Conditions of the Eye
Subtopic:
Eye Anatomy and Physiology
Eye Anatomy
The eye, our organ of sight, is a spherical structure known as the eyeball. This globe-shaped eyeball resides in the front portion of the bony cavity called the orbit, also known as the eye socket. The eyeball is securely positioned within this protective orbital cavity.
The eye houses the specialized sensory cells, or receptors, that detect light and enable vision. It also includes a refracting system, a series of transparent structures that work together to bend and focus incoming light rays precisely onto the receptors located in the retina at the back of the eye.
The Structure of the Eye
The eye is essentially a sphere, with the average adult eye measuring approximately 2.5cm in diameter.
Internally, the eye is divided into two main compartments.
The lens, along with the suspensory ligaments and the ciliary body, acts as a divider between these two internal spaces: the anterior chamber and the posterior chamber.
Anterior chamber. This space is filled with aqueous humour, a clear, watery fluid. This chamber is situated at the front of the lens. Furthermore, this chamber can be subdivided into the anterior and posterior cavities.
Posterior chamber. This chamber is located behind the lens and is filled with vitreous humour (also known as the vitreous body), a gel-like substance that helps maintain the eye’s shape.
The walls of the eye are composed of three distinct tissue layers:
The outer fibrous layer, which provides structural support and includes the sclera and cornea.
The middle vascular layer, also termed the uveal tract, which is responsible for nourishing the eye and comprises the choroid, ciliary body, and iris.
The inner nervous tissue layer, which is the retina, the light-sensitive component of the eye.
The outer fibrous layer
This outermost layer is made up of the sclera and the cornea.
The sclera, commonly known as the white of the eye, forms the tough, outer covering of the posterior and lateral sides of the eyeball. At the front of the eye, the sclera transitions into the cornea, a clear, transparent membrane.
The cornea’s transparency is due to its lack of blood vessels (avascularity) and the highly organized arrangement of its collagen fibers. The surface of the cornea is lined by the conjunctiva, a thin, clear membrane. The cornea is richly innervated with nerve endings originating from the trigeminal nerve, making it highly sensitive. Its firm, fibrous nature helps to maintain the overall shape of the eye. Crucially, light rays must pass through the cornea to reach the retina. The cornea has a convex shape on its anterior surface. A key function of the cornea is refracting (bending) incoming light rays, playing a vital role in focusing the light onto the retina.
The middle vascular layer
The middle vascular layer, also known as the uveal tract, is essential for the eye’s nourishment.
This layer is composed of the choroid, the ciliary body, and the iris. The choroid lines the inner surface of the sclera in the posterior part of the eye. The choroid is characterized by a rich network of blood vessels and typically appears chocolate brown due to the presence of pigment. The ciliary body is a forward extension of the choroid, where it connects to the suspensory ligaments. These ligaments extend from the ciliary body to the lens, holding it securely in place. The ciliary body receives its nerve supply from the 3rd cranial nerve (Oculomotor nerve).
The ciliary body also contains:
Ciliary muscles: These smooth muscles control the shape and thickness of the lens through contraction and relaxation, a process crucial for focusing.
Secretory epithelial cells (Ciliary glands): These specialized cells produce aqueous humour, which provides nutrients to the structures in the anterior chamber.
The iris is the visible, colored part of the eye located at the front. The iris extends anteriorly from the ciliary body, positioned behind the cornea and in front of the lens. It effectively divides the anterior chamber into anterior and posterior cavities. Within the iris are both circular and radiating muscle fibers. The contraction and relaxation of these muscles regulate the size of the pupil, controlling the amount of light entering the eye. The colour of the iris is genetically determined by the number of pigment cells present within it.
NB: The Oculomotor nerve is responsible for innervating the muscles of the iris and the ciliary body, which are classified as the intrinsic eye muscles.
The inner nervous tissue layer
The innermost layer of the eyeball is the retina.
This layer is the light-sensitive (photosensitive) component of the eye. It houses millions of sensory photoreceptor cells. These specialized cells are responsible for the crucial process of converting light into nerve impulses that the brain can interpret. The retina is composed of two distinct layers:
The pigmented outer layer: This layer lies adjacent to the choroid.
The innermost neural layer: This layer is in direct contact with the vitreous humour.
The light-sensitive layer contains the sensory receptor cells known as rods and cones. These cells contain photosensitive pigments that react to light, initiating the conversion of light rays into nerve impulses. Rod cells are predominantly found in the periphery of the retina and function optimally in low-light conditions. They are significantly more numerous than cone cells. Cone cells are concentrated near the center of the retina and are adapted for bright light conditions and color vision. Near the posterior pole of the retina is a region called the macula lutea, also known as the yellow spot. Within the yellow spot is a small area called the fovea centralis, which boasts the highest concentration of cone cells. This is the most critical part of the retina for achieving sharp, high-definition vision. The optic disc, also referred to as the blind spot, is a small area on the retina where the optic nerve exits the eye. This area lacks light-sensitive cells
Parts of the Eye and functions.
Eyebrows – These hair-covered ridges above the eyes serve to protect the eyeball by diverting sweat, dust, and other foreign bodies away from the eyes.
Eyelids – These movable folds act like curtains, providing a physical barrier to prevent injuries to the eye. They meet at the palpebral fissure. The eyelids contain various glands, including sebaceous glands, sweat glands, and accessory lacrimal glands, all associated with the conjunctival tissue.
Conjunctiva – This is a clear, delicate mucous membrane that lines the inner surface of the eyelids and covers the sclera. Being highly vascularised, it plays a crucial role in protecting the eye against infections. It also serves as a physical barrier and produces mucin (from goblet cells), which helps to lubricate the eyeball.
Sclera – This is the tough, white fibrous tissue forming the majority (approximately 5/6) of the eyeball’s outer layer. It protects the inner structures of the eye and is essential for maintaining the shape of the eyeball. It also serves as a pathway for blood vessels and nerves entering and exiting the eye.
Cornea – This clear, transparent dome-shaped structure covers the anterior 1/6 of the eyeball. It is composed of 5 layers and its functions include: protection of the eye due to its exposed position, acting as a significant refractive media of the eye (bending light), and helping to prevent aqueous humor from leaking out of the anterior chamber.
Anterior chamber – This space is located just behind the cornea. Its functions include acting as a refractive media, helping to maintain the shape and structure of the eyeball, and providing a fluid environment to bathe and nourish the nearby structures. It is also involved in the production and flow of aqueous humor.
Aqueous Humor is a fluid secreted by the epithelial cells of the ciliary body. It flows from the ciliary body through the suspensory ligaments into the posterior chamber, then through the pupil into the anterior chamber. From the anterior chamber, it drains through the trabecular meshwork into the canal of Schlemm (scleral venous sinus) and eventually enters the general circulation.
Iris – This is the thin, visible, contractile, and coloured part of the eye, featuring a central opening called the pupil. It divides the anterior segment of the eye into the anterior and posterior chambers. The iris controls the amount of light entering the eye by adjusting the pupil’s size and also plays a role in accommodation (adjusting focus).
Ciliary body – This structure is continuous with the choroid (the middle layer of the eyeball). It is responsible for suspending the lens via suspensory ligaments, which is important for accommodation, and also for the production of aqueous humor.
Choroid – This is the soft, brown, and highly vascularized part located behind the retina. Its primary function is to nourish the retina with its rich blood supply.
Lens – This is a transparent, highly elastic, biconvex structure situated immediately behind the pupil and in front of the vitreous body. Its thickness is controlled by the ciliary muscle via the suspensory ligaments, enabling focusing. Its functions include acting as a refractive media to bend light and also to absorb ultra-violet rays.
Retina – This is the innermost layer of the eyeball where images are formed. It contains key features like the macula, optic disk, and the photoreceptor cells, rods and cones. It consists of two main layers: the epithelial layer and the nervous layer. The retina absorbs light, stores and releases vitamin A, which is crucial for vision.
Vitreous body – This is a transparent, jelly-like substance that fills the space behind the lens. It helps to maintain the shape of the eyeball and also acts as a refractive medium.
Blood and Nerve supply to the eye
The primary blood supply to the eye is provided by the ciliary arteries and the central retinal artery. These are branches originating from the ophthalmic artery, which itself is a branch of the internal carotid artery, a major vessel supplying blood to the brain. Venous drainage, the removal of deoxygenated blood, is facilitated by the central retinal vein. Importantly, these blood vessels run alongside the optic nerve, sharing a common pathway to and from the eye.
The main nerve supply to the eye is through the optic nerve, also known as the 2nd cranial nerve. This crucial nerve is responsible for transmitting visual information to the brain. The individual retinal nerve fibres originate within the retina, the light-sensitive layer of the eye. These numerous fibres converge, coming together at the optic disc, a specific area on the retina, to form the optic nerve that carries visual signals to the brain for processing.
Physiology of Sight
The process of seeing begins when light rays emanating from objects in our environment enter the eye. These light rays are bent (refracted) as they traverse the various transparent structures, or clear media, of the eye. The differing densities of these media cause the light to change direction, ultimately focusing the rays precisely onto the retina.
Within the eye, the biconvex shape of the lens plays a vital role in this focusing process. The lens refracts the incoming light rays, bending them further to create a sharp image on the retina. However, before light reaches the lens, it first passes through the cornea, the eye’s clear outer layer. The cornea also contributes significantly to the refractive power of the eye, initiating the bending of light.
The lens is elastic, meaning it has the remarkable ability to change its shape. This change in shape alters the degree of light refraction, allowing the eye to achieve clarity of focus on objects at varying distances. This dynamic adjustment of the lens is known as accommodation.
Accommodation is a necessary function that enables us to see objects clearly regardless of their proximity. Without it, our vision would be consistently blurry for either near or far objects. The normal eye, when in a relaxed state, is set to bring rays of light from distant objects into sharp focus on the retina without any active adjustment.
However, for clear focusing on near objects, a coordinated, automatic response known as an autonomic reflex comes into play. This reflex involves three key actions: accommodation, miosis, and convergence, working together to bring close objects into sharp view.
Accommodation. This process describes the way the eye increases the bending power, or refractive power, of its lens to bring light rays from near objects into sharp focus on the retina. To achieve this, the ciliary muscle within the eye contracts, which in turn causes the lens to alter its shape, becoming more bulged. This increased curvature enhances its convexity and thus its refractive power, enabling clear vision at close range.
Miosis. This term refers to the constriction of the pupils. It is a reflexive action that naturally accompanies accommodation. By narrowing the pupil, miosis ensures that light rays are concentrated and pass more centrally through the lens. This minimizes peripheral light rays, which can be distorted, and optimizes the focusing of the image directly onto the retina.
Convergence (movement of the eyeballs). This action involves the coordinated, bilateral movement of both eyes simultaneously inward. This inward turning is essential in order to focus on a nearby object, such as when you are intentionally focusing on the tip of your own nose. This coordinated movement ensures that the image of the close object falls on the corresponding points on both retinas, preventing double vision.
The light-sensitive layer in the retina, which contains the specialized sensory photo receptor cells (rods and cones), performs the crucial task of converting the focused light rays into nerve impulses.
These nerve impulses are then transmitted along the visual pathways to the visual area located in the occipital lobe of the cerebrum, the brain’s visual processing center.
Here, in the visual cortex, the electrical signals are interpreted as the sensation of light and form. This raw sensory input is further processed into recognizable images of objects. These images are then assigned meaning through interaction with other areas of the brain. This interpretation heavily relies on information stored as memory within the association areas of the brain, allowing us to recognize and understand what we are seeing.
NB: It’s important to note that the images refracted onto the retina are actually upside down. However, the brain adapts to this inversion very early in life, learning to interpret these inverted signals so that objects are perceived as upright. This neural adaptation happens seamlessly, and we are typically unaware of this initial inversion.
Accessory Organs of the Eye
The eye, being a delicate sensory organ, benefits from several protective structures.
(1). The eye brows:
These are composed of numerous hairs that grow from the skin along the supraorbital margins of the frontal bone, which forms the upper edge of the eye socket. The primary function of the eyebrows is to protect the eye by physically intercepting sweat, dust, and other foreign bodies, preventing them from easily entering the eye.
(2). Eyelids and eyelashes:
These are two movable folds of tissue, positioned above and below the front of each eye. Within the eyelids are sebaceous glands, some of which empty their secretions into the hair follicles of the eyelashes. The eyelids contain two key muscles responsible for their movement: the Levator palpebrae superioris, which functions to raise the upper eyelid, and the Orbicularis oculi, which is responsible for closing the eyelids. The hair growing on the edge of the eyelids is known as eyelashes, which provide a physical barrier against debris. The inner surface of the eyelids is lined with a mucous membrane called the conjunctiva. This lining is a thin, transparent layer that coats the inner surface of the eyelid and also extends to cover the surface of the eyeball. The portion of the conjunctiva lining the eyelids is characterized by a highly vascularized columnar epithelium, indicating a rich blood supply. In contrast, the corneal conjunctiva, which covers the cornea, has an avascular stratified epithelium, meaning it lacks blood vessels. The points where the upper and lower eyelids meet at the corners of the eye are termed the medial canthus (closer to the nose) and the lateral canthus (further from the nose). Along the edges of the eyelids, known as the eyelid margins, are numerous sebaceous glands, specifically modified as meibomian glands. These glands secrete an oily substance called meibum, which is spread across the conjunctiva with each blink. This oily layer helps to delay the evaporation of tears, keeping the eye moist. A fundamental function of the eyelids and eyelashes is to protect the eye from injury by acting as a physical shield. The act of blinking, occurring approximately every 3 to 7 seconds, serves to distribute tears and oily secretions evenly over the cornea, which is essential for preventing the drying of the eyeball.
(3). Lacrimal apparatus:
The lacrimal apparatus is the system responsible for the production and drainage of tears from the front of the eyeball. This system includes:
Lacrimal gland and its ducts: The primary site of tear production.
Lacrimal canaliculi: Two small channels, the superior and inferior lacrimal canaliculi, located near the caruncle of the eye, that collect tears.
Lacrimal sac: A small reservoir where tears accumulate before draining further.
Nasolacrimal duct: The channel through which tears drain into the nasal cavity.
Each eye has a single lacrimal gland situated behind the supraorbital margin. Lacrimal glands are classified as exocrine glands, meaning they secrete substances onto an epithelial surface by way of ducts. They produce tears, a complex fluid composed of water, mineral salts, antibodies, and bactericidal enzymes like lysozyme. The tears exit the lacrimal glands through several small ducts and then spread across the front surface of the eye, underneath the eyelids, moving towards the medial canthus. At the medial canthus, the tears drain into two small openings called puncta, which are the entrances to the lacrimal canaliculi. These canaliculi are positioned one above the other, separated by a small, reddish nodule called the caruncle. From the canaliculi, the tears flow into the lacrimal sac, which is the expanded upper portion of the nasolacrimal duct. When foreign bodies or other irritants enter the eye, the secretion of tears is significantly increased, and the conjunctival blood vessels dilate as part of the inflammatory response. Tear production is also elevated during emotional states, such as when crying or laughing. Excess tears are removed from the eye via the lacrimal apparatus, flowing into the lacrimal sac and subsequently into the nasolacrimal duct, which empties into the nasal cavity.
The lacrimal apparatus plays several crucial roles. It provides a fluid that fills the conjunctival sac, the space between the inner surface of the eyelids and the outer surface of the eyeball. This fluid is a mixture of tears and the oily (meibum) secretions from the meibomian/tarsal glands. This mixture is spread across the cornea with each blink, effectively washing away irritants like dust. The tear fluid also provides oxygen and nutrients to the avascular corneal conjunctiva and facilitates the removal of waste products. The bactericidal enzyme lysozyme present in the tears helps to protect the eye by preventing microbial infection. The oiliness of the tear fluid helps to delay its evaporation, preventing the drying and friction of the conjunctiva. Additionally, the fluid prevents the eyelids from sticking together during sleep. The main functions of tears or tear fluid include: lubricating the eye to facilitate the exchange of oxygen and carbon dioxide, creating an optically smooth cornea surface for clear vision, cleansing the eye with the bactericidal enzyme lysozyme, and preventing the conjunctiva from drying out.
4). Extrinsic muscles :
These muscles, also known as the extraocular muscles, are responsible for controlling the movement of the eyeball. There are 6 extrinsic muscles attached to the outer surface of each eyeball:
Medial rectus: This muscle functions to rotate the eyeball inwards, towards the nose (adduction).
Lateral rectus: This muscle is responsible for rotating the eyeball outwards, away from the nose (abduction).
Superior rectus: This muscle’s primary action is to rotate the eyeball upwards (elevation). It also contributes to intorsion (rotating the top of the eye towards the nose) and adduction.
Inferior rectus: This muscle functions to rotate the eyeball downwards (depression). It also contributes to extorsion (rotating the top of the eye away from the nose) and adduction.
Function of the muscles
The primary function of these muscles is to enable a wide range of precise and coordinated movements of the eyeball. This flexible movement, achieved through the coordinated action of these muscles, allows us to direct our gaze in various directions. These movements are crucial for our ability to see objects in all directions within our field of vision. Furthermore, by enabling us to quickly shift our gaze, these muscles also play a protective function, allowing us to react to potential threats in our environment, thus protecting the eye and the whole body from potential harm.