Human Eye: Anatomy, Physiology, and Common Diseases
The Human Eye: Anatomy, Physiology, and Common Diseases
The senses are sight, hearing, taste, smell, and touch. An external stimulus triggers a sensory impression and is produced in the nervous system and the recipient organ.
The Sense of Sight
Sight is measured primarily by two parameters: the visual field (the portion of space that may be directed towards maintaining a fixed gaze) and visual acuity (the ability to distinguish two separate points that are very close together).
Anatomy and Physiology of the Eye
The eye consists of the eyeball, with its means of suspension, its motor apparatus, irrigation systems, and the cornea.
The Eyeball
The eyeball is located in a cavity of the face called the orbits. It is made up of three membranes: the sclera, the choroid, and the retina.
- The Sclera: This is a rigid, white covering that shapes the eye. It continues in front of the cornea and behind the optic nerve sheath.
- The Choroid (Uvea): This is a membrane that contains irrigated retinal vessels, the colored portion of the eye, and the central black pupil.
- The Iris: This muscle contracts the pupil (miosis) and another dilates it (mydriasis), regulating the amount of exterior light that reaches the retina.
The Process of Accommodation
The ciliary muscle shrinks and relaxes, changing the curvature of the lens. This allows for focusing on distant and close objects as necessary.
The Retina
The retina contains receptor membranes that receive visual stimuli; cones and rods.
- Cones: These are responsible for color vision.
- Rods: These perceive light.
Pathway of Light to the Retina
To arrive at the retina, light rays must traverse the cornea, the aqueous humor, the lens, and the vitreous body.
- Cornea: The transparent front of the sclera that concentrates light rays.
- Aqueous Humor: A liquid like water that fills the space between the lens and the cornea.
- Lens: Another lens that focuses images on the retina. It is elastic and varies its curvature depending on the distance to focus images sharply.
- Vitreous Body: A gelatinous mass that fills the eyeball behind the lens.
Auxiliary Portions of the Eye
- Ocular Muscles: The eye is moved by six muscles arranged radially.
- Lacrimal Irrigation: Keeps the cornea moist, which is essential for maintaining transparency.
- Protection of the Eyeball: The eyelids are reinforced by several layers of fibrous skin.
The Nervous Impulse
The nervous impulse originates in the receptors of the retina and is transmitted through the optic nerve.
Common Eye Diseases
- Myopia (Nearsightedness): The eyeball is too long, causing distant objects to appear blurry. It is corrected with concave lenses.
- Hyperopia (Farsightedness): The eyeball is too short, causing nearby objects to appear blurry. It is corrected with convex lenses.
- Astigmatism: The corneal curvature is not uniform, causing horizontal lines to appear blurry. It is corrected with cylindrical lenses.
- Strabismus: Medial deviation of the eye (converging) or lateral deviation (diverging). It is corrected with visual exercises.
- Presbyopia (Eyestrain): Loss of lens elasticity with age, preventing the focusing of nearby objects. It is corrected with convex lenses.
- Color Blindness: An inherited disease where colors cannot be differentiated. There is no treatment.
- Cataracts: The lens becomes opaque due to aging. It is corrected with surgery.
