Migration and Differentiation of Modern Humans The original Homo sapiens environment in eastern sub-Saharan Africa consisted of tropical and equatorial forests, savannahs, and riverine settings that suited their hunter-gatherer lifestyle. Over thousands of years, evolution had optimized their physical characteristics for sustained occupation of their surroundings. Their range extended from latitude 20 deg North to 35 deg South. Their heads tended to be elongated, to better support heat dissipation, and their skin, eye and hair color was likely dark, to protect against relatively high levels of UV radiation.
In a camera, the image is created on film; in the eye, the image is created on the retina, a thin layer of light sensitive cells at the back of the eye.
The lens of the eye bends, or refracts, light that enters the eye. The cornea, which is a clear, transparent covering in the front portion of the eye also contributes to focusing light on the retina.
Nerve fibers extending back from the retina's nerve cells come together behind the retina to form the optic nerve, a "cable" of nerve fibers connecting the eye with the brain. The optic nerve transmits messages about what we see from the eye to the brain. Like a camera, the human eye controls the amount of light that enters the eye through the lens under various lighting conditions.
One chart that is commonly used for measuring visual acuity is the Snellen chart, which contains letters of the alphabet arranged by line, with each line of letters from the bottom up increasing in size.
The letters on the lowest line are the smallest letters on the chart, and the letter at the top is the largest. When the Snellen chart is used, visual acuity is generally measured with a person seated 20 feet away from the chart. This means that at 20 feet the person can see the line of letters that people with normal sight see from 20 feet.
Why People Need Glasses—Refractive Errors Most often people need to wear eyeglasses to correct blurred or distorted vision caused by imperfections in the eyes' focusing mechanism.
These imperfections, which occur because light entering the eye is not brought into sharp focus on the retina, are known as common errors of refraction or refractive errors.
Refractive errors occur as a result of irregularities in the shape of the cornea, the actual size or shape of the eyeball itself, or the focusing capacity of the lens. Common refractive errors that are fully corrected with eyeglasses or contact lenses are not visual impairments because sight can be corrected to normal.
Nearly every person is likely to have a refractive error at some point in life, especially after age 40, and perhaps need to wear eyeglasses or contact lenses. The common refractive errors are: Myopia Nearsightedness Myopia is blurred vision that occurs when the eye's focusing mechanism brings light to a focus in front of the retina, usually because the eyeball is very elongated in shape.
Eyeglasses or contact lenses correct myopia but do not slow or alter its progression. Hyperopia Farsightedness Hyperopia is blurred vision that occurs when light is focused behind the retina,usually because the eyeball is short or small.
Eyeglasses or contact lenses correct hyperopia but do not slow or alter its progression.
Astigmatism Astigmatism refers to an irregularly curved cornea that distorts the focus of light entering the eye. Generally corrective lenses restore clear vision.
Presbyopia Presbyopia refers to the eye's loss of accommodation, the eye's focusing power and ability to adjust the focus of the eye on the distance between the individual and the object.
People with presbyopia, typically those age 40 and older, experience a progressive inability to focus for near vision viewing as the lens becomes less elastic with age.
Lenses with magnification are used to provide the correction needed. These lenses are commonly referred to as "reading glasses," or necessary magnification can be added to a person's regular eyeglasses as bifocals, or trifocals. Variable focus lenses are also available to correct presbyopia.
Visual Impairment Visual impairment describes vision that cannot be fully corrected by ordinary prescription lenses, medical treatment, or surgery. The term visual impairment includes conditions ranging from the presence of good usable vision, low vision, or to the absence of any sight at all--total blindness.
Many terms are used when people refer to visual impairment. These terms are explained below. Legal Blindness Legal blindness defines visual conditions that, when present, connote eligibility for government or other benefits and services.
Severe Visual Impairment Severe visual impairment is a term used by researchers at the National Center for Health Statistics NCHS to describe visual impairment in people who are unable to read ordinary newsprint even with correction.
This term, used primarily for studying visual impairment in the population, is not used in clinical references by eye care professionals.The entire length of all the eyelashes shed by a human in their life is over 98 feet with each eye lash having a life span of about 5 months. 4. To protect our eyes they are positioned in a hollowed eye socket, while eyebrows prevent sweat dripping into your eyes and eyelashes keep dirt out of your eyes.
Learn more about your eyes and cataracts, including the causes, symptoms, diagnosis, and treatment. But since cataracts and other conditions, A Visual Guide to Cataracts; The Eyes (Human. Prevent the eye from drying out.
The tear film allows atmospheric oxygen to dissolve and diffuse into the cornea A pigmented layer of muscular tissue that gives the eye its colour.
Characteristics of the HUMAN EYE (What can be seen from the outside) Structure. Eyelids. Eyelashes.
Sclera. Cornea. Conjunctiva. Tear gland. Iris. Pupil. Characteristics and Functions.
Thin folds of skin that cover and protect the cornea and conjunctiva from chemical and physical injury. Human eye: Human eye, specialized sense organ in humans that is capable of receiving visual images, which are relayed to the brain.
The anatomy of the eye includes auxillary structures, such as the bony eye socket and extraocular muscles, as well as the structures of the eye itself, such as the lens and the retina.
The eyes of the colour black give their holders a deep sense of spirituality. Their mind is intangible and ethereal. They can be good psychiatrists because the working of the human psychology intrigues them.