular matrix known as the "capsule," which ensures a polished optical surface and secures the lens in its position within the eye. Attached to the capsule near the lens's equator is a network of non-elastic microfibrils called "zonules," which extend to the ciliary muscle. When the ciliary muscle is at rest, the zonules are under significant tension, causing the lens to flatten. This configuration is optimal for focusing on distant objects in an emmetropic eye (free from refractive errors such as myopia or hyperopia).
• When the ciliary muscle contracts, it moves inward, releasing tension on the zonules and allowing the lens to adopt a more rounded shape, increasing its focal power for near objects.

The Posterior Segment of the Eye 

The Retina 

• The retina is a primarily transparent, thin tissue designed to capture photons of light and initiate image processing by the brain. It has an average thickness of 250 μm and consists of 10 layers. 
• From the surface of the retina to the back of the eye, these layers are: 
• The inner limiting membrane, the nerve fibre layer,   the ganglion cell layer 
• The inner plexiform layer (synapses between ganglion and bipolar or amacrine cells)
• The inner nuclear layer (horizontal, bipolar, amacrine, and inner plexiform cells, along with the retina-spanning glial cells)
• The outer plexiform layer (synapses between bipolar, horizontal, and photoreceptor cells), the outer nuclear layer (photoreceptor cells) 
• The components include the outer limiting membrane, the receptor layer consisting of outer and inner segments of the photoreceptor cells, and the retinal pigment epithelium (RPE). The RPE serves as the outermost layer of the retina. Its primary functions are to provide essential metabolic support to the receptor cells' outer segments and serve as a crucial light absorber, thereby reducing intraocular glare.
• The receptor layer has two types of receptors: rods and cones, named for their shapes. 
• The retina receives nourishment from two sources: the retinal vasculature serves the inner layers, while the choroidal vasculature, located between the RPE and the sclera, serves the metabolically active RPE and outer retinal layers. 

The Vitreous Humor

• The vitreous body is a gel-like structure that fills the posterior portion of the eye. It comprises collagen fibrils within a hyaluronic acid network and forms a clear gel.
• The vitreous body is loosely attached to the retina around the optic nerve head and the macula and more firmly attached at the ora serrata, just behind the ciliary body. 
• The vitreous body's anterior connections help separate the fluids in the anterior and posterior chambers. 
• The attachments around the optic nerve and macula help to keep the vitreous body in place against the retina.
• As we age, the vitreous humor starts to liquefy and contract. This can create pathways for aqueous humour from the front part of the eye (anterior chamber) to move into the back part (posterior chamber).

2.0Mechanism of Vision

• Objects reflect light that enters our eyes.
• Light rays pass through the cornea, the eye's front surface.
• They then traverse through a watery fluid called aqueous humour and enter the pupil.
• The lens, capable of changing thickness, bends the light to focus it onto the retina at the eye's rear.
• Light passes through a dense, transparent fluid known as vitreous humour on its way to the retina, which fills and maintains the eye's spherical shape.
• The light finally reaches the retina, where it triggers electrical impulses.
• These impulses are conveyed to the brain via the optic nerve and interpreted in the visual cortex, enabling us to perceive what we see.

3.0The Visual Pathway

• Each optic nerve extends from the eye, passing through the optic canal in the bony orbit. The optic nerves converge at the optic chiasma. 
• At this point, the ganglion cell axons from the temporal retina of both eyes stay on the same side, continuing into the optic tracts. 
• Specifically, the temporal ganglion cell axons of the right eye travel down the right optic tract, while those of the left eye follow the left optic tract. 
• In contrast, the ganglion cell axons from the nasal regions of the retinas cross over at the chiasma. Thus, only half of the nerve fibers from the eyes cross at this point.
• These nerve fibers proceed along the optic tracts until they reach the lateral geniculate nuclei, also known as the lateral geniculate bodies. 
• Here, the nerve fibers synapse with other neurons that form the optic radiations. These optic radiations then pass through the brain, eventually reaching the visual cortex, also known as the striate cortex. Located at the back of the brain's cortex, this is where visual information is processed.

4.0Accommodation and Focussing

• The ability to change the focal length of a lens by changing its curvature is called accommodation power. Only mammals and birds have this accommodation power in their eyes. 
• In normal conditions, the muscle fibres of the ciliary body remain relaxed, and the lens is stretched by its suspensory ligaments, which causes it to be flat. 
• A flat lens has a longer focal length. As a result, this eye can easily see long, distant objects. 
• To see nearby objects, the ciliary body's sphincter muscles contract, the ciliary body becomes broad, and suspensory ligaments become loose and relaxed. 
• This ligament relaxation causes the lens to become biconvex and reduce its focal length. As a result, animals can easily see nearby objects.

5.0Defects of Eye

Hypermetropia (Farsightedness)

• In this eye defect, a person can see objects placed at a far distance but cannot see objects close to them. 
• This defect is due to the small size of the eyeball or the lens's flatness. In this defect, an image is formed behind the retina. 
• To cure this defect, a person should wear convex lenses in spectacles. 

Myopia or Nearsightedness or Short Sightedness

• In this eye defect, a person can see objects near/close to them but cannot see objects placed at far distance. 
• This is due to the eyeball enlargement or increased lens convexity. 
• To cure this defect, people should wear concave lenses in spectacles.

Night Blindness

• This is due to a deficiency of vitamin A. 
• In this disorder, the synthesis of Rhodopsin is reduced; as a result, the person cannot see in dim light or at night. 

Glaucoma

• If the Schlemm canal is blocked in the eyeball, aqueous humour cannot return to the veins again. As a result, pressure is increased in the eye chambers, the retina is damaged, and the person becomes blind. 

Strabismus

• It is due to the loosening or contraction of any of the six skeletal muscles that give the eyeball the proper position in its orbit. Thus, the eyeball inclines towards one side of the orbit. It is strabismus or squint eyes. 
• The particular muscle may be cured by operation, and this defect is cured.

6.0Eye Muscle and Movement

• The eye consists of both extraocular and intraocular muscles. The extraocular muscles comprise four recti muscles and two oblique muscles. 
• The rectus muscles originate from the common tendinous ring, which is attached at the apex of the orbit near the optic foramina and the medial part of the superior orbital fissure. 
• The 6th cranial nerve innervates the lateral rectus (LR), the superior oblique (SO) by the 4th cranial nerve, and the 3rd cranial nerve supplies all other extraocular muscles. 
• The intraocular muscles include the sphincter pupillae, the dilator pupillae, and the ciliary muscles.

Recti and Oblique Muscles

• The extraocular muscles primarily contribute to maintaining postural tonicity and executing rapid contractions. They are also responsible for rotating the eye around its centre of rotation, located 13.5 mm behind the cornea and 1.6 mm nasal to the geometric centre of the eye.
• The extraocular muscles insert into the sclera at various distances from the limbus.