Anatomy of the Eye
The corneaThe cornea is the clear, transparent 'front window' of the eye through which light enters the eye. It handles about two-thirds of the focusing power of the eye and is critical for good vision. See Info on Eyes – Anatomy. is the transparent 'front window', through which light enters the eye. It has a very powerful ability to focus light onto the retinaThe retina is the receptor of light at the back of the eye. It fulfils the same function as the film in a film camera or the image sensor in a digital camera. The retina translates the images into electrical signals that are sent via the optic nerve to the visual cortex in the brain, where it is interpreted as the images we see. See Info on Eyes – Anatomy. of the eye. It is clear, spherical, and consists of 5 different layers.
The most superficial layer, the epithelium, is highly regenerative and heals quickly following superficial injuries. At the periphery of the cornea, a rim of specialized cells, the limbal stem cells, are responsible for the regeneration and renewal of the corneal epithelium. These cells undergo constant mitosis (or proliferation of cells), and slide over the cornea towards the center to replace cells that are shed due to the blinking action of the eyelids. These cells also form a barrier at the edge of the cornea, to prevent conjunctival epithelial growth onto the cornea.
The second layer of tissue is known as Bowman's layer, followed by the stroma, Descemet's membrane, and the endothelium on the internal aspect.
The corneal stromaThe cornea consists of 5 distinct layers. The third layer forms the main body of the cornea and is known as the corneal stroma. See Info on Eyes – Anatomy. consists, among other elements, of collagen fibres in geometric array. These fibres form a lattice, and are maintained in their highly organized configuration by chemical bonds among themselves.
The 3 central layers provide rigidity and is constructed in such a way that it transmits light. The internal layer of endothelium is a monolayer of cells with a highly specialized function to maintain the dehydrated status of the cornea, which among other unique properties thereof, is responsible for the clarity of the cornea.
The optical function of the cornea is to transmit and refract light (like a lens). It is the most powerful part of the refractive system of the eye, and provides more than two thirds of the eye's total refractive power. Light is focused by it onto the retina at the back of the eye, thereby creating an image of the original object, although inverted and smaller.
The cornea is in many ways a unique anatomical structure. Apart from it being transparent and the most dehydrated tissue in the human body, it also receives the most intense sensory nerve supply. This enables the brain to detect whether the surface of the cornea is compromised by dryness, injury or disease. Whenever the tearfilm breaks up or evaporates, these sensory nerves signal to the brain to effect tear secretion and the blinking reflex, thus maintaining the optical quality of the external surface of the cornea in perfect condition to allow a clear focus.
This is the coloured part of the eye, visible through the cornea. The central dark part in the iris is named the pupil. The pupil regulates the amount of light that enters the eye. The muscles of the iris cause the pupil to constrict or dilate, in order to control the amount of light entering the eye for optimal vision. Situated in the anterior segment of the eye, the iris forms the anatomical boundary between the anterior and posterior chamberThe posterior chamber is the section between the lens and the retina, which is filled with a jelly-like substance called the vitreous body. See Info on Eyes – Anatomy.s, the anterior chamberThe anterior chamber is the section in the eye between the cornea and the lens. See Info on Eyes – Anatomy. being the space in front of the iris extending to the inner aspect of the cornea, and the posterior chamber being the space between the back surface thereof and the front of the lensThe lens fulfils the same role as the lens in a camera. It handles about one-third of the focusing power of the eye and is critical for good vision.See Info on Eyes – Anatomy..
The crystalline lens
This is a biconvex, transparent structure, located behind the pupil, that finally focuses light onto the retina. It roughly contributes one third of the total refractive power of the eye.
The lens is suspended by delicate ligaments, the zonulesZonules are small ligament attachments between the natural lens and the cilliary body that keep the lens in position and transmit the force from the cilliary body to the natural lens, thereby changing its shape to enable changes in focus., which are attached to a surrounding circular muscle, the cilliary body.
Through the action of the cilliary body (transferred by the zonules to the lens), it has the ability to change its shape, and therefore makes the finer adjustments to the focus of the image perceived. This is what is understood by the term 'accommodationAccommodation is the physiological process by which the eye changes focus. A ring of muscle around the lens, called the ciliary body, changes the shape of the lens, which allows the eye to focus at different distances.'.
With advancing age, the lens becomes rigid and increases in size, with the result that the extent of accommodation deteriorates. Focusing on near objects therefore tends to become increasingly difficult as the person approaches the age of 45 - 50 years. This phenomenon is known as 'presbyopiaPresbyopia is an age-related condition. As we age, the natural lenses in our eyes gradually become inelastic and lose their ability to focus on nearby objects and fine print. From the age of 40 it becomes increasingly noticeable and people often jokingly complain that their 'arms are getting too short'. See Info on Eyes – Optics and Refractive Errors - Presbyopia..
When the lens becomes cloudy or opaque, it is named a 'cataractA cataract forms when the natural lens in the eye is, or is starting to become, opaque. If not treated, it can lead to blindness, which in most cases can be treated. See Cataract Centre – Understanding Cataracts.'.
This is the light-sensitive membrane that lines the inner aspect at the back of the eye. It contains photoreceptors (small, light-sensitive cells) that transform light energy into electrical impulses that are interpreted as images by the brain.
The retina consists of two embriologically different layers, the neurosensory layer (the part that is light-sensitive) and the retinal pigment epitheliumRetinal pigment epithelium (RPE) is the pigmented cell layer just outside the neurosensory part of the retina. This specialized tissue has numerous functions, including the nourishment and maintenance of the retinal visual cells. See Info on Eyes – Anatomy. (RPE). This layer of the retina is a pigmented cell layer just to the outside of the neurosensory retinaThe retina is the receptor of light at the back of the eye. It fulfils the same function as the film in a film camera or the image sensor in a digital camera. The retina translates the images into electrical signals that are sent via the optic nerve to the visual cortex in the brain, where it is interpreted as the images we see. See Info on Eyes – Anatomy. that interacts in various ways with the retinal photoreceptors to ensure its health and normal function. While it is firmly attached to the underlying, outer vascular choroidThe choroid is a delicate specialized vascular layer between the outer retina (RPE cells) and the sclera (the outer white 'wall' of the eye). It supplies the retina with oxygen, provides it with nutrients and removes the waste products of cell metabolism. layer, a potential space exists between the RPE and the overlying retinal photoreceptors. When a retinal detachment develops, these two cell layers of the retina (photoreceptors and RPE) separate from each other.
The macula ('yellow spot') is located at the central part of the retina and has the sharpest resolution power. The macula is responsible for vision during daytime, as well as perceiving colour. This part of the retina is critical for reading and the discrimination of fine detail.
The optic nerve
The eye is connected to the brain by means of the optic nerveThe optic nerve transmits the visual signals from the retina to the visual cortex in the brain, where it is interpreted as the images we see. See Info on Eyes – Anatomy.. It collects all the electrical activity from the retina, and transmits it to the visual cortexThe visual cortex is the part of the brain that translates the electrical signals from the retina into images we see. See Info on Eyes – Anatomy. in the brain, where these impulses are processed and perceived as visual images.
The visual cortex
This is the final destination of visual impulses. It is located at the back of the brain, in the occipital lobe, and handles the processing of electrical impulses from the eye. It is here where visual images are perceived and the process of 'vision' occurs. It is interesting to note that you don't actually 'see' with your eyes, but rather with your brain!